Fine-tuning: NCSU/NSF group learns how to control phase formation of titanium dioxide at room temperature
We have written several times in the past about the work of the NSF Center for Advanced Materials and Smart Structures at North Carolina State University and its intriguing work on oxide electronics. Recently, researchers at CAMSS, led by Jagdish (Jay) Narayan announced that they have devised a way to use titanium trioxide as a template for controlling the crystalline structure of titanium dioxide at room temperature. Their hope is that the new technique will make TiO2 more efficient in a range of electronic applications.
Controlling whether the TiO2 is in the anatase or rutile structure has great importance, depending on how it is to be used. The atomic structure of each phase determines the material’s optical, chemical and electronic characteristics. Rutile TiO2 is particularly important for electronic applications, and preventing the formation of the anatase phase would be a big boon to improving electronics efficiencies.
In an NCSU news release, Narayan notes that achieving the desired phase has bedeviled engineers. He says, ”Traditionally, it has been a challenge to stabilize titanium dioxide in the desired phase. The material tends to transform into the anatase phase below 500°C, and transform into the rutile phase at temperatures above 500°C. … 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. 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.”
In essence, the group leveraged knowledge of domain matching epitaxy to use a transient layer of Ti2O3 (on a sapphire substrate) to create a template for forming a rutile monolayer of TiO2. Actually, several monolayers of TiO2 are grown on the sapphire via pulsed laser deposition until a “critical thickness” is reached and the TiO2/Ti2O3/sapphire interfaces are formed. According to the NCSU release, the group attributes the formation of the Ti2O3 transient layer to the symmetry mismatch between the tetragonal structure of TiO2 and hexagonal structure of the sapphire.
In a paper on their work published in Applied Physics Letters (“Domain epitaxy in TiO2/α-Al2O3 thin film heterostructures”) the researchers note that “the matching of lattice parameters produces strain in the film and to some extent in the substrate. The pseudomorphic growth of the film continues until a “critical thickness” is reached where the strain energy becomes large enough to trigger the nucleation of misfit dislocations. These dislocations nucleate at the film surface and glide to the interface to relieve the misfit strains.
NCSU says these findings may be extremely useful in the development of photovoltaic cells, hydrogen production and optical communication technologies, and apparently 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.
Besides Narayan, the paper (doi:10.1063/1.4729937) was coauthored by M.R. Bayati and R. Molaei, PhD students at NC State; R. Narayan, a professor in the joint biomedical engineering department of NC State and the University of North Carolina at Chapel Hill; H. Zhou, a postdoctoral researcher at NC State; and S.J. Pennycook of Oak Ridge National Laboratory. The research was funded by the National Science Foundation.