Demonstration of Designer Strain Distributions in Two-Dimensional Semiconductors

Strain engineering of two-dimensional (2D) semiconductors has recently revealed exciting nanophotonic effects such as localized bandgap tuning, exciton funneling, and the creation of site-specific single photon emitters. While there has been significant progress in the theoretical understanding of nanoscale strain engineering over recent years, experimental progress is hindered by the presence of unintended strain in solid-state devices, blurring the relationship between the local strain environment and direct material response. Existing studies have mainly investigated the strain-induced nanophotonic effects of 2D semiconductors on isolated nanostructures such as nanopillars and nanotips. Here, we introduce an additional degree of freedom to strain engineering to investigate complex strain distributions by transferring 2D materials flakes onto nanostructures patterned in close proximity, enabling the study of a variety of strain distributions, such as uniaxial, biaxial, and triaxial strain within a single flake. The bright emission from thick flakes makes gallium selenide a fantastic material for understanding the relationship between local strain and optical response. Our findings reveal that finite strain distributions and resulting bandgap shifts occur in regions of gallium selenide suspended between closely spaced nanostructures, which is in agreement with strain distributions simulated using finite element analysis. Furthermore, by rotating nanopillar units within the same pattern, we enable studying strain effects as a function of alignment to the crystallographic directions. By carefully designing the strain distribution, we show that the energy, intensity, and localization of gallium selenide emission can be controlled. In addition, we demonstrate Raman shifts and emission shifts by strain engineering in tungsten disulfide. This research paves the way for designer strain distributions and tailorable nanophotonic behavior in two-dimensional materials.