chandra_synthesis and electronic transport in single walled carbon nanotubes

8/3/2019 Chandra_Synthesis and Electronic Transport in Single Walled Carbon Nanotubes

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Synthesis and Electronic Transport in Known Chirality

Single Wall Carbon Nanotubes

Bhupesh Chandra

Submitted in partial fulfillment of the

Requirements for the degree

of Doctor of Philosophy

in the Graduate School of Arts and Sciences

COLUMBIA UNIVERSITY

2009


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2009

Bhupesh Chandra

All Rights Reserved


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ABSTRACT

Synthesis and Electronic Transport in Known Chirality Single Wall Carbon

Nanotubes

Bhupesh Chandra

Carbon nanotubes are intriguing new materials with extraordinary electrical properties

originating from their quasi 1-dimensional structure and a strong, all carbon lattice. They

have great potential for use in electronic and energy related applications, such as low

resistance metal interconnects, optically transparent conducting films for display

applications, high surface area catalytic support, etc. However, a major roadblock for

practical usage of carbon nanotubes is the limited understanding of its synthesis and

electrical properties.

The first half of the thesis probes into the existing challenges of carbon nanotube

synthesis through chemical vapor deposition (CVD) process. Nanotubes grown with

various synthesis techniques are characterized with Rayleigh spectroscopy, enabling the

development of finely tuned recipes that deliver clean and unbundled single wall carbon

nanotubes. Two technological applications which directly utilize nanotubes coming from

CVD growth are discussed. The first application uses them as high surface area

electrodes in a enzyme mediated bio-fuel cell and the second application finds their use

as transparent and conducting films.

In the second half of the thesis, electrical transport results on chirality assigned nanotubes

are presented. Electronic measurements on a known structure carbon nanotube metal-

semiconductor heterojunction suggested the presence of quantum confinement at the

point of chirality change; this observation is explained through detailed theoretical

calculations. Finally, through an extensive study of electron-phonon scattering on known

structure single wall nanotubes, it is concluded that the substrate phonon modes have a

dominating effect in nanotube electron scattering. The role of substrate phonon scattering

is studied through experimenting with nanotube devices placed on multiple substrates

with different phonon energy modes. This result has far reaching implications in the field

of nanotube transport.


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Contents

1 Introduction to Carbon Nanotubes................................. 1

1.1 sp2 Hybridization ... 2

1.2 Graphene .... 3

1.3 Carbon Nanotubes .. 6

1.4 Nanotube Crystal Structure .... 7

1.5 Nanotube Electronic Dispersion .... 9

1.6 Nanotube Phonon Dispersion 12

1.7 Summary 14

Bibliography ..... 15

2 Carbon Nanotube Synthesis - Processes and Concepts. 17

2.1 Introduction 18

2.2 Chemical Vapor Deposition ... 20

2.2.1 Carbon Precursor 20

2.2.2 Temperature ... 22

2.2.3 Catalyst Nanoparticle . 23

2.2.4 Gas Flow Rates ... 24

2.3 CNT Growth Experiments . 25

2.3.1 Motivation and Aims .. 25

2.3.2 Growth Setup .. 26

2.3.3 Carbon Monoxide Growth . 27

2.3.4 Ethanol Growth ...... 30

2.3.5 Methane Growth ......... 34

2.3.6 Water-Assisted Ethylene growth ........ 36

2.3.7 Nanotube Growth- Fluid Flow Study ......... 37

2.3.8 Conclusion .. 41

Bibliography . 42

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3 Carbon Nanotube Synthesis - Technology Applications ... 46

3.1 Growth of CNTs on Carbon Toray Paper for Bio-Fuel Cell Applications. 47

3.1.1 Introduction 47

3.1.2 Experimental Setup and Process Parameters . 48

3.1.3 Growth Results ... 50

3.1.4 Summary 53

3.2 Growth of Transparent, Conducting CNT Films ... 54


3.2.2 Experiments 55

3.2.3 Measurements 58

3.2.4 Summary 61

Bibliography . 62

4 Introduction to Electron Transport in Carbon Nanotubes .......................... 65

4.1 Introduction ............ 66

4.1.1 Metallic and Semiconducting CNTs.. 66

4.1.2 Quantum Transport in a 1-d System .. 66

4.1.3 Schottky Barriers at the Nanotube- Metal Interface .. 67

4.1.4 Electronic Mean Free Path and Phase Coherence Length.. 68

4.1.5 Nanotube Band Diagrams-Field Effect Transistor Action 69

4.1.6 Coulomb Blockade in CNTs .. 71

4.2 Carbon Nanotube Device Fabrication 77


4.2.2 Rayleigh Scattering Spectroscopy . 78

4.2.3 Nanotube Transfer Printing ... 81

4.2.4 Metal Electrode Deposition ... 83

Bibliography . 85

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5 Molecular-Scale Quantum Dots from Carbon Nanotube Heterojunctions 87

5.1 Introduction ......... 88

5.2 Device Fabrication .. 88

5.3 Electronic Transport Measurements 90

5.4 Constant Interaction Model . 93

5.5 Sequential Tunneling ... 95

5.6 Theoretical Modeling .. 97

5.7 Explanation for Large Central Diamond and Diode-Like Behavior ... 102

5.8 Negative Differential Resistance (NDR) . 103

5.9 Conclusion ... 104

5.10 Acknowledgements . 105

Bibliography . 1066 Electron Phonon Scattering in Known Chirality Single Wall Carbon

Nanotubes... 110

6.1 Introduction ......... 111

6.2 Electron-Phonon Scattering. 112

6.3 Electron-Acoustic Phonon Scattering- Theoretical Background 113

6.4 Electron-Acoustic Phonon Scattering-Previous Experiments 115

6.5 Measurement Method . 117

6.6 Electrical Transport Measurements T=300K .. 118

6.7 Electrical Transport Measurements with Varying T .. 122

6.8 Analysis of Results . 125

6.9 Substrate Polarized Phonons (SPP) 126

6.10 Summary 133

Bibliography 135

Publications .

.

138

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List of Figures

1.1 sp2 hybridization of carbon and its derived materials 2

1.2 Structure of graphene in real and reciprocal space . 3

1.3 Graphene energy dispersion ... 5

1.4 Single and multiwall carbon nanotubes .. 6

1.5 Nanotube crystal structure .. 8

1.6 Band structure and Density of State (DoS) 10

1.7 Linear dispersion of metallic nanotubes near K-point ... 11

1.8 Nanotube phonon dispersion .. 13

2.1 Current CNT growth mechanism hypothesis . 19

2.2 A schematic of the nanotube growth wafer. 26

2.3 CVD growth system ... 26

2.4 Carbon monoxide grown nanotubes . 28

2.5 Structure of Fe-Mo catalyst nanocluster (Mueller Catalyst) . 30

2.6 Carbon nanotube CVD growth computer interface 32

2.7 Rayleigh scattering spectrum on two different suspended (16, 16) armchair

carbon nanotubes 33

2.8 Diameter and chirality distribution of single wall carbon nanotubes . 34

2.9 Methane growth results .. 35

2.10 Water-assisted ethylene growth results .. 37

2.11 Flow study surrounding a raised structure on silicon chip . 38

2.12 Flow movement in side plane of the slits and raised structure ... 39

2.13 Flow chart describing CVD growth parameters studied 41

3.1 Schematic of the nanotube growth system 48

3.2 Scanning electron images of CNTs grown on carbon paper .. 50

3.3 MWNT growth at different time intervals . 51

3.4 Effect of carbon nanotube growth time on surface area . 52

3.5 Various process parameters studied for carbon nanotube film growth .. 56

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3.6 SEM images of nanotube film growth on quartz substrates ... 57

