M. Sei

Polarized XANES studies on the rubbed polyimide for liquid crystal alignment; new applicability to the tribology of the polymer systems☆

Abstract The surface structure of rubbed polyimides, which is responsible for the homogeneous alignment of liquid crystal molecules, was examined with use of polarized XANES spectroscopy. Our first experiment was conducted on the odd-even effect of BPDA- Cn (bipenyl-3,3′,4,4′-tetracarboxylic dianhydride/ n -alkyl diamine) type polyimide; BPDA- Cn type polyimide with even number alkylene chain induces a large LC pretilt angle and the one with odd number alkylene chain does almost 0 degree LC pretilt angle. XANES spectra have successfully revealed the surface-structure difference between odd- and even-number BPDA polyimides, one is trans-cisoid type and the other is trans-transoid type, respectively.

Electronic Structures of Porphyrins and their Interfaces with Metals Studied by UV Photoemission

Abstract The electronic structures of porphyrins and their interfaces with metals were investigated by UV photoemission spectroscopy (UPS). The UPS spectral features of porphine, 5,10,15,20-tetraphenylporphynatozinc(II) (ZnTPP), 5,10,15,20-tetra(4-pyridyl)porphyrin (H2T(4-Py)P) and 5,10,15,20-tetraphenylporphyrin (H2TPP) could be assigned by comparison with MOPAC PM3 calculations. The electronic structures of the porphyrins can be regarded as the superposition of those of porphine and the substituents. The UPS results for ZnTPP/metal (Mg, Al, Ag, Au) interfaces indicated that the energies of the levels of ZnTPP relative to the Fermi level of substrate metals could be expressed as linear functions of the work function of metals with a shift of the vacuum level at interface. The slope of the linear functions was about unity. This indicates that the energy levels of ZnTPP are fixed to the vacuum level of substrate metal with constant interfacial dipole. The level positions of ZnTPP at the interface exhibited...

The electronic structure of porphyrin/metal interfaces studied by UV photoemission spectroscopy

The electronic structures of the interface between porphyrin and metal (Mg, Al, Ag and Au) were studied by UV photoemission spectroscopy (UPS). The samples were 5,10,15,20-tetraphenylporphynatozinc(ZnTPP), 5,10,15,20-tetra(4-pyridyl) porphyrin(H2T(4-Py)P) and 5,10,15,20-tetra-phenylporphyrin(H2TPP) which are known as organic semiconductors. We found that the energy levels of these porphyrins relative to the Fermi level of the substrate metals could be expressed as linear functions of the work function of metals with a shift of the vacuum level at interface (Δ). This indicates that the energy levels of these porphyrins are fixed to the vacuum level of substrate metal with an interfacial dipole layer. We also found that the magnitude of Δ is constant at ZnTPP/metal interfaces, but depends on the work function of the metal at H2T(4-Py)P and H2TPP/metal interfaces. This work function dependence of Δ might be explained by the existence of interface state.

The electronic structure of porphyrin/metal interfaces studied by ultraviolet photoelectron spectroscopy

Electronic structure of interfaces between 5,10,15,20-tetraphenylporphynatozinc (ZnTPP) and four metals (Mg, Al, Ag, and Au) were studied by ultraviolet photoelectron spectroscopy (UPS). The energy levels of ZnTPP relative to the Fermi level of substrate metals could be expressed as linear functions of the work function of metals with the shift of the vacuum level at interface (Δ). The slope of the linear functions was about unity. This indicates that the energy levels of ZnTPP are fixed to the vacuum level of substrate metal with constant interfacial dipole. 5,10,15,20-Tetra(4-pyridyl)porphyrin (H2T(4-Py)P)/metal and 5,10,15,20-tetraphenylporphyrin (H2TPP)/metal interfaces were also investigated, and similar linearity was observed between the energy levels of porphyrin and the work function of metal with the slope of much smaller than unity. This deviation of the slope from unity might be explained by the existence of interface state.

Polarized NEXAFS studies on the mechanical rubbing effect of poly(tetrafluoroethylene) oligomer and its model compound

The structural change of the evaporated perfluorotetracosane (PFT) n-CF3(CF2)22CF3 and poly (tetrafluoroethylene) (PTFE) (CF2)n films by the mechanical rubbing process was examined with use of NEXAFS spectroscopy. In the PTFE film, the chains were almost parallel to the surface and were uniaxially realigned along the rubbing direction, while the chains in the PFT film after evaporation were oriented normal to the surface, and the orientation was hardly affected by the rubbing process. The chains of the PFT evaporated on the rubbed PTFE film aligned along the rubbing direction.

The electronic structure and energy level alignment of porphyrin/metal interfaces studied by ultraviolet photoelectron spectroscopy

Abstract Electronic structures of 5,10,15,20-zinctetraphenylporphyrin (ZnTPP)/metal (Au, Ag, Al, Mg) interfaces prepared in ultrahigh vacuum condition were investigated by ultraviolet photoelectron spectroscopy (UPS). We found that the electronic energy levels of ZnTPP align with the vacuum level of substrate metal with a constant energy shift of the vacuum levels across the interface. These findings cannot be explained by the concepts of Fermi level alignment nor vacuum level alignment. We also found that sample exposure to oxygen induces energy level shift in close relation with change of substrate work function at oxygen exposure. The relation between the observed electronic structure and the electric characteristics of porphyrin/metal interfaces in solar cell is also discussed.

Ultrathin-film Local Oscillator for Determination of Complex Components of Second-Order Nonlinear Susceptibility

The second-harmonic generation (SHG) interferometry technique has to date been used to determine the phase of second-harmonic (SH) light generated in a single polarization combination of a polarizer and an analyzer. However, it is not easy to compare phases of the SH light generated by a plural polarization combination, because a thick local oscillator, such as an α-quartz single crystal, has several disadvantages for this purpose. The birefringence and optical rotatory power of the quartz crystal often require further consideration on the phase relation of the SH lights with different polarizations. Thus, the full determination of the complex χ-components of an organic ultrathin film by the use of the quartz crystal is impractical. After surveying SH active systems, we found that a 2-docosylamino-5-nitropyridine (DCANP) Langmuir-Blodgett (LB) film is the most applicable for the phase measurements. A few nanometers-thick DCANP LB film, which is much thinner than the wavelength of the SH light, yields a negligible phase delay even if birefringence is present. The SHG efficiency of the film can be easily controlled by changing the number of layers. Some experimental results are addressed to demonstrate the advantage of the DCANP LB film as a local oscillator.

Molecular orientation and electronic structure of evaporated porphyrin films

The molecular orientation in evaporated 5, 10, 15, 20-zinctetraphenylporphyrin (ZnTPP) and 5, 10, 15, 20-tetraphenylporphyrin (H2TPP) films was studied by X-ray-Absorption Near-Edge Structure (XANES) spectroscopy. For ZnTPP, the film deposited on Ag/Cu substrate at 367 K had highly oriented structure with the central molecular plane inclining at 25° ± 10° to the substrate surface. While ZnTPP film evaporated on the substrate at room temperature and H2TPP films showed little indication of regular orientation. Electronic structures of ZnTPP, H2TPP and porphine were studied by ultraviolet photoelectron spectroscopy. Reliable spectrum of solid porphine was obtained. The spectra of ZnTPP and H2TPP can be explained as the superposition of those of porphine and its substituents.