石井 久夫

Polyimide films irradiated with linearly polarized deep UV (LPDUV) light can align liquid crystals perpendicularly to its polarization direction. We report an experiment using near edge X-ray absorption fine structure (NEXAFS) spectroscopy with high surface sensitivity to elucidate the LC alignment mechanisms by total electron yield (TEY) and partial electron yield (PEY) methods. The dichroic ratio of O K-edge pi* peak intensity, originated from C=O bond, shows different UV exposure dependence for TEY and PEY cases. We can conclude that the LPDUV irradiated film possesses the depth dependence of anisotropy.

In this article, studies of two subjects recently carried out in our group on organic/metal interfaces are reported. The first is the examination of the change of the work function of various metals (Ca, Mg, Cu, Ag and Au) at the deposition of TPD [N,N ' -bis(3-methylphenyl)-N,N ' -diphenyl-[1,1 ' -biphenyll-4,4 ' -diamine (TPD)], which is used as a typical hole-injection layer. The work function change was pursued using Kelvin probe method and UV photoelectron spectroscopy (UPS) up to the film thickness of 100 nm. The results clearly showed that band bending leading to Fermi level alignment is not achieved, at least for systems prepared and measured under ultra high vacuum (UHV) conditions. The results of the exposure of the metal substrate to air was also examined. In the second work, we studied the mode of film growth, chemical interaction and electronic structure for interfaces formed by depositing p-sexiphenyl (6P) on metals (Au and Mg) and metals on 6P, using metastable atom electron spectroscopy (MAES), UPS and X-ray photoelectron spectroscopy MPS). The results strongly depended on the metal and the order of deposition. The Au surface was easily covered by 6P deposition (equivalent to 0.3 nm thickness), while Mg surface requires much larger amount of 6P. In contrast, metal atoms deposited on 6P diffused into the organic layer. In particular, the surface of 6P was exposed even after depositing Au equivalent to 200-nm thickness. The change of work function in these cases is interpreted based on these modes of interface formation. (C) 2001 Elsevier Science B.V. All rights reserved.

The molecular orientation at the surfaces of poly(9-vinylcarbazole) (PvCz) thin films was studied by angle-resolved ultraviolet photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The observed take-off angle (θ) dependence of photoelectron intensities from top π band peaks clearly at larger θ than the calculated one for the three-dimensional isotropic random orientation model. The results indicate that there are more pendant groups with large tilt angles than the three-dimensional isotropic random orientation model, which is in good agreement with the result obtained from NEXAFS spectroscopy. The surface electronic states of PvCz may be rather dominated by σ(C-H) states at the pendant carbazole group than π states © 2001 Elsevier Science B.V. All rights reserved.

A thiophene-terminated alkanethiolate self-assembled monolayer was studied by metastable atom electron spectroscopy (MAES)and infrared reflection absorption spectroscopy (IR-RAS). Since MAES is an extremely surface selective method, it allowed us to determine which part of the molecule is exposed to the outside of the surface in a monolayer. Theoretical simulation with partial exterior electron density suggested that not only the thiophene group but also a part of the main chain are exposed to the outside, indicating the bonding to the substrate with thiolate formation. The selection rules of IR-RAS demonstrate that the plane of the thiophene ring is almost lying parallel to the surface and that the alkyl chain aligns nearly along the surface normal. These results are consistent with the findings in our previous X-ray photoelectron spectroscopy study that the molecule is adsorbed with forming thiolate in a densely packed manner.

An ultraviolet photoelectron spectroscopic study was performed to examine the electronic structure of poly-(1,1,2,2-tetramethyldisilylenemethylene), [(SiMe2)(2)CH](n), which consists of repeating Si-Si-C units. We compared the observed spectra to reported gas-phase spectra of Si2Me6 and Si4Me10 and theoretical molecular orbital calculations. Theoretical calculations by the ab initio and PM3 methods agree well with the observed spectra, which enabled assignment of the observed spectral features. The broad peak derived from Si-Si bond orbitals in polycarbosilane suggests that Si-Si units, which are separated by carbon atoms, still interact with each other, to form sigma -conjugation through the backbone. The observed ionization threshold energy of the polycarbosilane[(SiMe2)(2)CH2](n), is 6.4 eV. The dispersion of the highest valence band of polycarbosilane is less than that of polysilane, indicating that the degree of sigma -conjugation of polycarbosilane is smaller than that of polysilane.

