The new technique involves using a focused laser beam to directly write on the surface of thin films and membranes made of amorphous TiO2 nanotubes. The heat from the beam can be used to control the local crystalline structure of the films, which in turn changes the refractive index of the materials.

The researchers, led by Ion Tiginyanu at the Technical University of Moldova and the Academy of Sciences, Moldova, recently found that they could make titania nanotubular membranes by anodizing titanium sheets below 0 °C in an electrolyte containing ethylene glycol and hydrofluoric acid. This technique produced ordered arrays of nanotubes arranged in a 2D hexagonal lattice.

Tiginyanu and colleagues have now discovered that they can make two different nanotube phases – anatase and rutile – by exposing the titania nanotube films and membranes to a focused laser beam. The laser treatment generates a combination of the two phases, or just the anatase phase. The good thing is that the writing appears to be permanent, and it is very difficult to modify the anatase phase once it is formed.

The team, which includes researchers from the University of New South Wales in Australia and the University of Bristol in the UK, found that nanotube structures with a rutile core and anatase shell are produced at laser beam power densities higher than 0.4 × 105 Wcm–2. However, they need to keep the laser beam at power densities below 1 × 105 Wcm–2 to preserve the initial morphology of the nanotube matrix.

“The amorphous matrix changes its phase to rutile or anatase thanks to the heat from the absorbed laser light,” explains Tiginyanu. “The central part of the laser spot, which is the hottest, generates the rutile structure while the peripheral areas of the spot (where the temperature is a little lower) is responsible for creating the anatase phase.” The researchers characterised the two different phases using micro-Raman and micro-cathodoluminescence scanning experiments and proved that the two structures had different refractive indices.

The modified titania nanotubes might be used to make novel optical waveguides, splitters, ring resonators and other photonic structures, says Tiginyanu. “Such maskless direct writing of optical waveguides in thin films and membranes consisting of orderly distributed TiO2 nanotubes could considerably widen the potential applications of these nanotubular structures,” he told nanotechweb.org.

The researchers report their work in the Journal of Applied Physics.

Further reading

FIB mills single-nanowire plasmonic gratings (Jun 2012)
Combined process advances silicon nanostructuring (Apr 2010)
FIB patterning controls position and dimensions of quantum dots (Apr 2012)
Surface writing produces designer nanostructures (Apr 2012)
Temperature orders TiO2 nanotubes (Apr 2010)
Nano-roof reveals dislocations (Jan 2011)