K.K. Okudaira

Does the molecular orientation induce an electric dipole in Cu-phthalocyanine thin films?

The effect of the molecular orientation on the molecular electronic structure is studied on the Cu-phthalocyanine∕graphite system via film thickness dependences of metastable-atom electron spectra and ultraviolet photoelectron spectra. We observed a decrease in the vacuum-level position and a corresponding band-bending-like shift in the highest occupied state only for thick films where the molecular tilt angle increases gradually with the film thickness. These shifts are explained by electric dipoles produced in the film by a gradient of the intermolecular electronic interaction along the surface normal due to the continuous increase in the molecular tilt angle. The result indicates that the change in the molecular orientation is an important origin of the band-bending-like shift in the molecular electronic states even if the molecule has no permanent electric dipole.

Characterization of 4-mercaptohydrocynnamic acid self-assembled film on Au(111) by means of X-ray photoelectron spectroscopy

Abstract Structure of the self-assembled film of 4-mercaptohydrocynnamic acid (HOOC(CH 2 ) 2 C 6 H 4 SH) deposited on Au(111) surface was characterized by X-ray photoelectron spectroscopy (XPS). From the XPS spectrum of S 2p, it was found that there are the two types of sulfur atoms, which correspond to sulfur in thiolate bonded to Au(111) and sulfur in unbound thiol at the surface. This result supports the bilayer formation of 4-mercaptohydrocynnamic acid molecules on Au(111) substrate. Furthermore, the XPS results of the O 1s region suggest that carboxylic anhydride is formed at room temperature.

Electronic density tailing outside π-conjugated polymer surface

Penning ionization electron spectroscopy (PIES) was adopted to examine surface electronic property of conjugated poly(3-hexylthiophene) (P3HT) aiming to detect the electronic density tailing outside a polymer surface. The electronic wave function of the highest occupied molecular orbital (HOMO) state is shielded by side hexyl chain for a P3HT film with edge-on conformation and was not detected by PIES, while it is tailing outside the polymer surface for face-on conformation and was observed clearly by PIES. The presence of HOMO electronic wave function outside the polymer surface makes it possible to form HOMO-HOMO overlapping with overlayer materials, and therefore more efficient charge transfer is expected in a heterojunction structure for device application.

PEEM and MEEM of chloroaluminum phthalocyanine ultrathin film on MoS2

Abstract Surface images of a chloroaluminum phthalocyanine (ClAlPc) film, which consists of monolayer and doublelayer regions, were observed by two surface sensitive microscopy, photoelectron emission microscopy (PEEM) using photons of the ionization threshold energy of the film and metastable electron emission microscopy (MEEM) using metastable He. In PEEM, an image contrast, which directly reflects slight difference of ionization threshold energies between monolayer and doublelayer regions, was obtained, while in MEEM little contrast was detected because of the use of all emitted electrons for the imaging. The PEEM result indicates that the ionization threshold energy of the film depends on the film thickness. Using PEEM, we can map the slight difference of electronic states due to the thickness difference of an organic thin film.

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.

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.

Penning ionization electron spectroscopy of titanyl phthalocyanine ultrathin films: electronic state and molecular orientation

Abstract Penning ionization electron spectroscopy (PIES) was used to investigate the molecular orientation, aggregation of molecules, and electronic structure of ultrathin films of titanyl phthalocyanine (OTiPc) vapor deposited on graphite and MoS 2 substrates. For the monolayer film, the molecular orientation was remarkably different between the two substrates. At room temperature, molecules spread over and almost cover the substrate on graphite, whereas they aggregate and form islands on MoS 2 . The PIES band due to the nonbonding orbital of the oxygen atom extending perpendicular to the molecular plane is extraordinary strong for the He * (2 3 S) metastable atom but not for the He * (2 1 S), indicating that the potential for the entrance channel is quite different between the two species.

Direct observation of S-Au bonding state of self-assembled monolayers by outermost-surface spectroscopy using metastable atom beam

Abstract Temperature dependencies of metastable atom electron spectra (MAES) and ultraviolet photoelectron spectra (UPS) were measured for 8-bromo-1-octanethiol (8BRT) SAMs prepared on Au (111) surfaces in ethanol (EtOH) and n-hexane solutions. For both 8BRT SAMs, it was found that molecules stand upright to the substrate and the film surfaces consist of Br atoms. However, we found that 8BRT SAM prepared in n-hexane solution mainly consists of physisorbed molecules and they almost desorb at 100°C, while 8BRT SAM prepared in EtOH solution involves both physisorbed and chemisorbed molecules. For the SAM prepared in EtOH solution, we observed the valence electronic states which can be ascribed to S–Au bond of the chemisorbed ones after the desorption of the physisorbed molecules.

Diffusion of chloroaluminum phthalocyanine on MoS2 surface detected by photoemission electron microscopy and metastable electron emission microscopy

Diffusion of a large organic semiconductor molecule, chloroaluminum phthalocyanine (ClAlPc), on a cleaved MoS2 surface was detected using photoemission electron microscopy (PEEM) and metastable electron emission microscopy (MEEM). The PEEM and MEEM images showed that a micropattern of ClAlPc ultrathin film prepared on the MoS2 surface by vacuum deposition shrinks with time and finally disappears even at room temperature at which the molecules do not evaporate. The results indicate that control of molecular diffusion is necessary for the preparation of stable micro or nanostructure of organic thin films.

CHARACTERIZATION OF SELF-ASSEMBLED MONOLAYER OF THIOPHENOL ON GOLD BY PENNING IONIZATION ELECTRON SPECTROSCOPY

Abstract The molecular orientation in self-assembled monolayers (SAMs) of thiophenol chemisorbed on polycrystalline gold from aqueous solutions has been investigated by Penning ionization electron spectroscopy (PIES) and ultraviolet photoelectron spectroscopy (UPS). The analysis of the relative band intensity of the Penning spectrum indicates that the phenyl ring in phenyl thiolate stands perpendicular to the substrate plane at room temperature. Upon heating, the phenyl ring becomes tilted in the layer.