Synthesis of Carbon Nanotubes (CNTs) as 
Thermal Interface Material 
 
by 
 
Baha Yakupoglu 
 
 
 
 
A thesis submited to the Graduate Faculty of 
Auburn University 
in partial fulfilment of the 
requirements for the Degree of 
Master of Science 
 
Auburn, Alabama 
December 14, 2013 
 
 
 
 
Keywords: Carbon nanotubes, CNT, Chemical Vapor Deposition, CVD, 
Sputtering Growth time 
 
 
Copyright 2013 by Baha Yakupoglu 
 
 
Approved by 
 
Hulya Kirkici, Chair, Profesor of Electrical and Computer Engineering 
Thomas Baginski, Profesor of Electrical and Computer Engineering 
Bogdan Wilamowski, Profesor of Electrical and Computer Engineering 
 
 
 
 
 
 
ii 
 
 
 
 
 
 
 
Abstract 
 
 
 
 Carbon nanotubes (CNTs) are promising materials for many potential applications. High 
electrical conductivity, superior mechanical strength, thermal conductivity, and chemical 
inertnes are some of the properties of CNTs. CNTs can carry high ? current densities, and they 
have been studied as cold ? cathode field emiters. Smal diameter and relatively long length of 
CNTs makes them perfect field emiters with high emision currents at low electric fields. 
 
In this thesis, electrical and thermal properties of CNTs considered. For this, selective 
and non ? selective multi ? wal carbon nanotubes (MWCNTs) are synthesized by Chemical 
Vapor Deposition (CVD) method. CNTs are synthesized on both Nickel (Ni) and Silicon (Si) 
substrates. In some cases, SiO
2
 under layer is employed on silicon wafer.  Iron (Fe), Nickel (Ni), 
and Carbon (C) are used as catalyst types onto Si and SiO
2
 coated Si substrates. The SEM 
images, growth conditions and their efects such as catalyst thickneses, DC/RF sputtering 
distance, sputtering presure, gas flow rate, and CVD temperature efects are discussed. The 
paterned CNT fabrication proces is also studied in this thesis. 
 
 
 
 
 
 
 
 
iii 
 
Acknowledgments 
 
 
 
 It gives me a great pleasure in acknowledging the support and help of Profesor Hulya 
Kirkici, who was not only an advisor, but also a motherly figure to me during this research. I 
would have not completed this study without her motivation, love, encouragement, guidance, 
suggestions, and valuable comments. It is a distinguishing opportunity to fel her support al the 
time for my profesional development. 
 
 I also would like to expres my many and sincere thanks to Profesor Thomas Baginski, 
and Profesor Bogdan Dan M. Wilamowski for their time and efort to review my thesis and 
being a part of my commite. It was also a great pleasure to be a part of their classes. 
 
 My thanks also go to all our lab mates and al research group members for establishing a 
fun environment during this research period. Special thanks to Roger Tsai, Huirong Li, Rujun 
Bai, and Ming Zhang for al their support. 
 
 Most of al, I?d like to thank my parents, Suheyla and Cevat Yakupoglu, and my uncle 
Semih Ozkurtaran for al their lifetime support, love, encouragement, and dedication.  
 
 Finaly, I?d like to thank my wife, Funda Yakupoglu. She was always here to cheer me up 
and stood by me through the good and bad.  
 
 
iv 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
To the memory of my mom 
who loved me with all her heart 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
v 
 
 
 
 
 
 
Table of Contents 
 
 
Abstract ......................................................................................................................................... ii 
Acknowledgments ....................................................................................................................... iii 
List of Tables .............................................................................................................................. vii 
List of Figures ............................................................................................................................ vii 
Chapter 1 Introduction  ................................................................................................................. 1 
Chapter 2 Literature Review ......................................................................................................... 5 
            2.1 Carbon Nanotube Structures ....................................................................................... 5 
            2.2 Carbon Nanotube Properties ....................................................................................... 7 
       2.2.1 Electrical Properties ........................................................................................... 8 
       2.2.2 Mechanical Properties ........................................................................................ 9 
       2.2.3 Chemical Properties ......................................................................................... 10 
 2.3 Carbon Nanotube Synthesis Techniques .................................................................. 11 
 2.4 CVD, thermal CVD, and PECVD ............................................................................. 15 
Chapter 3 Carbon Nanotubes Fabrication and Characterization ................................................. 19 
 3.1 Overview ................................................................................................................... 19 
 3.2 Wafer Cleaning and Catalyst Deposition .................................................................. 20 
            3.3 Paterning and Masking Proces ............................................................................... 2 
 3.4 Nickel as a Substrate to Grow CNTs ........................................................................ 24 
 3.5 CNT Growth Proces in CVD ................................................................................... 25 
vi 
 
Chapter 4 Results and Discussion ............................................................................................... 27 
            4.1 Catalyst Thicknes Efect ......................................................................................... 27 
            4.2 DC/RF Sputtering Distance and Presure Efect ...................................................... 30 
 4.3 Flow Rate Efect ....................................................................................................... 31 
            4.4 CVD Temperature Efect .......................................................................................... 31 
            4.5 SEM Images (Collection) ......................................................................................... 32 
Chapter 5 Conclusions ................................................................................................................ 40 
References   ................................................................................................................................. 41 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
vii 
 
 
 
 
 
 
List of Tables 
 
 
Table 2.1   Young?s modulus, tensile strength, and density of CNTs compared with the other  
                  materials .................................................................................................................... 10 
 
Table 4.1   Varying layer thickneses depending on the sputtering presure ............................. 31 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
vii 
 
 
 
 
 
 
List of Figures 
 
 
Figure 1.1    Atomic structure of carbon nanotubes a) Graphite latice b) Single ? waled CNT   
c) Multi ? Waled CNT .......................................................................................... 2 
Figure 1.2    a) arc ? discharge MWCNT TEM image     b) CVD MWCNT TEM image              
c) arc ? discharge MCNT AF image     d) CVD MCNT AF image   ...... 3 
 
