Comparative Study Of Electronic Properties In Pristine And Si-Doped Single-Walled Carbon Nanotube As Gas Sensor: A First Principle Study.

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UNIVERSITI TEKNIKAL MALAYSIA MELAKA

COMPARATIVE STUDY OF ELECTRONIC PROPERTIES IN

PRISTINE AND SI-DOPED SINGLE-WALLED CARBON

NANOTUBE AS GAS SENSOR: A FIRST PRINCIPLE STUDY

This report submitted in accordance with requirement of the Universiti Teknikal Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering

(Engineering Materials) (Hons.)

FARIZUL MUIZ BIN ALIAS B051210136

910718-11-5425

FACULTY OF MANUFACTURING ENGINEERING 2015

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UNIVERSITI TEKNIKAL MALAYSIA MELAKA

BORANG PENGESAHAN STATUS LAPORAN PROJEK SARJANA MUDA TAJUK: Comparative Study of Electronic Properties in Pristine and Si-Doped Single-Walled Carbon Nanotube as Gas Sensor: A First Principle Study

SESI PENGAJIAN: 2014/15 Semester 2

Saya FARIZUL MUIZ BIN ALIAS

mengaku membenarkan Laporan PSM ini disimpan di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut: 1. Laporan PSM adalah hak milik Universiti Teknikal Malaysia Melaka dan penulis. 2. Perpustakaan Universiti Teknikal Malaysia Melaka dibenarkan membuat salinan

untuk tujuan pengajian sahaja dengan izin penulis.

3. Perpustakaan dibenarkan membuat salinan laporan PSM ini sebagai bahan pertukaran antara institusi pengajian tinggi.

4. **Sila tandakan ( )

SULIT

TERHAD

TIDAK TERHAD

(Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia sebagaimana yang termaktub dalam AKTA RAHSIA RASMI 1972)

(Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/badan di mana penyelidikan dijalankan)

Alamat Tetap:

NO. 712, Jalan Sekolah Sungai Kechil, Nibong Tebal, 14300,

Seberang Prai Selatan, Pulau Pinang Tarikh: 2 July 2015

Disahkan oleh:

Cop Rasmi:

Tarikh: 2 July 2015

** Jika Laporan PSM ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh laporan PSM ini perlu dikelaskan sebagai SULIT atau TERHAD.

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DECLARATION

I hereby, declared this report entitled “Comparative study of electronic properties in pristine and Si-doped single-walled carbon nanotube as gas sensor: A first principle

study” is the results of my own research except as cited in references.

Signature : ………

Author’s Name : Farizul Muiz bin Alias

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APPROVAL

This report is submitted to the Faculty of Manufacturing Engineering of UTeM as a partial fulfillment to the requirements for the degree of Bachelor of Manufacturing Engineering (Engineering Materials) (Hons.). The member of the supervisory is as follow:

……….. (Dr. Mohd Asyadi Azam bin Mohd Abid)

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ABSTRAK

Nanotiub karbon berdinding tunggal dan silikon pendopan nanotiub karbon berdinding tunggal telah disiasat sebagai salah satu bahan untuk peranti pengesan gas. Gas yang telah dipilih ialah karbon dioksida, oksigen dan metanol disebabkan oleh penggunaan yang meluas dan dikawal dalam penggunaan dalam sector industri. Penjerapan dan ciri-ciri elektronik telah disiasat keatas nanotube karbon berdinding tunggal yang tulen dan yang telah didopan keatas reaksi gas kepada dinding nanotiub karbon berdinding tunggal. Semua pengiraan dijalankan menggunakan kajian prinsip pertama berteraskan teori fungsian ketumpatan (DFT) dimana telah dilaksanakan dalam kod komputer CASTEP and DMol3. Untuk pengoptimuman geometri, telah mengunakan korelasi pertukaran anggaran-anggaran fungsian seperti Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) supaya struktur menjadi tepat. Dengan menggunakan GGA, didapati bahawa ketepatan pengiraan sebanyak 80% ke 90% pada keseluruhan hasil. Dari segi tenaga serapan menunjukkan O2 dan CO2 mempunyai serapan yang lemah terhadap nanotube berdinding tunggal namun CH3OH dengan kadar -0.61 eV dan gas yang terletak pada silikon pendopan nanotiub karbon berdinding tunggal menunjukkan tenaga serapan yang tinggi menghasilkan reaksi dengan kadar diantara -1.6 eV ke -3.99 eV. Untuk pengiraan ciri-ciri elektronik, hasil menunjukkan penambahan state berdekatan valance band dan mengurangkan jurang band sebanyak 0.5 eV daripada strakture original nanotube berdinding tunggal sebanyak 0.6 eV sementara DOS menunjukkan jenis-P semikonduktur menghasilkan lebih sensatif gas sensor ketika diekposkan dengan gas. Oleh itu, dengan pencarian ini mampu menaikan usaha untuk mengoptimumkan pengunaan gas sensor dan akan menyumbang kepada lebih sensatif gas sensor.

