Megumi Akai-Kasaya

Enhanced fluorescence by surface plasmon coupling of Au nanoparticles in an organic electroluminescence diode

A significant increase in electroluminescence was achieved through coupling with localized surface plasmons in a single layer of Au nanoparticles. We fabricated a thin-film organic electroluminescence diode, which consists of an indium tin oxide (ITO) anode, a Au nanoparticle array, a Cu phthalocyanine hole transport layer, a tris(8-hydroxylquinolianato) aluminum (III) electron transport layer, a LiF electron injection layer, and an Al cathode. The device structure, with size-controlled Au particles embedded on ITO, can be used to realize the optimum distance for exciton-plasmon interactions by simply adjusting the thickness of the hole transport layer. We observed a 20-fold increase in the molecular fluorescence compared with that of a conventional diode structure.

Development of a scanning tunneling microscope for in situ experiments with a synchrotron radiation hard-X-ray microbeam.

A scanning tunneling microscope dedicated to in situ experiments under the irradiation of highly brilliant hard-X-rays of synchrotron radiation has been developed. In situ scanning tunneling microscopy (STM) observation was enabled by developing an accurate alignment system in ultrahigh vacuum. Despite the noisy conditions of the synchrotron radiation facility and the radiation load around the probe tip, STM images were successfully obtained at atomic resolution. Tip-current spectra were obtained for Ge nano-islands on a clean Si(111) surface by changing the incident photon energy across the Ge absorption edge. A current modification was detected at the absorption edge with a spatial resolution of the order of 10 nm.

Enhanced Red-Light Emission by Local Plasmon Coupling of Au Nanorods in an Organic Light-Emitting Diode

A significant increase in the quantum efficiency of an organic red-light-emitting diode was achieved through coupling with localized surface plasmons of Au nanorods with a length of 50–60 nm embedded on the substrate anode. We used 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM)-doped tris(8-hydroxylquinolianato)aluminum(III) (Alq3) as an emission layer. The fabricated diode structure consists of an indium tin oxide (ITO) anode, a Cu phthalocyanine (CuPc) hole transport layer, an Alq3 electron transport layer, a LiF electron injection layer, and an Al cathode. We observed a 3-fold increase in the intensity of molecular fluorescence compared with that of a conventional diode structure.

High-mobility organic single crystal transistors with submicrometer channels

We demonstrate high-performance electric-field effects in submicrometer-channel organic transistors with rubrene single crystals. Platinum source and drain electrodes are embedded in silicon dioxide gate insulators to reduce thickness of the dielectrics and to minimize the short-channel effect. The miniaturized devices exhibit typical output characteristics with Ohmic linear region, well-defined current saturation, and on-off ratio of 106. Mobility values are in the range of 0.1–0.3cm2∕Vs, which is comparable to those of the best submicrometer organic transistors. Anisotropy in the mobility is detected, indicating that bandlike transport is responsible for the high transistor performance of the short-channel devices.

Optimization of reproduced Morpho-blue coloration

Morpho butterflys metallic blue luster, which is produced from the butterflys scale, has a mysterious feature. Since the scale does not contain a blue pigment, the origin of the coloration is attributed to a microscopic structure that can also explain its high reflectivity. However, it appears blue from wide angular range, which contradicts obviously the grating or multilayer. The mystery of the lack of multi-coloration has recently been explained with a peculiar nano-structure, and experimentally proven by fabricating the optical film by controlling the parameters in nanoscale. The reproduced Morpho-blue was found to be important from viewpoint of a wide variety of applications. However, optical properties of the fabricated film were found to contain still some differences with that of the Morpho-butterfly, although the basic characteristics of the Morpho-blue itself was reproduced. In order to make the artificial Morpho-blue closer to the natural one than the prototype, we attempted to optimize the artificial film structure by controlling fabrication parameters. In this process, optical simulations and micro-structural observations were taken in account. By comparing a series of films fabricated with different nano-patterns, optimized parameters were semi-empirically obtained. Also the relationship between the structural parameters and the optical properties was analyzed. The reflective characteristics of the optimized film were found to reproduce the optical properties more closely to the natural Morpho-blue than the prototypes.

