Springer Handbookof Nanomaterials

Springer Handbook providesa concise compilation of approvedkey information on methods ofresearch, general principles, andfunctional relationships in physicaland applied sciences. The world’sleading experts in the fields ofphysics and engineering will be as-signed by one or several renownededitors to write the chapters com-prising each volume. The contentis selected by these experts fromSpringer sources (books, journals,online content) and other systematicand approved recent publications ofscientific and technical information.

The volumes are designed to beuseful as readable desk referencebook to give a fast and comprehen-sive overview and easy retrieval ofessential reliable key information,including tables, graphs, and bibli-ographies. References to extensivesources are provided.

123

HandbookSpringerof Nanomaterials

Robert Vajtai (Ed.)

With 685 Figures and 64 Tables

EditorRobert VajtaiRice UniversityDepartment of Mechanical Engineering and Materials Science6100 Main MS-321Houston, TX 77005-1827USA

ISBN: 978-3-642-20594-1 e-ISBN: 978-3-642-20595-8DOI 10.1007/978-3-642-20595-8Springer Dordrecht Heidelberg London New York

Library of Congress Control Number: 2013942548

c© Springer-Verlag Berlin Heidelberg 2013This work is subject to copyright. All rights are reserved, whether the wholeor part of the material is concerned, specifically the rights of translation,reprinting, reuse of illustrations, recitation, broadcasting, reproduction onmicrofilm or in any other way, and storage in data banks. Duplication of thispublication or parts thereof is permitted only under the provisions of theGerman Copyright Law of September 9, 1965, in its current version, andpermission for use must always be obtained from Springer. Violations areliable to prosecution under the German Copyright Law.The use of general descriptive names, registered names, trademarks, etc. inthis publication does not imply, even in the absence of a specific statement,that such names are exempt from the relevant protective laws and regulationsand therefore free for general use.

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61/3180/YL 5 4 3 2 1 0

V

Foreword

Nanomaterials are based on structures with character-istic features on the scale of nanometers. This size issmall if we compare with normal things around us, butit is not particularly small on the atomic scale. In fact,distances between individual atoms are typically a tenthof a nanometer (an Ångström), so a piece of a mater-ial with a side of a nanometer may contain hundreds oreven a thousand atoms. Therefore a nanomaterial usu-ally has some resemblance to a bulk material basedon the same atoms, but the normal material has beenmodified to reach superior properties such as highermechanical strength, different optical and magnetic per-formance, permeability to a fluid, or something else.Thus nanomaterials may allow us to obtain propertiesthat were previously impossible to achieve, impracticalto manufacture, or too expensive for use on a scale largeenough to be significant in daily life.

Among the general public, and even the scientificcommunity, nanomaterials are widely perceived as newin many different way – newly invented, newly used byhuman cultures, and newly studied.

In fact, nanomaterials are not new at all. Natureitself is filled with nanofeatures that have evolved inbiological systems, one well known example beingmoths’ eyes with nanostructured surfaces that provideantireflection and allow efficient use of feeble light.

Looking at history, we can also see that human be-ings have been using nanomaterials of various sorts fora very long time. Let us take three examples: nanocar-bons, nanometals, and nanoceramics.

Nanocarbons can be created in abundance on thenanometer scale when organic matter burns. Such car-bon nanoparticles were used by humans as far back asforty thousand years ago to depict and decorate. Theparticles were mixed with fat and used for painting inthe caves of Altamira and Lascaux in Spain and France,to mention two especially striking and well knowncases. This kind of carbon, in principle, is also an es-sential ingredient in ink and printing paste, and it wasused by monastic scribes and by Gutenberg and his fol-lowers to make texts of explosive cultural significanceand stunning beauty.

Nanometals have also been utilized for thousands ofyears. An example is the world famous Lycurgus glasscup, now in the holdings of the British Museum. This

Prof. Claes-GöranGranqvist

Department ofEngineering SciencesSolid State PhysicsUppsala UniversitySweden

cup, which was probably created inRome during the 4th century AD,contains embedded nanoparticles ofgold and silver. Because of theseparticles, the cup normally seems tobe a light green color, but it becomesruby red when light is shone throughit. The Lycurgus cup is a wonder ofcraftsmanship from Antiquity, and itis based on nanotechnology.

Finally, let’s consider nanoce-ramics. The world’s most widelyused artificial material is the nanoce-ramic cement, which was used ex-tensively by the Romans in con-structing buildings, baths and aque-ducts. Furthermore, recent archaeo-logical discoveries indicate that theRomans were not the first, thatthe Macedonians were using cementcenturies earlier.

Even research on nanomaterials is not as new as itseems. The term apparently began appearing in the titlesof scientific publications only 15 years ago. But today’snano was the subject of an older literature under theterm ultrafine.

As the examples above indicate, nanomaterials arewell rooted in the past. But they are also very much ofthe future. Let us consider a few specific examples.

Nanocarbons, used for cave painting and the print-ing of the Gutenberg Bible, are very much in focustoday, in the forms of fullerenes, nanotubes and nanodi-amonds, all of which offer a multitude of possibilitiesfor future technology. Two-dimensional carbon in theform of graphene has unique properties directly basedon quantum physics, and it may have important ap-plications in transparent electronics and elsewhere.Graphane, its hydrogenated cousin, is exciting in itsown right.

Nanometals, employed by the Romans to create theamazing Lycurgus glass cup, are the basis today formanifold applications, including thermal collectors thatharness the sun’s energy and innovative plasmonic solarcells. Indeed, plasmonics is becoming a household wordbecause of its relevance for light-emitting diodes, sen-

VI

sors and catalysts for chemical reactions, just to mentiona few technologies.

Thus many aspects of nanomaterials are indeed trulynew and are the subject of intense worldwide interest intoday’s academic and industrial research laboratories.This Handbook, which is a testimony to this growingbody of knowledge, presents welcome and authoritativesurveys over nanocarbons, nanometals and nanoce-ramics in its first three parts. Other sections covernanocomposites and nanoporous materials, as well asorganic and biological nanomaterials. Applications andimpacts are discussed at the end, together with impor-tant questions of toxicology, hazards and safety. These

issues are of great importance. We should remember theterrible impact that asbestos – in fact, a natural nano-material – had on human health before it was widelybanned. We certainly do not want to discover one daythat, in our quest for new materials to solve techno-logical problems, we have unleashed another dangerousnanomaterial into the world.

The editor and authors are to be congratulated onthe successful completion of this Handbook of Nano-materials. It will surely be a work of great and lastingimportance for the scientific community.

Uppsala, November 2012 Prof. Claes G. Granqvist

VII

Foreword

It has been more than a decade since President Bill Clin-ton talked about the promise of nanotechnology andthe importance of increasing investments in nanoscalescience and engineering research in a speech at the Cal-ifornia Institute of Technology on January 21, 2000.In his remarks, the President recalled Richard Feyn-man’s American Physical Society talk there in 1959.The following week, in his State of the Union Address,President Clinton announced his 21st Century ResearchFund, a $3 billion budget increase, which included themultiagency national nanotechnology initiative (NNI).The first year’s budget allocation to NNI was close tohalf a billion US$, nearly doubling what the agencieshad been spending on nanoscale research; and with thecontinuous support of succeeding administrations thebudget quadrupled in a decade. This strong federal sup-port, initially based on the promise of a revolutionarynew technology, was justified by steady scientific andtechnological advances at the nanometer scale and bythe growth of commercial applications, especially inbiotechnology and nanoelectronics, offering new waysto tackle disease and new industrial tools and toys.As President Clinton’s former science advisor, I amconfident that he is as pleased with the progress innanotechnology as are all of us – inside and outsidegovernment – who worked with him to develop andimplement the NNI.

One way to define nanotechnology, perhaps, is thatit is the knowledge and engineering (design and con-trol) of physical, chemical, and biological systems atthe nanometer (10−9 m) scale – from the size of indi-vidual molecules to dimensions of the order 100 nm.Nanotechnology is, by its nature, a field of synthesisand synergy often requiring physics, chemistry, biology,and almost all areas of engineering in the performanceof research and engineering design, for example invent-ing and optimizing the tools needed to synthesize andmanipulate matter at the nanometer scale. As with othernew fields, rapid advances in nanotechnology have ledto specialization into subdisciplines, one of the mostnatural and important being nanomaterials.

Nanomaterials science and engineering includes theproduction, properties, and applications of materials atthe nanometer scale; it is a part of nanotechnologyand at the same time, evidently, a subfield of materials

Prof. Neal Lane

Malcolm Gillis UniversityProfessor,Department of Physicsand Astronomy,Senior Fellow,James A. Baker III Institutefor Public PolicyRice UniversityHouston, Texas

science. The main goal of ma-terials science – macroscopic andnanoscale – is providing new andimproved building blocks for engi-neers in all fields. That said, nano-materials science has distinct fea-tures compared to the more maturescience and engineering of macro-scopic materials, the most salientbeing its revolutionary nature. Newmaterials and groups of materialswith surprising properties continueto be discovered – graphene andtopological insulators are two exam-ples from the recent past. As withall exploration at the frontiers ofknowledge, it is impossible to pre-dict what discoveries will be madeor how those discoveries might leadto applications, commercial or oth-erwise. But the history of scienceand technology suggests that someof those advances will surpass all our expectations. Al-ready we are seeing the benefits of nanotechnology incomputers and telecommunication devices, computerchips and sensors in automobiles, electric car batter-ies, medicines and sun creams, tablecloths and socks,tennis rackets, boats, golf clubs – and more. Giventhe likelihood that ongoing research will yield manymore nanomaterials, with surprising properties and, atthe same time, the continued exponential growth in thenumber of applications, it seems clear that nanomateri-als will, at some level, transform most aspects of ourlives. It is not too much of a stretch to suggest thatPresident Clinton’s policy decision to set up the NNI,which has supported thousands of scientists and en-gineers working in the field, has indeed helped moveus closer to realizing Feynman’s prediction – or, per-haps we should say his vision – of a revolutionary newtechnology. In the world of nanomaterials there is stillplenty of room at the bottom to use Richard Feynman’sfamous words.

A handbook, by one definition, is a compilationof knowledge about a particular field, collected intoa single volume publication that is convenient to use as

VIII

a ready reference. Since nanomaterials science can nowbe considered a self-sufficient discipline, a handbook isappropriate and timely. This new Springer Handbookof Nanomaterials targets several audiences: researchersworking in industry or academia, as well as gradu-ate students studying related fields. The organizationof the book follows the usual classification schemeof macroscopic materials science, with information ofa materials group – e.g., metals – collected together;

other aspects can be followed easily by using the well-developed index.

Putting together a handbook in a new field isa formidable challenge. I would like to congratulate theeditor and all of the authors who collaborated to plan,collect materials, and write this important groundbreak-ing Springer Handbook of Nanomaterials.

Houston, January 2013 Neal Lane

IX

Preface

Those who control materials, control technology, statedEiji Kobayashi, Senior Advisor of Panasonic Corpora-tion, explaining the importance of materials science andengineering. I would translate this quote to those whocontrol nanomaterials control nanotechnology; and,considering the effect of the development of nanomate-rials and nanotechnology on our global infrastructure,it is not too bold to state that those control technol-ogy at large, too. Nanomaterials have a determinant rolein many of advanced products around us. Stamp-sizedsound recording devices, modern passenger and fighterjets, spaceships and space stations, extreme tall build-ings and long bridges, none of these could be createdwithout these marvelous materials. As one could notforesee 50 years ago, the fast development that providedthe opportunity for these objects to be realized, now wecannot imagine our life without them.

The editor considers materials science as the knowl-edge of structure; properties of materials predicted orexplained with the help of this knowledge; experimen-tal and theoretical tools designed and established forpreparing, characterizing and modifying processes, andlast but not least showing application possibilities ofthe resulted materials. After defining nanomaterials wecan simply transpose this description for nanomaterialsscience. Materials are considered nanomaterials whentheir structure, processing, characterization or appli-cation differ from the macroscopic materials and thisdifference relates to the – normally sub-100 nm – fea-ture size. The description of the nanomaterials in thisSpringer Handbook follows the thorough but conciseexplanation of the synergy of structure, properties, pro-cessing, and applications. Specifically, our aim was topoint out the distinction between the properties of bulkand nanomaterials and the reasons for these differences.

To fulfill these goals, we provide a balanced reportof the literature of each materials group. The formatfollows the well-established structure of the SpringerHandbooks with chapters as the basic units that are or-ganized into several groups. In each chapter, authorscover materials of their expertise, however, they focusnot only on their own work, but report the interestingand important efforts in the community, establishing

a balance between references and scientific results re-ported in tables and figures. We describe nanomaterialsin textbook style for newcomers, encyclopedia-like ele-ments and – to follow the fast-space of new results –review or research papers for the experienced reader.Beyond scientific and moral correctness we also lookfor clarity by concise and easy-to-follow text, well-designed and clear figures which were all professionallydrawn by graphics designers.

The book is divided in Parts A to G and cov-ers carbon-based nanomaterials: fullerenes, nanotubes,nanofibers and nanodiamond, noble and common met-als and alloys, ceramic materials, crystalline and glassyoxides and other compounds; composites, hybrid struc-tures and solutions as well as porous metals, ceramicsand silicon; organic and bio-nanomaterials, bones andfibers and select applications, respectively. This higherlevel structure conforms to the macroscopic classifica-tion of materials and it is composed of chapters. Eachchapter is self-consistent and builds up of similar parts,history, definitions, production of the given materials,properties, and applications. All of these parts are richlyillustrated and consist of a balanced ratio of importantbasics and recent results.

My pleasant obligation is to thank all of the helpI received in planning and implementing the handbook.First of all, I need to acknowledge the diligent work ofthe authors in developing the chapters which involvesmore effort than a review paper, and the reward is notso immediate and evident. Their expertise, energy andtime are greatly appreciated. I also would like to thankthe advices and help of my colleagues at Rice Uni-versity and at Rensselaer Polytechnic Institute; as wellas Professors Thomas F. George, Bob Curl, PhaedonAvouris, Li Song and Jinquan Wei for keeping con-tact with many authors. The great workmanship of theSpringer publishing team and the continuous support ofthe managing editors Mayra Castro and Werner Skolautare also appreciated. I also need to thank my colleaguesand friends, Laszlo B. Kish, Claes-Goran Granqvist,Pulickel M. Ajayan and Richard W. Siegel that thecollaboration with them oriented me to nanomaterialsscience. Last, but not least, I thank for the help and pa-

X

tience of my wife, Agnes, without her I would have notbeen able to finish this job.

I wish the reader a pleasant and beneficial time whenusing the Springer Handbook of Nanomaterials, and

I hope that it serves as a frequently opened referencework.

Houston, November 2012 Robert Vajtai

XI

About the Editor

Robert Vajtai is a Faculty Fellow at Rice University, Houston, Texas, in the Depart-ment of Mechanical Engineering and Materials Science. His expertise covers synthesis,processing, characterization of physical and chemical properties of new, advanced ma-terial forms and structures. More specifically Dr. Vajtai’s interests are in nanostructuredmaterials, nanocomposites and nanomaterials; as well as their applications in thermalmanagement, energy storage, microelectromechanical systems, sensors and electronicdevices.

Dr. Vajtai received his scientific education in physics and his Ph.D. degree in solid-state physics from the University of Szeged (then named Jozsef Attila University),Hungary. From 1987 to 2002 he was a faculty member of the Department of Experi-mental Physics at the University of Szeged, Hungary. He was rewarded by the BolyaiFellowship of the Hungarian Academy of Sciences for 1999-2000. He spent sabbati-cals as a Fellow of the Swedish Institute in TheglosseintragAAngstrom Laboratory in Uppsala, Sweden, in the years 1998 and 1999;as an Eötvös Fellow at the EPFL in Lausanne, Switzerland in 1995/1996 and visitedthe Max Planck Institute in Göttingen, Germany, in 1993 via a Max Planck Fellowship.Before moving to Rice University in 2008, Dr. Vajtai spent eight years at the Rensse-laer Polytechnic Institute, Troy, New York, where he was a Laboratory Manager at theRensselaer Nanotechnology Center managing the carbon nanotechnology laboratories.

Dr. Vajtai started his research as a physicist studying laser-metal interaction,melting and oxidation of refractive metals and the nonlinear behavior of the far-from equilibrium processes and systems. Later he developed methods for pulse-probespectroscopy of biomaterials as well as OH radicals used for the study of organiccontamination of the atmosphere by airborne LIDAR systems. His research in ma-terials science started with the synthesis of nanometals and nanosized oxides for thedevelopment of sensors. This lead to a new method for the preparation of germaniumnanoparticles for building inverse opals used in infrared optical sensing. His most sig-nificant contribution is related to the synthesis of different forms of nanocarbons suchas carbon nanotubes, graphene and macroscopic systems designed and built from thesecarbon allotropes, e.g., electromechanical parts and nanotube wires. Recently, his inter-est extended to various atomically thin layers, hexagonal boron nitride, transition-metaldichalcogenides and oxides.

He has more than 145 journal publications in peer reviewed scientific journals andhe delivered numerous invited, keynote and plenary lectures on the topic.

Dr. Vajtai is a passionate teacher, he lectured physics, thermodynamics and elec-trodynamics courses with hundreds of experimental demonstrations; introductory andadvanced courses of materials science. He received several mentoring awards, amongthose the Siemens-Westinghouse Mentoring Award.

Robert Vajtai is a Faculty Fellow in the Department of Mechanical Engineering &Materials Science at Rice University. He received his undergraduate and Ph.D. degreesfrom the University of Szeged, then named Jozsef Attila University, Hungary.

XIII

List of Authors

Maya Bar-SadanBen-Gurion UniversityDepartment of ChemistryBe’er Sheba , Israele-mail: barsadan@bgu.ac.il

Giovanni BarcaroItalian National Research CouncilInstitute for the Physical and Chemical ProcessesVia Giuseppe Moruzzi 156124 Pisa, Italye-mail: barcaro@ipcf.cnr.it; giobarc@gmail.com

Paolo BettottiUniversity of TrentoDepartment of Physics, Nanoscience Laboratoryvia Sommarive 1438123 Povo, Italye-mail: bettotti@science.unitn.it

Alfredo CaroLos Alamos National LaboratoryMaterials Science and Technology DivisionLos Alamos, NM 87544, USAe-mail: caro@lanl.gov

Eunhyea ChungKorea Institute of Science and Technology (KIST)Center for Water Resource CycleHwarangno 14-gil 5, Seongbuk-guSeoul 136-791, Koreae-mail: echung@kist.re.kr

Suzanne A. Ciftan HensITC/International Technology Center8100 Brownleigh RoadRaleigh, NC 27617, USAe-mail: shens@itc-inc.org

Vicki L. ColvinRice UniversityOffice of Research, Chemistry6100 Main Str.Houston, TX 77005, USAe-mail: colvin@rice.edu

Rodolfo Cruz-SilvaShinshu UniversityResearch Center for Exotic NanocarbonsWakasato380-8553 Nagano, Japane-mail: rcruzsilva@shinshu-u.ac.jp

Pratap Kumar DeheriShayoNano Singapore Pte Ltd.609969 Singaporee-mail: dehe0001@e.ntu.edu.sg

Libo DengUniversity of ManchesterSchool of MaterialsOxford RoadManchester, M13 9PL, UKe-mail: libo.deng@manchester.ac.uk

Yi DingShandong UniversitySchool of Chemistry and Chemical Engineering27 South Shan Da RoadJinan, 250100, Chinae-mail: yding@sdu.edu.cn

Huanli DongChinese Academy of SciencesInstitute of Chemistry,Key Laboratory of Organic SolidsZhongguancun North First Street 2Beijing, 100190, Chinae-mail: dhl522@iccas.ac.cn

Mildred S. DresselhausMassachusetts Institute of TechnologyPhysics; Electrical Engineering77 Massachusetts AveCambridge, MA 02139, USAe-mail: millie@mgm.mit.edu

XIV List of Authors

Dimple P. DuttaBhabha Atomic Research CentreChemistry DivisionMumbai, 400085, Indiae-mail: dimpled@barc.gov.in

Hellmut EckertUniversity of Sao PauloDepartment of PhysicsAv. Trabalhador Saocarlense 400Sao Carlos, SP 13566-590, Brazile-mail: eckert@ifsc.usp.br

Morinobu EndoShinshu UniversityResearch Centre for Exotic Nanocarbons380-8553 Nagano, Japane-mail: endo@endomoribu.shinshu-u.ac.jp

Adam W. FeinbergCarnegie Mellon UniversityDepartment of Biomedical Engineering700 Technology Dr.Pittsburgh, PA 15219, USAe-mail: feinberg@andrew.cmu.edu

Alessandro FortunelliIPCF Consiglio Nazionale delle Ricerche (CNR)via Giuseppe Moruzzi 156124 Pisa, Italye-mail: alessandro.fortunelli@cnr.it

Yogeeswaran GanesanIntel Corporation5200 NE Elam Young ParkwayHillsboro, OR 97124, USAe-mail: yogiganesan@gmail.com

Wei GaoLos Alamos National LaboratoryCenter for Integrated NanotechnologyBikini Atoll RdLos Alamos, NM 87545, USAe-mail: wei.gaofanyi@gmail.com

Thomas F. GeorgeUniversity of Missouri-St. LouisOffice of the Chancellor, Center for NanoscienceOne University BoulevardSt. Louis, MO 63121, USAe-mail: tfgeorge@umsl.edu

Lei GongUniversity of ManchesterSchool of MaterialsOxford RoadManchester, M13 9PL, UKe-mail: lei.gong@stud.manchester.ac.uk

Takuya HayashiShinshu UniversityDepartment of Electrical and ElectronicEngineering380-8553 Nagano, Japane-mail: hayashi@endomoribu.shinshu-u.ac.jp

Wenping HuKey Laboratory of Organic SolidsInstitute of Chemistry, Chinese Academy ofSciencesZhongguancun North First Street 2Beijing, 100190, Chinae-mail: huwp@iccas.ac.cn

Erik H. HározRice UniversityElectrical and Computer Engineering6100 Main Str.Houston, TX 77005, USAe-mail: enano@rice.edu

Quentin JalleratCarnegie Mellon UniversityBiomedical Engineering700 Technology DrivePittsburgh, PA 15206, USAe-mail: qjallera@andrew.cmu.edu

Heli JantunenUniversity of OuluDepartment of Electrical EngineeringErkki Koiso-Kanttilankatu 3Oulu 90014, Finlande-mail: heli.jantunen@oulu.fi

Song JinUniversity of Wisconsin-MadisonDepartment of Chemistry1101 University Ave.Madison, WI 53706, USAe-mail: jin@chem.wisc.edu

List of Authors XV

Kaushik KalagaRice UniversityDepartment of Mechanical Enginnering &Materials Science6100 Main Str.Houston, TX 77005, USAe-mail: kalaga.kaushik@gmail.com

Ji-Hee KimRice UniversityDepartment of Electrical and ComputerEngineering6100 Main Str.Houston, TX 77005, USAe-mail: withphys@gmail.com

Yoong A. KimShinshu UniversityDepartment of Electrical and ElectronicEngineering380-8553 Nagano, Japane-mail: yak@endomoribu.shinshu-u.ac.jp

Ian A. KinlochUniversity of ManchesterSchool of MaterialsOxford RoadManchester, M13 9PL, UKe-mail: ian.kinloch@manchester.ac.uk

Imre Kiricsi (deceased)

Junichiro KonoRice UniversityElectrical and Computer Engineering & Physics andAstronomy6100 Main Str.Houston, TX 77005, USAe-mail: kono@rice.edu

Krisztián KordásUniversity of OuluDepartment of Electrical EngineeringErkki Koiso-Kanttilankatu 3Oulu 90570, Finlande-mail: lapy@ee.oulu.fi

Gábor KozmaUniversity of SzegedDepartment of Applied and EnvironmentalChemistryDugonics tér 136720 Szeged, Hungarye-mail: kozmag@chem.u-szeged.hu

Jarmo KukkolaUniversity of OuluDepartment of Electrical EngineeringErkki Koiso-Kanttilankatu 3Oulu 90014, Finlande-mail: jarmo.kukkola@gmail.com

Ákos KukoveczUniversity of SzegedDepartment of Applied and EnvironmentalChemistryRerrich Béla tér 1Szeged, Hungarye-mail: kakos@chem.u-szeged.hu

Vinod KumarBanaras Hindu UniversityDepartment of ZoologyLanka, Varanasi, 221005, Indiae-mail: mail2vinod2@gmail.com

Zoltán KónyaUniversity of SzegedDepartment of Applied and EnvironmentalChemistryRerrich Bela tér 16720 Szeged, Hungarye-mail: konya@chem.u-szeged.hu

Jaesang LeeKorea Institute of Science and Technology (KIST)Center for Water Resource CycleHwarangno 14-gil 5Seoul 136-791, Koreae-mail: lee39@kist.re.kr

Seunghak LeeKorea Institute of Science and Technology (KIST)Center for Water Resource CycleHwarangno 14-gil 5Seoul 136-791, Koreae-mail: seunglee@kist.re.kr

XVI List of Authors

Renat R. LetfullinRose-Hulman Institute of TechnologyPhysics and Optical Engineering5500 Wabash AvenueTerre Haute, IN 47803-3999, USAe-mail: letfullin@rose-hulman.edu

Roi LeviWeizmann Institute of ScienceDepartment of Materials and Interfaces234 Herzl StreetRehovot 76100, Israele-mail: roi.levi@weizmann.ac.il

Zuzanna A. LewickaRice UniversityDepartment of Chemistry6100 Main StreetHouston, TX 77005, USAe-mail: zuzannal@rice.edu

Longtu LiTsinghua UniversityMaterials Science & EngineeringBeijing, 100084, Chinae-mail: llt-dms@mail.tsinghua.edu.cn

Shaily MahendraUniversity of California, Los AngelesCivil and Environmental Engineering5732 Boelter HallLos Angeles, CA 90095, USAe-mail: mahendra@seas.ucla.edu

Balaji P. MandalBhabha Atomic Research CentreChemistry DivisionMumbai, 400085, Indiae-mail: bpmandal@barc.gov.i

Fei MengUniversity of Wisconsin-MadisonDepartment of Chemistry1101 University AvenueMadison, WI 53706, USAe-mail: fmeng@chem.wisc.edu

Guowen MengChinese Academy of SciencesInstitute of Solid State PhysicsHefei, Anhui 230031, Chinae-mail: gwmeng@issp.ac.cn

Younès MessaddeqUniversité LavalDepartment of Physics2375, rue de la TerrasseQuébec, Québec G1V 0A6, Canadae-mail: younes.messaddeq@copl.ulaval.ca

Jyri-Pekka MikkolaÅbo Akademi UniversityDepartment of Chemical EngineeringBiskopsgatan 8Åbo-Turku 20500, Finlande-mail: jyri-pekka.mikkola@chem.umu.se;jyri-pekka.mikkola@abo.fi

Melinda MohlUniversity of OuluDepartment of Electrical EngineeringErkki Koiso-Kanttilankatu 3Oulu 90570, Finlande-mail: memohl@ee.oulu.fi

Aarón Morelos-GómezShinshu UniversityInstitute of Carbon Science and Technology4-17-1 Wakasato380-8553 Nagano, Japane-mail: amorelos@shinshu-u.ac.jp

Stephen A. MorinHarvard UniversityChemistry and Chemical Biology12 Oxford StreetCambridge, MA 02138, USAe-mail: smorin@gmwgroup.harvard.edu

Marcelo NalinFederal University of Sao CarlosDepartment of ChemistryRodovia Washington Luiz, SP-310Sao Carlos, Sao Paulo, Brazile-mail: mnalin@ufscar.br

List of Authors XVII

Sebastien NanotRice UniversityElectrical and Computer Engineering & Physics andAstronomy6100 Main StreetHouston, TX 77098, USAe-mail: sebastien.nanot@gmail.com

Rachelle N. PalcheskoCarnegie Mellon UniversityBiomedical EngineeringPittsburgh, PA 15219, USAe-mail: rachelle@andrew.cmu.edu

Cary L. PintVanderbilt UniversityDepartment of Mechanical Engineering2301 Vanderbilt PlaceNashville, TN 37215, USAe-mail: Cary.L.Pint@vanderbilt.edu

Gael PoirierFederal University of AlfenasInstitute of Science and TechnologyRodovia José Aurélio Vilela 11999Poços de Caldas, MG CEP 37715-400, Brazile-mail: gael.poirier@unifal-mg.edu.br

Thalappil PradeepIndian Institute of Technology MadrasDepartment of ChemistryChennai, 600 036, Indiae-mail: pradeep@iitm.ac.in

István PálinkóUniversity of SzegedDepartment of Organic ChemistryDóm tér 86720 Szeged, Hungarye-mail: palinko@chem.u-szeged.hu

Raju V. RamanujanNanyang Technological UniversitySchool of Materials Science and Engineering50 Nanyang Ave.639798 Singaporee-mail: ramanujan@ntu.edu.sg

Sundara RamaprabhuIndian Institute of Technology, MadrasDepartment of PhysicsChennai, 600 036, Indiae-mail: ramp@iitm.ac.in

Jayshree RamkumarBhabha Atomic Research CentreAnalytical Chemistry DivisionMumbai, 400085, Indiae-mail: jrk@barc.gov.in

Arava L.M. ReddyRice UniversityDepartment of Mechanical Engineering andMaterials Science6100 Main Str.Houston, TX 77005, USAe-mail: leela@rice.edu

Vincent C. ReyesUniversity of California, Los AngelesDepartment of Civil and EnvironmentalEngineering5732 Boelter HallLos Angeles, CA 90095, USAe-mail: vincecreyes@ ucla.edu

Sidney J.L. RibeiroSao Paulo State University – UNESPInstitute of ChemistryAraraquara, SP 14801-970, Brazile-mail: Sidney@iq.unesp.br

William D. RiceLos Alamos National LaboratoryNational High Magnetic Field LaboratoryLos Alamos, NM 87545, USAe-mail: wdrice@lanl.gov

Silvia H. SantagneliSao Paulo State University – UNESPInstitute of ChemistryAraraquara, SP 14801-970, Brazile-mail: santagneli@iq.unesp.br

Preeti S. SaxenaBanaras Hindu UniversityDepartment of ZoologyLanka, Varanasi, Uttar Pradesh 221005, Indiae-mail: pssaxena@rediffmail.co

XVIII List of Authors

Olga A. ShenderovaInternational Technology Center8100 Brownleigh RoadRaleigh, NC 27617, USAe-mail: oshenderova@itc-inc.org

Rakesh ShuklaBhabha Atomic Research CentreChemistry DivisionMumbai, 400085, Indiae-mail: rakesh@barc.gov.in

Shashwat ShuklaNanyang Technological University639798 Singaporee-mail: shas0002@e.ntu.edu.sg

Theruvakkattil S. SreeprasadKansas State UniversityDepartment of Chemical Engineering1011 Durland HallManhattan, KS 66502, USAe-mail: sprasad@k-state.edu

Anchal SrivastavaBanaras Hindu UniversityDepartment of PhysicsLanka, Varanasi, Uttar Pradesh 221005, Indiae-mail: anchalbhu@gmail.com

Saurabh SrivastavaNational Physical LaboratoryBiomedical Instrumentation Section,New Rajender Nagar, New Delhi 110012, Indiae-mail: saurabhnpl@gmail.com

Yan SunBeihang University (BUAA)School of Biological Science and MedicalEngineering37 Xueyuan Road, Haidian DistrictBeijing, 100191, Chinae-mail: sunyan@buaa.edu.cn

Mária SzabóUniversity of SzegedApplied and Environmental Chemistry1 Rerrich Béle tér6720 Szeged, Hungarye-mail: szmaria@chem.u-szeged.hu

John M. SzymanskiCarnegie Mellon UniversityDepartment of Biomedical Engineering700 Technology DrivePittsburgh, PA 15219, USAe-mail: jszymans@andrew.cmu.edu

András SápiUniversity of SzegedDepartment of Applied and EnvironmentalChemistry1 Rerrich square6720 Szeged, Hungarye-mail: sapia@chem.u-szeged.hu

Reshef TenneWeizmann Institute of ScienceDepartment of Materials and InterfacesRehovot 76100, Israele-mail: Reshef.tenne@weizmnn.ac.il

Humberto TerronesPennsylvania State UniversityPhysics DepartmentS104 Davey LabUniversity Park, PA 16802, USAe-mail: hzt2@psu.edu

