High-resolution X-ray photoelectron spectroscopy of two-dimensional films of adenine on a highly oriented pyrolitic graphite [0 0 0 1] surface reveal bonding changes induced by exposure to water. The hydrogen-bond interactions between adenine molecules were replaced by adenine-water hydrogen bonds in a stepwise pattern. This reaction destroyed the film network and changed the chemical state of the nitrogen sites. The reaction with water molecules affected both the donor and acceptor states of the oxygen atoms, as compared to those in water.

The role of nitrogen in the charge transfer and storage capacity of lithium-intercalated heterocyclic oligophenylenes was investigated using photoelectron spectroscopy. The development of new occupied states at low binding energies in the valence band region, as well as core level chemical shifts at both carbon and nitrogen sites, demonstrates partial charge transfer from lithium atoms to the organic component during formation of the intercalated compound. In small compounds, i.e., biphenyl and bipyridine derivatives, the position of the nitrogen heteroatom significantly affects the spacing between gap states in the Li-intercalated film, yet it has minimal effects on the charge storage capacity. In larger, branched systems, the presence of nitrogen in the aromatic system significantly enhances the charge storage capacity while the Li-N bond strength at high intercalation levels is significantly weakened relative to the nitrogen-free derivative. These observations have strong implications towards improved deintercalation processes in organic electrodes in lithium-ion batteries. © 2007 American Institute of Physics.

Highly ordered, multilayer films composed of alternating, oppositely charged, polyionic copper phthalocyanines were prepared on HOPG [0001] substrates via layer-by-layer deposition from aqueous solution and characterized by scanning force microscopy and photoelectron spectroscopies. In films of up to four layers, individual layers alternate. Angle-resolved ultraviolet photoelectron spectra are consistent with a molecular orientation parallel to the substrate surface and indicate that structural order is reduced with film thickness © 2007 American Chemical Society.

The degree of crystallinity, the structure and orientation of crystallites, and the morphology of thin pentacene films grown by vapor deposition in an ultrahigh vacuum environment on polycrystalline copper substrates have been investigated by x-ray diffraction and tapping-mode scanning force microscopy (TM-SFM). Depending on the substrate temperature during deposition, very different results are obtained: While at 77 K a long-range order is missing, the films become crystalline at elevated temperatures. From a high-resolution x-ray-diffraction profile analysis, the volume-weighted size of the crystallites perpendicular to the film surface could be determined. This size of the crystallites increases strongly upon changing temperature between room temperature and 333 K, at which point the size of individual crystallites typically exceeds 100 nm. In this temperature region, three different polymorphs are identified. The vast majority of crystallites have a fiber texture with the (001) net planes parallel to the substrate. In this geometry, the molecules are oriented standing up on the substrate (end-on arrangement). This alignment is remarkably different from that on single-crystalline metal surfaces, indicating that the growth is not epitaxial. Additionally, TM-SFM images show needlelike structures which suggest the presence of at least one additional orientation of crystallites (flat-on or edge-on). These results indicate that properties of thin crystalline pentacene films prepared on technologically relevant polycrystalline metal substrates for fast electronic applications may be compromised by the simultaneous presence of different local molecular aggregation states at all temperatures. © 2006 American Institute of Physics.

The hole-vibration coupling of the highest occupied state in pentacene thin films on graphite was studied by high-resolution ultraviolet photoemission spectroscopy. It was found that vibration satellites in the film are more intense than that in the gas phase and the vibrational energy in the film is slightly lower than that in the gas phase. This demonstrates that the reorganization energy (hole mobility) in the pentacene thin film is slightly larger (smaller) than that expected from the photoemission spectrum of free pentacene molecules.

Structural and electronic properties of pristine and lithium-intercalated, phenyl-capped aniline dimers as a model for the lithium-polyaniline system have been studied by photoelectron spectroscopy and quantum chemical calculations. It was found that the electronic structure of reduced and oxidized forms of oligoanilines is only weakly affected by isomerism. Upon intercalation, charge transfer from the Li-atoms is remarkable and highly localized at N-atomic sites, where configurations are energetically favored in which both N atoms of the dimers are bound to Li atoms. Conversion of nitrogen sites is different for the two forms of aniline dimers and incomplete up to high intercalation levels, indicating a pronounced role of solid-state effects in the formation of such compounds. © 2006 American Chemical Society.

We used ultraviolet photoelectron spectroscopy (UPS) to investigate the energy level alignment at contacts between pentacene and Ag(1 1 1) in the presence of interfacial 6,13-pentacenequinone (PQ). Depending on the metal pre-coverage with PQ, we found evidence for three distinctly different interface morphologies and molecular orientations, accompanied by significant changes of the energy level alignment. Consequently, the hole injection barrier between pentacene and Ag(1 1 1) varied between 1.1 eV (pristine Ag) and 0.45 eV (5.4 nm PQ pre-coverage on Ag). In addition, our UPS results suggest that PQ can act as deep trap for electrons in a pentacene matrix. Depending on the exact mutual orientation of PQ and pentacene, the depth of these traps can be in the range of 0.2-0.75 eV. © 2006 Elsevier B.V. All rights reserved.

During the past three decades, ultraviolet photoelectron spectroscopy of polymer films has evolved from a sort of appearance-potential ( valence band edge) measurement, into a tool for studying the full valence band region of thin polymer films, including insulating polymers, semiconducting polymers and electrically conducting polymers. Progress may be loosely divided into several categories: (A) the melding of thin polymer film technology with ultra high vacuum technology and the widespread use of helium resonance lamps for studies of solid surfaces, (B) the combined approach of measurements and appropriate theoretical-computational methods, and (C) the advent of synchrotron radiation resulting in multi-photon spectroscopies, nominally in the area of the near UV. A coincident discovery of electrically conducting polymers, and especially the evolution of applications of semiconducting polymers, added technologically driven emphasis to this development of UPS for polymer materials. This contribution traces a limited number of highlights in the evolution of UPS of polymers, from the '70' s through to 2005.

Nonferromagnetic Mn2+ ions can be readily formed at the surface of half metallic La0.7 Sr0.3 MnO3 manganite as demonstrated by deoxygenating surface treatments. The 3 d5 contribution of these Mn2+ ions to the valence-band electronic structure has been characterized using Mn(2p) to 3d resonant photoemission measurements. The Mn2+ related 3d electrons were found to be stabilized by about 2 eV with respect to the mixed-valence 3d states, indicating their strong localization. Active participation of Mn2+ states in both spin and charge conductivity processes is therefore excluded. A two-channel picture, including independent Mn3+ Mn4+ and Mn2+ channels, emerges from detailed data analysis. Reversible Mn2+ formation and straightforward oxygen annealing effects point to a direct bonding between Mn2+ and oxygen vacancies that are most probably created at preexisting structural defects. The t2g and eg states of the mixed valence Mn3+ Mn4+ ions remain unaffected as the Mn2+ content increases, indicating a robust Mn3+ Mn4+ channel independent of structural defects. © 2006 The American Physical Society.

The vibrational coupling in the ground and excited states of positively charged naphthalene, anthracene, tetracene, and pentacene molecules is studied on the basis of a joint experimental and theoretical study of ionization spectra using high-resolution gas-phase photoelectron spectroscopy and first-principles correlated quantum-mechanical calculations. Our theoretical and experimental results reveal that, while the main contribution to relaxation energy in the ground state of oligoacene systems comes from high-energy vibrations, the excited-state relaxation energies show a significant redistribution toward lower-frequency vibrations. A direct correlation is found between the nature of the vibronic interaction and the pattern of the electronic state structure. © 2006 American Chemical Society.