We have used ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES) to study the changes in the valence electronic structure of oligophenylenes upon the deposition of alkali metals. We have fabricated thin films of p-quarterphenyl (4P) and p-sexiphenyl (6P) by thermal evaporation under ultrahigh vacuum conditions, resulting in partly oriented organic films. Subsequently, increasing amounts of alkali metals (Na, K, Rb, Cs in individual experiments) were deposited onto those in a stepwise manner. After each step, UPS and MAES spectra were recorded. We find new emissions within the formerly empty energy-gap of the organic materials induced by the charge transfer from the alkali metals. These are attributed to the formation of bipolarons. We find that the deexcitation probability of He-* (2(1)S) atoms on the doped organic surfaces exceeds that of He-* (2(3)S) by up to four times. (C) 2001 Elsevier Science B.V. All rights reserved.

Systems formed by depositing metals (Au and Mg) on p-sexiphenyl (6P) films were studied by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and metastable atom electron spectroscopy (MAES). The results of MAES indicate the penetration of metal into 6P for both Au and Mg deposition. Especially, 6P surface remains uncovered even when the average thickness of deposited Au reaches 125nm, An anomalous shift of spectra to the lower kinetic energy side, possibly due to some type of sample charging effect, was induced by the deposition of small amount of Au on 6P film, although conducting material was deposited.

In order to examine the validity of Mott-Schottky model at organic/metal interfaces, the position of the vacuum level (VL) of triphenyldiamine (TPD) film formed on five metal substrates was measured as a function of the film-thickness by Kelvin probe method in ultrahigh vacuum (UHV). At all the interfaces, sharp shifts of the VL were observed within 1 nm thickness. Further deposition of TPD up to 100 nm did not change the position of the VL indicating no band bending at these interfaces. These findings clearly demonstrate the Fermi level alignment between metal and bulk TPD solid in UHV is not established within a typical thickness of real devices.

Photogeneration of free polarons in conjugated polymers is discussed on the basis of (1) ps time-resolved experiments made by modified pump-and-probe technique with monitoring the photoconductivity changes induced by delayed probe pulses of red light, and (2) correlation between fluorescence decay curves and kinetics of accumulation of polaron pairs.

We report selected examples from our recent studies of organic-metal systems formed by vacuum-deposition using various electron spectroscopies, low-energy electron diffraction (LEED), infrared reflection-absorption spectroscopy (IR-RAS), and Kelvin probe. The systems and subjects are: (1) TPD (N,N'-bis(3-methylphenyl)-N,N'-diphenyl[1,1'-biphenyl]-4,4'diamine) -metal interfaces for examining possible band bending, (2) interfaces between p-sexiphenyl (6P) and metals, (3) preparation and characterization of oriented films of C44H90 and 6P on metal single crystal surfaces, including the determination of intramolecular energy band dispersion [E = E(k)] relation.

The interfaces of organic materials with other solids play important roles in the function of various organic devices such as organic light-emitting diodes (OLEDs), spectral sensitization in photography, organic solar cells, and electrophotography. Also they should be important in future molecular devices, both in the central part of the device and at the connection with outside circuits. However, serious experimental examination of such interfaces has started only recently. In this talk we focus our attention on the organic/metal interfaces, and summarize our understanding about (1) the energy level alignment right at the interface, and (2) possible band bending within an organic layer, mainly using the techniques of UV photoemission spectroscopy (UPS) and Kelvin probe method. As for (1), the formation of electric dipole layer was observed in most organic/metal interfaces, and its origin is discussed. As for (2), recent examination of the existence/absence of band bending in ultrahigh vacuum will be reported. It is also pointed out that there can be much effects of (i) atmosphere at sample preparation and measurements, and (2) chemistry and interdiffusion at the interface.