Mauricio TerronesPennsylvania State UniversityDepartment of Physics and Materials Science andEngineeringUniversity Park, PA 16802, USAe-mail: mut11@psu.edu

Nicholas A. ThompsonRice UniversityDepartment of Physics & Astronomy6100 Main Str.Houston, TX 77005, USAe-mail: nt7@rice.edu

Bipul TripathiBanaras Hindu UniversityDepartment of PhysicsLanka, Varanasi, Uttar Pradesh 221005, Indiae-mail: bipultripathi@gmail.com

List of Authors XIX

Ferdinando Tristán LópezShinshu UniversityFaculty of Engineering, Research Center for ExoticNanocarbons380-8553 Nagano, Japane-mail: ftristan@shinshu-u.ac.jp

Avesh K. TyagiBhabha Atomic Research CentreChemistry DivisionMumbai, 400085, Indiae-mail: aktyagi@barc.gov.in

Géza TóthUniversity of OuluDepartment of Electrical EngineeringErkki Koiso-Kanttilankatu 3Oulu 90570, Finlande-mail: geza@ee.oulu.fi

Robert VajtaiRice UniversityDepartment of Mechanical Engineering andMaterials Science6100 Main Str.Houston, TX 77005-1827, USAe-mail: robert.vajtai@rice.edu

Sofia M. Vega DíazShinshu UniversityResearch Center for Exotic Nanocarbons380-8553 Nagano, Japane-mail: smvd@shinshu-u.ac.jp

Aravind VijayaraghavanUniversity of ManchesterSchool of Computer ScienceManchester, M13 9PL, UKe-mail: aravind@cs.man.ac.uk

Xiaohui WangTsinghua UniversityDepartment of Materials Science & EngineeringBeijing, 100084, Chinae-mail: wxh@mail.tsinghua.edu.cn

Xuan WangRice UniversityDepartment of Electrica and Computer Engineering6100 Main Str.Houston, TX 77005, USAe-mail: xw13@rice.edu

Qiaoling XuChinese Academy of SciencesKey Laboratory of Materials Physics (CAS), AnhuiKey Laboratory of Nanomaterials andNanostructuresAnhui, 230031, Chinae-mail: qlxu@issp.ac.cn

Robert J. YoungUniversity of ManchesterSchool of MaterialsOxford RoadManchester, M13 9PL, UKe-mail: robert.young@manchester.ac.uk

Ling ZhangCarnegie Mellon UniversityDepartment of Biomedical Engineering700 Technology DrivePittsburgh, PA 15219, USAe-mail: zling03@gmail.com

Shaopeng ZhangTsinghua UniversityDepartment of Materials Science and EngineeringBeijing, 100084, Chinae-mail: zspthu@gmail.com

Zhonghua ZhangShandong UniversitySchool of Materials Science and EngineeringJingshi Road 17923Shandong, 250061, Chinae-mail: zh zhang@sdu.edu.cn

XXI

Contents

Foreword by Claes-Göran Granqvist ........................................................ VForeword by Neal Lane ............................................................................. VIIList of Abbreviations ................................................................................. XXIX

1 Science and Engineering of NanomaterialsRobert Vajtai ........................................................................................... 11.1 History and Definition of Nanomaterials ......................................... 21.2 Formation of Nanomaterials .......................................................... 61.3 Properties of Nanomaterials........................................................... 101.4 Typical Applications of Nanomaterials ............................................ 221.5 Concluding Remarks ...................................................................... 311.6 About the Contents of the Handbook ............................................. 31References .............................................................................................. 31

Part A NanoCarbons

2 Graphene – Properties and CharacterizationAravind Vijayaraghavan .......................................................................... 392.1 Methods of Production .................................................................. 422.2 Properties ..................................................................................... 502.3 Characterization ............................................................................ 582.4 Applications .................................................................................. 692.5 Conclusions and Outlook ................................................................ 74References .............................................................................................. 74

3 Fullerenes and Beyond: Complexity, Morphology,and Functionality in Closed Carbon NanostructuresHumberto Terrones .................................................................................. 833.1 Geometry and Structural Features of Fullerenes .............................. 853.2 Methods of Synthesis of Fullerenes and Proposed Growth Models .... 883.3 Physicochemical Properties of Fullerenes ........................................ 903.4 Applications of Fullerenes and Beyond ........................................... 923.5 Conclusions ................................................................................... 99References .............................................................................................. 99

4 Single-Walled Carbon NanotubesSebastien Nanot, Nicholas A. Thompson, Ji-Hee Kim, Xuan Wang,William D. Rice, Erik H. Hároz, Yogeeswaran Ganesan, Cary L. Pint,Junichiro Kono ........................................................................................ 1054.1 History .......................................................................................... 1064.2 Crystallographic and Electronic Structure ........................................ 106

XXII Contents

4.3 Synthesis ...................................................................................... 1114.4 Optical Properties .......................................................................... 1154.5 Transport Properties ...................................................................... 1234.6 Thermal and Mechanical Properties ................................................ 1284.7 Concluding Remarks ...................................................................... 135References .............................................................................................. 135

5 Multi-Walled Carbon NanotubesÁkos Kukovecz, Gábor Kozma, Zoltán Kónya ............................................. 1475.1 Synthesis ...................................................................................... 1485.2 Chemistry of MWCNTs ..................................................................... 1535.3 Properties ..................................................................................... 1575.4 Selected Applications..................................................................... 163References .............................................................................................. 169

6 Modified Carbon NanotubesAarón Morelos-Gómez, Ferdinando Tristán López, Rodolfo Cruz-Silva,Sofia M. Vega Díaz, Mauricio Terrones....................................................... 1896.1 Doped Carbon Nanotubes .............................................................. 1916.2 Defects in Carbon Nanotubes ......................................................... 1936.3 Nanotube Chemical Functionalization ............................................ 1976.4 Properties of Modified Carbon Nanotubes ....................................... 2036.5 Characterization of Modified Carbon Nanotubes .............................. 2086.6 Applications of Modified Carbon Nanotubes.................................... 2156.7 Toxicity and Biocompatibility ......................................................... 2186.8 Conclusions ................................................................................... 2206.9 Outlook and Perspectives ............................................................... 221References .............................................................................................. 221

7 Carbon NanofibersYoong A. Kim, Takuya Hayashi, Morinobu Endo, Mildred S. Dresselhaus ..... 2337.1 Similarity and Difference Between Carbon Fibers

and Carbon Nanofibers .................................................................. 2347.2 Growth and Structural Modifications of Carbon Nanofibers .............. 2387.3 Applications of Carbon Nanofibers.................................................. 2517.4 Conclusions ................................................................................... 257References .............................................................................................. 258

8 NanodiamondsOlga A. Shenderova, Suzanne A. Ciftan Hens ............................................. 2638.1 Stability of Diamond at the Nanoscale ............................................ 2648.2 Types of Nanodiamonds and Methods of Nanodiamond Synthesis ... 2678.3 Detonation Nanodiamond Processing and Modification .................. 2788.4 Fluorescent Nanodiamonds ........................................................... 284

Contents XXIII

8.5 Applications of Nanodiamond Particles .......................................... 2858.6 Future Directions of Production and Applications ............................ 292References .............................................................................................. 293

Part B NanoMetals

9 Noble Metal NanoparticlesTheruvakkattil S. Sreeprasad, Thalappil Pradeep ....................................... 3039.1 Historical Perspective of Gold and Silver NPs ................................... 3049.2 Diverse Nanostructures .................................................................. 3079.3 Common Synthetic Routes for the Preparation

of Noble Metal NPs ........................................................................ 3119.4 Properties of Noble Metal Nanoparticles ......................................... 3229.5 Postsynthetic Tuning of Properties ................................................. 3249.6 Functionalized Metal NPs ............................................................... 3439.7 Applications of Gold and Silver Nanoparticles ................................. 3479.8 New Gold and Silver Materials – Quantum Clusters ......................... 3639.9 Conclusions ................................................................................... 366References .............................................................................................. 367

10 Nanostructures of Common MetalsMelinda Mohl, Krisztián Kordás ................................................................ 38910.1 Post-Transition Metals ................................................................... 39010.2 Transition Metals ........................................................................... 39210.3 Concluding Remarks ...................................................................... 398References .............................................................................................. 399

11 Alloys on the NanoscaleGiovanni Barcaro, Alfredo Caro, Alessandro Fortunelli ............................... 40911.1 Concepts and Principles ................................................................. 41111.2 Preparation and Synthesis ............................................................. 41311.3 Characterization of Nanoparticles and Nanoalloys ........................... 41711.4 Properties ..................................................................................... 42411.5 Nanostructured Bulk Alloys ............................................................ 45011.6 Applications .................................................................................. 45711.7 Concluding Remarks ...................................................................... 458References .............................................................................................. 459

12 Magnetic Nanostructures: Synthesis, Properties, and ApplicationsShashwat Shukla, Pratap Kumar Deheri, Raju V. Ramanujan..................... 47312.1 Background .................................................................................. 47412.2 Atomic Origin of Magnetism ........................................................... 47512.3 Magnetic Length Scales and Origin of Nanomagnetic Behavior ......... 47812.4 Magnetic Nanostructures ............................................................... 48312.5 Conclusions ................................................................................... 505References .............................................................................................. 506

XXIV Contents

Part C NanoCeramics

13 Nanocrystalline Functional Oxide MaterialsRakesh Shukla, Dimple P. Dutta, Jayshree Ramkumar, Balaji P. Mandal,Avesh K. Tyagi ......................................................................................... 51713.1 Synthesis Methods......................................................................... 51813.2 Optical Properties of Oxide Nanomaterials ...................................... 52413.3 Sorbent Properties of Oxide Nanomaterials ..................................... 53213.4 Catalytic Properties of Oxide Nanomaterials .................................... 53613.5 Oxide Nanomaterials in Ionics........................................................ 53813.6 Conclusions ................................................................................... 541References .............................................................................................. 542

14 Piezoelectric NanoceramicsXiaohui Wang, Shaopeng Zhang, Longtu Li .............................................. 55314.1 Introduction to BSPT ...................................................................... 55414.2 Synthesis of BSPT Nanopowders via Sol–Gel Method ....................... 55514.3 Sintering of BSPT Nanoceramics ...................................................... 55614.4 Grain Size Effect on the Properties of BSPT Ceramics ........................ 56314.5 Summary ...................................................................................... 567References .............................................................................................. 568

15 Graphite OxideWei Gao .................................................................................................. 57115.1 Synthesis of Graphite Oxide ........................................................... 57215.2 Characterization, Chemical Structure and Properties........................ 57615.3 Applications .................................................................................. 58915.4 Concluding Remarks ...................................................................... 592References .............................................................................................. 592

16 Compound CrystalsRoi Levi, Maya Bar-Sadan, Reshef Tenne .................................................. 60516.1 Nanostructures .............................................................................. 60516.2 Synthetic Methods ......................................................................... 60816.3 Physical Properties ........................................................................ 61816.4 Applications .................................................................................. 62816.5 Conclusions ................................................................................... 630References .............................................................................................. 631

17 Growth of Nanomaterials by Screw DislocationFei Meng, Stephen A. Morin, Song Jin ....................................................... 63917.1 Classical Crystal Growth Theories .................................................... 64017.2 Theories for Screw-Dislocation-Driven Growth of Nanomaterials ..... 64217.3 Structural Characterization of these Nanomaterials ......................... 64517.4 Generality of Dislocation-Driven Nanomaterial Growth ................... 64917.5 Rational Growth of Dislocation-Driven Nanomaterials –

General Strategies ......................................................................... 658

Contents XXV

17.6 Applications .................................................................................. 65917.7 Summary and Perspectives ............................................................ 660References .............................................................................................. 661

18 Glasses on the NanoscaleHellmut Eckert, Sidney J.L. Ribeiro, Silvia H. Santagneli, Marcelo Nalin,Gael Poirier, Younès Messaddeq ............................................................... 66518.1 Studying Medium-Range Order in Glasses and Nanoceramics .......... 66618.2 Nanoceramics ............................................................................... 67618.3 Perspectives and Concluding Remarks ............................................ 684References .............................................................................................. 685

Part D NanoComposites

19 Carbon in PolymerRobert J. Young, Libo Deng, Lei Gong, Ian A. Kinloch................................. 69519.1 Materials Basics............................................................................. 69519.2 Carbon Nanotube Composites......................................................... 70219.3 Graphene Composites .................................................................... 71619.4 Conclusions ................................................................................... 722References .............................................................................................. 722

20 Nanoparticle DispersionsKrisztián Kordás, Jarmo Kukkola, Géza Tóth, Heli Jantunen, Mária Szabó,András Sápi, Ákos Kukovecz, Zoltán Kónya, Jyri-Pekka Mikkola.................. 72920.1 Stabilization of Nanoparticle Dispersions ........................................ 73020.2 Nanoparticle Dispersion in Practice ................................................ 73420.3 Dispersions of Carbon Nanomaterials ............................................. 74520.4 Drying Dispersions on Surfaces ....................................................... 75220.5 Concluding Remarks ...................................................................... 758References .............................................................................................. 758

Part E Nanoporous Materials

21 Nanoporous MetalsYi Ding, Zhonghua Zhang ........................................................................ 77921.1 Preparation of Nanoporous Metals ................................................. 77921.2 Properties of Nanoporous Metals .................................................... 78921.3 Applications .................................................................................. 80821.4 Concluding Remarks and Prospects ................................................ 810References .............................................................................................. 811

22 ZeolitesIstván Pálinkó, Zoltán Kónya, Ákos Kukovecz, Imre Kiricsi.......................... 81922.1 Common Zeolite Frameworks ......................................................... 822

XXVI Contents

22.2 Zeolite and Zeolite-Related Molecular Sieves .................................. 82322.3 Natural Zeolites: Occurrence and Formation .................................... 82522.4 Methods of Identification and Characterization .............................. 82822.5 Synthesis of Zeolitic Materials ........................................................ 83022.6 Ion Exchange, Sorption, and Diffusion in Microporous Materials ...... 83622.7 Acid–Base Properties of Zeolites ..................................................... 84122.8 Stability and Modification of Zeolite Structures ............................... 84322.9 Zeolites as Catalysts ....................................................................... 84622.10 Some Special Applications of Zeolites ............................................. 84822.11 Conclusions ................................................................................... 850References .............................................................................................. 850

23 Porous Anodic Aluminum OxideQiaoling Xu, Guowen Meng ...................................................................... 85923.1 Background .................................................................................. 85923.2 Preparation of AAO Templates ........................................................ 86023.3 Nanostructures Constructed in AAO Templates ................................. 86223.4 Conclusions and Outlook ................................................................ 879References .............................................................................................. 879

24 Porous SiliconPaolo Bettotti .......................................................................................... 88324.1 Basics of Porous Silicon Electrochemistry and Formation Models ...... 88424.2 Other Etching Methods .................................................................. 88624.3 Porous Silicon Structural Properties ................................................ 88724.4 Light Emission from Porous Silicon ................................................. 89024.5 Thermal and Electrical Properties ................................................... 89124.6 The Role of the Surface .................................................................. 89124.7 Applications of Porous Silicon ........................................................ 89224.8 Conclusions ................................................................................... 897References .............................................................................................. 898

Part F Organic and Bionanomaterials

25 Organic NanomaterialsHuanli Dong, Wenping Hu ....................................................................... 90525.1 Preparation/Synthesis of Organic Nanomaterials ............................. 90525.2 Properties of Organic Nanomaterials .............................................. 91025.3 Applications .................................................................................. 92525.4 Concluding Remarks ...................................................................... 930References .............................................................................................. 932

26 Nanocomposites as Bone Implant MaterialVinod Kumar, Bipul Tripathi, Anchal Srivastava, Preeti S. Saxena ............... 94126.1 The Quest for a Suitable Bone Implant ............................................ 94226.2 Bone ............................................................................................ 942

Contents XXVII

26.3 Existing/Conventional Bone Implant Materials and TheirShortcomings ................................................................................ 944

26.4 Major Challenges with Existing/Conventional Implant Materials ....... 94926.5 Nanotechnology and Tissue Engineering ......................................... 94926.6 Future Perspectives ....................................................................... 965References .............................................................................................. 965

27 Nanofiber BiomaterialsRachelle N. Palchesko, Yan Sun, Ling Zhang, John M. Szymanski,Quentin Jallerat, Adam W. Feinberg.......................................................... 97727.1 Methods of Production .................................................................. 98027.2 Properties of Nanofiber Biomaterials .............................................. 98627.3 Characterization of Nanofiber Biomaterials ..................................... 99327.4 Applications .................................................................................. 99927.5 Conclusions and Outlook ................................................................ 1005References .............................................................................................. 1006

Part G Applications and Impact

28 Nanostructured Materials for Energy-Related ApplicationsArava L.M. Reddy, Sundara Ramaprabhu.................................................. 101328.1 Energy-Related Carbon Nanotubes ................................................. 101328.2 CNTs as Support Material for Electrocatalysts in PEMFC ..................... 101628.3 CNTs as Supercapacitor Electrode Materials ..................................... 1023References .............................................................................................. 1032

29 Nanomaterials in Civil EngineeringJaesang Lee, Seunghak Lee, Eunhyea Chung, Vincent C. Reyes,Shaily Mahendra ..................................................................................... 103929.1 Applications of MNMs in Construction ............................................. 104129.2 Environmental Release of MNMs Used in Construction ..................... 104729.3 Potential Adverse Biological Impacts and Toxicity Mechanisms ........ 104929.4 Mitigation of Environmental and Health Impacts ............................ 105229.5 Conclusions ................................................................................... 1054References .............................................................................................. 1055

30 Plasmonic Nanomaterials for NanomedicineRenat R. Letfullin, Thomas F. George ........................................................ 106330.1 Introduction ................................................................................. 106330.2 Nanooptics – Lorenz–Mie Formalism .............................................. 106430.3 Optical Properties of Gold Nanoparticles in Biological Media............ 106530.4 Kinetics of Heating and Cooling of Nanoparticles ............................ 106730.5 Spatial Distribution of Temperature Fields Around the Nanoparticle . 107630.6 New Dynamic Modes in Selective Plasmonic Nanotherapy ............... 1083References .............................................................................................. 1095

XXVIII Contents

31 Carbon Nanotube Membrane FiltersAnchal Srivastava, Saurabh Srivastava, Kaushik Kalaga ............................ 109931.1 Types of Filtration ......................................................................... 110031.2 Mechanisms of Filtration ............................................................... 110131.3 Carbon Nanotube Membrane Filters ............................................... 110231.4 Future Research Perspectives ......................................................... 1112References .............................................................................................. 1112

32 Nanomaterial Toxicity, Hazards, and SafetyZuzanna A. Lewicka, Vicki L. Colvin ........................................................... 111732.1 Engineered Nanomaterials – General Overview............................... 111832.2 Occurrence of Engineered Nanoparticles in the Environment ........... 111932.3 Effects of Nanoparticles on Organisms ............................................ 112032.4 Nanoparticle Physicochemical Characteristics of Relevance

for Toxicology ................................................................................ 112432.5 Special Case – Sunscreens .............................................................. 113032.6 Conclusions ................................................................................... 1132References .............................................................................................. 1133

Acknowledgements ................................................................................... 1143About the Authors ..................................................................................... 1145Detailed Contents ...................................................................................... 1163Subject Index ............................................................................................. 1181

XXIX

List of Abbreviations

α-SMA α-smooth muscle actinp-NP p-nitrophenol0-D zero-dimensional1-D one-dimensional2-D two-dimensional2-PAM 2-pyridine-aldoxime methiodide2Q double-quantum3-D three-dimensional3Q triple-quantum4Hop 4-hexadecyloxyphenyl

A

AA ascorbic acidAAM anodized aluminum membraneAAO anodic aluminum oxideAAO anodized aluminum oxideAAS atomic absorption spectroscopyAb antibodyAC alternating currentAcac acetylacetoneACNT aligned carbon nanotubeACQ aggregation-caused quenchingAChE acetylcholine esteraseACP amorphous calcium phosphateAD arc dischargeAEE aggregation-enhanced emissionAES Auger electron spectroscopyAES 3-(2-aminoethylaminopropyl)trimethoxy-

silaneAFC alkaline fuel cellAFC antiferromagnetically coupledAFM atomic force microscopyAIE aggregation-induced emissionAIEE aggregation-induced enhanced

emissionALD atomic layer depositionAlPO aluminophosphateAM alveolar macrophageanti-EGFR anti-epidermal growth factor receptorAOC aromatic organic compoundsAPC antigen-presenting cellAPES aminopropyltrimethoxysilaneAPPES ambient pressure photoelectron

spectroscopyAPS 3-aminopropyltrimethoxysilaneAPT atom probe tomographyAPTES (aminopropyl) triethoxysilaneAPTS 3-aminopropyltriethoxysilaneAR analytical reagentAR aspect ratio

ARPES angle-resolved photoemissionspectroscopy

ASTM American Society for Testing andMaterials

ATQD N-(4-aminophenyl)-N ′-(4′-(3-triethoxy-silyl-propyl-ureido)phenyl-1,4-quinon-enediimine)

ATP adenosine-5′-triphosphateATRP atom-transfer radical polymerizationAWWA American Water Works Association

B

BASF Badische Anilin und Soda Fabrikbcc body-centered cubicBCF Burton–Cabrera–FrankBCP biphasic calcium phosphateBDAC benzyldimethylammoniumchlorideBDNF brain-derived neurotrophic factorBEP Brønsted–Evans–Polanyi relationsBES Office of Basic Energy SciencesBET Brunauer–Emmett–TellerBF bright fieldBFGF basic fibroblast growth factorBG back-gateBHJ bulk heterojunctionbioMEMS biological microelectromechanical

systemBMG bulk metallic glassBN boron nitrideBOM bubble overlapping modeBP buckypaperBPEA 9,10-bis(phenylethynyl)anthraceneBS black siliconBSA bovine serum albuminaBSI British Standards InstitutionBSPP bis(p-sulfonatophenyl) phenylphosphine

dihydrate dipotassiumBT barium titanateBT benzenethiolBTCP β-tricalcium phosphate

C

C16TAB hexadecyl trimethyl ammonium bromideC-PANI conductive camphorsulfonic acid-doped

emeraldine PANIC3DT 1,3-PropanedithiolCA contact angleCALPHAD calculation of phase diagramsCAM cluster aggregation mode

XXX List of Abbreviations

CBED convergent-beam electron diffractionCBEV coordination-dependent bond-energy

variationCCDB Cambridge crystallographic data baseCCG chemically converted grapheneCCT correlated color temperatureCCVD catalytic chemical vapor depositionCD cyclodextrinCFR continuous flow reactorCHP cyclohexylpyrrolidoneCHT chymotrypsinCIE International Commission on

IlluminationCIP current in the planeCMG chemically modified grapheneCMOS complementary

metal–oxide–semiconductorCMP chemical–mechanical planarizationCNF carbon nanofiberCNM carbon nanotube membraneCNT carbon nanotubeCN-TFMBE 1-cyano-trans-1,2-bis(3′,5′-bis-trifluoro-

methyl-biphenyl)ethyleneCO cuboctahedronCOD 1,5-cyclooctadieneCOLI collagen ICOLIV collagen IVCOST Cooperation in Science and TechnologyCOSY correlation spectroscopyCOT 1,3,5-cyclooctatrienecp close packedCP coherent phononCP cross polarizationCPP conduction perpendicular to planeCPP current perpendicular to the planeCPS collected photo signalCS cross sectionCS-PCL chitosan-graft-PCLCSA chemical shift anisotropyCSP colloidal silver preparationCT charge transferCTA+ cetyl-triamine cationCTA cetyltrimethylammoniumCTAB cetyltrimethylammonium bromideCV crystal violetCV cyclic voltammetryCVD chemical vapor depositionCW continuous-waveCuPC copper phthalocyanineCuTCNQ copper tetracyanoquinodimethane

D

D4R double four ringDAAQ 1,5-diaminoanthraquinoneDAFC direct alcohol fuel cell

DAPI 4′,6-diamidino-2-phenylindoleDAPRAL copolymer of maleic anhydride and

α-olefinDBR distributed Bragg mirrorDC dendritic cellDC direct currentDCE 1,2-dichloroethaneDD-PTCDI N ,N ′-di(dodecyl)-perylene-3,4,9,10-

tetracarboxylic diimideDDA discrete dipole approximationDDAB didecyldimethylammonium bromideDDC N ,N ′-dicyclohexylcarbodiimideDEFC direct ethanol fuel cellDEG diethylene glycolDF defluoridation capacityDF density functionDFAC direct formic acid fuel cellDFT density functional theoryDFTB density functional tight bindingDGU density-gradient ultracentrifugationDI deionizedDIC differential interference contrastDLC diamond-like carbonDLS dynamic light scatteringDLVO Derjaguin–Landau–Verwey–OverbeekDMA dimethylamideDMEU 1,3-dimethyl-2-imidazolidinoneDMF dimethylformamideDMFC direct methanol fuel cellDMPO 5,5-dimethyl-pyrroline N-oxideDMSA dimercaptosuccinic acidDMSO dimethyl sulfoxideDNA deoxyribonucleic acidDND detonation nanodiamondDOS density of statesDOX doxorubicindpa displacements per atomDPPTE 1,2-dipalmitoyl-sn-glycero-3-phospho-

thioethanolDQ double quantumDR draw ratioDRG dorsal root ganglionDRIFT diffuse reflectance infrared

Fourier-transformDSC differential scanning calorimetryDT decanethiolDTAB dodecyltrimethylammonium bromideDTE desaminotyrosyl-tyrosine ethyl esterDWCNT double-walled carbon nanotubeDWNT double-walled nanotubesDox doxorubicin

E

ECD electrochemical depositionECDL electrochemical double layer

List of Abbreviations XXXI

ECELL environmental cellECM extracellular matrixECP electronically conducting polymerECSA electrochemically active surface areaED electrodialysisED electron diffractionEDAX energy dispersive analysisEDC 1-ethyl-3-(3-dimethylaminopropyl)-

carbodiimideEDL electrical double layerEDLC electric double-layer capacitorEDS energy-dispersive x-ray spectroscopyEDTA ethylenediaminetetraacetic acidEDX energy-dispersive x-ray spectroscopyEELS electron energy-loss spectroscopyEFM electrostatic force microscopyEG evaporated goldEIS electrochemical impedance spectroscopyEL electroluminescenceELISA enzyme-linked immuno sorbent assayEM electromagneticEMI electromagnetic interferenceEOF electroosmotic flowEPA Environmental Protection AgencyEPR electron paramagnetic resonanceEPS extracellular polymeric substanceEQE external quantum efficiencyESC embryonic stem cellESR electron spin resonanceESR equivalent series resistanceETEM environmental TEMEXAFS extended x-ray absorption fine structureElAP(S)O element aluminophosphosilicateEPITH epithelial cells

F

f-SWCNT functionalized SWCNTFABMS fast atom bombardment mass

spectroscopyFBI Federal Bureau of InvestigationFBR fluidized bed reactorfcc face-centered cubicfct face-centered tetragonalFDA Food and Drug AdministrationFEB ferrocene/ethanol/benzylamineFES fluctuation-enhanced sensingFESEM field emission scanning electron

microscopeFET field-effect transistorFF fill factorFFT fast Fourier transformFGO functionalized GOFIB fibrinogenFIB focused ion beam

FIPOS full isolation by porous oxidized siliconFIT fluctuation-induced tunnelingFITC fluorescein isothiocyanateFLG few-layer grapheneFMR ferromagnetic resonanceFN fibronectinFND fluorescent carboxylated HPHT NDFND fluorescently enhanced NDfpRFDR finite-pulse radio frequency-driven

recoupling

G

GMR giant magnetoresistanceGN gold nanoparticleGNC gold nanoparticle clusterGNP gold nanoparticleGNP graphite nanoplateletGNR gold nanorodGNR graphene nanoribbonGO graphene oxideGOX glucose oxidaseGSH glutathioneGTBMD generalized tight-binding molecular

dynamics

H

HA humic acidHAADF high-angle annular dark fieldHATU 2-(7-aza-1H-benzotriazole-l-yl)-1,1,3,3,-

tetramethyluronium hexafluorophosphateHAZ heat-affected zoneHC hexagonal channelHCCN highly curved carbon nanostructureHCI highly charged ionhcp hexagonal close packedHDA hexadecylamineHDD 1,2-hexadecanediolHDDR hydrogenation–decomposition–

desorption–recombinationHDS hydrodesulfurizationHDT hexadecanethiolHEPES 4-(2-hydroxyethyl)-1-piperazineethane-

sulfonic acidHEV hybrid electric vehicleHF hydrofluoric acidHG hydrazinium grapheneHIV human immunodeficiency virusHL60 human promyelocytic leukemiaHMDA hexamethylenediamineHMO hydrous manganese dioxideHMOG heavy metal oxide glassHMTA hexamethylenetetramineHNS hot neutron source

XXXII List of Abbreviations

HOMO highest occupied molecular orbitalHOPG highly oriented pyrolytic graphiteHP Hall–PetchHPA hexylphosphonic acidHPC-Py pyrene-labeled hydroxypropyl celluloseHPHT high-pressure high-temperatureHPMC Hydroxypropylmethyl celluloseHPSMAP poly(styrene-co-maleic anhydride)

carrying pyreneHSMA hydrolyzed poly(styrene-co-maleic)

anhydriteh-PSMA hydrolyzed-poly(styrene-alt-maleic

anhydride)HREM high-resolution electron microscopyHRN helical rosette nanotubeHRP horseradish peroxidaseHRSEM high-resolution scanning electron

microscopeHRTEM high-resolution transmission electron

microscopyHSA human serum albuminhSKMC human skeletal muscle cellHTT heat treatment temperatureHWHM half-width at half-maximumHiPCO high-pressure carbon monoxide

I

IANH International Alliance for NanoEHS(environment, health, safety)

IC integrated circuitICP inductively coupled plasmaICP-MS inductively coupled plasma mass

spectrometryIE immersion–electrodepositionIF immunofluorescenceIF inorganic fullerene-like nanoparticleiFF isotactic polypropyleneIFSS interfacial shear strengthIg immunoglobulinIgG immunoglobulin GIh icosahedronIKVAV laminin derived self-assembling peptideIKVAV-PA IKVAV polyacrylamideIL interleukinIL ionic liquidIMR intramolecular rotationINCO International Nickel CompanyINT inorganic nanotubeIP iminopyrroleIPCE incident photon to charge carrier

efficiencyiPSC induced pluripotent stem cellIR infraredipr isolated pentagon rule

ISO International Standards OrganizationITO indium tin oxideIZA International Zeolite Association

K

KE Kirkendall effectKK Kramers–Kronig

L

LA longitudinal acousticLAM lamininLB Langmuir–Blodgett techniqueLB94 van Leeuwen–BaerendsLBL layer-by-layerLCD liquid-crystal displayLDA local density approximationLDOS local density of statesLED light-emitting diodeLEED low energy electron diffractionLIB lithium-ion batteryLMP Larson–Miller plotLN less nobleLPM large-pore mordeniteLPS lipopolysaccharideLSC limbal stem cellsLSP longitudinal surface plasmonLSPR localized surface plasmon resonanceLTA Linde type ALUMO lowest unoccupied molecular orbitalLYM lymphocytes

M

M metalloidMA mechanical alloyingMAE magnetic anisotropy energyMALDI-TOF matrix-assisted laser desorption/

ionization-time of flightMAPO metalaluminophosphateMAPSO metalaluminophosphosilicatesMAS magic angle spinningMBE molecular beam epitaxyMC metal clusterMCFC molten carbonate fuel cellMCL maximum contamination limitMCS ethylene glycol monomethyl etherMD molecular dynamicsMDA malondialdehydeMDA mercaptodecanoic acidMEA membrane electrode assemblyMEMS microelectromechanical systemMF mesoflowerMF microfiltration

List of Abbreviations XXXIII

MFC microbial fuel cellMFI melt–flow indexMFM magnetic force microscopyMGM metal-graphite multilayerMHAP micron particulate hydroxyapatiteML monolayerMN more nobleMNM manufactured nanomaterialsMNPM metallic nanoporous materialMO methyl orangeMOCVD metalorganic chemical vapor depositionMOF metal-organic frameworkMOKE magneto-optical Kerr effectMPB morphotropic phase boundaryMPC monolayer-protected clusterMPCF mesophase pitch-based carbon fiberMPS mercaptopropyltrimethoxysilaneMPTMS mercaptopropyltrimethoxysilaneMR magnetic resonanceMRAM magnetic random-access memoryMRI magnetic resonance imagingmRNA messenger RNAMRR material removal rateMRSw magnetic relaxation switchingMSA mercaptosuccinic acidMSC mesenchymal stem cellMSE mercurous sulfate electrodeMTBD [7-methyl-1,5,7-triazabicyclo[4.4.0]dec-

