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Search Results: 1 - 10 of 26687 matches for " Jae-Ung Lee "
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Resonant Raman and photoluminescence spectra of suspended molybdenum disulfide
Jae-Ung Lee,Kangwon Kim,Hyeonsik Cheong
Physics , 2015, DOI: 10.1088/2053-1583/2/4/044003
Abstract: Raman and photoluminescence (PL) measurements were performed on suspended and supported MoS2 up to 6 trilayers (TLs). The difference in the dielectric environment of suspended and supported MoS2 does not lead to large differences in the PL or Raman spectra. Most of the apparent difference in the PL intensity can be explained by the interference effect except for 1TL MoS2. The positions of the Raman peaks are not much different between suspended and supported MoS2. The relative intensities, on the other hand, show some differences between suspended and supported samples. In particular, the inter-layer vibration modes are enhanced near resonance with C excitons, with the breathing modes becoming relatively stronger. Up to 4 breathing modes are resolved in suspended 6TL MoS2.
Excitation energy dependent Raman spectrum of MoSe2
Dahyun Nam,Jae-Ung Lee,Hyeonsik Cheong
Physics , 2015, DOI: 10.1038/srep17113
Abstract: Raman investigation of MoSe2 was carried out with eight different excitation energies. Seven peaks, including E1g, A1g, E2g1, and A2u2 peaks are observed in the range of 100-400 cm-1. The phonon modes are assigned by comparing the peak positions with theoretical calculations. The intensities of the peaks are enhanced at different excitation energies through resonance with different optical transitions. The A1g mode is enhanced at 1.58 and 3.82 eV, which are near the A exciton energy and the band-to-band transition between higher energy bands, respectively. The E2g1 mode is strongly enhanced with respect to the A1g mode for the 2.71- and 2.81-eV excitations, which are close to the C exciton energy. The different enhancements of the A1g and E2g1 modes are explained in terms of the symmetries of the exciton states and the exciton-phonon coupling. Other smaller peaks including E1g and A2u2 are forbidden but appear due to the resonance effect near optical transition energies.
Estimation of Young's Modulus of Graphene by Raman Spectroscopy
Jae-Ung Lee,Duhee Yoon,Hyeonsik Cheong
Physics , 2012, DOI: 10.1021/nl301073q
Abstract: The Young's modulus of graphene is estimated by measuring the strain applied by a pressure difference across graphene membranes using Raman spectroscopy. The strain induced on pressurized graphene balloons can be estimated directly from the peak shift of the Raman G band. By comparing the measured strain with numerical simulation, we obtained the Young's modulus of graphene. The estimated Young's modulus values of single- and bi-layer graphene are 2.4\pm0.4 TPa and 2.0\pm0.5 TPa, respectively.
Excitation Energy Dependent Raman Signatures of ABA- and ABC-stacked Few-layer Graphene
The An Nguyen,Jae-Ung Lee,Duhee Yoon,Hyeonsik Cheong
Physics , 2014, DOI: 10.1038/srep04630
Abstract: The dependence of the Raman spectrum on the excitation energy has been investigated for ABA-and ABC- stacked few-layer graphene in order to establish the fingerprint of the stacking order and the number of layers, which affect the transport and optical properties of few-layer graphene. Five different excitation sources with energies of 1.96, 2.33, 2.41, 2.54 and 2.81 eV were used. The position and the line shape of the Raman 2D, G*, N, M, and other combination modes show dependence on the excitation energy as well as the stacking order and the thickness. One can unambiguously determine the stacking order and the thickness by comparing the 2D band spectra measured with 2 different excitation energies or by carefully comparing weaker combination Raman modes such as N, M, or LOLA modes. The criteria for unambiguous determination of the stacking order and the number of layers up to 5 layers are established.
Thermal conductivity of suspended pristine graphene measured by Raman spectroscopy
Jae-Ung Lee,Duhee Yoon,Hakseong Kim,Sang Wook Lee,Hyeonsik Cheong
Physics , 2011, DOI: 10.1103/PhysRevB.83.081419
Abstract: The thermal conductivity of suspended single-layer graphene was measured as a function of temperature using Raman scattering spectroscopy on clean samples prepared directly on a prepatterned substrate by mechanical exfoliation without chemical treatments. The temperature at the laser spot was monitored by the frequency of the Raman 2$D$ band of the Raman scattering spectrum, and the thermal conductivity was deduced by analyzing heat diffusion equations assuming that the substrate is a heat sink at ambient temperature. The obtained thermal conductivity values range from $\sim$1800 Wm$^{-1}$K$^{-1}$ near 325 K to $\sim$710 Wm$^{-1}$K$^{-1}$ at 500 K.
Anomalous excitonic resonance Raman effects in few-layer MoS2
Jae-Ung Lee,Jaesung Park,Young-Woo Son,Hyeonsik Cheong
Physics , 2015, DOI: 10.1039/C4NR05785F
Abstract: The resonance effects on the Raman spectra from 5 to 900 cm-1 of few-layer MoS2 thin films up to 14-layers were investigated by using six excitation energies. For the main first-order Raman peaks, the intensity maximum occurs at ~2.8 eV for single layer and at ~2.5 eV for few-layer MoS2, which correspond to the band-gap energy. At the excitation energy of 1.96 eV, several anomalous behaviors are observed. Many second-order peaks are anomalously enhanced even though the main first-order peaks are not enhanced. In the low-frequency region (<100 cm-1), a broad peak centered at ~38 cm-1 and its second order peak at 76 cm-1 appear for the excitation energy of 1.96 eV. These anomalous resonance effects are interpreted as being due to strong resonance with excitons or exciton-polaritons.
