Researchers led by T. Venky Venkatesan (first row, center), director of NUSNNI, have uncovered extraordinary properties of the semiconductor material strontium niobate. Photo: National University of Singapore.
Researchers led by T. Venky Venkatesan (first row, center), director of NUSNNI, have uncovered extraordinary properties of the semiconductor material strontium niobate. Photo: National University of Singapore.

Researchers from the National University of Singapore (NUS) have recently uncovered some extraordinary properties of strontium niobate, a unique semiconductor material that displays both metallic-type conduction and photocatalytic activity.

The novel properties were revealed in two studies conducted in collaboration with researchers from the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, and reported in papers in Nature Communicatons. They herald exciting opportunities for the creation of novel devices with unprecedented functionalities and a new family of photocatalytic materials.

“The key to these successful studies was the ability of the NUS Nanoscience and Nanotechnology Institute (NUSNNI) team to produce high-quality crystalline films of these materials that were then studied by a variety of measurements to provide scientific clues on how such materials perform under varying conditions,” said Wan Dongyang, a researcher from NUSNNI who was involved in both studies.

In the first study, a team led by assistant professor Andrivo Rusydi and director of NUSNNI T. Venky Venkatesan confirmed that strontium niobate is highly metallic due to a very large population of electrons in the material, which is typical of most metals. However, the team also found that strontium niobate is transparent at most photon energies, which is an exceptional property that is unlike most metals. Utilizing spectroscopic techniques, the research team discovered that this unique property arose from an intrinsic plasmonic absorption.

“From our studies, we found that this material is a semiconductor with a large bandgap of four electron volts,” said Teguh Citra Asmara, first author of the paper and a postdoctoral researcher at NUSNNI. “Based on our understanding of semiconductors and this material’s strong metallic behavior, we did not expect this material to absorb any visible photons, so the results we found are indeed surprising.”

“Plasmons are resonant oscillations of a collection of electrons and typically occur in a metallic solid,” explained Venkatesan. “Under the right conditions, photons can cause these plasmons to be excited in a solid and in this process the solid absorbs the photon energy. Before our team figured this out, this material was thought to consist of a smaller bandgap, of the order of two electron volts, and a secondary band above of comparable energy.”

In addition, the research team discovered a new family of plasmons that occurs at multiple frequencies. This new family of plasmons is observed even though strontium niobate is not a conventional metal.

“This novel discovery opens new research directions and paths for plasmonics research, and enables us to look into previously untapped insulating and strongly-correlated materials,” said Rusydi. “We are also currently studying the possible applications of this new type of plasmons.”

In the second study, another team of NUS researchers examined how strontium niobate catalyzes water. The team, supervised by Venkatesan, found that when strontium niobate is in contact with water under solar irradiation, it can split the water into oxygen and hydrogen. This study also involved researchers from Nanyang Technological University in Singapore.

“While this material converts water into hydrogen under solar irradiation, the mechanism behind this process was previously misinterpreted as being due to the high speed or mobility of the electrons in the material,” said Venkatesan. “Our group clearly showed that it was not the case. The measured electron mobility was significantly low, but the effect was enhanced by the resonant absorption of the solar photons by the intrinsic plasmons present in this material.”

These results strongly suggest a novel approach to designing catalysts for various applications, and the work could lead to new techniques for harvesting hydrogen – a sustainable fuel – from water.

“At NUSNNI, we have a group that has found a family of materials, aside from strontium niobate, that work equally well as plasmonic water splitters. Moving forward, we are working towards finding the right combination of photocatalytic processes for producing useful chemicals,” said Venkatesan.

This story is adapted from material from the National University of Singapore, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.