S. Narioka

Electronic structure of 8-hydroxyquinoline aluminum (alq3)/metal interfaces studied by UV photoemission

Abstract The electronic structures of tris(8-hydroxyquinoline) aluminum (Alq 3 ) / metal (Au, Al) interfaces as a model interface of organic electroluminescent (EL) devices were investigated by ultraviolet photoemission spectroscopy (UPS). We found abrupt shifts of the vacuum level of ca. 1 eV at the interfaces in contrast to the traditional assumption with a common vacuum level at the interface. The shift indicates the formation of interfacial dipole with the metal side negatively charged. At Alq 3 / Al interface, the estimated energy position of the lowest unoccupied molecular orbital (LUMO) was very close to the Fermi level of the substrate metal, corresponding to the electron-injecting nature of the interface. This is in contrast to the poor electron-injecting character expected by assuming a common vacuum level where Fermi level of the substrate should be around the center of the gap. The electronic structure of Alq 3 as a solid is also discussed in comparison with the results by semi-empirical molecular orbital calculation.

NEXAFS spectroscopic studies of molecular orientation in α-sexithienyl evaporated thin films on metal films

Abstract The molecular orientation of evaporated α-sexithienyl (6T) films on gold (Au), silver (Ag) and copper (Cu) films at room temperature were studied by polarized near edge X-ray absorption fine structure (NEXAFS) spectroscopy and infrared reflection absorption spectroscopy (IR-RAS). The polarized NEXAFS spectra of 6T on Ag and Cu films exhibit a strong angular dependence of C1s → π∗ resonance intensity. Combining the results with theoretical simulations and auxiliary information from IR-RAS, it was found that 6T molecules in the film on Ag and Cu films are highly oriented with their molecular axes inclining by about 70° to the substrate surface. On the other hand, NEXAFS of 6T on Au film showed only a little dependence, indicating that 6T molecules are in nearly random orientation.

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.

Determination of two-dimensional structures of ultrathin films of H2 and Cu phthalocyanine on MoS2 by angle-resolved ultraviolet photoemission and low energy electron diffraction

Abstract The angle-resolved ultraviolet photoelectron spectra of ultrathin films of metal-free phthalocyanine (H 2 Pc) and copper phthalocyanine (CuPc) on MoS 2 single crystal surfaces were measured using synchrotron radiation. The take-off and azimuthal angle dependencies of the photoelectron intensity were analyzed using independent-atomic-center approximation combined with molecular orbital calculation. Further, the low energy electron diffraction was observed for the same ultrathin films. The results indicate that H 2 Pc and CuPc molecules form a square lattice with respect to the three equivalent crystal axes of the MoS 2 surface, and at the lattice point lie flat on the substrate with a molecular azimuthal rotation of about 7°

Photoelectron spectroscopy of polysilanes, polygermanes, and related compounds

Summary form only given. Polysilanes and polygermanes have attracted attention as a new class of conducting polymers. In order to investigate their electronic strucure, we measured the whole valence band of five polysilanes, two polygermanes, three Si-Ge copolymers and tert-butyloctasilacubane by ultraviolet photoelectron spectroscopy (UPS). The observed spectra are of better quality than those previously reported for several compounds. The UPS spectra of the alkylpolysilanes and alkylpolygermanes indicate that the valence electronic structure can be ragarded as an overlap of those of the backbone and substituents. On the other hand the spectra of arylpolysilanes imply that the uppermost part of the valence band structure is slightly different from the superposition of those of the Si backbone and substituents. This result suggests /spl sigma/-/spl pi/ interaction between Si /spl sigma/HOVB states and /spl pi/HOMO stales of aryl group. The results of copolymers and tert-butyloctasilacubane are also discussed.

Photoelectron Angular Distribution of Thin Films of Copper Phthalocyanine on MoS2 Surfaces: Quantitative Determination of Molecular Orientation

Abstract The angle-resolved ultraviolet photoelectron spectra of ultrathin films of copper phthalocyanine (CuPc) on an MoS2 substrate were measured using synchrotron radiation. The azimuthal angle dependence of the photoelectron intensity was analyzed using independent-atomic-center approximation combined with molecular orbital calculation. The results indicate that CuPc molecules lie flat on the substrate and that the film grows epitaxially with respect to the three equivalent directions of the MoS2 surface.

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.

Study of α-Sexithienyl Thin Film by Polarized Near Edge X-ray Absorption Fine Structure

An ultrathin α-Sexithienyl (6T) film prepared by an organic molecular beam deposition method on a silver (Ag) film was studied by polarized near edge X-ray absorption fine structure (NEXAFS) spectroscopy using synchrotron radiation and IR-reflection absorption spectroscopy. The carbon K-edge NEXAFS spectrum of 6T was similar to those reported for poly-(3-methylthienylene) and thiophene. Polarized NEXAFS spectra of 6T exhibit strong angular dependence of 1 s→π* resonance intensity, showing that 6T molecules in the film deposited on the Ag film at room temperature had a highly oriented structure. To obtain quantitative information about molecular orientation, this dependence is analyzed by a comparison with theoretical calculation, indicating that the molecular axis is inclined by about 71° to the substrate surface. This angle is almost the same as that reported for 6T film on quartz.