3.7 Light transmission and sheet resistance measurements on as-grown nanotube

films 58

3.8 Comparison with results in literature . 60

4.1 Nanotube band diagrams .. 69

4.2 Capacitive model of nanotube device .. 72

4.3 Ground State lines of a nanotube QD ... 74

4.4 Stability diagram of the nanotube quantum dot 74

4.5 Quantization of nanotube bands ... 75

4.6 Quantum dot transport .. 76

4.7 Nanotube device fabrication process sequence 78

4.8 Single tube Rayleigh spectroscopy setup . 79

4.9 Nanotube Rayleigh scattering results ... 80

4.10 Nanotube transfer setup .... 82

4.11 Nanotube device on silicon wafer . 83

5.1 Device geometry and electrical measurements on semiconducting and

metallic sections 89

5.2 Electrical measurements on semiconducting and metallic sections . 90

5.3 Temperature dependent measurements across the heterojunction 91

5.4 Transport across heterojunction 93

5.5 Model of the metal-semiconductor nanotube HJ device .. 94

5.6 Rate equations for sequential tunneling through a spin- degenerate level ... 96

5.7 Coupling constants of metal nanotube and SC nanotube to QD .. 96

5.8 Single heterojunction structures ... 98

5.9 Calculated local density of states for different geometries ... 99

5.10 Defect migration ... 100

5.11 Interface DoS for double heterojunctions . 101

5.12 Band diagrams explaining that the central gap . 102

5.13 Explanation of the negative differential resistance (NDR) .. 104

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6.1 Emission and absorption phonons near K point ... 113

6.2 Conductance gate sweep for (21,9) nanotube ... 118

6.3 Conductance gate sweep for (26,11) nanotube . 119

6.4 Resistance vs. channel length for (26,11) nanotube at T=300K ... 120

6.5 Conductance gate sweep for (26,11) nanotube with changing temperatures ... 122

6.6 Resistance vs. channel length for (26,11) nanotube at different temperatures . 123

6.7 Resistivity vs. temperature for (26,11) nanotube .. 124

6.8 Experimental and theoretical resistivity values for (26,11) nanotube .. 129

6.9 Experimental and theoretical resistivity values for (21,15) and (24,12)

nanotube 130

6.10 Optical image of nanotube device with different substrate stripes ... 1316.11 Comparison of experimental and theoretical resistivity values for

semiconducting nanotube . 132

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List of Tables

6.1 Resistivity values for chirality assigned nanotubes at T=300K121

6.2 Parameters used for curve fitting data with SPP scattering...133

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Acknowledgements

Standing on the Shoulders of Giants

It was exactly six years ago when I accepted the offer to join PhD program at Columbia.

As a fresh undergraduate I did not know what was in store for me, PhD seemed too big a

decision, all I had was just a vague belief that things will turn out to be good. In all these

years that belief did not getshattered, but gradually turned into reality, of course, with the

help of numerous people that crossed my way during this course. This section is

dedicated to them.

I still remember my first day, when Prof. Vijay Modi, then the department chair,

introduced me to a new faculty, Prof. James Hone who later became my thesis advisor.

Working with Jim has been a tremendously rewarding experience. His insightfulness in

plethora of subjects helped me in taking on multiple projects. Apart from research work,

Jim showed me how to achieve a good work-life balance. I must admit that I got inspired

from him to go into Salsa dancing. Jim, thanks for giving me the absolute freedom and

support in pursuing my interests, you have been a wonderful advisor.

How could I forget my lab colleagues with whom I spent most of my time during PhD.

Robert Caldwell and Sami Rosenblatt, I will always miss our hour long discussions about

current politics and life in general. Changyao, thanks for being my buddy in working late

nights in lab and clean room, I am sure you will achieve great results with all the hard

work. Mingyuan, you have been a great lab partner and someone I always looked upon

for nanotube theory. I know you are the next one to graduate and I wish you all the luck

in your job search. Anurag, Yuyao, Changgu, Adam, Bonnie, Josh, its been wonderful

working with you guys all these years. Zhengyi and Nick, although I did not have a

chance to work with you, I appreciate your help during my last year at Columbia.

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My sincere regards go to Mark Hybertsen, Jeffrey Neaton and Joydeep Bhattacharjee,

without their help it would not have been possible to answer challenging theoretical

problems that crowded my projects. I also want to thank Philip Kim, and his research

group, especially Meninder Purewal, Yuri Zuev, Vikram Deshpande, and Mitsuhide

Takekoshi. My earnest regards goes to Prof. Tony Heinz and his group for help in

Rayleigh spectroscopy, special thanks to Matt Sfeir, Yang Wu, Hugen Yan, Christophe

Voisin, and Stephane Berciaud. I also want to acknowledge Limin Huang (Prof. Stephen

O Briens group), Gordana Dukovic (Prof. Louis Bruss group) and Inanc Meric (Ken

Shepards group) for assisting in nanotube growth, catalyst synthesis and device

fabrication.

List of people outside the research circle that helped me go through this long run is

endless. I would especially like to acknowledge my long time friends Rohit and Anuj. I

wont forget Anujs delicious cooking, and his nook for an evening tea (so British!).

Rohit and I shared the liking towards going out and enjoy the city life (and frequent visit

to Toms restaurant). Kanishka, Ritvik and Navya, I look forward to more outings and

parties with you guys. Kanak, you have been a very supportive roommate all this time. I

am sure we will spend more time together. My very special thank goes to my closest

friend Shaifali Agarwal. Shaifali, your unwavering support helped me pass through the

toughest of times, be it PhD completion or job search. This thesis would have looked

awful without your meticulous proof reading. I feel fortunate to have your company.

Lastly, this work could not have been possible without the support of my family members,

Mom, you and father are the sole inspiration behind this work. People say you cant do

that bad when both your parents are PhDs, now I believe they were right. Brother, I

always felt you as the safety net I could lean on if things go wrong, thanks for being there,

always. Bua Ji, your letters have always been a source of power and courage during PhD,

I hope to keep these values forever. And finally to my beautiful sister-in-law, your

regular phone calls always cheered me up for the day. Now, I can also put a Dr. before

my name as you do, well of course, I wont still be a real doctor.

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Dedicated to my mother and late father

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CHAPTER 1

______________________________

Introduction to Carbon Nanotubes

______________________________

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1.1 sp2

Hybridization

sp2

2pz 2pz

sp2

sp2

sp2 sp2sp2

sp2

2pz

2pz

*

sp2

2pz 2pz

sp2

sp2

sp2 sp2sp2

sp2

2pz

2pz

*

ba

Figure 1.1: sp2

hybridization of carbon and its derived materials. a) The three sp2

hybridized orbital are in-plane, with 2p orbital orthogonal to the plane, and * denotes

the bonding and anti bonding orbital. b) Graphene as the source of three different

materials- Fullerene (left), carbon nanotube (Center) and bulk graphite (right). Nature

Materials

Carbon is the sixth element of the periodic table. Each carbon atom has six electrons

which occupy 1s2, 2s

2and 2p

2atomic orbital. It can hybridize in sp (e.g. C2H2), sp

2(e.g.

graphite) or sp3

(e.g. CH4) forms. This property is unique to carbon in its particular group

in periodic table. Discoveries of very stable nanometer size sp2

carbon bonded materials

such as fullerenes[1], carbon nanotubes[2] and graphene[3] has stimulated the research in

this field. In the context of this thesis we will focus mainly on Carbon nanotubes. Since

carbon nanotubes derive most of the physical properties from graphene, it is therefore

useful to understand this material first.

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1.2 Graphene

K

M

b1

b2

Unit Cell

A B

a1

a2

Brillouin Zone

y

x

y

x

ky

kx

ky

kx

Figure 1.2: Structure of graphene in real and reciprocal space. a) Crystal structure

with positions of two equivalent atoms in the unit cell of graphene. The unit cell is

represented by dotted parallelogram. b) 2-D Brillouin zone of graphene with location of

high symmetry points K, M and ; b1 and b2 are the reciprocal unit vectors.

Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely

packed in a honeycomb crystal lattice[4]. A schematic showing the 1-D graphene lattice

is shown in Fig. 1.1. Several graphene layer stacked over each other gives bulk graphite.

