Substrate Patterning by Nanomachining for Controlled Carbon Nanotube Growth

A thesis submitted to the

Graduate School

of the University of Cincinnati

in partial fulfillment of the

requirements for the degree of

MASTER OF SCIENCE

in the Department of Mechanical and Materials Engineering

of the College of Engineering & Applied Sciences

by

Guangfeng Hou

Bachelor of Engineering in Mechanical Engineering

East China University of Science and Technology, Shanghai, China, 2009

July 2014

Committee Chair: Dr. Murali Sundaram ABSTRACT

Controlled growth of Carbon Nanotubes (CNTs) with appropriate properties has been of great interest both to researchers and industries, due to their wide applications. Because of its simplicity and scalability, catalytic chemical vapor deposition (CVD) is the most commonly used method to grow CNTs. In the CVD synthesis of CNTs, the choice of catalyst is one of the most important factors. Numerous efforts have been made to control the growth of CNTs by fine- tuning the related catalysts. However, it remains a challenge to control the properties of CNTs and there remain many unsolved issues in achieving the desired performance of the catalyst. In this thesis, various novel methods have been studied to control the growth of CNTs by controlling catalyst on proper substrates.

In this study, anodic aluminum oxide (AAO) membrane has been tested to grow CNTs with the confinement of its nano pores. The experimental work reveals that the maximum diameter of

CNTs grown from the pores is confined by the size of pores, providing upper limit for the diameter of grown CNTs.

Electro discharge patterning (EDP) has been attempted for the first time to deposit catalyst for growth of CNTs. It has been found that the catalyst material could be transferred to substrate, yielding CNT forests. Combined with program design, it is capable of producing various forest patterns of CNTs. EDP is a new versatile and robust method of patterning catalysts. One potential application is patterning catalyst on metals, growing CNTs to form metal/CNTs composites.

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Scanning probe microscopy (SPM) has been utilized for substrate patterning in order to control positioning of catalyst nano particles. This method provides the capability of positioning catalyst particles in nano scale. SPM nano manufacturing and capillary assembly have been combined to place catalyst nano particles in nano cavities and subsequent growth of CNTs.

CNTs successfully grew from the catalysts confined in nano cavities, which could be used to fabricate CNTs based nano devices.

In the CVD synthesis of CNTs, the catalyst-substrate interaction affects the growth of CNTs.

Effort has been made to investigate the catalyst-substrate interaction, and to control the catalyst atomic diffusion into the substrate. An analytical model has been developed to explain the catalyst size change during the CVD process. The experimental results reveals that the catalyst interdiffusion into the substrate could be controlled by using designed catalyst structures, and the height of grown CNT arrays could be increased.

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ACKNOWLEDGEMENTS

I would like to thank my advisor, Dr. Murali Sundaram, for his patient guidance, encouragement and financial support, which made this thesis possible. I am grateful for the opportunity to be his student. He has challenged me to grow in the academic field and taught me how to focus on the crucial aspects of research. I also want to express my gratitude to Dr. Schulz and Dr. Shanov for participating in the committee, as well as their support and suggestions for this study.

I would also like to thank all of my friends from Micro and Nano Manufacturing Lab for their suggestions and valuable inputs; especially Anne Brant, Abishek Balsamy Kamaraj, Sagil

James, Shiv Shailendar, Himanshu Ingale, Varun Kumar and Anudeep Reddy Boddapati.

Furthermore, I would like to thank the help from SNM group. They helped me through useful discussion and experiments. Dr. David S. Lashmore of University of University of New

Hampshire provided insightful comments for this work, which is highly appreciated.

The financial support from University of Cincinnati helped me through the work. This material is based upon work supported by the National Science Foundation.

Last but not least, I would like to thank my parents for their love. They always encourage me to pursue my dreams and provide me unconditional supports.

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TABLE OF CONTENTS

ABSTRACT ...... i ACKNOWLEDGEMENTS...... iv TABLE OF CONTENTS ...... v LIST OF FIGURES ...... vii LIST OF TABLES ...... ix 1 Introduction ...... 1 1.1 Motivation for research ...... 3 1.2 Outline ...... 4 2 Literature review ...... 5 2.1 Growth of CNTs by CVD ...... 5 2.2 Catalyst patterning techniques for growth of CNTs ...... 10 2.3 Challenges for controlled growth CNTs ...... 18 3 Growth of CNTs using AAO as template ...... 19 3.1 Experimental result ...... 20 3.2 Discussion ...... 22 4 Electrical discharge patterning of catalysts ...... 23 4.1 Introduction of EDP process ...... 24 4.2 Catalyst patterning process ...... 25 4.3 Growth of CNTs from electrical discharge patterned catalysts ...... 26 4.4 Discussion ...... 29 5 SPM assisted capillary assembly for catalyst patterning ...... 30 5.1 Introduction of SPM...... 30 5.2 Nano cavity machining by SPM ...... 33 5.3 Capillary force nano assembly for catalyst patterning ...... 35 5.4 Forces involved during the capillary force assembly process ...... 39 5.5 Discussion ...... 43 6 Influence of catalyst-substrate interaction on the growth of CNTs ...... 44 6.1 Catalyst-substrate interaction ...... 44 6.2 Catalyst diffusion and Ostwald ripening ...... 45

v

6.3 Modeling of catalyst diffusion ...... 46 6.4 Experimental investigation of catalyst interdiffusion ...... 53 6.5 Discussion ...... 61 7 Conclusions and Future Work ...... 62 7.1 Conclusion ...... 62 7.2 Future work ...... 62 Reference ...... 64

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LIST OF FIGURES

Figure 1: Various applications of CNT. (a) CMOS-compatible 150 nm interconnects[10]; (b) MWNT used as electrode[11]; (c) Solar cell using a SWNT-based transparent conductor[10]; (d) CNT bumps used for enhanced thermal dissipation in high power amplifiers[10]...... 2 Figure 2: World’ first carbon nanotube computer[12] ...... 3 Figure 3: Typical CVD process used for experimental CNTs synthesis [18] ...... 5 Figure 4 : Transmission electron microscopy images of (a) single-walled CNT [22], (b) double- walled CNT[23] and (c) multi-walled CNT [24]...... 6 Figure 5: Different CVD processes used for CNT synthesis [18] ...... 7 Figure 6: CVD furnace reactor used in this work for growth of CNTs ...... 8 Figure 7: Example of CNT growth condition used in this study ...... 9 Figure 8: Schematic of catalyst patterning by photolithography. (a) Creation of topography on SiO2. (b) Deposition of a metal film. (c) Formation of nanoparticles by annealing. (d) Deposition of a spin-on polymer. (e) Removal of external nanoparticles. (f) Growth of CNTs [37]...... 11 Figure 9: Example of catalyst patterning by E-beam lithography. (a) Catalyst stack and e-beam resist (HSQ) on top of an insulating substrate. (b) Exposure of the resist. (c) Development of the resist in TMAH. (d) Ion milling. (e) Cr wet etching. (f) CVD carbon nanotube growth [41]...... 13 Figure 10:Schematic of μCP for growing CNTs [15] ...... 14 Figure 11: Example of SPM local oxidation for growth of CNTs[39] ...... 16 Figure 12: DPN process for growth of CNTs [15] ...... 17 Figure 13：SEM image of AAO template[52] ...... 19 Figure 14: Schematic of growth of CNTs using AAO as template ...... 20 Figure 15: Catalyst particle on AAO surface by powder and suspension ...... 21 Figure 16: Growth of CNTs from AAO pores using iron suspension ...... 21 Figure 17: Schematic of EDP process ...... 23 Figure 18: Schematic Diagram of Traditional EDM Process ...... 24 Figure 19: Schematic of catalyst deposition by EDP process ...... 25 Figure 20: Iron catalyst on silicon substrate, (a) deposition spot with diameter around 4µm, (b) deposition spot with diameter around 1µm...... 25 Figure 21: SEM image of CNTs on EDP patterned substrates ...... 26 Figure 22: Growth of CNTs from iron patterned on silicon substrate ...... 27 Figure 23: EDS result of catalyst ...... 28 Figure 24: Iron particle and CNTs inside hole ...... 29 Figure 25: Schematic of proposed method ...... 30 Figure 26: Schematic of a typical SPM system ...... 32 Figure 27: Schematic of SPM nanoscale scratching process [64] ...... 32 Figure 28: An example pattern made using MATLAB program...... 33

