A new bilayer material, with each layer measuring less than one nanometer in thickness, someday could lead to more efficient and versatile light emission, such as bendy LED screens. Credit: Matthew Bellus
A new bilayer material, with each layer measuring < 1nm in thickness could lead to more efficient and versatile light emission. Researchers working at the Ultrafast Laser Lab at the University of Kansas successfully created the material by combining atomically thin layers of molybdenum disulfide and rhenium disulfide. “Both absorb light very well as semiconductors, and they’re both very flexible can be stretched or compressed,” said Hui Zhao, associate professor of physics and astronomy at KU. “The goal of this whole direction of research is to produce light-emitting devices, such as LEDs that are ultrathin – just a few nanometers thick – and flexible enough that you can bend it. We showed through this bilayer material, it can be achieved.”
“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,” he said. Previous studies have produced various bilayer materials by stacking different types of atomic sheets. However, in these materials, the electrons and the “seats” exist in different atomic layers. “Because electrons cannot easily find seats, light emission efficiency of these bilayer materials is very low – more than 100 times lower than in one atomic layer,” he said. But in the new material “All the electrons and their seats will be in their original layer, instead of separate. The light emission will be a lot stronger.”
Zhao et al created the new material using the same low-tech “Scotch tape” method pioneered in creating graphene, the single-atomic-layer material that won its creators the Nobel Prize for physics in 2010. “There’s a trick,” Zhao said. “You use Scotch tape to peel off a layer from the crystal and then you fold the tape a few times, so when you push the tape against a substrate and quickly peel it off, some of the material will be left on the substrate. Under a microscope, single-atom layer sections will have a different color because of their thickness.”
They stackedthe MoS2 layer on top of ReS2, with a precision better than one micrometer. The atomically thin sheets were connected by van der Waals force. After the samples were made, they used ultrafast lasers to observe motion of electrons and seats between the two atomic layers, and they saw clear evidence that both electrons and the seats can move from MoS2 to ReS2, but not along the opposite direction.
In doing so, the team confirmed theoretical calculations and predicted that bilayers formed by MoS2 and ReS2 would have promise as the basis for LED technology. The ultimate goal is to develop a method that allows precise control of the location of electrons and seats among different atomic layers so that the electronic and optical properties of the material can be controlled and optimized. “We’d someday like to see LEDs that are thinner, more energy efficient and bendable,” he said. “Think about a computer or phone screen if you could fold it a few times or and put it in your pocket.” http://news.ku.edu/2017/02/13/nanomaterial-fabricated-two-ultra-thin-sheets-holds-promise-led-breakthrough
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