Naoshi Takahashi

Direct conversion of silver complexes to nanoscale hexagonal columns on a copper alloy for plasmonic applications

We introduced a novel method for the rapid synthesis of silver nanohexagonal thin columns from an aqueous mixture of sodium thiosulfate (Na2S2O3) and silver chloride (AgCl) simply added to a phosphor bronze substrate. The reaction is based on galvanic displacement and the products are potentially useful for plasmonic applications.

Construction of a Compact Spin- and Angle-Resolved Photoelectron Spectrometer

A compact photoelectron spectrometer has been constructed to analyze energy, momentum and spin of the electronic states in condensed matter. The spectrometer consists of an electron energy analyzer, lens systems and a spin detector. The spin detector is a low-energy diffuse scattering type instrument with a potential of about 100–250 eV. The detector consists of a 90° deflector, a gold target, a four-section anode and lens systems. The detector is about 0.15×0.05×0.05 m3 in size and is rotatable along a single axis in the analysis chamber under ultra-high vacuum conditions. The parameters of the optical elements have been determined through optimization of the photoelectrons from GaAs(100) and with reference to the ray tracing calculation. The performance of the spin detector has been verified by observing the spin-polarized electrons from O/Cs/GaAs(100).

Photoelectron Spectroscopic Study of Coadsorbed States of Cs and O on GaAs(100)

The electronic states of O/Cs/GaAs(100) have been investigated by using ultraviolet photoelectron spectroscopy in order to understand the initial coadsorption kinetics on the way to the negative electron affinity formation. It was found that the energy positions of the Ga-3d and As-3d peaks shift to the lower binding energy side by the first Cs deposition, while they shift to the higher binding energy side during the n-th Cs deposition ( n≧3). It was also found that besides the appearance of oxidized As, the width of the Ga-3d peak becomes larger by the first oxygen exposure. In the case of the n-th Cs deposition and oxygen exposure, all photoelectron peaks and the work function made parallel shifts in energy, which can be well interpreted in terms of the change in band bending. These results indicate that the coadsorption kinetics can be classified into two stages: At first Cs deposition and oxygen exposure, both of Ga and As react with Cs and O, resulting in the rearrangement in the surface layers, whil...

Ultrathin epitaxial Fe films on vicinal GaAs(001): A study by spin-resolved photoelectron spectroscopy

Thin epitaxial Fe films have been grown on vicinal GaAs(001) substrates and their remanent magnetic properties and the degree of substrate atom diffusion investigated using synchrotron-based photoelectron spectroscopy. The vicinal Fe films, though exhibiting greater As diffusion than their singular homologues, displayed better film quality both from the structural and the magnetic points of view. The spin-resolved valence spectra of the vicinal films resemble those for crystalline bulk Fe at lower film thicknesses than for singular films.

Formation mechanism of plasmonic silver nanohexagonal particles made by galvanic displacement reaction

The galvanic displacement reaction (GDR) is a powerful method for the preparation of various plasmonic nanostructures within several minutes. However, the formation mechanism of the nanostructures, which retain plasmonic hotspots, still remains unclear. In this work, X-ray photoelectron spectroscopy (XPS) is applied to silver nanostructures made by newly-discovered GDR between Ag complex solutions and Cu alloy substrates, whose nanostructures form characteristic nanoscale hexagonal columns (NHCs) and generate strong surface-enhanced Raman scattering (SERS) signals of adsorbates. Detailed depth profiles by multi-element XPS analysis revealed that NHCs are made of Ag metallic cores covered with surface layers composed of copper sulfates and copper oxides, which prevent NHCs from fusion, resulting in highly concentrated stable hotspots. These findings explain why NHCs exhibit reproducible SERS signals and the proposed methodology gives new insights for efficient creation of plasmonic nanostructures in a few minutes.

Structure and element-specific magnetic moments of epitaxial face-centered-cubic FexNi1−x alloy ultrathin films

By core level x-ray photoelectron spectroscopy using an x-ray tube, ultrathin films of FexNi1−x alloys deposited epitaxially on a Cu(001) substrate have been investigated. Structural information is obtained by means of core level photoelectron diffraction. The angular dependent intensity scans reveal peaks corresponding to a face-centered-cubic lattice in the covered concentration range 0.08<x<0.9. Information on the local magnetic moments is obtained by observing the intensity asymmetry occurring when switching between directions of opposite magnetization (MLDAD). Both the Fe 2p3/2 and Ni 2p3/2 asymmetries show a maximum at about 50% Fe concentration.

Surface core exciton of alkali chloride film on Si(100)

Abstract Surface core exciton has been observed in the partial-photoelectron-yield spectra in the Cl-L 2,3 region of alkali chloride (KCl and NaCl) film deposited on the Si(100) surface. Compared to the bulk core exciton peaks, the surface core exciton peaks were shifted by 0.7 eV towards lower photon energy. This may be to the change in Madelung potential at the surface. The photoelectron spectra was measured with various photon energies near the Cl-L 2,3 edge for NaCl/Si(100), and it was found that the Auger spectra drastically change in shape at eht excitation energies corresponding to the bulk and surface core excitons.

SPIN-RESOLVED PHOTOELECTRON SPECTROSCOPY OF ULTRATHIN Fe FILMS ON GaAs(001)

Thin epitaxial Fe films were grown on singular and vicinal GaAs(001) substrates, and their magnetic and electronic structures were investigated by synchrotron-based spin-resolved and spin-integrated photoelectron spectroscopy with different Fe thickness. There were two types of substrates: one was a Si-doped n-type GaAs(001) surface with doping concentration of 2 × 1018cm-3 (singular substrate), and the other was orientated by 3° toward the (111)A direction (vicinal substrate). Spin polarization of the secondary electron peak at different growth stages of Fe coverage for the singular substrate sample and the vicinal one were measured. In the case of singular substrates, there was a dependence of their initial surface reconstruction, which is associated with complex domain structure, while no such the dependence was observed in the case of vicinal substrates. The result from the vicinal sample suggests the geometrical influence of the initial surface stoichiometry of the substrate.

Photoelectron and luminescence spectra of BaFCl, BaFBr, and CsTaF6

The photoelectron spectra and the excitation spectra of the ultraviolet luminescence in BaFCl anaFBr, and CsTaF6 have been investigated. It was found that the valence bands of BaFCl and CsTaF6 consist of mixing states of the constituents, whereas those of BaFBr are separated into two branches. Its was also found that the luminescence intensity of BaFBr is much enhanced around the excitation energy of the Ba-4d holes, while the enhancements in BaFCl and CsTaF6 due to the decay of the 4d holes are very small.

Nanosecond desorption of alkali fluorides excited by synchrotron radiation pulses

Abstract Desorption of the excited-state alkali atoms has been investigated on alkali fluoride crystals in the photon energy region of 20–75 eV, which includes the excitation energy for Li 1s-core excitons, Na 2p-core excitons, and valence excitations. The quantum yield of the excited-state alkali desorption was found to be comparable between valence and core-level excitations. Time response of desorption from LiF and NaF was also observed by using synchrotron radiation pulses. It was found that the time response consists of a nanosecond component and slower one, indicating that the fast desorption is due to the lattice instability induced by electronic transition and the slow one is due to the thermal instability of surface defects.