Radiation response of Y3Al5O12 and Nd3+-Y3Al5O12 to Swift heavy ions: insight into structural damage and defect dynamics.

Understanding the behavior of a material under irradiation is paramount to its application in the nuclear industry. The present work explores the radiation response of garnet Y3Al5O12 (YAG) and Nd3+-substituted Y3Al5O12 (Nd-YAG) under a 100 MeV Iodine beam at varying fluences to mimic the effect of fission fragments. This is relevant to the potential application of garnet as a host for minor actinide (MA) transmutation (Nd3+: surrogate for long-lived MA (Am3+, Np3+, Cm3+)). The un-irradiated and irradiated YAG and Nd-YAG samples were investigated by X-ray diffraction and Raman spectroscopy. Positron annihilation spectroscopy, thermal spike modelling and theoretical studies have been employed to understand the role of substitution and defect energetics in influencing this radiation response. Although both materials were not completely amorphized under the present irradiation conditions, a tremendous loss in crystallinity could be observed with increase in fluence, the damage being much more in Nd-YAG. Ion track radii of 2.17 nm and 2.91 nm were estimated for YAG and Nd-YAG respectively. Thermal-spike calculations show an increase in radiation-induced transient temperatures upon Nd-substitution that causes greater radiation damage in Nd-YAG. The enhancement in radiation-induced damage with increasing ion-fluence manifests in broadening and weakening of the Raman modes and XRD peaks. An increase in the average positron annihilation lifetime indicated the creation of oxygen vacancies. The defect formation energies of Y3Al5O12 have been theoretically estimated via density functional theory (DFT) and unfavorable energies required for creating cation pair anti-sites have been proposed as one of the possible reasons for the relatively poorer radiation response of YAG. The irradiation behavior of Y3Al5O12 has been compared with disordered fluorite (YSZ) and zirconate pyrochlores, which are well-researched ceramics for MA transmutation.

An assessment on crystallization phenomena of Si in Al/a-Si thin films via thermal annealing and ion irradiation.

In the present study, crystallization of amorphous-Si (a-Si) in Al/a-Si bilayer thin films under thermal annealing and ion irradiation has been investigated for future solar energy materials applications. In particular, the effect of thickness ratio (e.g. in Al : a-Si, the ratio of the Al and a-Si layer thickness) and temperature during irradiation on crystallization of the Si films has been explored for the first time. Two sets of samples with thickness ratio 1 : 1 (set-A: 50 nm Al/50 nm a-Si) and thickness ratio 1 : 3 (set-B: 50 nm Al/150 nm a-Si) have been prepared on thermally oxidized Si-substrates. In one experiment, thermal annealing of the as-prepared sample (of both the sets) has been done at different temperatures of 100 °C, 200 °C, 300 °C, 400 °C, and 500 °C. Significant crystallization was found to initiate at 200 °C with the help of thermal annealing, which increased further by increasing the temperature. In another experiment, ion irradiation on both sets of samples has been carried out at 100 °C and 200 °C using 100 MeV Ni7+ ions with fluences of 1 × 1012 ions per cm2, 5 × 1012 ions per cm2, 1 × 1013 ions per cm2, and 5 × 1013 ions per cm2. Significant crystallization of Si was observed at a remarkably low temperature of 100 °C under ion irradiation. The samples irradiated at 100 °C show better crystallization than the samples irradiated at 200 °C. The maximum crystallization of a-Si has been observed at a fluence of 1 × 1012 ions per cm2, which was found to decrease with increasing ion fluence at both temperatures (i.e. 100 °C & 200 °C). The crystallization of a-Si is found to be better for set-B samples as compared to set-A samples at all the fluences and irradiation temperatures. The present work is aimed at developing the understanding of the crystallization process, which may have significant advantages for designing crystalline layers at lower temperature using appropriate masks for irradiation at the desired location. The detailed mechanisms behind all the above observations are discussed in this paper.

Localized Surface Plasmon Resonance Studies on Pd/C Nano-Composite System: Effect of Metal Concentration and Annealing Temperature.

The effects of metal concentration and annealing temperature on the localized surface plasmon resonance (LSPR) properties of the Pd nanoparticles (NP) dispersed in carbon were investigated. The Pd/C nano-composite thin films with 7 to 39 atomic % concentration of metal content were deposited using the atom beam co-sputtering techniques and subjected to annealing at temperature varying from 300 °C to 600 °C. The UV-vis spectroscopy studies on as-prepared films displayed a Mie scattering profile, but not well-defined LSPR bands were observed for all the values of Pd concentration. This is attributed to the smaller size (3-4 nm) of Pd NPs and rough Pd/C interface, as confirmed from TEM studies. When samples were annealed at a temperature of 300 °C, three broad LSPR absorption bands in the visible region, along with a sharp peak at 210 nm, were observed and the effect of Pd concentration variation was insignificant on their position. The multiple LSPR bands were observed due to agglomeration NPs, which is consistent with earlier reports and is also observed in the TEM images. When annealing temperature was subsequently increased to 500 °C, a blue shift in the LSPR peak position with an increase in the Pd concentration was observed, which phenomena is attributed to the formation of bigger NPs with the formation of sharp NPs-interface at high temperature upon annealing. A monotonic increase in the magnitude and decrease in the FWHM with an increase in concentration suggested change in the dielectric function of sample due to the growth of NPs. This is further confirmed from XRD studies, where strain relaxation and grain growth were observed. The intensity of the SPR peak decreased with an increase in the annealing temperature. The LSPR peak disappeared on annealing at a temperature of 600 °C, suggesting the formation of continuous polycrystalline thin films of Pd. In summary, NPs size, metalmatrix interface, and concentration of metal play key roles in the tailoring the LSPR properties of the Pd.

