A groundbreaking method has been developed to harness high-energy electrons generated in light-absorbing semiconductors—so-called "hot electrons"—before they lose their surplus energy in chemical reactions. The technique is expected to dramatically improve the efficiency of next-generation eco-friendly photocatalytic technologies, such as solar-powered hydrogen production and carbon dioxide (CO2) reduction.
The Ulsan National Institute of Science and Technology (UNIST) announced on the 27th that a research team led by Chemistry Professor Jin Ho, in collaboration with a team led by Dr. Victor Klimov at the U.S. Los Alamos National Laboratory (LANL), has identified a "spin-exchange" reaction in which manganese (Mn) inside semiconductor quantum dots receives the energy of hot electrons and mediates the transfer of electrons to molecules.
Quantum dots, semiconductor particles a few nanometers (nm) in size, absorb light so that their internal electrons rise to a high-energy state, becoming hot electrons. When these electrons are passed to other molecules, those molecules gain electrons and can undergo photoreduction reactions. However, within ordinary quantum dots, hot electrons lose their energy in just a trillionth of a second and become "cooled electrons," making it extremely difficult to use them in actual chemical reactions.
To utilize hot electrons in photoreduction reactions before they lose their energy, the research team introduced magnetic manganese ions inside cadmium selenide (CdSe) quantum dots. The study found that the manganese ions undergo ultrafast spin exchange with electron-hole pairs containing hot electrons, first capturing energy that would otherwise dissipate as heat. Subsequently, as the manganese ions returned to their original spin state, electrons moved to the molecules and photoreduction reactions successfully occurred.
Spin exchange refers to a phenomenon in which the spins of electrons (a physical property with directionality) influence one another and change positions, transferring energy in the process.
To demonstrate this, the research team used "femtosecond transient absorption spectroscopy," which tracks the movement of electrons and energy by illuminating a sample with laser pulses at very short intervals. In experiments in which methyl viologen molecules—which are reduced when they receive electrons—were attached to the quantum dot surface, quantum dots containing manganese showed electron transfer speeds more than ten times faster than those without.
Until now, the light-induced electron transfer used in photocatalysts has been possible only when the energy levels between the semiconductor and the reacting molecule were properly aligned, imposing many limitations on system design. This study is highly significant in that it efficiently produced energetically unfavorable photoreduction reactions—previously nearly impossible—by using spin exchange as an intermediary.
Professor Jin Ho, the first author who led the research, explained, "Previously, if the energy levels between the semiconductor and the reacting molecule did not match, electron transfer was energetically unfavorable, making it difficult to trigger photoreduction reactions. This study demonstrated a new photocatalytic mechanism showing that utilizing the surplus energy of hot electrons through spin exchange can trigger reactions even under such conditions."
Professor Jin added, "It will play an important role in the design of next-generation, high-efficiency photocatalysts for solar-based hydrogen production and carbon dioxide conversion. Furthermore, we expect it to present new application possibilities in future quantum technology fields such as spintronics and quantum information devices."
The research results were published on June 26 in the internationally authoritative academic journal "Nature Communications."
