Probing Local Strain at MX_2?Metal Boundaries with Surface Plasmon-Enhanced Raman Scattering

摘要

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 MX_2 (M = Mo, W; X = S) layers by exploiting the strong local field enhancement at the boundary in surface plasmonenhanced Raman scattering (SERS). We show that the local mechanical strain splits both the in-plane vibration mode E_(2g)~1 and the out-of-plane vibration mode A_(1g) in monolayer MoS_2, and activates the in-plane mode E_(1g) that is normally forbidden in backscattering Raman process. In comparison, the effects of mechanical strain in thicker MoS_2 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?MX_2 boundaries, which locally modifies the electronic and phonon structures of MX_2, can have important effects on electrical transport through the metal?MX_2 contact.