High-Energy Spectroscopic Studies of the Electronic Structures of Organic Systems Formed from Carbon and Fluorine by UPS, Vacuum-UV Optical Spectroscopy, and NEXAFS: Poly(hexafluoro-1,3-butadience)[C(CF3)=C(CF3)]n, Fluorinated Graphites (CF, C2F, and C6F), Perfluoroalkanes n-CnF2n+2, Poly(tetrafluoroethylene) (CF2)n, and Fluorinated Fullerenes (C60Fx and C70Fx)

The interfaces of organic materials with other solids play important roles in the function of various organic devices such as organic light-emitting diodes (OLEDs), spectral sensitization in photography, organic solar cells, and electrophotography. Also they should be important in future molecular devices, both in the central part of the device and at the connection with outside circuits. However, serious experimental examination of such interfaces has started only recently. In this talk we focus our attention on the organic/metal interfaces, and summarize our understanding about (1) the energy level alignment right at the interface, and (2) possible band bending within an organic layer, mainly using the techniques of UV photoemission spectroscopy (UPS) and Kelvin probe method. As for (1), the formation of electric dipole layer was observed in most organic/metal interfaces, and its origin is discussed. As for (2), recent examination of the existence/absence of band bending in ultrahigh vacuum will be reported. It is also pointed out that there can be much effects of (i) atmosphere at sample preparation and measurements, and (2) chemistry and interdiffusion at the interface. © 2000 Materials Research Society.

Despite the fast-paced new developments and improvements of applications incorporating conjugated organic materials, rather little effort is made to obtain well-defined structures and thin films of these in order to exploit the highly anisotropic properties (e.g. conductivity, polarized light emission) of most conjugated organic materials. It has been shown that oligomers, which can easily be evaporated under ultrahigh vacuum conditions, exhibit heteroepitaxial growth on various substrates. Depending on the nature of the interaction of the organic molecules with the substrate different orientations can be obtained, as well as by simply changing the conditions of evaporation. Therefore, by going from amorphous or polycrystalline structures to highly textured or single crystalline ones new insights into the physical properties of the materials can be expected, and also new applications. We studied the formation of well-defined films of the electroluminescent oligomer p-sexiphenyl on various metal and semiconductor surfaces and determined the orientation of the molecules by means of low energy electron diffraction (LEED), infrared reflection absorption spectroscopy (IR-RAS) and atomic force microscopy (AFM). Depending on the substrate material we found that the molecules are oriented either parallel or perpendicular to the substrate surface. In addition to that, highly oriented 6P films were doped with potassium, and the change in the molecular vibrational signature was investigated with IR-RAS.

Metal deposition on a p-sexiphenyl (6P) film was studied by ultraviolet photoelectron spectroscopy (UPS), metastable atom electron spectroscopy (MAES), and X-ray photoelectron spectroscopy (XPS). The deposited metals were Au, Mg, and several alkali metals (K, Na, Rb, and Cs). No chemical reaction between 6P and Au or Mg was observed in the measured spectra, while additional gap states appeared in the UPS and MAES spectra by deposition of the alkali metals. The diffusion of Au and Mg atoms into the 6P film was observed in the MAES spectra. We found the trend of the vacuum level shift is different between the systems of the 6P on the Au and its reversed systems (Au on the 6P film), suggesting the different formation of the interface depending on the deposition sequence.

A long-chain n-alkane, tetratetracontane (n-C44H90), adsorbed on Ag(111) was studied by infrared reflection absorption spectroscopy. The first layer is physisorbed on the surface and assumes a "flat-on" structure with the molecular plane formed by the carbon atoms parallel to the surface. This structure gives CW stretching vibration bands at 2907 and 2814 cm(-1), which are attributed mainly to the CH bonds in the upper side of the molecular plane and those in the vicinity of the Ag(lll) surface, respectively. These frequencies are lower than those for the normal stretching modes of a methylene group, and this difference is ascribed to the interaction between the CH bonds on the metal side and the Ag surface. This "softening" of the CW bond in contact with the surface was explained by the interaction between the dipole and the image dipole induced in the metal substrate, and this frequency shift of the CH stretching vibration was simulated by electrodynamic calculations.

