Carrier Dynamics in the Space Charge Layer of MoS2 Flakes Studied by Time-Resolved ?-Surface Photovoltage

Carrier dynamics in the space charge layer (SCL) is an important topic in semiconductor science, especially for the promising two-dimensional semiconductor materials used in next-generation electronic and optoelectronic devices. Investigating these materials on both spatial and ultrafast time scales is crucial to promote their widespread application. Here, we systematically study the carrier dynamics of MoS2 flakes using femtosecond time-resolved spectroscopic photoemission electron microscopy. By acquiring a series of microarea photoelectron spectra at various time delays, we obtain the time-resolved microarea surface photovoltage (mu-SPV), which reflects the carrier dynamics in the SCL. Our findings show that the dynamics strongly depends on temperature and carrier density. At low temperature, the decay of SPV exhibits a significantly slow rate, that is limited by the low thermionic emission. A high pump fluence causes a large SPV maximum and a plateau that lasts only for tens of ps and is followed by a fast decay, while a low pump fluence results in a small SPV maximum with a plateau or a slow increase for longer than ns. These results arise from different competing mechanisms among carrier trapping, detrapping, diffusion, and electron-hole recombination. Our study demonstrates that time-resolved mu-SPV is an effective method for investigating charge carrier dynamics in layered semiconductor materials, offering valuable insight into the photophysical processes within the SCL.