We report x-ray photon correlation spectroscopy studies of the static structure factor and dynamic correlation function of a gold colloid dispersed in the viscous liquid glycerol. We find a diffusion coefficient for Brownian motion of the gold colloid which agrees well with that extrapolated from measurements made with visible light, but which was determined on an optically opaque sample and in a wave-vector range inaccessible to visible light. PACS numbers: 61.10.Lx, 05.40.+j, 42.25.Kb, 82.70.Dd Photon correlation spectroscopy (PCS) probes the dynamics of a material by analyzing the temporal correlations among photons scattered by the material. Visible light PCS has proven to be an indispensible technique for studying the long wavelength hydrodynamics of fluids, including simple liquids, liquid mixtures, liquid crystals, polymers, and colloids [1]. However, visible PCS cannot probe the short wavelength dynamics of materials or opaque materials at all. The new field of x-ray PCS (XPCS) offers an unprecedented opportunity to extend the range of length scales over which a material's low frequency (10 -3 to 10 6 Hz) dynamics can be probed down to interatomic spacings. It is clear that demonstration of the ability to make dynamic XPCS measurements would be a major step forward in this nascent field.Many of the important problems in the low frequency dynamics of condensed matter systems for which XPCS should be uniquely suited arise in disordered materials, in particular, liquids. These include studies of the dynamic structure factor of liquids down to interatomic length scales, density fluctuations in liquids undergoing a glass transition, internal conformational dynamics and reptation in polymers, and equilibrium concentration fluctuations in polymer blends near a phase separation critical point.Here we report results of equilibrium dynamic measurements on a disordered system using XPCS. Specifically, we measured the diffusion coefficient for Brownian motion of gold colloid particles dispersed in glycerol. Our results unequivocally demonstrate the feasibility of the XPCS technique, as well as illustrate the crucial importance of both matching the longitudinal coherence length of the x rays to the experimental requirements and the tremendous benefit of utilizing area detectors in XPCS measurements.While the principles of XPCS have been known for decades, the very low flux of coherent x rays available with previous sources has, until now, precluded its application as a practical technique. The critical development which has now made XPCS a feasible technique is the use of insertion devices at second and third generation synchrotron sources. The work reported here was done on the wiggler beam line X25 at the Brookhaven National Synchrotron Light Source (NSLS).Previous work has consisted of measurements of the static speckle patterns resulting from the structure of antiphase domains in Cu 3 Au [2] or from grazing incidence scattering from gold-coated polymer films of inhomogeneous thickness [3]. Identification of equ...

Core-shell magnetic nanoparticles have received significant attention recently and are actively investigated owing to their large potential for a variety of applications. Here, the synthesis and characterization of bimetallic nanoparticles containing a magnetic core and a gold shell are discussed. The gold shell facilitates, for example, the conjugation of thiolated biological molecules to the surface of the nanoparticles. The composite nanoparticles were produced by the reduction of a gold salt on the surface of pre-formed cobalt or magnetite nanoparticles. The synthesized nanoparticles were characterized using ultraviolet-visible absorption spectroscopy, transmission electron microscopy, energy dispersion X-ray spectroscopy, X-ray diffraction and super-conducting quantum interference device magnetometry. The spectrographic data revealed the simultaneous presence of cobalt and gold in 5.6±0.8 nm alloy nanoparticles, and demonstrated the presence of distinct magnetite and gold phases in 9.2±1.3 nm core-shell magnetic nanoparticles. The cobalt-gold nanoparticles were of similar size to the cobalt seed, while the magnetite-gold nanoparticles were significantly larger than the magnetic seeds, indicating that different processes are responsible for the addition of the gold shell. The effect on the magnetic properties by adding a layer of gold to the cobalt and magnetite nanoparticles was studied. The functionalization of the magnetic nanoparticles is demonstrated through the conjugation of thiolated DNA to the gold shell.

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Two samples of polymer-coated cobalt nanoparticles were synthesized and dispersed in agarose gel and water. The relaxivities r1 and r2 of the two samples were obtained at different temperatures (25, 37 and 40 degrees C) and magnetic field strengths (1.5 and 3 T). The average cobalt core diameters of the two samples were 3.3 and 3.9 nm (measured by transmission electron microscopy); the corresponding average total diameters (cobalt core + polymer coating) were 13 and 28 nm (measured by dynamic light scattering). The larger particles had the higher r1 relaxivity, whilst r2 was similar for the two samples. There was no significant change in r1 or r2 relaxivities with temperature but r1 at 1.5 T was approximately double the value at 3 T. The highest relaxivities were obtained at 1.5 T with values for r1 and r2 of 7.4 and 88 mM(-1) s(-1), respectively. These values are similar to those reported for iron oxide with larger core size, suggesting the potential of the cobalt nanoparticles for development and future use as a negative contrast agent.

We have measured accurate x-ray crystallographic data from the InSb(l 11)2x2 reconstructed surface using synchrotron radiation from the DORIS storage ring at DESY. We have analyzed these independently of all models and find a structure with seven atoms in the unit cell, implying that one site is unoccupied. The bonding configuration is reasonable for a III-V semiconductor surface, and is topologically identical (in projection) to the "vacancy-buckling" model proposed for the GaAs(lll)2x 2 surface. The differences between the InSb and GaAs structures are significant and reflect chemical trends in the periodic table.PACS numbers: 68.20.+t, 61.10.FrOrbital rehybridization is currently considered to be a major determinant of the atomic geometry in compound-semiconductor surfaces. The III-V(llO) surfaces, well understood from low-energy electrondiffraction (LEED) studies, 1 buckle under this influence to reduce the bond angles at the surface group-V element and increase those at the group-Ill element, demonstrating ;>-type and sp 2 -type bonding tendencies, respectively. 2 A buckling model has also been proposed by Haneman 3 to allow similar rehybridization in the reconstructed GaAsO 11)2x2 surface, but Chadi 4 has shown that the buckled arrangement is unstable to the removal of one surface Ga atom. The vacancy soformed has been identified in a recent LEED structure analysis of GaAs(lll), 5 which shows dramatic inplane buckling of the remaining seven atoms in the surface unit cell. Because glancing-incidence x-ray diffraction is highly sensitive to in-plane surface structures, 6 it is well-suited to this problem, and so we applied it to the InSb(lll)2x 2 reconstructed surface and determined the atomic arrangement. Our analysis includes no prior knowledge of these models and derives directly a seven-atom structure of the projected unit cell. We are able even to identify the atom types and conclude that it is an In atom missing, implicating "vacancy buckling" in InSb(lll) also.Precisely cut, oriented, and polished InSb crystals were cleaned in 2x 10~1 0 mbar by repeated cycles of sputtering with 500-eV Ar + ions and annealing at 420 °C for several hours. The (111) surface gave a sharp 2x2 LEED pattern, while the (111) surface was 3x3; the latter is discussed elsewhere and not considered further here. 7 Valence-band energy distribu-tions curves were recorded at /zv = 30 eV with use of synchrotron light from the storage ring DORIS at Deutsches Elektronen-Synchrotron in Hamburg; these were found to be much more sensitive to surface contamination than conventional Auger spectra, and showed the surface to be clean. The sample was then transferred in UHV to a small, ion-pumped diffraction cell with a 360° beryllium window and was mounted on a diffractometer on the adjacent beam line.The white synchrotron beam was vertically monochromated by parallel Ge(lll) crystals and filtered by total reflection from a horizontal flat Au-coated mirrror. The diffraction took place in the horizontal plane of an air table which was tilted wit...

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