Figure 2.1    sp
2
 hybridization of carbon and its derived materials a) The three sp
2
 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 diferent 
materials, fullerene (left), carbon nanotube (center) and bulk graphite (right) ...... 5 
Figure 2.2    a) A two ? dimensional honeycomb latice two show diferent types of tubules 
can be formed, and defined by chiral vectors      b) An example of (4,2)  
CNT construction????????????????????????..7 
 
Figure 2.3    Raman spectroscopy of 8 mins Carbon (C) sputtered on already 5 mins Fe sputtered 
Si substrate after CVD growth using ion laser excitation of a) 514 nm visible and 
b) 785 nm near infrared wavelengths (CVD growth conditions were 700
0
C, 70.8 
mTorr, and the Ar : C
2
H
2
 rate was 75scm : 20scm) ........................................... 9 
Figure 2.4    Currently used methods for Carbon Nanotubes Synthesis ..................................... 12 
Figure 2.5    Schematics of the arc-discharge apparatus employed for fulerene and nanotube 
production (b) image of the arc experiment betwen two graphite rods (courtesy of 
P. Redlich). The extreme temperature reached during the experiment is located 
betwen the rods (~3000?4000 K) ....................................................................... 13 
Figure 2.6    Schematic of Laser Ablation method ..................................................................... 14 
Figure 2.7    Thermal CVD schematic diagram to growth CNTs ............................................... 16 
Figure 2.8    Plasma enhanced CVD schematic diagram to growth CNTs ................................. 17 
Figure 3.1    Schematic and picture of DC/RF Sputtering Chamber, Plasma gun, and substrate 
        holder?????????????????????????????....21 
 
Figure 3.2    x2000 and x7000 SEM images of 5 minutes iron (Fe) sputtered Silicon substrate 
         surface??..............................................................................................................21 
ix 
 
 
Figure 3.3    The paterned Si substrate by 0.5 cm
2
 holes after the development step. .............. 22 
Figure 3.4    a) CNTs on Si substrate by 0.5 cm
2
 holes after 5 minutes Fe sputtering and 20 
minutes CVD growth. b) CNTs on Si substrate by 0.5 cm
2
 holes after 5 minutes Fe 
sputtering and 20 minutes CVD growth. (Growth conditions were 700
0
C, 70.8 
mTorr, and the Ar : C
2
H
2
 rate was 75scm : 20scm) ............................................ 23 
Figure 3.5    Illustrated flow chart of paterned CNT synthesis. a) Positive ? tone photoresist step. 
b) Mask alignment step. c) Dry and wet etching. d) 5 minutes Fe sputtering.            
e) Removing the PR. f) CVD growth ..................................................................... 24 
Figure 3.6    Fe catalyst sputtered for 5 minutes on Ni Foil (resolution x20k) ........................... 25 
Figure 3.7    A schematic diagram of the thermal CVD system ................................................. 26 
Figure 4.1    a) 10 nm deposition thicknes of Fe results with very dense CNTs on Si substrate.  
b) >10 nm deposition thicknes of Ni results with very dense CNTs on Si 
substrate. (Both pictures are taken after 20 minutes CVD growth. Growth 
conditions were 700
0
C, 70.8 mTorr, and the Ar : C2H2 rate was 75scm : 
20scm.)?????????????????????? ?????..27 
 
Figure 4.2    8 minutes C sputtering on already 5 minutes Fe sputtered Si substrate.  (Picture is 
taken after 20 minutes CVD growth. Growth conditions were 700
0
C, 70.8 mTorr,  
and the Ar : C
2
H
2
 rate was 75scm : 
20scm.)????????????????????????????...28 
 
Figure 4.3    a) Ni 5 minutes, and Fe 5 minutes sputtering on SiO
2
 b) Pretreated Ni 5 minutes, 
and Fe 5 minutes sputtering on SiO
2
 c) Ni 7 minutes sputtering, 8 hours annealing, 
and Fe 5 minutes sputtering respectively on SiO
2
 (Both pictures are taken after 20 
minutes CVD growth. Growth conditions were 700
0
C, 70.8 mTorr, and the Ar : 
C2H2 rate was 75scm : 20scm.) ......................................................................... 29 
Figure 4.4    Left side of the substrate respect to the Fig 3b -Ni 5 minutes, and Fe 5 minutes 
sputtered SiO
2
 sample (Picture is taken after 20 minutes CVD growth. Growth 
conditions were 700
0
C, 70.8 mTorr, and the Ar : C
2
H
2
 rate was 75scm : 20scm.)
 ................................................................................................................................ 30 
Figure 4.5    Randomly oriented CNTs grown of Fe catalyst sputtered for 5 minutes on Si 
substrate (resolution x10k) ..................................................................................... 33 
Figure 4.6    Randomly oriented CNTs grown of Fe catalyst sputtered for 1 minute on SiO
2
 
         substrate (resolution x10k)?????????????????????...33 
 
Figure 4.7   Randomly oriented CNTs grown of Fe catalyst sputtered for 5 minutes on SiO
2
 
substrate (resolution x10k) .................................................................................... 34 
x 
 
Figure 4.8    Ni catalyst sputtered for 5 minutes on Si substrate (resolution x50k) .................... 34 
Figure 4.9    Ni catalyst sputtered for 5 minutes on top of the Fe catalyst sputtered for 5 minutes 
Si substrate (resolution x30k) .............................................................................. 35 
Figure 4.10    Cross ? sectional image of Ni catalyst sputtered for 3 minutes on top of the Fe 
catalyst sputtered for 5 minutes Si substrate (resolution x20k) ........................... 35 
Figure 4.11    Ni catalyst for 3 minutes, Fe catalyst for 5 minutes, and C catalyst for 8 minutes 
            sputtered on Si substrate (resolution x30k)?????????.?????..36 
 
Figure 4.12    Cross ? sectional image of C catalyst sputtered for 8 minutes on top of the Fe 
catalyst sputtered for 5 minutes Si substrate (resolution x10k) ........................... 36 
Figure 4.13    Fe catalyst sputtered for 5 minutes on Ni Foil (resolution x20k) ......................... 37 
Figure 4.14    Ni catalyst sputtered for 5 minutes on top of the Fe catalyst sputtered for 5 minutes 
          SiO
2
 substrate (resolution x10k)??.....???????? ???????..37 
 