5-ene][bis(perfluoroethylsulfonyl)imide]MWCNT multiwalled carbon nanotubeMWNT multiwalled nanotubes

N

NaBBS sodiumbutylbenzene sulfonateNaDDBS sodium dodecylbenzene sulfonateNADH nicotinamide adenine dinucleotideNaOBS sodium octylbenzene sulfonateNaPSS polystyrene sulfonate sodium saltNBE near-band-edgeNC nanocrystallinenc-AFM noncontact AFMND nanodiamondNDO ozone-modified nanodiamondND-PTCDI N ,N ′-di(nonyldecyl)-perylene-3,4,9,10-

tetracarboxylic diimideNEMS nanoelectromechanical systemNEUT neutrophilsNEXAFS near-edge x-ray absorption fine structureNF nanofeaturesNF nanofiltrationNFA nanostructured ferritic alloyNG natural highly-oriented pyrolytic

graphite

NGF nerve growth factorNHAP nanohydroxyapatitenHAp nanohydroxyapatite particlen-HApC nanohydroxyapatite/chitosanNHS N-hydroxysuccinimidyl esterNIOSH National Institute for Occupational Safety

and HealthNIR near infraredNM noble metalNMP N-methyl-pyrrolidoneNmpd N-methylpyridiniumNmpr N-methylpyrroleNMR nuclear magnetic resonanceNO-IF nanooctahedra-IFNP nanoparticleNPG nanoporous graphiteNPG/GC NPG supported by glassy carbon

electrodeNPGC nanoporous gold compositeNPM nanoporous metalNPNT nanoporous nanotubeNPS nanoporous silverNR nanorodNSC neural stem cellNSM nanostructured materialsNT nanotubeNTS nanostructured transformable steelNV nitrogen-vacancyNW nanowire

O

O/F oxidant-to-fuelOCP open-circuit potentialOCT optical coherence tomographyODA octadecylamineODE octadeceneODF orientation distribution functionODPA octadecylphosphonic acidODS octadecyltrimethoxysilaneODS oxide dispersion strengthenedOER oxygen evolution reactionOFET organic field-effect transistorOh octahedronOL optical-limitingOLC onion-like carbonOLED organic light-emitting diodeOPD O-phenylenediamineOPH organophosphorus hydrolaseOPS oxidized PSOPV organic photovoltaicORR oxygen reduction reactionOSN organic solvent nanofiltrationOTM one-temperature model

XXXIV List of Abbreviations

P

P3HT poly(3-hexylthiophene)P3OT poly(3-octylthiophene)PA peptide amphiphilePA-6 prepared a nylon-6PAA poly(acrylic acid)PABS polyaminobenzene sulfonic acidPAFC phosphoric acid fuel cellPAGE polyacrylamide gel electrophoresisPAH polycyclic aromatic hydrocarbonPAN polyacrylonitrilePANI polyanilinePATS polythiophene derivativesPBO poly(p-phenylene benzobisoxazole)PBS phosphate buffered salinePC pentagonal columnPC photonic crystalPC polycarbonatePC principal componentPCA principal component analysisPCB polychlorinated biphenylPCE power conversion efficiencyPCF photonic crystal fiberPCL poly(ε-caprolactone)PCL-G PCL-gelatinPDDA poly(diallyldimethyl)ammonium

chloridePDDP 1-phenyl-3-((dimethylamino)styryl)-5-

((dimethylamino)phenyl)-2-pyrazolinePDEAEMA poly(2-diethylaminoethyl methacrylate)PDGF platelet-derived growth factorPDLC polymer-dispersed liquid-crystalPDMS polydimethylsiloxanePDOS phonon density of statesPE photoelectronPE polyethylenePEC photoelectrochemicalPECVD plasma-enhanced CVDPEDOT poly(3,4-ethylenedioxythiophene)PEEk produced poly(ether ether ketone)PEG polyethylene glycolPEI polyethyleneiminePEL permissible occupational exposure

limitPEMFC proton exchange membrane fuel cellPEN Project on Emerging NanotechnologiesPEO poly(ethylene oxide)PES potential energy surfacePET polyethylene terephthalatePFG pulsed-field-gradientPFM piezoelectric force microscopyPG PCL–gelatinPG proteoglycanPGA poly(glycolic acid)

PGLA copolymer of PGA and PLLAPGM platinum group metalpIh polyicosahedronPIPAAm responsive poly(N-isopropylacrylamide)PL photoluminescencePL-PEG phospholipid polyethylene glycolPLA poly-ethylene oxidePLA pulsed laser ablationPLE photoluminescence excitationPLGA poly(lactic-co-glycolic) acidPLLA poly(l-lactic) acidPM dipropylene glycol monomethyletherPMMA poly-methyl methacrylatePMN-PT PbMg1/3Nb2/3 O3-PbTiO3PmPV poly(m-phenylenevinylene-co-2,5-

dioctoxy-p-phenylenevinylene)PN phosphorus-nitrogenPNIPAm poly(N-isopropyl acrylamide)PNP plasmonicPP polypropylenepp peak-to-peakPPCP 1,2,3,4,5-pentaphenyl-1,3-

cyclopentadienePT PbTiO3PPE poly-p-phenyleneethynylenePPF propylene fumaratePPTA poly phenylene terephthalamidePPV poly-p-phenylenevinylenePPy polypyrrolePRR pattern recognition receptorPS polystyrenePS porous siliconPS-PFS poly(styrene-b-ferrocenyldimethylsilane)PSD photo signal detectorPSS poly(sodium 4-styrenesulfonate)PSS polystyrene sulfonatePSU polysulfonatePSU polysulfonePSVPh poly(styrene-co-vinyl phenol)Pt-NPG platinum-decorated nanoporous goldPt-NPGL platinum-plated nanoporous gold leafPTCDI N ,N ′-di(propoxyethyl)perylene-3,4,9,10-

tetracarboxylic diimidePTCE track-etched polycarbonatePTFE polytetrafluoroethylenePU polyurethanePV pervaporationPV photovoltaicPVA polyvinyl alcoholPVC polyvinylchloridePVD physical vapor depositionPVDF polyvinyldifluoridePVP polyvinyl pyrrolidonePW plane wavepzc point of zero charge

List of Abbreviations XXXV

PZN-PT PbZn1/3Nb2/3O3-PbTiO3PZT Pb(Zr,Ti)O3

Q

QC quantum clusterQD quantum dotQEXAFS quick EXAFSQHE quantum Hall effect

R

R6G rhodamine 6GRA right angleRBM radial breathing modeRCF rabbit corneal fibroblastRE rare-earthrebar reinforcement barREDOR rotational echo double resonanceRF radio frequencyRFDR radiofrequency-driven recouplingRFID radiofrequency identificationRGB red green blueRGD Arg-Gly-AspRGO reduced graphene oxiderhBMP-2 recombinant human bone morphogenic

protein-2RHE reversible hydrogen electrodeRIA radioimmuno assayRIE reactive-ion etchingRIR restriction of intramolecular rotationRJS rotary jet spinningRKKY Rudermann–Kittel–Kasuya–YosidaRM reactive millingRMS microscale surface roughnessRNA ribonucleic acidRO reverse osmosisROS reactive oxygen speciesRPC retinal progenitor cellsRRR redox replacement reactionRRS resonant Raman scatteringRT room temperatureRT-PCR real-time polymerase chain reactionR&D research and development

S

S–W Stone–WalesS/L solid/liquidSA sliding angleSA solar ablationSAED selected-area electron diffractionSAM self-assembled monolayerSANS small -angle neutron scatteringSAPO silicoaluminophosphateSAXS small-angle x-ray scattering

SBU secondary building unitSC simple cubicSC sodium cholateSCC stress corrosion crackingSCE saturated calomel electrodeSCR space-charge regionSD standard deviationSDBS sodium dodecylbenzene sulfateSDCH samaria-doped ceriaSDS sodium dodecyl sulfateSEC size exclusion chromatographySEI solid–electrolyte interphaseSEIRA surface-enhanced infrared absorptionSEM scanning electron microscopySES scanning electron spectroscopySERS surface-enhanced Raman scatteringSET single-electron transistorSF silk fibroinSFF solid freedom fabricationSFG sum-frequency generationSFM scanning force microscopySGS spaced superconducting electrodeSHE standard hydrogen electrodeSIM structured illumination microscopySIMS secondary-ion mass spectrometrysiRNA silenced RNASL superlatticeSLS solution–liquid–solidSMA shape-memory alloySMAD solvated metal atom dispersionSNR signal-to-noise ratioSOCT sodium octanoateSOFC solid oxide fuel cellSOI silicon-on-insulatorSP surface plasmonSP-STM spin-polarized scanning tunneling

microscopySPM scanning probe microscopySPM small-pore mordeniteSPP surface plasmon polaritonSPR surface plasmon resonanceSPS spark plasma sinteringSQ single quantumSQUID superconducting quantum interference

deviceSRNF solvent resistant nanofiltrationSS stainless steelSSA specific surface areaSSNMR solid-state nuclear magnetic resonanceSTEM scanning transmission electron

microscopySTM scanning tunneling microscopySTORM stochastic optical reconstruction

microscopySTS scanning tunneling spectroscopySWCNT single-walled carbon nanotube

XXXVI List of Abbreviations

SWNH single-wall nanohornSWNT single-walled nanotubeSXRD surface x-ray diffractionShdH Shubnikov–de HaasSi-MEMS silicon microelectromechanical systemSi-nc silicon nanocrystal

T

TA thioctic acidTA transverse acousticTAMRA tetramethylrhodamineTASA template-assisted self-assemblyTCNQ tetracyanoquinodimethaneTCO transparent conductive oxideTDABr tetradodecylammonium bromideTDDFT time-dependent density-functional-

theoryTDPA tetradecylphosphonic acidTE transition metal elementTEG tetra(ethylene glycol)TEM transmission electron microscopyTEOS tetraethyl orthosilicateTEP thermoelectric powerTFT thin-film transistorTG top gateTGA thermogravimetric analysisTGA thioglycolic acidTGF-β transforming growth factorTHF tetrahydrofuranTHPC tetrakismethyl)phosphonium chlorideTIC toxic industrial chemicalTIPS thermally induced phase separationTL transition-metal elementTMAH tetramethylammonium hydroxideTMR tunnel magnetoresistanceTNF-α tumor necrosis factorTNT 2-methyl-1,3,5-trinitrobenzeneTO truncated octahedronTOAB tetraoctylammonium bromideTOF turnover frequencyTOP trioctylphosphineTOPO trioctylphosphine oxideTPA tetrapropylammoniumTPD temperature programmed desorptionTPI 2,4,5-triphenylimidazoleTPL two-photon luminescenceTPP 1,3,5-triphenyl-2-pyrazolineTSP transverse surface plasmonTSW Thrower–Stone–WalesTTCP tetracalcium phosphateTWC three-way catalystThT thioflavin T

U

UF ultrafiltrationUHP ultrahigh pressureUHV ultrahigh vacuumUNCD ultrananocrystalline diamondUPD underpotential depositionUV ultravioletUV-VIS ultraviolet-visibleUVR ultraviolet radiation

V

vdW van der WaalsVGCF vapor-grown carbon fiberVHS van Hove singularityVLS vapor–solid–liquidVPC vacuum pyrolysis/carbothermalVRH variable range hoppingVS vapor–solidVSFG vibrational sum-frequency generationVSM vibrating sample magnetometryVSS vapor–solid–solidVan vancomycin

W

WAXD wide angle x-ray diffractionWC tungsten carbideWG waveguideWHO World Health Organization

X

XANES x-ray absorption near-edge spectroscopyXAS x-ray absorption spectroscopyxc exchange–correlationXPS x-ray photoelectron spectroscopyXRD x-ray diffraction

Y

YAB YAl3(BO3)4YAM Y4Al2O9Y-CNT Y-shaped carbon nanotube

Z

ZAP zone axis patternZHDS hydroxydodecylsulfateZHS zinc hydroxysulfateZLC zero-length-columnZSM zeolite sieve of molecular porosity

1

Science and E1. Science and Engineeringof Nanomaterials

Robert Vajtai

Nanomaterials possess different properties com-pared with macroscopic (bulk) materials built upfrom the same atoms or compounds. The produc-tion routes, characterization, and applications ofmaterials sized on the nanometer scale also differfrom the bulk.

In this chapter we define nanomaterials andthe specific science that describes them, and collectexamples of synthesis and applications of a rangeof these materials; we also dedicate an extendedpart of the chapter to material properties, e.g.,morphology, mechanical, electrical, magnetic,and optical properties. In both the general andspecific parts of the chapter, emphasis is placed onthe differences from the bulk phase of the samematerial and, if possible, the size dependence ofthe various material properties.

Following the handbook format, the chapteris concise and covers various common propertiesof nanomaterials and correlations with whichnanoscientists work; however, we insert specificparts which have some curiosity value, as well asseveral aspects of our own research.

1.1 History and Definition of Nanomaterials . 21.1.1 History of Nanomaterials ............... 21.1.2 Definition of Nanoscale

and Nanomaterials....................... 41.2 Formation of Nanomaterials .................. 61.3 Properties of Nanomaterials .................. 10

1.3.1 Morphology of Nanomaterials ........ 101.3.2 Bonds and Structures.................... 101.3.3 Mechanical Properties

of Nanomaterials ......................... 121.3.4 Electrical, Magnetic, and Optical

Properties ................................... 141.3.5 Thermal Properties ....................... 181.3.6 Chemical Properties, Reactivity,

and Functionalization................... 201.3.7 Behavior of Nanomaterials

in Corrosive Environments ............. 221.4 Typical Applications of Nanomaterials .... 22

1.4.1 Catalysts and Catalyst Templates .... 261.4.2 Energy Conversion and Storage ...... 261.4.3 Sensors Based on Nanomaterials .... 28

1.5 Concluding Remarks ............................. 311.6 About the Contents of the Handbook ..... 31References .................................................. 31

The techniques, recipes, and later science of making andtesting tools, artistic objects, and weapons are the maincriteria for the characterization and classification of hu-man historical ages – from the Stone Age through theBronze and Iron Ages to the Silicon Age we are livingin. During the history of mankind, the role of nanomate-rials has continuously increased, and the growth of thisfield is accelerating.

In any modern scientific approach, one should de-sign and synthesize materials with a high level of con-trol. The ultimate material design is when one can planthe structure of the produced materials atom by atom,with every defect, bond length, etc. (for example, 60 car-bon atoms, or 72, 80, or even a few million, and 80 boron

atoms built into a cage similar to carbon fullerene), andsynthesis should follow precisely this design.

In this chapter, we collect two sorts of informationabout nanomaterials. To a lesser extent, the commonproperties of nanomaterials that are discussed in thefollowing chapters are described, whereas in the mainpart of the chapter, the features and properties of vari-ous groups of materials are discussed. This introductorychapter is more exemplary than universal, and in mostcases not detailed. More detailed description of theproperties and materials discussed here can be found inthe later chapters. For navigation in the handbook pleaseuse the extended version of the table of contents and thewell-detailed subject index.

Introd

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R. Vajtai (Ed.), Springer Handbook of Nanomaterials, DOI 10.1007/978-3-642-20595-8_1, © Springer-Verlag Berlin Heidelberg 2013

2 Introduction

1.1 History and Definition of Nanomaterials

In this part of the chapter, we briefly mention severalimportant and interesting events from both ancient andmodern ages for nanomaterials production and appli-cation. We also present how our fellow scientists andgovernment organizations describe and define nanoma-terials.

1.1.1 History of Nanomaterials

Although we indubitably live not only in the Silicon butalso in the beginning of the Nano Age, almost everychapter in this book, as well as most of the comprehen-sive review papers in the literature, include historicalaspects. Usage of nanomaterials remounts to traditionalChinese medicine [1.6], and Mayan [1.7] and medievalItalian paints [1.8]. Nanomaterials such as the color-ful and magically healing inks made of colloid-sized(nano) gold particles were also used in artistic applica-tions such as the Lycurgus cup [1.9] and for producingboth small and large stained-glass windows for castlesand cathedrals.

In Fig. 1.1, we have collected several exam-ples of famous nanomaterials in chronological order(Fig. 1.1a–f). The timescale in Fig. 1.1g shows thesementioned examples and some other important mater-ials as well as important events. Figure 1.1a–c showspre-modern era examples of nanomaterial applications.The Lycurgus cup (Fig. 1.1a) includes gold nanoparti-cles which make its color green when we look at itin the usual way, in reflected light, but red in trans-mitted light [1.9]. Similarly, metal nanoparticles wereused in the magnificent south rose window of NotreDame Cathedral (Fig. 1.1b). The specific nanostruc-ture with cementite nanowires and carbon nanotubesof damascene-style steels (Fig. 1.1c) [1.10] invented inthe Mediterranean, and similar techniques used in Swe-den, are representative examples of the first manmadeengineered nanomaterials [1.10]. Of course, ancientand premodern technologies could not control materialproperties on the basis of knowledge of nanometer-scaleproperties; however, the recipes and methods for nano-material production were successful enough to provokeadmiration and sometimes fear in scientists of their age.At the same time, even with our modern knowledge,the admiration for these first craftsmen of nanomaterialsstill holds.

The history of nanomaterials took an interestingturn in the 20th century. First, the discovery of semi-conductor-based transistors [1.11] opened the road for

miniaturization and integration of these devices in thefirst chips [1.12]. In the same decade when the firstintegrated circuit (IC) was fabricated, Richard Feyn-man presented his famous lecture [1.13] about thehuge information storage capacity of materials if and(as he rather saw it) when one goes to the atomicscale, to store ultimately one bit of information inevery atom. He calculated that all of the informa-tion stored in the Encyclopedia Britannica would fitonto the head of a pin – assuming 120 dpi resolutionused in the original print and that the resolution isincreased 25 000-fold – without breaking the knownrules of physics, and he even envisioned several meth-ods for writing, multiplying, and reading informationat that density. In Fig. 1.1d,e, examples of nanomate-rials from the last 50 years are displayed. Figure 1.1dshows chromium nanocrystals [1.3] synthesized by theinert gas deposition method [1.3, 14]. Figure 1.1e dis-plays schematics for the newly discovered allotropesof carbon, i. e., nanodiamond, fullerene, nanotubes, andgraphene; these materials form such an important partof today’s nanomaterial science that at least one chap-ter in this handbook summarizes knowledge about eachof them. Figure 1.1f shows a transmission electronmicroscopy (TEM) image of gold nanorods and theirshape distribution [1.5]; these nanorods are useful innanomedicine, e.g., for imaging and curing cancer. Thebottom part of Fig. 1.1g shows a collection of severalhistorical and novel nanomaterials and related eventsdisplayed on a logarithmic scale, dating back from thepresent to 6000 BC. Even without displaying many im-portant discoveries in the last decades, Nobel and KavliPrizes awarded for achievements in nanomaterial sci-ence, and milestones of commercialization, an obviousexponential acceleration is visible.

Fig. 1.1a–g History of nanomaterial use. (a–c) Premodernexamples for nanomaterial applications. (a) The Lycurguscup photographed in reflective light and in transmission(after [1.1]). The south rose window of Notre DameCathedral in Paris, France. (c) Damascus saber (photoTina Fineberg) (after [1.2]). (d,e) Modern-age milestoneexamples of nanomaterials. (d) Metal nanoparticles, Crshown here, produced by the inert gas deposition tech-nique (after [1.3]); (e) allotropes of carbon: nanodiamond,fullerenes, nanotubes, and graphene [1.4]; and (f) goldnanorods [1.5]. (g) Timescale – measured back from to-day to 6000 BC on a logarithmic scale – of nanomaterialproduction and events related to nanomaterials �

Introd

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Science and Engineering of Nanomaterials 1.1 History and Definition of Nanomaterials 3

In this fast-paced and accelerating scientific and en-gineering nanoworld, one needs to follow several dozenjournals which deal with nanomaterials. In this chapter

a) b) c)

d)

g)

e) f)

Diamond

GraphiteAspect ratio

4.03.53.02.5 4.52

10

20

30

2.0

Abundance (%)

(10, 10) Tube

C60Buckminsterfullerene

Cr # 520

Ancient carbon black inks

Lycurgus cupStained-glass windows

Damascene saberFaraday's gold colloid

First Nobel Prize for nanomaterialsFeynman's speech

Metal nanoparticles preparedQuantum dotsC60 discovered

First nano journal

Healing nanogold

MWNT recognizedSWNT produced

Dawn of graphene

5 50

Years since event (plotted in 2012)

500 5000

20 nm

and in the whole Springer Handbook of Nanomateri-als, we try to provide a compass to identify the mostimportant features.

Introd

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4 Introduction

The scale of things – Nanometers and more

Things natural

Dust mite

≈ 60–120 μm wide

≈7–8 μm

Human hair

Red blood cells

≈ 5 mm

Ant

≈10–20 μm

Fly ash

≈ 2–1/2 nmdiameter

DNA

spacing 0.078 nm

Atoms of silicon

10 nm diameter

ATP synthase

Things manmade

200 μm

Head of a pin

Microelectromechanical system(MEMS) devices

Zone platex-ray lens

Self-assembled,nature-inspired

structure

Quantum corral of 48 iron atomson copper surface positioned one

at a time with an STM tipCarbon

buckyballCarbonnanotube

Nanotubeelectrode

Red blood cells

Pollen grain

10–100 μm wide

Outer ring spacing≈ 35 nm

Corral diameter ≈14 nm ≈1.3 nm diameter

≈1 nmdiameter

Many 10s of nm 1 μm

1 cm10–2 m

10–3 m

10–4 m

10–5 m

10–6 m

10–7 m

10–8 m

10–9 m

10–10 m

10 mm

0.1 mm100 μm

0.01 mm10 μm

0.1 μm100 nm

0.01 μm10 nm

0.1 mm

1 000 000 nm= 1 mm

1000 nm = 1 μm

1 nm

Nan

owor

ldM

icro

wor

ld

Ultr

avio

let

Soft

x-r

ayV

isib

leIn

frar

edM

icro

wav

e

The challenge

Fabricate and combinenanoscale buildingblocks to make usefuldevices, e.g., a photo-synthetic reactioncenter with integralsemiconductor storage.

1–2 mm

Fig. 1.2 Scale of things chart designed by the Office of Basic Energy Sciences (BES) for the US Department of Energy(ATP: adenosine-5′-triphosphate (after [1.15]))

1.1.2 Definition of Nanoscaleand Nanomaterials

As mentioned earlier, nanomaterials differ from thecorresponding bulk materials in many ways, and thisdissimilarity is the reason for the formation of a newdiscipline that describes the properties and behavior ofnanomaterials. To exhibit these unique nanoproperties,

materials need to have one, most obvious feature:at least one dimension should be on the nanoscale.In this section, we elaborate what nanoscale meansand describe the common properties and features thatmake nanomaterials different from their well-knownmacroscopic (bulk) counterparts. We also give def-initions of nanosize, nanoscale, and nanostructuredmaterials.

Introd

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Science and Engineering of Nanomaterials 1.1 History and Definition of Nanomaterials 5

Sizes Are Important: Usual Range of ValuesFirst of all we need to define the size range of struc-tures. The term nano comes from Greek and meansdwarf ; when used together with units of physical quan-tities, it expresses 10−9 times smaller than the unit.For nanomaterials, the nanoscale mainly means thatlength (e.g., size, diameter, edge) is measurable on thenanometer scale. Obviously, other physical quantities,such as area, volume, mass, and energy, may be veryfar from this particular prefix when we use the corre-sponding SI units. More exactly, most of the definitionsplace nanomaterials between the approximate limits of1 and 100 nm. One nanometer is also equal to 10 Å, us-ing the Angstrom, which is widely used in microscopyand atomic/molecular physics.

In Fig. 1.2, a comparative scale is displayed to showobjects with sizes that fall into our common under-standing (e.g., ants, pinhead, piece of human hair) tonanomaterials with few-nanometer feature size (e.g.,fullerene, nanotube, and DNA (deoxyribonucleic acid)).The figure also shows objects on the intermediatemicrometer scale (red blood cells and microelectrome-chanical systems (MEMSs)), and for comparison withanother well-known scale, the different ranges ofelectromagnetic waves. On this scale, nanomaterialscan be positioned between the wavelengths of visi-ble/ultraviolet light and x-ray radiation. Being smallerthan the shortest visible wavelengths limits the methodsavailable for determining the shapes of nanomaterialsby conventional light microscopy.

Fig. 1.3 Schematic of the top-downand bottom-up nanomaterial produc-tion procedures

Different from BulkThe most important question is why materials on this1–100 nm scale are distinguished and have their ownscience and engineering. In the size range below 1 nmwe can find molecules, atoms, elementary particles, etc.,which are different from the bulk but already have theirown disciplines. Multiple atoms or molecules behavedifferently from individual ones for some obvious rea-sons; e.g., larger size results in different behavior incollisions with atoms. Other changes with increasingnumber of atoms in the particle include perturbationof the atomic energy levels leading to energy bandstructures with fine structure; e.g., the energy differ-ence between neighboring states is E/2N , where E isthe width of the energy band and N is the number ofatoms included. The distinction between molecules andnanomaterials, however, is not sharp. C60 is considereda molecule and nanomaterial at the same time, and otherlarge molecules are also used as materials in molecularelectronics.

At the other end of the size scale, materials start tobe different from bulk below 100 nm size, because theeffects of quantum confinement on electrical, thermal,and optical properties become significant at about thissize.

Another, very important common feature of nano-materials (e.g., nanoparticles and nanocrystals) is thatthey have a very high fraction of their atoms on theirsurfaces. These atoms behave differently from the oneslocated inside the object or in ideal bulk crystals (in

Introd

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6 Introduction

100 nm

1 μm2 μm 200 nm

a) b)

c)

Fig. 1.4a–c Examples of top-downnanostructure fabrication methods.(a),(b) Nanoletters generated by directwriting with an AFM tip. (a) Theletters are made of SiO2 on a siliconwafer. (b) Structure after selectiveetching to remove the oxide [1.16].(c) High-aspect-ratio silicon pillarscreated by pattern generation by Ga-ion implantation in a focused ionbeam (FIB) setup and reactive plasmaetching of the wafer [1.17]

fact, the definition of an ideal crystal includes no phys-ical boundary or surface, as the crystal exhibits infiniteperiodicity) owing to the asymmetrical forces actingon them; there is a force acting on these atoms whichis directed into the particle. The integrated effect ofthe forces on every surface atom provides a surfacetension, the related pressure being so high that it canchange bond lengths in crystals. Through this, it also

changes mechanical, electrical, and thermal properties;e.g., the melting point of small particles can be con-siderably lower than the bulk value. In the remainderof this chapter we describe in slightly more detailthese changes in the physical properties of nanoma-terials as a function of feature size; later chapters inthis handbook provide more data for individual materialgroups.

1.2 Formation of Nanomaterials

Formation of nanosized materials – nanoparticles, nano-porous or nanostructured macroscopic materials – isachieved by two basic routes, namely the so-called top-down and bottom-up methods [1.20, 21]. In the former,macroscopic materials are used to fabricate nanomateri-als and nanostructures using – typically – very sophis-ticated methods. In the latter, nanoparticles and other

Fig. 1.5a–f Examples for bottom-up assembly of nanoparticles. (a–d) Different representations of an AuPd nanoparticle:(a) AFM topography of the surface of the sample covered by the nanoparticles; (b) high-resolution transmission electronmicroscopy (HRTEM) image of an individual nanoparticle; (c) HRTEM simulation of the same nanoparticle; (d) solid-ball atom model of an icosahedral nanoparticle (after [1.18]). (e,f) Explanation of the correlation between particle sizedistribution and the flow properties of the carrier gas. (e) Size distribution at medium inert gas drift conditions; (f) sizeand size distribution as a function of drift intensity (after [1.19]) �

nanomaterials are built up from their ultimate buildingblocks – atoms and molecules – via self-assembly pro-cesses. Figure 1.3 shows a comparative representationof top-down and bottom-up methods. In the top-downmethod we start from bulk materials and fabricate struc-tures or particles on the nanoscale, analogously to thecarving of a sculpture (or many small ones) from a mar-

Introd

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Science and Engineering of Nanomaterials 1.2 Formation of Nanomaterials 7

800

600

400

1000

200

600400200 800 x (nm)

z (nm)

3000

2500

2000

1500

1000

500

3500 10

1

0.1 2

1

0.01

0.001

0.0001

Number of particles

Particle radius (arb. units) Normalized drift (δ/δ0)

Normalized geometric mean (Š/L2)

Geom

etric standard deviation (σ)

N = 105

L = 200

L = 10L = 50L = 100L = 200L = 500

δ = 0.02

σ = 1.32Š = 9244

030 00025 00020 00015 00010 000 10 0001001010.1 10000.0150000 35 000

1 nm

1 nm1 nm

a) b)

c)

e) f)

d)

Š/L2

σ

Critical drift

Introd

uction

8 Introduction

Nature ofpatterns

Space group

Bluedomains:A block

Volumefractionof A block

Spheres(SPH) (3-D)

lm3m p6mm la3d Pn3m Pm

0–21% 21–33% 33–37% 37–50%

Cylinders(CYL) (2-D)

Double gyroid(DG) (3-D)

Double diamond(DG) (3-D)

Lamellae(LAM) (1-D)

a)

b)

150 nm

A

a b c d

e f g h

i j k l

C

B

Fig. 1.6a,b Block copolymer self-assembly used in bottom-up and top-down fabrication of nanostructures. (a) Schematic showingvarious block copolymer morphologies; blue color represents minority phase, with the matrix surrounding; schematic of a lineartriblock copolymer; triblock copolymer structures displayed by coding colors. (b) SEM image of the Co dot array developed bypoly(styrene-b-ferrocenyldimethylsilane) (PS-PFS) BCPs and Ne ion-beam etching (after [1.22–24])

ble boulder; in the bottom-up method we use techniquesto build up materials from their component parts, i. e.,atoms and molecules, analogously to the building ofa cathedral or forming a sculpture from clay.

As procedures used in synthesis and characteriza-tion are normally considered parts of nanotechnologyrather than nanomaterials science, being discussed indetail in the nanotechnology literature, we only givea very short description for the sake of completeness.

Top-down approaches include evolutionary tech-niques which are similar to those used in microtechnol-

ogy of integrated circuit fabrication (e.g., photolithog-raphy developed to < 20 nm resolution, nanoimprintlithography) along with several completely new meth-ods [e.g., e-beam and focused ion beam (FIB) lithog-raphy, direct writing with atomic force microscopy(AFM), and scanning tunneling microscopy (STM)].Figure 1.4 shows examples of these revolutionary meth-ods. Figure 1.4a presents a demonstration of nanoscaleoxidation of silicon in the shape of letters using anAFM tip [1.16], and the same pattern after etching outthe oxide by hydrofluoric acid (HF) (Fig. 1.4b). The

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Science and Engineering of Nanomaterials 1.2 Formation of Nanomaterials 9

a) b)

c)

Au substrate

Molecular transport

Alumina template

Seperated cupConnected cup Nanoring structure

AFM tip

Writingdirection

Water meniscus

ECD of NWs CVD of Y-CNTs

Y-CNTs

NWNW

200 nm

5 μm

200 nm

50 nm

5 nm

Metallayer

Fig. 1.7a–c Examples of nanomaterial production by methods which merge top-down and bottom-up features. (a) Dip-pen nanolithography, AFM structure deposition, and ink self-assembly on the surface (after [1.25]). (b) TEM image ofgold nanowires grown in an alumina template (after [1.26]); complex shapes and structures generated in predesigned andfabricated alumina template junctions (after [1.27]). (c) Schematics of low-aspect-ratio nanocup preparation on aluminatemplate and SEM images of the two sides of the freestanding nanocup layer (ECD: electrochemical deposition; NW:nanowire; Y-CNT: Y-shaped carbon nanotube) (after [1.28])

printing resolution of the letters corresponds to almost2 000 000 dpi, which is in the range of Feynman’s calcu-lations. Figure 1.4c shows high-aspect-ratio structuresfabricated by cryogenic plasma etching of gallium ion-implanted regions of a silicon wafer [1.17]. The authorsdefined the pattern by FIB lithography and used reac-tive etching, resulting in structures with 65 and 40 nmdiameters and 600 nm height (aspect ratio up to 15).

