Interlayer Exciton Transport Modulated by Twist-Angle-Dependent Moiré Potentials in WS2-WSe2 Heterobilayers

The nanoscale periodic potentials introduced by moiré patterns in semiconducting van der Waals heterostructures provide a new platform for designing exciton superlattices. T o realize these applications, a thorough understanding of the localization and delocalization of interlayer excitons in the moiré potentials is necessary. Here, we investigated interlayer exciton dynamics and transport modulated by the moiré potentials in WS2- WSe2 heterobilayers using experiments and theory. Experimental results verified the theoretical prediction of energetically favorable K-Q interlayer excitons and unraveled exciton-population dynamics that was controlled by the twist-angle-dependent energy difference between the K-Q and K-K excitons. Spatially- and temporally-resolved exciton- population imaging visualized exciton localization by twist-angle-dependent moiré potentials of ~100 meV and exciton delocalization by strong many-body interactions at densities >1012 cm-2.
The studied heterobilayers have two stacking orientations with twist angles of 𝜃 = 0° and 60°, which are energetically favorable in the modified two-step CVD growth. Both structures have type-II band aligment, resulting in the formation of spatially- indirect interlayer charge-transfer excitons, with electrons and holes residing in the WS2 and WSe2 layers, respectively. The lowest-energy transition is always K-Q and therefore K-Q interlayer excitons are expected to represent the ground state instead of the more commonly discussed K-K excitons. The spatial variations of the moiré potential for 0° are much stronger (deep potential)than for 60° (shallow potential). Thus, two predictionscan be made based on the DFT calculations: (i) the population dynamics of the K-K and K-Q excitons should be affected by the twist-angle-dependent energy difference between the two transitions; and (ii) the twist-angle- dependent moiré potentials should lead to different degrees of localization of the interlayer excitons in both systems. The validation of both predictions will be discussed in this presentation
The localization and delocalization of the interlayer excitons presented here have important implications for the potential applications of heterostructures; for long-range transport, more delocalized interlayer excitons are preferred and, therefore, deep moiré potentials should be avoided. On the other hand, for applications, such as, quantum emitters, deep moiré potential should be preferred, to localize excitons. We also stress that K-Q interlayer excitons are the ground state instead of the commonly assumed K-K excitons and should be considered when discussing interlayer excitons in the WS2-WSe2 systems.