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Browsing by Author "Foran, GJ"

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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
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
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.
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
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
Focusing monochromator and imaging-plate camera for grazing-incidence diffraction studies of thin films
(International Union of Crystallography, 1997-08-04) Foran, GJ; Garrett, RF; Gentle, IR; Creagh, DC; Peng, JB; Barnes, GT
A multiple-imaging-plate detector system and focusing monochromator have been developed and successfully applied to the time-resolved study of phase transitions in Langmuir–Blodgett films by grazing-incidence X-ray diffraction (GIXD). The monochromator described here combines fixed-exit-beam height with sagittal focusing of the second crystal. The design is similar to that of Matsushita et al. [Matsushita, Ishikawa & Oyanagi (1986). Nucl. Instrum. Methods, A246, 377–379], with the exception that the motion of the first crystal is achieved via a computer-controlled X-Y translation table rather than a set of cams. The second crystal is a ribbed Si(111) wafer mounted in a four-point bending mechanism. The first reported application of imaging plates to a GIXD study was carried out by our group and proved to be very successful in the determination of thin-film structure [Foran, Peng, Steitz, Barnes & Gentle (1996)[Foran, G. J., Peng, J. B., Steitz, R., Barnes, G. T. & Gentle, I. R. (1996). Langmuir, 12, 774-777.]. Langmuir, 12, 774–777]. To extend the capabilities of this system, an imaging-plate camera was designed and built which can accommodate up to 13 imaging plates (40 × 20 cm) inside the vacuum chamber of the main diffractometer at the Australian Beamline at the Photon Factory. © International Union of Crystallography
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
Investigations on gold nanoparticles supported on rare earth oxide catalytic materials.
(Elsevier, 2007-04-18) Bhargava, SK; Akolekar, DB; Foran, GJ
Supported gold nanoparticles rare earth (europium, dysprosium, samarium oxide, neodymium, gadolinium oxide and lanthanum oxide) materials were prepared using precipitation–deposition/co-precipitation methods. The techniques employed for the characterization of these materials were ICP-MS, TEM, XRD, BET, and XAS. Au L3-edge X-ray absorption spectroscopic measurements were carried out over a series of rare earth materials containing gold nanoparticles. The size of gold nanoparticles varied in the range of 2 to 6nm in the Eu/Dy/Sm/Nd/Gd/La materials. These materials possess surface areas ranging from 29 to 41m2/g with the high phase purity and crystallinity of the support (Eu/Dy/Sm/Nd/Gd/La) materials. An X-ray absorption fine structure (XANES, EXAFS) technique was used in obtaining critical information about the atomic distances, bonding and neighbouring environment for gold atoms in the rare earth Eu/Dy/Sm/Nd/Gd/La oxide materials for understanding the typical characteristics and structure of gold nanoparticles in these materials. © 2007, Elsevier Ltd.
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.
Ion-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, MC
Elemental 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
Ion-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, MC
Ion 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.
Multiple scattering effects on the EXAFS of Ge nanocrystals
(Institute of Physics, 2008-04-23) Araujo, LL; Foran, GJ; Ridgway, MC
We 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
New 36‐element pixel array detector at the ANBF — choosing the right detector for your beamline
(American Insitute of Physics, 2007-01-19) Foran, GJ; Hester, JR; Garrett. RF; Dressler. P; Fonne, C; Beau, JO; Lampert, MO
The Pixel Array Detector for XAFS data collection recently commissioned at the Australian National Beamline Facility is well matched to a busy second‐generation bending magnet source where both high and low‐flux applications are routinely encountered. In combination with the digital counting chain, throughput has improved by approximately a factor of 5. Detector resolution deteriorates slightly at high count rates. © 2007 American Institute of Physics.
Oxidation state of uranium in mantle melts
(Elsevier; Cambridge Publications, 2008-07) Berry, AJ; O'Neill, HSC; Foran, GJ
The oxidation state of U exerts a first order control on its partitioning during partial mantle melting. An understanding of U partitioning, and its abundance relative to other members of the U decay series, is important for interpreting U series disequilibrium, with implications for the rate of melt transport at mid-ocean ridges. U LIII-edge X-ray absorption near edge structure (XANES) spectra were recorded for a synthetic mid-ocean ridge basalt (MORB), and a number of CMAS (CaO-MgOAl2O3-SiO2) compositions, containing 0.5 wt % U3O8, equilibrated at 1400 ˚C and controlled oxygen fugacities (fO2)at one-atmosphere. Spectra were obtained for both quenched glasses, and in situ at magmatic temperatures (Berry et al. 2003). In situ measurements allowed changes in the spectra in response to the imposed fO2 to be followed in real time. The spectra were recorded in fluorescence mode at the Australian National Beamline Facility (beamline 20B), Photon Factory, Japan. The spectra exhibit systematic variations in absorption edge energy and crest intensity with fO2. These changes occur between -4 and +6 log units of the nickel-nickel oxide (NNO) fO2 buffer. The fO2 range of the spectral variation indicates that U4+ must oxidise to U6+ through two stepwise one-electron reactions, involving a U5+ intermediate, rather than a direct two-electron process. The results also suggest that U5+ may be the dominant oxidation state at the fO2 conditions of MORB generation. © 2008 Elsevier Ltd
Powder diffraction using imaging plates at the Australian National Beamline Facility at the Photon Factory
(American Institute of Physics, 1994-07-18) Garrett, RF; Cookson, DJ; Foran, GJ; Sabine, TM; Kennedy, BJ; Wilkins, SW
A novel x‐ray diffractometer was installed at the Australian National Beamline Facility at the Photon Factory, Japan, in October 1993. One of the major capabilities of the instrument is high speed high resolution powder diffraction using imaging plate detectors. The diffractometer combines a two circle goniometer and a large cassette in which imaging plates can be loaded covering 320° of 2θ. The diffractometer is enclosed in a large vacuum chamber and can be operated in air, vacuum, or helium. Recently, powder data has been obtained from rutile (TiO2) and NBS Si 640b at wavelengths from 0.62 to 1.9 Å using imaging plates, and has been used to characterize the performance of the instrument. The data has been refined using the Rietveld method and R values of under 2% obtained. The resolution of the system varies from a minimum of about 0.04° to around 0.25° at 2θ angles around 160°, which is the equal of most synchrotron based powder diffractometers, and only slightly worse than that obtained using an analyzer crystal and scintillation detector. Using the imaging plates, 160° of data is simultaneously acquired in an exposure of about 10 min, compared to conventional counter diffractometer scans which routinely exceed 10 hours. © 1995 American Institute of Physics.
Preferential 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, A
Relative 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.
Preferential 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, A
Extended 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
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
Stabilization of triam(m)inechloridoplatinum complexes by oxidation to PtIV.
(CSIRO Publishing, 2011-03-11) Daly, HL; Hall, MD; Failes, TW; Zhang, M; Foran, GJ; Hambley, TW
PtIV analogues of the active end groups {PtClN3} of multinuclear Pt anticancer drugs have been investigated. The crystal structure of trans,mer-[PtCl(OH)2(dien)]Cl shows that the bond lengths are similar to those in the dihydroxidoplatinum(iv) analogue of cisplatin. The axial ligands are shown to be the predominant influence on reduction potentials with the dihydroxido complex trans,mer-[PtCl(OH)2(NH3)3]Cl being the most resistant to reduction. X-ray absorption near-edge spectroscopy is shown to be suitable for monitoring the oxidation state of these complexes and reveals that trans,mer-[PtCl(OH)2(NH3)3]+ survives for more than 2 h in cancer cells. © 2011, CSIRO Publishing
Status 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, A
We 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.

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