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SLAC’s new electron camera visualizes ripples in 2-D material; support for future solar cells, electronics and catalysts

New research led by scientists from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University reveals how individual atoms move in trillionths of a second to form wrinkles on a three-atom-thick material. Visualized by a new “electron camera,” one of the world’s speediest, this unprecedented level of detail could guide researchers in the development of efficient solar cells, fast and flexible electronics and high-performance chemical catalysts.

As described in a paper published in the ACS journal in Nano Letters, the study 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.

This is the first published scientific result with our new instrument. It showcases the method’s outstanding combination of atomic resolution, speed and sensitivity.

—Xijie Wang, SLAC’s UED team lead

UED-MoS2-Simulations3
Illustrations (each showing a top and two side views) of a single layer of molybdenum disulfide (atoms shown as spheres). Top left: In a hypothetical world without motions, the “ideal” monolayer would be flat. Top right: In reality, the monolayer is wrinkled as shown in this room-temperature simulation. Bottom: If a laser pulse heats the monolayer up, it sends ripples through the layer. These wrinkles, which researchers have now observed for the first time, have large amplitudes and develop on ultrafast timescales. (SLAC National Accelerator Laboratory) Click to enlarge.

Monolayers, or 2-D materials, contain just a single layer of molecules. In this form they can take on new 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. However, no one has ever been able to study these motions on the atomic level and in real time before. Our results are an important step toward engineering next-generation devices from single-layer materials.

—Aaron Lindenberg, who led the research team

The researchers looked at molybdenum disulfide (MoS2), which is widely used as a lubricant but takes on a number of interesting behaviors when in single-layer form.

For example, 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.

Previous analyses showed that single layers of molybdenum disulfide have a wrinkled surface. However, these studies only provided a static picture. The new study reveals for the first time how surface ripples form and evolve in response to laser light.

Researchers at SLAC placed their monolayer samples, which were prepared by Linyou Cao’s group at North Carolina State University, 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. This technique is called 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.


UED-Setup-Labels
Ultrafast electron diffraction. A pulsed electron beam is created by shining laser pulses on a metal photocathode. The beam gets accelerated by a radiofrequency field and focused by a magnetic lens. Then it travels through a sample and scatters off the sample’s atomic nuclei and electrons, creating a diffraction image on a detector. Changes in these diffraction images over time are used to reconstruct ultrafast motions of the sample’s interior structure.

The superior performance of the new UED system is due to a very stable “electron gun” originally developed for SLAC’s X-ray laser Linac Coherent Light Source (LCLS), a DOE Office of Science User Facility. This electron source produces highly energetic electrons, packed into extremely short bunches. It spits out 120 of these bunches every second, generating a powerful electron beam that the researchers use to probe objects on the inside. (SLAC National Accelerator Laboratory) Click to enlarge.

Combined with theoretical calculations, these data show how the light pulses generate wrinkles that have large amplitudes—more than 15 percent 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.

—Aaron Lindenberg

Once scientists better understand monolayers of different materials, they could begin putting them together and engineer mixed materials with completely new optical, mechanical, electronic and chemical properties.

The research was supported by DOE’s Office of Science, the SLAC UED/UEM program development fund, the German National Academy of Sciences, and the US National Science Foundation.

Resources

  • Ehren M. Mannebach, Renkai Li, Karel-Alexander Duerloo, Clara Nyby, Peter Zalden, Theodore Vecchione, Friederike Ernst, Alexander Hume Reid, Tyler Chase, Xiaozhe Shen, Stephen Weathersby, Carsten Hast, Robert Hettel, Ryan Coffee, Nick Hartmann, Alan R. Fry, Yifei Yu, Linyou Cao, Tony F. Heinz, Evan J. Reed, Hermann A. Dürr, Xijie Wang, and Aaron M. Lindenberg (2015) “Dynamic Structural Response and Deformations of Monolayer MoS2 Visualized by Femtosecond Electron Diffraction” Nano Letters doi: 10.1021/acs.nanolett.5b02805

  • Weathersby, S. P. and Brown, G. and Centurion, M. and Chase, T. F. and Coffee, R. and Corbett, J. and Eichner, J. P. and Frisch, J. C. and Fry, A. R. and Gühr, M. and Hartmann, N. and Hast, C. and Hettel, R. and Jobe, R. K. and Jongewaard, E. N. and Lewandowski, J. R. and Li, R. K. and Lindenberg, A. M. and Makasyuk, I. and May, J. E. and McCormick, D. and Nguyen, M. N. and Reid, A. H. and Shen, X. and Sokolowski-Tinten, K. and Vecchione, T. and Vetter, S. L. and Wu, J. and Yang, J. and Dürr, H. A. and Wang, X. J. (2015) “Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory” Review of Scientific Instruments, 86, 073702 doi: 10.1063/1.4926994

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