Bongsoo Kim

Molecular orientation dependence of hole-injection barrier in pentacene thin film on the Au surface in organic thin film transistor

We have investigated the effects of a buffer layer insertion on the performance of the pentacene based thin film transistor with a bottom contact structure. When the pentacene molecules have a standing up coordination on the Au surface that is modified by the benzenethiol or methanethiol, the transition region in the pentacene thin film is removed along the boundary between the Au and silicon oxide region, and the hole-injection barrier decreases by 0.4eV. Pentacene on various surfaces showed that the highly occupied molecular level is 0.2–0.4eV lower in the standing up coordination than in the lying down coordination.

NEXAFS spectroscopy study of the surface properties of zinc glutarate and its reactivity with carbon dioxide and propylene oxide

Abstract The surface state of polycrystalline zinc glutarate (ZnGA) catalyst and its catalytic adsorption of carbon dioxide (CO 2 ) and propylene oxide (PO) were investigated by using near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The outermost layer of ZnGA catalyst was found to contain more hydrocarbon units (i.e., glutarate ligand component) than the inner layers. The ZnGA catalyst was found to reversibly react with CO 2 and to readily react with PO via adsorption onto the catalyst surface and insertion into the ZnO bond. Experiments in which the catalyst was treated with CO 2 followed by PO and vice versa showed that each of these components can replace the other component on the catalyst surface. This reversible adsorption and insertion of CO 2 and PO on the ZnGA surface provides a clue to the mechanism underlying the production of alternating poly(propylene carbonate) in the ZnGA-catalyzed copolymerization of CO 2 and PO. However, in comparison to CO 2 , PO was more easily adsorbed onto the ZnGA catalyst and inserted into the ZnO bond. As a consequence, PO significantly modified the catalyst surface. This suggests that the ZnGA-catalyzed copolymerization is initiated by PO rather than CO 2 .

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.

Core-level photoemission and near-edge x-ray absorption fine-structure studies of GaN surface under low-energy ion bombardment

We have investigated compositional changes on GaN surfaces under low-energy Ar ion bombardment using synchrotron-based high-resolution core-level photoemission measurements and near-edge x-ray absorption fine-structure (NEXAFS) spectroscopy. The low-energy ion bombardment of GaN produces a Ga-rich surface layer which transforms into a metallic Ga layer at higher bombarding energies. At the same time, the photoemission spectra around the N 1s core level reveal the presence of both uncoordinated nitrogen and nitrogen interstitials, which we have analyzed in more detail by x-ray absorption measurements at the N K-edge. We have proposed a mechanism for the relocation and loss of nitrogen during ion bombardment, in agreement with some recent experimental and theoretical studies of defect formation in GaN. We have also demonstrated that photoemission spectroscopy and NEXAFS provide a powerful combination for studying compositional changes and the creation of point defects at GaN surface.

BENDING MAGNET VUV BEAMLINE 2B AT THE POHANG LIGHT SOURCE

The SGM beamline (2B1), designed for photoemission spectroscopy, accepts a photon beam 10 mrad horizontally and 2 mrad vertically from a bending magnet source. The operating energy range of the monochromator is 12–1230 eV and the grating chamber has five exchangeable laminar gratings to cover the range. The monochromator employs a fixed entrance slit and a movable exit slit to satisfy the focusing condition during photon energy scan. The NIM beamline (2B2) is designed for 5–30 eV photon energy. The type of monochromator to be installed is 3 m normal incident, with off‐Rowland circle mounting. A gas‐phase experimental system, equipped with differential pumping stages, will be installed in the NIM beamline.

A novel Cr2O3 thin film on stainless steel with high sorption resistance

Abstract We report the ultrahigh-vacuum preparation of a Cr 2 O 3 film on a stainless-steel surface with marked sorption resistance. The thermal desorption spectra clearly show the water-repellent nature of the oxide film, which is almost total Cr 2 O 3 as indicated by the surface-sensitive photoemission spectrum. The film promises extreme high vacuum, and could also spell good news for avid computer fans.

Pentacene as protection layers of graphene on SiC surfaces

We report that pentacene can be used as a protection layer of graphene using synchrotron radiation-based photoemission spectroscopy. When pentacene was deposited on a single layer graphene, molecular states of pentacene were clearly observed, yet no change in the band structure of graphene could be identified. Unique electronic properties of graphene can be preserved in the presence of pentacene layers, and this finding can be exploited for fundamental research as well as application of graphene in electronic devices. After exposing the pentacene-covered graphene to air followed by a subsequent annealing under vacuum, band structure of graphene was completely maintained.

Compositional changes on GaN surfaces under low-energy ion bombardment studied by synchrotron-based spectroscopies

We have investigated compositional changes on GaN surfaces under Ar-ion bombardment using synchrotron-based high-resolution x-ray photoemission (PES) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The low-energy ion bombardment of GaN produces a Ga-rich surface layer which transforms into a metallic Ga layer at higher bombarding energies. At the same time, the photoemission spectra around N 1s core levels reveal the presence of both uncoordinated nitrogen and nitrogen interstitials, which we have analyzed in more details by x-ray absorption measurements at N K edge. We have demonstrated that PES and NEXAFS provide a powerful combination for studying the compositional changes on GaN surfaces. A mechanism for the relocation and loss of nitrogen during ion bombardment in agreement with some recent experimental and theoretical studies of defect formation in GaN has been proposed.

Photoemission spectroscopy of the evolution of ultra-thin Co films on Si(111) substrates upon annealing temperature

Abstract The evolution of the ultra-thin Co films on Si(111) substrates upon annealing temperature in the range from room temperature to 800°C was studied by means of photoemission spectroscopy (PES) with synchrotron radiation and low energy electron diffraction (LEED) techniques. 0.2–1.2 ML Co was evaporated onto Si(111)-7×7 at room temperature. The behavior of the Co–silicon interfaces upon the annealing temperature was examined using Si 2p core level shifts and valence band measurements. The Si 2p core level spectra were taken with the photon energy of 130 eV and we observed the core level shift upon the formations of CsCl-type CoSi and the CaF 2 -type CoSi 2 as a function of annealing temperature and of the thickness of Co. We also estimated the diffuse depth of Co depending on the annealing temperature.

Photoemission study of monolayer Co on Si(111) surface

Abstract The interaction of monolayer Co with the Si(111) surface is investigated by means of core-level and valence-band photoemission spectroscopy in the range of room temperature to 700°C. Upon increasing the annealing temperature, several formations of silicides are observed. This ultra-thin layer upon annealing at 300–400°C has electronic structure closely related to the metastable CsCl-type CoSi which was discovered by molecular beam epitaxy (MBE) grown onto the CoSi2 template to a thickness of ∼100 A. Further annealing up to 500°C leads to the e-CoSi-type electronic structure. Finally, the formation of CaF2-type CoSi2 is observed after annealing above 600°C. They lead also to an island formation above 700°C.