Yasuyuki Sainoo

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.

Local chemical reaction of benzene on Cu(110) via STM-induced excitation

We have investigated the mechanism of the chemical reaction of the benzene molecule adsorbed on Cu(110) surface induced by the injection of tunneling electrons using scanning tunneling microscopy (STM). With the dosing of tunneling electrons of the energy 2-5 eV from the STM tip to the molecule, we have detected the increase of the height of the benzene molecule by 40% in the STM image and the appearance of the vibration feature of the nu(C-H) mode in the inelastic tunneling spectroscopy (IETS) spectrum. It can be understood with a model in which the dissociation of C-H bonds occurs in a benzene molecule that induces a bonding geometry change from flat-lying to up-right configuration, which follows the story of the report of Lauhon and Ho on the STM-induced change of benzene on the Cu(100) surface. [L. J. Lauhon and W. Ho, J. Phys. Chem. A 104, 2463 (2000)]. The reaction probability shows a sharp rise at the sample bias voltage at 2.4 V, which saturates at 3.0 V, which is followed by another sharp rise at the voltage of 4.3 V. No increase of the reaction yield is observed for the negative sample voltage up to 5 eV. In the case of a fully deuterated benzene molecule, it shows the onset at the same energy of 2.4 eV, but the reaction probability is 10(3) smaller than the case of the normal benzene molecule. We propose a model in which the dehydrogenation of the benzene molecule is induced by the formation of the temporal negative ion due to the trapping of the electrons at the unoccupied resonant states formed by the pi orbitals. The existence of the resonant level close to the Fermi level ( approximately 2.4 eV) and multiple levels in less than approximately 5 eV from the Fermi level, indicates a fairly strong interaction of the Cu-pi(*) state of the benzene molecule. We estimated that the large isotope effect of approximately 10(3) can be accounted for with the Menzel-Gomer-Redhead (MGR) model with an assumption of a shallow potential curve for the excited state.

Epitaxial growth of CeO2(111) film on Ru(0001): scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS) study.

We formed an epitaxial film of CeO2(111) by sublimating Ce atoms on Ru(0001) surface kept at elevated temperature in an oxygen ambient. X-ray photoemission spectroscopy measurement revealed a decrease of Ce(4+)/Ce(3+) ratio in a small temperature window of the growth temperature between 1070 and 1096 K, which corresponds to the reduction of the CeO2(111). Scanning tunneling microscope image showed that a film with a wide terrace and a sharp step edge was obtained when the film was grown at the temperatures close to the reduction temperature, and the terrace width observed on the sample grown at 1060 K was more than twice of that grown at 1040 K. On the surface grown above the reduction temperature, the surface with a wide terrace and a sharp step was confirmed, but small dots were also seen in the terrace part, which are considerably Ce atoms adsorbed at the oxygen vacancies on the reduced surface. This experiment demonstrated that it is required to use the substrate temperature close to the reduction temperature to obtain CeO2(111) with wide terrace width and sharp step edges.

Inelastic tunneling spectroscopy using scanning tunneling microscopy on trans-2-butene molecule: Spectroscopy and mapping of vibrational feature

Inelastic tunneling spectroscopy (IETS) measurement using scanning tunneling microscopy (STM) with a commercially available STM set up is presented. The STM-IETS spectrum measured on an isolated trans-2-butene molecule on the Pd(110) shows a clear vibrational feature in d2I/dV2 at the bias voltage of 360 mV and -363 mV, which corresponds to the nu(C-H) mode (d2I/dV2 approximately 10 nA/V2). In addition, we have obtained an image by mapping the vibrational feature of nu(C-H) in d2I/dV2. The image is obtained by scanning the tip on the surface with the feedback loop activated while the modulation voltage is superimposed on the sample voltage. With the method that is readily performable with conventional software, we have clearly differentiated the molecules of trans-2-butene and butadiene through the mapping of the vibrational feature, demonstrating its capability of chemical identification in atomic scale.

