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March 06, 2013

C2013 American Chemical Society

Progress, Challenges, andOpportunities in Two-DimensionalMaterials Beyond GrapheneSheneve Z. Butler,," Shawna M. Hollen,," Linyou Cao, Yi Cui,^ Jay A. Gupta, Humberto R. Gutierrez,0

Tony F. Heinz, )Seung Sae Hong,^ Jiaxing Huang,z Ariel F. Ismach,# Ezekiel Johnston-Halperin,

Masaru Kuno,4 Vladimir V. Plashnitsa,4 Richard D. Robinson,1 Rodney S. Ruoff,# Sayeef Salahuddin,2

Jie Shan,3 Li Shi,O Michael G. Spencer,b Mauricio Terrones,0 Wolfgang Windl,9 and Joshua E. Goldberger,*

Department of Chemistry and Biochemistry and Department of Physics, The Ohio State University, Columbus, Ohio 43210, United States, Department of Materials

Science and Engineering, North Carolina State University, Raleigh, North Carolina 27607, United States,^Department of Materials Science and Engineering, Stanford

University, Palo Alto, California 94305, United States, )Department of Physics and Electrical Engineering, Columbia University, New York, New York 10027, United

States, zDepartment of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States,#Department of Mechanical Engineering

and the Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States, 4Department of Chemistry and

Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States, 1Department of Materials Science and Engineering, Cornell University, Ithaca,

New York 14853, United States, 2Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, United States,

3Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, United States, ODepartment of Mechanical Engineering, The University of Texas at

Austin, Austin, Texas 78712, United States, bDepartment of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14850, United States,0Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States, and 9Department of Materials Science and

Engineering, The Ohio State University, Columbus, Ohio 43210, United States. "These authors contributed equally.

T

wo-dimensional (2D) materials havehistorically been one of the most ex-tensively studied classes of materials

due to the wealth of unusual physical phe-nomena that occur when charge and heattransport is conned to a plane. Many ma-terials with properties dominated by theirtwo-dimensional structural units such asthe layered metal dichalcogenides (LMDCs),copper oxides, and iron pnictides exhibitcorrelated electronic phenomena such ascharge density waves and high-temperaturesuperconductivity.13 The (re)discovery4,5 ofsingle-layer graphene in 2004 by Novoselov

and Geim has shown that it is not onlypossible to exfoliate stable, single-atom orsingle-polyhedral-thick 2D materials fromvan derWaals solids, but that these materialscan exhibit unique and fascinating physicalproperties. In single-layer graphene's bandstructure, the linear dispersion at the K pointgives rise to novel phenomena, such as theanomalousroom-temperature quantumHalleffect, and has opened up a new categoryof Fermi-Diracphysics. Even at one-atom-thick, graphene is a fantastic electronic andthermal conductor, and graphene-basedmaterials have been proposed for a host of

* Address correspondence [email protected].

Received for review January 18, 2013and accepted March 6, 2013.

Published online

10.1021/nn400280c

ABSTRACT Graphene's success has shown that it is possible to

create stable, single and few-atom-thick layers of van der Waals

materials, and also that these materials can exhibit fascinating and

technologically useful properties. Here we review the state-

of-the-art of 2D materials beyond graphene. Initially, we will outline

the different chemical classes of 2D materials and discuss the various

strategies to prepare single-layer, few-layer, and multilayer assem-

bly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-

layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the di fferences that occur in the

electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface.

Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in eld-effect transistors, spin- and valley-tronics,

thermoelectrics, and topological insulators, among many other applications.

KEYWORDS: two-dimensional materials.graphene.nanosheets.graphane.van der Waals epitaxy.van der Waals solid

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applications ranging from transparent conductors tothermal interface materials to barristor transistor-likedevices.68 Furthermore, as single-layer graphene isentirely surface area, its properties and reactivity pro-foundly depend on the substrate, its local electronicenvironment, and mechanical deformations.

Still,there exists an entire periodic tableof crystalline

solid-state materials each having different electronic,mechanical, and transport properties, and the possibi-lity to create single-atom or few-atom polyhedral thick2D layers from any material remains. It was showndecades ago by Frindtet al.that layered van der Waalsmaterials, such as layered metal dichalcogenides,could be mechanically and chemically exfoliated intofew and single layers.9,10 This early work focused onattempts to obtain andcharacterizethese thinlayers.913

Experiments probing transport11 only scratched the sur-faceof theunique propertiesthese 2D materials exhibit. Itwas not until the recent surge of intense research ongraphene that the general potential of 2D materialsbecame apparent. Additionally, the past 8 years of grap-hene research has yielded many methods for synthesiz-ing, transferring, detecting, characterizing, and manipu-lating the properties of layered van der Waals materials.Furthermore, novel synthetic methods including topo-tactic, solution-based, solvothermal, and UHV surfaceepitaxial approaches have unleashed the potential tocreate new van der Waals solids and single-layer-thickmaterials. These established methods have enabled theeld of 2D materials beyond graphene to mature veryquickly.

Many novel materials that had been initially consid-

ered to exist only in the realm of theory have beensynthesized. These include groups IV and IIVI semi-conductor analogues of graphene/graphane (the sp2/H-terminated sp3 derivatives) such as silicene1417 andgermanane.18 Similar to graphene, the properties atthe single layer are also distinct from the bulk. Further-more, these 2D materials are useful building blocksthat can be restacked and integrated into compositesfor a wide range of applications.

Herein, we present a forward-looking review articlethat discusses the state-of-the-art of 2D materialsbeyond graphene. Initially, we will outlinethe differentchemical classes of 2D materials and discuss the

various strategies to prepare single-layer and multi-layer assemblies in solution, on substrates, and on thewafer scale. Additionally, we present an experimentalhow-to guide for identifying and characterizing single-layer-thick materials, as well as outline emerging tech-niquesthat yield both local andglobalinformation. Wedescribe the differences that occur in the electronicstructure between the bulk and the single layer anddiscuss various methods of tuning the properties bymanipulating the surface. Finally, we highlight theproperties and advantages of single-, few-, and many-layer 2D materials in eld-effect transistors, spin- and

valley-tronics, thermoelectrics, and topological insula-tors, among many other applications.

Structure and Synthesis of Two-Dimensional Materials. Thereliable synthesis of single- and few-layer 2D materialsis an essential first step for characterizing the layer-dependent changes in their properties, as well asproviding pathways for their integration into a multi-tude of applications. In general, there are three mainclasses of materials that can be prepared as a single-atom or single-polyhedral-thick layer.

Classes of Single- and Few-Layer Two-Dimensional

Materials. Layered van der Waals Solids. The most com-mon class of crystalline structures that can be exfo-liated as stable single layers are the layered van derWaals solids. These crystal structures feature neutral,

single-atom-thick or polyhedral-thick layers of atomsthat are covalently or ionically connected with theirneighbors within each layer, whereas the layers areheld together via van der Waals bonding along thethird axis. The weak interlayer van der Waals energies(4070 meV) enable the facile exfoliation of theselayers. The most common approaches for obtainingsingle- and few-layer-thick 2D materials from many ofthese solids include mechanical exfoliation of largecrystals using Scotch tape, chemical exfoliation bydispersing in a solvent having the appropriate surfacetension, and molecule/atom intercalation in order toexfoliate these layers and enable their dispersion in

polar solvents. This mechanical exfoliation process hasbeen used to prepare and study the properties of few-layer van derWaals materials, such as MoS2 and NbSe2,since the 1960s.9,11,19 The isolation of individual andfew layers using mechanical exfoliation remains themost powerful approach for studying their propertiessince it is considerably less destructive than the othermethods and has successfully been used to create large,10m single-layer flakes on a variety of substrates.

One of the most well-studied families of van derWaals solids is the layered metal chalcogenides (LMDCs),the most common being MoS2. Early transition metal

VOCABULARY:two-dimensional material- a material

in which theatomicorganizationand bond strength along

two-dimensions are similar and much stronger than along

a third dimension; van der Waals solid - a material whose

crystal structure features neutral, single-atom-thick or

polyhedral-thick layers of atoms with covalent or ionic

bonding along two dimensions and van der Waals bond-

ing along the third; nanosheet - stacked assembly or

hybrid composite formed from single to many layers of

two-dimensional materials; graphane- a single layer of a

two-dimensional hexagonal network of sp3-bonded car-

bon atoms in which every carbon is bonded to a terminal

hydrogen, alternatingly above and below the layer; van

der Waals epitaxy - the growth of a thin layer on the

surface of a substrate in which the materialsubstrate are

held together by weak van der Waals forces.

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dichalcogenides with stoichiometry MX2 (M = Ti, Zr, Hf,V, Nb, Ta, Re; X = S, Se, Te) crystallize into layered 2Dstructures in whichhexagonallypacked MX6 octahedra(for d0, d3, and some d1 metals) or trigonal prisms (ford1 and d2 metals) share edges with their six nearestMX6 neighborswithin each layer (Figure 1a).

1 There areover 30 different LMDCs which have many technolo-

gically interesting properties, and an emerging body ofexperimental work investigating the structure andproperties of single- and few-layer-thick derivativeshas evolved for many of these compounds (MoS2,WS2, and TiSe2).

