Isolating the Nonlinear Optical Response of a MoS$_2$ Monolayer under Extreme Screening of a Metal Substrate

TL;DR

This paper isolates and characterizes the nonlinear optical response of a MoS₂ monolayer on a gold substrate, revealing a linear spectral lineshape and a renormalized bandgap due to strong dielectric screening.

Contribution

It introduces a polarization-controlled FS-SFG technique to isolate the monolayer's optical response under extreme substrate screening, providing new insights into its quasiparticle bandgap.

Findings

Linear SFG spectral lineshape without exciton features

Estimated quasiparticle bandgap of about 1.65 eV

Strong dielectric screening causes bandgap renormalization

Abstract

Transition metal dichalcogenides (TMDCs) monolayers, as two-dimensional (2D) direct bandgap semiconductors, hold promise for advanced optoelectronic and photocatalytic devices. Interaction with three-dimensional (3D) metals, like Au, profoundly affects their optical properties, posing challenges in characterizing the monolayer's optical responses within the semiconductor-metal junction. In this study, using precise polarization-controlled final-state sum frequency generation (FS-SFG), we successfully isolated the optical responses of a MoS$_2$ monolayer from a MoS$_2$/Au junction. The resulting SFG spectra exhibit a linear lineshape, devoid of A or B exciton features, attributed to the strong dielectric screening and substrate induced doping. The linear lineshape illustrates the expected constant density of states (DOS) at the band edge of the 2D semiconductor, a feature often obscured by excitonic interactions in week-screening conditions such as in a free-standing monolayer. Extrapolation yields the onset of a direct quasiparticle bandgap of about $1.65\pm0.20$ eV, indicating a strong bandgap renormalization. This study not only enriches our understanding of the optical responses of a 2D semiconductor in extreme screening conditions but also provides a critical reference for advancing 2D semiconductor-based photocatalytic applications.