Quantum-Dot-Like States in Molybdenum Disulfide Nanostructures Due to the Interplay of Local Surface Wrinkling, Strain, and Dielectric Confinement

Abstract

The observation of quantum light emission from atomically thin transition metal dichalcogenides has opened a new field of applications for these material systems. The corresponding excited chargecarrier localization has been linked to defects and strain, while open questions remain regarding the microscopic origin. We demonstrate that the bending rigidity of these materials leads to wrinkling of the two-dimensional layer. The resulting strain field facilitates strong carrier localization due to its pronounced influence on the band gap. Additionally, we consider charge carrier confinement due to local changes of the dielectric environment and show that both effects contribute to modified electronic states and optical properties. The interplay of surface wrinkling, strain-induced confinement, and local changes of the dielectric environment is demonstrated for the example of nanobubbles that form when monolayers are deposited on substrates or other two-dimensional materials.

Imaging strain-localized excitons in nanoscale bubbles of monolayer WSe2 at room temperature

“…Using hyperspectral nanophotoluminescence (nano-PL) mapping, we achieve a sub-20 nm spatial resolution and resolve individual localized low-energy states that are separated by distances less than 50 nm within single nanobubbles at room temperature. Furthermore, we observe that these localized states form in doughnut-like patterns that are consistent with the first-principles predictions 27 that refine those of continuum plate and membrane models 9,10 . These LXs are also identified in nanobubbles of "fluxgrown" 1L-WSe2, which has a much lower defect density as compared to monolayers exfoliated from commercially available crystals (ca.…”

“…The correspondence between the theoretical analysis of the strain and resulting electronic states within the nanobubble and our nano-optical dataset presented in Fig. 3 strongly suggests that the experimentally localized states observed are the predicted quantum-dot-like states 27 . In principle, point defects in the 1L-WSe2 lattice can also localize excitons in a similarly inhomogeneous way.…”

“…However, multiple emitters per nanobubble are typically observed, which is at odds with a single localization site and suggests a possible role of crystallographic defect states or some other sub-nanobubble inhomogeneity 11,18,[22][23][24][25] . Recently, improved microscopic theoretical models predict that quantumdot-like electronic states in TMDs can form within nanobubbles in the absence of defects 26,27 . In particular, a first-principles approach that carefully considers the strain-induced atomic structure 27 (and which we employ here) predicts that strain maxima and multiple low-energy states form in a doughnut-like distribution near the nanobubble periphery.…”

“…Recently, improved microscopic theoretical models predict that quantumdot-like electronic states in TMDs can form within nanobubbles in the absence of defects 26,27 . In particular, a first-principles approach that carefully considers the strain-induced atomic structure 27 (and which we employ here) predicts that strain maxima and multiple low-energy states form in a doughnut-like distribution near the nanobubble periphery. This surprising strain distribution is an apparent result of atomic scale wrinkling around the edges of highly-strained nanobubbles, leading to more localized lower energy states relative to the case of a nanobubble with a smooth topography.…”

“…To gain more insight into the physical mechanism, we applied the theory of ref. [27], which considers the influence of local strain within the nanobubbles on electronic and optical properties. Our method is based on atomistic calculations, in which the relevant part of the nanostructure is modeled using a lattice of atomic sites.…”