Interaction between a gold substrate and monolayer MoS2: An azimuthal-dependent sum frequency generation study

Most of the monolayer transition metal dichalcogenides (TMDCs) are chemically stable and have a combination of electronic, linear, and nonlinear optical (NLO) characteristics not generally found in bulk solids. This combination of traits has driven intense interest in their application in a variety of optoelectronic and (photo)electrocatalytic contexts. To realize these applications, it is often necessary to bring the TMDC monolayer into contact with a metal surface. However, while the interaction between TMDCs and dielectric substrates has been intensely studied, much less has been reported on the interaction between TMDCs and metals. Here we use azimuthal-dependent sum frequency generation (SFG) spectroscopy to study the interaction of monolayer ${\mathrm{MoS}}_{2}$ and gold by comparing the second-order NLO response of ${\mathrm{MoS}}_{2}$ on $\mathrm{Si}/{\mathrm{SiO}}_{2}$ as a function of azimuthal angle. In contrast to the well-known sixfold symmetric pattern of ${\mathrm{MoS}}_{2}$ on $\mathrm{Si}/{\mathrm{SiO}}_{2}$ in all polarization combinations, both the symmetric pattern and relative intensities of the azimuthal-dependent SFG response of ${\mathrm{MoS}}_{2}$ on Au depend strongly on polarization. Analysis of the components and magnitudes of the second-order nonlinear susceptibility (${\ensuremath{\chi}}^{(2)}$) of the ${\mathrm{MoS}}_{2}$/${\mathrm{SiO}}_{2}$, ${\mathrm{MoS}}_{2}$/Au, and gold substrate reveals a strong interaction between monolayer ${\mathrm{MoS}}_{2}$ and gold substrate broadly consistent with prior theoretical studies. Our measurement of the photon energy dependent nonlinear optical signal symmetry on ${\mathrm{MoS}}_{2}/\mathrm{Au}$ is strong evidence that we observe a mode-specific electronic effect that depends on the exciton resonance. In addition our approach allows the quantification of the ${\ensuremath{\chi}}^{(2)}$ of monolayer ${\mathrm{MoS}}_{2}$ on ${\mathrm{SiO}}_{2}$ and Au, $(7.9\ifmmode\pm\else\textpm\fi{}1.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20}$, and $(1.4\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}\phantom{\rule{4pt}{0ex}}{\mathrm{m}}^{2}$/V, respectively. The current study demonstrates that the interaction between Au and a model TMDC monolayer substantially alters both the magnitude and symmetry of the NLO response. Such effects are important in application of these materials in optoelectronic devices and offer a contact-free probe of substrate-induced changes in the monolayer's electronic structure.