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We analyzed the vacuum level shift (Δ) induced by the dipole layer at the interfaces between perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and noble metals (Au, Ag and Cu). The variation of Δ observed by ultraviolet photoelectron spectroscopy does not show a simple dependence on the metal work function, which contradicts the prediction by the induced density of interface states (IDIS) model proposed by Vazquez et al. [H. Vazquez, F. Flores, R. Oszwaldowski, J. Ortega, R. Perez and A. Kahn, Appl. Surf. Sci 234 (2004) 107]. We found that two factors, (1) the energy separation between the lowest unoccupied molecular orbital (LUMO) of PTCDA and the metal d-band states, which results in the attractive effect due to the orbital hybridization, and (2) the coupling matrix element between the adsorbate states and the metal d-band states, which result in the repulsive effect due to the orbital orthogonalization between the adsorbate states and the metal d-band states, have a clear correlation with the Δ formation. Our results indicate that the interactions between the molecular orbitals of PTCDA and the metal d-band states play an important role in determining the interfacial electronic structure, which has not been taken into account within the framework of the IDIS model.

We report the electronic structure and the charge transport mechanism of highly ordered films of pentacene on Cu(1 10) surface studied by angle-resolved ultraviolet photoemission spectroscopy using synchrotron radiation. For a flat-lying monolayer film, we observed the evidences of (i) formation of the interface states and (ii) two-dimensional intermolecular band dispersion of the resultant interface states, which may originate from the hybridization between the molecular orbitals and the wave function of the substrate. For an upright-standing multilayer film, we observed the two-dimensional intermolecular band dispersion, which originates from the intermolecular π-π interaction. The observed effective masses of the hole for different azimuths demonstrate the presence of the anisotropy of the hole mobility in pentacene crystals also at higher temperatures.

The molecular arrangement in the monomolecular layer (1 ML) film of hexadecafluorozincphthalocyanine (F 16 ZnPc) vacuum-deposited on GeS(0 0 1) in ultrahigh vacuum were studied by low energy electron diffraction (LEED), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS). From the LEED analysis, we found that the F 16 ZnPc film grows epitaxially on the substrate surface and that the lattice structure in the 1 ML region is commensurate with the lattice of the surface. The tilt angle between the molecular plane and the substrate surface is 12.5° ± 2.5° deduced from the NEXAFS and ARUPS results. We also determined the azimuthal angle of the adsorbed F 16 ZnPc molecule with respect to the surface orientation of GeS(0 0 1) by comparing the ARUPS spectra with the theoretical simulation. Finally, we suggested a possible full film structure.

The electronic structure at the interface formed between an organic film and a metal electrode plays a crucial role in the performance of electronic devices using organic semiconductors such as electroluminescent displays (light-emitting diodes), field-effect transistors, and photovoltaic cells. In particular, the energies of the highest-occupied and lowest-unoccupied molecular orbitals relative to the Fermi level of metals, i.e., the energy level alignment at organic/metal interfaces, are of fundamental importance in discussing the barrier heights for the charge injection and separation at the interface. However, it is generally not easy to discuss the interfacial electronic structure precisely since the interface energetics is usually modified due to various interface-specific phenomena. In this article, we introduce the recent progress in studies of the energy level alignment at organic/metal interfaces: (1) basic understanding, (2) interface dipole layer, and (3) interface states.

We report our study on the current-voltage characteristics of single layer devices employing pure and heavily tetrathianaphthacene (TTN)-doped tris(8-hydroxy-quinoline) aluminum (Alq3) films as active layers. Sandwich structures with magnesium (Mg) top and bottom electrodes were employed in the devices. In the undoped device, the current increases exponentially with the applied voltage, and is concluded to be limited by the carrier injection at the Mg/Alq3 interface with a small injection barrier. With the same device structure, both the bulk-doped and interface-doped devices show a bulk-limited current flow, which is much larger than that in the undoped device and behaves as trap-free space-charge-limited current (SCLC) with a field-independent mobility. The improvement of the current is ascribed to the large enhancement of carrier injection by the doping. An increase of the magnitude of the interfacial dipole has been proposed as the possible cause for the enhancement of carrier injection.

