Once the electrons of ZnO QDs are transferred to graphene, they can recirculate in the graphene channel within the lifetime. If the lifetime is long enough, it forms a gain system which could be utilized as highly sensitive light detectors or sensors. To determine the lifetime of the transferred electrons, we switched on and off the laser alternatively to measure the changes in the electronic characteristics. All the measurements are conducted at V G = 0 V to remove the artifacts of the gate-voltage introduced molecule adsorption/desorption. [ 27 ] The source-drain voltage V DS is set to be 1 mV. As shown in Figure 2 d, when the device is exposed to air, during UV light irradiation (325, air), an increase of the sourcedrain current is observed, confi rming that electrons were transferred from the QDs to graphene. After the UV light is switched off, the current drops to its original level. From the falling edge of the curve, we get the carriers’ lifetime τ by fi tting it with an exponential function exp(-t/ τ ) and obtain τ = 5 s, as shown in the top inset in Figure 2 d. This approach has been widely used and proved to be an effi cient way to determine the carriers’ lifetime. [ 21 , 22 , 26 ] Then we put the device in the vacuum condition with 325 nm laser irradiation (325, VA), and in air with 445 nm laser (445, air). As expected, the device has no noticeable response when we switch on and off the light, as shown in Figure 2 d.