Study Helps Promote Gibbs’ Classical Nucleation Theory of Material Formation

A latest collaborative research headed by a research group at the Department of Energy’s Pacific Northwest National Laboratory, University of California, Los Angeles and the University of Washington could offer engineers innovative design rules for developing microelectronics, tissues, and membranes and pave the way for better production techniques for new materials.

Meanwhile, the study, published online in the journal Science on December 6^th, 2018, helps promote a scientific hypothesis that has not been proven for more than a century. According to the study, quite similar to the way children follow a rule to line up single file after recess, certain materials use a fundamental rule to assemble on surfaces one row at a time.

Nucleation—that basic formation step—is ubiquitous in ordered structures across nature and technology, from rock candy to cloud droplets. However, in spite of certain predictions made by the American scientist J. Willard Gibbs in the 1870s, researchers have still not come to an agreement on how this basic process occurs.

Gibbs’ theory for materials that form row by row is verified by the new study. Headed by Jiajun Chen, UW graduate student working at PNNL, the study unravels the fundamental mechanism, which bridges a basic knowledge gap and opens innovative pathways in materials science.

Chen made use of small protein fragments known as peptides that exhibit specificity, or distinctive belonging, to a material surface. The UCLA associates have been discovering and using material-specific peptides such as these as control agents to force nanomaterials to grow into specific shapes, for example, those preferred in semiconductor devices or catalytic reactions. The researchers discovered this while analyzing the way a particular peptide—a peptide with a strong binding affinity toward molybdenum disulfide—interacts with the material.

It was complete serendipity. We didn’t expect the peptides to assemble into their own highly ordered structures.

James De Yoreo, Materials Scientist, PNNL

De Yoreo is the co-corresponding author of the paper and Chen’s doctoral advisor.

That may have occurred since “this peptide was identified from a molecular evolution process. It appears nature does find its way to minimize energy consumption and to work wonders.”

Yu Huang, Study Co-Corresponding Author, Professor of Materials Science and Engineering, UCLA

For liquid water to be transformed into solid ice, it is necessary to create a solid-liquid interface. Gibbs’ classical nucleation theory proposes that even though the conversion of water into ice saves energy, energy is spent in creating the interface. The intricate part is the initial start—this is precisely when the new ice particle’s surface area is large than its volume, so it costs more energy to create an ice particle than is saved.

According to Gibbs’ theory, it is estimated that if the materials can grow in one dimension, that is, row by row, there would be no such energy penalty. Then, the materials can prevent what researchers term the nucleation barrier and are free to self-assemble.

Recently, there has been a debate over the nucleation theory. Some scientists have discovered evidence that the fundamental process is, in fact, more intricate compared to that hypothesized in Gibbs’ model.

However, “this study shows there are certainly cases where Gibbs’ theory works well,” stated De Yoreo, who is also a UW affiliate professor of both chemistry and materials science and engineering.

Earlier research works had already demonstrated that certain organic molecules, for instance, peptides like the ones in the Science paper, can self-assemble on surfaces. However, at PNNL, De Yoreo and his colleagues investigated further and found a means to gain insights into how molecular interactions with materials have an influence on their nucleation and growth.

The peptide solution was exposed to fresh surfaces of a molybdenum disulfide substrate by the researchers, and the interactions were measured using atomic force microscopy. Then, the measurements were compared with molecular dynamics simulations.

De Yoreo and his colleagues came to a conclusion that even in the earliest stages, the peptides got attached to the material one row at a time, barrier-free, similar to the predictions of Gibbs’ theory.

The high-imaging speed offered by the atomic force microscopy enabled the scientists to observe the rows quite similar to the way they were forming. The outcomes demonstrated that the rows were ordered right from the start and grew at the same speed without regard to their size—an important piece of evidence. They also formed new rows once sufficient peptide was in the solution for existing rows to grow; that is possible only if row formation is barrier-free.

This row-by-row process offers clues for designing 2D materials. At present, at times, designers have to put systems far out of equilibrium, or balance, to form specific shapes. De Yoreo stated that this is challenging to control.

But in 1D, the difficulty of getting things to form in an ordered structure goes away. Then you can operate right near equilibrium and still grow these structures without losing control of the system.

James De Yoreo, Materials Scientist, PNNL

It could modify assembly pathways for those engineering microelectronics or even bodily tissues.

Huang and her colleagues at UCLA have shown the innovative opportunities for devices based on 2D materials created through interactions in solution. However, she stated that the existing manual processes applied to develop such materials have limitations, such as scale-up capabilities.

Now with the new understanding, we can start to exploit the specific interactions between molecules and 2D materials for automatous assembly processes.

Yu Huang, Study Co-Corresponding Author, Professor of Materials Science and Engineering, UCLA

De Yoreo told that the next step is to create artificial molecules with the same properties as the peptides analyzed in the new paper—only more rugged.

At PNNL, De Yoreo and his colleagues are on the search for stable peptoids, which are as easy to produce as peptides but can better deal with the chemicals and temperatures used in the processes to develop the intended materials.

Enbo Zhu, Zhaoyang Lin, and Xiangfeng Duan at UCLA; Juan Liu and Hendrik Heinz at the University of Colorado, Boulder; and Shuai Zhang at PNNL are the co-authors of the study. Simulations were carried out using the Argonne Leadership Computing Facility, a Department of Energy Office of Science user facility. The study was funded by the National Science Foundation and the Department of Energy.

Video credit: PNNL