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Figure 29: Various nano cavities on silicon substrate by diamond coated SPM probes: (d) cavities of 217 nm depth and 170nm diameter; (h) cavities with average 25nm depth and 270nm diameter. Scale bar of 100nm...... 34 Figure 30: Eleven by eleven cavity array manufactured on silicon by SPM: (a) Designed pattern (b) SPM image of dots (c) SEM image of the same feature ...... 34 Figure 31: Schematic of capillary force assisted catalyst assembly ...... 35 Figure 32: Magnetic NP on featured silicon substrate ...... 36 Figure 33: 10 nm magnetic NP assembly result: (a) before NP assembly (b) after NP assembly and ultrasonic removal. Scale bar of 500nm...... 37 Figure 34: Topography file of nano cavity before(a) and after(b) catalyst patterning; square of 2 by 2 um; cavity depth of 10nm and diameter of 150nm...... 37 Figure 35: CNTs on modified surface: (a) modified silicon surface; (b) CNT growing on modified area; (c) CNT SEM image, diameter around 21 nm; (d) SPM image of grown CNT, scale bar of 30 nm ...... 38 Figure 36: 10nm magnetic NP assembly and Growth of CNTs result: (a) Nano cavities on silicon substrate (b) magnetic NP assemblyed on the cavities (c) (d)(e)(f)Growth of CNTs (Square: 2X2µm; Cavity depth: 10nm; Cavity width: 150nm; CNT Diameter: ~30nm) ...... 39 Figure 37: Schematic of capillary assembly process ...... 40 Figure 38: Capillary force and attachement force acted on particle ...... 41 Figure 39: Schematic of supported metal nanoparticle with radius of R and contact angle θ. γsupport γmetal and γinterface are the surface energies of the support, metal particle and interface respectively...... 47 Figure 40: Schematic of surface and interdiffusion between catalyst nanoparticle and support layer...... 48 Figure 41: Catalyst metal particle growth rate plot when C=0...... 52 Figure 42: Catalyst metal growth rate under different conditions ...... 53 Figure 43: Single layer catalyst design ...... 54 Figure 44: SEM image of CNTs from designed substrate ...... 55 Figure 45: Measurement of height by optical microscope and SEM ...... 55 Figure 46: Height of grown CNT arrays of single layer catalyst design...... 56 Figure 47: Various multilayer catalyst design ...... 58 Figure 48: Height of grown CNT arrays of multilayer catalyst design...... 59 Figure 49: CNT array height result from single and multilayer catalyst design ...... 60

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LIST OF TABLES

Table 1: Comparison of SPM and other microscopy techniques [62] ...... 31 Table 2: Substrate and catalyst preparations ...... 56

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1 Introduction

The Carbon nanotubes (CNTs) are tubular structure composed of carbon atoms. Their diameter is in the range of nanometer order, and length of several micrometers/centimeter. They have extraordinary mechanical and electronic properties such as low density, high aspect ratio, one dimensionality, high strength and high electrical and thermal conductivity. Various methods have been utilized to produce carbon nanotubes. Among them, three key growth techniques are electric arc discharge, laser vaporization, and chemical vapor deposition. Chemical vapor deposition (CVD) has been the most popular CNT production method because of its simplicity and up-scalability [1, 2]. The CVD method can be used to grow both single walled nanotube

(SWNT) and multi walled nanotube (MWNT). In these processes, CNTs can be formed from carbon source in presence of catalysts and certain energy input. The catalysts, normally composed of metal nanoparticles, are needed to accelerate the decomposition of the gaseous carbon precursor and enable the growth of nanotubes[3]. Fe, Co and Ni, are commonly used as catalysts because of their high carbon solubility and high carbon diffusion rate at high temperatures[1].

Devices consisting of CNTs have extraordinary properties which could enable a wide range of applications in nanoelectronics[4, 5], sensors[6, 7], bio devices, field emitters[8] and composites[9] (Figure 1).

1

a b

c d

Figure 1: Various applications of CNT. (a) CMOS-compatible 150 nm interconnects[10]; (b) MWNT used as electrode[11]; (c) Solar cell using a SWNT-based transparent conductor[10]; (d) CNT bumps used for enhanced thermal dissipation in high power amplifiers[10].

One specific example is the assembly of CNTs to make novel nano devices. Two major strategies have been widely employed to achieve this target. The first one is to grow and assemble CNTs selectively on catalyst patterned substrates. The other is to post-assemble CNTs after their growth. One recent example is the carbon nanotube computer by Shulaker et al.[12], which has the potential to outperform silicon by improving the energy efficiency by more than an order of magnitude (Figure 2). However, since the post-assembly method requires the treatment and manipulation of CNTs, it may lead to the destruction of their intrinsic properties[13]. Using catalyst patterned substrate to get desired CNTs is a more effective method of fabricating these nano devices.

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Figure 2: World’ first carbon nanotube computer[12]

1.1 Motivation for research

The aforementioned advantageous properties of CNTs ideally expected in an atomically- perfect CNTs is far from the properties of CNTs currently produced. It is still not possible to produce CNTs with well-defined properties in large quantities by a cost-effective technique[14], such as location, orientation and geometry [15, 16].

Understanding and control of the growth of CNTs process, especially the catalyst and growth condition control is essential to produce CNTs with superior properties.

Precise control on direct growth of CNTs on catalyst patterned substrate is one promising way to overcome this barrier. Catalyst patterning on substrate is indispensable for controlling the growth of CNTs. If successful, it is capable of gaining precise control over CNTs’ diameter, length, growth direction and even mechanical and electrical properties.

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Although several studies have been conducted by various research groups to control the

CNTs by patterning catalysts, there is still a lack of knowledge on how the catalyst interacts with

CNT and substrates, and there is still a long way to successfully control the properties of grown

CNTs.

As mentioned before, many applications rely on the patterned growth of CNTs with controlled properties. This work focuses on the catalyst patterning by nanomachining to control the growth of CNTs.

In this study, various novel catalyst patterning methods are proposed and tested to grow

CNTs by CVD. Electrical discharge patterning has been used for the first time to pattern catalyst for growth of CNTs. SPM assisted capillary assembly has been utilized to explore the capability of patterning catalysts on substrates. The interaction between catalyst and substrate has also been studied.

1.2 Outline

The thesis is organized as follows: Chapter 1 provides an introduction, followed by a literature review of catalyst patterning techniques in Chapter 2. Chapter 3 addresses growth of

CNTs using Anodic Aluminum Oxide (AAO) as a template. Chapter 4 describes novel EDP method and related growth of CNTs results, and Chapter 5 provides information of SPM assisted capillary assembly of catalyst patterning. Chapter 6 covers the efforts to control the catalyst- substrate interaction, followed by the conclusion in Chapter 7.

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2 Literature review

As stated before, CVD is one of the most popular and widely used methods to synthesize

CNTs with the help of proper catalysts. The process consists of the catalytic conversion of a gaseous hydrocarbon precursor into a solid material (CNTs) at the surface of catalyst particle.

The catalyst composition, morphology and sate are critical for growth of CNTs. This chapter reviews the growth of CNTs process by CVD, and various catalyst patterning techniques.

2.1 Growth of CNTs by CVD

Chemical Vapor Deposition may be defined as the deposition of a solid material from vapor phase on a heated surface during chemical reaction [17]. In CVD process, precursor gas/gases are introduced into a reactor where heated objects will be coated. There will be chemical reactions happening on and near the high temperature object, and then a thin film will be deposited on the surface. During the process, normally there are by-product gases generated, which will be exhausted. The schematic of typical CVD process is shown in Figure 3.

Figure 3: Typical CVD process used for experimental CNTs synthesis [18]

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CVD synthesis has been utilized as the most popular synthesis method in industry and academia due to its product quality and quantity[19], with better performance than the discharge method[20] and laser vaporization technique[21]. Typical CNTs grown by CVD are shown in

Figure 4.

a b c

Figure 4 : Transmission electron microscopy images of (a) single-walled CNT [22], (b) double-walled CNT[23] and (c) multi-walled CNT [24].

