It has been proposed that oxygen molecules can grab the electrons from the n-type metal oxide semiconductors, adsorb on their surface, and thus change the carrier densities. [ 26 ] To better understand the charge transfer mechanism of the device, we put the sample in vacuum (VA) conditions (10 − 6 mbar). Figure 2 c illustrates that under the vacuum condition, with 325 nm light irradiation, the CNP of graphene undergoes a signifi cant shift towards about −40 V because of electron transfer, as shown in curve 2. However, it does not return after we switch off the laser, shown by curve 3. To further confi rm the results, we have also done the control experiments in both nitrogen and oxygen environment, see Figure S2 in Supporting Information. In nitrogen, similar to the vacuum case, the UV light causes a left shift of CNP, and the CNP would not come back after we switch off the UV light; while in oxygen, it resembles the case in the air, the CNP would undergo a left shift with irradiation of UV light, and come back to its original place after we remove the UV light. We therefore believe that oxygen molecules play an important role in this electron transfer process, because in the vacuum condition the oxygen molecules are pumped away after the UV induced desorption.