Anomalous polarization dependence of Raman scattering and crystallographic orientation of black phosphorus
Jungcheol Kim,Jae-Ung Lee,Jinhwan Lee,Hyo Ju Park,Zonghoon Lee,Changgu Lee,Hyeonsik Cheong
Physics , 2015, DOI: 10.1039/C5NR04349B
Abstract: We investigated polarization dependence of the Raman modes in black phosphorus (BP) using five different excitation wavelengths. The crystallographic orientation was determined by comparing polarized optical microscopy with high-resolution transmission electron microscope analysis. In polarized Raman spectroscopy, the B2g mode shows the same polarization dependence regardless of the excitation wavelength or the sample thickness. On the other hand, the Ag1 and Ag2 modes show a peculiar polarization behavior that depends on the excitation wavelength and the sample thickness. The thickness dependence can be explained by considering the anisotropic interference effect due to birefringence and dichroism of the BP crystal, but the wavelength dependence cannot be explained. We propose a simple and fail-proof procedure to determine the orientation of a BP crystal by combining polarized Raman scattering with polarized optical microscopy.
Polarization dependence of double resonant Raman scattering band in bilayer graphene
Jae-Ung Lee,Ngor Mbaye Seck,Duhee Yoon,Seon-Myeong Choi,Young-Woo Son,Hyeonsik Cheong
Physics , 2014, DOI: 10.1016/j.carbon.2014.02.007
Abstract: The polarization dependence of the double resonant Raman scattering (2D) band in bilayer graphene (BLG) is studied as a function of the excitation laser energy. It has been known that the complex shape of the 2D band of BLG can be decomposed into four Lorentzian peaks with different Raman frequency shifts attributable to four individual scattering paths in the energy-momentum space. From our polarization dependence study, however, we reveal that each of the four different peaks is actually doubly degenerate in its scattering channels, i.e., two different scattering paths with similar Raman frequency shifts for each peak. We find theoretically that one of these two paths, ignored for a long time, has a small contribution to their scattering intensities but are critical in understanding their polarization dependences. Because of this, the maximum-to-minimum intensity ratios of the four peaks show a strong dependence on the excitation energy, unlike the case of single-layer graphene (SLG). Our findings thus reveal another interesting aspect of electron-phonon interactions in graphitic systems.
Photocurrent generation at ABA/ABC lateral junction in tri-layer graphene photodetector
Minjung Kim,Seon-Myeong Choi,Ho Ang Yoon,Sun Keun Choi,Jae-Ung Lee,Jungcheol Kim,Sang Wook Lee,Young-Woo Son,Hyeonsik Cheong
Physics , 2015, DOI: 10.1016/j.carbon.2015.09.095
Abstract: Metal-graphene-metal photodetectors utilize photocurrent generated near the graphene/metal junctions and have many advantages including high speed and broad-band operation. Here, we report on photocurrent generation at ABA/ABC stacking domain junctions in tri-layer graphene with a responsivity of 0.18 A/W. Unlike usual metal-graphene-metal devices, the photocurrent is generated in the middle of the graphene channel, not confined to the vicinity of the metal electrodes. The magnitude and the direction of the photocurrent depend on the back-gate bias. Theoretical calculations show that there is a built-in band offset between the two stacking domains, and the dominant mechanism of the photocurrent is the photo-thermoelectric effect due to the Seebeck coefficient difference.
Genetic polymorphism and natural selection in the C-terminal 42?kDa region of merozoite surface protein-1 among Plasmodium vivax Korean isolates
Jung-Mi Kang, Hye-Lim Ju, Yoo-Mi Kang, Dong-Hyun Lee, Sung-Ung Moon, Woon-Mok Sohn, Jae-Won Park, Tong-Soo Kim, Byoung-Kuk Na
Malaria Journal , 2012, DOI: 10.1186/1475-2875-11-206
Abstract: A total of 149 P. vivax-infected blood samples collected from patients in Korea were used. The region flanking PvMSP-142 was amplified by PCR, cloned into Escherichia coli, and then sequenced. The polymorphic characteristic and natural selection of PvMSP-142 were analysed using the DNASTAR, MEGA4 and DnaSP programs.A total of 11 distinct haplotypes of PvMSP-142 with 40 amino acid changes, as compared to the reference Sal I sequence, were identified in the Korean P. vivax isolates. Most of the mutations were concentrated in the 33?kDa fragment (PvMSP-133), but a novel mutation was found in the 19?kDa fragment (PvMSP-119). PvMSP-142 of Korean isolates appeared to be under balancing selection. Recombination may also play a role in the resulting genetic diversity of PvMSP-142.PvMSP-142 of Korean P. vivax isolates displayed allelic polymorphisms caused by mutation, recombination and balancing selection. These results will be useful for understanding the nature of the P. vivax population in Korea and for development of a PvMSP-142 based vaccine against P. vivax.
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