Temperature-dependent EXAFS analysis of embedded Pt nanocrystals.

The vibrational and thermal properties of embedded Pt nanocrystals (NCs) have been investigated with temperature-dependent extended x-ray absorption fine structure (EXAFS) spectroscopy. NCs of diameter 1.8-7.4 nm produced by ion implantation in amorphous SiO(2) were analysed over the temperature range 20-295 K. An increase in Einstein temperature (∼194 K) relative to that of a Pt standard (∼179 K) was evident for the smallest NCs while those larger than ∼2.0 nm exhibited values comparable to bulk material. Similarly, the thermal expansion of interatomic distances was lowest for small NCs. While the amorphous SiO(2) matrix restricted the thermal expansion of interatomic distances, it did not have a significant influence on the mean vibrational frequency of embedded Pt NCs. Instead, the latter was governed by finite-size effects or, specifically, capillary pressure.