Figure 4.15    Ni catalyst sputtered for 7 minutes on top of the Fe catalyst sputtered for 5 minutes 
SiO
2
 substrate (resolution x20k) .......................................................................... 38 
Figure 4.16    Ni catalyst sputtered for 10 minutes on top of the Fe catalyst sputtered for 5  
           minutes SiO
2
 substrate (resolution x20k)??????????????.?..38 
 
Figure 4.17    Side image of Fe catalyst sputtered for 5 minutes on .5 cm
2
 x .5 cm
2
 paterned Si 
substrate (resolution x10k) ................................................................................... 39 
Figure 4.18    Cross ? sectional image of Fe catalyst sputtered for 5 minutes on .35 cm
2
 x .35cm
2
 
paterned Si substrate (resolution x10k) ............................................................... 39 
 
 
 
 
 
! 1!
Chapter 1 
 
Introduction 
 
 
 Carbon nanotubes (CNTs) are a kind of macromolecule of carbon, which can be 
thought of as graphitic sheets with a hexagonal latice that are rolled into a smoothly 
continuous cylinder. Due to their exceptional electrical, mechanical, and chemical 
properties, CNTs have motivated a wide area of research, since their first discovery in 
1990s, in both science, and engineering on their fabrications, and applications [1]. CNTs 
have many structures depending on thickneses, lengths, number of layers and the type of 
helicities. They are clasified acording to these structures, and their electrical 
characteristics rely on these categories. Their structure is typicaly categorized as single ? 
wal carbon nanotubes (SWCNTs), and multi ? wal carbon nanotubes (MWCNTs) with 
respect to the graphitic layers as ilustrated in Figure 1.1 [2]. The simplest MWCNT is 
caled double ? wal CNT (DWCNT), which is formed only from two layers. Moreover, 
the disymmetry (wrapping angle) of the configuration of the CNTs is taken into 
consideration and afects their density, latice structure, conductivity, and whether they 
act as metals or as semiconductors. I?ll describe these forms by chiral vectors in chapter 
2.  
The strength and the remarkable physical properties of CNTs like persistency, 
stifnes?etc. can be unfurled by finding the correct form. In fact, they can be up to a 
hundred times stronger than stel, while 6 times lighter than it [3]. These special 
properties open a wide range of aspects to CNTs as practical and commercial products at 
new, and existing applications such as field efect / single ? electron transistors [4 ? 8], 
conductive plastics, biosensors [9], structural composite materials, radar ? absorbing 
! 2!
coating, atomic force microscope (AFM) tips [10], bateries with improved lifetime, ultra 
capacitors, extra strong fibers, and field emiters [11].  
 
 
Figure 1.1 Atomic structure of carbon nanotubes a) Graphite latice b) Single ? 
waled CNT c) Multi ? Waled CNT [2] 
 
 Nanotubes have very impresive electric field emision properties, and they are 
extremely conductive. Their turn ? on voltage can be as low as 1 ? 3 V/um, and emision 
current could be up to 0.1 mA [12]. Thus, they are also a promising material in high 
current density applications, and lightweight packaging as a cold ? cathode field emision 
source. In addition, they can carry very large (up to 100 MA/cm
2
 [13]) current densities, 
and they sustain this superconductivity, even with the transition of temperatures up to 5 K 
[14]. They have thermal conductivity up to 3000 W/m K [15].  
 There are a couple of well ? known techniques that are used to synthesize CNTs 
including arc ? discharge, laser ablation, and chemical vapor deposition (CVD). Arc ? 
discharge and laser ablation are the first used methods to produce CNTs. These methods 
produce relatively les defective CNTs in comparison to the other techniques. However, 
CVD method is more preferable and popular nowadays, due to relatively cheap and easy 
! 3!
production of very large quantities of CNTs. Unfortunately, the CVD technique presents 
large quantities of defects, and CNTs? chemical, electrical, and mechanical properties 
suffer from these defects. Yet, above advantages, plus easy control of the reaction course, 
and high purity of the obtained material [16] make this technique the most suitable one 
for a potential industrial level production.  Transmision electron microscope (TEM) and 
atomic force microscope (AFM) pictures of arc ? discharge MWCNTs, and CVD 
MWCNTs as shown in Figure 1.2 [17]. 
 
 
Figure 1.2 a) arc ? discharge MWCNT TEM image   b) CVD MWCNT TEM image     
c) arc ? discharge MWCNT AF image     d) CVD MWCNT AF image  [37] 
 
 Randomly aligned carbon nanotubes were synthesized and were studied by using 
chemical vapor deposition technique in this thesis. Iron (Fe), Nickel (Ni), and Carbon (C) 
are used as catalysts. Growth conditions, catalyst thicknes efect, DC / RF sputtering 
! 4!
distance and presure efect, flow rate efect, and CVD temperature efect are 
investigated. Furthermore, using photolithography steps, diferent sizes (0.5 cm
2
 by 0.5 
cm
2
, and 0.35 cm
2
 by 0.35 cm
2
) of paterned CNTs are also succesfully synthesized. 
! 5!
 Chapter 2 
 
 
2.1 Carbon Nanotube Structures 
 
 Carbon is an active element to produce diferent molecular compounds and 
crystalized solids. Fullerene, CNT, and graphite are a couple of examples of carbon ? 
bonded materials that have unique nanometer sizes of sp
2
 forms as sen in Figure 2.1 [20] 
 
 
 
Figure 2.1 sp
2
 hybridization of carbon and its derived materials a) The three sp
2
 
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 diferent 
materials, fullerene (left), carbon nanotube (center) and bulk graphite (right) [20] 
 
