Online monitoring of alloyed bimetallic nanoparticle formation by optical spectroscopy

An extension of the Mie [Ann. Phys. 25, 377 (1908) ] theory for the calculation of optical absorption spectra in bimetallic nanoparticles has been developed. The nanoparticle dielectric function is assumed to be a weighted linear combination of dielectric functions for single particles. Accordingly, analytical expressions for the resonance light absorption frequency, the spectrum maximum value, and the full width at half maximum have been derived, taking into account the interband transitions in the dielectric functions. Experiments have been performed on polymer-embedded Ag/Au nanoparticles prepared by reducing the presence of poly(vinyl pyrrolidone) at room temperature. Experimental absorption spectra have been compared to numerical curves derived by the model in the case of Ag/Au systems at different relative compositions, and they are in good agreement. The time dependence of both Ag/Au nanoparticle size and chemical composition during the growth process has also been investigated.