Arrays of randomly oriented zinc oxide (ZnO) nanowires were fabricated on silicon wafers via a simple thermal evaporation method. During the fabrication, the temperature around the substrate was below 500°C. The products were analyzed by conventional means and determined to be single crystals of wurtzite-type ZnO that grew along the c-axis. These nanowires were 10-100nm in diameter and 10-100um in length, suggesting a possible high field enhancement factor. The dependence of the field emission current on the anode-cathode voltage (I-V behavior) of the ZnO nanowire arrays was measured in a lab-built ultrahigh vacuum system with a base pressure of 10-7Pa. After surface cleaning by heat treatment, two characteristic electric fields, under which 10uA/cm2 and 1mA/cm2 current densities were extracted, were measured to be 4.0 and 4.7V/um, respectively. As observed with a transparent anode, emission occurred uniformly over the whole sample surface. A 72h-long test on emission stability was performed under a constant voltage of 2.75kV. The current dropped occasionally to approximately 80% of the initial value during the test owing to the poor adherence of the nanowires to the substrate. These preliminary results have shown the perspective of, as well as a major drawback to, a ZnO nanowire array being developed into a cold electron source to be used in future electronic devices

Arrays of zinc oxide (ZnO) nanowires and nanobelts were synthesized by the thermal evaporation of mixed powders of ZnO and graphite. Neither catalyst nor vacuum environnmnt was involved in the fabrication. For comparison, the ZnO nanowires were grown on a pre deposited transitional ZnO fihi1 on a brass substrate and the ZnO nanobelts were grown directly on a Si substrate. Their field emission properties were systematically measured. Current density of 10uA/cm2 was achieved at the fields of 5.7 and 6.2V/um from the nanowires and nanobelts, respectively. Also, the emission sites were found to distribute uniformly on the whole cathode. In the preliminary test on the stability, the ZnO nanobelts, which were sharp at the tip but wide at the root, exhibited better robustness than the ZnO nanowires. The post test scanning electron microscopy (SEM) observation showed that the degradation of their field emission capability resulted from the breaking of the nanowires, which was tentatively attributed to the resistive heating during the field emission. In contrast, the shedding of the ZnO from the substrate was not so serious as imagined.