Scientists identify new type of semiconductor nanocrystals

Scientists at the U.S. Naval Research Laboratory (NRL) have identified a new class of semiconductor nanocrystals with bright ground-state excitons, a major advance in the field of optoelectronics, reported in a paper published in ACS Nano, a journal of the American Chemical Society (ACS).

This groundbreaking theoretical research has the potential to revolutionize the development of highly efficient light-emitting devices and other technologies.

Typically, the lowest energy excitons in nanocrystals emit little light and are called “dark” excitons. Dark excitons slow down light emission and therefore limit the performance of nanocrystal-based devices such as lasers and light-emitting diodes (LEDs). Scientists have long sought to overcome dark excitons.

“We sought to find new materials in which the exciton order is inverted, with the lowest energy exciton being the brightest,” said Dr. John Lyons from the Department of Theory of Advanced Functional Materials. “Using criteria based on our theoretical model, we searched open-source databases of materials and identified over 150 targets. We further refined this list through advanced first-principles calculations, ultimately narrowing it down to 28 candidate bright excitonic nanomaterials.”

More detailed modelling of these materials shows that at least four of them are capable of generating bright ground-state excitons within the nanocrystals. “This discovery, in collaboration with Professor David Norris at the Federal Institute of Technology (ETH) Zurich and Dr Peter Sercel at the Centre for Hybrid Organic-Inorganic Semiconductors for Energy (CHOISE), could pave the way for the development of ultra-bright and efficient light-emitting devices, lasers and other technologies,” Lyons said.

Dr. Alexander Efros, senior scientist in the Materials Science Department and lead author of the paper, elaborated on the implications of the research.

“In our work, we have identified several bright excitonic materials that can emit light across a wide spectrum, from infrared to ultraviolet,” says Efros. “This versatility makes these materials extremely useful for optoelectronic applications. The ability to design nanocrystals with bright exciton states across this wide range opens new avenues for creating better, more efficient LEDs, solar cells, and photodetectors.”

By solving the dark exciton problem, NRL scientists hope to inspire the larger nanomaterials community to tackle long-stalled bright excitonic nanostructures. Three of these materials are currently being grown at NRL as part of the Nanoscience Laboratory Program’s Bright Nanocrystal Emitters Initiative, with the goal of conclusively demonstrating bright exciton behavior in the laboratory and leveraging it for future naval technologies.

“Our results demonstrate the power of combining high-throughput computational screening, pen-and-paper theory, and high-precision calculations of electronic structure,” said Dr. Michael Swift. “No single technique is enough on its own, but by combining them we have discovered new ultra-bright nanocrystals and unlocked the power of bright excitons across an unexplored class of materials.”

The Advanced Functional Materials Theory Division conducts fundamental and applied research into the functional, structural, biological and electronic materials systems. The division develops new ways to simulate materials and systems, including original development of computational and theoretical techniques, modification of existing approaches, and application of established methodologies to new materials and disciplines. The division’s goal is to use theory and simulation to understand, improve, and develop materials critical to the current and future Navy.