Influence of the effective layer thickness on the ground-state and excitonic properties of transition-metal dichalcogenide systems

The effective dimensionality of a semiconductor strongly influences its excitonic properties. Due to their unit-cell thickness, monolayer materials are the almost perfect realization of a two-dimensional system. However, the unit-cell dimensions can easily exceed the nominal value of the exciton Bohr radius. Employing a fully microscopic theory based on the equation-of-motion approach, it is shown that the effective finite-layer thickness in transition metal dichalcogenides (TMDCs) is responsible for the observed nonhydrogenic exciton series, not only in monolayers, but also in few-layer and bulk systems. The theoretical results identify the resonances in bulk TMDCs as due to quasi-two-dimensional intra- and interlayer excitons.