Address: 1-1 Namiki, Tsukuba-city, Ibaraki 305--44, Japan
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Papers by Tadaaki NAGAO
Moiré nanosphere lithography: use colloidal moiré patterns as masks
Proceedings of SPIE, Aug 28, 2015
Nanosphere lithography (NSL) uses self-assembled layers of monodisperse micro-/nano-spheres as ma... more Nanosphere lithography (NSL) uses self-assembled layers of monodisperse micro-/nano-spheres as masks to fabricate plasmonic metal nanoparticles. Different variants of NSL have been proposed with the combination with dry etching and/or angled-deposition. These techniques have employed to fabricate a wide variety of plasmonic nanoparticles or nanostructures. Here we report another promising extension - moiré nanosphere lithography (MNSL), which incorporates in-plane twisting between neighboring monolayers, to extend the patterning capability of conventional NSL. In conventional NSL, the masks, either a monolayer or bilayer, are formed by spontaneous self-assembly. Therefore, the resulted colloidal crystal configurations are limited. In this work we used sequential stacking of polystyrene nanosphere monolayers to form a bilayer crystal at the air/water interfaces. During this layer-by-layer stacking process, a crystal domain in the top layer gains the freedom to positon itself in a relative angle to that in the bottom layer allowing for the formation of moiré patterns. Subsequent O2 plasma etching results in a variety of complex nanostructures that have not been reported before. Using etched moiré patterns as masks, we further fabricated the corresponding gold nanostructures and characterized their scattering optical properties. We believe this facile technique provides a new strategy to fabricate novel and complex plasmonic nanostructures or metasurfaces.
In situ Surface-Enhanced Infrared Absorption Spectroscopy for the Analysis of the Adsorption and Desorption Process of Au Nanoparticles on the SiO<sub>2</sub>/Si Surface
Langmuir, Apr 26, 2007
The adsorption and desorption of Au nanoparticles (AuNP) in colloidal D2O suspension on the (3-am... more The adsorption and desorption of Au nanoparticles (AuNP) in colloidal D2O suspension on the (3-aminopropyl)triethoxysilane treated SiO2/Si surface was investigated by in situ attenuated total reflection surface enhanced infrared absorption (ATR-SEIRA) spectroscopy with a liquid flow cell. With increasing surface density of AuNP, the absorption of the vibrational modes of D2O and of the citrate molecules covering the AuNP increases due to SEIRA. Repulsive electrostatic Coulomb forces between the AuNP lead to the saturation of the AuNP surface density at submonolayer coverage. We show that the adsorption kinetics can be investigated by monitoring in situ the molecular vibrational modes of D2O and the citrate molecules. Furthermore, we clarify that the adsorption process can be described very well by a diffusion-limited first-order Langmuir kinetics model. When exposing a saturated AuNP submonolayer to 2-aminoethanethiol (AET)/D2O solution, the AuNP are removed from the surface and the IR absorption of the D2O vibrational modes become weaker again. Taking into account the time dependencies of the OD and the CH peaks, we propose a microscopic model where the AET molecules quickly adsorb on the AuNP by replacing most of the precovering citrate molecules exposed to the AET solution. As this takes place, the AuNP agglomerate-as we could detect with scanning electron microscopy-and are finally removed from the surface.
STM study of Ag film initial stages growth on a GaN(0001) surface grown by MBE
The adsorption and growth of Ag on the GaN(0001) pseudo 1x1-Ga surface was studied by using a com... more The adsorption and growth of Ag on the GaN(0001) pseudo 1x1-Ga surface was studied by using a combined scanning tunneling microscopy (STM)-molecular beam epitaxy (MBE) system. Ag shows high mobility and huge diffusion length on the surface, which results in the formation of large monolayer Ag islands at submonolayer coverage. We report the observation of a drastic change of the Ag growth mode on the GaN(0001) surface, from Stranski-Krastanov growth at low Ag flux (similar to0.8 ML/min) to layer-by-layer growth at a high flux (1 ML/sec). Based on this finding a new approach to obtain flat epitaxial Ag film on the GaN(0001) surface, by using high Ag flux, is demonstrated. In addition, by annealing the Ag film-covered GaN(0001) surface we found a novel unreconstructed Ag-terminated GaN(0001)-1 x 1 and explain its structure by a T-1-site adatom model.
