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Synthesis and Characterization of Carbon Based One-Dimensional Structures Tuning Physical and Chemical Properties
HamidReza Barzegar
Doctoral Thesis
Department of Physics
Umeå University
Umeå 2015
This work is protected by the Swedish Copyright Legislation (Act 1960:729)
ISBN: 978-91-7601-191-1
Electronic version available at http://umu.diva-portal.org/
Printed by: Print & Media
Umeå, Sweden 2015
To my wife
i
Abstract
Carbon nanostructures have been extensively used in different
applications; ranging from electronic and optoelectronic devices to energy
conversion. The interest stems from the fact that covalently bonded carbon
atoms can form a wide variety of structures with zero-, one- and two-
dimensional configuration with different physical properties. For instance,
while fullerene molecules (zero-dimensional carbon structures) realize
semiconductor behavior, two-dimensional graphene shows metallic behavior
with exceptional electron mobility. Moreover the possibility to even further
tune these fascinating properties by means of doping, chemical modification
and combining carbon based sub-classes into new hybrid structures make
the carbon nanostructure even more interesting for practical application.
This thesis focuses on synthesizing SWCNT and different C60 one-
dimensional structures as well as tuning their properties by means of
different chemical and structural modification. The purpose of the study is to
have better understanding of the synthesis and modification techniques,
which opens for better control over the properties of the product for desired
applications.
In this thesis carbon nanotubes (CNTs) are grown by chemical vapor
deposition (CVD) on iron/cobalt catalyst particles. The effect of catalyst
particle size on the diameter of the grown CNTs is systematically studied and
in the case of SWCNTs it is shown that the chirality distribution of the grown
SWCNTs can be tuned by altering the catalyst particle composition. In
further experiments, incorporation of the nitrogen atoms in SWCNTs
structures is examined. A correlation between experimental characterization
techniques and theoretical calculation enable for precise analysis of different
types of nitrogen configuration in SWCNTs structure and in particular their
effect on growth termination and electronic properties of SWCNTs are
studied.
C60 one-dimensional structures are grown through a solution based
method known as Liquid-liquid interfacial precipitation (LLIP). By
controlling the crystal seed formation at the early stage of the growth the
morphology and size of the grown C60 one-dimensional structures where
tuned from nanorods to large diameter rods and tubes. We further introduce
a facile solution-based method to photo-polymerize the as-grown C60
nanorods, and show that such a method crates a polymeric C60 shell around
the nanorods. The polymeric C60 shell exhibits high stability against common
hydrophobic C60 solvents, which makes the photo-polymerized nanorods
ideal for further solution-based processing. This is practically shown by
decoration of both as grown and photo-polymerized nanorods by palladium
nanoparticles and comparison between their electrochemical activities. The
ii
electrical properties of the C60 nanorods are also examined by utilizing a field
effect transistor geometry comprising different C60 nanorods.
In the last part of the study a variant of CNT is synthesized in which large
diameter, few-walled CNTs spontaneously transform to a collapsed ribbon
shape structure, the so called collapsed carbon nanotube (CCNT). By
inserting C60 molecules into the duct edges of CCNT a new hybrid structure
comprising C60 molecules and CCNT is synthesized and characterized. A
further C60 insertion lead to reinflation of CCNTs, which eventually form
few-walled CNT completely filled with C60 molecules.
iii
Sammanfattning
Kolnanostrukturer har funnit tillämpningar i en rad olika områden såsom
elektronik och optoelektronik men även inom tillämpningar för
energiomvandlingar och energilagring. Det stora intresset för
kolnanostrukturer har sitt ursprung i att de kan forma kovalent bundna
strukturer av noll-dimensionell, en-dimensionell, eller två-dimensionell
karaktär som alla har vitt skilda fysikaliska egenskaper. Exempelvis uppvisar
fullerener (noll-dimensionell kolnanostruktur) halvledaregenskaper medan
grafen (två-dimensionell) besitter metalliska ledningsegenskaper med
exceptionellt hög konduktivitet. Dessutom kan de unika egenskaperna hos
dessa material ytterligare modifieras genom dopning, reaktioner med andra
molekyler och atomer, samt genom att kombinera olika slags
kolnanostrukturer till nya hybridmaterial, vilket kan leda till ytterligare
intressanta egenskaper som lämpar sig för praktiska tillämpningar.
Denna avhandling fokuserar på att syntetisera enkelväggiga kolnanorör
(SWCNT), och olika en-dimensionella C60-strukturer, samt att kontrollera
deras egenskaper genom kemisk eller strukturell modifiering. Målet med
studien är att få en bättre förståelse för dessa processer vilket ger en ökad
kontroll av hur egenskaperna hos kolnanostrukturer kan kontrolleras och
därigenom öka deras implementerbarhet för olika tillämpningar. Kolnanorör
(CNTs) tillverkas genom kemisk ångdeposition, eng. chemical vapor
deposition (CVD) med katalyspartiklar av järn/kobolt. Effekten av hur
katalyspartiklarnas storlek påverkar diametern hos de tillverkade
kolnanorören studeras systematiskt. Vidare visas att de enkelväggiga
kolnanorörens kiralitet kan påverkas och kontrolleras genom att förändra
katalyspartiklarnas komposition. I andra experiment studeras dopning av
kväve i kolnanorören. Genom att jämföra experimentella och teoretiska data
kan en detaljerad analys över olika kväveatomers kemiska inbindning i
kolnanorören beskrivas, i synnerhet med avseende på kväveatomernas effekt
på kolnanorörens elektroniska egenskaper och deras tillväxt. En-
dimensionella C60-strukturer tillverkas genom en lösningsbaserad metod.
Genom att kontrollera nukleationsstadiet för kristallerna i ett tidigt skede
kan morfologi och struktur kontrolleras så att tillväxten kan styras från små
C60-stavar till större stavar och C60-tuber. I en separat studie introducerar vi
en lösningsbaserad metod att fotopolymerisera C60-stavarna. Vi visar att vår
metod skapar ett polymeriserat skal i stavarna vilket skyddar dem från att
lösas upp i vanliga hydrofobiska lösningsmedel. Därigenom blir de
polymeriserade C60-stavarna möjliga att använda i olika lösningsbaserade
metoder, vilket demonstreras i experiment där olika typer av stavar
dekoreras med palladium-nanopartiklar och karaktäriseras med
elektrokemiska metoder. Elektriska egenskaper, i synnerhet
iv
elektronmobilitet studeras också för C60-stavarna fält-effekt-
transistormätningar.
I avhandlingens sista del studeras en variant av flerväggiga kolnanorör
med stor diameter och få väggar. Under vissa kriterier kollapsar dessa till en
struktur som kan liknas vid två på varandra packade grafenskikt men med en
speciellt formad kanal längs med de parallella långsidorna. Genom att fylla
dessa kanaler med C60 molekyler skapar vi en ny hybridstruktur. Vi visar att
de kollapsade kolnanorören kan ”pumpas” upp till sin ursprungliga
cylindriska form genom att fortsätta fylla rören med C60 molekyler.
v
List of publications
This thesis is based on the following publications:
(Reprints made with the permission from the publishers)
I. Simple Dip-Coating Process for the Synthesis of Small Diameter
Single-Walled Carbon Nanotubes-Effect of Catalyst Composition
and Catalyst Particle Size on Chirality and Diameter
Barzegar H R, Nitze F, Sharifi T, Ramstedt M, Tai C W,
Malolepszy A, Stobinski L and Wågberg T. 2012 J. Phys. Chem. C
116 12232-9.
II. Doping Mechanism in Small Diameter Single-Walled Carbon
Nanotubes: Impact on Electronic Properties and Growth Selectivity
Barzegar H R, Gracia-Espino E, Sharifi T, Nitze F and Wågberg T.
2013 Nitrogen J. Phys. Chem. C 117 25805-16.
III. Water Assisted Growth of C60 Rods and Tubes by Liquid-Liquid
Interfacial Precipitation
Barzegar H R, Nitze F, Malolepszy A, Stobinski L, Tai C W and
Wågberg T. 2012 Method Molecules 17 6840-53.
IV. On the Fabrication of Crystalline C60 Nanorod Transistors from
Solution
Larsen C*, Barzegar H R*, Nitze F, Wågberg T and Edman L.
2012 Nanotechnology 23 344015. (* Authors equally contributed to
the manuscript)
V. Solution-Based Phototransformation of C60 Nanorods: Towards
Improved Electronic Devices
Barzegar H R*, Larsen C*, Edman L and Wågberg T. 2013
Particle & Particle Systems Characterization n/a-n/a. (* Authors
equally contributed to the manuscript)
vi
VI. Palladium Nanocrystals Supported on Photo-transformed C60
Nanorods: Effect of Crystal Morphology and Electron Mobility on
the Electrocatalytic Activity towards Ethanol Oxidation
Barzegar H R*, Hu G Z*, Larsen C, Jia X E, Edman L and
Wågberg T. 2014 Carbon 73 34 40. (* Authors equally contributed
to the manuscript)
VII. C60/Collapsed Carbon Nanotube Hybrids - A Variant of Peapods
Barzegar H R, Gracia-Espino E, Yan A, Ojeda-Aristizabal C, Dunn
G, Wågberg T, and Zettl A. 2014 submitted to Nanoletter
This thesis is also influenced by the following work:
- In-situ TEM Study of Collapsing, Reinflating and Twisting of Multi-
Walled Carbon Nanotubes
Yan A*, Barzegar HR*, Ojeda-Aristizabal C, Dunn G, Wågberg T
and Zettl A. In manuscript form. (* Authors equally contributed to
the manuscript)
Other work I have contributed to but not directly contributed to this thesis:
1. Nitze F, Sandström R, Barzegar HR, Hu GZ, Mazurkiewicz M,
Malolepszy, Stobinski, L, Wågberg T. Direct support mixture painting,
using Pd(0) organo-metallic compounds – an easy and environmentally
sound approach to combine decoration and electrode preparation for
fuel cells. J Mater. Chemistry 2, 20973-20979.
2. Hu G Z, Nitze F, Gracia-Espino E, Ma J, Barzegar H R, Sharifi T, Jia
X, Shchukarev A, Lu L, Ma C, Yang G, Wågberg T. Small palladium
islands embedded in palladium-tungsten bimetallic nanoparticles form
catalytic hot-spots for oxygen reduction. Nature Communication 5,
5253 (2014).
vii
3. Hu G Z, Nitze F, Jia X, Sharifi T, Barzegar H R, Gracia-Espino E,
Wågberg T. Reduction free room temperature synthesis of a durable and
efficient Pd/ordered mesoporous carbon composite electrocatalyst for
alkaline direct alcohols fuel cell. Rsc Advances 4, 676-682 (2014).
4. Sharifi T, Gracia-Espino E, Barzegar H R, Jia X E, Nitze F, Hu G Z,
Nordblad P, Tai C W and Wågberg T. Formation of nitrogen-doped
graphene nanoscrolls by adsorption of magnetic gamma-Fe2O3
nanoparticles. Nature Communications 4, (2013).
5. Sharifi T, Nitze F, Barzegar H R, Tai CW, Maruzkiewicz M, Maloepszy
A, Stobinski L, Wågberg T. Nitrogen doped multi walled carbon
nanotubes produced by CVD-correlating XPS and Raman spectroscopy
for the study of nitrogen inclusion. Carbon 50, 3535-3541 (2012).
6. Jia X E, Hu G Z, Nitze F, Barzegar H R, Sharifi T, Tai C W, Wågberg
T. Synthesis of Palladium/Helical Carbon Nanofiber Hybrid
Nanostructures and Their Application for Hydrogen Peroxide and
Glucose Detection. Acs Applied Materials & Interfaces 5, 12017-12022
(2013).
7. Nitze F, Barzegar H R, Wågberg T. Easy synthesis of Pd fullerene
polymer structures from the molten state of
tris(dibenzylideneacetone)dipalladium(0). Phys Status Solidi B 249,
2588-2591 (2012).
8. Hu G, Nitze F, Sharifi T, Barzegar HR, Wågberg T. Self-assembled
palladium nanocrystals on helical carbon nanofibers as enhanced
electrocatalysts for electro-oxidation of small molecules. Journal of
Materials Chemistry 22, 8541-8548 (2012).
9. Hu G, Sharifi T, Nitze F, Barzegar H R, Tai CW, Wågberg T. Phase-
transfer synthesis of amorphous palladium nanoparticle-functionalized
viii
3D helical carbon nanofibers and its highly catalytic performance
towards hydrazine oxidation. Chemical Physics Letters 543, 96-100
(2012).
10. Hu G Z, Nitze F, Barzegar H R, Sharifi T, Mikolajczuk A, Tai C W,
Borodzinski A, Wågberg T. Palladium nanocrystals supported on helical
carbon nanofibers for highly efficient electro-oxidation of formic acid,
methanol and ethanol in alkaline electrolytes. J Power Sources 209,
236-242 (2012).
ix
Table of Contents
1. Introduction 1
2. Fullerene 3
2.1. Photo-polymerization of fullerene 3
2.2. One-dimensional fullerene structures 4
3. Carbon nanotubes 5
3.1. Single-walled carbon nanotubes 5
3.2. Multi-walled carbon nanotubes 7
3.3. Synthesis of carbon nanotubes 8
3.3.1. Chemical vapour deposition 8
3.4. Doping of carbon nanotube 9
3.5. Application and challenges 11
3.6. Carbon nanotube hybrid nanostructures 11
3.7. Collapsed carbon nanotubes 12
4. Experimental methods 13
4.1. Single-walled carbon nanotube growth 13
4.1.1. Catalyst particle preparation 13
4.1.2. Growth of pristine carbon nanotubes 15
4.1.3. Growth of nitrogen doped single-walled carbon nanotubes 15
4.2. Synthesis of one-dimensional fullerene structures 16
4.2.1. C60 Nanorods 16
4.2.2. Water assisted growth of C60 nanorods and tubes 16
4.2.3. Photo-polymerization of C60 nanorods 16
4.2.4. Functionalization of C60 nanorods with Pd nanoparticles 17
4.3. Synthesis of C60/CCNT hybrid structures 17
5. Characterization methods 19
5.1. Atomic force microscopy 19
5.2. Transmission electron microscopy 19
5.3. Scanning electron microscopy 21
5.4. X-ray photoelectron spectroscopy 21
5.5. Raman spectroscopy 21
5.5.1. Raman spectroscopy of single-walled carbon nanotubes 22
5.6. X-ray diffraction 24
5.7. Tehrmogravimetric analysis 24
5.8. Dynamic light scattering 24
6. Results and discussions 25
x
6.1. Catalyst particle preparation for carbon nanotube growth 25
6.1.1. Analysis of size and distribution of catalyst particles 25
6.2. Carbon nanotube growth 27
6.3. Growth and characterization of nitrogen doped
single-walled carbon nanotubes 29
6.3.1. Analysis by X-ray photoelectron spectroscopy 29
6.3.2. Theoretical Modeling of pyridinic and pyrrolic nitrogen 31
6.3.3. Radial breathing mode analysis 33
6.3.4. Analysis of D- and G´- bands 35
6.4. One-dimensional C60 structures 36
6.4.1. C60 nanorods 36
6.4.2. C60 tubes 37
6.4.3. Characterization of the grown C60 structures 38
6.4.4. Solution based photo-polymerization of C60 nanorods 40
6.4.5. Electronic characterization of C60 nanorods 42
6.4.6. C60 nanorods as a support for Pd catalyst particles 43
6.4.6.1. Electrocatalytic activity of Pd-C60 nanorods
toward ethanol oxidation 54
6.5. C60/Collapsed carbon nanotube hybrid nanostructure 46
7. Conclusion 51
8. Summary of the appended articles 53
9. Acknowledgments 57
10. References 58
xi
Most commonly used abbreviations
AFM Atomic force microscopy
BM-Pd Benzyl mercaptan functionalized Pd
CVD Chemical vapor deposition
CNT Carbon nanotube
CCNT Collapsed carbon nanotubes
CV Cyclic Voltammetry
DLS Dynamic light scattering
EELS Electron energy loss spectroscopy
FFT Fourier transform
HRTEM High resolution transmission electron microscopy
LLIP Liquid-liquid interfacial precipitation
MWCNT Multi-walled carbon nanotube
m-DCB m-dichlorobenzene
Nitrogen doped N-doped
RBM Radial breathing mode
SAED Selected area electron diffraction
SDS Sodium Dodecyl Sulfate
STM Scanning electron microscopy
SWCNT Single-walled carbon nanotube
TEM Transmission electron microscopy
TGA Thermogravimetric analysis
vHS van Hove singularity
XPS X-ray photo spectroscopy
XRD X-ray diffraction
xii
1
1. Introduction
Historically the research on carbon materials (graphite and artificial
diamond) goes back to 19th century, however it was the advent of fullerenes,
a zero dimensional structure, in 1985 by Kroto and Smalley[1] which made
carbon based material a more interesting subject for nano-research. This
advent lead Iijima[2] in 1991 to discover MWCNTs, a new class of one
dimensional carbon nanomaterial which exhibits extraordinary properties.
