Tunable exciton funnel using Moiré superlattice in twisted van der Waals bilayer.

A spatially varying bandgap drives exciton motion and can be used to funnel energy within a solid (Nat. Photonics 2012, 6, 866-872). This bandgap modulation can be created by composition variation (traditional heterojunction), elastic strain, or in the work shown next, by a small twist between two identical semiconducting atomic sheets, creating an internal stacking translation u(r) that varies gently with position r and controls the local bandgap Eg(u(r)). Recently synthesized carbon/boron nitride (Nat. Nanotechnol. 2013, 8, 119) and phosphorene (Nat. Nanotechnol. 2014, 9, 372) may be used to construct this twisted semiconductor bilayer that may be regarded as an in-plane crystal but an out-of-plane molecule, which could be useful in solar energy harvesting and electroluminescence. Here, by first-principles methods, we compute the bandgap map and delineate its material and geometric sensitivities. Eg(u(r)) is predicted to have multiple local minima ("funnel centers") due to secondary or even tertiary periodic structures in-plane, leading to a hitherto unreported pattern of multiple "exciton flow basins". A compressive strain or electric field will further enhance Eg-contrast in different regions of the pseudoheterostructure so as to absorb or emit even broader spectrum of light.