Bottom-up approaches are quite similar to the for-mation of macroscopic crystals. However, the thermo-dynamics of the processes involved for nanomaterialsdiffer from the bulk; as we show later, the free en-ergy, chemical potential, phase diagrams, and kineticsof phase transformations are distinct. Nucleation [1.29]and diffusion are the key elements in the formationof particles; however, other parameters, such as forcedor natural drift of the carrier gas in the synthesis(Figs. 1.1 and 1.5) of nanoparticles by the inert gas de-

position method [1.3, 14, 18, 30, 31], also play a veryimportant role in determining the mean particle sizeand distribution width [1.19, 32]. One of the mostwidely used bottom-up methods is chemical vapor de-position (CVD), where nanomaterials grow on catalystlayers; references to illustrate such materials can betaken from thousands of related papers for single-wallednanotubes (SWNTs) [1.33, 34], multi-walled nanotubes(MWNTs) [1.35], and h-BN [1.36] deposition.

Solution-based methods are governed by the con-centration of the components used, the temperature ofthe reaction, and the amount of surfactant; examplesinclude seed-mediated growth of copper nanoparti-cles [1.37], synthesis of shape-controlled gold nanopar-ticles [1.38], and specifically the Kirkendall effectapplied for preparation of hollow particles [1.39, 40].

Sonothermal synthesis [1.41] and exfoliation oftwo-dimensional (2-D) materials – for making nanoma-

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10 Introduction

terials from bulk – are considered bottom-up methods,as we do not have control over the exact manner offormation or properties of each of the particles viamacroscopic equipment or tools.

Another possibility is to combine the advantagesof the bottom-up and top-down methods. A fewsuch examples are displayed in Figs. 1.6 and 1.7.An interesting way of increasing the resolution ofphotolithography beyond its physical limitations isby using directed self-assembly and self-orientationof block copolymers [1.42]; structures derived from

this technique are displayed in Fig. 1.6. Figure 1.7ashows schematics of dip-pen lithography; in this case,the AFM manipulation is the top-down componentwhile self-assembly of the printed materials is thebottom-up component. In the widely used methodof membrane-based deposition [1.26], we have con-trol over the shape of the alumina template troughparameters such as the etching time, acid type,and temperature; as examples, nanowire, nanojunc-tion nanocup, and nanoring fabrication are collected inFig. 1.7b–c.

1.3 Properties of Nanomaterials

When nanomaterials are studied, with few exceptions,the goal of the investigation is to reveal the values ofphysical properties similar to those used to character-ize bulk phases of materials. In most cases though, thevalues of these properties are very different from thosein the bulk, depending on the size and morphology ofthe nanomaterials; this is similar to the way in whichthe properties of bulk materials also depend on theircrystal system (e.g., face-centered cubic (fcc) versusbody-centered cubic (bcc)).

1.3.1 Morphology of Nanomaterials

Not only the overall size, but also the shape ofnanomaterials is a factor governing other proper-ties; the aspect ratio, porosity, and surface roughnessall change the surface-to-volume ratio and therebyother properties. Shape is such an important prop-erty that classification of nanomaterials can be basedon their dimensionality or aspect ratio. Figure 1.8shows schematics of nanomaterials limited to zero,one, two, and three dimensions. Zero-dimensional (0-D)objects have nanometer feature size in every direc-tion; one-dimensional (1-D) objects have nanometersize in two directions but larger, e.g., micrometer,length in the third; two-dimensional (2-D) objectsare atomically thin sheets of materials, while three-dimensional (3-D) nanomaterials are nanoporous ornanostructured materials. Figure 1.8b, c, and d showexamples of 0-D, 1-D, and 2-D materials, respec-tively. Quantum dots [1.43], nanoparticles [1.3, 14],and fullerenes [1.44] are 0-D objects; the exampleshown here is a series of TEM images of iron ox-ide nanoparticles with particle diameters of 6–13 nm,where the diameter was controlled with 1 nm accu-racy [1.45, 46]. The one-dimensional material shown

in Fig. 1.8c is a GaAs nanowire mat; the inset showsan individual wire [1.47]. The most famous 1-D mater-ials are the single-walled nanotubes (SWNTs) [1.48,49]and multi-walled carbon nanotubes (MWCNTs) [1.50].Two-dimensional materials include graphene [1.51],boron nitride (h-BN) [1.36], and graphene oxide [1.52].In Fig. 1.8d, TEM images of BCN of two and threeatomic layer thickness are shown [1.53]. Typical 3-Dnanostructured materials are nanoporous metals and ce-ramics, aerogels, and zeolites [1.54]. Figure 1.8e showsGleiter schematics of different kinds of 3-D mater-ials [1.55], and Fig. 1.8e,f illustrates the self-assemblyof microparticles from azobenzene thiol-functionalizednanoparticles [1.56].

1.3.2 Bonds and Structures

The compressive strain in bulk solid materials is rel-atively small, even in the case of high applied stress,because the bonds in solids are strong and changingtheir length requires high pressure and a substantialamount of energy. Quite amazingly, the asymmetrycaused by the forces near to surface atoms in solidschange the bond length of nanomaterials at least on thescale that we are able to achieve by applying outsideforces. The importance of the effect of these changes inbond length increases with the surface atom to volumeatom ratio (dispersion) and accordingly on the particlesize; it is determinant in clusters and nanoparticles at thelower end of the nanoscale, normally between 1–5 nm(about 50–5000 atoms).

Table 1.1 presents a collection of data for bondlength relaxation [1.57], data based on references [1.58–70]. It is clear that bond contraction is between 4%and 30%, and, surprisingly for a bulk material scientist,there is a 30% change in the bond length of diamond,

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Science and Engineering of Nanomaterials 1.3 Properties of Nanomaterials 11

a)

b)c) d)

3-D 2-D 1-D 0-D

e) f)

Chemicalcompositionof crystallites

Shape ofcrystallites

Layer-shaped

SameDifferent

for differentcrystallites

Families of NSM

Cat

egor

ies

of N

SM

Composition ofboundaries and

crystallitesdifferent

Crystallitesdispersed in matrix

of differentcomposition

Rod-shaped

Equiaxedcrystallites

Fig. 1.8a–f Classification of nanomaterials based on their shape, dimensionality, and structure. (a) Simple schematicsshowing 1-D, 2-D, and 3-D nanomaterials. (b) 0-D structures: TEM images of iron oxide nanoparticles with particlediameters of 6, 7, 8, 9, 10, 11, 12, and 13 nm (after [1.45, 46]). (c) 1-D structures: SEM images of GaAs nanowireswith an inset of the TEM image of an individual nanowire (after [1.47]). (d) 2-D structures: atomic layers (two andthree) of BCN (after [1.53]). (e) Schematics of classification of different 3-D (bulk-like) nanostructured materials (NSM)(after [1.55]). (f) 3-D structure: a microobject built up from self-organized nanoparticles (after [1.56, 71])

too. The table summarizes data for covalent, metallic,and ionic bonds. While the contraction of ionic bondsis slightly less than that for the other two bond types, itis also obvious that the phenomenon is universal for allkinds of bonds. Table 1.1 also names the changes in thephysical properties – energy related to bonds, magnetic

momentum, and hardness – caused by the bond lengthcontraction and reported in the referenced literature.

Figure 1.9 illustrates how the bond length and bondpotential depend on nanoparticle size and shape. Fig-ure 1.9a shows the relative change in the lattice constantfor several metal nanoparticles – gold, copper, platinum,

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12 Introduction

Table 1.1 Bond lengths for typical bonds (covalent, metallic, and ionic) and effect on several physical properties (af-ter [1.57], data from [1.58–70])

Bond nature Medium c1 = d1/d Effect

Covalent Diamond {111} [1.72] 0.7 Surface energy decrease

Metallic Ru [1.73] and Co [1.74] 0.9

Re [1.75] (1010) surfaces 0.9 Atomic magnetic momentum is increased by 25–27%

[1.76–78]

Fe-W, Fe-Fe [1.79] 0.88

Fe(310) [1.77], N(210) [1.78] 0.88

Al(001) [1.80] Cohesive energy rises by 0.3 eV/bond [1.81]

Ni, Cu, Ag, Au, Pt and 0.85–0.9

Pd dimer bond [1.80] Single-bond energy increases 2–3-fold [1.80]

Ti, Zr [1.80] 0.7

V [1.80] 0.6

Ionic O−Cu(001) [1.82–84] 0.88–0.96

O−Cu(110) [1.82, 83] 0.9

N−Ti/Cr [1.85] 0.86–0.88 N-TiCr surface is 100% harder that the bulk [1.85]

Extraordinary (Be, Mg) (0001) Zn, Cd and >1 No indication of effects on physical properties is yet given

cases Hg dimer bond [1.80]

silver, and aluminum – in the size range of 1–20 nm.Figure 1.9b illustrates how the shape and position ofthe bond potential change for ≈ 15% lattice contrac-tion. The shorter bond length corresponds to a deeperand narrower potential well, resulting in stronger ma-terials and lower thermal expansion. Figure 1.9c showsnanochain formation for gold, in which chain formationis successful, and for copper, in which chains do notform. The bond energy for atoms in the nanometal chainstructures is 2–3 times larger than for bulk materials.

1.3.3 Mechanical Propertiesof Nanomaterials

Nanomaterials and nanostructured materials have dis-tinct mechanical properties for several reasons: Firstly,the shorter bond length results in stronger and stiffermaterials, as mentioned in the previous section; Fur-thermore, the limited size of the units of the materialdiminishes the probability of certain defects; e.g., grainboundaries are very rare in small nanoparticles. It is wellknown that carbon–carbon bonds in the hexagonal lat-tice of graphite are the strongest ones in any solid, andsecond only to the N−O bond overall. On the macro-scopic scale, however, we do not consider graphite anextraordinarily strong material as its bonds in the c-crystal direction are weak, and the lateral strength inthe a–b plane is diminished greatly by the fact that thelayer is not continuous, so pulling forces easily disinte-

grate the crystal. On the nanoscale, the hexagonal sheetsof carbon are separated, and strength is measured on in-dividual flakes and domains. The role of point defectsin nanomaterials, from the point of view of mechanicalstrength, is not as critical as for discontinuities in bulkmaterials. Graphene and carbon nanotubes have highstrength and Young’s modulus owing to these strongin-plane bonds.

It has been shown that metals with grain structureshave an optimum grain size for minimum creep [1.89];the optimum grain size for most metals is on thefew-nanometer scale, corresponding to the limit ofHall–Petch hardening by grain size reduction. Similarly,in the case of ceramics, the maximum Vickers hardnessis measured at the optimum grain size that falls in therange of 10–50 nm [1.90, 91].

Figure 1.10 demonstrates several basic mechani-cal properties of nanomaterials. Figure 1.10a and bshow the particle size dependence of hardness for

Fig. 1.9a–c Typical effects of properties of bonds innanoparticles as a function of particle size. (a) Dependenceof the bond length on particle diameter for several metals(after [1.86]). (b) Increase of the bond energy density withdecreasing particle size and the consequently shorter bondlength (after [1.57, 87]). (c) Bond strength as a function ofdimensionality. Chain formation of gold atoms and failureto achieve the same with copper. Binding energies of bulkand atomic 1-D chain structures (after [1.88]) �

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Science and Engineering of Nanomaterials 1.3 Properties of Nanomaterials 13

Ag

0

Au

Cu

Pt

Al

ε (%)

–1

–2

–3

0

–1

–2

–3

0.0

–0.3

–0.6

–0.9

–1.20.0

–0.5

–1.0

–1.5

–0.2

–0.4

–0.6

0 20D (nm)

105 15

a) b)

c)

0.5

Atomic potential

0

0.6 0.8 1 1.2 1.4 1.6 1.8 2

EC

EB (d)

EB (di)

ci–m–0.5

–1

–1.5

1

–1

–0.5

–2

–1.5 AuCuPtPdNiAg

–3 2.2 2.6 2.4 2.8 3.0 3.2 3.42

–2.5

0

0.5 Bulk fcc

Nearest-neighbor bond length (Å)

Binding energy per bond (eV)

1

–1

–0.5

–2

–1.5 AuCuPtPdNiAg

–3 2.2 2.6 2.4 2.8 3.0 3.2 3.42

–2.5

0

0.5 Chain

Nearest-neighbor bond length (Å)

Binding energy per bond (eV)

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14 Introduction

metals and intermetallic alloys [1.92], respectively. Inthe case of metals, there is an obvious trend for thehardness to be 3–10 times higher for the smallestnanoparticles than for those larger than 100 nm, lat-ter is the value corresponding to the bulk material.For the alloys, a trend of increasing hardness is vis-ible for the 100 to ≈ 40 nm range; however, in manycases this trend becomes softening for further diminish-ing nanoparticle sizes. Figure 1.10c,d shows bucklingof carbon nanotubes [1.93–95]. While SWNTs havevery high Young’s modulus and relatively high stiff-ness against tensile forces [1.96], they can be deformedby compressive forces, which makes them ideal high-resolution yet gentle and forgiving AFM tips [1.95].Figure 1.10e,f demonstrates the in-plane mechanicalstrength of a graphene sheet based on nanoidentationcarried out by an AFM tip [1.97].

1.3.4 Electrical, Magnetic,and Optical Properties

Nanoscale materials, especially with dimensionality ofzero to two, exhibit intriguing electromagnetic behav-ior, as the most important effects and properties arederived from the quantum confinement of the wavefunction of the charge enclosed in the particles. In thenanometer-sized dimensions of the material, the wavefunction has a limited number of solutions. This re-sults in changes in several electrical properties: withthe decreasing feature size the bandgap becomes wider,the conductivity decreases and the density of states de-creases. Optical properties change accordingly, as theabsorbed and emitted photons depend on the energydifference between the states among the bands or intheir fine structure. Beyond this, quasiparticles are gen-erated due to the confined space. An electron–hole pair,i. e., an exciton, carries energy in nanomaterials with-out motion of net charge. Plasmons, surface plasmons(polaritons), and polarons play a key role in nanoma-terial interactions with optical phonons, e.g., causingthe color dependence of gold colloids as a function ofparticle size. Magnetic properties are also governed bythe size of the nanomaterials; e.g., superparamagneticmaterials exist below the size of the domain size.

Electrical PropertiesElectrostatic forces play a major role in nanoparticleformation and also have an important effect on nano-material properties [1.71]. Most of the larger structuresbuilt up from nanomaterials are held together by vander Waals forces. One of the advantages attributed to

the high aspect ratio of 1-D structures is the field en-hancement of the electrostatic potential, which enablesa low turn-on voltage for field emission [1.98, 99].

Nanomaterials, especially 1-D structures such asnanowires and nanotubes, conduct electrical current dif-ferently compared with bulk conductors. The limitedsize in the direction perpendicular to the axis and theatomically organized structure of the 1-D objects –crystals or nanotubes, e.g., molecules – result in weakphonon–charge carrier interaction and ballistic trans-port. In the ballistic regime of electrical conduction, theresistance does not depend on the length of the conduc-tor but rather on the number of conductance channelsused, following Landauer’s law [1.100]. Often, there isa single channel, and the quantum nature of the conduc-tance as a function of the number of channels can bedemonstrated, e.g., for carbon nanotubes [1.101]. How-ever, in other reports, e.g., on nanotubes used to buildfield-effect transistors (FETs), the conductance mecha-nism is considered to be diffusive [1.102].

Another important effect in devices made of nano-sized conductors and semiconductors is the Coulombblockade that occurs when new charge carriers cannotenter the conduction channel while another charge car-rier occupies it, as demonstrated for single-moleculedevices [1.103]. This phenomenon enables the function-ing of single-electron transistors (SETs) even at roomtemperature [1.104]. SETs are also useful to investigatespin–spin interactions between localized and mobileelectrons, i. e., the Kondo effect, more effectively thanin macroscopic systems [1.105].

High current-carrying capacity – i. e., no failurecaused by electromigration – of individual carbon nano-tubes, both single-walled (4 × 109 A cm2) [1.106] andmulti-walled (> 1010 A cm2 at 250 ◦C) [1.107], and wiresmade of nanotubes has also been demonstrated. Sincecarbon atoms are bound into the nanotube by covalentbonds, the charge carriers and high temperature cannoteasily move them away from their original location. Thestate-of-the-art specific conductivity of carbon nanotube

Fig. 1.10a–d Selected mechanical properties of nanoma-terials. (a,b) Hardness of metals and intermetallic alloysas a function of grain size in nanophase materials (af-ter [1.92]). (c),(d) Molecular dynamics computation of thebehavior of a carbon nanotube under compressive force.The structure in the strain energy curve corresponds tothe modes of buckling (after [1.93–95]). (e,f) Measure-ment and values of the Young’s modulus for a suspendedgraphene sheet on an alumina template by AFM (af-ter [1.97]) �

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Science and Engineering of Nanomaterials 1.3 Properties of Nanomaterials 15

a)12

4

6

2

Cu

Cu

Se

Pd

NiNi

AgFe

Fe

00.1

100 15 5

0.2 0.3 0.4 0.5 0.60

8

10

d –1/2 (nm–1/2)

d (nm)

Har

dnes

s (G

Pa)

14

12

4

6

2

Nb3Sn

FeCuSiBFeMoSiBFeMoSiBFeSiBNiZr2

NiZr2

TiAlTiAl

Ni-PNi-P

Nb3AlNb3AlTiAlNb

00.1

100b)

15 5

0.2 0.3 0.4 0.5 0.60

8

10

d –1/2 (nm–1/2)

d (nm)

Har

dnes

s (G

Pa)

c) d)

0.006

Strain energy

0.004

0.002

00 0.05 0.10 0.15

Strain εe)

0.5 μm

1 μm 1.5 μm

f)

15

10

5

0235 268 469

E2-D(N/m)436402369335302

0.7 0.8 1.4

Effective Young’s modulus (TPa)

Cou

nts

1.31.21.11.00.9

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16 Introduction

50004.2 K

350 K

6000Mobility (cm2/Vs)

Carrier density (1012 cm–2)

4000

3000

2000

0

1000

–3 –2 –1 0 1 2 3

e)100

Current gain (h21

)

Frequency (GHz)

10

11 10 100

f)

3-D 2-D

Energy

Density of states

200I (μA)

Δt (s)

150

100

50

00 0.05 0.10 0.15

InitialThinned(3 steps)

Thinned(10 steps)

0.20

Density of states

Energy

Metal SemiconductorBulk Nanocrystal Atom Bulk Nanocrystal Atom

1-D 0-D

a) c)b)

500

G (μS)

VTG

(V)

400

300

200

0

100

–2.5 –1.5 –0.5 0.5 1.5 2.5 3.5 4.5

20 VBG

10 VBG

–5 VBG

–15 VBG

15 VBG

5 VBG

–10 VBG

–20 VBG

d)

5.0Conductance (G

0)

Depth (nm)

4.0

3.0

2.0

1.0

00 1000 2000 3000

Holes Electrons

1/f

TG

SD

BG

Dev1: Vd= 2.0 V

Dev2: Vd= 2.0 V

Dev1: Vd= 2.5 V

Dev3: Vd= 2.0 V

Fig. 1.11a–f Examples for electrical properties of nanomaterials. (a) Schematics to represent changes in the DOS of nanomaterialsas a function of dimensionality (after [1.109]). (b) Quantized decrease of the current when shells of a multi-walled nanotube burneddown consecutively and the schematics of the nanotube with partially removed walls (after [1.110]). (c) Conductance of carbonnanotubes when a parallel circuit was built from one to four individual tubes. As the voltage was low (100 mV), each nanotubeadded only one channel to the conductance and the measured values were nG, where n is the number of nanotubes/channels.A larger number of nanotubes resulted in conductance up to 1000 G (after [1.111]). (d) Transistor characteristics of a FET devicewhere the active element was a single-layer graphene channel. The figure displays the transconductance, charge carrier mobility,(e) and current gain (f) as a function of top and bottom gate voltage, carrier density and temperature, frequency and drain voltage,respectively (after [1.112])

fibers surpasses the values for all conventionally appliedmetallic conductors (Cu, Al, Au, etc.) [1.108].

Figure 1.11 shows a collection of electrical proper-ties. Figure 1.11a shows a schematic of the dependence

of the density of states (DOS) on the dimensionality ofthe nanomaterials; the larger the number of quantum-confined dimensions, the higher the number of discretestates in the DOS [1.109]. Figure 1.11b shows how the

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Science and Engineering of Nanomaterials 1.3 Properties of Nanomaterials 17

resistance of a carbon nanotube increases as it loses car-bon layers from its wall; the resistance, and accordinglythe conductivity, also changes in quanta, illustratingthe independence of the conductance channels in eachlayer [1.110]. Results of a similar experiment are shownin Fig. 1.11c. Here, the conductance of a nanotubebundle was measured when one, two, three, and fournanotubes were included sequentially in a circuit in-cluding a contact made of mercury [1.111]. In bothcases, the size of the quantized steps of the conduc-tance change – following the Landauer law – is equal toG0. By immersing the nanotube bundle further into themercury electrode, the authors measured conductanceas high as ≈ 1000 G0. In Fig. 1.11d–f, the character-istics of a FET based on a single-layered graphenesheet are shown [1.112]. First, the structure of thedevice and the conductance as a function of the topgate (TG) voltage are displayed at different back-gate(BG) voltages. The electron and hole mobility were cal-culated from Hall-effect measurements, being higherthan 5000 cm2 V−1 s−1 for low temperatures and car-rier densities, and around 3000 cm2 V−1 s−1 at roomtemperature (RT). The high-frequency behavior of thedevice shows 1/ f dependence of the current gain asa function of frequency, and the cutoff frequency is inthe range of 100 GHz.

Magnetic PropertiesThe magnetic properties of nanomaterials dependstrongly on the characteristic size, i. e., the diameter ofnanoparticles or the grain size of nanostructured ma-terial, which is normally small in comparison with themagnetic domain size of the material. The configura-tion of thin films and few-domain nanoparticles wasinvestigated by Kittel [1.113]. Figure 1.12a presentsthese configurations and the energy density for thevarious configurations in thin films, showing that (forsmall size) the single-domain system is energetically fa-vorable. Nanomaterials can be classified according tothe type of interaction among the magnetic particles,extending from no interaction in a well-distributednanoparticle system to strongly interacting nanostruc-tured materials [1.114] (Fig. 1.12b). Ferrofluids consistof particles surrounded by surfactants, capping agents,and solvents, forming an independent system, while al-loys with magnetic constituents are strongly correlated.

Superparamagnetic materials behave similarly toparamagnetic materials. However, the magnetizationvalue is considerably higher and the particles keeptheir magnetization values for a measurable timescale,e.g., several minutes. The magnetization curve of su-

perparamagnetic materials does not show hysteresis,and the exact shape depends on the particle size or itsdistribution.

By introducing defects into the lattice of carbon,e.g., implanting nitrogen or carbon atoms into nanodi-amond, ferromagnetic behavior is observable [1.115].Also, ferromagnetism is measured on monoatomiccobalt structures at low temperature (10 K), showing theinteraction between neighboring atoms [1.116]. DuringMackay transformation and Bain transformation, thecrystal structure is changing, and the magnetic proper-ties follow the crystal symmetry [1.117], as shown inFig. 1.12c.

Figure 1.12d shows the grain size dependence ofthe magnetization of Ni films of different grain sizes,i. e., the magnetization hysteresis loops, the coerciveforce values, and the relative change in the magne-tization [1.118]. In all three cases, the changes aresignificant below feature size of 10 nm.

Optical PropertiesIt is a common misbelief that nanomaterials cannot beseen by the naked eye or be detected by optical mi-croscopy. This misbelief is based on the fact that thenanoscale is defined as 1–100 nm while the wavelengthof visible light ranges between 400 to 800 nm; however,this scale difference only means that size and shape can-not be resolved by visible-range photons. In fact, thereare plenty of interactions between atoms, molecules,and nanomaterials with light, and some of these inter-actions are applicable to detect the size and morphologyof nanomaterials. All of these detection methods need tobe first used in parallel with other approaches to mea-sure, e.g., nanoparticle size, and after this calibration,the optical method itself can serve as a fast and easymeasurement approach. In the case of gold nanoparti-cles or thin layers of graphene, practiced researchers cantell their size or thickness by eye alone, in the latter casewith subnanometer, atomic accuracy. In this section, themain features of the optical properties of nanomateri-als are described, while detailed description of theseproperties for specific nanomaterials is included in therelevant chapters of this handbook.

Optical properties – similarly to the electrical ones –are governed by quantum confinement: the lower di-mensionality and smaller size result in a larger energydifference between neighboring discrete energy levelsin the DOS, and accordingly higher excitation energy(Fig. 1.13) [1.109]. Inasmuch, the smaller the particle,the shorter the wavelength in the absorption spectrum,and the color shifts from red to blue. This interac-

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18 Introduction

tion with photons can be interpreted by introducinga quasiparticle known as the surface plasmon (SP) torepresent the oscillations in the confined space [1.43].The absorption spectrum has a maximum when the SP–photon interaction is strongest, at the surface plasmonresonance (SPR) wavelength. The SPR for differentnanoparticles depends – beyond the size – also on thematerial and shape of the nanoparticles. Figure 1.13ademonstrates visible-range colors caused by SPRs ofdifferent energy for several basic shapes of gold, sil-ver, and alloy nanoparticles [1.119]. Figure 1.13b showsa particular case from [1.119], where the resonancewavelength shifts when the silver nanoshell particlesare coated with a gold layer of increasing thickness.Figure 1.13c presents a silver nanoprism preparationmethod where the size distribution and accordingly theabsorption spectrum are well controlled by the wave-length of light used in the photo-induced reaction.Another interesting phenomenon is the shift in the ab-sorption of gold nanotriangles from 800 nm to 600 nmwith the extent of the truncation of the tips [1.120].

Along with other microscopy techniques, the opti-cal property corresponding to Raman scattering [1.121,122] provides insightful information into the localbonding system of nanomaterials through phonon–photon interactions. It is used for characterizing a widerange of materials from metals to oxides; one of thecharacteristic examples is its application in studyingcarbon nanomaterials. Figure 1.13d shows spectra ofcarbon structures for comparative analysis. Graphenehas two characteristic peaks, the ratio of the intensityof the G and G′ peaks depends on the number of layersin the 2-D structure. In the case of carbon nanotubes, theD and G peaks dominate the spectra, while for SWNTs,the radial breathing mode (RBM) is also present andprovides information about the diameter and (to someextent) the chirality of the nanotubes. Shifts, shoulders,and new peaks in the spectra can be interpreted as sig-nals caused by defects in the bonds or in the structure.

Fluorescence [1.123] and bandgap photolumines-cence are also characteristic properties of nanomaterials,the latter being applied, e.g., to study the diameterand chirality of SWNTs in solutions [1.124, 125]. Theschematics of the band structure with the first and sec-ond van Hove optical transitions and the measuredvalues on a Kataura plot are displayed in Fig. 1.13e.

1.3.5 Thermal Properties

Thermal conductivity, specific heat, melting point, andglass-transition temperature are just a few examples

Fig. 1.12a–d Selected magnetic features in nanomateri-als. (a) Domain configurations studied by Kittel. For thinfilms the single-domain configuration has lowest energyfor thickness below 300 nm. Several domain configura-tions for nanoparticles also were suggested in this paper,which was published in 1946 [1.113]. (b) Schematics forthe main types of nanostructures for magnetic behavior,including independent ultrafine particles (e.g., ferroflu-ids), core–shell particles with magnetic core, particles asfillers in a matrix, and small crystallites in a noncrys-talline matrix (after [1.114]). (c) Mackay transformationshows the magnetization changes as a function of crys-tal structure. Schematics of icosahedron, cuboctahedron,and fcc with inscribed body-centered tetragonal Bain celland the magnetization as a function of cluster size aredisplayed (after [1.117]). (d) Magnetic hysteresis loops,coercive forces, and relative changes compared with thebulk value for Ni film with different grain sizes is displayed(after [1.118]) �

of thermal properties that strongly depend on the par-ticle or feature size of nanomaterials. Phase diagramsof nanomaterials depend on the particle size, shape, andeven their environment.

The Gibbs–Thomson equation describes the de-creasing tendency of melting point with decreasingparticle size. For spherical particles, for instance, itpredicts that the difference in the melting point hasinverse dependence on the diameter of the particle. Fig-ure 1.14a shows the melting point of gold particles asa function of particle size D [1.90, 126]. Below 20 nmthe decrease is observable, and for particles smaller than3 nm the melting temperature decreases by ≈ 600 K.Figure 1.14b illustrates an unusual behavior. In specificcases, indium particles embedded into an aluminum ma-trix show a melting point increase with decreasing grainsize; this exceptional behavior is caused by the interac-tion with the matrix: here, for smaller grain size, thesurface energy of the grain decreases.

Figure 1.14c shows experimental demonstrationsof the theoretically predicted, unusually high ther-mal conductivity [1.127, 128] of carbon nanotubesand graphene, ≈ 6000 W/(m K). Values of thermalconductivity measured on a thick layer of alignedSWNTs range up to 240 W/(m K) in the direction ofalignment, and approximately ten times lower in theperpendicular direction. A similar trend was observedfor the heat conductivity of aligned carbon nanotubeforests [1.129]. For the measurement of thermal con-ductivity in graphene sheets, the shift in the positionof the Raman G peak was investigated. This shift

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Science and Engineering of Nanomaterials 1.3 Properties of Nanomaterials 19

bcc (bulk)

C

BA

C

B

A

C

B

A

b

c

aa' b'

c'

D+ + + +– – – –

– – – –+ + + +

I

II

III

45°

T

Film thickness (cm)10–3

1000

100

10

10–7

Energy per unit area (erg/cm2)

10–6 10–5

Case II

3 × 10–5cm

Case I

Case III

10–6

Cluster size (atoms)

BccMackay transformedIcosahedralCuboctahedralBcc (nonmagic)BccIcosahedralExp.

600

3.5

3.0

2.5

2.0

1.5

0

Magnetic moment (μB/atom)

200 400

H (kOe)

D = 7.9 nmD = 5.5 nmD = 3.2 nm

D = 10 nm

D = 3.1 nm

1.0

4

2

0

–2

–4

–1.0

4πM (kG)

–0.5 0.50.0D (nm)

25

–20

0

–40

–60

–80

–1000

MS tailoring (%)

5 201510

a)

b)

d)

c)

L

L

A

B

C

D

D (nm)10

0.3

0.2

0.1

0.00.25

MC (kOe)

5 7.5

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20 Introduction

corresponds to ≈ 5000 W/(m K) heat conductivity. An-other interesting hybrid system is displayed in panelFig. 1.14d, where covalently bonded carbon nanotubepillars connect parallel graphene sheets to each other.By changing the geometry of this unique system, i. e.,the length and separation of the pillars, one can tune fordifferent vertical and lateral thermal conductivities.

1.3.6 Chemical Properties, Reactivity, andFunctionalization

Chemical approaches are used in almost all nanomate-rial preparation processes. As prepared, nanomaterialshave extraordinary properties; however, for many ap-plications they need to be modified, and accordinglyfurther chemical treatment is needed. Some nanomateri-als, e.g., fullerenes and nanotubes, can be considered asmolecules, and their chemistry is quite well defined. Inthe case of metallic and ceramic nanomaterials, the re-actions and products depend on the size and exact shapeof the nanomaterials used. In the narrower definition,we consider only modifications to the strong bonds –covalent, ionic, and metallic – of the materials; in thewider definition, modifications to weak interactions –hydrogen bonding and van der Waals interaction –among parts of the new nanomaterial are also consid-ered. Exchange reactions and chemical transformationsof nanoalloys also represent a vast field of nanomaterialchemistry [1.131].

In the scope of this short introduction, a fewexample reactions are presented, where changes in co-valent bonds are dominant. Small pieces of graphene,also called graphene molecules, can be handled bytypical methods of organic chemistry; they can be dec-orated, or connected to each other, and their lateral sizecan be controllably increased or decreased [1.132]. InFig. 1.15a, schemes of several graphene molecules aredisplayed, demonstrating differently sized, bare, and R-functionalized samples. The figure also presents a largermolecule, namely a nanopropeller, which is built fromthree graphene flakes connected to each other. The colorof these materials depends on the size of the molecules;with increasing size, the color shifts from blue in thedirection of red.