CNT structures are determined by the wrapping angle of the hexagonal graphene 
sheets. Chiral vectors can describe these forms. They can be either ?arm ? chair?, ?zig ? 
! 6!
zag? or several non ? standard chiral structures. The chiral vector equation can be 
described as R = na
1
 + ma
2
 (where the n, and m are integers), and can be explained 
easily with Figure 2.2a. The condition for the chirality convention should be !?!?
0. We begin with describing the tube axis by drawing two lines (the green lines). Then, 
we arbitrarily choose a point on one of the lines and cal it A. Then, we draw our 
armchair line (yelow line) betwen two tube axis lines, which separates the honeycombs 
to two equal halves. After we draw the armchair line, we choose another point on the tube 
axis line that corresponds to a carbon atom, and also is closest to the armchair line, and 
cal it B. The red line, which connects these two points, wil be our chiral vector R. The 
angle betwen the chiral vector R, and the armchair line is caled the wrapping angle, ?. 
If the chiral vector R is on the armchair line, in other words if the wrapping angle ?=0
0
, 
the tube is caled ?armchair?. If the wrapping angle ?=30
0
, the tube is caled ?zig ? zag?. 
Lastly, for 0
0 
<!? < 30
0
, it is caled chiral tube. The values of the n, and m influence the 
chirality of the tube, which affect the conductivity, density etc. of the CNTs.  
The diameter of the tube can be described using the relationship: 
!=
!
!
=
!!
!
+!"+!
!
!
 
and the wrapping angle can be described as: 
!"#?=
!
!
!!
=
2!+!
2!
!
+!"+!
!
 
When the diference n-m is divisible by three, the SWCNT is considered to be a metallic 
nanotube. Otherwise, it is considered to be a semiconducting one. In most of the cases, 
while the n, and m are random numbers, the formed nanotubes can be expected to be 2/3 
is semiconducting, and the final third metalic. [21] 
! 7!
 
Figure 2.2 a) A two ? dimensional honeycomb latice two show diferent types of 
tubules can be formed, and defined by chiral vectors           b) An example of (4,2) CNT 
construction 
 
2.2 Carbon Nanotube Properties 
 Three properties of CNTs, which are electrical, chemical, and mechanical, are the 
most important and widely researched for the manufacturing and the application 
purposes.  
! 8!
2.2.1 Electrical Properties 
  
The properties of CNTs show excelent features such as conductivity, and 
resistivity. There are a couple of diferent techniques that are in use to determine these 
properties. Some of the common ? use of them are Raman spectroscopy [22], four ? point 
probe technique [23], electron spin resonance (ESR) [24], scanning tunneling microscopy 
(STM) [25], atomic force microscopy (AFM), and electron energy loss spectroscopy 
(EELS) [26]. Figure 2.3 shows a CNT structure that was studied under Raman 
spectroscopy using ion laser excitation of both 514 nm visible and 785 nm near infrared 
wavelengths. Those samples were fabricated in ? house by CVD method. 
 
 Ebbesen et al. [23] measured the resistivities of MWCNTs by using the four ? 
point probe technique where the tungsten wires used as probes. The results are ranged 
betwen 5.1x10
-6
 ? ? cm to 1.2x10
-4
  ? ? cm. The resistivities of SWCNTs are also 
measured by using the same technique by Smaley et al. [27]. The results range from 0.34 
!? ? m to 1 !? ? m. Moreover, CNTs can carry up to 100 MA/cm
2
 current densities 
[28], and high thermal conductivity of ~200 W/m K has been measured by Benes et al. 
[29]. 
! 9!
  
 
Figure 2.3 Raman spectroscopy of 8 mins Carbon (C) sputtered on already 5 mins Fe 
sputtered Si substrate after CVD growth using ion laser excitation of a) 514 nm visible 
and b) 785 nm near infrared wavelengths (CVD growth conditions were 700
0
C, 70.8 
mTorr, and the Ar : C
2
H
2
 rate was 75scm : 20scm) 
 
 
2.2.2 Mechanical Properties 
 The CNTs are very strong materials. They have great stifnes and tensile 
strength. Lots of measurements have been made to calculate the Young?s modulus, and 
the strength of CNTs [30], [31]. Furthermore, the comparison of these calculations with 
! 10!
the other materials has been shown [32]. Table 2.1 shows The Young?s modulus, tensile 
strength and densities of SWCNTs and MWCNTs compared with the other materials. 
The average Young?s modulus of CNTs is around 1.8 TPa, which is a high value 
considering that the maximum value of the Young?s modulus of the stel is .186 TPa. 
Since MWCNTs are formed from multiple layers of SWCNTs, and the highest value of 
Young?s modulus of SWCNTs are calculated depending on the coaxial intertube coupling 
of Van der Wals Force [33], the modulus value is higher for the MWCNTs. 
 
Table 2.1 Young?s modulus, tensile strength, and density of CNTs compared with 
the other materials [32] 
 
2.2.3 Chemical Properties 
 Although CNTs show low chemical reactivity, oxidation is an important chemical 
reaction, which is efective on CNTs at high temperatures over 750
o
C. The oxidation 
begins from the tips of the CNTs, and goes inwards layer by layer at the MWCNTs. Since 
it begins from the tips and afects the layers, it is possible to have open and thinner CNTs 
! 11!
after this reaction. This proces can be resulted with diferent oxides groups [34] that 
enhance CNTs chemical reactivity and change their weting, opening etc. properties. 
  Some promising applications that exist because of the chemical properties of 
CNTs such as absorbing molecules with non ? covalent forces are cancer therapy, and 
drug delivery [35], and [36]. Biosensors are another important area that CNTs can be 
used to fabricate ultrasensitive levels. Shibayama et al. reported a glucose biosensor with 
a sensitivity of 40uA mM-1 cm-2, a correlation coeficient of .992, a linear response 
range of .025 ? 1.9 mM, a detection limit of 6.2uM at S/N=3, +.8 V vs Ag/AgCl [37]. 
These ultrasensitive biosensors can also be used in DNA detection [38].  
 Furthermore, CNTs can be fabricated as the darkest material, which makes them a 
perfect candidate for the future solar ? cel applications.  
 
2.3 CNT Synthesis Techniques 
 The first methods to growth CNTs were arc ? discharge and laser ablation ones. 
Nowadays, these proceses yield their place to Chemical Vapor Deposition (CVD) 
technique. CVD method is useful for both controlling CNTs orientation and length, and 
also for alowing large area procesing relatively cheaply. There are also a couple of 
asisted approaches to CVD method such as thermal chemical vapor deposition, plasma 
enhanced chemical vapor deposition (PECVD), radio frequency chemical vapor 
deposition (RF ? CVD), microwave plasma enhanced chemical vapor deposition 
(MPECVD), hot filament chemical vapor deposition, water ? asisted chemical vapor 
deposition, and oxygen ? asisted chemical vapor deposition as shown in Figure 2.4 [19]. 
! 12!
 
