“It’s a new kind of material.”

These are the words of researcher Zahra Fakhraai. Together with her team, she discovered a liquid phase that was unknown to science until recently. And that may have many applications. For example, not only could better OLED screens or more stable glass be produced, we may also be able to further unravel the mysterious nature of glass physics through this discovery.

glass physics

At school you learn that matter can be in three different states: gas, liquid and solid. But surprisingly, glass does not adhere to these rules. The structure of glass is very similar to the liquid phase, but its properties are similar to solids. This so-called ‘glass phase’ is still a great mystery to scientists. For example, the material feels hard, but it does not have an ordered crystal structure. From the outside, the material behaves like a solid, but the inside looks as disorderly as a liquid.

The problem

And here we also run into some problems. Glass from which ultra-thin films (layers) are made is widely used in applications such as OLED screens. But these thin films behave more like a liquid at low temperatures. This resulting material may be prone to droplet formation or crystallization, at the expense of stability. “Chemists have come up with creative ways to anchor the molecules,” Fakhraai says in conversation with Scientias.nl. “But technical properties, such as conductivity, suffer as a result.” However, in an effort to address these problems, scientists made a remarkable discovery.

New liquid phase

Instead of cooling liquid to produce glass, the researchers used vapor deposition. Here, a material is directly transformed from a gas into a solid. The researchers made a surprising discovery. Because by using vapor deposition, the team turned out to gain access to another, until recently unknown liquid phase. “Until now, we have only observed this phase in extremely thin films (below 50 nanometers),” Fakhraai says. “Once we exceed this thickness, the phase becomes unstable (similar to melting ice). We suspect this is why this phase has not been observed in the past.”

How it works

It means that the newly discovered phase cannot be reached simply by allowing a liquid to cool. “That’s because the movement of the molecules is getting too slow,” explains Fakhraai. “As a result, the opportunity to form the new liquid phase is lost. The molecules then get stuck in the regular glass phase.” Thus, the only way to perceive the newly discovered phase is through deposition directly from the vapor phase. “We keep the carrier substrate at a very low temperature (again, imagine that you are directly forming ice from water vapor on a very cold surface), Fakhraai explains further. “Once we have formed this phase, we have the opportunity to study its properties by changing the temperature of the substrate. We managed to do this by producing substrates with a temperature gradient. On cold substrates we produce films directly in this phase. We produce the regular glass phase on warmer substrates. Then we can see at what temperature there is a transition between these phases and how the properties differ in films of the same thickness but formed at different temperatures.”

“This phase of glass has an incredibly high density”

New material

What is particularly promising about the newly discovered liquid phase is that researchers are now able to create very thin films of glass with a higher density and stability. “This phase of glass, which is only stable in thin films, has an incredibly high density,” Fakhraai explains. “It even has a higher density than crystal. Density plays an important role in many other physical properties of glass, such as elasticity, conductivity, thermal conductivity, speed of sound, etc. Although we have not yet measured these properties, we expect that this phase will allow us to produce more stable thin films with which such properties can be improved. Conductivity is especially important, because the molecule we used in our research is widely used in organic electronic materials (OLEDs). So understanding this phase also helps us to improve technical properties.”

Applications

And that offers unprecedented possibilities. Perhaps even the development of completely new devices. “It’s really a new kind of material,” Fakhraai says. Although the researchers are still skeptical about the applications. “First, we need to verify whether the newly discovered phase also exists in other molecules,” the researcher continues. “We have preliminary data showing that this is indeed the case. We can then start applying it where nanoscale glass is usually used. For example, to package the Moderna and Pfizer vaccines, you need glass that can get very cold and won’t shatter. The fact that the technology for this now exists shows that we are on the right track to develop such glass.”

Unraveling the mystery

In addition to its interesting applications, the study also another implication, which is “what our data means for verifying glass transition theories,” Fakhraai said. “During many years several theories have been discussed. To my knowledge, no study has yet looked into the possibility of a new, low-temperature phase in thin films. Developing theories and models of this phenomenon may clarify the nature of glass physics in general. In addition, this can help us distinguish between different theories about the glass phase. A theory that can deal with boundary conditions at the nanoscale and predict glass properties on different surfaces is currently lacking. We therefore hope that our experimental data can be used as a framework for developing such theories.”

The researchers plan to continue their experiments in the lab to learn more about the crucial parameters that lead to the unique new phase transition. And that may help solve some of the other remaining mysteries about glass. “Our hope is that this fundamental understanding will lead to more applications and a better ability to fabricate glass thin films with improved properties,” Fakhraai sums up. “Understanding the structure and properties better opens up many new possibilities when designing new products.”