Studies of the chemical vapor deposition method of generating graphene

Project Abstract

Graphene has garnered significant attention in the scientific community due to its exceptional properties and wide range of potential applications. One of the most common methods for producing graphene is chemical vapor deposition (CVD), which involves the catalytic decomposition of hydrocarbon precursors to form graphene on a substrate. This research project aims to investigate the process of graphene synthesis using the chemical vapor deposition method. The study involves a detailed analysis of the parameters that influence the quality and properties of the produced graphene, such as precursor gases, temperature, pressure, and catalyst materials. By systematically varying these parameters, the goal is to optimize the CVD process for efficient and high-quality graphene production. The experimental setup includes a high-temperature furnace for heating the substrate and precursor gases to the required temperature for graphene growth. Different catalyst materials, such as copper, nickel, and platinum, are tested to determine their effectiveness in promoting graphene growth. The use of different hydrocarbon precursors, such as methane and ethylene, allows for a comparison of their impact on the quality and structure of the synthesized graphene. Characterization techniques such as Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) are employed to analyze the structural and morphological properties of the synthesized graphene. Raman spectroscopy provides information on the number of graphene layers, defect density, and structural quality, while SEM and AFM images offer insights into the surface morphology and uniformity of the graphene films. The results of this study contribute to a better understanding of the CVD method for graphene synthesis and provide valuable insights into the optimization of process parameters for achieving high-quality graphene films. The findings can potentially lead to advancements in various graphene-based applications, including electronics, sensors, energy storage, and composites. Overall, this research project delves into the intricate process of chemical vapor deposition for graphene synthesis, focusing on the influence of key parameters on the quality and properties of the produced graphene. The systematic investigation and analysis of these parameters offer valuable insights for improving the efficiency and quality of graphene production through CVD, paving the way for further advancements in graphene-based technologies.

