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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