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ABSTRACT

A pristine single-walled carbon nanotube (SWCNT) and silicon-doped SWCNT have been investigated as materials for gas sensor. The gases that had been chosen were carbon dioxide, oxygen, and methanol due to their widely use and controlled in industrial sector. Adsorption and electronic properties were investigated on the pristine and doped SWCNT upon the gas reacted to SWCNT wall. All the calculation were performed using a first principle study base on density functional theory that has been implemented in CASTEP and DMol3 computer code. For geometry optimization, exchange correlation functional approximations such as Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) have been utilized in order to get accurate structural information. Using GGA showing improvement in accuracy of data by 80% to 90% in overall calculation. In term of adsorption energy indicates that the O2 and CO2 are weakly adsorbed on the pristine SWCNT but CH3OH with value of -0.61 eV and all other gases attached on Si doped SWCNT showing higher amount of adsorption energy that promote to chemisorption reaction with value range of -1.6 eV to -3.99 eV. As the electronic properties been calculate, the result indicate there is additional state produce at valance band and reducing the band gap structure in silicon-doped SWCNT of 0.5 eV from the original pristine structure of 0.6 eV while DOS showing a p-type semiconductor properties producing a more sensitive electronic properties upon doping and exposure of gases. Thus, these finding will be able to gear up efforts in optimizing the usage of gas sensor and hence will substantially contribute to the more sensitive gas sensor.

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iii

DEDICATION

To everyone that contributes to this research, my family and my friend that has been helping me all along.

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ACKNOWLEDGEMENT

First of all I would like to express my gratitude to Allah S.W.T. for blessing on me throughout this while in completing this final year report. Therefore, I would like to take this opportunity to express our gratitude to all those who helped me either directly or indirectly in carrying out this research.

A million awards to my supervisor, Dr. Mohd Asyadi Azam bin Mohd Abid for his unfailing patience, concerned, advice and encouragement throughout this research. Besides that, not to forget Dr. Mohamad Fariz bin Mohamad Taib from UiTM Shah Alam and Mrs. Husna A. Hamid for given guidance and support to me that enabled me to complete my PSM log book and report as well as my research.

Last but not least, thanks also to all the parties involved in providing encouragement to me while completing this research, including both my parents and all staffs of FKP for giving me guidance to accomplish my research. Not forgotten, I would like to thank to all my friends that have helped me a lot by giving some ideas and suggestions to accomplish this research.

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TABLE OF CONTENT

Abstract i

Abstrak ii

Dedication iii

Acknowledgement iv

Table of Content v

List of Tables viii

List of Figures ix

List of Abbreviations, Symbols and Nomenclatures xi

CHAPTER 1: INTRODUCTION

1.1 Background 1

1.2 Problem statement 3

1.3 Objectives 5

1.4 Scope 5

CHAPTER 2: LITERATURE REVIEW

2.1 Introduction 7

2.2 The nature of Carbon 7

2.3 Carbon nanotube 9

2.3.1 Single-walled carbon nanotube (SWCNTs) 10

2.3.2 Chirality of SWCNT 10

2.3.3 Multi-walled carbon nanotube (MWCNTs) 12

2.4 Doping system of CNT 13

2.4.1 Si atom doping system 14

2.5 CNTs based gas sensor 15

2.6 The nature of gas molecules 18

2.6.1 Carbon dioxide 18

2.6.2 Oxygen 19

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2.7 Theoretical studies of nanotube on gas adsorption 19