2022 Other families of van der Waalssolids that have been exfoliated into single layersinclude hexagonal boron nitride,23 vanadium oxidederivatives, and other chalcogenides including Bi2Te3,Sb2Te3, and -FeSe.

24,25 Many novel van der Waalscompounds can be created viathe topotactic deinter-calation of precursor solids. For example, the layeredCaGe2and CaSi2Zintl phases can be topochemicallydeintercalated in aqueous HCl to produce layeredhydrogen-terminated or half-hydrogen-terminated/half-hydroxy-terminated GeH and SiH0.5(OH)0.5, re-spectively(Figure 1b).18,2628 These group IV graphaneanalogues2932 are a particularly intriguing class ofsystems due to the possibility of utilizing covalentchemistry to modulate and tune the properties. Asanother example, recently, the exfoliation of metalcarbides such as Ti3AlC2using HF to produce neutrallayers of Ti3C2(OH)2 has been demonstrated.

33 Newlayered van der Waals solids are constantly being dis-covered by the solid-state community. In 2012, ReN2was synthesized for the rst time using high-pressure

techniques and was found to have the MoS2 crystalstructure type.34

Neutral, dimensionally reduced hybrid organic/in-organic van der Waals derivatives of nonlayered solidshave also recently been discovered. Dimensional re-duction refersto thecreation of novel crystal structuresof metal-anion (M-X) frameworks by the addition of areagent that disrupts the polyhedral connectivityalong one or more dimensions while retaining a de-gree of the metal coordination geometry and generalpolyhedral connectivity.35 In these hybrid van derWaals solids, stoichiometric equivalents of neutral or-ganic ligands bind to the metal and disrupt the M-X-M

framework without changing the electron count andrelative metal-anion radii. For example, it has beenshown that almost every IIVI semiconductor thattypically crystallizes into the three-dimensional spha-lerite or wurtzite lattices, such as ZnS, ZnSe, and ZnTe,can be converted into atomically thin 2D crystallineframeworks when synthesized via solution-phase sol-vothermal techniques in the presence of alkylamineligands.3640 The bulk sphalerite or wurtzite structureconsists of a corner-sharing metal-anion tetrahedral.These dimensionally reduced structures can be envi-sioned as a single (110) plane of corner-sharing

metal-anion tetrahedra where every metal is bondedto three anions andcapped witha short-chainalkylamineand every anion is bonded to three different metals(Figure 1c). The exfoliation into single-layer-thick deriva-tives of these hybrid materials, such as ZnSe, has beenrecently demonstrated.41

Layered Ionic Solids. The second class of materialsthat can be prepared as single or few layers featuresbulk crystal structures withcharged2D polyhedral layers

that are typically held together with strongly electro-positivecations orstrongly electronegativeanions suchashalides, or OH. To enable their dispersion as singlelayers in solution, these cations/anions are typicallyexchanged with bulky organic cations/anions, such astetrabutylammonium/dodecyl sulfate. These materials canthen be easily dispersed onto substrates, with the majorityof materials depositing as single to few layers. There arenumerous examples of oxide materials that have beenprepared this way including (1) cation-exchanged layersfrom RuddlesdenPopperperovskite-typestructures, suchas KLn2Ti3O10, KLnNb2O7, RbLnTa2O7, and KCa2Nb3O10

Figure 1. (a) Crystal structures of the 1T and 2H crystalstructuresoftheMX2 family(X=yellowsphere).Themetalisin octahedral coordination in the 1T structure, and trigonalprismatic coordination in the two layers per unit cell 2Hcrystal structure. (b) Deintercalation of CaGe2 in aqueousHCl at low temperatures results in GeH (calcium = yellow,germanium = purple, hydrogen = black). (c) Crystal struc-tureof ZnSe(butylamine)(Zn = gray,Se = orange, N = green,C = black). (d) Crystal structure of KCa2Nb3O10, a Ruddle-sdenPopperperovskite phase thatcan be exfoliated uponreplacing the K cation (orange) with an organic cation.

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(Figure 1d)4247 (Ln = lanthanide ion); (2) cation-exchanged layered metal oxides such as LiCoO2andNa2Ti3O7;

48,49 (3) halide- or hydroxide-exchanged layersderived from metal hydroxides, such as Ni(OH)2xorEu(OH)2.5Cl0.5.

50,51 Additionally, many neutral layeredtransition metal oxides such as MnO2 can undergochanges in oxidation state and become protonated in

aqueous acidic solutions.52 This proton can then besubstituted for bulky organic cations. The observedthicknesses from atomic force microscopy (AFM) forthese classes of materials are typically larger than theirexpected values due to the presence of hydrationlayers or intercalating ions, which results in a surfacesheet interface with a thickness between 0.5 and1.0 nm. The lateral sizes of exfoliated layers from ionicmaterials typically depend on the size of the startingcrystalline material, and single-layer flakes in the rangeof tens of micrometers have been observed.48

Surface Growth of Nonlayered Materials. The de-position of materials on substrates offers the potentialto grow and study the properties of single- to few-atom-thick materials beyond those existing as layeredbulk crystals. For example, recently, it was demonstratedthat monolayers of silicon deposited on Ag(111) or ZrB 2organizeintoa puckered hexagonal graphene-likelatticewith sp2 bonding configuration.17,53 Thissilicene materialshares a similar band structure to graphene; how-ever, the interactions with the substrate induce a bandgap opening at the K point. Additionally, ultrathin in-sulators such as Cu2N,

54,55 Al2O3,56 NaCl,57 MgO,58 TiO2,

59

andmetal adlayers have been prepared in this fashion onmetal substrates. Unlike exfoliation, the choice of sub-

strate has been typically limited to metals due to theprevalence of STM as a characterization tool.

Solution-Phase Growth. Solution-phase methods suchas solvothermal or colloidal growth reactions offer afacile production method to synthesize gram scalequantities of 2D materials with precise thicknessesand basal-plane sizes.6064 Recently, general colloidalsynthetic methods have been developed to prepareLMDCs such as TiS2, VS2, ZrS2, HfS2, NbS2, TaS2, TiSe2,VSe2, and NbSe2viathe reaction of metal halides andcarbon sulfide or elemental selenium in the presenceof primary amines.65 The colloidal materials typicallyrange in lateral dimensions from 10 to 100 nm and

have thicknesses from single sheets to tens of nano-meters. As an example, TiS2 nanosheets that havelateral lengths of 500 by 500 nm with thicknesses onthe order of 5 nm have been created (Figure 2). Thesemethods use abundant low-cost precursors and mildcolloidal growth conditions.66 However, strategies forcontrolling the thicknesses and lateral dimensionshave yet to be established.

One of the key steps in using solution-phase meth-ods to grow transition LMDCs is the generation ofchalcogenide anions in the appropriate oxidation state(S2, Se2,andTe2). To grow suldes, primary amines

and sulfur are typical reactants because they formsulfur-containing alkylammonium polythioamine,67

polythiobisamine,68,69 and alkylammonium poly-sulde70 complexes. These sulfur-containing species,in turn, exhibit favorable decomposition kinetics andproduce H2S when heated, which reactswith themetalprecursor.

Vapor Deposition. Vapor deposition stands as anappealing, versatile synthetic strategy. However, the

development of a controlled synthetic method of 2Dmaterials such as 2D chalcogenides by vapor deposi-tion requires a better understanding of the fundamen-tals involved. While vapor deposition has beenextensively used for the growth of thin films andnanomaterials such as nanowires,71 nanotubes,72 andgraphene,73 knowledge obtained from these materialsmay not simply apply to 2D materials. Unlike typicalnanomaterials whose growth is primarily governed bya catalyst, the vapor deposition growth of 2D chalco-genides is often noncatalytic.24,74,75 Without the dom-inance of catalysts, the growth of 2D nanosheets issubject to strong influences of many experimental

parameters that may play only a negligible role incatalyzed growths. For example, the diffusion of sourcematerial vapor through the gas flow boundary layerstrongly controls the vapor deposition growth of GeSnanosheets (Figure 3).74

The growth of single-layer substrate-wide 2D ma-terials is essential for commercialization and wouldalso benet fundamental studies of single-layer phe-nomena. The key to the preparation of substrate-widesingle-layer van der Waals 2D materials and their hetero-

junctions is monolayer (ML) epitaxy. van der Waalsforces have associated energies of 4070 meV, which

Figure 2. (a,b) Low-resolution TEM images of TiS2 layers.The inset in (a) shows animage ofmultipleTiS2 nanosheets,and the inset in (b) a selected area electron diffractionpattern of a single TiS2 layer. (c,d) High-resolution TEMimages showing the edge and basal plane of a single TiS2layers.66

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are much smaller than covalent bonding energies of2006000 meV. We infer that epitaxy of crystalline 2Dmaterials on crystalline substrates is dened by strongbonding at the reactive edges of the single-crystaldomains of the material and weak interlayer forcesbetween the sheets. This van der Waals epitaxy occursbetween 2D sheets of the same material (homoepitaxy)aswellas2Dsheetsofdifferentmaterials(heteroepitaxy).

Because of the weak interlayer forces, epitaxy is possibleeven if there is signicant lattice mismatch between thematerials.25,75 The resulting heterojunctions do not sufferfrom the interfacial defects that are generated during 3Dheteroepitaxy due to the large stresses generated bybent or broken interlayer bonds.76,77 van der Waalsheteroepitaxy has also been shown to be possible with3D substrates that have been suitably passivated.76,77

Table 1 shows a listing of crystalline 2D heterojunctionsthat have been produced using van der Waals epitaxy.

