Tetsuhiro Sekiguchi

Photon stimulated ion desorption studies using pulsed synchrotron radiation

Photon stimulated ion desorption from small molecules on Si(100) surfaces and from polymer thin films was studied using time‐of‐flight (TOF) mass spectrometric techniques. The design and operation of a TOF mass spectrometer during single bunch operation of the Photon Factory 2.5 GeV storage ring is described. Experimental results for H2O/Si(100), DCOOD/Si(100), polymethylmethacrylate and polymethylacrylate thin films are demonstrated as examples of the capabilities of the apparatus.

X-ray photoelectron spectroscopic observation on B–C–N hybrids synthesized by ion beam deposition of borazine

B–C–N hybrid thin films were grown from ion beam plasma of borazine (B3N3H6) on graphite substrate at room temperature, 600 °C, and 850 °C. The films were characterized in situ by x-ray photoelectron spectroscopy (XPS). XPS study suggested that B and N atoms in the deposited films are in a wide variety of chemical bonds, e.g., B–C, B–N, N–C, and B–C–N. The substrate temperature and ion fluence were shown to have a significant effect on the coordination and elemental binding states on the B–C–N hybrids. It was found that B–C–N hybrid formation is enhanced at high temperature, and this component is dominantly synthesized at low fluence. The results imply that it is possible to control the composition of B–C–N hybrid by changing the ion fluence and the temperature during ion implantation.B–C–N hybrid thin films were grown from ion beam plasma of borazine (B3N3H6) on graphite substrate at room temperature, 600 °C, and 850 °C. The films were characterized in situ by x-ray photoelectron spectroscopy (XPS). XPS study suggested that B and N atoms in the deposited films are in a wide variety of chemical bonds, e.g., B–C, B–N, N–C, and B–C–N. The substrate temperature and ion fluence were shown to have a significant effect on the coordination and elemental binding states on the B–C–N hybrids. It was found that B–C–N hybrid formation is enhanced at high temperature, and this component is dominantly synthesized at low fluence. The results imply that it is possible to control the composition of B–C–N hybrid by changing the ion fluence and the temperature during ion implantation.

Ion desorption from H2O chemisorbed on Si(100) by O 1s electron excitation at room temperature

The photon stimulated ion desorption (PSID) of H+ and O+ from monolayer H2O adsorbed on the Si(100) has been studied using pulsed synchrotron radiation in the 510–620 eV range. Ions were detected and mass analyzed by means of time‐of‐flight spectroscopy. Relative yield spectra of these ions indicated characteristic behavior near the O K edge. The predominant H+ fragment ion shows a sharp rise at about 530 eV and two broad peaks (535 eV, 558 eV), while O+ shows a delayed threshold at about 570 eV and gradual increase up to 700 eV. Above the O K edge, these ion yields were found to deviate significantly from the O(KVV) Auger electron yield. This was interpreted by the fact that ion desorption was enhanced by multiple ionization and is strongly affected by the decay process with the surface. In order to elucidate the mechanism of the PSID, the technique of photoion–photoion coincidence between H+ and O+ ions was also applied. The results suggested the formation of multiply charged intermediates, i.e., OH4+ i...

Selective adsorption of atomic hydrogen on a h-BN thin film

The adsorption of atomic hydrogen on hexagonal boron nitride (h-BN) is studied using two element-specific spectroscopies, i.e., near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray photoelectron spectroscopy (XPS). B K-edge NEXAFS spectra show a clear change in the energy region of the π* band before and after reaction with atomic deuterium. On the other hand, N K-edge NEXAFS spectra show only a little change. B 1s XPS spectra show a distinct component at the low binding energy side of a main component, while N 1s XPS spectra show peak broadening at the high binding energy side. These experimental results are analyzed by the discrete variational Xα method with a core-hole effect and are explained by a model in which hydrogen atoms are preferentially adsorbed on the B sites of h-BN. Based on the experimental and theoretical results, we propose a site-selective property of BN material on adsorption of atomic hydrogen.

Photon stimulated ion desorption from condensed thiophene photoexcited around the S1s-edge

Photon stimulated ion desorption and Auger decay spectra were measured for condensed thiophene around the S1s-edge. Three kinds of ions, H+, S+, and S2+, were observed as main desorbed species. Partial ion yields (PIY) have been determined for these three fragment ions as a function of the photon energy. It was found that the indirect x-ray induced electron stimulated desorption mechanism plays an important role in H+ ion desorption. For S+ ion desorption, on the other hand, a pronounced enhancement at ∼3eV away from the first core-to-valence resonance was observed in the PIY curve. On the basis of the results for the Auger decay spectra, the enhancement of S+ desorption at ∼3eV away from the first resonance is interpreted in terms of the effective screening of positive holes due to the existence of Rydberg electrons.