There are several different CVD platforms available with minor variations in their energy source to initiate the growth of CNTs. These various CVD processes have been classified according to their process parameters and energy used (shown in Figure 5). All these CVD processes have been reported to successfully grow CNTs. Commonly used CVD processes are microwave CVD[25, 26], low pressure CVD [27], inductive plasma CVD[28],and hot filament

(HF) CVD[29].

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Figure 5: Different CVD processes used for CNT synthesis [18]

The main aspect of CVD synthesis of CNT consist of[30]: hydrocarbon precursors used, catalyst particles catalytic activity, interaction of catalyst particle with hydrocarbon, interaction of catalyst with support. This thesis focuses on catalysts patterning on the substrate, and their interaction with substrate.

In this study, a CVD reactor (Figure 6) has been used to grow CNT from supported catalyst prepared by various methods.

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Figure 6: CVD furnace reactor used in this work for growth of CNTs

The CVD synthesis process generally involves the heating of the catalyst while controlling the flow of various gases. The heating process used in this study is shown in Figure 7. Argon has been used as inert gas and ethylene as hydrocarbon. Normally the growth temperature is around

700º -780º for designed growth time.

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900 Argon+Hydrogen+Ethylene+Water 800 Hydrogen CNT Growth Period 700

600

C)  500 Argon 400

Temperature( 300 Cooling

200

100

0 0 50 100 150 200 250 300 350 400 Time(min.)

Figure 7: Example of CNT growth condition used in this study

There are many parameters influencing CVD growth performance, including catalyst composition and preparation, hydrocarbon precursors, catalyst-support interactions, and furnace pressure, and temperature[31]. Among these, catalysts’ properties and their interaction with support is of utmost importance since initializes the CNT growth.

Catalyst in growth of CNTs involves nanometer-size metal particles which could accelerate the hydrocarbon decomposition on the catalyst particle. Transitional metals are normally used since they have stronger adhesion with the growing CNTs and thus are more efficient for forming high-curvature CNTs[32]. Various research groups recorded that the catalyst particle size dictate the CNT diameter [33-36]. The critical size threshold for growth of single CNT has been observed by Teo et al[36]. In most cases, the catalyst particles are deposited on support materials such as SiO2 or Al2O3, for better growth result.

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2.2 Catalyst patterning techniques for growth of CNTs

In order to fabricate precisely patterned catalyst substrates, a number of techniques have been explored, such as photolithography, microcontact printing, electron-beam lithography/ion-beam lithography, and dip-pen lithography. a) Photolithography

Photolithography is widely used in microelectronics. Due to its capability to pattern over large areas, photolithography could pattern catalyst and thus the growth of CNTs on a wafer- scale. There are two steps in the patterning of catalyst by conventional photolithography, patterning of photoresist and coating of catalyst (e.g. spin-casting of Fe iron solution). Figure 8 shows the typical process of photolithography catalyst patterning [37]. The deep ultraviolet

(DUV) photolithography was used to pattern Fe/Mo catalysts on full 4 in. Si/SiO2 wafers [15].

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Figure 8: Schematic of catalyst patterning by photolithography. (a) Creation of topography on SiO2. (b) Deposition of a metal film. (c) Formation of nanoparticles by annealing. (d) Deposition of a spin-on polymer. (e) Removal of external nanoparticles. (f) Growth of CNTs [37].

Photolithography has proved to be successful for patterning catalysts, but it also suffers from some limitations, such as the requirement of sophisticated equipment (e.g., photo mask, clean room). As a multiple-step processes, it is time consuming, and the photoresist and etching solution may leave some contamination. Moreover, photolithography does not have the resolution to produce particle level nanometer-scale patterns, patterning catalyst at the single- particle level at specific locations for growth of CNTs is a challenging task for photolithography

[15, 38-40]. b) Electron-beam lithography and focused ion beam lithography

Electron-beam lithography (EBL) is a technique of using electron beams to scan a surface covered with resist film. Patterns are then formed after the lift-off process, and the electron beams are issued as the source energy to pattern resists on surfaces. It is analogous to

11 photolithography, because both are energy-driven patterning of surfaces. However, since the electron wavelength is much smaller than light, EBL can generate nano patterns, with defined shapes and substantially smaller sizes [41]. Figure 9 is an example of electron-beam lithography used for CNT catalyst patterning.

Similar to EBL, focused ion beam lithography (FIB) uses high-energy ion beam instead of electron beam to pattern surfaces [15]. These two patterning lithographic techniques are advantages for depositing catalyst with nano resolution at specific locations for growth of CNTs.

Despite their widespread use, these methods suffer from the low throughput, high cost, and complicated experimental conditions, making them unsuitable for large-area patterning of catalyst and wafer-scale CNT arrays [15, 42].

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Figure 9: Example of catalyst patterning by E-beam lithography. (a) Catalyst stack and e- beam resist (HSQ) on top of an insulating substrate. (b) Exposure of the resist. (c) Development of the resist in TMAH. (d) Ion milling. (e) Cr wet etching. (f) CVD carbon nanotube growth [41].

c) Microcontact printing

Microcontact printing (μCP) has been reported to be a suitable micro fabrication tool for patterning small molecules and nanomaterial in the micrometer scale over a large surface area.

The schematic of μCP is shown in Figure 10. In brief, the patterned stamp with raised and recessed features was coated with desired materials and then was brought in contact with a substrate to transfer materials. Through the transferring, the patterns were formed. Some

13 advantages of this technique include the low cost and easy fabrication of stamp and the easy operation of the printing process. It has been proven that catalyst nanoparticles can be directly patterned on wafers by μCP for growth of CNTs [15].

Figure 10:Schematic of μCP for growing CNTs [15]

µCP is fundamentally different from traditional photolithography techniques. First, only a polymer stamp, poly(dimethylsiloxane) (PDMS), with a predesigned pattern is required. In contrast to the sophisticated equipment required in photolithography, this provides almost universal accessibility. The stamps can be conveniently fabricated and repeatedly used. Second, the µCP technique is compatible with a variety of materials. Third, this technique is suitable for printing on planar, curved, 3D and flexible substrates, making it possible to fabricate 3D and flexible electronics. It is anticipated that µCP will continue to be utilized in laboratory research owing to its simplicity and flexibility for easily creating patterns and tailoring their surface functionalities [42].

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However, the masks used to make the micro patterned stamps are expensive, and each pattern modification (i.e., shape, size, spacing) requires re-designing of the mask. Moreover, it is not easy to generate sub-500 nm features using the conventional PDMS stamp [16, 40]. Moreover, it is usually difficult to control the distinct position [43]. d) SPM-based techniques

The scanning probe microscopy (SPM) provides the ability to observe surface structures at atomic resolution. It is also a powerful tool to modify the sample surface. Various methods have been developed to pattern surfaces using SPM. SPM-based techniques can be divided into two major categories, namely constructive and destructive, in which the probe tip is used to generate patterns by delivering materials onto surface and damage or scratch the surface, respectively [42].

SPM local anodic oxidation is an electrochemical process, which could fabricate nanoscale patterns by oxidizing the sample surface through anodic reaction between tip and the surface.

The SPM tip is used as a cathode, and the water meniscus between tip and sample surface function as the electrolyte. The OH- ions from the water provide the oxidant for the electrochemical reaction. SPM local oxidation can change the morphology and properties of metallic or semiconductor surfaces for fabrication of nano devices. An example of SPM local oxidation used for growth of CNTs is shown in Figure 11[39].

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Figure 11: Example of SPM local oxidation for growth of CNTs[39]

One limitation of SPM local oxidation is that the tip and substrate should be conductive or semi conductive. Occasionally, the tip has to be coated with conductive material in order to achieve the desired oxidation results. The oxide thickness is limited by the electric field decay, and the oxide growth process self-terminates from the oxidation process [44].

Dip-pen nanolithography (DPN) is a widely used SPM-based non constructive technique for surface modification, which will be discussed in this section. DPN has been one of the most popular Atomic force microscopy (AFM) nanolithography techniques since its invention, in which materials initially on the tip are transferred to the surface while scanning in either static or dynamic mode. In this process, the SPM probe is analogous to an ink pen. The ‘‘ink’’ usually consists of nanoparticles suspended in liquid, or inorganic or biological molecules in a solvent.

An example of DPN for growth of CNTs is shown in Figure 12[15].

