Meltdown of palladium nanoparticles due to prolonged hydrogen ion bombardment

Nanometric crystalline particles of palladium are known to form on polycrystalline palladium under the bombardment of low-energy hydrogen ions. Mostly, the Pd particles disperse on an amorphous medium or in a matrix protruding from the target surface. Here evidence is presented that Pd nanoparticles originally present on the matrix surface are liquefied by bombarding hydrogen ions. The macroscopic target temperature for this to occur is around one-third of the melting point of Pd. This phenomenon is too unfamiliar to be described in terms of existing scientific models and an as-yet unknown process might underlie its occurrence.     

Influence of a surfactant on single ion track etching: Preparing and manipulating cylindrical micro wires

The influence of the alkali resistant surfactant Dowfax 2A1 on single ion track etching in 30 μm polycarbonate foils is studied at low etch rate (5 M NaOH at 41.5 ± 2 °C) using electro conductivity measurements. At surfactant concentrations above 10⁻⁴ vol.% break-through times are predictable (Δt/t < 0.25). At high surfactant concentrations (?0.1 vol.%) the formation of cylindrical channels is favoured. The shape of these channels (length ? 26 μm, diameter ? 1.8 μm) is verified by electro-replication and SEM observation of the resulting wires. Agreement of radii is better than 0.1 μm. Depending on the current limit set during electro replication compact or hollow cylinders can be obtained. A technique for localizing and manipulating individual micro wires by their head buds is described.     

Formation of metastable conductive nanowire in a thiospinel compound CuIr2S4 induced by ion irradiation

We observed an increase in the conductivity of a thiospinel compound, CuIr₂S₄, induced by H⁺ and He⁺ irradiation with energies of 1-2 MeV. It was indicated that the metastable conductive phase was produced by electronic excitation due to the ion beam and this phase was similar to the X-ray-induced phase. Conductivity as a function of ion fluence was analyzed by a simple model where the ion-induced change occurred in a cylindrical region around an ion trajectory. The cross-sectional area of the cylinder was obtained by analyzing the conductivity as a function of ion fluence for each ion, and it was found that an impinging ion produced a nanowire in the conductive phase. In addition, the yield of the Ir dimer displacement, which was related to the increase in conductivity, was considerably high. The ion irradiation effect reported in this paper is unique with regard to the high yield and low linear energy transfer (LET) in the formation of the conductive-phase nanowire. Both these unique aspects could be ascribed to the low band-gap energy and strong electron-lattice interaction of this compound.     

Ion beam induced defects in solids studied by optical techniques

Optical methods can provide important insights into the mechanisms and consequences of ion beam interactions with solids. This is illustrated by four distinctly different systems.X- and Y-cut LiNbO₃ crystals implanted with 8 MeV Au³⁺ ions with a fluence of 1 × 10¹⁷ ions/cm² result in gold nanoparticle formation during high temperature annealing. Optical extinction curves simulated by the Mie theory provide the average nanoparticle sizes. TEM studies are in reasonable agreement and confirm a near-spherical nanoparticle shape but with surface facets. Large temperature differences in the nanoparticle creation in the X- and Y-cut crystals are explained by recrystallisation of the initially amorphised regions so as to recreate the prior crystal structure and to result in anisotropic diffusion of the implanted gold.Defect formation in alkali halides using ion beam irradiation has provided new information. Radiation-hard CsI crystals bombarded with 1 MeV protons at 300 K successfully produce F-type centres and V-centres having the structure as identified by optical absorption and Raman studies. The results are discussed in relation to the formation of interstitial iodine aggregates of various types in alkali iodides. Depth profiling of and aggregates created in RbI bombarded with 13.6 MeV/A argon ions at 300 K is discussed.The recrystallisation of an amorphous silicon layer created in crystalline silicon bombarded with 100 keV carbon ions with a fluence of 5 × 10¹⁷ ions/cm² during subsequent high temperature annealing is studied by Raman and Brillouin light scattering.Irradiation of tin-doped indium oxide (ITO) films with 1 MeV protons with fluences from 1 × 10¹⁵ to 250 × 10¹⁵ ions/cm⁻² induces visible darkening over a broad spectral region that shows three stages of development. This is attributed to the formation of defect clusters by a model of defect growth and also high fluence optical absorption studies. X-ray diffraction studies show evidence of a strained lattice after the proton bombardment and recovery after long period storage. The effects are attributed to the annealing of the defects produced.     

