Development and Characterization of Novel Optoelectronic Devices Based on Composite Nanomaterials

Low dimensional metal oxide materials have promising applications in high performance optoelectronic devices due to their intrinsic merits such as high surface to volume ratio, tunable detection wavelengths via changing the size of nanoparticles, high quantum, relatively low cast and compatible with surface coating processes. In this dissertation, novel UV photodetectors were developed for potential biohazard detection system applications, which can be realized via detection of the signature emissions from the fluorescence groups inside the bacteria. Two material systems were studied for UV photodetectors: l) low dimensional metal oxide materials and 2) composite nanomaterials with improved carrier separation and transport. In addition to the development of high performance UV photodetectors, this dissertation also investigated surface plasma resonance (SPR) effect of ZnO based composite nanomaterials for potential LED and solar cell applications.;Two photodetectors were fabricated from low dimensional metal oxide materials. First, a bandpass photodetector with high responsivity in near-UV region was developed for possible detection of emission from NADH group inside the E-Coli that have an emission peak at 450 nm. The UV photodetector achieved high photoresponsivity in both UV and visible regions. However, the transient response of the photodetector is very slow, which is a general characteristic for metal oxides based photodetectors. The slow transient response greatly limits the metal oxide materials' applications for frequency modulated biohazard detection systems. To overcome the slow transient response, a photodiode based on ZnO nanowires (NWs)/p-Si heterojunction structure was fabricated using a simple hydrothermal method followed by post growth hydrogen treatment. The hydrogen treatment can significantly increase the free carrier concentration in ZnO NWs and hence improve the conductivity. The heterojunction photodiode demonstrated greatly improved transient response as compared to traditional ZnO NWs based photodetectors. The photoresponsivity spectra of the device in the UV and visible regions are dependent on the polarity of the applied voltages, which is due to the large valance band offset and carrier transportation mechanism inside the heterojunction structure.;In addition to hydrogen doping method for transient response improvement, we also investigated using composite nanomaterials that combines the high carrier mobility of carbon nanomaterials with the high photoresponsivity of low dimensional metal oxides. The composite nanomaterials demonstrated significantly enhanced carrier transport and collection efficiency, which greatly improves the performance of the UV photodetectors in terms of both photoresponsivity and response speed. Four different UV photodetectors were developed. First, multiwalled carbon nanotubes (MWCNTs) network as electrode was deposited on ZnO NWs/p-Si heterojunction photodiode grown by hydrothermal process. Fast transient response with rise time 0.09 s and fall time 0.08 s was observed for the photodiode with MWCNTs network. The photodiode also showed higher responsivity in the UV region (4.7A/W at 365 nm, 2V) compared with the reference sample. For the second UV photodetector, the typically slow time response of ZnO nanoparticle detectors was greatly improved by 40 times after applying of the solution processed graphene quantum dots (GQDs) in the active region. Furthermore, a maximum photoresponsivity improvement of 260% was realized for the photodetector with 4 depositions of GQDs. The enhanced performance of the UV photodetectors fabricated from GQDs and PVA-ZnO nanoparticles are attributed to enhanced carrier separation and more efficient carrier transport through GQDs. Third, a general strategy was developed to achieve the self-assembled wrapping of graphene sheets around ZnO nanoparticles (NPs), ZnO NWs, In2O3 NPs and porous Co2O 3 NPs. The UV photodetector fabricated from ZnO NPs-graphene core/shell structure demonstrated fast transient response and high photoresponsivity. The high performance of the photodetector is attributed to improved structural integrity and carrier transport efficiency through graphene encapsulation. Finally, an UV photodetector was fabricated from WO3 nanodiscs/reduced graphene oxide (WO3 NDs/RGO) composite material, which was synthesized via a facile three-step synthesis. The photodetector showed high photocurrent to dark current ratio (600) under UV illumination at 340 nm when biased at 20 V. In addition, fast transient response with response time on the order of milliseconds was observed for the WO3 NDs/RGO composite photodetector, which is attributed to improved carrier transport efficiency through RGO. A maximum photoresponsivity of 6.4 A/W at 347 nm was observed under 20 V bias.;As for the SPR effects, three different material systems were investigated for potential LED and solar cell applications: PVA-ZnO/MWCNTs/PVA-ZnO, SiO 2-Au core-shell/PVA-ZnO, and pure Au nanoparticles embedded organic polymer solar cells. The PVA-ZnO/MWCNTs/PVA-ZnO composite structures showed greatly enhanced band edge emission (>300%) that originates from SPR between ZnO and MWCNTs. As for the SiO2-Au core-shell structures, the SPR peaks can be tuned in a wide range (554 nm--631 nm) by adjusting the size and the shape of Au nanoparticles attached to the SiO2 surface. The core-shell structures provide an effective approach for simultaneously suppressing defect level emission and enhancing near band edge emissions of the ZnO nanoparticles by defect-induced surface plasmon resonance. The superior SPR tunability of the core-shell structure, together with the low cost and flexibility of the approach, makes it a nanomaterial of high potential for future optoelectronics. Finally, SPR effect of pure Au NPs for inverted solar cell application was demonstrated. After embedded with a thin layer of Au NPs, a maximum efficiency improvement of 14.29% was achieved.