A new material that is both highly transparent and electrically conductive could make large screen displays, smart windows and even touch screens and solar cells more affordable and efficient, according to the Penn State materials scientists and engineers who discovered it.
Indium tin oxide, the transparent conductor that is currently used for more than 90 percent of the display market, has been the dominant material for the past 60 years. However, in the last decade, the price of indium has increased dramatically. Displays and touchscreen modules have become a main cost driver in smartphones and tablets, making up close to 40 percent of the cost.
While memory chips and processors get cheaper, displays get more expensive from generation to generation. Manufacturers have searched for a possible ITO replacement, but until now, nothing has matched ITO's combination of optical transparency, electrical conductivity and ease of fabrication.
A team led by Roman Engel-Herbert, assistant professor of materials science and engineering, reports today (Dec 15) online in Nature Materials a new design strategy that approaches the problem from a different angle. The researchers use thin -- 10 nanometer -- films of an unusual class of materials called correlated metals in which the electrons flow like a liquid.
While in most conventional metals, such as copper, gold, aluminum or silver, electrons flow like a gas, in correlated metals, such as strontium vanadate and calcium vanadate, they move like a liquid. According to the researchers, this electron flow produces high optical transparency along with high metal-like conductivity.
"We are trying to make metals transparent by changing the effective mass of their electrons," Engel-Herbert said. "We are doing this by choosing materials in which the electrostatic interaction between negatively charged electrons is very large compared to their kinetic energy. As a result of this strong electron correlation effect, electrons 'feel' each other and behave like a liquid rather than a gas of non-interacting particles. This electron liquid is still highly conductive, but when you shine light on it, it becomes less reflective, thus much more transparent."
To better understand how they achieved this fine balance between transparency and conductivity, Engel-Herbert and his team turned to a materials theory expert, Professor Karin Rabe of Rutgers University.
"We realized that we needed her help to put a number on how 'liquid' this electron liquid in strontium vanadate is," Engel-Herbert said.
Rabe helped the Penn State team put together all the theoretical and mathematical puzzle pieces they needed to build transparent conductors in the form of a correlated metal. Now that they understand the essential mechanism behind their discovery, the Penn State researchers are confident they will find many other correlated metals that behave like strontium vanadate and calcium vanadate.
Lei Zhang, lead author on the Nature Materials paper and a graduate student in Engel-Herbert's group, was the first to recognize what they had discovered.
"I came from Silicon Valley where I worked for two years as an engineer before I joined the group," said Zhang. "I was aware that there were many companies trying hard to optimize those ITO materials and looking for other possible replacements, but they had been studied for many decades and there just wasn't much room for improvement. When we made the electrical measurements on our correlated metals, I knew we had something that looked really good compared to standard ITO."
Currently indium costs around $750 per kilogram, whereas strontium vanadate and calcium vanadate are made from elements with orders of magnitude higher abundance in the earth's crust. Vanadium sells for around $25 a kilogram, less than 5 percent of the cost of indium, while strontium is even cheaper than vanadium.
"Our correlated metals work really well compared to ITO," said Engel-Herbert. "Now, the question is how to implement these new materials into a large-scale manufacturing process. From what we understand right now, there is no reason that strontium vanadate could not replace ITO in the same equipment currently used in industry."
Along with display technologies, Engel-Herbert and his group are excited about combining their new materials with a very promising type of solar cell that uses a class of materials called organic perovskites. Developed only within the last half dozen years, these materials outperform commercial silicon solar cells but require an inexpensive transparent conductor. Strontium vanadate, also a perovskite, has a compatible structure that makes this an interesting possibility for future inexpensive, high-efficiency solar cells.
Engel-Herbert and Zhang have applied for a patent on their technology.
Along with Zhang and Engel-Herbert, Hai-Tian Zhang, Craig Eaton, Yuanxia Zheng and Matthew Brahlek, all students or postdoctoral Fellows in Engel-Herbert's group, worked on this paper, "Correlated metals as transparent conductors." Others from Penn State and the Materials Research Institute on this project were Moses Chan, Evan Pugh professor of physics, and his postdoctoral Fellow, Weiwei Zhao; and Venkatraman Gopalan, professor of materials science and engineering and his student Lu Guo.
With Rabe was her student Yuanjun Zhou from Rutgers University. Anna Barnes, Hamna Haneef and associate professor Nikolas Podraza of Univesity of Toledo also worked on this project.
The Office of Naval Research, the National Science Foundation and the Department of Energy funded this work. Fabrication of the correlated metals was performed at the Materials Research Institute in the laboratory facilities of Penn State's Millennium Science Complex.
A'ndrea Elyse Messer | EurekAlert!
Scientists announce the quest for high-index materials
24.07.2017 | Moscow Institute of Physics and Technology
ADIR Project: Lasers Recover Valuable Materials
24.07.2017 | Fraunhofer-Institut für Lasertechnik ILT
3-D shape acquisition using water displacement as the shape sensor for the reconstruction of complex objects
A global team of computer scientists and engineers have developed an innovative technique that more completely reconstructs challenging 3D objects. An ancient...
Physicists have developed a new technique that uses electrical voltages to control the electron spin on a chip. The newly-developed method provides protection from spin decay, meaning that the contained information can be maintained and transmitted over comparatively large distances, as has been demonstrated by a team from the University of Basel’s Department of Physics and the Swiss Nanoscience Institute. The results have been published in Physical Review X.
For several years, researchers have been trying to use the spin of an electron to store and transmit information. The spin of each electron is always coupled...
What is the mass of a proton? Scientists from Germany and Japan successfully did an important step towards the most exact knowledge of this fundamental constant. By means of precision measurements on a single proton, they could improve the precision by a factor of three and also correct the existing value.
To determine the mass of a single proton still more accurate – a group of physicists led by Klaus Blaum and Sven Sturm of the Max Planck Institute for Nuclear...
The research team of Prof. Dr. Oliver Einsle at the University of Freiburg's Institute of Biochemistry has long been exploring the functioning of nitrogenase....
A one trillion tonne iceberg - one of the biggest ever recorded -- has calved away from the Larsen C Ice Shelf in Antarctica, after a rift in the ice,...
21.07.2017 | Event News
19.07.2017 | Event News
12.07.2017 | Event News
24.07.2017 | Power and Electrical Engineering
24.07.2017 | Materials Sciences
24.07.2017 | Materials Sciences