Laser-Induced Inter-Diffusion in AuAg Core-Shell Nanoparticles

JOSE´ H. HODAK; ARNIM HENGLEIN; MICHAEL GIERSIG; GREGORY V. HARTLAND

JOURNAL OF PHYSICAL CHEMISTRY B

Año: 2000 vol. 104 p. 11708 - 11718

1089-5647

The preparation of gold-silver nanoparticles with a core-shell structure by radiation chemistry is described.The optical properties of particles containing Au cores and Ag shells are compared to those of the reversesystem for a variety of overall particle compositions. Nanosecond and picosecond laser-induced heating (at532 nm) is used to melt the AucoreAgshell particles into homogeneous alloyed nanoparticles. The transitionfrom the kinetically stable core-shell structure to the alloy is demonstrated by TEM and by the spectralchanges accompanying melting. It is found that the particles must accumulate many laser pulses to completelymix into the alloy. In the case of nanosecond excitation, alloying and reshaping from faceted and irregularparticles into smooth spheres occurs at absorbed energies of 5-6 mJ/pulse, and fragmentation takes place athigher energies, >10 mJ/pulse. In the case of 30 ps laser excitation, the thresholds for alloying/reshaping andfragmentation are lower: 1 and 4 mJ/pulse, respectively. The higher energy threshold for nanosecond excitationcompared to the picosecond case is due to dissipation of the absorbed energy to the solvent during excitation,which is estimated to occur on a 100-200 ps time scale. Thus, the temperatures reached in the particles bynanosecond excitation are lower than those achieved by picosecond excitation for equal pulse energies.