Optical Based Techniques for 2D Layered Materials

Two-dimensional materials are the thinnest unsupported crystalline solids that do not exhibit surface dangling bonds. The unique structure of these materials including graphene and its successors leads to novel optical, electrical properties in comparison to their bulk counterparts. The changes in the structural and physical properties thus highly influence the performance of the resulting devices. Particularly, they are characterized by intralayer covalent bonding and interlayer van der Waals bonding with superior interlayer (compared to intralayer) transport of fundamental excitations (charge, heat, spin, and light). These atomic sheets afford the ultimate thickness scalability for semiconductor devices while simultaneously providing an unmatched combination of device physics and mechanics [Akinwande et al., “Two-dimensional flexible nanoelectronics,” Nat. Commun. 5, 5678 (2014)]. Hence, these 2D layers could act as building blocks for future optoelectronic and photonic devices. Even though their unique structure confers various optoelectronic capabilities, the same structure impedes their characterizations as they are transparent and have a nanometre-scale thickness. The future application of these nanosheets will be dictated by our precise understanding of their optoelectronic properties through standardized characterization techniques. Among all the available characterization techniques, optical investigations are a powerful tool as the interaction between incident light beam and the material can provide us with information about the optoelectronic properties of the materials. The simplicity and the non-destructive nature of these techniques make them an important characterization tool. This chapter deals with the systematic study of various optical methods which are useful in investigating materials of the 2D family. The initial stage in characterizing 2D material is to locate them and count number of layers in the nanosheets. The first section describes the use of optical microscopy as an imaging technique and its usefulness in determining the thickness/layer number in a 2D layer stack. Methods to investigate nonlinear optical properties of 2D materials is discussed in the next section. Photoluminescence emission studies combined with density functional theory can be utilized to characterize the band structure of the 2D materials. Thus, the third section of this chapter describes the use of optical absorption and photoluminescence technique to investigate their electronic properties. Systematic discussion is put forward for the methods to ascertain particle size and surface charge of the materials in the last section.