Understanding how atoms move is fundamental to making and using materials. Atoms on the surface of some glasses move at nearly the same rate as atoms on the inside. But for other glasses, surfaces atoms move a million times faster. Researchers in the Wisconsin MRSEC IRG 1 have combined experiments, simulations, and data-centric methods to understand why some surfaces are so much faster than others. They found that atoms in glasses move by breaking out of a “cage” of nearby atoms. On the surface, that cage can be weaker than inside the glass, allowing for faster motion. They also discovered a relationship that predicts surface motion from more accessible data about bulk motion. Their results unify behavior for glasses of organic molecules, metals, and oxides and make creating glasses for applications like light-emitting diodes, quantum computers, and hard coatings easier.
Glasses have disordered arrangements of atoms without the repeating patterns that crystals have. However, there are small-scale patterns of atoms that touch each other that strongly affect the energy of the glass, how the atoms move when they get hot, and other properties like strength and response to an electric field. Unfortunately, there are many possible patterns and many slight variations of each one, so studying them is like sorting the grains of sand on a beach by size and color by hand–it’s an impossible task. Wisconsin MRSEC IRG 1 uses machine learning to sort the sand. They have developed algorithms to find small-scale atomic patterns in large simulations of glasses and link them to the glass’ energy. Ongoing studies have connected patterns to atomic motions, which provides a path to simulations of glasses over long times and low temperatures that are currently impossible.
The generation and sharing of a large dataset created as part of his study has won Zheng Yu the 2021 Wisconsin MRSEC Excellence in Open Science Prize. A graduate student in Dr. Bu Wang’s lab at the Grainger Institute for Engineering, Yu generated the data as part of his work investigating the relationship between structure and stability in specialized glasses using computer simulations and machine learning.
Using physical vapor deposition, researchers produced glassy films that are smooth and uniform, but which also have the molecules aligned with one another and organized in layers. This added structure could make the glass more efficient for conductors and expand the range of materials that can be used in future organic electronics. The colorful images in the figure show measurements using synchrotron x-rays that contrast the disordered starting material and the ordered glass.
IRG 1 showed how the atoms around the defects can restrict their ability to jump between configurations and how defects can talk to each other via sound waves. Both phenomena keep the defects from interfering with sound waves allowing the waves to travel long distances.
Creating glassy thin films of organic molecules at different temperatures changes both their stability in the glassy state – their density, and how hard they are to melt – and it changes how the molecules in the films are arranged – whether they tend to lie down flat on the surface or stand straight up. Wisconsin MRSEC researchers have shown that these changes in stability and average molecular orientation also change the mechanical properties of the film, including how stiff it is and how hard it is.