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 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! REFERENCES [1] Iijima, S., ?Helical microtubules of graphitic carbon? Nature 354, pp.56 ? 58 1991 [2] Yograj Singh Duksh, Brajesh Kumar Kaushik, Sankar Sarkar, Raghuvir Singh, "Performance comparison of carbon nanotube, nickel silicide nanowire and copper VLSI interconnects: Perspectives and challenges ahead", Journal of Engineering, Design and Technology, Vol. 8 Iss: 3, pp.334 ? 53, 2010 [3] Thomas, Jim, et al. "Nanotechnology." The Ecologist. May, 2003. [4] S. J. Tans, A. R. M. Verschueren, and C. Dekker, ?Room ? temperature transistor based on a single carbon nanotube?, Nature (London), vol.393, 49, 1998 [5] A. Javey, J. Guo, Q. Wang, M. Lundstrom, H. J. Dai, ?Ballistic carbon nanotube field ? efect transistor?, Nature, vol.424, pp.654 ? 57, 2003 [6] C. Lu, Q. Fu, S. Huang, and J. Liu, ?Polymer Electrolyte ? Gated carbon nanotube field ? efect transistor?, Nano Let, vol.4, pp. 623 ? 27, 2004 [7] S. J. Tan, M. H. Devoret, H. J. Dai, A. Thes, R. E. Smaley, L. J. Gerligs, and C. Dekker, ?Individual single wall carbon nanotubes as quantum wires?, Nature (London), vol.386, pp.474 ? 77, 1997 [8] M. Bockrath, D. H. Cobden, and P. L. McEuen, Science 290, 1552, 2000 [9] J. Kong, N. R. Franklin, C. Zhou, M. G. Chapline, S. Peng, K. Cho, and H. Dai, ?Nanotube molecular wires as chemical sensors?, Science, vol.287, pp.622 ? 25, 2000 [10] H. J. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert, and R.E. Smaley, ? Nanotubes as Nanoprobes in scanning probe microscopy?, Nature (London), vol.384, pp.147 ? 150, 1996 [11] W. A. deHer, A. Chatelain, and D. Ugarte, ?A carbon nanotube field ? emision electron source?, Science, vol.270, pp.1179 ? 80, 1995 [12] W. Zhu, C. Bower, G.P. Kochankski, and S. Jin, ?Electron field emision from nanostructured diamond and carbon nanotubes?, Solid ? State Electronics, vol.45, pp.921 ? 28, 2001 [13] B.Q. Wei, R. Vajtai, P.M. Ajayan, ?Reliability and current carrying capacity of carbon nanotubes?, Appl Phys Let, vol.79, no.8, pp.1172 ? 74, 2001 ! 42! [14] Z.K. Tang, L.Y. Zhang, N. Wang, X.X. Zhang, G.H. Wen, G.D. Li et al. ?Superconductivity in 4 Angstrom single-walled carbon nanotubes?, Science, vol.292 no.5526, pp.2462 ? 65, 2001 [15] P. Kim, L. Shi, A. Majumdar, P.L. McEuen, ?Thermal transport measurements of individual multi ? walled nanotubes?, Phys Rev Let, vol.87, pp.215502?216606, 2001 [16] Zhu, Y.J., Lin, T.J., Liu, Q.X., Chen, Y.L., Zhang, G.F., Xiong, H.F., Zhang, H.Y., "The efect of nickel content of composite catalysts synthesized by hydrothermal method on the preparation of carbon nanotubes", Mater. Sci. Eng. B- Solid State Mater. Adv. Technol., vol.127 no.2-3, pp.198-202, 2006 [17] J.P. Salvetat, A.J. Kulik, J.M. Bonard, G.A.D. Briggs, T. Stockli, K. Metenier et al., ?Elastic modulus of ordered and disordered multi ? walled carbon nanotubes?, Adv. Mater, vol.11, no.2, pp.161 ? 65, 1999 [18] Jonathan N. Coleman, Umar Khan, Werner J. Blau, Yurii K. Gun?ko, ?Small but strong: A review of the mechanical properties of carbon nanotube?polymer composites?, Carbon, vol.44, Iss.9, pp.1624 ? 