2.8 Computational molecular modelling 21

2.8.1 First principles of quantum theory 21

2.8.2 Density functional theory 23

2.8.2.1 The Hohenberg-Kohn Theorems 24

2.8.2.2 Kohn-Sham Method 24

2.8.2.3 Exchange-Corellation Functional 25

2.8.3 Local density approximation 26

2.8.4 Generalized gradient approximation 26

2.8.5 K-Point Sampling 27

2.8.6 Materials studio software 28

2.8.6.1 CASTEP (Cambridge serial total energy package) 30 2.8.6.2 Dmol3 (density functional calculation on

molecules)

2.8 Summary of computational theory 32

CHAPTER 3: METHODOLOGY

3.1 Introduction 33

3.2 The computational materials studio software 34

3.3 Computer system specification 38

3.4 Building structure of single-wall carbon nanotube - STEP 1 39

3.5 Structure refinement – STEP 2 44

3.6 Calculation of material properties using quantum based software (CASTEP and DMol3) – STEP 3

3.7 Analysis structure – STEP 4 48

3.7.1 Adsorption Energy 48

3.7.2 Electronic band structure 49

3.7.1.1 Band structure 49

3.6.1.2 High symmetry of Brillouin zone 50

3.7.3 Density of state 52

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vii

4.1 Geometry properties 54

4.1.1 Pristine SWCNT and Si-doped SWCNT 56

4.1.2 O2 on pristine SWCNT and Si-doped SWCNT 58 4.1.3 CO2 on pristine SWCNT and Si-doped SWCNT 60 4.1.4 CH3OH on pristine SWCNT and Si-doped SWCNT 62

4.2 Adsorption energy analysis 65

4.3 Electronic properties 68

4.3.1 Pristine and Si doped SWCNT electronic properties analysis 68

4.3.2 O2 adsorption 71

4.3.3 CO2 adsorption 74

4.3.4 CH3OH adsorption 77

CHAPTER 5: CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusion 80

5.2 Recommendations 81

REFERENCES 82

APPENDICES

Appendix A: Adsorption energy calculations 94

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viii

LIST OF TABLES

2.1 Cross section view of CNT 12

3.1 Overview of materials studio system requirement 37

3.2 Convergence thresholds in DMol3 44

3.3 Brillouin Zone symmetry points 50

3.4 Brillouin Zone SWCNT structure path 51

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LIST OF FIGURES

2.1 Summary of different carbon form 8

2.2 Graphite rolled up to form CNT 9

2.3 Single-walled carbon nanotube 10

2.4 The chiral vector of CNT 11

2.5 Multi-walled carbon nanotube 13

2.6 Si doping on SWCNT 15

2.7 Typical CNTs gas sensor 17

2.8 Homogeneous gas concepts 26

2.9 The time and size in materials multi-scale characterization 31

3.1 General step for quantum modelling 35

3.2 Molecular modelling using first principle 36

3.3 Four main step in quantum modelling 36

3.4 PSM work flow chart 37

3.5 Creating structural in visualizer 40

3.6 Structure of (8, 0) zigzag single-walled pristine carbone nanotube 41 3.7 A (8, 0) zigzag single-walled Silicon doped single-walled 42

carbon nanotube

3.8 Gas molecules (a) O2 (b) CO2 (c) CH3OH 42

3.9 The side view upon gasses location above SWCNT where 43 (a) O2 (b) CO2 (c) CH3OH

3.10 DFT exchange-correlation energy 44

3.11 Parameter setup in DMol3 and CASTEP 47

3.12 Band structure of metal, semiconductor and insulator 49

3.13 Brillouin Zone band structure 50

3.14 SWCNT structure Brillouin Zone 52

3.15 Example of DOS of various CNTs structure 53

4.1 Possible adsorption location for gases (a) Pristine SWCNT 55 (b) Si doped SWCNT

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4.2 Geometry structure of (a) Pristine SWCNT; (b) Si doped SWCNT 56 4.3 Representing the side view of (a) Pristine SWCNT; 57