Figure 2.4 Currently used methods for Carbon Nanotubes Synthesis [19] 
 
 Arc discharge and laser ablation methods occur at high temperatures. Synthesized 
CNTs with these techniques usualy have fewer defects compared with CVD technique. 
Both techniques condense carbon atoms from evaporation of solid carbon sources onto 
substrates. Inert gases like helium or argon are usualy introduced in arc ? discharge 
technique, and CNTs are formed through arc ? vaporization of two-graphite rods placed 
end to end that act as anodes and cathodes. They are separated by about 1 m at low 
presure. The important factors that afect the type, and quality of the formed CNTs are 
gas presure, plasma uniformity, and deposit temperature formed on the electrode with 
this technique. An arc ? discharge apparatus schematic, and an experiment picture are 
shown in Figure 2.5 [39] 
! 13!
 
Figure 2.5 (a) Schematics of the arc-discharge apparatus employed for fulerene and 
nanotube production (b) image of the arc experiment betwen two graphite rods (courtesy 
of P. Redlich). The extreme temperature reached during the experiment is located 
betwen the rods (~3000?4000 K) [39] 
 
 
! 14!
 Laser ablation is one of the other good methods to synthesize high quality and 
pure CNTs. It is first used by Smaley et al. [40]. As in arc ? discharge method, there are 
lots of critical parameters that afect the formed tubes such as laser properties, inert gas 
presure, temperature, and target material structure. [19] The basic mechanism and 
principle of this method is carbon target vaporization as in arc ? discharge technique. The 
diference is that the laser provides the energy that hits the graphite target, which is 
located in the oven, and contains catalyst materials mostly nickel (Ni) or Cobalt (Co). 
The synthesized CNTs pas the chamber with the help of the inert gas flow, and are 
collected on a cold finger. A schematic of this method is shown in Figure 2.6. 
 
 
Figure 2.6 Schematic of Laser Ablation method [41] 
 
  
 
! 15!
2.4 CVD, Thermal CVD, and PECVD 
 Chemical vapor deposition method is the most common, and relatively economic 
technique so far that is used to grow CNTs. Although the CNTs synthesized by this 
technique suffer from very large quantities of defects, economic reasons, simplicity, 
controllable growth, and mas production capabilities give a priority to this method.  
 Catalyst preparation, and actual growth are two important steps in CVD method. 
The preparation of the catalyst involves sputtering of transition metals like iron (Fe), and 
nickel (Ni) on to the substrate materials as it is studied in this thesis. Also, gaseous phase 
activated carbon is required for actual growth. It is common to use gaseous carbon 
sources such as acetylene, methane, and carbon monoxide (CO), and we can clasify the 
methods in use to activate these molecules as plasma enhanced CVD and thermal CVD 
[42]. Thermal CVD uses conventional heat sources to heat the furnace to the desired 
temperature such as infrared lamp, resistive, or inductive filaments. Since the catalyst has 
an influential efect on both growth rate and produced CNTs [43], diferent techniques 
are studied at the preparation level of catalyst such as metalic aloys. For instance, 
Cobalt (Co), and Fe aloy gives 10 ? 100 times more SWCNTs than pure iron [44]. The 
catalyst dimensions also afect the CNT production. While the large particles can be 
useful to synthesize MWCNTs, dimensions that are too large can result in carbon fibers 
or filaments [42]. Moreover, metal particle concentration, and deposition temperature 
have efects on CNTs growth too [45]. A schematic ilustration of thermal CVD can be 
sen in Figure 2.7. 
 
 
! 16!
 
Figure 2.7 Thermal CVD schematic diagram to growth CNTs [41] 
 
 In the plasma enhanced CVD method, high voltage and high frequency are 
applied to both electrodes, and CNTs are synthesized in a furnace by glow discharge.  
Plasma has been used as an activator and decomposer of the reactants in the gas phase, 
and a source to generate glow discharge in PECVD technique. Electrical discharges at 
DC, RF, and MW frequencies, and hot ? filament are mostly used to generate the plasma. 
There are two paralel plate electrodes located in the PECVD chamber, and the substrate 
is placed on the grounded electrode. It is important to keep the p x d equation constant 
(where d is the distance betwen the electrodes, and p is the presure of the chamber) to 
be able to maintain the discharge. It is important to have the desired substrate temperature 
stabled in order to increase the nucleation density. A separate heater might be used for 
that reason. A schematic of the PECVD can be sen in Figure 2.8 [46]. 
 
! 17!
 
Figure 2.8 Plasma enhanced CVD schematic diagram to growth CNTs [46] 
 
 The chamber presure can typicaly take a value betwen 1 to 20 mTorr during the 
experiment. The fed gas can be acetylene, methane, ethylene, ethane, or carbon 
monoxide, and enters the chamber from one of the paralel plate electrodes that does not 
hold the substrate. The energy of the plasma helps the carbon(C) molecules turn into 
reactive atoms, and CNTs are grown on the metal or metal aloy catalyst particles that 
sputtered on the substrate. Extremely dense vertical CNTs can be synthesized by PECVD 
method, since a local electrical field can be generated betwen the substrate holder and 
the plasma. The other advantage of the PECVD technique is the CNT growth can be 
achieved at the low temperatures in comparison to the other growth mechanisms. This 
! 18!
property gets the PECVD method one step forward for the possible integration of CNTs 
with semiconductor device applications. 
 