Vitamin C hinders radiation cross-linking in aqueous poly(vinyl alcohol) solutions

Poly(vinyl alcohol) (PVA) is a promising semi-crystalline material for biomedical applications. It is soluble in water and can be formed into hydrogels by freezing and thawing or crystallizing from an aqueous theta solution such as that of polyethylene glycol (PEG). Radiation cross-linking caused by sterilization or high dose irradiation of concentrated PVA solutions could compromise some properties of these hydrogels. Therefore, we hypothesized that radiation cross-linking of PVA solutions and PVA-PEG theta gels could be prevented by using the antioxidant vitamin C as an anticross-linking agent. Our hypothesis tested positive. Vitamin C concentrations of 0.75 and 4.5 mol/mol of PVA repeating unit could prevent cross-linking in 17.5 wt/v% PVA solutions made with PVA molecular weight of 115,000 g/mol irradiated to 25 and 100 kGy, respectively. Vitamin C also prevented cross-linking in 25 kGy irradiated PVA-PEG theta gels containing up to 5 wt% PEG and decreased the viscosity of those up to 39 wt%.     

Enhanced beam deflection in bent crystals using multiple volume reflection

This paper presents simulations of the trajectories of high-energy ions through several bent crystal layers. At certain layer alignments volume reflection occurs from each layer and the resultant multiple volume reflection angle is correspondingly increased, along with the range of entrance angles over which ions undergo volume reflection. Another feature is that the range of entrance angles for which bent crystal channeling occurs is also increased in passing through several bent layers. The use of several bent crystal layers to produce multiple volume reflection provides an alternative approach to the design of a space shield or radiation protection at accelerators based on bent crystals.     

Optimization of nanopores obtained by chemical etching on swift-ion irradiated lithium niobate

The morphology of the nanopores obtained by chemical etching on ion-beam irradiated LiNbO₃ has been investigated for a variety of ions (F, Br, Kr, Cu, Pb), energies (up to 2300 MeV), and stopping powers (up to 35 keV/nm) in the electronic energy loss regime. The role of etching time and etching agent on the pore morphology, diameter, depth, and shape has also been studied. The transversal and depth profiles of the pore have been found to be quite sensitive to both irradiation and etching parameters. Moreover, two etching regimes with different morphologies and etching rates have been identified.     

Near 100% deflection efficiency in bent crystals using multiple volume reflection

Simulations of the trajectories of high-energy ions through several bent crystal layers show that ions can be reflected from each layer, resulting in a multiple volume reflection angle which is correspondingly increased in proportion to the number of layers. At certain configurations of a 20 layer structure we achieve a 100% deflection efficiency of high-energy ions through an angle which is 20 times greater than the critical angle. The range of entrance angles over which ions undergo volume reflection is 40 times greater than the critical angle. The use of several bent crystal layers to produce multiple volume reflection provides an alternative approach to beam steering which overcomes limitations of bent crystal channeling such as narrow angular acceptance and low deflection efficiency. This study shows that multiple volume reflection can be used for high efficiency collimation in high-energy colliders such as the LHC and ILC and for space and radiation shields based on bent crystals.     

Grain rotation in nanocrystalline layers under influence of swift heavy ions

Significant changes in texture occur in nanocrystalline Ti, TiN and NiO layers during irradiation with 350 MeV Au ions. The angle between ion beam and layer normal Θ was between 30° and 70°. The major effect is a collective rotation of the nanocrystals. In case of ω layers the texture rotated by more than 70° at an ion fluence . In addition to grain rotation, the layers exhibit a shear motion like that observed previously with amorphous materials. Below about the grain growth is small and grain rotation is reversible, i.e. reversing the sign of Θ and applying the same Φt, the grains roll back into their original orientation. The second observed effect is the alignment of the grains, whose coalescence eventually leads to a mosaic crystal. However, grain rotation is absent or immeasurably small in micro-crystalline titanium. An attempt to understand the processes in nanocrystalline materials in terms of amorphous grain boundaries, as well as the disclination dipole diffusion along the grain boundary was made.     

Ion track formation in low temperature silicon dioxide

Low temperature silicon dioxide layers (LTO), deposited on crystalline silicon substrates, and thermally densified at 750 °C for 90 min or 900 °C for 30 min, jointly with thermally grown silicon dioxide layers, were irradiated with low fluence 11 MeV Ti ions. A selective chemical etch of the latent tracks generated by the passage of swift ions was performed by wet or vapour HF solution. The wet process produced conically shaped holes, while the vapour procedure generated almost cylindrical nanopores. In both cases thermal SiO₂ showed a lower track etching velocity V_t, but with increasing the densification temperature of the LTO samples, the V_t differences reduced. LTO proved to be suitable for wet and vapour ion track formation, and, as expected, for higher densification temperatures, its etching behaviour approached that of thermal silicon dioxide.     