52, 2006 [19] J. Prasek, J. Drbohlavova, J. Chomoucka, J. Hubalek, O. Jasek, V. Adam, R. Kizek, ?Methods for carbon nanotubes synthesis-review?, Journal of Materials Chemistry, vol.21, no.40, pp.15872 ? 84, 2011 [20] Bhupesh Chandra. PhD thesis, Columbia University, 2009 [21] M. S. Dreselhaus, G. Dreselhaus, and Ph. Avouris. ?Carbon nanotubes, Topics in Applied Physics? 2001 [22] M.S. Dreselhaus, G. Dresehaus, R. Saito, and A. Jorio. ?Raman spectroscopy of Carbon !Nanotubes?, Physics Reports, 2004 [23] T. W. Ebbesen, H. Lezec, H. Hiura, J. W. Bennet, H. F. Ghaemi, and T. Thio, ?Electrical conductivity of individual carbon nanotubes?, Nature, 382, 54, 1996 [24] Mayumi Kosaka, Thomas W. Ebbesen, Hidefumi Hiura, and Katsumi Tanigaki. ?Electron !spin resonance of carbon nanotubes?, Chemical Physics Leters, 1994 [25] L. P. Biro, TS. Lazarescu, Ph. Lambin, P. A. Thiry, A. Fonseca, J. B. Nagy, and A. A. Lucas. ?Scanning tunneling microscope investigation of carbon nanotubes produced by catalytic decomposition of acetylene?, Physical Review B, 1997 [26] Ryuichi Kuzuo, Masami Terauchi, and Michiyoshi Tanaka, ?Electron Energy-Loss Spec!tra of Carbon Nanotubes?, Japanese Journal of Applied Physics, 1992 [27] H. S. Nalwa, editor, ?Handbook of nanostructured materials and nanotechnology?, Academic Pres, Vol. 5, 2000 ! 43! [28] B.Q. Wei, R. Vajtai, P.M. Ajayan ?Reliability and current carrying capacity of carbon nanotubes?, Applied Physics Leters, vol.79 no.8, pp.1172?1174, 2001 [29] J. Hone, M.C. Llaguno, M.J. Biercuk, A.T. Johnson, B. Batlogg, Z. Benes et al. ?Thermal properties of carbon nanotubes and nanotube-based materials?, Applied Physics A ? Materials Science & Procesing, vol.74, no.3, pp. 339?343, 2002 [30] M. Yu, O. Lourie, M.J. Dyer, T.F. Kely, R.S. Ruoff ?Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load?, Science, vol.287, pp. 637?640, 2000 [31] J.P. Salvetat, G.A.D. Briggs, J.M. Bonard, R.R. Bacsa, A.J. Kulik, T. Stockli et al. ?Elastic and shear moduli of single-walled carbon nanotube ropes?, Physical Review Letters, vol.82, no.5, pp. 944?947, 1999 [32] A. Thes, R. Le, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Le, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, and R.E. Smaley, ?Crystalline Ropes of Metallic Carbon Nanotubes?, Science, 273, 483, 1996! [33] A. J. Cheng, ?Cold Cathodes for Applications in Poor Vacuum and Low Presure Gas Environments: Carbon Nanotubes vs. Zinc Oxide Nano needles?, Master Thesis, Auburn University, 2006 [34] H. Hiura, T. W. Ebbesen, K. Tanigaki, ?Opening and purification of carbon nanotubes in high yields?, Adv. Mater. 7, 275 (1995) [35] H. TA, and J. Hil, ?Modeling the Loading and Unloading of Drugs into Nanotubes?, Small, Vol. 5, pp. 300-08, 2009 [36] N. W. S. Kam, M. O?Connel, J. A. Wisdom, H. Dai, ?Carbon Nanotubes as multifunctional biological transporters and near-infrared agents for selective cancer cel destruction?, PNAS, Vol. 102, pp. 11600, 2005 [37] H. Muguruma, Y. Matsui, Y. Shibayama,?Carbon Nanotube-Plasma Polymer-Based Amperometric Biosensors: Enzyme-Friendly Platform for Ultrasensitive Glucose Detection?, Japanese Journal of Applied Physics, Vol. 46, Issue 9A, pp. 6078, 2007 [38] J. Clendenin, J. Kim, S. Tung, ?An aligned Carbon Nanotube Biosensor for DNA Detection?, Proc of 2007 2 nd IEE Conference on Nanotechnology, pp. 1028, 2007 [39] M Terrones, ?Science and technology of the twenty-first century: Synthesis, Properties and Applications of Carbon Nanotubes?, Annu. Rev. Mater.Res. , Vol. 33, pp. 419?501, 2003 ! ! 