Sukmin Chung

Effects of metal penetration into organic semiconductors on the electrical properties of organic thin film transistors

To investigate the effects of metal penetration into organic semiconductors on the electrical properties of organic thin film transistors, gold was deposited onto pentacene films at various deposition rates. The sharp interface between the gold electrode and the pentacene film that results from a fast deposition rate was found to produce lower contact resistance and an increase in the field-effect mobility.

Number of graphene layers as a modulator of the open-circuit voltage of graphene-based solar cell

Impressive optical properties of graphene have been attracting the interest of researchers, and, recently, the photovoltaic effects of a heterojunction structure embedded with few layer graphene (FLG) have been demonstrated. Here, we show the direct dependence of open-circuit voltage (Voc) on numbers of graphene layers. After unavoidably adsorbed contaminants were removed from the FLGs, by means of in situ annealing, prepared by layer-by-layer transfer of the chemically grown graphene layer, the work functions of FLGs showed a sequential increase as the graphene layers increase, despite of random interlayer-stacking, resulting in the modulation of photovoltaic behaviors of FLGs/Si interfaces.

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.

Effect of the Cr-rich oxide surface on fast pumpdown to ultrahigh vacuum

The outgassing characteristic of an ordinary stainless-steel ultrahigh vacuum (UHV) chamber was determined using an extremely dry nitrogen venting system, and compared with that of an extreme high vacuum (XHV) chamber. A marked sorption resistance effect of the vacuum surface of the XHV chamber was found. The outgassing rate of the XHV chamber was about 100 times lower than that of the ordinary UHV chamber in the unbaked case. It is believed that the recent achievement of fast pumpdown to UHV is the result of not only the reduction of the water content of the venting gas, but also the low gas-sorbability of the vacuum surface.

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.

Outgassing rate characteristic of a stainless‐steel extreme high vacuum system

Using an extremely dry nitrogen venting system, fast return to ultrahigh vacuum condition was achieved. The apparatus is similar to that of our earlier extreme high vacuum publication. The system pumps to 1.1×1−10 and 1.4×10−11 Torr in 5 and 50 h, respectively, after the start of pumpdown and without bakeout. The pressure‐time curve for pumpdown shows that the system pressure is determined by surface outgassing which decays algebraically as t−α, where the slope α changes from 2.13 for p≥10−9 Torr, 1.24 for 10−10≤p≤10−9, and 0.82 for p≤10−10 Torr. However, α=0.5, which would be expected for bulk diffusion, was not observed. Curve fitting yields desorption energies of 19, 20, 21, 22, and 23 kcal/mol for five types of adsorption sites on the surface for water. Analysis suggests that the elimination of water with desorption energies of 20 and 21 kcal/mol is essential for rapid pumpdown to the 10−10 Torr decade. A desorption energy of 10.4 kcal/mol for hydrogen was obtained by outgassing rate measurement. This...

Early stages of collapsing pentacene crystal by Au

The characteristic feature of metal contacts with gold on organics is deterioration of the organic crystals during the contact formation. 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 report how the collapse of the pentacene crystal is initiated even by a few Au atoms. Our results indicate that the gentle decoupling of intra and intermolecular 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.

Direct observation of oxygen-induced structural changes in stainless-steel surfaces

Oxidation of stainless was performed at 450 °C and oxygen partial pressures of 10−9–10−4 Torr. Atomic force microscopy (AFM) images and surface-sensitive photoemission spectroscopy spectra clearly show that, at 450 °C, oxygen pressures lower than the critical pressure (pc′=∼1×10−8 Torr) favor the formation of a smooth Cr2O3 oxide film whose AFM images clearly show a markedly smooth surface with no distinct grains. When oxidized at oxygen partial pressures higher than the critical pressure, the oxide film formed exhibits a rough surface with distinct grains that contain a significant amount of iron and manganese.

Creation of extreme high vacuum with a turbomolecular pumping system: A baking approach

Pressures in the low 10−12 Torr range are produced by a 200 °C bakeout with a turbomolecular pumping system. A small molecular drag pump in conjunction with the membrane pump turns out to be an effective means of achieving low backing pressures. The stainless‐steel test chamber of 6800 cm2 surface area has been prebaked to 450 °C. An ultimate pressure of 1×10−12 Torr is obtained on the extractor gauge and to date this is the lowest pressure attained with turbomolecular pumps only. This system offers simplicity of arrangement, ease of operation, rather low cost, and fast pumpdown to ultrahigh vacuum. The pressure reaches 2×10−9 and 4×10−11 Torr at 2 and 83 h, respectively, after startup without bakeout when vented to atmospheric pressure using extremely dry nitrogen.

The change of the chemical composition on a stainless-steel surface depending on the oxygen treatment and the annealing temperature and time, using photoemission spectroscopy

Abstract Recent studies have shown that an oxidized vacuum chamber has much better outgassing characteristics than an unoxidized one. However, the chemical composition of the vacuum chamber surface has not been fully characterized yet. Therefore, we employed photoemission spectroscopy using synchrotron radiation to study the stainless-steel surface as a function of oxygen exposure and annealing temperature. 143 eV photon energy was used to increase the surface sensitivity and the intensity of the Fe 3p and Cr 3p. This experiment was done at the 2B1 spherical grating monochromator (SGM) beamline at the Pohang Accelerator Laboratory.