Resolving contradictory estimates of band gaps of bulk PdSe₂: A Wannier-localized optimally-tuned screened range-separated hybrid density functional theory study

Palladium diselenide (PdSe₂)—a layered van der Waals material—is attracting significant attention for optoelectronics due to the wide tunability of its band gap from the infrared through the visible range as a function of the number of layers. However, there continues to be disagreement over the precise nature and value of the optical band gap of bulk PdSe₂, owing to the rather small value of this gap that complicates experimental measurements and their interpretation. Here, we design and employ a Wannier-localized optimally tuned screened range-separated hybrid (WOT-SRSH) functional to investigate the electronic band structures and optical absorption spectra of bulk and monolayer PdSe₂. In particular, we account carefully for the finite exciton center-of-mass momentum within a time-dependent WOT-SRSH framework to calculate the indirect optical gap and absorption onset accurately. Our results agree well with the best available photoconductivity measurements, as well as with state-of-the-art many-body perturbation theory calculations, confirming that bulk PdSe₂ has an optical gap in the mid-infrared (upper bound of 0.44 eV). More generally, this work further bolsters the utility of the WOT-SRSH approach for predictive modeling of layered semiconductors.