Tadaaki Nagao

Structures and electronic transport on silicon surfaces

By utilizing a variety of surface superstructures formed on silicon surfaces and atomic layers grown on them, close correlations between the atomic-scale structures and electrical conduction phenomena at the surfaces have been revealed. State-of-art techniques for analyzing and controlling atomic/electronic structures of surfaces are leading to an understanding of the novel electronic transport properties at surfaces. For example, the electrical conduction through surface-state bands, which are inherent in the surface superstructure, has been confirmed in in-situ measurements. An important phenomenon has also been found, where adatoms donate carriers into the surface-state band, resulting in a remarkable enhancement in electrical conductance. The nucleation of the adatoms diminishes such a doping effect. Furthermore, electrical conduction through atomic layers grown on the surfaces, whose growth structures are sensitive to the substrate surface structures, will be also discussed. In this review, we emphasize that the surface electronic transport properties are closely related to the atomic structures and atomistic dynamics on surfaces. The ultimate two-dimensional electron systems, consisting of the surface-state bands and grown atomic layers, are expected to provide a new stage in surface physics, as well as a precursory stage leading to atomic-scale electronics devices.

Longitudinal and transverse coupling in infrared gold nanoantenna arrays: long range versus short range interaction regimes

Interaction between micrometer-long nanoantennas within an array considerably modifies the plasmonic resonant behaviour; for fundamental resonances in the infrared already at micrometer distances. In order to get systematic knowledge on the relationship between infrared plasmonic resonances and separation distances dx and dy in longitudinal and transverse direction, respectively, we experimentally studied the optical extinction spectra for rectangularly ordered lithographic gold nanorod arrays on silicon wafers. For small dy, strong broadening of resonances and strongly decreased values of far-field extinction are detected which come along with a decreased near-field intensity, as indicated by near-field amplitude maps of the interacting nanoantennas. In contrast, near-field interaction over small dx does only marginally broaden the resonance. Our findings set a path for optimum design of rectangular nanorod lattices for surface enhanced infrared spectroscopy.

Electrochemical synthesis of mesoporous gold films toward mesospace-stimulated optical properties

Mesoporous gold (Au) films with tunable pores are expected to provide fascinating optical properties stimulated by the mesospaces, but they have not been realized yet because of the difficulty of controlling the Au crystal growth. Here, we report a reliable soft-templating method to fabricate mesoporous Au films using stable micelles of diblock copolymers, with electrochemical deposition advantageous for precise control of Au crystal growth. Strong field enhancement takes place around the center of the uniform mesopores as well as on the walls between the pores, leading to the enhanced light scattering as well as surface-enhanced Raman scattering (SERS), which is understandable, for example, from Babinet principles applied for the reverse system of nanoparticle ensembles.

Surface-Plasmon-Enhanced Photodriven CO2 Reduction Catalyzed by Metal-Organic-Framework-Derived Iron Nanoparticles Encapsulated by Ultrathin Carbon Layers.

Highly efficient utilization of solar light with an excellent reduction capacity is achieved for plasmonic Fe@C nanostructures. By carbon layer coating, the optimized catalyst exhibits enhanced selectivity and stability applied to the solar-driven reduction of CO2 into CO. The surface-plasmon effect of iron particles is proposed to excite CO2 molecules, and thereby facilitates the final reaction activity.

Monitoring the Presence of Ionic Mercury in Environmental Water by Plasmon-Enhanced Infrared Spectroscopy

We demonstrate the ppt-level single-step selective monitoring of the presence of mercury ions (Hg2+) dissolved in environmental water by plasmon-enhanced vibrational spectroscopy. We combined a nanogap-optimized mid-infrared plasmonic structure with mercury-binding DNA aptamers to monitor in-situ the spectral evolution of the vibrational signal of the DNA induced by the mercury binding. Here, we adopted single-stranded thiolated 15-base DNA oligonucleotides that are immobilized on the Au surface and show strong specificity to Hg2+. The mercury-associated distinct signal is located apart from the biomolecule-associated broad signals and is selectively characterized. For example, with natural water from Lake Kasumigaura (Ibaraki Prefecture, Japan), direct detection of Hg2+ with a concentration as low as 37 ppt (37 × 10−10%) was readily demonstrated, indicating the high potential of this simple method for environmental and chemical sensing of metallic species in aqueous solution.

