The Ulsan National Institute of Science and Technology (UNIST) announced on Aug. 7 that professor Kwon Oh-hoon at its School of Natural Science and his research team has developed an analysis method by which how a subnanometer-level substance changes in structure can be seen every femtosecond by means of an ultrafast transmission electron microscope. The team used the method for a real-time observation of how rod-shaped gold nanoparticles change in response to external energy and published the result of the observation on the Aug. 7 edition of Matter, a sister journal to Cell.
Techniques for analyzing the structures of substances have been consistently developed to the point of being able to observe single atoms. However, a substance constantly changes inside every femtosecond and an analysis method for responding to an extremely transient reaction is needed to grasp physical properties with accuracy and precision. Essential to that end is a higher time resolution.
Although optical microscopes recently reached a femtosecond-level time resolution, the devices still have their own limitations in terms of the minimum observable size and are incapable of distinguishing subnanometer-level substances. On the other hand, the electron microscope allows such substances to be observed while achieving a femtosecond-level time resolution by adjusting the speed of electron beams.
The research team succeeded in observing the vibration of the nanoparticles on a femtosecond basis by means of the ultrafast transmission electron microscope, which projects an electron beam every femtosecond. The team irradiated the nanoparticles with laser to generate the acoustic vibration, conducted the observation by means of electron beams projected on a femtosecond basis, and combined the resultant successive images to produce a single image.
In addition, the team used a camera for direct electron detection as a detector to increase the limit of detection approximately tenfold. An optical microscope allows an image to be checked immediately by using transmitted or reflected light, but the electron microscope requires a detector so that an electron containing the shape of a sample is converted into a photon and the photon is converted back into an electron for electric signal-to-image conversion. The research team lowered the limit of the minimum detectable signal by simplifying that process.