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Wednesday, February 15, 2017

Kansas EPSCoR Track 2 Researcher and University of Kansas Physicist, Dr. Hui Zhao, discovers a new bilayer material


Taken from the cover of Nanoscale Horizons,
Volume 2 Number 1 January 2017
  A member of the Kansas NSF EPSCoR Track 2 “Collaborative Research: Imaging and Controlling Ultrafast Dynamics of Atoms, Molecules and Nanostructures” research team, Dr. Hui Zhao and his team at the Ultrafast Laser Lab at the University of Kansas have successfully created a new bilayer material. This material was developed by combining atomically thin layers of molybdenum disulfide and rhenium disulfide. Each layer of the new material measures less than one nanometer in thickness. According to Zhao, both molybdenum disulfide and rhenium disulfide “absorb light very well as semiconductors, and they’re both very flexible and can be stretched or compressed.” The goal of this type of research is to produce more efficient and versatile light emission devices, such as LEDS that can bend and that are just a few nanometers thick.  By creating the bilayer material, Zhao achieved the results he pursued.  In order to better explain this scientific breakthrough, Zhao used the following comparison:

“One can think of a material as a classroom full of students — which are the electrons — one on each seat,” he said. “Sitting on a seat, a student — or electron — can’t move freely to conduct electricity. Light can provide enough energy to stand up some of the students, who can now move freely and, as electrons, to conduct electricity. This process is the foundation for photovoltaic devices, where the energy of sunlight is captured and converted to electricity.” The emission of light involves the inverse process, in which a standing electron sits down in a seat, releasing its kinetic energy in the form of light.  “To make a good material for light emission devices, one needs not only the electrons that carry energy, but also the ‘seats’ — called holes — for the electrons to sit down.”

   Previous scientific studies, including some by Zhao, had already used the method of stacking different types of atomic sheets to create bilayer materials; however, those materials had the electrons and the “seats” located in different atomic layers.  Because it is difficult for electrons to find "seats," light emission efficiency of prior bilayer materials was very low – more than 100 times lower than if both electrons and “seats” were located in one atomic layer. Within this newly produced bilayer material, the electrons and their seats will be located in their original layer, instead of separate layers. Thus, the new bilayer material will produce light emissions that are much stronger.
   A cover story appearing in the January 2017 edition of the peer-reviewed journal Nanoscale Horizons by Zhao and his fellow researchers Matthew Bellus, Samuel Lane, Frank Ceballos and Qiannan Cui, all KU physics graduate students, and Ming Li and Xiao Cheng Zeng of the University of Nebraska-Lincoln, titled Type-I van derWaals heterostructure formed by MoS2 and ReS2 monolayers details the how the low tech “Scotch tape” method was used to create the new bilayer material.

Funding for this Research partially provided by the Kansas and Nebraska NSF EPSCoR Track 2 Grant #1430519 titled: "Imaging and Controlling Ultrafast Dynamics of Atoms, Molecules, and Nanostructures."  The grant's educational objectives are designed to enhance STEM education in Kansas by supporting activities that will lead to an expanded STEM workforce or prepare a new generation for STEM careers in the areas of atomic/molecular/optical science.

For additional information on this discovery, go to KU Today article, Using 'Scotch tape' and laser beams, researchers craft new material that could improve LED screensat at https://goo.gl/w1HKyh