Browsing by Author "Ridgway, MC"
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- ItemAnisotropic vibrations in crystalline and amorphous InP(American Physical Society, 2009-05) Schnohr, CS; Kluth, P; Araujo, LL; Sprouster, DJ; Byrne, AP; Foran, GJ; Ridgway, MCThe 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
- ItemAtomic-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, MCExtended 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
- ItemChanges 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, DJChanges 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.
- ItemComparison 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, MCInP 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
- ItemFcc-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, MCWe 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
- ItemInfluence 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, MCThe 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
- ItemIon irradiation effects on metallic nanocrystals(Taylor & Francis, 2007-07) Kluth, P; Johannessen, B; Giulian, R; Schnohr, CS; Foran, GJ; Cookson, DJ; Byrne, AP; Ridgway, MCWe 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.
- ItemThe ion-irradiation tolerance of the pyrochlore to fluorite Ho(x)Yb(2-x)TiO5 and Er2TiO5 compounds: a TEM comparative study using both in-situ and bulk ex-situ irradiation approaches(Elsevier, 2018-08-15) Aughterson, RD; Lumpkin, GR; Smith, KL; de los Reyes, M; Davis, J; Avdeev, M; Ridgway, MC; Cairney, JMWe refine the crystal structures of a systematic series of compounds with the general composition Ho(x)Yb(2-x)TiO5 (x = 2, 1.6, 1.2, 1, 0.8, 0.4, 0) and Er2TiO5 and find a transition from defect-pyrochlore to defect-fluorite structure with increasing ytterbium content, decreasing lanthanide radius. Short-range structure modulations consisting of pyrochlore-like nano-domains are systematically characterised using transmission electron microscopy. We test the Kr2+ 1 MeV ion-irradiation response of Ho2TiO5, HoYbTiO5, Yb2TiO5, and Er2TiO5, via the crystalline to amorphous transition observed by using the in-situ TEM approach. The critical dose of amorphisation, Dc, was measured at various temperatures and used to calculate the critical temperature for maintaining crystallinity, Tc. A trend of lower Tc values with decreasing lanthanide radius is found. We describe a new approach for determining Tc values using cross-sectional TEM analysis of ex-situ bulk irradiated, 1 MeV Se+, samples; Ho2TiO5, HoYbTiO5 and Yb2TiO5. The results of Dc and Tc values using the two approaches vary; however the trends across the sample system remain the same. © 2018 Published by Elsevier B.V.
- ItemIon-irradiation-induced amorphization of Cu nanoparticles embedded in SiO2(American Physical Society, 2007-11) Johannessen, B; Kluth, P; Llewellyn, DJ; Foran, GJ; Cookson, DJ; Ridgway, MCElemental Cu nanoparticles embedded in SiO2 were irradiated with 5 MeV Sn3+. The nanoparticle structure was studied as a function of Sn3+ fluence by extended x-ray absorption fine structure spectroscopy, small-angle x-ray scattering, and transmission electron microscopy. Prior to irradiation, Cu nanoparticles exhibited the face-centered-cubic structure. Upon irradiation at intermediate fluences (1×1013 to 1×1014 ions/cm2), the first nearest neighbor Cu-Cu coordination number decreased, while the Debye-Waller factor, bondlength, and third cumulant of the bondlength distribution increased. In particular, at a fluence of 1×1014 ions/cm2 we argue for the presence of an amorphous Cu phase, for which we deduce the structural parameters. Low temperature annealing (insufficient for nanoparticle growth) of the amorphous Cu returned the nanoparticles to the initial preirradiation structure. At significantly higher irradiation fluences (1×1015 to 1×1016 ions/cm2), the nanoparticles were dissolved in the matrix with a Cu coordination similar to that of Cu2O. © 2007, American Physical Society
- ItemIon-irradiation-induced porosity in GaSb and InSb(Australian Institute of Physics, 2005-01-31) Kluth, SM; Johannessen, B; Kluth, P; Glover, CJ; Foran, GJ; Ridgway, MCIon irradiation of crystalline GaSb and InSb can yield not only amorphisation, as commonly observed in semiconductors, but also porosity. Extended x-ray absorption fine structure spectroscopy, electron microscopy and Rutherford backscattering spectrometry have been used to determine the exact nature of and relationship between these two transformations. In both materials, low dose, room temperature implantation produces spherical voids yet the material remains crystalline. With increasing implant dose, the porous layer eventually evolves into a network of straight rods 15nm in diameter. We suggest the porosity arises from preferential clustering of interstitials into extended defects and vacancies agglomerating to form voids.
- ItemMeasurement 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, MCIn 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.
