Ion Tracks and Nanohillocks Created in Natural Zirconia Irradiated with Swift Heavy Ions.

Natural monoclinic zirconia (baddeleyite) was irradiated with 340 MeV Au ions, and the irradiation-induced nanostructures (i.e., ion tracks and nanohillocks) were observed using transmission electron microscopy. The diameter of the nanohillocks was approximately 10 nm, which was similar to the maximum molten region size calculated using the analytical thermal spike model. Ion tracks were imaged as strained regions that maintained their crystalline structure. The cross-sections of most of the ion tracks were imaged as rectangular contrasts as large as 10 nm. These results strongly indicated that the molten region was recrystallized anisotropically, reflecting the lattice structure. Furthermore, low-density track cores were formed in the center of the ion tracks. The formation of low-density track cores can be attributed to the ejection of molten matter toward the surface. A comparison of the ion tracks in the synthetic zirconia nanoparticles and those in larger natural zirconia samples showed that the interface between the strained track contrast and the matrix was less clear in the former than in the latter. These findings suggest that the recrystallization process was affected by the size of the irradiated samples.

Ag nanoparticles embedded in Nd:YAG crystals irradiated with tilted beam of 200 MeV Xe ions: optical dichroism correlated to particle reshaping.

We report on the fabrication of reshaped Ag nanoparticles (NPs) embedded in a Nd:YAG crystal by combining Ag ion implantation and swift heavy Xe ion irradiation. The localized surface plasmon resonance (LSPR) effect is proved to be efficiently modulated according to the phenomenon of polarization-dependent absorption. The LSPR peak located at 448 nm shows red shift and blue shift at 0° and 90° polarization, respectively, which is in good agreement with calculation by discrete dipole approximation. Based on the near-field intensity distribution, the interaction between reshaped NPs shows a non-ignorable effect on the optical absorption. Furthermore, the polarization-dependence of the photoluminescence (PL) intensity is analyzed, which is positively related to the modulated LSPR absorption. It demonstrates the potential of the enhancement of PL intensity by embedded plasmonic Ag NPs. This work breaks the conventional view of the quenching effect of NPs by ion irradiation and opens a new way to realize the modulation of optical dichroism.

Dynamic nuclear polarization of electron-beam irradiated polyethylene by pairs of alkyl free radicals.

We compared dynamic nuclear polarization (DNP) behavior of electron-beam irradiated polyethylene (e-PE) with that of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-doped polystyrene (T-PS). Growth and decay rates of nuclear polarization P(n) of e-PE increased linearly with increasing concentration C(e) of free radicals, whereas those of T-PS increased with square of C(e). From these results, we suggest that the pair of alkyl radicals produced by the radiolysis, -CH(2)-CH(2)-CH(2)-→-CH(2)-CH()-CH(2)-+H(), and the subsequent hydrogen abstraction reaction, H()+-CH(2)-CH(2)-CH(2)-→H(2)+-CH(2)-CH()-CH(2)- in e-PE plays an important role on DNP by the cross effect, whereas T-PS is polarized by a pair of unspecified TEMPO radicals accidentally located nearby. We propose that higher P(n) would be expected by optimizing the inter-radical distance within the pair in e-PE and other irradiated samples.

Thermal stability of embedded metal nanoparticles elongated by swift heavy ion irradiation: Zn nanoparticles in a molten state but preserving elongated shapes.

Solid Zn and V nanoparticles (NPs) embedded in silica were elongated by swift heavy ion (SHI) irradiation with 200 MeV Xe(14+) ions to a fluence of 5.0 × 10(13) ions cm(-2). Isochronal annealing was carried out in a vacuum from 200 to 1000 °C in steps of 100 °C for 10 min each. The degree of shape elongation was evaluated at room temperature (RT) by two different optical methods: linear dichroism spectroscopy and birefringence spectroscopy. In the as-irradiated state, the samples showed an absorption band at 5 eV due to radiation-induced defects in the silica in addition to the anisotropic absorption due to the elongated metal NPs. After annealing at 400 °C the defect band had completely disappeared, while the degree of shape elongation was almost unchanged or rather slightly increased in both the Zn and V NPs. The elongation of the Zn NPs slightly decreased but maintained a certain value after annealing at 500 °C, which is much higher than the melting point (MP) of Zn NPs (~420 °C). This observation indicates that shape elongation is mostly maintained even if the Zn NPs are in the molten state to some extent during annealing. The elongation of the Zn NPs was almost eliminated after annealing at 600 °C. In the case of the V NPs, elongation was maintained up to 800 °C but mostly eliminated at 900 °C. Since the recovery temperature of 900 °C from the elongated to the spherical shape is much lower than the MP of bulk V (1890 °C), we consider that the elongation is eliminated without melting of V NPs, i.e. via solid state mass transportation. The melting of NPs is not the key factor for the recovery to the spherical shape.