Understanding motions of thin layers may help design solar cells, flexible electronics and catalysts of the future. New research shows how individual atoms move in trillionths of a second to form wrinkles on a 3-atom-thick material. It was made possible with SLAC’s instrument for ultrafast electron diffraction (UED), which uses energetic electrons to take snapshots of atoms and molecules on timescales as fast as 100 quadrillionths of a second.
Monolayers, or 2D materials, contain just a single layer of molecules. In this form they can take on new and exciting properties such as superior mechanical strength and an extraordinary ability to conduct electricity and heat. But how do these monolayers acquire their unique characteristics? Until now, researchers only had a limited view of the underlying mechanisms.
“The functionality of 2-D materials critically depends on how their atoms move,” said SLAC and Stanford researcher Aaron Lindenberg. They looked at molybdenum disulfide, MoS2, widely used as a lubricant but takes on a number of interesting behaviors when in single-layer form >150,000X thinner than a human hair. Eg the monolayer form is normally an insulator, but when stretched, it can become electrically conductive. This switching behavior could be used in thin, flexible electronics and to encode information in data storage devices. Thin films of MoS2 are also under study as possible catalysts that facilitate chemical reactions. In addition, they capture light very efficiently and could be used in future solar cells.
Because of this strong interaction with light, researchers also think they may be able to manipulate the material’s properties with light pulses.The study reveals for the 1st time how surface ripples form and evolve in response to laser light. SLAC team placed monolayer samples into a beam of very energetic electrons. The electrons, which come bundled in ultrashort pulses, scatter off the sample’s atoms and produce a signal on a detector that scientists use to determine where atoms are located in the monolayer = “ultrafast electron diffraction”. The team then used ultrashort laser pulses to excite motions in the material, which cause the scattering pattern to change over time.
“Combined with theoretical calculations, these data show how the light pulses generate wrinkles that havelarge amplitudes – more than 15 % of the layer’s thickness – and develop extremely quickly, in about a trillionth of a second. This is the first time someone has visualized these ultrafast atomic motions,” Lindenberg said. Once they understand monolayers of different materials, they could begin putting them together and engineer mixed materials with completely new optical, mechanical, electronic and chemical properties. https://www6.slac.stanford.edu/news/2015-09-10-slac-ultrafast-electron-camera-visualizes-ripples-2-d-material.aspx
Researchers have used SLAC’s experiment for ultrafast electron diffraction (UED), one of the world’s fastest ‘electron cameras’ to take snapshots of a three-atom-thick layer of a promising material as it wrinkles in response to a laser pulse. Understanding these dynamic ripples could provide crucial clues for the development of next-generation solar cells, electronics and catalysts. Credit: SLAC National Accelerator Laboratory
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