These layers are loosely bonded to each other and hence can slip freely, making the

extraction of graphene possible from graphite crystal through mechanical/chemical

exfoliation[5]. Due to the presence of sp2

hybridized carbon atoms in one plane, graphene

is atomically two dimensional. Fig. 1.2 shows the unit cell and the Brillouin zone of

graphene; a1 and a2 are the unit vectors in real space and b1 and b2 are the reciprocal

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lattice vectors. In a hexagonal lattice like this, the real space unit vectors can be written

as:

=

=22

3

22

321

aaa

aaa ,,,

rr

(1.1)

Where a is the lattice constant given by CCa 3 where is the carbon-carbon bond

length (0.144 nm). In the Brillouin zone (Fig. 1.2b) there are three high symmetry points

at the center, corners and center edge. The approximate energy dispersion relation for pi-

orbitals calculated through tight binding calculation in the Brillouin zone gives [6]:

CCa

21

2

24

22

341

/

coscoscos),(

+

+=

akakaktkkE

yyxyx (1.2)

It is important to understand that in such sp2

hybridize system only electrons contribute

to electronic transport; hence the above energy dispersion describes the electronic band

structure of graphene. Fig. 1.3 shows the energy dispersion for graphene plotted in the

Brillouin zone using equation 1.2. The upper half of energy curve describes anti-bonding

(*) orbital, while the lower part describes the bonding () orbital. One of the most

striking feature of graphene is that and * bands are degenerate at the K points in the

Brillouin zone, through which Fermi energy passes. Due to the symmetry requirements of

two equivalent carbon atoms in the unit cell of hexagonal lattice, graphene possesses a

zero gap at the K point, making it a zero band gap semiconductor.

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Figure 1.3: Graphene energy dispersion. The three dimensional graph has kx, ky and E

as axes. The location of high symmetry points K, M and is also shown. The Fermi

energy passes through K points (E=0). Its interesting to note that near the K point the

graphene dispersion becomes linear (massless).

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1.3 Carbon Nanotubes

Figure 1.4: Single and multiwall carbon nanotubes. Hypothetically one can visualize

the formation of single wall carbon nanotube through rolling single graphene sheet into a

cylinder. For multiwall nanotubes bi-layer graphene sheet will be the starting material

A single wall carbon nanotube is technically defined as a cylinder made up of rolled up

sheet of graphene as described artistically in Fig. 1.1. However, rolling up graphene is not

the actually way a nanotube forms; its actual synthesis process is explained in chapter 2.

The diameter of carbon nanotubes typically vary from 0.7-3 nm. Due to such small

diameters, nanotubes become quasi one dimensional. They can posses a single shell or

multiple shells, as depicted in Fig 1.4. Tubes with single shell are called single wall

carbon nanotubes (SWNT) while once with more than one shell are multiwall carbon

nanotubes (MWNT). The length of nanotubes can be up to centimeters, giving them an

astonishing length/diameter ratio of 107.

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Carbon nanotubes (CNT) were first discovered in the black soot product from a CVD

process[2, 7]. Since then, their synthesis techniques have evolved considerably. The last

10 years have seen tremendous research in both nanotube synthesis[8] and their potential

use in electronic circuits[9], composites, thin films[10], etc. Electronic properties of

nanotubes have attracted their use as metallic wires and as semiconducting channels in

field effect transistors. Processes have been developed to separate semiconducting from

metallic nanotube through solution based techniques. However CVD growth of specific

chirality nanotube is still not possible. On the industrial front, carbon nanotubes have

found use in making composites and gas sensors. This stems from their extraordinary

mechanical properties and high surface area.

1.4 Nanotube Crystal Structure

Unlike graphene, nanotubes can exhibit remarkably different physical properties

depending upon their structure. The crystal structure of a nanotube depends upon the axis

along which the cylinder is formed from the graphene sheet. Fig 1.5 describes the vectors

on graphene plane that are important in understanding the formation of nanotube from a

graphene sheet. The vector OB perpendicular to the nanotube axis is called the chiral

vector (Ch); the vector OA, which is parallel to the axis is termed as the translational

vector (T), this is the unit vector of 1-D carbon nanotube. The chiral vector is denoted by

nmwhereamanCh 021 ,rrr (1.3)

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a1

a2

5a1

5a2

+21

55 aaCh

Figure 1.5: Nanotube crystal structure. Formation of a (5,5) CNT is depicted on a

graphene plane. The shaded gray rectangle represents the unit cell of the (5,5) CNT. The

angle between the unit vectora1 and the chiral vector (Ch) is the chiral angle (0


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1.5 Nanotube Electronic Dispersion

For understanding the electronic band structure of nanotube the reciprocal lattice or k-

space is considered. The nanotube reciprocal lattice vectors, K1 (circumferential

direction) and K2 (along the nanotube axis) are obtained by the relation ijji KR 2 where Ri is the lattice vector in real space. Note that real space lattice vectors for

nanotube are Ch and T.

The electronic structure of carbon nanotubes can be obtained from the dispersion of

graphene through applying periodic boundary conditions along chiral vector Ch. The

main difference being that in CNTs electronic wave functions become quantized along

Ch as the circumferential length becomes comparable to Fermi wavelength (F). This

concept can be used to obtain the one dimensional dispersion for nanotubes through

taking out periodic slices from the graphene dispersion relation. This is summarized in

the equation 1.4[6]:

Tk

TandNwhereK

K

KkEkE graphene


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dispersion. Using this approach the dispersion and density of states of (5,5) nanotube and

(5,4) nanotube is plotted in Fig. 1.6.

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-10

-8

-6

-4

-2

0

2

4

6

8

10 (5,5)

E(k)/t(eV)

k

-8 -6 -4 -2 0 2 4 6 8

(5,5)

DensityofStates(eV-1)

E (eV)

-8 -6 -4 -2 0 2 4 6 8

(5,4)

DensityofState(eV-1)

E (eV)

-0.10 -0.05 0.00 0.05 0.10-4

-2

0

2

4(5,4)

E(k)/t(eV)

k

ab

c d

Figure 1.6: Band structure and Density of States (DoS). a) (5,5) armchair nanotube,

shows conduction and valence band crossing at E=0 (Fermi Energy). b) DoS plot on the

right shows constant states near Fermi energy. At higher energies, van hove singularities

are seen as a signature of the one dimensional band structure. c) (5,4) semiconducting

nanotube. d) DoS graphs show zero states in a finite energy window near the Fermi

energy. This is the semiconducting band gap.

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Whenever the crossing plane passes through the high symmetry K point, the nanotube

will have a finite density of states at the Fermi energy (E=0) and show metallic behavior.

In the event the cutting plane does not passes through the K point, the resultant nanotube

possesses a finite band gap and it will behave as a semiconductor, as shown for the case

of (5,4) nanotube in Fig. 5c,d. The energy gap of a semiconducting nanotube depends

only on its diameter, and is given byt

CC

gd

atE

= , here t denotes the overlap betweenthe wave functions used in the tight binding and is the carbon-carbon atomic

distance in the graphene lattice. Semiconducting and metallic CNTs can directly be

identified with the chiral indices (n,m). In general, if n-m=3q where q is a whole number,

then the tube is metallic, otherwise semiconducting. One important property of metallic

nanotubes is its linear (massless) dispersion near the K point. This is shown in Fig 1.7.

CCa

E

K1

K2

E

k

Cutting Plane passes through K point

Graphene bands near K point Metallic CNT bands near K point

Figure 1.7: Linear dispersion of metallic nanotubes near K-point. Green and red

depicts metallic and semiconducting nanotube dispersion respectively.

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It has some fascinating consequences. For example, due to massless dispersion, metallic

nanotubes becomes insensitive to any long range disorders such as localized charges near

the tube and hence their mean free paths become order of magnitude higher than

semiconducting nanotubes[11].

