Probing Local Strain at MX₂–Metal Boundaries with Surface Plasmon-Enhanced Raman Scattering

Interactions between metal and atomically thin two-dimensional (2D) materials can exhibit interesting physical behaviors that are of both fundamental interests and technological importance. In addition to forming a metal–semiconductor Schottky junction that is critical for electrical transport, metal deposited on 2D layered materials can also generate a local mechanical strain. We investigate the local strain at the boundaries between metal (Ag, Au) nanoparticles and MX2 (M = Mo, W; X = S) layers by exploiting the strong local field enhancement at the boundary in surface plasmon-enhanced Raman scattering (SERS). We show that the local mechanical strain splits both the in-plane vibration mode E2g(1) and the out-of-plane vibration mode A1g in monolayer MoS2, and activates the in-plane mode E1g that is normally forbidden in backscattering Raman process. In comparison, the effects of mechanical strain in thicker MoS2 layers are significantly weaker. We also observe that photoluminescence from the indirect bandgap transition (when the number of layers is ≥2) is quenched with the metal deposition, while a softened and broadened shoulder peak emerges close to the original direct-bandgap transition because of the mechanical strain. The strain at metal–MX2 boundaries, which locally modifies the electronic and phonon structures of MX2, can have important effects on electrical transport through the metal–MX2 contact.