Template technique controls crystalline structure of titanium dioxide for photonic uses

Raleigh, NC--Researchers from North Carolina State University have developed a new technique for controlling the crystalline structure of titanium dioxide at room temperature.¹ The development should make titanium dioxide more efficient in a range of applications, including photovoltaic cells, hydrogen production, antimicrobial coatings, smart sensors and optical communications technologies.

“We have now developed a technique that precisely controls the phase, or crystalline structure, of titanium dioxide at room temperature—and stabilizes that phase, so it won’t change when the temperature fluctuates,” says Jay Narayan, a professor of materials science and engineering at NC State and coauthor of a paper describing the work. "This process, called phase tuning, allows us to fine-tune the structure of the titanium dioxide so that it has the optimal structure for a desired application.”

Two phases of TiO₂

Titanium dioxide (TiO₂) most commonly comes in one on of two major phases—rutile, or anatase. The arrangement of atoms dictates the material’s optical, chemical, and electronic properties. As a result, each phase has different characteristics. The rutile phase is better suited for use in photovoltaic cells, smart sensors, and optical communication technologies, and smart sensors. The anatase phase has characteristics that make it better suited for use as an antibacterial agent and for applications such as hydrogen production.

Traditionally, it has been a challenge to stabilize TiO₂ in the desired phase, as the material tends to transform into the anatase phase below 500 °C, and into the rutile phase at temperatures above 500 °C, notes Narayan.

The NC State process starts with a widely available sapphire substrate that has the desired crystalline structure. The researchers then grow a template layer of titanium trioxide (Ti₂O₃) on the substrate. The structure of the Ti₂O₃ mimics the structure of the sapphire substrate. The TiO₂ is then grown on top of the Ti₂O₃ template layer.

The structure of the TiO₂ differs from the Ti₂O₃ -- but is dictated by the structure of that template layer. This means that one can create the TiO₂ in any phase simply by modifying the structure of the Ti₂O₃ and sapphire substrate. This works because of a process called domain-matching epitaxy (DME). In DME, the crystalline lattice planes in the template layer line up with the lattice planes of the material being grown on that template.

The researchers have also demonstrated how this technique can be used with silicon computer-chip substrates, which can be integrated into electronics such as smart sensors.

REFERENCE:

1. M. R. Bayati et al., Applied Physics Letters, published online June 20, 2012.