Bright and dark excitons in 1L WSe₂: a time-dependent density-functional theory analysis

Intra-valley bright and intra- and inter-valley dark excitons play an important role in the ultrafast dynamics and photoluminescence (PL) properties of 2D transition metal dichalcogenides (TMDs). Their predictive modeling relies on a microscopic understanding of their properties using accurate ab initio tools. We present results of time-dependent density-functional theory study of the excitonic properties of single layer (1L) WSe₂ obtained with local and long-range exchange-correlation kernels. The nanoquanta kernel is found to provide the best agreement for exciton binding energies with those obtained from solutions of the Bethe-Salpeter equation and experimental data. From calculated transition contribution map we establish the weight of different direct electron-hole excitations (defined by the momentum of the excited electron) in each excitonic state. We also analyze possible hybridized excitonic states and estimate the lifetime of each exciton, including effects of brightening of dark excitons that significantly affect the PL spectrum. Obtained results may help to better understand physical properties of 1L WSe₂, one of the most actively studied TMDs.