Nariaki Okubo

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.

Irradiation effects on reduced activation ferritic/martensitic steels?tensile, impact, fatigue properties and modelling

At temperatures below 400 °C, irradiation often causes hardening and reduction of elongation as well as toughness degradation to a considerable degree. Data, however, indicate that these changes remain in manageable ranges for ITER-TBM application. Moreover, the saturation tendency of these changes with neutron dose suggests that some of the reduced activation ferritic/martensitic steels are feasible even for future DEMO applications. It is also stressed that the development of a design methodology that is compatible with the large irradiation induced property changes is essential to enable these applications. Modelling activities for the macroscopic mechanical response are expected to play key roles in design methodology development. Macroscopic models of plasticity (a constitutive equation) and cyclic softening behaviour after irradiation are discussed. The significance of the models for estimating microstructural change during irradiation and beneficial effects of the heat treatment for irradiation performance are also introduced.

Counterevidence to the ion hammering scenario as a driving force for the shape elongation of embedded nanoparticles

Counterevidence is provided in the ion-hammering scenario as a driving force for the shape elongation of embedded nanoparticles (NPs) under swift heavy ion irradiation (SHII). Ion-induced compaction and the hammering, which are both induced in silica under SHII, dominate at low and high fluences, respectively, causing a crossover between them around a fluence of ∼4 × 1012 ions/cm2. Nevertheless, the shape elongation of NPs detected by the optical dichroism exhibits nearly linear dependence in a wide fluence range between ∼1 × 1011 and 2 × 1013 ions/cm2, indicating that the hammering does not play an important role.Counterevidence is provided in the ion-hammering scenario as a driving force for the shape elongation of embedded nanoparticles (NPs) under swift heavy ion irradiation (SHII). Ion-induced compaction and the hammering, which are both induced in silica under SHII, dominate at low and high fluences, respectively, causing a crossover between them around a fluence of ∼4 × 1012 ions/cm2. Nevertheless, the shape elongation of NPs detected by the optical dichroism exhibits nearly linear dependence in a wide fluence range between ∼1 × 1011 and 2 × 1013 ions/cm2, indicating that the hammering does not play an important role.

Experimental evidence of crystalline hillocks created by irradiation of CeO₂ with swift heavy ions: TEM study.

In this study, CeO2 was irradiated with 200 MeV Au ions at oblique incidence. Observation of as-irradiated samples by transmission electron microscopy (TEM) shows that hillocks are created not only at the wide surfaces, but also at the crack faces of the thin samples. Since the hillocks created at the crack faces can be imaged by TEM, their shape and crystallographic features can be revealed. From the images of hillocks created at the crack faces, many of the hillocks are found to be spherical. We present the first experimental evidence that hillocks created for CeO2 irradiated with swift heavy ions have a crystal structure whose lattice spacing and orientation coincide with those of the matrix. The mechanism of spherical crystalline hillock formation is discussed based on the present results.

Optical birefringence of Zn nanoparticles embedded in silica induced by swift heavy-ion irradiation

Zn nanoparticles (NPs) embedded in a silica matrix subjected to irradiation with swift heavy ions of 200 MeV Xe¹⁴⁺ have been found to undergo shape elongation from spheres to prolate-spheroids while maintaining the major axes of the NPs in parallel alignment. The directionally-aligned Zn spheroids enable acquisition of optical properties, such as linear dichroism and birefringence. In this paper, the birefringence of the Zn spheroids was evaluated by the crossed-Nicols (XN) transmittance, where a sample was inserted between a pair of optical polarizers that were set in an orthogonal configuration. Linearly-polarized light aligned by the first polarizer was transformed to an elliptic polarization by the birefringence of the Zn spheroids. The existence of the birefringence was confirmed by the non-zero transmittance of the second polarizer in the orthogonal configuration. The sample irradiated with a fluence of 5.0 × 10¹³ ions/cm² exhibited a maximum XN transmittance of 2.1% at a photon energy of ~4 eV. The XN transmission was observed down to a fluence of 1.0 × 10¹² ions/cm², but reduced below the detection limit at a fluence of 1.0 × 10¹¹ ions/cm². The possible application of the elongated Zn NPs as a polarizer with nanometric thickness working in the near- and mid-ultraviolet region is discussed.

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.

Swift heavy ion irradiation to ZnO nanoparticles: Steep degradation at low fluences and stable tolerance at high fluences

A mono-layer of ZnO nanoparticles (NPs), each of which does not mostly overlap with one another, was formed on the surface of a silica glass by implantation with 60 keV Zn+ ions and subsequent ther...