NORMAN, Okla. – A team of University of Oklahoma materials scientists has done what many in the field thought impossible: magnetize quantum dots by "doping" them with manganese.
The implications span everything from how we power our homes to how we build computers, scan for diseases, grow crops and illuminate our world.
"It's surprisingly difficult to integrate manganese, a good magnetic dopant, into cesium lead bromide nanoparticles," said assistant professor Yitong Dong , who describes his team's discovery in the Journal of the American Chemical Society. Cesium lead bromide nanoparticles (CsPbBr3) are a perovskite crystalline material used in a variety of commercial and consumer technologies.
"Our paper details a method to do it efficiently and consistently," Dong said. "We've doped the undopable."
The breakthrough comes just two years after the Nobel Prize in chemistry was awarded for the discovery of quantum dots – microscopic semiconductor crystals billionths of a meter wide. To visualize their size, the Nobel Foundation noted that the difference between a quantum dot and a soccer ball is comparable to the difference between a soccer ball and the Earth.
Despite their small scale, quantum dots are powerhouses of modern innovation. Their color can be tuned by adjusting their size, making them vital components in television displays, computer monitors and LED lighting. Their versatility also makes them essential ingredients in solar cells, biomedical imaging, quantum communication and even next-generation computing.
Dong's group works with a particularly promising class of nanoparticles: cesium lead bromide perovskite quantum dots, known for their bright emission and low-cost fabrication.
Researchers have long sought to incorporate manganese – an optically and magnetically active dopant – into dots to enhance their luminescent efficiency and usefulness. However, previous methods of dot synthesis faced challenges in adding enough manganese to make the dots more practically useful.
Dong's team found a workaround by removing positively charged cations in the form of cesium from the dots and creating a bromide-rich solution environment. When they added manganese cations to the process, the fast-growing crystals were regulated and the dots could absorb the magnetic cations into their structure, displacing about 40% of the lead ions.
"Essentially, the crystals swallowed the manganese, which resulted in successful dots doping with very high concentrations," Dong said.
The resulting dots glowed orange when excited, shifting from blue before doping. Typically, quantum dots change color when their size changes. In Dong's research, this was introduced through chemical alteration. The manganese-doped dots also glowed brightly, with near 100% efficiency.
With further development on this method, Dong said, the advances would have several practical benefits. Humans prefer the low energy of orange light over high energy blues, and many crops absorb warmer orange hues more effectively, making manganese-doped quantum dots ideal for use in indoor and agricultural lighting. The improved optical properties could also increase efficiency in solar cells, Dong said.
At scale, the dots Dong's team produced would be cheaper than conventional methods because they don't need to be coated with another material to protect their surface.
And that's only the beginning.
Because the manganese-doped dots are magnetic, their behavior could open the door to a new class of technologies – from spin-electronics to enhanced medical imaging.
The potential extends to quantum computers. Magnetically doped quantum dots could serve as building blocks for qubits – manipulated with light rather than electricity – a major advantage, Dong said, since quantum dots are more stable under optical excitement.
Dong emphasized that, despite the optimism, more work remains to control the doping in dots of varying sizes and to study the properties of doped manganese ions. Still, he believes this discovery represents the arrival of a powerful new class of materials.
"We're so excited that a new family of materials can join this field," Dong said. "They're cheap, scalable and amazingly efficient without extensive engineering. With doping, they can be even more versatile."