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Figure 12: DPN process for growth of CNTs [15]

Unlike the traditional lithographic methods, DPN is a maskless and single-step direct- writing method, and can be carried out under moderate operating conditions which does not require high vacuum or high-energy ions or beams. This eliminate the possibility of cross- contamination and sample-damage. More importantly, DPN is capable, in principle, of delivering any solution precisely to a specifically designated location, where “inks” may form any desired pattern with feature sizes down to sub-100 nm. This is crucial for nano device fabrication in complex integration systems [16].

DPN consequently provides a number of advantages in patterning catalytic precursors for subsequent CVD growth of carbon nanotubes. However, the reliability and reproducibility of the technique depends critically on the properties of the ink used in the process; as well as ambient conditions such as humidity and temperature during the patterning process. Additionally, the sizes of the catalyst particles that are formed on the substrate determine the yield of CNTs after the CVD process. The water meniscus between the tip and substrate is commonly believed to be responsible for ink transport, the dimension of which is largely influenced by the relative humidity [40, 42].

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2.3 Challenges for controlled growth CNTs

Although the previously mentioned techniques have been explored for growth of CNTs, they are still far from achieving the goal of controlling the properties of grown CNTs. Some challenges include:

1. The successful growth of single CNT with controlled properties from patterned catalyst.

2. The inefficient, time-consuming preparation behind catalyst patterning. A fast and robust

catalyst patterning method is needed.

3. The ability to control the catalyst-substrate interaction.

To address these challenges, this work explores three novel catalyst controlling methods.

SPM capillary force assembly has been successfully utilized to grow a single CNT. Electrical discharge patterning has been used for the first time to pattern catalysts, providing a robust catalyst patterning method. The interaction between catalyst and substrate has been studied.

Details of these studies will be described in subsequent chapters.

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3 Growth of CNTs using AAO as template

Anodic aluminum oxide (AAO) is well-organized ceramic structure with nano pores

(Figure 13). AAO could be fabricated by electrochemically oxidizing aluminum with control of the pore size by varying the magnitude of the anodization voltage. In this study, the AAO with pore diameter of 200 nm has been purchased from Sigma-Aldrich. Due to its structure uniformity,

AAO has been utilized to grow controlled CNTs serving as templates. It is one way of growing regularly aligned CNTs that are required in nano electronic devices. It has been reported that various metallic nanowires could be synthesized by the electrodeposition of materials in the the nano channels, such as Ag[45], Au[46], Co[47], Ni[48] and Pd [49]. Few reports that the CNTs diameter and length could be controlled using AAO templates [50, 51] with small diameter.

Synthesis of long large-diameter CNT/CNF has been attempted in this study.

Figure 13：SEM image of AAO template[52]

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3.1 Experimental result

AAO could be fabricated with pores of varying sizes in the nano scale (200 nm diameter in this study). By utilizing the well-defined pores in AAO, the ability to grow CNTs confined by the pore size has been achieved (Figure 14).

Figure 14: Schematic of growth of CNTs using AAO as template

Iron nano powder and iron suspension have been employed to place nanoparticles inside pores of AAO. The results are shown in Figure 15. From the results, the iron suspension yields more uniform catalyst deposition than the iron powder catalyst (which forms larger catalyst particles due to particle agglomeration). Thus, iron suspension has been chosen in later experiments.

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Figure 15: Catalyst particle on AAO surface by powder and suspension

Figure 16: Growth of CNTs from AAO pores using iron suspension

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From Figure 16, CNTs have been successfully grown from AAO pores. The pores provide sufficient confinement for growth of CNTs. The growth varies from one or several

CNTs per hole, which depends on the amount of catalysts inside each particular pore.

3.2 Discussion

AAO has been used to grown CNT from its 200 nm pores; their maximum diameter has found to be confined with the pore size. The upper limit of the grown CNTs has been controlled.

The productivity can be further improved by tuning the deposition process parameters, such as the concentration of the catalyst suspension and the magnitude of magnetic field applied. Smaller pore sizes may be used to grow lower diameter CNTs.

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4 Electrical discharge patterning of catalysts

Electrical discharge has been used in producing carbon nanotubes [53] and machining of carbon nanotube forests [54]. CNTs are also used as electrode in EDM process [55]. However, electro discharge has not yet been reported in literature for catalyst patterning. The capability of patterning iron as a catalyst for growth of CNTs by electrical discharge patterning (EDP) is explored in this study.

Figure 17: Schematic of EDP process

The EDP process is illustrated on Figure 17. Catalyst wire is maintained in the vicinity of the substrate. With the electrical discharge produced by power supply, plasma forms between the wire and substrate, attracting catalyst particles to the substrate surface. The result of the process depends on several parameters, such as the voltage, tool diameter, and distance between tool and substrate.

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4.1 Introduction of EDP process

EDP is based on the Electrical Discharge Machining (EDM) technique. EDM is a non- contact electrothermal process, which can be used to drill holes, generate profiles and make complex shaped dies. The schematic of EDM process is shown in Figure 18. Conversion of electrical energy to thermal energy is achieved through repeated occurrence of sparks between the tool and the work piece; both are immersed in a dielectric medium and separated by a small distance (spark gap) results in the material removal from workpiece as well as tool by melting, evaporation and, in special cases spalling. The material removed is the major source of debris particles. The common dielectric fluids used are kerosene, paraffin, and light hydrocarbon oils. A necessary condition for producing a discharge is the ionization of the dielectric medium and splitting molecules into ions and electrons (i.e., formation of plasma).

Figure 18: Schematic Diagram of Traditional EDM Process

For the quite different purpose of catalyst patterning, EDP is employed to deposit catalyst material from the tool to the substrate, as shown in Figure 19. The plasma forms between catalyst tool and substrate, followed by spark emergence. The electric discharges produced at the gap

24 remove the catalyst tool material via melting and evaporation, which is then latter deposited on the substrate and used for growth of CNTs.

Figure 19: Schematic of catalyst deposition by EDP process

4.2 Catalyst patterning process

Several deposition results are shown in Figure 20. The setup is combined with a precise micro stage which has the programming capability to control the pattern of deposition. Shown below are dot arrays fabricated by the setup. The dot has a diameter around 2µm. The substrates with the designed catalyst pattern will be used to grow CNTs by CVD.

a b

Figure 20: Iron catalyst on silicon substrate, (a) deposition spot with diameter around 4µm, (b) deposition spot with diameter around 1µm .

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4.3 Growth of CNTs from electrical discharge patterned catalysts

Figure 21 is SEM image of one EDP catalyst dot. The substrate is conductive silicon and catalyst used is iron. The CNT forest grown there provide the evidence of iron catalyst. Figure 22 are more growth of CNTs result.

Figure 21: SEM image of CNTs on EDP patterned substrates

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Figure 22: Growth of CNTs from iron patterned on silicon substrate

From Figure 21 and Figure 22, the morphology of grown CNTs shows that EDP could be used to provide unique CNT metal composites. CNTs grew on the plasma-defined area, and from the catalyst material. To confirm the existence of catalyst removal from the catalyst tool and deposition on the surface, energy-dispersive x-ray spectroscopy (EDS) analysis has been carried out using an environmental SEM. The result (Figure 23) clearly demonstrates the existence of catalyst material for the growth of CNTs.

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Figure 23: EDS result of catalyst

Another notable phenomenon is the formation of small holes under sparse plasma interactions. In Figure 24, multiple micro holes have been formed after EDP patterning and CVD synthesis, resulting in a large growth of CNTs. It is speculated that catalyst is preferable to stay inside the holes, yielding more CNTs (Figure 24). This may help to investigate the catalyst deposition mechanism during the EDP process, and help to improve the catalyst pattering result.

A possible cause is that the formation of plasma and initial removal of catalyst material from the tool. Because the tool is located a distance above the substrate, the catalyst material can travel to the substrate and surface. The dielectric fluid was observed to reduce the catalyst material present on the surface, leaving a greater quantity inside the holes.

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Figure 24: Iron particle and CNTs inside hole

4.4 Discussion

EDP is a versatile and robust catalyst patterning method at ambient conditions, which has been successfully employed to grow bulk CNTs. The growth catalyst is determined by the material transfer of the catalyst electrode. It is the first time electrical discharge has been utilized to pattern catalysts. This inexpensive technique could be used to make micrometer-scale and even nano scale patterned surfaces for the growth of CNTs. It has been confirmed that the catalyst material removed from the catalyst tool helps to promote the growth. There are many potential applications of EDP-patterned CNT, such as production of various CNT/metal composites by using different substrates and catalysts.

