石井 久夫

We demonstrated full determination of second-order nonlinear susceptibility of a 4'-n-octyl-4-cyanobiphenyl (8CB) liquid crystal (LC) monolayer adsorbed on a second-harmonic (SH) active polyimide (PI) substrate. In order to separate the SH signal of the LC film from that of the PI film, we adopted an interferometry technique of second-harmonic generation (SHG) using an ultra-thin film local oscillator. We have found a variety of phases in the components of susceptibility; those of chi(zu) and chi(izi), are almost the same but the phase of chi(zzz) differs by 80 degrees from the other two. The phases of the components of the surface susceptibility tensor are not always identical. This fact indicates that the surface SH response is more complicated than what we expected.

The electronic structure and orientation of tetratetracontane (n-C44H90, TTC) monolayer film on Cu(100) were studied by angle-resolved UPS (ARUPS). We observed a 2 x 1 LEED pattern at room temperature. This indicates that the TTC molecule lies on the Cu(100) surface with its chain axis parallel to the Cu(110) direction. The application of the dipole selection rules to normal emission spectrum revealed that the C-C-C plane of TTC is parallel to the surface (flat-on orientation). We also examined the dependence of the photoemission spectra on the take off angle, The simulated spectra for flat-on orientation based on independent atomic center (IAC) approximation combined with ab initio MO calculations are in good agreement with the observed spectra. These results verify the deduced molecular orientation and demonstrate the reliability of theoretical simulation with IAC approximation. (C) 1998 Elsevier Science B.V.

Recent studies of large functional organic materials by ultraviolet photoelectron spectroscopy (UPS) are reviewed focusing on the studies of interfaces including organics and the development of angle-resolved UPS (ARUPS) studies. The development of organic electroluminescent devices stimulated the studies on metal/organic, organic/metal, and organic/organic interfaces. Much chemistry occurs when metal vapor is deposited on organics, while interfaces formed by depositing on metals have much smaller reactivity, and are more suitable for studying the fundamental aspects of metal/organic contact. In contradiction to the traditional assumption of vacuum level alignment of vacuum level, vacuum level shifts at the interface were found in most interfaces. From the studies of various systems, it was found that both electronic polarization by image charge and charge transfer across the interface can occur. The effect of oxygen was also examined and ascribed to the change of the metal surfaces. The ARUPS studies of oriented samples have become a powerful tool by the development of reasonably reliable theoretical methods based on the independent-atomic-center (IAC) approximation. The emission angle dependence of peak intensities has been used for determining the molecular orientations, for detailed studies of molecular electronic structures of model systems of polymers, and the study of peak intensity oscillation with photon energy for fullerenes. (C) 1998 Elsevier Science B.V.

Interfacial electronic structures related to organic electroluminescent (EL) devices were studied by UV photoemission spectroscopy (UPS). The two classes of interfaces studied were: (1)interfaces in a typical multilayer device Al/Alq(3)/TPD/ITO, where Alq(3) is tris(8-hydroxyquinolino)-aluminum, TPD is N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine, and ITO is indium tin oxide, and (2) TTN/metals and TCNQ/metals interfaces, where TTN is tetrathianaphthacene and TCNQ is tetracyanoquinodimethane. The UPS studies of the specimen formed by the successive deposition of TPD, Alq(3), and Al on ITO revealed interfacial energy diagrams, with the vacuum level shift of -0.25 eV (downward) and -0.1 eV (downward) at the TPD/ITO and the Alq(3)/TPD intel faces, respectively. The deposition of TTN and TCNQ on metals showed opposite direction of the shift of the vacuum level, with the positive and negative charge at the vacuum side. This can be explained by considering the charge-transfer between the metal and the organic molecule, with these directions being consistent with the electron donating and accepting ability of these molecules.

Ionization threshold energies of 26 substituted polyacetylenes were determined by ultraviolet photoelectron spectroscopy. Examination of the data obtained, along with data for other polymers, and the energy of the lowest energy electronic excitation revealed that the threshold energies correspond to the photoemission from the highest occupied orbitals in the pi-conjugated main chains. For the majority of the polymers, the main origin of the significant variation of threshold energy and electronic excitation energy is ascribed to the change of the degree of pi-conjugation due to steric hindrance of substituents. Calculations with Huckel theory gave semiquantitative agreement with the observed results. Polymers with Cl substituents at the main chain also showed a large inductive effect by the electron-withdrawing Cl atom. The HOMO of these polymers was found to be derived from the pi-conjugated main chain.

