A joint research team from Korea and Japan has successfully managed to freely form and control the quasiparticle called “exciton,” which appears as an intermediate in chemical reactions, within a single molecule. The research findings are expected to contribute to the development of next-generation energy devices, the exploration of the fundamental principles of various chemical bonds, and the search for new quantum materials.
Professor Yoo-Soo Kim from the Department of Chemistry at Gwangju Institute of Science and Technology (GIST) and his team, including Professor Hiroshi Imada and researchers from the RIKEN Institute in Japan, have developed a technique to ultra-rapidly observe and control the fleeting chemical changes occurring in a single molecule. The results were published in the international journal “Science” on the 6th (local time).
An exciton is a quasiparticle in which a negatively charged electron and a positively charged hole coexist as a pair. Although quasiparticles are not precisely particles, they are treated as a single entity representing the collective movement exhibited by multiple interacting particles.
When a material absorbs energy, electrons inside the material rise to higher energy levels, creating holes at their original energy levels. Although the electron and hole have different electric polarities and attract each other, an exciton is a state where they are not fully bonded.
Excitons serve as a junction where light and electricity convert. When excitons are created with light energy and the electron and hole separate to flow towards opposite electrodes, a photoconversion phenomenon, used in devices like solar cells, occurs. Conversely, when excitons are created with electrical energy and the electron and hole recombine, light is emitted equivalent to the energy difference between them, a fundamental principle of displays. By creating excitons and extracting electrons for reactions, desired chemical reactions can be precisely controlled and efficiency increased.
Excitons play a significant role in recent advanced quantum material research. By analyzing changes that occur after excitons, which interact sensitively with electrons within quantum materials, are projected, the properties of these quantum materials can be indirectly understood.
The observation of excitons utilized the THz-STM method, combining terahertz (THz) light—capable of distinguishing ultra-short time durations at the picosecond (ps, one trillionth of a second) level—and scanning tunneling microscopy (STM). STM uses a fine probe with a tip as thin as a single atom to observe structures at the nanometer (1nm is one-billionth of a meter) level. Traditional THz-STMs were limited to measuring current changes when charges were injected into molecules.
The research team implemented THz-optical STM equipment with added photon detection technology and analyzed a single molecule of palladium (Pd) phthalocyanine. They succeeded in freely forming excitons at desired moments by continuously injecting charges into the molecule while controlling the waveform of THz light. Professor Kim stated, “This is the first time we’ve succeeded in real-time observation and control of the light emitted as the exciton dissipates, within picosecond time scales and nanometer dimensions.”
This research outcome is a product of the organic consolidation of strengths from universities, research institutes, and businesses in both Korea and Japan, highlighting not only the impact of the research content but also its significant implications in terms of science and technology diplomacy.
Professor Kim remarked, “The field of optical control of single molecules is a fiercely competitive area globally, serving as a core foundational technology for future quantum information and energy conversion sectors.” He added, “In this context, it’s an achievement that Korea and Japan’s leading researchers and institutions have crossed borders to collaborate like a single team and taken a step ahead.”
Reference materials include doi.org/10.1126/science.ads2776.