Probing photoelectric characteristics of graphene
由於石墨烯能隙為零，所以石墨烯的光電流並不大，溫度80 K時其值約36.2 nA，其增益比例為1.88 %。而石墨烯/硫化鉛薄膜照光會產生p型摻雜，代表硫化鉛受到光所產生之電子-電洞對，只有電洞傳輸至石墨烯上，因此光電流會受到閘極偏壓的改變而有所影響。在300 K時沒有光反應、在80 K時光反應非常強烈，代表傳輸至石墨烯上之電洞數目跟溫度成反比，當閘極偏壓為-35 V，增益比例為15.77 %、閘極偏壓為35 V，增益比例為-10.66 %。
In this experiment, we probe into photocurrent of graphene devices and graphene/PbS thin film devices. Graphene flakes were made by mechanical exfoliation and PbS thin films were deposited by thermal evaporation. Due to its feature of a gapless semiconductor, graphene does not exhibit considerable photocurrent effects. The photocurrent of graphene flakes is about 36 nA and the enhancement ratio due to blue light excitation is 1.88 % at 80 K. In contrast, the PbS deposited graphene flakes may show higher photocurrent. It is argued that, after light illumination, the graphene is p-type doped. It implies that only holes transport to graphene after the generation of electron-hole pairs in PbS by light illumination. Therefore, the photocurrent is strongly dependent on the gating field. Though it does not show visible photoresponsivity at 300 K, the photoresponsivity increases largely at 80 K. When the gate biases are applied at -35 V and 35 V, the enhancement ratios raise up to 15.77 % and -10.66 %, respectively. The photocurrent of PbS is strongly dependent on temperature, and enhancement ratio is inversely proportional to the temperature. On the other hand, the graphene/Pbs heterostructures do not exhibit appreciable gating effects. It is argued that the holes and electrons will recombine and the holes transporting to graphene will be reduced much more. From the photocurrent measurements, we observed that the recombination rate of PbS is proportional to temperature and we proposed the photocurrent transmission mechanism of graphene/PbS heterostructure devices.