T Comaschi

Publications (3)2.55 Total impact

• Article: XAFS study of the thermal behaviour of gold bulk and clusters

  T Comaschi, A Balerna, S Mobilio
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  ABSTRACT: Materials showing negative thermal expansion (NTE) coefficient over large temperature ranges are nowadays of great interest for their possible applications. Small nanoparticles show changes in their properties with respect to the corresponding bulk, mainly due to the high surface to volume ratio and to the confinement of electrons in a small volume. In the present paper we report a x-ray absorption fine structure (XAFS) study on the thermal expansion coefficient of an Au foil and of Au nanoparticles of very small dimensions ranging from 2.4 nm and 5.0 nm. Their L3 edge has been investigated in the temperature range 20K – 300K and a very accurate data analysis has been performed taking into account the presence of asymmetry effects. All clusters showed a thermal trend of the first shell distance significantly different from that of the bulk. The larger clusters were characterized by a reduction of the thermal expansion coefficient with respect to bulk; in the smallest samples the crossover from a thermal expansion to a NTE effect was observed. A simple model, based on the contribution of localized states induced by the finite size of the clusters, qualitatively accounts for the observed behaviour.
  Journal of Physics Conference Series 11/2009; 190(1):012122.
• Article: Thermal dependent anharmonicity effects on gold bulk studied by extended x-ray-absorption fine structure.

  T Comaschi, A Balerna, S Mobilio
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  ABSTRACT: The structural parameters of the first five coordination shells of an Au bulk obtained from high accuracy L(3)-edge extended x-ray absorption fine structure (EXAFS) spectra in the temperature range 20-300 K are reported. Good agreement with previously reported studies is found. The effective second and third order force constants evaluated using EXAFS data are compatible with those calculated from phonon dispersion curves. A careful comparison of the variations of the EXAFS first shell distance with x-ray diffraction data provided the mean squared relative displacement of the atomic vibrations perpendicular to the first interatomic bond. An alternative new approach that is useful in achieving this parameter when x-ray diffraction data are not available is proposed.
  Journal of Physics Condensed Matter 08/2009; 21(32):325404. · 2.55 Impact Factor
• Article: Temperature dependence of the structural parameters of gold nanoparticles investigated with EXAFS

  T. Comaschi, A. Balerna, S. Mobilio
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  ABSTRACT: The L3 edge of Au nanoparticles, having sizes ranging from 2.4 to 5.0 nm, have been investigated by x-ray absorption fine structure spectroscopy in the temperature range of 20–300 K. Data were recorded at the European Synchrotron Radiation Facility with a very good signal to noise ratio. To achieve a very high accuracy in the determination of the first shell distance, a very careful data analysis was performed also taking into account the presence of asymmetry effects. In all samples, the temperature dependence of the first neighbor distance results is different from that of the macrocrystalline counterpart. In the largest size samples, a reduction of the thermal expansion was found, whereas in the smallest ones, the presence of a crossover from an initial thermal expansion to a thermal contraction was observed. Calculations based on a simple model show that localization effects that increase as the nanoparticle size decreases can explain the reported thermal effects.
  Phys. Rev. B. 77(7).