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Dive into the research topics where Mun Seok Jeong is active.

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Featured researches published by Mun Seok Jeong.


Scientific Reports | 2015

Control of Photoluminescence of Carbon Nanodots via Surface Functionalization using Para-substituted Anilines

Woosung Kwon; Sungan Do; Ji-Hee Kim; Mun Seok Jeong; Shi-Woo Rhee

Carbon nanodots (C-dots) are a kind of fluorescent carbon nanomaterials, composed of polyaromatic carbon domains surrounded by amorphous carbon frames, and have attracted a great deal of attention because of their interesting properties. There are still, however, challenges ahead such as blue-biased photoluminescence, spectral broadness, undefined energy gaps and etc. In this report, we chemically modify the surface of C-dots with a series of para-substituted anilines to control their photoluminescence. Our surface functionalization endows our C-dots with new energy levels, exhibiting long-wavelength (up to 650 nm) photoluminescence of very narrow spectral widths. The roles of para-substituted anilines and their substituents in developing such energy levels are thoroughly studied by using transient absorption spectroscopy. We finally demonstrate light-emitting devices exploiting our C-dots as a phosphor, converting UV light to a variety of colors with internal quantum yields of ca. 20%.


ACS Nano | 2015

Synthesis of Centimeter-Scale Monolayer Tungsten Disulfide Film on Gold Foils

Seok Joon Yun; Sang Hoon Chae; Hyun Ok Kim; Jin Cheol Park; Ji-Hoon Park; Gang Hee Han; Joo Song Lee; Soo Min Kim; Hye Min Oh; Jinbong Seok; Mun Seok Jeong; Ki Kang Kim; Young Hee Lee

We report the synthesis of centimeter-scale monolayer WS2 on gold foil by chemical vapor deposition. The limited tungsten and sulfur solubility in gold foil allows monolayer WS2 film growth on gold surface. To ensure the coverage uniformity of monolayer WS2 film, the tungsten source-coated substrate was placed in parallel with Au foil under hydrogen sulfide atmosphere. The high growth temperature near 935 °C helps to increase a domain size up to 420 μm. Gold foil is reused for the repeatable growth after bubbling transfer. The WS2-based field effect transistor reveals an electron mobility of 20 cm(2) V(-1) s(-1) with high on-off ratio of ∼10(8) at room temperature, which is the highest reported value from previous reports of CVD-grown WS2 samples. The on-off ratio of integrated multiple FETs on the large area WS2 film on SiO2 (300 nm)/Si substrate shows within the same order, implying reasonable uniformity of WS2 FET device characteristics over a large area of 3 × 1.5 cm(2).


ACS Nano | 2011

Laser thinning for monolayer graphene formation: heat sink and interference effect.

Gang Hee Han; Seung Jin Chae; Eun Sung Kim; Fethullah Güneş; Il Ha Lee; Sang Won Lee; Si Young Lee; Seong Chu Lim; Hae Kyung Jeong; Mun Seok Jeong; Young Hee Lee

Despite the availability of large-area graphene synthesized by chemical vapor deposition (CVD), the control of a uniform monolayer graphene remained challenging. Here, we report a method of acquiring monolayer graphene by laser irradiation. The accumulation of heat on graphene by absorbing light, followed by oxidative burning of upper graphene layers, which strongly relies on the wavelength of light and optical parameters of the substrate, was in situ measured by the G-band shift in Raman spectroscopy. The substrate plays a crucial role as a heat sink for the bottom monolayer graphene, resulting in no burning or etching. Oscillatory thinning behavior dependent on the substrate oxide thickness was evaluated by adopting a simple Fresnels equation. This paves the way for future research in utilizing monolayer graphene for high-speed electronic devices.


Applied Physics Letters | 2009

Directional control of surface plasmon polariton waves propagating through an asymmetric Bragg resonator

Soo Bong Choi; D. J. Park; Y. K. Jeong; Y. C. Yun; Mun Seok Jeong; Clare C. Byeon; Ju Hyung Kang; Q.-Han Park; D. S. Kim

We have demonstrated the directional control of surface plasmon polariton (SPP) waves propagating through an asymmetric plasmonic Bragg resonator using femtosecond temporal-phase control via the resonant coupling of SPPs and the interference of SPPs. The near-field images display significant temporal-phase dependence, switching between left and right propagation after the Bragg resonator. Our results would be a key step toward the control of surface plasmon propagation direction in nanoscaled plasmonic applications


Applied Physics Letters | 2001

Spatially resolved photoluminescence in InGaN/GaN quantum wells by near-field scanning optical microscopy

Mun Seok Jeong; Jinwoo Kim; Y.-W. Kim; J. O. White; Eun-Kyung Suh; C.-H. Hong; H. J. Lee

Spatially and spectrally resolved photoluminescence (PL) from InGaN/GaN quantum wells is obtained using near-field scanning optical microscopy (NSOM). Samples displaying high macroscopic PL intensity revealed nonuniform intensity and linewidth but nearly uniform peak position. It suggests that the contrast in the NSOM image reflects nonuniform distribution of dislocations or defects which act as nonradiative recombination centers. The formation of quantum dots with size of 30±25 nm and their size-dependent interaction with dislocations were observed in plan-view transmission electron microscopy. It is likely that the high luminescence efficiency is due to the efficient localization of excitons in high-density quantum dots located in regions with fewer dislocations.


