Synthesis of periodically structured titania nanotube films and their potential for photonic applications.

8 Nanoscience and nanotechnology have attracted much interest in recent years because materials exhibit novel properties when structured at nanometer dimensions. In particular, titania (TiO 2 ) nanotubes, a unique functional material combined with directionality and dimension, have been regarded as having unlimited potential. Although many successes have already been achieved, for instance, as solar-energy materials, [ 1–5 ] for use in batteries, [ 6–8 ] and as a tool for sensing, [ 9–11 ] signifi cantly less research has focused on the potential of such structures for manipulating photons in terms of an optical material—“photonic crystals” (PCs)—which can open a new range of possibilities for fl exible materials synthesis and new phenomena generation. Three-dimensional (3D) PCs, which have enormous omnidirectional bandgaps, became an emerging fi eld in the 1990s, since when various synthesis approaches have been proposed and demonstrated, such as self-assembly, [ 12–16 ] lithography, [ 17–19 ] and direct laser writing. [ 20–22 ] However, most of these methods bear inherent limitations that may hinder their practical deployment, for instance, complicated and costly micromanipulation techniques, the limited tunability of the stop band, and the diffi culty of integration. Herein, we present a material assembly route that can bridge the gap between TiO 2 nanotubes and photonics. The main benefi t of this method for 3D PC manufacture is its high effi ciency, good quality, and continuous production, thereby promoting a new application of TiO 2 nanotubes for photon manipulation. To fabricate nanostructured TiO 2 arrays, anodization is a simple, straightforward, and self-driven approach among several synthesis strategies, [ 23–25 ] both on Ti foils and Ti thin fi lms. [ 26 , 27 ]