The effect of surface defects on the optical and electrical properties of ZnO nanorods

University of Technology Sydney. Faculty of Science.This thesis reports on the effect of native point defects on both the NH₃ gas sensing properties and the surface electronic structure of ZnO nanorods. Low temperature hydrothermal growth at 90°C was utilised to synthesize ZnO nanorods approximately 55 ± 5 nm in diameter. The type and density of intrinsic surface defects on the ZnO nanorod was controlled using post growth annealing in O₂ gas and Zn vapor environments. Low voltage cathodoluminescence (CL) spectroscopy confirmed that the heat treatment process produced different surface defect structures. The as-grown, O₂, and Zn annealed nanorods exhibited broad CL peaks centered at 1.90 eV (YL), 1.70 eV (RL), and 2.44 eV (GL), which were attributed to O interstitials or LiZn deep acceptors, acceptor-like VZn complexes, and donor-like VO related centers, respectively. The first part of this thesis focuses on the influence of deep level surface defects on NH₃ gas response of ZnO nanorods. Electrical and gas sensing measurements revealed that the NH₃ gas sensitivity was 4.1 for the as-grown (YL), 22.6 for O₂ anneal (RL), and 1.4 for Zn vapor anneal (GL) samples. Hydrogen plasma treatment quenched the RL and inverted the ammonia electrical response due to the incorporation of shallow hydrogen donors. Changes to the gas sensing response were attributed to a shift in the ZnO Fermi level position relative to the ammonia gas chemical potential due to the formation near surface donor or acceptor centers. In the second part of this thesis the effect of native point defects on the surface electronic structure of all ZnO nanorod samples was studies utilizing X-ray photoemission spectroscopy (XPS), X-ray Absorption Near-Edge Structure (XANES), electron spin resonance (ESR), low voltage CL spectroscopy, sub band gap photoluminescence (PL) spectroscopy and electrical I-V measurements. Correlative characterization measurements established that the degree of surface band bending and surface conductivity is controlled by the amount of adsorbed ambient gas species (O₂, H₂O and OH), which is mediated by type and concentration of intrinsic surface point defects. The last part of this thesis concentrates on the chemical origin of RL peak and its optical properties. A wide range of analysis techniques, including PL and CL spectroscopy, XANES and ESR was used to comprehensively characterize the RL emission. From these data, the RL has been assigned to highly lattice coupled VZn-related acceptor-like center. No correlation was found between the RL and nitrogen impurities

Development Of Nanoparticle Based Alternative for Metal Additive Manufacturing

NPS NRP Executive SummaryProject Summary: The research conducted had the objective of developing an additive manufacturing (AM) strategy to produce metal or alloy parts through the use of layer-by-layer extrusion of small particle paste formulations along post-processing steps. That is, we aimed to hybridize additive manufacturing approaches with known powder metallurgy (PM) processes to produce alloys of naval relevance. The work that we are reporting herein generated metal parts using, instead of expensive direct sintering equipment (metal tri dimensional (3D) printer), a conventional 3D printer (as the ones used for polymeric filaments). The later was equipped with an extruder capable of deliver paste composed of metal nano or micron particles and binding media that evaporated after the printing operation was completed. The 3D parts produced using those paste formulations had similar characteristics that the ones observed in green specimens generated by PM. Using the same type of post-treatments that AM and PM routes employ, such as annealing and hot isostatic pressing (HIP), we were able to generate 3D specimens of NiTi and NiCu alloys. We demonstrated that the new process could successfully generate solid specimens, which, after HIP operations, showed mechanical robustness. This new approach could be easier to adapt than laser or e-beam sintering routes and has potential to be used for metal/alloy parts that do not require stringent load bearing specifications.N4 Materials Readiness and LogisticsNPS-18-N337-AApproved for public release; distribution is unlimited

Tailoring deep level surface defects in ZnO nanorods for high sensitivity ammonia gas sensing

© 2014 American Chemical Society. The influence of deep level surface defects on electrical and gas sensing properties of ZnO nanorods NH3(g) sensors was studied. ZnO nanorods 50-60 nm in diameter were synthesized via low-temperature hydrothermal growth at 90°C on sapphire substrates. The as-grown nanorods exhibited a cathodoluminescence (CL) peak centered at 1.90 eV (YL), attributed to LiZn deep acceptors or O interstitials. Subsequent annealing in O2 at 1 atm and Zn vapor at 650°C produced broad CL peaks centered at 1.70 eV (RL) and 2.44 eV (GL), respectively. The RL and GL have been ascribed to acceptor-like VZn and donor-like VO related centers, respectively. Electrical and gas sensing measurements established that the NH3 gas response sensitivity was 22.6 for O2 anneal (RL), 1.4 for Zn vapor anneal (GL), and 4.1 for the as-grown (YL) samples. Additionally, treatment in H-plasma quenched the RL and inverted the NH3 electrical response due to the incorporation of H donors. Changes in the gas sensing response are explained by a shift in the position of the ZnO Fermi level relative to the chemical potential of NH3 gas due to the creation of near surface donor or acceptors. These data confirm that ZnO nanorods arrays can be tailored to detect specific gas species. (Chemical Equation Presented)