The tautomerism in solid phase was studied by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the N K- and O K-edges and X-ray photoelectron spectroscopy (XPS) in the N1s and O1s regions for five N- salicylideneaniline derivatives: N,N'-disalicylidene-1,6-pyrenediamine (DSPY), N,N'-disalicylidene-pphenylenediamine (BSP), N-(2-hydroxy-l- naphthylidene)-l-pyrenylamine (NPY), N,N'-di(2-hydroxy-l-naphthylidene)p- phenylenediamine (DNP), and the complex between DNP and 7,7,8,8- tetracyanoquinodimethane (DNP-TCNQ). The NEXAFS spectral features were assigned by (1) the comparison with the core excitation spectra of reference compounds and (2) ab initio MO calculations of the core-excited states by the improved virtual orbital method using the relaxed Hartree-Fock orbitals. The tautomeric structures were deduced by the intensity of peaks characteristic of the tautomers in OH- and NH-forms. The XPS spectra gave structures consistent with those by NEXAFS and also allowed quantitative estimation of the relative tautomer populations. The results range from the dominance of the OH-form in DSPY and BSP to a comparable mixture of the OH- and NH-forms in DNP and DNP-TCNQ. These structures are consistent with the previous estimation by X-ray diffraction (XD), except for DNP-TCNQ where XD suggested the NH-form dominance. The good correlation of the population by XPS with the C-O bond length by XD supports the present conclusion for DNP-TCNQ. These results demonstrate the usefulness of NEXAFS as well as XPS for studying the tautomerism in hydrogen bonded systems.

The surface freezing effect of n-alkane has been studied by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. Samples used in the experiment were n-C50H102 (pentacontane) 2500Å thick films evaporated on Cu substrate. An ordered structure has been observed at a 1.3°C higher temperature than the bulk melting point. Angle resolved measurement has revealed that the molecular long axis of pentacontane in the surface freezing layer is almost perpendicular to the surface.

NEXAFS spectra of three condensed aromatic hydrocarbons, chrysene, perylene, and coronene, were measured and analyzed in particular emphasis on the effect of core excitonic effect as the origin of modification of the observed spectra from density of unoccupied states (DOUS). The comparison of the observed spectra with reported inverse photoemission spectra and calculated DOUS showed poor correspondence, indicating large deviation between the NEXAFS spectra and DOUS. On the other hand, the simulation of the spectra by ab inito MO calculations, using the hole potential method, reproduced the experimental results well. The analysis of the results of MO calculations revealed that core excitonic effect significantly depends on the site of the excited atom and the character of the unoccupied orbital, making the observed spectra much different from the DOUS.

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.

Electronic structures of N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1' (TPD)/metal and N,N'-diphenyl-1,4,5,8-naphtyltetracarboxylimide (DP-NTCI)/metal interfaces were directly investigated as a model interface of organic electroluminescent (EL) devices using UV photoemission spectroscopy (UPS). At the organic/metal interfaces, abrupt shift of vacuum level was observed, in contrast to the traditional assumption of common vacuum level at the interface. For understanding EL devices, we see the necessity of the direct observation of the interfacial electronic structure by UPS or other techniques for understanding EL devices.

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 (Delta). 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 Delta 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 Delta might be explained by the existence of interface state.

Ultraviolet photoelectron spectra were measured using synchrotron radiation for four kinds of pi-conjugated polymers, poly(pyridine-2,5-diyl)[Ppy], poly(2,2'-bipyridine-5,5'-diyl)[Pbpy], poly(2-methylantraquinone-1,4-diyl)[P(2Me-1,4-AQ)], and poly(antraquinone-1,5-diyl)[P(1,5-AQ)], which exhibit n-type electrically conducting properties. Upon K-doping of Pbpy, two new states appeared in the originally empty energy gap. This finding indicates that the bipolaron bands are formed in K-doped Pbpy. While K-doped Ppy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K-doped Pbpy. The UPS spectra of K-doped P(2Me-1,4-AQ) also show similar gap states, while it is almost absent in K-doped P(1,5-AQ). The changes in electronic structure of these polymers are discussed based on the conformational difference between these polymers.

The electronic structures of model interfaces of organic electroluminescent (EL) devices were investigated with UV photoemission spectroscopy (UPS). At model interfaces of an Al/Alq(3)/TPD/Au device (Alq(3):tris(8-hydroxyquinolino) aluminum, TPD: N,N-1-diphenyl-N,N-1-(3-methylphenyl)-1,1(1)-biphenyl-4,4(1)- diamine) and DP-NTCI/metal interfaces (DP-NTCI: N,N-1-diphenyl-1,4,5,8-naphtyltetracarboxylimide), the vacuum level shifts were observed, indicating the invalidity of the traditional energy level alignment model where a common vacuum level is assumed at organic / metal interface. On the other hand, the results for Alq(3)/TPD interface showed little vacuum level shift, leading to apparent applicability of the traditional model. The observed interfacial electronic structures corresponds well to the injecting nature of the interface. The effect of air exposure to the interfacial electronic structure and the relation between the vacuum level shifts and the substrate work function were discussed, together with the results for porphyrin/metal interfaces.