1.6 Nanotube Phonon Dispersion

Phonons are quantized lattice vibrations. Similar to electronic dispersion, the energy

dispersion for phonons (except for radial breathing phonon mode) can also be derived

from graphene through the zone folding technique. This is summarized equation 1.5.

1

2

2

21K

K

Kkww mD

m

D

r

r

r

Here, 61,......m denotes the modes of vibration (1.5)

A detailed calculation of phonon dispersion of graphene and nanotube can be found in

reference 1. Figure 1.8 shows the phonon dispersion of a (10,10) nanotube very near the

point. The three lines that intersect the zero energy are the acoustic modes of the

nanotube. Overall these lines correspond to four acoustic modes, since one of them

represents a doubly degenerate Transverse Acoustic (TA) mode in the x and y direction.

The other two modes are longitudinal acoustic (LA) in z direction and the twisting mode

(TW). In addition to acoustic modes there are various optical modes for example E2g

mode (17.0 cm-1

) and radial breathing mode (165 cm-1

) near k=0.

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o

Ao

A

Figure 1.8: Nanotube phonon dispersion. It is obtained through zone folding the

graphene phonon dispersion. Inset shows the phonon density of states of nanotubes with

the van hove singularities. The phonon density of states of graphene is shown with a

dotted line for comparison [12].

From the phonon dispersion relation it is possible to obtain the phonon density of states

(DoS), shown in the Fig. 1.8 (inset). The nanotube phonon DoS is constant below 2.5

meV and then increases as higher sub bands enter. Just like electronic density of states,

phonons also possess one dimensional Van Hove singularities near each sub band edge

[13].

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1.7 Summary

Most of the nanotube properties are derived from graphene, as briefly explained in this

chapter. However, contrary to graphene, carbon nanotubes exist in various crystal

structures depending upon their direction of roll up in the graphene plane. This results in

a variety of nanotube types represented by indices (n,m). Electronically, nanotubes exist

as metals as well as semiconductors, depending again upon the chiral indices (n,m). The

chapter also explained briefly the phonon band structure of nanotubes. This concept will

be helpful in later chapters when the origin of resistivity due to scattering of electrons due

to various phonon modes in the nanotube is discussed.

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6. Saito, R., G. Dresselhaus, and M.S. Dresselhaus, Physical Properties of Carbon

Nanotubes. 1998: Imperial College Press.

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8. Dai, H.J., Nanotube growth and characterization. Topics in Applied Physics,

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9. Ouyang, M., J.L. Huang, and C.M. Lieber, Fundamental electronic properties

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CHAPTER 2

______________________________

Carbon Nanotube

Synthesis - Processes and Concepts

______________________________

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2.1. Introduction

The first experimental evidence of Carbon nanotubes (CNTs) came in 1991[1] in the

form of multi wall nanotubes (MWNT). This prompted a sudden increase in nanotube

synthesis research. In 1993, the first experimental evidence of single wall nanotube tubes

(SWNT) came [2, 3]. Since then, the synthesis methods for CNTs have been developed

tremendously. This chapter explains the basics of CNT growth as well as describes

several unique CNT growth processes developed throughout this study.

Production methods for carbon nanotubes (CNTs) can be broadly divided into two

categories: chemical and physical depending upon the process used to extract atomic

carbon from the carbon carrying precursor. Chemical methods rely upon the extraction of

carbon solely through catalytic decomposition of precursors on the transition metal

nanoparticles, whereas physical methods also use high energy sources, such as plasma or

laser ablation to extract the atomic carbon. Traditionally, physical methods give bulk

quantities of CNTs which could then be treated chemically to remove any carbon soot or

nanoparticles present in the mixture[4]. These two approaches can further be

characterized according to the use of other important aspects of the synthesis process,

such as type of precursor and transition metal nanoparticles used. In spite of being

thoroughly investigated for the last 10 years, CNT growth process has remained

somewhat controversial. Although the exact dynamics of the growth is not yet clear,

consensus has been reached on a hypothesis which works pretty well. It is so far been

understood that the nanotubes grow from the over saturation of the transition metal

nanoparticles with carbon atoms. A sketch of process is shown in Fig. 2.1.

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Atomic Carbon from precursor gas

Nanotube growth direction

Catalyst Nanoparticle

Atomic Carbon from precursor gas

Nanotube growth direction

Catalyst Nanoparticle

Figure 2.1: Current CNT growth mechanism hypothesis. Transition metal

nanoparticle acting as catalyst gets saturated with atomic carbon to grow nanotubes.

The over-saturation of nanoparticles with carbon atoms results in the production of

different type of molecular carbon species, like graphitic carbon, carbon filaments,

multiwall carbon nanotubes, single wall carbon nanotubes and most recently, graphene.

Selecting the right conditions to grow any of these materials (especially, nanotube and

graphene) has remained a trial and error approach. This has resulted in a plethora of CNT

growth papers, each having a specifically tuned recipe for growth, which is very difficult

to reproduce in any other system or setup.

The holy grail of controlling the growth has focused the synthesis effort more towards

chemical vapor deposition (CVD) process where the yield of tube growth is not that high

and it can finely be controlled through the use of catalyst nanoparticles placed on a

silicon chip. The next few sections will detail CVD process and advancements.

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for higher rate of nanotube growth, very high temperatures can initiate self dissociation of

gases which can cause catalyst poisoning. Apart from the temperature, the dissociation

rates of the precursor can also be controlled by the partial pressure of the species. For

example, at high pressure CO dissociation rate increases, hence to achieve higher

nanotube yield high pressure CO growth processes are used. A well known example is

the HiPCO process from Smalleys group[16] at Rice University. Similarly, low pressure

growth has also been used to decrease catalyst poisoning to achieve ultra long CNTs [6].

Another important parameter associated with the precursor is its feed rate. At very high

temperatures where the precursor is near self-dissociation, the reaction rate gets limited

by the precursor feed rate in the system. High feed rates can increase the rate of growth

but just like high temperatures can also result in more of carbon soot formation and hence

catalyst poisoning. Precursor feed rate is also coupled with the size of catalytic

nanoparticles. For example in a study by Cheung et al[17], it is shown that different

carbon species can be produced with the same diameter catalyst nanoparticles using

different carbon carrier gas flow rates. At higher flow rates, larger diameter nanoparticles

will grow more since the smaller ones will quickly get poisoned and vice versa for lower

flow rates.

In a typical growth scenario, carbon precursors are premixed with other gases such as H2

or other carbon carriers (e.g. C2H5OH+CH3OH)[18]. This is done to have finer control on

the reaction rates inside the CVD chamber. For example, if the product after dissociation

is H2, then a fine control of premixed hydrogen can be used to check on the dissociation

rate of carbon precursor. This is discussed in detail in the succeeding experimental

section. Carbon precursor gases can themselves produce unwanted nanotubes due to the

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presence of premixed nanoparticles coming from the contamination in the gas

cylinder[19]. Gradual changes in growth patterns with change in cylinder pressure have

been reported before. It is explained in chapter 3 that how this shortcoming can be used

for achieving ultra dense nanotube growth on carbon fibers. This suggests that for a well-

controlled CVD system the use of cleaner gases or gas filters is essential in achieving

reproducible CNT growth results.

2.2.2 Temperature

The ideal temperature for CNT growth depends on several factors, mainly carbon

feedstock, catalyst, and the type of CNTs desired (single or multi wall). Nanotubes are

typically grown in a temperature range from 550oC to 1000oC [10]. In addition to

changing the reaction kinetics during growth, the temperature plays an important role in

the pre-growth treatment of the catalyst. Small catalytic nanoparticles readily oxidize

readily under ambient conditions; therefore, to bring them back to native state a

controlled reduction step is required at moderate temperatures (~700oC)[20].

Temperature can also significantly affect the growth depending upon ramping rates.

