  Gold nanoclusters are grown by 1.8 MeV Au++ implantation in silica matrix and
subsequent air annealing in the temperature range of 873K-1273K. Post-annealed
samples show plasmon resonance in the optical region (~ 2.38-2.51 eV) for
average cluster radii ~1.04-1.74 nm. A blueshift of the plasmon peak is
observed with decreasing cluster size in the annealed samples. Similar trend of
blueshift with decreasing cluster size in case of Au nanoclusters embedded in
the porous alumina matrix [B. Palpant, et al., Phys. Rev. B 57, 1963 (1998)]
has convinced us to assume a possible role of 'rind' like porosity at the Au
nanocluster-matrix interface with available open volume defects in the
amorphous silica matrix.
