Studies of the chemical vapor deposition method of generating graphene

Thesis 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>