| Zinc oxide nanostructures, such as ZnO nanowires (NWs) and quantum dots (QDs) were fabricated via solution routes and incorporated with graphene to form high-performance ZnO/graphene nanohybrid ultraviolet optoelectronic devices. Structures such as vertically aligned ZnO-NWs/graphene heterojunction nanohybrids combine the superior sensitivity of crystalline ZnO-NWs with high charge mobility of graphene, allowing high device performance surpassing their conventional counterparts'. Controlling the ZnO nanostructure morphology and its interface with graphene is important to optimization of the optoelectronic processes including exciton dissociation, charge transfer and transport, which in turn affects the ZnO/graphene nanohybrids device performance. This thesis explored two types of ZnO/graphene nanohybrids: one with vertically aligned ZnO-NWs grown directly on graphene and the other, with porous ZnO printed on graphene. In the former, we have found that a ZnO seed layer facilitates growth of a dense array ZnO-NWs of radii approaching the Debye length (~20 nm) that is desired for optimal surface electron depletion effect. In contrast, a seedless process resulted in a lower density of ZnO-NWs of a larger diameter on the order of sub-to-few micrometers. Consequently, higher UV photoresponsivity up to 728 A/W was obtained on ZnO-NW/graphene nanohybrids obtained in the seeded process, which is anticipated from the larger surface-to-volume ratio and hence more enhanced photoconductive gain from the surface electron depletion. However, a strong charge trapping effect was also introduced by the seed layer at the ZnO-NW/graphene interface, leading to much slower photoresponse. In the printed ZnO nanohybrids, inks of zinc acetate precursor without (ZnOPr) and with crystalline ZnO QDs (ZnOPrQDs) have been explored. The former exhibited a micro-porous structure while the latter, nanoporous with feature size comparable to the ZnO's Debye length. Without graphene, higher UV photoresponsivity of 383.6 A/W and the on/off ratio of 2470 were observed in nanoporous ZnO photoconductors as expected, which are significantly better than that of 14.7 A/W and 949 in the microporous counterparts. With graphene, the ZnO/graphene exhibited a photoresponse of ~1000 A/W and a photoconductive gain of 1.8x10 4, a whole order of magnitude better than without graphene. These nanostructures have demonstrated high performances and printability for sensor applications. |