Effect of substrates on the molecular orientation of silicon phthalocyanine dichloride thin films

Molecular orientations of silicon phthalocyanine dichloride (SiPcCl2) thin films deposited on three different substrates have been measured by near-edge x-ray absorption fine structure (NEXAFS) spectroscopy using linearly polarized synchrotron radiation. The substrates investigated were highly oriented pyrolitic graphite (HOPG), polycrystalline gold and indium tin oxide (ITO). For thin films of about five monolayers, the polarization dependences of the Si K-edge NEXAFS spectra showed that the molecular planes of SiPcCl2 on three substrates were nearly parallel to the surface. Quantitative analyses of the polarization dependences revealed that the tilted angle on HOPG was only 2?, which is interpreted by the perfect flatness of the HOPG surface. On the other hand, the tilted angle on ITO was 26?. Atomic force microscopy (AFM) observation of the ITO surface showed that the periodicity of the horizontal roughness is of the order of a few nanometres, which is larger than the molecular size of SiPcCl2. It is concluded that the morphology of the top surface layer of the substrate affects the molecular orientation of SiPcCl2 molecules not only for mono-layered adsorbates but also for multi-layered thin films.

Molecular ordering effect of regioregular poly(3-hexylthiophene) using sulfur K-edge X-ray absorption spectroscopy

The electronic structure of the unoccupied conduction band of regioregular poly(3-hexylthiophene) (RR-P3HT) was investigated by X-ray absorption spectroscopy (XAS) near the sulfur K-edge. Angle-dependent XAS studies revealed that polymer chains in films are well aligned and oriented edge-on with respect to the Si substrate. It was clearly observed that the bottom of the conduction band near the Fermi level in a π–π stacking film is lower (0.3 eV) than that in a powder. It was demonstrated that π–π stacking interactions improve the power conversion efficiency through the reduction of the lowest unoccupied molecular orbital (LUMO) energy level.

Orientation of one-dimensional silicon polymer films studied by X-ray absorption spectroscopy

Molecular orientations for thin films of one-dimensional silicon polymers grown by vacuum evaporation have been assigned by near-edge X-ray absorption fine structure (NEXAFS) using linearly polarized synchrotron radiation. The polymer investigated was polydimethylsilane (PDMS) which is the simplest stable silicon polymer, and one of the candidate materials for one-dimensional molecular wire. For PDMS films deposited on highly oriented pyrolytic graphite (HOPG), four resonance peaks have been identified in the Si K-edge NEXAFS spectra. Among these peaks, the intensities of the two peaks lower-energy at 1842.0 eV and 1843.2 eV were found to be strongly polarization dependent. The peaks are assigned to the resonance excitations from the Si 1s to σ* pyz and σ* px orbitals localized at the Si-C and Si-Si bonds, respectively. Quantitative evaluation of the polarization dependence of the NEXAFS spectra revealed that the molecules are self-assembled on HOPG surface, and the backbones of the PDMS are oriented nearly parallel to the surface. The observed orientation is opposite to the previously observed results for PDMS on the other surfaces such as oxide (indium tin oxide) and metal (polycrystalline copper). The flat-lying feature of PDMS observed only on HOPG surface is attributed to the interaction between CH bonds in PDMS and p orbitals in HOPG surface.

Metal-molecular interface of sulfur-containing amino acid and thiophene on gold surface

Chemical-bonding states of metal-molecular interface have been investigated for L-cysteine and thiophene on gold by x-ray photoelectron spectroscopy (XPS) and near edge x-ray adsorption fine structure (NEXAFS). A remarkable difference in Au-S bonding states was found between L-cysteine and thiophene. For mono-layered L-cysteine on gold, the binding energy of S 1s in XPS and the resonance energy at the S K-edge in NEXAFS are higher by 8–9 eV than those for multi-layered film (molecular L-cysteine). In contrast, the S K-edge resonance energy for mono-layered thiophene on gold was 2475.0 eV, which is the same as that for molecular L-cysteine. In S 1s XPS for mono-layered thiophene, two peaks were observed. The higher binging-energy and more intense peak at 2473.4 eV are identified as gold sulfide. The binding energy of smaller peak, whose intensity is less than 1/3 of the higher binding energy peak, is 2472.2 eV, which is the same as that for molecular thiophene. These observations indicate that Au-S interface behavior shows characteristic chemical bond only for the Au-S interface of L-cysteine monolayer on gold substrate.

X-Ray Photoelectron Spectroscopic Observation on the Formation of Carbon Nitride Thin Films Produced by Low-Energy Nitrogen Ion Implantation.

X-ray photoelectron spectroscopy (XPS) spectra of N 1s and C 1s were measured for carbon nitride thin films prepared by low-energy nitrogen ion implantation in graphite. To assign the XPS spectra, we also measured XPS spectra for some standard materials which have sp2 or sp3 C-N bond configurations. In N 1s XPS spectra for the ion-implanted graphite, we found three clear peaks at the binding energies of EB=398.3, 400.3, and 402.6 eV. However, in spite of the difference in bonding systems, all the carbon nitride compounds showed similar N 1s binding energies corresponding to the second peak (EB=400.3 eV). Furthermore, we found a broad structure which resulted from the formation of C-N bonds at a binding energy 2 eV higher than that of the graphite peak, by removing the damage effect from the C 1s XPS spectra for the ion-implanted graphite. Based on these results, we propose an assignment of XPS spectra for the ion-implanted graphite.