Assembling Structure of Single-Walled Carbon Nanotube Thin Bundles Jinyong Wang, Zhong Jin, Jin Cheng, and Yan Li* Beijing National Laboratory for Molecular Sciences, National Laboratory of Rare Earth Material Chemistry and Application, Key Laboratory for the Physics and Chemistry of NanodeVices, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China ReceiVed: February 12, 2009; ReVised Manuscript ReceiVed: March 20, 2009 Atomic force microscopy (AFM), high resolution transmission electron microscopy, and Raman spectroscopy were used to study the assembling structure of thin bundles containing only a few single-walled carbon nanotubes (SWCNTs). The normal close-packed bundles as well as two novel kinds of bundles including spiral bundles and the ribbon-like bundles were observed for both free-standing SWCNTs and SWCNTs on substrates. Molecular mechanics calculation was employed to study the energetic competition between these three types of bundles. It was found that the free-standing spiral bundles and ribbon-like bundles are metastable compared with the close-packed bundles. However, ribbon-like bundles are more likely to be formed on substrates. This indicates that it is not reliable to assess whether the tested nanotube is individual through the height profile of the AFM image. 1. Introduction Single-walled carbon nanotubes (SWCNTs) have been re- searched extensively for their unique electronic, optical, and mechanical properties. Because of the strong intertube interac- tion, which can be as high as 1 eV/nm, 1 the bulk SWCNTs tend to self-assemble into bundles. The bulk SWCNT bundle normally has a triangular lattice structure in a honeycomb array with the uniform intertube spacing of ∼0.34 nm. 2-4 This kind of close-packed structure is energetically favorable when the bundle is composed of many tubes. The packing structure greatly affects the properties, especially the mechanical properties of the SWCNTs. Besides the close-packed large bundles, there are still some small bundles composed of only a few tubes. This kind of bundles exist in the as-prepared SWCNT samples, and more frequently, they are present in the SWCNT dispersions in various solvents. 5-7 SWCNT thin bundles have been found in wide applications. They can act both as the reinforcement materials in SWCNT-polymer composites 8,9 and the high resolution scanning probes. 10,11 SWCNT films on substrate, which are widely used in optoelectronic devices, chemical and biological sensors, field emission devices, and electrodes, are composed of both individual SWCNTs and thin SWCNT bundles. 6,12-14 Thin SWCNT bundles each containing only a few SWCNTs exist widely. However, studies on such kinds of bundles are still very few. It is well accepted that the properties of nanostructures depend on their structures. 15,16 However, for the thin bundles, their assembling structures are still unclear, let alone how the assembling structures influence the properties. It remains an open question as to whether the SWCNTs in the thin bundles still arrange in a triangular lattice. In this article, we present a comparison of experimental and theoretical evidence showing that SWCNTs in a thin bundle are not necessarily packed in a triangular lattice. They can assemble into ribbon-like or spiral structures. These kinds of assemblies enriched the aggregation structures of SWCNTs. In addition, the observation of flat ribbon structures composed of a few paralleled SWCNTs indicates that the widely used criterion of individually dispersed SWCNTs by the height measurement with atomic force microscope (AFM) is not reliable. 2. Experimental Section Ultralong SWCNT bundles on substrate were grown on SiO 2 / Si substrate by the CVD method at 900 °C. Cu was used as the catalyst and CH 4 as the stock gas. 17 Free-standing SWCNT bundles were directly grown on copper grids coated with SiO 2 using Fe as catalyst and CH 4 (500 sccm) as carbon source at 900 °C. 18 To coat the copper grids with SiO 2 , a SiO 2 sol was prepared by mixing together tetraethyl silicate (10 mL), ethanol (30 mL), water (3.5 mL), and 0.5 M HCl (∼0.1 mL) and stirring for 2 h. Then the SiO 2 sol was spin-coated onto the copper grids and dried at 80 °C for 30 min before further usage. Raman measurements were performed under ambient condi- tions using a helium-neon laser (633 nm, 1.96 eV excitation) in the backscattering configuration on a Jobin-Yvon HR800 spectrometer. The laser power was carefully controlled to avoid sample heating. The AFM images were taken on a SPI3800/ SPA400 scanning probe microscope (SEIKO Instruments) with commercial Si cantilevers operated at tapping mode. The high resolution transmission electron microscopy (HRTEM) images were obtained on a Hitachi 9000 TEM. 3. Results and Discussion Besides the bundles with triangular lattice structure, which are very similar to the bulk assemblies 3,19 and have been observed and studied by many groups in previous studies, 20-22 two new types of SWCNT thin bundles were found in our study. One is of spiral structure, and the other is of ribbon-like structure. Both types have not been reported in the previous study, which may indicate that they only exist in the thin bundles. Figure 1a and b shows the AFM image and the height profile of a thin SWCNT bundle grown on the SiO 2 /Si substrate. The periodic fluctuation in its height indicates the spiral structure * To whom correspondence should be addressed. Phone/Fax: +86-10- 62756773. E-mail: yanli@pku.edu.cn. J. Phys. Chem. C 2009, 113, 8132–8135 8132 10.1021/jp901303b CCC: $40.75  2009 American Chemical Society Published on Web 04/15/2009