Preparation and characterization of gold and ruthenium colloids in thin zinc oxide films

Gold and Ruthenium colloids have been introduced into thin films of ZnO by spin coating metal ion-doped solutions of zinc acetylacetonate onto quartz or Pt substrates, followed by heating to 400 degreesC. Both types of him exhibit a blue tint due to Mie light absorption of the metal nanoparticles. Analysis of the visible absorption spectra suggests particle radii on the order of 2 nm. However, electron micrographs reveal some larger particles and particle aggregates as well. While the Au colloid absorption band peaks at approximately 575 nm, Raman excitation profiles of the colloidal films exhibit red-shifted maxima near 700 nm. We attribute the striking Raman intensity enhancement at an excitation wavelength of similar to 700 nm to Surface-enhanced Raman (SER) scattering of the E-1 mode of ZnO. Ru colloids also induce SER scattering from a residual RuO2 layer that apparently coats the Ru metal particle. In this case, the SER scattering is greatest when the laser excitation is tuned into the maximum of the Ru colloid absorption band, in accord with the so-called electromagnetic theory of SER scattering. While not all of the Ru in Ru-doped films is prepared in the zero oxidation state, XPS measurements show that all of the gold in the gold-doped films is in the zero oxidation state. (C) 2000 Elsevier Science B.V. All rights reserved.