Collections of copper nanocrystals characterized by different sizes and shapes: Optical response of these nanoobjects

The use of reverse micelles makes it possible to produce copper nanocrystals characterized by various sizes and shapes. By increasing the water content, the average size of the metallic nanocrystals increases from 3 to 13 nm. Structural investigations clearly indicate the formation of pure copper with various crystallographic structures. The absorption spectra of the corresponding colloidal solutions exhibit two plasmon modes in the UV-visible range, around 560 and 640 nm. The progressive emergence of the first mode (Mie's resonance of a perfect sphere) with the increase in the mean particle size is attributed to a mere finite-size effect. The second resonance is explained in terms of nanocrystal shape.