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 minuscule structures.
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.
The results of this research were published in Advanced Materials.