29

5 SPM assisted capillary assembly for catalyst patterning

SPM can be used for tip-based nanofabrication on various materials. The proposed method is illustrated in Figure 25. First, an SPM was used to fabricate nano cavities on silicon substrate with the help of MATLAB program. Once the proper cavities are made, catalyst nano particles will be assembled into the cavities by capillary force nano assembly, followed by CVD growth of CNTs and characterization.

Figure 25: Schematic of proposed method

5.1 Introduction of SPM

Scanning Probe Microscopy (SPM), invented by Bining, Quate and Gerber in 1986 [56], has the capability of nano scale characterization in vacuum, air or in liquids with sub-nanometer resolution (Table 1). The comparison of SPM with Transmission electron microscopy (TEM),

Scanning electron microscope (SEM) and optical microscope is shown in Table 1. Aside from

30 high-resolution imaging, the SPM allows electrical property measurement [57], magnetic property measurement [58], nano-object manipulation [59, 60] and nano manufacturing [61].

SPM TEM SEM Optical

Max Atomic Atomic 1’s nm 100’s nm Res Typical Cost 100-200 >500 200-400 10-50 (K$) Imaging environ. Air, fluid, vacuum vacuum vacuum Air, fluid

In-situ Yes NO No Yes

In-fluid Yes No No Yes

Sample prep. Easy Difficult Easy Easy

Table 1: Comparison of SPM and other microscopy techniques [62]

The SPM can be described as a “blind-man stick” [63]: In the same way as a blind man reconstructs in his brain the street surface by scanning the sidewalk with his stick – either by tapping or by gently moving the stick from point to point, line after line – the SPM reconstructs digitally the sample surface through submicrometric scanning with a probe.

The main components of SPM include: probe, optical detection system, piezo crystal scanner and feedback system. The signals in the SPM system are also labeled in Figure 26.

31

Figure 26: Schematic of a typical SPM system Many SPM based nano fabrication techniques have been developed with different degrees of similarities. One example is mechanical scratching (Figure 27), which constitutes the mechanical removal process of various materials. The materials underneath are directly extracted or removed by tip scratching or plowing. In the process, a certain amount of force is applied on the tip by controlling the feedback system.

Figure 27: Schematic of SPM nanoscale scratching process [64]

32

5.2 Nano cavity machining by SPM

To fully control the SPM-fabricated features, a MATLAB program has been designed to generate C code to run the SPM probe in predesigned pattern. Figure 28 is an example of features the program could produce.

Figure 28: An example pattern made using MATLAB program

Below Figure 29 are some SPM images of manufactured cavities on silicon. These results reveal that the SPM has the capability to fabricate various cavities with different parameters which will later be used for holding catalyst particles.

33

a b c d

e f g h

Figure 29: Various nano cavities on silicon substrate by diamond coated SPM probes: (d) cavities of 217 nm depth and 170nm diameter; (h) cavities with average 25nm depth and 270nm diameter. Scale bar of 100nm.

Scanning electron microscopy (SEM) has been used to verify the nano cavities. The SPM and SEM images are shown below. The machined cavities later are latter used to growth CNTs.

(Figure 30).

a b c

Figure 30: Eleven by eleven cavity array manufactured on silicon by SPM: (a) Designed pattern (b) SPM image of dots (c) SEM image of the same feature

34

5.3 Capillary force nano assembly for catalyst patterning

Figure 31 shows the process of capillary force assembly. The substrate with SPM manufactured cavities was placed vertically along the wall of beaker. A syringe pump was used to draw the catalyst suspension out of the beaker, thus lowering the liquid level. The capillary force during the contact line movement pushes the catalyst particles into the nano cavities, which are then used to grow the CNTs by CVD. Note that the concentration of the catalyst suspension and speed of the liquid removal is important to get good result.

Figure 31: Schematic of capillary force assisted catalyst assembly

A select portion of the experiment results is shown below. Figure 32 shows assembly results with high concentration of catalyst suspension (density of 3.12×1016/ml). It is evident there are too many catalyst particles were deposited on the substrate surface, indicating a lower concentration is preferred.

35

a b

c d

Figure 32: Magnetic NP on featured silicon substrate

Figure 33 and Figure 34 are SPM height/topography images of assembled catalyst particles inside cavities. After comparing pre- and after-assembly images, it can be determined whether or not the cavity is filled with catalyst particles.

36

a b

Figure 33: 10 nm magnetic NP assembly result: (a) before NP assembly (b) after NP assembly and ultrasonic removal. Scale bar of 500nm.

a

b

Figure 34: Topography file of nano cavity before(a) and after(b) catalyst patterning; square of 2 by 2 um; cavity depth of 10nm and diameter of 150nm.

The CNTs grown from the assembly sample are shown below. They are grown using ethylene under 750ºC for 25 minutes. Figure 35 is sample with higher catalyst suspension concentration, which yields a relatively large amount CNTs covering the area. Conversely, in

Figure 36, the CNTs are only found growing in the cavity. 37

a b

c d

Figure 35: CNTs on modified surface: (a) modified silicon surface; (b) CNT growing on modified area; (c) CNT SEM image, diameter around 21 nm; (d) SPM image of grown CNT, scale bar of 30 nm

38

a b c

d d f

Figure 36: 10nm magnetic NP assembly and Growth of CNTs result: (a) Nano cavities on silicon substrate (b) magnetic NP assemblyed on the cavities (c) (d)(e)(f)Growth of CNTs (Square: 2X2µm; Cavity depth: 10nm; Cavity width: 150nm; CNT Diameter: ~30nm)

5.4 Forces involved during the capillary force assembly process

In this section, the capillary assembly process has been investigated, and the forces determining the process are identified. This helps to understand the process and find possible methods to achieve effective assembly.

Figure 37 illustrates the capillary assembly process; the receding suspension line moves down through its duration. The schematic indicates that the nanoparticle is pushed into the cavity.

In reality, the thermal fluctuation of these nanoparticles has to be overcome. Considering of the nano scale size of these particles, it is not trivial for the influence of thermal fluctuation. Hence, for successful assembly, there must be certain forces overcoming this fluctuation.

39

Figure 37: Schematic of capillary assembly process

Normally used catalyst suspension involves nanoparticle colloids. There is always constant erratic Brownian motion of the colloids, which is caused by fluctuating momentum from solvent molecules to the suspended particles. The root-mean-square displacement motion

1/2 of the particles in time t could be quantified using Stokes-Einstein diffusion coefficient

D0=kT/6πηR[65]:

1 1 1 2 2푘푇 < 푅 >2= ( )2 = (2퐷 푡)2 (1) 6πηR 0

Where k is Boltzman’s constant, T the absolute temperature, η the solvent viscosity and R the particle radius.

The forces acting on the particles include: Derjaguin-Landau-Verwey-Overbeek (DLVO) force for electrostatic and Van der Waals force, surface tension (capillary) force, particle weight and buoyancy force[66]. For particles smaller than 1 μm in diameter, the gravitational and buoyancy forces are negligible. The electrostatic force and Van der Waals force normally are

40 typically significantly small than capillary forces[67]. Thus, the capillary force remains as the dominant force for assembly, which is shown in Figure 38.

Figure 38: Capillary force and attachement force acted on particle

In the assembly process, the contact line moves across the particle. The average attachment force caused by capillary force could be expressed by[68]:

퐹 = 2휋푅훾푠푖푛2(훽)푐표푠훼 (2) 푎푡푡 2

where R is radius of particle, γ is liquid surface tension, α substrate contact angle and β particle contact angle.

The effect of thermal fluctuation is around kT level, here the work done by capillary interaction force is calculated during the process, comparing it with thermal fluctuation [69] to decide whether it is possible to achieve assembly.

41

The particle is influenced by thermal fluctuation and attachment force Fatt. Consider a particle in vicinity of the substrate with distance from substrate S=R (rough estimation based on the meniscus shape). To be more precise, S should be the distance of particle from substrate when attachment force starts acting on the particle, which is affected by the meniscus shape.

