To eliminate possible artifacts from the gate hysteresis, all of the resistance-voltage curves were acquired on the same measurement cycle while we scanned the bias from positive to negative values. As we see in Figure 2 a, the charge neutrality point (CNP) was initially at 18 V, shown by curve 1, due to the doping effects from the air and substrate. After we placed ZnO QDs on the device, the CNP was shifted to about 8 V, as shown by curve 2, consistent with a slightly higher working function of ZnO QDs compared to graphene, and the carrier mobility was slightly decreased from 3600 cm 2 V . 1 s . 1 to 3000 cm 2 V . 1 s . 1 due to the disorder introduced by the QDs.