Metal-assisted exfoliation of MoS2

This work investigates how metal contact affects interlayer coupling in few-layer MoS2 flakes. Interfacial modifications are probed using ultra-low-frequency (ULF) Raman spectroscopy, which, in combination with a modified linear-chain model, can quantify changes to the van der Waals coupling between layers. Across contacts with Au, Pd, Ni, Co and Ti, universal decoupling effects are observed, which weaken the MoS2/ MoS2 interface nearest to the metal. The degree of decoupling increases with thickness, reaching a maximum at a few layers, which varies for different metals. This finding is corroborated by the observation of bubbles on annealed samples, which form only under thicker crystals and burst to expose monolayer, providing direct evidence of mechanical weakening. The effect does not depend on metal thickness or morphology and occurs even on atomically flat Au films, indicating that it is intrinsic to the metal/MoS2 interaction. We ascribe the decoupling effect to strain induced by the metal in the attached layer. In multilayer flakes, this strain is transferred inefficiently to the remaining layers, leading to a lattice registry mismatch and reduced coupling. This interpretation is supported by the observation that under sufficient externally applied strain, multilayer flakes undergo similar decoupling effects. These findings, which further establish ULF Raman as a uniquely sensitive probe for measuring subtle interfacial effects, demonstrate that metal contacts directly modify the interlayer coupling of 2D materials, with implications for exfoliation processes and other important metal/2D material interfaces such as electrodes.