Another example of well-defined molecules whichare considered nanomaterials is the fullerenes. Thesecan be modified in two ways: additional atoms are ei-ther included inside the cage of the fullerene or added asoutside ligands, being called endohedral (inner) and ex-ohedral (outer) fullerenes, respectively. An uncommonexample is that of B80 [1.133], a fullerene decorated by

Fig. 1.13a–e Selected optical properties of nanomaterials.(a) Typical surface plasmon resonance spectral rangesof silver and gold nanoparticles having various mor-phologies, compositions, and structures (after [1.119]).(b) Ultraviolet-visible (UV-Vis) extinction spectra andphoto of solution of silver nanoshells coated with differentthicknesses of gold, and solid gold colloids; TEM image ofthe shell and solid particles (after [1.119]). (c) TEM imageof silver nanoprisms and extinction spectra of nanoprismsprepared with illumination by various laser wavelengths(after [1.130]). (d) Raman shift measured on different typesof graphene-related nanocarbons. The main features (RBMand disorder-induced D, D′, and D + D′ bands; first-orderRaman-allowed G band; and second-order Raman over-tones G′ (2iTO) and 2G) are labeled in some spectra, butthe assignment applies to all of them. The analysis of thefrequency, line shape, and intensity of these features atdifferent exciting laser wavelengths provides a great dealof information about each respective sp2 carbon structure(after [1.122]). (e) Method for identifying single-walledcarbon nanotube diameters by spectrofluorometry (Katauraplots). First and second van Hove optical transitions aredisplayed as a function of structure for semiconductingnanotubes (after [1.125]) (HOPG: highly oriented pyrolyticgraphite; SWNH: single-walled nanohorn) �

alkali ligands, which has been suggested for use as aneffective hydrogen storage material (Fig. 1.15b) [1.134].

Carrying out covalent chemistry of SWNTs isnecessary for achieving properties important for theformation of composites, preparation of solutions,etc. [1.135]. In most cases, the first chemical reactionaims to break the strong carbon–carbon sp2 bonds in thesidewalls of the nanotubes by oxidation or fluorination.Afterwards, the attached OH or F groups can be substi-tuted with alkyl or aromatic groups. Figure 1.15 showsdifferent kinds of nanotube reaction products. Variousgroups can be introduced onto the sidewall or defectlocations in the sidewall, or in the cap; surfactants orpolymers can be attached to the surface via noncovalentbonds, and the cage of the nanotube can also be filledwith different chemicals (in this schematic, C60).

Producing nanotubes with given chirality by directgrowth or separation methods is still an underdevel-oped area and a challenging task. Functionalizationof metallic nanotubes [1.136] with side-groups, whichmakes them heavier than unfunctionalized semicon-ductor nanotubes, offers a way for more selective andhigher yield separation.

Another important area of nanocarbon chemistryis the oxidation, reduction, and functionalization of

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Science and Engineering of Nanomaterials 1.3 Properties of Nanomaterials 21

514500488470

550600633

1000λ (nm)

400

Extinction

800600

800Wavelength (nm)

10 nm

400 nm 750 nmLight color

Silver rods

Silver spheres

Gold/silver alloyed spheres

Silver cubes

Gold spheresGold rods

Gold shell with hollow interiors

Silver plates

0.6

0.5

0.4

0.3

0.7

0.2

300

Extinction (arb. units)

700600500400

a)

b)

c)

d)e)

(d)(c)(b)(a) (e)

(d)(c)

(b)(a)

(e)

514500488470 550 600 633

4000

Graphene

HOPG

RBM

G

C2

V2

V1

C1

E22S

E22S

E33S

E44S

E11S

E11S

G'

SWNT

SWNH

Amorphous carbon

Damaged graphene

Raman shift (cm–1)0 3000 2.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

4.5

0.00.5 2.01.0 1.520001000

Intensity

Density of electronic states

Energy

Nanotube diameter (nm)

Eii (eV)

DD + D'

D'G

2GG'

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22 Introduction

graphene. Graphite oxide is often produced to aid ex-foliation of layers of graphene into graphene oxide(GO), after which reduction procedures are needed toyield graphene as a product (reduced graphene oxide,RGO) [1.137, 138]. Figure 1.15d displays the consec-utive steps applied in one of the possible reductionprocedures of GO, as well as a photo of the solutions ateach step and the C 1s x-ray photoelectron spectroscopy(XPS) peak in the spectra of these materials.

1.3.7 Behavior of Nanomaterialsin Corrosive Environments

Some of the ancient and medieval applications of nano-materials pointed towards corrosion prevention. TheChinese heiqigu are black bronze mirrors with a surfacecoating made of SnO2 nanoparticles, most probablydoped with Cu, Fe, Pb, and Si [1.145,146]. Similarly, asmentioned in Sect. 1.1.1, Mayan blue paint was not onlya rare and beautiful color for its time, but also had corro-sion resistance and retained its properties for centurieswhile buried in soil [1.7].

Figure 1.16a shows an example of preventing analuminum alloy from corroding in an NaCl-containingwater environment [1.147]. The scanning electronmicroscopy (SEM) and TEM images show the mor-phology of halloysite (aluminosilicate) nanocontainerswhich hold and release anticorrosion agents, normallypolymers, onto the surface when the pH of the envi-ronment changes. The photos show extensive surfacecorrosion of the specimen in a dilute solution whencoated without the nanomaterial included but minimaldamage in ten times higher concentration salt solutionwith the filled halloysite additive.

Superalloys are designed and manufactured not onlyfor mechanical strength and creep resistance but alsoto avoid corrosion in high temperature and harsh en-vironmental conditions. They themselves may or maynot consist of nanostructured materials. Figure 1.16bshows an effective nanomaterial coating for corrosionprevention of a Ni superalloy (K52) [1.148] depositedby sputtering a nanostructured layer of the same alloy

Fig. 1.14a–d Thermal properties of nanomaterials. (a) Melt-ing point of gold nanoparticles as a function of size,showing decrease with smaller sizes (after [1.90, 126]).(b) Melting point of indium nanograins in aluminum ma-trix; the direction of deviation from the bulk value dependson the method of preparation (after [1.139]). (c) Ther-mal conductivity of SWNTs and graphene. The thermalconductivity of aligned nanotubes is within an order ofmagnitude of in-plane values for graphite or diamond (af-ter [1.140, 141]). Raman shift of the G peak position ofa graphene sample versus change in total dissipated power.Values calculated for thermal conductivity are higher thanfor CNTs (after [1.142]). (d) Pillared graphene structureproposed originally for hydrogen storage (after [1.143]).Thermal conductivity of the structure along the grapheneplane direction is decreased while the vertical thermal con-ductivity is increased by increasing the number of pillars(after [1.144]) �

onto the surface of the specimen. The SEM and TEMimages show the morphology of the deposited layer; ithas a microgranular structure built up from sub-10 nmparticles. The samples were oxidized in air at high tem-perature, and the oxidation kinetics showed parabolicgrowth of the oxide layer (Wagner regime) for both theuncoated and coated alloy, however the rate of oxidationwas higher for the uncoated sample at all tempera-tures investigated. The surface of the oxidized samplewas coated by nanowire structures, mainly alumina andTiO2 nanowires, while the uncoated sample had a mi-crogranular oxide layer. In a hot salt environment, thetrends were different: the coated sample started to cor-rode faster, and after an initial fast scaling, the processslowed down.

In several cases, nanomaterials exhibit the same orworse corrosion resistance than bulk materials. This ef-fect is mainly attributed to the high concentration ofdefects in these nanoparticles and nanowires. In [1.149],silver nanowires were investigated, and the authorsfound faster sulfide formation in nanowires than forbulk silver and that the reaction mechanism was thesame.

1.4 Typical Applications of Nanomaterials

Nanomaterials are not a sanctified object of nano-science; they have already found applications in a verywide range of engineering fields. From mechanical

engineering to bioengineering, we use nanomaterialsregularly in our computers, stain-resistant clothes, andsuntan creams. Other applications such as nanomaterial-

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Science and Engineering of Nanomaterials 1.4 Typical Applications of Nanomaterials 23

a) b)

1300

500

1000

3000 200

D (Å)15010050

T (K)220

140

160

180

200

1200 100

Melting point bulk Melt-spun

Ball-milled

D (nm)60 804020

Tm (°C)

T0 =156.6°C

c)

d)

250

50

100

150

200H-aligned SWNTs

Aligned

Unaligned

00 400

T (K)150 200 250 300 35010050

(W/(mK))5000

1000

2000

3000

4000

6

0

2

4

–6

–2

–4

1500 1650Raman shift (cm–1)

1600 0 4Power change on the sample (mW)

3211550

Intensity (arb. units) G peak position shift (cm–1)

Excitation: 488 nmAmbient: room temperature

G peak

Laser power (mW)

0.9502.168

Suspended graphene ≈1583 cm–1 Suspendedgraphene

Experimental dataLinear extrapolation

Linear regression Y = A + B × XParameter Value ErrorA 1.19548 0.27638B –1.29207 0.11447

Thermal conductivity (W/(m K))

300

400

500

350

450

100

200

150

0

50

250

Graphite sheet length (nm)80 90 100403020 110706050

0

Thermal conductivity (W/(m K))

25

20

80

0

40

100

60

Minimum interpillar distance (Å)105 15 20

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24 Introduction

a)

c) d)

b)

C42

R

R R

R

RR

R

R R

R

R

R

RR

R

R R

R

R

R

R

R

RR

C60

SWNT

C78 B80Na12(H2)72

GO: 4.08×10–1 S/m CCG1: 8.23×101 S/mNaBH4/H2O

80 °C, 1 h

1100 °C

Annealingin Ar/H2 15 min

278

GPCCG3CCG2CCG1GO

296

294

292

290

288

286

284

282

280

298

CCG3: 2.02×104 S/m CCG2: 1.66×103 S/m

B80Li(H2)2

B80K12(H2)72

B80Na(H2)6

HAMB

B80K(H2)8

Conc. H2SO4180 °C, 12 h

Fig. 1.15a–d Chemical properties of nanomaterials. (a) Graphene molecules demonstrating various sizes and shapes.Graphene molecules are mostly 2-D materials; however, they can be organized into larger, 3-D structures; a nanopro-peller is shown as an example. The size dependence of the color of planar polycyclic aromatic hydrocarbons (PAHs)is also demonstrated (after [1.132]). (b) Modifications of the B80 molecule by forming complexes with alkali elements.The complexes are suggested as effective hydrogen storage materials up to 11 wt. % capacity (after [1.134]). (c) Variousways of functionalization of SWNTs: attaching molecules to the defect locations in the side-walls, connecting moleculesby covalent bonds, functionalization with surfactants, functionalization with polymers, and endohedral functionalizationwith fullerenes (after [1.150]). (d) Steps of a GO reduction procedure showing the schematics, the corresponding bond-ing energies in XPS, and photos of the solutions for each step. The vials contain GO in deionized (DI) water, GO indimethylformamide (DMF), CCG3 (CCG: chemically converted graphene) in DI water, and CCG3 in DMF from left toright (after [1.137])

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Science and Engineering of Nanomaterials 1.4 Typical Applications of Nanomaterials 25

a)

a1) a2) a3)

b1) b2) b3)

b)

1 μm 200 nm 100 nm

20 μm 10 μm 100 nm

0.3

0.2

0.1

0.4800 °C

Cast alloy

Nanocoating

Time (h)

Mass change (mg/cm2)

0.080604020 100

0.8

0.6

0.4

0.2

1.0900 °C

Cast alloy

Nanocoating

Time (h)

Mass change (mg/cm2)

0.0806040200 100

2.5

2.0

3.5

3.0

1.0

1.5

0.5

4.01000 °C

Cast alloy

Coating

Time (h)

Mass change (mg/cm2)

0.0806040200 100

Fig. 1.16a,b Applicationof nanomaterials forpreventing corrosion.(a) SEM and TEM imagesof halloysite nanotubenanocontainers (top row).Photos of aluminum spec-imens after 2 weeks ofexposure to corrosivemedium (bottom row):a1) blank aluminum alloyimmersed in 0.3% NaCl,a2) blank aluminum al-loy immersed in 0.3%NaCl saturated with 2-mercaptobenzothiazole,and a3) aluminum alloycoated with halloysite-doped sol–gel film afterimmersion in 3% NaClsolution (after [1.147]).(b) Top row: Layers ofnanostructured materialson its bulk substrate forK52 alloy. SEM imagesfor top b1) and cross-section b2) morphologiesare shown along withthe TEM showing thestructure of the sputterednanocoating b3). Bottomrow: Oxidation kinetics ofthe bulk and coated alloysexhibited faster oxidationrate for the bulk samplesat the three tempera-tures (800 ◦C, 900 ◦C,1000 ◦C) investigated(after [1.148])

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26 Introduction

filled composites and medicines released from nanocap-sules will soon belong to this category. In scientificcommunications, a vast number of applications havebeen proposed in diverse fields. Several chapters inthe final part of this handbook are devoted to ap-plications, and in all the other parts, each chaptercontains a short section showcasing applications ofthe nanomaterials considered. Here, we provide onlya glimpse on applications, grouped into the categoriesof catalysts and catalyst templates, nanomaterials inenergy conversion, and storage and sensors based onnanomaterials.

1.4.1 Catalysts and Catalyst Templates

Heterogeneous catalytic reactions need high surfacearea for obvious reasons, as the process can be real-ized only at surfaces, and all materials in the volumerepresent unnecessary weight and cost. There is anotherreason why nanomaterials can behave differently fromtheir larger-grain counterparts; i. e., they have differentselectivity features. In surface catalysis it is well knownthat different crystal planes and different defect sites onthe surface (e.g., steps, kinks) promote different reac-tions for the same conditions [1.151]. It has also beenpointed out that, in homogeneous catalysis, edge andcorner atoms play an exceptional role; however, theirratio can change during the reaction, and in fact surfacereconfiguration is common [1.155]. In nanomaterials,the number of this kind of defects is naturally higher,depending also on their size and shape, so control ofthese parameters provides well-regulated reactions ina multichannel process. Figure 1.17a displays an exam-ple of how the size and shape of platinum nanoparticlesgovern the completeness of a pyrrole hydrogenationreaction [1.151]. Figure 1.17b shows how the size ofa cobalt catalyst changes the yields of Fischer–Tropschsynthesis [1.152]. The yield is proportional to the dis-persion of cobalt atoms, and the site-time yield isaccordingly constant, which means that the size of theparticles has an important effect; however, it does notchange the surface features, so the authors could notfind a shape dependence

Figure 1.17c introduces a specific, electrochemical,use of gold–platinum nanocatalyst used in Li–air andZn–air batteries [1.153]. It is clear that these batterieshave much higher specific energy storage capacity thanconventionally used ones. The battery works similarlyto Li-ion batteries inside, however they have a specialporous anode where the air – in some cases oxygen orozone – enters. Here, a catalyst is needed for efficient

conversion of Li ions to lithium oxide during discharg-ing, and lithium oxide to lithium ions during chargingof the battery. In the nanosized alloy catalyst the plat-inum part is responsible for the reduction reaction andthe gold part helps the oxidation.

1.4.2 Energy Conversion and Storage

Harvesting clean energy – mainly by converting nat-urally present energy to electricity – from abundantresources is one of the most important tasks to be ac-complished by modern engineering. As clean energysources (e.g., solar, wind) are not correlated with en-ergy demand in time and non-localized energy sourcesare needed in transportation, too, efficiency in bothenergy harvesting and energy storage is necessary. En-ergy harvesting involves macroscopic equipment, e.g.,wind turbines, however in many methods nanomaterialsare used to improve efficiency. The two most relevantfields are thermo- and piezoelectric conversion and solarenergy conversion by solar cells. Thermoelectric nano-materials convert thermal energy between two locationsat different temperatures. The measure of the efficiencyof a material in thermoelectric conversion is the ZTparameter, the ratio of the Seebeck coefficient and thethermal and electrical conductivity of the material. Su-perlattices are the most efficient conversion devices,however their fabrication is costly and scaling up iscomplicated. Work has been carried out for efficientuse of simpler systems, e.g., silicon nanowires [1.79].Another way to apply nanomaterials in energy con-version is by using quantum dots – nanoparticles – insolar cells in order to trap more photons or capturemore energy from photons [1.156]. Particular cases of

Fig. 1.17a–c Application of nanomaterials as catalysts.(a) Pyrrole hydrogenation reaction catalyzed by platinumnanoparticles having different size; the reaction product isa mix of pyrrolidine and n-butylamine for small (1–2 nm)particles and mostly n-butylamine for larger (3–5 nm) par-ticles; the amount of butane and ammonia slowly increaseswith increasing particle size (after [1.151]). (b) CO con-version time yield and turnover rate in a Fischer–Tropschreaction as a function of cobalt dispersion [1.152]. (c) En-ergy density for several commonly investigated batteries;schematic of a possible configuration for Li–air or Zn–airbattery, showing the location of the catalyst; charge–discharge characteristics of a Li–air cell displaying the roleof the PtAu catalyst particle in the reduction and oxida-tion steps (ORR: oxygen reduction reaction; OER: oxygenevolution reaction) (after [1.153, 154]) �

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Science and Engineering of Nanomaterials 1.4 Typical Applications of Nanomaterials 27

1

a)

c)

b)

+2H2N

H H

N +H2 +H2 +NH3

Pyrrole

908070605040302010

100

0

Pyrrolidine

Pyrrolidine

n-Butylamine

n-Butylamine

Butane andAmmonia

Butane and Ammonia

Pyrrolidine

n-Butylamine

Butane and Ammonia

+H2N

5Pt size (nm)

5 nm Pt nanocubes

Theoretical specific energy (Wh/kg)Practical specific energy (Wh/kg)

5 nm Pt nanopolyhedra

0

Selectivity (%)

4

25

20

15

10

5

30

0

100

10

0.12 0.140.10.05 0.080.040.02Co dispersion

Cobalt fractional dispersion

0

4.5

4.0

3.5

3.0

2.5

5.0(a) (b)

2.0

0 –400 01200Q(mAh/gcarbon)

E vs. Li (VLi)

xLi++O2+xe–

Carbon

PtAu/C

OER @ Pt

Ar-filled

ORR @ AuxLi++O2+xe–

LxO2 O2-filled

LxO2

800800

0.12 0.14 0.16 0.180.10.05 0.080.040.020

Cobalt-time yield (104 s–1)

Site-time yield (103 s–1)

321

90

70

50

30

10

12 000

10 000

6000

4000

2000

Discharge

Lithiummetal

Air

Organicelectrolyte

Catalysts

Porous carbon

e–e–

Li+O2

8000

0

420Pt size (nm)

Selectivity (%)

410400390380

Lead/a

cidNi/C

d

Ni/MH

Li-ion

Zn-air

Li-air

TiO2

TiO2

SiO2

SiO2

Al2O3 Other

TiO2

TiO2

SiO2

SiO2

Al2O3 Other

Co

Co-Ru

Co

C-Ru

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28 Introduction

clean energy conversion that use nanomaterials are dis-played in Fig. 1.18a. The measurements show a strongdependence of the reduction rate achieved with TiO2on the gold particle size; the gold template modifies theFermi level, and the shift in the Fermi energy increaseswith decreasing particle size. This reaction is used inphotoelectrochemical energy conversion. It was alsoshown that the solar energy conversion is dependent onthe configuration of the optically transparent electrode(OTE/SnO2/(H2PCnMPC+C60)m), where the lengthof the linker (the number of fullerene molecules) playsthe most important role [1.72]. For energy storage,we have collected examples from our previous workin Fig. 1.18b. The SEM images show an MnO2 struc-ture prepared by hydrothermal synthesis [1.73]. Thenanowires obtained have potential applications in super-capacitor devices. We also present an atomically thin,transparent, flexible supercapacitor based on single- orfew-layer graphene electrodes. The transparency of theGO and RGO layers are demonstrated placed them ontoa printout; here the thickness of the multilayered filmswas set to ≈ 10 nm by consecutive dip-coating steps.The last part (Fig. 1.18b) shows a supercapacitor arraydesigned on a monolithic GO piece. Parts of the GOwere reduced by a laser beam, and these RGO partsserve as electrodes while the unreduced GO acts asa solid electrolyte.

1.4.3 Sensors Based on Nanomaterials

Nanomaterials are excellent choices for use as activeelements of chemical (gas) sensors. To achieve highsensitivity, the material needs to have high dispersion,as surface atoms interact and undergo changes whereasatoms inside the material do not change, meaning that,the higher the surface atom ratio, the larger the relativechange in the measured physical property of the ma-terial. Similarly to applications as catalysts, in sensorsnot only the size of the particles is important but alsotheir shape. As the shape, number, and configurationof edge and corner atoms change, further differencesoccur in the signals measured. Figure 1.19a demon-strates the effect of various shapes when nanoparticles,nanowires, and nanoplatelets of tungsten oxide wereused [1.77]. In this case, the nanowires have the high-est sensitivity, followed by the platelets and finally thenanoparticles.

Fluctuation-based sensing or fluctuation-enhancedsensing (FES) provides selectivity via investigationof fluctuations in resistance, voltage or current. The

Fig. 1.18a–c Application of nanomaterials in energy con-version and storage. (a) Dependence of photoreductionefficiency of TiO2 particles on the size of a goldnanoparticle used as a substrate. The smaller the par-ticle, the higher the reduction efficiency. Schemes of(H2PCnMPC + C60)m configurations are also displayed.These particles are applicable for photo-induced cur-rent; the photocurrent for the visible spectrum is dis-played for different configurations of the structure ofOTE/SnO2/(H2PCnMPC + C60)m with parameters of[H2P] = 0.19 mM, n = 15, Cn = [C60] = 0.38 mM (af-ter [1.72, 74]). (b) Evolution of MnO2 morphology asa function of the dwell time of hydrothermal synthe-sis. Supercapacitors prepared with nanowire electrodeshave 150–200 F/g specific capacitance (after [1.73]).(c) Physical appearance of nanomaterial supercapacitors;transparent, flexible supercapacitor with graphene elec-trodes (after [1.75]) and laser-defined GO electrode–RGOelectrolyte (after [1.76]) (IPCE: incident photon to chargecarrier efficiency; PVA: polyvinyl alcohol) �

method is based on measurement of a signal as a func-tion of time, analysis of this signal, and in most casesfast Fourier transformation (FFT), and finally interpre-tation of the power spectral density in characteristicfrequency ranges to identify the chemical environ-ment [1.78, 81]. This interpretation can be a simplelinearization, application of a neural network, or useof principal component analysis (PCA) plots. All theseinterpretation methods allow selectivity for differentgases and for different concentrations. Any kind of sen-sor can be essentially used with this method; however,sensors made of nanomaterials have the advantage ofmuch stronger FES signals and significantly more in-formation content [1.80, 82, 83]. Carbon nanotubes areespecially useful and important building blocks of FESdevices, as they can be functionalized with appropriatechemical groups that make them selective for particu-lar materials [1.84]. Functionalization is especially wellstudied in the case of CNTs for use in biosensors [1.85].Figure 1.19b displays a print made with a carbon nano-tube ink. A similar printing method and ink were usedto create the sensors for the devices providing the sig-nals in the lower row. The fluctuation power spectrawere measured using a Taguchi-type sensor, and showmeasurable differences between three different gasescompared with a synthetic air reference. The PCA plotsdemonstrate the sensitivity and selectivity of the printedcarbon nanotube sensors. Signals of 30 ppb concentra-tion are shown to be selective for the measured gases.

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Science and Engineering of Nanomaterials 1.4 Typical Applications of Nanomaterials 29

Electrical contact

a)

b)

c)

15

Reduction efficiency (%)

Au

Au

TiO2

10

0

5

25

20

No Au 8 nm Au 5 nm Au 3 nm Au 300 nm

NHCO

(CH2)5(CH2)5

1 h 6 h 18 h

NHCO 30.94 Å

10.05 Å4.24 Å

17.3 Å

50 d

c

b

a

50

50

50

IPCE (%)

10

0

60

Wavelength (nm)1000800600400

GO film

2-D supercapacitor

2-D supercapacitor

2-D supercapacitor

2-D supercapacitor

2-D su

2-D su

2-D su

2-D su

acitor

acitor

acitor

acitor

RGO film

Gold current collector

Graphene

Gold current collector

Graphene

PVA-H3PO4 (Gel electrolyte)

100 nmX50.000 100 nmX50.000 100 nmX50.000

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30 Introduction

a)

b)

1600

1200

1000 ppm

500 ppm

100 ppm50 ppm

10 ppm 1 ppm

800

400

0

2000

Time (s)20001200 16008000

Res

pons

e

400

200025003000

15001000500

0

3500

Temperature (°C)250200150

WO2.72 nanowiresWO 3 nanoplatelets

WO 3 nanoparticles

50R

espo

nse

100

3.0

2.5

2.0

1.5

3.5

Log (concentration (ppm))2 30

Log

(re

spon

se)

1

0

5

–5

PC–1×10–14 (99.41 %)0 5 10 15–15

0.1 ppm

N2OH2S

0.1 ppm

PC–2×10–14 (0.16 %)

–5–10

0

5

–5

PC–1×10–15 (98.59 %)–5 0 5 10–20

30 ppb

N2O

H2S

30 ppb

PC–2×10–15 (0.34 %)

–10–15

100Frequency (Hz)

NO 10 ppm

SO2 500 ppm

H2 500 ppm

Synthetic air

1 10

100 nm 300 nm

5 nm

50 nm

10–21

10–23

10–25

10–19

10–27

R–2 normalized power spectrum (V2/Ω2)

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Science and Engineering of Nanomaterials References 31

Fig. 1.19a,b Application of nanomaterials in sensors. (a) SEM and TEM images of tungsten oxide nanoplatelets andTEM of WO2.72 nanowires. Measuring different concentrations of H2S using a nanoplatelet sensor. Gas sensing mea-surement: sensor signal as a function of H2S concentration at 250 ◦C. Comparative graphs showing sensitivity of differentnanostructures as a function of temperature and concentration (after [1.77]). (b) Demonstration of selectivity achievedby the fluctuation-enhanced sensing method. An image printed using nanotube solution as ink, similar to the ones usedto print circuits for sensors (after [1.157]). Linearized spectra measured on thick-film nanoparticle sensors (after [1.81]).PCA plots of FES of N2O (circles) and H2S (squares) measured with an MWCNT sensor showing both sensitivity andselectivity (PC: principal component) (after [1.83]) �

1.5 Concluding Remarks

Nanomaterials differ in many regards from their macro-scopic – bulk – allotropes, and accordingly they callfor special scientific and engineering approaches. Thelarge number of scientific articles, encyclopedias, andhandbooks in related topics, as well as textbooks andfull courses, show the growing importance of thisfield.

In this chapter we could give only a glimpse ofthe numerous unique and interesting, mainly univer-sal properties of nanosized materials. This similarity of

properties of materials as a function of their charac-teristic size clearly shows that nanomaterials representa one-of-a-kind, coherent group of materials. The laterchapters in this handbook elaborate on the synthesis,specific properties, and applications of the various ma-terial groups. Our main objective in this chapter wasto collect and present material properties in a way thatwould introduce novice readers to a new topic, and atthe same time serve as a reference for the everyday workof well-established scientists and engineers.

1.6 About the Contents of the Handbook

The structure of the handbook follows the classificationof important material groups. Part A describes carbon-based nanomaterials from fullerenes to nanotubes andnanofibers, as well as nanodiamonds. Part B introducesnoble and common metals and alloys. Part C describesceramic materials, crystalline and glassy oxides, andother compounds. Part D describes composites andhybrid structures where nanomaterials serve as fillers

and building blocks, as well as solutions, so-callednanoinks. Part E deals with porous nanomaterials, met-als, ceramics, and silicon. Part F presents examples oforganic and bio-nanomaterials, bones, and fibers. Fi-nally, Part G contains several chapters on the topics ofapplications in nanomedicine, civil engineering, toxi-cology, and hazards, as well as energy harvesting andenergy storage.

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37

NanoCarbPart APart A NanoCarbons

2 Graphene – Properties and CharacterizationAravind Vijayaraghavan, Manchester, UK

3 Fullerenes and Beyond:Complexity, Morphology, and Functionalityin Closed Carbon NanostructuresHumberto Terrones, University Park, USA

4 Single-Walled Carbon NanotubesSebastien Nanot, Houston, USANicholas A. Thompson, Houston, USAJi-Hee Kim, Houston, USAXuan Wang, Houston, USAWilliam D. Rice, Los Alamos, USAErik H. Hároz, Houston, USAYogeeswaran Ganesan, Hillsboro, USACary L. Pint, Nashville, USAJunichiro Kono, Houston, USA

5 Multi-Walled Carbon Nanotubes

Ákos Kukovecz, Szeged, HungaryGábor Kozma, Szeged, HungaryZoltán Kónya, Szeged, Hungary

6 Modified Carbon NanotubesAarón Morelos-Gómez, Nagano, JapanFerdinando Tristán López, Nagano, JapanRodolfo Cruz-Silva, Nagano, JapanSofia M. Vega Díaz, Nagano, JapanMauricio Terrones, University Park, USA

7 Carbon NanofibersYoong A. Kim, Nagano, JapanTakuya Hayashi, Nagano, JapanMorinobu Endo, Nagano, JapanMildred S. Dresselhaus, Cambridge, USA

8 NanodiamondsOlga A. Shenderova, Raleigh, USASuzanne A. Ciftan Hens, Raleigh, USA

39

Graphene – P2. Graphene – Properties and Characterization

Aravind Vijayaraghavan

Graphene is the two-dimensional allotrope of car-bon, consisting of a hexagonal arrangement ofcarbon atoms on a single plane. This chapter ex-plores the history of graphene, as the theoreticalbuilding block for other carbon allotropes as well asits rise as a material in its own right in recent years.Graphene can be fabricated by different methodsincluding mechanical exfoliation, chemical vapordeposition, and decomposition of SiC, althoughbulk-quantity production of pristine grapheneremains a challenge. The atomic and electronicstructure of graphene is described, highlightingthe strong correlation in graphene between struc-ture and properties, as is the case with othercarbon allotropes. Graphene exhibits a numberof unique and superlative electronic and opticalproperties. The intrinsic properties of graphenecan be tailored by nanofabrication, chemistry,electromagnetic fields, etc. Various applicationsof graphene have been proposed in electronic,optoelectronic, and mechanical products. In ad-dition, graphene has emerged as a candidate inchemical, biochemical, and biological applica-tions. Derivatives of graphene such as grapheneoxide or graphane are also of interest in terms ofboth fundamental properties and applications.

2.1 Methods of Production.......................... 422.1.1 Micromechanical Cleavage

of Graphite (Scotch Tape Technique) 422.1.2 Chemical Vapor Deposition (CVD) .... 43

2.1.3 Decomposition of Carbides ............ 442.1.4 Exfoliation by a Solvent ................ 462.1.5 Synthetic Production Route ........... 492.1.6 Graphene Nanoribbon (GNR).......... 492.1.7 Derivatives of Graphene ................ 50

2.2 Properties ............................................ 502.2.1 Structure and Physical Properties ... 502.2.2 Mechanical Properties................... 512.2.3 Electronic Properties ..................... 522.2.4 Optical Properties ......................... 552.2.5 Thermal and Thermoelectric

Properties ................................... 562.2.6 Chemical Properties ...................... 562.2.7 Properties of Graphene Derivatives. 57

2.3 Characterization ................................... 582.3.1 Optical Characterization ................ 582.3.2 Transmission Electron Microscopy ... 582.3.3 Scanning Probe Techniques ........... 602.3.4 Angle-Resolved Photoemission

Spectroscopy (ARPES)..................... 602.3.5 Raman Spectroscopy ..................... 612.3.6 Electrical Characterization ............. 632.3.7 Photocurrent Microscopy ............... 69

2.4 Applications ......................................... 692.4.1 Structural and Electrical Composites 692.4.2 Transparent Conducting Films ........ 692.4.3 Sensors ....................................... 722.4.4 Electronic Applications .................. 732.4.5 Photonics and Optoelectronics ....... 74

2.5 Conclusions and Outlook ....................... 74

References .................................................. 74

As a three-dimensional (3-D) material, carbon exists asthree predominant allotropes: diamond, graphite, andamorphous carbon (historically knows as carbon black).These are distinguished by their crystalline structureand the hybridization of the carbon atoms therein.

