Determination of the roles of strain and tearing in single photon emission from nanoindented WSe$_2$
Single-photon emission in two-dimensional single-layer WSe2 is attractive for the on-demand generation of quantum states of light. The electronic states that are responsible for single-photon generation preferentially form in regions of localized tensile strain, enabling deterministic positioning and strain engineering. Nanoindentation of single-layer WSe2 yields controlled and reproducible deformations that generate the localized strain needed to activate the single-photon-emitting states. However, using nanoindentation both for investigating structure-property relationships and for manufacturing quantum light sources based on WSe2 is hindered by key questions on the structural integrity of the indented 2D material, the resulting strain generated, and the sub-micron location of the emitters. In this work, we study the structure of indented single-layer WSe2 using a fabrication process that inverts the indents into protruding pillars that can be probed using electron microscopy. We explicitly identify strain relaxation of the indented single-layer WSe2 due to tearing and confirm that single-photon-emitting states still form in these systems, likely at the extremities of the tear. For indents that are confirmed to be intact (i.e., not torn), we assess the ability to strain engineer the single-photon emitters. While strain does not strongly affect the emission energy or the brightness, we find that increased strain reduces the spatial density of emitters. This trend indicates that an optimal amount of strain is needed for emitter formation and/or the emitters preferentially form on the periphery of the indent. Our investigation provides insight into the most relevant structure-property relationships for using strain to engineer quantum light sources in single-layer WSe2 and other 2D semiconductors.