Ramping up the furnace temperature at different rates is shown to have profound effect

on the CNT growth kinetics. For example, Lius research group at Duke University has

shown that fast heating[8] of the growth tube produces long and aligned CNTs. This was

attributed to the effect of temperature on the flow conditions near the surface which cause

catalyst particles to leave the substrate and grow long tubes. The role of temperature is

becoming more important as nanotube world looks towards growing graphene through

CVD process [21-23].

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2.2.3 Catalyst Nanoparticle

Catalysts are transitional metal nanoparticles obtained through various sources, such as

metal salts, evaporated metal films, etc. Two factors that define a catalyst particle are its

size and composition. Nanoparticles of Fe, Co, Mo, Ni, Cu, Au, etc. [10] have been tried

as catalyst either in pure metallic form or as alloys. These transition metal nanoparticles

have common advantages of high carbon solubility, carbon diffusion rates and high

melting temperatures. Various approaches are used to obtain the nanoparticles; some of

the most common ones use metal salts (nitrates, sulfates and chlorides), where stable

nanoparticles complexes are formed in a suitable solvent. Evaporated film of metal can

also produce uniform nanoparticles upon controlled annealing treatments. Organic

carriers have also been used in the formation of very small size nano clusters; one of such

catalyst is used for the current research[24]. Sizes of these particles are reported to be

anywhere from 1 nm to 15 nm. There are strong indications of dependence of CNT

diameter on the catalyst particles size; hence narrowing the catalyst diameter can help in

a controlled diameter nanotube growth. Also of tremendous importance is the method

used to apply the nanoparticles on the growth substrate, which affects their resultant

morphology significantly. Most nanoparticles are suspended in solvents, which after

drying out on the growth substrate give a coffee stain effect; this causes the nanoparticles

to clump before growth, making their average size high. Several innovative methods are

in use to get around this problem, for example, mixing the nanoparticles with polymers

before application on substrates has significantly helped in achieving well-separated

particles of uniform size[25]. Another promising approach is to mix the particles with

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PMMA or photo resist and pattern them using lithography techniques. Every approach

has its pros and cons and work best for a particular type of CNT growth.

2.2.4 Gas Flow Rates

Flow rate of carbon precursor affects the rate of carbon dissociation during the growth. In

addition to affecting reaction rates, gas flow conditions are also responsible in defining

CNT length and orientation during growth. The effect of flow conditions such as

turbulence, stable boundary layer, etc. on the tube growth was first demonstrated by

Lius[8] research group for CO growth. Longer tubes are regularly seen to grow more

from wafer edges than the middle of the wafer. They are also found to be aligned to the

gas flow direction. The observation of wavy tubes is attributed to the unsteady flow

conditions near the growth wafer (excluding the quartz crystal aligned growth, where the

mechanism is faulty crystal steps). A recent publication on suspended growth of

nanotubes found local thermal oscillations to play a big role in obtaining aligned

growth[26]. In order to stabilize the near surface flow several techniques have been tried,

such as bringing down the quartz tube cross section and decreasing the flow rates to very

low values. The large variability in nanotube lengths could be attributed to the type of

growth steps they go through. It is now generally accepted that nanotube growth

processes are of two kinds: 1. tip based, 2.root-based. In the former process the catalyst

nanoparticle remains in the end of the CNT, flowing along with the flow while growing

CNT in the back. In the latter process the particle stays on the surface while the tube

grows up due to stress induced by carbon atoms saturating the catalyst from below (Fig.

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1.1). Tip-based process achieves very long length tubes since metal particle gets saturated

in air while flowing. More of fluid flow studies will be discussed in the coming sections.

2.3 CNT Growth Experiments

2.3.1. Motivation and Aims

Since the start, CNT growth processes have been riddled with problems relating to

control over type of CNTs grown and process reproducibility. One major issue with

understanding the growth results is a lack of good characterization techniques. Imaging

techniques such as Atomic Force Microscopy (AFM) and Scanning Electron Microscopy

(SEM) have a high throughput and are used frequently for characterizing grown

nanotubes. However, they have big error margins in measuring nanotube diameters and

cleanliness. It is also almost impossible to distinguish a double wall nanotube, or a small

nanotube bundle from a single tube, using these techniques. Current project aims to

characterize grown CNTs much more accurately using new techniques such as Rayleigh

spectroscopy on suspended nanotubes (described in section 4.2.2). The high throughput

and accuracy of this technique promises to give reliable information about type of

nanotubes grown through every growth process. Rayleigh spectroscopy was used through

out this project to check if the tubes are single or bundled, whether they have amorphous

carbon deposition or not, and if they are single then their diameter and chirality. So much

information about every growth done was not available before. Therefore the use of this

technique with CNT growth gave the impetus for a detailed CNT synthesis study. Fig.2.2

shows a schematic the substrate used for growing nanotubes in this study.

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Apart from studying the synthesis process, the use of chirality assigned single tubes in

several other projects called for highly tuned recipes to achieve specific goals. These

goals can be summarized in following points:

1. Ultra long nanotubes for electron transport measurements such as resistivity and

mean free path determination. .

2. It was found through Rayleigh spectroscopy that more than 99% of long tubes are

bundles of 2-5 tubes. Finding ways to grow single tubes through changing growth

parameters was very important.

3. Clean, amorphous carbon free nanotubes for good electrical measurements.

Gas Flow Direction

Figure 2.2: A schematic of the nanotube growth wafer. Suspended nanotubes are

grown over a slit, etched through a silicon wafer for Rayleigh characterization.

2.3.2 Growth Setup

Fig. 2.3 shows the schematic of the CVD growth system. This consists of a one inch

quartz tube inside an oven with a temperature controller capable of reaching 1200 oC.

Figure 2.3: CVD growth system. Inside the quartz tube, a quartz sample holder is used

to carry the growth substrate. Flow of gases is controlled through electronic mass flow

controllers.

feed gases

vent

Flow Controllers

feed gases

vent

Flow Controllers

Carbon Nanotube

Gas Flow Direction

Carbon Nanotube

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A variety of parameters were studied for the CVD growth process. This included several

carbon precursors, catalyst nanoparticles, various temperature ranges and flow conditions.

The effect of each parameter change on the resultant CNT growth is observed and

recorded. The next few sections will describe the variety of growth processes classified

on the basis of carbon feedstock.

2.3.3 Carbon Monoxide Growth

Carbon monoxide is a highly efficient feedstock for CNT growth, both for bulk

synthesis[27]

and for direct CVD growth on substrates. For direct growth onto substrates,

CO is mixed with H2 to optimize nanotube growth and minimize the deposition of

amorphous carbon. A significant advantage of CO as a feedstock is that the process

window for CNT growth is very wide: good growth is generally achieved for hydrogen

concentrations of ~20% to ~70% at 900 C[28].

Another advancement in CO CVD has

been the efficient production of long SWNTs by a fast heating technique[8]. In this

method, a substrate with catalyst is inserted directly into the hot zone of a furnace under

flow of feedstock gas. Under these conditions, a significant proportion of CNTs evidently

grow by tip growth mechanism described previously. This allows for very fast nanotube

growth and alignment of the tubes with the gas flow in the CVD reactor. Iron

nanoparticle catalysts (diameter ~ 2 nm) are deposited on Si/SiO2 substrates, either

randomly or in patterned areas. The substrates are then inserted into a 1 diameter quartz

tube that fits into a tube furnace (18 hot zone). The quartz tube is long enough (48) so

that the sample can be moved in and out of the hot zone by simply sliding the tube. The

substrates are first placed in the hot zone of the furnace and kept in pure oxygen at 650

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C for 30 minutes to burn off the organic capping layers on the nanoparticles and any

residual photo resist on the substrate. The substrate is then removed from the hot zone.

The oxygen flow is turned off and replaced (after Argon purge) with CO/H2 at flow rates

of 1000 SCCM for both gases. The oven is heated to 900 C, allowed to stabilize, and the

sample is quickly inserted into the hot zone. The substrate is maintained at the growth

temperature for 20-30 minutes, followed by slow cooling in pure H2 to room temperature.

