Transition metal dichalcogenide (TMDC) monolayers, as two-dimensional (2D) direct bandgap semiconductors, hold promise for advanced optoelectronic and photocatalytic devices due to their exceptional properties. To realize these applications, it is often necessary to bring the TMDC monolayer into contact with a metal surface to facilitate charge carrier injection and extraction from TMDC monolayers. The effects of the mutual interaction on the optical and electronic properties of both the monolayer and the substrate are therefore of crucial importance for the performance of the devices. To understand how a TMDC (MoS₂) monolayer interacts with a metallic substrate (Au), I employed the azimuth- and polarization-dependent sum frequency generation (SFG) spectroscopy to explore the interaction from the perspectives of symmetry, spectrum, and dynamics.

In contrast to the well-known sixfold symmetric pattern of MoS₂ on dielectric substrates in all polarization combinations, I found both the symmetric pattern and relative intensities of the azimuth-dependent SFG response of MoS₂/Au depend strongly on polarization. A full polarization analysis revealed a lowering of symmetry from the D_3h to the C_3v point groups after changing substrate from SiO₂ to Au for the MoS₂ monolayer. Analysis of the components and magnitudes of the second-order nonlinear susceptibility reveals a strong interaction between monolayer MoS₂ and Au substrate. The measurement of the photon energy dependent optical symmetry on MoS₂/Au confirmed a mode-specific electronic effect.

Through precise polarization-controlled, I isolated the optical responses of a MoS₂ monolayer from a MoS₂/Au junction. The resulting SFG spectra exhibit a linear lineshape, devoid of A and B exciton features, attributed to the strong dielectric screening and substrate induced doping. The linear lineshape is consistent with the expected constant density of states at the band edge of the 2D semiconductor, as corroborated by our theoretical calculations. Extrapolation yields the onset of a direct quasiparticle bandgap of about 1.65±0.20 eV, indicating a strong bandgap renormalization.

In the ultrafast dynamics domain, sub-20 fs injection of hot electrons across the MoS₂/Au interface and their subsequent relaxation at the conduction band minimum (CBM) of MoS₂ were observed under a pump-probe scheme. The relaxation processes driven by an internal electric field show distinct timescales, including a fast return around 2 ps and a slower, trap-state-mediated return around 60 ps. Additionally, the presence of MoS₂-modified Au was confirmed by distinct dynamics compared to pure Au.

These insights into optical symmetry, spectral characteristic, and dynamic behavior provide important experimental evidence for our understanding of 2D semiconductor-metal interfaces and assist the development of TMDC/metal based optoelectronic devices and photocatalysts.