Vacuum-deposited films of a long-chain n-alkane, n-C44H90, tetratetracontane (TTC), on a Au(lll) surface were investigated by infrared reflection absorption spectroscopy. We found four different structures for TTC on the Au surface. One is a "flat-on" structure formed by molecules in an alll-trans conformation with the molecular plane composed of carbon atoms parallel to the surface. This structure was found in the first monolayer physisorbed on the surface. The second structure includes the gauche conformation, which shows a change in the orientation of the methylene groups with an increase in the thickness of TTC. The third structure is the crystalline state formed by all-trans molecules, which was found in the third and thicker layers with the molecular long axis parallel to the surface. These three structures were found for films deposited and measured at room temperature. With an increase in temperature after deposition at room temperature, the first and second structures were easily converted to another structure. This fourth structure consists of TTC molecules in the vicinity of the surface with their molecular axes parallel to the surface and their molecular planes tilted from the surface. The molecules in this structure interact with the Au(111) surface through some of their methylene groups, while the molecules in the flat-on structure contact the surface through all of their methylene groups. The appearance of these two structures is controlled by the balance of two kinds of interaction: methylene group - Au surface interaction and the interaction among TTC molecules. In the second layer, the formation of TTC molecules including a gauche conformation is ascribed to the geometrical incompatibility between flat-on TTC and crystalline TTC. © 2000 American Chemical Society.

We have performed in the present work time-resolved experiments on poly(3-dodecyl-thiophene) (P3DDT) and poly(2,5-dioctyloxy-p-phenylene vinylene) (OO-PPV) films by directly probing the formation of charge carriers responsible for the cw photoconductivity within the time domain of -10 ps to 1 ns. Laser light pulses of 400 nm wavelength, 150 fs width, induced photoconductivity in a sample with a frequency 1 kHz. Red 800 nm light pulses delayed in respect to blue ones were revealed to affect the photoconductivity. The effect of the second pulses increased with the delay time. Red light induced changes of the photoconductivity were positive in OO-PPV, and negative in P3DDT. These results are rationalized as an evidence of delayed not immediate formation of free charge carriers. The carriers seem to be formed within 10 ps after the pumping pulse. A mechanism of formation of free polarons from polaron pair is suggested, which has permitted to explain main feature of the results including different signs of the effect of the red light in different polymers.

Energy structures of molecular semiconductors are investigated under air on the basis of the Kelvin probe (KP) method; a critical examination of the applicability of this method for direct determination of work functions is performed. It is revealed that vacuum-sublimed films of three phthalocyanines (p-type), two porphyrins (p- and n-type) and a perylene derivative (n-type) in contact with metals form ideal Schottky barriers whose heights are simply determined by the difference in work function between a molecular solid film and a metal. Based on this finding (achievement of a Schottky-Mott rule), a common work function of 4.8 +/- 0.05 eV for p-type molecular semiconductors and 4.1-4.3 eV for n-type ones are deduced. On the other hand, work function values measured in air directly with the KP method (phi(KP)) are strongly dependent on the metal used as substrate. This metal dependence is ascribed to nonalignment of Fermi levels between a molecular solid and a contacting metal under the open-circuit situation of the KP configuration. Indeed, phi(KP) values of phthalocyanine films corrected for the Fermi energy difference at the metal/phthalocyanine junction become independent of the respective substrate metal, although they are still by 0.1-0.2 eV greater than the value of 4.8 eV evaluated from the Schottky-Mott rule. Possible reasons for the nonalignment of Fermi levels and the remaining discrepancies are discussed in connection with incorporation of O-2 and/or H2O into molecular semiconductors. Influences of him thickness and illumination on phi(KP) are also examined in order to clarify junction properties at metal/molecular solid contacts. Based on these results, use of the Schottky-Mott rule is highly recommended for determination of work functions of molecular semiconductors exposed to air. (C) 2000 Elsevier Science S.A. All rights reserved.