Electronic structures of model interfaces of organic electroluminescent (EL) devices and porphyrin/metal interfaces were investigated by UV photoemission spectroscopy (UPS). At all the measured interfaces, shift of the vacuum level was observed, showing the formation of an interfacial electric dipole layer. For Alq3 (tris(8-hydroxyquinolino) aluminum), TPD (N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine), and DP-NTCI (N,N'-diphenyl-1,4,5,8-naphthyltetracarboxylimide)/metal interfaces, interfacial energy diagrams determined by UPS correspond well with the actually observed carrier-injecting character at the interfaces. For ZnTPP (5,10,15,20-zinc-tetraphenylporphyrin), H2TPP (5,10,15,20- tetraphenylporphyrin), and H2T(4-Py)P (5,10,15,20-tetra(4-pyridyl) porphyrin)/metal interfaces, the shifts of the vacuum level as well as the energies of the levels in porphyrins could be expressed as a linear function of work function of the metal substrate. The slope of the linear function depended on the compound. These findings are in contrast to the traditional assumption of common vacuum level at the interfaces. For ZnTPP/metal interfaces, sample exposure to oxygen induced energy level shift in close relation with the change of the substrate work function at oxygen exposure. The present results have clearly demonstrated that direct observation of the interfacial electronic structure by microscopic method such as UPS is necessary for understanding the organic electronic devices such as EL devices and organic solar cells. © 1997 IEEE.

In order to clarify electronic structures of molecular semiconductor/metal interfaces, a Schottky-Mott rule is examined for vacuum-sublimed films of two kinds of porphyrins, which have similar chemical structures, but opposite conductance types. The result shows that Schottky barrier heights are simply determined by the difference in work function between the porphyrin solids and metals irrespective of the conductance types of the porphyrin semiconductors, indicative of negligible influence of surface states on the Schottky barrier formation. Measurements of photocurrent generation efficiencies at these Schottky junctions indicate that a surface recombination process is not a major deactivation route for electron-hole pairs generated in the molecular semiconductors by light. (C) 1996 American Institute of Physics.

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.

The electronic structure of oriented thin films of sexiphenyl (6P) as a short model compound of poly(p-phenylene) (PPP) was investigated by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation. The sample film of 6P was prepared by vacuum evaporation onto a heated Ag substrate. Infrared reflection-absorption spectroscopy, x-ray diffraction, and the observed selection rules for ARUPS indicated that the 6P molecules are highly oriented aligning its molecular axis parallel to the surface normal. In the normal emission spectra of ARUPS, the band intensity of the HOMO and the next HOMO was enhanced at h nu=28 similar to 40eV. This enhancement can be explained in terms of the wave number(k)-conservation rule in the one-dimensional intramolecular band with some initial state broadening of k (or relaxation of k-conservation rule) due to the small number of repeating units. This indicates that wave number k along the molecular axis serves as a reasonably good quantum number even in a system consisting of only six repeating units.

Polarized vacuum-UV absorption and reflection spectra were measured for oriented films of polytetrafluoroethylene (PTFE) (CF2)(n) and its model compound perfluorotetracosane (PFT) n-C24F50. The lowest energy excitation leading to an intense peak was found to be polarized along the molecular chain, and its energy decreases with the chain length (8.05 eV for PFT and 7.66 eV for PTFE). The polarization and chain-length dependence are discussed with the aid of information by theoretical calculation, NEXAFS, and electron transmission spectroscopy.

Two different series of smectic liquid-crystalline biphenyl carboxylates have been prepared and their smectic properties investigated. The 'lateral' alkyl chain at the branching carbon was varied from methyl to hexyl for the homologous series of 4-(1-alkylheptyloxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylates. The antiferroelectric phase (S-CA((*))) was observed for the homologous series except the pentyl homologue. The smectic layer thicknesses for the series were found to increase as ascending the 'lateral' alkyl chain, indicating that the 'lateral' alkyl group of the series is in fact along the molecular long axis to some extent, which can occur if the molecule has the 'bent-shaped' structure.

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 nibbing process. The chains of the PFT evaporated on the rubbed PTFE film aligned along the rubbing direction.

The electronic structures of five polysilanes and octa(t-butyl)octasilacubane were investigated by ultraviolet photoemission spectroscopy (UPS). The spectral features were assigned by comparison with spectra of the constituents parts. The UPS spectra of the polyalkylsilanes indicate that the valence electronic structure can be regarded as an overlap of those of the Si-backbone and substituents. In the case of polyarylsilanes, all the spectral features except those in the uppermost energy region correspond one by one to each band in the spectra of the substituents as in the case of polalkylsilanes. The deviation of the spectral features in the uppermost energy region from those of the constituents suggests sigma-pi interaction between the pi HOMO states of benzene and the Si sigma HOVB states of the Si backbone. The results of octa(t-butyl) octasilacubane are also discussed.

