UT researchers give nanosheets local magnetic properties

Two-dimensional crystals are very suitable for creating high-quality magnetic thin films. This appears from two recent publications written by scientists from the University of Twente's MESA+ research institute. The researchers show that by growing the magnetic layers on various 2D crystals, better known as nanosheets, you can control the preferred direction of the magnetism very locally.

In an article published in Advanced Functional Materials, they present this method to create magnetic patterns on the micrometer scale. In Angewandte Chemie, they demonstrate that you can make the nanosheets in less than a minute, while the synthesis process had been known to be very slow. The magnetic films can be deployed for many different applications, such as new generations of smartphones.

With pulsed laser deposition (PLD) you can achieve controlled growth of thin layers of certain materials. Here, a material is heated rapidly with a powerful laser beam, so that it evaporates and a plasma is created. This spreads quickly in a vacuum chamber and is deposited on a substrate where it forms a thin layer. In this way you can control the thickness of the layer and you can form smooth and thin layers, often with special properties that are interesting for use in electronics and electro-mechanics, for example. For such applications, it is however essential that you can also make patterns in the layered materials. This is not easy, especially because the substrate needs to be heated to temperatures above 500° C during the PLD process. Many of the existing methods are therefore not adapted to existing manufacturing methods for microstructures.

Use of nanosheets

The UT researchers have now developed a new method, in which they make use of nanosheets obtained from three-dimensional crystals with a layered structure. If you dissolve these crystals in a special liquid, they spontaneously disintegrate into individual nanosheets. It was long thought that the crystal disintegration process could take weeks. However, the researchers have now shown that the nanosheets are already able to form within a few seconds, which opens the way for the production of nanosheets on a large scale.

Based on the solution, various nanosheets can be introduced in micro-patterns on a substrate. These patterns form the starting point for the growth of thin magnetic layers of magnetic LaSrMnO3 at high temperatures by means of PLD. Depending on the type of nanosheet the structure of the magnetic film assumes a specific orientation, and thus determines the magnetism of the film at that location. The process is monitored by means of, for example, electron backscatter diffraction (EBSD); a technique that makes it possible to 'reveal' the structure in the patterns.





Caption: EBSD image showing the local structure of a thin film. The left half of these images shows the preferred direction of the LaSrMnO3-film perpendicular to the growth direction, while the right half shows the directions in the plane with the contours of the individual nanosheets clearly visible. The distance between two lines in the pattern is a few micrometers.

FUNCTIONAL PROPERTIES

The researchers show that you can use the micro patterns to control the functional properties of a material in detail. In addition to magnetism, it is possible to pattern other properties at the micrometer scale. An important step has thus been taking in bridging the gap between scientific research into artificial layered crystals and their ultimate application. The group from Twente plays a leading role in this worldwide. 

RESEARCH

The research was performed by scientists from the Inorganic Materials Science department of UT research institute MESA+. It forms part of the TOP project funded by the Netherlands Organisation for Scientific Research (NWO) and the Chinese Scholarship Council. The research involved close cooperation with the Condensed-Matter and Medical Physics group at the University of California (UC, Irvine). 

University of Twenty

Nanoscale Coatings Improve Stability and Efficiency of Devices for Renewable Fuel Generation

A graphic representation of how atomic layer
deposition can aid renewable hydrogen fuel
generation. Two papers published in
Proceedings of the National Academy of
Sciences show how atomic layer deposition
can make water-splitting devices more stable and
more efficient.

Splitting water into its components, two parts hydrogen and one part oxygen, is an important first step in achieving carbon-neutral fuels to power our transportation infrastructure – including automobiles and planes.

Now, North Carolina State University researchers and colleagues from the University of North Carolina at Chapel Hill have shown that a specialized coating technique can make certain water-splitting devices more stable and more efficient. Their results are published online this week in two separate papers in the Proceedings of the National Academy of Sciences.

Atomic layer deposition, or “ALD,” coats three-dimensional structures with a precise, ultra-thin layer of material. “An ALD coating is sort of like the chocolate glaze on the outside of a Klondike bar – just much, much thinner,” explains Dr. Mark Losego, research assistant professor of chemical and biomolecular engineering at NC State and a co-author on the work. “In this case, the layers are less than one nanometer thick – or almost a million times thinner than a human hair.”

Although extremely thin, these coatings improve the attachment and performance of surface-bound molecular catalysts used for water-splitting reactions in hydrogen-fuel-producing devices.

In the first paper, “Solar water splitting in a molecular photoelectrochemical cell,” the researchers used ALD coatings on nanostructured water-splitting cells to improve the efficiency of electrical current flow from the molecular catalyst to the device. The findings significantly improved the hydrogen generating capacity of these molecular-based solar water-splitting cells.

In the second paper, “Crossing the divide between homogeneous and heterogeneous catalysis in water oxidation,” the researchers used ALD to “glue” molecular catalysts to the surface of water-splitting electrodes in order to make them more impervious to detachment in non-acidic water solutions. This improved stability at high pH enabled a new chemical pathway to water splitting that is one million times faster than the route that had been previously identified in acidic, or low pH, environments. These findings could have implications in stabilizing a number of other molecular catalysts for other renewable energy pathways, including the conversion of carbon dioxide to hydrocarbon fuels.

“In these reports, we’ve shown that nanoscale coatings applied by ALD can serve multiple purposes in water-splitting technology, including increasing hydrogen production efficiency and extending device lifetimes,” Losego said. “In the future, we would like to build devices that integrate both of these advantages and move us toward other fuels of interest, including methanol production.”

NC State’s Gregory Parsons, Alcoa Professor of Chemical and Biomolecular Engineering, and Ph.D student Berc Kalanyan co-authored both papers with Losego. Thomas J. Meyer, the Arey Distinguished Professor of Chemistry at UNC-Chapel Hill, is the corresponding author on both papers; UNC researchers Dr. Aaron K. Vannucci and Dr. Leila Alibabaei were leading authors. The research was funded by the U.S. Department of Energy, the Research Triangle Solar Fuels Institute and the University of North Carolina Energy Frontier Research Center.

Source: http://news.ncsu.edu/releases/nanocoatings-improve-efficiency-stability/