Direct pattern transfer onto poly(vinylidene fluoride) was achieved by using x-ray photons from a synchrotron radiation source. Quadrupole mass spectrometry and ultraviolet photoemission spectroscopy, combined with ab initio molecular orbital calculations, were employed to investigate the mechanism of direct photomicromachining. The mass spectrometry identified H-2, F, and HF as the etched products, with no carbon containing species being detected. The changes in photoemission spectra due to photodegradation were analyzed by comparison with ab initio molecular orbital calculations. This analysis indicated that a high degree of conjugation is generated in the degraded polymer due to the loss of fluorine atoms. It is concluded that the mechanism of direct photomicromachining is ascribable to the shrinking of the irradiated polymer region due to defluorination and the generation of conjugation. (C) 2000 American Institute of Physics. [S0021-8979(00)03208-4].

We developed an apparatus of differential thermal analysis (DTA) capable of simultaneous surface specific ultraviolet (UV) photoemission measurements to investigate thin-film phase transitions. The apparatus was installed in a vacuum chamber of 10-6Torr range for thermal isolation and the measurements of UV photoemission. As a sample substrate, we used a thin (10 μm) copper sheet supported by two wires for optimal thermal resistivity. The performance of the apparatus was examined using a 650-A-thick pentacontane (n-C50H102) film, which may exhibit a unique monolayer phase transition known as surface freezing. We observed two anomalies of DTA curve around the bulk melting temperature, one of which is apparently due to the bulk melting. Since the temperature dependence of the surface specific UV photoemission measurements showed corresponding changes in photoemission current, we could conclude that the other phase transition peak originates from the surface freezing effect. This demonstrates that our DTA-UV apparatus is sufficiently sensitive to examine such monolayer phase transitions.

The vacuum level shift Delta was determined by Kelvin probe method for TPD/metal systems prepared in ultrahigh vacuum (TPD: N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine). The energy of the vacuum level sharply changed at the initial stage of depositing TPD on the metal substrates, and further bending was not observed up to 100 nm thickness of TPD. This result is consistent with our previous studies of ultraviolet photoelectron spectroscopy. These results indicate the invalidity of traditional model with a common vacuum level at organic/metal interface. We discussed the dependence of the vacuum level shift on the work function Phi(m) of the metal substrate (Au, Cu, Ag, Mg, and Ca). A liner correlation between Delta and Phi(m) was observed only in the region of 3.8 eV < Phi(m) < 4.5 eV, indicating that Fermi level alignment is not achieved at least for some interfaces. We found that Delta for TPD film on the air-exposed metal substrate is smaller than that of TPD on clean metals and was observed a liner relation between Phi(m) and Delta except for the Cu substrate.

The electronic structure and molecular orientation of a tetratetracontane (n-C44H90 TTC) ultrathin film on a Cu(100) surface were studied by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation. A well-oriented thin film of TTC was successfully prepared by vacuum evaporation in ultrahigh vacuum at room temperature. We observed a (2×1)-like low-energy electron-diffraction (LEED) pattern for the deposited TTC film. This result indicates that the TTC molecules lie on the Cu(100) surface in two types of domains, rectangular to each other, in which the alkyl-chain axes are along the [110] and [11¯0] directions of the Cu(100) surface. The application of the dipole selection rules to the normal-emission ARUPS spectrum revealed that the C–C–C plane of TTC is parallel to the Cu(100) surface plane (flat-on orientation). The intramolecular energy-band dispersion of TTC was examined by changing the take-off angle of emitted electron along the [110] direction of the Cu(100) surface. The observed results support the conclusion about the direction of alkyl-chain axes by LEED observation. In order to analyze the molecular orientation more quantitatively, we also performed theoretical simulations of the angle-resolved photoemission spectra using the independent-atomic-center (IAC) approximation combined with ab initio molecular-orbital (MO) calculations for various molecular orientations. The simulated spectra for flat-on orientation are in excellent agreement with the observed spectra. These results once again verify the deduced molecular orientation, and also demonstrate the reliability of theoretical simulation with the IAC/MO approximation for compounds without a p-electron system. Furthermore, we observed a work function change of about -0.3 eV by adsorption of TTC. Such a decrease of the work function indicates the formation of a dipole layer at the interface, in contrast to the traditional picture of energy-level alignment assuming a common vacuum level at the organic/metal interface. © 1999 The American Physical Society.

