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Title: Carbon-semiconductor hybrid nanostructures and their photoresponse behaviors
Authors: Zhan, Zhaoyao
Keywords: DRNTU::Engineering::Mechanical engineering
Issue Date: 2013
Abstract: Carbon nanomaterials (including carbon nanotube and graphene) and semiconductor nanoparticles have attracted considerable research effort in past two decades in terms of both fundamental and application research. Semiconductor nanoparticles have large optical absorption cross-sections and tenable electronic properties. Carbon nanomaterials are unique in terms of structure, carbon nanotube (CNT) bears a hollow one-dimensional structure; and graphene is an one-atom-thick sheet and has extremely high specific surface area; both of them are presenting excellent charge transport capability. The combination of them will provide an ideal model for nanoscale optoelectronic devices. The key step in the operation of this kind of hybrid optoelectronic devices is the interfacial charge transfer between carbon nanomaterials and semiconductors nanoparticles. The fast transfer and transport of charge will help to separate bound excitons (electron-hole pairs generated by light excitation), and suppress the recombination. Basing on the understanding on excitonic solar cells, the diffusion length of exciton is only in a range less than 100 nm before energy quenching; thus it is very important to dissociate the exciton effectively before recombination.' A hybrid nanostructure is extremely beneficial for exciton dissociation for following reasons. First, there is a builtin electric field at the interfacial area, which helps to separate the bound exciton; then, the hybrid nanostructure bears an extremely high interfacial area which provides countless sites for exciton dlssoclation." According to the understanding on electrons transport in nanoparticle structures, under illumination, the photogenerated electrons repeatedly interact with a distribution of traps as they undertake a random hopping through the nanoparticles. A photogenerated electron may experience a million trapping events before being collected by electrode." The longer transporting time in nanoparticles will increase the possibility of recombination. Thus, if the exciton is separated at the interfacial area of the hybrid nanostructure, with one kind of charge{electron or hole) injected into a better transport pathway, say, carbon nanomaterial, it will transport to the collecting electrode very fast, lowering the possibility of recombination. Thus the recombination is not a dominant problem in hybrid nanostructures. In this thesis, we utilized facile methods to synthesize several typical carbon-semiconductor hybrid nanostructures. We will investigate the charge transfer mechanisms in the hybrid materials by studying their photoresponse behaviors. We first studied the photoresponse behavior of multiwalled carbon nanotube-Zinc Oxide (MWNT-ZnO) hybrid nanostructures and charge transfer mechanism. Based on the mechanism, conversion of conductance from n-typed to p-typed is realized. The conductance of the hybrids could be strongly tuned by the amount of ZnO loading: in the case of shorter deposition time or fewer ZnO nanoparticles (NPs), the photogenerated electrons in ZnO transfer into the MWNT, resulting in a decrease in conductance of MWNT-ZnO hybrids; While in the case of longer deposition time or more ZnO NPs, the photogenerated electrons will not only decrease the conductance of holes in the MWNT matrix, but also convert the conductance of hybrids from p-typed to n-typed. A device structure based on another hybrid system, MWNT-CuS, was then designed to study the charge transfer behaviors. By using an asymmetrical contact structure, the hybrid devices present significant rectification in I-V curves, and show predominant photoresponse to light illumination. The strong photoresponse to light illumination is a result of dissociation of photogenerated excitons by the built-in electric field across the Schottky junctions at the MWNT/CuS interfaces due to the intimate contact resulted from in situ synthesis. The asymmetrical contacts provide transport loops for both electrons and holes. Finally the charge transfer and photoresponse behaviors in reduced graphene oxide-ZnO (rGO-ZnO) hybrid nanostructures were investigated in detail. Under visible light illumination, a significant zero-bias photocurrent was observed in rGO-ZnO hybrids device. This photocurrent could be attributed to the fast charge transfer at rGO-ZnO interfaces, effective charge transport in rGO sheets, and continuous adsorption and desorption of ionized surface oxygen. The work function difference (Fermi energy) between the ZnO and rGO provides the driving force for charge separation. The charge-transfer phenomenon in these hybrid nanostructures demonstrates the carrier separation capability for optoelectronic devices. This investigation of charge transfer behavior in carbon nanomaterials based hybrid nanostructures will provide knowledge for the design of high performance optoelectronics devices, such as solar cells, photodetectors and so on.
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Appears in Collections:MAE Theses

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