Surface plasmon resonance in nanocrystalline silver in a ZnO matrix

Silver nanoparticles embedded in ZnO matrix were deposited onto fused silica substrates using high pressure (~40 Pa) d.c. sputtering techniques. The particle size in the films was tailored by varying the system pressure and substrate temperature, while the metal volume fraction was controlled by adjusting the relative time of sputtering of the targets. Blue-shift of the surface plasmon resonance peak was observed with the reduction in size and volume fraction of metal particles. A surface plasmon peak in the absorption spectra was found to be absent in the films with particle size and metal concentration below a critical value. A sharp absorption edge in the absorbance spectra within the UV-VIS range indicated semiconducting behavior of the ultrafine silver particles. Films deposited at lower substrate temperature showed a narrow distribution of nanoparticles, nearly spherical in shape. Increase in substrate temperature resulted in a non-uniform size and shape in the films due to the agglomeration of the nanoparticles. These size and shape distributions have a profound effect on the optical absorbance spectra and result in a broad and asymmetric surface plasmon band. A shape distribution introduced in the Maxwell-Garnett or Bruggeman effective medium theory was found to give a reasonable description of the experimentally observed optical absorption spectra.