CNTs became even more interesting after observation of SWCNTs[3] and by
the prediction of their electrical properties.[4] The main reason for the high
interest in SWCNTs is because of their diversity in structure, resulting in
different electrical properties, which can range from high conducting
metallic behaviour to large band gap semiconductor. In addition, their one-
dimensional morphology make them interesting for practical electronic and
energy applications. The family of carbon nanostructures became complete
by finding the two-dimensional (2D) carbon nanostructure in 2004 known
as graphene.[5, 6] It is noteworthy that the most recently isolated carbon
nanostructure can be seen as the building block of the previously discovered
ones, through stacking (graphite), wrapping (fullerene) and rolling (CNTs)
as shown in figure 1.[7] Like carbon nanotubes, graphene also shows
different electrical properties depending on the size and configuration of
carbon atoms at the edges of graphene sheet.
Figure1. Schematic indicating relation between graphene sheet and a fullerene C60, CNT and graphite.[7]
2
Besides the three main members of carbon nanostructures, numerous
other carbon nanostructures, such as graphene whiskers, carbon onions,
carbon nanofibers and ordered mesoporous carbon have been also studied.
Due to their high electrical conductivity and high surface area, the last two
structures, have proven to be good supports for metal nano catalysts in
electrocatalytic application.[8-11] Thus it is clear that the term “Carbon
nanostructure” refers to a wide variety of carbon-based materials. Although
they are all made of carbon, depending on atomic configuration and size,
they show different properties. The structural diversity of carbon
nanostructures –realizing different properties- as well as high abundance of
carbon in the earth and on the earth’s crust have made them interesting for
both scientists and industries. One of the main focuses in the field of carbon
nanostructure is to tune their physical or chemical properties for certain
practical applications. There are different ways to tune the properties of
carbon nanostructure such as; size tuning, surface modification,
incorporation of guest atoms (doping) or by combining the properties of two
different known carbon nanostructure in controlled way (hybrid carbon
nanostructures). For example it has been shown that the incorporation of
nitrogen atoms in the structure of MWCNTs significantly increase their
electrochemical reactivity, and that they even can be used directly as a metal-
free catalyst for energy conversion applications in fuel cells.[12] In the case
of SWCNT the nitrogen incorporation also modifies their electrical
properties so that they might transform from metallic to semiconductor
SWCNT or vice versa.[13] Also hybrid carbon nanostructures, mixing for
example fullerenes and SWNTs represent a way to tune the material
properties. It has been shown that the encapsulation of C60 inside SWCNTs
(peapods) results in slightly different electrical properties compared to the
parents materials.[14]
This thesis deals with the synthesis of carbon-based materials; namely
CNTs, one-dimensional C60 structures and hybrid carbon nanostructures,
comprising both CNT and C60 molecules. In the case of SWCNTs the study
mainly focuses on their selective growth as well as nitrogen incorporation in
their structure. The study on the C60 one-dimensional structures focuses on
an efficient synthesis method to control the morphology and size of the
grown structures as well as the possibility to modify their properties, in
particular lowering their solubility, for further solution-based process
application. The last part of the study focuses on a different phase of CNT in
which CNT possess a transition from tubular to flat ribbon shape structure.
Further by insertion of C60 molecules into open duct edges of CCNT a new
hybrid nanostructure is synthesized.
3
2. Fullerenes
Fullerene is the general name for a class of hollow carbon molecules
containing different number of carbon atoms. The most stable molecules of
this family is known as Buckministerfullerene (C60) containing 60 carbon
molecules. The carbon atoms in C60 form a soccer ball shaped structure built
up by 20 hexagons and 12 pentagons without any dangling bond (figure 2).
The first experimental observation of
C60 was made in 1985 by laser vaporization
of graphite1 but later C60 was synthesized
in much larger amount by arc-discharge
vaporization of graphite.[15] Carbon
atoms in C60 molecules are sp2 hybridized,
where each carbon atom is bonded to
neighboring atoms by two single bonds
and one double bond. It should however
be noted that due to high curvature in
fullerene molecules the π electrons of
neighboring atoms interact with each
other which results in an admixture of sp2
and sp3 character. The carbon to carbon diameter of a C60 is 0.7 nm, but
taking the π orbitals into account, the electron cloud of C60 has a real
diameter of 1 nm (also called van der Waals diameter). At ambient
conditions pristine C60 molecules form a face centered cubic crystal structure
with a lattice constant of 14.17 Å, and a nearest neighbor distance (center to
center) of about 1 nm.
2.1. Photo-polymerization of fullerene A fascinating property of C60 is that under certain conditions the C60
molecules can form covalent bonding with each other to form; dimers (two
C60s are bonded together), linear chains, and even two- or three-dimensional
polymeric phases.[16, 17] The polymerization of C60 was first observed by
Rao et al.[18] in 1993 by irradiation of a C60 thin films by ultra-violet (UV) or
visible light, so-called photo-polymerization. The formation of covalent
bonding between two C60 requires that two carbon-carbon double bond in
two molecules are oriented parallel to each other and that the distance
between two molecules is less than 4.2 Å.[19] The bonding between two
molecules occurs via a photochemical reaction known as “2+2
cycloaddition”. Although there are a rather high number of double bonds on
a C60 molecule (30), it is interesting to note that a free rotation of the
molecules is a prerequisite to form the cycloaddition bridge bond. The
Figure 2. Schematic of a fullerene
C60 molecule.
4
polymerization of C60 molecules decrease the C60-C60 distance in crystals and
also decrease the solubility of the C60 in common hydrophobic solvent of C60.
The later mechanism can be used to protect the C60 crystal structure in some
solution-based process application.[20] The photo-polymerization and its
effect on some of the practical applications will be discussed in detail in
paper V and VI.
2.2. One-dimensional fullerene structures Homogeneous or blend thin film of C60 and its derivative PCBM are widely
used in organic electronic devices such as solar cells[21-25] and integrated
electronic circuits.[26-31] However for such applications and other energy
related application high surface area and good crystallinity is very
advantageous.
Figure 3. Scanning electron microscopy image of (A) C60 nanorods and (B) C60 nanotubes
synthesized by LLIP method.
One-dimensional crystalline C60 structure, often referred to as C60
nanorods, C60 nanowhisker, or C60 tubes (figure 3) can be grown through
different approaches such as slow evaporation,[32, 33] the use of
templates,[34] vapor-solid processes,[35] fast solvent-evaporation
techniques[36] and liquid-liquid interfacial precipitation (LLIP).[37-40]
Among these methods the LLIP method is of particular interest due to
several controllable parameters, which in turn affect the final product. In
this method a poor solvent of C60 (such as ethanol or isopropanol alcohol) is
gently added to a solution of C60 in a strong solvent such as toluene or m-
dichlorobenzene (m-DCB). If the interface is left undisturbed during growth
process, then it is referred as static LLIP method.[41] The LLIP method can
also be assisted by hand shaking or weak ultrasonication, which result in a
diffuse interface between two solvents. Due to diffusion of two solvents into
each other (close to the interface) the C60 become oversaturated and
consequently C60 crystal seeds are formed. Continued diffusion of two
solvents into each other results in more free C60 molecules which settle down
on crystal seeds and depending on the size of the crystal seeds as well as
A B
5
miscibility of two solvent, C60 rods or tube can be grown at the interface of
two solvents. It has been reported that the morphology as well as the
dimensions of the grown crystals can be affected by changing the involved
solvents[36, 42], changing the solvent ratio (ratio of the poor solvent to good
solvent)[43-45] or growth temperature[46] during the growth. Growth
mechanism of C60 structures using LLIP method is explained more
extensively in next chapter as well as in paper III.
3. Carbon nanotubes
Carbon nanotubes are perhaps the most amazing member of carbon
nanostructures with exceptional properties such as high strength, even
higher than the strongest steel, high thermal conductivity and in particular
interesting electrical properties which varies from metallic behavior (with
high electrical conductivity) to large band gap semiconductors. CNTs are
divided into two main categories; single- and multi-walled carbon
nanotubes. A third sub-class that often also is differentiated from the single
and multi-walled carbon nanotubes is the double walled CNT, since it has
certain unique properties.[47] In the following sections I will introduce some
general aspects of single- and multi-walled CNTs.
3.1. Single-walled carbon nanotubes Although a SWCNT is simply defined as a rolled up graphene sheets the
resulting SWCNT can have different properties depending on the way that
graphene sheet is rolled. The main reason for this difference can be
explained by the unit cell of the formed SWCNT.
Figure 4. Schematic showing (A) chiral vector Ch, translational vector T and chiral angle θ
for (4,2) SWCNT in a graphene sheet, (B) the corresponding (4,2) tube, (C) and (D) zigzag and
armchair SWCNT. The unit cell of each tube is displayed in red color indicating different
number of carbon atoms per unit cell.
6
A SWCNT is defined by a vector in a 2D graphene sheet known as chiral
victor “Ch”, which goes along the complete circumference of the “opened”
SWCNT, as shown in figure 4. The chiral vector is defined in terms of
graphene unit cell vectors a1 and a2:
𝐶ℎ = 𝑛𝑎1 + 𝑚𝑎2 (1)
,where n and m are integers defining the number of unit vectors along a1 and
a2 directions.
Since the chiral vector forms the circumference of the SWCNT the
diameter of the tube d can be derived in terms of Ch and consequently
integers n and m:[48]
𝑑 = 𝐶ℎ
𝜋=
√3 𝑎𝑐𝑐(𝑚2+𝑛2+𝑚𝑛)1
2⁄
𝜋 (2)
In equation (2) acc is the carbon-carbon bond length in graphene sheet
(1.421 Å in graphite).[48] The angle between the Ch and the a1 direction is
called chiral angle (θ) and may take any value from 0 to 30 degree. Two
special cases are defined referring to chiral angles 0° or 30°. The tubes with
such angles are called zigzag or armchair tubes respectively, originating from
the zigzag and armchair pattern of the carbon atoms at the edges of the tube,
as shown in figure (C) and (D). The unit cell of a SWCNT can be visualized in
a 2D graphene sheet by the help of translation vector (T). The translation
vector is perpendicular to the chiral vector and connects a point in chiral
vector to the nearest equivalent point in the 2D graphene sheet. The area of
the rectangle defined by T and Ch (unit cell vectors of SWCNT in real space)
is equivalent to the unit cell of the corresponding SWCNT. The chiral vector
may have different length and direction, which in turn results in a large
number of possible SWCNTs structures. Since the structure and properties
of a SWCNT are fully recognized in terms of the integers n and m, usually a
SWCNT is denoted as (n,m) tube. As shown in Figure 4, the unit cell of each
SWCNT (indicated in red color) contains different number of carbon atoms.
In fact this is the main reason for diverse electrical properties of SWCNTs.
The electronic band structure of SWCNTs is extracted from the band
structure of 2D graphene. This is done by projecting the first Brillouin Zone
(BZ) of SWCNT to the first BZ of 2D graphene. Due to quantum confinement
of SWCNTs along its diameter (originating from the periodic boundary
conditions for the electron wave function along the circumference) the wave
vectors in reciprocal space take discrete value in the circumferential
direction. In contrast, for a tube with infinite length, the wave vector is
continuous in the direction along the tube axis. Therefore the allowed wave
vectors for SWCNTs generate a set of discrete lines, known as cutting lines,
7
in the reciprocal space of SWCNT. The cutting lines can be projected into 2D
BZ of graphene and consequently on its electron dispersion for valence and
conduction band as shown in figure 5. The electronic band structure of
SWCNTs is then defined as the intersection of the cutting lines with the
electron dispersion relation of 2D graphene.
Figure 5. (A) The cutting line projected on conduction and valence band of 2D graphene in
its first Brillouin zone for (4,2) tube. The electronic band structure (B) and density of states (C)
of (4,2) tube.[48]
The valence and conduction band of 2D graphene are degenerated (two
states with same energy, meaning that they will touch at certain points) at
the vertex of the BZ. Therefore, if one of the cutting lines passes through one
of the degenerated points, then the SWCNT exhibit metallic behavior,
otherwise it is a semiconductor with a band gap depending on where the
cutting lines lie on the BZ of 2D graphene.[48-50]
Another interesting and important property of SWCNT, as a 1D structure,
is the appearance of sharp singularities, known as van Hove singularities
(vHS), in its electronic density of states (DOS). Due to the vHS, SWCNTs
have strong response to optical absorption or emission for certain
wavelength. This will be explained more extensively in the resonance Raman
spectroscopy section.
3.2. Multi-walled carbon nanotubes A MWCNT is formed by a set of concentric SWCNTs with inter wall
distance of 3.4 Å.[51] In contrast to SWCNTs, which can have metallic or
semiconducting behavior the MWCNTs, are almost always metallic.
Advantages with MWCNTs are their relatively high conductivity, high elastic
modulus, high surface area and that their synthesis process is easier than
that for the SWCNTs.
(A) (B) (C)
8
3.3. Synthesis of carbon nanotubes The first and oldest known process for synthesizing CNTs is electric arc-
discharge process,[2, 52] a method which were already used for fullerene
synthesis.[15] A more advanced synthesis process, which still technically
resembles the arc-discharge process, is known as the laser pulse ablation
method.[53, 54] In fact it utilizes a high power laser beam instead of arc-
discharge to vaporize carbon. However both arc-discharge and laser ablation
techniques suffer from large amount of side products such as amorphous
carbon and carbon onions. So far the most promising technique to synthesize
large scale CNTs in more controlled manner is CVD.[55-58] Although the
CVD grown CNTs, in particular MWCNTs, have graphitic walls with lower
crystallinity [59] compared to the arc-discharge and laser ablation methods,
the possibility to mass synthesize CNTs at low temperature with high purity
make CVD a superior approach for CNT growth. In addition CVD involves
several controllable parameters to achieve desirable product such as growth
of vertically aligned CNTs[60] even on a pre-patterned arrays of
electrodes.[61, 62] In the following section the CVD method is explained in
detail.
3.3.1. Chemical vapor deposition Chemical vapor deposition is a catalyst-based method in which a carbon
precursor (for example a hydrocarbon gas: acetylene, methane or vapor of
alcohols) is catalytically decomposed at appropriate temperature inside a
reaction chamber (usually a quartz tube). A CVD setup is schematically
shown in figure 6. Transition metals such as Fe, Co, Mo, Ni or a mixture of
two are often used as catalyst material and act as a medium to transport
carbon atoms to carbon nanotubes at appropriate temperature. The catalyst
has to be in the form of nanoparticles and it can be applied either by
precipitation out of gas phase, as a powder or pre-deposited on substrates
such as silicon wafers, quartz plates or porous substrates.[60, 63, 64]
There are several experimental and theoretical attempts to find an
explanation for catalytic based growth of CNTs, and in fact scientists have
suggested several different growth mechanisms for CNTs depending on the
examined experimental conditions and growth methods.[65, 66] A generally
accepted growth mechanism for the CVD method can however be explained
by three main steps:[67, 68] i) catalytic decomposition of carbon precursor
over the surface of catalyst particles at appropriate temperatures, ii)
diffusion and dissolution of carbon atoms through or on the catalyst particles
to form a supersaturated solution, and iii) precipitation of carbon atoms in
tubular form which is energetically more favorable over graphitic sheets as
there are no dangling bonds in the tubular form.[59, 68]
9
It is well known that there is
a relation between catalyst
particle size and the diameter of
the growing CNT.[67-70] The
CVD process makes it possible
to effectively use this concept to
control the diameter of the
growing CNTs. This effect is
extensively explained in paper I.
