The Optically Detected Coherent Lattice Oscillations in Silver and Gold Monolayer Periodic Nanoprism Arrays: The Effect of Interparticle Coupling

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Abstract
Using femtosecond transient spectroscopy, we studied the optically detected laser-induced coherent phonon oscillation of monolayers of periodic arrays of prismatic-shaped silver and gold nanoparticles, assembled by using the technique of nanosphere lithography. In this method, the same size of polystyrene sphere and the same vacuum conditions are used. Under these circumstances, the gold nanoprisms formed are found to have sharper tips than the corresponding silver nanoprisms. For both gold and silver nanoparticles, the surface plasmon absorption maximum is found to depend linearly on size. The coherent lattice oscillation periods are also found to depend linearly on size. However, although the observed dependence for the silver nanoparticle is found to follow the calculated dependence of a single particle on size (based on a one-dimensional standing wave model), the gold nanoparticle deviates from this model, and the deviation is found to increase with the size of the nanoparticles. This deviation can be explained by considering interparticle coupling. A simple interparticle lattice oscillating dipolar coupling model of the dimer is found to qualitatively account for both the sign and the size dependence of the deviation. The absence of this deviation in the silver nanoparticle arrays is blamed on the weak interparticle coupling due to their rounded tips and the possibility of oxidation of their surfaces.