Toyoaki Eguchi

Size-specific catalysis by single-size palladium nanoclusters.

Organic thin film electronics place a high demand on bottom-up technology to form a two-dimensionally (2D) functional unit consisting of a single molecular crystalline layer bound to a layered structure. As the strong interaction between a substrate and molecules makes it difficult to evaluate the electronic properties of organic films, the nature of electronic excited states has not been elucidated. Here, we study a 2D crystalline anthracene monolayer electronically decoupled by alkanethiolates on a gold substrate using scanning tunneling microscopy and time-resolved two-photon photoemission spectroscopy and unravel the geometric/electronic structures and excited electron dynamics. Our data reveal that dispersive 2D excited electrons on the surface can be highly coupled with an annihilation of nondispersive excitons that facilitate electron emission with vibronic interaction. Our results provide a fundamental framework for understanding photoexcited anthracene monolayer and show how the coupling between dispersive and nondispersive excited states may assist charge separation in crystalline molecular layers.

We have employed a two-photon photoelectron emission microscopy (2P-PEEM) to observe the photocarrier electron dynamics in an organic thin film of fullerene (C₆₀) formed on a highly oriented pyrolytic graphite with a spatial resolution of ca. 135 nm. In this approach, photocarrier electrons in C₆₀ single-layer islands generated by the first pump photon are detected by the second probe photon. These spectromicroscopic observations conducted over a 100 × 100 nm² region of C₆₀ islands consistently reproduced the macroscopic two-photon photoemission spectrum of fully covered C₆₀ monolayer film, where the energy of photocarrier electron in the islands was +0.9 eV relative to the Fermi level. Time-resolved 2P-PEEM revealed that the photocarrier electron decayed from the monolayered C₆₀ islands into the substrate with a time constant of 470 ± 30 fs.

Time-resolved two-photon photoemission (TR-2PPE) spectroscopy is employed to probe the electronic states of a C₆₀ fullerene film formed on highly oriented pyrolytic graphite (HOPG), acting as a model two-dimensional (2D) material for multi-layered graphene. Owing to the in-plane sp²-hybridized nature of the HOPG, the TR-2PPE spectra reveal the energetics and dynamics of photocarriers in the C₆₀ film: after hot excitons are nascently formed in C₆₀ via intramolecular excitation by a pump photon, they dissociate into photocarriers of free electrons and the corresponding holes, and the electrons are subsequently detected by a probe photon as photoelectrons. The decay rate of photocarriers from the C₆₀ film into the HOPG is evaluated to be 1.31 × 10¹² s⁻¹, suggesting a weak van der Waals interaction at the interface, where the photocarriers tentatively occupy the lowest unoccupied molecular orbital (LUMO) of C₆₀. The photocarrier electron dynamics following the hot exciton dissociation in the organic thin films has not been realized for any metallic substrates exhibiting strong interactions with the overlayer. Furthermore, the thickness dependence of the electron lifetime in the LUMO reveals that the electron hopping rate in C₆₀ layers is 3.3 ± 1.2 × 10¹³ s⁻¹.

Chemical characterization was performed for an alkali-like superatom consisting of a Ta-encapsulating Si₁₆ cage, Ta@Si₁₆, deposited on a graphite substrate using X-ray photoelectron spectroscopy (XPS) to element-specifically clarify the local electronic structure of the cage atoms. The XPS spectra derived from Ta 4f and Si 2p core levels have been well modeled with a single chemical component, revealing the formation of a symmetric Si cage around the Ta atom in the deposited nanoclusters. On chemical treatments by heating or oxygen exposure, it is found that the deposited Ta@Si₁₆ is thermally stable up to 700 K and is also exceptionally less reactive toward oxygen compared to other Ta–Si nanoclusters, although some heat degradation and oxidation accompany the treatments. These results show the promising possibility of applying Ta@Si₁₆ as a building block to fabricate cluster-assembled materials consisting of naked nanoclusters.