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▲ Metal insulator phase change from vanadium dioxide nanoparticles (image/data provided by UNIST) © Patent News |
A higher resolution imaging technique that can directly observe and control sudden changes that occur during ‘trillionths of a second’ has been successfully implemented.
UNIST Department of Chemistry Professor Oh-Hoon Kwon’s team used Korea’s only ‘four-dimensional high-speed transmission electron microscope’ to investigate the very fast phase change process of a metal insulator from vanadium dioxide (VO2) nanoparticles in a femtosecond. 10-15It was captured directly in real time and space with a second level of accuracy.
Vanadium dioxide exhibits a metal-insulator phase change phenomenon at 68 degrees Celsius, and is evident as a key next-generation material such as optical sensors and fast-switching devices. However, because this phase change process occurs in a very short time period of a femtosecond, it was impossible to observe it directly at the nanoparticle level with current imaging techniques.
Ultrafast transmission electron microscopy produces femtosecond pulses of photoelectrons at a photocathode and accelerates them to high energy, resulting in bigometers shorter than the atomic size.10-12m) with high spatial and temporal resolution. However, each electron that makes up the photoelectron pulse has a negative charge, so it exhibits the property of repelling each other. As a result, the photoelectron pulse passes through the barrel of the microscope lens and gradually spreads out in space and time, leading to a decrease in resolution.
The research team used a commonly used energy filter in a different way to overcome the limitations of transmission electron microscopy and photograph the phase change process of vanadium dioxide. First, part of the photoelectron pulses that propagated through spacetime on reaching the microscope camera were filtered out with an energy filter. Afterwards, the image was reconstructed with some of the filtered photoelectrons, and the phase shift during the moment down to a femtosecond was clearly captured. This is a result of using the physical law that photoelectrons with the same energy exist in the same space and time after acceleration.
By using the energy filter in this way, the extremely fast phase change process of each individual nanoparticle that makes up the vanadium dioxide nanoparticle cluster can be captured at once. In particular, the research team also confirmed for the first time direct evidence that vanadium dioxide nanoparticles made on a graphene substrate have a different structure, so that they can go through a ‘meta-stable state’ in the middle of a phase change.
Dr. Kim Ye-jin (currently a postdoctoral researcher at the California Institute of Technology (Caltech)), first author, said, “Many efforts have been made to improve the time resolution of ultrafast transmission electron microscopes.” It has been proven that the process of material change that takes place at the nanometer level can be clearly photographed.”
Professor Kwon Oh-hoon said, “This is the first research in advanced imaging that experimentally implements femtosecond imaging techniques based on universal knowledge of physics.” It will be able to contribute to increasing its use.”
The results of the research were published on January 27 at 2:00pm (local time) in ‘Science Advances’, a sister journal to the world-renowned academic journal Science. (Title of paper: Femtosecond-Resolved Imaging of Single-Particle Transition in Energy-Filtered Ultrafast Electron Microscopy)
