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.