In addition the possibility to
apply different gases like Ar and H2 as well as manipulating the pressure
inside the reaction chamber gives more opportunities to affect the final
product. For example by adding a source of different atoms such as nitrogen
or boron it is possible to dope the growing CNT by foreign guest atoms.
Doped CNT structures has rendered enormous interest due to the possibility
to fine tune CNT properties for various applications. The doping mechanism
will be explained in following section and paper II.
3.4. Doping of carbon nanotubes Doping of carbon nanostructure such as graphene and CNTs is a well-
known approach to tailor their electrical and optical properties as well as
their electrochemical activity. This is mainly due to different number of
valence electron of doping elements compared to carbon atom. The most
common candidates (chemical elements) for doping carbon nanostructures
are boron, nitrogen and phosphorus.[71-83] In this thesis I will focus on
nitrogen doped (N-doped) SWCNTs.
The nitrogen doping of CNTs and their applications have attracted
researcher’s interest within last decade.[84-88] One of the most fascinating
applications of N-doped carbon nanostructures is their implementation as
organic catalyst in energy conversion processes, such as oxygen reduction
reactions[89, 90] and water oxidation,[91, 92] opening up for the possibility
to utilize N-doped CNTs to make platinum free fuel cells.[12] In the case of
SWCNTs the nitrogen doping is more interesting due to the possibility for
altering their electrical properties, which would open an alternative route to
produce SWCNT samples with all having, for example, semiconducting
properties.
The most effective way to incorporate nitrogen in CNT structures is by
adding a nitrogen precursor during CVD growth of CNTs. For example in
addition to the carbon precursor a gas or solvent vapor containing nitrogen
atoms such as ammonia or pyridine can be used as a source of nitrogen
atoms in CVD process.[93]
Reaction
chamber Catalyst
Oven
Figure 6. Schematic of a CVD setup.
10
There are at least three recognized and well-defined types of nitrogen
functionalities in graphitic structures; quaternary (graphitic), pyridinic and
pyrrolic (in six or five member ring)[94, 95] as shown in figure 7.
Figure 7. Different types of nitrogen incorporation in CNT structure (A) quaternary, (B)
pyridinic, (C) pyrolic in six-member ring, (D) pyrollic in five-member ring.
The only type of nitrogen incorporation, which preserves the hexagonal
structure of the graphitic sheet, is the quaternary. In contrast the other two
types show deviation form hexagonal structure by creating vacancies and
pentagons. The MWCNTs typically can have more defective sites, sometimes
even leading to cap formation of inner walls (bamboo CNTs), and therefore
their nitrogen uptake can be as high as 15 to 20%.[96] In contrary the
nitrogen incorporation in SWCNTs is limited to about 3%.[13] Since
different types of nitrogen incorporation have different effect on electrical
and chemical activity of the doped structures[95] it is indeed important to
track the type of nitrogen incorporation in carbon nanostructures. So far the
most typical characterization techniques for N-doped graphitic structures
are based on analysis of binding energies by X-ray photoelectron
spectroscopy (XPS). However the reported binding energies for different
types of nitrogen incorporation in graphitic structures vary in a relatively
wide range in different studies; mainly from 398.1-399.3 eV for pyridinic,
399.8-401.2 eV for pyrrolic and 401.1-402.7 eV for quaternary nitrogen
functionalities.[84, 97, 98] In addition it has been theoretically calculated
that for N-doped SWCNTs, due to curvature, the nitrogen binding energies
can be different from the one in a flat graphitic sheet.[99] Therefore a
correlation of spectroscopy and atomic resolution microscopy or theoretical
calculation can help to improve the understating of nitrogen doping
mechanism as well as the nitrogen configuration in graphitic structure. The
nitrogen doping mechanism in SWCNTs is studied both experimentally and
theoretically in paper II.
11
3.5. Application and challenges Due to low cost, easy synthesis and possibility of large-scale synthesis,
CVD grown MWCNTs has already found their place in market. However they
are mainly used as a powder (unorganized) in bulk composites and thin films
to enhance the electrical and mechanical properties of the desired
structures.[100] In addition, the catalyst particle impurity in CVD grown
CNTs is a common problem for many applications. Nevertheless the
observation that the addition of CNTs as an active material has led to an
increase in performance in many commercially interesting applications such
as lithium ion batteries[101-103] and fuel cells electrodes[103] is likely to be
followed by commercial products.
In the context of this discussion it is interesting to point out that the
commercialization of SWCNTs are even more challenging. The first reason is
their synthesis condition, which require more precise control compared to
MWCNTs and consequently increase the cost for bulk synthesis. As
previously mentioned, the electrical properties of SWCNTs largely depend on
their structural parameters. Both metallic and semiconducting SWCNTs
have highly desirable properties for certain applications. In the case of
semiconducting SWCNTs a great interest lies in implementing them in
electronic circuits (field effect transistor)[104, 105] or photovoltaic devices.
However, typically all synthesis process of SWCNTs, including CVD, result in
a mixture of semiconductor and metallic SWCNTs with different diameter
and chiralities. Therefore large efforts have been made to selectively post
purify the SWCNTs for certain chirality or diameter.[106-110] In another
approach SWCNTs has been cut to several small pieces and each peace used
as a seed to grow SWCNT with the same chirality.[111] However the
difficulties of these methods, high cost and also the possibility that post
purification process affect the properties of the tubes, have encouraged
scientist to control the chirality of SWCNTs during the growth (selective
growth). Over last decade there have been large number of experimental and
theoretical works, studying the effect of different growth parameters on the
chirality of the grown SWCNTs, such as growth temperature,[112, 113]
catalyst composition,[114] catalyst supports,[113, 115, 116] carbon
precursor,[117, 118] catalyst pre-treatment,[119-121] pressure[122] and effect
of applying additional gases during the growth.[119] This extensive works
have led to great progress and in some advanced procedures; it is now
possible to grow SWCNTs with up to 90 to 95% selectivity of metallic or
semiconducting properties.[123] The effect of catalyst composition on the
chirality of the CVD grown SWCNTs is explained in paper I.
3.6. Carbon nanotube hybrid nanostructures
Hybrid nano-structures are of great interest due to possibility for
engineering new materials with tunable physical and chemical properties. An
12
interesting candidate to synthesize such hybrid materials is CNT. This is due
to its clean and homogenous quasi one-dimensional inner cavity with high
aspect ratio, which opens up for encapsulation of different guest material. In
addition, due to nanometer or sub nanometer distance between guest
material and graphitic walls of CNT, the two materials can interact with each
other and exhibit new properties for the hybrid material. Smith et al[124]
introduced such material by encapsulation of C60 in SWCNTs, which for
obvious reasons was named peapods, and thereafter similar structure were
reported for double- and multi-walled CNTs.[125, 126] Besides C60, several
other molecules or inorganic materials (for instance metals) can be
encapsulated in the inner cavity of the carbon nanotubes.[127-129] The
cylindrical confinement defined by CNT walls restricted the encapsulated
material to have special dimension and/or packing, which do not exist in
regular (non-capsulated) form.[125, 127, 130-132] In addition graphitic walls
of the CNT can protect the material from possible reaction (in particular
oxidation) with the surrounding environment.[132] Out of all possible
materials, encapsulation of fullerene and its derivatives has attracted most
attention during last two decades, perhaps since it opens up for numerous
fundamental studies on their structure and electronic properties. As an
example the molecular orbitals of encapsulated C60 can potentially modify
the electronic band structure of SWCNTs.[14, 133, 134]
3.7. Collapsed carbon nanotubes A carbon nanotube may experience a transition from tubular (inflated
state) to collapsed ribbon-like configuration due to the lack of energetic
stability. It has been theoretically proven that for a CNT with sufficiently
small diameter (<R1) only the inflated state is stable. However, above a
critical diameter R2 the collapsed state is stable and the inflated state is
metastable (both R1 and R2 are increasing functions of number of tube
walls).[135, 136] A third region is defined between R1 and R2 in which the
tube is stable in inflated state but metastable in collapsed state.[136-141] It
has been predicted that, in the bistable region, the two states could be
degenerated by applying thermal energy[142-144] or by charge transfer to
the collapsed tube.[137] Such transition can potentially make CCNTs
applicable for electrical-mechanical transducer devices.
The two main factors that define the stable configuration for a CNT are
curvature energy and the van der Waals interaction between opposing
graphene sheets. If the van der Waals force (for sufficiently large diameter
tube) overcomes the curvature energy the collapsed state is energetically
favorable. Therefore the collapse may be induced by mechanical
perturbation or by thermal fluctuations, which bring opposing walls together
and thereafter the tube collapses in a zipping fashion due to the van der
Waals force. Collapsed carbon nanotubes (CCNTs) are an analogy of
13
graphene nanoribbons (bilayer or multilayer) but with perfectly bonded
edge-atoms, which due to strain energy, two curved open channels are
formed along the two edges of the CCNT.[145]
CCNTs were first experimentally observed by Chopra et al. [135] in 1995
and later synthesized by either direct growth of large-diameter single- or
double-walled tubes, (which are prone to collapse)[146-149] or by solution-
based extraction of inner cores of MWCNTs through sonication.[150] The
extraction of the inner walls creates single- or few-walled tubes with very
large inner diameter compare to the parent CNT, which tend to collapse. The
later method has been proven to be a promising method for large fabrication
of CCNTs with different diameters and number of walls.
14
4. Experimental methods
4.1. Single-walled carbon nanotube growth SWCNTs were grown by CVD method using a mixture of iron and cobalt
as catalyst particles. In this section I will first describe the procedure to
prepare the catalyst particles and thereafter describe the CVD process to
grow SWCNTs on the prepared catalyst particles.
4.1.1. Catalyst particle preparation
Catalyst particles were deposited on silicon
(Si) wafer by using a custom-made dip-coating
setup (figure 8). A precise motor pulls the
substrate vertically by the help of a spindle. The
vertical motion of the motor can be controlled
from few millimeters per minute to maximum 10
cm/min by the help of a controller. A solution of
iron (III) nitrate and cobalt (II) nitrate in
ethanol (ethanol method) or a dispersion of
catalyst particles in acetone, also here a mixture
of iron and cobalt (acetone method) was used as
catalyst precursor. A piece of Si wafer
(approximate dimensions of 8 mm x 3.5 mm x
0.5 mm) was ultrasonically cleaned in an
acetone bath and thereafter treated by
ultraviolet ozone cleaner to produce a
hydrophilic surface. The cleaned substrate was
then dip-coated using catalyst precursor solution
or dispersion and the effect of precursor
concentration and withdrawal velocity on
catalyst particle size and distribution was
studied.
In the ethanol method the size of the catalyst particle on the Si wafer could
be controlled by withdrawal speed and solution concentration. On the other
hand in acetone method the iron (III) nitrate did not dissolve completely in
acetone and instead formed iron hydroxide particles with size up to several
micrometer. By adding hydrochloric acid (HCL 37%) to the dispersion
during sonication the size of the iron hydroxide particles were tuned. The
size of iron hydroxide particle in dispersion was investigated by dynamic
light scattering (DLS) and by AFM on dip-coated Si wafer. We concluded
that by using the ethanol method the catalyst particles formed on the silicon
wafer whereas in the acetone method the catalyst particle formed mainly
already in the dispersion. (paper I)
Figure 8. Photograph of
custom-made dip-coating
setup.
15
4.1.2. Chemical vapor deposition growth of pristine carbon
nanotubes
Prior to CVD process the dip-coated Si wafer was baked in air at 400 °C to
form metal oxide particle. The Si wafer was then placed in reaction chamber
in the CVD setup and heated up to 800 °C in argon atmosphere at ambient
pressure. The catalyst particles were then reduced by further heat treatment
by Varigon gas and finally the growth of multi- or single-walled CNTs
(depending on the size of the used catalyst particles) was achieved by
introducing carbon precursor (acetylene) to the reaction chamber. To avoid
any oxidation of the grown CNTs the product was cooled down to 170 °C in
argon atmosphere before taking the sample out from the reaction chamber.
The growth parameters used for synthesize of multi- and single-walled CNTs
are summarized in table 1 and 2 respectively (paper I).
Table 1. Growth parameters used for synthesize of MWCNTs
Temperature
[° C]
Time
[min]
Ar
[ml/min]
Varigon
[ml/min]
Acetylene
[ml/min]
Heating R.T – 800 Ap. 30 180 0 0
Pretreatmnt 800 10 125 50 0
Growth 800 25 125 50 3.8
Cooling 800-170 - 180 0 0
Table 2. Growth parameters used for synthesize of SWCNTs
Temperature
[° C]
Time
[min]
Ar
[ml/min]
Varigon
[ml/min]
Acetylene
[ml/min]
Heating R.T – 800 Ap. 30 200 0 0
Pretreatment 800 10 140 60 0
Growth 800 15 140 60 3.8
Cooling 800-170 - 200 0 0
4.1.3. Growth of nitrogen doped single walled carbon nanotubes
The catalyst particles prepared by ethanol method were employed also for
the growth of N-doped SWCNTs. The growth parameters are same as table II
except that in the growth step, in addition to the acetylene gas, ammonia gas
(as a nitrogen precursor) was introduced into the reaction chamber. While
the flow rate of acetylene was kept constant, the flow rates of ammonia gas
was changed between 10 to 36 ml/min, in five different experiments, in such
a way that the total ammonia content in the reaction chamber (ammonia
feed) was 0, 3.6, 6.5, 9.5, and 13.5%. The nitrogen incorporation in the
grown SWCNTs was then studied regarding the type and total concentration.
(paper II)
16
4.2. Synthesis of one-dimensional fullerene structures
4.2.1. C60 Nanorods
C60 one-dimensional structures were
synthesized by Liquid-Liquid interphase
method (LLIP). In all experiments, prior to
use, C60 powder was degassed at 150 °C in
vacuum for 12 h. In a typical experiment 10
ml of ethanol was gently added to 1 ml
solution of C60 in m-dichlorobenzene (m-
DCB, 1 mg/ml) in a way that a clear
interface was defined between the two
phases, as shown in figure 9(A). Thereafter
the solution was sonicated using power
setting 1 or 2 in ultrasonic bath, (USC300D
from VWR) in such a way that a third
yellowish phase was appeared in the vial, as
shown in figure 9(B). The blend solution
was then stored at room temperature for 3-
7 days before further characterization or use. The synthesized structures
were then characterized regarding diameter distribution, crystal structure,
solvent incorporation as well as their electrical properties (paper III and IV).
4.2.2. Water assisted growth of C60 nanorods and tubes
Water was used as an additive to the ethanol in the LLIP growth of C60
one-dimensional structures and its effect on the size and morphology of the
grown structure was studied (paper III). In a typical experiment 10 ml of
ethanol was mixed with distilled water (with the desired volume ration of
water to ethanol) and the mixture was gently added to 1 ml of C60 solution in
m-DCB (concentration of 1 mg/ml). The blend solution was then stored at
room temperature for one week. Five different volume ratio of water to
ethanol; 0:100, 2:100, 5:100, 10:100 and 20:100 were used in this set of
experiments and the grown structure were examined and compared with
respect to their size and morphology.
4.2.3. Photo- polymerization of C60 nanorods
The aim of photo-polymerization of C60 nanorods was to decrease their
solubility in order to be able to implement them in applications, which
involve solvents that weakly could dissolve C60. In order to have a
homogeneous polymerization of C60 nanorods we introduced a solution
based photo-polymerization method. In a typical experiment a dispersion of
the synthesized nanorods in the original blend solvent (mixture of ethanol
Figure 9. Interface between
ethanol and C60 solution in m-DCB (A)
before and (B) after sonication.
