Temperature dependence of van Hove singularity excitons in atom thick semiconductors*

Atomically thin semiconducting transition metal dichalcogenides (TMDs) are emerging as a new platform for exploring two-dimensional exciton physics. These excitons play a crucial role in determining the light-matter interactions of a semiconducting material, such as absorption, photoluminescence, and electroluminescence. In addition to direct-band gap transition excitons (known as A- and B- excitons), there also exists a pair of van Hove singularity (vHS) assisted excitonic transitions, known as the C- and D- excitons. Currently it is not known how these vHS excitons modify at different temperature. To bridge the knowledge gap, we probed the temperature dependence of both direct-band gap and vHS excitons via photocurrent spectroscopy. We observed that all the excitonic peaks shift to lower energy as the temperature decreases. Moreover, we observed that the rate of shift for vHS excitons differs significantly from the rate of shift for the direct-band gap excitons. This study advances our understanding of the intrinsic properties of excitons in 2D TMDs.