Thin bismuth film as a template for pentacene growth

Thin Bi(001) films grown by ultrahigh vacuum deposition on Si(111)‐7×7 surfaces at room temperature, were annealed at ∼400K in order to improve their morphology by reducing the step density on the surface. Annealed, well-ordered Bi(001) films have been subsequently used as substrates for growth of pentacene (Pn). It has been determined using low-energy electron microscope that Pn nucleates on Bi(001) into a highly ordered, crystalline layer, with Pn molecules “standing up” on the Bi surface, and the (001) plane on the growth front. Moreover, the Pn layer is aligned with the Bi(001) surface having a “point-on-line” commensurate relationship with the substrate. The Pn∕Bi(001) film crystallizes in a bulk-like structure directly from the first Pn layer. Formation of the thin film phase reported for the Pn growth on SiO2 and other inert substrates was not observed in our experiments.

Effective decoration of Pd nanoparticles on the surface of SnO2 nanowires for enhancement of CO gas-sensing performance

Decoration of noble metal nanoparticles (NPs) on the surface of semiconducting metal oxide nanowires (NWs) to enhance material characteristics, functionalization, and sensing abilities has attracted increasing interests from researchers worldwide. In this study, we introduce an effective method for the decoration of Pd NPs on the surface of SnO2 NWs to enhance CO gas-sensing performance. Single-crystal SnO2 NWs were fabricated by chemical vapor deposition, whereas Pd NPs were decorated on the surface of SnO2 NWs by in situ reduction of the Pd complex at room temperature without using any linker or reduction agent excepting the copolymer P123. The materials were characterized by advanced techniques, such as high-resolution transmission electron microscopy, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The Pd NPs were effectively decorated on the surface of SnO2 NWs. As an example, the CO sensing characteristics of SnO2 NWs decorated with Pd NPs were investigated at different temperatures. Results revealed that the gas sensor exhibited excellent sensing performance to CO at low concentration (1-25ppm) with ultrafast response-recovery time (in seconds), high responsivity, good stability, and reproducibility.

Large surface-state conductivity in ultrathin Bi films

In situ microscopic-four-point probe conductivity measurements were performed for ultrathin Bi films on Si(111)-7×7. From the extrapolation of thickness-dependent conductivity and decrease in conductivity through surface oxidization, we found clear evidence of large surface-state conductivity (σSS∼1.5×10−3Ω−1∕◻ at room temperature) in Bi(001) films. For the thinnest films (∼25A), the transport properties are dominated by the highly inert surface states that are Rashba spin-split, and this suggests the possibility of using these Bi surface states for spintronics device application.

Morphology of ultrathin manganese silicide on Si(111)

Abstract Aggregates of Mn atoms adsorbed on Si(111)-(7×7) react with the substrate and form very flat islands terminated with a well ordered ( 3 × 3 ) surface ranging up to several hundreds of angstroms. The Si atoms in the DAS (dimer-adatom-stacking fault) structure mostly adjacent to step edges are stripped off to form the flat silicide islands. Island height and diameter of the silicide islands on the (7×7) surface are found to change drastically by changing the annealing time, annealing temperature, and the Mn coverage after room temperature deposition. Mn adsorption and silicide formation of some noble metal precovered Si(111) surfaces were also studied. Different morphologies were observed on each surface [( 3 × 3 )- Ag , ( 3 × 3 )- Au , (6×6)- Au ] , which indicates the different diffusion lengths and reactivity of the Mn atoms on each surface.

Surface-State Bands on Silicon –Si(111)-√3×√3-Ag Surface Superstructure–

After reviewing the atomic and electronic structures of the Si(111)-?3??3-Ag surface, which have recently been clarified after much research, we describe the experimental confirmations of electrical conduction through its surface-state band. A newborn method, micro-four-point probe, is introduced for conductivity measurements with high surface sensitivity.