Science for our
Nation's
Energy Future

Energy Frontier Research Center

Community Website
Frontiers in
Energy Research
Newsletter
Winter 2018

Accelerating New Materials Discovery with Help from Computers

Modern-day alchemists are identifying new materials using guidance from computer predictions

Bor-Rong Chen

A transformation is taking place in materials science where the discovery process will happen not in the lab, but on the screen of a computer. Image courtesy of Nathan Johnson, PNNL

Technologies such as batteries, LEDs, and solar cells are constructed from materials that have the desired application-specific properties. Despite the significant demands for finding new functional materials, the discovery process relies largely on trial-and-error experiments in which materials are made, properties measured, and then suitable applications considered. However, a transformation is taking place in materials science where the discovery process will happen not in the lab, but on the screen of a computer. The Center for Next Generation of Materials Design (CNGMD), an Energy Frontier Research Center, has developed this new paradigm for materials discovery and development. In the new paradigm, fast-processing computation and predictive theory identify promising new functional materials and guide their synthesis.

Metastable materials are not the most stable but are promising. The atomic arrangements of most of the materials around us are usually in the most stable configuration. However, the atomic arrangement in a metastable material is not the most stable one! Given enough time, a metastable material may transform to a more stable configuration. Sometimes, the barriers are high enough that the transformation will never happen. Many metastable materials may therefore have sufficient lifetimes so that they are viable for technological applications. A familiar example is a diamond, in which the carbon atoms are arranged in a metastable configuration compared to that in graphite, which is the most stable form of carbon. However, the transformation from diamond to graphite requires a huge amount of energy, and it takes an extremely long time under normal temperature and pressure. This is why they say “diamonds are forever” and can be used as a stable material for all intents and purposes. Functionally, a diamond is different from graphite in many ways. For example, a diamond is optically transparent, has uniquely high hardness, and is electrically insulating. In fact, in many cases, metastable materials such as diamonds have the best combination of properties for a specific application. Therefore, understanding how to better make metastable materials with desirable functional properties is an active and exciting current research area.

Until recently, the discovery of metastable materials has been largely guided by empirical experimental observations combined with material science intuition. Now, among others, theorists and experimentalists in the CNGMD are working together to purposefully design new metastable materials as well as potential routes for their synthesis. Using this approach, CNGMD scientists have deliberately synthesized several metastable materials, including improving the synthesis for a known form of titanium dioxide that was difficult to achieve and creating zinc-manganese-oxide alloy structures that have never been made before.

Synthesizing hard-to-make metastable materials. Metastable materials are often more difficult to synthesize than their stable counterparts. This is both due to the difficulty in targeting a particular structure and identifying the synthetic pathway to that specific polymorph. Take, for example, brookite, which is one of several known forms of titanium dioxide. Compared to its more stable counterparts, rutile and anatase, brookite is more promising for accelerating light-driven chemical reactions (that is, photocatalysis). However, even though brookite is a naturally occurring mineral, the synthesis of compositionally pure brookite films has been a long-standing challenge. Until recently, the highest fraction of brookite achieved using pulsed laser deposition (PLD, which is a growth technique using laser to vaporize a target material and then coat it onto a surface) was only 45 percent, with the remainder consisting of rutile and anatase. Through the application of theory-guided synthesis, researchers at the CNGMD have now achieved 95 percent fraction brookite in a thin film on glass using PLD.

While previous experimental efforts suggest that sodium ions help to promote brookite growth, calculated formation energies suggest otherwise. After calculating the formation energies of various titanium dioxide structures with sodium incorporation, surprisingly, sodium incorporation does not stabilize brookite at all! This contradictory result motivated the team to design experiments to explore the formation mechanism of brookite.

A 60-nanometer titanium dioxide thin film under an optical microscope—100 times for (a) and 20 times for (b)—that contains rutile (R), brookite (B), and anatase (A) crystals. Image courtesy of Haggerty et al., Scientific Reports (2017), DOI: 10.1038/s41598-017-15364-y

To investigate the growth of brookite, the team grew thin, amorphous titanium dioxide films with thicknesses ranging from 20 to 80 nanometers (which is about 1,000 times thinner than human hair) on different types of glasses with and without the presence of sodium. The researchers heat treated and analyzed the structure of these titanium dioxide films. Interestingly, regardless of the substrate used, sodium or not, 70 to 95 percent of brookite fraction was achieved when the thickness of the film was between 45 and 65 nanometers. The combination of computational and experimental results confirms that brookite growth does not require the presence of sodium ions. The experimental results further suggest controlling the thickness of the film can provide a novel route to brookite growth, as brookite was preferentially formed in an intermediate film thickness region. Overall, this discovery takes the CNGMD a step closer to developing methods to selectively and predictively synthesize a desired metastable material.

