Controlling thin films with atomic 'spray painting'

Researchers were able to determine the effects of iron on the material's function through high-precision synthesis and measurements in oxide thin films. This was for example, in the ability to convert water into oxygen in a fuel cell. Pacific Northwest National Laboratory
Modern electronics and high-quality mirrors would not be possible without thin films. Thin films of various materials are required for semiconductor chips in cell phones and computers. These thin films can include metal oxides, which contain at least one metal along with oxygen.

Thin metal oxide films are more than just an electronic layer. They are useful in energy storage, sensing, catalysis, as well as catalysis. Understanding the materials at atomic level is essential to create thin films that can replace liquid layers in batteries and promote chemical transformations. Researchers at Pacific Northwest National Laboratory (PNNL) are studying how metal oxide thin films can be used to generate clean energy.

Scott Chambers, PNNL Materials Scientist and Laboratory Fellow, said that he likes to imagine what he does as "spray painting a target using atoms." His team uses molecular beam epitaxy to deposit elements atom-by-atom on top of solid crystals. Researchers can create high-quality, thin films of crystal with exact control over their structure and composition.

Some thin films conduct electricity differently than others. Scientists can alter the way that different films react to an electric current by stacking them.

Peter Sushko (PNNL materials scientist) stated that "our ability to develop advanced technologies in energy depends on our ability to make thin layers of material."

Advanced synthesis equipment is required to produce extremely thin oxide films. The equipment will be moved to a larger lab in PNNL's Energy Sciences Center. Two molecular beam epitaxy system and one pulsed laser deposit instrument are currently used by the Atomically Precise Materials group. Another pulsed laser deposit instrument is planned to increase the team's ability to create different types of experimental thin films.

Even small changes in thin films can have significant effects

Le Wang, a PNNL materials scientist led a recent study that used atomically-perfect thin films to create stable, high-performance catalysts. The ability of thin films to convert water into oxygen can be affected by changing the composition of lanthanum iron oxide (LaNi 1x Fe x 3 or LNFO). This is crucial for clean energy production. LNFO can be used to replace or reduce costly precious metal-based catalysts.

Research has shown that lanthanum nickel dioxide can generate more oxygen if it contains some nickel. The exact cause of this increased activity is not known.

Le Wang uses an Xray photoelectron spectrum instrument to measure composition and electronic structure synthesized thin film. Pacific Northwest National Laboratory

This uncertainty was addressed by the PNNL research team using its high-precision instruments and films. This work was published in Nano Letters and provides clear evidence and explanations as to why nickel and iron mix leads to more efficient oxygen production.

Researchers at PNNL created a variety of LNFO thin films of high quality, ranging from pure lanthanum Nickel Oxide to lanthanum Iron Oxide and a wide range of other compositions. Researchers also created several high-quality standards using the same equipment. This allowed them to pinpoint small but significant changes in the electronic structure.

These small changes revealed that the iron was transferring electrons to the Nickel in a process called charge transfer. This charge transfer makes it easier to convert water into oxygen. Researchers were able to identify this previously unknown charge transfer and gain insight into the reasons LNFO is a better catalyst.

Modeling for atomic-scale understanding

In its research, the team employs a multi-pronged approach. The team synthesizes new materials and analyzes their structure in the lab. Benchtop experiments are limited in the amount of detail they can provide. What is the secret weapon of the team? Theory.

Combining theory with experiments leads to deeper understanding of oxide thin films. Computational modeling gives insight into the motion of atoms on film surfaces and the way electrons rearrange at smaller scales than the diameters of an atom. Researchers wanted to test if LNFO atoms showed the same subtle signs as the ones they saw in the lab.

Wang said, "It was an exciting moment when we received Peter [Sushko]’s calculations back and they matched so well our experimental data." "Those results validated our argument regarding the importance of charge transfer LNFO."

The future of thin films

The ESC will continue this research with large windows highlighting a new high-visibility laboratory space. Visitors to the ESC lobby can see researchers working on new samples. Sushko stated, "We are excited about the new window into our science that the move will offer visitors to the ESC." "We're all excited to be in the same place, not only because of the increased laboratory and the additional instrumentation.

Next? Researchers plan to replace lanthanum partially with strontium in the same system of films, creating an oxide that contains four different metals. This will allow the team to better understand the structure and properties of complex oxide film structures. This will help to guide future synthesis efforts and design better catalysts.

Explore more Scientists are taking a new step towards microelectronics development

More information: Le Wang and colleagues, Understanding the Electronic Structure Evolution in Epitaxial LaNi1xFexO3 Thin films for Water Oxidation. Nano Letters (2021). Information from Nano Letters: Le Wang et al., Understanding the Electronic Structure Evolution in Epitaxial LaNi1xFexO3 Thin films for Water Oxidation. (2021). DOI: 10.1021/acs.nanolett.1c02901

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