Nobuo Ueno

Charged and metallic molecular monolayers through surface-induced aromatic stabilization

Large π-conjugated molecules, when in contact with a metal surface, usually retain a finite electronic gap and, in this sense, stay semiconducting. In some cases, however, the metallic character of the underlying substrate is seen to extend onto the first molecular layer. Here, we develop a chemical rationale for this intriguing phenomenon. In many reported instances, we find that the conjugation length of the organic semiconductors increases significantly through the bonding of specific substituents to the metal surface and through the concomitant rehybridization of the entire backbone structure. The molecules at the interface are thus converted into different chemical species with a strongly reduced electronic gap. This mechanism of surface-induced aromatic stabilization helps molecules to overcome competing phenomena that tend to keep the metal Fermi level between their frontier orbitals. Our findings aid in the design of stable precursors for metallic molecular monolayers, and thus enable new routes for the chemical engineering of metal surfaces.

INNER-SHELL EXCITATION AND SITE SPECIFIC FRAGMENTATION OF POLY(METHYLMETHACRYLATE) THIN FILM

Soft x‐ray excitations in the 250–600 eV photon energy range on poly(methylmethacrylate) (PMMA) result in ionic fragmentation of the original polymer with the most intense ions corresponding to CH+3, H+, CH+2, CH+, CHO+, and COOCH+3. The photon energy dependence of ion desorption from thin films of PMMA was measured to investigate the primary steps in radiation induced decomposition following carbon and oxygen 1s electron excitations using monochromatic pulsed‐synchrotron radiation. It was clearly found that the decomposition depends on the nature of the electronic states created in the excited species. The fragmentation pattern changes depending on the transitions of the 1s electron to a Rydberg orbital, an unoccupied molecular orbital or the ionization continuum. Moreover, the fragmentation occurs specifically around the site of the atom where the optical excitation takes place. Excitations from carbon and oxygen 1s to σ* states seem to be specially efficient for ion production as observed in the case o...

VALENCE BANDS OF ORIENTED FINITE LINEAR-CHAIN MOLECULAR-SOLIDS AS MODEL COMPOUNDS OF POLYETHYLENE STUDIED BY ANGLE-RESOLVED PHOTOEMISSION

Abstract Angle-resolved photoemission spectra were measured using synchrotron radiation of two kinds of oriented model compounds of polyethylene with their molecular axes perpendicular to the substrate surface, i.e. evaporated films of hexatriacontane CH 3 (CH 2 ) 34 CH 3 and Langmuir-Blodgett films of Cd arachidate (CH 3 (CH 2 ) 17 COO) 2 Cd. Both films show similar photoelectron energy distribution curves determined by the long-alkyl chain. The intramolecular energy-band dispersion of polyethylene was determined from the photon-energy dependence of the normal-emission spectra. This is the first direct observation of an energy-band dispersion in organic solids. The upper bands formed by C 2p and H 1s electrons extend from 8.8 to 15.5 eV below the vacuum level, and the deeper-lying bands originating from C 2s electrons lie from 17.5 to 24.7 eV. The band structure obtained is compared to results from XPS and ESR studies. Furthermore, the experimentally determined band structure is discussed in detail in view of theoretical calculations for polyethylene. Ab initio and extended Huckel calculations give a good description of the experimental results.

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.

Band gap states of copper phthalocyanine thin films induced by nitrogen exposure

The impact of 1 atm N2 gas exposure on the electronic states of copper phthalocyanine thin films was investigated using ultrahigh-sensitivity ultraviolet photoelectron spectroscopy. The highest occupied molecular orbital band of the film showed a drastic reversible change in the bandwidth and band shape as well as in the energy position upon repeated cycles of N2 exposure and subsequent annealing. Furthermore, two types of gap-state densities with Gaussian and exponential distributions appeared after the exposure and disappeared due to the annealing. These changes are ascribed to a weak disorder in the molecular packing structure induced by N2 diffusion into the film.

Very narrow photoemission bandwidth of the highest occupied state in a copper-phthalocyanine monolayer

Abstract We observed a very narrow bandwidth of the highest occupied molecular orbital (HOMO) state in ultraviolet photoemission spectra (UPS) of copper-phthalocyanine monolayer deposited on graphite. The HOMO band in UPS consists of three components which may originate from the vibrational coupling. The full width at half maximum of each component was found to be ∼150 meV at 295 K. This HOMO-bandwidth leads to an estimation that the lifetime of the HOMO hole should be at least longer than 2.2 fs, which may be dominated by the electron transfer rate from the substrate to the molecule.

Structure of copper- and H2-phthalocyanine thin films on MoS2 studied by angle-resolved ultraviolet photoelectron spectroscopy and low energy electron diffraction

Angle-resolved ultraviolet photoelectron spectra (ARUPS) of copper phthalocyanine (CuPc) and metal-free phthalocyanine (H2Pc) films (thickness from monolayer to 50–80 A) on cleaved MoS2 substrates were measured using monochromatic synchrotron radiation. Observed take-off angle (θ) and azimuthal angle (φ) dependencies of the top π 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°, −37°, or −67° 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...

Intermolecular energy-band dispersion in oriented thin films of bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) by angle-resolved photoemission

Angle‐resolved ultraviolet photoemission spectra using synchrotron radiation were measured for oriented thin films of bis(1,2,5‐thiadiazolo)‐p‐quinobis(1,3‐dithiole) (BTQBT) on graphite. From the photon energy dependence of normal emission spectra, the energy‐band dispersion of π‐bands were observed for the highest (HOMO) and next highest (NHOMO) bands. This is the first observation of intermolecular dispersion in a single‐component organic molecular crystal. The results demonstrate that the BTQBT molecules have a strong intermolecular interaction, which can be derived from the introduction of a covalent interaction between sulfur atoms in addition to the usual intermolecular interaction by van der Waals forces.

Photoemission study of direct photomicromachining in poly(vinylidene fluoride)

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 H2, 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.

Molecular Orientation and Aggregation of Titanyl Phthalocyanine Molecules on Graphite Substrates: Effects of Surface Topography of the Substrate

Penning ionization electron spectroscopy (PIES) was used to investigate the effects of crystallographic inperfection of the substrate surface on organic ultrathin-film growth. For titanyl phthalocyanine (OTiPc) evaporated on graphite, it was found that the molecular orientation and aggregation in the film depend significantly on the type of graphite substrate. On a highly oriented pyrolytic graphite (HOPG), OTiPc film prepared by 1-monolayer-equivalence (MLE) deposition consists of islands of double layers, while on Grafoil, the molecules do not aggregate as on the HOPG, and form a monolayer. This large difference originated from the surface topography of the two graphite substrates.