Vibrational properties of Au and Cu nanocrystals formed by ion implantation

An and Cu Nanocrystals (NCs) were fabricated using ion implantation into SiO2 and subsequent thermal annealing. Vibrational properties were studied using extended x-ray absorption fine structure (EXAFS) spectroscopy at measurement temperatures between 15 and 300 K. Einstein temperatures were extracted and show a significant decrease for 3.3 nm Cu NCs as compared to bulk Cu. An NCs, however, show no significant change in the Einstein temperature even at a size of 2.5 nm. The influence of the competing processes of softening of the vibrational density of states due to under-coordinated surface atoms and stiffening of the NC core atoms due to surface tension effects are discussed.