Large-Area CVD Growth of Graphene and Hexagonal

Boron Nitride. It is worthwhile to re-evaluate the suc-cessful vapor deposition growth of single-layer gra-

phene in order to understand the extent to which wecan transfer these methods to the substrate-widegrowth of other van der Waals sytems. Thin carbonfilms were originally grown on single-crystal transitionmetals such as platinum by exposing the metal surfaceto a hydrocarbon at high temperature in ultrahighvacuum (UHV) conditions.7880 LEED patterns observedby the Somorjai group7880 were assigned in 19694 tobeingfrom single- andfew-layer graphene (SLG andFLG,respectively). Theformationof these graphenelayerswaslater explained by the dissociation of the hydrocarbon onthe metal surface, carbon diffusion into the bulk of the

metal, and its segregation during cooling or by carbonsupersaturation.81,82 There is an interest in syntheticapproaches for the formation of large-area graphene,for basic research as well as a variety of applications,and this has driven the re-evaluation of FLG growth ontransition metals by the diffusionsegregation tech-nique. The growth of FLG on Ru,83 Ir,84 Co,85,86

Ni,73,81,82,87 Pt,85 and Pd85 by chemical vapor deposi-tion (CVD) has been reported. Control of the qualityand number of layers grown has proven challenging,

and often inhomogeneous films areobtained. FLG wasalso achieved by the sublimation of silicon in SiCcrystals.88

The successful synthesis of SLG with high homo-geneity and reproducibility was achieved in 2009 bylow-pressure CVD on copper foils with methane as thecarbon source.73 Aspects of the growth kinetics andmechanisms were elucidated using an isotope-label-ing technique in which the Cu is exposed sequentiallyto 13CH4 and then normal methane.

8 9 1 3C-labeledgraphene can be readily distinguished from normalgraphene by Raman spectroscopy and mapping(Figure 4a). A comparison of the growth of monolayer

graphene and FLG on Cu versus Ni showed that thegraphene growth on Cu is surface-mediated; that is,dissociation of the hydrocarbon followed by carbonspecies diffusion on the surface leads to nucleation,islandgrowth, andnally completion of a monolayer.89

This is rationalized by the extremely low carbon solu-bility in Cu even at the growth temperature of about1040 C that inhibits the diffusion ofC intothebulk Cu,making Cu foil an excellent substrate forgrowth of large-area SLG. Kinetic studies on the growth of graphene ledto the conclusion that the graphene grain size could beincreased by raising the growth temperature and

TABLE 1. Two-dimensional Heterojunctions Produced by

van der Waals Epitaxy

layer substrate ref

graphene sapphire 322

C60 MoS2 323

MoS2 graphene 75

GaS GaSe, Si(111) 324

GeS SiO2/Si 74

TaS2 mica 325

WS2 graphite 326,327

HfS2 WSe2 328

CdS InSe/H-S(111) 329

SnS2 WSe2, MoS2, MoTe2, GaSe, WSe2 330,331

SnSe2 SnS2,WSe2, MoS2, MoTe2, GaSe 330,331

Bi2Se3 graphene/SiC, Si(111) 238,332

BiTe2Se h-BN 333

ZnSe InSe, GaSe 334

MoSe2 SnS2, MoS2, GaAs(111) 335

GaSe GaAs(111), Si(111) 336,337

NbSe2 mica, GaAs(111) 338

CdTe MoSe2, WSe2 339PbTe Si(111) 340

Figure 3. (a) Schematic illustration of experimental setupfor synthesis by a noncatalytic vapor deposition process.The precursor vapor can be introduced from outside orgenerated insidethe tube furnace.(b) TEMimage of a typicalGeS nanosheet. Inset is an electron diffraction pattern of thenanosheet.74

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reducing the partial pressure of the hydrocarbon, andgrain size was thereby increased, at rst, from a fewmicrometers to tens of micrometers.90 Thedomain sizeof SLG has since been increased to several hundredmicrometers91,92 and even millimeters93 (Figure 4c).One measure of graphene quality is themagnitude of

the carrier mobility, and larger grain size has beencorrelated with higher carrier mobility values.90,93

Goodconditions to grow SLG include (1) hydro-carbon dissociation (catalytic or thermal); (2) a catalystfavoring growth of graphene; and (3) nucleation,growth, and completion of the monolayer lm byhaving all reactions occurring on the surface of thesubstrate. Indeed, SLG was shown to grow in UHVconditions on single-crystal metals with high carbonsolubility4,82 and more recently by low-pressure CVDon Ni lms94 by controlling the kinetic factors duringthegrowth and, in particular, having thereaction occur

only on the metal substrate surface. Other relevantparameters are the sticking coefficient of the carbonspecies on the metal surface and these species' abilityto diffuse on the surface.

In contrast to graphene, other layered systems arecomposed of two or more elements, making the

synthesis more complex. Hexagonal boron nitride(h-BN) might be the most studied layered material aftergraphene, although reports on the CVD synthesis ofMoS2 are emerging.

95 The synthesis of single- and few-layer h-BN was achieved rst in UHV CVD systems onsingle-crystal metals.96 Recently, the synthesis of sin-gle-layerh-BN and few-layer h-BN wasreportedin CVDsystems using solid23,97 and gaseous precursors.98

Submonolayer domains on Cu foils were achievedusing very low partial pressure of ammonia borane23

(Figure 4d), while control over thenumber of layers wasachieved using diborane and ammonia on Ni foils,98 as

Figure 4. (a) Schematic illustration showing the dissociationdissolutionsegregation on Ni and the surface-mediatedgrowth of monolayer graphene on Cu. The Raman mapping of the G band for graphene grown on Cu (bottom left) showsgraphene areas enriched with 13C (dark) and normal C (bright); note that the distribution of13C is essentially random for thelmgrownontheNifoil(bottomright); 89 scalebarsare5m.(b) Increasingthe domainsizeby raising thetemperature (T)andlowering both the precursor gas ow (JMe) and the methane partial pressure (PMe) as compared to an earlier protocol, SEMimages; scale bars are10m.91 (c)SEM imagesshowing furtherincreaseof the graphene domainsize which wasattributed tocontrolling the surface roughness of the Cu foils.93 (d) AFM and SEM images showing submonolayer h-BN transferred to aSiO2/Siand Cu foil,respectively, in which triangular domains canbe observed.The atomicmodelfor such domains isdepictedon the right.23 (e) Control of the number of h-BN layers on Ni foils by control of the reaction time. TEM images for samplesgrown for 1, 5, 15, and 30 min.98

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depicted in Figure 4e. Despite such achievements, thegrowth mechanism for h-BN remains unclear. Thegrowth of SLG was signicantly improved in only afew years by understanding some of the key parametersfor growth. Can we apply similar methodologies to thegrowth of single or multiple layers of other layeredmaterials? It calls for further nucleation and growth

studies as well as an adequate set of characterizationtools comparable to those usedto characterizegraphene.

Multilayer Assemblies. As will be discussed later,there are numerous applications for multilayer assem-bliesand hybrid composites from single- to many-layer(thickness


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structural or spectroscopic information with propertiesat the local scale.

The rst challenge in single-layer characterization issingle-layer preparation and detection. Mechanicalexfoliation from bulk crystals is typically the simplest

method for transferring 2D sheets of layered van derWaals solids such as graphene,5,20,120,121 metal chal-cogenides,122 and GeH18 to any substrate. Exfoliationtypically produces 10m sized akes ranging in thick-ness from single to few layers. Optical microscopy isinitially the most powerful high-throughput methodfor initially identifying single- and multiple-layerakes.20,58,122,123 Dielectric-coated SiO2/Si substratesare the most common substrates used to visualizeand locate single and few layers (Figure 6a,e,f). Thecolor of the dielectric-coated wafer depends on aninterference effect from reection offof the two sur-faces of the dielectric. Single- and few-layer akes on

the surface of the dielectric modify the interferenceand create a color contrast between the ake and thesubstrate.123,124 For optimal contrast, the thickness ofthe dielectric coating needs to be within 5 nm of theideal value. However, since the index of refraction ofmany novel materials is unknown, it is often necessaryto rst experimentally exfoliate onto a range of sub-strates having different dielectric thicknesses to initi-ally determine the optimal thickness.

Visualizingsingle layers on othersubstrates withoutthe need for interference methods has also beenexplored. Some metal dichalcogenides (MoS2, WS2)

have direct band gaps only as single layers, enabl-ing their direct visualization by uorescence micros-copy.125 However, as the number of layers increases,these materials' band gaps become indirect, signi-cantly reducing their uorescence.

