Using ultrafast electron diffraction, scientists have been able to capture atomic wrinkles forming in 2D materials in response to laser pulses. Illustration: SLAC National Accelerator Laboratory.
Using ultrafast electron diffraction, scientists have been able to capture atomic wrinkles forming in 2D materials in response to laser pulses. Illustration: SLAC National Accelerator Laboratory.

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. These atomic wrinkles were captured by a brand new ‘electron camera’, one of the world's speediest, which could help researchers develop a new generation of efficient solar cells, fast and flexible electronics, and high-performance chemical catalysts.

As reported in Nano Letters, this breakthrough 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," says scientist Xijie Wang, SLAC's UED team lead. "It showcases the method's outstanding combination of atomic resolution, speed and sensitivity."

"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,’ reports SLAC Director Chi-Chang Kao.

Monolayers, or two-dimensional (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 2D materials critically depends on how their atoms move," explains 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 properties when in single-layer form.

For example, the monolayer form is normally an insulator, but can become electrically conductive when stretched. 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. "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," says Stanford researcher Ehren Mannebach, the study's lead author.

Previous analyses showed that single layers of MoS2have a wrinkled surface. However, these studies only provided a static picture. By taking advantage of UED, the new study reveals for the first time how surface ripples form and evolve in response to laser light.

To conduct UED, researchers at SLAC place 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 reveals where atoms are located in the monolayer. The team use the 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 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," Lindenberg said.

Once scientists better understand the characteristics of monolayers made from different materials, they can begin putting them together to engineer mixed materials with completely new optical, mechanical, electronic and chemical properties.

This story is adapted from material from the SLAC National Accelerator Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.