We reviewed our recent work on the interface structures between metal and oxide substrate by means of total reflection fluorescence XAFS method. Total reflection fluorescence XAFS gives important information about the 3 dimensional local structure around the deposited metal even if the metal has no long range order. Two systems are described, Cu on TiO2(110) and Ni on Al2O3(0001). Cu on TiO2(110) prepared by CVD method and subsequent reduction at 363 K is in a trimer structure on a TiO2(110) surface. Ni on Al2O3(0001) prepared by a vacuum evaporation was also studied by this method. The initial adsorption site of Ni is determined at a 3 fold hollow site with Ni-O distances at 0.195 nm.
Time resolved dispersive XAFS (DXAFS) is a useful method to elucidate changing structures and is applicable to investigate the structures of short living species during chemical reactions. We applied this method to the chemical study of catalysts preparation. Recent results of our DXAFS study on the reduction process of Cu-ZSM−5 and on the decarbonylation of Mo(CO)6 in the cages of Y-type zeolite are describe in this article as well as a brief description of DXAFS measurement and DXAFS data analysis.
A novel experimental technique, soft X-ray energy dispersive surface XAFS (X-ray absorption fine structure), has recently been developed, by combining a new soft X-ray beamline with a position sensitive electron energy analyzer. The new technique enables us to obtain a NEXAFS (near-edge X-ray absorption fine structure) spectrum in one shot, without scanning the X-ray photon energy. The accumulation time for one NEXAFS spectrum for submonolayer adsorbate is reduced to 10−30 s, while it takes typically several minutes in the case of conventional method. Therefore it becomes possible to trace chemical reactions occurring on metal and semiconductor surfaces, and the local structure and electronic state during the surface reaction can be investigated. Some examples for the time-resolved observation of surface chemical reaction are demonstrated, together with the principle of the energy dispersive surface XAFS measurement.
The molecular orientation and the molecule-substrate interface structure of self-assembled monolayers of alkanethiol and its related molecules have been investigated by use of X-ray absorption fine structure (XAFS). The molecular orientation is mainly dependent on the molecular density, while for the ordering of molecular arrangement, which is necessary for self-assembly, the key factor is the balance between structures demanded by intermolecular and molecule/substrate interactions. The internal degree of freedom of the molecule plays an important role in the balancing. In the case of alkanethiols this condition is fulfilled at the saturated coverage, while in the case of a thiophene-substituted alkanethiol, the interaction between thiophene moieties prevent its monolayer from self-assembling at the saturated coverage but leads to form a self-assembled monolayer at a lower coverage. The molecular orientation in a monolayer exerts a strong effect on the orientation in a multilayer that is formed on the monolayer. This suggests that the structure of three-dimensionally self-assembled molecular aggregation can be controlled by using self-assembled monolayers as the template.
Chemical bonding at polymer surface is an important factor because it relates to the fundamental properties of polymers such as wettability and adhesion specially in industrial applications. In this study, NEXAFS (Near Edge X-ray Absorption Fine Structure) spectroscopy using synchrotron radiation was applied to the characterization of polymer surfaces. The spectrum provided a good deal of information related to chemical bond that is difficult to obtain by X-ray photoelectron spectroscopy. The usefulness of NEXAFS spectroscopy as a method of polymer surfaces characterization is demonstrated.
We demonstrated that fluorescence XAFS measurements can reveal the local structures around dilute elements in thin semiconductor layers. In the GaAs samples doped with Er and O, majority of the Er atoms substituted Ga sublattices with adjacent two O atoms and two As atoms (Er-2O center). In the GaInN/GaN samples, it was shown that the segregation of InN phase occurred in the GaInN layer at a higher In-content (x = 0.30), although the segregation was not observed at a lower In-content (x = 0.05). XANES spectrum in the Tb-implanted SiO2 sample was observed by detecting X-ray-excited visible luminescence. The spectrum was quite similar to that was measured by detecting fluorescence X-ray.
For a site-selective analysis of trap centers on semiconductor surfaces, a new X-ray absorption spectroscopy using scanning probe detection of capacitance (Scanning capacitance microscope - X-ray absorption fine structure; SCM-XAFS method) is proposed. Since capacitance is sensitive to localized electron, X-ray induced photoionization of trap center can be detected in terms of capacitance change of diode structure. The X-ray photon energy dependence of the capacitance involved in a point contact diode of GaAs sample and Au-coated Si tip of SCM indicates XAFS spectra of the trap centers in the local region. The correspondence of the Fermi level to the specific trap level enhances the SCM-XAFS signal from the trap center, resulting that the site-selective observation of the trap center can be realized by bias control. The double resonance of the surface site selection (Fermi energy resonance) and the resonant X-ray absorption of the selected site (photon energy resonance) enhances the capacitance signal, indicating that the resonant X-ray absorption peaks can be assigned to the surface sites.
Analysis of core-loss spectra in electron energy-loss spectroscopy associated with transmission electron microscopy (TEM-EELS) is introduced as a tool giving information equivalent to X-ray absorption fine structure (XAFS) on nano-scale areas. After briefly describing the principle of TEM-EELS, including extended energy-loss fine structure (EX-ELFS) and energy-loss near edge structure (ELNES), merits and demerits of TEM-EELS are compared to those of XAFS analysis. A new analysis method applying the ‘wavelet transformation’ is introduced. This method is effective particularly to isolate the signals from the original EXELFS spectrum containing noises and artifacts. Furthermore, information on higher-order coordination shells and many-body correlations can be extracted from the EXELFS interference function, using the wavelet transformation. As an application example of this method, we show characterization of local structural relaxation of amorphous silicon by intense X-ray illumination. Finally some future prospects of TEM-EELS are briefly mentioned.
Isotope abundance and sensitivity factors were taken into consideration in the inference of Ga+ primary ion TOF-SIMS fragment pattern of inorganic compounds. As most elements have isotopes, the isotope abundance in the inference of TOF-SIMS fragment pattern should be considered. In this experiment, Cu metal and its compounds that include isotopes: 63Cu and 65Cu in the ratio of 69.2 vs. 30.8, are employed as measuring samples. Further, a sensitivity factor experimentally obtained is attempted to use.