Carbon atoms in diamond are all sp3 hybridized andarranged in diamond cubic structure which comprisestwo interpenetrating face-centered cubic (fcc) lattices.Graphite has a layered structure, where the sp2 hy-bridized carbon atoms are arranged in a hexagonal

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R. Vajtai (Ed.), Springer Handbook of Nanomaterials, DOI 10.1007/978-3-642-20595-8_2, © Springer-Verlag Berlin Heidelberg 2013

40 Part A NanoCarbons

a) b) c) d)

Fig. 2.1a–d Allotropes of sp2 carbon: (a) graphite (3-D), (b) graphene (2-D), (c) carbon nanotube (1-D), and (d) fullerene (0-D)(courtesy K.S. Novoselov)

lattice in each plane, while the planes themselves areAB (Bernal) stacked and held together by van derWaals forces. Amorphous carbon, as the name indicates,does not have long-range crystalline order, althoughlocally the atoms are bound together covalently andcomprise a mix of sp2 and sp3 carbons. While di-amond can be reduced in size to the nanoscale toform nanodiamond, it is graphite that can be truly re-duced to lower-dimensional allotropes. A single layer ofgraphite is defined as graphene, the topic of this chapter.Graphene has been used as the building block to con-ceptually visualize carbon allotropes such as graphite,carbon nanotubes, and fullerenes; it was believed thatsuch a freestanding, two-dimensional (2-D) structurewould not be stable. Carbon nanotubes (CNT) form itsone-dimensional (1-D) counterpart, while fullerenes arethe zero-dimensional (0-D) allotropes. These variousforms of carbon are summarized in Fig. 2.1. It is im-portant to note that CNTs or fullerenes are not uniquestructures, but rather describe a family of structures,which are described in detail in subsequent chapters.Nonetheless, their structure and properties are all de-rived from graphene.

Despite serving as the fundamental building blockfor these carbon allotropes, graphene remained a con-cept until 2004. It was substantially predated by itsrelated allotropes, fullerenes being discovered and de-scribed in 1985 [2.1], while carbon nanotubes weresynthesized and their atomic structure elucidated in1991 [2.2]. The term “graphene” was coined in1987 [2.3], to describe one of the two alternating lay-ers in graphite intercalation compounds (GIC), the otherlayer being the intercalating agent. It was postulated thatindependent, freestanding graphene would not becomea physical reality since it would voluntarily transforminto a more stable allotrope in an attempt to minimize itssurface energy. Supported monolayers of carbon, how-

ever, had been previously synthesized and described,including epitaxial graphene which has been knownsince the 1970s. Chemical derivatives of graphite suchas graphite oxide can be traced back to the 1950s andcan exist as single layers (graphene oxide), althoughthese are not truly two-dimensional layers due to out-of-plane atoms which stabilize their structure.

Since graphene occurs naturally as a constituent ofbulk graphite, it appeared to be the logical place to startthe hunt for freestanding graphene. This effort culmi-nated in the successful exfoliation of a single sheet ofcarbon atoms by Andre Geim and Kostya Novoselovat the University of Manchester in 2004 [2.4], usinga technique referred to as micromechanical cleavage,or colloquially, the Scotch tape method. This discov-ery and the subsequent investigations into the propertiesof graphene were rewarded with the Nobel Prize inPhysics in 2010 for Geim and Novoselov. The existenceof graphene, however, does not contradict the physicsthat predicted that it could not exist. It was discoveredthat graphene is not truly flat; there exist atomic-scaleripples in the carbon sheet, which accommodate theexcess surface energy, thereby stabilizing the 2-D struc-ture of graphene. For this reason, it might be argued thatgraphene is a quasi-2-D material. However, it has beenshown to exhibit a range of properties that are unique to2-D physics, and therefore graphene will be identifiedas a 2-D material for the rest of this chapter, withoutqualification.

A surge of research into the structure and propertiesof graphene ensued, and graphene did not disappoint.Almost immediately, an anomalous quantum Hall effectwas reported in graphene, which also serves as directexperimental evidence for the electrons in graphenebehaving as massless Dirac fermions, confirming theo-retical predictions. Graphene yielded record high valuesfor various properties, such as tensile strength, car-

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Graphene – Properties and Characterization 41

a)

b)

8000

6000

4000

2000

0

Publ

icat

ions

per

yea

r

1985 1990 1995 2000 2005 2010

Robert Curl, Harold

Kroto, and Richard

Smalley report discovery

of C60 (fullerenes)

Andre Geim and

Kostya Novoselov

report measurements

on graphene flakes

Geim and Novoselov

win Nobel Prize in

Physics

Carbon nanotubes

Graphene

Fullerenes

Carbon nanotubes

Graphene

Fullerenes

0

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ear 2000

1600

1200

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1990 1995 2000 2005 2010 2015

Carbon nanotubes

(CNTs) used in

lithium-ion batteries

and sporting

equipment

CNT composites

used in car parts

and tested in aircraft

Four of today's

large-scale CNT

manufacturers

founded

Today's major

graphene

manufacturers

founded

Graphene's future• Touchscreens• Capacitors• Fuel cells• Batteries• Sensors• High-frequency circuits• Flexible electronics

Sumio Iijima reports

measurements of

multiwalled carbon

nanotubes, sparking

widespread interest

Curl, Kroto, and

Smalley win Nobel

Prize in Chemistry

Fig. 2.2a,b Time line of nanocarbon allotropes and the (a) number of publications and (b) number of patent applications per yearon each topic (after [2.5])

rier mobility, thermoelectric power, etc. Scientists havealso succeeded in transferring epitaxial graphene fromits native metal or silicon carbide substrate onto othersubstrates of interest or as freestanding structures.Since the early 1990s, carbon nanotubes have beendescribed as rolled-up graphene sheets. This descrip-tion has come full circle, with the recent unzipping ofcarbon nanotubes to yield graphene. While the num-ber of publications on carbon nanotubes has leveledoff in recent years at about 8000 papers per year, thepublications on graphene are only just starting their ex-ponential growth at a rate faster than that which wasenjoyed by carbon nanotubes in their first few years(Fig. 2.2; over 3000 papers were published in the fieldof graphene in 2010 [2.5]). A similar trend is also ob-served in patent applications based on carbon nanotubesand graphene.

Any chapter on graphene would not be completewithout discussion of its other, quasi-2-D derivativesbased on the fundamental graphene structure. Perhapsthe most interesting of these is the case where twographene layers are AB stacked to form a bilayer.Unlike graphene, which has zero electronic bandgapand is therefore a quasimetal, this bilayer structurecan have a bandgap. A bandgap is critical for elec-tronic applications, and one of the most active areasof research in graphene is currently the generation ofa stable, true, electronic bandgap in graphene. Theoxidized derivative, graphene oxide (GO), has been al-luded to before, and serves as a useful intermediatein a chemical route for graphite exfoliation. Hydro-genated graphene, christened graphane, and fluorinatedgraphene or fluorographene have been recently pro-duced and characterized.

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42 Part A NanoCarbons

2.1 Methods of Production

Historically, graphene supported on substrates such asmetals and SiC was synthesized first; however, suchgraphene was not liberated from the substrate supportto form a truly two-dimensional structure. Freestand-ing graphene was not a reality until the ground-breakingpublication of Novoselov and Geim in 2004. Con-sequently, this micromechanical cleavage method ispresented first, followed by the epitaxial synthesis ofgraphene as well as chemical vapor deposition onmetallic substrates. Graphene can also be exfoliatedfrom graphite by sonochemical means in a solvent,followed by a purification step to extract the mono-layers. Recently, islands of nanographene have alsobeen synthesized by chemical routes, and this bottom-up approach is discussed. Finally, separate sections aredevoted to production of graphene nanoribbons (GNR)and derivatives of graphene.

2.1.1 Micromechanical Cleavage of Graphite(Scotch Tape Technique)

Micromechanical cleavage (or exfoliation), as the nameimplies, refers to thinning down of graphite by me-chanically reducing the number of layers in a repeatedfashion. Graphite is known to cleave preferentiallyalong the interlayer direction where layers are held to-gether by weak van der Waals forces, rather than acrossthe strong covalent bonds that bind the atoms withina layer. The most common procedure to accomplishthis is using adhesive tape. Attaching a thick graphiteflake to adhesive tape on both its exposed faces, andthen peeling the two pieces of tape apart, results intwo thinner flakes of graphite, stuck to each of the ad-hesive tape pieces. Repeating this process a sufficientnumber of times would in principle result in a singlesheet of carbon atoms, i. e., graphene, adhered to theadhesive tape. This is still not freestanding graphene,since it is supported by the tape, and must be transferredto a suitable weakly coupling substrate or suspendedacross supports. The genericized Scotch trademark fortransparent adhesive tape has subsequently lent its nameto the micromechanical exfoliation of graphite. Differ-ent variations of this method exist.

One of the earliest such efforts was undertaken byFernandez-Moran, who succeeded in thinning graphitedown to ≈ 15 layers (5 nm) over a millimeter size, toserve as a support membrane for transmission elec-tron microscopy [2.6]. This result remained relativelyunknown outside the electron microscopy commu-

nity, until the interest of condensed matter physiciststurned to graphene and other lower-dimensional car-bons. The first successful thinning down of graphiteto its monolayer graphene form involved a wet/drymethod [2.4]. The surface of highly oriented pyrolyticgraphite (HOPG) was first patterned into square mesas,which were pressed into wet photoresist. After baking,the mesas attached to the now dry photoresist, and couldbe detached from the bulk of the HOPG. Scotch tapewas used to repeatedly peel off layers of graphite fromthe mesas, thinning them down, until only very thinlayers remained in the photoresist. The photoresist wasthen dissolved in acetone to release these thin flakes,which float on the solvent surface. The flakes werecollected onto Si/SiO2 wafer pieces dipped into the sol-vent. Thicker flakes adhering to the silicon could becleaned off by sonication in 2-propanol, while thinnerflakes were reported to adhere strongly to the substratedue to capillary forces.

In the resultant sample, the flakes of graphene, bi-layer graphene, and few-layer graphene (FLG) must bedistinguished from among a sea of flakes of variousthicknesses. Fortunately, at certain thicknesses of theSiO2 layer that covers the silicon wafer, for example,300 nm, the interference contrast generated by grapheneflakes on the surface makes them relatively easy tospot and identify by optical microscopy. A trainedeye can, in fact, distinguish between graphene, bilayergraphene, and thicker flakes. This is described in de-tail in Sect. 2.2.4 on optical properties of graphene.Figure 2.3 shows the appearance of micromechani-cally cleaved graphene flakes of different thicknesses inan optical microscope [2.7], atomic force microscope(AFM) [2.8], and transmission electron microscope(TEM) [2.9]. These, along with Raman spectral map-ping, are routinely used to characterize graphene flakes,and are described in Sect. 2.3 dedicated to graphenecharacterization.

The procedure has since evolved into a completelydry technique. It has been shown that rubbing thefreshly cleaved surface of a layered material such asgraphite on another solid surface results in a vari-ety of flakes, among which monolayer flakes can beinvariably found [2.8]. Alternatively, HOPG is me-chanically cleaved repeatedly between two pieces ofadhesive tape until the surface of the tape is cov-ered by a layer of relatively thin graphite [2.10]. Nospecific criterion exists at the time of writing for theoptimum degree of such exfoliation. Researchers rely

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Graphene – Properties and Characterization 2.1 Methods of Production 43

a) b) c)

300 nm SiO2

5 µm White light

0A 9A 13A

Fig. 2.3a–c Monolayer graphene flake as seen in (a) an optical microscope [2.7], (b) an atomic force microscope [2.8],and (c) a transmission electron microscope (after [2.9])

on personal experience or historic parameters specificto their laboratory to carry out this procedure. Oncea satisfactory degree of exfoliation has been accom-plished with the adhesive tape, it is pressed againstthe surface of a desired substrate, such as Si/SiO2wafer. Again, various scientist-specific parameters ex-ist for this process, such as duration, force, applicationand peeling-off procedure, etc. At the end of this pro-cedure, the surface of the graphene as well as thesubstrate is often contaminated with adhesive residuefrom the tape, which has been shown to limit thecarrier mobility in the graphene flake. Efforts to re-move this residue have included annealing at 200 ◦Cin a reducing atmosphere of Ar and H2 [2.11], anneal-ing in vacuum at 280 ◦C [2.12], and current-inducedJoule heating after graphene device fabrication [2.13].Variations such as applying an electric field perpen-dicular to the substrate during the transfer from theadhesive tape have also been explored [2.14]. Covalentlinkers such as perfluorophenylazide between grapheneand SiO2 have been explored to aid in the exfolia-tion process to increase monolayer yield [2.15]. Theresidue issue can also be completely avoided by evap-orating a thin film of gold on the HOPG surface priorto the transfer step, to avoid direct contact betweenthe adhesive and the graphene [2.16]. The gold canbe subsequently dissolved in a suitable etchant withoutaffecting the graphene. The lack of standardization is in-dicative of the early stage of current graphene research,and is perhaps an indication that micromechanical ex-foliation is not destined to evolve into a large-scale orindustrial method for graphene production. Competingtechniques, discussed subsequently, have reached much

higher degrees of standardization and reproducibility,albeit with drawbacks of their own.

It should be noted that the best quality of graphenecurrently available is indisputably that produced by mi-cromechanical cleavage, where the graphene quality isdefined in terms of crystalline domain size, numberof defects, carrier mobility, etc. The choice of initialgraphite material having large grain size, using freshlycleaved graphite for further exfoliation, and the cleanli-ness and quality of the adhesive tape and SiO2 substrateare all variables which significantly affect the qual-ity of the final flakes obtained. Flakes of graphenehundreds of μm across are routinely produced in labora-tories all over the world by this method, predominantlyfor fundamental research purposes. In some cases,millimeter-size flakes have also been reported. Whilethis method has not matured for commercial applica-tions, ventures such as Graphene Industries have beenestablished to sell micromechanically cleaved grapheneflakes.

2.1.2 Chemical Vapor Deposition (CVD)

The study of the deposition of thin graphitic layerson metal substrates by CVD dates back to the late1960s [2.17, 18]. One of the motivations for this was infact to eliminate the formation of graphitic structures onmetals such as platinum which results in degradation ofcatalytic activity. CVD is currently the preferred routefor large-scale fabrication of carbon nanotubes, andtherefore has generated substantial excitement as a po-tential method for large-scale production of graphene.In general, this involves thermal decomposition of

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44 Part A NanoCarbons

gaseous hydrocarbon sources followed by dissolutionand recrystallization of the cracked carbon on the sur-face of metallic substrates. The solubility of carbon invarious metals, such as rhodium, ruthenium, iridium,and rhenium, has been measured [2.19], along with theobservation that excess carbon dissolved in such metalsat high temperatures can segregate as graphite on thesurface upon cooling. Various metallic substrates andcarbon feedstock have been explored in the effort togrow monolayer graphene, and some of the significantdevelopments are mentioned next.

As early as 1991, monolayer graphene wasgrown on Pt(111) by hydrocarbon decomposition at800 ◦C [2.20], resulting in islands of 20–30 nm size dis-tributed uniformly over the surface. Upon annealing athigher temperatures, the graphene was found to accu-mulate into large, regularly shaped islands on terracesand step edges. Recently, a variation in which a beam ofmethane molecules with high kinetic energy (670 meV)impacting a Pt(111) surface at 890 K resulted in largedomains of monolayer graphene covering the entirePt surface [2.21]. Ni soon followed, requiring a min-imum temperature of 600 ◦C, and monolayer graphiteon Ni(111) was shown to have an arrangement wherebyone carbon atom in a unit cell of the graphite over-layer is located at the on-top site of the topmost Niatoms, while another carbon atom exists at the fcc hol-low site [2.22]. Carbon has been shown to segregate onthe surface of Ru(0001) as monolayer graphene [2.23],when annealed at 1400 K. STM reveals a (11 × 11)structure with good rotational alignment and structuralperfection, a well-defined periodicity of ≈ 30 Å, andlarge domain sizes exceeding 100 μm. Graphene hasalso been grown by thermal decomposition of benzeneon Ir(111) [2.24].

At present, the two predominant metallic substratesfor CVD graphene growth are Ni and Cu. Large-size (cm2) films of monolayer and few-layer graphenehave been grown on Ni [2.25], with monolayer re-gions as large as 20 μm in size. Cu does even better,and predominantly monolayer graphene covering manycm2 is grown in various laboratories using methaneCVD [2.26]. The solubility of C in Cu appears to makethe process self-limiting, and at most 5% of the sur-face is covered by small islands of bilayer graphene.Most importantly, methods have been developed to de-tach these films from the metallic substrate and transferthem intact onto dielectric substrate (Fig. 2.4), wherethey can be lithographically patterned and processedfor electronic or optical applications [2.25]. In anothervariation, graphene is grown on a thin copper film on

arbitrary substrates, and the Cu dewets and evaporatesduring the growth process itself, leaving behind thegraphene film intact on the substrate [2.27]. The firsttruly large-area production of graphene has recentlybeen reported using a continuous CVD deposition andtransfer process [2.28].

In general, for substrates with small lattice mis-match (< 1%) such as Co(0001) and Ni(111), com-mensurate superstructures are formed, while substrateswith larger mismatches such as Pt(111), Ir(111), andRu(0001) yield incommensurate moiré superstructures.The first graphene monolayer on the metal surface hasstrong interaction with the substrate, and the spacing be-tween the two is much shorter than between two layersof graphite (3.35 Å). For the case of Ru it is 1.45 Å, andfor Ni it is 2.11 Å.

2.1.3 Decomposition of Carbides

The second substrate-supported route for grapheneproduction involves thermal decomposition of surfacelayers of carbides such as SiC. About 250 differentcrystal structure of SiC are known, but α-SiC is themost commonly encountered polymorph and, until re-cently, the primary focus of epitaxial graphene growth.α-SiC has a hexagonal crystal structure, similar towurtzite. The (0001) (Si) and (0001̄) (C) faces of 6H-(α-SiC) [2.29–31] and 4C-(α-SiC) [2.32] have been shownsuitable for the growth of epitaxial graphene.

6H-(α-SiC) is first cleaned for 20 min at 850 ◦Cunder a Si flux to prevent Si sublimation during thecleaning step. At higher annealing temperatures in ul-trahigh vacuum (UHV), the surface of SiC undergoesvarious reconstructions, until the graphitization tem-perature when the surface graphene layers form. TheSi surface of the hexagonal SiC undergoes the fol-lowing reconstructions: 3 × 3 at 850 ◦C under Si flux,√

3 ×√

3R30◦ below 1000 ◦C, 6√

3 × 6√

3R30◦ (6R3)at 1150 ◦C, and graphitization at 1350 ◦C. On this face,the C atoms are in epitaxy with the SiC underneath af-ter graphitization. The surface is passivated by the firstC layer, the interface extends to two C layers, and sub-sequent C layers are decoupled from the substrate andexhibit the properties of graphene. Graphitization oc-curs at 1150 ◦C on the C face of the hexagonal SiC,and the C layer occurs on a SiC 2 × 2 native reconstruc-tion. This reconstruction saturates the dangling bondstates, so that the first C layer already exhibits grapheneproperties. However, this C layer is no longer epitax-ial with the underlying SiC, so the long-range orderof the SiC substrate no longer imposes itself upon the

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Graphene – Properties and Characterization 2.1 Methods of Production 45

a) b) c) d)3L2L1L

GrapheneSiO2

Glass

Graphene

1 cm

Fig. 2.4a–d Graphene films transferred onto (a) a SiO2/Si substrate and (b) a glass plate. (c) Scanning electron micro-graph (SEM) image of graphene transferred onto SiO2/Si (285 nm-thick oxide layer), showing wrinkles as well as two-and three-layer regions. (d) Optical microscope image of the same regions as in (c) (after [2.26])

a) b) c)

8.0 μm1.400 1.600 1.800 2.600 2.800

Raman shift (cm–1)

Inte

nsity

(ar

b. u

nits

)

(1356 ± 5) cm–1D-peak

(1592 ± 5) cm–1G-peak

(2706 ± 5) cm–12-D-peak

37 cm–1

(1596 ± 5) cm–1(2717 ± 5) cm–1

54 cm–1

Fig. 2.5 (a) AFM image of graphene on 6H-SiC(0001) formed by annealing in Ar (p = 900 mbar, T = 1650 ◦C). (b) Lowenergy electron diffraction (LEED) pattern at 74 eV showing the diffraction spots due to the SiC(0001) substrate (bluearrows) and the graphene lattice (red arrows). The extra spots are due to the (6

√3 × 6

√3) interface layer. (c) Comparison

of Raman spectra of Ar-grown (red) and UHV-grown (blue) epitaxial graphene (after [2.33])

C layer. Therefore, it has not been possible to accom-plish both long-range ordering as well as decouplingfrom the surface simultaneously using 6H-(α-SiC). Epi-taxial graphene has also been grown on SiC(0001) in Aratmosphere [2.33], at close to atmospheric pressure anda significantly higher annealing temperature of 1650 ◦C,resulting in morphologically and electronically superiorgraphene compared with vacuum annealing (Fig. 2.5).

Perhaps the greatest limitation of SiC as a sub-strate for graphene growth is cost. α-SiC wafers arerelatively expensive, at about USD 300 for a 50 mmwafer. Cubic 3C-SiC (β-SIC), however, can be growndirectly on the surface of Si wafers of 300 mm andlarger, and is therefore a more commercially viablesubstrate. It was believed that, due to its cubic struc-

ture, β-SiC would be unsuitable for graphene growth.However, recently scientists have succeeded in grow-ing graphene on the Si-rich (100) surface of β-SiCby a series of annealing cycles with temperature in-creasing from 1200 to 1550 K [2.34]. They foundthat the strong lattice mismatch between graphene andunderlying SiC results in very weak coupling simi-lar to the (0001̄) C face of α-SiC. However, it wasfound that graphene growth on β-SiC was guided alongthe [110] crystallographic direction despite the latticemismatch, raising hopes that both substrate–graphenedecoupling as well as substrate-guided large domainsize might be simultaneously achievable after furtherprocess optimization and characterization of grapheneon β-SiC.

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46 Part A NanoCarbons

Ribbons of graphene, a few nanometers wide, de-velop an electronic bandgap due to confinement effects,which is absent in larger dimensions of graphene, whichis a zero-bandgap semimetal. The importance and meth-ods of inducing a bandgap in graphene are discussedlater. Here, we briefly discuss how SiC decompositioncan be used to grow graphene nanoribbons [2.35]. It isknown that the (0001) face of both 6H and 4C α-SiCwith vicinal miscuts towards 〈11̄00〉 displays bunch-ing of parallel steps into (11̄0n) nanofacets up to 4–5unit cells in height and oriented at an angle of ≈ 25◦to the basal plane. The α-SiC (0001̄) face generallydoes not show preferential orientation for nanofacets,but step-bunched (11̄0n) nanofacets can be induced bysuitable pretreatment. It has also been observed thatgraphene grown on the (0001) and (0001̄) faces of α-SiC are continuous over these steps. Controlled facetscan be achieved by conventional photolithography andmicrofabrication. Few-layer graphene is shown to growselectively on these facets. Facets of other crystallo-graphic orientations are possible, and it is expected thatthe graphene quality, properties, and growth mechanismwill depend significantly on the crystallographic sur-face. However, these preliminary results indicate that,with further research and optimization, the ideal facetsand growth conditions might be determined for large-scale controlled growth of graphene nanoribbons.

Interestingly, there was significant research into thegrowth and characterization of graphene on other metalcarbides [2.36–38], such as TiC, TaC, and HfC, as earlyas the 1980s. Then, it was referred to as monolayergraphite. Graphene has been grown on the (100) and(111) faces of these carbides by heating them to 1700 Kin UHV. As with SiC, graphene nanoribbons as narrowas 1.3 nm with well-defined edge structure have beengrown, for instance, on TiC (755) surface [2.39]. In allthese cases, a significant degree of hybridization be-tween the graphene π-electrons and the electronic bandsof the substrate carbide was reported, similar to some ofthe crystallographic faces of SiC. Despite the revelationthat the graphene is significantly decoupled from certainother SiC faces, similar exploration into other metalliccarbides remains pending.

2.1.4 Exfoliation by a Solvent

Exfoliation of graphene from graphite involves over-coming the interlayer van der Waals bonds. This isthe same interaction in play between individual CNTsin a bundle. Just as ultrasonication in a solvent hasbeen used to overcome this weak force and separate

and disperse individual CNTs from a bundle, it hasalso been used to individualize graphene layers fromgraphite. This process can be facilitated if the inter-layer attraction can be compromised by intercalates.The resultant flakes of graphene in a solvent can bestabilized to prevent aggregation, and separated intofractions which are enriched in particular graphenethicknesses.

Graphite Intercalation CompoundsDue to the nature of hybridization of carbon atoms ingraphite, it is capable of reactivity involving incorpo-ration of atoms, ions, or molecules in its lattice whileleaving its basic structure unchanged. Such graphite in-tercalation compounds (GIC) [2.40] may be broadlyclassified into those with homopolar bonding andpolar bonding. Graphite oxide (GO) and graphite flu-oride (GF) are examples of homopolar bonding, whilepotassium-, rhodium- and cesium-graphite are exam-ples of polar bonding. GICs were well studied asearly as the 1950s and are a staple of chemistry text-books. Here, we restrict our discussion to exfoliationof GICs, in particular graphite oxide, and its reductionto graphene, which has been achieved with varying de-grees of success. A family of GICs with interhalogencompounds offers control over the stage of intercalationand subsequently the layer distribution in the resultantgraphene.

Graphite can be oxidized to GO in various ways.In the modified Staudenmaier method [2.41] a mixtureof 97% sulfuric acid and fuming nitric acid is cooleddown to 5 ◦C in an ice bath, graphite in flake or powderform is added, followed by repeated additions of potas-sium perchlorate every hour over a period of 3 days. Theresulting solution, sometimes referred to as graphiticacid, is filtered and washed until the pH of the fil-trate reaches 5 or more. The Brodie method [2.41] isidentical, except that only nitric acid is used, and thepotassium perchlorate is added every hour for 3 h. Inthe Hummers method [2.42, 43], graphite is oxidizedin a mixture of concentrated sulfuric acid, sodium ni-trate, and potassium permanganate at 45 ◦C for 2 h.At this stage, the material is often referred to as ex-pandable graphite, reasons for which are explained inthe next section. In the electrochemical method [2.41],a graphite sheet electrode is anodically polarized inperchloric acid with a platinum wire as counterelec-trode. When dried, the above methods result in a powderconsisting of graphite oxide flakes. Graphite fluoridehas also been used as a starting point for graphenedispersions. Graphite can be fluorinated under fluo-

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Graphene – Properties and Characterization 2.1 Methods of Production 47

a) b)

GO With NaOH With KOH

c)

Fig. 2.6 (a) HOPG before (top) and after (bottom) oxidation and expansion (after [2.45]). (b) Deoxygenation of exfoliated GOunder alkaline conditions (after [2.46]). (c) AFM image of exfoliated monolayer graphene oxide sheets (after [2.47])

rine pressure of 200 mmHg, in a temperature range of375–640 ◦C [2.44].

Interhalogen compounds such as IBr and ICl alsoform GICs, offering control over the stage of intercala-tion. Stage I GIC refers to intercalation of every layerof graphite, while stage II GICs only have every sec-ond layer of graphite intercalated, and stage III GICshave every third layer intercalated, etc. As discussedin subsequent sections, bilayer and trilayer grapheneare electronically distinct from monolayer graphene andin certain instances, such as semiconductor electronics,might prove superior to monolayer graphene. Exfolia-tion of graphene from stage II and stage III GICs hasbeen shown to yield solutions of predominantly bilayerand trilayer graphene [2.48], and is currently the onlylarge-scale route available to synthesize these multi-layer graphenes.

From GIC to GrapheneUsually, the next step involves expansion of intercalatedgraphite by decomposing and expelling the intercalate.Rapid annealing of expandable graphite to 1050 ◦C gen-erates high-pressure gaseous decomposition productswhich force the individual layers apart. This resultsin a ≈ 100-fold expansion in the interlayer spacing ingraphite, and the material is now referred to as expandedgraphite (Fig. 2.6a) [2.45, 47]. Similarly, interhalogenGICs can be expanded by expelling the entrapped in-tercalants.

Expanded graphite or graphite oxide is dispersedin a solvent by ultrasonication, resulting in grapheneor GO solutions, respectively. In the case of GO, thepredominant product in solution is monolayer GO,while stage II and III GICs of interhalogen com-pounds yield solutions of predominantly bilayer andtrilayer graphenes. Alternatively, GO can be interca-lated and exfoliated, for instance, by tributylammoniumcations [2.49]. The phenol, carbonyl, and epoxy groups

resulting from the oxidation of graphite ensured col-loidal stability in polar solvents [2.50]. Polymers,surfactants, DNA, etc. can be used to provide additionalstabilization of the GO flakes in colloidal suspension.Edge-selective diazonium functionalization [2.51] hasalso been demonstrated as a way to stabilize high-concentration graphene solutions without the stabilizingagents perturbing the bulk structure of the graphenesheets.

Exfoliated GO can be subsequently reduced to yieldreduced GO. It cannot be referred to as graphene atthis stage due to the incomplete nature of the reductionprocess. One route involves reduction in water with hy-drazine hydrate [2.52, 53] or dimethylhydrazine [2.54].A reduced GO suspension can also be obtained byheating the exfoliated GO suspension under stronglyalkaline conditions by addition of NaOH at 50–90 ◦C(Fig. 2.6b) [2.46]. Alternately, the flakes can be de-posited in a substrate and reduced by hydrazine vaporsor hydrogen plasma [2.55]. All these methods resultin the formation of unsaturated and conjugated carbonatoms, which results in electrical conductivity and Ra-man signatures intermediate between those of GO andpristine graphene.

GF can be reacted with n-butyl and n-hexyllithium reagents in hexane at 0 ◦C. The alkyl lithiumreagent replaces the fluorine functionalization dur-ing this process. The product can then be dispersedin ethanol by sonication. The alkyl functionalization,followed by a subsequent annealing step, partially re-stores the pristine graphene structure similar to reducedGO [2.56]. A one-step electrochemical approach hasbeen demonstrated to form ionic liquid functional-ized graphite sheets, which are then exfoliated intofunctionalized graphene dispersed in polar aprotic sol-vents [2.57].

It is also possible to exfoliate noncovalent GICs toyield graphene flakes that do not suffer the disadvantage

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48 Part A NanoCarbons

DGU

f4

f10

f16

f22

f28

Mean flake thickness (nm)1.0 2.0 3.0

1.0

0.8

0.6

0.4

0.2

0.0

Relative frequency

f4f16f28

a) b) c)

Fig. 2.7 (a) Schematic illustration of ordered sodium cholate encapsulation of graphene sheets and a photograph of an unsortedaqueous graphene suspension with graphene loading of ≈ 0.1 mg/ml. (b) Photograph of a centrifuge tube following DGU markedwith the main bands of monodisperse graphene. (c) Mean flake thickness histogram measured by AFM of sorted graphene takenfrom the locations marked in panel (b) (after [2.59])

of high defect density; for instance, alkali-metal GICshave been shown to readily and spontaneously exfoliatein N-methyl-pyrrolidone (NMP), yielding a stable so-lution of negatively charged graphene sheets. Graphenecan also be noncovalently functionalized and exfoliatedwith 1-pyrenecarboxylic acid by continuous sonica-tion in water [2.58]. Interhalogen compounds do notcovalently functionalize the graphite upon intercala-tion, and therefore the resultant solution is one of puregraphene, without any need for further reduction orreconversion.

Exfoliation Without IntercalationIn an effort to avoid disruption to the desirable structureand properties of graphene, efforts have been under-taken to directly exfoliate and disperse graphite ina solvent by ultrasonication, as has been successfullydemonstrated for debundling CNTs. Systematic studyhas been undertaken in the case of CNTs to explore theirsolubility in various solvents without the assistance ofstabilizing agents such as surfactants. It has emergedthat certain solvents such as N-methyl-pyrrolidoneand N ,N-dimethylamide (DMA) are ideally suited todissolve CNTs in significant concentration [2.60]. Dis-solution of CNTs in aqueous media is only possibleusing stabilizing surfactants; however, these solutionshave emerged as the premier option among researchers,since the adsorbed surfactants can be easily desorbed ordisintegrated if and when required.

