Direct probing of ultrafast hot-carrier injection and photo-induced degradation at Au/WS₂ interfaces using time-domain THz emission spectroscopy

We investigate interfacial charge carrier dynamics and photo-induced degradation in monolayer WS2 on gold (Au) and fused silica (SiO2) substrates using time-domain THz emission spectroscopy, with a focus on the role of interface morphology. For excitation with photon energies above the band gap of WS2, we observe a stronger net transient photocurrent — dominated by hole transfer from WS2 to Au — in samples with a rugged, discontinuous van der Waals (vdW) interface. This counterintuitive result arises from asymmetric charge flow across the tunneling barrier, whereas smoother, uniform interfaces facilitate balanced electron–hole transfer, which cancels the net current. Inefficient charge separation in WS2 on insulating SiO2 is further linked to rapid photo-induced degradation under ambient conditions, attributed to exciton recombination at defect sites, which in turn can initiate surface chemical reactions. This study highlights the critical importance of metal–semiconductor interface morphology in governing both the dynamics of photo-generated charge carriers and sample stability of 2D material-based devices.