G. de M. Azevedo

Structural perturbations within Ge nanocrystals in silica

Extended x-ray absorption fine structure (EXAFS) spectroscopy was used to identify structural perturbations in Ge nanocrystals produced in silica by ion implantation and annealing. Although the nanocrystals retained tetrahedral coordination, both the short- and medium-range orders were perturbed relative to bulk crystalline material. Equivalently, the nanocrystal interatomic distance distribution deviated from that of bulk crystalline Ge, exhibiting enhanced structural disorder of both Gaussian and non-Gaussian forms in the first, second, and third nearest-neighbor shells. The relative influences of nanocrystal size, bonding distortions, multiple phases, and a matrix-induced compression were considered.

Structural characterization of Cu nanocrystals formed in SiO2 by high-energy ion-beam synthesis

Cu nanocrystals (NCs) were produced by multiple high-energy ion implantations into 5‐μm-thick amorphous silica (SiO2) at liquid-nitrogen temperature. The Cu-rich SiO2 films were subsequently annealed to reduce irradiation-induced damage and promote NC formation. The NC size distribution and structure were studied utilizing a combination of Rutherford backscattering spectroscopy, x-ray diffraction, cross-sectional transmission electron microscopy, and extended x-ray-absorption fine-structure (EXAFS) spectroscopy. We present results derived from all four techniques, focussing on EXAFS measurements to study the local atomic structure surrounding Cu atoms, and comparing NC samples with bulk standards. Using a unique sample preparation method, we drastically improve the signal-to-noise ratio to extract high-quality EXAFS data to enable the determination of a non-Gaussian bond length distribution via the third-order cumulant. We quantify subtle concentration- and annealing-temperature-dependent changes in the C...

Impact-parameter dependent energy loss of screened ions

Abstract We describe a simple model for the electronic energy loss as a function of the impact parameter for screened projectiles at high velocities. The physical inputs are the projectile screening potential, the electronic density and the oscillators strengths for the target electrons. An excellent agreement with full first-order Born calculations is obtained. The results are then used to compute the angular dependence of the electronic energy loss under channeling conditions.

Bond length contraction in Au nanocrystals formed by ion implantation into thin SiO2

P. K. is grateful to the Humboldt Foundation in Germany for support. P.K., B.J., A.C., C.J.G., G.d.M.A., G.J.F., and M.C.R. were supported by the Australian Synchrotron Research Program.

High temperature emissivity, reflectivity, and x-ray absorption of BiFeO3

We report on the lattice evolution of BiFeO3 as function of temperature using far infrared emissivity, reflectivity, and x-ray absorption local structure. A power law fit to the lowest frequency soft phonon in the magnetic ordered phase yields an exponent β=0.25 as for a tricritical point. At about 200 K below TN∼640 K it ceases softening as consequence of BiFeO3 metastability. We identified this temperature as corresponding to a crossover transition to an order-disorder regime. Above ∼700 K strong band overlapping, merging, and smearing of modes are consequence of thermal fluctuations and chemical disorder. Vibrational modes show band splits in the ferroelectric phase as emerging from triple degenerated species as from a paraelectric cubic phase above TC∼1090 K. Temperature dependent x-ray absorption near edge structure (XANES) at the Fe K edge shows that lower temperature Fe3+ turns into Fe2+. While this matches the FeO wustite XANES profile, the Bi LIII-edge downshift suggests a high temperature very c...

Common structure in amorphised compound semiconductors

Abstract Extended X-ray absorption fine structure has been utilised to determine the structural parameters of the amorphous III–V semiconductors GaP, GaAs, InP and InAs. The amorphous phase was formed by ion implantation to inhibit preparation-specific artefacts associated with the alternative fabrication techniques of sputtering and evaporation. Relative to crystalline standards, increases in bond length and Debye–Waller factor were apparent for all materials with a slight reduction in coordination number (3.8–4 atoms). Similarly, homopolar bonding in the amorphous phase was measurable for all materials. For example, In–In bonding in amorphous InP comprised ∼18% of the total In bonds and demonstrated the necessity of a ring distribution containing both even and odd members.

Gettering of Pd to implantation-induced nanocavities in Si

The authors thank the Australian Research Council for their financial support. G.deM.A. acknowledges the Brazilian agency CNPq ~Conselho Nacional de Desenvolvimento Cienti´fico e Tecnolo´gico! for a postdoctoral fellowship.

Angular dependent energy loss of 0.8–2.0 MeV He ions channeled along the Si〈1 0 0〉 direction

Abstract We present measurements of the electronic stopping power in the 0.8–2.0 MeV energy range of 4 He 2+ ions channeled through the Si〈1 0 0〉 axis as a function of the incident angle. The measurements were carried out using the Rutherford Backscattering (RBS) technique with a SIMOX sample that consists of a single Si crystal on the top of a 500 nm buried layer of SiO 2 built into a Si〈1 0 0〉 wafer. The measured ratios between the angular dependent and random stopping powers have almost the same width near the maximum of the 4 He 2+ stopping power and decrease for higher energies.

ELECTRONIC STOPPING POWER OF (1 0 0) AXIAL CHANNELED 7LI IONS IN SI CRYSTALS

Abstract We report measurements on the electronic stopping power of 7 Li ions along the 〈1 0 0〉 Si direction in the energy region between 1 and 8 MeV. The measurements were carried out using the standard RBS/Channeling technique with SIMOX targets. The results show that the stopping power decreases for increasing energies, from 48 eV/A at 1 MeV down to 19 eV/A at 8 MeV. The general features obtained for the stopping power of 7 Li resemble those obtained for He ions measured previously.

Investigation of indirect structural and chemical parameters of GeSi nanoparticles in a silica matrix by combined synchrotron radiation techniques

The formation of GeSi nanoparticles on an SiO2 matrix is studied here by synchrotron-based techniques. The shape, average diameter and size dispersion were obtained from grazing-incidence small-angle X-ray scattering data. X-ray diffraction measurements were used to obtain crystallite sizes and composition via resonant (anomalous) measurements. By using these techniques as input for extended X-ray absorption fine structure analysis, the local composition surrounding the Ge atoms is investigated. Although the results for each of the methods above are commonly analyzed separately, the combination of such techniques leads to an improved understanding of nanoparticle structural and chemical properties. Crucial indirect parameters that cannot be quantified by other means are accessed in this work, such as local strain, the possibility of forming core–shell structures, the fraction of Ge atoms diluted in the matrix (not forming nanoparticles), the amorphous and crystalline Ge fractions, and the relative population of nanoparticles with single and multiple crystalline domains.