(b) Si doped SWCNT

4.4 The binding energy of pristine SWCNT with O2 58 4.5 The binding energy of Si doped SWCNT with O2 58 4.6 Geometry structure of O2 adsorbed on a) Pristine SWCNT; 60

b) Si doped SWCNT 60

4.7 The binding energy of pristine SWCNT with CO2 61 4.8 The binding energy of Si doped SWCNT with CO2 61 4.9 Geometry structure of CO2 adsorbed on a) Pristine SWCNT; 62

b) Si doped SWCNT

4.10 The binding energy of pristine SWCNT with CH3OH 62 4.11 The binding energy of Si doped SWCNT with CH3OH 63 4.12 Geometry structure of CH3OH adsorbed on a) Pristine SWCNT; 64

b) Si doped SWCNT

4.13 Pristine SWCNT (a) Band structure (b) DOS 69

4.14 Si doped SWCNT (a) Band structure; (b) DOS 70

4.15 Electronic properties of O2 on Pristine SWCNT 72 (a) Band structure; (b) DOS

4.16 Electronic properties of O2 on Si doped SWCNT 73 (a) Band structure; (b) DOS

4.17 Electronic properties of CO2 on Pristine SWCNT 75 (a) Band structure; (b) DOS

4.18 Electronic properties of CO2 on Pristine SWCNT 76 (a) Band structure; (b) DOS

4.19 Electronic properties of CH3OH on Pristine SWCNT 78 (a) Band structure; (b) DOS

4.20 Electronic properties of CH3OH on Pristine SWCNT 79 (a) Band structure; (b) DOS

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LIST OF ABBREVIATIONS, SYMBOLS AND

NOMENCLATURES

1D - One dimensional

2D - Two dimensional

3D - Three dimensional

µm - Micrometer

Ĥ - Hamiltonian operator

Ψ - Wavefunction

Å - Angstrom

a - lattice vectors

B - Boron

BZ - Brillouin-zone

C - Carbon

CASTEP - Cambridge serial total energy package

Ch - Chiral vector

CB - Conduction band

CH3OH - Methanol

CNT - Carbon nanotube

CO - Carbon monoxide

CVD - Chemical vapor disposition

DFT - Density functional theory

DFPT - Density functional perturbation theory DMol3 - Density functional calculation on molecules

DOS - Density of state

E - Energy

e - Electron charge

Ead - Adsorption energy

eV - Electron volt

FFT - Fast Fourier transform

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xii

HCP - High performance computer

IR - Infrared

LDA - Local density approximation

NH3 - Ammonia

NO2 - Nitrogen dioxide

nm - Nanometer

M - Electron mass

MS - Materials studio

MWCNT - Multi-walled carbon nanotube

O2 - Oxygen

P - Momenta of ions

PC - Personal computer

PW91 - Perdew and Wang

r - Radius

rc - Cut-off radius

SCF - Self consistence field

Si - Silicon

SWCNT - Single-walled carbon nanotube

USP - Ultra-soft pseudopotential

VB - Valance band

Viz. - Visualizer

WDA - Weight density approximation

XC - Exchange-correlation

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1.1 Background Study

Nowadays, gas sensors are becoming more essential measurement for the used in industries and factories as well as in the daily life. Many gases can be a perfect example of a gas that can be harmful such as carbon monoxide, carbon dioxide and many others. Due to this causes, detectors for this gas can be found in many mass market retailers and industries across the country. As a results, gas sensors are being extensively studied to develop a sensitive, low cost and reliable gas sensor.

