Nanoscale density fluctuations in swift heavy ion irradiated amorphous SiO2

We report on the observation of nanoscale density fluctuations in 2 μm thick amorphous SiO₂ layers irradiated with 185 MeV Au ions. At high fluences, in excess of approximately 5 × 10¹² ions/cm², where the surface is completely covered by ion tracks, synchrotron small angle x-ray scattering measurements reveal the existence of a steady state of density fluctuations. In agreement with molecular dynamics simulations, this steady state is consistent with an ion track “annihilation” process, where high-density regions generated in the periphery of new tracks fill in low-density regions located at the center of existing tracks.The authors acknowledge the Australian Research Council and the Australian Synchrotron Research Program for financial support and thank the staff at the ANU Heavy Ion facility for their continued technical assistance. O.P., F.D., and K.N. acknowledge financial support from the Academy of Finland under its Centre of Excellence program as well as the OPNA project, and grants of computer capacity from CSC

Formation and structural characterization of Ni nanoparticles embedded in SiO₂

Face-centered cubic Ni nanoparticles were formed in SiO₂ by ion implantation and thermal annealing. Small-angle x-ray scattering in conjunction with transmission electron microscopy was used to determine the nanoparticle size as a function of annealing temperature, whereas the local atomic structure was measured with x-ray absorption spectroscopy. The influence of finite-size effects on the nanoparticle structural properties was readily apparent and included a decrease in coordination number and bond length and an increase in structural disorder for decreasing nanoparticle size. Such results are consistent with the non-negligible surface-to-volume ratio characteristic of nanoparticles. In addition, temperature-dependent x-ray absorption spectroscopy measurements showed the mean vibrational frequency (as obtained from the Einstein temperature) decreased with decreasing nanoparticle size. This reduction was attributed to the greater influence of the loosely bound, under-coordinated surface atoms prevailing over the effects of capillary pressure, the former enhancing the low frequency modes of the vibrational density of statesThis work was financially supported by the Australian Synchrotron and the Australian Research Council with access to equipment provided by the Australian Nanofabrication Facility

Nano-porosity in GaSb induced by swift heavy ion irradiation

Nano-porous structures form in GaSb after ion irradiation with 185 MeV Au ions. The porous layer formation is governed by the dominant electronic energy loss at this energy regime. The porous layer morphology differs significantly from that previously reported for low-energy, ion-irradiated GaSb. Prior to the onset of porosity, positron annihilation lifetime spectroscopy indicates the formation of small vacancy clusters in single ion impacts, while transmission electron microscopy reveals fragmentation of the GaSb into nanocrystallites embedded in an amorphous matrix. Following this fragmentation process, macroscopic porosity forms, presumably within the amorphous phase.The authors thank the Australian Research Council for support and the staff at the ANU Heavy Ion Accelerator Facility for their continued technical assistance. R.C.E. acknowledges the support from the Office of Basic Energy Sciences of the U.S. DOE (Grant No. DE-FG02-97ER45656)

Pt nanocrystals formed by ion implantation: a defect-mediated nucleation process

The influence of ion irradiation of SiO₂ on the size of metalnanocrystals (NCs) formed by ion implantation has been investigated. Thin SiO₂ films were irradiated with high-energy Ge ions then implanted with Pt ions. Without Geirradiation, the largest Pt NCs were observed beyond the Pt projected range. With irradiation, Ge-induced structural modification of the SiO₂ layer yielded a decrease in Pt NC size with increasing Ge fluence at such depths. A defect-mediated NC nucleation mechanism is proposed and a simple yet effective means of modifying and controlling the Pt NC size is demonstrated.The authors thank the Australian Research Council for financial support

Swift heavy-ion irradiation-induced shape and structural transformation in cobalt nanoparticles