We report on the current–voltage ( J – V ) characteristics of single-layer devices employing pure and heavily TTN-doped Alq 3 films as active layers. In the undoped device, the current is limited by the carrier injection at the Mg/Alq 3 interface with a small injection barrier. In the uniformly-doped device, the current flow is much larger and behaves as space-charge-limited current (SCLC). This means a conversion from injection-limited to bulk-limited current. As the interface- and uniformly-doped devices show similar magnitude of current, the improvement of the current is ascribed to the large enhancement of electron injection due to the doping-induced modification of the interfacial electronic structure.

The effect of n-type doping of hexadecafluorophthalocyaninatozinc (F 16 ZnPc) with an electron-donating organic molecule tetrathianaphthacene (TTN) was investigated by ultraviolet photoemission spectroscopy (UPS). We measured the UPS spectra over a wide range of the film thickness for pristine F 16 ZnPc, pristine TTN, and TTN-doped F 16 ZnPc. From the change of the energy of the vacuum level (VL), we could deduce the energy level alignment at organic/metal interfaces. The results for pristine F 16 ZnPc showed (1) abrupt shift of VL at small thickness corresponding to dipole layer formation, and (2) more gradual shift of VL, which might correspond to the band bending to align the Fermi level by residual impurity acting as unintentional dopant. For F 16 ZnPc doped with TTN, the region (2) is merged into the region (1), indicating that the thickness of the band-bending region of the doped F 16 ZnPc was much reduced from that of pristine F 16 ZnPc. This demonstrates that F 16 ZnPc was effectively doped with carriers from the TTN molecules. It was also found that the energy of the highest occupied molecular orbital of F 16 ZnPc was shifted to higher binding energy by doping TTN molecules. These results suggest that TTN acts as a good donor in F 16 ZnPc.

The electronic structure at highly ordered pentacene monolayer prepared on Cu(110) substrate was studied by angle-resolved ultraviolet photoemission spectroscopy. The valence-level photoemission line shape showed the evidences of (i) formation of the interface states and (ii) two-dimensional energy-band dispersion of the resultant interface states. The lattice constant deduced from the observed energy-band dispersion is consistent with the reported one based on the low-energy electron diffraction experiments. Thus, the observed energy-band dispersion can be ascribed to the in-plane intermolecular energy-band dispersion in the pentacene monolayer on Cu(110). These phenomena may originate from the hybridization between the molecular orbital and the wave function of the substrate surface. Furthermore, work-function change of about $\ensuremath{-}0.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ by adsorption of pentacene was observed from the shift of the secondary-electron cutoff. Such a decrease of the work function indicates the formation of a dipole layer at the interface with the molecule positively charged. This direction is opposite to the naive expectation from the electron transfer from the substrate to the molecule, which was suggested from the previous work of core-level photoemission spectroscopy [McDonald et al., Surf. Sci. 600, 1909 (2006)]. This unexpected result may originate from the charge redistribution at the interface due to the induced image charge in the metal and the push back of electrons spilled out from the metal surface by the adsorbed molecules, which may overwhelm the effect of electron transfer.

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.

This study investigates electron transport properties of a Joule heated multi-wall carbon nanotube at temperatures as high as about 2100 K. Results show that the electrical resistance is temperature-dependent. The temperature dependence of the resistance is inferred to originate from the temperature-dependent intrashell resistance and intershell conductance. The resistance variation was analyzed using a resistive transmission line model. Results well reproduced those obtained using a double shell conduction model where the electrons transport in the outermost shell and the second inner shell.

The n-type doping ability of the electron-donating organic molecule tetrathianaphthacene (TTN) was examined by ultraviolet photoemission spectroscopy (UPS) for two kinds of matrix organic semiconductors, hexadecafluorophthalocyaninatozinc (F16ZnPc) and tris(8-hydroxyquinoline)aluminum (Alq3). The observed dependence of the UPS spectra suggests that TTN acts as a good donor in F16ZnPc, while the effect was not so significant for Alq3. These results and the consideration of the energy parameters of these molecules indicate that a close match between the highest occupied molecular orbital of the dopant and the lowest unoccupied molecular orbital of the matrix is important for efficient n-type doping.