Basically, chemical gas sensor is well defines as a device that is able to change the physical properties upon exposure to gas molecules through electrical conductance. An electrical signal is measured upon the change in these properties and the results are then used to detect the gas molecules. Fast response time and high accuracy are the two basic requirements for gas sensors. According to Kerotocenkov (2013), the size, cost and weight are the main constraints along producing sensor with capability to measure the physical properties over a certain lifetime. In contrast, commercially market available for gas sensors have advantages in lower cost, smaller size and easy to operate. Typical gas sensors can be from in a variety of size micrometers to centimeters while the size can be reduced due to rapid large scale ups of such productions (Yamazoe, 2005). These all make these gas sensors viable for real time in situ environments. Meanwhile, the

INTRODUCTION

CHAPTER 1

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2 major drawbacks for gas sensors include stability, sensitivity, selectivity, and reproducibility which could render these gas sensors unsuitable for long term usage.

Nanotechnology is known as any process or product that has sub micrometer dimensions. Since 1990s, there is a rapid growth in research interest of nanotechnology including increasing in high performance of nanomaterials and nanoscale electronic devices. (Pandyan, 2012). The discovery of carbon nanotubes (CNTs) has been attracted considerable attention since they have a wide range of applications in nanometer-scale electronic devices. Furthermore, carbon material in sensor technology becoming an ideal component, since they have a very good sensitivity on sensing with interesting electronic properties of the materials (Roberts et al, 2009). As a result, recent advancements in nanotechnology have provided opportunities to improve sensor performances in detecting gases dramatically. Considerable interests have been on new room temperature operated gas sensing materials. Due to their ability to change the electrical properties upon exposure to the gas molecule and high surface, CNTs have been a promising as new gas sensing material combining with outstanding properties which include high performance, fast respond upon exposure of gas and wide selectivity of gases that able to be detected (Jung et al, 2014). Therefore, a huge effort in research has been carried out in new gas sensors as the demand of gas detection technologies is increased. It is indeed not surprising to see the sensor research community as a beneficiary due to advanced nanomaterial developments as carbon nanotubes have been one of the mostly studied nanomaterial.

Quantum calculations using first principle study base on the density functional theory (DFT) provide a great opportunity to prescribe the initial and the fundamentals properties in materials. Furthermore, the atomic view can be accurately visualized in detail. Besides that, the first principle calculations require less empirical data with cost efficient comparing to the experiment laboratory work which will help in term of reasonable and affordable in large scale material design.

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3 The main focus of this study is to investigate and understand the pristine and doped zigzag (8, 0) single-walled carbon nanotube (SWCNT) reacted toward gas adsorption. The zigzag (8, 0) SWCNT was choosen because, according to Iijima and Ichihashi (1993), the (8, 0) CNTs were dominant during synthesized and having a semiconductor nature. In addition, the gasses consists of O2, CO2, and CH3OH that will react with pristine SWCNT, but the doping system in SWCNT anticipated to enhance the adsorption energy as well as the electronic properties upon adsorption of gases which will promote an impact in sensitivity of the gas sensor technology. Furthermore, the gases were chosen due to the wide range of industrial application and some of them having human and environmental issues.

1.2 Problem statement

There have been a high interest studies toward gas sensing application using CNT. However, there are less fundamental understanding in the interaction behavior classes of gas molecules (O2, CO2, and CH3OH) with pristine CNT and doped CNT system (Zanolli et al., 2011). Ironically, there are only investigation on the amount of gas that can be adsorbed by the CNTs but no identification on binding distance and stable position of the gas reacted toward the CNTs. Therefore, upon understanding the sensitivity of the gas molecule adsorption, the geometry and adsorption behavior (adsorption energy) becoming essential. According to G. Guo et al. (2007), pure CNTs are sensitive only toward gas molecules such as NO2, SO2, and O2 which have strongly chemical interaction with CNTs. Thus, it is highly recommended to find the effective ways to improve the sensitivity of CNTs upon other molecules, in particular industrial application as well as the human and environmental effects gases. Furthermore, pristine SWCNT having a weak Vander wall forces and it cannot detect some of gas molecules such as carbon monoxide and ammonia (Pandyan et al., 2012).

