This shows how the lattice dynamics of gallium telluride can be studied by ultrafast electron diffraction. Image: Qingkai Qian, Penn State.
This shows how the lattice dynamics of gallium telluride can be studied by ultrafast electron diffraction. Image: Qingkai Qian, Penn State.

Layered van der Waals materials are of high interest for electronic and photonic applications, according to researchers at Penn State and SLAC National Accelerator Laboratory. These researchers have now provided new insights into the interactions of layered materials with laser and electron beams, which they report in a paper in ACS Nano.

Two-dimensional (2D) van der Waals materials are composed of strongly bonded layers of molecules with weak bonding between the layers. The researchers used ultrafast pulses of laser light that excited the atoms in the lattice of the 2D material gallium telluride and then exposed the lattice to an ultrafast pulse of an electron beam. This allowed them to study the lattice vibrations in real time using electron diffraction, leading to a better understanding of these materials.

"This is a quite unique technique," said Shengxi Huang, assistant professor of electrical engineering at Penn State and corresponding author of the paper. "The purpose is to understand fully the lattice vibrations, including in-plane and out-of-plane."

One of their interesting observations is the breaking of a law that applies to all material systems. Friedel's Law posits that in the diffraction pattern pairs of centrosymmetric Bragg peaks should be symmetric, directly resulting from the Fourier transformation. In this case, however, the pairs of Bragg peaks show opposite oscillating patterns.

The researchers call this phenomenon the dynamic breaking of Friedel's Law. It is a very rare, if not unprecedented, observation in the interactions between the beams and these materials.

"Why do we see the breaking of Friedel's Law?" Huang said. "It is because of the lattice structure of this material. In layered 2D materials, the atoms in each layer typically align very well in the vertical direction. In gallium telluride, the atomic alignment is a little bit off."

When the laser beam shines onto the material, the heating generates the lowest-order longitudinal acoustic phonon mode, which creates a wobbling effect in the lattice. This can affect the way electrons diffract in the lattice, leading to the unique dynamic breaking of Friedel's law.

This technique is also useful for studying phase change materials, which absorb or radiate heat during phase change. Such materials can generate the electrocaloric effect in solid-state refrigerators. It will also be of interest to people who study oddly structured crystals and the general 2D materials community.

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