Effects of ion irradiation on the structural transformation of sol-gel derived TEOS/MTES thin films

Ion beam processing of organic/inorganic thin films has been shown to be an effective means in converting polymeric films into their final ceramic-like state. In this study, hybrid sol-gel derived thin films based on TEOS (tetraethylorthosilicate) Si(OC₂H₅)₄ and MTES (methyltriethoxysilane) CH₃Si(OC₂H₅)₃ were prepared and deposited on Si substrates by spin coating. After the films were allowed to air dry, they were heat treated at 300 °C for 10 min. Ion irradiation was performed at room temperature using 125 keV H⁺ and 250 keV N²⁺ ions with fluences ranging from 1 × 10¹⁴ to 5 × 10¹⁶ ions/cm². FT-IR and Raman spectroscopies were used to quantify the chemical structural transformations which occurred including the evolution of the organic components, the cross-linking of silica clusters, and the clustering of carbon.     

Radiation-induced grafting of inorganic particles onto polymer backbone: A new method to design polymer-based nanocomposite

Polypropylene/polyhedral oligomeric silsesquioxane (PP/POSS) nanocomposites were prepared by in situ radiation-induced grafting of POSS onto PP. Radiation-induced grafting of POSS was confirmed by FT-IR spectroscopy. The mechanical property of PP/POSS nanocomposites increased with the increase in POSS content and with the increase in absorption dose up to 5 kGy, above which it started to decrease. The reduction of mechanical property at high doses can be attributed to the chain scission of PP by radiation. The degree of reduction in decomposition temperature of irradiated PP/POSS nanocomposites was found to be much less than that of neat PP due to the covalent grafting of POSS onto PP by radiation.     

Dislocation mobility study of heavy ion induced track damage in LiF crystals

The track damage created in LiF crystals by swift U, Xe and Kr ions with a specific energy of 11.1 MeV/u was studied using dislocation mobility measurements, track etching, SEM, AFM and optical microscopy. The results demonstrate high sensitivity of dislocation mobility to track core damage. The relationship between the energy loss of ions, dislocation mobility and track structure is discussed.     

Photoconduction investigations in 75 MeV oxygen ion irradiated kapton-H polyimide

Photoconduction behaviour of 75 MeV oxygen ion irradiated (Fluences: 1.8 × 10¹¹, 1.8 × 10¹² and 1.8 × 10¹³ ions/cm²) kapton-H polyimide film in the visible region has been investigated at different temperatures ranging 400-2500 °C and at various electric fields ranging 40-600 kV/cm. A photoinduced exciton formation is the major source for providing charge carriers through thermolization and field-assisted dissociation processes. An attempt has been made to fit the field dependence of the steady state photocurrent to one of the several possible conduction mechanisms. In the high and low fluence (1.8 × 10¹³ and 1.8 × 10¹¹ ions/cm²) irradiated samples there exists a possibility of Poole-Frankel type of photoconduction mechanism, whereas at intermediate fluence (1.8 × 10¹² ions/cm²) a Schottky type photoconduction mechanism may be operative. The log I_ps versus 1/T plots consist of two straight lines with a knee point around 800-1000 °C. The activation energy estimated from the slope of these lines is field dependent varying from 0.40 to 0.73 eV and 0.18 to 0.23 eV above and below the knee point, respectively. This indicates the presence of more than one type of trapping levels in irradiated kapton-H polyimide.     

Birefringence in optical fibers formed by proton implantation

Birefringence can be induced in silica-based optical fibers by ion implantation. In the present research, protons were implanted in single-mode optical fibers with two different energies, one being the energy with which the protons can just reach the center of the optical fiber core and the other being a slightly lower energy. The degree of birefringence was evaluated by measuring reflection spectra of Bragg gratings formed at the proton-implanted region of the optical fibers. The results confirmed that birefringence is induced by unidirectional densification along the projected range of protons formed in the fiber core and by densification of the fiber cladding. The induced birefringence reached three to ten times higher than that of a conventional birefringent fiber. The birefringence caused by ion implantation can be a versatile tool for manufacturing various optical fiber devices.     