Designing never-before-synthesized alloys with new properties. Conventionally, alloys are created by mixing two or more metals or oxides with similar crystal structures together. The properties of alloys can be tuned based on the composition of the pure parent materials. In contrast, CNGMD theorists computationally explored alloys created by combining materials with dissimilar structures. Surprisingly, they found that new properties not found in the parent materials could emerge, and that such materials should be synthesizable under non-equilibrium synthesis methods.

At first, blending materials with different structures together may sound counterintuitive, and thermodynamically it is. However, when the CNGMD scientists calculated the behavior of a mixture of manganese oxide (rock salt structure) and zinc oxide (wurtzite structure), they found some very interesting results. As anticipated, the two oxides have a poor thermodynamic miscibility, meaning they do not mix well together, using equilibrium synthesis methods. However, over a large window of temperature and composition conditions, the alloy would be stable against small local composition fluctuations, meaning that if the structure could somehow be made, it would not spontaneously phase separate. Guided by these calculations, the team experimentally synthesized brand-new manganese-zinc-oxide (MnxZn1-xO) alloys using non-equilibrium thin film growth methods.

Using high-throughput calculations, CNGMD scientists found the temperature and composition region for possible metastable MnO-ZnO alloys (blue diamond). Out of the region, MnO and ZnO separate from each other (red diamond). Image courtesy of Holder et al., Science Advances (2017), DOI: 10.1126/sciadv.1700270

Property calculations predicted that the structural and optical properties of this new type of alloy can be controlled in a non-linear or even discontinuous fashion by changing the ratio between the two end points (pure manganese oxide and pure zinc oxide), a result that was confirmed experimentally. This successful proof-of-principle experiment now opens up new materials design strategies to create alloys over wide compositional and structural ranges to enable new functional materials.

Can calculations further guide synthesis? Using state-of-the-art, high-throughput computational methods, theorists can now provide meaningful guidance for the synthesis of targeted functional materials, including those that have never been made before. Accordingly, scientists in the CNGMD are now looking to integrate experimental and computational approaches to advance the design and theory-guided synthesis of functional metastable materials.

Acknowledgments

Haggerty et al. This work was supported as part of the Center for Next Generation of Materials Design, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Basic Energy Sciences. Use of the Stanford Synchrotron Radiation Lightsource, at SLAC National Accelerator Laboratory, was supported by the Department of Energy, Office of Science, Basic Energy Sciences. DK acknowledges computational resources provided by the Extreme Science and Engineering Discovery Environment, which is supported by National Science Foundation.

Holder et al. This work was supported by the Department of Energy, Office of Science, Basic Energy Sciences, as part of the Energy Frontier Research Center titled Center for Next Generation of Materials Design. High-performance computing resources were sponsored by the Department of Energy’s Office of Energy Efficiency and Renewable Energy and are located at the National Renewable Energy Laboratory.

More Information

Haggerty JES, LT Schelhas, DA Kitchaev, JS Mangum, LM Garten, W Sun, KH Stone, JD Perkins, MF Toney, G Ceder, DS Ginley, BP Gorman, and J Tate. 2017. “High-Fraction Brookite Films from Amorphous Precursors.” Scientific Reports 7:15232. DOI: 10.1038/s41598-017-15364-y

Holder AM, S Siol, PF Ndione, H Peng, AM Deml, BE Matthews, LT Schelhas, MF Toney, RG Gordon, W Tumas, JD Perkins, DS Ginley, BP Gorman, J Tate, A Zakutayev, and S Lany. 2017. “Novel Phase Diagram Behavior and Materials Design in Heterostructural Semiconductor Alloys.” Science Advances 3(6):e1700270. DOI: 10.1126/sciadv.1700270

About the author(s):

  • Bor-Rong Chen is a postdoctoral researcher at the SLAC National Accelerator Laboratory. As a member of the Center for Next Generation of Materials Design (CNGMD) Energy Frontier Research Center, she investigates the formation and evolution of oxides during water-based chemical reactions in real time by using X-ray analysis techniques. Her goal is to understand the nature of reaction pathways and to design more effective ways for synthesizing new materials. Chen has been studying materials science since 2005, and she graduated with a Ph.D. in materials science and engineering from Northwestern University in 2017.

More Information

Haggerty JES, LT Schelhas, DA Kitchaev, JS Mangum, LM Garten, W Sun, KH Stone, JD Perkins, MF Toney, G Ceder, DS Ginley, BP Gorman, and J Tate. 2017. “High-Fraction Brookite Films from Amorphous Precursors.” Scientific Reports 7:15232. DOI: 10.1038/s41598-017-15364-y

Holder AM, S Siol, PF Ndione, H Peng, AM Deml, BE Matthews, LT Schelhas, MF Toney, RG Gordon, W Tumas, JD Perkins, DS Ginley, BP Gorman, J Tate, A Zakutayev, and S Lany. 2017. “Novel Phase Diagram Behavior and Materials Design in Heterostructural Semiconductor Alloys.” Science Advances 3(6):e1700270. DOI: 10.1126/sciadv.1700270

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.