Pre-Aged Organic Glass Could Extend Lifetime of Cellphone Displays

For three decades, glass expert Mark Ediger has been studying the basic properties of organic glass to discover new ways to regulate the placement of atoms or molecules and to slow down substance degradation, particularly substances that lack the stiffness of a crystal.

This sample of organic glass, made in Ediger’s lab, has been “pre-aged,” meaning it deforms more slowly than most glasses. PHOTO: MARK EDIGER

“Glass seems to work pretty well,” says Ediger, indicating the windows that overlook the UW–Madison power plant on Dayton Street. The windows in Ediger’s office are the only bit of glass, and during the discussion of contemporary glass, constant references are made toward these windows, which paradoxically, are not the kind of glass that interests Ediger.

If you ask an ordinary person, ‘What is glass?’ they will point to a window, but glass is a much broader category of materials. A plastic ruler is a polymer glass. The fuselage of the Boeing 787 aircraft is a polymer glass reinforced with carbon fiber. And the display of a Samsung smartphone is made of OLEDs (organic light-emitting diodes) that utilize organic glasses.

Mark Ediger, Professor of Chemistry, UW–Madison

In scientific terms, glass is a solid, non-crystalline material composed of atoms or molecules sandwiched together in many different configurations. Crystals can only withstand a single packing arrangement between adjacent crystals.

Positioning himself in the window’s sunlight, Ediger explained that the science of silicate glass is already extremely advanced. Made mostly from sand, silicate glass is the stuff of beer bottles, light bulbs, and windows. However, there are many other glasses that exist at the periphery of modern science. For instance, Ediger is interested in glasses that are made from organic molecules, which are compounds based on carbon. Carbon is an element that exists at the core of the varied group of molecules in the universe.

When compared to crystals, glasses are more flexible.

For any given organic molecule, you only have a few crystal structures to pick from; you are out of luck if one of them does not have the properties you want. But molecules in glass are really flexible about their local environment and who they are willing to hang out with, so for organic molecules, there are an infinite number of glasses we can make. Some glasses might, for example, resist water uptake or be exceptionally hard or resist degradation by light.

Mark Ediger, Professor of Chemistry, UW–Madison

As seen in majority of glasses, OLEDs contain light-making molecules that are oriented in a random way, “but you want those molecules positioned so the light is aimed toward your eye,” he says.

One way to boost efficiency and extend the lifetime of a cellphone battery is through sophisticated manufacturing methods to regulate the orientation of OLED molecules. Molecules tend to move continuously in glass, and this will ultimately reduce the performance of the OLEDs that are developed to light up cellphone displays each year. However, glasses that are relatively stable can extend the lifetime of such displays.

If we tame them completely, we get a crystal, which we don’t want, but we have found a middle way to produce materials that are in many respects better than traditional glass, but are not crystals.

Mark Ediger, Professor of Chemistry, UW–Madison

While crystals have their own applications, for instance the silicon crystal forms the basis for computer chips, glass is even better to utilize light for digital communication or view through a window.

An optical fiber has to be able to carry a signal what — 60 miles — without being scattered by the boundaries between crystals. There’s no way you could do that with crystals.

Mark Ediger, Professor of Chemistry, UW–Madison

Back in 2007, in the Science journal, Ediger along with his colleagues published an important development to seek a middle ground between the amorphous anarchy of a glass and the stiff repetition of a crystal. This article elucidated how organic glass was developed through vapor deposition of organic molecules on a cold plate within a vacuum chamber.

Initially we did not know what we were making, we just knew that it was bizarre, unexpected.

Mark Ediger, Professor of Chemistry, UW–Madison

As predicted, the novel material did not behave under neutron irradiation.

We raised the temperature to something we figured would start the molecules moving, but we had to raise the temperature another 25 degrees (Celsius) before the molecules started to move. It turned out we had made a form of glass in which the molecules were so much better packed that they did not move until we reached a much higher temperature.

Mark Ediger, Professor of Chemistry, UW–Madison

Ediger discovered a technique to “pre-age” glass and prevent the degradation induced by slight rearrangements of atoms over time.

Our contribution was showing that there is this interesting space between traditional glass and crystals,” he says. “You are putting order in, but if you put in too much order, it becomes a crystal, and you have gone too far.

Mark Ediger, Professor of Chemistry, UW–Madison

Ultimately, Ediger discovered that molecules settle into a comfortable position when the cold-plate deposition process is used, and another molecule bury these group of molecules, locking them in place. This method is similar to the rapid production of an old vintage wine, Ediger explains.

These materials are effectively thousands or millions of years old. They have already had a chance to find a packing arrangement they are pretty happy with, and will stay there for a very long time.

Mark Ediger, Professor of Chemistry, UW–Madison

Ediger explained that since he was a graduate student at Stanford University, he wanted to bring some form of order to an amorphous material.

Glass was the control for an experiment I was doing. Glass was supposed to be boring and stable, but I was surprised at how much the molecules were rattling around. [His fascination with glass grew when his first UW–Madison graduate student, Pat Hyde] pointed out that we could do experiments to understand glass in a new way. In one way or another, that conversation has been responsible for at least half of what I’ve done at UW–Madison. At a great university you have bright, interesting young people who have good ideas, and they can move science in a direction it would not otherwise go.

Mark Ediger, Professor of Chemistry, UW–Madison

Ediger, a Kansas native, calls himself as a practical person, and explains that his main aim is to understand the rules that control the formation and transformation of glass over time. In Ediger’s laboratory, the cross-pollination between practicalities and theory still continues, as Yue Qiu, a graduate student, employs vacuum deposition process to prepare a new group of compounds that could prove handy for OLEDs.

Yue’s work answers a fundamental question and also can be useful in practice. OLEDs deteriorate over time, so cellphone displays get dimmer, and Yue’s work might eliminate that. [Qiu] is also asking an important fundamental question that applies very broadly to many aspects of glass technology. How do you pack the molecules in a glass so tightly that light cannot cause the molecules to rearrange? You could improve cellphone displays by trial and error, but that would be a long process. If we can identify the principles, we could cut years from that.

Mark Ediger, Professor of Chemistry, UW–Madison

Source: http://www.wisc.edu/

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