Hiroyuki S. Kato

The electronic structure of oxygen atom vacancy and hydroxyl impurity defects on titanium dioxide (110) surface

Introducing a charge into a solid such as a metal oxide through chemical, electrical, or optical means can dramatically change its chemical or physical properties. To minimize its free energy, a lattice will distort in a material specific way to accommodate (screen) the Coulomb and exchange interactions presented by the excess charge. The carrier-lattice correlation in response to these interactions defines the spatial extent of the perturbing charge and can impart extraordinary physical and chemical properties such as superconductivity and catalytic activity. Here we investigate by experiment and theory the atomically resolved distribution of the excess charge created by a single oxygen atom vacancy and a hydroxyl (OH) impurity defects on rutile TiO(2)(110) surface. Contrary to the conventional model where the charge remains localized at the defect, scanning tunneling microscopy and density functional theory show it to be delocalized over multiple surrounding titanium atoms. The characteristic charge distribution controls the chemical, photocatalytic, and electronic properties of TiO(2) surfaces.

Homoepitaxial growth of high-quality zn-polar ZnO films by plasma-assisted molecular beam epitaxy

High-quality ZnO films have been grown on Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy. With increasing O/Zn ratio from the stoichiometric to the O-rich flux condition, the growth mode and the surface morphology changed from three-dimensional growth with a rough surface to two-dimensional growth with a smooth surface. The minimum linewidth from the (1010) ω-rocking curve was 100 arcsec, and the n = 2 state of A-exciton was clearly observed in the photoluminescence at 4.2 K. Due to the reduction in the edge-type threading dislocation density, the residual carrier concentration in these homoepitaxial ZnO films was as low as 2.2×1016 cm-3, which is one order of magnitude lower than that previously reported for heteroepitaxial ZnO films.

Effect of O/Zn Flux Ratio on Crystalline Quality of ZnO Films Grown by Plasma-Assisted Molecular Beam Epitaxy

The effect of O/Zn flux ratio on the crystalline quality of ZnO films grown at 700°C by plasma-assisted molecular beam epitaxy was investigated. Zinc beam flux (FZn) was varied from 2.2≤FZn≤8.3 A/s with an O2 flow rate of 3 sccm and RF power of 300 W. The surface morphology of the ZnO layers strongly depended on FZn. ZnO epilayers grown under stoichiometric flux conditions (i.e., FZn=5.1 A/s) had high crystalline quality, as was confirmed by using X-ray diffraction, photoluminescence (PL), and Hall-effect measurements: the full width at half maximum (FWHM) of a skew symmetric (1010) X-ray rocking curve was 720 arcsec; the dominant neutral donor bound exciton emission intensity in the PL spectra became maximum with the narrowest FWHM; the electron mobility was a maximum of 130 cm2V-1s-1; and a residual carrier concentration of 1.2×1017 cm-3 was achieved. We demonstrated that stoichiometric ZnO films have the lowest dislocation density and the highest electron mobility compared with ZnO films grown under nonstoichiometric flux conditions.

Effects of ZnO/MgO Double Buffer Layers on Structural Quality and Electron Mobility of ZnO Epitaxial Films Grown on c-Plane Sapphire

High-electron-mobility ZnO epilayers were grown on c-plane sapphire with ZnO/MgO double-buffer layers by plasma-assisted molecular beam epitaxy. Precisely controlled low growth rate of the double-buffer layers was crucial to the improvement of electrical properties. Both X-ray diffraction ω rocking curve measurement and calculated electron mobilities revealed that the improvement of electron mobility of ZnO films is due to a decrease in dislocation density. The highest electron mobility of 137 cm2V-1 s-1 in as-grown ZnO film was achieved at room temperature.

Study of the adsorption structure of NO on Pt(111) by scanning tunneling microscopy and high-resolution electron energy-loss spectroscopy

The structure of nitric oxide (NO) on a Pt(111) surface was studied by scanning tunneling microscopy (STM ) and high-resolution electron energy-loss spectroscopy (HREELS ). The coexistence of two species is observed by STM after saturating the surface with NO at 70 K and annealing to 215 K. Two pairs of NMO and PtMN stretching vibrational frequencies are observed in HREELS spectra after annealing, which are assigned to the two species observed by STM. Comparison between the spectra of HREELS and infrared absorption spectroscopy (IRAS) indicates that IRAS is less sensitive to the lower-frequency mode. A new model for the adsorption structure of NO on the Pt(111) surface is proposed and discussed.