For the full benefit of the silicon chip industry and to further shift the photoresponse cut-off ... more For the full benefit of the silicon chip industry and to further shift the photoresponse cut-off wavelength of the silicon photodetectors, highperformance Ag-doped Si p-n photodiodes with an extended infrared photoresponsivity are constructed on the bulk silicon wafer by a facile thermal diffusion process at 550 ○ C for different annealing periods of 5, 10, and 15 minutes under an argon atmosphere. These Si-compatible p-n photodiodes revealed an obvious zero-bias room temperature photoresponsivity with a threshold photon energy at a longer wavelength compared to the photoresponsivity cut-off wavelength of the commercial Si photodiode of the Hamamatsu Photonics Co (model: S2281/ -04). The photoresponsivity has decreased with the annealing time increase however; the detectivity has been improved by the significant drop in leakage current and noise power. The outcomes indicate that this study paves the way for developing cost-effective Si-compatible p-n junction photodiodes, with an obvious zero-biased room-temperature photoresponsivity of a comparable intensity and longer cut-off wavelength compared to the commercial Hamamatsu Si photodiode.
Improvement of Photocatalytic Efficiency by Adding Ag Nanoparticles and Reduced Graphene Oxide to TiO2
JSAP-OSA Joint Symposia 2017 Abstracts
Titanium dioxide (TiO2) is the commonly used photocatalyst. However, because only a small ultravi... more Titanium dioxide (TiO2) is the commonly used photocatalyst. However, because only a small ultraviolet portion of solar spectrum can excite the electron-hole pairs resulting from the large band gap (3.2 eV) [1] and the recombination rate is high, its efficiency is restrained. To overcome this drawback, we added silver nanoparticles and reduced graphene oxide (RGO) to construct the ternary plasmonic catalyst to improve the catalytic performance of TiO2 nanopowder (P25). We prepared three different geometries of Ag nanostructures including sphere, decahedron and prism because the plasmon resonance properties of Ag could be controlled by the morphology of Ag nanoparticle, which shows characteristic strong localized surface plasmon resonance (LSPR) leading to an increase in light absorption [2]. The incorporated RGO inhibited the charge recombination and enhanced the electron-hole separation. In this study, Ag nanodecahedrons/P25/RGO and Ag nano-prisms/P25/RGO hybrid photocatalysts posse...
Photothermal heating with metallic nanostructures has the unique property of generating heat at t... more Photothermal heating with metallic nanostructures has the unique property of generating heat at the nanoscale owing to plasmon resonances. In this study, the heat transfer of anodic aluminum oxides (AAOs) coated with plasmonic titanium nitride (TiN) of 80 nm thickness are experimentally, numerically, and analytically studied, wherein TiN photothermally generated heat. High optical absorptance and photothermal heating efficiency are observed for the samples with pore sizes in the range of 161–239 nm, and the sample with the pore size of 239 nm exhibits the highest absorptance and photothermal heating efficiency. In addition, the numerical and analytical heat transfer analyses using the effective thermal conductivities for AAO-TiN samples are in reasonable agreement with experimental results, indicating the validity of effective thermal conductivities, which consider the periodic nature. These results can be extended to design other optically absorbing periodic structures for photothe...
In transition metal dichalcogenides, valley depolarization through intervalley carrier scattering... more In transition metal dichalcogenides, valley depolarization through intervalley carrier scattering by zone-edge phonons is often unavoidable. Although valley depolarization processes related to various acoustic phonons have been suggested, their optical verification is still vague due to nearly degenerate phonon frequencies on acoustic phonon branches at zone-edge momentums. Here we report an unambiguous phonon momentum determination of the longitudinal acoustic (LA) phonons at the K point, which are responsible for the ultrafast valley depolarization in monolayer MoSe2. Using sub-10-fs-resolution pump-probe spectroscopy, we observed coherent phonons signals at both even and odd-orders of zone-edge LA mode involved in intervalley carrier scattering process. Our phonon-symmetry analysis and first-principles calculations reveal that only the LA phonon at the K point, as opposed to the M point, can produce experimental odd-order LA phonon signals from its nonlinear optical modulation. O...