The effect of hydrogen on the nanotube growth can be attributed to two factors. First, the

direct hydrogenation of CO produces carbon as well as water (H2O), which helps in

cleaning the catalyst particles off any amorphous carbon deposition. The second reported

effect of H2 is that its adsorption on the catalyst nanoparticles increases the catalytic

dipropotionation of CO [16] which increases the nanotube growth rates. In addition to

this hydrogen also provided a reducing atmosphere during the processs. Figure 2.4a

shows arrays of long SWNTs that can be achieved by fast-heating CO CVD.

Figure 2.4: Carbon monoxide grown nanotubes. a) Scanning Electron Microscope

(SEM) images tubes grown through fast heating method. b) Tubes grown through

patterning local catalyst islands.

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It is also possible to grow long nanotubes from individually patterned catalyst pads, as

shown in Fig. 2.4b. The catalyst pads were defined by photolithography employing a

two-layer resist consisting of a layer of SU-8 photo resist on top of PMMA. Because

PMMA is soluble in SU-8 developer, patterning the top SU-8 layer also selectively

removes the underlying PMMA. After deposition of catalyst, the bilayer is lifted off in

acetone. The PMMA acts as a release layer, while the SU-8 prevents stray redeposition of

the catalyst onto the substrate.

Although carbon monoxide showed a very consistent and dense growth it suffered from

some major drawbacks:

1. The self decomposition of CO resulted in considerable amorphous carbon

deposition in the quartz tube. This threatened the cleanliness of the grown

nanotubes.

2. Due to the hazardous nature of the gas, the CVD setup has to be enclosed and

continuously vented.

3. The presence of iron-carbonyl as impurity inside the gas frequently gave

spontaneous gas phase growth of nanotubes. In order to reduce this, the gas had to

be passed through a preheating oven to deactivate the impurity metal

contamination through carbon crackdown, making the process more complex.

4. The metal impurities in the gas were destroying the mass flow controllers, since

these controllers used an active micro heater inside them to calibrate flow rates.

These disadvantages shifted the focus towards ethanol based process, which was

already proven to be clean and non-hazardous [29].

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2.3.4 Ethanol Growth

Ethanol process is a low temperature method for growing single wall CNTs[30]. The

method uses Ar as carrier gas to introduce ethanol in the growth chamber as a carbon

feedstock. The amount of ethanol taken away by per unit volume of gas is controlled by

keeping the bubbler in an ice bath which controls the ethanol vapor pressure.

Traditionally, ethanol growth process was used with Co catalyst either in solution phase

(Co salt) or in evaporated thin film form[15]. Both the approaches worked well for

growing ultra long nanotube bundles; though single tubes were rare. This may be due to

the clumping of catalyst nanoparticles in solution phase or during the growth process. In

order to solve this problem a new type of catalyst was prepared from a published

nanoparticle cluster synthesis process[31]. The catalyst nanoparticle consisted of Fe and

Mo atoms arranged in a cage like structure. The exact structure is shown in Fig. 2.5.

Figure 2.5: Structure of Fe-Mo catalyst nanocluster (Mueller Catalyst), green spheres on

the outside wire frame are 30 FeIII atoms, encapsulated inside are 12 {(Mo)(Mo)5}

pentagons. Image taken from ref [31].

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The circumferential atoms are bonded with acetate ligands making the cluster readily

soluble in hydrophilic solvents like water, ethanol. The goal was to apply this catalyst on

silicon oxide substrate without letting it clump once solvent is evaporated. This was

achieved through mixing it with a surfactant polymer (Pluronic F-127) before its

application on growth wafers.

The high viscosity of surfactant polymer prevented the catalyst from clumping after the

solvent is evaporated. The polymer is then burned in air to get mono dispersed cluster of

nano-particles. An important step in this process is diluting the catalyst solution

sufficiently so that resultant nanoparticle clusters are too well separated to clump even at

high growth temperatures. In addition to addressing the catalyst problem various small

changes to the CVD system helped in achieving the desired growth.

1. Replacement of the Teflon tubing with the No-Ox tubing helped in diffusion of

oxygen inside the growth system. Oxygen diffusion through sintered polymer

tubes such as Teflon was found to be a major source of irreproducibility in the

growth results.

2. Use of leak proof quartz tube fittings helped in checking for any potential leakage

at the quartz tube-gas inlet joint.

3. With the use of industry standard mass flow controllers the growth system was

made completely computer controlled. This resulted in the execution of precise

values of flow rate and temperature that eventually made recipes repeatable. An

image of the computer interface of the CVD growth system is shown in Fig. 2.6.

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Figure 2.6: Carbon nanotube CVD growth computer interface. The program controls

the flow rates of gases, oven temperature as well as process timings.

Encouraging results were obtained using the new catalyst nanoparticles with ethanol

CVD process. Furthermore, optimized flow and temperature conditions were achieved by

testing CNT growths at different conditions. One major issue with ethanol grown CNTs

was their cleanliness. The CNTs easily get coated with amorphous carbon if the growth

conditions are not ideal. It was found that above 860oC, self-dissociation of ethanol

caused significant amorphous carbon deposition over the CNTs. This effect could be

observed through Rayleigh scattering as shown in Fig. 2.7. Typically, single nanotubes

with higher amorphous carbon deposition show a higher light scattering intensity in the

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background than the cleaner tube. Using this feedback from the Rayleigh technique it was

possible to get a finely tuned recipe that grows cleaner CNTs.

Figure 2.7: Rayleigh scattering spectrum on two different suspended (16, 16) armchair

carbon nanotubes, spectrum (a) has less signal to background ratio than spectrum (b).

This suggests that the (16,16) nanotube in (b) is cleaner than in (a).

The growth of clean, single walled CNTs has made possible to gather sufficient number

of suspended tube samples on TEM compatible chips for preparing the database which

that relates the tube chirality with Rayleigh scattering peaks. This database is updated

continuously and is currently used to directly assign the tube indices by obtaining the

Rayleigh scattering peak position. Chirality and diameter values assigned using such a

database for nanotubes grown through ethanol CVD process are shown in Fig. 2.8.

Nanotube diameter distribution ranges from 1.2 to 2.8 nm with a clear peak at ~ 2nm.

This is two to three times higher than what is expected from the size of the catalyst

nanoclusters, this indicates toward possible clumping of two to three catalyst

nanoparticles during growth. The chirality distribution is more towards higher chiral

2.0 2.2 2.4 2.6

E(eV) 1.6 1.8 2.0 2.2 2.4 2.6

Intensity

(a.u.)

E(eV)

a b

2.0 2.2 2.4 2.6

Intensity

(a.u.)

E(eV) 1.6 1.8 2.0 2.2 2.4 2.6

Intensity

(a.u.)

E(eV)

a b

Intensity

(a.u.)

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angles, something which is still not understood, although there have been theoretical

predictions of near-armchair tubes being more stable in structure than tubes of other

chiralities.

12 16 20 24 28

0

5

10

15

20

No.oftubes

Diameter (A)

0 5 10 15 20 25 30

0

4

8

12

16

No.oftubes

Chiral Angle (degree)

Figure 2.8: Diameter and chirality distribution of single wall carbon nanotubes grown

through ethanol CVD process using Mueller catalyst.

The ethanol growth process could also be tuned to the length requirement of CNTs. Ultra

long nanotubes (~ 10 cm) were produced through increasing the growth temperature

(~885 oC) and using higher gas flow rates.

2.3.5 Methane Growth

The very first CVD growth of nanotube achieved on silicon wafers used CH4 as the

feedstock[12]. The interest in CH4 based growth increased with the reporting of ultra long

and aligned nanotube growth through ultra low flow of methane[32]. This approach was

counterintuitive to the high flow rate growth and the fast heating growth mechanism

suggested previously. Since CH4 doesnt dissociate that easily, it requires high

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temperature during growth. It is hypothesized that due to the use of high temperature,

CH4 process grows smaller diameter nanotubes than the ethanol based growth.

