Pristine, adherent ultrathin gold nanowires on substrates and between pre-defined contacts via a wet chemical route† Paromita Kundu, a U. Chandni, b Arindam Ghosh b and N. Ravishankar * a Received 8th September 2011, Accepted 9th November 2011 DOI: 10.1039/c1nr11264c We demonstrate a simple strategy of obtaining clean, ultrathin single crystal Au nanowires on substrates and interconnecting pre- defined contacts with an insight into the growth mechanism. The pristine nature enables electron transport measurement through such ultrathin wires and opens up possibilities of exploring its properties for a wide range of applications. A key step for fabricating nanodevices for applications is to develop strategies towards integrating nanostructured metal and semi- conductor/insulating entities. 1–4 Gold nanostructures with controlled shapes have attracted considerable interest for interdisciplinary research owing to their intrinsic low resistivity, chemical stability, ligand binding capability, biocompatibility as well as interesting electronic and optical properties 5 and are ideal candidates for SERS, 6,7 heterogeneous catalysis, 8,9 nanomedicine and drug delivery, 10 biosensing, 11 gas-sensing, 12 electrocatalysis 13 and as inter- connects in nanoelectronic devices. 14 For all these applications, the key requirement is to obtain pristine (surfactant-free), stable and adherent nanostructures on substrates. Current methods of drop casting chemically synthesized nanostructures on substrates lead to poor adherence of the nanostructures to the substrate, thereby precluding the use of cleaning procedures to remove the surfactants and hence there is an urgent need to develop alternate, general strategies to obtain clean and adherent nanostructures on substrates for measurements/applications. Ultrathin single crystalline Au nanowires (NW) represent a fasci- nating system and have been synthesized recently. 15 While theoretical predictions exist about the striking electronic/transport behavior expected in this size regime, 16 experiments are very limited 17 owing to difficulties in growth, characterization and making contacts for device applications. Single crystal Au nanowires with controlled diameter and without grain boundaries may be preferred over thicker poly- crystalline wires as interconnects in devices to overcome problems associated with electromigration. 18 For biological/biosensor applica- tions or SERS applications, pristine wires on substrates are required. 19 The possible emergence of non-metallic behavior in the 2 nm diameter wires could also possibly lead to interesting catalytic properties in these wires supported on various substrates. 20 In spite of the recent spate of activity on synthesis of ultrathin wires, 15,21,22 investigating their properties and exploiting them for applications have remained a challenge owing to the oleyl amine (OA) capping and the extreme fragility of the wires. Though several treatments have been reported for removing OA capping from nanoparticles, problems of wire aggregation and stability in solution remain 23 (ESI, Fig. S1†). Hence, a new strategy needs to be devised for interrogating properties of individual wires and for exploiting them for various applications. Here, we demonstrate a simple wet chemical approach for inte- grating ultrathin Au nanowires on various substrates, while retaining the dimension and crystallinity of the wires as obtained in the solution phase. This method significantly enhances the yield of wires on substrates and allows for effective cleaning of the nanowires without dislodging them off the substrate (ESI, Fig. S2†). Heterogeneous nucleation of Au(+1) intermediate 24 followed by its conversion to fine Au particles on the substrate is the key towards improved adhesion of the wires onto the substrate. 25 The ability to obtain pristine surfaces by cleaning makes it an ideal system for investigating the intrinsic behavior of the molecular scale Au nanowires as well as enables the use of such wires for applications. We have investigated the detailed mechanism of wire formation on the substrate which is important to control the yield, population and length of the wires for targeted applications. We also demonstrate that the wires can be grown to interconnect pre-fabricated Au pads on the substrate with control over the number and the length of wires. Electrical characterization has been done to validate the protocol which indicates that the wires do indeed form good and stable contacts, thereby enabling transport measurements on such wires for the first time. Our method thus opens up possibilities of using such ultrathin nanowires on substrates for sensor and catalysis applications. For growing Au nanowires on the substrate, we followed a similar solution-phase synthesis route reported earlier 15 with the RCA- cleaned substrate dipped in solution right from the start of the reaction; long wires grow on different substrates under similar conditions of temperature as illustrated in Fig. 1a–e. SEM images show wire bunches where the individual wires are not resolved. The single crystalline nature of the wire with h111i as the growth direction is confirmed by high-resolution TEM as shown in Fig. 1f. The wires grown on C-coated Cu grids or silicon nitride membranes show the a Materials Research Centre, Indian Institute of Science, Bangalore, India. E-mail: nravi@mrc.iisc.ernet.in; Fax: +91 08023607316; Tel: +91 08022933255 b Department of Physics, Indian Institute of Science, Bangalore, India † Electronic supplementary information (ESI) available: Experimental section, additional figures and device fabrication. See DOI: 10.1039/c1nr11264c This journal is ª The Royal Society of Chemistry 2012 Nanoscale, 2012, 4, 433–437 | 433 Dynamic Article Links C < Nanoscale Cite this: Nanoscale, 2012, 4, 433 www.rsc.org/nanoscale COMMUNICATION Published on 01 December 2011. Downloaded by Forschungszentrum Julich Gmbh on 08/08/2014 10:32:49. 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