Tai-Hee Kang

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.

Temperature dependent structural changes of graphene layers on 6H-SiC(0001) surfaces

We investigated the electronic and structural properties of graphene layers grown on a 6H-SiC (Si-terminated) substrate by using core level photoemission spectroscopy (CLPES), low energy electron diffraction (LEED), and near edge x-ray absorption fine structure (NEXAFS). The angle between the plane of the graphene sheet and the SiC substrate was measured by monitoring the variation of the π* transition in the NEXAFS spectrum with the thickness of the graphene layers. As the thickness of the graphene layers increased, the angle gradually decreased.

Surface property change of graphene using nitrogen ion

We introduced nitrogen ions to modify the graphene surface and its property changes were investigated. A graphene layer grown on 6H-SiC(0001) was irradiated with 100 eV nitrogen ions. Surface property changes were studied using photoemission spectroscopy (PES), near edge x-ray adsorption spectroscopy (NEXAFS), and atomic force microscopy(AFM). N 1s core level spectra show that three kinds of nitrogen species, nitrogen gas, graphite-like and pyridine-like nitrogen were induced on the nitrogen ion implanted graphene surface.

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.

Temperature-induced phase transitions of the Si(1 0 0) and (1 1 3) surfaces

Abstract We report in this paper the effect of adatoms and structural vibrations on temperature-induced phase transitions of surfaces at elevated temperatures. In the case of Si(1xa00xa00) surface, a temperature-induced metallization takes place at about 600 K without any change in low energy electron diffraction pattern. Photoemission spectroscopy reveals that the metallization is not related to the (instantaneous) symmetrization of asymmetric dimers. Si adatoms produced at elevated temperatures are suggested to play a role of donor, giving rise to the metallization. On the other hand, the Si(1xa01xa03) surface exhibits a structural phase transition between 3×2 and 3×1 phases at about 800 K. The local structure of the 3×2 surface at room temperature seems to be the same as that of the 3×1 surface at 800 K. We propose that the temperature-induced phase transition on Si(1xa01xa03) is an order–disorder transition due to the thermal fluctuation of two types of tetramer, which can be easily understood within the two-dimensional Ising model.

Atom-selective loss structures: evidence for semiconducting property of Na/Si(100) surfaces

Abstract We report why Na adsorption on the Si(1xa00xa00) surface leads to satellites in photoemission spectra at room temperature. Adsorbate-related peaks, Naxa02p and Naxa0LVV, lose their kinetic energy by about 2.0 eV, while a satellite is found at a loss energy of about 3.0 eV. In contrast to previous reports, we ascribe the loss structures as being due to atom-selective interband transitions. The satellites are suggested as evidence for the semiconducting nature rather than for the metallic character of the Na/Si(1xa00xa00) surface.

Initiation of Collapsing Pentacene Crystal by Au

Metal contacts with gold on organics are an essential factor in organic electronics. The unveiled key challenge is to probe dynamic details of the microscopic evolution of the organic crystal when the atomic Au is introduced. Here, we show how the collapse of the pentacene crystal is initiated even by a few Au atoms. Our photoemission and x‐ray absorption results indicate that the gentle decoupling of intra and inter‐molecular π‐π interactions causes the localization of the lowest unoccupied molecular orbital as well as the removal of cohesive forces between molecules, leading to the subsequent crystal collapse.