Nanoscale PAPER Cite this: Nanoscale, 2014, 6, 13651 Received 11th July 2014, Accepted 7th September 2014 DOI: 10.1039/c4nr03913k www.rsc.org/nanoscale Enhanced plasmonic properties of gold-catalysed semiconductor nanowires† Denys Naumenko,*‡ a,b Valentina Zannier,‡ a,c Vincenzo Grillo, d Damiano Cassese, a,c Giacomo Priante, a Simone dal Zilio, a Silvia Rubini a and Marco Lazzarino a A key challenge for the development of plasmonic nanodevices is their integration into active semi- conducting structures. Gold-catalysed semiconductor nanowires are promising candidates for their bottom-up growth process that aligns a single gold nanoparticle at each nanowire apex. Unfortunately these show extremely poor plasmonic properties. In this work, we propose a way to enhance their plasmonic resonance up to those of ideal and isolated gold nanoparticles. A suitable purification protocol compatible with GaAs and ZnSe molecular beam epitaxy of nanowires is used to produce plasmonic active nanowires, which were used to enhance the Raman signal of pentacene and graphene oxide. Enhancement factors up to three orders of magnitude are demonstrated. Introduction Semiconductor nanowires (Sm NWs) are promising self- assembled nanostructures which combine semiconductor pro- perties with a high aspect ratio nanometric size. Many appli- cations of nanowires such as single nanowire transistors, 1,2 quantum point contacts, 3 Josephson junctions, 4 and core– shell light emitting diodes 2,5 have been demonstrated. In recent years, the rapid expansion of plasmonics into the nanoscience 6–8 allowed to combine the photonic and plasmo- nic properties of Sm NWs and nanoscale metal structures, 9 resulting in the development of ultracompact and high-per- formance couplers, 10 light-harvesting systems, 11 and high- efficiency emitters. 12 In these examples, Sm NWs are used to modulate the plasmonic response of a separate plasmonic active component, 10 to convert an external plasmonic field into a photocurrent 11 or into tunable light emission, 12 but in all of them the coupling is the result of a two step fabrication process in which the NWs and the plasmonic resonators are fabricated separately in two distinct steps. In contrast, a limited number of papers have addressed the plasmonic pro- perties of the intrinsically self-assembled gold nanoparticles – semiconductor NWs (Au NP–Sm NWs) coupled systems, which arise from the growth of metal-catalysed Sm NWs. Indeed in the vast majority of Sm NWs, due to the peculiar growth process that requires a metal (very often Au) 13 nanoparticle catalyst, each NW has on its top a metal nanoparticle. 14 The use of gold-catalysed Si NWs for locally enhancing the electro- magnetic field was proposed by Christiansen and co- workers 15,16 in a tip enhanced configuration, while a few other papers proposed Si NWs as high aspect ratio supports for silver deposition, to be used as surface enhancement Raman scattering (SERS) substrates. 17–19 So far, however, little or no success has been reported in the direction of using the catalyst nanoparticles as field enhancers. 20,21 The main reason for this failure probably stems from the NW growth process itself. According to the vapour–liquid solid model, 14 the metal par- ticle adsorbs the semiconductor precursors till the eutectic composition is reached and the particle melts. The growth pro- ceeds, with precursors added to the liquid particle from the gas phase, and the semiconductor precipitates to the solid phase. When the growth stops, the metal particle is far from consisting of pure metal, rather it is very close to the eutectic composition. In the case of Au, one of the most important materials for plasmonic applications, 8,22 this implies usually a reduction of free electrons and, therefore, a reduced plasmon resonance. 23,24 In particular, the intensity of plasmon reson- ance in bimetallic AuCu, AuZn, and AuAl NPs is reduced and/ or frequency-shifted with respect to pure gold NPs. 25–27 This † Electronic supplementary information (ESI) available: SEM characterization of: (i) transferred ZnSe NWs onto the glass substrates, (ii) etched gold NPs of ZnSe NWs in Ar+ plasma, and (iii) self-catalysed GaAs NWs. Simulation of extinction spectra. Statistical characterization of plasmon-active sites at the focal plane with a length of NWs. Photoluminescence on ZnSe NWs. B 3g Raman mode of pentacene as an indicator of molecule orientation. The dependence of graphene oxide Raman response on Au-catalysed ZnSe NWs on the number of its layers. Deposition geometry of the analytes used on well-oriented Au-catalysed NWs. See DOI: 10.1039/c4nr03913k ‡ D.N. and V.Z. contributed equally to this work. a IOM-CNR Laboratorio TASC, AREA Science Park, Basovizza, 34139 Trieste, Italy. E-mail: naumenko@iom.cnr.it; Tel: +39 040 375 6462 b AREA Science Park, Padriciano 99, 34149 Trieste, Italy c PhD School in Nanotechnology, University of Trieste, Piazzale Europa 1, 34128 Trieste, Italy d CNR-IstitutoNanoscienze, Centro S3, Via G Campi 213/a, 41125 Modena, Italy This journal is © The Royal Society of Chemistry 2014 Nanoscale, 2014, 6, 13651–13659 | 13651