Optoelectronic properties of density-controlled ZnO nanopillar arrays

An effective method of controlling the density of ZnO nanopillars (ZnO-NPs) by using the self-assembled silver nanoislands as the growth catalyst is demonstrated. We were able to vary the density of the ZnO-NPs to within the range of ∼10-30 μm ^-2 by properly manipulating the size distribution of the silver nanoislands partially covering the Si substrates. Continuous silver film can also be used as the catalyst to facilitate the growth of ZnO islands and pillars on Si substrates, albeit with much less control of the resultant density and morphological appearances. The field-emission measurements indicate that the performance of field emission for ZnO-NPs can be improved markedly by reducing the density of the ZnO-NPs. The turn-on field of the low-density ZnO-NPs was as low as 2.39 V/μm with a corresponding current density of 10 μA/cm ² and a field enhancement factor (β value) of ∼3500. The enhancement of field-emission characteristics is attributed to the much reduced field screen effect by properly controlled density of the ZnO-NPs.