  The electron-hole separation efficiency is a key factor that determines the
performance of two-dimensional (2D) transition metal dichalcogenides (TMDs) and
devices. Therefore, searching for novel 2D TMD materials with long timescale of
carrier lifetime becomes one of the most important topics. Here, based on the
timedomain density functional theory (TD-DFT), we propose a brand new TMD
material, namely janus-MoSTe, which exhibits a strong build-in electric field.
Our results show that in janus-MoSTe monolayer, the exciton consisting of
electron and hole has a relatively wide spatial extension and low binding
energy. In addition, a slow electron-hole recombination process is observed,
with timescale on the order of 1.31 ns, which is even comparable with those of
van der Waals (vdW) heterostructures. Further analysis reveals that the
extremely long timescale for electron-hole recombination could be ascribed to
the strong Coulomb screening effect as well as the small overlap of
wavefunctions between electrons and holes. Our findings establish the build-in
electric field as an effective factor to control the electron-hole
recombination dynamics in TMD monolayers and facilitate their future
applications in light detecting and harvesting.
