Would you like a computer built into your wristwatch? How about camouflage uniforms that change with their surroundings? Perhaps holiday-light displays that flash changing messages might be nice? These are just a few of the elastic, light-weight electronic devices physicists at The University of Texas at Dallas are developing by weaving organic light-emitting displays (OLEDs) into textiles and clothing.
Soon a person may reach into his or her pocket, pull out a handkerchief-like piece of plastic material, unfurl it to four times its size and place it on a wall to make electronic media presentations.
The woven display might project Powerpoint presentations, websites, maps or other electronic media. It would be reusable – easily updated with fresh data at the possessor’s whim.
Fourth-year physics doctoral candidate Chris Williams conducts research on OLEDs. He said such devices right now are among the most commercially promising applications to come from the intersection of physics, chemistry, engineering, and nanotechnology.
Polymers, diodes and chips, oh my!
A diode is an electronic device that has two electrodes or terminals and is used to convert alternating electric current into direct current, depending on which way the voltage is biased or supplied.
Most work in organic LEDs stems from discoveries by Dr. Alan G. MacDiarmid, the Nobel Prize-winning professor here at UTD who created the prototype conducting polymer in 1977, as well as work done by scientists at Eastman-Kodak in the early Eighties.
Polymers are long chains of carbon atoms with side groups typically composed of hydrogen, oxygen or nitrogen. Side groups containing other elements are also possible, but less common, Chris said.
“For a polymer to be able to conduct electric current, it must consist alternately of singe and double bonds between the carbon atoms. It must also be ‘doped,’ meaning electrons are removed (through oxidation) or introduced (through reduction). These ‘holes’ or extra electrons can move current along the molecule – it becomes electrically conductive,” a press release on MacDiarmid’s contribution to science said.
Chris added, “So although they are plastic – most people don’t think of plastic as being conductive – these materials are. Certain plastics and certain polymers can carry current and also emit light.”
The physicist’s contribution involves analyzing the materials and understanding their properties and mechanics – how they behave and function – to determine ways to maximize efficiency.
Physicists devise simple layered structures of metals and organic materials that can be placed on plastic substrates, creating a flexible, low-cost alternative to liquid crystal displays (LCDs).
Simple, low-temperature deposition methods – spin casting, vacuum evaporation, and sublimation, physical vapor deposition – make processing OLEDs commercially attractive when compared with LCDs, Chris said.
“With the polymeric materials, I dissolve dry polymer in a suitable solvent. I then take a substrate – either glass or plastic – and put it on a motorized plate. The plate pulls vacuum on the substrate to hold it in place and then begins to rotate. I take a dropper and drop some of the solution onto the substrate and the solution spreads out and makes a nice thin uniform film.
“With small molecules, I put them into a tungsten boat inside a vacuum chamber and apply voltage across the boat. It’s resistively heated like a light bulb filament. The material, which is in a powder form, heats up and evaporates within the vacuum. I place the substrate above it so that some of that evaporated material accumulates on the substrate,” Chris said.
The Clean Room and other neat facilities
“The Clean Room itself has everything you can think of – characterization equipment, deposition equipment, vacuum chambers, photo and electron-beam lithography. Just about any semiconductor process can be done in the Clean Room,” Chris said.
He had nothing but praise for his work environment.
“The facilities here – the NanoTech facilities in particular – are top notch. The machine I work on is a state-of-the-art deposition system. It actually consists of two vacuum chambers – one for organic deposition and one for metal materials. Each one is enclosed in its own glove box purged with argon so that the chamber opens up and is vented from the glove box.
“The devices and materials are prevented from being exposed to oxygen and moisture. We can take the devices out, package them, seal them up and never expose them to atmosphere. That alone helps us to eliminate the degradation of devices and eliminates problems associated with atmospheric exposure,” Chris said.
Prior to UTD
Chris did his undergraduate work at Virginia Tech, in Blacksburg, where he majored in materials science and engineering.
“In my first summer job at Virginia Tech, I was doing what I am doing now, research on organic light-emitting diodes. I got my start working on that and working on a vacuum system – one a little more primitive than the one I’m working on now. But nonetheless it gave me a background in vacuum science, which has been very valuable,” he said.
A Virginia Tech professor helped Chris get a job working for Dr. Anvar Zakhidov, professor of physics at UTD. At the time, Zakhidov was at Honeywell International, which is headquartered in Morristown, New Jersey.
“My professor was interested in having me go up there for the summer between my sophomore and junior years to work on organic light-emitting diodes,” Chris said.
Falling in with Zakhidov and meeting Baughman
When Chris graduated, he had some decisions to make.
“I was looking to either stay where I was in Blacksburg, working as a grad student, or to explore other options.
“Around that time I learned that Dr. Zakhidov, along with Dr. Ray Baughman, had left Honeywell, and were starting a nanotech research facility at UTD. I was very interested in that and got in contact with Dr. Zakhidov. He said they’d be working on OLEDs and that I should join them. And so I did.
“That following January, just three weeks after I graduated, I started here as a student. It’s been great. I picked the right place,” Chris said.
“I think UTD is great, especially now,” Chris said. “I guarantee that the carbon nanotube sheets that were recently developed are not going to be the last big discovery made here. In another year, maybe less, I'm sure there will be another round of interviews for Drs. Ray Baughman and Anvar Zakhidov related to something new. There are some big things going on here.”
Because of Chris’ work with OLEDs and carbon nanotube sheets, he’s recently received some media attention. He has been interviewed and taped by The Associated Press concerning his research. And Chris hit another milestone last year: his first journal publication.
“I have authored my first publication recently in the Journal of Luminescence. The work was related to effects of organic dopants on the luminescence of OLED structures. We use these dopant materials to increase the conductivity of the organic layers.
“We seek higher efficiency devices with lower turn-on voltage, but we’ve found that the presence of these dopants in some cases can negatively effect the emission of the materials by quenching. We wanted to understand what’s going on in those cases,” Chris said.
Chris said that his work can often lead to more questions than answers.
“We want to do a second paper to study the effects of dopants on both electro luminescence and photo luminescence. That’s hopefully going to be published in Advanced Materials, a journal of materials science.
“We also are going to do a follow-up paper related to an article in Science about the carbon nanotube sheets. I built the structures – the polymeric LEDs – on top of these carbon nanotube sheets.
“We are going to do another paper that’s going to involve both small molecule and polymeric devices. We’re going to look at the injection properties and study more of the physics of what’s going on as far as charge injection and emission,” Chris said.
A bright and luminous future
When asked about his aspirations beyond UTD, Chris wasted no time.
“I want to go to work for a company. I want to go into industrial research, either for a small startup firm or a large established company. I’m wide open to the possibilities. Eventually, I’d like to wind up back in academia as a professor at some point and teach.
- Updated: October 12, 2011