Fluorescence Quenching Microscopy (FQM) usesan organic fluorophore whose fluorescence is quen-ched in the presence of single- or few-layer sheets(Figure 6f). Many 2D materials feature either lowenough band gaps or the appropriate band alignmentto quench nearby fluorophores viaa FRET mechanismor charge transfer mechanism. One can locate singlelayers of 2D materials on a substrate by monitoring thereduction in fluorescence intensity of a thin fluoro-phore layer that was spun-coat on top.126128

AFM is a powerful technique to determine layerthickness (Figure 6b,d) with a precision of 5%.114,118,129

However, discrepancies arise from differences in the

interactions of the tip with the sample and sub-strate.130 For example, the thickness of a single layercan be better determined by measuring the height of asecond layer on a first layer, rather than the heightbetween a single layer and the substrate because in theformer the tiplayer interactions are constant. To controlthese differences and obtain the most accurate heightprofile of a single layer, one must first optimize the tipsurface distance to exclude any hysteretic artifacts.130

Raman Spectroscopyis a useful method for finger-printing a material and layer-dependent changes ofthe vibrational structure (Figure 6c). It is necessary to

Figure 6. (a) Optical micrograph of thinlms of MoS2150 (b) AFM ofMoS2.

150 (c) Raman spectra of single-layer, few-layer, andbulk MoS2 lms.

150 (d) AFM of graphene,126 (e) optical micrograph of graphene,126 (f) FQM image of graphene using PVP/uorescein.126 Scale bars are 10m.

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use low power (e.g., 16 kW/cm2 for Bi2Te3,131 or

40 kW/cm2 for GeH) when obtaining Raman spectraof single layers to prevent sample decomposition. TheStokes/anti-Stokes peakratio can be usedto determinethe local temperature.132 Raman spectra can alsodetect vibrational modes that are active due to sym-metry breaking in single-layer films and enhanced

vibrations in out-of-plane modes when the single layeris suspended.131 When varying the layer number be-tween bulk, few, and single layers, the Raman spectradiffer in spectral width and intensity due to differinginterlayer interactions.122,129 Substrates that have vibra-tional modes that overlap with those in the material ofinterest should be avoided. Vibrational spectroscopy,in general, and Raman spectroscopy, in particular,could be used to detect isotopic-enriched 2D materialsto study the growth mechanism as demonstrated andthus be used to study the growth mechanism.89

Transmission Electron Microscopy (TEM)can pro-vide detailed information on the nature of crystallinity,layer sizes, interlayer stacking relationships, and ele-mental composition (Figure 7a,b).63,114,119,122,133135 Inthe case of graphene, selected area electron diffrac-tion can distinguish between monolayer and multi-layer graphene when the intensity ratio of the (100)to (110) diffraction spots is larger or smaller than 1,respectively. This deviation in diffraction intensity canalso distinguish between single- and multilayer flakesof other 2D materials when there is a change ininterlayer registration. Recent advances have shownthat dark-field microscopy can quantitatively deter-mine thelayernumber andstackingorderof multilayer

graphene.136

Scanning transmission electron micro-scopy (STEM), using high-angle annular dark-field de-tector coupled with electron energy loss spectroscopycan be used to visualize individual atoms in an isolatedlayer. Here a sub-angstrom electron beam is rasteredacross a sample and is scattered when it hits a parti-cular atom. Figure 7c shows a STEM image of carbonand oxygen substitutions in a single layer of boronnitride. Low accelerating voltages (


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nanosheets, the presence of low-angle basal reflectionsin the pattern indicates an ordered arrangement of re-stacked nanosheets along the stacking axis, and like-wise, the absence of intense basal reflections indicatesrandom orientations for aggregated nanosheets.111

The orientation of films on substrates favors the align-ment of the thin axis. Broadening signatures of specificpeaks under this orientation can indicate the thinaxis.147 To gain more insight, the nanosheet XRDpattern can be modeled by the structure factor, theLorentz polarization factor, the Laue interference func-tion, and the temperature factor.114,146 Structure factorsfor single and multiple layers can be calculated assum-ing atomic positions, unit cell structure, and orientationto a substrate.114 Results of this modeling can helpdetermine the fundamental units of the nanosheets(e.g., atomic make up of termination layers), the spacingbetween sheets, and if the nanosheets are restacked. In-

plane XRD can be used successfully to observe hkreflections (iflis the thickness axis) of the nanosheets'2D unit cell.114,119 This is a useful method to observemissing peaks from the naturally textured orientation ofnanosheets on a substrate. Unit cell parameters can beextracted from refinements of these data andcomparedto bulk values.

Characterization Opportunities. The developmentand implementation of new characterization techni-ques that can rapidly and nondestructively probe theatomic structure, defects,and properties of single-layermaterials is necessary to advance the field. Since the

electronic properties of materials are often dominatedby minority defects, the discovery, availability, andimplementation of techniques that can identify min-ority species, defects, and edge states, such as next-generation aberration-corrected STEMas wellas STM,willprove incredibly valuable. The emergence of combinedspectroscopic andimaging techniquessuchas AFM-TERS

can significantly aid in the identification and character-ization of single-layer-thick sheets while minimizing sub-strate signals. Furthermore, to refine the growth of high-quality single-layer materials for electronic applications,the implementation of feedback loops between synth-esis and device fabrication will be necessary.

Properties, Integration, and Devices. The unique quali-ties of 2D materials, such as their reduced dimension-ality and symmetry, lead to the appearance ofphenomena that are very different from those of theirbulk material counterparts. This difference is perhapsmost glaring in the transformation of the band struc-ture as the single layer is approached. Additionally,since a 2D material is entirely made up of its surface,the interface between the surface and the substrateand the presence of adatoms and defects can drama-tically alter the material's inherent properties. The two-dimensional nature of these materials also plays anentirely mechanical role as they are inherently flexible,strong, and extremely thin. These materials exhibitother unique and potentially useful properties includ-ing high electron mobilities, topologically protectedstates, tunable band structures, and high thermalconductivities. The development of 2D materials isexpected to improve current device technology, and

their novel transport and topological properties mightprovide additional opportunities for spintronic devicesand quantum computing.148 Even multilayer assem-blies of 2D materials have a broad base of potentialapplications including batteries, supercapacitors, andp-n junctions. This section provides a detailed over-view of the novel properties of 2D materials beyondgraphene and discusses their potential for deviceapplications and integration.

Changes in Electronic Structure at the Single Layer:

MoS2, a Case Study. The layered metal dichalcogenides(LMDCs) constitute one of the most studied families ofvan der Waals solids beyond graphite.148 These mate-

rials display remarkably diverse electronic properties,ranging from metals to semiconductors to insulators.They also exhibit interesting strongly correlated elec-tron phenomena, such as charge density waves andsuperconductivity. Amongthem,molybdenumdisulfide(MoS2; semiconductor), has attracted particular atten-tion. Several distinctive properties, related both to itsoptical149161 and transport characteristics,20,148,162173

havebeendemonstratedin MoS2 at the mono- and few-layer level. Examples of these properties include theexistenceof a directgap,125,149 unlikethe bulk material,with strong excitonic effects156 and the possibility of

Figure 8. XRDpatterns of niobate nanosheets of (top) bulk,(middle) (solid line) wet aggregates of nanosheets and(dashed line)calculateddiffraction pattern fora singlelayer,(bottom) and dried aggregates of nanosheets.146 Reprintedwith permission from ref 146. Copyright 2002 Elsevier.

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full control of thevalleyand spin occupation by opticalpumping with circularly polarized light.153155,158 MoS2has also been shown to yield field-effect transistorswith very high current onoff ratios at monolayerthickness163 and to function within a vertical graphene-MoS2-graphene tunneling transistor strucure.

171 Theseproperties, in combination with the promise of synthesisof large-area high-quality samples (notably by recentlydeveloped CVD methods174,175), suggest interestingpos-sibilities for applications of this and related materials inelectronics and optoelectronics. Here we summarizesome of the recent progress in understanding theelectronic properties of ultrathin MoS2layers.

The overall physical structure of the MoS2mono-layer is similar to that of the graphene honeycomblattice. For MoS2, however, the A and B sublattices,rather than both being occupied by C atoms, areoccupied either by Mo atoms or by a pair of S atoms(Figure 12c), with the entire MoS2monolayer structureforming a triplelayer of atoms. Thedifference betweenthe A and B sublattices lifts the degeneracy in theelectronic structure at the K (K0) point in the Brillouinzone. This degeneracy is responsible for the distinctiveDirac cone dispersion relation in graphene. In contrast,in monolayerMoS2, a substantial energygap is present.

The bulk MoS2 crystal, consisting of MoS2 mono-layers arranged with Bernal stacking, is known to be anindirect gap semiconductor, with a band gap of 1.29eV.176 The calculated band structure for bulk MoS2shows that the valence and conduction band extremaoccurawayfromtheK(K0) point (Figure 9a).125 Throughcharacterizationby absorption,photoluminescence(PL),and photoconductivity spectroscopy (Figure 9ce) asa function of thickness, Mak and co-workers149 havetraced the effect of quantum connement on theelectronic structure of MoS2 layers. With decreasingthickness, the indirect band gap, which lies below thedirect gap in the bulk material, shifts upward in energy

bymorethan0.6eV.Thisleadstoacrossovertoadirectgap material in the limit of monolayer thickness sincethe electronic states near the K point are more localizedwithin the layer and shift up in energy only modestlywith decreasing layer thickness (Figure 9a,b). The resultof this crossover in the band structure is the emergenceof brightphotoluminescence, which is absentfor thickerlms, in the monolayer material125,149 (Figure 9c). Thedirect gap character of monolayer MoS2has also beenconrmed byab initiocalculations.125,177181

The ability to control the material's electronic struc-ture through thesample thickness is notuniqueto MoS2.