A separate growth system with ultra low manual flow controllers was designed to make

methane growth possible. Mueller catalyst was used for this process. The growth

temperature was tested in the range of 940-980 oC. Methane flow rate was at ~2 sccm

while hydrogen at ~4 sccm. Further details of the process are provided in reference[32].

Fig.2.9 shows the resultant nanotube growth on silicon wafers.

2.2 2.4 2.6

Intensity

(a.u.)

Energy (eV)

a b

Figure 2.9: Methane growth results. a) SEM image of ultra long nanotubes (> 2mm)

are grown over silicon wafers, the tubes are crossing a slit on the wafer as shown in the

right SEM image. b) Rayleigh spectrum of one of the single, suspended tube section

grown through methane process.

The nanotubes were very straight and ultra long. These tubes were grown crossing the

slits on the silicon wafers for Rayleigh characterization. Through Rayleigh scattering

technique, it is found that nearly all of these long tubes were big bundles of single wall

nanotubes. Also they did not seem to be very clean. The Rayleigh spectrum of one of the

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single tubes obtained from this growth process is shown in Fig. 2.8b. Due to the virtual

absence of any single tube, the process was not continued.

2.3.6 Water-Assisted Ethylene Growth

Ethylene growth combined with water vapors has been proven to grow ultra tall vertical

nanotube forests, which can be used for applications such as nanotube yarns and

composites. One of the salient features of this process was the use of water during growth,

which was carried by argon gas into the growth chamber. It is hypothesized that water

helped in cleaning the catalyst hence increasing its lifetime[6]. Thereby tubes can be

grown for a longer time. The growth process to achieve these tall nanotube forests is as

follows:

1. Substrate preparation: 10 nm of aluminum oxide is deposited through Atomic

Layer Deposition (ALD) system on a Si/SiO2 wafer. Thereafter, 1 nm thin film of

iron as a catalyst is evaporated using Electron Beam Evaporation (EBE).

2. Prepared substrate is placed in the quartz growth tube on a quartz sample holder

and the oven is heated to a temperature of 750oC in a gas flow of Ar (600 sccm)

and hydrogen (400 sccm).

3. At 750oC, ethylene (150 sccm) mixed with argon (40 sccm) coming separately

through the water bubbler is introduced in the growth tube. The growth is done

for 15-25 minutes. Once the growth is over, the system is cooled down to room

temperature in Argon (600 sccm) and hydrogen (400 sccm).

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Figure 2.10: Water-assisted ethylene growth results. Left SEM image shows the ultra

tall vertical CNTs forest. A high magnification SEM image of these tubes is shown in the

right.

Millimeter tall and straight nanotube forest can be seen in Fig.2.10. A closer look through

scanning electron microscope suggests that the CNTs are not as aligned as they looked at

lower magnifications. The forest contains a directional growth CNTs which meshed up

with each other.

Such growth has been utilized for making nanotube composites and probing the photo

mechanical response of CNTs. These CNTs are also finding research applications in

fabricating flow channels for understanding transport of water inside carbon nanotubes.

2.3.7 Nanotube Growth: Fluid Flow Study

Gas flow conditions play an enormous role in deciding the length and alignment of CNTs

during CVD growth. Ultra high flow rates, fast heating approach[8], as well as ultra low

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flow rates[32] of gases have been tried to achieve directionally uniform, ultra long

growth of CNTs. Although there are several dissimilarities among these approaches, they

all agree on the fact that CNTs grow in the direction of the gas flow. A major concern

was that these ultra long nanotubes were rarely single. Through Rayleigh spectroscopy

it is found that more that 99% of long nanotube were bundle of single wall tubes.

One approach to reduced bundling is to separate the catalyst particles far away so that the

CNT grown from them dont bundle up due to close proximity. This was achieved by the

use of Mueller catalyst and polymer combination as explained in section 2.2.4. However,

it was found that the tubes still get bundled because of flow conditions near the substrate,

influenced heavily by surface geometry.

a b

Figure 2.11: Flow study surrounding a raised structure on silicon chip. a) SEM

image of the nanotube growth obtained through a raised pillar. Crossing of tubes suggests

that nanotubes follow the flow lines while growing. b) Simulated flow lines around the

structure.

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Fig. 2.11a shows the SEM image of a pillar, etched on the surface of the silicon wafer.

Catalyst is stamped on the surface of the pillar and CNTs are grown from it. As seen

clearly in the image, tubes do tend to bundle up following the flow lines around the raised

structure. The flow lines around the pillars are shown in Fig. 2.11b. The fluid flow

simulations are done using COMSOL FemLabTM finite element analysis software.

a

b

Figure 2.12: Flow movement in side plane of the slits and raised structure. a) SEM

image of the tubes following a parabolic trajectory while crossing the slit. b) Color shows

the average flow velocity with red being the higher. The flow stream lines are represented

by red lines. The white line represents a particles trajectory with the mass of a catalyst

nanoparticle. The particles origin was set very close to the surface. This suggests that

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with the right flow lines, particles from the surface can lift up to give ultra-long

nanotubes.

Fig. 2.12a shows the SEM image of grown tubes in the side plane. The trajectory that

tubes follow when grown over the slits from a catalyst particles right underneath the

pillar is highlighted through the arrows. This matches well with flow lines simulated in

the Fig. 2.12b. The silicon chip in the model of Fig. 2.12b is tilted at an angle to show

the leading edge effects, which can make a catalyst particle pulled up from a leading edge

and follow the flow streamline while simultaneously growing the CNT from back (tip-

based growth). This also explains why most of the long tubes grow from the wafer edges.

It is also evident that CNTs follow the flow streamlines during growth. Since the flow

velocity near to the surface is very low, there is no fluid turbulence. Therefore, the tube

directionality is determined mainly by local flow perturbations created by the raised

structures near the surface.

With the knowledge of flow simulation results, raised platforms are designed across the

slits to help grow unbundled tube. These platforms served multiple functions:

1. With platform so close to the slit, its edge can be used to grow long, flow aligned

tubes. This is opposed to using the chip edge which is far away from the slit. This

decreases the chances of tube bundling on way to the slit.

2. Less catalyst is needed, because most of the tubes that grow from platform edge

cross the slits. The high growth efficiency through edge allows the use ultra low

catalyst density (~ 10000 nanoparticles/ mL) hence directly reducing bundling.

3. Once characterized through Rayleigh, it was much easier to transfer suspended

tubes onto the target chip with raised platforms across the slit.

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Developing an understanding of the nanotube growth dynamics and its relation with local

flow conditions helped in achieving unbundled, yet straight and clean single wall carbon

nanotubes. More work towards studying local flow conditions along with temperature

gradients occurring near the growth substrate can deliver better results.

2.3.8 Conclusion

This study contributes significantly to the understanding of nanotube growth through

CVD process. A wide growth parameter range is covered, as shown in Fig. 2.13 below.

Growth Parameters Studied

TemperatureCarbon Feedstock Catalyst

Figure 2.13: Flow chart describing CVD growth parameters studied

The effect of local flow conditions in the resultant CNT growth is also studied through

experiments as well as theoretical models. Overall, the research furthers the

understanding of various ways through which multiple CVD growth parameters can be

chosen and controlled to attain the type of growth desired.

Fe, Co, Mo Salts

Fe Nanoparticles

FeCl3

Fe-Mo Complex

Low (


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CHAPTER 3

______________________________

Carbon Nanotube

Synthesis - Technology Applications

______________________________

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3.1 Growth of CNTs on Carbon Toray Paper for Bio-

Fuel Cell Applications

3.1.1 Introduction

Since their discovery in 1991[1], CNTs have been extensively researched for their

potential applications in semiconductor devices, solar cells, composites, displays, etc[2].

