Effect of adsorbates on field-electron emission from ZnO nanoneedle arrays

We studied the influence of adsorbates on field emission (FE) properties of well-aligned ZnO nanoneedle arrays with varying initial electric field, vacuum gap, and ambient pressure. The FE current exhibits hysteresis upon a loop of applied voltage. On applying an initial electric field of 3.85×106 V/m, the turn-on voltage increases ∼50%, Fowler-Nordheim plots show better linearity, as well as the hysteretic behavior can be annihilated. It is found that the FE current depends on the vacuum gap and the ambient pressure. The FE current increased sensitively with increasing the ambient pressure P when P<∼7×10−5 Pa, and it saturates in the pressure range of 7×10−5–3×10−4 Pa above which the FE current drops. The phenomena can be explained by the adsorbate effects. The adsorbate states modify the effective work function of the samples, and two emission regimes, adsorption-controlled and desorption-controlled processes, were proposed. Our results are useful for practical applications as the I–V characteristics can be affected by environmental parameters, especially the ambient pressure.