Hybrid graphene–quantum dot phototransistors with ultrahigh gain

Graphene is an attractive material for optoelectronics1 and photodetection applications2,3,4,5,6 because it offers a broad spectral bandwidth and fast response times. However, weak light absorption and the absence of a gain mechanism that can generate multiple charge carriers from one incident photon have limited the responsivity of graphene-based photodetectors to ∼10−2 A W−1. Here, we demonstrate a gain of ∼108 electrons per photon and a responsivity of ∼107 A W−1 in a hybrid photodetector that consists of monolayer or bilayer graphene covered with a thin film of colloidal quantum dots. Strong and tunable light absorption in the quantum-dot layer creates electric charges that are transferred to the graphene, where they recirculate many times due to the high charge mobility of graphene and long trapped-charge lifetimes in the quantum-dot layer. The device, with a specific detectivity of 7 × 1013 Jones, benefits from gate-tunable sensitivity and speed, spectral selectivity from the short-wavelength infrared to the visible, and compatibility with current circuit technologies. A phototransistor in which electric charges are absorbed by colloidal quantum dots and circulated in graphene exhibits high values for gain, responsivity and specific detectivity.