Tunable Exciton Funnel Using Moire Super lattice 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 E-g(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. E-g(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 E-g-contrast in different regions of the pseudoheterostructure so as to absorb or emit even broader spectrum of light