TEMPLATE-SYNTHESIZED NANOSCOPIC GOLD PARTICLES - OPTICAL-SPECTRA AND THE EFFECTS OF PARTICLE-SIZE AND SHAPE

We have prepared nanoscopic gold cylinders of controlled radius and aspect ratio via electrodeposition of the metal within the pores of anodically-grown porous aluminum oxide membranes. The nanocylinder radii are determined by the pore dimensions of the host alumina which, in turn, depend on anodization conditions. The particle aspect ratios were controlled by varying the amount of Au deposited within the pores. The optical spectra of the gold nanocylinder/alumina composites exhibit strong absorption bands in the visible spectrum. The extinction maxima (lambda(max)) values for gold particles approaching spherelike geometry agree well with Mie theory calculations. The blue shift of lambda(max) as the particle aspect ratio is increased is in qualitative agreement with Maxwell-Garnett theory. We propose a simple modification of Maxwell-Garnett theory that addresses both size and shape effects.