Simple and rapid synthesis of ultrathin gold nanowires, their self-assembly and application in surface-enhanced Raman scatteringw

One-dimensional gold nanowires (AuNWs) have attracted considerable interests because of their high aspect ratios, unusual physical properties and potential applications in nanoelectronics, photonics and sensors. The large anisotropy of AuNWs and their subsequent assembly ability make them especially important building blocks for the linkage of nanoscale electronics and molecular devices. Many standard approaches have been extensively developed to produce AuNWs with desired shapes on the basis of particle assembly, surfactant mediation, template assistance or physical deposition. However, most of these early reported techniques usually resulted in the production of polycrystalline or relatively large diameter (45 nm) nanowires with poor morphology and rough surfaces. The most recent reports by Xia, Yang and others delineated the synthesis of ultrathin AuNWs (o5 nm) by using oleylamine (OA) or combined chemicals as both one-dimensional growth templates and reducing agents. The complexes of AuCl (or HAuCl4) and OA through aurophilic interactions could be reduced to form the ultrathin AuNWs. Despite their significant success with respect to the better control of the dimension of the nanowires and well-defined crystalline structures, a novel and more simplified method for the rapid synthesis of ultrathin AuNWs in large quantities remains highly desirable as most current approaches required a lengthy reaction time (usually up to several days), high temperature treatment, or complicated and multistep manipulation to improve the productivity and quality of the ultrathin AuNWs. In this paper, we present a quick and simple method for preparing ultrathin single crystalline AuNWs in a solution of HAuCl4, OA and triisopropylsilane (TIPS) at room temperature within a few hours. This process involved the addition of OA as a stabilizer and one-dimensional growth template, and TIPS as a highly effective reducing agent. Especially when deposited onto the substrates such as a copper grid or silicon wafer, the ultrathin nanowires could easily self-assemble into two-dimensional network structures with quantities of closely packed parallel nanowires. These assembled structures could serve as an active substrate for surface-enhanced Raman scattering (SERS) application (Scheme 1). In a typical synthesis, 100 mL OA was mixed with 3 mg HAuCl4 in 2.5 ml hexane, followed by the addition of 150 mL TIPS to form a yellow solution. Then the reaction proceeded at room temperature without stirring for 4B5 hours until the color gradually changed to dark red. The final products were centrifuged, washed by ethanol and finally redispersed in hexane for further investigation. Control experiments in different synthetic conditions revealed that TIPS was crucial to accelerate the formation of the AuNWs at room temperature (Fig. S1, ESIw). A large-scale experiment was also successfully conducted by using the same approach (Fig. S2, ESIw). Energy disperse X-ray spectroscopy (EDX) confirmed that the nanowires were composed of pure gold (Fig. S4, ESIw), and Fourier-transform infrared (FTIR) analysis (Fig. 1(B)) demonstrated that the nanowires were mainly capped with amine groups. Transmission electron microscopy (TEM) analysis of the ultrathin AuNWs (Fig. 2) demonstrated that the nanowires displayed a relatively high level of flexibility with low thickness down to 1.8 nm and length up to 2 mm (Fig. S3(D), ESIw). Upon evaporation of the solvent, the long and ultrathin nanowires tended to form ‘‘stacked’’ parallel bundles on the substrate. The different layers of parallel bundles were further