Application of simple mechanical polishing to fabrication of nanogap flat electrodes

We fabricated nanogap flat electrodes with smooth boundaries between metal electrodes and an insulating substrate for the purpose of evaluating the electrical transport properties of a single molecule and molecular assemblies. We proposed a simple fabricating procedure that combined a lift-off process containing electron beam lithography and reactive ion etching with a mechanical grinding process. The fabrication of flat electrodes with a 200 nm gap, whose step is less than ~1 nm, has been demonstrated using this procedure.

Polaron injection into one-dimensional polydiacetylene nanowire

A polydiacetylene nanowire fabricated on a highly ordered pyrolytic graphite surface was studied by scanning tunneling microscopy/spectroscopy (STM/STS). The spectroscopy of individual polydiacetylene nanowires revealed the theoretically predicted π-band and band edge singularities, which are characteristics of a one-dimensional π-conjugated polymer. Furthermore, under a high electric field intensity applied to the polymer wire, the spectrum shows a narrow band gap caused by polaron injection. When a much higher electric field intensity was applied to the wire, an avalanche current flow that rises abruptly has been observed. We consider that such a large current flow originates from an electrical property of the polymer wire, which was changed by a dynamical modulation of the injected polaron.

Structure of Atomically Smoothed LiNbO3 (0001) Surface

LiNbO3 (LN) is a substrate material used extensively for various optical or piezoelectric devices. We investigated an atomic-scale smoothing process of a LN single-crystal (0001) surface. Atomic force microscopy (AFM) observation revealed a surface morphology flattened from threefold symmetry to a step-terrace structure. The marked change of the surface was found to be dominated by a balance of surface diffusion and crystal growth within a period of 1–5 hours. An atomically flat surface with a straight and single-step height of 0.22 nm was obtained for a conventional LN substrate on the market. Next, we evaluated the surface composition and structure of the atomically flat LN (0001) surface by coaxial-impact collision ion scattering spectroscopy (CAICISS). We found that the atomically smoothed surface was terminated by a Nb atomic layer in which the Nb atoms were not shifted from the bulk position.

Coulomb Blockade in a Two-Dimensional Conductive Polymer Monolayer.

Electronic transport was investigated in poly(3-hexylthiophene-2,5-diyl) monolayers. At low temperatures, nonlinear behavior was observed in the current-voltage characteristics, and a nonzero threshold voltage appeared that increased with decreasing temperature. The current-voltage characteristics could be best fitted using a power law. These results suggest that the nonlinear conductivity can be explained using a Coulomb blockade (CB) mechanism. A model is proposed in which an isotropic extended charge state exists, as predicted by quantum calculations, and percolative charge transport occurs within an array of small conductive islands. Using quantitatively evaluated capacitance values for the islands, this model was found to be capable of explaining the observed experimental data. It is, therefore, suggested that percolative charge transport based on the CB effect is a significant factor giving rise to nonlinear conductivity in organic materials.

Scanning Tunneling Microscopy Combined with Hard X-ray Microbeam of High Brilliance from Synchrotron Radiation Source

In situ scanning tunneling microscopy (STM) with highly brilliant hard X-ray irradiation was enabled at SPring-8. To obtain a good signal-to-noise ratio for elemental analysis, an X-ray beam with a limited size of 10 µm was aligned to a specially designed STM stage in ultrahigh vacuum. Despite various types of noises and a large radiation load around the STM probe, STM images were successfully observed with atomic resolution. The use of a new system for elemental analysis was also attempted, which was based on the modulation of tunneling signals rather than emitted electrons. Among tunneling signals, tunneling current was found to be better than tip height as a signal to be recorded, because the former reduces markedly the error of measurement. On a Ge nanoisland on a clean Si(111) surface, the modulation of tunneling current was achieved by changing the incident photon energy across the Ge absorption edge.