We have observed enormous shifts of the optical-absorption peak during the reduction of gold-sulfide particles (${\mathrm{Au}}_2$S) to gold particles. A two-step colloidal method is used for the nanoparticle synthesis. We can explain our findings by assuming the colloidal particles have a gold coating on the surface. This is also consistent with our transmission-electron-microscopy figures, displaying a core-shell structure, and electron-diffraction data. The optical-absorption peak initially shifts toward the red and at later times toward the blue wavelengths. By controlling the initial size of the gold-sulfide particles, the resonance shift is correlated with a theoretical model that includes both quantum confinement and the resonance effects (the so-called surface-plasmon resonance). The use of metal-coated particles with a nonmetallic core material offers two advantages for studying quantum confinement. First, the particles are initially large, and have a large polarizability and consequently a large absorption cross section, and second, the thin metal layer confines the electron in one dimension and can extend itself in the other two dimensions.

• Received 18 January 1994

DOI:https://doi.org/10.1103/PhysRevB.50.12052