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4 It has been widely studied that the electronic properties of CNTs having a semiconducting nature and very sensitive to only certain gases. Moreover, the range of gases CNTs sensors can be detected are limited to gas molecules that possess large binding energies and charge transfers toward nanotube (Wang and Yeow, 2009). According to Adjizian et al. (2014), the special characteristics of CNT due to they possess high surface area to interact with gases and the electrical conductivity change upon the gas adsorption at room temperature. A promising approach to enable single-walled carbon nanotubes (SWCNTs) to detect gas molecules is by doping (heteroatom substitution), when heteroatom impurities are being introduced (such as Silicon) or forming active sites in tube walls, the sensitivity of adsorption on SWCNTs can be improved (Bai et al., 2007).

Moreover, the use of different doping atom (Si) will produce different electrical properties for semiconductor SWCNT that also help in producing a sensitive gas sensor, but there are limited understanding on the changing in electronic properties of doped CNT (Llobet, 2013). Furthermore, the electronic properties of gas adsorbates upon molecule adsorption producing different electronic properties with different type of gas interaction. The effect of the electronic properties through doping and gas adsorption can be observed in the change in the electrical resistance. Indeed, electronic charge transfer between the gas molecule and the doped CNT will affect the position of the Fermi energy and effect the conductivity of gas detection (Zanolli et al., 2011). The need to understand the adsorption behavior and the electrical properties of doped SWCNT upon gas adsorption is worth it to study in suggesting the good sensitivity of gas sensor as it will show the electron charge transfer concepts upon adsorption of gases molecule.

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1.3 Objectives

The main objectives of this project are:

i. To understand the fundamental insight into the molecular interactions between common gaseous adsorbates pristine SWCNT and Si-SWCNT models.

ii. To investigate electronic properties of pristine and silicon doped (8, 0) zigzag SWCNT upon gas molecular adsorption.

1.4 Scope

This project will mainly focus on understanding the right adsorption in term of high adsorption energy of these molecules base on the geometrical configuration and structural stability of doped SWCNT. After completing the adsorption energy and geometrical analysis, there are also several measurements need to be account in order to achieve high sensitivity which is electronic properties. Furthermore, from electrical properties involved the study of changing in band structure and density of states using density functional theory based on computational numerical base to achieve the best type of doped SWCNT related to gas sensor application. In order to optimize the structure of pristine and doped SWCNT upon gas adsorption and understanding their electronic properties, DFT theory were executed. Chapter 1 offers an overview to the project by describing the important of SWCNT as gases sensor and explain the basic of using computational as to design a new material base on the quantum theory. Furthermore, the project problem were identified and proposed solutions that need to be gain are established in problem statement. Chapter 2 provides review on SWCNT material as well as Silicon doped SWCNT upon gases molecules. Also, this chapter brief the first principle study of adsorption behavior between pristine and doped SWCNT on exposure to gases molecule will be explained in this chapter. More, DFT also been reviewed in detail starting from Schrodinger equation until energy cut-off for using basis set of plane wave with Local Density Approximate (LDA) and Generalized Gradient

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6 Approximate (GGA) exchange correlation. Chapter 3 describe the methodology on computational and understanding the usage of first principle calculation that been applied in this project. The computational methods setting parameters are shown and well explained to ensure the calculation technique are the most accurate with cut-off energy and convergence the k-point. The structures of pristine SWCNT and doped SWCNT are respectively built by using Material Studio Visualizer. The adsorption energy and electronic properties are perform by using DMol3 and CASTEP computer modules.

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2.1 Introduction

This chapter provides an overview on the basic understanding of carbon nanotube (CNT) and will be discuss in order to understand the molecular structure of the materials. The literature of doping system in CNT and an understanding the nature of gas molecules and adsorption for CNT are also stated with regard to cover the scope of our research. In addition, the density functional theory (DFT) with first principle study and as one of the mathematical and physic tools to investigate the electronic properties of pristine and doped CNT upon gas adsorption are also reported in this chapter.

