Quantum thermal contraction of Au nanoparticles

We report on the successful fabrication of a fine Au powder with a mean particle size of 4 nm. A crossover from a positive thermal expansion at low temperatures to a negative thermal expansion at high temperatures was observed in the fcc cell parameter at around 125 K. Using a discrete electronic energy level scheme for nanoparticles, we introduce a weakly temperature dependent level spacing Δ(T) to show how the potential energy is lowered by lattice expansion at low temperatures and by contraction at high temperatures. Anomalies were also observed in the AC magnetic susceptibility and the heat capacity measurements, which can be interpreted by the same model.