Unconventional exciton behavior and carrier multiplication in MoTe2.

A comprehensive understanding of exciton generation and transport in van der Waals layered transition metal dichalcogenides is essential for advancing high-performance optoelectronic devices. However, the nanometer-scale range and short lifetimes of excitons impede reliable characterization. Here, we overcome these limitations using a direct nanoscale photocurrent imaging technique that integrates a conductive atomic force microscope with continuous-wave excitation. By mapping local photocurrents in MoTe2/graphene heterostructures, we visualize exciton distribution and generation at the nanoscale. We observe a diffraction-like photocurrent pattern, indicating lateral redistribution of active excitonic carriers from the excitation center. Notably, the active excitonic-carrier region is strongly confined near the top few layers due to an electronic potential barrier. The highly confined excitons contribute to a high quantum yield (∼95%) by carrier multiplication in bulk MoTe2. Consequently, the enhanced exciton generation in a few layers can occur by unusually large exciton binding energy, lowered dielectric screening, strengthened carrier extraction from decreased barrier height, and the longer photocarrier lifetime of bulk MoTe2.