Chapter 1 Introduction

Transcription

1 Chapter 1 Introduction In our planet carbon forms the basis of all organic molecules which makes it the most important element of life. It is present in over 95% of the known chemical compounds overall (Falcao and Wudl 2007). Carbon can be found in many forms (about 10 million compounds) and is a very abundant element in our lives. It s very complex behaviour has been extensively studied throughout the years. Carbon has been identified as a key component of natural compounds (such as DNA) and processes (such as photosynthesis). Carbon also takes part in reactions in the universe (collision of alpha particles on sun). Carbon is regarded as one of the most versatile elements in the periodic table forming a wide variety of structures. Carbon is unique in possessing allotropes of each possible dimensionality, 3D diamond, 3D graphite, 2D graphene, 1D nanotube and 0D fullerenes. The carbon allotropes i.e. those materials which consist entirely of carbon atoms, can be divided into three classes according to the type of bonds linking the atoms together. This type of bonding depends upon hybridization of carbon atomic orbitals which is usually sp 2 or sp 3. When the carbon atoms have sp 3 hybridization diamond structure is formed. In case of sp 2 hybridization one of graphitic material is formed (i.e. fullerenes, nanotube, graphene or graphite). Diamond is the strongest of all bulk materials and has the highest thermal conductivity ever measured. Graphite, a three dimensional (3D) allotrope of carbon, became widely known after the invention of pencil in 1564 (Petroski 1989). Its usefulness as an instrument for writing comes from the fact that graphite is made out of stacks of graphene layers. Fullerenes (Andreoni 2000) are molecules where carbon atoms are arranged spherically. Fullerenes can be obtained from graphene with the introduction of pentagons and hence fullerenes can be thought as wrapped-up graphene. Carbon nanotubes (Saito Dresselhaus and Dresselhaus 1998, Charlier et al. 2007) are obtained by rolling graphene along a given direction and reconnecting the carbon bonds. The last one, graphene, forms the basis of all these other graphitic allotropes (see figure 1.1).

2 Figure 1.1 Graphene from graphite. Graphite is a basic material found in nature. When taken apart graphite sheets become graphene. A rolled up layer of graphene forms a carbon nanotube, folded up it becomes a small football, fullerene (The Nobel Prize In Physics 2010, The Royal Swedish Academy Of Sciences, Graphene Graphene is a single layer of graphite. It has been used and studied as building block for carbon based materials for decades. This 2D configuration consists of a real atomic monolayer of carbon atoms organized into a honey comb lattice. More than seventy five years ego, Landau (Landau 1937) and Peierls (Peierls 1935) argued that 2D crystals could not exist. Mermin further developed this theory which was supported by some experimental results (Venables et al. 1984). These experimental observations of a decreasing melting temperature for thin films with a decreasing thickness seemed to provide strong support for this theory. Indeed, the melting temperature of thin films rapidly decreases with decreasing thickness, and the films become unstable (segregate into islands or decompose) at a thickness of, typically, dozens of atomic layers. For this reason, atomic monolayers have so far been

3 known only as an integral part of larger 3D structures, usually grown epitaxially on top of monocrystals with matching crystal lattices. Without such a 3D base, 2D materials were presumed not to exist. But it was a surprise in 2004 when several 2D materials were observed by Geim et al.at in Manchester University (Novosolov et al. 2004). Among these 2D materials were graphene, the structurally similar boron nitride, and others, but only graphene has been intensively studied till now. There has been a long debate on the cause of the apparent stability of graphene monolayers but there seems to have emerged a consensus that the 2D crystals become intrinsically stable by a soft crumpling in the third dimension (Meyer et al., Nelson et al. 2004). Figure 1.2 Graphene. The almost perfect web is only one atom thick. It consists of carbon atoms joined together in a hexagonal pattern similar to chicken wire. (Meyer 2009). This 3D warping (observed on a lateral scale of 10 nm) (Meyer et al. 2009) leads to a gain in elastic energy but suppresses thermal vibrations (anomalously large in 2D), which above a certain temperature can minimize the total free energy (Nelson et al. 2004). Today it is the thinnest and most attractive nanomaterial in the universe and the strongest ever measured (Geim 2009). It has excellent electrical, thermal and optical properties (Abergel et al. 2010).

