Substrate impact on the thickness dependence of vibrational and optical properties of large area MoS2 produced by gold-assisted exfoliation

The gold-assisted exfoliation is a very effective method to produce large-area (cm2-scale) membranes of molybdenum disulfide (MoS2) for electronics. However, the strong MoS2/Au interaction, beneficial for the exfoliation process, has a strong impact on the vibrational and light emission properties of MoS2. Here, we report an atomic force microscopy, micro-Raman, and micro-photoluminescence (μ-PL) investigation of 2H-MoS2 with variable thickness exfoliated on Au and subsequently transferred on an Al2O3/Si substrate. The E2g-A1g vibrational mode separation Δω (typically used to estimate MoS2 thickness) exhibits an anomalous large value (Δω ≈ 21.2 cm-1) for monolayer (1L) MoS2 on Au as compared to the typical one (Δω ≈ 18.5 cm-1) measured on 1L MoS2 on Al2O3. Such substrate-related differences, explained in terms of tensile strain and p-type doping arising from the MoS2/Au interaction, were found to gradually decrease while increasing the number of MoS2 layers. Furthermore, μ-PL spectra for 1L MoS2 on Au exhibit a strong quenching and an overall redshift of the main emission peak at 1.79 eV, compared to the 1.84 eV peak for 1L MoS2 on Al2O3. After PL spectra deconvolution, such redshift 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 the MoS2/Au interface.