Controlling the growth of nanomaterials is an important step towards device applications. Sang Hyun Lee at Tohoku University and colleagues1 have found a new approach to synthesizing ordered arrays of nanorods in precisely defined areas. Their method uses a simple technique to polarise the substrate, thereby controlling the areas in which zinc oxide nanorods grow.

Zinc oxide crystals can have either zinc-polar or oxygen-polar surfaces. In zinc-polar surfaces, each oxygen atom has three dangling bonds pointing upwards from the surface, and one bond pointing down to a zinc atom. In oxygen-polar surfaces, each oxygen atom has only one dangling bond, pointing upwards. These dangling bond arrangements alter the polarity of the zinc oxide surfaces.

The researchers used magnesium oxide buffer layers to control the polarity of zinc oxide that grows on sapphire substrates. Notably, oxygen-polar zinc oxide films grew on a ~2.7nm thick buffer layer, whereas magnesium oxide buffer layers thicker than 2.7nm produced growth of zinc-polar zinc oxide films.

Fig. 1: Scanning electron microscopy image of zinc oxide nanorod arrays on periodically polarity inverted (PPI) template areas (left) and of zinc oxide nanowall arrays (right).

Lee and colleagues used buffer layers of different thicknesses to create stripes of zinc-polar and oxygen-polar zinc oxide and then coated the whole template with a thin gold film. The different polarities of the underlying zinc oxide layers promoted the growth of different forms of zinc oxide deposited on top of the gold. Nanorods grew only in the zinc-polar areas, whilst thin zinc oxide films were formed over oxygen-polar regions. The final structure consisted of periodic stripes of nanorods (Left hand side of Fig. 1).

Lee envisages a number of potential applications for the nanoarrays. “The periodic nanostructures are applicable to photonic devices such as nanolasers, photonic crystals and extraction layers in light-emitting diodes,” he says. “I also believe that the structures could be useful for remote energy conversion systems and as templates for light harvesters and biosensors.”

For future work, Lee plans to extend the range of nanostructures that can be formed using this method, such as their recent work on the creation of nanowalls (right hand side of Fig. 1). Two-dimensional patterns are another goal, for photonic device applications.