Ultrathin seed-layer for tuning density of ZnO nanowire arrays and their field emission characteristics

We report on how to tune the density of zinc oxide (ZnO) nanowire arrays in a wide range (more than 5 orders of magnitude) in a low temperature (80 degrees C solution-phase growth process. A model based on the coexistence of nanowire growth and etching of the ultrathin (< 3.5 nm) ZnO seed-layer was proposed to explain the effect. It was demonstrated that when the seed-layer thickness changes from 1.5 to 3.5 nm, the nanowire density increases from 6.8 x 10(4) to 2.6 X 10(10) cm(-2). This significant variation of density with the seed-layer thickness was found to happen only when the layer thickness is within a few nanometers. If it is too thick (> 3.5 nm), the variation is very narrow, and if it is too thin (< 1.5 nm), no nanowires can grow. In addition, the density variation was accompanied by the change of both diameter and height of the nanowires, which leads to a change in aspect ratio. Both changes in density and aspect ratio were found to obviously affect the field emission characteristics. It was demonstrated that optimal conditions can be found to grow ZnO nanowire films with better field emission characteristics.