Seung Jin Chae - Academia.edu (original) (raw)

Papers by Seung Jin Chae

Journal of Information Display

From the very first time that graphene was used as a transparent electrode for OLED applications,... more From the very first time that graphene was used as a transparent electrode for OLED applications, the emergence of active-matrix (AM)-graphene OLED displays has been envisioned. Realizing this expectation, however, turned out to be difficult. Two obstacles are the growth and transfer of a large-area graphene film and the patterning of a graphene film into pixels. To solve these problems, a process of patterning a graphene film without surface contamination was developed. The fabrication of OLED panels by the patterned graphene anode on Gen 2(370 × 470 mm)-sized and flexible substrates was successfully demonstrated. In this work, oxide TFT arrays were combined as a switching backplane, and a pixelated graphene OLED was used as an emissive layer, to realize AM-graphene OLED displays. To explore the technical feasibility of flexible AM-graphene OLED displays, the aforementioned components were formed on a flexible substrate. For commercial-level production, all the processes that were used were chosen to be compatible with the conventional display processes.

Precise control of morphologies of one-dimensional (1D) or 2D nanostructures during growth has no... more Precise control of morphologies of one-dimensional (1D) or 2D nanostructures during growth has not been easily accessible, usually degrading the device performance and therefore limiting applications to various advanced nanoscale electronics and optoelectronics. Graphene could be a platform to serve as a substrate for both morphology control and direct use of electrodes due to its ideal monolayer flatness with π electrons. Here, we report that by using graphene directly as a substrate, vertically well-aligned ZnO nanowires and nanowalls were obtained systematically by controlling Au catalyst thickness and growth time, without invoking significant thermal damage on the graphene layer during thermal chemical vapor deposition of ZnO at high temperature of about 900 o C. We further demonstrate a piezoelectric nanogenerator that was fabricated from the vertically aligned nanowire-nanowall ZnO hybrid/graphene structure generates a new type of direct current.

Journal of Applied Physics, Jun 15, 2009

Single-walled carbon nanotube field-effect transistors were irradiated with 20 keV electrons usin... more Single-walled carbon nanotube field-effect transistors were irradiated with 20 keV electrons using an e-beam lithography exposure method. Analysis of conduction data in the temperature range from 25 to 300 K indicated the creation of insulating regions containing traps along the nanotube channel. Further analysis of semiconducting and metallic nanotube devices shows dramatic differences in the effect of the electron exposure on the hopping defect barrier heights. Barriers for metallic nanotubes saturate at significantly larger values than semiconducting nanotubes due to shorter localization lengths. The limited and near constant density of states at the Fermi level induces a larger hopping length to localization length ratio, further limiting current and increasing measured trap heights. Poole-Frenkel hopping with an adjustment for electron localization is utilized to explain the inconsistencies. n-type and p-type barriers in the nanotube devices displayed exponential dependence on applied gate voltage bias, with the peak barrier height in the metallic device defining a switch of majority carrier.

CLEO: 2014, 2014

ABSTRACT We discuss the origin of negative dynamic terahertz (THz) conductivity in graphene. By p... more ABSTRACT We discuss the origin of negative dynamic terahertz (THz) conductivity in graphene. By performing series of fluence and photon-energy dependent studies, we show that the nonequilibrium THz dynamics is strongly affected by the polycrystalline plasmons.

Nano, 2011

One-step roll-to-roll lamination transfer of graphene was achieved by poly(ethylene co-vinyl acet... more One-step roll-to-roll lamination transfer of graphene was achieved by poly(ethylene co-vinyl acetate) as a binding material which has thermoplastic property and wide rubbery-plateau region between T g ~T m . The transferred graphene film has a transmittance of 96.7% at 550 nm and 1.96 kΩ/sq of sheet resistance measured by simple 4-probe method. Standard deviation was found to be 0.198 kΩ/sq.

RSC Adv., 2014

We report high-quality GaN crystals grown directly on graphene layers without a buffer layer by m... more We report high-quality GaN crystals grown directly on graphene layers without a buffer layer by metal–organic chemical vapour deposition.

