## 3 Results
The field emission and emission-stimulated desorption results from the samples are shown in Figure 4.  
The relationship between the emission current \(I\) and anode voltage \(V\) is shown in Figure 4a. It is visible that the emission current increases exponentially with anode voltage increasing. The turn-on voltage is lower than 300 V, which is corresponding to the turn-on field of 0.5 V/\(\upmu\)m. The low turn-on field should be attributed to the high aspect ratio and small tip radius at the apex. To further analyze  
Figure 3: Energy dispersive spectroscopy (EDS) spectrum of ZNWs. the emission properties, the classic Fowler-Nordheim (FN) law [21; 22] was induced and used to describe the relationship between \(\ln(I/V^{2})\) and \(1/V\), which is shown in Figure 4b. The approximate linear FN plot demonstrates that the current consists of field emission electrons. It is worth noting that the field emission measurement was performed after a proper aging process (maintaining emission current at 10 \(\mu\)A for several tens of minutes). The emission current is not high. This is because the applied electric field was not high enough. On the other hand, the effective emission area was small due to the different morphology and field screening effects. The variation of gas pressure during the field emission process was investigated and shown in Figure 4c,d. Figure 4c shows that the gas pressure increases with emission current increasing. When the anode voltage decreases, the emission current decreases and the amount of residual gas decreases (see Figure 4d). The variation of gas pressure during electron emission process should be attributed to the gas release. In general, the release of the residual gas derives from three aspects. The first is from anode. The second is from internal surface of the vacuum chamber or lead wire. The third is from the sample of ZnO nanomaterials. As the vacuum chamber with two parallel plates had undergone heat treatment and ESD before field emission measurement for removal of contaminants and degassing, it can be inferred that the residual gas released from the sample. The gas desorption from the field emitters may be caused by several effects including thermal effects and surface state variations, all of which is stimulated by the emission process. Therefore, emission-stimulated desorption was defined and investigated in the way of monitoring the variation of pressure. The gas released from the ZnO nanomaterials is beneficial for electron emission since the work function of ZnO will decrease slightly with gas desorption. The effects of adsorption-desorption on the work function of emitters have been explored by [PERSON] et al. [23]. The effects of work function on the field emission properties can be found based on the FN equation. Although the ion pump of the vacuum system was working all the time, there was also some residual gas which was not taken away in time. Therefore, the gas released from the ZnO surface may go through several cycles of adsorption and desorption before being pumped out totally. Figure 4: Field emission and emission stimulated desorption characteristics of ZnO nanomaterials. (**a**) Current–voltage plots of the field emission from ZnO nanomaterials. (**b**) The corresponding Fowler–Nordheim (FN) plots. (**c**) Pressure variation during field emission with anode voltage increasing. (**d**) Pressure variation during field emission with anode voltage decreasing. The stability test was performed, and the corresponding results are shown in Figure 5. Figure 5a shows current-voltage characteristics of the sample of ZnO nanomerials for a 10-times cyclic test. It can be found that the emission current was improved once cycle at a time. This is because the emission-stimulated desorption of the residual gas from ZnO surface will help to reduce the barrier which blocks the electron emission. A low turn-on voltage in the range of 0.5-0.9 V/um was obtained.