Here S=R is used as example to explain the principle involved in the process. The work done by attachment force is expressed by the following relation:

푾 = 푭 × 푺 = ퟐ흅푹ퟐ휸풔풊풏ퟐ(휷)풄풐풔휶 (3) 풂풕풕 풂풕풕 ퟐ For effective assembly, the attachment force work should overcome the thermal fluctuation energy kT so that the particle could reach and stay on the substrate. On the other hand, the attachment force should not be to too high so as to pin the particle on the surface, which is observed in this study. Although there is no exact threshold that determines when pining, it is assumed that attachment energy one order of magnitude higher than thermal fluctuation energy would cause catalyst pining on flat substrate surface.

Based on the aforementioned discussion, the effective assembly would happen when the following condition is satisfied:

ퟏ 풌푻 < 푾풂풕풕 < ퟏퟎ 풌푻 (4)

i.e. 1 푘푇 < 2휋푅2훾푠푖푛2(훽)푐표푠훼<10 kT 2

For example, a 10 nm diameter iron particle, using γ=67.91 mN/m[70](water surface tension at 50ºC), β=72.5ºC[71], T=323.15 K (50ºC), k=1.381×10-23 J/K, yields the following results:

42

72.5 1.381 × 10−23 × 323.15 < 2π × 25 × 10−18 × 67.91 × 10−3푠푖푛2 ( ) 푐표푠훼 2

< 10 × 1.381 × 10−23 × 323.15

89.3º < 훼 < 89.9º

A better estimation could be achieved if the average height of the meniscus (and thus the average of distance of particle from the substrate) could be calculated. The implies the existence of an optimal range of contact angle for given nanoparticles.

5.5 Discussion

The SPM-assisted capillary assembly process has been designed and successfully tested in placing catalyst particles in cavities for the growth of CNTs. Single CNT was successfully grown. This method has the potential to investigate the influence of different catalyst types and amounts on the yield of CNTs, with the proper control of process parameters. The grown and patterned CNTs could be used in various applications, such as nano electronics. A mathematical model has been built to explain the assembly process, depending on the work done by capillary force, the ability of a nanoparticle to overcome thermal fluctuation is evaluated.

43

6 Influence of catalyst-substrate interaction on the growth of CNTs

After the catalyst has been patterned on substrate, catalytic chemical vapor deposition (CVD) was used to grow CNTs with the help of catalysts. From the catalysis perspective, three interactions: catalyst particle-hydrocarbon, catalyst particle-support, and hydrocarbon-support, should be considered for the synthesis of CNTs. The catalytic properties of the catalyst-substrate system greatly depend on the interaction between the catalyst and support material[72].

It has been reported [3, 73-78] that catalyst diffusion is one major growth of CNTs termination mechanism. To grow CNTs with good properties, the catalyst substrate interactions, particularly the catalyst diffusion, should be studied.

6.1 Catalyst-substrate interaction

The same catalyst behaves differently depending on the support materials used. Some materials are commonly used as substrates in CVD synthesis, including quartz, silicon dioxide, silicon carbide, and alumina.

It has been reported that the catalytic behavior of various catalyst particles could be ceased due to metal–substrate chemical reaction (bond formation)[33]. This may be the reason why silicon supports are not often used for Growth of CNTs, since silicide would form, ceasing the catalyst activity. Typically a diffusion buffer layer of SiO2 or Al2O3 is employed. It should be noted that the formation of alloy compounds with the oxide layer can still occur at high temperatures and long treatment times [3].

For different buffer layers, Arcos et al.[79] found that alumina materials are better catalyst support than silica under some circumstances and proposed that it is due to stronger metal- support interaction in the alumina. This yields high metal dispersion and thus a high density of

44 catalytic sites. These interactions would help to prevent the metal catalysts from enlarging and becoming undesired large clusters, which otherwise will cause to the formation of graphite particles, amorphous carbon and defective CNTs[33]. Mattevi et al.[80] proposed that the stronger interaction of Fe with Al2O3 anchor Fe particles to the oxide surface and limit their coarsening. Gao et al. [81]performed MD simulation and experiment to study the diffusion of Fe on different substrates; it was found to be relatively low on alumina, which was proposed to be the reason behind high CNTs growth on alumina.

However, others have noticed gold clusters are active when deposited on TiO2, but inactive on Al2O3; thus proposing that strong metal-support interactions will cease the catalytic activity of metal particles on certain supports, due to charge redistribution and atom diffusion[30].

Based on literature, there is no established knowledge on how the catalyst-substrate interaction would influence the growth of CNTs. Among the debate, catalyst diffusion and morphological change are important topics.

6.2 Catalyst diffusion and Ostwald ripening

Kim et al. [82] found that a period beyond five minutes of thermal annealing of Fe thin film to form catalyst particles, would result in particle size and density to decrease. This is caused by

Fe diffusion into the alumina layer, which induces mass loss from the catalyst and eventual termination of growth of CNTs. It is reported that interdiffusion of Fe into Al2O3 can be initiated at temperatures as low as 600 ºC[83], which is lower than typical CVD growth temperatures.

Thus, the catalyst diffusion into support buffer layer must be evaluated.

Cho et al. [84] examined the growth and termination of vertically aligned long carbon nanotube arrays, and found catalyst metals could diffuse/migrate into the support buffer layer;

45 catalyst on the substrate were found to decrease more than 50% percent by atomic number percent. It was proposed that the diffusion of small catalyst particles into sublayers contributes to the growth of CNTs’ termination. Several other groups [73-75] have also found the experimental evidence of catalyst diffusion into support layer and hypothesized it is the reason for growth of

CNTs termination.

Aside from the mechanism of catalyst diffusion into the support layer, catalyst surface diffusion is another possible source for growth termination reported by many researchers [3, 75-

78] due to its ability to decrease catalyst density and increase average catalyst particle size.

Surface diffusion is also called Ostwald ripening, which describes the phenomenon that large nanoparticles grow in size at the expense of small nanoparticles.

Based on the above discussion, it is important to investigate the catalyst diffusion regarding its role in catalyst deactivation. In the following section, a model for the catalyst surface diffusion and interdiffusion is described.

6.3 Modeling of catalyst diffusion

To study the catalyst diffusion, it is assumed that the catalyst particles are supported on a substrate. As shown schematically in Figure 39, a supported catalyst particle in a spherical segment could be described by the radius of curvature R, contact angle θ, exposed area

2 2 As=2πR [1-cosθ] and interface area Aint=πR sin2θ.

46

Figure 39: Schematic of supported metal nanoparticle with radius of R and contact angle θ. 후퐬퐮퐩퐩퐨퐫퐭 후퐦퐞퐭퐚퐥 and 후퐢퐧퐭퐞퐫퐟퐚퐜퐞 are the surface energies of the support, metal particle and interface respectively.

Whether the catalyst particle is solid or liquid is still a debatable issue. The melting point of this nanoparticle in CVD environment is a key factor, which could be influenced by three effects

[30]. First, the presence of carbon could decrease the melting point by up to a few hundred degrees. Second, for a particle in the range of 1-10 nm in diameter, the equilibrium vapor pressure is largely increased; the melting point of a nanoparticle is significantly differs from that of bulk material. Third, the catalyst-substrate interaction could also modify the melting point.

From high resolution TEM images [85, 86], it is reported that the catalyst particles behave as liquid, yet maintains a crystalline structure.

For a metallic nanoparticle on a buffer layer, in the simplified case of absence of surface energy anisotropies of the metal, a mechanical equilibrium for this system can be expressed by

Young’s equation:

휸풔풖풑풑풐풓풕 = 휸풎풆풕풂풍풄풐풔휽 + 휸풊풏풕풆풓풇풂풄풆 (5)

47

For Fe islands on SiO2, the following condition takes place: −1

This indicates that the surface energy of SiO2 is less than that of the Fe/SiO2 interface[87].

Experimentally, the work of adhesion (Wad) rather than the interface free energy could be measured. Wad could be calculated by the formula:

푾풂풅 = 휸풎풆풕풂풍 + 휸풔풖풑풑풐풓풕 − 휸풊풏풕풆풓풇풂풄풆 = 휸풎풆풕풂풍(ퟏ + 풄풐풔휽) (6) The adhesion energy is a potentially critical factor in the CNTs’ growth mode. For nanoparticles with lower adhesion energy with support, tip growth mode is more effective, since it is easier for nanoparticle to be lifted off the support. For catalyst particles with higher adhesion energy, bottom growth is preferred.

