Daryush Ila

Resolving the electronic and nuclear effects of MeV ions in polymers

Abstract The electronic and nuclear stopping effects produced by MeV ion bombardment in polyvinylidene chloride (PVDC) and polyethylene (PE) were separated by stacking thin films of the polymers. The resulting layered system consisting of each polymer was bombarded with 3.5 and 5.0 MeV alpha particles. A layered system was selected such that the first layers experienced most of the effects of the electronic energy deposited and the last layers received most of the effects of the nuclear stopping. The electrical conductance and the changes in the chemical structure were measured by direct resistivity measurements, Raman microprobe analysis and FTIR. Post-irradiation analyses of the films indicated differences in these properties in the various film layers which are attributed to the individual effects of the stopping powers in the polymer films.

Nano-cluster engineering: A combined ion implantation/co-deposition and ionizing radiation

We have used the energy deposited due to the electronic excitation by post-implantation irradiation to induce the nucleation of nano-clusters of Au in silica. We have produced the Au/silica by two methods. (A) MeV Au implantation into silica, (B) producing thin films of a combined Au and silica on a silica substrate, using co-deposition of gold and silica. The process of ion beam assisted nucleation of nano-clusters was used to reduce the threshold implantation dose, or the Au concentration in the silica host, required to produce Au nano-crystals by at least two orders of magnitude. In this presentation, we applied a similar technique, post-irradiation electronic excitation, to films produced by both ion implantation of Au into silica as well as to films produced using co-deposition of gold and silica. By a co-deposition technique, gold and silica (co-deposited at various concentrations) are grown, then post-irradiated. The resultant Au nano-cluster formation was observed and studied using optical spectroscopy, X-ray diffraction, RBS and TEM.

Effect of accelerated protons on the molecular-topological structure and thermal stability of poly(vinylidene fluoride)

The bombardment of poly(vinylidene fluoride) (PVDF) with accelerated protons in vacuum results in the release of gaseous products and the destruction of cluster junctions in its pseudo-network structure, which pass to the crystalline ones of three modifications with different temperatures and rates of melting, the molecular mass of crystallized chains, and their weight fractions. The composition of gaseous products indicates that, upon proton bombardment, the detachment of fluorine or hydrogen atoms yielding H2 and HF occurs as the main process, and the scission of the carbon chain hardly takes place in this case.

MeV silver ion implantation induced changes in optical properties of MgO (100)

Abstract The implantation of 1.5 MeV Ag ions at 1.2 × 10 17 ion/cm 2 into MgO(100) single crystals produces optical absorption color centers which can be reduced by heat treatment at temperatures above 1000°C. We observed the formation of both F-centers and V-centers. We observed optical absorption due to the formation of silver nanoclusters at heat treatment temperatures above 550°C and due to F-center aggregates at heat treatment above 800°C. Upon further heat treatment, the color centers diminish while absorption due to the Ag nanoclusters is enhanced. Using Mies theory we confirmed that Ag is in the form of nanoclusters and by using Rutherford backscattering spectrometry (RBS) and ion channeling, we confirmed that these Ag metallic clusters have, in fact, orientations similar to that of the host substrate, MgO. The measured optical absorption due to Ag nanocrystals, 430 nm, is in agreement with the theoretical prediction.

Ion beam promoted lithium absorption in glassy polymeric carbon

Abstract Glassy Polymeric Carbon (GPC) samples prepared from a precursor possess accessible pore volume that depends on the heat treatment temperature [G.M. Jenkins and K. Kawamura, Polymeric Carbons - Carbon Fiber, Glass and Char (Cambridge University Press, Cambridge, 1976) p. 140]. We have shown that lithium percolates without diffusion into the accessible pores of GPC samples immersed in a molten lithium salt bath at 700°C [D. Ila, G.M. Jenkins, L.R. Holland, A.L. Evelyn and H. Jena, Vacuum 45 (1994) 451]. Ion bombardment with 10 MeV Au atoms increases the total pore volume available for lithium occupation even for samples normally impermeable to lithium. The lithium concentration depth profile is measured using Li 7 (p,2α) nuclear reaction analysis. We will report on lithium percolation into GPC prepared at temperatures between 500°C and 1000°C and activated by a 10 MeV gold ion bombardment.

