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 E_2g–A_1g 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.