  The phase transitions among polymorphic two-dimensional (2D) transition metal
dichalcogenides (TMDs) have attracted increasing attention for their potential
in enabling distinct functionalities in the same material for making integrated
devices. Electron-injection to TMDs has been proved to be a feasible way to
drive structural phase transition from the semiconducting H-phase to the
semimetal dT-phase. In this contribution, based on density-functional theory
(DFT) calculations, firstly we demonstrate that hole-injection drives the
transition of the H-phase more efficiently to the metallic T-phase than to the
semimetallic dT-phase for group VI-B TMDs (MoS2, WS2, and MoSe2, etc.). The
origin can be attributed to the smaller work function of the T-phase than that
of the dT-phase. Our work function analysis can distinguish the T and dT phases
quantitatively while it is challenging for the commonly used crystal field
splitting analysis. In addition, our analysis provides a unified understanding
for both hole- and electron-injection induced phase transitions for 2D
materials beyond TMDs, such as the newly synthesized MoSi2N4 family. Moreover,
the hole-driven T-phase transition mechanism can explain the recent experiment
of WS2 phase transition by hole-doping with yttrium (Y) atoms. Using 1/3
Y-doped WS2 and MoSe2 as examples, we show that the Mo and W valency increases
to 5+. These above findings open up an avenue to obtain the metallic T-phase,
which expands the possible stable phases of 2D materials.
