Laser-induced inter-diffusion in AuAg core-shell nanoparticles

The preparation of gold-silver nanoparticles with a core-shell structure by radiation chemistry is described. The optical properties of particles cont aining Au cores and Ag shells are compared to those of the reverse system f or a variety of overall particle compositions. Nanosecond and picosecond la ser-induced heating (at 532 nm) is used to melt the AU(core)Ag(shell) parti cles into homogeneous alloyed nanoparticles. The transition from the kineti cally stable core-shell structure to the alloy is demonstrated by TEM and b y the spectral changes accompanying melting. It is found that the particles must accumulate many laser pulses to completely mix into the alloy. In the case of nanosecond excitation, alloying and reshaping from faceted and irr egular particles into smooth spheres occurs at absorbed energies of 5-6 mJ/ pulse, and fragmentation takes place at higher energies, >10 mJ/pulse. In t he case of 30 ps laser excitation, the thresholds for alloying/reshaping an d fragmentation are lower: 1 and 4 ml/pulse, respectively. The higher energ y threshold for nanosecond excitation compared to the picosecond case is du e to dissipation of the absorbed energy to the solvent during excitation, w hich is estimated to occur on a 100-200 ps time scale. Thus, the temperatur es reached in the particles by nanosecond excitation are lower than those a chieved by picosecond excitation for equal pulse energies.