heterobilayer

Stacking transition metal dichalcogenide (TMDC) monolayers with an angle of twist [1,2] create artificial superlattices. In such twisted heterobilayers, the emergence of the moiré pattern forms spatially periodic deep potential traps, giving rise to moiré sub-bands. Due to type II band alignment in these heterobilayers, electrons and holes reside in different layers, leading to interlayer moiré-trapped bound excitonic states.

In this work, we create closely spaced moiré traps using a twisted heterobilayer of WS₂/WSe₂ with a twist angle of θ≈59°. We observe three equally spaced emission peaks arising from the moiré sub-bands. The lowest (highest) energy moiré exciton exhibits localized (delocalized) character, as identified by their sublinear (linear) power law and slow (fast) decay time. We probe the interaction among these moiré sub-band states using steady-state and time-resolved photoluminescence measurements with gate voltage and optical power as varying parameters. Our main observations are: (1) With increasing positive V_g (n-doping), the lifetime of the moiré-trapped exciton comes down from 100 ns to 10 ns, with a suppressed steady-state PL intensity – indicating an enhanced nonradiative process due to spatial flattening of the conduction band. (2) With an increase in V_g, the moiré trapped excitons exhibit an unconventional Stark shift. (3) With an increase in the incident optical power, the moiré localized exciton energy peak shows a spectral blue shift up to 20 meV, owing to coulomb interaction from neighboring moiré-trapped excitons.

References

1. Kha Tran et al., Nature, 567, 71-75, 2019.

2. Fengcheng Wu et al., Phys. Rev. B 97, 035306, 2018.