Low temperature thermal evaporation growth of aligned ZnO nanorods on ZnO film: a growth mechanism promoted by Zn nanoclusters on polar surfaces

ZnO aligned nanorods have been obtained on ZnO films by vapour phase at relatively low temperature (<500 degrees C). In this temperature range, which is for example interesting for glass-compatible processes for solar cell production, it is generally impossible to obtain a standard vapour-solid growth of these ordered nanostructures without the use of metal-organic precursors or metal catalysts. However, it is pointed out that, by means of a deposition of Zn nanoclusters on the polar surface of ZnO film grains, it is possible to obtain vertically aligned ZnO nanorods even at 480 degrees C. Zn nanocluster formation and rod growth can be achieved in a few minutes in a single process, without introducing any foreign element that might produce undesired material contamination and affect the physical properties of these nanostructures. The growth mechanism is discussed in detail, making comparisons with the results achieved on different substrates and under different growth conditions. Thanks to the comprehension of the growth mechanism, very uniform deposition of nanorods has been obtained on two square centimetres substrates in a small laboratory-scale reactor.