  Graphene has emerged as a novel platform for opto-electronic applications and
photodetector, but the inefficient conversion from light to current has so far
been an important roadblock. The main challenge has been to increase the light
absorption efficiency and to provide a gain mechanism where multiple charge
carriers are created from one incident photon. Here, we take advantage of the
strong light absorption in quantum dots and the two-dimensionality and high
mobility of graphene to merge these materials into a hybrid system for
photodetection with extremely high sensitivity. Exploiting charge transfer
between the two materials, we realize for the first time, graphene-based
phototransistors that show ultrahigh gain of 10^8 and ten orders of magnitude
larger responsivity compared to pristine graphene photodetectors. These hybrid
graphene-quantum dot phototransistors exhibit gate-tunable sensitivity,
spectral selectivity from the shortwave infrared to the visible, and can be
integrated with current circuit technologies.
