Probing Evolution of Local Strain at MoS₂-Metal Boundaries by Surface-Enhanced Raman Scattering

Strain usually exists in two-dimensional (2D) materials and devices, and its presence drastically modulates their properties. When 2D materials interface with noble metals, local strain and surface plasmon can couple at the metal-2D material boundaries, delivering a lot of intriguing phenomena. Current studies are mostly focused on the explanations of these strain-related phenomena based on a static point of view. Although strain can typically be relaxed in many environments, the time evolution of strain at metal-2D material interfaces remains largely unknown. In this work, we investigate the evolution of local strain at Ag-MoS₂ boundaries by surface-enhanced Raman scattering. With the split of MoS₂ Raman peaks as an indicator of local strain, it is found that the originally localized strain at Ag-MoS₂ boundaries evolves and relaxes with time into a delocalized strain in MoS₂ plane. The time to start the strain relaxation depends on the number of layers of MoS₂ flakes, suggesting that the relaxation may result from the mechanical instability of the interface between the topmost MoS₂ layer and the underlying materials. The relaxation occurs in a certain period of time, i.e., ∼70 days for 1L and ∼30 days for 3L. Accompanying the strain relaxation, surface sulfurization of Ag also occurs, a process that reduces the strength of locally enhanced electric field. Our results not only provide a deep understanding of strain evolution at metal-MoS₂ interfaces but also shed light on the optimization of MoS₂-based device fabrications.