! 19!
Chapter 3 
CNTs Fabrication and Characterization 
 
3.1 Overview 
Randomly aligned Carbon Nano Tubes (CNTs) are synthesized by using chemical 
vapor deposition technique in this thesis.  Silicon (Si) and Nickel (Ni) are used as the 
substrate to grow CNTs.  A metal catalysis is deposited onto the substrate using a radio 
frequency/direct current (RF/DC) sputtering system.  The sputtering chamber is capable 
of reaching 5x10
-6
 Torr presure and both RF and DC sputtering are feasible.  However, 
in this work only the DC sputtering is utilized because of the choice of the metal target 
materials.  The silicon substrate is n-type silicon with<100> orientation and has SiO
2
 pre 
? coating on it.  In some cases, the substrate has been treated with heat or by other means.  
The substrate is clasified as ?untreated?, when no treatment is applied.  Thin films of 
Iron (Fe), Carbon (C) and Nickel (Ni) are deposited as catalysts or underlayers for the 
catalyst onto substrates.  The distance betwen the sputtering gun and the sample was 
kept constant at 20 cm, and the presure was kept at 7 mTorr during the metal deposition.  
Argon (Ar) gas is used as the background gas forming the plasma for deposition.  
Depending on the desired thicknes of the metal film, the sputtering deposition time is 
varied.  After the deposition, the substrate with the catalyst layer(s) transferred to the 
thermal chemical vapor deposition (CVD) chamber.  The CVD chamber is capable of 
reaching 1100
0
C maximum temperature, but it was kept constant at 700
0
C during the 
CNT growth proces.  The chamber is a quartz tube with diameter of 2 inch, with heating 
! 20!
element wrapped around it is forming the furnace. Tube chamber is pumped down to a 
base presure of 10
-3
 Torr using a mechanical pump initialy, and then, filed with the 
fed gas.  A mixture of argon (Ar) and acetylene (C
2
H
2
) fed gases are introduced to the 
chamber with a flow rate of 75 scm / 20 scm respectively.  Gas flow is controlled by a 
mas flow controller and a presure of 70.8 mTorr is maintained in the chamber for 20 
minutes for chemical vapor deposition of CNTs.  
 
3.2 Wafer Cleaning and Catalyst Deposition 
 The Si wafer is cleaned with diluted HF solution and DI water before the 
sputtering proces to remove the impurities and native oxides. Then, it is put in an oven 
for dehydration bake at 120
0
C for 20 minutes, in order to get rid of al the moisture on the 
wafer surface. It is then ready to load into the sputtering chamber. A 2-inch diameter Fe, 
Ni, and C targets are used as the catalysts to deposit onto the surface of wafer using DC 
sputtering. The substrate is mounted 20 centimetres over the plasma (ion) gun, which is 
an ion source without a filament, and works with microwave plasma discharge principle. 
A picture and an ilustration of the sputtering chamber are shown in Figure 3.1. The 
chamber is then pumped down to the presure of 5x10
-6
 Torr. Argon is fed in to the 
chamber, and the chamber presure is maintained at 7 mTorr. The plasma is generated 
with DC/RF power that is applied to the plasma gun. The metal atoms sputtered from the 
target to the substrate, and a metal layer is obtained on the substrate surface at the end of 
the proces. Sputtering times are varied betwen 1 to 15 minutes depending on the 
required catalyst thicknes. An SEM image of 5 minutes Fe catalyst uniformly sputtered 
onto a Si substrate is shown in Figure 3.2.  
! 21!
 
 
Figure 3.1 Schematic and picture of DC/RF Sputtering Chamber, Plasma gun, and 
substrate holder 
 
 
 
 
Figure 3.2 x2000 and x7000 SEM images of 5 minutes iron (Fe) sputtered Silicon 
substrate surface [54] 
 
 
 
! 22!
3.3 Paterning and Masking Proces 
Paterned CNTs are also synthesized by using chemical vapor deposition 
technique on Si substrate. Two diferent sizes of masks are used to generate the paterns. 
The first mask had 0.5 cm
2
 by 0.5 cm
2
 holes, and the second one had 0.35 cm
2
 by 0.35 
cm
2
 holes. The Si wafers are first cleaned in Hydrogen Fluoride (HF): Water (1: 50, 
respectively) solution, and then are put in the oven for dehydration bake for 20 minutes at 
120
0
C. Hexamethyldisilazane (HMDS) vapor prime is applied for 5 minutes as an 
adhesion promoter to the photoresist (PR). Positive ? tone photoresist (AZ 5214) is then 
applied to the wafers for 30 seconds at 3000 rpm spin speed. After the photoresist step, 
the wafers are soft baked at 105
0
C for 1 minute. The proper mask is selected and the 
wafers are exposed to the UV light for 10 seconds in order to remove the PR. AZ 400K 
solution is used as the developer, and the wafers are developed for 18 seconds in 2: 1 
(Water: AZ400K) concentration. The paterns then become visible as sen in Figure 3.3.  
 
 
Figure 3.3 The paterned Si substrate by 0.5 cm
2
 holes after the development step. 
(This paterned wafer is fabricated in ? house) 
! 23!
 
A hard bake at 120
0
C for 1 minute follows after the development step. The wafers are 
exposed to the plasma for 14 seconds in order to clean the wafer surface. The wafers are 
then etched 20 ?m by using wet and dry etching, and are put into the sputtering chamber. 
The similar sputtering and CVD furnace steps are followed, and CNTs are synthesized at 
the desired paterns as sen in Figure 3.4. 
 
 
 
 
Figure 3.4 a) CNTs on Si substrate by 0.5 cm
2
 holes after 5 minutes Fe sputtering and 
20 minutes CVD growth. b) CNTs on Si substrate by 0.5 cm
2
 holes after 5 minutes Fe 
sputtering and 20 minutes CVD growth. (Growth conditions were 700
0
C, 70.8 mTorr, 
and the Ar : C
2
H
2
 rate was 75scm : 20scm) 
 
 
An ilustrated flow chart of the al explained steps can be sen in Figure 3.5. 
! 24!
 
Figure 3.5 Illustrated flow chart of paterned CNT synthesis. a) Positive ? tone 
photoresist step. b) Mask alignment step. c) Dry and wet etching. d) 5 minutes Fe 
sputtering. e) Removing the PR. f) CVD growth 
 
 
3.4 Nickel as a Substrate to Grow CNTs 
 A Nickel foil is also used as a substrate. Fe is used as the catalyst to deposit onto 
the surface of the Ni foil using DC sputtering. Diferent sputtering times are used 
betwen 1 to 15 minutes in order to experience the diferent catalyst thicknes affects. It 
is concluded that, although some formations of CNTs found with carbon clusters under 
the SEM, the results were very poor. It?s decided to use SiO
2
 under layer to improve the 
results; however, this experiment is not included in this thesis. Figure 3.6 shows Fe 
catalyst sputtered for 5 minutes on Ni Foil. 
 
