Molecular Nanowires of 1 nm Diameter from Capillary Filling of Single-Walled Carbon Nanotubes Ching-Hwa Kiang,* Jong-Suk Choi, Todd T. Tran, and Alfred Dirk Bacher Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90095 ReceiVed: April 29, 1999; In Final Form: July 7, 1999 Molecular nanowires inside single-walled carbon nanotubes are produced by capillary filling. Bismuth was drawn into single-walled carbon nanotubes, where it formed single-crystal nanowires of nanometer dimensions. Metal was introduced in its gas, solution, and solid phases, with the solution phase process the most efficient and versatile method of filling. The majority of fillings are one-dimensional nanowires with high length to diameter ratios. The strong capillary effect in single-walled carbon nanotubes should allow these materials to host a wide variety of nanoscale materials. Introduction Since the discovery of single-walled carbon nanotubes in 1993, 1-3 this new class of materials has demonstrated a potential to make a major contribution to a variety of nanotechnology applications, including molecular electronics, 4 hydrogen storage media, 5 and scanning probe microscope tips. 6 Carbon nanotubes can be expected to provide a basis for a future generation of nanoscale devices, 6-9 and it has been predicted that modifica- tion 10-12 of single-walled carbon nanotubes will lead to an even more diverse range of applications. For example, the electrical property of empty single-walled carbon nanotubes is extremely sensitive to their structure and the existence of defects, which imposes great difficulty for using unfilled nanotubes in electronic device applications. The property of filled single-walled carbon nanotubes, on the other hand, will be dominated by the filling materials, and therefore, filled nanotubes will be more robust in applications such as nanoelectronics. In this work, we present the synthesis of Bi nanowires of 1 nm diameter by filling single- walled carbon nanotubes, since the electronic transport properties of one-dimensional Bi wires have been studied extensively, and the magnetoresistance is expected to be influenced by the dimensionality. 13 The technique may be used to synthesize other materials such as magnetic nanowires, which should allow studies of magnetism in one dimension. There have been reports on filling of multiwalled 14-16 and single-walled 17 carbon nanotubes. The percentage of filled tubes was either very low 14,17 or with very low length to diameter ratios. 15-17 In addition, it is practically impossible to synthesize uniform multiwalled carbon nanotubes, and hence the filled tubes differ both in diameters and the number of carbon layers, which may affect the properties of these materials. An efficient and reproducible method to filling single-walled carbon nano- tubes will be the key to future advances of this field. Experimental Methods and Results We synthesized single-walled carbon nanotubes with the arc method. 18 In brief, we used an electric arc running a dc current at 95 A under 400 Torr helium to vaporize the carbon and the catalyst. 2,19,20 The electrodes were 6 mm diameter graphite rods, with the anode containing mixtures of, in atomic percentage, graphite powder (90%), cobalt catalyst (5%), and bismuth cocatalyst (5%). 18,21,22 Bismuth was used to improve the catalytic properties of the primary catalyst for producing high-yield, large- diameter single-walled carbon nanotubes and as a source material for the nanowires. This method produces 20 g/h of raw soot with high content of single-walled carbon nanotubes (∼70%). High-resolution transmission electron microscopy (HRTEM) studies were done on Phillips CM200 and CM300 microscopes equipped with Gatan slow-scan cameras and image filters. The nanotube diameter (D) distribution was measured from HRTEM images, which shows 70% of the nanotubes have 0.7 nm < D < 2 nm, 15% have 2 nm < D < 3 nm, and 15% have 3 nm < D < 7 nm. Filling of the nanotubes with bismuth metal occurred during the high-temperature gas phase reaction when the bismuth metal incorporated into the composite anode was vaporized with carbon and cobalt in a helium atmosphere. Figure 1 is a single- walled carbon nanotube entirely filled with a single crystal of bismuth of 1 nm inner diameter and 30 nm in length. While * To whom correspondence should be addressed. E-mail: chk@ chem.ucla.edu. Figure 1. (a) High-resolution transmission electron micrograph of a 1.4 nm diameter single-walled carbon nanotube filled with a single- crystal bismuth nanowire. The filling extend throughout the entire stretch of the nanotube. (b) The middle section of the tube shown in (a), with lattice fringes clearly seen. The lattice spacing d ) 0.33 nm corresponds to the (012) spacing of a bulk bismuth crystal. 7449 J. Phys. Chem. B 1999, 103, 7449-7451 10.1021/jp991424m CCC: $18.00 © 1999 American Chemical Society Published on Web 08/18/1999