17
and m-DCB ratio if 1 to 10) was exposed to a green laser beam (wavelength of
532 nm and power density = 15 mW/mm2) for 20 h.
Figure 10. Schematics showing the set-up used for photo-polymerization.
As schematically shown in figure 10 the vial, containing 1 mg nanorods in
11 ml blend solvent, was vertically fixed and the laser beam was directed onto
the bottom of the vial by using a mirror. The spot size of the laser was
adjusted to be equal to the inner diameter of the vial by using optical lenses.
The vial was covered by aluminum foil (except at the bottom surface) in
order to trap the scattered laser beam inside the vial and allow for an
efficient photo-exposure (paper IIV).
4.2.4. Functionalization of C60 nanorods with Pd nanoparticles
In order to examine the properties of C60 nanorods as a support for metal
nano-catalyst in electrochemical reaction the as-grown, annealed (will be
explain in result section) and photo-polymerized C60 nanorods were
decorated by Pd nano-particles. The Pd nano-particles were prepared by the
so-called phase transfer method.[9] In a typical experiment sodium
tetrachloropalladate (II) in dimethyl sulfoxide (DMSO), concentration of 1.4
mg/ml, was reduced by hydrazine. The reduced Pd particles were
functionalized by addition of 25 ml of toluene containing 0.25 ml phenyl
mercaptan while the mixture was stirred for 1 h. The DMSO and phenyl
mercaptan were removed from the mixture by adding mixture of water and
ethanol. Thereafter the benzyl mercaptan functionalized Pd (BM-Pd)
nanoparticles were transferred to toluene phase. The BM-Pd nanoparticles
were dried and re-dispersed in ethanol (1 mg/ml). Thereafter 1 ml of the
prepared MB-Pd particle dispersion was added to a 3 ml of C60 nanorod
dispersion in ethanol (1 mg/ml).
4.3. Synthesis of C60/CCNT hybrid structures The CCNTs were synthesized through sonication of MWCNT
dispersion.[150] Prior to sonication the caps of highly crystalline arc-
Al-foil wrapping
Mirror
Lens
20 h
18
discharge grown MWCNTs were removed by thermal oxidation for 30 min at
700° C in an Argon/Oxygen (ratio of 1 to 4) environment. Extraction of inner
walls of uncapped CNTs and C60 intercalation in CCNTs were performed
either in a single step or in separate sonication steps. In the first approach
the heat-treated MWCNTs were mixed with C60 with mass ratio of 1:1 or 1:3.
Thereafter the C60/MWCNTs mixture was dispersed in n-hexane (with a C60
concentration of 0.3 mg/ml) using an ultrasonic sonicator for 1 to 3 hours.
The amount of solvent was tracked during sonication and fresh solvent was
added as necessary. The sample was then collected by filtration and washed
by toluene to remove the free C60 molecules from the surface of CCNTs. The
collected material was dispersed in 1% weight per volume solution of Sodium
Dodecyl Sulfate (SDS) in water and sonicated for 30 min. The dispersion was
centrifuged for 1 h at 20,000g. The supernatant was mixed with methanol
and the precipitated material was collected. In the second approach the
uncapped CNTs were first dispersed in 1% weight per volume solution of
SDS in water and sonicated for 1 h to synthesize CCNTs. The CCNTs were
then separated by centrifugation and filtration followed by overnight heat
treatment in a vacuum oven at 200° C to remove the residual solvent in the
CCNTs. The insertion of C60 in CCNTs was performed in the same manner
described above by dispersing and sonication of CCNT and C60 mixture in n-
hexane.
19
5. Characterization methods
5.1. Atomic force microscopy Atomic force microscopy (AFM) monitors the topography of a surface by
scanning the force exerted between a sharp tip, attached to a cantilever, and
the surface of the sample. A laser beam is focused on the cantilever and
reflected on a photodiode as shown in figure 11. The photodiode tracks the
deflection of laser beam during scanning and consequently the deflection of
cantilever caused by interaction between tip and surface of the sample.
Figure 11. Schematic showing the principle of operational for an AFM.
There are three different operation modes for AFM:
1. Contact mode
2. Non-contact mode
3. Tapping mode
In contact mode the tip is brought close enough to the surface of the
sample so that the repulsive force dominates the tip-sample interaction. In
contrast, in non-contact mode, the cantilever oscillates with low amplitude
near its resonance frequency and the tip is brought to the surface of the
sample where the attractive force dominates the tip-sample interaction.
Similar to non-contact mode the tapping mode also utilizes an oscillating
cantilever but here the dominating tip-sample interaction is the repulsive
force and with this technique the tip constantly touches the surface of the
sample.
5.2. Transmission electron microscopy The limitation of optical microscopy, which is imposed by the wavelength
of the visible light, was a great motivation for scientist to replace the photons
by another entity with wave-particle duality nature, but with shorter
wavelength. Following the work by Louis de Broglie, in 1925, who
theoretically proved that electrons have wave-like nature, electrons became
the best candidate for more precise microscopy. After massive work by Knoll
20
and Ruska in 1932, the first electron microscope became commercially
available in 1939. The innovation of electron microscopy had great influence
on material science research and Ruska was therefore awarded the Nobel
Prize in 1986 for his invention. The principal of a transmission electron
microscopy (TEM) is close to the optical microscope, however in TEM a
beam of electrons (instead of photons) from an electron gun is focused on
the sample by using different electromagnetic lenses (instead of optical
lenses) and apertures.
Figure 12. (Right) A photograph of transmission electron microscope and (left) a schematic
of different parts showing the principle of imaging.
The main parts of a TEM are the electron gun, magnetic lenses, apertures,
sample holder, projection screen and CCD camera as shown in figure 12. The
magnetic lenses and apertures guide the electrons with particular
wavelength to the sample. The electrons will scatter by the sample as a
function of both thickness and mass of the sample. Thicker region of the
sample or higher mass elements (with higher atomic number) more strongly
scatter electrons off the axis of the incoming beam and consequently less
number of electron will projected on the screen from thicker or higher mass
regions. This causes different contrast on the screen and is known as mass-
thickness contrast image formation. The image in TEM can also form by a
phase difference between the scattered electron waves, however such an
analysis requires a coherent beam. The phase difference includes
information about the atomic mass, thickness and even orientation of
different part of the sample. Besides imaging, TEM can also be used to study
the crystal structure of the sample by analyzing the diffracted electrons, as
well as studying the elemental composition of the sample by tracking the
energy of the scattered electrons.
21
In TEM the wavelength of electrons depends on their energy and
theoretically a 100 eV electron beam has enough short wavelength to resolve
an atom. In practice however, there are other parameters, which limit the
resolution of the TEM, in particular imperfect magnetic lenses which cause
astigmatism of electron beam. Therefore in recent decades there was a great
effort to improve the quality of the magnetic lenses as well as designing
different correctors to improve the resolution of the TEM.[151]
5.3. Scanning electron microscopy Scanning electron microscopy (SEM) utilizes a highly focused electron
beam, which scans the surface of the desired sample. The electrons interact
with the sample and generate different signals such as; secondary electrons
(due to ionization of atoms in the sample), back-scattered electrons (the
reflected electrons) and transmitted electrons. The secondary electrons,
coming from the ionized atoms at the surface, are used to create high-
resolution images. The intensity of the back-scattered electrons depends on
the atomic number of the atoms and can thereby be used for elemental
analysis of the sample. The SEM imaging (secondary electron imaging) is
mainly used to study the topography of the sample and is more efficient for
conductive material, since charge accumulation in insulators disturb the
imaging. The SEM can also work in transmission mode in which the
transmitted electrons are used for imaging.
5.4. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) is widely used for surface analysis
of a sample in terms of its chemical composition. In XPS the sample is
irradiated with monochromatic X-rays in an ultra-high vacuum chamber.
The X-rays ionized the surface atoms and electrons from the core orbital
(inner shell) are emitted with specific kinetic energies. The binding energy of
the emitted electrons (known as photoelectrons) can be measured as the
difference between the X-ray energy and kinetic energy of the
photoelectrons. The XPS counts the number of photoelectrons with specific
energy by using an electron energy analyzer. Since every element has a
particular binding energy associated with each core atomic orbital, every set
of peaks in a photoelectron spectrum can be related to a specific element in
the sample. In addition the intensity of the peaks are related to the
concentration of the elements in the sample.
5.5. Raman spectroscopy Raman spectroscopy is based on inelastic scattering of light, known as
Raman scattering after it was first observed experimentally by Sir
Chandrasekhra Venhata Raman in 1928. Fundamentally, when photons
interact with the molecules through an inelastic scattering the scattered
22
photons have either slightly higher or slightly lower energy than the incident
one. The change in photon’s energy is related to the absorption or release of
a phonon with energy equal to the energy difference between two vibrational
states of the molecules. In a Raman spectrometer a sample is exposed to
photons with certain frequency (from a laser source) and thereafter Raman
spectrometer measures the change in frequency of the scattered photons
with respect to the incident photons. Traditionally this change is represented
as change in wave number in Raman spectra. If Raman scattering results in
transition from lower vibrational state to the higher state, by absorbing a
phonon, the process called Stokes scattering, in contrast if it results in
transition from higher vibrational state to the lower one, by releasing a
phonon, the process is called anti-Stokes scattering, the two process are
schematically shown in figure 13.
Figure 13. Schematic showing the Stokes and anti-Stokes scattering in Raman scattering
accompanied by absorbing and releasing a phonon respectively.
Since molecules are usually at the ground vibrational state at ambient
temperatures, the probability of Stokes scattering is higher than that of anti-
Stokes scattering and usually Stokes scattering is monitored in Raman
spectrometers. In fact the Raman scattering by itself is a process with very
low probability and only one photon out of about 106-108 photons results in
Raman scattering.[152] However the situation is different for SWCNTs due
to appearance of vHSs in their DOS. If the energy of the incident photons
matches with the energy difference between two vHSs (revealing an allowed
optical transition) then the Raman process will significantly enhanced and
the process is called incident resonance Raman scattering. Similar resonance
condition may happens if the energy of the incident photons plus the energy
of a Raman active phonon matches the energy difference between vHSs, this
process is called scattered resonance Raman scattering.[153]
5.5.1. Raman spectroscopy of SWCNTs
Since SWCNTs satisfy the condition for resonance Raman scattering a
relatively low power of excitation source and short measurement time results
in strong Raman signals. In addition several Raman futures of SWCNTs
make it possible to gain different information about the examined sample
23
such as; purity of the sample, defects (imperfection of hexagonal structure
and presence of guest atoms), chirality and diameter (and consequently
metallic or semiconductor nature of the tube) and charge transfer. Thus
Raman spectroscopy is known as a nondestructive, relatively simple, and
powerful technique to characterize SWCNTs. The main features in Raman
spectrum of SWCNTs are: the tangential mode (G-band), defect active mode
(D-band), G’-band and radial breathing modes (RBM) which give useful
information about electronic properties, structural defects, charge defects,
diameter and chirality of SWNTs, these features are shown in figure 14.[48,
154, 155]
Perhaps the most
interesting feature in Raman
spectrum of SWCNT is the
RBM, which appear between
120 to 360 cm-1. The
vibrations of carbon atoms
along the radial direction of
SWCNT are responsible for
these features. The frequency
of RBM (ωRBM) has been
found to be inversely
proportional to the diameter
of the tube (dt) through the
following empirical equation:
𝜔𝑅𝐵𝑀 =𝐴
𝑑𝑡+ 𝐵 (3)
in which A and B are experimentally determined parameters. For an isolated
SWCNT the parameter B is zero; however in SWCNT bundles, where tubes
interact with each other, the parameter B is considered as a correction in
equation (3) for tube-tube interaction. By using ωRBM (tube diameter) in
combination with Raman excitation energy (photon energy) one can
determine the chirality of the tube and consequently the metallic or
semiconductor nature of the examined tube. However since the gap between
vHSs (from valence to conduction band) in DOS of SWCNTs depends on the
diameter of the tube it is important to have the right excitation energy to
satisfy the resonance Raman condition for each SWCNT. Thus the best way
to characterize a sample is to use different Raman excitation energy to track
all SWCNTs in the examined sample.
The G-band is a first order Raman scattering (involves one phonon
scattering) and its frequency is charge sensitive which gives the possibility to
track the total charge of the SWCNTs, for instance in studying doped
G-b
an
d
D-b
an
d
RB
M G
’-b
an
d
Inte
ns
ity (
a.
u.)
Raman Shift (cm-1
)
Figure 14. Typical Raman spectrum of
SWCNT sample, different features such as D- G-
G’- Bands and RBM are shown in the spectrum.
24
SWCNTs.[48] The D-band and G’-band are second order Raman scattering
(involves two phonon scattering). The D-band is sensitive to defects in
graphitic structures and its relative intensity to G-band usually used to
determine the crystallinity of the sample. The G’-band is also charge
sensitive and its shift can be used to compare the total charge of two sample
with similar carbon nanotubes (same diameter and chirality). The practical
use of RBM, D-band and G’-Band are discussed in paper I and II.
5.6. X-ray diffraction X-ray diffraction (XRD) is the most powerful method to study crystal
structure of materials in the form of single crystal or a fine powder. X-rays
consist of high-energy photons with small wavelength, comparable to the
size of atoms. Therefore when a material is irradiated by x-ray the scattered
photons carry information about the atoms and the constrictive or
destructive interference of the scattered photons from different atoms can be
used to gain information about the lattice spacing and in general crystal
structure of the material.
5.7. Thermogravimetric analysis The idea behind thermogravimetric analysis (TGA) is quite simple; the
mass of the sample is monitored as it is heated up (to the desired
temperature) in a closed environment. The TGA utilizes a high-precision
balance and usually few milligrams of material is enough for complete
analysis. The test can be done in different environment such as an inert gas
(like Ar) or reactive gases such as oxygen or hydrogen for instance, to check
the mass change due to evaporation, oxidation or other reactions.
5.8. Dynamic light scattering Dynamic light scattering (DLS) also known as photon correlation
spectroscopy (PCS) and quasi-elastic light scattering (QELS) technique can
be used to measure the size of the particles (down to 1 nm) suspended in a
liquid. The basic idea is to shine a monochromatic light such as a laser to a
dispersion containing particles and detect the scattered light at different
angle. The particles in the suspension are in Brownian motion and particles
with different sizes have different motion. The DLS uses this concept to
measure the size of the particles.
25
6. Results and discussions
6.1. Catalyst particle preparation for carbon nanotube growth
This section is based on the result of study presented in paper I.[156]
As described in section 4.1.1 the catalyst particles were prepared on Si
wafer by dip-coating method and the catalyst precursor (solution containing
iron and cobalt ions) was prepared using two different methods; the ethanol
and the acetone methods.
6.1.1. Analysis of size and distribution of catalyst particles
When a perfect solution of iron (III) nitrate and cobalt (II) nitrate is used
for dip-coating (ethanol method) the size and distribution of the catalyst
particle on Si wafer is affected by solution concentration and withdrawal
velocity. Figure 15(A) displays an AFM image of a typical substrate coated
with iron/cobalt catalyst particles. The analysis of AFM images indicate that
close to the edges of the substrate the catalyst particle size distribution was
less homogeneous, which is in line with previous studies.[157] In addition
the size distribution is more homogenous at the middle part of the coated
region (indicated as zone 2 in figure 15(B)) compared to the other two parts.
Therefore zone 2 of each prepared substrate was always used for further
analysis and for CNT growth.
Figure 15. (A) A typical AFM image of catalyst particles on the dip-coated substrate. (B)
Schematic showing the dimensions of the coated region (shaded region) which is divided into
three equal zones. (C) Average particle diameter versus withdrawal velocity.
As demonstrated in figure 15(C) the average particle size on the substrate
increases with increasing withdrawal velocity and/or solution concentration.