 
! 25!
 
Figure 3.6 Fe catalyst sputtered for 5 minutes on Ni Foil (resolution x20k) 
 
3.5 CNT Growth Proces in CVD 
 After the sputtering proces the silicon substrate is loaded in the thermal CVD 
reactor to synthesize CNTs. The quartz tube furnace is the chamber of the system. An 
adjustable resistive heater is used to heat the furnace to the experiment temperatures. The 
experiment temperatures are measured by a thermocouple that is connected to the 
substrate holder. Acetylene/Argon gas mixture is introduced in to the chamber with a 
flow rate of 20 scm / 75 scm respectively. The flow rates inside the thermal CVD 
system are measured by a flow meter that is connected to the system through the feding 
gas cylinders. The presure in the chamber is controlled by a valve that is connected to a 
presure gauge and a mechanical pump. The experiment conditions of synthesis of CNTs 
were maintained at a growth temperature of 700?C and at the growth presure of 70.8 
Torr. An ilustration of the system is shown in Figure 3.6 [47]. 
 
! 26!
 
Figure 3.7 A schematic diagram of the thermal CVD system [47] 
! 27!
Chapter 4 
 
Results and Discusion 
 
 
 
4.1 - Catalyst Thicknes Effect 
 
 
The catalyst thicknes is one of the critical parameters that afect CNT density as 
well as diameter and length. It was found that les than 7 nm thicknes of Fe results in the 
growth of inhomogeneous regions of randomly aligned CNTs. On the other hand, more 
than 12 nm thicknes of Fe results in clustered carbon/graphite growth.  Two SEM 
pictures are sen in Figure 4.1 for Fe, and Ni catalysts with diferent catalyst thickneses.  
 
 
 
Figure 4.1  a) 10 nm deposition thicknes of Fe results with very dense CNTs on Si 
substrate. b) <10 nm deposition thicknes of Ni results with mostly carbon clusters and 
weak CNTs on Si substrate. (Both pictures are taken after 20 minutes CVD growth. 
Growth conditions were 700
0
C, 70.8 mTorr, and the Ar : C2H2 rate was 75scm : 
20scm.) 
 
 
 
 
 
! 28!
 It was also found from the results that a secondary level of C deposited over Fe 
improves the CNT growth as sen in Figure 4.2. 
!
 
 
Figure 4.2 8 minutes C sputtering on already 5 minutes Fe sputtered Si substrate.  
(Picture is taken after 20 minutes CVD growth. Growth conditions were 700
0
C, 70.8 
mTorr, and the Ar : C
2
H
2
 rate was 75scm : 20scm.) 
 
 Likewise, the Ni thicknes diferences were concluded with distinctive results. It 
was found that if the Ni thicknes is more than 10 nm, there is no uniform CNT growth 
achieved, but mostly carbon clusters with a small amount of random growth CNT islands 
as shown in Figure 4.1b. Although CNTs were grown les than stated thicknes, the 
results were poor. That might be due to the Nickel Silicide (NiSi) formation favorable for 
temperatures of over 300
0
C as explained by Ducati et al [48].  So, in order to prevent this 
formation SiO
2
 underlayer is used betwen the Ni catalysis and the substrate. 
 Although the results were improved as shown in Figure 4.3a, the results were not 
at the desired level. Therefore, an 8 hours annealing proces at 300
0
C was applied to 
substrate with the SiO
2
 coating as a pretreatment.  Then a layer of Fe was sputtered on 
top of the Ni deposited substrate that has the SiO
2
 as the under-layer.  Those treatments 
! 29!
improved the density of CNTs and helped the growth with the same synthesizing 
conditions explained above.  One example is shown in Figure 4.3b. The efect of the 
annealing proces betwen the Ni and Fe sputtering also was studied, however, this 
treatment did not improve the results as it is sen in Figure 4.3c. 
 
Figure 4.3: a) Ni 5 minutes, and Fe 5 minutes sputtering on SiO
2
 b) Pretreated Ni 5 
minutes, and Fe 5 minutes sputtering on SiO
2
 c) Ni 7 minutes sputtering, 8 hours 
annealing, and Fe 5 minutes sputtering respectively on SiO
2
 (All pictures are taken after 
20 minutes CVD growth. Growth conditions were 700
0
C, 70.8 mTorr, and the Ar : C2H2 
rate was 75scm : 20scm.) 
! 30!
 
 
It is concluded that although the pretreatment improved the density of CNTs, the 
formation was inhomogeneous on some regions of the substrate as shown in Fig 4.4. 
 
 
Figure 4.4: Left side of the substrate respect to the Fig 3b -Ni 5 minutes, and Fe 5 
minutes sputtered SiO
2
 sample. -  (Picture is taken after 20 minutes CVD growth. Growth 
conditions were 700
0
C, 70.8 mTorr, and the Ar : C
2
H
2
 rate was 75scm : 20scm.)  
 
4.2 - DC/RF Sputering Distance and Presure Efect 
 
Sputtering distance was kept constant at 20 cm in this paper, and sputtering presure 
controlled from 3 mTorr to 15 mTorr ranges. Sputtered catalyst thickneses for Fe were 
measured and given at Table 1 for t = 5 minutes. It is studied that higher than 7 mTorr 
presures are caused larger thickneses.  This larger catalyst thicknes resulted with 
graphite growth instead of CNTs. In contrast, if the catalyst is deposited at the presures 
of lower than 7 mTorr, then the formation of CNTs were sen, however, the tubes were 
weak (thin), and inhomogeneously formed on the surface.  The measured catalyst 
! 31!
thickneses corresponding these deposition conditions were lower at these relatively low 
presures. 
 
 
 
Table 4.1 Varying layer thickneses depending on the sputtering presure. 
 
