Browsing by Author "Schnohr, CS"

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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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    Atomic-scale structure of Ga1-xInxP alloys measured with extended x-ray absorption fine structure spectroscopy
    (American Physical Society, 2008-09) Schnohr, CS; Araujo, LL; Kluth, P; Sprouster, DJ; Foran, GJ; Ridgway, MC
    Extended x-ray absorption fine structure spectroscopy was used to measure the interatomic distance distributions of the first three nearest-neighbor (NN) shells around Ga and In atoms in Ga1-xInxP. The first NN shell has a composition-dependent bimodal distance distribution with a relaxation parameter of epsilon= 0.80 +/-0.04 similar to other III-V ternary alloys. The second NN shell distance distribution remains multimodal, corresponding to the three different cation-cation pairs but is closer to the virtual-crystal approximation (VCA). The third NN shell mean distance is well approximated by the VCA although the distribution is significantly broadened. Predictive model calculations are discussed in detail where good agreement with experimental results is found. Like in Ga1-xInxAs, lattice mismatch is accommodated in Ga1-xInxP by both bond-length and bond-angle relaxations although primarily via the latter. © 2008, 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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    Influence of annealing conditions on the growth and structure of embedded Pt nanocrystals
    (American Institute of Physics, 2009-02-15) Giulian, R; Araujo, LL; Kluth, P; Sprouster, DJ; Schnohr, CS; Johannessen, B; Foran, GJ; Ridgway, MC
    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 scattering measurements demonstrate that the annealing ambient has a significant influence on NC size. Samples annealed in either Ar, O-2, or forming gas (95% N-2: 5% H-2) at temperatures ranging from 500 degrees C-1300 degrees C form spherical NCs with mean diameters ranging from 1-14 nm. For a given temperature, annealing in Ar yields the smallest NCs. O-2 and forming gas ambients produce NCs of comparable size though the latter induces H chemisorption at 1100 degrees 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. © 2009, American Institute of Physics
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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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    Size-dependent characterization of embedded Ge nanocrystals: structural and thermal properties
    (American Physical Society, 2008-09) Araujo, LL; Giulian, R; Sprouster, DJ; Schnohr, CS; Llewellyn, DJ; Kluth, P; Cookson, DJ; Foran, GJ; Ridgway, MC
    A combination of conventional and synchrotron-based techniques has been used to characterize the size-dependent structural and thermal properties of Ge nanocrystals (NCs) embedded in a silica (a-SiO2) matrix. Ge NC size distributions with four different diameters ranging from 4.0 to 9.0 nm were produced by ion implantation and thermal annealing as characterized with small-angle x-ray scattering and transmission electron microscopy. The NCs were well represented by the superposition of bulklike crystalline and amorphous environments, suggesting the formation of an amorphous layer separating the crystalline NC core and the a-SiO2 matrix. The amorphous fraction was quantified with x-ray-absorption near-edge spectroscopy and increased as the NC diameter decreased, consistent with the increase in surface-to-volume ratio. The structural parameters of the first three nearest-neighbor shells were determined with extended x-ray-absorption fine-structure (EXAFS) spectroscopy and evolved linearly with inverse NC diameter. Specifically, increases in total disorder, interatomic distance, and the asymmetry in the distribution of distances were observed as the NC size decreased, demonstrating that finite-size effects govern the structural properties of embedded Ge NCs. Temperature-dependent EXAFS measurements in the range of 15-300 K were employed to probe the mean vibrational frequency and the variation of the interatomic distance distribution (mean value, variance, and asymmetry) with temperature for all NC distributions. A clear trend of increased stiffness (higher vibrational frequency) and decreased thermal expansion with decreasing NC size was evident, confirming the close relationship between the variation of structural and thermal/vibrational properties with size for embedded Ge NCs. The increase in surface-to-volume ratio and the presence of an amorphous Ge layer separating the matrix and crystalline NC core are identified as the main factors responsible for the observed behavior, with the surrounding a-SiO2 matrix also contributing to a lesser extent. Such results are compared to previous reports and discussed in terms of the influence of the surface-to-volume ratio in objects of nanometer dimensions. © 2008, American Physical Society
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    Temperature-dependent EXAFS analysis of embedded Pt nanocrystals
    (Institute of Physics, 2009-04-15) Giulian, R; Araujo, LL; Kluth, P; Sprouster, DJ; Schnohr, CS; Foran, GJ; Ridgway, MC
    The vibrational and thermal properties of embedded Pt nanocrystals (NCs) have been investigated with temperature-dependent extended x-ray absorption fine structure (EXAFS) spectroscopy. NCs of diameter 1.8-7.4 nm produced by ion implantation in amorphous SiO2 were analysed over the temperature range 20-295 K. An increase in Einstein temperature (similar to 194 K) relative to that of a Pt standard (similar to 179 K) was evident for the smallest NCs while those larger than similar to 2.0 nm exhibited values comparable to bulk material. Similarly, the thermal expansion of interatomic distances was lowest for small NCs. While the amorphous SiO2 matrix restricted the thermal expansion of interatomic distances, it did not have a significant influence on the mean vibrational frequency of embedded Pt NCs. Instead, the latter was governed by finite-size effects or, specifically, capillary pressure. © 2009, Institute of Physics