Figure 40: Schematic of surface and interdiffusion between catalyst nanoparticle and support layer.

There exist two kinds of diffusion for supported catalysts, surface diffusion and interdiffusion. The processes are illustrated in Figure 40.

48

For surface diffusion, Ostwald ripening is the most commonly used mechanism proposed to describe the kinetic process. Wynblatt and Gjostein[88] derived kinetic equations for evolution of island radii from a sample of initial size distribution. In Ostwald ripening, metal monomers detach preferentially from small metal islands, diffuse randomly across the support substrate, and attach preferentially to larger metal islands. The diffusing adatoms are assumed to have sites on the surface of both the metal particles and the support substrate. There are two different rate- limiting regimes: the ripening is rate limited either by a detachment/attachment process of metal atoms at the edge of the island or by surface diffusion of metal adatoms form one island to another.

Aside from the surface diffusion, interdiffusion of catalyst into substrate has been reported by several groups [74, 82, 89]. Sakurai et al.[90] considered the surface diffusion and interdiffusion of iron nanoparticle for the catalyst size growth in the CVD process. However, their surface diffusion model is simplified from [91], which assumes that the interface transfer is much faster than surface transfer. For typical coinage metals the activation barrier for emission of adatoms onto oxide substrates is much higher (~2.5 eV) than adatom surface diffusion (0.2-0.5 eV)[92].

Therefore, it is more reliable to expect the surface transfer is faster than interface transfer, and consider the interface adatom as the limiting process.

Ostwald ripening could be modeled by mean-field surface diffusion methods. The relation controlling nanoparticle radius r is given by [92, 93]

풅풓 푲풄풐풏풔 ퟐ휸휴 ퟐ휸휴 = [풆풙풑 ( ∗ ) − 풆풙풑 ( )] (7) 풅풕 풓 풓 풌푩푻 풓풌푩푻

49

Where Kcons is constant, γ metal surface energy, Ω the atomic volume of the bulk metal, 푘퐵 is

Boltzmann’s constant, T is temperature, 푟∗ is critical radius which is relating a NP in equilibrium neither decreasing nor increasing in size.

2γΩ 2γΩ Since ∗ and are small[88], equation (7) can be simplified by using: 푟 푘퐵푇 푟푘퐵푇

2γΩ 2γΩ 2γΩ 2γΩ exp ( ∗ ) = 1 + ∗ and exp ( ) = 1 + 푟 푘퐵푇 푟 푘퐵푇 푟푘퐵푇 푟푘퐵푇

Thus equation (7) becomes,

풅풓 푲풄풐풏풔 ퟐ휸휴 ퟐ휸휴 푲풄풐풏풔 ퟐ휸휴 ퟏ ퟏ 푲 풓 = [ ∗ − ] = [ ∗ − ] = ퟐ [ ∗ − ퟏ] (8) 풅풕 풓 풓 풌푩푻 풓풌푩푻 풓 풌푩푻 풓 풓 풓 풓

The rate can be finally expressed as:

풅풓 = 푲 ( 풓 − ퟏ) (9) 풅풕 풓ퟐ 풓∗

Where 퐾 = 2γΩ퐾푐표푛푠 푘퐵푇

According to this model, for nanoparticle with size smaller than r*, dr/dt is negative, so the radius would decrease. With a smaller radius, the dr/dt would still be negative, which means the particle would continue to shrink. This process would continue until the small particle decreases and disappears. For big nanoparticle with size larger than r*, the growth rate dr/dt is positive, which indicates the particle will grow, after which the growth will still be positive. The model predicts the large nanoparticle will keep growing up infinitely, which obviously will not happen.

There are two possible reasons, the first one being a finite availability of small particle proving adatoms. After certain time, the small nanoparticles will shrink to disappear, and there will be no

50 adatoms attaching to the big particle. The second reason may be the interdiffusion of nanoparticle into the support layer, which will cause mass loss of the nanoparticle.

The interdiffusion of catalyst particle could be incorporated into equation (9) by considering subsurface flux [90],

풅풓 = 푲 ( 풓 − ퟏ) − 풋푺 (10) 풅풕 풓ퟐ 풓∗ 흏풏/흏풓 Where j is the flux of interdiffusion into support layer, S is the vertical cross section of the nanoparticle, and n is the number of atoms in the nanoparticles.

2 4πr 2 3 Since n = and 푆 = π(푟푠푖푛휃) , where 훼 = (2 − 3푐표푠휃 + 푐표푠 휃)/4 and Vm is molar 3Vm volume. This gives the following relation,

풅풓 = 푲 ( 풓 − ퟏ) − 푪 (11) 풅풕 풓ퟐ 풓∗

2 where 퐶 = 푗푉푚푠푖푛 휃/(4훼). Note that C could be treated as valid when the interdiffusion doesn’t dominate the diffusion process.

If C=0, a typical graph representing dr/dt is shown on Figure 41. From the graph, the critical size r* is the cutoff value for the catalyst particle.

51

15

10

5

0

-5

-10

dr/dt -15

-20

-25

-30

-35

-40 0 1 2 3 4 5 6 7 8 r/r*

Figure 41: Catalyst metal particle growth rate plot when C=0.

If C is nonzero, the catalyst metal particle growth rate corresponds to the plot shown in

Figure 42.

If C

C

Considering the energy perturbation due to environment change, the particle size may oscillate

52 according to this stable range. In this case, the proposed model could predict the particle size range, which is essential for the later growth of CNTs.

If C>K/4r*^2, the growth rate is negative and thus all the catalyst particles will continue shrink and disappear. In this case, the interdiffusion may dominate the catalyst particle change.

15

10 C=0 5 r1 r2 0

-5 C

-10 2

dr/dt C>K/4r* -15

-20

-25

-30

-35

-40 0 1 2 3 4 5 6 7 8 r/r*

Figure 42: Catalyst metal growth rate under different conditions

6.4 Experimental investigation of catalyst interdiffusion

In previous section, it is assumed that the interdissufion of catalyst into the support layer is slow and at constant speed. In reality, the interdiffusion of catalyst is complicated. More factors including the substrate, growth time, pre-treatment should be considered. In the following section, a series of experiments was designed to investigate the catalyst-substrate interaction, especially the catalyst interdiffusion. 53

Figure 43 illustrates the designed single layer process. 10 nm Al and 1 nm Fe were deposited by e-beam evaporation on the substrate in sequence. During the later annealing at 840ºC, the iron catalyst will diffuse into the support and form a complicated mixer[79]. Subsequently, another iron catalyst (1.5 nm) layer was deposited on the surface, providing the extra catalyst source for the growth of CNTs.

Figure 43: Single layer catalyst design

Figure 44 contains growth results of CNTs from the designed samples, which were grown using ethylene under 750ºC for 30 minutes. The height of CNT arrays are measured by an optical microscope (Figure 45). For each sample, five height values have been obtained and averaged into onevalue. The result is shown in Figure 46. The height of CNT arrays is highly related with the active catalysts on the surface. By measuring the CNT array height, the available active catalyst could be estimated, thus evaluating the catalyst interdiffusion state.

54

Figure 44: SEM image of CNTs from designed substrate

Figure 45: Measurement of height by optical microscope and SEM

55

3500

3000

2500 m) μ 2000

1500

CNT height ( height CNT 1000

500

0 A1 A2 A3 B1 B2 B3 Samples

Unpretreatment+30minutes@750C Unpretreatment+60minutes@780C Pretreatment+60minutes@780C Pretreatment +120minutes@780C