We report uniaxially oriented nickel aluminum (NiAl) films grown by DC magnetron sputtering under... more We report uniaxially oriented nickel aluminum (NiAl) films grown by DC magnetron sputtering under in situ heating. The films self-organize in (110) orientation with relatively low surface roughness and tight grain boundaries in columnar structure. The electrical carrier concentration and resistivity are on the order of 10 21 cm −3 and 10 −5 cm, respectively. The plasmonic performance, as indicated by the dielectric function, is comparable to that of a NiAl single crystal and exceeds those of conventional refractory materials (molybdenum, tungsten, titanium nitride) at visible to NIR wavelengths. This work paves the way to superalloy-based plasmonic nanostructures for photothermal energy applications.
Photoelectrochemical (PEC) reaction pioneered by Fujishima and Honda has widely emerged as a pote... more Photoelectrochemical (PEC) reaction pioneered by Fujishima and Honda has widely emerged as a potential approach to convert solar energy into chemical energy[1]. Various photoelectrode materials have been explored, among which SrTiO3 (STO) has gained importance since it can decompose H2O into H2 and O2 without applying an external bias potential as its conduction band level is higher than the redox potential for the H2 evolution[2]. To improve the PEC activities of STO, doping and/or coupling with metal nano-particle has been adopted. It has been shown that coupling with metal nanoparticle like Au/Ag promotes light absorption in the visible spectral region, which is termed as hot electron injection that enhances anodic photocurrent[3]. On the other hand, although it has been demonstrated that Rh-Doping induces P-type nature[4], there have been very few efforts towards enhancing cathodic photocurrent.
Plasmonic nanoparticles/reduced graphene oxide sensitized hierarchical TiO2 nanorods for solar water splitting
Metallic nanoparticles exhibiting distinctive localized surface plasmon resonance (LSPR) have bee... more Metallic nanoparticles exhibiting distinctive localized surface plasmon resonance (LSPR) have been widely employed for photoelectrochemical (PEC) water splitting, due to their remarkable optical scattering as well as near-field nanofocusing of light, which efficiently enhance the photoactivity of semiconductors [1].
Fermi level dependence of ultrafast optical responses of single-layer graphene has been investiga... more Fermi level dependence of ultrafast optical responses of single-layer graphene has been investigated using sub-10-fs pump-probe spectroscopy under bias voltages. We observe the ultrafast thermalization of photoexcited carriers, whose dynamics can be modulated via bias-induced change of the Fermi level. The relaxation time and the amplitude of the electronic response are maximized when the Fermi level reaches approximately half of the excitation photon energy. From the analysis of the pump-pulse-induced optical conductivity change, we find that the bias-induced blocking of the relaxation pathways and the pump-induced change of the electronic temperature and the Fermi level significantly contribute to the observed ultrafast optical modulation. The results demonstrate the controllability of the ultrafast optical responses in single-layer graphene, which could be useful for future ultrafast electro-optic graphene devices.
Photochromic microresonator arrays with whispering gallery mode fingerprints are successfully pre... more Photochromic microresonator arrays with whispering gallery mode fingerprints are successfully prepared, which function as high-security optical authentication microdevices.
