Browsing by Author "Byrne, AP"

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    Anisotropic vibrations in crystalline and amorphous InP
    (American Physical Society, 2009-05) Schnohr, CS; Kluth, P; Araujo, LL; Sprouster, DJ; Byrne, AP; Foran, GJ; Ridgway, MC
    The temperature-dependent evolution of atomic vibrations in crystalline and amorphous InP has been studied using extended x-ray absorption fine-structure (EXAFS) spectroscopy. Measurements were performed at the In K edge for temperatures in the range of 20-295 K. In crystalline InP, the first nearest-neighbor (NN) EXAFS Debye-Waller factor, representative of the correlated mean-square relative displacement (MSRD) parallel to the bond direction, is considerably smaller than the uncorrelated mean-square displacement (MSD) determined from x-ray diffraction measurements. In contrast, the MSRD perpendicular to the bond direction agrees well with the MSD. This clearly demonstrates that vibrations of two neighboring atoms relative to each other are strongly reduced along the bond direction but are unhindered perpendicular to it, consistent with the well-known behavior of III-V semiconductors where bond bending is energetically favored over bond stretching. With increasing interatomic distance, the correlation of atomic motion quickly vanishes as manifested by increased EXAFS Debye-Waller factors. For the third NN shell the value closely approaches the MSD demonstrating the nearly uncorrelated motion of atoms only three shells apart. In the amorphous phase, only information about the first NN shell is accessible although the latter is now comprised of both P and In atoms. The EXAFS Debye-Waller factors are significantly higher than in the crystalline phase but exhibit a very similar temperature dependence. This results from strongly increased structural disorder in the amorphous phase whereas the thermally induced disorder is very similar to that in crystalline InP. A correlated Einstein model was fitted to the Debye-Waller factors yielding Einstein temperatures that vary as functions of the vibrational phase difference and reduced mass of the atomic pair, and represent a measure of the strength and thermal evolution of the corresponding relative vibrations. © 2009, American Physical Society
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    Changes in metal nanoparticle shape and size induced by swift heavy-ion irradiation
    (Elsevier, 2009-03) Ridgway, MC; Kluth, P; Giulian, R; Sprouster, DJ; Araujo, LL; Schnohr, CS; Llewellyn, DJ; Byrne, AP; Foran, GJ; Cookson, DJ
    Changes in the shape and size of Co, Pt and An nanoparticles induced by swift heavy-ion irradiation (SHII) have been characterized using a combination of transmission electron microscopy, small-angle X-ray scattering and X-ray absorption near-edge Structure. Elemental nanoparticles of diameters 2-15 nm were first formed in amorphous SiO2 by ion implantation and thermal annealing and then irradiated at room temperature with 27-185 MeV Au ions as a function of fluence. Spherical nanoparticles below a minimum diameter (4-7 nm) remained spherical under SHII but progressively decreased in size as a result of dissolution into the SiO2 matrix. Spherical nanoparticles above the minimum diameter threshold were transformed to elongated rods aligned with the ion beamdirection. The nanorod width saturated at an electronic energy deposition dependent value, progressively increasing from 4-6 to 7-10nm (at 518 keV/nm, respectively) while the nanorod length exhibited a broad distribution consistent with that of the unirradiated spherical nanoparticles. The threshold diameter for spherical nanoparticle elongation was comparable to the saturation value of nanorod width. We correlate this saturation value with the diameter of the molten track induced in amorphous SiO2 by SHII. In summary, changes in nanoparticle shape and size are governed to a large extent by the ion irradiation parameters. © 2009, Elsevier Ltd.
