Donor-acceptor-pair transitions in suspended monolayer WS₂

In conventional semiconductors, the donor-acceptor-pair (DAP) transitions could dominate the photoluminescence, when the sample is cooled to low temperature and the excitation is weak. Here we report observation and characterization of DAP transitions in a suspended monolayer WS₂ using steady-state and time-resolved photoluminescence spectroscopy. Without substrate induced doping in suspended monolayer WS₂, we observed an ultra-broad emission centered at 1.83 eV and with bandwidth of more than 300 meV. The bandwidth is much larger than generally observed bound excitons. These sub-band states show a sub-linear power-law dependence with an index of 0.3 at 12 K. In the DAP transitions, the recombination centers are spatially separated, and their emission energy depends on their relative distances. We observed that the board emission spectra shift to higher energy as the excitation power increased. Time-resolved photoluminescence spectra show the broad emission shifts from higher energy to lower one along the intensity decay. This work provides further deep understanding of trap states and decay channels in transition metal dichalcogenides.