Crossover between Photochemical and Photothermal Oxidations of Atomically Thin Magnetic Semiconductor CrPS₄

Many two-dimensional (2D) semiconductors represented by transition metal dichalcogenides have tunable optical bandgaps in the visible or near IR-range standing as a promising candidate for optoelectronic devices. Despite this potential, however, their photoreactions are not well understood or controversial in the mechanistic details. In this work, we report a unique thickness-dependent photoreaction sensitivity and a switchover between two competing reaction mechanisms in atomically thin chromium thiophosphate (CrPS₄), a two-dimensional antiferromagnetic semiconductor. CrPS₄ showed a threshold power density 2 orders of magnitude smaller than that for MoS₂ obeying a photothermal reaction route. In addition, reaction cross section quantified with Raman spectroscopy revealed distinctive power dependences in the low and high power regimes. On the basis of optical in situ thermometric measurements and control experiments against O₂, water, and photon energy, we proposed a photochemical oxidation mechanism involving singlet O₂ in the low power regime with a photothermal route for the other. We also demonstrated a highly effective encapsulation with Al₂O₃ as a protection against the destructive photoinduced and ambient oxidations.