Etched Titania Films Could Speed Production of Integrated Photonics

Tokyo Metropolitan University researchers have achieved high-throughput production of thin, ordered, titania through-hole membranes. The method relies on anodization — a process that increases the thickness of a natural oxide layer on the surface of metal parts, in this case mask-etched titanium — and charts a course for industrial production of ordered titania membranes for integrated photonics.

Also known as titanium dioxide, titania is commonly used to make photonic crystals, as the reflective layers in mirrors, as a coating for self-cleaning and anti-fogging surfaces, as a pigment, and as a UV light absorber (often as the active ingredient in sunscreens). In the presence of light, it also accelerates chemical reactions, making it a valuable substance for industrial applications such as helping break down harmful pollutants in the air when embedded in building materials.

Top: A new high-throughput process for making ordered through-hole membranes from titania. Bottom left: Scanning electron micrograph of titania through-hole membrane. Bottom right: Cross-sectional scanning electron micrograph of through-hole membrane. Courtesy of Tokyo Metropolitan University.

Anodization in the newly introduced method, described by Takashi Yanagishita and Hideki Masuda, occurred on mask-etched titanium. The scientists delivered an application of heat to an etched titanium template, on which they grew a titania layer with ordered arrays of holes. The delivery of heat changed the amorphous and disordered titania structure into a crystalline form.

In then applying anodization in a second instance, the researchers were able to convert part of the produced layer back to an amorphous state. They then gradually dissolved the amorphous portion to free the film from the template, without changing the template’s shape. Because disordered and crystalline titania dissolve differently, the team was able to use acid to selectively dissolve the layer that remained in contact with the template. The remaining free titania layer possessed the same through-hole pattern present at the beginning of the process.

Researchers have developed methods for creating porous titania films in laboratory settings, by patterning holes tens of nanometers in width onto thin layers of titania situated in ordered arrays. Though effective, the process does not enable scientists to build the films at scale, necessary to their application in photonics technology.

The team led by Takashi Yanagishita and Hideki Masuda previously introduced a “stamping” method for patterning on titanium metal before using anodization to grow a layer of titania. Holes formed the same pattern as those made artificially on the metal, though the individual stamps dissipated quickly due to the hardness of the titanium.

The new method avoids stamping altogether, instead relying on the delivery of heat.

The template pattern on the metal also remained intact, meaning that the template can be reused after removal of the film. The researchers also tested with different spacings, decreasing physical separation to 100 nm.

The researchers said that the scalability and high throughput of their protocol could expedite the speed with which the advance supports commercial production. They said they are optimistic the method will work with a wide range of additional nanostructured materials, with varying functions.

The work was supported by the Light Metal Educational Foundation and a grant from the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research Program (KAKHENI).

The research was published in RSC Advances (www.doi.org/10.1039/D0RA07650C).