  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.
