Substrate impact on the thickness dependence of vibrational and optical properties of large area $Mo{S}_2$ produced by gold-assisted exfoliation

Abstract

The gold-assisted exfoliation is a very effective method to produce large-area ($c{m}^2$-scale) membranes of molybdenum disulfide ($Mo{S}_2$) for electronics. However, the strong $Mo{S}_2/Au$ interaction, beneficial for the exfoliation process, has a strong impact on the vibrational and light emission properties of $Mo{S}_2$. Here, we report an atomic force microscopy (AFM), micro-Raman ($\hat{I}¼-R$) and micro-Photoluminescence ($\hat{I}¼-PL$) investigation of $Mo{S}_2$ with variable thickness exfoliated on Au and subsequently transferred on an $A{l}_2{O}_3/Si$ substrate. The $E_{2g}$ - $A_{1g}$ vibrational modes separation $\hat{I}”\hat{I}¼$ (typically used to estimate $Mo{S}_2$ thickness) exhibits an anomalous large value ($\hat{I}”\hat{I}¼=21.2c{m}^{-1}$) for monolayer (1L) $Mo{S}_2$ on Au as compared to the typical one ($\hat{I}”\hat{I}¼=18.5c{m}^{-1}$) measured on 1L $Mo{S}_2$ on $A{l}_2{O}_3/Si$. Such substrate-related differences, explained in terms of tensile strain and p-type doping arising from the $Mo{S}_2/Au$ interaction, were found to gradually decrease while increasing the number of $Mo{S}_2$ layers. Furthermore, $\hat{I}¼-PL$ spectra for 1L $Mo{S}_2$ 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 $Mo{S}_2$ on $A{l}_2{O}_3$. 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 $Mo{S}_2/Au$ interface.

14 pages, 4 figures