- ItemMultiple scattering effects on the EXAFS of Ge nanocrystals(Institute of Physics, 2008-04-23) Araujo, LL; Foran, GJ; Ridgway, MCWe present a detailed extended x-ray absorption fine structure (EXAFS) spectroscopy study on the influence of multiple scattering effects on the analysis of bulk polycrystalline Ge (c-Ge) and of four Ge nanocrystal (NC) distributions with mean sizes from 4 to 9 nm. A complete description of the EXAFS signal up to the third shell of nearest neighbours for both c-Ge and Ge NCs is only achieved by including at least two double scattering and one triple scattering path. Unlike reports for bulk semiconductors and Ge-Si quantum dots, our results show that including only the most prominent double scattering path is insufficient for accurately ascertaining the structural parameters of the second and third shells, leading to unphysically small coordination numbers for the NCs. The same is observed when no multiple scattering paths are taken into account. The size dependence of the interatomic distance distributions up to the third shell of nearest neighbours has been determined for the first time. A greater reduction in coordination numbers and higher structural disorder were observed for the outer shells, reflecting the increase of the surface-to-volume ratio and reinforcing the presence of an amorphous Ge layer between the SiO2 matrix and the NCs. © 2008, Institute of Physics
- ItemPreferential amorphisation of Ge nanocrystals in a silica matrix(Australian Institute of Physics, 2005-01-31) Ridgway, MC; Azevedo, GDM; Elliman, RG; Wesch, W; Glover, CJ; Miller, R; Llewellyn, DJ; Foran, GJ; Hansen, JL; Nylandsted Larsen, ARelative to bulk crystalline material, Ge nanocrystals in a silica matrix exhibit subtle structural perturbations including a non-Gaussian inter-atomic distance distribution. We now demonstrate such nanocrystals are extremely sensitive to ion irradiation. Using transmission electron microscopy, Raman spectroscopy and extended x-ray absorption fine structure spectroscopy, the crystalline-to-amorphous phase transformation in -8 nm diameter nanocrystals and bulk crystalline material has been compared. Amorphisation of Ge nanocrytals in a silica matrix was achieved at an ion dose -100 times less than that required for bulk crystalline standards. This rapid amorphisation of Ge nanocrystals is attributed to the preferential nucleation of the amorphous phase at the nanocrystal/matrix interface, the pre-irradiation, higher-energy structural state of the nanocrystals themselves and an enhanced nanocrystal vacancy concentration due to the more effective trapping of irradiation-induced interstitials at the nanocrystal/matrix interface and inhibited Frenkel pair recombination when Ge interstitials are recoiled into the matrix. To demonstrate the significance of the latter, we show ion irradiation of -2 nm diameter nanocrystals yields their dissolution when the range of recoiled Ge atoms exceeds the nanocrystal bounds.
- ItemPreferential amorphisation of Ge nanocrystals in a silica matrix(Elsevier, 2004-09-05) Ridgway, MC; Azevedo, GDM; Elliman, RG; Wesch, W; Glover, CJ; Miller, R; Llewellyn, DJ; Foran, GJ; Hansen, JL; Nylandsted Larsen, AExtended X-ray absorption fine structure and Raman spectroscopies have been used to compare the crystalline-to-amorphous phase transformation in nanocrystalline and polycrystalline Ge. We demonstrate Ge nanocrystals are extremely sensitive to ion irradiation and are rendered amorphous at an ion dose ∼40 times less than that required to amorphise bulk, crystalline standards. This rapid amorphisation is attributed to the higher-energy nanocrystalline structural state prior to irradiation, inhibited Frenkel pair recombination when Ge interstitials are recoiled into the matrix and preferential nucleation of the amorphous phase at the nanocrystal/matrix interface. © 2005 Elsevier B.V
- ItemThe response of complex ceramic oxides exposed to ion-irradiation, compared using two TEM characterisation techniques; bulk, ex-situ, and thin crystal, in-situ(Australian Microscopy and Microanalysis Society, 2015-02-12) Aughterson, RD; Cairney, JM; Ridgway, MC; Zaluzec, NJ; Lumpkin, GRThe response of materials exposed to high energy particles has been an active area of research for several decades, due both to potentially improved and detrimental effects on the material properties. For the nuclear industry ion-irradiation has been used to simulate recoil damage from alpha decay, and exposure to neutrons in fission and fusion reactors [1]. Whilst there are many changes to the host material caused via the impact of accelerated ions the particular focus of this research is on the crystalline to amorphous transition. The amorphisation of the host material can lead to detrimental effects on its properties such as swelling, embrittlement, and micro-cracking leading to eventual structural failure. The complex ceramic oxides chosen for this study, Ln2TiO5 (Ln = lanthanides and yttrium), have several uses within the nuclear industry. The compound Dy2TiO5 has been used within Russian WWER type reactors for two decades due to its good resistance to irradiation induced swelling and structural failure [2]. Of particular interest are the cubic symmetry compounds with defect fluorite structure, which gives good radiation response. Previous studies have indicated that the series ofLn2TiO5 compounds may take on a variety of crystal symmetries depending on the lanthanide size and fabrication conditions used [3, 4].Previous to any ion-irradiation exposure the materials of interest were tested for homogeneity of stoichiometry