Self-Directed Chain Reaction by Small Ketones with the Dangling Bond Site on the Si(100)-(2 × 1)-H Surface: Acetophenone, A Unique Example

Using scanning tunneling microscope (STM) at 300 K, we studied the growth of one-dimensional molecular assemblies (molecular lines) on the Si(100)-(2 x 1)-H surface through the chain reaction of small ketone (CH 3COCH 3, PhCOPh, and PhCOCH 3) molecules with dangling bond (DB) sites of the substrate. Acetone and benzophenone show the growth of molecular lines exclusively parallel to the dimer row direction. In contrast, acetophenone molecules show some molecular lines perpendicular, in addition to parallel, to the dimer row direction. Most of the molecular lines perpendicular to the dimer row direction were grown by self-turning the propagation direction of a chain reaction from parallel to perpendicular directions relative to the dimer row. A chiral center created upon adsorption of an acetophenone molecule allows the adsorbed molecules to align with identical as well as alternate enantiomeric forms along the dimer row direction, whereas such variations in molecular arrangement are not observed in the case of acetone and benzophenone molecules. The observed molecular lines growth both parallel and perpendicular to dimer row directions appears to be unique to acetophenone among all the molecules studied to date. Hence, the present study opens new possibility for fabricating one-dimensional molecular assemblies of various compositions in both high-symmetry directions on the Si(100)-(2 x 1)-H surface.

Impact of Mixture Gas Plasma of N2 and O2 as the N Source on ZnO-Based Ultraviolet Light-Emitting Diodes Fabricated by Molecular Beam Epitaxy

ZnO-based double-heterostructure ultraviolet light-emitting diodes (LEDs) were fabricated on n-type Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy employing mixture gas plasma of N2 and O2 as the N source. By using mixture gas plasma of N2 with a little added O2, the number of N atoms increased and N2 molecules decreased, as confirmed by optical emission spectrometry. The fabricated LEDs had an ultraviolet near-band-emission of around 380 nm. The integrated electroluminescence intensity of the LED fabricated using N2 and O2 plasma was more than 10 times higher than that of an LED fabricated using N2 plasma.

Deposition and crystallization studies of thin amorphous solid water films on Ru(0001) and on CO-precovered Ru(0001).

The deposition and the isothermal crystallization kinetics of thin amorphous solid water (ASW) films on both Ru(0001) and CO-precovered Ru(0001) have been investigated in real time by simultaneously employing helium atom scattering, infrared reflection absorption spectroscopy, and isothermal temperature-programmed desorption. During ASW deposition, the interaction between water and the substrate depends critically on the amount of preadsorbed CO. However, the mechanism and kinetics of the crystallization of approximately 50 layers thick ASW film were found to be independent of the amount of preadsorbed CO. We demonstrate that crystallization occurs through random nucleation events in the bulk of the material, followed by homogeneous growth, for solid water on both substrates. The morphological change involving the formation of three-dimensional grains of crystalline ice results in the exposure of the water monolayer just above the substrate to the vacuum during the crystallization process on both substrates.

First-principles calculations of hydrogen diffusion on rutile TiO2(110) surfaces.

Density functional calculations are performed to study the H-atom diffusion on titanium dioxide (110) surface in the cases of water-molecule dissociation and splitting of the adjacent hydroxyl OH pair. It is shown that, when a water molecule is adsorbed at a surface oxygen-vacancy site, a fragment H atom of the water molecule tends to diffuse toward the nearest-neighboring bridging-oxygen sites by using a straight-line or relay-point path. As the result, a pair of surface hydroxyl OH is formed on the same oxygen row. In a thermal process, on the other hand, such OH pair favorably splits only by using a relay-point path, i.e., by transferring one H atom from a bridging-oxygen site to a next-neighboring one along the same oxygen row by way of another in-plane oxygen site. We found that the latter splitting reaction is activated around room temperature.

Stepwise morphological change of porous amorphous ice films observed through adsorption of methane

Morphological change of amorphous ice films of D2O has been studied through adsorption of methane using thermal desorption spectroscopy (TDS) and infrared reflection absorption spectroscopy under ultrahigh vacuum. The investigated ice films were prepared under several different conditions; first, water (D2O) molecules are evaporated onto a Ru substrate at 25 K, and then subjected to an annealing process at various temperatures prior to methane deposition. On ice annealed at low temperatures, two desorption species of methane were observed in TDS: one was derived from methane adsorbed near the ice surface and the other was attributed to the desorption of methane encapsulated in ice during heating. Only the former species was observed when the annealing temperature exceeded 60 K. This indicates that reconstruction of ice occurs below 60 K, which inhibits the encapsulation of methane molecules from the amorphous ice. On the other hand, infrared spectra of ice covered with methane show that the micropores in ...