WS₂/WSe₂ Nanodot Composite Photodetectors for Fast and Sensitive Light Detection

Owing to the exotic optical properties, semiconducting transition metal dichalcogenides (TMDs) have been targeted for sensitive photodetection with a high gain. Yet, it remains a challenge to achieve fast response dynamics while retaining a high responsivity. In this work, we demonstrate a 2D composite photodetector comprising of graphene-contacted monolayer WS2 stacked atop dispersed WSe2 nanodots. Upon light illumination, excess photoexcited electrons are generated and drift to a WS2 channel due to the type-II band alignment at the WS2/WSe2 heterojunction, leaving photoexcited holes in WSe2 nanodots. Recirculation of electrons across the WS2 channel provides a photogain during the lifetime of holes that remain trapped in WSe2 nanodots. The composite photodetector can reach a high responsivity of 8.4 × 102 A/W with a gain of 2.0 × 103 electrons per photon and, meanwhile, a fast response time in a few milliseconds. It also exhibits a high detectivity of 9.7 × 1011 Jones and an on-to-off current ratio of 105. This performance enhancement is ascribed to the devised concept of using CVD-grown WSe2 nanocrystals as a photosensitizer, which reduces the contact barrier by WSe2-nanodot photodoping and provides rapid excitonic dynamics through the size-controllable WSe2/WS2 heterojunction.