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    Comparison of the atomic structure of InP amorphized by electronic or nuclear ion energy-loss processes
    (American Physical Society, 2008-02) Schnohr, CS; Kluth, P; Byrne, AP; Foran, GJ; Ridgway, MC
    InP was amorphized by ion irradiation in two very different regimes: (i) 185 MeV Au irradiation, where the energy loss was predominantly via inelastic processes (electronic stopping), or (ii) Se irradiation in an energy range of 0.08-7 MeV, where elastic processes (nuclear stopping) were dominant. The structural parameters of the amorphous phase were determined for as-irradiated and thermally relaxed samples using extended x-ray absorption fine structure spectroscopy. Despite the fundamentally different energy transfer mechanisms, no significant difference in the atomic structure of the two amorphized samples was observed. We attribute this result to a common "melt and quench" process responsible for amorphization. In fact, the measured structural parameters for the amorphized samples, including the fraction of homopolar In-In bonding, were consistent with simulations of the amorphous phase produced by assuming a quench from the melt. © 2008, American Physical Society
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    Fcc-hcp phase transformation in Co nanoparticles induced by swift heavy-ion irradiation
    (American Physical Society, 2009-09) Sprouster, DJ; Giulian, R; Schnohr, CS; Araujo, LL; Kluth, P; Byrne, AP; Foran, GJ; Johannessen, B; Ridgway, MC
    We demonstrate a face-centered cubic (fcc) to hexagonally close-packed (hcp) phase transformation in spherical Co nanoparticles achieved via swift heavy-ion irradiation. Co nanoparticles of mean diameter 13.2 nm and fcc phase were first formed in amorphous SiO2 by ion implantation and thermal annealing and then irradiated at room temperature with 9-185 MeV Au ions. The crystallographic phase was identified with x-ray absorption spectroscopy and electron diffraction and quantified, as functions of the irradiation energy and fluence, with the former. The transformation was complete at low fluence prior to any change in nanoparticle shape or size and was governed by electronic stopping. A direct-impact mechanism was identified with the transformation interaction cross-section correlated with that of a molten ion track in amorphous SiO2. We suggest the shear stress resulting from the rapid thermal expansion about an ion track in amorphous SiO2 was sufficient to initiate the fcc-to-hcp phase transformation in the Co nanoparticles. © 2009, American Physical Society
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    Ion irradiation effects on metallic nanocrystals
    (Taylor & Francis, 2007-07) Kluth, P; Johannessen, B; Giulian, R; Schnohr, CS; Foran, GJ; Cookson, DJ; Byrne, AP; Ridgway, MC
    We have investigated structural and morphological properties of metallic nanocrystals ( NCs) exposed to ion irradiation. NCs were characterized by transmission electron microscopy in combination with advanced synchrotron-based analytical techniques, in particular X-ray absorption spectroscopy and small-angle X-ray scattering. A number of different effects were observed depending on the irradiation conditions. At energies where nuclear stopping is predominant, structural disorder/amorphization followed by inverse Ostwald ripening/dissolution due to ion beam mixing was observed for Au and Cu NCs embedded in SiO2. The ion-irradiation-induced crystalline to amorphous transition in the NCs, which cannot be achieved in the corresponding bulk metals, was attributed to their initially higher structural energy as compared to bulk material and possibly preferential nucleation of the amorphous phase at the NC/SiO2 interface. At very high irradiation energies (swift heavy ion irradiation), where the energy loss is nearly entirely due to electronic stopping, a size-dependent shape transformation of the NCs from spheres to rod like shapes was apparent in Au NCs. Our preliminary results are in good agreement with considerations on melting of the NCs in the ion track as one mechanism involved in the shape transformation. © 2007, Taylor & Francis Ltd.
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    Measurement of latent tracks in amorphous SiO2 using small angle x-ray scattering
    (Elsevier, 2008-06) Kluth, P; Schnohr, CS; Sprouster, DJ; Byrne, AP; Cookson, DJ; Ridgway, MC
    In this paper we present preliminary yet promising results on the measurement of latent ion tracks in amorphous, 2 mu m thick SiO2 layers using small angle X-ray scattering (SAXS). The tracks were generated by ion irradiation with 89 MeV An ions to fluences between 3 x 10(10) and 3 x 10(12) ions/cm(2). Transmission SAXS measurements show distinct scattering from the irradiated SiO2 as compared to the unirradiated material. Analysis of the SAXS spectra using a cylindrical model suggests a core-shell like density distribution in the ion tracks with a lower density core and a higher density shell as compared to unirradiated material. The total track radius of similar to 48 angstrom is in very good agreement with previous experiments and calculations based on an inelastic thermal spike model. © 2008, Elsevier Ltd.
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    Shape transformation of Pt nanoparticles induced by swift heavy-ion irradiation
    (American Physical Society, 2008-09) Giulian, R; Kluth, P; Araujo, LL; Sprouster, DJ; Byrne, AP; Cookson, DJ; Ridgway, MC
    Pt nanoparticles (NPs) formed by ion-beam synthesis in amorphous SiO2 were irradiated with Au ions in the energy range of 27-185 MeV. Small-angle x-ray scattering (SAXS) and transmission electron microscopy were used to characterize an irradiation-induced shape transformation within the NPs. A simple yet effective way of analyzing the SAXS data to determine both NP dimensions is presented. A transformation from spherical to rodlike shape with increasing irradiation fluence was observed for NPs larger than an energy-dependent threshold diameter, which varied from 4.0 to 6.5 nm over 27-185 MeV. NPs smaller than this threshold diameter remained spherical upon irradiation but decreased in size as a result of dissolution. The latter was more pronounced for the smallest particles. The minor dimension of the transformed NPs saturated at an energy-dependent value comparable to the threshold diameter for elongation. The saturated minor dimension was less than the diameter of the irradiation-induced molten track within the matrix. We demonstrate that Pt NPs of diameter 13 nm reach saturation of the minor dimension beyond a total-energy deposition into the matrix of 20 keV/nm(3). © 2008, American Physical Society