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March 13th, 2012

ORNL group finds new way to tune band gaps in metal oxide films

Published on March 13th, 2012 | By: pwray@ceramics.org

ORNL’s material scientists developed a synthesis strategy for discovering novel complex-oxide thin films for stronger solar light absorption. Credit: ORNL.

The research into metal oxide thin films for electronic applications continues to bear fruit. Last week I wrote about successful efforts to convert and a single layer of nickel oxide from its natural p-type state to an n-type, essentially “sketching” electronics into the top layer of a homogenous thin film with a laser. It’s hoped that this accomplishment will usher in new era for electronic fabrication and durability.

This week, the news is about successful efforts of optimizing — by a significant amount — the band gap in metal oxide thin films, a development that should open new applications in optoelectronic devices and energy materials.

One of the reasons metal oxides are attractive is because they tend to perform well in harsh, high-temperature environments and can have intriguing ferroelectric properties. A group of researchers in the Materials Science and Technology Division at the Oak Ridge National Lab have been working on trying to find a way to optimize the band gap in metal oxides without compromising the mechanical and physical properties. Now they report that they have learned how to build films layer-by-layer of ferroelectric bismuth titanate with site-specific substitutions of lanthanum cobaltite to control and gain a big improvement in band gap.

Controlling the band gap is key because it determines the upper wavelength limit of light absorption in a material. Achieving wide band gap tunability is highly desirable for developing, for example, more efficient photovoltaic cells that have stronger solar light absorption.

The ORNL group’s layer-by-layer growth technique was developed by Ho Nyung Lee, and he and his colleagues have achieved a 30 percent reduction in the band gap of bismuth titanate. Their findings are outlined in Nature Communications in the paper, “Wide band gap tunability in complex transition metal oxides by site-specific substitution,” (doi:10.1038/ncomms1690).

In this paper, the authors note that “site-specific substitution with the Mott insulator lanthanum cobaltite, its band gap can be narrowed as much as 1 electron volt, while remaining strongly ferroelectric. We find that when a specific site in the host material is preferentially substituted, a split-off state responsible for the band gap reduction is created just below the conduction band of bismuth titanate. This provides a route for controlling the band gap in complex oxides.” [emphasis added]

In an ORNL news release, Lee says, “Our approach to tuning band gaps is based on atomic-scale growth control of complex oxide materials, yielding novel artificial materials that do not exist in nature. This ‘epitaxy’ technique can be used to design entirely new materials or to specifically modify the composition of thin-film crystals with sub-nanometer accuracy.”

Using these methods, the group says that the band gaps in complex metal oxides should be able to be continuously controlled over 1 electron volt by the site-specific alloying. A patent is pending for this technology.

In the news release, Michelle Buchanan, associate lab director for ORNL’s Physical Sciences Directorate, says, “This work exemplifies how basic research can provide technical breakthroughs that will result in vastly improved energy technologies.”

Besides improved solar cells, ORNL says that the techniques also should lead to gains in light emitting diodes, displays and other electronic devices.

Optical measurements were performed in part at the Center for Nanophase Materials Sciences, a DOE-BES user facility at ORNL.

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