The $L_3$ edge of Au nanoparticles, having sizes ranging from $2.4\text{to}5.0\mathrm{nm}$, have been investigated by x-ray absorption fine structure spectroscopy in the temperature range of $20–300\mathrm{K}$. 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.

• Received 26 July 2007

DOI:https://doi.org/10.1103/PhysRevB.77.075432