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High-Speed Imaging of Second-Harmonic Generation in MoS2 Bilayer under Femtosecond Laser Ablation by Kim, Young Chul;Yoo, Hoseong;Nguyen, Van Tu;Lee, Soonil;Park, Ji-Yong;Ahn, Yeong Hwan Nanomaterials, 2021 https://doi.org/10.3390/nano11071786 Abstract We report an in situ characterization of transition-metal dichalcogenide (TMD) monolayers and twisted bilayers using a high-speed second-harmonic generation (SHG) imaging technique. High-frequency laser modulation and galvano scanning in the SHG imaging enabled a rapid identification of the crystallinity in the TMD, including the orientation and homogeneity with a speed of 1 frame/s. For a twisted bilayer MoS2, we studied the SHG peak intensity and angles as a function of the twist angle under a strong interlayer coupling. In addition, rapid SHG imaging can be used to visualize laser-induced ablation of monolayer and bilayer MoS2 in situ under illumination by a strong femtosecond laser. Importantly, we observed a characteristic threshold behavior; the ablation process occurred for a very short time duration once the preheating condition was reached. We investigated the laser thinning of the bilayer MoS2 with different twist angles. When the twist angle was 0°, the SHG decreased by approximately one-fourth of the initial intensity when one layer was removed. Conversely, when the twist angle was approximately 60° (the SHG intensity was suppressed), the SHG increased abruptly close to that of the nearby monolayer when one layer was removed. Precise layer-by-layer control was possible because of the unique threshold behavior of the laser-induced ablation. Keywords: second-harmonic generation; transition-metal dichalcogenides; twisted bilayer; laser ablation
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2
- 작성자박은영
- 작성일2022-01-20
- 417
- 동영상동영상
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Nondestructive Tomographic Imaging of Rust with Rapid THz Time-Domain Spectroscopy by Kim, Hwan Sik;Baik, Seung Yeob;Lee, Joong Wook;Kim, Jangsun;Ahn, Yeong Hwan Applied Sciences-Basel, 2021 https://doi.org/10.3390/app112210594 Abstract In this study, we developed a rapid nondestructive tool for testing rust spread in a metal covered by a paint layer by using a THz time-domain spectroscopy system at a speed of 100 Hz/pixel. Time-of-flight imaging helps identify rust formation by exclusively obtaining the reflection from the steel below the paint surface. The use of frequency-selective imaging allows us to manipulate the contrast in rust imaging. Higher contrast is generally obtained when monitoring using the higher frequency component. In addition, we monitored the spread of rust in a steel plate under the influence of two different chemical solutions: NaCl and acid. We found that in the early stages, the decrease in THz reflection was governed by the high-frequency components due to the formation of lepidocrocite, whereas the low-frequency component develops as the proportion of hematite increases with time. Keywords: terahertz imaging; nondestructive testing; rust
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0
- 작성자박은영
- 작성일2022-01-20
- 392
- 동영상동영상
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Low Dark Current and Performance Enhanced Perovskite Photodetector by Graphene Oxide as an Interfacial Layer by Ali Hassan;Muhammad Azam;Yeong Hwan Ahn;Muhammad Zubair;Yu Cao;Abbas Ahmad Khan Nanomaterials, 2021 https://doi.org/10.3390/nano12020190 Abstract Organic-inorganic hybrid perovskite photodetectors are gaining much interest recently for their high performance in photodetection, due to excellent light absorption, low cost, and ease of fabrication. Lower defect density and large grain size are always favorable for efficient and stable devices. Herein, we applied the interface engineering technique for hybrid trilayer (TiO2/graphene oxide/perovskite) photodetector to attain better crystallinity and defect passivation. The graphene oxide (GO) sandwich layer has been introduced in the perovskite photodetector for improved crystallization, better charge extraction, low dark current, and enhanced carrier lifetime. Moreover, the trilayer photodetector exhibits improved device performance with a high on/off ratio of 1.3 × 104, high responsivity of 3.38 AW−1, and low dark current of 1.55 × 10−11 A. The insertion of the GO layer also suppressed the perovskite degradation process and consequently improved the device stability. The current study focuses on the significance of interface engineering to boost device performance by improving interfacial defect passivation and better carrier transport. Keywords: photodetector; perovskite; graphene oxide; low dark current; defect passivation
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-2
- 작성자박은영
- 작성일2022-01-20
- 383
- 동영상동영상
[논문] 안영환, 이순일 - High-Speed Imaging of Second-Harmonic Generation in MoS2 Bilayer under Femtosecond Laser Ablation 위치 확인