44! [40] T. Guo, P. Nikolaev, A. Thes, D. T. Colbert and R. E. Smaley, ?Fullerene Nanotubes in Electric-Fields?, Chem. Phys. Leters, Vol. 243, pp. 49?54, 1995 [41] M. Daenen, R.D. de Fouw, B. Hamers, P.G.A. Janssen, K. Schouteden, M.A.J. Veld, ?The Wondrous World of Carbon Nanotubes?, project, 2003! [42] M.L. Terranova1, V. Sesal, and M. Rossi, "The World of Carbon Nanotubes: an overview of CVD Growth Methodologies", Chemical Vapor Deposition, Vol. 12, pp. 315?325, 2006 [43] M. Su, B. Zheng, and J. Liu, ?A Scalable CVD Method for the Synthesis of Single Walled Carbon Nanotubes with High Catalyst Productivity?, Chem. Phys. Leters, Vol. 322, pp.321-26, 2000 [44] H. Lee, Y.S. Kang, P.S. Lee, and J. Y. Lee, ?Hydrogen Storage in Ni Nanoparticle- Dispersed Multi-walled Carbon Nanotubes?, J. Aloys Compd., Vol. 330, pp. 569-572, 2005 [45] Ph. Mauron, Ch. Emenegger, A. Z?ttel, Ch. N?tzenadel, P. Sudan, L. Schlapbach, ?Synthesis of Oriented Nanotube Films by Chemical Vapor Deposition?, Carbon 40, pp. 1339-1344, 2002 [46] M. Meyyappan, editor, ?Carbon nanotubes science and applications?, CRC Press, 2004 [47] Y. Tzeng, C. Liu, C. Cutshaw, Z. Chen, ?Low temperature CVD Carbon Coatings on Glass Plates for Flat Panel Display Applications? Mat Res. Soc. Proc, Vol 621, 2000 [48] M. Chhowala, K. B. K. Teo, C. Ducati, N. L. Rupesinghe, G. A. J. Amaratunga, A. C. Ferrari, D. Roy, J. Robertson, and W. I. Milne, ?Growth proces conditions of vertically aligned carbon nanotubes using plasma enhanced chemical vapor deposition? Journal of Applied Physics, Vol. 90, No. 10, pp5308-17, 2001 [49] Bower Chris, Zhu Wei, Jin Sungho, Zhou Oto, ?Plasma-induced alignment of carbon nanotubes? Journal of Applied Physics Vol. 77, No. 6, pp830-32, 2000 [50] Chunsheng Du, Ning Pan, ?CVD growth of carbon nanotubes directly on nickel substrate? Materials Leters, Vol.59, pp1678-82, 2005 [51] Chung ? Nan Tsai, ?Selective and Non-selective Synthesis of Carbon Nanotubes (CNTs) by Chemical Vapor Deposition (CVD) Characterization: Catalysts and Underlayers Effects on Field Emision Properties?, Master Thesis, Auburn University, 2012 [52] V. I. Merkulov, D. H. Lowndes, Y. Y. Wei, G. Eres, and E. Voelkl, ?Patterned growth of individual and multiple vertically aligned carbon nanofibers? Journal of Applied Physics, Vol. 76, No. 24, pp3555-58, 2000 ! 45! [53] Hou T. Ng, Bin Chen, Jesica E. Koehne, Alan M. Casel, Jun Li, Jie Han, and M. Meyyappan, ?Growth of Carbon Nanotubes: A Combinatorial Method To Study the Efects of Catalysts and Underlayers? Journal of Physical Chemistry, Vol. 107, No. 33, pp8484-89, 2003 [54] Haitao Zhao, ?Design and Construction of CNTs Triggered Pseudospark Switch? PhD Disertation, Auburn University, 2012 [55] S. Hofmann, M. Cantoro, B. Kleinsorge, C. Casiraghi, A. Parvez, and J. Robertson, ?Efects of catalyst film thicknes on plasma-enhanced carbon nanotube growth?, Journal of Applied Physics Vol. 98, No. 3, 034308, 2005