Understanding Optical Properties of Glass can Help Make Stable Glasses

It is a known fact that glass is everywhere. Whether someone is gazing out a window or scrolling down a smartphone, chances are that a glass layer exists between the person and the item being viewed.

Penn researchers found that certain optical properties in stable glasses were due to the layer-by-layer nature of the deposition that allows molecules to pack more tightly in the direction normal to the surface during the deposition. (Credit: University of Pennsylvania)

Although glass has been used for nearly 50 centuries now, there are many hidden facts about glass, for example, the manner in which different types of glasses are formed and the way they attain specific characteristics. In-depth knowledge of these facts can culminate in various inventions in the field of technology (e.g. glass with disparate mechanical characteristics and also scratch-free coatings).

For some years, scientists from the University of Pennsylvania have been investigating the characteristics of stable glasses, or closely packed types of glasses synthesized by coating molecules in vapor phase onto a cold substrate.

There have been a lot of questions about whether this is analogous of the same amorphous state of naturally aged glasses such as amber, which are formed by just cooling a liquid and aging it for many, many years.

Zahra Fakhraai, who is an associate professor of chemistry in Penn’s School of Arts & Sciences

To solve the questions, Fahkraai and Tianyi Liu, a PhD student, worked in cooperation with Patrick Walsh, a chemistry professor, who formulated and created an innovative and unique molecule with a precisely round spherical shape. Fakhraai stated that these distinctive molecules do not align themselves with any substrate upon being deposited. Due to this fact, the scientists anticipated that the glasses would be isotropic and amorphous, that is, that the particles forming the atoms—be it colloids, atoms, or grains—are ordered without any overarching order or pattern.

It is an astonishing fact that the scientists observed that the stable glasses were birefringent, or the refractive index of light was observed to be different in directions normal and parallel to the substrate, which is normally not anticipated in a round material. The outcomes of the study were reported in the Physical Review Letters journal.

Due to the birefringence, light shone in one direction will break differently when compared to light that shines from another direction. This is the effect that is normally used in liquid crystal displays: altering the material orientation makes light to differently interact with it, thus creating optical effects. In majority of the deposited glasses, this is caused by aligning of the molecules in a specific direction while being condensed from the vapor phase into an intense glassy state.

According to Fakhraai, the birefringence patterns exhibited by the stable glasses were unusual because the scientists did not anticipate the round molecules to be orientated inside the material.

Collaborating with James Kikkawa, a physics professor, and Annemarie Exarhos, a PhD student, who together carried out photoluminescence experiments to observe the orientation of the molecules, as well as Joseph Subotnick, a chemistry professor, and Ethan Alguire, a PhD student, who together assisted in performing the simulations for observing the crystal structure and computing the refractive index of the crystal which enabled them to calculate the math related to the extent of birefringence or ordering in the amorphous state, the scientists ascertained their guess that the material lacked orientation.

Although the researchers measured zero order in the glass, there still was a specific amount of birefringence equivalent to having nearly 30% of the molecules to be perfectly ordered. In their experiments, the researchers discovered that this was caused as a result of the layer-by-layer form of the deposition that enables the molecules to get tightly packed in the direction normal to the surface upon being deposited. The density the glass is directly proportional to the birefringence value. The process can be regulated by altering the substrate temperature that governs the densification degree.

We were able to show that this is a unique kind of order that is emergent from the process. This is a new sort of packing that’s very unique because you don’t have any orientation, but you can still manipulate the molecular distances on average and still have a random but birefringent packing overall. And so this teaches us a lot about the process of how you can actually access these lower state phases but also provides a way of engineering optical properties without necessarily inducing an order or structure in the material.

Fakhraai.

As the stressors are differently dispersed in and out of plane, the mechanical characteristics glasses can differ, which may come in handy for developing coatings and in the field of technology. The orientation of a glass or the layering in the glass may even be regulated to provide specific characteristics, for example, anti-scratch properties.

We expect that if we were to indent the glass surface with something,” stated Fakhraai, “it would have different toughness versus indenting it on the side. This could change its fracture patterns or hardness or elastic properties. I think understanding how shape, orientation and packing could affect the mechanics of these coatings is one of the places where interesting applications could emerge.”

Fakhraai further added that the most exhilarating part of this study is the basic facet of being in a position to demonstrate that high-density amorphous phases do exist. She is also confident that she along with other scientists will be in a position to use the knowledge gained by analyzing these systems to ascertain the properties of highly aged glass.

This tells us that we can actually make glasses that have packings that would be relevant to very well-aged glass,” stated Fakhraai. “This opens up the possibility of better fundamentally understanding the process by which we can make stable glasses.”

National Science Foundation grants DMR-11-20901, DMR-1206270, CHE-1152488, and DMREF-1628407 funded the study.

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