4 1.2 Functionalized Graphene The chemical modification of graphene can herald another avenue for making new materials with novel properties. Sofo et al. (Sofo et al. 2007) predicted through firstprinciple total energy calculations that graphene could be fully hydrogenated to form a stable two dimensional hydrocarbon. They called the new hydrocarbon as graphane and proposed it to have potential applications in hydrogen storage and nanoelectronics. This theoretical prediction has been very quickly translated into reality through the co-operative research efforts of Geim s (Elias et al. 2009) group at Manchester University with scientists at Cambridge University, UK and Radboud University, The Netherlands. Geim s group has experimentally demonstrated for the first time that a single layer of graphene can be hydrogenated to graphane. Figure 1.3 Graphene hydrogenation (A) A graphene layer, where delocalized electrons are free to move between carbon atoms, is exposed to a beam of hydrogen atoms. (B) In non-conductive graphane, the hydrogen atoms bond their electrons with electrons of carbon atoms and pull the atoms out of the plane. (Savchenko 2009). The experimental work also showed that the process of hydrogenation is reversible, making graphane a potential candidate for hydrogen storage systems. Since upon hydrogenation, graphene, a semi-metal turns into an insulator, it is a good candidate for investigating the nature of metal insulator transition (MIT). Graphane has the same honeycomb structure as graphene, except that it is "spray-painted" with hydrogen atoms that attach themselves to carbon (see figure 1.3). Graphane is

5 thermodynamically stable as comparable to hydrocarbons, more stable than metal hydrides and more stable than graphene by ~ 0.15 ev (Elias et al. 2009). It has large hydrogen storage capacity 7.7 wt% which exceeds the Department of Energy (DOE) 2010 target of 6%, thus it is a promising candidate for hydrogen storage. 1.3 Motivation Due to low mobility of charge carriers the silicon technology is not suitable for nano scale devices. Alternatively to replace silicon by elements of group III and other group V elements the fabrication of such A III B V based devices is very expensive additionally silicon and A III B V technologies have poor compatibility in integrated circuits (Berashevich and Chakraborty 2010). However, after the discovery of graphene, carbon electronics with a possibility of ballistic transport has emerged as a more promising technology for fabrication of nano scale devices. In the last few years, graphene has been considered as one of the most sensational materials for its many exotic properties and it is an excellent candidate for future electronics due to the extraordinarily high mobility of its charge carriers. The absence of a band gap hinders its applications in microelectronic devices, but the suitable band gap can be introduced by chemical functionalization of graphene which results into the tuning of electronic and magnetic properties for vast applications in microelectronics. The few other aspects like, (a) potential use of graphene for hydrogen storage (Sofo et. al 2007, Boukhvalov et. al 2008, Deng et.al 2004, Rojas and Leiva 2007, Ataca et. al 2008, Lin et. al 2008 and Chan et. al 2008) (b) to make graphene magnetic for potential use in spintronics (Boukhvalov et. al 2008, Boukhvalov et. al 2008a, Roman et. al 2006, Roman et. al 2007, Yazyey and Helm 2007 and Wu, Liu and Jiang 2008) (c) low reflectance, high carrier mobility & high absorption to fabricate optoelectronic devices (Berashevich and Chakraborty 2010) and (d) to create electrochemical & biosensors (Liang and Zhi 2009) are also motivations to study chemical functionalization of graphene. The hydrocarbon compound Graphane, is an emerging material for applications in electronic and photonics. Graphane has opened up increasingly fertile possibilities in hydrogen storage and two dimensional electronics. 1.4 Aim & Objectives Graphene is a promising carbon-based material which was mechanically exfoliated from graphite in 2004, the 440 years after the invention of pencil. It represents a new

6 class of materials which are only one atom thick and it has gained attention due to unique electronic and magnetic properties. It offers new inroads into low dimensional properties and continues to provide fertile ground for many applications. Recent computational and experimental studies have shown that hydrogenation of pristine graphene results in a metal-semiconductor transition. In the present work our main aim is the study of functionalized graphene modeled in chair conformation using the method of numeric localized atomic orbitals, pseudopotentials and DFT as implemented in SIESTA code. The objectives of this study are the following: 1. Calculation of Electron Density of States for Nanostructured Functionalized Graphene. 2. Calculation of Band Structure for Nanostructured Functionalized Graphene. 3. Calculation of Magnetic Properties for Nanostructured Functionalized Graphene. 4. Calculation of Optical Properties for Nanostructured Functionalized Graphene. 5. Calculation of Transport Properties for Nanostructured Functionalized Graphene. 1.5 Outline of thesis: This thesis has been written as per format provided by Shoolini University and is divided into five chapters. In Chapter 1 we present the introduction to graphene and functionalized graphene, motivation, aim and objectives of the thesis. The review of literature on graphene and functionalized graphene, including experimental and theoretical work, is presented in Chapter 2. Chapter 3 covers the methodology used in this work. In Chapter 4 we present our results with discussion. Chapter 5 contains conclusions and future scopes. After chapter 5 we have given the list of references used in this work. In addition the thesis contains three Appendices. Appendix 1 contains the input files for the generation and testing of pseudopotentials of carbon and hydrogen. Appendix 2 contains some of the input files used during this study. Appendix 3 contains the optimized nano structures which have been investigated during this study. At the end we have given the list of publications and detail of seminars/conferences/workshops attended.