Heat conduction in nano-environment observed in cooling processes of colloidal silver nanoparticles in water

Ultrafast dynamics of colloidal silver nanoparticles in an aqueous solution was investigated by observing the ultrafast lensing effects caused by a photo-induced refractive index change. The ultrafast response consisted of an instantaneous peak by the optical Kerr effect and a subsequent bi-exponential decay component. The decay component was independent of the relative polarization planes of pump and probe beams and had temperature-dependent time constants ranging from 0.4 to 2.2 ps and from 10 to 100 ps. A minimum at around 7°C was found for the time constant of the fast component. The results suggest that the structure of water molecules adjacent to the silver surface is affected by hydrogen bonds in a temperature-dependent manner similar to that of the molecular structure of bulk water.