Figure 9. Calculated band structures of (a) bulk and (b) monolayer MoS2.125 The solid arrows indicate the lowest-energy

transitions. (c) PL spectra for mono- and bilayer MoS2samples. Inset: PL quantum yield of thin layers of MoS2for number oflayersN= 16 in log scale. (d) Absorption spectra (left axis) normalized by the layer number Nin the photon energy rangefrom 1.3 to 2.4 eV for mono- and bilayer MoS2. The corresponding PL spectra (right axis, normalized by the intensity of thepeak (A) areincluded forcomparison. Thespectra aredisplaced alongthe vertical axis forclarity. (e) Band gapenergy of thin layersofMoS2 for N= 16.The band gap values were inferred from theenergy of the indirect gap PL feature I for N= 26 and from theenergy of the PL peak A for N= 1. As a reference, the (indirect) band gap energy of bulk MoS2is shown as dashed line.

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Similar changes in the electronic band structure as afunction of sample thickness have been predicted forother semiconducting LMDCs compounds MX2(M = Mo, W and X = S, Se, Te).182 Furthermore, recentcalculations have shown that theelectronic andopticalproperties of monolayer and few-layer MoS2 can becontrolled through strain engineering.183186

Another important aspect of the properties of 2Dsemiconducting LMDCs such as MoS2is the importantrole of many-body electronic interactions. In particular,the optical properties are dominated by excitonictransitions. The enhanced Coulomb interactions exist-ing in these materials can be attributed to reduceddielectric screening in these atomically thin systems.The absorption spectrum of the undoped MoS2mono-layer (Figure 9d) shows pronounced absorption peaksfrom excitonic transitions, rather than the steps thatwould be expected for absorption arising fromband-to-band transitions in a 2D system with a direct gap. Thetwo features, known as the A (1.9 eV) and B (2.1 eV)excitons,125,149,187 arise from transitions from the twohighest-lying spin-split valence bands to the lowestconduction bands.

The inuence of doping on theoptical properties ofmonolayer MoS2 has been studied recently by Maket al.using FET structures156 for electrostatic control ofthe doping level. By analyzing the absorption andemission line shapes at low temperature, Mak et al.have identied, in addition to thepresence of theusualneutral excitons, negative trions, quasi-particles com-posed of two electrons and a hole. The trion bindingenergy has been determined to be 20 meV, which far

exceeds that observed in conventional 2D semiconduc-tor quantumwells and renders trions signicant even atroom temperature. Similar phenomena have also re-cently been observed in MoSe2FETs by Rosset al.

157

Because of symmetry, crystals frequently have in-dependent, degenerate valleys in the conduction orvalence band. Access to this so-called valleydegreeof freedom has attracted interest both from the pointof view of fundamental physics and a possible newcontrol variable for new classes of electronic devices,commonly called valley-tronics and discussed inmore detail below. To date, however, it has provendifficult to selectively populate a given valley to pro-

duce valley polarization. MoS2 monolayers with adirect energy gap at the K and K0 points in the Brillouinzone have recently been predicted to be suitable forthecontrol of valley populationusing optical pumpingwith circularly polarized light.154,188 Because of thebroken inversion symmetry, strong spinorbit interac-tions split the spin-degenerate valence bands by 160meV in MoS2monolayers. This broken spin degener-acy, in combination with time-reversal symmetry, im-plies that the valley and spin of the valence bands areinherently coupled. When optical transitions betweenthe upper valence band and the conduction band

(corresponding to the A exciton) are induced by circu-larly polarized photons, excitation occurs only at eithertheKorK0 valley, depending on thehelicity of thelight.This excitation process, because of the coupled valleyand spin degree of freedom, also gives rise to photo-generated particles of a well-dened spin state. Thesetheoretical predictions have been recently veried by

several experimental groups153155,158 through a de-monstrationof the retentionof the circular polarizationstate of light emission induced by resonant excitationwith circularly polarized light. The important role ofinversion symmetry in establishing valley polarizationwasfurther conrmed by a comparison of thebehaviorof MoS2bilayers in which inversion symmetry, unlikefor the case of the monolayer, is present. More generaldiscussion of 2D materialsfor spin- andvalley-tronics isprovided in later sections of this review.

Edge Effects on the Electronic Structure. As we dis-cuss in the previous section, LMDCs exhibit novelphysicochemical properties when the bulk (3D) formis reduced to a monolayer (2D). The most notoriouschange is related to the electronic band structure,which can change from an indirect band gap in 3D toadirectbandgapina2Dmonolayer. 135,149 Ifthemono-layer has finite lateral dimensions, contributions fromthe edges might become important and new phenom-ena are expected.135,189 Although a monolayerof LMDCsis actually composed of three atomic layers in whicheach layer of metal atoms is sandwiched between twolayersof chalchogenatoms (X-M-X), theprojection in theplane exhibits a hexagonal or honeycomb-like crystalstructure similar to graphene; hence the edge termina-

tions canbe zigzag, armchair, chiral, or a mixture of thesetypes.

Both the structural and chemical terminations ofthe edge are crucial to determine the local physico-chemical properties of these systems. First principlescalculations have predicted that diverse edge passiva-tion in metal dichalcogenides could produce differentspins states that modify the edge electronic and mag-netic properties.135,189,190 In addition, high spin densitycould be localized surrounding metal vacancies.191 Theexperimental exploration of edges in inorganic 2Dmaterials is still at an early stage. From the experi-mental point of view, triangular clusters of MoS2on Au

substrates have been studied by STM,139,192,193 reveal-ing metallic electronic states localized close to theedges. For perfect continuous edges, those metallicstates can be viewed as one-dimensional conductingwires.192 Thesame group studied thesize dependenceof thecluster morphology and electronic structure andconsistently found sulfur-terminated edges in single-layer clusters.139 Enhanced catalytic activity has alsobeen correlated with the density of edge sites in MoS22D clusters.194

To date, MoS2 has been the testing groundfor many experiments in monolayers of metal

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dichalcogenides.135,139,149,150,152,155,163,173,174,194,199202

However,othercompounds ofthisfamily arealso goodcan-didates to study newandexciting properties.135,191,199202

Recently, the optical behavior of WS2 triangular mono-layers was investigated by Raman spectroscopy andmicro-PL.135 Monolayers of WS2 exhibited a single PLpeak between 1.9 and 2.0 eV, corresponding to a 2Dexciton associated with the direct band gap transition. Aremarkable enhancement of the PL intensity was ob-served in regions close to the edges of the 2D trian-

gular clusters (Figure 10a). PL mappings of the WS2triangular island (Figure 10b) showing the peak posi-tion and absolute maximum intensity, respectively, areshown in Figure 10c,e. In general, the spectrum closerto the edge shows both a signicantly stronger PLsignal when compared to the central region, as well asa red shift of the PL peak. TEM analysis revealed that thetriangular islands possess mainly zigzag edges, and thesynthesis conditions provided a sulfur-rich environment.Determining the exact mechanism for PL enhancementdemandsa one-to-one correlation between the chemicalstructure of the edge and its optical response. The mainchallenge consists of implementing experiments that

reveal the exciton dynamics, such as temperature-dependent PL and/or time-resolved PL, with spatialresolution of the order of tens of nanometers.

Surface Tuning of Band Structure. The need to tunethe optical and electronic properties of graphene forapplications has created a large number of proposedmethods to engineer its band structure by introducingdefects and/or dopants.203205 Both p-doping andn-doping of graphene have been reported, the latterby doping with nitrogenspecies204,206,207 andthe formerby functionalization of graphenewith hydrogen, oxygen,hydroxyl groups, or carboxylic groups.208 However,

adsorbed molecules have been found to have anextremely high mobility and low adhesion in ambientatmosphere,209 which makes reliable device fabrica-tion challenging. Chemisorbed molecules change theband structure dramatically from a near-zero gapsemimetal to a wide gap semiconductor, turning gra-phene into a highly effective insulator.210,211 The car-

rier mobility has also been shown to be decreased bymore than 3 orders of magnitude to 10 cm2/V3 s.Overall, tuning the electronic and carrier properties ofgraphene in a useful range hasbeen a very challengingtask, stimulating the search for alternative 2D semi-conductors with better tunability.

Even though the graphene-like band structure ofsilicene and germanane and the explanation for puck-ered versus at 2D sheets was reported many yearsago,212 much less effort has been spent in examiningthe tunability of these alternative elemental 2D mate-rials, which have been successfully synthesized re-cently.15,17,18,53,211 Traditional density functional theo-ry (DFT) suggests that a 2.75 eV direct band gap isformed aftercomplete hydrogenation of silicene,213218

whereas other types of functional groups (F, Cl, Br, andI)leadtodifferent band gapvalues.219 H-terminated Gegraphane analogueshave been calculated to also havea direct band gap of 1.7 eV.220 Here we summarizerecent theoretical results that explain how to control-lably manipulate the relative band energies and localextrema of a covalently modiable group IV silicongraphane analogue by termination with different or-ganic substituents.

