Modification of the quasi two-dimensional surface state on Au(111) surface with adsorption of n-alkane molecule, tetratetracontane: TTC was investigated by angle-resolved photoemission spectroscopy (ARUPS) and X-ray photoemission spectroscopy (XPS). From ARUPS the energy band structure of the surface state of Au(111) surface is found to be significantly affected by TTC adsorption. The modified surface potential on Au(111) surface by the adsorption confines the surface electrons into the quantum well newly established between TTC wide energy gap and sp band gap of the Au(111) surface increasing its localization nature. Clear chemical shift in C 1s core level was observed in XPS spectrum for TTC layer right at the interface and small back-donation from Au(111) surface to TTC unoccupied states is suspected. On the other hand, a large interface dipole layer about 0.72 eV was observed by TTC adsorption. From these results, Pauli repulsion effect is expected not to have dominant effect on such large dipole formation on TTC adsorption.

We constructed an ultrahigh vacuum (UHV) system with which in-situ conductivity and Seebeck coefficient measurements can be performed, including the detailed examination of the oxygen effect. It can also carry out the deposition of metal electrodes and organic films under UHV condition through measurements. The conductive characteristics of fullerene (C60) films were examined by the measurement at conductivity and Seebeck coefficient under UHV. We observed that the conductivity at room temperature was higher than 1×10−3 S cm−1. This value is two orders of magnitude in conductivity higher than the Hamed report. [A. Hamed et al., Phys. Rev. B 47, 10873 (1993).] Seebeck coefficient S was also measured. Its sign was negative, indicating electron conduction for C60 film. After the measurements under UHV, oxygen was introduced into the chamber by a variable leak valve. The conductivity decreased drastically by increasing the pressure of oxygen. It decreased more than five orders from under UHV to 1 atom oxygen. The temperature dependence of the conductivity was also examined. The conductivity was thermally activated. Activation energies of the conductivity increase by increase oxygen pressure. This means that the oxygen act as carrier traps. As an n-type dopant, acridine orange base (AOB) was co-deposited into C60 films. The magnitude in the conductivity increased but not drastic. The conductivity of doped film also decreased by increasing the pressure of oxygen. The conductivity even under 6.6×10−2 Pa, however, was high enough. Though AOB was not effective dopant against C60 film in UHV condition, AOB doped C60 film became resistant to oxygen.

We report the fabrication and characterization of the bottom contact organic thin film transistors and inverter based on a heterostructure of C60 on pentacene. The transistor shows ambipolar transport characteristics with high electron and hole mobilities of 0.23 cm2V−1s−1 and 0.14 cm2V−1s−1, respectively. After exposure to air, the n-channel in C60 is completely degraded whereas the p-channel in pentacene keeps working. Both the n-channel and the p-channel are stable in N2 atmosphere. The inverter exhibits typical transfer characteristics, which are interpreted by the distribution of the accumulated electrons and holes depending on the bias conditions. These results suggest a potential way to fabricate organic complementary circuits with a single organic heterostructure device.

Electronic structures of the ionic liquids were studied by ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The sample materials contain 1-buthyl-3-alkylimidazolium ion [Cnmim]+ (n=4, 8, 10) as the cation in combination with fluorine-containing anions (tetrafluoroboronate BF4−, hexafluorophosphate PF6−). Comparing the calculated density of states with the observed spectra, we found that the ionization thresholds of these ionic liquids are determined by the highest occupied molecular orbitals (HOMO) of the cation, although the calculated HOMOs of the isolated anions are higher than that of isolated cation. The combination of the UPS and IPES results reveals that the band gaps of these ionic liquids are determined by only cation.

We investigated the electronic structure and the energy level alignment at the interfaces between perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and metals (Au, Cu and Co) by ultraviolet photoelectron spectroscopy (UPS). In the case of PTCDA/Au interface, the apparent interface states and charge transfer (CT) states were not observed in its UPS spectra. In the case of PTCDA/Cu and Co interfaces, however, we found that the CT from metal to PTCDA occurred, and that the apparent CT states were observed at PTCDA/Cu interface.

The effect of oxygen doping on titanyl phthalocyanine (TiOPc) films was investigated by ultraviolet photoelectron spectroscopy (UPS). The energy of the electronic levels of TiOPc films probed by UPS was clearly different between the film deposited in ultrahigh vacuum (UHV) and in oxygen atmosphere. The film deposited in UHV showed downward band bending characteristic of n-type semiconductor. On the other hand, the film deposited in oxygen atmosphere showed upward band bending characteristic of p-type semiconductor. Such trends are in excellent correspondence with reported field effect transistor characteristics. In order to examine the Fermi level (EF) alignment between TiOPc film and the substrate, the HOMO energy of TiOPc relative to the EF of the metal substrate was determined for various substrates. Although EF alignment was not achieved for the TiOPc film prepared in UHV, possibly because of insufficient charge density in the TiOPc film, it was achieved in the case of TiOPc film exposed to oxygen, probably by p-type doping effect of oxygen.

