Raman spectra evidence for the covalent-like quasi-bonding between exfoliated MoS₂ and Au films

Gold-enhanced mechanical exfoliation method attracts broad interests in recent years, which has been widely used for preparing large-area and high-quality 2D single crystals. Even many calculations predict that there is strong interaction between Au film and the exfoliated 2D crystals, direct experimental evidence is still lacking. Here, we perform Raman spectroscopy measurements for few layer MoS₂ with and without Au film underneath. The main peaks of MoS₂ on Au film show no obvious change at higher frequency, however, the breathing and shear modes at low-frequency are suppressed, especially for breathing modes. In contrast, both breathing modes and shear modes can be detected on suspended MoS₂ and the samples are transferred from Au film to SiO₂/Si. These comparison results provide direct evidence for the existence of covalent-like quasi-bonding at the interface of Au film and the exfoliated MoS₂ crystal. This MoS₂/Au interface interaction presents a special pinning-effect for low-frequency rigid vibration. Similar pinning-effect is also discovered in WS₂/Au system. Our work reports the suppression of low-frequency Raman modes of MoS₂, WS₂ on Au film, which will deliver new inspiration for studying other interactions between layered materials and solid surfaces.