Raman Spectroscopy on Few layer graphene

Abstract

Graphene is a two-dimensional allotrope of carbon exhibiting planer honeycomb lattice structure.1 A carbon atom in graphene makes three σ-bonds with the neighbouring atoms in the plane and one π-bond perpendicular to the plane of these carbon atoms. The delocalized electron due to side overlapping of π-bond in graphene gives high intrinsic carrier mobility. The maximum carrier mobility of single layer suspended graphene is 200,000 cm2V-1s-1,2 whereas the maximum carrier mobility of silicon is 1,400 cm2V -1s-1. 3 Hence, graphene is an excellent substitute for high-frequency FET switch over silicon FET switch. The three σ-bonds (tight bonds) make graphene the strongest material among all other materials. The free-standing singlelayer graphene shows Young's modulus as high as 1TPa, while that of carbon nanotube is reported to be around 270-950 GPa. 4 This implies that graphene can be a promising material for nanoelectromechanical systems (NEMS). The distinct electronic and mechanical properties of graphene have attracted a lot of attention for developing alternative graphene-based alternative, which helps address the technical limits of current technologies.

Few layers graphene (FLG) have been studied both theoretical and experimentally.6,7 These studies show that the number of layers and staking orders of FLG has a strong influence on its characteristic properties. Hence, identifying the structural differences in FLG is a crucial requirement in developing FLG-based devices. The characteristic properties of monolayer and bilayer graphene have been extensively studied. However, there are very few reports on three or more-layers of graphene. Here, we have focused on identifying and fabricating few layers graphene (FLG) devices and studying their electrical and optical characteristics. Different techniques have been reported in the literature for the synthesis of graphene. Mechanical exfoliation is the method that we have used here. The structural and electronic characteristics of graphene are studied using Raman spectroscopy. The thesis is arranged as follows. Chapter 2 explains the fabrication of graphene devices. Mechanical exfoliation technique used to obtain few layers of graphene flakes and optical microscopy used to locate graphene flakes is briefly explained. I have described the fabrication of graphene devices for electrical measurements. The electron-beam lithography method is used to fabricate these devices. Chapter 3 discusses Raman spectroscopy to identify the number of layers of graphene flakes. In this section, I have compared peaks up to five layers of graphene flakes. Also, statistical analysis is performed to obtain the number of layers in the graphene flakes. Chapter 4 discusses electrical measurements on few layers graphene FET device. Chapter 5 describes the exfoliation of graphene flakes on flexible substrates and Raman spectroscopy measurements. Chapter 6 provides the summary of the results