Heterojunctions of π-conjugated molecules are fundamentals of electronic functionalities in organic semiconductor devices, yet the leading principles determining the structures of the molecular heterojunctions are still unestablished. In the present study, a well-defined organic “complementary” pn-heterojunction of perfluoropentacene (C₂₂F₁₄) overlayers formed on the single crystal surface of pentacene (C₂₂H₁₄) was examined by means of grazing-incidence x-ray diffraction and atomic force microscopy. It is revealed that the perfluoropentacene molecules crystallize epitaxially to align their nearest-neighbor direction ({010} axis) along the {110} axis of the pentacene single crystal.

We carried out near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) measurements of water-adsorbed ultrathin GeO₂ films on Ge substrates, and the results were compared with those for SiO₂ films on Si. We obtained NAP-XPS spectra at relative humidity (RH) values of up to ∼15% and showed that the GeO₂/Ge structures attract more water molecules than the SiO₂/Si structures at RH above ∼10⁻⁴%. This is probably because water molecules infiltrate the GeO₂ films to form hydroxyls. Then we revealed positive charging of the water-adsorbed SiO₂ films by their interaction with X-rays. For the water-adsorbed GeO₂ films, we observed greater positive charging of the films, of which origin is discussed.

Electronegativity is one of the fundamental concepts in chemistry. Despite its importance, the experimental determination has been limited only to ensemble-averaged techniques. Here, we report a new methodology to evaluate the electronegativity of individual surface atoms by atomic force microscopy. By measuring bond energies on the surface atoms using different tips, we found characteristic linear relations between the bond energies of different chemical species. Using Paulingʼs equation for polar covalent bond, we successfully quantify their electronegativity values. Moreover, we demonstrate that the method is sensitive to variation of the electronegativity of given atomic species on a surface due to different chemical environments. Our findings open up new ways of analyzing surface chemical reactivity in atomic scale.

Here we report our recent progress as to single crystalline metal oxide nanowires towards the nano-device applications. As the basis of such fundamental science, we focus on (1) formation technology of highly crystalline nanowires, (2) evaluation technology of single nanowire physical properties, and (3) control of spatial nanowire positions. Especially, I focus on our recent progress of our research group, which are related to single crystalline nanowires comprised of functional metal oxide materials and their memristive devices.

As a support of catalytic Co nanoparticles used for the growth of high-density single-walled carbon nanotubes (SWCNTs), conductive Co silicide films are superior to insulating SiO₂ films. A Co silicide films is formed through reaction between a Co thin film and the underlying Si wafer. During this process, however, a TiN cap film must be formed to prevent the Co thin film from forming Co nanoparticles and must be etched after the formation of the silicide film, which makes the formation process complicated. In this study, we optimized the thickness of the Co film and formed a silicide film without using a TiN cap film to simplify the formation process. As a result, the formation of the Co nanoparticles was greatly suppressed during the formation of a silicide film. Using the Co silicide film obtained after the formation process at 650^oC for 10 min as the support of catalytic Co nanoparticles, vertically aligned CNTs film with a thickness of 5.3 μm were obtained by hot-filament chemical vapor deposition (CVD) with ethanol steam at 450^oC for 30 min. This thickness is approximately 1.8-fold that previously reported.