Now we control phenomena that occur at the molecular level [Now in Science]
An international research team has succeeded in real-time control of the quantum state of a single molecule that occurs in a 'snap' moment of one trillionth of a second. This means that humanity can control molecules that are invisible to the eye. A technology that controls energy conversion and chemical reactions occurring at the molecular level in real time
by combining a scanning tunneling microscope (STM), which can visualize and observe substances that are thinner than a hair in the nanometer (nm, 1 nm is one billionth of a meter), and terahertz (THz) light with an extremely short time scale in the picosecond (ps, 1 ps is one trillionth of a second) unit, has been developed through joint research between Korea and Japan
. [Photo = GIST]
The Gwangju Institute of Science and Technology (GIST, President Lim Ki-chul) announced on the 7th that the Department of Chemistry Professor Kim Yu-soo (Director of the Quantum Conversion Research Group, Institute for Basic Science) and Professor Hiroshi Imada, together with the Physical and Chemical Research Institute (RIKEN), Yokohama National University, the University of Tokyo, Hamamatsu Photonics KK, and the University of Ulsan, have developed a technology that can observe and control phenomena occurring at the molecular level in real time and at
ultra-high speed. The phenomenon of charge transfer between molecules and electrodes (charge exchange) is one of the fundamental molecular science phenomena that occurs in chemical reactions occurring on the surface of organic devices or catalysts.
During this charge transfer process, transient intermediate states such as charge states or excitons (quasi-quantum states in which negatively charged electrons and positively charged holes coexist) are formed. These states have a very short lifetime, on the order of picoseconds (ps, 1ps is one trillionth of a second). It's fleeting. In order to investigate their characteristics, it is necessary to control the charge at ultra-high speed.
Recent advances in optical technology have enabled ultrafast charge control using optical pulses in the terahertz (THz) region with a short time span of picoseconds. By combining terahertz (THz) pulses with scanning tunneling microscopy (STM), it has become possible to inject charges into materials at the nanometer (nm) level.
Existing THz-STMs can only measure currents due to charge manipulation, which limits their ability to investigate changes in molecular states that occur when charges are injected into molecules.
The research team developed a device that combines optical technology with STM (optical STM) and succeeded in observing various quantum phenomena at the single-molecule level more precisely.
Using a THz-optical STM device that combines optical STM and THz pulses, the research team conducted an experiment targeting a single Pd phthalocyanine molecule containing a palladium (Pd) atom at its center.
The research team was able to detect the luminescence of the molecule in the wavelength band near 660 nm by irradiating the STM with THz pulses. This result means that excitons were formed when charges were injected into the frontier orbitals (HOMO and LUMO) of the Pd phthalocyanine molecule, and luminescence occurred.
When the current was also measured while measuring the luminescence, almost no current flowed. This indicates that charges were exchanged only between the STM probe and the molecule, and that there was almost no forward current passing through the molecule.
The research team then investigated how the luminescence phenomenon changed when the waveform of the THz pulse was changed. The waveform of the THz pulse can be expressed as a physical quantity called the carrier envelope phase (the phase of the optical electric field oscillation (carrier) with respect to the envelope (envelope) of the optical pulse).
The research team developed a 'THz phase shifter', an optical device that can change the carrier envelope phase, and controlled the waveform of the THz pulse. As a result of measuring the luminescence intensity emitted from the molecule while changing the carrier envelope phase, it was observed that the luminescence intensity changed as the waveform of the THz pulse changed.
It was found that the luminescence intensity reached its maximum when the phase was near 210°.
Based on these research results, the research team concluded that it is possible to control the state of molecules and form excitons through ultrafast and continuous charge injection using THz pulses.
Professor Kim Yu-su said, "Through this study, we have established a method to measure and control the quantum state of molecules with extreme spatiotemporal resolution by combining THz pulses and optical STM." He continued, "This time, we were limited to detecting the light emitted from the molecule, but if combined with other laser light sources, it will open the way to measuring various optical phenomena such as Raman scattering and photoluminescence with high temporal resolution."
This research result (paper title: Ultrafast on-demand exciton formation in a single-molecule junction by tailored terahertz pulses) was published in the international scientific journal Science on March 7.
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