Optical properties of ion track assisted Au nanochain like aggregates

The amorphous silica (a-SiO2) was irradiated with 6 MeV Si+ ions with fluences of 1 × 1011, 5 × 1011 and 1 × 1012 ions/cm2 and etched with HF (hydro fluoric acid) for 6 min to produce ion tracks. The etched samples were coated with 12 nm thick gold films and were treated with rapid thermal processing (RTP) at 850 °C for 2 min in the nitrogen atmosphere. The RTP treated samples were etched with aqua-regia to remove the gold which has not diffused into the ion tracks. These samples were characterised with optical absorption and photoluminescence (PL) spectroscopy. The surface plasmon resonance (SPR) peak, which is characteristic of Au nano-aggregates was observed. SPR peaks at 2.17 and 2.28 eV are assigned to the dipolar and quadropolar resonance of linear chain of nanoclusters respectively and the resonance peak at 2.35 eV is attributed to non-linear aggregation of gold nanoparticles. The blue bands above ~ 3 eV are observed in PL which are close to the reported values for Au nanoclusters. Also the green PL band is observed around 2.6 eV and the green emission arises from the radiative recombination of sp-band electrons with d-band holes. The intensity of the green PL band is mainly due to the local electric field connected with the Au-particle plasmons oscillations. The mechanism behind the formation of linear Au nanochain is discussed.