Sieved graphite powder was dispersed in NMPby bath sonication. The macroscopic particles andaggregates were sedimented by mild centrifugation(500–2000 rpm), resulting in a homogeneous dark dis-persion which was found to contain a high fraction

of monolayer and few-layer graphene flakes [2.61].Other solvents such as DMA, γ-butyrolactone, and 1,3-dimethyl-2-imidazolidinone yield similar results. Theprocedure has been adopted successfully for usingwater as solvent, in the presence of sodium dode-cylbenzene sulfonate or sodium cholate as stabilizingsurfactant [2.62]. The predominant drawback of thisprocess lies in the fact that the sonication breaks upthe graphene into particularly small fragments, withthe monolayer flakes having lateral dimensions of,on average, 100 nm. This is similar to the case ofCNTs, where sonication appears to cut them downto ≈ 200 nm [2.63]. The aqueous graphene dispersioncan now be processed by density-gradient ultracentrifu-gation (DGU) using iodixanol as density medium toyield fractions enriched in particular graphene thick-nesses (Fig. 2.7) [2.59]. Highly enriched solutions ofmonolayer and bilayer graphene with the above sizelimitation are now available for research purposes fromcommercial sources such as Nanointegris, but not yet inindustrial quantity.

Single- and few-layer graphene sheets with sizesup to 0.1 mm have been fabricated by quenching hotgraphite in ammonium hydrogen carbonate aqueoussolution [2.64]. Few-layer graphene has also beenproduced by immersing and intercalating graphitein supercritical CO2 for 30 min followed by rapidlydepressurizing the supercritical fluid to expand and ex-foliate the graphite [2.65]. The expanding CO2 gascontaining the graphene flakes was collected directlyin an aqueous solution containing stabilizing surfactantto avoid aggregation. Other supercritical fluids, such asethanol, NMP, and DMF, can also be used to exfoliategraphite into graphene [2.66].

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a)

Si Wafer

500 nm

[1120]–

b)

3 nm20 nm

c) d)

500 nm

e) W-sub 10 nm

Fig. 2.8a–e Graphene nanoribbons formed by various means. (a) Nanoparticle cutting (after [2.67]); (b) synthesized frompolyphenylene precursors (after [2.68]); (c) etching of carbon nanotubes embedded in a polymer (after [2.69]); (d) unzippingof carbon nanotubes (after [2.70]); (e) chemical exfoliation in DCE with PmPV (after [2.71])

2.1.5 Synthetic Production Route

If the reduction of bulk graphite into graphene is viewedas a top-down approach, then the chemical synthesis ofgraphene from smaller aromatic hydrocarbons will con-stitute the bottom-up approach. If graphene is regardedas a polycyclic aromatic hydrocarbon (PAH), one of thelargest of these synthesized involves 222 carbon atomsor 37 benzene units in a hexagonal structure, 3 nmin diameter [2.72], from an oligophenylene precursorwhich was planarized by oxidative cyclohydrogenation.These structures have also shown a high tendency toself-assemble on surfaces [2.73] and could potentiallyact as precursors for larger synthetic graphene.

2.1.6 Graphene Nanoribbon ( GNR)

The techniques described here have been suitably mod-ified and developed with particular focus on formingvery narrow ribbons of graphene with widths of tensof nanometers and with well-defined edge structureand orientation (Fig. 2.8). This is of particular impor-tance in electronic applications, since such nanoribbonsof graphene are one of the means to engineer anelectronic bandgap in otherwise gapless graphene, asdiscussed in detail in Sect. 2.3.6. Once graphene flakeshave been deposited onto a substrate, nanoribbonscan be fabricated on it using standard and noncon-ventional lithography and etching processes, such aselectron-beam lithography [2.74] and nanowire lithog-raphy [2.75], respectively.

Nickel [2.76] and silver [2.77] nanoparticles havebeen shown to act as a knife for cutting patterns in sur-face graphite layers of HOPG. The cutting proceedsvia catalytic hydrogenation of the graphene lattice, and

preferentially along crystallographic directions. Theparticles can become deflected into proceeding alonga different direction if they come within close proxim-ity of each other, of defects in the graphene lattice, orof previously formed cuts. The result is a complex pat-tern of cuts which border various well-defined shapes ofthe surface graphene layers, including instances wheretwo parallel cuts result in a narrow ribbon betweenthem. These shapes can be transferred onto arbitrarysubstrates using the mechanical exfoliation methods de-scribed earlier [2.67]. The graphene can also be cutinto ribbons after they have been transferred onto anyarbitrary substrate [2.78].

Expanded graphite was dispersed in a 1,2-dichloro-ethane (DCE) solution containing a polymer poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinyl-ene) (PmPV) by sonication for 30 min followed by cen-trifugation to remove larger aggregates. The supernatantafter sonication was shown to contain an appreciablefraction of graphene nanoribbons and related morpholo-gies such as ribbons with kinks, bends, and nonparallelsides [2.71]. The exact mechanism or variation to theliquid-phase exfoliation procedures described earlierthat results in the significant yield of nanoribbons in thiscase is not clearly understood.

Graphene nanoribbons, 8–12 nm in length and2–3 nm width, have also been synthesized by surface-assisted coupling of molecular precursors into linearpolyphenylenes and their subsequent cyclohydrogena-tion [2.68].

CNTs have been described as rolled-up graphenesheets, and now graphene nanoribbons have been madefrom unraveling CNTs. Oxidized nanoribbons were ob-tained by suspending CNTs in concentrated sulfuricacid followed by treatment with 500 wt.% KMnO4 for

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50 Part A NanoCarbons

1 h at 22 ◦C and 1 h at 55–70 ◦C. The process, describedas CNT unzipping, could occur as a linear longitudinalcut or in a spiral manner depending on the chirality ofthe CNT [2.70]. CNTs have also been converted to GNRby controlled plasma etching of CNTs that are partiallyembedded in a polymer film [2.69].

2.1.7 Derivatives of Graphene

Graphane, a fully saturated hydrocarbon derived fromgraphene, with formula CH, was predicted to bestable based on first-principles total-energy calcu-lations [2.79]. Experimentally, it was later shownthat graphene can be hydrogenated and converted tographane using a low-pressure (0.1 mbar) hydrogen-argon mixture (10% H2) with direct-current (DC)plasma for 2 h [2.80]. The hydrogenation is stable but

reversible, and the graphane can be reconverted tographene by annealing at 450 ◦C in Ar atmospherefor 24 h. The reconverted graphene, however, containsremnant defects just as vacancies and oxygenated orhydrogenated carbon atoms. In the case of substrate-supported graphene, only one side is hydrogenated,while both sides can be hydrogenated in the case ofsuspended graphene.

Graphene can also be fluorinated with xenon di-fluoride. When one side is exposed, F coveragesaturates at 25% (C4F), whereas fluorination of bothsides results in perfluorographene [2.81] and fluoro-graphene [2.82], which are the nonstoichiometric andstoichiometric variations. Nonstoichiometric and mul-tilayered fluorographene can also be exfoliated fromgraphite fluoride [2.56, 83]. Hydrazine treatment hasbeen shown to reverse the fluorination.

2.2 Properties

While its very existence as a freestanding two-dimensional material is a feather in graphene’s cap, it isthe properties of graphene that make it the truly excep-tional material that has stoked feverish research in thisfield. The individual properties of pristine graphene arediscussed first, while the properties of graphene deriva-tives such as GO, graphane, and fluorographene arediscussed in the final section.

2.2.1 Structure and Physical Properties

Graphene shares most of its structure and physical prop-erties with graphite, its parent material. The carbonatoms are arranged in a two-dimensional hexagonallattice (Fig. 2.9b), which can also be constructed astwo interpenetrating triangular sublattices, which takesparticular significance in bilayer and other multilayergraphenes. The carbon atoms are sp2 hybridized, andthe in-plane carbon–carbon bond length is a = 1.42 Å.The remaining p-orbital is oriented perpendicular tothe plane of carbon atoms and delocalizes to formthe π (valence) and π∗ (conduction) electronic bandswhich are discussed in detail in Sect. 2.2.3. The car-bon layers are usually stacked in an ABAB (Bernal)stacking; however, in certain few-layer graphenes suchas that grown by CVD, the layers are rotated withrespect to this standard arrangement. The interplanespacing is 3.45 Å. A staggered ABCABC (rhombo-hedral) arrangement is also possible, but has not

been realized by any of the graphene productionroutes.

Two-dimensional structures such as graphene havebeen postulated to be intrinsically unstable, and ac-cording to the Mermin–Wagner theorem [2.84], long-wavelength fluctuations destroy the long-range order of2-D crystals. Even 2-D crystals embedded in 3-D spacehave a tendency to crumple. The puzzling stability ofsuspended 2-D graphene sheets has been attributed tointrinsic microscopic undulations in which the surfacenormal varies by several degrees and the out-of-planedeformation reaches 1 nm [2.9,85]. This observation byTEM is discussed further in Sect. 2.3.2 and also con-forms to atomistic Monte Carlo simulations. Similarcorrugation has also been reported on graphene sup-ported on SiO2 substrates, where it is a superpositionof intrinsic rippling as well as extrinsic undulationsimposed by the substrate surface morphology [2.86].Periodic ripples have also been observed on weaklycoupled graphene monolayers on substrates such asRu(0001) [2.87]. Corrugations in substrate-supportedgraphene are primarily observed by STM and are dis-cussed further in Sect. 2.3.3. In addition to ripples,substrate-supported graphene also exhibits ubiquitouswrinkles which could be several nanometers in width.Scrolling has been occasionally observed at the edgesof graphene flakes, both suspended [2.88] and sub-strate supported [2.89], and this appears to rely on thefabrication method. Scrolling occurs when graphene is

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Graphene – Properties and Characterization 2.2 Properties 51

a) b)

A

A

BB

1.42 Å

BA

Fig. 2.9 (a) SEM of graphene suspended over a macroscopic hole of a Cu TEM grid (after [2.90]). (b) Structure ofgraphene (after [2.92])

subjected to liquid-phase processing during microfab-rication, while its solid-phase or gas-phase processingappears to avoid this [2.90], and it is possible to ob-tain large free-standing sheets of monolayer graphene(Fig. 2.9a). The ripples in graphene also result in pertur-bations in the electronic structure, and many electronicand chemical properties of graphene have been at-tributed to these ripples, rather than being intrinsic tographene. However, it has been shown that graphenedeposited on atomically flat terraces of cleaved micasurfaces is flat down to the atomic scale [2.91]. Theheight variation observed by AFM was less than 25 pm,and such ultraflat graphene is expected to permit ex-ploration of various intrinsic physical and chemicalproperties of graphene.

2.2.2 Mechanical Properties

Carbon materials have made it a habit of setting recordsfor their intrinsic mechanical properties. Diamond is thehardest known natural material, and is assigned a gradeof 10 (highest) on the Mohs scale of mineral hard-ness [2.93]. Similarly, the record for tensile strengthhas been held by CNTs; a Young’s modulus of 1 TPaand tensile strength of 150 GPa coupled with elonga-tion to failure as high as 20% have been experimentallyreported [2.94].

The earliest experimental indication for the extraor-dinary stiffness of graphene was the observation thatgraphene beams supported on only one end do not scrollor fold, quite unlike the papery or cloth-like appearanceof graphene. If the effective thickness of monolayergraphene is estimated to be 0.23 Å from elastic the-ory, a bending rigidity of 1.1 eV [2.85], and a Young’s

modulus of 22 eV/Å2 from the elastic modulus of bulkgraphite [2.95], the lengths of unsupported grapheneobserved in TEM samples have been 106 times largerthan its effective thickness. Suspended graphene cangain additional thickness from large-scale corrugationsby a factor of (H/a)2, where H is the characteristicheight of the corrugations. In addition to supporting itsown weight, suspended graphene has been shown tosupport significant extra load such as copper nanoparti-cles [2.90], as well as surviving accidental shocks suchas during handing.

Direct measurements of the elastic properties ofgraphene have been conducted by nanoindentationof suspended graphene layers in an AFM [2.96,97]. Details of the measurement technique are foundin Sect. 2.3.3. Measurements conducted on few-layergraphene of less than 8 nm thickness yielded springconstants of 1–5 N/m. A Young’s modulus of 0.5 TPawas extracted by fitting the data to a model for dou-bly clamped beams under tension. For measurements onmonolayer graphene, the force–displacement character-istics yield second- and third-order elastic stiffness of340 and −690 N/m, respectively. The breaking strengthwas found to be 42 N/m, which represents the intrin-sic strength of a defect-free sheet. This correspondsto Young’s modulus E = 1.0 TPa, third-order elasticstiffness of 2.0 TPa, and intrinsic strength of 130 GPa.These figures mean that graphene is the strongest ma-terial ever measured.

Nonlinear finite elasticity theory for graphene res-onators for both electrostatic and electrodynamic caseshas been developed and agrees well with experimentson graphene resonators [2.98]. The dynamic response ofclamped graphene resonators resembles that of coupled

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52 Part A NanoCarbons

a) b) c)

Au

Si

SiO2

Au

5 μm

Q = 125

–0.2

0

0.2

0.4

0.6Current (nA)

Frequency (MHz)

III

10025 7550

Fig. 2.10 (a) Schematic of graphene resonator, with electrostatic actuation and electrical readout. (b) SEM image of sucha resonator. (c) The graphene resonance (I) at 65 MHz. Resonances of metal beams (II) are also visible below 25 MHz.Inset: the graphene resonance at low driving power, and Lorentzian fit (red line) with Q = 125 (after [2.102])

Duffing-type resonators. Similarly, a continuum platemodel for the vibration of multilayered graphene sheets,including the van der Waals (vdW) interaction betweenthe layers, suggests that the lowest natural frequenciesare identical for various numbers of layered graphenes.Higher resonance frequencies, however, depend on thevdW interaction and are different for different layeredgraphenes [2.99]. In general, natural resonance fre-quencies in the THz regime are expected for grapheneresonators, due to the combination of their extremethinness and extraordinary stiffness. Experimentally,the mechanical vibrations in electrostatically actuatedgraphene resonators have been imaged by a specialmodification of atomic force microscopy [2.100]. Res-onance frequencies in the tens of MHz have beenrecorded in graphene resonators (Fig. 2.10), with qual-ity factors as high as 4000 at room temperature [2.101]and 10 000 at 5 K [2.102].

2.2.3 Electronic Properties

Electronically, monolayer, bilayer, and trilayer graph-ene are electronically distinct materials. Beyond threelayers, graphene’s electronic properties tend towardsthose of bulk graphite. In certain aspects, graphene ofup 10 layers might exhibit deviation in electronic prop-erties from bulk graphite and could be referred to asgraphene, but beyond 10 layers all graphenes are indis-tinguishable from graphite.

Monolayer GrapheneThe electronic structure of graphene was first describedin 1946 [2.103], as a theoretical building block to de-

scribe graphite. The valence and conduction bands ofgraphene are conical valleys that touch at the high-symmetry K and K′ points of the Brillouin zone. Nearthese points, the energy varies linearly with the mag-nitude of momentum, i. e., follows a linear dispersionrelation. In neutral graphene, this point of intersectioncoincides with the charge neutrality point, and is re-ferred to as the Dirac point.

In every other material known to condensed matterphysicists, the electrons behave as and can be describedby the Schrödinger equation. In graphene, on the otherhand, electrons have been shown to behave as rela-tivistic particles, and should be described by the Diracequation [2.104–106]. The interaction of electrons withthe periodic potential of the graphene hexagonal latticeresults in quasiparticles, which can be viewed as elec-trons devoid of their rest mass m0 and therefore calledmassless Dirac fermions. The linear energy dispersionmeans that the speed of electrons in graphene is a con-stant, independent of momentum, as in the case of thespeed of photons. The velocity of electrons in grapheneis ≈ 106 m/s, about 300 times slower than the speed oflight (photons).

The electronic states near the Dirac point arecomposed of states belonging to the two graphenesublattices, and as a result the quasiparticles possesspseudospin, similar to the electron’s spin [2.107, 108].As a result, these Dirac fermions are said to be chi-ral. Another relativistic feature of these quasiparticlesis the Klein paradox [2.107], wherein they tunnelthrough a potential barrier of any height and widthwith a transmission probability of 1 or without a re-flected component. As a result, electrons in graphene

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Graphene – Properties and Characterization 2.2 Properties 53

a) b) c)

E = ED

π*

π

π*

π

π*

π

Fig. 2.11a–c Electronic structure of (a) monolayer, (b) symmetric bilayer, and (c) asymmetric bilayer of graphene (af-ter [2.109])

can propagate over (relatively) vast distances of the or-der of microns through the graphene lattice, even in thepresence of lattice defects or other external perturbingpotentials [2.4].

Bilayer GrapheneIf the hexagonal atomic structure of graphene is com-posed of nonidentical elements, such as in boron nitride,the lateral in-plane symmetry is broken and a largebandgap is formed between the π and π∗ states. Thisis the case in bilayer graphene, where the AB (Bernal)stacking between the two graphene renders the two car-bon atoms inequivalent and results in two graphenesublattices. As a result, the unit cell of bilayer graphenecontains four atoms, and two additional bands result (πand π∗ states). If the inversion symmetry between thetwo layers is broken, then an energy gap between thelow-energy valence and conduction bands forms at theDirac point (Fig. 2.11).

The first experimental demonstration of this ef-fect was performed on bilayer graphene synthesizedon SiC (6H, (0001) orientation) [2.109]. The as-growngraphene is n-doped due to the depletion of the sub-strate’s dopant carriers. At low temperature, the SiCdopant electrons are frozen out and the substrate actsas a nearly perfect insulator while the excess electronsin graphene retain their high mobility. In this case, thesymmetry of the bilayers is broken by the dipole fieldcreated between the depletion layer of the SiC and theaccumulation of charge on the graphene layer next tothe interface. Further n-type doping can be introducedby deposition of potassium atoms onto the vacuumside, which donate their lone valence electrons to thegraphene layer, forming another dipole. The bindingenergy–momentum dispersion relation of π, π∗, and σ

states along high-symmetry directions was measured by

angle-resolved photoemission spectroscopy (ARPES)(Fig. 2.12). The relative potential of the top and bottomgraphene layers is varied by changing the doping levelby potassium adsorption. An apparent gap at the K pointappears in the as-prepared graphene, disappearing andreappearing with increasing level of K doping.

The electronic gap in bilayer graphene can thus becontrolled by applying an external transverse electricfield, such as by a gate bias, making it the only knownsemiconductor material with a tunable energy gap. Us-ing a tight-binding model, the value of the gap wasextracted as a function of electron density, showing thatit can be tuned to values larger than 0.2 eV, using fieldsof ≈ 1 V/m.

The two key semiconductor parameters, the elec-tronic bandgap and carrier doping concentration, canalso be independently tuned by using a dual-gate config-uration. Reliable determination of the bilayer bandgaphas been carried out in such a configuration usinginfrared microscopy [2.110]. Figure 2.13 shows thegate-modulated bilayer absorption spectra at the chargeneutrality point. The two features present in the spectra,a peak below 300 meV and a dip around 400 meV, arisefrom different optical transitions between the bilayerelectronic bands. Transition I shows pronounced gatetunability up to 250 meV at 3 V/nm, since it accountsfor the bandgap-induced spectral response.

By examining the electronic band structure ofgraphene around the K point within a tight-bindingapproach, it has been shown that a single graphenelayer is a zero-gap semiconductor with a linear Dirac-like spectrum around the Fermi energy, while graphiteshows semimetallic behavior with band overlap of about41 meV. Bilayer graphene has a parabolic band struc-ture around the Fermi energy and is a semimetal likegraphite; however, the band overlap is only 0.16 meV.

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54 Part A NanoCarbons

a) b) c)

Momentum

0.0125 e– 0.0350 e–0.005 e–

Bin

ding

ene

rgy

(eV

)0.2

0.0

–0.4

–0.6

–0.8

–1.0

–1.2

–0.2

–1.40.1 Å–1

Fig. 2.12a–c Evolution of gap closing and reopening by changing the doping level by potassium adsorption. Experimentaland theoretical bands (solid lines) for (a) as-prepared graphene bilayers and b,c with progressive adsorption of potassiumare shown. The number of doping electrons per unit cell, estimated from the relative size of the Fermi surface, is indicatedat the top of each panel (after [2.109])

Theory

Δ = 250 MeV

Δ = 190 MeV

Δ = 145 MeV

Δ = 105 MeV

c)

12

8

4

0

600Energy (meV)

200 400

Absorption difference (%)b)

12

8

4

0

600Energy (meV)

200 400

Absorption difference (%)

Experiment

D−

= 3.0 V/nm

D−

= 1.9 V/nm

D−

= 1.4 V/nm

D−

= 1.0 V/nm

a)

EF

IV V

Δ

II

I

III

Fig. 2.13a–c Infrared spectroscopy to probe bilayer energy gap opening at strong electrical gating. (a) Allowed opticaltransitions between different subbands of a graphene bilayers. (b) Gate-induced absorption spectra at the charge neutralitypoint for different applied displacement fields D̄. Curves are offset from zero for clarity. (c) Theoretical prediction of gate-induced absorption spectra based on a tight-binding model where the bandgap value is taken as an adjustable parameter.The fit provides an accurate determination of the gate-tunable bandgap at strong electrical gating (after [2.110])

This overlap increases with the number of graphenelayers, and for 11 or more layers it is smaller than 10%.

SuperconductivityAndreev reflection at a metal–superconductor junc-tion involving graphene is fundamentally different fromnormal metals [2.111]. In weakly doped graphene,

electron–hole conversion involves electrons from theconduction band being converted into a hole from thevalence band. This interband conversion is associatedwith specular reflection instead of the retroreflectionfound in normal metals where electron–hole conver-sion occurs within the conduction band (Fig. 2.14). TheJosephson effect has also been experimentally stud-

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Graphene – Properties and Characterization 2.2 Properties 55

a) b) c)

Superconductor

Andreev retroreflection Specular Andreev reflection

30

20

xh

ehe

y

Superconductor

10

00–10 10 0–350 350 750

V (µV)–750

dV/dI (kΩ)

2Δ/3

2ΔΔ

0.8

1.0

1.2

B (mT)

l (nA)

Fig. 2.14 (a) Andreev retroreflection (left) at the interface between a normal metal and a superconductor, and specularAndreev reflection (right) at the interface between undoped graphene and a superconductor. Arrows indicate the directionof the velocity, and solid or dashed lines distinguish whether the particle is a negatively charged electron (e) or a positivelycharged hole (h) (after [2.111]). (b) Josephson effect in graphene: dV/dI (I, B) at T = 30 mK (yellow-orange is zero, thatis, the supercurrent region, and red corresponds to finite dV/dI ) (after [2.112]). (c) dV/dI versus V , showing multipleAndreev reflection dips below the superconducting energy gap. The dips in dV/dI occur at values of V = 2Δ/en, wheren is an integer number (after [2.112])

ied in macroscopic junctions consisting of a graphenelayer contacted by two closely spaced superconduct-ing electrodes (SGS) [2.112, 113]. A supercurrent isobserved, which can be carried either by electronsin the conduction band or by holes in the valanceband, as determined by the gate voltage. A finite su-percurrent is also observed at zero charge density atthe charge neutrality point, indicating phase-coherentelectronic transport at the Dirac point. The diffusivejunction model has been shown to yield quantitativeagreement with experiments [2.114], while a ballis-tic SGS model is inconsistent with the data. Thisis attributed to potential fluctuations in graphene dueto the influence of the substrate as well as metal-lic leads. Crossed Andreev reflection in graphene–superconductor–graphene junctions [2.115] and An-dreev reflection in graphene nanoribbons [2.116] havebeen theoretically investigated, but experimental confir-mation remains pending.

2.2.4 Optical Properties

Successful exfoliation of monolayer graphene dependson the recognition of the optical properties of graphenemore than the exfoliation procedure [2.7]. The choice of300 nm-thick SiO2 on Si substrate allowed optical iden-tification of the exfoliated monolayer graphene, whichwould otherwise have been invisible and not practi-cally detectable; for instance, only flakes thicker thanten layers can be found in white light on top of 200 nmSiO2, which also marks the commonly accepted tran-

sition from graphene to bulk graphite. The contrast ofa graphene flake depends not only on the SiO2 thicknessbut also on the wavelength λ of light used. Figure 2.15summarizes the expected contrast as a function of SiO2thickness as well as wavelength of monochromatic il-lumination, derived using Fresnel theory. It was alsoinferred that the complex refractive index of graphene isthe same as that of bulk graphite, n = 2.6−1.3i, whichis independent of λ. This can be explained by the factthat the optical response of graphite with the electricfield parallel to graphene planes is dominated by thein-plane electromagnetic response. Since changes in thelight intensity due to graphene are relatively minor, theobserved contrast can be used to determine the numberof graphene layers.

The absorbance of light by monolayer and bilayergraphene has been measured to be 2.3 and 4.6%, re-spectively, in the visual regime (450–750 nm), and thisextends linearity up to five layers. The optical trans-parency of noninteracting graphene is solely determinedby the fine structure constant of quantum electrodynam-ics (α = e2/�c = 1/137), which describes the couplingbetween light and relativistic electrons [2.117, 118].This is because, as discussed in the previous sec-tion, the electrons in graphene behave as relativisticDirac particles and electron–electron Coulomb interac-tions can be neglected. The high-frequency (dynamic)conductivity G for Dirac fermions in graphene is a uni-versal constant equal to e2/4�. The universal G impliesthat observable quantities such as graphene’s opticaltransmittance T and reflectance R are also universal

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56 Part A NanoCarbons

410 nm

300 nm SiO2(a) (b) (c) 200 nm SiO2

470 nm

White light5 μm White light

530 nm 590 nm 650 nm λ=710 nm

λ=560 nm

410 nm 470 nm 530 nm 590 nm 650 nm λ=710 nm

200100

Blu

eG

reen

Red

300SiO2 thickness (nm)

0

0.10

0.05

0.15

0.00

λ (nm)

500

400

600

700

Fig. 2.15 Left: Color plot of contrast as a function of wavelength and SiO2 thickness. The color scale on the right showsthe expected contrast. Right: Graphene crystallites on 300 nm SiO2 imaged with white light (panel a), green light, andanother graphene sample on 200 nm SiO2 imaged with white light (panel c). Single-layer graphene is clearly visiblein the left image (panel a), but even three layers are indiscernible on the right (panel c). Image sizes are 25 × 25 μm2.Top and bottom panels show the same flakes as in (panel a) and (panel c), respectively, but illuminated through variousnarrow bandpass filters with bandwidth of ≈ 10 nm (after [2.7])

and given by T ≡ (1+2πG/c)−2 = (1+1/2πα)−2 andR ≡ 1/4π2α2T for normal light incidence. This yieldsgraphene’s opacity (1− T ) ≈ πα = 2.3%.

2.2.5 Thermal and ThermoelectricProperties

CNTs are known to have very high thermal conduc-tivity K with the experimentally determined value ofK ≈ 3000 W/(m K) at room temperature for an individ-ual multi-walled CNT [2.119] and K ≈ 3500 W/(m K)for an individual single-walled CNT [2.120]. These val-ues exceed those of the best bulk crystalline thermalconductor, diamond, which has thermal conductivity inthe range K = 1000–2200 W/(m K) [2.121].

The first experimental determination of the ther-mal conductivity of suspended monolayer graphenepegged the value at 5300 W/(m K) and a phonon meanfree path of 775 nm near room temperature [2.122],which was extracted from the dependence of the RamanG peak frequency on the excitation laser power and in-dependently measured G peak temperature coefficient.Interestingly, this value is higher than the bulk graphitelimit of K ≈ 2000 W/(m K) [2.123]. It has been experi-mentally shown that the room-temperature thermal con-ductivity decreases from ≈ 2800 to ≈ 1300 W/(m K) asthe number of graphene layers in few-layer graphene(FLG) increases from two to four [2.124]. The ob-served evolution from two-dimensional graphene tobulk graphite is explained by the cross-plane couplingof the low-energy phonons and changes in the phonon

Umklapp scattering, since more states are available forscattering owing to the increased number of phononbranches.

The thermoelectric power (TEP) is the voltagedeveloped across a sample when a constant tem-perature gradient is applied. TEP of 80 μV/K wasrecently measured in graphene at room temperature(300 K) [2.125]. Similar to the quantum Hall effect inelectronic transport, quantized TEP has also been ob-served in graphene at high magnetic fields [2.125]. TheTEP can be tuned in graphene, even to negative values,under the application of a gate bias or chemical poten-tial [2.126]. Very large TEP values have been predictedfor graphene nanoribbons, for instance, 4 mV/K fora 1.6 nm-wide ribbon [2.127]. In comparison, the high-est value experimentally reported so far is 850 μV/K fortwo-dimensional electron gases in SrTi2O3 heterostruc-tures [2.128], while only a few μV/K has been reportedfor bulk graphite [2.123]. The TEP power of SWNTshas been theoretically and experimentally shown to be60 μV/K [2.129], inferior to that of graphene. A giantthermoelectric coefficient of 30 mV/K was reported ina nanostructure consisting of metallic electrodes peri-odically patterned over graphene, deposited on a silicondioxide substrate [2.130].

2.2.6 Chemical Properties

The chemistry of graphene is dominated entirely byits surface, since every carbon atom is a surface atomtwice over, forming a part of two surfaces. For nanorib-

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Graphene – Properties and Characterization 2.2 Properties 57

c) IntensityPristine Functionalized

D

n = 1 D/G = 0.185

60 000

30 000

01100 2000 2900

Raman shift (cm–1)

Intensity

15 000

10 000

5000

01100 2000 2900

Raman shift (cm–1)

d) IntensityPristine Functionalized

D

n = 2 D/G = 0.012

40 000

20 000

01100 2000 2900

Raman shift (cm–1)

Intensity

50 000

25 000

01100 2000 2900

Raman shift (cm–1)

e) IntensityPristine Functionalized

D

n = ∞ D/G ≈ 0

60 000

30 000

01100 2000 2900

Raman shift (cm–1)

Intensity

50 000

25 000

01100 2000 2900

Raman shift (cm–1)

D

10 000

5000

01300 1600

a)

L1L3

L2

b)

10 μm 10 μm

Fig. 2.16 (a) Microscopic images of single-layer (right), bilayer (left), and (b) multilayer (n ≈ ∞) graphene. (c–e) Raman spectraof pristine (left) and functionalized (right) sheets: (c) spot L1 on single sheet with inset showing expanded 1300–1700 cm−1

region, (d) spot L2 on bilayer, and (e) spot L3 on multilayer (n ≈ ∞, graphite). There is no D peak for the pristine samples (leftspectra). The D/G ratio after reaction of single layer (0.185) is about 15 times higher than that for a bilayer (0.012) and greaterfor other multilayers (≈ 0). Reactions all performed at 35 ◦C with 17 mM 4-nitrobenzene diazonium water with 1 wt.% sodiumdodecyl sulfate (SDS) (after [2.131])

bons of graphene, the edges play an increasing role indetermining their reactivity.

It has been shown that, for electron transfer chem-istries, single graphene sheets are almost 10 times morereactive than bilayer or multilayer graphene (Fig. 2.16)according to the relative intensity of the disorder (D)peak in the Raman spectrum examined before and afterchemical reaction [2.131, 132]. Substrate-induced dop-ing of the graphene resulting in electron-rich regionshas been proposed to explain this trend. The effect ofdoping is greatest in monolayers because the screeninglength in the c-axis in graphite and graphene is only 5 Å,comparable to the interlayer spacing of 3.5 Å [2.133,134]. Similarly, the reactivity of edges is at least twotimes higher than the reactivity of the bulk graphenesheet [2.131]. Predictions based on Gerischer–Marcuselectron transfer theory and tight-binding approxi-mations predict that armchair and zigzag graphenenanoribbons (GNRs) have opposite trends in reactiv-ity, with the former increasing with width and the latterdecreasing. In zigzag ribbons the major reactivity con-tribution comes from edge states [2.135]. This reactivitytrend for zigzag GNR is reversed for very narrow rib-bons due to the presence of large semiconducting gapswith correspondingly low reactivities.

Graphene can be readily functionalized throughdiazonium or nitrene [2.136] reactions, which can in-troduce reactive species covalently linked to graphene.These groups then serve as templates for further chem-istry and grafting of functional groups, for instance,through an azide linker. Chemical functionalization ofgraphene can be monitored through its effect on the con-ductivity of graphene, serving as a means to control theelectrical transport properties of graphene [2.137, 138].Furthermore, p-doped and n-doped regions in graphenecan be generated by suitably functionalizing them, forinstance, with diazonium salts and polyethylene imine,respectively [2.139]. Such chemical modification canalso be performed on different parts of a single sheetto form p–n junctions in graphene [2.140].

