8. Optical and Electrical Properties of Transition Metal Dichalcogenides (Monolayer and Bulk)

In this chapter, we discuss the electronic band structure, electrical, and optical properties of transition metal dichalcogenides. The different crystallographic structures for transition metal dichalcogenides are presented along with a discussion of the chemical bonding. Many of the transition metal dichalcogenides consist of van der Waals bonded monolayers where the monolayers consist of trilayers with a transition metal atom layer between a top and bottom chalcogenide layer. Often these monolayers have a trigonal prismatic arrangement of chalcogenide atoms around the metal atoms. A tight binding model for three of the \(d\) orbitals of the transition metal atoms provides a useful description of the highest energy valence band and lowest energy conduction bands of trigonal prismatic monolayer transition metal dichalcogenide. The impact of spin orbit coupling on the band structure is shown. We discuss how the electronic band structure due to the honeycomb lattice of many transition metal dichalcogenides monolayers interacts with spin orbit coupling resulting in differences in optical transitions between the \(K\) and \(K^{\prime}\) locations in the Brillouin zone. We present photoluminescence spectra demonstrating these differences. We also show theoretical and experimental dielectric function data for a variety of monolayer, multilayer, and bulk transition metal dichalcogenides. We show how Raman spectroscopy is sensitive to the layer structure. We also discuss the observation of superconductivity of TMD materials. A summary of the point group and space group symmetry and Raman Tensors of transition metal dichalcogenides is provided.