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.