Direct probing of ultrafast interfacial charge carrier dynamics and photo-induced degradation at WS₂/Au contacts by time domain THz emission spectroscopy

Two-dimensional transition metal dichalcogenides (TMDCs) combine sizeable band gaps, strong spin-orbit coupling, and large exciton binding energies, making them prime candidates for optoelectronic applications. Yet, forming reliable electrical contacts remains challenging due to Schottky barriers, Fermi-level pinning, and environmental degradation. To elucidate interfacial charge injection and stability, we directly measure charge transfer times and efficiencies at monolayer (ML) WS₂-Au junctions using time-domain THz emission spectroscopy (TES) under above- and below- band gap excitation. For sub-bandgap excitation, interfacial currents stem from hot-electron injection from Au to WS₂, showing faster and more efficient transfer for samples with uniform, strongly coupled contacts. Under above-bandgap excitation, hole injection from WS₂ to Au dominates, yielding higher net photocurrents in discontinuous, rugged interfaces. This counterintuitive observation is attributed to an asymmetric charge flow across the tunneling barrier, whereas uniform contacts lead to a more balanced electron and hole transfer and a canceled net current. Finally, TES reveals that WS₂ on an insulating SiO₂ substrate rapidly degrades under ambient illumination due to confined exciton recombination, while WS₂ on Au remains stable owing to efficient carrier extraction that suppresses photo-induced reactions. This highlights the critical role of metal-TMDC interface morphology in governing charge transport and material durability.