Imaging sound movements of atomic units with an optical microscope
An international team of researchers, led by Dr. Hyunmin Kim of Companion Diagnostics and Medical Technology Research Group at DGIST developed an imaging technique to monitor the sound movement of an atomically thin substance at a high-resolution of 300-nanometer. The technology is expected to be used in the development of new materials, solar cells, and catalyst, etc.
The research team, through a joint research project with Professor Jong-Hyun Ahn's research team at Yonsei University, has presented a 'Transient Second-Harmonic Generation (TSHG) Pulse Imaging System', which can analyze the ultra-fast dynamics of light interacting with molybdenum disulfide (MoS2), a typical 2D atom laminating structure, at a resolution of 300 nanometer (nm).
This research is particularly significant, as the existing equipment used for measuring ultrasonic waves generated by the vibration of ultrafast electrons and lattice had limitations in application due to the noise ratio compared to low signal and spatial resolution. Despite the limitations, the research team succeeded to develop the microscope with the improved optical resolution and it may lead to quick and accurate analysis of material characteristics in the mass production era of semiconductor 2D materials.
TSHG Imaging Technology developed by the researchers can measure sound generation at the level of 1011Hz unit (1Hz vibrates once per one second), which is generated by the reaction of a lattice and electron moved by a pump pulse having a different wavelength, using the generation of a wavelength that is half of the pro pulse wavelength at the point where symmetry is broken on a crystal substance.
Previously, to measure the ultrafast electron movement of a femto-second (10-15 second) unit in a 2D atomic unit structure or the generation of related sound, a pulse wave in pump-probe had to be exposed to a material. The change in the absorption or reflection of the probe pulse generated was measured for analysis. However, the signals were small, so the measurement time had to be extended and a high-performance signal amplifier had to be used to increase the Signal-to-Noise ratio.
Also, the laser used in this method had high energy so it could cause sample damage and a detachable state of the molecules if the focus size of the laser was adjusted to below a micrometer (?). There were also limitations in analysis if the sample size was small.
In this study, to decrease the laser focus size while reducing damage to the sample, Dr. Kim and his team decreased the laser output used in an existing transient-absorption spectroscope by thousands to tens of thousands of times, and applied a high-performance scanning system to visualize it in real-time.
The research team increased the substance penetration level of the laser using a near-infrared ray pulse length of 1.04? size as a probe pulse and located the secondary harmonic pulse length on the visible ray section of green color (520nm). Using this method, theymaximized efficiency to analyze the movement of electrons to the ionization energy section of the dense energy band of the 2D substance when combined with the pump pulse.
According to the research team, it is proved that the new imaging technology is useful to analyze various atomic structures such as hexagon and triangular stars, by combining the Second-Harmonic Generation of Pulse Imaging System with a 4-wave mixed pulse imaging function and applying it to lamination structural analysis of molybdenum disulfide manufactured using the chemical vapor deposition (CVD) method.
In addition, the TSHG technique is expected to contribute to research on related materials. The research can be applied to studies of electron lifespan that determine the efficiency of energy materials and catalysts such as 2D materials and perovskite and quantum dots.
Dr. Kim said, "The electron-hole movement analysis of materials which are mass-produced using the 'Transient Second-Harmonic Generation of Pulse Imaging Technology" can be visualized simultaneously, which will contribute greatly to the development of source technology based on new nano-materials. We will research and develop super-precision energy and optical elements by expanding the high-resolution real-time analysis technology we have secured to the analysis of physical lattice constraint environments."
This study was featured on the cover of the world-class scholarly journal Advanced Materials on February 13th and in the online version of 'NPG Asia Materials', a sister journal of the world-class scholarly journal Nature, on February 9th.
For more information, please contact:
Senior Researcher Hyunmin Kim
Companion Diagnostics and Medical Technology Research Group
Daegu Gyeongbuk Institute of Science and Technology (DGIST)
E-mail: [email protected]
Research Paper on Journal of Advanced Materials https://doi.org/10.1002/adma.201705190
Research Paper on Journal of NPG Asia Materials https://doi.org/10.1038/am.2017.226
Houk Jang, Krishna P. Dhakal, Kyung-Il Joo, Won Seok Yun, Sachin M. Shinde, Xiang Chen, Soon Moon Jeong, Suk Woo Lee, Zonghoon Lee, JaeDong Lee, Jong-Hyun Ahn,* Hyunmin Kim, "Transient SHG Imaging on Ultrafast Carrier Dynamics of MoS2 Nanosheets,"Advanced Materials, online Published on Feburary 13th, 2018.
Sachin M. Shinde, Krishna P. Dhakal, Xiang Chen, Won Seok Yun, Jae Dong Lee, Hyunmin Kim*, Jong-Hyun Ahn*, "A Stacking-Controllable Interlayer Coupling and Symmetric Configuration of Multilayered MoS2,"NPG Asia Materials, online Published on Feburary 9th, 2018.