To develop the relationship between the band

structure and the terminating functional group onsilicene, Restrepo et al.221 used DFT calculations withspinorbit coupling and a hybrid HSE06 exchange-correlation functional,222,223 which features exact ex-change not included in previous calculations. Silicane(H-terminated silicene) is found to have an indirectband gap of 2.94 eV with the conduction band mini-mumattheMpointandadirectgapof3.14eVatthepoint (Figure 11). The local conduction band minimumat originates from the SiH*-bonds, whereas theglobal minimum at M corresponds to the SiSi *-bonds.221 These two conduction band minima reactdifferently to manipulation of the silicane atomic

structure. Strain that increases the SiSi bond lengthslowers the minimum at while having a nearly neg-ligible effect on the minimum at M. This leads to achange from indirect to direct band gap when theground-state SiSi bond length of 2.36 is increasedto 2.40 . This change also causes the effective mass atthe conduction band minimum to decrease from 0.18to 0.087 me(48%). Changes in the SiH bond lengthalso mostly affect the and not the M point states butare smaller than those induced by varying the SiSibond length. This high sensitivity of the point mini-mum to changes in the bond between the Si atoms

Figure 10. (a) Room-temperature PL spectra at two differ-ent positions (smallcircles inc and e) onthe islandshown in(b). (b) SEM images of a large WS2 triangular island. (c,e)Corresponding micro-PL mappings of the PL peak positionand the absolute intensity at the peak maximum, respec-

tively. (d) Top-view atomistic model of a small WS2clusterwith sulfur-saturated zigzag edges.135

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and the terminating hydrogen atom suggests that it ispossible to tune the band gap using different termi-nating substituents. Small organic moieties such asmethyl or ethynyl groups have also been calculated todecrease the conduction band energies and result indirect band gaps or equal conduction band minimumenergies at M and (Figure 11), with varying effectivemasses.221 These results show that in contrast to gra-

phene, where functionalization leads to drastic changesin the semimetallic band structure, silicene functiona-lization should allow for a wide continuous tunabilityin band and conduction properties and offer excit-ing prospects for the engineering of new electronicmaterials.

Two-Dimensional Materialsfor Spin-and Valley-tronics.

Expanding charge-based microelectronics to take ad-vantage of the spin and valleydegrees of freedom isexpectedto increase computing power, reduce energyconsumption, and enable the development of entirelynovel devices.224 Graphene is an attractive material forspintronic applications because of its tunable carrier

concentration and intrinsically low spinorbit interac-tion and hyperfine couplings, which are expected tolead to long spin lifetimes.225 Here, graphene serves asan important test case for the expansion of spintronicsto other 2D materials beyond graphene.

Many benchmarks toward spin functional gra-phene devices have already been met, the most im-portant of which is the demonstration of room-temperature gate-tunable spin transport, as shown inFigure 12a.226,227 These experiments used lateral spinvalves (Figure 12a, inset), which have ferromagneticelectrodes to preferentiallyinject spin-up or spin-down

electrons. The spin diffuses from the injection site andcan be detected nonlocally by an electrode outside ofthe current path. Record spin lifetimes in graphene of6.2 ns228 at 20 K have been observed (Figure 12b).Essential to this achievement was overcoming the so-calledconductance mismatch problem at the injectioninterface with the incorporation of a high-quality, half-monolayer tunnel barrier of MgO.229 Despite these

successes, measured spin lifetimes in graphene areorders of magnitude lower than those expected fromab initio calculations.230 A central challenge in gra-phene spin-transport studies is to nd the source ofthis additional spin scattering. Recent measurementsof spin transportin bilayer graphene spin valvesas wellas those in single-layer graphene spin valves dopedwith adatoms indicate that local inhomogeneities inthe electronic environment are primarily responsiblefor the low measured spin lifetimes. If true, then whileunfortunate for applications requiring robust spincoherence this extreme sensitivity of spin transportto adsorbates and screening might be intentionally

exploited for novel functionality.The spin properties of graphene provide a frame-

work for the exploration of spin functionality in other2D materials. MoS2is a specic example of how thatfunctionality can be expanded. MoS2exhibits strongspinorbit coupling, which can signicantly suppressspin scattering fora certain spin direction(Figure12c).231

Though similar to graphene in other ways, the strongspin coupling in MoS2makes this material insensitive tospin scattering where graphene is sensitive. Achievingtunable spin scattering through creative combination ofgraphene and MoS2could provide a spectrum of new

Figure 11. DFT results calculated with HSE06 hybrid potentials for silicene functionalized with (a) H, (b) half H and halfhydroxyl (OH), (c) methane CH3, and (d) acetylene C2H.

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spin functionality. Even more, MoS2has the potential tolinkspin andorbital degrees of freedom, combining twoof the most exciting new ideas for novel approaches toelectronics: spintronics and valley-tronics.

Valley-tronics, as described above, is the manipu-lation of the population of degenerate low-energy

valleys that areexhibited in theband structureof somematerials, including graphene and MoS2. Of particularrelevance for electronic transport, these valleys areseparated widely enough in momentum space thatintervalley scattering is strongly suppressed.231,232 Thepotential for computing with valley-tronics was rst

Figure 12. (a) Nonlocal magnetoresistance scans of single-layer graphene spin valves measured at room temperature. Theblack (red) curve shows the nonlocal resistance as the in-plane magnetic eld is swept up (down). The nonlocal MR (RNL) of130 is indicated by the arrow. Inset: Nonlocal spin transport measurement on this device with a spacing ofL= 2.1m andSLGwidthofW=2.2m.229 (b) Hanlemagnetoresistance measurement(with eld applied out-of-plane) of a bilayer graphenespin valve at the charge neutrality point gate voltage measured at a temperature of 20 K (inset at 300 K). The spin lifetime(s =6.2ns)anddiffusionconstant( D =0.0047m

2/s)aredeterminedbya ttotheHanlecurve.228 (c) (Left,top) Representationof the trigonal prismaticstructure of monolayer MoS2 where each layer is a honeycomb lattice structure with each sublatticeoccupied by a Mo and two S atoms. (Left, bottom) The lowest-energy conduction bands and the highest-energy valencebands labeled by the z-component of their total angular momentum near the K and K0 point of the Brillouin zone. The spin

degeneracy at the valence band edges is lifted by spin

orbit interactions. The valley and spin degrees of freedom arecoupled. Under left-circularly polarized excitation, only the K-valley is populated, whereas under right-circularly polarizedexcitation, only the K-valley is populated.156 (Right) Illustration of polarized spins (blue arrows) excited by right-circularlypolarized light (red beam) populating the K0-valley in an MoS2sample. (d) Band lines near the Fermi level of BN/grapheneheterobilayers with specic stacking patterns (shown on the right), calculated using local density approximation functional.The interlayer distances arex=5,3.3,3,2.7,and2.4andtheFermilevelissetto0. 240 Panel d reprintedwith permission fromref 233. Copyright 2011 American Institute of Physics.

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discussed for 2D electron gas devices, like AlAs andSi,233 but maybe better realized with 2D materialswithappropriate band structures. Because inversion sym-metry is broken in MoS2, the valley and spin degrees offreedom are coupled and new optical selection rulesenable excitation of specic spin carriers into a specicvalley, as illustrated in Figure12c.156,231 This spin-valley

coupling forbids independent spin or valley ipping,ensuring robust indices useful for spintronic and valley-tronic applications. There is likely a lot of functionalityyet to be discovered in 2D materials beyond graphene.Not only in MoS2, just one of many transition metaldichalcogenides,148 but similar phenomena may alsobe exploited in non-MX2materials such as silicene orgermanane.

Finally, mechanically assembled heterostructuresof van der Waals bonded 2D materials might beexpected to show combined functionality of the in-dividual layers.234 The unique properties of the inde-pendent layers, which could include spin or chargedensity waves, superconductivity, and carefully chosenband structures, coupled with an anticipated high rateof electron tunneling between layers could lead toemergent properties.235 Not being limited by epitaxy,like traditional thin lm heterostructures, a nearlyinnite number of 2D material heterostructures withvarious properties can be imagined. Historically, hetero-structures such as semiconductor superlattices have ledto the development of novel technologies such asdielectric mirrors,236 microwave metamaterials,237 andnegative refraction index materials.238

Emergent functionality in 2D material heterostruc-

tures is anticipated theoretically. Density functionaltheory calculations predicted a band gap in gra-phene/BN bilayers.239 More recently, rst principlescalculations show that the band gap and effectiveelectron mass in graphene/BN heterostructures mightbe tuned by the interlayer spacing and stacking ar-rangement of the individual layers, as shown inFigure 12d.240 Experimentally, graphene/BN hetero-structures have proved important for achieving highelectron mobility in graphene since the underlying BNlattice creates a better dielectric environment andreducing sensitivity to charged point defects in theunderlying SiO2.

241,242 These heterostructures have

also led to an understanding of the observedminimalconductivity in graphene,243 which results fromcharge puddles created by the nearby SiO2 sub-strate.244,245 Additionally, tunneling eld-effect transis-tors made from graphene/BN or graphene/MoS2het-erostructures have been demonstrated to have highon/offratios (up to 10000).171 While these heterostruc-tures do not yet show emergent phenomena, theyrepresent important technological advances.