It is also found that the lower withdrawal velocities result in more
homogenous catalyst particle distribution on the substrate, thus it is more
reasonable to increase the size of the catalyst particles by increasing the
solution concentration rather than the withdrawal velocity.
8 mm
Zone 3
Zone 1
Zone 2
3 cm
(B)
Avera
ge
Dia
mete
r (n
m)
Withdrawal Velocity (cm/min)
(C) (A)
26
When acetone is used as a solvent the cobalt (II) nitrate is completely
dissolved in acetone, whereas iron (III) nitrate do not dissolve completely
but rather forms a suspension of iron hydroxide particles with average size of
several micrometers. The iron hydroxide particles can be, partially or
completely, dissolved by adding hydrochloric acid in a controlled manner,
while the suspension is sonicated. This effect was used to manipulate the size
of the catalyst particles in the suspension. To facilitate further discussion the
RHCl/SP is defined as the volume ratio of the HCl to suspension containing
catalyst particle (when the concentration of iron (III) nitrate and cobalt (II)
nitrate was 12.5 mg/ml). The size distribution of iron hydroxide particles in
the suspension was determined by DLS technique.
Figure 16. DLS evaluation of particle size in the suspension (prepared by mixing iron (III)
nitrate and cobalt (II) nitrate in acetone) for two different values of RHCl/SP. (A) and (B)
particle size versus intensity of the scattered light, (C) and (D) particle size versus volume of the
particles.
The graphs in figure 16(A) and (B) show the intensity of the scattered light
versus the particle size for two different value of RHCl/SP. Since larger
particles scatter more light the intensity of the scattered light does not give
quantitative information regarding the number of particles with specific size.
Therefore for further quantitative analysis the volume of particles versus
particle size is presented in figure 16(C) and (D). It is clear that as RHCl/SP
increases the average particle size become smaller. In fact higher value of
RHCl/SP (≥ 2 × 10-3), for given concentration of catalyst precursor, completely
dissolve the iron hydroxide particles and results in perfect solution. It should
be noted that in this method the catalyst particle were mainly formed in
solution phase and by the dip-coating these particles were “fished up” and
deposited on the substrate. Furthermore a combination of iron hydroxide
Inte
nsit
y (
%)
RHCl/SP
=1.25x10-3
(A)
RHCl/SP
=1.5x10-3
(B) (b)
Size (d. nm)
Vo
lum
e (
%)
(D)
Size (d. nm)
(E)
27
particle and cobalt ions in acetone results in slightly different catalyst
composition on the dip-coated substrate. Such that the ratio of iron to cobalt
on substrate, dip-coated by using a perfect solution is close to 1 (0.99 ± 0.04)
and increases to 1.12 ± 0.03 for a substrate dip-coated by using a suspension
containing iron hydroxide particle and cobalt ions (RHCl/SP= 1.5 × 10-3). The
above methods and the resulting catalyst particles are discussed more
extensively in paper I.
6.2. Carbon nanotube growth By using the methods discussed in
previous section catalyst particles
with different sizes and
compositions were used to grow
carbon nanotubes. In general
catalyst particles with average size
larger than 10 nm results in the
growth of MWCNTs. Figure 17
displays TEM image of MWCNTs
grown on catalyst particles with
average size of 40 ± 5 nm resulting
in tubes with average diameter of
21± 7 nm. This was an effect
observed in different samples,
meaning that the average diameter
of the grown tubes were consistently
40 to 50% smaller than the average size of the used catalyst particles. We
rationalize this effect by a combination of change in the catalyst particle size,
due to evaporation of residue solvents at high temperature as well as a
stretching of the catalyst particles inside the grown MWCNT, as shown in the
insets of figure 17. The latter effect is in agreement with in situ observations
reported earlier.[158]
By using catalyst particles with smaller average diameter the growth of
few-walled carbon nanotubes were observed and further decrease of catalyst
particle size to below 10 nm results in the growth of SWCNTs. Figure 18(A)
displays the high frequency Raman spectra of three SWCNT samples
(recorded by using 633 nm Raman excitation wavelength) grown on catalyst
particles prepared by three different methods. The EtOH denotes the sample
grown on catalyst particle prepared by ethanol method (perfect solution, see
section 6.1). AC(m) and AC(h) denote the samples grown on catalyst particle
prepared by acetone method with RHCl/SP= 1.5 × 10-3 (see figure 16 (B) and
(D)) and high amount of HCl (perfect solution) respectively. The RBMs of the
samples, recorded with three different Raman excitation wavelengths are
also presented in figures 18(B) to (D). For comparison all spectra in figure
Figure 17. TEM image of the grown
MWCNTs the insets show the catalyst
particle starching inside the grown
MWCNTs.
28
18(A) are normalized with respect to the G-band intensity. Since the D-band
is a defect related Raman feature the ratio of integrated are of G-band to D-
band is frequently used as degree of crystallinity.[159-161] The small D-band
in AC(h) and in particular in EtOH spectra suggests a high purity SWCNT
sample, free of amorphous carbon and MWCNTs.[154, 162-164] However the
D-band is more intense and broader in spectrum AC(m), when iron
hydroxide particles were involved in catalyst particle preparation.
Figure 18. (A) High frequency Raman spectra of three different SWCNT samples (recorded
by 633 nm Raman excitation wavelength) grown on catalyst particles prepared with ethanol
method (denoted by EtOH), acetone method with RHCl/SP= 1.5 × 10-3 (denoted by AC(m)) and
acetone method with high amount of HCl (perfect solution) (denoted by AC(h)). (B) to (D) RBM
frequencies of three samples recorded by three different Raman excitation wavelengths.
This is due to the presence of a relatively few number of catalyst particles
with larger enough diameter to nucleate the growth of MWCNTs (see DLS
measurement in figure 16(D)). Interestingly a comparison of the RBM of
three samples reveals that the sample prepared by iron hydroxide particles
shows higher relative abundance for four specific chiralities; (7,5), (8,5),
(12,1) and (12,0) compared to other two samples. This evaluation was based
on the integrated area of the RBMs peaks, which has been frequently used
Inte
ns
ity (
arb
. U
nit
)
(A) (B)
Inte
ns
ity (
arb
. U
nit
)
Raman Shift (cm-1
)
(C)
Raman Shift (cm-1
)
(D)
29
for quantitative analysis of SWCNTs in the sample.[165-167] This effect can
be rationalized by difference in catalyst particle composition. As discussed in
previous section the implementation of iron hydroxide particles in catalyst
particle preparation results in slightly different ratio of iron to cobalt
compare to the catalyst particles prepared by perfect solution of iron and
cobalt ions. Ding et al.[168] have theoretically shown that the growth of
SWCNTs is dependent on the adhesion force between catalyst particle and
carbon atoms, so that too high or too low adhesion force terminates the
growth of SWCNTs. On the other hand the adhesion force affects the number
of carbon atoms at the interface between catalyst particle and growing tube
and consequently it dictates the chirality of the growing SWCNT.
6.3. Growth and characterization of nitrogen doped single-
walled carbon nanotubes
This section is based on the result
of study presented in paper II.[13]
The catalyst particles prepared by
ethanol method (section 6.1) was used
to grow N-doped SWCNTs and
ammonia gas (section 4.1.3) was used
as nitrogen precursor in reaction
chamber. Figure 19 shows Raman
spectra of five different samples
(recorded by 633 nm Raman
excitation wavelength) when the
ammonia content in the reaction
chamber (ammonia feed) was 0, 3.6,
6.5, 9.5 and 13.5%. As the ammonia
feed increases the integrated area
under the D-band increases, indicating a lower crystallinity due to an
increase in amorphous carbon or growth of MWCNTs. The decrease in
crystallinity is more pronounced at highest ammonia content. In addition at
highest ammonia content the intensity of the RBMs decreases significantly
as compared to the intensity of the Si peak (substrate) denoted by asterisk in
the spectra. We rationalize this effect by a decrease in number of grown
SWCNTs on this sample; a conclusion that is further supported by
theoretical calculation and will be explained later in this thesis.
6.3.1. Analysis by X-ray photoelectron spectroscopy
In order to track the nitrogen incorporation in the grown SWCNTs all
samples were examined by XPS. Figure 20(A) and (B) indicate typical high-
resolution N 1s and C 1s spectra of the synthesized SWCNTs deconvoluted by
five- and two-model peaks respectively. The first peak in N 1s spectrum at
Raman Shift (cm-1
)
Inte
ns
ity (
a.
u.)
633 nm
Figure 19. Raman spectra of five
different SWCNT samples where the
ammonia feed in the reaction chamber
was 0, 3.5, 6.5, 9.5 and 13.5%.
30
398.4 eV is assigned to the nitrogen atoms in pyridinic structures and the
second peak at 400.9 eV can be attributed to pyrrolic nitrogen incorporation.
As shown in figure 20(C) by increasing ammonia feed in the reaction
chamber the total nitrogen content in the grown SWCNTs increases. At 3.2%
the nitrogen content however reaches a maximum and thereafter a further
increase in ammonia feed lead to a decrease in nitrogen content of the grown
SWCNTs. We also observed (comparing the N 1s signals for the five different
samples) that at low nitrogen content the pyridinic nitrogen dominate over
pyrrolic one. By increasing the nitrogen content the ratio of the two types
reaches almost to one (where the total nitrogen content has its maximum
value) and thereafter the pyrrolic type is dominating.
Figure 20. Typical high resolution of (A) N 1s peak, deconvoluted by a two-peak model. (B)
C 1s spectrum of the sample deconvoluted by a five Gaussian peak-model. (C) Total nitrogen
content and integrated area of the peaks corresponding to pyridinic and pyrrolic like nitrogen
incorporation as a function of ammonia feed. (D) Integrated area of three peaks at 284.5, 285.8,
and 287 eV versus ammonia feed.
It should be noted that there are some inconsistences regarding the
binding energy of pyrrolic nitrogen in graphitic structure (in particular in
SWCNTs) and the reported binding energy for pyrrolic and quaternary
nitrogen to some extent overlap with each other.[84, 97-99] In SWCNTs the
situation is even more complicated, where the nitrogen binding energy is
Binding Energy (eV)
Inte
ns
ity (
a.
u.)
(B) 284.5 eV
285.8 eV
287 eV
289 eV
290.5 eV
Binding Energy (eV)
Inte
ns
ity (
a.
u.)
Pyridinic
Pyrrolic
(A)
Ammonia content (%)
No
rmalized
are
a (
%)
(D)
Ammonia feed (%)
Nit
rog
en
co
nte
nt
(%) (C)
31
shown to be chirality dependent.[99] In our analysis we used the features in
C 1s spectra of the examined sample as well as theoretical calculations (will
be explained later in the text) to support our assignment of the 400.9 eV
peak to pyrrolic nitrogen. The peak at 284.5 eV in C 1s spectrum is attributed
to carbon atoms in sp2 configuration and the peaks at 285.8 eV and 287 eV
are frequently ascribed to carbon atoms in defective regions and carbon
atoms bonded to nitrogen atoms [84, 97, 98] as well as to carbon atoms with
more pronounced sp3 configuration.[169] By analyzing the intensity of the
peaks in C 1s spectra (figure 20(D)) we find that as the nitrogen content
increases the carbon atoms with sp2 nature decrease and in contrast the
signs of defective regions and sp3 carbon increases. On the other hand the
pyrrolic nitrogen in five member ring induces curvature to the structure of
SWCNTs and consequently increase the admixture of both sp2 and sp3
hybridization as well as more structural defects in SWCNTs structure. The
increase in sp3 nature as well as increase in D-band intensity at high
ammonia feed can therefore be explained by the increase in pyrrolic like
nitrogen incorporation.
6.3.2. Theoretical modeling of pyridinic and pyrrolic nitrogen
In order to explain the limit in nitrogen uptake by grown SWCNTs and to
study the effect of nitrogen atoms on the growth properties of SWCNTs we
utilize theoretical modeling for nitrogen incorporation in SWCNT structures.
We consider four different SWCNTs for this part of study: two metallic tube;
(12,0) and (8,5) and two semiconductor ones; (7,5) and (12,1).
Figure 21. Final geometry of (8,5) tube; (A) pristine and N-doped with pyridinic nitrogen
with total concentration of (B) 1.2%, (C) 3.0%, (D) 4.3%, (E) 5.5%.
The theoretical calculation shows that the pyridinic nitrogen can be
introduced to the SWCNTs structure with concentration higher than the
(A) 0% (B) 1.2% (C) 3.0% (D) 4.3% (E) 5.5%
32
experimentally observed one (3.2 %) while the tube still preserves its tubular
form. Figure 21 presents the result for (8,5) tube for different concentration
of pyridinic like nitrogen (up to 5.5 %). Although the nitrogen atoms break
the tube symmetry and introduce corrugation to the tube structure the
nitrogen defects are stable and there is no signs of unfavorable configuration
that may lead to instability of the SWCNT structure. Similar results were
obtained for other examined SWCNTs.
The situation is different for pyrrolic nitrogen. To obtain more precise
result the pyrrolic nitrogen were introduced in the (12,0) tube first as
periodic system where there is no free edges or dangling bond at the two
ends of the tube. The calculation indicated that the pyrrolic nitrogen in
SWCNT wall is unstable and after geometrical optimization they transfer to
pyridinic nitrogen even when the carbon atoms are passivated with hydrogen
atoms. The initial and final structures for (12,0) tube are shown in figure
22(A) and (B).
Figure 22. (A) (12,0) tube, as a periodic system, initially doped with pyrrolic nitrogen and
its final structure after geometrical optimization. (B) Same as (A) but with passivated carbon
atoms. (C) Initial and final structure of (12,0) tube doped with pyrrolic nitrogen when the
system is non-periodic.
In contrary in non-periodic system, where tube has edges and has the
possibility to bend or form cap, the pyrrolic nitrogen are stable at the edges
of SWCNT and either form caps or bend the tube by creating pentagonal
rings, as shown in figure 22(C). The above results explain the experimentally
observed limit of nitrogen uptake in SWCNTs. At high ammonia feed the
probability to form pyrrolic nitrogen increases (experimentally when pyrrolic
is dominated over the pyridinic nitrogen) and consequently terminate the
growth by cap formation. Due to termination of SWCNT growth the
probability to form amorphous structure in the sample increases. This is in
line with the result of Raman measurement (figure 19) where the intensity of
the RBMs decreases significantly at highest value of ammonia feed and a
large D-band appears in the Raman spectrum. Since nitrogen atoms in
(A) (B) (C)
33
amorphous material tend to pair and leave the material as N2 [170] the total
nitrogen content of the sample at high ammonia feed decreases.
6.3.3. Radial breathing mode analysis
To study the effect of nitrogen incorporation on chirality distribution we
performed a quantitative analysis of different chiralities by relating the
integrated area of each peak in the RBMs frequencies to the abundance of
the corresponding tube. For better statistics the RBM frequencies were
collected from 50 different spots of each sample and the averages were used
for analysis. Three different Raman excitation wavelengths (488, 633 and
785 nm) were used to collect signals from most of the grown SWCNTs as
shown in figure 23.
The RBM analysis indicate that as the ammonia feed increases the growth
of some of the SWCNTs are hindered while few other tubes remain stable. In
other words the nitrogen incorporation narrows the chirality distribution in
the sample. In contrast to previous reports we see however no evidence that
nitrogen incorporation promote the growth of small diameter tubes.[71, 161,
171-173] To explain the lower probability of SWCNTs with certain chirality at
high ammonia feed we propose two different possibilities. The first
Figure 23. Integrated area of
RBMs for five different samples,
normalized to the total RBM area for
specific Raman excitation wavelength,
recorded by (A) 488, (B) 633 and (C)
784 nm Raman excitation wavelength.
The arrows indicate the RBM, which
increase or decrease as ammonia feed
increases.
Raman Shift (cm-1
)
Inte
gra
ted
are
a (
%)
Diameter (nm)
(A) 488nm
(12,4)
(9,6)
(12,0)
(8,5)
633 nm
Inte
gra
ted
are
a (
%)
Diameter (nm)
Raman Shift (cm-1
)
(B)
(7,5)
(12,1)
Diameter (nm)
Raman Shift (cm-1
)
Inte
gra
ted
are
a (
%) (C)
785 nm
34
explanation is based on the energy formation of different types of nitrogen
incorporation in SWCNTs.