Their unique quasi-one dimensional structure gives them extraordinary electronic (high

current carrying capability) as well as mechanical properties[3]. In this work, we examine

CNTs as high surface area support for one such application: a bio-fuel cell. Bio-fuel cells

utilize enzymes as catalysts; however, because of the relatively low enzymatic activity,

large amounts of them are required. In a previous work, research has employed paper

made up of carbon fibers mixed in a binder material as a high surface area catalyst

support[4] to enhance the activity. The aim here is to utilize the extraordinary high

surface area per unit volume of CNTs to further enhance the fuel cell reaction rates

Presently, carbon nanotubes are grown by various chemical vapor deposition (CVD)

techniques, viz. arc discharge process, plasma enhanced chemical vapor deposition

(PECVD), high pressure carbon monoxide process (HiPCO), etc. All these processes use

some kind of catalyst and carbon carrier gas combination for the nanotube growth [5].

The main disadvantage with all the above listed processes is inefficient use of resources

(gases, high power consumption) and long time of operation (typically 4-5 hours per

growth). The next section describes a newly developed approach to grow CNTs on

carbon TorayTM

paper through utilizing joule heating and gas phase catalyst.

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3.1.2 Experimental Setup and Process Parameters

CVD growth of CNTs on carbon fibers process involves the local heating of growth

substrate, which is in contrast to the regular CVD processes in which the whole growth

chamber is heated [6, 7]. The method is fast, and excellent for the application involving

growing multiwall carbon nanotubes (MWNT) through a porous, conducting carbon

paper. A schematic of such a growth setup is shown in Fig 3.1.

To Pump/ExhaustGas Inlet

Pressure Gauge

Electrode

Glass Flask

Carbon paper

Power supply electrodeTo Pump/Exhaust

Gas Inlet

Pressure Gauge

Electrode

Glass Flask

Carbon paper

Power supply electrode

Figure 3.1: Schematic of the nanotube growth system.

The carbon paper (Toray inc., 90m thickness, 40mmx20mm) is clamped between two

copper electrodes over a 20mm x 13 mm rectangular slot made in a ceramic plate. Carbon

carrier gases containing a gas phase catalyst (Fe (CO)5) is passed through the porous

paper via a passage below it. Carbon paper is sandwiched between two identical ceramic

plates placed over the gas inlet. This arrangement makes sure that the gases pass

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through the paper, to help achieve CNT growth throughout the paper thickness. The

growth assembly is enclosed in a leak proof glass enclosure as shown in Fig 3.1. The

chamber is first evacuated to the base pressure of 50 m-torr. It is then purged with argon

gas at a steady flow rate of 2500 sccm, until the pressure reaches close to atmosphere.

Finally, the chamber is pumped back to the base pressure and the throttle valve connected

to the pump is closed. This process makes the chamber environment free of any traces of

oxygen. Thereafter, a mixture of approximately 75% CO, 25% H2 (both Ultra high purity

grade) is fed into the chamber at a flow rate of 300 sccm and 100 sccm respectively. It

takes 8-10 minutes for the pressure in the chamber to reach 350 torr; here, the throttle

valve for pump is opened slowly to make the pressure constant inside the chamber.

Current is then passed through the carbon paper by an external DC power supply

(Agilent E3633A) till the carbon paper reaches the growth temperature. Most of the

heating takes place in the paper only since it is the most resistive part in the circuit (sheet

resistance 1.4 / ). The selection of CO as the carbon feedstock is due to its longer

CNT growth temperature window than other gases.

The effect of hydrogen on the nanotube growth can be attributed to two factors: first, the

direct hydrogenation of CO produces carbon as well as water (H2O). This increases the

reaction of carbon production as the byproduct H2O helps in cleaning the catalyst

particles off any amorphous carbon deposition[8]. The second reported effect of H2 is

that its adsorption on the catalyst nanoparticles increases the catalytic dipropotionation of

CO through Boudouard reaction [9, 10] which increases the nanotube growth rates.The

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catalyst used in these experiments is gas-based iron penta-carbonyl (Fe(CO)5), trace

amounts of which is present as impurities in the carbon monoxide.

3.1.3 Growth Results

Growth of CNTs is characterized by Scanning electron microscopy. The overall surface

area of nanotube grown substrate is measured with electrochemical capacitive

measurements as well as with nitrogen physiosorption measurements (BET).

Figure 3.2: Scanning electron images of CNTs grown on carbon paper

Fig. 3.2 shows a high magnification SEM image of the CNTs grown on an individual

carbon fiber. The average length of the CNTs is less than 500 nm. Scanning electron

microscope images of the CNTs grown over the carbon fibers for different time intervals

is shown in Fig. 3.3. Density of nanotubes increases continuously with growth time; this

is mainly due to the use of fresh gas phase catalyst coming in continuously with the

incoming gas. This would not have been possible with the substrate-based catalyst due to

steady deactivation of catalyst nano-particles with growth time. After 24 minutes of

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growth nanotubes significantly start filling up the voids between these carbon fibers as

seen in Fig. 3.3.

15 sec 30 sec

45 sec24 min 28 min30 min

Figure 3.3: MWNT growth at different time intervals. Very thin layer of nanotube is

seen at just the beginning of growth (t=15 sec). However as the growth time increases the

tube density increases too. After 30 minutes the tube density becomes high enough to

start filling the void space between the carbon fibers.

Fig. 3.4 shows capacitive and BET surface area measurements of nanotubes grown over

Toray paper. Capacitive measurements give lesser surface area estimates than nitrogen

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physiosorption (BET) measurements because of inaccessibility of large electrolyte ions to

pores smaller than their size. The rate of surface area increase is highest for the first few

minutes of the growth. After 30 minutes of growth the overall surface area is increased

by a factor of 180 as compared to bare Toray paper. Increasing the growth time more

than 30 minutes does not result in any appreciable increase in surface area due to increase

in clumping of CNTs, which decreases the effective area available for nitrogen

physiosorption in BET measurements. This also decreases the pore size for ions to go

through in an electrochemical capacitive surface area measurement setup.

Figure 3.4: Effect of carbon nanotube growth time on surface area

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3.1.4 Summary

Use of gas phase catalyst with local resistive heating of the substrate grows ultra-dense

CNTs on a carbon fiber substrate. The CNT mats increase the surface area of the carbon

fiber paper by more than two orders of magnitude. This promises an improvement in the

catalytic efficiency of bio-fuel cells where carbon substrates are used as enzyme support.

Study of the nanotube growth time on increasing surface area suggests an ideal time after

which the surface area saturates. Study of various other growth conditions like

temperature, flow rates, catalyst concentration can help in growing longer tubes which

can fill the voids between the carbon fibers with less clumping; this has the potential of

further improving the surface area of the carbon paper.

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3.2 Growth of Transparent, Conducting CNT Films

3.2.1 Introduction

Thin, transparent and electrically conducting films are frequently used in consumer

appliances, such as LCD touch screen displays, organic LEDs, etc. Their future use in

thin film solar cells, e-paper and foldable displays is imminent. However, current thin

film materials used for this purpose (such as Indium Tin Oxide or ITO) are very

expensive and not flexible enough to allow for such advanced applications.

CNTs can act as a viable transparent, conducting thin film material because of its

extraordinary electronic properties such as high conductivity and mobility, as well as

excellent mechanical and electronic stability towards deformation. A random network of

CNTs deposited on a substrate is a quasi 2-D network of metallic and semiconducting

wires. Due to 1-Dnature of each wire, the network is transparent for infrared and visible

wavelength regions. The reflectivity of these films is also very low. One interesting

aspect of the nanotube network is that its DC conductivity is decoupled from its optical

transparency. This means that the conductivity of a network can be independently

increased by growing high quality nanotubes of same density so that the optical

transparency remains unchanged[11].

Most of the previous work on using carbon nanotube films is focused on solution based

tubes, which are filtered through a membrane and deposited on the transparent quartz

substrate [12-15]. This approach has the advantages of being easy and scalable. However,

there are several shortcomings too, such as:

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1. Carbon nanotubes

chandra_synthesis and electronic transport in single walled carbon nanotubes

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