S. Hasegawa

Angle-resolved ultraviolet photoelectron spectroscopy of thin films of bis(1,2,5-thiadiazolo)-p-quinobis (1,3-dithiole) on the MoS2 surface

In this paper we report on the angle-resolved ultraviolet photoelectron spectroscopy on ultrathin films of bis(1,2,5-thiadiazolo)-p-quinobis (1,3-dithiole) (BTQBT) deposited on a MoS2 surface with synchrotron radiation, and describe the quantitative analysis of the angular distribution of photoelectron from the highest occupied state. The observed angular distributions were better explained by those calculated with the single-scattering approximation combined with molecular orbital calculation considering intramolecularly scattered waves than the previously used independent-atomic-center approximation combined with molecular orbital calculation. Further, the low-energy-electron diffraction measurements were performed on the film. From the low-energy-electron diffraction, the two-dimensional lattice of the ultrathin films on the MoS2 was found to be MoS2(0001)−(13×13, R=±13.9°)-BTQBT, and from the analysis of the photoelectron angular distributions, it was found that at the lattice points the molecules lie...

Electronic structure of Alq3/LiF/Al interfaces studied by UV photoemission

Abstract The electronic structure of tris (8-hydroxyquinolino) aluminum (Alq 3 /LiF/Al system was studied in relation to the enhancement of electron-injection efficiency by the insertion of LiF insulating layer at Alq 3 /Al contact, using UV photoemission spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and metastable atom electron spectroscopy (MAES). The observed energy separation between the HOMO of Alq 3 and the Fermi level of Al substrate increased from 2.7eV to 3.0eV by inserting 0.5nm thick LiF layer. This result indicates that the LiF layer induces the decrease of the electron injection barrier. We also found extra states probably caused by the interaction at the Alq 3 /Al interface.The spectral intensity of this extra state decreased with increasing LiF thickness, and vanished at 0.5nm.

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...

Electronic structure and molecular orientation of well-ordered polyethylene oligomer (n-C44H90) on Cu(100) and Au(111) surfaces studied by UV photoemission and low energy electron diffraction

Abstract 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 2×1-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.

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.

Structure of ultrathin films of chloroalminium phthalocyanine on MoS2 studied by means of penning ionization electron spectroscopy, angle-resolved UPS and LEED

Abstract The structure of ultra-thin films (0.5–5 monolayers) of chloroalminium phthalocyanine (ClAlPc) on an MoS 2 substrate was studied by Penning ionization electron spectroscopy (PIES), angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and low-energy electron diffraction (LEED). The change in the molecular orientation was sensitively detected during the variation of the thickness and temperature of the film.

Pendant group orientation of poly(2-vinylnaphthalene) thin film surface studied by near-edge x-ray absorption fine structure spectroscopy (NEXAFS) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS)

Angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy were applied to the investigation of the tilt angles of the naphthalene pendant groups at the surface of a poly(2-vinylnaphthalene) thin film. In contrast to NEXAFS, which provides only an average determination of the tilt angle, ARUPS combined with a sophisticated analysis of photoelectron angular dependence offers more detailed information. It was concluded that the naphthalene pendant groups are tilted randomly at the polymer surface, and that the tilt angle distribution is well described as a three-dimensional isotropic random orientation.

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°

Comparison between calculated and measured photoemission spectra of C60 thin films

Abstract The photon energy hν dependences of angle-resolved ultraviolet photoelectron spectra of C 60 were measured in the range of hν = 20–120 eV. They exhibited the same intensity oscillations as reported previously, and were compared with the calculated results. In the calculation we used the single-scattering approximation for the final state with the result of the STO-5G molecular orbital calculation for the initial state. The calculated hν dependences for the HOMO and NHOMO states agreed well with the measured results. This indicated that the oscillations originate from the interference of photoelectron waves emanating from the 60 carbon atoms. Farther, the analytical calculation with the simplified initial state revealed that the spherical shell like molecular structure of C 60 with its large radius is the essential factor for the oscillations.