Figure 24. Formation energies for different nitrogen incorporations in four different
SWCNTs. The notations in the graph refer to pristine tube (N0Vc0), quaternary nitrogen (N1Vc0),
pyrrolic nitrogen (N1H1Vc1), pyridinic nitrogen (N1Vc1) and two pyrrolic nitrogen (N2Vc2).
Figure 24 present the energy formation of different types of nitrogen
incorporation for the four SWCNTs under examination. The notations in the
graph refer to pristine tube (N0Vc0), quaternary nitrogen (N1Vc0), pyrrolic
nitrogen (N1H1Vc1), pyridinic nitrogen (N1Vc1) and two pyrrolic nitrogen
(N2Vc2). The (12,0) tube shows lower formation energy for all different
nitrogen defects, interestingly this tube is one of the inhibited tubes at high
ammonia feed. Due to lower formation energy of nitrogen defects the tube
accepts more nitrogen and as a result the probability of cap formation due to
pyrrolic defect increases which eventually lead to growth termination. The
second explanation is based on the adhesion force between carbon/nitrogen
atoms and catalyst particle at the early stage of the growth.
Figure 25. (A) Schematic diagram showing the adhesion energy for pristine and N-doped
(12,0)-SWNT. (b) Calculated adhesion energy for pristine (black) and N- doped SWNTs (light
gray) the (12,0)-, (8,5)-, (7,5)-, and (12,1)-SWNT.
As explained in section 6.2 the adhesion force strongly affects the growth
and chirality of the SWCNTs. Our calculations reveal that nitrogen addition
(A) (B)
35
at the interface between the catalyst particle and growing SWCNT results in
lower adhesion force between the growing SWCNT and catalyst particle. For
example the adhesion force of (12,0) tube decreases by 16% as a result of
introducing nitrogen. Figure 25(B) indicate similar results obtained for the
other SWCNTs under examination. It is clear from the graph in figure 25(B)
that the change in adhesion force (due to nitrogen incorporation) is chirality
dependent. These changes are comparable to the previous calculation where
small change in adhesion force between bimetallic catalyst and carbon atoms
explained the selective growth of SWCNTs.[168] Thus the nitrogen
incorporation may move out some of the chiralities form the “stable growth
window” defined by adhesion force.
6.3.4. Analysis of D- and G´- bands
In this part of the study we examine the D- and G´- bands of the grown N-
doped SWCNTs and the possible relation to the type of nitrogen doping. D-
band is a double resonance Raman process involving one elastic and one
inelastic scattering event thus the phonon contributing to this process may
have slightly different energy. Therefore in general it is reasonable to
consider two main peaks for the D-band of SWCNTs.[174] The G´- band is
also a double resonance Raman process but involving two inelastic scattering
events and only one of the initial or final states in scattering process will
match with the real electronic state. Therefore it is expected to observe only
one peak in the G´- bands of pure SWCNTs.[174] However the G´- bands is a
charge sensitive Raman feature and it has been reported that charge defects
renormalize the electron and phonon energy, which results in a new peak in
the G´-band (or a shift of its position) in charged SWCNTs. For instance a
downshift (upshift) of the G´-band is observed by n-doping (p-doping) of
SWCNTs.[155]
Figure 26. (A) Typical D- and G′-band in Raman spectrum of N-doped SWCNTs,
deconvoluted by using a four and five Lorentzian peaks model, respectively. The integrated area
of the peaks under the (B) D-band and (C) G′-band versus ammonia feed during the growth.
Figure 26(A) shows the D- and G´-bands in a typical Raman spectrum of
an N-doped SWCNT sample, recorded by 633 nm Raman excitation
(C)
Ammonia feed (%)
Inte
gra
ted
are
a (
%) (B)
Inte
gra
ted
are
a (
%)
Ammonia feed (%) Raman Shift (cm-1
)
(A) Gʹ-band
D-band Inte
nsit
y (
a.
u.)
36
wavelength. The D- and G´-bands can be deconvoluted by using a four and
five Lorentzian-peak model respectively. The main contribution to the D-
band is by two peaks at 1301 and 1314 cm-1. As shown in figure 26(B) the
integrated area under the peak at 1301 cm-1 follows a similar trend as the
total nitrogen content of the sample (figure 20(C)) suggesting that its
contribution to the D-band is affected by the presence of nitrogen atoms in
SWCNT structure.
The trend of the peaks in G´-band with respect to the type of nitrogen
defect is also interesting. As shown in figure 26(A) the G´-band has two main
contributions from 2598 and 2616 cm-1 peaks. From figure 26(C) it is clear
that the integrated area under the low frequency peak at 2598 cm-1 (sign of
negative charge defect) is inversely proportional to the pyridinic defects in
the sample. To explain this behavior we examine the effect that different
nitrogen defects have on the electronic band structure of the SWCNTs. The
results indicated that pyrrolic nitrogen defects generate new band in the
band structure of pristine tube and decrease the band gap. On the other hand
substitution and pyridinic nitrogen defects generate n-type and p-type
behavior respectively. It is therefore plausible that the integrated area of the
low frequency peak at 2598 cm-1 is inversely proportional to amount of
pyridinic nitrogen due to p-type nature of this defects. In conclusion it seems
clear that the analysis of the low frequency peak in the G´-band of N-doped
SWCNTs has to be done with respect to the type of nitrogen defects.
6.4. One-dimensional C60 structures The section is based on studies presented in papers III and IV.[175, 176]
6.4.1. C60 nanorods
C60 nanorods were grown by LLIP method at the interface between a
solution of C60 in m-DCB and ethanol, as described in section 4.2.1. Due to
low solubility of C60 in ethanol C60 molecules form clusters at the interface of
two solvents, which later act as crystal seeds. By further diffusion of two
solvents the C60 molecules oversaturate in the blend solution, close to the
interface, and crystallize in the form of rods on the crystal seeds. When the
interface between two solvents is left undisturbed the as-grown nanorods
show a broad diameter distribution. In contrast by sonicating the interface a
large number of small crystal seeds with homogenous size form at the
interface, which results in the growth of nanorods with homogenous
diameter distribution. Figure 27(A) presents a typical TEM image of the as-
grown C60 nanorods. A statistical analysis on more than 500 nanorods
(figure 27(B)) revealed an average diameter of 172 nm and an aspect ratio of
the order of 103.
37
Figure 27. (A) Typical TEM image of C60 nanorods grown by LLIP method when the
interface was sonicated. (B) A histogram showing the diameter distribution of the as-grown
nanorods, the blue line represents a Gaussian fit of the data.
6.4.2. C60 tubes
C60 tubes were synthesized by LLIP method at the interface between a
solution of C60 in m-DCB and mixture of ethanol and water. Five different
volume ratios of water to ethanol (W/E equal to 0:100, 2:100, 5:100, 10:100,
20:100) were used to examine the effect of the water on the grown
structures, as explain in 4.2.2.
The first noticeable effect of the water addition is the increase in the
dimensions of the grown C60 structure.
Figure 28(A) shows the average diameter of the grown structure versus
W/E; the error bars in the image show the standard deviation. Similar trend
(B)
Co
un
t
Diameter (nm)
(A)
Water/Ethanol ratio
Avera
ge d
iam
ete
r μ
m
(A) (B)
(C)
Figure 28. (A) Average
diameter of the grown C60 structure
versus the volume ratio of water to
ethanol. SEM image of C60 tubes
grown when the volume ratio of
water to ethanol was (B) 2:100 and
(C) 5:100.
38
was observed for the length of the grown structures. It should be noted that
due to the hydrophobicity of m-DCB the time that two solvents diffuse to
each other increases as the water content in ethanol increases. By adding
water to ethanol a growth of C60 tubes could be observed and as the ratio of
water to ethanol increases the relative abundance of C60 tubes in the sample
increases. Figure 28(B) and (C) display SEM images of C60 tubes grown with
W/E ratio of 2:100 and 5:100. The analysis of the SEM images reveal that as
the diameter of the tube increases the wall thickness does not change
significantly.
In fact by adding water to ethanol large chains of alcohol and water
molecules form in the mixture.[177] This, in addition to the slow diffusion of
m-DCB in mixture of water and ethanol, result in formation of large crystal
seeds at the interface. Also, in such system the rate at which the C60
molecules oversaturates is low and therefore the C60 molecules will have
more time to settle at the preferred circumferential edge sites, which have
been reported to have higher free energy.[178, 179] Concurrently, the C60 has
a certain diffusion length on the crystal seed, independent of the crystal seed
size,[178] and therefore large crystal seeds result in C60 tubes with outer
diameter close to the seed size but with a remained wall thickness. In
conclusion the main reason for the growth of C60 tubes is the formation of
large crystal seed. To investigate this effect more extensively the interface
between C60 solution and water ethanol mixture (w/E: 20:100) was
sonicated. Such system results in the growth of C60 nanorods with
homogenous diameter distribution similar to the structure explained in
section 6.4.1, in line with our hypothesis.
6.4.3. Characterization of the grown C60 structures
The results presented below are based on examination of C60 nanorods,
but similar results were obtained for C60 tubes as well.
Different characterization techniques such as TGA, Raman spectroscopy,
XRD and FTIR revealed that the m-DCB molecules were incorporated in the
structure of as-grown C60 nanorods. Figure 29 shows the Raman spectra
recorded on as-grown nanorods (upper trace (i)) and pristine C60 powder
(lower blue trace (ii)). The Raman spectrum of as-grown C60 nanorods shows
three peaks at 341, 353 and 535 cm-1 (marked by asterisks) in addition to 10
distinct modes (8Hg and 2Ag) characteristic of pristine C60. In accordance
with the previous reports[180] we assigned these peaks to the presence of m-
DCB molecules in the structure of as-grown C60 nanorods. In addition, as
shown in the insets in figure 29, the Hg(1) and Ag(1) modes of as-grown C60
nanorods are shifted to lower frequencies by three and two cm-1, respectively
compared to their position in the spectrum of pristine C60. This is in
agreement with the previous studies where downshifts have been assigned to
interactions between C60 and solvent molecules.[32, 36, 180]
39
Figure 29. Raman spectra recorded on as-grown C60 nanorods (i) and pristine C60 (ii). The
asterisks indicate the peaks corresponding to the presence of m-DCB molecules in the nanorod
structure. The insets indicate the downshift in the Ag(1) and Hg(1) modes due to the presence of
m-DCB molecules in the as-grown nanorod structure.
The presence of solvent molecules in the as-grown C60 nanorods was
further confirmed by TGA analysis in which a total of 10.7 % weight loss was
recorded between the 85 to 171°C. Based on the TGA result and for further
analysis the solvent molecules were removed from the as-grown C60
nanorods by 24 h vacuum oven heat treatment at 200 °C (annealed
nanorods).
Figure 30. XRD pattern of (A) as-grown C60 nanorods assigned to a hcp crystal structure.
(B) Annealed nanorods (i), pristine C60 (ii). HRTEM image of (C) as-grown nanorods (B)
annealed nanorods. Insets in (C) and (D) indicate the corresponding SAED pattern.
Inte
ns
ity (
a.
u.)
Raman Shift (cm-1
)
(C)
Inte
ns
ity a
. u
.
(A)
2θ degree
Inte
ns
ity a
. u
.
(B) (D)
40
From XRD the crystal structure of the as-grown C60 nanorods was
determined to be hexagonal close packed (hcp) with a unit cell size of
a=b=23.17 Å and c=10.18 Å. The XRD pattern of as-grown nanorods is
shown in figure 30(A), the peaks are assigned based on different crystal
planes of a hcp crystal structure. Figure 30(B) reveals the XRD pattern of
annealed C60 nanorods (upper trace). Due to annealing (solvent removal) the
crystal structure of the as-grown C60 nanorods transforms to the original
crystal face centered cubic (fcc) structure of pristine C60 with a unit cell size
of a=b=c=14.13 Å. For comparison the XRD pattern of pristine C60 is also
presented in figure 30(C) (lower trace). It should be noted that due to solvent
removal the annealed nanorods become porous with a polycrystalline
structure. Figure 30(C) and (D) represent the HRTEM image and
corresponding selected area electron diffraction (SAED) pattern (insets) of
the as-grown and annealed nanorods respectively. Both HRTEM image and
distinct, sharp spots in the SAED pattern of the as-grown nanorods indicate
a single domain crystal. In contrast the porosity and clearly visible crystal
domains are observed in the HRTEM image of the annealed nanorods. The
polycrystalline structure of the annealed nanorods is further evidenced by
streaks in the SAED pattern.
6.4.4. Solution based photo-polymerization of C60 nanorods
This section is based on the study
presented in paper V.[181]
As explained in section 4.2.3 the photo-
polymerization of C60 nanorods were
conducted by exposing a dispersion of
nanorods to a green laser beam.
Figure 31 shows a photograph of two
vials containing the as-grown C60
nanorods (left vial) and photo-exposed
nanorods (right vial) dispersion with the
same concentration. It is clear from the
photograph that photo-exposure has
resulted in a color change from yellow-
brown to more orange-brown. It is known
that the polymerization of C60 lead to a
narrowing of the band gap[182] thus the color change could be a sign of
polymerization. In order to track the possible polymerization, the photo-
exposed nanorods were examined by Raman spectroscopy. The Ag(2) Raman
mode of C60 at 1469 cm-1 is sensitive to intermolecular bonds between
neighboring C60 molecules such that a downshift to 1465 cm-1 is a sign of C60
dimers formation and its further downshift to 1460 and 1456 cm-1 are signs
of linear and branch polymeric chain respectively.[183-187]
As-grown Exposed
Figure 31. Photograph of vials
comprising dispersion of as-grown
(left) and photo-exposed C60
nanorods.
41
Figure 32. Raman spectra of photo-exposed nanorods (upper trace) and of pristine C60
(lower trace) recorded by (A) 514 nm and (B) 785 nm Raman excitation wavelength.
Figure 32(A) displays the Ag(2) Raman mode of photo-exposed C60
nanorods (upper trace) and pristine C60 (lower trace) recorded by 514 nm
Raman excitation wavelength. The Ag(2) mode of photo-exposed nanorods is
significantly broader than the one for pristine C60 and can be deconvoluted
by a four-peak model located at 1468, 1463, 1458 and 1452 cm-1. The new
peaks in Ag(2) clearly show the formation of C60 dimers, linear polymers and
branch polymers in the structure of photo-exposed nanorods (from here on
referred to polymerized nanorods). Since C60 has high absorption in the
visible range the 514 nm laser beam will be mainly absorbed by the first few
layers of C60 molecular at the surface of the nanorods. Therefore the Raman
spectrum recorded by this laser mainly collects data from the surface of the
nanorods. To examine the nature of the polymeric structure in the C60
nanorods a near-infrared laser with 785 nm wavelength was employed.
Figure 32(B) presents the Raman spectra of polymerized nanorods (upper
trace) and pristine C60 (lower trace) recorded by 785 nm Raman excitation
wavelength. A laser with such a long wavelength will only be weakly
absorbed by the C60 molecules[28, 188] at the surface and consequently it
will collect data from inner part of the nanorods. Deconvulotion of Ag(2)
Raman mode (by four-model peak) reveals that indeed the peak at 1469 cm-1
has the highest contribution to this mode, strongly suggesting that the C60
molecules at the center of the C60 nanorods are in monomeric state. In
conclusion the Raman data show that the exposure of nanorods to the green
laser results in a polymerized C60 shell, but with a bulk interior comprising
C60 molecules in monomeric state. As a consequence of the polymerized shell
the solubility of C60 nanorods in common hydrophobic C60 solvent
significantly decrease. Figure 33 shows a photograph of two vials containing
the as-grown C60 nanorods (left vial) and polymerized (right vial) in m-DCB
with the same concentration. The as-grown nanorods dissolve immediately
Inte
ns
ity (
a.u
.)
