Fabrication, Characterization, and Optical Properties of Gold Nanoparticle/Porous Alumina Composites: The Nonscattering Maxwell−Garnett Limit

We have been exploring the optical properties of nanoscopic gold particles prepared by electrochemically depositing Au within the pores of nanoporous alumina membranes. Maxwell−Garnet (MG) effective medium theory has been used as a guide for modeling the optical properties of these Au nanoparticle/alumina membrane composites. MG theory is, however, rigorously applicable only in the limit of metal nanoparticles with infinitesimally small diameters. As a result, the position of the plasmon resonance absorption predicted theoretically using MG theory was always blue-shifted relative to the position of the experimental absorption band. The smallest diameter particles investigated in this prior work had diameters of 60 nm. Au nanoparticles with smaller diameters (52 nm down to 16 nm) were prepared, and the optical properties of composites containing these nanoparticles are described here. The λmax values for the smallest diameter particles are essentially identical with the values predicted by MG theory. Hence, we have succeeded in preparing Au nanoparticles that experimentally achieve the MG-predicted plasmon resonance absorption limit.