Controlling the size and alignment of ZnO microrods using ZnO thin film templates deposited by pulsed laser ablation

The production of dense arrays of well aligned ZnO nano- and microrods with a controllable distribution of diameters is demonstrated. The rods were grown using a hydrothermal method, on pre-deposited ZnO thin films exhibiting a range of different grain sizes. These template ZnO thin films were deposited by pulsed laser ablation, at 193 nm, in a low background pressure of O-2; average grain sizes ranged from similar to10 nm (room temperature deposition) to similar to50 nm when deposited at a substrate temperature (T-sub) of 500 degreesC. The morphology of the ZnO microrod arrays grown onto these ZnO thin films was found to depend on the T-sub used during film deposition. Increasing T-sub resulted in a lower density of larger nanorods, with a more pronounced [0001] alignment. The diameters of the rods produced are typically an order of magnitude greater than the grain size of the template ZnO film. Thus the rods cannot be a direct continuation of the grain structure of the template films. Rather, we suggest that the rod diameter is controlled by the density of sites at which the initial nucleation occurs, which is expected to be higher on the smaller grain size films deposited at lower temperatures. The factor controlling the final size of the rods is thus the available free space into which they can grow, which is smaller at the higher nucleation densities. The increased extent of [0001] texturing of the thin film templates deposited at higher T-sub is proposed as the reason for the improved [0001] alignment of rods grown on these templates.