September 10, 2015

SLAC’s Ultrafast ‘Electron Camera’ Visualizes Ripples in 2-D Material

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.
(SLAC National Accelerator Laboratory)
Understanding Motions of Thin Layers May Help Design Solar Cells, Electronics and Catalysts of the Future

(September 10, 2015)  New research led by scientists from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University shows how individual atoms move in trillionths of a second to form wrinkles on a three-atom-thick material. Revealed by a brand 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.

The breakthrough, accepted for publication Aug. 31 in Nano Letters, could take materials science to a whole new level. 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.

“This is the first published scientific result with our new instrument,” said scientist Xijie Wang, SLAC’s UED team lead. “It showcases the method’s outstanding combination of atomic resolution, speed and sensitivity.”

SLAC Director Chi-Chang Kao said, “Together with complementary data from SLAC’s X-ray laser Linac Coherent Light Source, UED creates unprecedented opportunities for ultrafast science in a broad range of disciplines, from materials science to chemistry to the biosciences.” LCLS is a DOE Office of Science User Facility.

This animation explains how researchers use high-energy electrons at SLAC to study
faster-than-ever motions of atoms and molecules relevant to important materials
properties and chemical processes.

Extraordinary Material Properties in Two Dimensions

Monolayers, or 2-D 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.

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)

“The functionality of 2-D materials critically depends on how their atoms move,” said SLAC and Stanford researcher Aaron Lindenberg, who led the research team. “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.” The research team looked at molybdenum disulfide, or MoS2, which is widely used as a lubricant but takes on a number of interesting behaviors when in single-layer form – more than 150,000 times thinner than a human hair.

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.

Visualization of laser-induced motions of atoms (black and yellow spheres) in a molybdenum
disulfide monolayer: The laser pulse creates wrinkles with large amplitudes – more than
15 percent of the layer’s thickness – that develop in a trillionth of a second.
(K.-A. Duerloo/Stanford)

Because of this strong interaction with light, researchers also think they may be able to manipulate the material’s properties with light pulses.

“To engineer future devices, control them with light and create new properties through systematic modifications, we first need to understand the structural transformations of monolayers on the atomic level,” said Stanford researcher Ehren Mannebach, the study’s lead author.

Electron Camera Reveals Ultrafast Motions

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.

To study ultrafast atomic motions in a single layer of molybdenum disulfide,
researchers followed a pump-probe approach: They excited motions with a laser pulse
(pump pulse, red) and probed the laser-induced structural changes with a subsequent
electron pulse (probe pulse, blue). The electrons of the probe pulse scatter off the
monolayer’s atoms (blue and yellow spheres) and form a scattering pattern on the detector
– a signal the team used to determine the monolayer structure. By recording patterns at
different time delays between the pump and probe pulses, the scientists were able to
determine how the atomic structure of the molybdenum disulfide film changed over time.
(SLAC National Accelerator Laboratory)

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.

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