Ion Tiginyanu's team at the Technical University of Moldova and the Academy of Sciences, Moldova, began by anodizing titanium sheets below 0 °C in an electrolyte containing ethylene glycol and hydrofluoric acid. This technique produces self-organized surface nucleation layers with ordered arrays of nanochannels distributed in a 2D hexagonal lattice.

Usually, such electrochemical processes result in random pitting of the surface when applied to semiconductor wafers or metal sheets. Ordering comes later, thanks to interactions between growing pores or tubular structures and the regular structure is effectively "buried" under the overlying disordered nucleation layer.

Micro-cavity effect
Tiginyanu and colleagues discovered that each pore at the surface represents a starting point for the subsequent growth of a double-walled titania nanotube. These individual nanotubes can then be easily detached from the network and studied separately. Indeed, the Moldova team found that individual titania tubes luminescence thanks to a micro-cavity effect, where light follows closed trajectories inside the tubular structures. This is the first time that such an effect has been observed in nanotubes and means that these structures might come in useful for micro-laser applications.

The researchers also found that they could vary the inner diameter of the nanotubes from around 10 nm to more than 250 nm, by increasing the electrolyte temperature. Being able to vary the diameter of nanotubes is important for tailoring their characteristics, explains Tiginyanu.

"The new findings are very promising for expanding the applications areas of titania nanotubular structures," he told "For example, they might be used to develop cost-effective photonic elements based on negative refractive index materials – in particular flat and concave focusing devices with super-resolution."

The work was published in Physica Status Solidi – Rapid Research Letters.