Nonlinear optical waveguides produced by MeV ion implantation in LiNbO3

Abstract We analyze microstructure, linear and nonlinear optical properties of planar waveguides produced by implantation of MeV Ag ions into LiNbO3. Linear optical properties are described by the parameters of waveguide propagation modes and optical absorption spectra. Nonlinear properties are described by the nonlinear refractive index. Operation of the implanted crystal as an optical waveguide is due to modification of the linear refractive index of the implanted region. The samples as implanted do not show any light-guiding. The implanted region has amorphous and porous microstructure with the refractive index lower than the substrate. Heat treatment of the implanted samples produces planar light-guiding layer near the implanted surface. High-resolution electron microscopy reveals re-crystallization of the host between the surface and the nuclear stopping region in the form of randomly oriented crystalline grains. They make up a light-guiding layer isolated from the bulk crystal by the nuclear stopping layer with low refractive index. Optical absorption of the sample as implanted has a peak at 430 nm. This peak is due to the surface plasmon resonance in nano-clusters of metallic silver. Heat treatment of the samples shifts the absorption peak to 545 nm. This is more likely due to the increase of the refractive index back to the value for the crystalline LiNbO3. The nonlinear refractive index of the samples at 532 nm (of the order of 10−10 cm2 W−1) was measured with the Z-scan technique using a picosecond laser source. Possible applications of the waveguides include ultra-fast photonic switches and modulators.

POST BOMBARDMENT ENHANCED OPTICAL ABSORPTION IN GOLD IMPLANTED SILICA

Abstract We have demonstrated that the post bombardment of 3.6 MeV Au implanted suprasil by 1.2 MeV boron, 2.0 MeV boron and 0.9 MeV alpha particles can induce the formation of Au nanoclusters. We have also demonstrated that post bombardment increases the annealing temperatures necessary for Au cluster formation beyond 1000°C. The optical absorption band characteristic of the Au implanted suprasil was used to monitor the formation of the metallic clusters before and after post bombardment as well as at each annealing temperature.

RBS and Raman spectroscopy study of heat-treatment effect on phenolformaldehyde resin☆

Abstract Phenolformaldehyde resin, which is used to make items of glassy carbon hollow ware, contains residual alkaline catalyst and other impurities. Pyrolysis reduces the concentrations of hydrogen and oxygen during condensation and dehydrogenation reactions, which occur between 150 and 1000°C as the resin is converted to a polymeric glassy carbon. Molded samples heat-treated from 150 to 600°C and sprayed samples heat-treated to 1000 and 2500°C were studied using Rutherford backscattering spectrometry (RBS) and Raman spectroscopy. Raman spectroscopy was used to study the structural changes induced by heat treatment. Raman spectroscopy showed that most carbonization occurred at 500 to 600°C while the RBS data from resin samples at each temperature showed that oxygen and sodium concentrations were reduced at higher temperatures.

Enhanced tissue adhesion by increased porosity and surface roughness of carbon based biomaterials

Abstract We present recent results using ions of C, O, Si, Fe, Zn and Au at energies between 100 keV and 10 MeV to increase the roughness and porosity of the partially and fully cured precursor phenolic resins. The fully cured phenolic resin is called glassy polymeric carbon (GPC). GPC is chemically inert, biocompatible and useful for medical applications, such as heart valves and other prosthetic devices. Ion implantation enhances biological cell/tissue growth on, and tissue adhesion to, prosthetic devices made from GPC. We have previously shown that increased porosity of GPC is also useful for drug delivery devices. The porosity of the ion implanted partially and fully cured precursor phenolic resins was measured by introducing lithium from a molten LiCl salt into each sample. By using Li(p,2α) nuclear reaction analysis (NRA) we measured the concentration of Li retention in the pre- and post-implanted samples. The surface roughness was measured using optical microscopy. The curing process was monitored using micro-Raman microscopy. We have correlated the NRA measurements of increased pore availability with the observations of increased surface roughness.

Vanishing optical isolation barrier in double ion‐implanted lithium niobate waveguide

Isolation properties at different propagation directions have been studied for an optical barrier between two superposed planar waveguides produced by MeV He+ implantation in LiNbO3. The barrier exists while the modes of the double structure propagate along the optic axis. The spacing between the modes measured with a prism coupler features efficiently isolated waveguides. The barrier ‘‘vanishes’’ when the directional angle between an extraordinary wave and the optic axis exceeds 50°. In this case the mode spacing shows strong coupling between the waveguides that makes the double structure similar to a single waveguide with double thickness.