Moir\'e Potential Impedes Interlayer Exciton Diffusion in van der Waals Heterostructures

The properties of van der Waals (vdW) heterostructures are drastically altered by a tunable moire superlattice arising from periodic variations of atomic alignment between the layers. Exciton diffusion represents an important channel of energy transport in semiconducting transition metal dichalcogenides (TMDs). While early studies performed on TMD heterobilayers have suggested that carriers and excitons exhibit long diffusion lengths, a rich variety of scenarios can exist. In a moire crystal with a large supercell size and deep potential, interlayer excitons may be completely localized. As the moire period reduces at a larger twist angle, excitons can tunnel between supercells and diffuse over a longer lifetime. The diffusion length should be the longest in commensurate heterostructures where the moire superlattice is completely absent. In this study, we experimentally demonstrate that the moire potential impedes interlayer exciton diffusion by comparing a number of WSe2/MoSe2 heterostructures prepared with chemical vapor deposition and mechanical stacking with accurately controlled twist angles. Our results provide critical guidance to developing 'twistronic' devices that explore the moire superlattice to engineer material properties.