Scientific reports, 2013

Oscillator-strength sum rule in light-induced transitions is one general form of quantum-mechanic... more Oscillator-strength sum rule in light-induced transitions is one general form of quantum-mechanical identities. Although this sum rule is well established in equilibrium photo-physics, an experimental corroboration for the validation of the sum rule in a nonequilibrium regime has been a long-standing unexplored question. The simple band structure of graphene is an ideal system for investigating this question due to the linear Dirac-like energy dispersion. Here, we employed both ultrafast terahertz and optical spectroscopy to directly monitor the transient oscillator-strength balancing between quasi-free low-energy oscillators and high-energy Fermi-edge ones. Upon photo-excitation of hot Dirac fermions, we observed that the ultrafast depletion of high-energy oscillators precisely complements the increased terahertz absorption oscillators. Our results may provide an experimental priori to understand, for example, the intrinsic free-carrier dynamics to the high-energy photo-excitation,...

Optics express, Jan 7, 2011

We report GaN-based near ultraviolet (UV) light emitting diode (LED) that combines indium tin oxi... more We report GaN-based near ultraviolet (UV) light emitting diode (LED) that combines indium tin oxide (ITO) nanodot nodes with two-dimensional graphene film as a UV-transparent current spreading electrode (TCSE) to give rise to excellent UV emission efficiency. The light output power of 380 nm emitting UV-LEDs with graphene film on ITO nanodot nodes as TCSE was enhanced remarkably compared to conventional TCSE. The increase of the light output power is attributed to high UV transmittance of graphene, effective current spreading and injection, and texturing effect by ITO nanodots.

Nano Letters, 2012

We present terahertz spectroscopic measurements of Dirac fermion dynamics from a large-scale grap... more We present terahertz spectroscopic measurements of Dirac fermion dynamics from a large-scale graphene that was grown by chemical vapor deposition and on which carrier density was modulated by electrostatic and chemical doping. The measured frequency-dependent optical sheet conductivity of graphene shows electron-density-dependence characteristics, which can be understood by a simple Drude model. In a low carrier density regime, the optical sheet conductivity of graphene is constant regardless of the applied gate voltage, but in a high carrier density regime, it has nonlinear behavior with respect to the applied gate voltage. Chemical doping using viologen was found to be efficient in controlling the equilibrium Fermi level without sacrificing the unique carrier dynamics of graphene.

Journal of the American Chemical Society, 2012

Coexistence of both edge plane and basal plane in graphite often hinders the understanding of lit... more Coexistence of both edge plane and basal plane in graphite often hinders the understanding of lithium ion diffusion mechanism. In this report, two types of graphene samples were prepared by chemical vapor deposition (CVD): (i) well-defined basal plane graphene grown on Cu foil and (ii) edge plane-enriched graphene layers grown on Ni film. Electrochemical performance of the graphene electrode can be split into two regimes depending on the number of graphene layers: (i) the corrosion-dominant regime and (ii) the lithiation-dominant regime. Li ion diffusion perpendicular to the basal plane of graphene is facilitated by defects, whereas diffusion parallel to the plane is limited by the steric hindrance that originates from aggregated Li ions adsorbed on the abundant defect sites. The critical layer thickness (lc) to effectively prohibit substrate reaction using CVDgrown graphene layers was predicted to be ∼6 layers, independent of defect population. Our density functional theory calculations demonstrate that divacancies and higher order defects have reasonable diffusion barrier heights allowing lithium diffusion through the basal plane but neither monovacancies nor Stone-Wales defect.

Japanese Journal of Applied Physics, 2011

We report the enhanced light output power of GaN-based light-emitting diode (LED) by using graphe... more We report the enhanced light output power of GaN-based light-emitting diode (LED) by using graphene film as a transparent conducting electrode. Monolayer graphene was synthesized on copper foil by using chemical vapor deposition method and directly transferred onto the GaN-LED as a top electrode. Compared to the conventional LEDs using indium tin oxide (ITO) layer for an electrode material, the light output power of LED with graphene electrode was improved by 25%. This was attributed excellent graphene characteristics of high electrical conductivity, high optical transmittance of nearly 97% over a wide range of infrared, visible, and ultraviolet region and large area uniformity with fewer defects.

Applied Physics Letters, 2011

Applied Physics Letters, 2013

Applied Physics Express, 2012

We report the implementation of Au-doped graphene film as a transparent and current-spreading ele... more We report the implementation of Au-doped graphene film as a transparent and current-spreading electrode (TCSE) in GaN-based near-ultraviolet (UV) light-emitting diode (LED) to achieve good UV emission efficiency. The TCSE effects of Au-doped graphene film were clearly seen in both the electroluminescence (EL) and current-voltage (I-V) characteristics. The EL output power of 380-nm wavelength UV-LEDs with Au-doped graphene film was enhanced by about 20% at an injection current of 20 mA compared with that of conventional UV-LEDs. The increase of the light output power is attributed to the high UV transmittance of graphene, effective current spreading, and injection.

Advanced Materials, 2011

Direct integration of vertically aligned carbon nanotube (VACNTs) patterns on a graphene layer co... more Direct integration of vertically aligned carbon nanotube (VACNTs) patterns on a graphene layer combined with a liquid crystal cell can be utilized as an optical array. The hybrid structures have high transparency and conductivity and, as reported by Sun IL Kim, Jong Min Kim, Young Hee Lee, and co-workers, clear diffraction patterns are obtained by modulating an external electric fi eld.

Advanced Functional Materials, 2012

ABSTRACT In spite of recent successful demonstrations of flexible and transparent graphene heater... more ABSTRACT In spite of recent successful demonstrations of flexible and transparent graphene heaters, the underlying heat-transfer mechanism is not understood due to the complexity of the heating system. Here, graphene/glass defoggers are fabricated and the dynamic response of the temperature as a function of input electrical power is measured. The graphene/glass defoggers reveal shorter response times than Cr/glass defoggers. Furthermore, the saturated temperature of the graphene/glass defoggers is higher than for Cr/glass defoggers at a given input electrical power. The observed dynamic response to temperature is well-fitted to the power-balance model. The response time of graphene/glass defogger is shorter by 44% than that of the Cr/glass defogger. The convective heat-transfer coefficient of graphene is 12.4 × 10−4 W cm−2 °C−1, similar to that of glass (11.1 × 10−4 W cm−2 °C−1) but smaller than that of chromium (17.1 × 10−4 W cm−2 °C−1). The graphene-based system reveals the lowest convective heat-transfer coefficient due to its ideal flat surface compared to its counterparts of carbon nanotubes (CNTs) and reduced graphene oxide (RGO)-based systems.

ACS Nano, 2011

The doping/dedoping mechanism of carbon nanotubes (CNTs) with AuCl(3) has been investigated with ... more The doping/dedoping mechanism of carbon nanotubes (CNTs) with AuCl(3) has been investigated with regard to the roles of cations and anions. Contrary to the general belief that CNTs are p-doped through the reduction of cationic Au(3+) to Au(0), we observed that chlorine anions play a more important role than Au cations in doping. To estimate the effects of Cl and Au on CNTs, the CNT film was dedoped as a function of the annealing temperature (100-700 °C) under an Ar ambient and was confirmed by the sheet resistance change and the presence of a G-band in the Raman spectra. The X-ray photoelectron spectroscopy (XPS) analysis revealed that the doping level of the CNT film was strongly related to the amount of adsorbed chlorine atoms. Annealing at temperatures up to 200 °C did not change the amount of adsorbed Cl atoms on the CNTs, and the CNT film was stable under ambient conditions. Alternatively, Cl atoms started to dissociate from CNTs at 300 °C, and the stability of the film was degraded. Furthermore, the change in the amount of Cl atoms in CNTs was inversely proportional to the change in the sheet resistance. Our observations of the Cl adsorption, either directly or mediated by an Au precursor on the CNT surface, are congruent with the previous theoretical prediction.