The electronic structure of oriented thin films of sexiphenyl (6P) as a short model compound of poly(p-phenylene) (PPP) was investigated by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation. The sample film of 6P was prepared by vacuum evaporation onto a heated Ag substrate. Infrared reflection-absorption spectroscopy, x-ray diffraction, and the observed selection rules for ARUPS indicated that the 6P molecules are highly oriented aligning its molecular axis parallel to the surface normal. In the normal emission spectra of ARUPS, the band intensity of the HOMO and the next HOMO was enhanced at hv=28 similar to 40eV. This enhancement can be explained in terms of the wave number(k)-conservation rule in the one-dimensional intramolecular band with some initial state broadening of k (or relaxation of k-conservation rule) due to the small number of repeating units. This indicates that wave number k along the molecular axis serves as a reasonably good quantum number even in a system consisting of only six repeating units.

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.

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 of oriented thin films of sexiphenyl (6P), which is a short model compound of poly(p-phenylene), was studied by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation. Oriented thin films of 6P were prepared by vacuum evaporation on a heated Ag substrate. Infrared reflection-absorption, x-ray diffraction, and the observed selection rules for ARUPS indicated that the axes of 6P molecules are oriented perpendicular to the substrate surface. The dependence of normal emission intensity of the highest occupied molecular orbital (HOMO) and the next 4OMO in these films on photon energy h, with a maximum at h=28-40 eV, was explained in terms of the wave number (k) conservation rule in the one-dimensional intramolecular band with some initial state broadening of k (or relaxation of the k-conservation rule) due to the small number of repeating units. This indicates that the wave number k along the molecular axis serves as a reasonably good quantum number even in a system of only six repeating units. The quantitative simulation for the h dependence of normal emission using the independent-atomic-center (IAC) approximation combined with MO calculations gave a good description of the observed experimental data. © 1995 The American Physical Society.

Ultraviolet photoelectron spectra were measured for five polysilanes, two polygermanes, three Si-Ge copolymers and tert-butyloctasilacubane. The UPS spectra of the polyalkrylsilanes and polyalkylgermanes indicate that the valence electronic structure can be regarded as an overlap of those of the backbone and substituents. On the other hand, the spectra of polyarylsilanes imply that the uppermost part of the valence band structure is slightly different from the superposition of those of the Si backbone and substituents due to sigma-pi interaction. The results of copolymers and tert-butyloctasilacubane are also discussed.

Three different series of smectic liquid-crystalline biphenyl carboxylates have been prepared and their smectic properties investigated. The 'lateral' alkyl chain at the branching carbon was varied from methyl to hexyl for the homologous series of 4-(1-alkylheptyloxycarbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylates (series A). A smectic phase which has the same structure as the antiferroelectric phase (SCA*) or 'anticlinic' phase (SCA′) was observed for all the members of the series except the pentyl homologue. The smectic layer thicknesses for the homologous series were found to increase as ascending the 'lateral' alkyl chain, indicating that the 'lateral' alkyl group of the series is in fact directed along the molecular long axis to some extent, which can occur if the molecule has a 'bent' structure. A 'swallow-tailed' compound, 4-(1-propylbutyloxycarbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylate, and the related 'ring-tailed' compounds, 4-(1-cycloalkyloxycarbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylates, were also prepared (series B). The swallow-tailed compound showed an SCA′ phase, whereas the related ring-tailed compounds exhibited an ordinary Sc phase, indicating that steric interactions via the terminal alkyl chains stabilize the SCA′ phase. The biphenyl carboxylates possessing a 2-alkyloctyl group at the branching chiral centre, i.e., 4-(2-methylalkyloxycarbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylates (series C), were found to exhibit no SCA* phase.

機能性有機分子薄膜など有機固体の電気・光物性は,価電子帯のエネルギーバンド構造に深くかかわる。シンクロトロン放射光を用いて紫外光の波長を連続的に変化させながら角度分解紫外光電子分光(ARUPS)スペクトルを測定すると,エネルギーバンド構造を直接測定することができる。本稿では,分子間の強い相互作用にもとづく分子間エネルギーバンド分散や,高分子に特有な分子内エネルギーバンド分散などの実測例を紹介する。また,光電子強度の角度依存性の定量的解析から,有機薄膜中の分子配向角を求めた結果について述べる。

UV photoelectron spectra were measured for ten polymers with main chains composed of Si and Ge atoms. The spectra of poly(dialkylsilane)s were described well as the overlap of the spectra of constituent parts, while polysilanes with aryl side groups showed the effect of sigma-pi mixing. The spectra of polygermanes and Si-Ge block copolymers are fairly similar to those of corresponding polysilanes, while Si-Ge random copolymers show indications of the effect of the presence of many Si-Ge bonds.