Topological Insulators. On the surface of bulk ma-terials, either dangling bonds or band bending by anelectrostatic potential can result in the appearance of

2D electronic states. Topological insulators (TIs) haveunusual bulk properties that allow them to accommo-date 2D electronic states on their surface. 246 In TIs,strong spinorbit effects create metallic electronicstates on the materials' surfaces, while the bulk of thematerial remains insulating. Furthermore, the surfacebands of TIs have an interesting electronic property in

which the spin of a surface electron is always perpendi-cular to its momentum.247 This extraordinary spin prop-erty does not allow electrons to be backscattered byimpurities,248 suggesting electronic channels of highmobility.

Following therst observation of this surface bandfrom a BiSb alloy,249 many materials, especiallylayered van der Waals materials, have been reportedto be topological insulators. Layered binary chalcogen-ides, such as Bi2Se3, Bi2Te3, and Sb2Te3, are particularlyinteresting candidates for applications250 due to theirrelatively large bulk band gap (0.2 eV) and simpleband structure, which is similar to the Dirac band ofgraphene (Figure 13a,b). The existence of surfacestates in this family of materials is conrmed experi-mentally251254 (Figure 13c). However, the surfaceelectron transport is not yet straightforward to observemainly due to the large contribution of bulk electrons.Imperfections in the bulk crystal structure create vastamounts of additional electrons, overwhelming thesurface electron transport.253,254 Therefore, the TIsneed to be engineered to reduce the bulk contributionin order to achieve transport dominated by surfaceelectrons.

Lowering the dimensionality of these TIs might

provide a route to study their surface state. Reducingsample size can suppress bulk carriers by increasing asurface-to-volume ratio.254 Also, low-dimensional TIswill allow the electron chemical potential to be modu-lated by an electric eld, which can also reduce bulkcarriers. TI nanodevices can be prepared from CVD-grown nanostructures254256 (Figure 13d,e) and via themechanical exfoliationof single crystals (Figure 13f).257

Field-effect measurements have successfully demon-strated themodulationof thecarrier typesby gate voltage,showing a potential for electronic applications.255257

To control bulk electrons in TIs, compensationaldoping253,255,256 and proper surface encapsulation256

are crucial.The unusual properties of the TI surface states

suggest many applications not feasible in conventionalmaterials. The intrinsic spin properties of TIs are attrac-tive for spintronics applications.258 Also, superconduct-ing TIs are expected to generate exotic quantummodes, potentially useful for quantum computa-tion.258,259 Additionally the surface states can play arole in energy science by enhancing thermoelectriceffects in Bi2Te3 and Bi2Se3, which are well-knownthermoelectric materials.260 Finally, this newly discov-ered electronic state, often called graphene with a

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spin texture, is a platform for a number of interestingstudies and applications.

Thermal and Thermoelectric Properties. Recent ad-vances in the synthesis and isolation of graphene andother atomic layered materials have enabled funda-mental studies of unique thermal and thermoelectrictransport properties of 2D systems, as well as thepotentialapplications of these 2D materials for thermalmanagement and thermoelectric energy conversion.Because of the absence of interlayer phonon scatter-ing, the thermal conductivity of an ideal 2D system,such as suspended SLG or h-BN, isexpectedto behigher

than the basal-plane values of their three-dimensionalstacks, namely, graphite and bulk h-BN. Although theresults from several micro-Raman measurements261,262

of suspended SLG are in agreement with these theore-tical predictions, other Raman measurements263,264 ofsuspended SLG samples have yielded values compar-able to or lower than the basal-plane values of graphite,as shown in Figure 14. In addition, further experimentsare needed to verify recent theoretical predictions thatflexural phonons (out-of-plane modes) make a largecontribution to the thermal conductivity in suspendedfew-layer graphene and h-BN.265,266

In addition, both theoretical and experimental stud-ieshavefoundconsiderablereductionofthebasal-planethermal conductivity in graphene or h-BN supportedon or encased in an amorphous material267269 orcontaminated by polymer residue,270 as shown inFigure 14. In particular, the interface interaction withan amorphous material has been found to decreasethe basal-plane thermal conductivity with decreasinglayer thickness of FLG268,271 or suspended few-layerh-BN with polymer residue.272 This trend is opposite tothat suggested for suspended FLG or h-BN.261,265,266

Thesendings indicate that the basal-plane values for

high-quality graphite and h-BN bulk crystals present apractical limit in the thermal conductivity of few-layergraphene and h-BN when these 2D layered materialsare used as heat spreaders in contact with a heat sourceand a heat sink or as nanollers to increase the effectivethermal conductivity of a composite. However, evensupported SLG can possess a basal-plane thermal con-ductivity higher than that of copper.267 Moreover,further theoretical and experimental research may po-tentially nd new approaches to increasing the basal-plane thermal conductivity of supported graphene andh-BN by choosing the right support material or tuning

Figure 13. (a) Electronic structure of Bi2Se3reconstructed by angle-resolved photoemission spectroscopy (ARPES), showingbulk conduction band (upper pocket), bulk valence band (lower pocket), and surface-state band (linear band). 251 Reprintedwith permission from ref251.Copyright 2009 NaturePublishing Group.(b) Crystal structure ofBi2Se3 (layered structure).Eachquintuple layer is made of ve atomic layers of bismuth (Bi) and selenium (Se).254 (c) Magnetoresistance of a Bi2Se3nanoribbon in parallel magnetic eld, showing periodic oscillations from the AharonovBohm interference of the surfaceelectrons.254 (d) Optical microscopy image of topological insulator (Bi

xSb2xTe3) nanoplates (510 nm thickness) on SiO2

substrate.255 Thescalebar indicates5 m. (e) Scanning electron microscopyimage of topological insulator (Bi2Se3) nanowiresand nanoribbons (50200 nm thickness).256 The scale bar indicates 10 m. (f) Optical microscopy image of a topologicalinsulator (Bi2Se3) ake on SiO2 substrate from mechanical exfoliation of a bulk crystal.

257 The scale bar indicates 2 m.Reprinted with permission from ref 257. Copyright 2011 Nature Publishing Group.

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the interface roughness and interaction. In particular,superior electron mobility has been found in graphenesupported on h-BN compared to graphene supportedon amorphous SiO2.

241 It remains to be investigated

whether graphene supported on atomically smoothh-BN may possess superior thermal transport propertiescompared to graphene supported on SiO2.

The interface thermal resistance is as important inthermal transport as the basal-plane thermal conduc-tivity of 2D layered materials. Although thecross-planethermal conductivities of graphite and h-BN are muchsmaller than their basal-plane values, the cross-planediffusive thermal resistance of few-layer graphene andh-BN is expected to be smaller than the interfacethermal resistance because of the small thickness.The thermal interfaceresistance of graphene in contactwith a dielectric or metal has been measured to be in

the range of (15) 108 K m2 W1 at room tem-perature.273276 These values are not very large for thevan der Waals interface, likely because graphene israther conformal to the surface roughness.277 In addi-tion, interface phonon transport between highly ani-sotropic graphene and a dissimilar material remains aninteresting subject for theoretical study.278

Compared to graphene that is a semimetal, h-BNis an insulator, which can be advantageous for itsapplications as a heat spreader in direct contact withhigh-power density semiconductor nanodevices. More-over, other 2D layered materials such as MoS2,

133

silicene,53 and germanane provide different bandgap values suitable for various functional devices. Inthese functional devices made of 2D layered materials,local heating can become an important issue similar tothe challenge faced by silicon nanoelectronic devices,especially if the in-plane thermal conductivity of these2D materials is suppressed by phonon scattering by

interface disorder or interactions with another materi-al. However, little is currently known about thethermaltransport properties of these 2D materials and hetero-structure devices171 made of multiple 2D materials.

The possibly suppressed thermal conductivity () of2D layered materials with surface disorder or perturba-tion is desirable for thermoelectric materials. Duringthe past two decades, nanostructures have been ac-tively investigated for suppressing the phonon con-tribution to the thermal conductivity of thermoelectricmaterials.279 In such efforts, it is critical to control theinterface charge states and scattering in the nanos-tructured materials, so that these effects do not resultin a considerable reduction of the charge mobility,power factor,S2, andgure of merit,Z=S2/, whereS is the Seebeck coefficient and is the electricalconductivity.280 Moreover, research along this direc-tion was initially stimulated by theoretical predictionsof power factor enhancement in 2D and one-dimen-sional quantum structures.281,282 Recent theoreticalstudies have further suggested that the power factorcan be enhanced by the coupling of the two topolo-gical surface states in few quintuple layers of Bi2Te3and Bi2Se3.

283 A careful control of the surface qualityand surface band bending will be required for these

effects due to quantum connement and topologicalsurface states to be veried in experiments. Thesechallenges suggest abundant opportunities for re-search in thermal and thermoelectric properties of2D materials beyond graphene.

Applications to Transistor Scalability. Progress inthe semiconductor industry has been driven by acontinuous reduction of the transistor geometry. Cur-rent silicon-based field-effect transistors (FETs) feature22 nm gate lengths, and further size reductions moti-vate the need to search for alternative device conceptsand materials. A conventional FET consists of a semi-conducting channel that is connected to source and

drain electrodes and can be switchedon and off bytheapplication of a field via a gate electrode that isseparated by a dielectric material (Figure 15a). Desir-able features of switching transistors include high on/off current ratios, low power consumption, and fastswitching times.

