Highly mismatched alloys, which are still in the experimental stages of development, are combinations of elements that won't naturally mix together using conventional crystal growth techniques. Professor Rachel Goldman compares them to some extent to homogenized milk, in which the high-fat cream and low-fat milk that would naturally separate are forced to mix together at high pressure.
New mixing methods such as "molecular beam epitaxy" are allowing researchers to combine disparate elements. The results, Goldman says, are more dramatic than smooth milk.
"Highly mismatched alloys have very unusual properties," Goldman said. "You can add just a sprinkle of one element and drastically change the electrical and optical properties of the alloy."
Goldman is a professor in the departments of Materials Science and Engineering, and Physics. Her team included other U-M physicists and engineers as well as researchers from Tyndall National Institute in Ireland.
Solar cells convert energy from the sun into electricity by absorbing light. However, different materials absorb light at different wavelengths. The most efficient solar cells are made of multiple materials that together can capture a greater portion of the electromagnetic radiation in sunlight. The best solar cells today are still missing a material that can make use of a portion of the sun's infrared light.
Goldman's team made samples of gallium arsenide nitride, a highly mismatched alloy that is spiked with nitrogen, which can tap into that underutilized infrared radiation.
The researchers used molecular beam epitaxy to coax the nitrogen to mix with their other elements. Molecular beam epitaxy involves vaporizing pure samples of the mismatched elements and combining them in a vacuum.
Next, the researchers measured the alloy's ability to convert heat into electricity. They wanted to determine whether its 10 parts per million of nitrogen were distributed as individual atoms or as clusters. They found that in some cases, the nitrogen atoms had grouped together, contrary to what the prevailing "band anti-crossing" model predicted.
"We've shown experimentally that the band anti-crossing model is too simple to explain the electronic properties of highly mismatched alloys," Goldman said. "It does not quantitatively explain several of their extraordinary optical and electronic properties. Atomic clusters have a significant impact on the electronic properties of alloy films."
If researchers can learn to control the formation of these clusters, they could build materials that are more efficient at converting light and heat into electricity, Goldman said.
"The availability of higher efficiency thermoelectrics would make it more practical to generate electricity from waste heat such as that produced in power plants and car engines," Goldman said.
This research is newly published online in Physical Review B. The paper is entitled "Nitrogen composition dependence of electron effective mass in gallium arsenide nitride."
This research is funded by the National Science Foundation, the Science Foundation Ireland, and the Center for Solar and Thermal Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy.Michigan Engineering:
Rachel GoldmanContact: Nicole Casal Moore
Nicole Casal Moore | EurekAlert!
Machine-learning predicted a superhard and high-energy-density tungsten nitride
18.07.2018 | Science China Press
In borophene, boundaries are no barrier
17.07.2018 | Rice University
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
18.07.2018 | Life Sciences
18.07.2018 | Materials Sciences
18.07.2018 | Health and Medicine