and crystallography. Characterisation was carried out via backscattered electron imaging (Z contrast) to highlight any variations in elemental composition. This was followed up with multiple spot analyses using energy dispersive x-ray spectroscopy to confirm the homogeneous nature of the material plus the stoichiometry. X-ray diffraction was used to determine the long range symmetry of the test materials plus confirm the single structure nature. Any materials found to have more than one crystal structure type or greater than 5% secondary phase were rejected for further ion-irradiation based experiments. The preliminary study of ion-irradiation response was carried out using the in-situ approach where the test materials were exposed to 1 MeV Kr2+ ions and monitored for their transition from crystalline to amorphous state. The in-situ ion-irradiation was carried out using the intermediate voltage electron microscope (IVEM)-Tandem facility at Argonne National Laboratory. The critical dose of irradiating ions, Dc, required to render the Ln2TiO5 completely amorphous was determined by monitoring selected area electron diffraction patterns for loss of diffraction spots (Bragg maxima) and replacement with diffuse rings (refer to Figure 1). Further bulk Se+ ion-irradiation was carried out at the Australian National University using the TANDEM, heavy ion accelerator. The damage penetration depth was characterised ex-situ using cross-sectional TEM. The cross-sectional damage depth profile of the bulk sample was compared with simulation, SRIM (Stopping Range of Ions in Matter), based calculations and a critical dose of amorphisation value attained. By using these two TEM characterisation approaches the thin crystal in-situ results can be compared with the more “realistic” bulk approach. ©2015 Australian Microscopy and Microanalysis Society
- ItemShape 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, MCPt 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
- ItemSize-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, MCA 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
- ItemStatus of the x-ray absorption spectroscopy (XAS) beamline at the Australian synchrotron(American Institute of Physics, 2007-02-02) Glover, CJ; McKinlay, J; Clift, M; Barg, B; Boldeman, JW; Ridgway, MC; Foran, GJ; Garrett, RL; Lay, PA; Broadbent, AWe present herein the current status of the X-ray Absorption Spectroscopy (XAS) Beamline at the 3 GeV Australian Synchrotron. The optical design and performance, details of the insertion device (Wiggler), end station capabilities and construction and commissioning timeline are given.
- ItemStructural and vibrational properties of co nanoparticles formed by ion implantation(American Institute of Physics, 2010-01-01) Sprouster, DJ; Giulian, R; Araujo, LL; Kluth, P; Johannessen, B; Cookson, DJ; Foran, GJ; Ridgway, MCWe report on the structural and vibrational properties of Co nanoparticles formed by ion implantation and thermal annealing in amorphous silica. The evolution of the nanoparticle size, phase, and structural parameters were determined as a function of the formation conditions using transmission electron microscopy, small-angle x-ray scattering, and x-ray absorption spectroscopy. The implantation fluence and annealing temperature governed the spherical nanoparticle size and phase. To determine the latter, x-ray absorption near-edge structure analysis was used to quantify the hexagonal close packed, face-centered cubic and oxide fractions. The structural properties were characterized by extended x-ray absorption fine structure spectroscopy (EXAFS) and finite-size effects were readily apparent. With a decrease in nanoparticle size, an increase in structural disorder and a decrease in both coordination number and bondlength were observed as consistent with the non-negligible surface-area-to-volume ratio characteristic of nanoparticles. The surface tension of Co nanoparticles calculated using a liquid drop model was more than twice that of bulk material. The size-dependent vibrational properties were probed with temperature-dependent EXAFS measurements. Using a correlated anharmonic Einstein model and thermodynamic perturbation theory, Einstein temperatures for both nanoparticles and bulk material were determined. Compared to bulk Co, the mean vibrational frequency of the smallest nanoparticles was reduced as attributed to a greater influence of loosely bonded, undercoordinated surface atoms relative to the effect of capillary pressure generated by surface curvature. © 2010, American Institute of Physics
- ItemStructural characterization of Ge nanocrystals in silica amorphised by ion irradiation(Elsevier, 2008-06) Araujo, LL; Giulian, R; Johannessen, B; Llewellyn, DJ; Kluth, P; Azevedo, GDM; Cookson, DJ; Ridgway, MCGe nanocrystals (NCs) grown by ion implantation in amorphous silica matrices were irradiated with 5 MeV Si ions over a different fluence range (2 x 10(11)-2 x 10(13) cm(-2)) than previously reported. Size and depth distributions as well as structural disorder in the NCs were measured by RBS, TEM, SAXS and EXAFS. The EXAFS results show that the embedded Ge NCs are rendered amorphous at fluences similar to 40 times lower than bulk crystalline Ge (c-Ge). No significant changes in the size or depth distribution of the NCs are observed for all irradiation fluences. Compared to c-Ge, the higher-energy structural state of the NCs prior to irradiation and the presence of the nanocrystal/matrix interface are considered the main causes for the peculiar amorphisation behavior of embedded Ge NCs. © 2008, Elsevier Ltd.