Symmetric and asymmetric growth of ZnO hierarchical nanostructures: nanocombs and their optical properties

One-dimensional nanostructure growth behaves very differently on the ±(0001) polar surfaces of wurtzite ZnO, resulting in asymmetric growth in these two directions. In this report we show that by tuning the degree of supersaturation, the growth asymmetry can either be rationally utilized or broken to obtain single- or double-sided ZnO nanocombs, respectively. The nanowires grown on the two polar surfaces of the double-sided nanocombs are of comparable length, suggesting some kind of symmetric growth, but they are distributed on the two sides in an asymmetric manner. More interestingly, the two sides of the double-sided nanocomb exhibit distinct optical properties. Possible mechanisms are proposed to explain the growth and optical properties of the double-sided nanocombs.