Vibrational Properties of Nanoscale Materials: From Nanoparticles to Nanocrystalline Materials

The vibrational density of states (VDOS) of nanoclusters and nanocrystalline materials are derived from molecular-dynamics simulations using empirical tight-binding potentials. The results show that the VDOS inside nanoclusters can be understood as that of the corresponding bulk system compressed by the capillary pressure. At the surface of the nanoparticles the VDOS exhibits a strong enhancement at low energies and shows structures similar to that found near flat crystalline surfaces. For the nanocrystalline materials an increased VDOS is found at high and low phonon energies, in agreement with experimental findings. The individual VDOS contributions from the grain centers, grain boundaries, and internal surfaces show that, in the nanocrystalline materials, the VDOS enhancements are mainly caused by the grain-boundary contributions and that surface atoms play only a minor role. Although capillary pressures are also present inside the grains of nanocrystalline materials, their effect on the VDOS is different than in the cluster case which is probably due to the inter-grain coupling of the modes via the grain-boundaries.Comment: 10 pages, 7 figures, accepted for publication in Phys. Rev.

Annealing of gold nanostructures sputtered on polytetrafluoroethylene

Gold nanolayers sputtered on polytetrafluoroethylene (PTFE) surface and their changes induced by post-deposition annealing at 100°C to 300°C are studied. Changes in surface morphology and roughness are examined by atomic force microscopy, electrical sheet resistance by two point technique, zeta potential by electrokinetic analysis and chemical composition by X-ray photoelectron spectroscopy (XPS) in dependence on the gold layer thickness. Transition from discontinuous to continuous gold coverage takes place at the layer thicknesses 10 to 15 nm and this threshold remains practically unchanged after the annealing at the temperatures below 200°C. The annealing at 300°C, however, leads to significant rearrangement of the gold layer and the transition threshold increases to 70 nm. Significant carbon contamination and the presence of oxidized structures on gold-coated samples are observed in XPS spectra. Gold coating leads to a decrease in the sample surface roughness. Annealing at 300°C of pristine PTFE and gold-coated PTFE results in significant increase of the sample surface roughness

VARIATION DE LA MAILLE CRISTALLINE DE PETITS CRISTAUX D'OR PAR EFFET DE TAILLE

La variation de la maille cristalline de très petits cristaux d'or (25 Å à 140 Å de diamètre) a été mesurée en fonction de la taille des cristaux par diffraction électronique. Les résultats montrent que la maille cristalline diminue linéairement avec l'inverse de la taille des cristaux, ce qui est en accord qualitatif avec les résultats de C. W. Mays et al. Cependant, en interprétant cette contraction de maille en terme de la contrainte superficielle de l'or, nous obtenons une valeur de 3,83 Nm-1 soit le triple de la valeur obtenue par ces auteurs. L'importance de la soustraction du fond continu intervenant dans la détermination du diamètre des anneaux de diffraction est mise en évidence. Les résultats sont discutés en fonction de la structure et de la taille des cristaux.Electron diffraction has been used to measure variation in the crystal lattice parameter of very small gold particles (25 Å to 140 Å in diameter) as a function of crystal size. These results are qualitatively in agreement with those obtained by C. W. Mays et al. When this lattice contraction is interpreted in terms of the surface stress of gold, we obtain a value of 3.83 Nm-1 which is three times greater than the value found by Mays et al. Our results demonstrate the importance of substrating the continuous background in the determination of the diameter of the diffraction rings. The results are discussed as a function of the structure and size of the crystals