Project Overview

<p> 1.1 INTRODUCTION<br>1.1.1 Carbon Materials<br>Group IVA, consists of carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb).<br>Carbon is the chief constituent of coal, and it forms the backbone of the hydrocarbon<br>molecules in oil and natural gas. The element, carbon, is one of the most versatile<br>elements in the periodic table in terms of the number of compounds it may form.<br>Carbon also occur widely in carbonate rocks, such as limestone, dolomite and marble.<br>Basically, carbon has 3 allotropes i.e. diamond, carbon nanotubes and fullerene. Each<br>of these carbon allotropes has dierent features due to the bonding between carbon<br>atoms. Carbon has four valence electrons with an electronic conguration of 1s22s22p2.<br>It may form virtually an innite number of compounds. This is largely due to the types<br>of bonds it can form and the number of dierent elements it can join in bonding.[1]<br>Carbon Bonding<br>Bonding in any element will take place with only the valence shell electrons. Carbon<br>may form single, double and triple bonds. The valence shell electrons are found in<br>the incomplete, outermost shell. In the ground state (lowest energy state), two of the<br>electrons are in the 1s orbital (K shell), two are in the 2s orbital (L shell) while the<br>third pair is in the 2p orbital (L shell). The 1s electrons are considered to be the core<br>electrons and are not available for bonding. There are two unpaired electrons in the<br>2p orbitals, so if carbon were to hybridize from this ground state, it would be able to<br>1<br>Figure 1.1: Orbital diagram of carbon at ground state.<br>form at most two bonds. Recall that energy is released when bonds are formed, so<br>it would be to carbon’s benet to try to maximize the number of bonds it can form.<br>For this reason, carbon will form an excited state by promoting one of its 2s electrons<br>into its empty 2p orbital and hybridize from the excited state. By forming this excited<br>state, carbon will be able to form four bonds. Since both the 2s and the 2p orbital are<br>half-lled, the excited state is relatively stable.[1]<br>1.1.2 Forms of carbon<br>Carbon sits directly above silicon on the periodic table and therefore both have 4<br>valence electrons. However, unlike silicon, carbon’s 4 valence electrons have very similar<br>energies, so their wavefunctions mix easily facilitating hybridization. In carbon, these<br>valence electrons give rise to 2s, 2px, 2py, and 2pz orbitals while the 2 inner shell<br>electrons belong to a spherically symmetric 1s orbital that is tightly bound and has<br>an energy far from the Fermi energy of carbon. For this reason, only the electrons in<br>the 2s and 2p orbitals contribute to the solid-state properties of graphite. This unique<br>ability to hybridize sets carbon apart from other elements and allows carbon to form<br>0D, 1D, 2D, and 3D structures.[2]<br>Diamond<br>The three dimensional form of carbon is diamond. It is sp3 bonded forming 4 covalent<br>bonds with the neighboring carbon atoms into a face-centered cubic atomic structure.<br>Because the carbon-carbon covalent bond is one of the strongest in nature, diamond has<br>a remarkably high Youngs modulus and high thermal conductivity. Undoped diamond<br>2<br>has no free electrons and is a wide band gap ( 5:5eV) insulator. The exceptional<br>physical properties and clever advertising such as Diamonds are forever contribute<br>to its appeal as a sought after gem. When properly cut and polished, it is set to make<br>beautiful pieces of jewellery. One of the most famous of these is the Hope Diamond. For<br>many of the large, high quality crystals used to make jewelry, diamond must be mined.<br>The smaller defective crystals are used as reinforcement in tool bits which utilize its<br>superior hardness for cutting applications. [2] The high thermal conductivity of dia-<br>mond makes it a potentially useful material for microelectronics where heat dissipation<br>is currently a major problem. However, diamonds scarcity makes this unappealing.<br>To this end, scientists and engineers are trying to grow large diamond wafers. One<br>method to do so is chemical vapor deposition (CVD) where solid carbon is deposited<br>from carbon containing gases such as methane or ethylene. By controlling the growth<br>conditions, it is possible to produce defect free diamonds of limited size.[2]<br>Fullerenes and carbon nanotubes<br>More exotic forms of carbon are the low dimensional forms known as the fullerenes<br>which consist of the 0 dimensional C60 molecule and its 1 dimensional derivative, carbon<br>nanotubes. A single walled carbon nanotube is a single layer of graphite, referred to<br>as graphene, rolled into a cylindrical tube with a 1nm diameter Carbon nanotubes<br>can be metals or semiconductors and have mechanical properties similar to diamond.<br>They attracted a lot of attention from the research community and dominated the<br>scientic headlines during the 1990s and early 2000. This interest in nanotubes was<br>partly responsible for the resurgent interest in graphene as a potentially important and<br>interesting material for several applications.[2]<br>Graphene and Graphite<br>Graphene and Graphite are the two dimensional sp2 hybridized forms of carbon found<br>in pencil lead. Graphite is a layered material formed by stacks of graphene sheets sepa-<br>rated by 0:3 nm and held together by weak van der Waals forces. The weak interaction<br>between the sheets allows them to slide relatively easily across one another. This gives<br>pencils their writing ability and graphite its lubricating properties, however the nature<br>of this interaction between layers is not entirely understood. Another frictional eect<br>3<br>believed to be important is the registry of the lattice between the layers. A mismatch<br>in this registry is believed to give graphite the property of superlubricity where the<br>frictional force is reduced considerably. A single 2-D sheet of graphene is a hexagonal<br>structure with each atom forming 3 bonds with each of its nearest neighbors. These<br>are known as the bonds oriented towards these neighboring atoms and formed from 3<br>of the valence electrons. These covalent carbon-carbon bonds are nearly equivalent to<br>the bonds holding diamond together giving graphene similar mechanical and thermal<br>properties as diamond. The fourth valence electron does not participate in covalent<br>bonding. It is in the 2pz state oriented perpendicular to the sheet of graphite and<br>forms a conducting band. The remarkable electronic properties of carbon nanotubes<br>are a direct consequence of the peculiar band structure of graphene, a zero bandgap<br>semiconductor with 2 linearly dispersing bands that touch at the corners of the rst<br>Brillouin zone .[16] Bulk graphite has been studied for decades but until recently there<br>were no experiments on graphene. This was due to the diculty in separating and<br>isolating single layers of graphene for study.[2]<br>1.1.3 Mass production of graphene<br>The main problem with graphene is to nd a way to produce graphene in/on a large<br>scale and at a low cost, consequently the full potential of graphene for applications<br>will not be realized until their growth can be further optimized and controlled. Re-<br>producibility of the graphene production is also another problem studied by many<br>researchers. Among the dierent techniques that have been applied for the mass pro-<br>duction of graphene, chemical vapor deposition (CVD) appears to be the most promis-<br>ing method owing to its relatively low cost and potentially high yield production. The<br>CVD method seems to be the best because of the lower reaction temperature. Their<br>future use will also strongly depend on the development of simple, ecient and inex-<br>pensive technologies for large scale production.<br>1.1.4 Problem statement<br>There has been great progress in both the production and application of graphene since<br>it discovery in 2004. Till now, graphene has been commonly synthesized using four<br>4<br>dierent methods namely, arc discharge, epitaxial growth on silicon carbide, exfoliation<br>of graphite oxide and mechanical exfoliation. The chemical vapor deposition (CVD)<br>method has shown to be a promising method to synthesize graphene on a large scale.<br>However, the problems encountered in the CVD method are the many factors that<br>in uence the production of the dierent forms of graphene such as types of catalyst,<br>carbon source, ow rate of precursors and the operating temperature. Among these<br>parameters, the types of catalyst and carbon source are the most critical factors in u-<br>encing the types and structures of graphene produced. Hence, a detailed study on the<br>eect of the types of catalyst and carbon source on the formation of dierent types<br>and structure of graphene will be undertaken.<br>1.1.5 Scope of research<br>The scopes of this study are listed as below:<br>1. To prepare series of substrate supported catalysts.<br>2. To synthesize graphene from dierent precursors via chemical vapor deposition<br>(CVD).<br>3. To characterize the as-synthesized graphene using:<br>a Field Emission Scanning Electron Microscopy (FESEM),<br>b Atomic Force Microscopy (AFM),<br>c Raman spectroscopy,<br>d Transmission Electron Microscopy (TEM).<br>1.1.6 Research Objectives<br>The objectives of this research are:<br>1. To synthesize graphene using dierent types of transition metals as catalysts and<br>dierent carbon sources by the chemical vapor deposition (CVD) method.<br>2. To characterize the as-synthesized graphene samples.<br>5 <br></p>