Crossover between Photochemical and Photothermal Oxidations of Atomically Thin CrPS₄

Two-dimensional (2D) semiconductors represented by transition metal dichalcogenides have optical bandgaps that can be tuned via thickness, strain, dielectric screening and intercalation, and thus stand promising candidates for optoelectronic devices. Despite the potential, however, many including phosphorene and CrI₃ lack photostability even in the ambient air. Here, we studied photoreaction mechanisms of 2D chromium thiophosphate (CrPS₄), an antiferromagnetic semiconductor, using Raman spectroscopy and atomic force microscopy (AFM). Few-layer CrPS₄ underwent photodegradation which accelerated with increasing partial pressure of O₂. Topographic analysis by AFM showed that 2D CrPS₄ is photo-oxidized at a laser power density two orders of magnitude lower than that required for MoS₂ obeying a photothermal mechanism. Based on various control experiments over photon energy, power density and humidity, we propose that the low-power photoreaction of CrPS₄ is a one-photon photochemical oxidation mediated by CrPS₄-sensitized generation of singlet O₂ and is switched over to a photothermal oxidation in a high-power regime. Additionally, the efficacy of thin Al₂O₃ films will be discussed as protecting layers of 2D CrPS₄.