We have also taken an AFM image of our device after ZnO QDs were sprayed on graphene, see Figure S1 in Supporting Information. It turns out ZnO QDs form many discrete islands with thickness of several tens of nanometers, which may be due to the aggregation of the ZnO QDs. Using the same spray method, we made a control device of ZnO QDs alone to investigate its optoelectrical properties. In the dark condition, the resistance is more than 10 10 Ohms. When we shine 325 nm laser on the device, this resistance value doesn’t change at all. This result confi rms that the electronic transportation properties we measured for the hybrid structures are indeed mainly from graphene, not from the pure QDs. Meanwhile, the transport characteristics of our device in Figure 2 a are the typical graphene ambipolar fi eld effect, which is very different from the n-type semiconductor behavior in ZnO QDs. [ 25 ]