Two-dimensional materials have an innate advan-tage over bulk materials, like Si, for device scalabilitybecause of their favorable electrostatic properties. Inparticular, due to their dimensionality, these materialscan approach theideal effective screening length,,andcan be operated beyond the quantum capacitance

Figure 14. Comparison of reported experimental thermalconductivity values of graphite, graphene, and h-BN. Thesuspended SLG data are from Balandin e t a l.,319 Chenet al.,262 Lee et al.,264 and Faugeras et al.263 The data forSLGand 8-layer graphene (8LG) supported on SiO2 arefromSeol et al.267 and Sadeghi et al.,271 respectively. The data forpyrolytic graphite (PG) are the recommended values byTouloukian.320 The bulk h-BN data are from Sichel et al.321

The data for the 11- and 5-layer suspended h-BN withpolymer residue are from Jo et al.272

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limit(QCL). To reduce the channel length in today'stypical silicon-on-insulator transistor, the semiconduc-tor thickness must also be reduced to ensure that 2Delectrostatic screening effects are minimized.284 Elec-

trostatic potential variations along the channel arescreened out within a material-dependent .285,286 Toprevent the electriceldof thedrain from being totallyscreened in a small channel, like those of ultrascaledMOSFETs, it is desirable for to be as small as possible.While the exact form of depends on the detailsof thedevice structure, for single and double gate geome-tries, it can be approximated by285

ffiffiffiffiffiffiffiffiffiffiffiffiffiffisdsdox

ox

s (1)

Here,sand dsdenote the dielectric constant and

thickness of the semiconductor, respectively, and oxanddoxare the respective quantities for the dielectricoxide. In either case, minimizing ds will minimize .Thus, single-layer-thick 2D materials represent the bestpossible scenario that nature has to offer.

A second advantage of 2D material-based transistors istheir ability to operate beyond the QCL, the limit at whichthe oxide capacitance is equal to the quantum capaci-tance, the capacitance due to gate-voltage-induced freecharges in the channel.287,288 In the QCL regime, theintrinsicgate delay is reduced (), and the P3 (where Pisthe dynamic power) now decreases linearly withchannel

length in the QCL regime.289,290 Thus the energy con-sumption required forswitching is minimized in the QCL.In addition, many 2D materials from the dichalcogenidefamily,andeven silicaneandgermanane, possessa larger

band gap than bulk Si. This property could result in amuchlower direct source-to-drain leakage current,whichis especially important for ultrascaled transistors.291

Recognizing the potential impact of 2D materialsfor transistorapplications, a signicant amountof workhas been devoted to making these transistors in thepast fewyears.163,167,292296 Recently,the performanceof MoS2 transistors has beentheoretically examined

164

using real space, self-consistent, and fully parallel non-equilibrium Green's function transport simulationwithin a k3p approach and band structuretted torstprinciples calculations. Figure 15b shows the draincurrent (ID) versusgate voltage (VG) characteristic for

a simulated MoS2transistor. The maximum ON currentcan be as high as 2 mA/m, and the OFF current couldbe as small as 10 fA/m, thereby giving an incrediblylarge ON/OFF ratio (1010). Figure 15c shows thesubthreshold swing as a function of gate oxide thick-ness. These simulation results show fantastic immunityto short channel effects. Note that almost an idealsubthreshold swing (60 mV/decade) is achievable witha reasonably large gate oxide thicknessof around 5 nmeven for a 30 nm channel length device. This result canbe attributed to the fact that the screening lengthisvery small as expected for a 2D material. Figure 15d

Figure 15. (a) Schematic illustration of cross sectionof MoS2 FET.163 Reprinted with permission from ref163. Copyright 2011

Nature Publishing Group. (b) Transfer characteristic of a monolayer MoS2 transistor in both log and linear scales. (c)Subthreshold swing as a function of gate oxide thickness for two different gate lengths. (d) Capacitance vs voltagecharacteristics. Inset shows the change in surface potential swith gate voltage Vg.

164

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shows the capacitancevoltage characteristic. Notethat the total capacitance (shown in blue) is muchsmaller than the oxide capacitance (black dashed line)and closer to the quantum capacitance (shown in solidred). This means that the device is working close to theQCL. These simulations conrm the fantastic electro-static control, QCL operation, and low leakage current

for a 2D material.Multilayer Assemblies and Their Applications. Nano-

sheets (Figure 5), referring torestacked or assembled2D materials, provide new opportunities and advan-tages for devices. For many applications, these nano-sheets and their composites have inherent and pre-dicted advantages: they can be solution processed,they can display new behavior due to their extremelyhigh surface area, and as free-standing multilayer assem-blies, they are flexible119,297299 and can be manipulatedeither mechanically or by deposition on templates. Mix-ing of nanosheets with other materials, including metalnanoparticles, polymers, and small molecules, can be aroute to interesting solution-phase nanocomposite hy-brids, such as superlattice-like assemblies.99101

Reassembly of nanosheets into multilayer lms canlead to improved properties in the areas of super-capacitors,300 pseudocapacitors,301 photoconductivematerials,302,303 and heterojunction photodiodes304

as new magnetic materials305 and as magneto-opticalcomponents.306,307 Additional applications include thefollowing:

Batteries: The morphological and size advantagesof nanosheets have shown improved performance asbattery electrodes. For example, reassembled octatita-

nate nanosheets have better reversible capacities thanelectrodes madefrombulk octatitanate, presumably dueto the ability of these nanosheets to better withstanddamage caused by lithium insertion andextraction.300,308

Magnetic Properties: Ferromagnetic nanosheetscandisplay uniqueproperties such as anisotropic mag-netization andstronger influences fromsurfaceatoms.309

For instance, gigantic magneto-optical effects have beenobserved in Ti0.8Co0.2O2 nanosheets. Because thesenanosheets are composed entirely of surface atoms,surface cobalt spins and their local ferromagnetic cou-plings are much stronger than those that occur in dilutemagnetic semiconductors and bulk magnets.309

Photoconductors: Restacked nanosheets havehigher surface area than their bulk precursors, and thisaspect, coupled with the finite thickness effects onelectronic structure, the reactivity, and the openframe-work, can lead to enhancements in nanosheet photo-activity in solar energy conversion111 and in capac-itance.301 Furthermore, it has been recently demon-strated that a vertically oriented 2D organic/inorganiclamellar photodetector architecture can have bet-ter detectivities than sensitized nanoparticle thinfilms.310 This is because the excellent 2D conductionchannel allows for reduced majority carrier transit

times, and the atomic-scale thickness of the 2D ma-terial enables a high sensitization ratio.

High Dielectric Constant Materials: As high-k di-electrics, nanosheets can be used where thin filmsfrom vapor deposition cannot. Metal oxide thin filmsgrown by conventional vapor deposition techniquesoften show decreasing dielectric constants with de-creasing film thickness.118,311,312 One reason for thedegradation in dielectric properties is the high tem-peratures that complex vapor processing requires, oftenabove 600 C, which reduces the polarizability due tononstoichiometry.312 Free-standing and stacks of nano-sheets, however, have been shown to have low leakagecurrent densities and high dielectric constants for thick-nessesdownto10nm(Figure16a).117,118,312 Reassemblednanosheets also have the advantage of electronic isola-tion of each nanosheet in these multilayer assemblies.309

Liquid Crystals: Inorganic magnetic nanoparticlesof magnetite have been mixed with niobate nano-sheets and placed in a magnetic field to guidethe nanoparticles' orientation.111,313 Light scatteringshows orientational dependence on the magnetic field(Figure 16b). Potassium niobate nanosheets have beensuccessfully functionalized with silane and dispersed inorganic solvents to create liquid crystals.314 The nano-sheet colloids were dispersed in chloroform and ex-hibited birefringence characteristics of a nematicphase.111,314 Liquid-crystal properties of nanosheets havealso been reported for nanosheets made of niobates315

andKCa2Nb3O10,316amongothers.Thelateral dimensions,

aspect ratio, and colloid concentration of the nano-sheets strongly dictate the liquid-crystal properties.315

OPPORTUNITIES AND OUTLOOK

In this nascent eld of 2D materials, the salientdifferences between bulk and few or single layers are

just starting to be understood. New fundamentalstudies can address single-layer scale differences inmany-body interactions; phonon transport, such asexural phonon modes; interfacial electronelectron,electronphonon, electronmagnon, etc. coupling;excitonic and other quasiparticle properties; the nature

Figure16. (a) High-k propertiesof titania nanosheet assem-blies compared to conventional thin lms. The values forthe multilayered titania nanosheets are larger than whathas been reported for any other high-k material. 312 Rep-rinted with permission from ref 312. Copyright 2006 JohnWiley and Sons. (b) Optical micrograph of a liquid crystalmadefromFe3O4Ca2Nb3O10 nanosheets in pyridine underan inhomogeneous magnetic eld.313

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33. Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Heon,M.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W. Two-Dimen-sional Nanocrystals Produced by Exfoliation of Ti3AlC2.

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38. Huang, X.; Li, J.; Fu, H. The First Covalent OrganicInorganic Networks of Hybrid Chalcogenides: StructuresThat May Lead to a New Type of Quantum Wells. J. Am.Chem. Soc.2000,122, 87898790.

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2d materials review

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