Light-induced symmetric changes in small crystals allow researchers to create materials with customized properties

Imagine building a Lego Tower with perfectly aligned blocks. Each block represents a small crystal atom known as a quantum dot. When you hit a tower, external forces can shift atoms with quantum dots, breaking their symmetry and affecting their properties, so that blocks can be shifted and their structure can be altered.

Scientists have learned that deliberately breaking symmetry, or restoring symmetry, can be created with quantum dots to create new materials with unique properties. In a recent study, researchers at the U.S. Department of Energy (DOE) Argonne National Laboratory have discovered a way of using light to change the arrangement of atoms in these minimal structures.

Quantum dots made from semiconducting materials such as lead sulfide are known for their unique optical and electronic properties due to their small size, and can revolutionize fields such as electronic devices and medical imaging. By leveraging the ability to control the symmetry of these quantum dots, scientists can tailor materials to have specific light and electrical-related properties. This research opens new possibilities for designing materials that can perform tasks that previously thought were impossible and offers a path to innovative technology.

Lead sulfide is usually expected to form a cubic crystal structure characterized by high symmetry similar to table salts. In this structure, lead and sulfur atoms must place themselves in a very ordered lattice, just as they alternate red and blue Lego blocks.

However, previous data suggest that lead atoms are not exactly where they are expected. Instead, they were slightly out-of-center, leading to structures with less symmetry.

“Changing symmetry can change the properties of the material, and it’s like a whole new material,” explained Argonne physicist Richard Scheller. “There is a lot of interest in the scientific community in order to find ways to create a state of matter that is not produced under normal conditions.”

Using advanced laser and X-ray techniques, the team studied how the structure of lead sulfide quantum dots changed when exposed to light. At DOE’s SLAC National Accelerator Laboratory, we observed the behavior of these quantum dots in an incredibly short time frame using a tool called Megaelectronvolt Ultrafast Electron Diffraction (MEV-ued).

Meanwhile, Advanced Photon Source (APS), a DOE Science user facility at Argonne, conducted ultrafast total X-ray scattering experiments using Beamline 11-ID-D to study temporary structural changes up to 1/100 million seconds on a timescale. These x-ray measurements benefited from a recent APS upgrade. This provides a high-energy X-ray beam up to 500 times brighter than before.

Additionally, at the Nanoscale Materials Center, another DOE Science user facility in Argonne, the team quickly performed absorption measurements of less than a trillion, less than two trillion, less than two trillion, less than two trillion, to understand how electronic processes change when symmetry changes. These state-of-the-art facilities at Argonne and SLAC played a key role in helping researchers learn more about the symmetry and optical properties of quantum dots on extremely fast timescales.

Using these techniques, researchers observed that when quantum dots are exposed to short bursts of light, the symmetry of the crystal structure changes from a disordered state to a more organized state.

“When quantum dots absorb light pulses, the excited electrons shift the material into a more symmetrical arrangement, and lead atoms return to their central position,” said Burak Guzelturk, APS physicist.

The return of symmetry directly influenced the electronic properties of quantum dots. The team noticed a decrease in bandgap energy. This is the difference in the energy required to jump from a state in which electrons are in a semiconductor material to another state. This change can affect the extent to which the crystal exerts electricity and responds to external forces such as electric fields.

Furthermore, the researchers also investigated how quantum dot size and their surface chemistry influence temporary changes in symmetry. By adjusting these factors, you can control symmetry shifts and fine-tune the optical and electronic properties of quantum dots.

“We assume that the crystal structure doesn’t actually change, but these new experiments show that the structure is not always static when light is absorbed,” Schaller said.

The findings of this study are important for nanoscience and technology. Light pulses can be used to change the symmetry of quantum dots, allowing scientists to create materials with specific properties and functions. To enable LEGO bricks to be converted into infinite structures, researchers are learning how to “build” quantum dots with the properties they want, paving the way for new technological advancements.

The results of this study were published in Advanced Materials.