The emerging technologies and production methods for the ZnO nanostructures application such as Ar and O2 plasma etching, spin coating, ultra-fast chemical solution growth, and dense nanorod array, are introduced at first. Some important effects, ZnO nanostructures growth mechanisms, control ZnO nanostructure morphology and ZnO nanostructures application are organized, compared, and discussed as well.
The effects of emitter morphologies on field emission characteristics and mechanism of chemical and plasma etching are studied in Chapter 2. The sharp tip structured ZnO nanorods by employing etching process. Through plasma etching, the 110o tip angle ZnO nanorods were obtained. In order control emitter morphologies, we proposed two-step etching process, a combination of chemical etching process and plasma etching process were able to easily control tip angle of the nanorods. After such plasma etching and two-step etching processes, the field emission properties of ZnO vertically aligned nanorod emitters were significantly enhanced (turn-on field:
1.70 V/μm) and the stability of their turn-on fields was improved (turn-on field:
1.60±0.41 V/μm for 5000 s).
The 85o tip angle ZnO nanorods exhibited lower turn-on field and field emission stability for 5000 s, however, the large dispersion of turn-on field and unknown environmental stability in tip angle (85o) ZnO nanorods limit their development in realizing the practical field emission application. In Chapter 3, the effects of oxygen plasma treatments, at various etching times on the field emission characteristics (turn-on field dispersion and environmental stability) of ZnO nanorod emitters are
demonstrated. The synthesized process is simple, low cost and enable to large scale production. The (002) orientation and tip-structured morphology of the nanorod emitters were proved by XRD pattern, FE-SEM and TEM observations. The PL spectrum reveals that the green emission peak that occurred form oxygen vacancies after oxygen plasma treatment is lowered and obviously this phenomenon is due to reduced the concentration of oxygen vacancies of the nanorod emitters. The tip structured (100º) nanorod emitters also can be formed after oxygen plasma treatment.
Thus, through oxygen plasma treatment, the tip (100º) structured ZnO nanorod emitters have enhanced performance including lower turn-on (2.42 V/μm) and threshold fields (3.61 V/μm), higher field enhancement factor (2268), good stability characteristics over 104 s (turn-on field, threshold field and field enhancement factor are 2.34±0.21 V/μm, 3.87±0.18 V/μm and 2252.8±107.5, respectively) at room temperature. In addition, the tip structured ZnO nanorod emitters can successfully and stably operate up to 100 ℃ without notable degradation of emission properties.
Therefore, oxygen plasma treatment could be used improve electron field emission and light emitting applications.
Chemical etching and plasma etching are reported to improve and stabilize field emission behavior of ZnO nanorod emitters, but it requires to carefully controlling the chemical etching conditions such as solution concentration, temperature and etching time to form the convex morphology, which is not appropriate for the field emission application. In Chapter 4, ZnO nanotip (20º) emitters were successfully fabricated by employing the combination of chemical solution growth and oxygen plasma treatment.
The synthesized process is simple, fast, lower cost and enable to large scale production. The nanotip emitters exhibit the (002) highly preferred orientation and small tip angle morphology proved by XRD patterns, FE-SEM and TEM observations.
The nanotip emitters have turn-on field of 1.07 V/μm, threshold electric field of 1.63
V/μm, field enhancement factor of 4735 and exhibit stable and reproducible field emission properties at 25-100 ℃. Such good performances is attributed to reduced oxygen vacancy concentration, small tip angle of 20º, smooth edge surface of the emitter and a better crystallinity obtained for ZnO nanotip (20º) emitters after the oxygen plasma etching process. The ZnO nanotip emitters have high potential for application in electron field emission and light emitting devices in the future.
The effects of SnO2 thickness of ZnO-SnO2 core-shell nanowires on gas sensor characteristics and mechanism of adsorption and desorption are studied in Chapter 5.
The ZnO-SnO2 core-shell nanowires had a single crystal ZnO cores and amorphous SnO2 shell layer by two-step chemical growth. Through spin coating, the SnO2 shell layer uniformly coated onto the ZnO nanowires and the various SnO2 shell layer thickness were obtained by adjusting spin coating times. The H2 gas sensors were fabricated by these core shell nanowires using silver layer as the electrode. High sensitivity (80 %) at both low H2 concentration (25 ppm) and low working temperature (250 ℃) was obtained. The core shell nanowire sensor with good sensitivity has high potential in gas sensor application.
The effects of length of ZnO nanorods on PET substrate for UV photodetector application and mechanism of adsorption and desorption are studied in Chapter 6. We successfully develop a simple method to fabricate high performance ZnO UV photodetector on the flexible PET substrate. Through chemical solution method, the ZnO nanorods uniformly grow on the PET substrate and the various ZnO nanorods lengthes are obtained by adjusting growth time. The ZnO-10/PET structure reveals good mechanical stability through bending test. Upon UV illumination, the ZnO-10 nanorods photodetector exhibits high sensitivity at low UV power density (25 μW/cm2), fast recovery time (120 sec), good orientation properties, reproducible photoresponse (20 cycles) and multi-level photoresponse. The ZnO-10/PET
photodetector made by a simple process has high potential for practical applications.
The resistive switching behavior of Ga-doped ZnO (GZO) nanorod thin films with various Ga/Zn molar ratios are investigated in Chapter 7. We successful fabricated of compact GZO nanorod thin film devices to avoid the short circuit between top and bottom electrodes without any embedded process and demonstrate the reversible and stable bipolar RS characteristics. The formation and rupture of conducting filaments in the grain boundaries between GZO nanorods and ZnO seed layer can accurately explain the switching mechanism of the GZO nanorod thin film devices. A 1D nanorod provides a straight and extensible conducting filament along the nanorod side wall, resulting in stable RS behavior. The results of this study confirm that the compact GZO nanorod thin films structure is a promising candidate for RRAM applications.