Layered 2D semiconductors for ultrafast photonic applications

Abstract

As a class of semiconductors, transition metal dichalcogenides (TMDs) have the formula MX 2 , where M stands for a transition metal (i.e., Mo, W, Ti, Nb, etc.) and X stands for a chalcogen (i.e., S, Se or Te). TMDs show graphene-like layered structure. Strong covalent bonds in layers and weak van der Waals interaction between layers allow TMDs to form a robust 2D nanostructure. In a TMD monolayer, the single transition metal layer is sandwiched between the two chalcogen layers. Owing to the specific 2D confinement of electron motion and the absence of interlayer coupling perturbation, 2D layered TMDs show unique photonics-related physical properties, e.g., 1) Sizable and layer-dependent bandgap, typically in the 1–2 eV range; 2) Indirect-to-direct bandgap transition as the decreasing of the number of monolayer; 3) Fairly good photoluminescence and electroluminescence properties; 4) Remarkable excitonic effects, i.e., high binding energy, large oscillator strength and long lifetime.