Raman Shift (cm-1
)
λRaman
= 785 nm
1469
1460
1454
1464
(B) (A)
Raman Shift (cm-1
)
Inte
ns
ity (
a.u
.)
λRaman
= 514 nm
1458
1468
1463
1452
42
in m-DCB and make a clear purple solution while the polymerized rods give
a brown foggy dispersion.
Figure 33. (A) Photograph of two vials containing (left) as-grown nanorods and (right)
photo-exposed nanorods dispersed in m-DCB. The as-grown nanorods completely dissolved in
m-DCB resulting in a clear purple solution, while the polymerized nanorods in m-DCB has a
brown foggy appearance. (B) TEM image of the photo-exposed nanorods, following 10 days of
storage in m-DCB. The inset shows higher magnification TEM image.
TEM analysis (figure 33(B)) confirmed that even after 10 days of storage
the polymerized nanorods remain robust without any surface damage.
6.4.5. Electronic characterization of C60 nanorods
The electronic properties of synthesized nanorods were characterized
using field-effect transistor (FET) geometry. A p-type Si wafer with 200 nm
silicon oxide was used as substrate. A set of interdigited source and drain
electrodes (15 nm Au on top of 15 nm Cr), with the channel length of 20 μm,
were patterned on top of the SiO2 using lift-off lithography. The nanorods
were applied on the electrodes by either drop-casting or by dip-coating the
substrate using a dispersion of nanorods. In the dip-coating process a
withdrawal velocity of 3 cm/min and a dip angle of 70° was used in order to
get a set of relatively aligned nanorods crossing the channels. To avoid
oxygen contamination the drop-casting/dip-coating process was done under
inert N2 atmosphere.
As-grown, annealed and polymerized nanorods were separately used to
fabricate the FET and their electrical properties were studied with respect to
their structural properties. The mobility of the examined nanorods and their
average diameter are presented in table III. The as-grown nanorods exhibit a
higher electron mobility compared to the annealed and polymerized
nanorods. The lower electron mobility of annealed nanorods is most
probably related to their polycrystalline nature. It is also clear from the data
for the as-grown nanorods that as the average diameter decreases from 250
(B) (A)
43
to 172 nm the average mobility increases by one order of magnitude, in line
with the previous studies where an inverse relation between mobility and
diameter of C60 nanorods were observed.[189, 190]
Table 3. The mean value of the measured electron mobility of as-grown, annealed and
polymerized nanorods together with their average diameter and crystal structure
Nanorod
treatment Ø [nm]
Crystal
structure Crystallinity
Existence of
guest
molecules
µn (cm2/Vs)a
As-grown 250 hcp Single-crystal Yes 4.4(±2.7)·10-2
172 hcp Single-crystal Yes 3.0(±2.9)·10-1
Annealed 250 fcc Polycrystalline No 8.8(±2.6)·10-3
Photo-
polymerized 172 hcp Single-crystal Yes 4.7(±3.9)×10-3
a Mean values with standard deviation in parenthesis.
Following polymerization the mobility of nanorods decreases by a factor
of 50, which is in agreement with previous result for photo-exposed C60
nanorods[191] and C60 thin films.[26] It should be noted that the FET
measures the electron transport capability of the first few monolayers of
active material positioned on top of the gate-insulator substrate.[192]
Therefore it is reasonable to expect that the monomeric domains at the
center of polymerized nanorods exhibit higher electron mobility similar to
the as-grown nanorods. This will practically be shown in next section.
6.4.6. C60 nonorods as a support for Pd catalyst particles
This section is based on the study presented in paper VI.[193]
As discussed in section 4.2.4 the C60 nanorods were decorated by
functionalized Pd nanoparticles (Pd-C60 nanorods) through a phase transfer
solution process. The motivation for this study was twofold; i) to examine the
practical application of polymerized C60 nanorods (with low solubility) for
solution based processes and ii) to examine the ability of the C60 nanorods as
support for metal nano-catalyst.
Three different batches of C60 nanorods; as-grown, annealed (porous, with
polycrystalline nature due to solvent removal) and polymerized nanorods
were used as a support for Pd nano-particles. Figure 34(A), (C) and (E) show
as-grown, polymerized and annealed (porous) Pd-C60 nanorods. It is clear
from figure 34(A) that the surfaces of the as-grown nanorods are partially
dissolved during the decoration process (in which nanorods were dispersed
in ethanol) and suffer a damaged/porous surface. The damaged/porous
surface result in inhomogeneous distribution and agglomeration of Pd
44
nanoparticles over the surface as shown in HRTEM image in figure 34(B).
The inset in the figure shows the corresponding fast Fourier transform (FFT)
pattern, the presence of streaks reveal that the structure has more
polycrystalline nature. In contrast to the as-grown nanorods, the
polymerized ones remain robust during decoration process and as a result
the Pd nanoparticles are homogenously distributed over their surface, as
clearly confirmed by TEM images in figure 34(C) and (D). The inset in figure
34(D) shows the corresponding FFT pattern with sharp bright spots
indicating that the nanorod structures remain intact during the decoration
process.
Figure 34. Low and high-resolution TEM images of (A)-(B) as-grown Pd-C60, (C)-(D)
polymerized Pd-C60 and (E)-(F) porous Pd-C60 nanorods. The insets in (B), (D) and (F) shows
the corresponding FFT pattern indicating a crystalline structure for the Pd-C60 polymerized and
polycrystalline structure for Pd-C60 as-grown and porous nanorods.
Figure 34(E) and (F) show the annealed nanorods decorated by Pd
nanoparticle. Similar to the Pd-C60 as-grown nanorods the porous surface of
the annealed nanorods also result in inhomogeneous distribution of the Pd
nanoparticles. It should be noted however that there is a structural
difference between the as-grown and annealed Pd-C60 nanorods, namely that
the annealed nanorods are solvent free whereas the as-grown nanorods
contain m-DCB molecules.
(F)
(E) (C) (A)
(D) (B)
45
6.4.6.1. Electrocatalytic activity of Pd-C60 nanorods toward
ethanol oxidation
A series of cyclic voltammetry (CV) was performed to investigate the
electrocatalytic activity of different synthesized Pd-C60 nanorods; first in a
blank KOH solution to determine the electrocatalytic active area (figure
35(A)) and second in alkaline solution (0.5-M ethanol in 1-M KOH) to
investigate the electrocatalytic activity of decorated nanorods towards
ethanol oxidation (figure 35(B)).
Figure 35. CV results of Polymerized (curve a, black line), as grown (curve b, blue line) and
porous (curve c, red line) Pd-C60 nanorod electrodes in (A) blank solution containing 1 M KOH
(Scan rate: 0.05 V/s) and (B) in 1 M KOH containing 0.5 M ethanol (Scan rate: 0.05 V/s).
As shown in figure 35(A) the peak current density of Pd reduction on
polymerized Pd-C60 nanorods is significantly higher than that of the as-
grown and annealed ones. The electrochemical active surface area of the
electrodes[194] can be calculated to 0.028, 0.072 and 0.026 cm2 for as-
grown, polymerized and annealed Pd-C60 nanorods respectively. Thus the
electroactive surface area of Pd nanoparticles on polymerized Pd-C60
nanorods is 2.57 and 2.77-fold higher than that of as grown and annealed Pd-
C60 nanorods. This indicates that the Pd particles are more homogenously
distributed at the surface of polymerized C60 nanorods compared to the as-
grown and annealed ones, in line with the TEM observations. Figure 35(B)
reveals the CV results of electrodes modified by Pd decorated nanorods in 1-
M KOH containing 0.5-M ethanol. The polymerized Pd-C60 nanorods
electrodes shows an onset potential (Eonset) of ethanol oxidation of -0.46 V,
which is lower than that of the as-grown (about -0.39 V) and porous (about -
0.42V) Pd-C60 nanorods. In addition the normalized current density of the
ethanol oxidation peak for the polymerized Pd-C60 nanorords electrode is 0.7
mA/cm2, which is significantly higher than that of the as-grown (0.29
mA/cm2) and porous (0.37 mA/cm2) nanorods. In conclusion the
polymerized Pd-C60 nanorods show a significantly higher electrocatalytic
activity towards ethanol oxidation compared to other two samples. As
Cu
rren
t (μ
A)
Potential /V (vs. Hg/HgO)
Cu
rren
t (μ
A)
Potential /V (vs. Hg/HgO)
46
discussed in the previous section the electron mobility of the polymerized
nanorods is smaller than that of the as-grown and annealed ones with factors
of 64 and 34 but it is still by far the most efficient support for Pd
nanoparticles. This could be rationalized by homogeneous distribution of Pd
particles and a well-preserved crystalline state of the polymerized nanorods.
It however also strongly indicates that the monomeric C60 molecules at the
center of the polymerized nanorods (as discussed in previous section) can
contribute with a high electron mobility, which would correspond to values
in line with the high electron mobility observed for the as-grown nanorods.
This also gives information about differences in the localization of charge
transfer for the FET process and the electrocatalytic process. While the
former is strictly confined to the surface, the latter is rather a bulk
conduction.
6.5. C60/Collapsed carbon nanotube hybrid nanostructure
This section is based on the study
presented in paper VII
As explained in section 4.3 the
CCNTs were synthesized by sonication
of highly crystalline, uncapped arc-
grown MWCNTs. The sonication
extracts the inner walls of the tube and
creates single- to few-walled CNTs
with large inner diameter. Such a tube
is energetically unstable in tubular
form and prone to collapse.
Figure 36 shows a high resolution
TEM image of a four-walls CCNT with
the width of 21 nm. The CCNT is bent
in the middle where eight graphene
layers can be observed. The width of
the CCNT does not change at the bent
region, indicating that the CNT is collapsed all the way through its length.
The strain energy of the curved edges in a CCNT (which opposes the
attractive van der Waals forces of the opposing graphene sheets) forms edge
ducts with the teardrop shape cross-section (as shown schematically in right
side of figure 37), running all the way through the edges of CCNT.
Figure 36. HRTEM image of a four-
walled CCNT, the tube is bent at the
middle where its cross section with eight
graphitic walls is clear. Cross section of
four-walled CCNT is schematically shown
in the inset.
47
Figure 37. (Left) A C60 molecule with (carbon center to carbon center) diameter of 0.7 nm.
The van der Waals interaction range of C60 is shown by shadow, indicating a diameter of about 1
nm. (Right) A cross section view of edge duct of a two-walled CCNT with theoretically calculated
dimensions of the duct (L and d).
The theoretically calculated dimensions of the edge duct, using a simple
continuum elasticity model, predict a duct bulb height d = 0.78 nm and bulb
width L = 1.81 nm for a double-walled CCNT. Similar results for single- to
four-walled tube are presented in table 4.
Table 4. Theoretically determined width (L) and height (d) of the singe- to four-walled
CCNT. For comparison the available experimentally measured values are also presented in the
table.
CCNT
n
Theoretical
calculation Experiment
L (nm) d (nm) L (nm) d (nm)
n=1 1.65 0.71 - -
n=2 1.81 0.78 - -
n=3 1.99 0.86 - -
n=4 2.19 1.03 2.5 0.94
A C60 molecule has a diameter of 0.7 nm and a van der Waals diameter of
approximately 1 nm (as schematically shown in figure 37 on the left).[125] It
has been shown that insertion of C60 in a SWCNT with diameter smaller than
1.25 nm is strongly endothermic,[195] therefore if the CCNT was completely
rigid it would be extremely difficult to insert C60 into the edge ducts.
However the opposite walls in a CCNT are attached to each other due to
graphitic bonding energy. Thus C60 molecules can unglue the opposite walls
and insert into the edge ducts. It should be noted that the dimensions of the
duct bulb increase slightly with number of CCNT walls but as can be seen in
table 4 even for a four-walled CCNT the d is smaller than the threshold value
for C60 insertion.
We experimentally examined the insertion of C60 in the duct edges of
CCNT. A solution-based method was used for C60 insertion, in which a
d =
0.7
8 n
m
L = 1.81 nm
48
dispersion of C60 clusters and CCNTs in n-hexane was sonicated for one to
three hours as explained in section 4.3.
Figure 38. HRTEM image and theoretical modeling (top view and duct cross section) and
duct dimension of (A) a double-walled CCNT filled with linear chain of C60 molecules. (B) a five-
walled CCNT filled with staggered C60 configuration. (C) a three-walled CCNT showing dimers
as a result of C60 close packing in a staggered configuration in combination with duct pressure,
elevated temperature and/or electron beam. Dashed box in C(i) indicates a C60 pair forming
dimers. The scale bar in all images is 5 nm.
Figure 38A(i) shows a TEM image of the edge region of a double-walled
CCNT filled with a linear chain of C60 molecules. The distance between C60
molecules in linear chain has experimentally found to be 0.98 nm, which is
in argument with the previously observed one in conventional peapods.[124]
Figures 38A(ii) and (iii) represent the results of theoretical modeling for
linear chain of C60’s inside the duct of a double-walled CCNT. As can be seen
the bulb dimension increased after C60 insertion; the bulb height (d)
increased from 0.78 to 1.32 nm and bulb width (L) increased from 1.81 to
2.18 nm. Consistent with experiment, the calculated C60-C60 distance is
found to be 0.97 nm. Interestingly both theoretical and experimental result
showed that the C60 molecules can open up the bulb even more and form a
staggered C60 configuration at the edges of CCNTs. Figure 38B(i) shows such
a C60 configuration in a five-walled CCNT. The result of theoretical modeling
for staggered C60 configuration at the edge of a double-walled tube are
presented in figure 38B(ii) and (iii), indicating an increase in the duct size.
Also the theoretical modeling predicts slightly smaller C60 distance along the
width of the CCNT as shown in figure 38B(ii). This is reasonable since the
C60 molecules experience anisotropic confinement due to teardrop shape of
A
B
C
ii
i
d =
1.3
2 n
m
L = 2.18 nm
iii
d =
1.3
9 n
m
L = 3.4 nm
iii
L = 3.29 nm
d =
1.4
3 n
m iii
i
i
ii
ii
49
the duct and C60’s are always pushed toward the edges of CCNTs. As a result
of smaller distance the probability of dimerization for C60 molecules along
the width of the CCNT increases, such an effect is shown in figure 38C(i) in
which a pair of C60 is indicated by a rectangle. The C60 distance in pairs is
experimentally found to be 0.9 nm however C60 distance along the duct axis
is still 0.98 nm. Such a pair with short C60-C60 distance represents a C60
dimer[196, 197] and is likely obtained by duct pressure in combination with
elevated temperature and/or electron beam. It should be noted that although
the duct size slightly increases as the number of walls in CCNT increases
(table 4), due to teardrop shape cross section of the duct the C60 molecules in
staggered configuration are still pushed toward the edges in CCNTs. It is also
found that the C60 configuration in the duct is independent of the number of
walls in CCNTs, as manifested by observing both linear chain and staggered
C60 side by side along the edge of a CCNTs, as well as observing staggered
configuration along the edges of single-walled CCNTs. The configuration of
C60 at the edge is rather dependent on the relative concentration of C60 to
CCNTs in hexane and/or the sonication time, meaning that longer process
time or higher concentration of C60 in the dispersion result in more
staggered configurations (more TEM images are presented in supporting
information of paper VII).
Taking this a step further, we observe that a prolonged processing time
results in complete reinflation of the CCNTs. Figure 39(A) shows a double-
walled CCNTs with linear C60 chains on the left side where it possess a width
of 7.83 nm. On the right side however C60 molecules can be observed even at
the center and the width of the CCNT decreases to 6.82 nm.
Figure 39. HRTEM image of (A) a two-walled CCNTs in intermediate stage of reinflation.
(B) a competently reinflated three-walled CCNT. The schematics represent the possible cross
section of the tube in A an B. Scale bar is 5 nm.
A
B
50
A simple calculation (considering the curvature at the two edges) reveals a
diameter of, approximately, 5.8 nm for the original tube (before collapsing).
