In this thesis, ZnO nanostructures with controlled morphologies and qualities were synthesized by hydrothermal and chemical vapor deposition (CVD) methods. The optical properties of individual ZnO nanowires (NWs) were investigated by high resolution near-field scanning optical microscopy. The electrical transport properties of individual ZnO NWs were characterized by fabricating individual NW field-effect transistors (FETs) via the standard photolithography and electron beam lithography techniques.

The morphologies and crystal structures of as-grown ZnO nanostructures were first analyzed by scanning electron microscopy and transmission electron microscopy. ZnO NW arrays grown by the hydrothermal method can form on flexible substrates in large scale. However, hydrothermally grow...[ Read more ]

In this thesis, ZnO nanostructures with controlled morphologies and qualities were synthesized by hydrothermal and chemical vapor deposition (CVD) methods. The optical properties of individual ZnO nanowires (NWs) were investigated by high resolution near-field scanning optical microscopy. The electrical transport properties of individual ZnO NWs were characterized by fabricating individual NW field-effect transistors (FETs) via the standard photolithography and electron beam lithography techniques.

The morphologies and crystal structures of as-grown ZnO nanostructures were first analyzed by scanning electron microscopy and transmission electron microscopy. ZnO NW arrays grown by the hydrothermal method can form on flexible substrates in large scale. However, hydrothermally grown ZnO NWs had high concentrations of native defects, which was revealed by the strong defect-related emission in the photoluminescence (PL) spectra and the high electron concentration. ZnO NWs from the CVD method can grow on metallic or ZnO seeds coated on silicon substrates by the vapor-liquid-solid and vapor-solid mechanisms. The CVD grown ZnO NWs showed a high quality as revealed by the strong ultraviolet (UV) emission in the PL and a relatively low electron concentration.

To improve the optical and electrical properties of hydrothermally grown ZnO NWs, we developed a sealed post annealing treatment method. By the treatment, the doping concentration, optical and electrical properties of hydrothermally grown ZnO NWs can be effectively modified without changing the structures of ZnO NWs. The present treatment effectively compensated the native point defects in hydrothermally grown ZnO NWs.

By investigating individual bent ZnO NWs, we found that the strain significantly changed the PL properties of the bent NWs. Then we proposed a model to qualitatively explain the redshift in the PL. In this model, the strain induced piezotronic effect plays an important role in the PL of the bent NWs. The piezoelectric field largely modified the spatial distribution of photo-excited carriers in bent ZnO NWs. This effect, together with strain-induced changes in energy band structure due to the piezoresistive effects, resulted in a net redshift of free exciton PL emission from the bent region of NWs. The bending-induced variations in phonon energy were also revealed by the study of longitudinal optical phonon-exciton interaction.

To explore the applications of ZnO nanostructures in optoelectronic devices, we fabricated ZnO quantum dots (QDs) decorated graphene based UV photodetectors. The hybrid nanostructures took advantages of ZnO in UV sensing and graphene in high carrier conductivity. The ZnO/graphene hybrid photodetectors had a high UV photoconductive responsivity and gain. By studying the working mechanism, we found the oxygen-assisted charge transfer between ZnO QDs and graphene played an important role in the performance of the devices.