Single-molecule reactions and spectroscopy via vibrational excitation

Inelastic tunnelling spectra of single C4 hydrocarbon molecules adsorbed on the Pd(110) surface are presented. Experimental evidence is given that the symmetry of the molecular orbital into which the tunnelling electron first enters determines which vibrational modes are excited. The action spectrum for cis–2–butene exhibits most of the vibrational modes that are expected to be excited except for ? (C=C), which may be because the molecule is p bonded to the substrate, thus making the lifetime of the excited state short.

Atomically resolved Larmor frequency detection on Si(111)-7×7 oxide surface

We demonstrate that the electron spin resonance–scanning tunneling microscope can detect the Larmor frequency of a single spin with the atomic scale resolution on the oxygen-adsorbed Si(111)-7×7 surface. The spin signal from the Si dangling bond was detected on the oxygen-induced bright Si adatom with the Larmor frequency corresponding to g∼2.00. However, no peak was detected on the dark adatom that is tied with an oxygen atom and the dangling bond is disappeared.

Scanning Tunneling Microscope Study of Surface Morphology Variation of CeO2(111) with Changing Annealing Condition

The surface morphology variation of CeO2(111) with changing annealing condition was examined by scanning tunneling microscopy (STM). Hexagonal islands and pits containing two types of step edge have been found as characteristic features in a short-time-annealed surface. We observed that the triangular pits having only one type of step edge increase in number with annealing time, while the hexagonal pits and islands decrease and the deviated hexagonal pits increase. Step edges of the hexagonal pits that have higher coordinative saturation get preferential thermodynamic stability to form triangular pits, the most stable surface structures on the CeO2(111) surface in the equilibrium stage of annealing. These results have important implications for controlling catalytic activity on ceria.

Characteristic Configuration of Cis-2-butene Molecule on Pd(110) Determined by Scanning Tunneling Microscopy

Adsorption of cis-2-butene molecules on the Pd (110) surface was studied by scanning tunneling microscopy (STM) at 4.7 K for the first time. In contrast to the fact in the previous work that trans-2-butene molecules are adsorbed at the on-top site of the Pd(110) surface with its C=C double bond being almost parallel to the [110] axis, cis-2-butene molecules were found to be adsorbed slightly off the on-top site. In addition, the C=C bond was not parallel to the [110] axis, but to four directions equivalent to the structure of the Pd(110) surface. Furthermore, at high bias voltages for the STM measurement, the cis-2-butene molecule became unstable and moved in two different manners around the palladium atom to which the molecule was bonded, suggesting the existence of two different mechanisms underlying the motion on the surface.

Spontaneous Fluctuation between Symmetric and Buckled Dimer Domains of Si(100) at 80 K

At 80 K, not all the dimers of Si(100) appear buckled in the scanning tunneling microscopy (STM) images but a certain number of the dimers are observed in a symmetric configuration. We report on observations of a two-dimensional spontaneous fluctuation of the symmetricbuckled dimer domains at some particular locations. We interpret the spontaneous fluctuation to be induced by the competition of several antiphased c(4×2) buckled domains to expand. The fluctuation of domains was interpreted by two mechanisms: a fast switching between buckled dimer domains; and symmetric dimers induced by migration of P defects.

Electronic Structure of the Si(100) Surface A Defects Analyzed by Scanning Tunneling Spectroscopy at 80 K

The A defects on the Si(100) surface can be classified as A 1 ,A 2, and A3 at low temperatures. We carried out scanning tunneling microscopy observations and scanning tunneling spectroscopy measurements at»80 K to study their electronic structures. We found that the A1 defect is semiconductive similar to the A defect at room temperature (RT), while the A2 and A3 defects exhibited states in the surface band gap at 80 K. On comparing these results with the theoretical models, we concluded that the A1 defect correspond to the Rebonded vacancy model. The broken vacancy model and the twisted vacancy models are the possible candidates for the A2 and A3 defects, respectively.