## 3 Results  
Various ZnO nanostructures were obtained on silicon substrates by using water assisted zinc source method. The representative SEM images (Figure 2) taken from different area of the substrate have revealed the different morphologies of ZnO nanostructure arrays. The ZnO nanowires (ZNWs) (see Figure 2a) with around 12.7 \(\upmu\)m long were synthesized under a more traditional VSL process. The ZNWs were distributed in very high coverage density. Hexagonal prismatic nanorods of ZnO (see Figure 2b) have orientation almost perpendicular to the substrate, and are aligned in higher coverage density. The ZnO nanoneedles (see Figure 2c) with around 5.6 \(\upmu\)m length and diameter of 102 nm sharp head were also synthesized due to a relatively high oxygen concentration in the local area. The ZnO crystals would grow into thicker needle-like structures when the concentration of oxygen is higher. ZnO comblike structures (see Figure 2d) were also created due to the polar surface growth [18]. The variation of crystal growth should be attributed to the spatial change of Zn and O\({}_{2}\) concentration distribution inside the quartz vial. It can be found from the direction of gas flow (see Figure 1) that the gas pressure is different at the different location in the small quartz vial. The vapor inside the quartz vial would be brought out finally. Therefore, the closer the substrate gets to the export of the quartz vial, the more products of ZnO nanostructures will be obtained. Figure 2: Scanning electron microscopy (SEM) images of ZnO nanomaterials grown on silicon substrate. (**a**) ZnO nanowires (ZNWs), (**b**) ZnO nanorods with hexagonal crystal structure, (**c**) ZnO nanoneedles, (**d**) ZnO nanocombs. The vapor concentration of zinc and oxygen plays an important role in the synthesis of ZnO nanomaterials which is determined by the relative rate of surface reaction with respect to the mass transfer. At the part of the substrate closed to zinc source, where oxygen had the lowest but zinc vapor had the highest concentration, the surface-reaction-limited situation dominated the reaction. In other words, the mass transfer rate was higher, which is beneficial for the growth of nanostructure with high aspect ratio. At the area of the substrate closed to the port of the quartz vial, a smaller concentration difference between zinc vapor and oxygen leaded to a higher nucleation rate which made nanowires grow into branch. The mass-transfer-controlled process dominated the reaction. Conclusively, to obtain certain kind of ZnO nanostructure, it is necessary to balance a proper mass-transfer process and surface-reaction effects. The EDS spectrum of the ZNWs is depicted in Figure 3. Only the peaks associated with Zn and O atoms are seen in the EDS spectrum demonstrated qualitatively that the nanomaterials are indeed ZnO. Field emission properties of the ZnO nanomaterials were measured using a two-parallel-plate configuration in an ultrahigh vacuum (UHV) chamber with a pressure better than 2 \(\times\) 10\({}^{-6}\) Pa at room temperature. The sample with different morphologies was loaded on a stainless-steel holder using conducting glue to act as cathode. Another parallel nickel disk with diameter of 40 mm served as the anode at a fixed cathode-anode spacing of 0.6 mm during all the measurements. Both of the steel holder and nickel anode plates had been undergone heat treatment (\(\sim\)400 \({}^{\circ}\)C) before mounting to remove contaminants and degassing. Before field emission measurement, vacuum bakeout (\(\sim\)200 \({}^{\circ}\)C) was performed for degassing from the inside surface of the chamber and lead wires, then electron stimulated desorption (ESD) [19; 20] was used to outgassing from anode again. To examine the stability of the ZnO nanomaterials, field emission characteristics were measured two years later. A dc voltage was applied between the sample and anode to supply a strong electric field to extract electrons out of the ZnO nanomaterials under tunneling effect.