Editorial Feature

Controlling Thin Film Structure with Molecular Beam Epitaxy

Researchers at Pacific Northwest National Laboratory (PNNL) used high-precision synthesis and measurements of oxide thin films to find out how iron affects how they function. Their paper, published recently in Nano Letters, paves the way for new technology that could harness metal oxide thin films in clean energy production.

thin films, films, molecular beam epitaxy, epitaxy, oxide, energy production

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Stacks of Applications for Thin Film Materials

We use thin-film materials in numerous commercial and specialist devices, from most modern consumer electronics to high-quality mirrors for research. Semiconductor chips in our cell phones and computers use thin films made from various materials, including metal oxides (materials that contain oxygen and at least one metal).

But metal oxide thin films are used in many more applications than just modern electronics. Sensing, catalysis, and energy storage uses for these materials have all been demonstrated in recent years.

To realize the wealth of opportunities for thin-film applications in technology, we need a better understanding of these materials at the atomic level of scale.

Improving this understanding could have a major impact on human-induced climate change. Advanced energy technologies using thin-film materials are on the horizon, and researchers are currently focused on unlocking them.

The recent PNNL-based research shows how iron affects oxide thin films, for instance in how they convert water to oxygen in fuel cells.

Combining Theory and Experimentation

The interdisciplinary team combined theory with experimentation to generate deep insights into oxide thin films. Computational modeling simulated lanthanum nickel-iron oxide (LaNi1-xFexO3 or LNFO) thin films and showed how electrons might rearrange at nanoscales.

These subtle changes of charge transfer predicted by the computer modeling were also observable in the laboratory experiments.

Synthesizing Thin Films

The researchers had to use advanced synthesis equipment to make extremely thin oxide films with high enough precision for their experiments. PNNL’s new Energy Sciences Center (ESC) will house this technology in the future.

PNNL’s Atomically Precise Materials team uses two molecular beam epitaxy systems and one pulsed laser deposition instrument to synthesize thin films for this kind of research. At the ESC, there will be one more pulsed laser deposition instrument to increase the team’s capacity for creating experimental thin films even further.

Molecular Beam Epitaxy

PNNL materials scientists likened molecular beam epitaxy to “spray painting a target with atoms.” The technique deposits elements one atom at a time on top of a solid crystal substrate.

This means that researchers can make high-quality, crystalline thin films while precisely controlling their composition and structure.

AlScN Piezoelectric Thin Film to Fabricate a 3.5-GHz Solidly Mounted Resonator

Some thin films always conduct electricity, while others do not. But scientists can change how thin films respond to an electrical current by stacking them up in layers using molecular beam epitaxy.

Harnessing Atomically Precise Thin Films

The PNNL team harnessed atomically precise thin films to make stable high-performance catalysts, using these techniques. They discovered that varying the molecular composition of LNFO thin films can affect their ability to convert water into oxygen.

LNFO thin films’ ability to convert water into oxygen could be used for clean energy production in the future. LNFO could replace or reduce the need for catalysts typically studied for this application, which are based on expensive precious metals.

Scientists had already shown that replacing some of the nickel in lanthanum nickel oxide with iron makes it better at generating oxygen. But the reason for this phenomenon was unclear.

PNNL researchers used high-precision films and instrumentation to find clear evidence to explain why mixing nickel and iron leads to more efficient oxygen formation.

Synthesizing LNFO with molecular beam epitaxy enabled the scientists to identify small changes that showed iron transferring electrons to nickel with charge transfer. This charge transfer had not been observed before, and its discovery can be the basis of a better understanding of LNFO’s catalytic properties.

The Future of Thin Film Research

The new ESC facility at PNNL will host an expanded laboratory as well as more laser deposition instrumentation for the PNNL team.

The building includes large windows to showcase the important work carried out there. Anybody who enters the lobby of the new building will be able to watch researchers making new samples in the laboratory.

The ESC will also bring disparate fields of study together in one place. This will further encourage the kind of interdisciplinary collaboration – between experimental materials scientists, theoretical physicists, data scientists, engineers, and a host of other disciplines converging on issues of energy and sustainability – that enabled and enhanced the recent PNNL research.

In the new ERC labs, PNNL scientists are planning to replace some of the lanthanum in LNFO with strontium, using the same file system. This would create an oxide material with four different metals in the mix.

Gaining an understanding of these processes in a research setting can guide new synthesis efforts to design and manufacture even better catalysts for the large-scale sustainable energy production the planet needs.

References and Further Reading

Mundy, B. (2021). Controlling Thin Films with Atomic “Spray Painting.” Pacific Northwest National Laboratory. Available at: https://www.pnnl.gov/news-media/controlling-thin-films-atomic-spray-painting.

Pilkington, B. (2019). Green Thin Film Technology. AZO Cleantech. Available at: https://www.azocleantech.com/article.aspx?ArticleID=998.

Wang, L. et al. (2021). Understanding the Electronic Structure Evolution of Epitaxial LaNi1–xFexO3 Thin Films for Water Oxidation. Nano Letters. Available at: https://doi.org/10.1021/acs.nanolett.1c02901.

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Ben Pilkington

Written by

Ben Pilkington

Ben Pilkington is a freelance writer who is interested in society and technology. He enjoys learning how the latest scientific developments can affect us and imagining what will be possible in the future. Since completing graduate studies at Oxford University in 2016, Ben has reported on developments in computer software, the UK technology industry, digital rights and privacy, industrial automation, IoT, AI, additive manufacturing, sustainability, and clean technology.

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