  Defect induced trap states are essential in determining the performance of
semiconductor photodetectors. The de-trap time of carriers from a deep trap
could be prolonged by several orders of magnitude as compared to shallow trap,
resulting in additional decay/response time of the device. Here, we demonstrate
that the trap states in two-dimensional ReS2 could be efficiently modulated by
defect engineering through molecule decoration. The deep traps that greatly
prolong the response time could be mostly filled by Protoporphyrin (H2PP)
molecules. At the same time, carrier recombination and shallow traps would
in-turn play dominant roles in determining the decay time of the device, which
can be several orders of magnitude faster than the as-prepared device.
Moreover, the specific detectivity of the device is enhanced (as high as ~1.89
x 10^13 Jones) due to the significant reduction of dark current through charge
transfer between ReS2 and molecules. Defect engineering of trap states
therefore provides a solution to achieve photodetectors with both high
responsivity and fast response.
