Surface Stress and Size Effect on the Lattice Parameter in Small Particles of Gold and Platinum

Small particles of gold and platinum with mean sizes ranging from 30 to 400 A have been studied by Scanning High Energy Electron Diffraction. From the measurement of the diameters of (220) and (422) rings at different temp., the lattice contraction has been determined as a function of temp. and as a function of size. These experimental data are used to deduce the thermal expansion coefficient and the surface stress coefficient of the particles. In the case of Pt, the values of the surface stress coefficients have been found to be g sub (220) = 3.86 plus/minus 0.7 N/m and g sub (422) = 4.44 plus/minus 1 N/m. These values are slightly greater that the value of the surface tension of bulk Pt ( gamma = 2.6 to 3.5 N/m). This result indicates that the surface of the particles is in a state of weak dilatation. For Au, both values g sub (220) = 3.08 plus/minus 0.7 N/m and g sub (422) = 3.19 plus /minus 1 N/m are clearly larger than the surface tension of bulk gold ( gamma = 1.7-2 N/m): the resulting dilatation of the surface is in accordance with the polytetrahedral structure of the gold particles. In both metals, the thermal expansion coefficient has been found to be size independent. 17 ref.--AA