2.2 The nature of carbon

Carbon is an element having 6 as an atomic number with a symbol C. From the periodic table, the element of carbon belong to the group 14 which is respected having a

non-metallic group with a six elections with two of the electron filling the 1s orbital another remaining electron fill the sp2_{for double bonding or sp}3_{for single bonding as} well as the sp hybrid orbital for triple bonding. According to Zhang (2012), in comparison with different bonding state will introduce into certain structural arrangement in which sp bonding will give rise to one dimensional chain structure, sp2 bonding will produce planar structure and sp3_{bonding to tetrahedral structure.}

LITERATURE REVIEW

CHAPTER 2

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8 The short lived nature of atomic carbon is effect the stability, because of that carbon is stabilized in different molecular configurations with different structures called allotropes. Moreover, the several best allotropes are graphite, nanoform, and amorphous carbon with recent addition of fullerenes and carbon nanotube (D'Souza and Kadish, 2012). The carbon tree family which is shown Figure 2.1 shows that the range in carbon nanoform family of is significantly larger than mention because of unique bonding versatility of carbon.

Figure 2.1: Summary of different carbon form

Carbon Diamond

(sp3₎ Graphite _(sp2₎

Turbostratic Rhombohedral Hexagonal

Carbynes

(sp) Amorphous (sp3_/sp2₎ _{(quasi sp}Nanoform 2₎

Fullerenes Nanotubes

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react with pristine SWCNT, but the doping system in SWCNT anticipated to enhance the adsorption energy as well as the electronic properties upon adsorption of gases which will promote an impact in sensitivity of the gas sensor technology. Furthermore, the gases were chosen due to the wide range of industrial application and some of them having human and environmental issues.

1.2 Problem statement

system (Zanolli et al., 2011). Ironically, there are only investigation on the amount of gas that can be adsorbed by the CNTs but no identification on binding distance and stable position of the gas reacted toward the CNTs. Therefore, upon understanding the sensitivity of the gas molecule adsorption, the geometry and adsorption behavior (adsorption energy) becoming essential. According to G. Guo et al. (2007), pure CNTs are sensitive only toward gas molecules such as NO2, SO2, and O2 which have strongly

chemical interaction with CNTs. Thus, it is highly recommended to find the effective ways to improve the sensitivity of CNTs upon other molecules, in particular industrial application as well as the human and environmental effects gases. Furthermore, pristine SWCNT having a weak Vander wall forces and it cannot detect some of gas molecules such as carbon monoxide and ammonia (Pandyan et al., 2012).

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5 1.3 Objectives

The main objectives of this project are:

i. To understand the fundamental insight into the molecular interactions between common gaseous adsorbates pristine SWCNT and Si-SWCNT models.

ii. To investigate electronic properties of pristine and silicon doped (8, 0) zigzag SWCNT upon gas molecular adsorption.

1.4 Scope

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7 2.1 Introduction

2.2 The nature of carbon

Carbon is an element having 6 as an atomic number with a symbol C. From the periodic table, the element of carbon belong to the group 14 which is respected having a

non-metallic group with a six elections with two of the electron filling the 1s orbital another remaining electron fill the sp2_{for double bonding or sp}3_{for single bonding as}

well as the sp hybrid orbital for triple bonding. According to Zhang (2012), in comparison with different bonding state will introduce into certain structural arrangement in which sp bonding will give rise to one dimensional chain structure, sp2

bonding will produce planar structure and sp3_{bonding to tetrahedral structure.}

LITERATURE REVIEW

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Figure 2.1: Summary of different carbon form Carbon

Diamond

(sp3₎ Graphite _(sp2₎ Turbostratic Rhombohedral Hexagonal

Carbynes

(sp) Amorphous (sp3_/sp2₎ _{(quasi sp}Nanoform 2₎ Fullerenes Nanotubes

Comparative Study Of Electronic Properties In Pristine And Si-Doped Single-Walled Carbon Nanotube As Gas Sensor: A First Principle Study.

UNIVERSITI TEKNIKAL MALAYSIA MELAKA