Absorption spectra of Ni nanoparticles in silica glass (SiO2) fabricated by negative-ion implantation of 60 keV Ni to 4x1016 ions/cm2 were determined from three sets of spectra, i.e., transmittance, reflectance of implanted-surface side and that of rear-surface side, of the same samples, to exclude incoherent multiple reflection (ICMR) due to substrates. Although the absorption spectrum of as-implanted state is smeared with defect absorption, two absorption bands at 3.3 and 6.0 eV due to Ni nanoparticles are observed after annealing at 800°C in vacuum. However, a predicted peak energy from a criterion for surface plasmon resonance (SPR), εm'(ω) + 2 εd'(ω) = 0, was in 2.8 eV, far away from the observed peaks. Another criterion, (εm' + 2εd')2 + (εm'')2 = minimum, gives the peak energy of 5.9 eV. From decomposition of the dielectric constants into free- and bound-electron contributions, we conclude that the 3.3 eV peak is SPR-like, although the contribution of the bound-electrons to the 3.3 eV peak is not small. Size dependence also supports the assignment of the 3.3 eV peak. The large contribution of the bound electrons is due to a nature of the partially filled 3d orbitals of Ni. This is contrast to the closed 3d orbitals of Cu, and probably is the origin of the broad peak width.
Linear and nonlinear optical properties of Ta nanoparticle composites, fabricated by negative ion implantation, were studied in the visible range. Negative Ta ions of 60 keV were implanted into amorphous SiO2 and crystalline TiO2 with a total dose of 3× 1016 ions/cm2. Optical absorption evidently indicated a surface plasmon peak around 2 eV and the peak resulted from formation of nanoparticles embedded in the matrix. The plasmon peak shifted with dependening on the refractive index of the substrates. The laser-induced transient absorption was measured with the technique of pump-probe femtosecond spectroscopy. The transient absorption of the Ta nanoparticle composite in SiO2 recovered in several picoseconds due to energy transfer from the excited electrons to the lattice via the electron-phonon interaction. The transient response was comparable to that of the noble metals. The electron-phonon coupling was evaluated by the two-temperature model and the coupling constant yielded a value of g = 3.1 × 1018 W/m3K. The Ta nanoparticle composite has the advantage of thermal stability in comparison with Cu nanoparticle composites.
The magnetic nanoparticles are fabricated in silica glass (SiO2) using high-flux implantation of nickel negative-ions of 60 keV. Photo-absorption measurements and the cross-sectional transmission electron microscopy (XTEM) observation confirm formation of metallic Ni nanoparticles in SiO2, and exclude possible formation of Ni silicides (Ni3Si, Ni2Si, NiSi) and oxides (NiO) as major products. The mean diameter of the nanoparticles was in ~2.9 nm, and the depth distribution was similar to the prediction from the TRIDYN code with taking account of the sputtering. Temperature- and field- dependences of magnetization show that the nanoparticles are in the super-paramagnetic state with a blocking temperature of ~27 K.
Steady-state and laser-induced transient absorption around the surface plasmon resonance of copper nanoparticle composites, fabricated by ion implantation, have been studied by optical measurements. Negative ion implantation has been applied to generate the Cu nanoparticles in amorphous SiO2, crystalline MgOn(Al2O3), LiNbO3, SrTiO3 and TiO2 with various refractive indices and optical energy gaps. The surface plasmon resonance in the steady-state absorption resulted from formation of nanoparticles in the substrates and shifted to red with increasing refractive index of the matrix. The nanoparticle fabrication by the negative ion implantation was succeeded in all the insulating substrates used, and it is capable to tune the resonance band to 1.7 - 2.2 eV (730 - 560 nm) by selecting of the matrix. However, there remained a problem that the plasmon band in LiNbO3, SrTiO3 and TiO2 with narrow energy gap overlapped radiation-induced defect band. Laser-induced transient absorption was measured with a technique of pump-probe femtosecond spectroscopy. The bleaching plasmon band recovered in several picoseconds due to energy transfer from the excited electron system to the phonon system via the electron-phonon interaction. The transient absorption is also affected by radiation damage in the matrices with the narrow energy gap.
High-energy ion implantation is one of the unique methods to fabricate nano-scale structures, taking advantage of the spatial controllability and the non-equilibrium atomic injection. Metal-ion implantation into a transparent insulator creates a metal nanoparticle composite, which is promising as a nonlinear optical material with ultrafast response. Radiation damage of substrates and/or nanoparticles, which is inherent in ion implantation, is a drawback for optical performance of the nanoparticle composites. It is desired to annihilate radiation damage without melting the matrix. We have applied laser irradiation of sub-gap energy during heavy-ion implantation. Copper ions of 3 MeV and laser of a sub-gap energy (2.3 eV) irradiated insulators of a-SiO2 and spinel MgO•2.4(Al2O3). The dose rate varied up to 10 (mu) A/cm2 for Cu ions of 3 MeV and up to 0.2 J/cm2•pulse at 10 Hz for YAG-SHG laser. Only when ions and photons were simultaneously irradiated at the higher photon intensity (> 0.1 J/cm2•pulse), the insulators were effectively bleached in the optical absorption spectra. As well as the bleaching, precipitation enhancement and atomic desorption took place. The results indicate importance of dynamical electronic excitation during ion irradiation and that the photon irradiation enhances atomic displacements either at the surface or in the bulk.