Tuning the lateral density of ZnO nanowire arrays and its application as physical templates for radial nanowire heterostructures

The lateral density of ZnO nanowire arrays grown with pulsed laser deposition (PLD) can be tuned from 1 to 10−2 μm−2 by introducing a ZnO nucleation layer and optimizing the distance between the substrate and the ablated target. High-density (∼10 μm−2) nanowire arrays can be grown on sapphire substrates with or without gold catalysts. However, if a ZnO wetting layer was adopted, the density of ZnO nanowires could be controlled with high reproducibility. The decreasing growth density is attributed to a competition between the two-dimensional film epitaxy and one-dimensional nanowire growth. The dependence of nanowire density on the substrate–target distance mainly arises from the expansion dynamics of the plasma plume and the chamber geometry. Using low-density nanowires as templates, a general PLD route was developed to grow radial nanowire heterostructures. Here we demonstrate MgZnO/ZnO/MgZnO nanowire quantum wells and ZnO/ZnO:P core–shell nanowire p–n junctions.