The electronic structure and molecular orientation of tetratetracontane (n-C44H90) films on Cu(100) and Au(111) surfaces were investigated by angle-resolved UV photoemission spectroscopy (ARUPS) and low energy electron diffraction (LEED). The observed ARUPS spectra showed the drastic take-off angle dependence due to intramolecular band dispersion. A 2x1-like LEED pattern was observed for both substrates. From these results and theoretical simulation of ARUPS spectra based on independent-atomic center (IAC) approximation, we found that the C-C-C plane of the adsorbed TTC molecule is parallel to the substrate surface and its molecular axis is along a [110] direction for both substrates. We also measured the work function change by adsorption of TTC. The observed values were c.a. -0.3eV and -0.7eV for Cu(100) and Au(111) systems, respectively. Such decrease of the work function indicates the existence of a dipole layer at the interfaces in contrast to the traditional picture of energy level alignment at organic/metal interface assuming a common vacuum level at the interface. The dipole formation in such physisorbed systems can be explained by the polarization of the TTC molecule due to an image force. (C) 1999 Elsevier Science B.V. All rights reserved.

The molecular orientation at the surfaces of poly(2-vinylnaphthalene) thin films was studied by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The NEXAFS study showed that pendant naphthalene groups are oriented with a tilt angle (β) of 57°, while the ARUPS study gave information on the tilt-angle distribution as well as the mean value (∼58°) of β. The mean value of β determined by ARUPS agrees well with that found in the NEXAFS study. © 1999 Elsevier Science B.V. All rights reserved.

We have performed differential thermal analysis (DTA) and surface-specific ultraviolet (UV) photoemission measurements to investigate the surface freezing effect of n-alkanes. The samples used in the experiments were pentacontane (n-C50H102: 650 Å) and tetratetracontane (n-C44H90: 600 Å) films evaporated on a copper substrate. We have observed a monolayer phase transition by DTA and proved that the transition is of first order. Since the temperature dependence of the surface-specific UV photoemission measurement showed a stepwise increase and decrease of the photoemission current at the transitions observed in DTA, we could conclude that the monolayer phase transition peak originates from the surface freezing effect. © 1999 Elsevier Science B.V. All rights reserved.

Adsorption of a long chain n-alkane, tetratetracontane (n-C(44)H(90)), on an Ag(111) surface was studied by infrared reflection absorption spectroscopy (IRAS). The first layer physisorbed on the Ag(lll) surface assumes a 'flat on' structure with the molecular plane formed by carbon atoms parallel to the surface. The structure gave CH stretching vibration bands at 2907 and 2814 cm(-1). With the aid of auxiliary IRAS data of n-C(44)H(90) on Au(111), these peaks were attributed to the CH bonds in the upper side of the molecular plane (HC/Ag) and those in the vicinity of the Ag(111) surface (CH/Ag), respectively. The difference in frequency was ascribed to the interaction of the latter CH bonds with Ag. This 'softening' of the CH bond was explained by dipole-image dipole interactions with the Ag surface. (C) 1999 Elsevier Science B.V. All rights reserved.

p-Sexiphenyl(6P)/metal (Au, Mg) interfaces were studied by ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and metastable atom electron spectroscopy (MAES). The vacuum level shifts (Δ) at the interface were observed in both 6P/Au (Δ=-0.8±0.1 eV) and 6P/Mg (Δ=-0.4±0.1 eV) systems. The MAES spectra indicate that the 6P molecules are on the substrate with their mean molecular plane parallel to the substrate surface in both systems, and that the 6P film on Mg substrate has some disorder. Thickness dependence of UPS, MAES, and XPS spectra suggests no significant chemical interaction for 6P on Au, Mg as well as Au, Mg on 6P interfaces. In the system of Au on 6P/Au, the spectral feature of 6P in MAES spectra did not disappear even when the thickness of evaporated gold reached 20 nm, indicating a penetration of Au atoms (or the clusters) into the 6P film. © 1999 Elsevier Science B.V. All rights reserved.

