Limits of funneling efficiency in non-uniformly strained 2D semiconductors

Photoexcited electron-hole pairs (excitons) in transition metal dichalcogenides (TMDC) experience an effective force when these materials are non-uniformly strained. In the case of strain produced by a sharp tip pressing at the center of a suspended TMDC membrane, the excitons are transported to the point of the highest strain at the center of the membrane. This effect, exciton funneling, can be used to increase photoconversion efficiency in TMDC, to explore exciton transport and to study correlated states of excitons arising at their high densities. Here, we analyze the limits of funneling efficiency in realistic device geometries. The funneling efficiency in realistic monolayer TMDCs is found to be low, $\lt$5% both at room and low temperatures. This results from dominant diffusion at room temperature and short exciton lifetimes at low temperatures. On the other hand, in TMDC heterostructures with long exciton lifetimes the funneling efficiency reaches ~50% at room temperature, as the exciton density reaches thermal equilibrium in the funnel. Finally, we show that Auger recombination limits funneling efficiency for intense illumination sources

Non‐uniform strain engineering of 2D materials

2D materials are elastic substances that can sustain high strain. While the response of these materials to spatially uniform strain is well studied, the effects of spatially non-uniform strain are understood much less. In this review, we examine the response of two different 2D materials, transition metal dichalcogenides and graphene, under non-uniform strain. First, we analyze pseudo-magnetic fields formed in graphene subjected to highly localized non-uniform strain. Second, we discuss the effect of non-uniform strain on excitons in non-uniformly strained TMDC. We show that while transport or “funneling” of excitons is relatively inefficient, a different process, a strain-related conversion of excitons to trions is dominant. Finally, we discuss the effects of uniform and non-uniform strain in a graphene-based phononic crystal. We find that uniform strain can be used to broadly tune the frequency of the phononic bandgap by more than 350 % and non-uniform strain smears that bandgap