Enhanced room temperature ferromagnetism and green photoluminescence in Cu doped ZnO thin film synthesised by neutral beam sputtering.

The Cu (3 to 15 at%) is incorporated into ZnO thin film by atomic beam co-sputtering has been investigated for enhancement in room temperature ferromagnetism and green photo-luminance. These Cu-ZnO thin films examined with Raman spectroscopy, X-Ray Diffraction (XRD), UV-Visible spectroscopy, Hall measurement, magnetic force microscopy (MFM) and magnetic hysteresis. Raman spectroscopy, XRD confirms wurtzite structure and improvement in the crystallinity of ZnO upto 7% Cu. Further increase in Cu concentration results in growth in Cu nanoparticles. On increasing Cu concentration, there is decrement in transparency and increase in band gap with increase in n-type carrier concentration as confirmed from UV-Visible and Hall measurement studies. Magnetic measurement exhibited unique feature of room temperature ferromagnetic ordering in undoped and doped sample upto 3% Cu. The enhancement in magnetic moment as well as green emission in photoluminescence response with increase in Cu doping indicates that generation of large defects in ZnO by Cu doping, which can be attributed to combined effect of the presence of oxygen vacancies and/or structural inhomogeneity as well as formation of bound magnetic polarons. Importantly, synthesised Cu doped ZnO thin films can be used as spin LEDs and switchable spin-laser diodes.

Effect of grain size and microstructure on radiation stability of CeO2: an extensive study.

To investigate the variation in the radiation stability of ceria with microstructure under the electronic excitation regime, ceria samples sintered under different conditions were irradiated with high energy 100 MeV Ag ions. The ceria nanopowders were synthesized and sintered at 800 °C (S800), 1000 °C (S1000) and 1300 °C (S1300), respectively. The samples with widely varying grain size, densities and microstructure were obtained. The pristine and irradiated samples were studied by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). None of the samples amorphized up to the highest fluence of 1 × 10(14) ions per cm(2) employed in this study. XRD and Raman studies showed that the sample with lowest grain size suffered maximum damage while the sample with largest grain size was most stable and showed little change in crystallinity. Raman spectroscopy indicated the enhanced formation of Ce(3+) and related defects in the sample with larger grain size after irradiation. The most intriguing result was the absence of Ce(3+)-related defects in the sample with lowest grain size which actually showed maximum damage upon irradiation. The XPS studies on S800 and S1300 provided concrete evidence for the presence of Ce(3+) and oxygen ion vacancies in S1300. The grain boundaries and grain size dependent stability have been discussed.

Shape elongation of Zn nanoparticles in silica irradiated with swift heavy ions of different species and energies: scaling law and some insights on the elongation mechanism.

Zinc nanoparticles (NPs) embedded in silica were irradiated with swift heavy ions (SHIs) of seven different combinations of species and energies. The shape elongation induced by the irradiations was evaluated by optical linear dichroism (OLD) spectroscopy, which is a sensitive tool for determining the change in the mean aspect ratio (AR) of NPs. Although the mean AR change indicated a linear fluence dependence in the low- and medium-fluence regions, it indicated a nonlinear dependence in the high-fluence region. The data reveal that the elongation efficiency of Zn is correlated with the electronic stopping power 'Se in silica' and is not correlated with either the 'Se in Zn' or the nuclear stopping power. The elongation efficiency plotted as a function of the 'Se in silica' revealed a linear relationship, with a threshold value of ∼2 keV nm(-1), which is the same dependence exhibited by the ion-track formation in silica. The log-log plot showed that the elongation efficiency increased linearly with Se above a critical value of ∼3 keV nm(-1) and steeply decreased with Se to the power of 5 below the critical Se. The steep decrease can be ascribed to the discontinuous nature of the ion tracks, which is expected at Se ∼ 2-4 keV nm(-1) in silica. The fluence Φ dependences of AR - 1 under various irradiations are well-normalized with the electronic energy deposition of SHIs, i.e., the product of Se and Φ, with a Se greater than the same critical value of ∼3 keV nm(-1). The normalized data above the critical value fell on a linear relation, AR(Φ) - 1 ∝ SeΦ, for SeΦ < 2 keV nm(-3) and a sublinear relation, AR(Φ) - 1 ∝ (SeΦ)(1/2) for SeΦ > 2 keV nm(-3). On the basis of these experimental results, we discuss some insights into the elongation mechanism.

Setup for in situ x-ray diffraction study of swift heavy ion irradiated materials.

An in situ x-ray diffraction (XRD) setup is designed and installed in the materials science beam line of the Pelletron accelerator at the Inter-University Accelerator Centre for in situ studies of phase change in swift heavy ion irradiated materials. A high vacuum chamber with suitable windows for incident and diffracted X-rays is integrated with the goniometer and the beamline. Indigenously made liquid nitrogen (LN2) temperature sample cooling unit is installed. The snapshots of growth of particles with fluence of 90 MeV Ni ions were recorded using in situ XRD experiment, illustrating the potential of this in situ facility. A thin film of C60 was used to test the sample cooling unit. It shows that the phase of the C60 film transforms from a cubic lattice (at room temperature) to a fcc lattice at around T=255 K.