We examined Langmuir Blodgett (LB) films of PFODA (perfluorooctadecanoic acid) prepared at various surface pressures with NEXAFS (Near Edge X-ray Absorption Fine Structure) spectroscopy. Experimental results revealed that the sample without the rubbing treatment possesses (1) no anisotropy along the dipping direction with almost vertical alignment to the substrate surface, (2) higher degree of order of the chain for the sample prepared under the higher surface pressure. Moreover, it was found that the rubbing process induces a uni-directional tilt angle of the chain without changing the magnitude of the degree of order.

Angle-resolved ultraviolet photoelectron spectra (ARUPS) of copper phthalocyanine (CuPc) and metal-free phthalocyanine (H2Pc) films (thickness from monolayer to 50-80 Angstrom) on cleaved MoS2 substrates were measured using monochromatic synchrotron radiation. Observed take-off angle (theta) and azimuthal angle (phi) dependencies of the top pi band intensity were analyzed quantitatively by the single-scattering approximation theory combined with molecular orbital calculations. The analysis indicated that the molecules lie flat on the MoS2 surface in monolayer films of CuPc and H2Pc. The azimuthal orientation of the molecules (angle between molecular axis and surface crystal axis of MoS2), was found to be about -7 degrees, -37 degrees, or -67 degrees for both monolayer films of CuPc and H2Pc. In the azimuthal orientation, the analyses indicated that there are only molecules with conterclockwise rotation, although both clockwise and counterclockwise rotations are expected. From the low energy electron diffraction, the two-dimensional lattice structure of the monolayer film was obtained. On the basis of these two kinds of experimental results, the full structure of the monolayer film, the two-dimensional lattice and the molecular orientation at the lattice points, was determined. Furthermore, for the thick films it is found from the analyses of ARUPS that CuPc and H2Pc molecules tilt about 10 degrees from the surface plane. (C) 1999 American Institute of Physics. [S0021-8979(99)01308-0].

We performed X-ray Absorption Near Edge Structure (XANES) and Atomic Force Microscope (AFM) measurements for the system of multilayers, HTC (hexatriacontane, n-C36H74)/polyimide/Si, to determine an effective escape depth of photoelectrons, L-eff, in HTC films for total electron yield detection at C K-edge. Quantitative degrees of superposition of C K-edge spectra of a saturated hydrocarbon and a polyimide with aromatic rings were examined to obtain L-eff for HTC films. L-eff of n-alkane at the C K-edge has been determined to be is 35 Angstrom and spectroscopy agree closely.

The tautomeric structures in solid phase were studied by near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) for five N-salicylideneaniline derivatives. The observed features in the NEXAFS spectra 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. The relative intensity of the observed peaks characteristic of each tautomer enabled us to deduce the tautomeric structures of these compounds. The XPS spectra allowed quantitative estimation of the relative tautomer populations. These structures are consistent with the previous estimation by Xray diffraction. These results demonstrate the usefulness of NEXAFS as well as XPS for studying the tautomerism in hydrogen bonded systems.

The surface-freezing effect of pentacontane (n-C50H102) and tetratetracontane (n-C44H90) films evaporated on a copper substrate has been investigated by differential thermal analysis (DTA) simultaneously with measurements of surface-specific ultraviolet (UV) photoemission. Two anomalies in the DTA curve were observed near the bulk melting temperature, one of which has been attributed to bulk melting. Since the temperature dependence of the surface-specific UV photoemission measurement showed a corresponding stepwise increase and decrease in the photoemission current at the two anomalies observed in the DTA, we have concluded that the other phase transition peak originates from surface freezing. (C) 1999 Elsevier Science B.V. All rights reserved.