The shape and structural evolution of Co nanoparticles embedded in SiO₂ and subjected to swift heavy-ion irradiation have been investigated over a wide energy and fluence range. Modifications of the nanoparticle size and shape were characterized with transmission electron microscopy and small-angle x-ray scattering.Nanoparticles below a threshold diameter remained spherical in shape and progressively decreased in size under irradiation due to dissolution.Nanoparticles above the threshold diameter transformed into nanorods with their major dimension parallel to the incident ion direction. Modifications of the atomic-scale structure of the Co nanoparticles were identified with x-rayabsorption spectroscopy. Analysis of the x-rayabsorption near-edge spectra showed that prior to irradiation all Co atoms were in a metallic state, while after irradiation Co atoms were in both oxidized and metallic environments, the former consistent with dissolution. The evolution of the nanoparticle short-range order was determined from extended x-ray absorption fine structure spectroscopy. Structural changes in the Co nanoparticles as a function of ion fluence included an increase in disorder and asymmetric deviation from a Gaussian interatomic distance distribution coupled with a decrease in bondlength. Such changes resulted from the irradiation-induced decrease in nanoparticle size and subsequent dissolution.This work was financially supported by the Australian Synchrotron and the Australian Research Council with access to equipment provided by the Australian Nanofabrication Facility. ChemMatCARS Sector 15 is principally supported by the NSF/ DOE under Grant No. NSF/CHE–0822838

Energy dependent saturation width of swift heavy ion shaped embedded Au nanoparticles

The transformation of Aunanoparticles (NPs) embedded in SiO₂ from spherical to rod-like shapes induced by swift heavy ion irradiation has been studied. Irradiation was performed with ¹⁹⁷Au ions at energies between 54 and 185 MeV. Transmission electron microscopy and small angle x-ray scatteringmeasurements reveal an energy dependent saturation width of the NP rods as well as a minimum size required for the NPs to elongate. The NP saturation width is correlated with the ion track diameter in the SiO₂. NP melting and in-plane strain in the irradiatedSiO₂ are discussed as potential mechanisms for the observed deformation.P.K. and M.C.R. thank the Australian Research Council for support. P.K., R.G., D.J.S., and M.C.R. were supported by the Australian Synchrotron Research Program, funded by the Commonwealth of Australia via the Major National Research Facilities Program

T-cell production of matrix metalloproteinases and inhibition of parasite clearance by TIMP-1 during chronic Toxoplasma infection in the brain

Chronic infection with the intracellular protozoan parasite Toxoplasma gondii leads to tissue remodelling in the brain and a continuous requirement for peripheral leucocyte migration within the CNS (central nervous system). In the present study, we investigate the role of MMPs (matrix metalloproteinases) and their inhibitors in T-cell migration into the infected brain. Increased expression of two key molecules, MMP-8 and MMP-10, along with their inhibitor, TIMP-1 (tissue inhibitor of metalloproteinases-1), was observed in the CNS following infection. Analysis of infiltrating lymphocytes demonstrated MMP-8 and -10 production by CD4+ and CD8+ T-cells. In addition, infiltrating T-cells and CNS resident astrocytes increased their expression of TIMP-1 following infection. TIMP-1-deficient mice had a decrease in perivascular accumulation of lymphocyte populations, yet an increase in the proportion of CD4+ T-cells that had trafficked into the CNS. This was accompanied by a reduction in parasite burden in the brain. Taken together, these findings demonstrate a role for MMPs and TIMP-1 in the trafficking of lymphocytes into the CNS during chronic infection in the brain

The influence of annealing conditions on the growth and structure of embedded Pt nanocrystals

The growth and structure of Pt nanocrystals (NCs) formed by ion implantation in a-SiO₂ has been investigated as a function of the annealing conditions. Transmission electron microscopy and small-angle x-ray scatteringmeasurements demonstrate that the annealing ambient has a significant influence on NC size. Samples annealed in either Ar, O₂, or forming gas (95% N₂: 5% H₂) at temperatures ranging from 500 °C–1300 °C form spherical NCs with mean diameters ranging from 1–14 nm. For a given temperature, annealing in Ar yields the smallest NCs. O₂ and forming gas ambients produce NCs of comparable size though the latter induces H chemisorption at 1100 °C and above, as verified with x-ray absorption spectroscopy. This H intake is accompanied by a bond-length expansion and increased structural disorder in NCs of diameter >3 nm.We thank the Australian Synchrotron Research Program and the Australian Research Council for financial support