Surface-State Assisted Carrier Recombination and Optical Nonlinearities in Bulk to 2D Non-Layered PtS

Cooperite, or platinum sulfide (PtS), is a rare mineral that generally exists as microscale, irregularly-shaped crystallites. The presence of impurities, in both naturally-occurring and synthesized samples, has hindered the study of its optical properties in the past. In this work, we prepare large-scale, uniform PtS films in bulk to 2D form through the thermally assisted conversion (TAC) method. An abnormal trend is observed in linear spectral studies whereby the optical bandgap narrows as the film thickness decreases. A model based on the continuous distribution of carriers in real space, which can be regarded as a quantum well normal to the plane is used to describe the thickness-dependent carrier recombination phenomenon. In the nonlinear optical measurements, PtS exhibits ultrafast saturable absorption and self-defocusing properties in the visible region, which are dominated by the resonant electronic nonlinearities.