Building blocks of innovation: Light-induced symmetry changes in tiny crystals

By Amber Rose | March 24, 2025

Breakthrough allows researchers to create materials with tailored properties, unlocking unprecedented control over their optical and electronic properties.

Imagine building a Lego tower with perfectly aligned blocks. Each block represents an atom in a tiny crystal, known as a quantum dot. Just like bumping the tower can shift the blocks and change its structure, external forces can shift the atoms in a quantum dot, breaking its symmetry and affecting its properties.

Scientists have learned that they can intentionally cause symmetry breaking — or symmetry restoration — in quantum dots to create new materials with unique properties. In a recent study, researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have discovered how to use light to change the arrangement of atoms in these miniscule structures.

“We often assume the crystal structure doesn’t really change, but these new experiments show that the structure isn’t always static when light is absorbed.” — Richard Schaller, Argonne physicist

Quantum dots made of semiconductor materials, such as lead sulfide, are known for their unique optical and electronic properties due to their tiny size, giving them the potential to revolutionize fields such as electronics and medical imaging. By harnessing the ability to control symmetry in these quantum dots, scientists can tailor the materials to have specific light and electricity-related properties. This research opens up new possibilities for designing materials that can perform tasks previously thought impossible, offering a pathway to innovative technologies.

Typically, lead sulfide is expected to form a cubic crystal structure, characterized by high symmetry similar to that of table salt. In this structure, lead and sulfur atoms should arrange themselves in a very ordered lattice, much like alternating red and blue Lego blocks.

However, previous data has suggested that the lead atoms were not precisely where they were expected to be. Instead, they were slightly off-center, leading to a structure with less symmetry.

“When symmetries change, it can change the properties of a material, and it’s almost like a brand-new material,” Argonne physicist Richard Schaller explained. ​“There’s a lot of interest in the scientific community to find ways to create states of matter that can’t be produced under normal conditions.”

The team used advanced laser and X-ray techniques to study how the structure of lead sulfide quantum dots changed when exposed to light. At DOE’s SLAC National Accelerator Laboratory, they used a tool called Megaelectronvolt Ultrafast Electron Diffraction (MeV-UED) to observe the behavior of these quantum dots in incredibly short timeframes, down to a trillionth of a second.

Meanwhile, at the Advanced Photon Source (APS), a DOE Office of Science user facility at Argonne, they conducted ultrafast total X-ray scattering experiments using Beamline 11-ID-D to study temporary structural changes at timescales down to a billionth of a second. These X-ray measurements benefited from the recent APS upgrade, which delivers high-energy X-ray beams that are up to 500 times brighter than before.

Additionally, at the Center for Nanoscale Materials, another DOE Office of Science user facility at Argonne, the team performed fast — again, less than a trillionth of a second — optical absorption measurements to understand how the electronic processes change when the symmetry changes. These state-of-the-art facilities at Argonne and SLAC played a crucial role in helping researchers learn more about controlling symmetry and the optical properties of the quantum dots on very fast timescales.

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

“When quantum dots absorb a light pulse, the excited electrons cause the material to shift to a more symmetrical arrangement, where the lead atoms move back to a centered position,” said Burak Guzelturk, a physicist at the APS.

The return of symmetry directly affected the electronic properties of the quantum dots. The team noticed a decrease in the bandgap energy, which is the difference in energy that electrons need to jump from one state to another within a semiconductor material. This change can influence how well the crystals conduct electricity and respond to external forces, such as electric fields.

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

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

This study’s findings are important for nanoscience and technology. Being able to change the symmetry of quantum dots using just light pulses lets scientists create materials with specific properties and functions. Just as Lego bricks can be transformed into endless structures, researchers are learning how to ​“build” quantum dots with the properties they want, paving the way for new technological advancements.

Other contributors to this work include Jin Yu, Olaf Borkiewicz, Uta Ruett and Xiaoyi Zhang from Argonne; Joshua Portner, Justin Ondry and Ahhyun Jeong from the University of Chicago; Samira Ghanbarzadeh, Thomas Field, Jihong Ma and Dmitri Talapin from the University of Vermont; Mia Tarantola, Eliza Wieman and Benjamin Cotts from Middlebury College; Alicia Chandler from Brown University; Thomas Hopper and Aaron Lindenberg from Stanford University; Nicolas Watkins from Northwestern University; and Xinxin Cheng, Ming-Fu Lin, Duan Luo, Patrick Kramer, Xiaozhe Shen and Alexander Reid from SLAC National Accelerator Laboratory. 

The results of this research were published in Advanced Materials. This study was funded by DOE’s Office of Basic Energy Sciences and partially supported by DOE’s Office of Science, Office of Workforce Development for Teachers and Scientists under the Science Undergraduate Laboratory Internships Program.

About Argonne’s Center for Nanoscale Materials

The Center for Nanoscale Materials is one of the five DOE Nanoscale Science Research Centers, premier national user facilities for interdisciplinary research at the nanoscale supported by the DOE Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge, Sandia and Los Alamos National Laboratories. For more information about the DOE NSRCs, please visit https://​sci​ence​.osti​.gov/​U​s​e​r​-​F​a​c​i​l​i​t​i​e​s​/​U​s​e​r​-​F​a​c​i​l​i​t​i​e​s​-​a​t​-​a​-​G​lance.

About the Advanced Photon Source

The U. S. Department of Energy Office of Science’s Advanced Photon Source (APS) at Argonne National Laboratory is one of the world’s most productive X-ray light source facilities. The APS provides high-brightness X-ray beams to a diverse community of researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. These X-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being. Each year, more than 5,000 researchers use the APS to produce over 2,000 publications detailing impactful discoveries, and solve more vital biological protein structures than users of any other X-ray light source research facility. APS scientists and engineers innovate technology that is at the heart of advancing accelerator and light-source operations. This includes the insertion devices that produce extreme-brightness X-rays prized by researchers, lenses that focus the X-rays down to a few nanometers, instrumentation that maximizes the way the X-rays interact with samples being studied, and software that gathers and manages the massive quantity of data resulting from discovery research at the APS.

This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology by conducting leading-edge basic and applied research in virtually every scientific discipline. Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.

The U.S. Department of Energy’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://​ener​gy​.gov/​s​c​ience.