Yoshihiko Takeda

High-Current Heavy-Ion Accelerator System and Its Application to Material Modification

A high-current heavy-ion accelerator system has been developed to realize intense particle fluxes for material modification. The facility of a tandem accelerator attained 1 mA-class ion current both for negative low-energy ions and positive high-energy ions. A negative ion source of the key device is of plasma-sputter type, equipped with multi-cusp magnets and Cs supply. The intense negative ions are either directly used for material irradiation at 60 keV or further accelerated up to 6 MeV after charge transformation. Application of negative ions, alleviating surface charging, enables us to conduct low-energy high-current irradiation to insulating substrates. Since positive ions over MeV are irrelevant to the Coulomb repulsion, the facility as a whole meets high-current irradiation into insulators over a wide energy range. Application of high flux ions provides technological merits not only in efficient implantation but also in different material kinetics. Other characteristics of the system are co-irradiation of intense laser and in-situ detection of kinetic processes. For the material modification, we present nanoparticle fabrication in insulators, and synergistic phenomena by co-irradiation of ions and photons.

X-Ray Emission Induced by 60 keV High-Flux Copper Negative-Ion Implantation

X-ray emission induced by high-flux 60 keV Cu negative ion implantation into silica glasses (a-SiO2) has been studied in the energy range of 0.6–20 keV. At low ion fluxes, the emission spectrum consists of a strong Cu(L) line at 0.95 keV only, without Cu(K) and Si(K) lines. The result is explained by the electron promotion through the quasi-molecule formation. With increasing ion flux, new peaks appear at 1.8, 2.5 and 3.2 keV. These peaks are ascribed to the sum-peak effect under high-flux implantation. Judging from the cross sections and the time dependence, the observed Cu(L) X-ray is concluded to be generated via Cu–O collisions.

Spectral investigation of nonlinear local field effects in Ag nanoparticles

The capability of Ag nanoparticles to modulate their optical resonance condition, by optical nonlinearity, without an external feedback system was experimentally demonstrated. These optical nonlinearities were studied in the vicinity of the localized surface plasmon resonance (LSPR), using femtosecond pump-and-probe spectroscopy with a white-light continuum probe. Transient transmission changes ΔT/T exhibited strong photon energy and particle size dependence and showed a complex and non-monotonic change with increasing pump light intensity. Peak position and change of sign redshift with increasing pump light intensity demonstrate the modulation of the LSPR. These features are discussed in terms of the intrinsic feedback via local field enhancement.

Time of flight-secondary ion mass spectrometry analysis of protein adsorption on a polyvinylidene difluoride surface modified by ion irradiation

We investigated the effects of nanoscopic surface modification of polyvinylidene difluoride (PVDF) and low-density polyethylene (LDPE) by plasma-based ion implantation on protein adsorption with time of flight-secondary ion mass spectrometry (ToF-SIMS) analysis. The chemical composition of the LDPE and PVDF surfaces was changed by ion irradiation. In particular, irradiation substantially decreased the number of CH and CF bonds on the PVDF surface, but only slightly decreased that of CH bonds for LDPE. These decreases may reflect a higher hydrogen recombination rate of the LDPE than the PVDF surface. An increase in oxygen was observed on both the LDPE and PVDF surfaces following ion irradiation, but was saturated after irradiation of 1×1015cm-2 on the PVDF surface. The hydrophilicity of the ion-irradiated LDPE surface was promoted with an increase of the total ion fluence. Ion irradiation also changed the surface properties of PVDF to become more hydrophilic, but the variation did not correlate with the total ion fluence presumably due to the presence of fluorine atoms and the saturation of oxidation. Both bovine serum albumin (BSA) and collagen adsorption were suppressed on the LDPE surface by ion irradiation, which may have resulted from a decrease of the hydrophobic interaction. By contrast, ion irradiation increased protein adsorption on the PVDF surface, and BSA was adsorbed more than collagen, whereas there was no difference in the adsorption between BSA and collagen on the ion-irradiated LDPE surface. Moreover, the adsorption of BSA decreased on the oxygen- and fluorine-rich PVDF surface. These results indicate that the nanoscopic composition changes on the PVDF surface affect the adsorption behavior of BSA. Specifically, ferroelectric property on the PVDF surface was changed by ion irradiation and the nanoscopic change in polarity presumably affected the protein adsorption. Our findings suggest that selective adsorption control of protein can be achieved by ion irradiation to PVDF surface.

Experimental dispersion of the third order optical susceptibility of Ag nanoparticles

We have experimentally investigated the dispersion of the third order optical susceptibility χ(3) of silver nanoparticles embedded in silica glass in the vicinity of the surface plasmon resonance. The dispersion of the real and imaginary parts of the effective third order optical susceptibility χeff(3) was evaluated from the effective refractive index using a spectroscopic ellipsometry and the transient transmission and reflection changes using a femtosecond pump probe spectroscopy. The Imu2009χeff(3) exhibits a minimum value of −1.3×10−17u2009u2009m2/V2 at 3.03xa0eV. The results demonstrate that the local field factor greatly contributes to the dispersion of χeff(3) for Ag nanoparticles.

RADIATION PHOTONICS: A CASE OF METAL-NANOPARTICLE COMPOSITES

A multilayer non-linear optical structure based on the metal nanoparticles has been fabricated by sequential vapor deposition of silica and implantation of 60 keV Cu- ions. The multilayer structure showed an enhanced non-linear optical response.

Quantum Size Effects in the Intrinsic Nonlinearity of Metal Plasmonic Nanoparticles

We clarified the quantum size effects of the intrinsic nonlinearity in metal plasmonic nanoparticles. Understanding the underlying mechanisms in the nonlinear regime is a crucial step for controlling the light at nanoscale.

In-Situ Spectroscopy of Ion-Induced Photon Emission During Metal Nanoparticle Formation in Silica Glass with High-Flux Cu Implantation

Ion-induced photon emission from a silica glass irradiated with high-flux Cu ions was measured in a wavelength range from 450 nm to 800 nm, while nanoyarticles spontaneously formed in the silica glass. Current density was varied up to 100 µA/cm 2 at a constant total dose of 3×10 6 ions/cm 2 . The photon emission primarily arose from the vicinity of the substrate surface and consisted of sharp peaks due to neutral and singly-ionized species, Cu(I), Cu(II) and Si(II) ions, as well as a broad-band background. Intensity of Si(II) and Cu(I) increased with increasing current density. On the other hand, Cu(II) did not show a monotonic increase, decreasing around 100 µA/cm 2 . Measurements of in-situ EDX and ex-situ RBS were also conducted to study the relevant mechanisms. The ion-induced photon emission was attributed to recombination processes of sputtered ions and electrons in the plasma, induced by the high-flux Cu beam.