Microchannel Networks for Nanowire Patterning

One-dimensional (1D) nanostructures, such as nanowires, nanotubes, and molecular wires are currently being investigated in great detail for their unique electronic and mechanical properties and their potential implementation as devices.1-4 Integration of the nanotubes and nanowires into useful devices requires placing them in specific positions with desired configurations reproducibly.1,4 This, however, remains to be a major challenge in the field. We describe a strategy for aligning and patterning conductive [Mo3Se3]∞ nanowires on substrates using microchannel networks.5 This strategy relies on the solvent evaporation induced self-assembly of [Mo3Se3]∞ infinite chains within the microchannels. Aligned [Mo3Se3]∞ wires6,7 as thin as several nanometers can be readily patterned using micrometer-sized channels. Multilevel cross-bar junction configurations and nano-to-macroscale connections were also demonstrated. Optical microscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) studies, and electrical measurements show that these wires are highly oriented, crystalline, and conductive. The current process in constructing and measuring nanotube or other nanowire devices has been mainly involved the deposition on prefabricated electrodes in hope of getting tubes/wires at the right place and configuration.1 Atomic force microscope (AFM) has also been used to push or deposit nanotubes into desired configurations.8,9 Unfortunately, these device fabrication methods have the intrinsic limitations of being highly serendipitous or timeconsuming. We wish to develop simple and parallel methods for this purpose using soft lithography. Soft lithography has been extensively used to create microstructures with lateral dimensions of 30 nm to 500 μm.5 The patterned materials include metals, polymers, colloids, proteins,10 and ceramics.5,11 Generally, the patterned feature size reflects the actual dimension of the surface features on the micromolds. Here we show [Mo3Se3]∞ molecular wires can be patterned on a nanometer scale using micromolds having micrometer-sized channel network. [Mo3Se3]∞ molecular wires, 6 A in diameter, are infinite chains of polycondensed Mo6Se8 polyhedra.7 They have been experimentally determined to be highly conductive with a resistivity of 10-2-10-4 Ω‚cm at room temperature.6,12 Individual [Mo3Se3]∞ molecular wires can be obtained as a solution (10-4-10-6 M) by dissolving single crystals of LiMo3Se3 in the polar solvent dimethyl sulfoxide or N-methylformamide.7 Microchannel networks were formed between a poly(dimethylsiloxane) (PDMS) micromold and a silicon/glass substrate. The microchannels have variable height of 1-4 μm, width of 1-10 μm, and length of 5-10 mm. A droplet (∼0.1-10 μL) of the wire solution was placed at the open end of the microchannels, and the channels were filled within minutes via capillary action. The solution-filled microchannel network was placed in a vacuum to evaporate the solvent. We found that molecular wires tend to form nanowire bundles and align along the corners of the microchannels upon solvent evaporation and PDMS micromold removal, leaving an otherwise clean contact area. Cross-polarized optical microscopy studies indicate the wires are continuous up to several millimeters and are well aligned (Figure 1a). FESEM studies (Figure 1b-d) show the width of nanowires varies from 10 to 200 nm, which is largely determined by the molecular wire solution concentration and the microchannel volume. This feature size is significantly smaller than the microchannel size. Each nanowire generally consists of several thinner wires with diameter of ∼10 nm or less (Figure 1c). Double helix formation between these nanowires was (1) Fuhrer, M.; Nygard, J.; Shih, L.; Forero, M.; Yoon, Y.; Mazzoni, M. S. C.; Choi, H. J.; Ihm, J.; Louie, S. G.; Zettl, A, McEuen, P. L. Science 2000, 288, 494-496. (2) 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, 391394. (3) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435446. (4) Rueches, T.; Kim, K.; Joselevich, E.; Tseng, G. Y.; Cheung, C.; Lieber, C. M. Science 2000, 289, 94-97. (5) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550-575. (6) (a) Venkataraman, L.; Lieber, C. M. Phys. ReV. Lett. 1999, 83, 53345336. (b) Venkataraman, L. Thesis, Harvard University, 1999. (7) Tarason, J. M.; DiSalvo, F. J.; Cheve, C. H.; Carroll, J.; Walsh, M.; Rupp, L. J. Solid State Chem. 1985, 58, 290-300. (8) Lefebvre, J.; Lymch, J. F.; Llaguno, M.; Radosavljevic, M.; Johnson, A. T. Appl. Phys. Lett. 1999, 75, 3014-3016. (9) Cheung, C. L.; Hafner, J. H.; Odom, T. W.; Kim, K.; Lieber, C. M. Appl. Phys. Lett. 2000, 76, 3136-3138. (10) Bernard, A.; Renault, J. P.; Michel, B.; Bosshard, H., R.; Delamarche, E. AdV. Mater. 2000, 12, 1067-1070. (11) (a) Qin, D.; Xia, Y.; Xu, B.; Yang, H.; Zhu, C.; Whitesides, G. M. AdV. Mater. 1999, 11, 1433-1437. (b) Aizenberg, J.; Braun, P. V.; Wiltzus, P. Phys. ReV. Lett. 2000, 84, 2997-3000. (c) Aizenberg, J.; Black, A. J. J.; Whitesides, G. M. Nature 1999, 398, 495-498. (12) Golden, J. H.; DiSalvo, F. J.; Frechet, J. M. J.; Silcox, J.; Thomas, M.; Elman, J. Science 1996, 273, 782-785. Figure 1. (a) Cross-polarized optical image of nanowire patterns on glass substrate. (b-d) FESEM images of wire arrays on silicon substrate. Arrow in c indicates the thinner wires are pulling together to form a thicker bundle. 10232 J. Am. Chem. Soc. 2000, 122, 10232-10233