Strong substrate dependence of interfacial electronic structure of 11, 11, 12, 12-tetracyanonaphtho-2, 6-quinodimethane (TNAP) thin film was observed by infrared reflection-absorption spectroscopy and ultraviolet photoemission measurements. It was found that an apparent charge transfer interaction exists in the interface between the TNAP film and Ag substrate and Cu complex is formed by deposition of TNAP onto Cu substrate.

In various organic electronic devices, interfaces formed by organic layers can play important roles. We have been studying various organic interfaces for clarifying their structure and electronic structure. In this talk, we will report our recent study of the effect of various types of doping for a variety of dopants – residual impurity, atmospheric gases, and metallic and organic intentional dopants. In particular, detailed and quantitative information about the effect of oxygen from the viewpoint of electronic structure was obtained for titanyl phthalocyanine (TiOPc), and the results corresponded well with the recent report of atmospheric effect on orga nic field effect transistor.

Interfacial band offset and band bending of organic semiconductors are critical to understand and improve organic photovoltaic cells. In this study, the energy level alignment of fullerene(C60) / metal-free phthalocyanine (H2Pc) interface which is one of the model interfaces of organic photovoltaic cells has been investigated using UV and X-ray photoemissions. For both ‘H2Pc on C60’ and “C60 on H2Pc' interfaces, 0.3 eV downward energy level shift was observed in XPS at the interface formation. This energy shift is quite steep in contrast to the band bending observed for C60/metal interfaces in our previous study, where thickness of 500nm was required to achieve 0.21eV band bending to get Fermi level alignment between metal electrode and C60. To clarify the origin of the band bending, the effect of the insertion of C60-H2Pc co-deposited layer between C60 and H2Pc layers was also investigated. The result suggested that possible doping of H2Pc to C60 is not main origin of the observed energy shift. We also found that the vacuum level shift at H2Pc/C60 interface is strongly dependent on the deposition sequence of the interface formation.

Recently we found high and persistent spontaneous buildup of the surface potential upon vacuum vapor deposition of tris(8-hydroxyquinolinato) aluminum(III)(Alq3) on Au substrate under dark conditions (28 V for 560 nm thick Alq3 film). Such giant potential is removed by visible-light irradiation. These natures of the film suggest the possible applications to various organic devices such as memory devices. In this study, we investigated the retention time of the giant potential and the mechanism of the light-induced depolarization in order to discuss the feasibility of the device applications. The observed decay rate of the surface potential in vacuum condition was roughly 10 % loss in 10 years, which is enough for memory devices. As to the decay rate by light-irradiation, the observed rate was successfully reproduced by a theoretical simulation based on the photo-induced randomization of oriented Alq3 molecules.

We report on the recent progress of our continued effort for the preparation and characterization of well oriented films of tetratetracontane (n-C44H90) and perfluorotetracosane (n-C24F50), which are good model molecules of fundamental polymers, polyethylene (CH2)n and poly(tetrafluoroethylen) (CF2)n, vacuum-deposited on metal surfaces. When the surface of the metal substrate was oxidized, the deposited chain molecules are aligned with their molecular axes vertically standing on the surface, while they lie flat on the surface when atomically clean Cu(100) surface was used. In the case of n-C44H90 on Cu (100), low-energy electron diffraction (LEED) studies revealed that the molecules were also azimuthally aligned with the chain axis almost parallel with the <100> and <010> axis, and infrared reflection-absorption spectroscopy (IR-RAS) enabled detailed studies of the film structure at increasing film thickness._By applying the technique of angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) to these flat-lying molecules, the intramolecular energy-band dispersion relation could be determined, The results agreed well with the results deduced from the previous work using vertically standing molecules assuming an inner potential V0. of –5 eV, confirming the validity of the estimation of V0. Excellent agreement was found between the observed and simulated spectra using the independent-atomic-center (IAC) approximation. Also the value of the vacuum level shift by molecular deposition was deduced for TTC/Cu(100) system to be – 0.3 eV. As for n-C24F50, the E(k) relation deduced in our previous study using vertically standing molecules showed discrepancy with band calculations. The detailed reexamination of the experimental data using IAC calculations for the previous report gave more realistic estimation of V0, and the newly deduced E(k) relation using this data showed good correspondence with theoretical band calculations, thus removing the difficulty in the previous work.

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.

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 (IRRAS) 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.

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 AI/AIq3TPD/ITO, where Alq3 is tris(8-hydroxyquinolino)- aluminum, TPD is N,N×-diphenyl-NN×-(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, Alq3, 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 Alq3 / TPD interfaces, 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 chargetransfer between the metal and the organic molecule, with these directions being consistent with the electron donating and accepting ability of these molecules.