A Brief Review of the Chemical Structure and Raman Spectrum of Mono- and Multilayer Molybdenum- and Tungsten-Based Transition Metal Dichalcogenides

The electrical characteristics of graphene have reinvigorated research into inorganic, two-dimensional materials for use in various fields such as optoelectronics, renewable energy, and energy storage. However, graphene has limited use in the aforementioned fields given that it lacks an inherent energy bandgap. To circumvent this one exception in an otherwise groundbreaking electronic material, researchers have increasingly focused attention on transition metal dichalcogenides (TMDCs). The TMDCs we shall discuss are layered materials that with recent developments can be synthesized as monolayers. These layered TMDCs consist of three different atomic layers that make up a single monolayer TMDC. As TMDCs move from bulk form to monolayer form, they also transition from an indirect bandgap to a direct bandgap. These monolayer TMDCs, among other electronically useful properties, exhibit a direct bandgap of 1.8 eV which, in turn, allows them to be used in various applications such as photodetectors, energy storage, and transistors, to name a few. Within these applications, the deposition or growth of monolayer or few-layer TMDCs continues to be a challenge. The more common deposition methods such as chemical vapor deposition (CVD) and electrophoretic deposition (EPD) have pros and cons, and each are better suited for different applications. Raman spectroscopy is a useful tool in determining whether a deposition has produced monolayer TMDCs and the quality of the deposition.