Optical properties of ion track assisted Au nanochain like aggregates S. Amirthapandian, P. Magudapathy, B.K. Panigrahi ⁎, K. Saravanan, K.G.M. Nair, C.S. Sundar Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India abstract article info Available online 4 March 2009 PACS: 61.80.Jh 73.20.Mf Keywords: Ion track Gold Nanochain PL SPR The amorphous silica (a-SiO 2 ) was irradiated with 6 MeV Si + ions with fluences of 1×10 11 , 5×10 11 and 1 × 10 12 ions/cm 2 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. © 2009 Published by Elsevier B.V. 1. Introduction The interaction of light with small noble metal nanoclusters embedded in dielectric media is investigated extensively for many years [1]. Recently there has been a great interest in optical properties of metal nanoclusters due to their fascinating surface plasmon resonance (SPR) properties [2,3]. The SPR property of nanoclusters, has applications in data storage, light generation, biomedical and non- linear optics [1,4]. The resonance frequency of the metal nanoclusters depends on the size, shape, dielectric constants of metal and medium, and interparticle interactions. In a collection of particles (aggregates), the shift in the plasmon bands are expected and it is due to the electromagnetic coupling among the particles [5]. The photoluminescence efficiency of metal surface (like gold) is ~ 10 -10 i.e., only one photon is emitted per 10 10 electron-hole pairs excited. However, noble metal nanostructures show better PL effi- ciency as compared to metal films. Direct ion implantation of metals is a well known route for the formation of metal nanoclusters embedded in dielectric media [6,7]. In particular, a green PL band was observed in Au nanowhiskers and the PL emission was attributed to the radiative recombination of sp-band electrons with d-band holes and enhance- ment in PL intensity is due to local electric field associated with gold particle plasmon oscillations [8]. Earlier, blue luminescence was reported for Au nanoclusters [6], Au nano islands (b 10 nm) [9], and Au colloidal suspensions [10]. However, recently the one-dimensional assemblies of metal nanoparticles and their associated tunable surface plasmon properties are of particular interest in the area of metal nanomaterials [11–13]. In the present work, a novel method of preparing Au nanochain by the use of ion beam induced track is demonstrated and the Au nanochain like aggregates are characterised by optical absorption and photoluminescence measurements. 2. Experimental Fig. 1 shows the schematic representation of the synthesis route of Au nanochain like aggregates. The amorphous silica substrates were cut into 10×10 mm pieces and irradiated with 6 MeV Si + ions with various ion fluences 1 × 10 11 , 5 × 10 11 and 1 × 10 12 ions/cm 2 with a base vacuum of 3 ×10 -7 mbar. The samples are labelled as Au111 (1×10 11 ions/cm 2 ), Au511 (5×10 11 ions/cm 2 ) and Au112 (1×10 12 ions/cm 2 ). The 6 MeV Si + ion irradiation was carried out by using 1.7 MV Tandetron accelerator at IGCAR, Kalpakkam. The irradiated samples were etched with HF (hydro fluoric acid) for 6 min to produce ion tracks. Further, the etched samples were coated with 12 nm thick gold films by using thermal evaporation. During gold film growth, the base vacuum of the chamber is 1 × 10 -6 mbar. These samples 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 is not diffused into the ion tracks. The UV–VIS spectroscopic measurements were recorded in the energy range of 1.5–3.5 eV using Shimadzu PC-3101 spectrophotometer. The Surface & Coatings Technology 203 (2009) 2687–2689 ⁎ Corresponding author. Tel.: +91 44 274 80 085; fax: +91 44 274 80 081. E-mail address: bkp@igcar.ernet.in (B.K. Panigrahi). 0257-8972/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2009.02.095 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat