Cheolho Jeon

Low-temperature synthesis of graphene on nickel foil by microwave plasma chemical vapor deposition

Microwave plasma chemical vapor deposition (MPCVD) was employed to synthesize high quality centimeter scale graphene film at low temperatures. Monolayer graphene was obtained by varying the gas mixing ratio of hydrogen and methane to 80:1. Using advantages of MPCVD, the synthesis temperature was decreased from 750 °C down to 450 °C. Optical microscopy and Raman mapping images exhibited that a large area monolayer graphene was synthesized regardless of the temperatures. Since the overall transparency of 89% and low sheet resistances ranging from 590 to 1855 Ω∕sq of graphene films were achieved at considerably low synthesis temperatures, MPCVD can be adopted in manufacturing future large-area electronic devices based on graphene film.

Synthesis of Bandgap-Controlled Semiconducting Single-Walled Carbon Nanotubes

Bandgap-controlled semiconducting single-walled carbon nanotubes (s-SWNTs) were synthesized using a uniquely designed catalytic layer (Al(2)O(3)/Fe/Al(2)O(3)) and conventional thermal chemical vapor deposition. Homogeneously sized Fe catalytic nanoparticles were prepared on the Al(2)O(3) layer and their sizes were controlled by simply modulating the annealing time via heat-driven diffusion and subsequent evaporation of Fe at 800 degrees C. Transmission electron microscopy and Raman spectroscopy revealed that the synthesized SWNTs diameter was manipulated from 1.4 to 0.8 nm with an extremely narrow diameter distribution below 0.1 nm as the annealing time is increased. As a result, the bandgap of semiconducting SWNTs was successfully controlled, ranging from 0.53 to 0.83 eV, with a sufficiently narrow energy distribution, which can be applied to field-effect transistors based on SWNTs.

Band bending of LiF/Alq3 interface in organic light-emitting diodes

The insertion of LiF for an interlayer material between the Al cathode and tris-(8-hydroxyquinoline) aluminum (Alq3) in the organic light-emitting diodes (OLEDs) provides an improved device performance. The highly occupied molecular orbital (HOMO) level lowering in the Alq3 layer induced by a low-coverage LiF deposition results in the reduction of electron injection barrier height. We investigated the electronic structure of the interface between the ultrathin LiF and the Alq3 layer, using synchrotron x-ray photoelectron emission spectroscopy. The results revealed that the major origin of the HOMO level lowering is not the chemical bonding of dissociated fluorine in the Alq3 layer but the band bending caused by charge redistribution driven by work function difference between LiF and Alq3 layer.

Field emitter density control effect on emission current density by Ag–Cu alloy coating on carbon nanotubes

We have investigated the morphological evolution and improvement in field emission properties of carbon nanotube (CNT) emitters coated with an Ag–Cu alloy (ACa). Vertically aligned multiwalled CNTs (MWCNTs) were synthesized by direct current-plasma enhanced chemical vapor deposition. The MWCNTs were then coated with ACa by dc-magnetron sputtering and then annealed. Scanning electron microscopy revealed an increase in the size of the ACa droplets on the CNTs after thermal annealing, and a decrease in the emitter density with increasing deposition time. The emitter density was controlled by the amount of ACa with high surface tension and annealing. A lower turn-on voltage (1.18 V/μm) and higher emission current density of 588.9 μA/cm2 at 5.0 V/μm were achieved from the sample containing ACa droplets with an average radius of 500 nm.

Homogeneous and stable p-type doping of graphene by MeV electron beam-stimulated hybridization with ZnO thin films

In this work, we demonstrate a unique and facile methodology for the homogenous and stable p-type doping of graphene by hybridization with ZnO thin films fabricated by MeV electron beam irradiation (MEBI) under ambient conditions. The formation of the ZnO/graphene hybrid nanostructure was attributed to MEBI-stimulated dissociation of zinc acetate dihydrate and a subsequent oxidation process. A ZnO thin film with an ultra-flat surface and uniform thickness was formed on graphene. We found that homogeneous and stable p-type doping was achieved by charge transfer from the graphene to the ZnO film.

Growth of Millimeter-Scale Vertically Aligned Carbon Nanotubes by Microwave Plasma Chemical Vapor Deposition

Millimeter-scale, vertically aligned carbon nanotubes (CNTs) were grown at a relatively low temperature of 700 °C by the microwave plasma chemical vapor deposition. Oxygen and water were used to investigate their role in the growth rate and crystallinity of CNTs. Scanning electron microscopy and transmission electron microscopy were adopted to observe the height and number of walls of the CNTs, respectively. The addition of oxygen significantly increased the growth rate of CNTs (~23 µm/min) for 70 min, which is three and two times higher than that of normal CNTs and water-assisted CNTs, respectively. Furthermore, the high growth rate was maintained for 220 min and the crystallinity of CNTs, which was evaluated by the G to D band ratio in Raman spectra, was also greatly improved only by introducing oxygen.

Enhanced Photocatalytic Performance Depending on Morphology of Bismuth Vanadate Thin Film Synthesized by Pulsed Laser Deposition

We have fabricated high quality bismuth vanadate (BiVO4) polycrystalline thin films as photoanodes by pulsed laser deposition (PLD) without a postannealing process. The structure of the grown films is the photocatalytically active phase of scheelite-monoclinic BiVO4 which was obtained by X-ray diffraction (XRD) analysis. The change of surface morphology for the BIVO4 thin films depending on growth temperature during synthesis has been observed by scanning electron microscopy (SEM), and its influence on water splitting performance was investigated. The current density of the BiVO4 film grown on a glass substrate covered with fluorine-doped tin oxide (FTO) at 230 °C was as high as 3.0 mA/cm2 at 1.23 V versus the potential of the reversible hydrogen electrode (VRHE) under AM 1.5G illumination, which is the highest value so far in previously reported BiVO4 films grown by physical vapor deposition (PVD) methods. We expect that doping of transition metal or decoration of oxygen evolution catalyst (OEC) in our BiVO4 film might further enhance the performance.

Formation of Frustrated Lewis Pairs in Ptx‐Loaded Zeolite NaY

The formation of a frustrated Lewis pair consisting of sodium hydride (Na(+) H(-) ) and a framework-bound hydroxy proton O(H(+) ) is reported upon H2 treatment of zeolite NaY loaded with Pt nanoparticles (Ptx /NaY). Frustrated Lewis pair formation was confirmed using in situ neutron diffraction and spectroscopic measurements. The activity of the intrazeolite NaH as a size-selective catalyst was verified by the efficient esterification of acetaldehyde (a small aldehyde) to form the corresponding ester ethyl acetate, whereas esterification of the larger molecule benzaldehyde was unsuccessful. The frustrated Lewis pair (consisting of Na(+) H(-) and O(H(+) )) generated within zeolite NaY may be a useful catalyst for various catalytic reactions which require both H(-) and H(+) ions, such as catalytic hydrogenation or dehydrogenation of organic compounds and activation of small molecules.

Strain Relaxation of Graphene Layers by Cu Surface Roughening

The surface morphology of copper (Cu) often changes after the synthesis of graphene by chemical vapor deposition (CVD) on a Cu foil, which affects the electrical properties of graphene, as the Cu step bunches induce the periodic ripples on graphene that significantly disturb electrical conduction. However, the origin of the Cu surface reconstruction has not been completely understood yet. Here, we show that the compressive strain on graphene induced by the mismatch of thermal expansion coefficient with Cu surface can be released by forming periodic Cu step bunching that depends on graphene layers. Atomic force microscopy (AFM) images and the Raman analysis show the noticeably longer and higher step bunching of Cu surface under multilayer graphene and the weaker biaxial compressive strain on multilayer graphene compared to monolayer. We found that the surface areas of Cu step bunches under multilayer and monolayer graphene are increased by ∼1.41% and ∼0.77% compared to a flat surface, respectively, indicating that the compressive strain on multilayer graphene can be more effectively released by forming the Cu step bunching with larger area and longer periodicity. We believe that our finding on the strain relaxation of graphene layers by Cu step bunching formation would provide a crucial idea to enhance the electrical performance of graphene electrodes by controlling the ripple density of graphene.

In Situ X-ray Photoemission Spectroscopy Study of Atomic Layer Deposition of TiO 2 on SiliconSubstrate

In-situ X-ray photoemission spectroscopy (XPS) has been used to investigate the initial stages of TiO2 growth on a Si(001) substrate by atomic layer deposition (ALD). The core level spectra of Si 2p, C 1s, O 1s, and Ti 2p were measured at every half reaction in the titanium tetra-isopropoxide (TTIP)–H2O ALD process. The ligand exchange reactions were verified using the periodic oscillation of the C 1s concentration, as well as changes in the hydroxyl concentration. XPS analysis revealed that Ti2O3 and Si oxide were formed at the initial stages of TiO2 growth. A stoichiometric TiO2 layer was dominantly formed after two cycles and was chemically saturated after four cycles.