The release of industrial untreated wastewater creates a hazardous impact on the environment. In ... more The release of industrial untreated wastewater creates a hazardous impact on the environment. In this regard, the development of environment friendly catalyst is of paramount importance. Here, we report a highly efficient and reusable core-shell TiN/SiO2/Cr-TiO2 (TSCT) photocatalyst which is composed of SiO2-cladded titanium nitride (TiN) nanoparticles (NPs) decorated with Cr-doped TiO2 NPs for the removal of organic contaminant from water. The TiN NPs serve as the main light absorber component with excellent visible light absorption along with Cr-TiO2 NPs. The TSCT shows remarkable improvement in the photodecomposition of methylene blue (MB) over Cr-TiO2 and TiO2 NPs. An efficient structural design is proposed by adopting calcium alginate beads (P-Marimo beads) as transparent scaffold for supporting our TSCT which exhibits floatable nature on the water surface and realizes easy handling as well as excellent reusability for multipurpose water purification. Surprisingly, our TSCT is found to keep its catalytic activity even after the
We report results of terahertz Faraday and Kerr rotation spectroscopy measurements on thin films ... more We report results of terahertz Faraday and Kerr rotation spectroscopy measurements on thin films of Bi1-xSbx, an alloy system that exhibits a semimetal-to-topological-insulator transition as the Sb composition x increases. By using a single-shot time-domain terahertz spectroscopy setup combined with a table-top pulsed mini-coil magnet, we conducted measurements in magnetic fields up to 30 T, observing distinctly different behaviors between semimetallic (x < 0.07) and topological insulator (x > 0.07) samples. Faraday and Kerr rotation spectra for the semimetallic films showed a pronounced dip that blue-shifted with the magnetic field, whereas spectra for the topological insulator films were positive and featureless, increasing in amplitude with increasing magnetic field and eventually saturating at high fields (>20 T). Ellipticity spectra for the semimetallic films showed resonances, whereas the topological insulator films showed no detectable ellipticity. To explain these observations, we developed a theoretical model based on realistic band parameters and the Kubo formula for calculating the optical conductivity of Landau-quantized charge carriers. Our calculations quantitatively reproduced all experimental features, establishing that the Faraday and Kerr signals in the semimetallic films predominantly arise from bulk hole cyclotron resonances while the signals in the topological insulator films represent combined effects of surface carriers originating from multiple electron and hole pockets. These results demonstrate that the use of high magnetic fields in terahertz magnetopolarimetry, combined with detailed electronic structure and conductivity calculations, allows us to unambiguously identify and quantitatively determine unique contributions from different species of carriers of topological and nontopological nature in Bi1-xSbx.
Thermochromic material vanadium dioxide (VO 2) has been extensively studied as a smart window mat... more Thermochromic material vanadium dioxide (VO 2) has been extensively studied as a smart window material, which could change its optical properties with temperature due to its phase transition. A large percentage of the visible and thermal infrared light are expected to transmit in its cold dielectric state, while VO 2 can regulate infrared transmittance in its hot metallic state. In this study, we proposed a periodic hexagonal array of VO 2based disks to achieve improvement of visible transmittance and maintain the outstanding solar-heat modulation ability. The VO 2 thin film (50 nm) on our proposed textured substrate showed~5% higher transmittance in the visible region at 555 nm than the planar VO 2 thin film. This work provides an insight into the structural design of VO 2-based smart window with enhanced visible light transmittance and effective infrared shielding at high temperatures.
Using terahertz-pump and terahertz-probe spectroscopy, we investigated terahertz-induced carrier ... more Using terahertz-pump and terahertz-probe spectroscopy, we investigated terahertz-induced carrier generation processes in Bi1-xSbx thin films. The field dependence of the terahertz-induced transmittance change indicates distinct nonlinearity related to the Zener tunneling in narrow band-gap materials.
Terahertz-field-induced carrier generation processes were investigated in Dirac electron systems,... more Terahertz-field-induced carrier generation processes were investigated in Dirac electron systems, single-crystalline bismuth antimony alloy thin films (Bi1-xSbx; ). This investigation was performed by precisely tuning, via the substituent ratio x, the band structure of the films from that associated with a semimetal to that characteristic of a narrow-gap semiconductor. Terahertz-field-induced absorption was clearly observed within a few picoseconds after the terahertz pump-pulse illumination of Bi1-xSbx semimetal and semiconductor samples. The field-strength dependence of the induced absorption was compared with the calculated Zener tunneling probability in the Dirac-like band dispersion. Through this comparison, the mechanism of the induced absorption was attributed to the carrier generation via the terahertz-field-induced Zener tunneling. The tunneling occurred in sub-picosecond timescales even at room temperature, demonstrating that Bi1-xSbx thin films are promising for future high-speed electronics and the investigation of universal ultrafast tunneling dynamics.
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Papers by Tadaaki NAGAO