Abstract
In this Ph.D. thesis, composites of ultrananocrystalline diamond (UNCD), thin film metallic glass (TFMG), and a forest of zinc oxide (ZnO) nanostructures (nanorods and nanotubes) have been developed for gas sensing and optoelectronic applications. The composite-structures provide a number of synergetic properties including a large surface area, optimized crystallinity, work function, electrical properties, and surface passivation layers. The ZnO based composite-structures offer good performance in applications for hydrogen gas sensors and ultraviolet (UV) photodetectors.
Simple two-step hydrothermal method in conjunction with microwave plasma enhanced chemical vapor deposition (MPECVD) system have been applied to synthesize ZnO nanorods and UNCD films, respectively. Additionally, radio frequency (RF) magnetron sputtering technique was used for the deposition of TFMG. The grown ZnO-based composite-structures in this thesis emphasized controlling the surface morphology of UNCD films beneath ZnO nanostructures. Defect-controlled gas sensitivity were investigated by varying deposition time of UNCD films. It is found that a partially grown UNCD films that were deposited for 7.5 min are the optimized underlayers for hydrogen gas sensing applications. These optimized ZnO-based composite structures exhibit extremely higher gas response than that of the pristine ZnO sensor. The reason for the improvement in sensing performance of the composite-structures have been investigated in terms of the defect-related sensitivity and energy band theory of the heterostructures.
Furthermore, in this thesis, a metal-semiconductor-metal (MSM) UV photodetectors have been fabricated by using a silicon substrate on top of which a silver paste was deposited as an electrode. Bilayer of TFMG/ZnO nanotubes were grown on UNCD films and this ZnO-based composite-structures have enhanced their optoelectronic properties. The nanostructures are characterized by SEM, EDX, TEM, HRTEM, XRD, Raman, and PL spectroscopy. From the characterization of the materials, we observed that these nanostructures emit a ~ 377 nm (UV) usually called the near band edge emission and a band green band related to the defects and surface states at about 600 nm.
The ZnO-based composite-structures are successfully synthesized at low manufacturing temperatures. This dissertation gives an overview of the optical properties of composites of UNCD, TFMG, and ZnO nanostructures and proposes an effective way to enhance the efficiency of optoelectronic and hydrogen gas sensing devices. The fabrication of ZnO-based composite- structures will be useful for the development of next-generation devices.

Abstract
In this Ph.D. thesis, composites of ultrananocrystalline diamond (UNCD), thin film metallic glass (TFMG), and a forest of zinc oxide (ZnO) nanostructures (nanorods and nanotubes) have been developed for gas sensing and optoelectronic applications. The composite-structures provide a number of synergetic properties including a large surface area, optimized crystallinity, work function, electrical properties, and surface passivation layers. The ZnO based composite-structures offer good performance in applications for hydrogen gas sensors and ultraviolet (UV) photodetectors.
Simple two-step hydrothermal method in conjunction with microwave plasma enhanced chemical vapor deposition (MPECVD) system have been applied to synthesize ZnO nanorods and UNCD films, respectively. Additionally, radio frequency (RF) magnetron sputtering technique was used for the deposition of TFMG. The grown ZnO-based composite-structures in this thesis emphasized controlling the surface morphology of UNCD films beneath ZnO nanostructures. Defect-controlled gas sensitivity were investigated by varying deposition time of UNCD films. It is found that a partially grown UNCD films that were deposited for 7.5 min are the optimized underlayers for hydrogen gas sensing applications. These optimized ZnO-based composite structures exhibit extremely higher gas response than that of the pristine ZnO sensor. The reason for the improvement in sensing performance of the composite-structures have been investigated in terms of the defect-related sensitivity and energy band theory of the heterostructures.
Furthermore, in this thesis, a metal-semiconductor-metal (MSM) UV photodetectors have been fabricated by using a silicon substrate on top of which a silver paste was deposited as an electrode. Bilayer of TFMG/ZnO nanotubes were grown on UNCD films and this ZnO-based composite-structures have enhanced their optoelectronic properties. The nanostructures are characterized by SEM, EDX, TEM, HRTEM, XRD, Raman, and PL spectroscopy. From the characterization of the materials, we observed that these nanostructures emit a ~ 377 nm (UV) usually called the near band edge emission and a band green band related to the defects and surface states at about 600 nm.
The ZnO-based composite-structures are successfully synthesized at low manufacturing temperatures. This dissertation gives an overview of the optical properties of composites of UNCD, TFMG, and ZnO nanostructures and proposes an effective way to enhance the efficiency of optoelectronic and hydrogen gas sensing devices. The fabrication of ZnO-based composite- structures will be useful for the development of next-generation devices.

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