Therefore the tube on the right side of the image is not completely reinflated
and has a cross section similar to the schematic shown in figure 39(A). A
theoretical modeling reveals that if a CCNT is filled with a single layer of C60
molecules, its width slightly increases compared to C60/CCNT with linear C60
chain (more information regarding such a model is presented in supporting
information of paper VII). This excludes the possibility of having a single
layer of C60 molecules in the right side of the CCNT in figure 39(A) and
signals that the C60 molecules are in a three-dimensional configuration
rather than a two-dimensional. Figure 39(B) shows TEM image of a different
CCNT completely filled with C60 molecules. The reinflated tube in figure
39(B) has a diameter of 10 nm, which is above the critical diameter for a
three-walled tube (above that the tube will collapse).[136]
51
7. Conclusion and outlook
The results presented in this thesis give insights into tuning the properties
of carbon nanostructures, in particular SWCNTs and C60 one-dimensional
structures, by means of controlling the structure during the growth or post
structural modification.
A clear relation between the diameter of the CVD grown CNTs and the size
of the catalyst particle is illustrated in paper I. It is also shown that the
chirality distribution of the grown SWCNT can be tuned by changing the
catalyst composition of the catalyst particle. This is in agreement with the
previous theoretical calculation where this effect is explained by the
adhesion force between the catalyst particle and precipitated carbon
atoms.[168]
N-doped SWCNTs were grown by CVD and it is found that the nitrogen
content, under examined experimental condition, is limited to 3.2%. The
correlation of XPS, Raman spectroscopy and theoretical calculation assist in
a better assignment of the type of nitrogen incorporation in grown SWCNTs.
For more precise assignment of the types of nitrogen incorporation a
correlation between atomic resolution microscopy and spectroscopy seems
to be helpful. However with the current available electron microscope it
seems challenging since the nitrogen configuration can easily be affected by
electron beam. The limitation of nitrogen uptake of SWCNT is related to the
type of nitrogen incorporation. The pyrrolic nitrogen is not energetically
preferred at the walls of the tube, instead it is stable at the edges where it
induced curvature to the structure by forming pentagonal rings and finally
terminates the growth by cap formation. Therefore it might be possible to
increase the nitrogen content of SWCNTs by controlling the type of the
nitrogen content, in particular by avoiding pyrrolic nitrogen.
The nitrogen doping modifies the electronic band structure of the tubes
and depending on the type of nitrogen incorporation it may switch its n-type
nature to p-types or vice versa, or remain unchanged. It is also to some
extent possible to track the type of nitrogen doping by features in Raman
spectrum of N-doped SWCNTs, however for a precise assignment more
theoretical calculation is required.
A growth model for C60 nanorods and tubes is presented in paper III. By
changing the size of the crystal seed, in the early stage of the growth, it is
possible to change the size and morphology of the grown structure from
small diameter C60 nanorods to large diameter C60 tubes. By creating crystal
seeds with homogenous size it is possible to grow C60 nanorods with
homogenous diameter however with the presented method the minimum
value obtained for average diameter found to be 170 nm. The growth of C60
structure with much smaller diameter would be beneficial for applications
52
where a crystalline material with high surface area is desirable, such as
photovoltaic devices.
Solution based photo-polymerization of C60 structures create a
polymerized shell around the structures which decrease their solubility and
opens up for further practical application. Although the photo-polymerized
shell possess significantly lower electron mobility, the C60 monomers at the
center of the structure still show high electron mobility, acquiring a high
bulk charge transfer for the whole structure.
The encapsulation of C60 molecules in the edge ducts of CCNTs with
different configuration creates new carbon hybrid nanostructures, and the
possibility of changing the number of C60 molecules inside the CCNTs with
different number of walls and different width opens for engineering
materials with different properties. In particular the reinflation of the CCNT
by C60 molecules creates small diameter C60 structure, confined by few
graphitic layers, which does not exist in non-constrained conditions. It
would be interesting to examine the electrical properties of such materials in
particular possible superconductive behavior (after doping with alkaline
metals). The CCNTs can potentially be reinflated by insertion of different
guest material, such a large inner cavity with few graphitic layer may create a
cell for specific electron microscopy studies.
53
8. Summary of the appended articles
Paper I: Simple Dip-Coating Process for the Synthesis of Small Diameter
Single-Walled Carbon Nanotubes-Effect of Catalyst Composition and
Catalyst Particle Size on Chirality and Diameter
Barzegar H R, Nitze F, Sharifi T, Ramstedt M, Tai C W, Malolepszy A,
Stobinski L and Wågberg T 2012 J. Phys. Chem. C 116 12232-9.
Contribution: All results, measurements, analysis and manuscript
preparation, excluding XPS measurements.
The study presents CVD growth of CNTs on catalyst particles deposited on
silicon wafer by a simple dip-coating method. A solution containing Fe and
Co ions was used as catalyst precursor. By adjusting the dip-coating
parameters and solution concentration the catalyst particle size was tuned on
the substrate and consequently the diameter of the grown CNT was altered
from small diameter MWCNT to small diameter SWCNTs. The grown
SWCNTs were analyzed by Raman spectroscopy and the chirality
distribution is studied. By slight change in the ratio of Fe to Co the chirality
distribution become narrower and mainly four chiralities are predominated
in the sample compared to the case when ratio of Fe to Co was one to one.
Paper II: Doping Mechanism in Small Diameter Single-Walled Carbon
Nanotubes: Impact on Electronic Properties and Growth Selectivity
Barzegar H R, Gracia-Espino E, Sharifi T, Nitze F and Wågberg T 2013
Nitrogen J. Phys. Chem. C 117 25805-16.
Contribution: All experimental results, measurements, analysis and
partially manuscript preparation, excluding XPS measurements and
theoretical modeling.
N-doped SWCNTs were synthesized by CVD method on Fe/Co catalyst
particles. Acetylene and ammonia gases were used as carbon and nitrogen
precursors respectively. The nitrogen content of the grown SWCNT was
tuned by changing the flow rate of ammonia gas during the growth (while
acetylene flow kept constant). It is found that nitrogen atoms incorporate
into the grown SWCNT structure as pyridinic and pyrrolic functionalities
and the total nitrogen content of the tube is limited to 3.2 %. Correlation
between experimental measurements (Raman spectroscopy and XPS) and
theoretical calculation showed that at low nitrogen content (below 3.2 %),
when the pyridinic nitrogen is dominated over pyrrolic one, the SWCNTs
preserve their tubular form. In contrary at 3.2% the pyrrolic starts to
54
dominate and induce curvature to the SWCNTs, which finally terminate the
growth. RBMs in Raman spectra of the samples reveal that at high nitrogen
content the growth of some of the tubes were hindered, resulting in narrower
chirality distribution. This effect is explained by both stability of the tube
under nitrogen doping as well as by interaction of carbon/nitrogen atoms
with catalyst particles at the early stage of the growth. Furthermore the effect
of different nitrogen incorporation on the electronic band structure of the N-
doped SWCNT is studied.
Paper III: Water Assisted Growth of C60 Rods and Tubes by Liquid-
Liquid Interfacial Precipitation
Barzegar H R, Nitze F, Malolepszy A, Stobinski L, Tai C W and Wågberg
T 2012 Method Molecules 17 6840-53.
Contribution: All results, measurements, analysis and manuscript
preparation, excluding FTIR.
The study presents a simple and efficient method to control the dimension
and morphology of the fullerene (C60) structures grown by LLIP method. The
C60 nanorods were grown at the interface between ethanol and a solution of
C60 in m-DCB. The size and morphology of the grown structure were tuned
from C60 nanorods to C60 tubes by addition of water to the ethanol. It is
found that the diffusion rate of the two solvents at the interface decreases by
addition of water to the ethanol. This results in creation of large crystal seeds
at the interface and consequently in the growth of C60 tube. Different
characterization techniques reveal that m-DCB molecules are incorporated
in the grown structures, resulting in a solvated structure.
Paper IV: On the fabrication of crystalline C60 nanorod transistors from
solution
Larsen C*, Barzegar H R*, Nitze F, Wågberg T and Edman L 2012
Nanotechnology 23 344015. (* Authors equally contributed to the
manuscript)
Contribution: Sample preparation and characterization; contribute to
device fabrication and manuscript preparation.
C60 nanorods were grown by LLIP method. The grown nanorods have
diameters mainly between 200 and 250 nm, with an aspect ratio of the order
of 1000. From XRD and SAED it is found that the as-grown C60 nanorods are
perfectly crystalline all along their length in a hcp crystal structure. The
55
incorporated solvent molecules in the structures were removed by vacuum
annealing of the nanorods at 200 °C. This results in porous C60 nanorods
(due to solvent removal) and crystal structure transform to fcc with
polycrystalline nature. The electrical properties of the as-grown and
annealed nanorods were examined on FET substrate, in which the nanorods
were aligned to cross the channel between inter-digited source and drain
electrodes. The results indicate that the polycrystalline structures (annealed
nanorods) have lower electron mobility compared to the as-grown one.
Paper V: Solution-Based Phototransformation of C60 Nanorods: Towards
Improved Electronic Devices
Barzegar H R*, Larsen C*, Edman L and Wågberg T 2013 Particle &
Particle Systems Characterization n/a-n/a. (* Authors equally contributed
to the manuscript)
Contribution: All results, measurements and manuscript preparation
excluding device fabrication (FET).
The study presents a facile solution-based method for photo-
polymerization of C60 nanorods. C60 nanorods were grown by sonication
assisted LLIP method, comprising an average diameter of 170 nm and aspect
ratio of the order of 1000. A dispersion of the as-grown nanorods in a
mixture of ethanol and m-DCB was exposed to green laser light (λ = 532 nm,
power density = 15 mW mm-2) for 20 h. Raman spectroscopy of the photo-
exposed sample indicates clear signs of dimerized and polymerized C60
molecules. By comparison of Raman spectra recorded with different
excitation wavelength it is found that the photo exposure results in a
polymerized C60 shell at the surface of the nanorods, encapsulating monomer
C60s at the center. It is also shown that the polymerized shell protects the
nanorods from being dissolved in common hydrophobic solvent for C60s.
Furthermore the electronic properties of the photo-polymerized nanorods
are examined by using a FET geometry showing an average electron mobility
of 4.7 × 10-3 cm2 V-1 s-1. This is lower than that the one for as-grown nanorods
by a factor of 50. Since the FET measures the electron-transport capacity of
the first few layer of material on top of the gate surface; it is predicted that
the monomer C60 molecules at the center of the nanorods still exhibit high
electron mobility.
Paper VI: Palladium nanocrystals supported on photo-transformed C60
nanorods: Effect of crystal morphology and electron mobility on the
electrocatalytic activity towards ethanol oxidation
Barzegar H R*, Hu G Z*, Larsen C, Jia X E, Edman L and Wågberg T
2014 Carbon 73 34 40. (* Authors equally contributed to the manuscript)
56
Contribution: Sample preparation and analysis excluding Pd decoration
and CV measurement.
Different C60 nanorods; as-grown, photo-polymerized and porous
(annealed) ones are decorated by functionalized Pd particles with an average
size of 4.78 ± 0.66 nm. The decoration process involves a dispersion of
nanorods in ethanol. The as-grown nanorods are partially dissolved in
ethanol, resulting in porous surface. The decoration of the as-grown and
annealed nanorods results in inhomogeneous distribution of Pd particles. In
contrary the polymerized shell of the photo-polymerized nanorods results in
homogenous Pd particle distribution. The electrocatalytic activity of different
prepares samples are tested towards ethanol oxidation. Although the photo-
polymerized nanorods have lower electron mobility than the as-grown and
porous ones; it is found that the photo-polymerized Pd decorated nanorods
has the highest electrocatalytic activities towards ethanol oxidation
compared to the other two samples. This is rationalized by better
crystallinity and higher electron mobility of the monomer C60 molecules at
the center of the photo-polymerized nanorods.
Paper VII: C60/Collapsed Carbon Nanotube Hybrids - A Variant of
Peapods
Barzegar H R, Gracia-Espino E, Yan A, Ojeda-Aristizabal C, Dunn G,
Wågberg T, and Zettl A 2014 submitted to Nanoletter
Contribution: All results, measurements and manuscript preparation
excluding theoretical modeling and calculation.
The study present a hybrid nanostructure synthesized by encapsulation of
C60 molecules at the edge ducts of a collapsed carbon nanotubes (CCNTs).
The CCNTs are obtained by sonication of highly crystalline MWCNTs, which
extract the inner walls of the tube in a telescopic fashion. This creates single-
to few-walled tubes with large inner diameter, which are prone to collapse.
The encapsulation of C60 is also done through a solution-based process. A
correlation between the experimental data (TEM characterization) and
theoretical calculation indicate that C60 molecules slightly open up the
graphitic walls of the CCNTs to fit in the edge ducts and form a linear C60
chain along the edge of the CCNTs. By increasing the relative concentration
of the C60 to CCNTs during the filling process as well as by increasing the
process time it is found that CCNT can accommodate more C60 in the ducts,
resulting in a zigzag packing along the two edges of CCNT. Furthermore it is
shown that the C60 molecules may completely reinflate the CCNT by packing
C60 molecules in its inner cavity.
57
9. Acknowledgements
I would like to thank all of the people who have contributed and supported
me during my PhD studies, scientific or personally.
First of all I would like to express my gratitude to my supervisor Thomas
Wågberg for all his support and guidance during these four years of PhD
study in Umeå. The opportunity to be a PhD student in your research group,
where I had the chance to learn and developed so many skills, really means a
lot to me. You have created various opportunities for me. Indeed I could not
be at this stage without your support.
I would like to thank my co-supervisor Ludvig Edman for all his support,
and the various encouraging scientific discussions.
I would like to express my gratitude to professor Alex Zettl at University of
California at Berkeley for giving me the opportunity to be in his group for
one year.
I would like to thank all members of Thomas Wågberg’s group, former
and present; Florian Nitze - you always help me as a supervisor during my
master program and a colleague during my PhD studies- thanks for all your
help. Tiva Sharifi, an old friend as well as a colleague during my PhD studies,
thanks for all scientific discussions and contributions in different projects
and of course all enjoyable coffee times we had in the physics department.
Special thanks to Eduardo Gracia Espino, a perfect collaborator, I really
enjoyed all of the scientific discussions we had on different projects; your
works always helped me to have better understanding of the experimental
results. Guangzhi Hu, thanks for all scientific discussion, collaporations and
of course the fishing lessons. Xueen Jia a nice colleague with lots of energy,
thanks for all of the positive energy you brought to the lab.
I would like to thank all of the group members of Ludvig Edman’s group
for their help. Special thanks to Christian Larsen; a nice and calm
collaborator; thanks for all scientific discussion on joined projects as well as
your help during teaching.
I would also like to thank all group member of Zettl group, who warmly
welcomed me to the group during my visit at UC Berkeley, in particular
Haider Rasool, Claudia Ojeda-Aristizabal, Mehdi Jamei, Gabriel Dunn, Aidin
Fathalizadeh and Aiming Yan.
I would like to acknowledge all members of the Department of Physics in
Umeå University for such a great and pleasant work environment. Special
thanks to Katarina Hassler and Lena Burström for their help with the
administrative work, especially for their assistance with invoices and travel
related issues. I was always amazed with your patient and effort to fix
mistakes in my reimbursement forms. Jörgen Eriksson, the first person to
58
call for practical issues and of course a quick response, thanks for all your
help. Hans Forsman, Leif Hassmyr and Gabriella Allansson, I don’t think I
could have enjoyed my teaching experience in Umeå without your help. I am
really thankful for your assistances. Special thanks to Lena Åström and
Tomas Gustafsson for their nice works in the workshop.
I would like to acknowledge all my great friends from Umeå. Warmest
thanks to Farzad Amirvaresi and Fariba Khojasteh for their great support
which truly influenced my educations in Umeå.
Finally my wife, Alieyh, my life in Umeå started with you, we went
through all the steps side by side to become successful. This thesis is
indirectly your work as well so I would like to dedicate this to you. Thank you
for your love and support during all these years.
59
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