2.2.7 Properties of Graphene Derivatives

Graphane was theoretically predicted to take one of twoconfigurations: a chair conformer with the hydrogenatoms alternating on both sides of the plane for the twographene sublattices, and a boat conformer with the hy-drogen atoms alternating in pairs [2.79]. These chairand boat conformers have a direct electronic bandgapof 3.5 and 3.7 eV, respectively. Graphane is a com-

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58 Part A NanoCarbons

pletely insulating material; its resistivity changes bytwo orders of magnitude with decreasing temperaturefrom 300 to 4 K, and its carrier mobility decreases to≈ 10 cm2/(V s) at liquid-helium temperatures for typi-cal carrier concentrations of 1012 cm−2 [2.80].

Similarly, fluorographene [2.82] is a high-qualityinsulator with large optical bandgap of > 3 eV androom-temperature resistivity of > 1012 Ω. The Young’s

modulus of fluorographene was measured to be 0.3 TPa,which is about 30% the stiffness of graphene. Simi-larly, fluorination reduces graphene’s intrinsic breakingstrength by 2.5 times. However, fluorographene isable to sustain the same ultimate strain of 15% asgraphene. Fluorographene is also strongly hydrophobic,and can be considered the two-dimensional equivalentof Teflon.

2.3 Characterization

Each of the properties discussed in the previous sec-tion has to be measured and correlated using multiplecharacterization techniques, which are discussed in thissection; for instance, electronic properties of graphenehave to be independently verified by ARPES, opticalspectroscopy, and electronic transport. This is essen-tial, since just one measurement, for instance, electronictransport, might not be able to sufficiently distinguishbetween an electronic bandgap and a mobility gap. Sim-ilarly, mechanical properties have to be confirmed bya combination of tensile testing, electromechanical res-onance, and Raman spectroscopy.

2.3.1 Optical Characterization

Based on the optical properties of graphene discussedin an earlier section, and the fact that green light is mostcomfortable for the eyes, optimal SiO2 thicknesses of90 and 280 nm can be recommended [2.7]. Similarly,it has been shown that graphene can be observed on50 nm Si3N4 using blue light and on 90 nm poly-methylmethacrylate (PMMA) using white light [2.7]. Opticalcontrast can similarly be used to identify graphene ox-ide on Si/SiO2 substrates, as well as to visualize itsconversion to reduced GO upon annealing, since boththe effective index of refraction and the effective extinc-tion coefficient increase [2.141].

Rayleigh scattering can identify the number ofgraphene layers as well as probe their dielectric con-stant [2.142]. Rayleigh imaging relies on elasticallyscattered incident photons, while Raman spectroscopy,which is discussed later, collects inelastically scatteredphotons. For graphene on Si/SiO2 substrate, underwhite-light illumination combined with interferomet-ric detection, the contrast can be tailored by adjustingthe SiO2 thickness and the light modulations dependstrongly on the graphene thickness. Up to six layers, thegraphene behaves as a superposition of single sheets andthe monochromatic contrast increases linearly.

2.3.2 Transmission Electron Microscopy

Transmission electron microscopy (TEM) is one ofthe most direct observation techniques to elucidate thestructure of graphene. High-resolution TEM can resolveindividual carbon atoms as well as adatoms, defects,and other anomalies in graphene (Fig. 2.17) [2.88]. Thehigh-energy electrons in a TEM can also be used toengineer defects such as vacancies, cause edge recon-structions and graphene sublimation, as well as observethem in situ [2.10]. Various techniques have been devel-oped to transfer micromechanically cleaved grapheneflakes onto TEM grids. If folds occur in the trans-ferred graphene flake, observation of the folded edgecan yield information about the number of layers inthe graphene flake [2.9]; a monolayer fold edge turnsup as a single dark line, while a bilayer fold edgeappears as two dark lines and so on, in analogy tosingle-walled and multi-walled carbon nanotubes. Inaddition, nanobeam electron diffraction (Fig. 2.17) canalso be used to quantify the layering in graphene [2.9].Monolayer graphene can be distinguished from higher-layered graphenes by the anomalous intensity ratio ofthe diffraction peaks; its 01̄10 peaks being more intensethan the 12̄10 peaks. When measured as a function of in-cidence angle, it probes the whole 3-D reciprocal space.The total (integrated) intensity of the 01̄10 and 12̄10peaks of monolayer graphene varies weakly with tilt an-gle and no minima in intensity are observed, since theintensities in reciprocal space for monolayer grapheneare continuous rods. In contrast, the total intensity of bi-layer graphene diffraction peaks varies strongly with tiltangle, including minima at certain angles where somepeaks vanish [2.143]. However, while the total intensityin monolayer graphene only decreases slightly, signif-icant peak broadening is observed with increasing tiltangle. This effect is most pronounced in monolayers,and decreases with increasing thickness of the grapheneflake. This is attributed to nanoscale corrugations in 2-D

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Graphene – Properties and Characterization 2.3 Characterization 59

d)

400

200

600

00 1.4

Distance (Å–1)1.210.80.40.40.2

Intensity (arb. units)c)

100

50

150

00 1.4

Distance (Å–1)1.210.80.40.40.2

Intensity (arb. units)

e)

2000

4000

0–30 30

Tilt angle (deg)20100

0–110

0–100

–1010

–10–20

Intensity (arb. units)

0–110

–1010

–1100

f)

10 000

5000

15 000

00 50

Tilt angle (deg)40302010

Intensity (arb. units)

0° 0°–1

–120

0–110

1–210

–1010

–2110

Tilt axis

1–100

–1

–120 0

–110

1–210

–1010

–1210

–2110

a) b)

Fig. 2.17 (a) Atomic-resolution TEM of graphene (after [2.88]). (b) Nanobeam electron diffraction patterns of monolayerand bilayer graphene. Relative intensities of 11̄00 and 12̄10 peaks in (c) monolayer and (d) bilayer graphene. Variation ofintensity of the 11̄00 peaks with tilt angle for (e) monolayer and (f) bilayer graphene (after [2.143])

graphene, with the surface normal deviating on averageby ±5◦ in monolayers and ±2◦ in bilayers. Consider-ing that the spatial extent of these corrugations cannotbe drastically smaller than the coherence length of thediffracted electrons and that a large number of orienta-tions should be included within the submicron electron

beam in order to yield a smooth Gaussian shape of thediffraction peak, it is estimated that the corrugationsoccur on length scales of 10–25 nm. This nanoscale cor-rugation extending into the third dimension squares theexistence of 2-D graphene with the theoretical predic-tion that perfect 2-D atomic crystals cannot exist.

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60 Part A NanoCarbons

a) b)

c) d)

1 μm

0.5 μm

1.5 μm

I

III

II

Fig. 2.18a–d Images of suspended graphene membranes. (a) Scan-ning electron micrograph of a large graphene flake spanning anarray of circular holes (1 and 1.5 μm in diameter). Area I showsa hole partially covered by graphene, area II is fully covered, andarea III is fractured from indentation. Scale bar 3 μm. (b) Noncon-tact AFM image of one membrane, 1.5 μm in diameter. The solidblue line is a height profile along the dashed line. The step height atthe edge of the membrane is 2.5 nm. (c) Schematic of nanoindenta-tion on suspended graphene membrane. (d) AFM image of fracturedmembrane (after [2.96])

2.3.3 Scanning Probe Techniques

Scanning probe techniques discussed here in the contextof graphene characterization include atomic force mi-croscopy (AFM), electrostatic force microscopy (EFM),and scanning tunneling microscopy (STM) and spec-troscopy (STS).

AFM in tapping mode is commonly used to measurethe thickness of graphene flakes on substrates; how-ever, the correlation between measured thickness andactual thickness as well as number of layers is chal-lenging [2.144]. Electrostatic interactions between thetip and the graphene, adsorbed moisture, and incorrectchoice of AFM parameters such as free amplitude val-ues can all influence the final measured thickness ofa graphene flake. Therefore, while using AFM to char-acterize graphene flakes, an internal reference such asa fold in the flake, or a second characterization toolsuch as Raman spectroscopy, is often used to correlatethe measured thickness and number of layers, and this

process needs to be repeated at least for every differ-ent substrate and processing conditions involved in thegraphene preparation.

In addition, AFM is used to measure the flatness ofgraphene on various substrates, and it is revealed that ul-traflat graphene can be obtained on mica surface [2.91]with standard deviation of height and height correla-tion length of 24.1 pm and 2 nm, respectively, comparedwith 154 pm and 22 nm, respectively, for SiO2 substrate.AFM can also be used in nanoindentation mode to probethe stiffness of suspended graphene (Fig. 2.18) [2.96].This technique has the advantage that the sample geom-etry can be precisely defined and the sheet is clampedaround the entire hole circumference; a Young’s modu-lus of 1 TPa and intrinsic breaking strength of 42 N/mhave been measured. EFM has been used to confirm thatthe surface potential of few-layer graphene increaseswith film thickness, approaching bulk graphite valuesfor five or more layers [2.145]. This is a measure of theextent of the electrostatic interaction between grapheneand the substrate, and the screening of these perturba-tions by underlying graphene layers.

STM can image graphene with atomic resolution,and the correlation of the graphene hexagonal lattice tothe direction of the edge of an exfoliated flake revealsthe orientation of the edge as being either armchair orzigzag [2.146]. It has been shown that, in mechanicallyexfoliated graphene flakes, a majority of edges followeither of these orientations and intersect at angles thatare multiples of 30◦. STM imaging of graphene grownepitaxially on SiC [2.147] or metallic substrates [2.20,23,87,148] reveals the superlattice structure and the ex-tent of coupling between the graphene and substrate.STM can also be used to locate and characterize pointdefects in the graphene lattice [2.149, 150]. STS can beused to probe the atomically resolved local electronicstructure of graphene [2.147,151,152]. A prominent gapin the tunneling spectrum unique to graphene has beenobserved and attributed to a phonon-mediated inelastictunneling process.

2.3.4 Angle-Resolved PhotoemissionSpectroscopy (ARPES)

ARPES is a direct experimental technique that hasmeasured the electronic density of states in graphenewith both energy and momentum information. Theshape of the π and π∗ bands near EF at the K-pointfrom ARPES reveals the transition from 2-D to bulkcharacter from one to four layers of graphene [2.153].Fermi velocities and effective masses of the electrons

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Graphene – Properties and Characterization 2.3 Characterization 61

Edgegraphite

Edge1 layer

D1

D

D2

c)514.5 nm

2000

1500

1000

500

2500

01400

Raman shift (cm–1)1300

Intensity (arb. units)

1350

b)514 nm

2800Raman shift (cm–1)

2600

Intensity (arb. units)

Graphite

10 layers

5 layers

2 layers

1 layer

2700

a)

40 000

30 000

20 000

10 000

50 000514 nm

Graphite

Graphene

3000Raman shift (cm–1)

1500

Intensity (arb. units)

2000 25000

d)

Monolayer

Ele

ctro

n en

ergy

q = Exchangedphonon momentum

q

b

a

cπ*

π

εL

εL= Laser energy Fermi level

Bilayer q1B

q1A

εL

q2A

q2B

εL

K M K'Γ

Fig. 2.19 (a) Comparison between Raman spectra of graphene and graphite. (b) Evolution of 2D peak shape with numberof layers of AB-stacked graphene. (c) Comparison of D peak between graphene and graphite. (d) Scattering processingcausing the 2D peak components in monolayer and bilayer graphene (after [2.156])

can also be measured. ARPES on epitaxial AB-stackedbilayer graphene on SiC has revealed that the magni-tude of the gap between the valance and conductionbands can be varied by controlling the carrier density,for instance, with a transverse electric field. On the otherhand, APRES also reveals that individual graphene lay-ers of multilayer graphene grown on SiC(0001̄) behaveas decoupled monolayers with independent linearly dis-persed bands at the K-point [2.154]. ARPES also al-lows studies of electron–electron, electron–phonon, andelectron–plasmon interactions, and indicates that allthree must be considered on an equal footing in under-standing the quasiparticle dynamics in graphene [2.155].Ab initio simulations of the ARPES intensity spectra of

graphene has been able to reproduce key experimentalobservations including the indication of a mismatch be-tween the upper and lower halves of the Dirac cone.

2.3.5 Raman Spectroscopy

The three significant Raman spectral features ingraphene are the G peak at ≈ 1580 cm−1, the D peakat ≈ 1350 cm−1, and the 2D peak at ≈ 2700 cm−1, asseen in Fig. 2.19 [2.156]. The G peak is due to the E2gmode, i. e., in-plane vibrations of the carbon atoms. TheD peak and 2D peak are strongly dispersive, with exci-tation energy due to the Kohn anomaly at the K-point,while the G peak is not.

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62 Part A NanoCarbons

a)

As-deposited grapheneGated graphene [12]Suspended graphene [19]

p-doping

n-doping

2700

2694

2688

2682

2676

26701580 1590 1600 1610

Pos(G) (cm–1)

Pos (2D) (cm–1)

= 514 nm

b)

Disorder

Doping

60

80

40

20

8

6

1575 1580 1590 15951585 1600Pos(G) (cm–1)

FWHM (G) (cm–1)

Disordered graphene [19]

As-deposited graphene

Gated graphene [11]

Suspended graphene [19]

UV-disordered graphene

Disordered graphite [17]

Fig. 2.20 (a) Pos(G) as a function of Pos(2D) for as-deposited graphene and gated graphene. (b) Pos(G) as a functionof FWHM(G) for as-deposited graphene, compared with disordered graphene, disordered graphite, and gated graphene.The dotted lines are only a guide to the eye (after [2.159])

The 2D peak is the second order of the zone-boundary phonons and therefore does not requiredefects. For monolayer and few-layer graphene, the 2Dpeak serves as a fingerprint for identification [2.156].In general, the 2D peak of graphite has four compo-nents: 2-D1B, 2-D1A, 2-D2A, and 2-D2B. Monolayergraphene has a single sharp 2D peak, dominated by the2-D1A component. In bilayer graphene, the 2-D1A and2-D2A peaks have higher relative intensity comparedwith the other two, and the 2D peak appears up-shiftedand broader compared with monolayer graphene. Inmonolayer graphene, there is only one phonon sat-isfying the double-resonance conditions for the 2DRaman peak. In bilayer graphene, the interaction be-tween graphene layers causes the π and π∗ electronicbands to split into four bands. According to densityfunctional theory dipole matrix elements, the incidentlight couples more strongly to two among four possi-ble optical transitions (Fig. 2.19). The excited electronscan be scattered by phonons with momenta q1B, q1A,q2A, and q2B. The corresponding processes for holesare associated to identical phonon momenta, resultingin four components to the 2D peak of bilayer graphene.As the number of layers further increases, the 2D1peaks reduce in intensity, and beyond five layers, itresembles the 2D peak of bulk graphite. It is also im-portant to note here that non-AB stacked graphene, suchas multilayer CVD graphene, also shows a single 2Dpeak [2.157, 158]. However, this can be distinguished

from monolayer graphene by its full-width at half-maximum (FWHM) of 50 cm−1, which is twice that ofmonolayer graphene.

A similar observation can also be made for theD peak [2.156]. The D peak of monolayer grapheneis a single sharp peak, while in bulk graphite it canbe resolved into two peaks, D1 and D2. The D peakis observed in defective graphene, and prominently atthe edges of graphene flakes. In carbon nanotubes, con-finement and curvature split the two degenerate modesof the G peak into G+ and G− components, whereasonly one G peak is observed in graphene. The D peakarises from a double-resonance process involving elec-tron scattering by zone-boundary phonons as well asdefects in graphene. Since these do not satisfy the Ra-man selection rule, they do not occur in the Ramanspectra of defect-free graphene. A similar process in-volving intravalley scattering gives rise to a D′ peak at≈ 1620 cm−1 in defective graphene.

The Raman spectrum of graphene also respondsto doping, i. e., changes in the Fermi surface ofgraphene [2.160, 161]. Graphene can be doped inten-tionally or unintentionally, by electron transfer fromadsorbed chemical species, and by modulation of theelectronic band structure by a gate voltage or interac-tion with the substrate. The G peak upshifts for bothhole and electron doping. The position of the 2D peak,however, decreases monotonically with increasing elec-tron concentration or decreasing hole concentration,

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Graphene – Properties and Characterization 2.3 Characterization 63

T (K)

4

6

2

0

n0(T)/n0(4K)

300100Vg (V)

0.5

0

–100

RH(kΩ/T)

100500–50

Vg (V)

3

0–100

σ (mΩ–1)

1000

Vg (V)

8

6

4

2

0–100

Š (kΩ)

d)

c)

b)a)

100500–50

δε

εF

εF

εF

Fig. 2.21a–d Field effect in few-layergraphene. (a) Typical dependences ofgraphene’s resistivity ρ on gate volt-age for different temperatures (T = 5,70, and 300 K for top to bottomcurves, respectively). (b) Exampleof changes to the film’s conductivityσ = 1/ρ (Vg) obtained by invertingthe 70 K curve (dots). (c) Hall co-efficient RH versus Vg for the samefilm at T = 5 K. (d) Temperature de-pendence of carrier concentration n0

in the mixed state for the film in (a)(open circles), a thicker FLG film(squares), and multilayer graphene(d ≈ 5 nm, solid circles). Red curvesin b–d are the dependences calculatedfrom the model of a 2-D semimetal il-lustrated by insets in (c) (after [2.4])

and the 2D peak can be used to distinguish betweenelectron and hole doping. The changes in the G peakposition are related to the nonadiabatic Kohn anomalyat the Γ point, while the shift in the 2D peak posi-tion is due to electron–electron scattering in additionto electron–phonon scattering. Taken together, a plotof 2D versus G peak positions can be used to distin-guish between electron and hole doping in graphene(Fig. 2.20a) [2.159]. Doping trends can also be ob-served in the FWHM and intensities of Raman peaks.Raman peaks in graphene can also be shifted by bi-axial strain induced, for example, by interaction withthe substrate [2.162, 163]. It has been proposed thatthe correlation between normalized shift of 2D andG peak positions, i. e., Δ2D/2D0 and ΔG/G0, where2-D0 and G0 are the peak positions for undopedgraphene, indicates whether the shift arises from dop-ing or strain [2.164]. When Δ2D/2D0 versus ΔG/G0is plotted from Raman spectra obtained at a num-ber of points on a graphene samples, a linear fit withslope close to 1.58±0.18 indicates that strain playsa predominant role, while a smaller slope indicates in-

creasing influence of doping. Highly doped samplesyield a slope of < 1. In another approach, if the plotof FWHM versus position of the G peak is monoton-ically decreasing, it is related to doping, while if it ismonotonically increasing, it is caused by strain or dis-order (Fig. 2.20b) [2.161]. Under uniaxial strain, theG peak splits into two bands, G+ and G−, analogousto the effect of curvature on the G peak of carbon nano-tubes [2.165].

2.3.6 Electrical Characterization

The first electrical characterization of graphene wascarried out on micromechanically cleaved few-layergraphene flakes (Fig. 2.21) [2.4]. The sheet resistivityρ of FLG flakes varies with applied gate voltage Vg,exhibiting a sharp peak of several kiloohms and decay-ing to ≈ 100 Ω at high Vg. The conductivity σ = 1/ρ

increases linearly with Vg on either side of the resis-tivity peak (conductivity valley). The Hall coefficientRH reverses sign at the same VG as the resistivitypeak. This resembles an ambipolar semiconducting

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64 Part A NanoCarbons

B (T)

1.5

1.0

0.5

0

Šxx (kΩ) Šxy (kΩ)a)

10

+80 V+80 V

+100 V

–25 V

–100 V

6 842

1

2

3

4

Vg (V)

80

40

60

20

0

BF(T)b)

1000 50–50–100

T (K)

10

0

Δ (Ω)

10050

–100 V at 9 T

Fig. 2.22 (a) Examples of ShdH oscillations in a graphene device for different gate voltages; T = 3 K, and B is themagnetic field. (b) Dependence of the frequency of ShdH oscillations BF on gate voltage. Solid and open symbols arefor samples with δε ≈ 6 and 20 meV, respectively. Solid lines are guides to the eye. The inset shows an example of thetemperature dependence of amplitude % of ShdH oscillations (circles) (after [2.4])

field-effect transistor (FET), except that there is no zero-conductance region since there is no bandgap. Thistunability of conductivity is achieved by transformingthe graphene by electric-field doping from a completelyelectron to completely hole conductor, passing througha mixed state where both electrons and holes con-tribute equally. This behavior holds true for monolayergraphene as well as undoped bilayer graphene; how-ever, doped bilayer graphene has an intrinsic bandgapand will be discussed subsequently. In both the elec-tron and hole regions, RH decreases with increasingcarrier concentration as 1/ne as expected, and the resis-tivity follows the standard ρ = 1/neμ relation, whereμ is the carrier mobility. Also significant is the mini-mum conductivity of graphene at the charge neutralitypoint (σmin), which has been shown to be about 4e2/h.This σmin is not related to the physics of the Diracpoint singularity, but instead related to charge-densityinhomogeneities (electron–hole puddles) induced bythe substrate or charged impurities [2.166, 167]. In-variably, the position of the charge neutrality point (ρpeak) is shifted to large positive VG, leaving the ungatedgraphene as a hole metal. This large shift is attributedto unintentional doping of the graphene by adsorbedspecies such as water. The peak position can also beshifted by intentional doping [2.166] or removal ofdopants by thermal or current-induced annealing [2.13],but such charged impurities do not affect σmin. The

mobility and minimum conductivity also decrease asa result of defects, which can be induced in a controlledfashion to study this effect, for instance, by ion irradia-tion [2.168] or exposure to atomic hydrogen [2.169].

The earliest determination of carrier mobility byfield-effect and magnetoresistance measurements infew-layer graphene yielded ≈ 10 000 cm2/(V s), whichwas independent of the absolute temperature, indicatingthat it was limited by scattering defects. For multilayergraphene, the mobility reached 15 000 cm2/(V s) at300 K and 60 000 cm2/(V s) at 4 K. Substrate-inducedcharge puddles are significantly reduced in suspendedgraphene, and low-temperature mobility approaching200 000 cm2/(V s) has been reported [2.170, 171]. Insuch devices, the conductivity of suspended grapheneat the Dirac point is strongly temperature dependentand approaches ballistic values at liquid-helium temper-atures [2.171].

Graphene flakes also exhibit pronounced Shub-nikov–de Haas (ShdH) oscillations in both longitudinalresistivity ρxx and Hall resistivity ρxy (Fig. 2.22). Theoscillations depend only on the perpendicular compo-nent of the magnetic field B cos Θ, where Θ is the anglebetween the magnetic field and the graphene. The fre-quency of the SsdH oscillations BF depends linearly onVG, indicating that the Fermi energies εF of holes andelectrons are proportional to their concentration n. Thisis different from the 3-D dependence εF proportional

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Graphene – Properties and Characterization 2.3 Characterization 65

Vg (V)

8

6

–2

–4

4

2

0

–6

σxy(4e2/h)a)

100

T = 4 K

Doped

Pristine

B = 12 T

0 50–50–100n (1012cm–2)

0.09

0.06

0.03

mc/meb)

80 4

Screened

Unscreened

–4–8

Fig. 2.23 (a) Measured Hall conductivity of pristine (undoped) and chemically doped bilayer graphene (n0 ≈5.4 × 1012 cm−2), showing a comparison of the QHE in both systems. (b) Cyclotron mass versus n, normalized to thefree electron mass me. Experimental data are shown as open circles. The inset shows an electron micrograph (in falsecolor) of the Hall bar device with a graphene ribbon width of 1 μm (after [2.172])

to n2/3, proving the 2-D nature of charge carriers ingraphene.

QHE has been observed in graphene even at roomtemperature, since the electrons suffer little scatteringdue to their relativistic nature and have a large cyclotrongap which exceeds the thermal energy kBT by a factorof 10.

Under the influence of strong magnetic field, elec-trons in a two-dimensional system such as graphenedevelop strong Coulomb interactions between them,leading to correlated states of matter such as a frac-tional quantum Hall liquid. This collective behavior ingraphene was predicted to yield the fractional quantumHall effect (FQHE); however, due to prevalent disor-der effects, observation of this remained elusive in earlymeasurements. The FQHE was eventually reported insuspended graphene devices when a plateau at fillingfactor ν = 1/3 was observed above a magnetic field aslow as 2 T [2.173, 174]. An insulating state was alsoobserved at magnetic fields B > 5 T and filling factorsν < 0.15, which has been attributed to symmetry break-ing of the zeroth Landau level by electron–electroninteraction.

The first electron transport measurement of the tun-able bandgap in bilayer graphene was conducted on

micromechanically cleaved graphene on an oxidizedsilicon wafer (300 nm SiO2) [2.172]. The silicon sub-strate was used as a back gate to modulate the carrierdensity n, while doping from adsorbed NH3 on the ex-posed graphene surface was used to mimic a top gateand open a bandgap corresponding to a fixed electrondensity n0. Under applied magnetic field, a plateau atzero Hall conductivity σxy = 0 occurs in biased bilayergraphene, as a result of the gap opened between the va-lence and conduction bands. Plateaus at σxy = 4Ne2/hoccur as expected, including at N = 0 (Fig. 2.23a). Thisis the standard integer QHE that is expected for an am-bipolar semiconductor with energy gap larger than thecyclotron energy. This plateau is absent in monolayerand unbiased bilayer graphene, which show an anoma-lous double step of 8e2/h at n = 0, indicative of themetallic state at the charge neutrality point. A huge peakin the longitudinal resistivity ρxx at n = 0 was also ob-served, exceeding 150 kΩ at 4 K compared with 6 kΩ

for the unbiased case. From Shubnikov–de Haas mea-surements, the cyclotron mass, mc, in biased bilayergraphene is found to be an asymmetric function of thecarrier density, which is a clear signature of a bandgap(Fig. 2.23b). Measurements with dual-gated bilayergraphene have also confirmed these results [2.175].

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66 Part A NanoCarbons

a) b)Eg (meV)

100

10

10 30 60 90

W (nm)

P2P3

P1

P4D1D2

Eg (meV)

100

10

10 30 60 90

θ (deg)

Fig. 2.24(a) SEM im-age of a setof graphenenanoribbon de-vices of varyingwidth. (b) De-pendence ofenergy gap onribbon widthand orientation(inset). Dashedline shows thepredicted em-pirical scaling(after [2.177])

Due to the linear dispersion relation in graphene,intrinsic electron scattering by acoustic phonons is in-dependent of carrier density and only contributed 30 Ω

to the room-temperature resistivity of graphene. Thiswould yield an intrinsic mobility of 2 × 105 cm2/(V s) atcarrier density of 1 × 102/cm2, which would be make itthe highest known mobility, superior to those of InSband semiconducting single-walled carbon nanotubes.However, a strong temperature dependence of mobil-ity is observed in substrate-supported graphene devices,suggestive of extrinsic scattering, which limits the mo-bility to about 4 × 104 cm2/(V s) [2.176].

Various approaches have been proposed to increasethe performance of graphene devices, in particularto reduce impurity scattering and enhance mobility.Ultrahigh current density-induced removal of adsor-bents, photoresist, or e-beam resist residue can be usedto clean graphene in situ during transport measure-ments [2.13]. A parylene-coated SiO2 substrate usedas a dielectric stack for back-gating yields a stablecharge neutrality point and low hysteresis [2.178], sincethe hydrophobic nature of the parylene surface sup-presses moisture-related doping and charge-injectioneffects and yields mobilities of up to 10 000 cm2/(V s).Similar results have been achieved by utilizing anorganic polymer buffer between graphene and conven-tional top-gate dielectrics [2.179]. It was demonstratedthat merely changing the dielectric to a high-k dielec-tric or media does not increase the carrier mobilitybeyond ≈ 10 000 cm2/(V s), suggesting that Coulombscattering is not the dominant limitation beyond thisregime [2.180]. Phonon scattering or resonant scattererswith energy close to the Dirac point have been proposedas alternate mechanisms.

As discussed previously, another route to openinga bandgap in graphene is to exploit lateral confine-ment of charge carriers in a graphene nanoribbon,which creates an energy gap near the charge neutralitypoint [2.177]. Graphene nanoribbons of varying widthsand different crystallographic orientations have beenfabricated by lithographic patterning of monolayer ex-foliated graphene (Fig. 2.24). An energy gap is observedfor narrow ribbons, which scales inversely with ribbonwidth. Energy gaps in excess of 100 meV were observedfor widths less than 20 nm, which could have poten-tial technological relevance. It has also been shown thatedge states do not contribute to the dominant electricalnoise at low frequencies for nanoribbons as narrow as20 nm [2.181]. However, the lack of a well-defined crys-tallographic structure of lithographically etched edgesmeans that the effect of orientation is not observed inthe bandgap produced in such nanoribbons.

It is also possible to fabricate quantum dot (QD)devices entirely out of graphene using a similar litho-graphic procedure [2.182–184]. Such a device consistsof a graphene island connected to the source and drainvia two narrow graphene constrictions and three fullytunable graphene lateral gates (Fig. 2.25). Larger QDs(> 100 nm) show conventional single-electron transis-tor characteristics, with periodic Coulomb blockadepeaks. For smaller QDs, the peaks become stronglynonperiodic, indicating a strong contribution fromquantum confinement. The narrow constrictions remainconductive and show a confinement gap of ≈ 0.5 eV.This can be extended to a double QD system (Fig. 2.25)where the coupling of the dots to the leads and betweenthe dots is tuned by graphene in-plane gates [2.185].This structure has been proposed for the realization of

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Graphene – Properties and Characterization 2.3 Characterization 67

a) d)

e)

b)

GraphenePG

GCGL

CL

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Ti/Au

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Fig. 2.25 (a) SEM of an all-graphene single quantum dot device (after [2.184]). (b) SEM of an all-graphene double quantum dotdevice (after [2.185]). (c) Conductance of a graphene single QD device over a wide range of gate voltages at T = 4 K. (d) Zoomin to low-conductance region showing Coulomb blockade oscillations (after [2.182]). (e) Coulomb diamonds showing differentialconductance as a function of gate voltage and drains–source bias

spin qubits from graphene QDs [2.186]. It has beenshown that, in an array of many qubits, it is possibleto couple any two of them via Heisenberg exchangewhile the others are decoupled by detuning. This uniquefeature is a direct consequence of the quasirelativisticnature of carriers in graphene.

One of the important considerations in electronicdevice performance is the signal-to-noise ratio, whereusually the low-frequency 1/ f noise dominates. Inmonolayer graphene, the 1/ f noise follows Hooge’sempirical relation with a noise level comparable tocarbon nanotube and bulk semiconductor devices. How-ever, in bilayer graphene, the 1/ f noise is stronglysuppressed and obeys a unique dependence on carrierdensity, due to effective screening of carrier scatteringby external impurities [2.134]. In monolayer graphene,the noise amplitude is minimum at the Dirac point andincreases with increasing carrier density. However, inbilayer graphene, the noise amplitude achieves a max-imum at the Dirac point and decreases with increasingcarrier density. In both cases, the noise is independentof carrier type.

In an effort to realize commercial viability ofgraphene electronics, reduced graphene oxide (RGO)

has been explored as an alternative to pristine mono-layer graphene. However, measurements in individualmonolayer RGO flakes have yielded conductivitiesranging between 0.05 and 2 S/cm and field-effectmobilities of 2–200 cm2/(V s) at room temperature.Conductivity decreases by up to three orders of mag-nitude when measured down to 4 K, following a T−1/3

dependence, suggesting variable-range hopping con-duction between regions of highly reduced (nearlypristine) graphene islands separated by defective orpoorly reduced regions.

SpintronicsWhen graphene devices are fabricated with ferromag-netic electrodes, such as the soft magnetic NiFe orCo, it is possible to inject spin-polarized current intothe graphene. A thin Al2O3 or MgO tunnel barrieris used at the ferromagnet–graphene interface. High-quality graphene enjoys ballistic transport with spinrelaxation lengths between 1.5 and 2 μm even at roomtemperature, which is only weakly dependent on chargedensity [2.187]. The switching fields of the elec-trodes can be controlled by in-plane shape anisotropy.Graphene spin valves have been constructed using ei-

PartA

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