Efficiently controlling heat generation has become a major challenge as the semiconductors used in smartphones, computers, and similar devices have become higher-performance and more miniaturized. The way heat is transmitted is determined by the vibration of the atoms that constitute a material, but it is not easy to directly examine, at the atomic scale, how neighboring atoms vibrate in coordination with one another.
A research group made up of Director and Professor Naoya Shibata, JSPS Research Fellow Koudai Tabata, Associate Professor Taketo Seki, and Project Associate Professor Ryo Ishikawa, all of the Institute of Engineering Innovation, School of Engineering, The University of Tokyo, have succeeded in an atomic-scale double-slit experiment(1) that treats neighboring atoms as “two slits,” and demonstrated that it is possible to read out the vibration of atoms from the fringe pattern formed by electrons.
The double-slit experiment makes use of a phenomenon where waves passed through two narrow slits overlap and create a pattern of bright and dark fringes. Since the British physicist Thomas Young first performed it using light in the early 19th century, it (Young’s experiment) has been known as the most fundamental experiment demonstrating the wave nature of light. If this experiment could be reduced to the atomic scale inside a material, it would allow direct investigation of the arrangement and motion of atoms at the level of a single atomic bond, but an atomic-scale double-slit experiment of this kind has never been realized until now.
The research group successfully observed atomic-scale double-slit interference (1) with an electron beam by utilizing a phenomenon where, when an electron beam narrowly focused by a scanning transmission electron microscope (STEM) (2) is incident between two neighboring silicon atomic columns (3) separated by only 136 pm (picometer: 1 pm is one trillionth of a meter), the atom pairs behave like a double slit. This scale corresponds to shrinking Young’s experiment by roughly seven orders of magnitude (one ten-millionth).
Furthermore, by closely examining how the fringe pattern appears, the researchers found that it is possible to read out the degree to which neighboring atoms vibrate in the same direction. This represents a new method for investigating the strength of bonding between atoms and how heat is conducted within a material.
This technique is a new measurement method for gaining information about phonons (lattice vibrations)(4), which affect the rigidity of atomic bonds and how heat is conducted, by focusing on individual atomic bonds one at a time. Going forward, applying this method to research and development of semiconductor materials may make it possible to investigate, at the atomic level, regions where heat tends to accumulate or where it does not flow easily. In the future, this is expected to contribute to heat-dissipation design in semiconductor devices and to the development of materials that use heat efficiently.
(1) Double-slit experiment, interference
The double-slit experiment makes use of a phenomenon where waves passing through two slits overlap, creating a striped pattern (interference fringes) made up of alternating regions of reinforcement (bright lines) and regions of cancellation (dark lines). This phenomenon is observed across waves in general, and serves as a classic example demonstrating wave nature.
(2) Scanning transmission electron microscope (STEM)
A device that observes the structure of a sample by scanning a narrowly focused electron beam (electron probe) over the sample and detecting the transmitted and scattered electrons. The electron probe can be focused down to 100 pm or less, enabling observation at a resolution capable of distinguishing individual atoms.
(3) Atomic column, dumbbell structure
When a crystal is viewed from a specific direction, the row of atoms that line up overlapping along the direction of travel of the electron beam is called an atomic column. When silicon is viewed from the [110] crystallographic direction, two atomic columns lie close together in a pair, and due to the shape this is called a dumbbell structure. In this research, this pair was used as a natural double slit.
(4) Phonon (lattice vibration)
A phonon is a concept which treats the way a large number of atoms in a crystal vibrate collectively as though they were particles. Phonons are responsible for a material’s thermal conduction and specific heat.
The research was conducted jointly with Associate Professor Toma Susi of the University of Vienna.
The results were achieved as part of the research project “SHIBATA Ultra-atomic Resolution Electron Microscopy” supported by the Japan Science and Technology Agency (JST) under the Strategic Basic Research Program ERATO
This project aims to realize the simultaneous observation of atomic-scale structure and electromagnetic field distribution across a temperature range from ultra-low to high temperatures, and thereby to construct a new measurement technique—one that could be called ultra-atomic-resolution electron microscopy—surpassing conventional atomic-resolution electron microscopes, and capable of directly “seeing” the origins of material and biological functions.
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