4.3 - Flow Rate Effect 
 
 
A flowing mixture of acetylene (C
2
H
2
) and argon (Ar) is used to grow CNTs. It is 
decided that best results achieved by the flow rate of C
2
H
2
, 20 scm; and Ar, 75 scm. 
The chamber presure is fixed to 70.8 mTorr during the growth proces, and temperature 
of 700
0
C is used for Fe, Ni, and secondary layered (Fe - C and Ni ? Fe) catalyst ? coated 
Si substrates. Although some carbon clusters are presented, the tube density was the 
highest. The growth time was 20 minutes, and it is only afected the carbon nanotubes? 
lengths.  
 
4.4 - CVD Temperature Effect 
 
 
Most of the CNT synthesis is carried out at the deposition temperature of 700
0
C.  
Deposition temperatures of lower than 700
o
 C are resulted with very low density CNT 
growth.  It is speculated that the decomposition of acetylene (C
2
H
2
) is not eficient at 
these low temperatures. Furthermore, at higher temperatures than 700
0
C, CNT deposition 
! 32!
proces is resulted with weak CNT growth too. So, it can be asumed that the growth rate 
of CNTs is as a function of temperature initialy increases, and then decreases as the 
temperature increases beyond 700
o
 in our system. 
 
4.5 ? SEM Images (collection) 
 
 
 In this section, the scanning electron microscope (SEM) images of CNT samples 
with diferent growth conditions and underlayers are presented. The Figure 4.5, 4.6, and 
4.7 SEM images show that the samples with 5 minutes Fe sputtering time on Si substrate, 
and with 1 and 5 minutes Fe sputtering time on SiO
2
. Figure 4.8, 4.9, and 4.10 show that 
Ni sputtering on Si substrate results with carbon clusters. That might be due to the Nickel 
Silicide (NiSi) formation favorable for temperatures of over 300
0
C as it is stated earlier at 
this section, and explained by Ducati et al [48].  Figure 4.11 shows that Ni sputtering on 
top of the 5 minutes Fe sputtering can be resulted with some weak formations of CNTs. 
The sample with 5 minutes Fe and 8 minutes C sputtering time can yield very good-
quality, and almost verticaly aligned MWCNTs as shown in Figure 4.12. Figure 4.13 
shows an experiment to growth CNT on Ni Foil. A succesful CNT growth on SiO
2
 
substrate with 5 minutes Ni and 5 minutes Fe sputtering can be sen in Figure 4.14. When 
the Ni sputtering time is increased to 7 minutes and then 10 minutes as it sen in Figure 
4.15 and 4.16 respectively, the catalyst thicknes goes too thick. Therefore, some carbon 
clusters wil be formed instead of CNTs. Lastly, the cross-sectional, and side SEM 
images of randomly oriented MWCNTs of paterned Si substrates are presented in Figure 
4.17 and 4.18. 
! 33!
 
Figure 4.5 Randomly oriented CNTs grown of Fe catalyst sputtered for 5 minutes on 
Si substrate (resolution x10k) 
 
 
 
 
Figure 4.6 Randomly oriented CNTs grown of Fe catalyst sputtered for 1 minute on 
SiO
2
 substrate (resolution x10k) 
! 34!
 
Figure 4.7 Randomly oriented CNTs grown of Fe catalyst sputtered for 5 minutes on 
SiO
2
 substrate (resolution x10k) 
 
 
 
 
Figure 4.8 Ni catalyst sputtered for 5 minutes on Si substrate (resolution x50k) 
! 35!
 
Figure 4.9 Ni catalyst sputtered for 5 minutes on top of the Fe catalyst sputtered for 5 
minutes Si substrate (resolution x30k) 
 
 
 
 
Figure 4.10 Cross ? sectional image of Ni catalyst sputtered for 3 minutes on top of the 
Fe catalyst sputtered for 5 minutes Si substrate (resolution x20k) 
 
! 36!
 
Figure 4.11 Ni catalyst for 3 minutes, Fe catalyst for 5 minutes, and C catalyst for 8 
minutes sputtered on Si substrate (resolution x30k) 
 
 
 
 
Figure 4.12 Cross ? sectional image of C catalyst sputtered for 8 minutes on top of the 
Fe catalyst sputtered for 5 minutes Si substrate (resolution x10k) 
 
! 37!
 
Figure 4.13 Fe catalyst sputtered for 5 minutes on Ni Foil (resolution x20k) 
 
 
 
 
Figure 4.14 Ni catalyst sputtered for 5 minutes on top of the Fe catalyst sputtered for 5 
minutes SiO
2
 substrate (resolution x10k) 
 
! 38!
 
Figure 4.15 Ni catalyst sputtered for 7 minutes on top of the Fe catalyst sputtered for 5 
minutes SiO
2
 substrate (resolution x20k) 
 
 
 
Figure 4.16 Ni catalyst sputtered for 10 minutes on top of the Fe catalyst sputtered for 
5 minutes SiO
2
 substrate (resolution x20k) 
 
! 39!
 
Figure 4.17 Side image of Fe catalyst sputtered for 5 minutes on .5 cm
2
 x .5 cm
2
 
paterned Si substrate (resolution x10k)!
!
!
!
!
 
Figure 4.18 Cross ? sectional image of Fe catalyst sputtered for 5 minutes on .35 cm
2
 x 
.35cm
2
 paterned Si substrate (resolution x10k) 
! 40!
Chapter 5 
 
 
 
Conclusions 
 
 
 
 In this thesis, selective and non ? selective randomly oriented multi ? wal carbon 
nanotubes are synthesized. Their electrical and thermal properties are also investigated. 
Diferent metals namely Fe, Ni and C are sputtered as catalysts on Si, Ni, and SiO
2
 
coated Si substrates. From the experimental results, it is concluded that the catalyst 
thicknes, sputtering distance and background presure during the sputtering are key 
parameters to grow CNTs. The CNTs are grown by chemical vapor deposition (CVD) 
method.  The CVD furnace temperature and gas flow rates also played important roles in 
the synthesize proces. Moreover, results show that the electrical and thermal properties 
of CNTs change depending on the growth proces and conditions. 
 
The paterned CNTs are also studied and the fabrication proces is clearly 
explained in this work. Advantages of CVD technique are sen in the experiment results. 
Controllable growth and mas production capabilities give a priority to this technique. It 
can be safely said that this technique is the most probable method to synthesize CNTs for 
many future applications.  
!
! 41!
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