  Defects in conventional semiconductors substantially lower the
photoluminescence (PL) quantum yield (QY), a key metric of optoelectronic
performance that directly dictates the maximum device efficiency.
Two-dimensional (2D) transition metal dichalcogenides (TMDCs), such as
monolayer MoS2, often exhibit low PL QY for as-processed samples, which has
typically been attributed to a large native defect density. We show that the PL
QY of as-processed MoS2 and WS2 monolayers reaches near-unity when they are
made intrinsic by electrostatic doping, without any chemical passivation.
Surprisingly, neutral exciton recombination is entirely radiative even in the
presence of a high native defect density. This finding enables TMDC monolayers
for optoelectronic device applications as the stringent requirement of low
defect density is eased.
