Visualization of Dark Excitons in Semiconductor Monolayers for High-Sensitivity Strain Sensing

Transition-metal dichalcogenides (TMDs) are layered materials that have a semiconducting phase with many advantageous optoelectronic properties, including tightly bound excitons and spinvalley locking. In tungsten-based TMDs, spin- and momentumforbidden transitions give rise to dark excitons that typically are optically inaccessible but represent the lowest excitonic states of the system. Dark excitons can deeply affect the transport, dynamics, and coherence of bright excitons, hampering device performance. Therefore, it is crucial to create conditions in which these excitonic states can be visualized and controlled. Here, we show that compressive strain in WS2 enables phonon scattering of photoexcited electrons between momentum valleys, enhancing the formation of dark intervalley excitons. We show that the emission and spectral properties of momentum-forbidden excitons are accessible and strongly depend on the local strain environment that modifies the band alignment. This mechanism is further exploited for strain sensing in two-dimensional semiconductors, revealing a gauge factor exceeding 104.

Automated summary

Transition-metal dichalcogenides (TMDs) are layered materials with advantageous optoelectronic properties. In tungsten-based TMDs, dark excitons, which are optically inaccessible but represent the lowest excitonic states, can be formed by phonon scattering of photoexcited electrons under compressive strain in WS2. The emission and spectral properties of these dark excitons strongly depend on the local strain environment, making it a promising mechanism for strain sensing in two-dimensional semiconductors, with a gauge factor exceeding 104.