Platinum diselenide
(PtSe2) is an emerging class of two-dimensional (2D) transition-metal
dichalcogenide (TMD) crystals recently gaining substantial interest,
owing to its extraordinary properties absent in conventional 2D TMD
layers. Most interestingly, it exhibits a thickness-dependent semiconducting-to-metallic
transition, i.e., thick 2D PtSe2 layers, which are intrinsically
metallic, become semiconducting with their thickness reduced below
a certain point. Realizing both semiconducting and metallic phases
within identical 2D PtSe2 layers in a spatially well-controlled
manner offers unprecedented opportunities toward atomically thin tailored
electronic junctions, unattainable with conventional materials. In
this study, beyond this thickness-dependent intrinsic semiconducting-to-metallic
transition of 2D PtSe2 layers, we demonstrate that controlled
plasma irradiation can “externally” achieve such tunable
carrier transports. We grew wafer-scale very thin (a few nm) 2D PtSe2 layers by a chemical vapor deposition (CVD) method and confirmed
their intrinsic semiconducting properties. We then irradiated the
material with argon (Ar) plasma, which was intended to make it more
semiconducting by thickness reduction. Surprisingly, we discovered
a reversed transition of semiconducting to metallic, which is opposite
to the prediction concerning their intrinsic thickness-dependent carrier
transports. Through extensive structural and chemical characterization,
we identified that the plasma irradiation introduces a large concentration
of near-atomic defects and selenium (Se) vacancies in initially stoichiometric
2D PtSe2 layers. Furthermore, we performed density functional
theory (DFT) calculations and clarified that the band-gap energy of
such defective 2D PtSe2 layers gradually decreases with
increasing defect concentration and dimensions, accompanying a large
number of midgap energy states. This corroborative experimental and
theoretical study decisively verifies the fundamental mechanism for
this externally controlled semiconducting-to-metallic transition in
large-area CVD-grown 2D PtSe2 layers, greatly broadening
their versatility for futuristic electronics.