The properties of thin titanium oxide films have been studied

January 21, 2020

A team of scientists from the Research and Education Center "Functional Nanomaterials" of Kant Baltic State University works on the development of new prospective nanomaterials. Together with foreign colleagues they have recently discovered a method for synthesizing titanium oxide (Ti2O3) thin films. Some of the new materials are considerably different from their bulk analogs and show the required conductivity within a wider range of temperatures. In the future they may be used to create effective catalysts that would not depend on temperature. The results of the study were published in the Thin Solid Films journal.

Some titanium oxides are known for their unique properties, such as increased photocatalytic activity (i.e. they effectively use light to speed up chemical reactions). Titanium oxide-based coatings are able to clean themselves under the influence of light. Moreover, they can potentially be used to purify air and water from harmful substances and to desalinate sea water. The same property has recently been found in nanomaterials based on titanium oxide (III). Among other things, titanium oxide (III) is able to change phase from a semiconductor to a metal. In the course of such a phase change it considerably increases its electrical conductivity when heated. Not only the level of conductivity, but even the relative position of atoms is subject to changing. One of the main goals of modern science is to look for structure-property patterns of this kind.

In this work the authors used the magnetron sputtering method to obtain thin titanium oxide films. In other words, the scientists bombarded a titanium target with charged particles. The ablated atoms were deposited on a special substrate and reacted with free oxygen radicals that were formed when oxygen molecules collided with the charged particles. As a result of this reaction thin oxide films were formed. Based on different oxygen concentrations and substrate temperatures, the scientists managed to obtain TiO, Ti2O3 and TiO2 oxides and among them - two completely different Ti2O3 structures that differed by the positions of atoms in the crystal lattice. One had a trigonal lattice similar to that of corundum, and the other one - an orthorhombic lattice that was obtained in its pure form. To do so, the scientists modified the sputtering method by using titanium oxide instead of titanium metal to ablate atoms in a completely oxygenless environment.

The scientists studied the properties of different types of titanium oxide (III) and discovered differences between the orthorhombic film produced by magnetron sputtering and another one described in 2017. The film created by the team remained a semiconductor within the whole range of described temperatures (from -268°? to 300°?) while the previously known structure became a conductor at temperatures above 100?? and a superconductor at temperatures close to absolute zero (below -265°?). It also turned out that the conductivity of the corundum-type film was similar to that of a bulk Ti2O3 heated to 170-200°?. At these temperatures the bulk gains metal properties, while the film preserved high conductivity within the range from -268°? to several hundred degrees Celsius.

"Compared to already known structures manufactured using other methods, our films maintain their properties within a wider range of temperatures. Thus, the corundum-type film remains a metal conductor at low temperatures, and the orthorhombic film - a semiconductor at high temperatures," said Petr Shvets, a Candidate of Physics and Mathematics, and a senior researcher at the Research and Education Center "Functional Nanomaterials".
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The participants of the project also represented Trinity College (Dublin, Ireland), Bundeswehr University Munich (Germany), and Dublin City University.

Immanuel Kant Baltic Federal University

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