Low sensitivity of acoustic breathing mode frequency in Co nanocrystals upon change in nanocrystallinity

Cobalt nanocrystals (NCs) with narrow size distribution and polycrystalline structure in their native form are synthesized in reverse micelles. After annealing at 350 °C, these NCs are transformed into single crystalline phase with hexagonal close-packed structure. The vibrational dynamics of NCs differing by their nanocrystallinity is studied by femtosecond pump-probe spectroscopy. By recording the differential reflectivity signal in the native and annealed Co NCs, the frequency of their fundamental breathing acoustic mode can be measured in the time domain. A small decrease of the breathing mode frequency is observed in single crystalline Co NCs compared to that measured in polycrystals, indicating low sensitivity of their fundamental radial mode upon change in crystallinity. This result is in agreement with predictions from calculations using the resonant ultrasound approach.