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Research on carbon materials-based thin films with low light reflectance has received paramount importance in order to have high absorber coatings for stray light control applications. We report here a...

Coplanar heterojunctions composed of van der Waals layered materials with different structural polymorphs have drawn immense interest recently due to low contact resistance and high carrier injection rate owing to low Schottky barrier height. Present research has largely focused on efficient exfoliation of these layered materials and their restacking to achieve better performances. We present here a microwave assisted easy, fast and efficient route to induce high concentration of metallic 1T phase in the original 2H matrix of exfoliated MoS layers and thus facilitating the formation of a 1T-2H coplanar superlattice phase. High resolution transmission electron microscopy (HRTEM) investigations reveal formation of highly crystalline 1T-2H hybridized structure with sharp interface and disclose the evidence of surface ripplocations within the same exfoliated layer of MoS. In this work, the structural stability of 1T-2H superlattice phase during HRTEM measurements under an electron beam of energy 300 keV is reported. This structural stability could be either associated to the change in electronic configuration due to induction of the restacked hybridized phase with 1T- and 2H-regions or to the formation of the surface ripplocations. Surface ripplocations can act as an additional source of scattering centers to the electron beam and also it is possible that a pulse train of propagating ripplocations can sweep out the defects via interaction from specific areas of MoS sheets.

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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.