Spatially heterogeneous ultrafast interfacial carrier dynamics of 2D-MoS₂ flakes

Understanding the ultrafast carrier dynamics of transition metal dichalcogenides is critical for future applications in high-speed electronic and optoelectronic devices. In this work, we used a combination of femtosecond (fs) time-resolved micro-area photoelectron spectroscopy and photoemission electron microscopy (PEEM) to obtain the space-resolved surface photovoltage (SPV) and photoelectron intensity dynamics of two-dimensional (2D) molybdenum disulfide (MoS₂) flakes, both of which exhibited high spatial heterogeneity and defect effects. Additionally, surface S vacancy defects were characterized by spatially resolved X-ray photoelectron spectroscopy. The SPV relaxation dynamics indicated that the charge carrier lifetime in the space-charge layer (SCL) ranged from several to tens of nanoseconds and was dominated by the thermionic emission process. The defects, which acted as electron-hole recombination centers, greatly shortened the charge carrier lifetime by almost one order of magnitude. Furthermore, three relaxation processes were observed in the photoelectron intensity dynamics, namely, two fast processes with rates of 3–6 ps (ps) and 60–100 ps, which were largely affected by the defect density, and one slow relaxation process with a rate of 2–5 ns (ns). However, these were not related to the defect density and were attributed to the transportation of electrons towards the bulk. Therefore, our results provide a deeper understanding of the interfacial carrier dynamics of 2D-MoS₂ materials and the effects of defects on charge carrier lifetime in the SCL.