Journal of Applied Physics | 2010

Luminescence and Raman studies of YNbO4 phosphors doped by Eu3+, Ga3+, and Al3+

Mihail Nazarov; Young Jin Kim; Eun Young Lee; Kyoung-In Min; Mun Seok Jeong; Su Woong Lee; Do Young Noh

Efficient phosphors based on YNbO4 and doped by Eu3+, Ga3+, and Al3+ were synthesized with different fluxes under different thermal conditions and investigated under x-ray excitation and Raman spectroscopy. The samples codoped by Al3+ and Ga3+ show higher intensity under x-ray excitation in comparison with YNbO4:Eu3+ phosphors. The model of redistribution energy transfer from D51 level to F7J is proposed. Monoclinic fergusonite crystal structure and excellent luminescent properties under x-ray excitation allow recommend these phosphors as good candidates for different applications.


Scientific Reports | 2015

Carrier localization in In-rich InGaN/GaN multiple quantum wells for green light-emitting diodes

Hyun Jeong; Hyeon Jun Jeong; Hye Min Oh; Chang-Hee Hong; Eun-Kyung Suh; Gilles Lerondel; Mun Seok Jeong

Carrier localization phenomena in indium-rich InGaN/GaN multiple quantum wells (MQWs) grown on sapphire and GaN substrates were investigated. Temperature-dependent photoluminescence (PL) spectroscopy, ultraviolet near-field scanning optical microscopy (NSOM), and confocal time-resolved PL (TRPL) spectroscopy were employed to verify the correlation between carrier localization and crystal quality. From the spatially resolved PL measurements, we observed that the distribution and shape of luminescent clusters, which were known as an outcome of the carrier localization, are strongly affected by the crystalline quality. Spectroscopic analysis of the NSOM signal shows that carrier localization of MQWs with low crystalline quality is different from that of MQWs with high crystalline quality. This interrelation between carrier localization and crystal quality is well supported by confocal TRPL results.


Optics Express | 2009

Terahertz near-field enhancement in narrow rectangular apertures on metal film

D. J. Park; Soo Bong Choi; Y. H. Ahn; Fabian Rotermund; Ik-Bu Sohn; Chul Kang; Mun Seok Jeong; D. S. Kim

We report huge field accumulations in rectangular aperture arrays on thin metal film by using shape resonance in THz frequency region. A huge far-field transmission enhancement is observed in samples of various widths ranging from 10 mum to 1.8 mum which correspond to only an order of lambda/100. Theoretical calculations based on vector diffraction theory indicates 230 times near-field enhancement in case of the 1.8 mum wide rectangular aperture. Transmission measurement through the single rectangular aperture shows that the shape resonance, not the periodicity, is mainly responsible for the transmission enhancement and the corresponding field enhancement.


Nature Communications | 2016

Unusually efficient photocurrent extraction in monolayer van der Waals heterostructure by tunnelling through discretized barriers

Woo Jong Yu; Quoc An Vu; Hyemin Oh; Hong Gi Nam; Hailong Zhou; Soonyoung Cha; Joo Youn Kim; Alexandra Carvalho; Mun Seok Jeong; Hyunyong Choi; A. H. Castro Neto; Young Hee Lee; Xiangfeng Duan

Two-dimensional layered transition-metal dichalcogenides have attracted considerable interest for their unique layer-number-dependent properties. In particular, vertical integration of these two-dimensional crystals to form van der Waals heterostructures can open up a new dimension for the design of functional electronic and optoelectronic devices. Here we report the layer-number-dependent photocurrent generation in graphene/MoS2/graphene heterostructures by creating a device with two distinct regions containing one-layer and seven-layer MoS2 to exclude other extrinsic factors. Photoresponse studies reveal that photoresponsivity in one-layer MoS2 is surprisingly higher than that in seven-layer MoS2 by seven times. Spectral-dependent studies further show that the internal quantum efficiency in one-layer MoS2 can reach a maximum of 65%, far higher than the 7% in seven-layer MoS2. Our theoretical modelling shows that asymmetric potential barriers in the top and bottom interfaces of the graphene/one-layer MoS2/graphene heterojunction enable asymmetric carrier tunnelling, to generate usually high photoresponsivity in one-layer MoS2 device.


ACS Nano | 2015

Semiconductor–Insulator–Semiconductor Diode Consisting of Monolayer MoS2, h-BN, and GaN Heterostructure

Hyun Jeong; Seungho Bang; Hye Min Oh; Hyeon Jun Jeong; Sung-Jin An; Gang Hee Han; Hyun Kim; Ki Kang Kim; Jin Cheol Park; Young Hee Lee; Gilles Lerondel; Mun Seok Jeong

We propose a semiconductor-insulator-semiconductor (SIS) heterojunction diode consisting of monolayer (1-L) MoS2, hexagonal boron nitride (h-BN), and epitaxial p-GaN that can be applied to high-performance nanoscale optoelectronics. The layered materials of 1-L MoS2 and h-BN, grown by chemical vapor deposition, were vertically stacked by a wet-transfer method on a p-GaN layer. The final structure was verified by confocal photoluminescence and Raman spectroscopy. Current-voltage (I-V) measurements were conducted to compare the device performance with that of a more classical p-n structure. In both structures (the p-n and SIS heterojunction diode), clear current-rectifying characteristics were observed. In particular, a current and threshold voltage were obtained for the SIS structure that was higher compared to that of the p-n structure. This indicated that tunneling is the predominant carrier transport mechanism. In addition, the photoresponse of the SIS structure induced by the illumination of visible light was observed by photocurrent measurements.

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Hyun Jeong

Gwangju Institute of Science and Technology

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Eun-Kyung Suh

Chonbuk National University

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Hye Min Oh

Sungkyunkwan University

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Kyoung-Duck Park

University of Colorado Boulder

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Clare C. Byeon

Gwangju Institute of Science and Technology

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Do-Kyeong Ko

Gwangju Institute of Science and Technology

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Soo Bong Choi

Incheon National University

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