Dark exciton based strain sensing in tungsten-based transition metal dichalcogenides

The recent emergence of atomically thin two-dimensional (2D) materials has opened up possibilities for the design of ultrathin and flexible nanoelectronic devices. As truly 2D materials, they exhibit an optimal surface-to-volume ratio, which results in an extremely high sensitivity to external changes which can not be achieved by conventional semiconductors. This makes these materials optimal candidates for sensing applications. Here, we propose a dark exciton based concept for ultrasensitive strain sensors. By investigating both dark and bright excitons in tungsten-based monolayer transition metal dichalcogenides, we demonstrate that the dark-bright-exciton separation can be controlled by strain, which has a crucial impact on the activation of dark excitonic states. The predicted opposite strain-induced shifts for dark and bright excitons result in a pronounced change in photoluminescence stemming from dark excitons even at very small strain values. The predicted high optical gauge factors of up to 8000 are promising for the design of optical strain sensors.