Recent progress in the study of the energy level alignment and band bending at organic interfaces is reviewed. taking the examples mainly from the results of the group of the authors using ultraviolet photoelectron spectroscopy (UPS). metastable atom electron spectroscopy (MAES), and Kelvin probe method (KPM). As for the energy level alignment light at the interface, the formation of an electric dipole layer is observed for most of the organic/metal interfaces, even when no significant chemical interaction is observed. The origin of this dipole layer is examined by accumulating the data of various combinations of organics and metals, and the results indicate combined contribution from (i) charge transfer (CT) between the organic molecule and the metal, and (2) pushback of the electrons spilled out from metal surface, for the case of nonpolar organic molecule physisorbed on metals. Other factors such as chemical interaction and the orientation of polar molecules are also pointed out. As for the band bending, the careful examination of the existence/absence of band bending of purified TPD* molecule deposited on Various metals in ultrahigh vacuum (UHV) revealed negligible band bending up to 100 nm thickness, and also the failure of the establishment of Fermi level alignment between organic layer and the metals. The implications of these findings are discussed, in relation to the future prospects of the studies in this field. (*: N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine).

Carbon K-edge NEXAFS spectra of chrysene, perylene, and coronene were measured using synchrotron radiation, and the results were analyzed by ab initio molecular orbital (MO) calculations. The spectra do not agree well with the calculated density of unoccupied states (DOUS), indicating significant core-hole effect leading to deviation of the NEXAFS spectra from the DOUS. On the other hand, the observed spectra were well simulated by theoretical calculations taking this effect into account by the improved virtual orbital method. This allowed a detailed analysis of the core-hole effect, which affects both transition energy and intensity. During the course of this analysis, the core-hole effect in benzene was also analyzed. It was found that the magnitude of the core-hole effect is strongly dependent on the combination of the excited site and final vacant orbital. This dependence could be semiquantitatively explained in terms of the combination of the excitation site and the orbital patterns of the final state. The severe deviation of the NEXAFS spectra from the DOUS is ascribed mainly to such variation of the core-hole effect, with additional effect by the site-dependent core ionization energy. In some cases, even the transitions to the same vacant orbital contribute to different spectral features, or an apparently single spectral feature consists of transitions to different vacant orbitals. Thus direct information about DOUS of a molecule with such inequivalent sites is not obtainable from the NEXAFS spectrum, although smaller deviation is suggested for other systems with single site, such as the C atoms in C-60 and graphite, and the N atoms in Zn tetraphenyl porphyrin. (C) 1998 American Institute of Physics. [S0021-9606(98)00147-0].

Near edge X-ray absorption fine structure (NEXAFS) spectra were obtained using total electron yield detection for poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) and their model compounds 4,4'-biphenyldicarboxylic acid dimethyl ester (PAM), 4,4'-biphenyldicarboxylic acid (PCA), and terephthalic acid (TPA). The spectra of PET and PET were interpreted with the comparison of the spectral features of model molecules and the polarized NEXAFS spectra of oriented films of PAM, PCA, and PET. From the polarization dependence, the peaks previously ascribed to a C 1s --> pi*(ring) or pi*(ring/C=O) excitation at 289.8 eV and an O 1s --> 3p/Rydberg or --> pi*(C=O) transition at 536.4 eV were reassigned to a C 1s --> sigma*(O-CHx) transition and an O 1s --> sigma*(O-CHx) transition, respectively. The present results demonstrate that the polarization dependence of the NEXAFS spectra of oriented small analogous molecules are useful for the assignment of the NEXAFS spectra of polymers.

Electronic structures of evaporated films of five organic light-emitting and carrier-injecting materials for organic electroluminescent devices were studied by ultraviolet photoemission spectroscopy. The compounds examined were (i) light-emitting materials tris(8-hydroxyquinolino) aluminum (Alq(3)) and 1,2,3,4,5-pentaphenylcyclopentadiene, (ii) a hole-injecting material N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine, and (iii) electron-injecting materials N, N'-diphenyl-1,4,5,8 -naphthyletracarboxyldiimide and 1,3,5-tris(5-phenyl-1,3,4-oxadiazol-2-yl)benzene. The spectral features corresponding to the top parts of the valence states, which dominate the electric properties of the materials, were assigned by the comparison with the simulated density of states obtained from PM3 molecular orbital calculations. Using these calculations, the evolution of the electronic structure of each molecule from those of constituent parts was discussed. The characters of the unoccupied states obtained by these calculations were also presented. Using these data, the correlation between the ionization threshold energies determined by ultraviolet photoemission spectroscopy and the carrier-injecting and light-emitting properties were discussed. (C) 1998 American Institute of Physics.