Satoshi Kera

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.

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.

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.

Electron affinity of pentacene thin film studied by radiation-damage free inverse photoemission spectroscopy

The electron affinity of pentacene thin films has been evaluated during the last decades, but it is still under controversial due to varieties of film quality and radiation damages of the films introduced during inverse photoemission spectroscopy (IPES) experiment together with insufficient energy resolution of the instruments. We employed the near-ultraviolet IPES with a better energy resolution 0.27 ∼ 0.32 eV and using lower energy electron beams (0 eV ≤ Ei ≤ 4.9 eV) to study the unoccupied states of pentacene thin film. Due to a large mean-free-path of the electron in this energy region, the threshold electron affinity of the bulk of pentacene film was precisely determined to be 2.70 ± 0.03 eV. Using the threshold ionization energy of 4.90 ± 0.05 eV determined by ultraviolet photoemission spectroscopy, the band-gap energy of the pentacene film is obtained to be 2.20 ± 0.06 eV.

Accessing surface Brillouin zone and band structure of picene single crystals

We have experimentally revealed the band structure and the surface Brillouin zone of insulating picene single crystals (SCs), the mother organic system for a recently discovered aromatic superconductor, with ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction with a laser for photoconduction. A hole effective mass of 2.24m(0) and the hole mobility μ(h)≥9.0 cm(2)/V s (298 K) were deduced in the Γ-Y direction. We have further shown that some picene SCs did not show charging during UPS even without the laser, which indicates that pristine UPS works for high-quality organic SCs.

Reversible Single‐Molecule Switching in an Ordered Monolayer Molecular Dipole Array

Making electronic devices using a single molecule has been the ultimate goal of molecular electronics. For binary data storage in particular, the challenge has been the ability to switch a single molecule in between bistable states in a simple and repeatable manner. The reversible switching of single molecules of chloroaluminum phthalocyanine (ClAlPc) dipolar molecules within a close-packed monolayer is demonstrated. By pulsing an scanning tunneling microscopy tip, read-write operations of single-molecular binary bits at ~40 Tb/cm(2) (~250 Tb/in(2)) are demonstrated.

Halide-Substituted Electronic Properties of Organometal Halide Perovskite Films: Direct and Inverse Photoemission Studies

Solution-processed perovskite solar cells are attracting increasing interest due to their potential in next-generation hybrid photovoltaic devices. Despite the morphological control over the perovskite films, quantitative information on electronic structures and interface energetics is of paramount importance to the optimal photovoltaic performance. Here, direct and inverse photoemission spectroscopies are used to determine the electronic structures and chemical compositions of various methylammonium lead halide perovskite films (MAPbX3, X = Cl, Br, and I), revealing the strong influence of halide substitution on the electronic properties of perovskite films. Precise control over halide compositions in MAPbX3 films causes the manipulation of the electronic properties, with a qualitatively blue shift along the I → Br → Cl series and showing the increase in ionization potentials from 5.96 to 7.04 eV and the change of transport band gaps in the range from 1.70 to 3.09 eV. The resulting light absorption of MAPbX3 films can cover the entire visible region from 420 to 800 nm. The results presented here provide a quantitative guide for the analysis of perovskite-based solar cell performance and the selection of optimal carrier-extraction materials for photogenerated electrons and holes.

Orientational ordering of nonplanar phthalocyanines on Cu(111): strength and orientation of the electric dipole moment.

In order to investigate the orientational ordering of molecular dipoles and the associated electronic properties, we studied the adsorption of chlorogallium phthalocyanine molecules (GaClPc, Pc=C32N8H16(-2) on Cu(111) by using the x-ray standing wave technique, photoelectron spectroscopy, and quantum mechanical calculations. We find that for submonolayer coverages on Cu(111) the majority of GaClPc molecules adsorb in a Cl-down configuration by forming a covalent bond to the substrate. For bilayer coverages the x-ray standing wave data indicate a coexistence of the Cl-down and Cl-up configurations on the substrate. The structural details established for both cases and supplementary calculations of the adsorbate system allow us to analyze the observed change of the work function.

Characterization of ultrathin films of titanyl phthalocyanine on graphite: PIES and UPS study

Abstract Penning ionization electron spectroscopy (PIES) and ultraviolet photoelectron spectroscopy (UPS) were used to characterize ultrathin films (0.2–3 MLE) of titanyl phthalocyanine (OTiPc) deposited on a graphite substrate. The change in the molecular orientation at the outermost surface layer was selectively probed by PIES during the variations of the thickness and temperature of the films.