Substrate impact on the thickness dependence of vibrational and optical properties of large area $MoS_2$ produced by gold-assisted exfoliation

TL;DR

This study investigates how gold-assisted exfoliation affects the vibrational and optical properties of monolayer and few-layer MoS2, revealing substrate-induced strain, doping, and emission shifts.

Contribution

It provides detailed analysis of substrate effects on MoS2 properties using AFM, Raman, and photoluminescence, highlighting the impact of gold interaction on vibrational and optical behavior.

Findings

Anomalously large vibrational mode separation in monolayer MoS2 on Au

Red-shift and quenching of photoluminescence on Au substrate

Substrate-induced strain and doping effects decrease with layer number

Abstract

The gold-assisted exfoliation is a very effective method to produce large-area ($cm^2$-scale) membranes of molybdenum disulfide ($MoS_2$) for electronics. However, the strong $MoS_2/Au$ interaction, beneficial for the exfoliation process, has a strong impact on the vibrational and light emission properties of $MoS_2$. Here, we report an atomic force microscopy (AFM), micro-Raman ($\mu-R$) and micro-Photoluminescence ($\mu-PL$) investigation of $MoS_2$ with variable thickness exfoliated on Au and subsequently transferred on an $Al_2O_3/Si$ substrate. The $E_{2g}$ - $A_{1g}$ vibrational modes separation $\Delta\mu$ (typically used to estimate $MoS_2$ thickness) exhibits an anomalous large value ($\Delta\mu=21.2 cm^{-1}$) for monolayer (1L) $MoS_2$ on Au as compared to the typical one ($\Delta\mu=18.5 cm^{-1}$) measured on 1L $MoS_2$ on $Al_2O_3/Si$. Such substrate-related differences, explained in terms of tensile strain and p-type doping arising from the $MoS_2/Au$ interaction, were found to gradually decrease while increasing the number of $MoS_2$ layers. Furthermore, $\mu-PL$ spectra for 1L $MoS_2$ on Au exhibit a strong quenching and an overall red-shift of the main emission peak at 1.79 eV, compared to the 1.84 eV peak for 1L $MoS_2$ on $Al_2O_3$. After PL spectra deconvolution, such red shift was explained in terms of a higher trion/exciton intensity ratio, probably due to the higher polarizability of the metal substrate, as well as to the smaller equilibrium distance at $MoS_2/Au$ interface.