Morphological change in FePt nanogranular thin films induced by swift heavy ion irradiation

We have investigated morphology change of FePt nanogranular films (FePt)₄₇(Al₂O₃)₅₃ under irradiation with 210 MeV Xe ions. Here, electron tomography technique was extensively employed to clarify three-dimensional (3D) structure in irradiated specimens, in addition to conventional transmission electron microscopy (TEM) techniques such as bright-field observation and scanning TEM energy dispersive X-ray spectroscopy (STEM-EDX) analysis. The ion irradiation induces the coarsening of FePt nanoparticles with elongation along the beam direction. Electron tomography 3D reconstructed images clearly demonstrated that when the fluence achieves 5.0 × 10¹⁴ ions/cm², well-coarsened FePt balls have been formed on the irradiated surface, and the particles in the film interior have been deformed into rods along the ion trajectory. The alloy particles become inhomogeneous in composition after prolonged irradiation up to 1.0 × 10¹⁵ Xe ions/cm². The particle center is enriched with Pt, while Fe is slightly redistributed to the periphery.     

Formation of dislocation loops in silicon by ion irradiation for silicon light emitting diodes

We have studied the influence of the ion species, ion energy, fluence, irradiation temperature and post-implantation annealing on the formation of shallow dislocation loops in silicon, for fabrication of silicon light emitting diodes. The substrates used were (1 0 0) Si, implanted with 20-80 keV boron at room temperature and 75-175 keV silicon at 100 and 200 °C. The implanted fluences were from 5 × 10¹⁴ to 1 × 10¹⁵ ions/cm². After irradiation the samples were processed for 15 s to 20 min at 950 °C by rapid thermal annealing. Structural analysis of the samples was done by transmission electron microscopy and Rutherford backscattering spectrometry. In all irradiations the silicon substrates were not amorphized, and that resulted in the formation of extrinsic perfect and faulted dislocation loops with Burgers vectors a/2〈1 1 0〉 and a/3〈1 1 1〉, respectively, sitting in {1 1 1} habit planes. It was demonstrated that by varying the ion implantation parameters and post-irradiation annealing, it is possible to form various shapes, concentration and distribution of dislocation loops in silicon.     

Quasi-static heating of stack targets with intense ion beams for equation of state measurements

A novel target configuration for equation of state measurements using intense ion beams is proposed. It is based on a stack consisting of several thin foils which expand quasi-statically when heated with the ion beam. The merging of the single foils causes a sharp increase of the expansion velocity. Together with the known energy deposition of the ion beam this allows direct determination of equation of state data. The parameters of the stack target were optimized with hydrodynamic calculations. Finally, a simple analytical model to predict the expansion velocity was developed.     

Correlation between ion beam parameters and physical characteristics of nanostructures fabricated by focused ion beam

We report a study of the physical characteristics of the pillars of C, Pt and W grown by 10-30 keV Ga focused ion beam (FIB) as a function of Ga ion flux, and present a quantitative analysis of the elements using energy-dispersive analysis of X-rays (EDAX). All the FIB grown pillars exhibit a rough morphology with whisker like protrusions on the cylindrical surface and broadening of the base as compared to the nominal size. For a constant fluence, the height of the pillar initially increases and then reduces after going through a maximum as a function of ion flux in all the cases. The compositional analysis shows good metallic quality for Pt structures but reveals significant contamination of Ga in C and Ga and C in W structures at higher ion fluxes. Explanation to all these observations has been sought in the light of secondary ion and electron effects and the different processes involved which lead to the FIB induced deposition.     

Irradiation induced dislocation loop and its influence on the hardening behavior of Fe-Cr alloys by an Fe ion irradiation

Nano indentation analysis and transmission electron microscopy observation were performed to investigate a microstructural evolution and its influence on the hardening behavior in Fe-Cr alloys after an irradiation with 8 MeV Fe⁴⁺ ions at room temperature. Nano indentation analysis shows that an irradiation induced hardening is generated more considerably in the Fe-15Cr alloy than in the Fe-5Cr alloy by the ion irradiation. TEM observation reveals a significant population of the a₀<1 0 0> dislocation loops in the Fe-15Cr alloy and an agglomeration of the 1/2a₀<1 1 1> dislocation loops in the Fe-5Cr alloy. The results indicate that the a₀<1 0 0> dislocation loops will act as stronger obstacles to a dislocation motion than 1/2a₀<1 1 1> dislocation loops.