Metal Nanowire Formation Using Mo3Se3- as Reducing and Sacrificing Templates

Recent research in the field of nanometer-scale electronics has focused on two fundamental issues: the operating principles of small-scale devices1 and schemes that lead to their realization and eventual integration into useful circuits.2 The availability of a nanoscale toolbox is the key for this field of research. Among the many potential building blocks within this nanoscale toolbox, nanowires are considered one of the key components because they can be used as interconnects and other functional devices in nanoelectronics.3 Unfortunately, although several processes have been developed for the syntheses of semiconductor nanowires,4 few methods have been developed for preparing free-standing, uniform metal nanowires. Among them, template synthesis (in porous matrixes such as porous Al2O3 films5 and mesoporous silica6) and step-edge decoration7 are considered as effective approaches. The step-edge decoration method was recently employed to produce Mo nanowires of 100 nm thickness.7 Metal nanowires have also been prepared by using DNA8 and carbon nanotubes as templates.9 Herein we report a simple chemical process for synthesizing long, free-standing metal nanowires. LiMo3Se3 nanowires are used as both reducing agents and sacrificial templates in this study to yield continuous metal nanowires. The metal nanowires generally have diameters of 10100 nm and lengths of several micrometers. These metal nanowires display small ohmic resistances at room temperature, indicating that these wires could prove useful as interconnects in nanoelectronic circuits. Previously, DNA has been used as a biotemplate for making metallic nanowires,8 although the continuity of the resulting metal nanowires has been problematic. Since DNA itself does not possess any reducing/oxidizing capability, the synthesis generally is a two-step process, which involves metal activation followed by chemical reduction. Nevertheless, the construction of electronic circuits based only on native DNA remains problematic, mainly due to the high resistance of DNA that limits its potential applications in this regard. Recently, Braun and co-workers presented a new approach by fixing DNA between two contacts and utilizing it as a template for the construction of a silver nanowire.8 This technique uses the molecular recognition properties of the molecule for the defined buildup of a nanostructure and installs its electrical functionality by the directed construction of a metallic wire on the biotemplate. However, the reported 100 nm thick silver wires displayed a nonconducting gap for small bias voltages. In this study, we choose [Mo3Se3]∞ molecular chains as our experimental system for two important reasons. First, these molecular chains are obtained by dissolution of crystals of quasi1D materials LiMo3Se3 in polar solvent.10 Each chain is made of staggered stacks of triangular Mo3Se3 with a diameter of 0.5 nm. The molecular chain itself is a reducing agent and can be readily oxidized. Second, these individual molecular wires form uniform nanowire bundles with diameters of 2-100 nm in certain polar solvents such as methanol and DMSO. Hence, these nanowires possess dual functionalities, being reductive one-dimensional templates. In fact, the redox chemistry of LiMo3Se3 was previously studied by Tarascon, and it was demonstrated that insertion and extraction of Li/Mo6Se6 is reversible.11 To form uniform metal nanowires, a redox reaction is carried out using LiMo3Se3 nanowires as the reducing agents to reduce aqueous metal ions (e.g., AuCl4, Ag+, PdCl4, PtCl4). Metal ions are reduced and deposited directly on the nanowire templates, while the nanowire templates are oxidized into Mo3Se3 and eventually dissolve in water (Figure 1). Consequently, metal nanowires with diameters of 2-100 nm can be obtained through this cooperative chemical templating process. Metal nanowires of Au, Ag, Pt, and Pd can be readily synthesized. In a typical experiment, a LiMo3Se3 solution was prepared by dissolving 5 mg of LiMo3Se3 in 20 mL of DMSO. A 2 mL drop of metal ion solution (0.005 wt %) was added to this DMSO solution. Using the Au nanowire system as an example, evidence of the reaction is apparent immediately after the solution mixing. The overall solution color develops a pink tint, and a peak at 526 nm appears in the UV-vis absorption spectra, indicating that Au nanocluster nucleation and growth has occurred. Figure 2 shows transmission electron microscopy images taken before (a) and after (b-d) the templating reaction for the Au nanowire system. Before the reaction, the LiMo3Se3 molecular chains self-assemble into nanowire bundles of 10-100 nm in diameter. After the redox reaction, Au nanowires of similar diameters and morphology are obtained. These Au nanowires are continuous and polycrystalline. Figure 2d shows a high-resolution TEM image of an individual Au nanowire with diameter of 15 nm. It can be seen that the Au nanocrystalline domains fuse together at the interface and form a continuous and robust (1) Dekker: C. Phys. Today 1999, 52, 22. (2) (a) Hu, T.; Odom, W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (b) Collier, C. P.; Wong, E. W.; Behloradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (c) Rueches, T.; Kim, K.; Joselevich, E.; Tseng, G. Y.; Cheung, C.; Lieber, C. M. Science 2000, 289, 94. (3) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (4) Wu, Y.; Yang, P. J. Am. Chem. Soc. 2001, 123, 3165. (5) (a) Foss, C. A.; Tierney, M. J.; Martin, C. R. J. Phys. Chem. 1992, 96, 9001. (b) Preston, C. K.; Moskovits, M. J. Phys. Chem. 1993, 97, 8495. (c) Whitney, T. M.; Jiang, J. S.; Searson, P. C.; Chien, C. L. Science 1993, 261, 1316. (6) Huang, M.; Choudrey, A.; Yang, P. Chem. Commun. 2000, 12, 1603. (7) Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120 and references therein. (8) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (9) Zhang, Y.; Dai, H. Appl. Phys. Lett. 2000, 77, 3015. (10) Tarascon, J. M.; DiSalvo, F. J.; Carrol, C. H. J.; Walsh, M.; Rupp, L. J. Solid State Chem. 1985, 58, 290. (11) Tarascon, J. M.; Hull, G. W.; DiSalvo, F. J. Mater. Res. Bull. 1984, 19, 915. Figure 1. Schematic illustration of metal nanowire templating reaction between LiMo3Se3 nanowires and metal ions. 10397 J. Am. Chem. Soc. 2001, 123, 10397-10398