Electronic structure evolution and exciton energy shifting dynamics in WSe2: from monolayer to bulk

Exciton related processes in two-dimensional (2D) transition metal dichalcogenides (TMDCs) play important roles in their optoelectronic applications. In this work, through broadband transient absorption spectroscopy, the electronic band structure evolution, exciton energy shifting dynamics and power-dependence spectral characteristics of WSe2 layers, including monolayer, bilayer, tri-layer and bulk WSe2 under 400 nm and 800 nm excitations are investigated. Particularly, under 400 nm excitation, due to the hot-exciton effect, the A-exciton energy shifting dynamics in WSe2 layers have been analysized in detail, where thicker WSe2 samples possess slower hot-exciton cooling lifetimes, and the exciton recombination approaches are affected by the band structure and interlayer interactions, in comparison with that under 800 nm excitation. The power-dependence spectral evolution in WSe2 layers suggests that the charged states like trions could be facilitated in tri-layer WSe2 (or thicker samples) at the same experimantal conditions. These findings in WSe2 layers could provide a deep insight into the hot-exciton related processes in 2D TMDCs from transient experiments ponit of view.