Figure 46: Height of grown CNT arrays of single layer catalyst design

Substrate Catalyst A1 Silica 10 nm Al +1 nm Fe on Silica A2 Silica 10 nm Al +1 nm Fe on Silica+ annealing A3 Silica 10 nm Al +1 nm Fe on Silica+ annealing+1.5 nm Fe B1 Silicon 10 nm Al +1 nm Fe on Silica B2 Silicon 10 nm Al +1 nm Fe on Silica+ annealing B3 Silicon 10 nm Al +1 nm Fe on Silica+ annealing+1.5 nm Fe C1 Silica 1 nm Fe+ 10 nm Al+ 1 nm Fe C2 Silicon 1 nm Fe+ 10 nm Al+ 1 nm Fe C3 Silica 1 nm Fe+ 10 nm Al2O3+ 1 nm Fe C4 Silicon 1 nm Fe+ 10 nm Al2O3+ 1 nm Fe Table 2: Substrate and catalyst preparations The CNT growth result is shown above (Figure 46) with catalyst conditions (Table 2). For samples with catalyst deposited on silica (A1-A3). There is a height-changing pattern for all four growth conditions: A1 (10 nm Al + 1 nm Fe) behaves best, yielding highest CNT arrays. After annealing (A2), the height of CNT arrays decrease significantly by more than 50%. For example, for samples underwent 4 hours annealing at 450ºC, the height of A2 decreased by 75.8% than that of A1. The sample with extra 1.5 nm catalyst (A3) grew longer CNT arrays than annealed 56 samples (A2). This height-changing pattern supports the previous assumption. As mentioned earlier, the annealing process could accelerate the catalyst interdiffusion into the substrate, thus causing loss of catalyst on the surface which is responsible for the growth of CNTs. Therefore a drop of grown CNTs’ height appears due to loss of catalyst during annealing. The depositing of extra 1.5 nm catalyst on the surface after annealing (A3) provides more active catalyst on surface during CVD synthesis. Then no wonder this sample (A3) helps recover the height of CNT arrays from annealing samples (A2). It is worth mentioning that even after putting 1.5nm iron catalyst, the performance of the sample (A3) is still inferior to the pure sample (A1). This indicates that the catalyst activity may be increase even higher if more extra catalyst is deposited beyond 1.5 nm.

The same height-changing pattern exists on the silicon substrates (B1-B3). However, it is noticeable that for samples with extra 1.5 nm Fe (A3 and B3), silica gives better result than silicon in terms of recovering ability from annealed samples (A2 and B2). This indicates depositing extra catalyst results in different behavior, depending on the substrate material used.

The reason may be due to more stability of silica. Although the Fe-Al interaction is more important than Al-Si/Silica interaction, the later interaction did affect the growth of CNTs, more specifically, silica yields better result than silicon.

In the current experiments, samples A1 and A4 yield highest CNT arrays. In order to achieve better result, it is reasonable to provide more extra catalyst so that the performance could match even surpass the unannealed samples (A1). One way of providing more catalyst is depositing two catalyst layers. Figure 47 illustrates the four designed multilayer catalyst structure by varying the substrate and support layer. Based on the first design, catalyst interdiffusion into the substrate causes the deactivation of the catalyst. It is assumed that, by depositing another catalyst layer

57 under the support layer of aluminum and alumina, the bottom catalyst would diffuse upward into the support layer, and reduce the catalyst loss from the surface catalyst.

Figure 47: Various multilayer catalyst design

The result of the CNT array height from multilayer catalyst design is shown below

(Figure 49). Note that the anneal in the Sample Notes means 840ºC for 5minutes when preparing the deposition layers. After that, before CVD, some sample went to annealing in air at

450ºC for 4 hours, which is designed to accelerate the diffusion from bottom catalyst layer.

58

4000 3500

3000 m) μ 2500 2000 1500

CNT height ( height CNT 1000 500 0 C1 C2 C3 C4 Samples

Unpretreatment+30minutes@750C Unpretreatment+60minutes@750C Pretreatment+60minutes@750C Pretreatment+120minutes@750C

Figure 48: Height of grown CNT arrays of multilayer catalyst design

The activity of different multilayer catalyst structure are different. Generally, the Fe-Al-

Fe structure is better than Fe-Al2O3-Fe structure. From sample C1 to C4, there is a trend of height decreasing. Although for the unannealed+60minutes@780ºC condition, C3 and C4 gave similar result, the trend is also valid. The Fe-Al-Fe structure (C1 and C2) yield better result than

Fe-Al2O3-Fe structure (C3 and C4), for both silicon and silica substrate. The reason may be due to the stability of aluminum and alumina, and different dissolubility of iron in them. Because the alumina is more stable with lower interdiffusion rate, it is easier for catalyst under Al layer to diffuse upward into the surface than diffusing through alumina layer, thus providing more active catalyst.

For multilayer catalyst structures. The difference of substrate for Fe-Al-Fe is more obvious than Fe-Al2O3-Fe structure. For alumina support layer, the difference from silica and silicon substrate is limited. However, for aluminum support layer, the variance from silica and

59 silicon substrates is larger. The height variance of C1 and C2 is much obvious than height variance of C3 and C4. The reason may be the stability of alumina, so it is not much affected by the substrate.

4000

3500

3000 m)

μ 2500

2000

1500 CNT height ( height CNT 1000

500

0 A1 A2 A3 B1 B2 B3 C1 C2 C3 C4 Samples

Unpretreatment+30minutes@750C Unpretreatment+60minutes@750C Pretreatment+60minutes@750C Pretreatment+120minutes@750C

Figure 49: CNT array height result from single and multilayer catalyst design

The CNT array grown form single and multilayer catalyst design under same growth conditions is shown in Figure 49. For the highest CNT array, there is no obvious winner and their performance is similar. Samples A1, B1 and C1 yield highest CNTs with small variance.

However for a longer growth time (120 minutes), the height of C1 outperform A1 and B1 by

32.46%. Height of Sample C2-C4 increased to comparable value with Sample A1 and B1. For example, sample C2 yields CNT array with height of 2230μm and A1 of 2585.1μm height from first design. For shorter growth time, C2-C4 are normally around half of the height of A1 and B1 for growth time of 30 and 60 minutes. This implies that the diffusion of bottom catalyst

60 is greatly influenced with growth time. For a longer growth time, they could be diffused more upward to the surface, providing more active catalyst.

6.5 Discussion

Catalyst-substrate interaction is a complex yet important basis for controlling growth of

CNTs. After designed experiments, it was shown that:

a) Catalyst-substrate interaction, particularly interdiffusion, could be controlled by adding

extra catalysts on the surface. Height of CNT arrays could be increased;

b) The extra catalyst could also be added from the bottom by multilayer catalyst method. It

is better to use silica as support and Al as intermediate layer for growing longer CNT

arrays. Or a thinner intermediate layer may be helpful for the bottom catalyst to diffuse

upward into surface;

c) For multilayer catalyst, a longer growth time could benefit the bottom catalyst diffusion

into the surface.

This method is useful for controlling the CNT property and should be considered when

designing catalysts. If combined with other catalyst design approaches, such as binary

catalyst and proper pre-treatment, the properties of grown CNT would be highly improved.

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7 Conclusions and Future Work

7.1 Conclusion

Three novel catalyst engineering methods have been proposed and studied in this work.

AAO has been utilized to grow CNTs with maximum diameter confined by the pore size.

Varying number of CNTs grew form a pore, probably due to presentence of high amount of iron.

Electrical discharge patterning technique successfully patterned catalyst for CNTs growth.

This is the first time it has been used for catalyst patterning. Deposition pattern size down to 1μm was achieved and a mixture of CNTs and substrate was achieved.

Single CNT has been successfully grown by SPM assisted capillary assembly for catalyst patterning, which is a universal method which works for any catalyst suspension. This eliminates the need for high cost lithography equipment and provides a quick test platform for various catalysts. A mathematical model explaining the assembly process was created.

The interaction between catalyst and substrate has been investigated. An analytical model has been created to investigate catalyst surface diffusion and interdiffusion behavior. A novel method of controlling the interdiffusion has been proposed and experimentally tested. The results proved that by designed approaches, single layer and multilayer design, the interdiffusion could be controlled, and the height of CNT arrays was increased.

7.2 Future work

CNTs grown by AAO template could be further controlled in the length, yield, tube numbers per hole, crystallinity by catalyst engineering and tuning growth condition.

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Currently, the EDP patterned catalyst are typically manufactured on the micro scale.

However, by reducing the catalyst tool size and controlling the process parameters, it is possible to achieve nano scale catalyst deposition, which is worthwhile to implement. If realized, it will provide a nano scale patterning technique which is robust and provides vast applications.

The catalyst-substrate interaction control method could be combined with other catalyst design techniques, such as binary catalysts, to improve the grown CNTs’ properties.

The catalyst-substrate interaction in SPM-assisted assembly and EDP patterning technique can be analyzed to understand their effect on the growth of CNTs under different pattering methods.

Finally, there are a large number of applications for the patterned CNTs, including nano electronics, nano sensors, and photonics. This work contributes to the fabrication of nano devices-a reality in the future.

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