Electronic Structures of Single-Walled Carbon Nanotubes Studied by NMR
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cylinders formed by rolling a graphite sheet with certain rules. Because of the unique geometric shape and various topological structures, these tiny tubings exhibit a marvelous variety of electronic properties. As shown by theoretical calculations [1, 2], the electronic property of an individual single-walled carbon nanotube depends on its diameter and chirality. For example, an individual SWNT can be either metallic or semiconducting. The SWNT material is considered to be very promising for the future industry of miniature electronics. For metallic SWNTs and semiconducting SWNTs, significantly different patterns of spikes in the DOS are predicted. Recent scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements [3, 4] on individual SWNTs support the theoretical calculation. The optical spectroscopy studies on bulk SWNT samples have also revealed such DOS spikes. However, neither technique measures quantitatively the DOS at the Fermi level, g(EF). Furthermore, the distribution of chirality in a bulk sample is not known and the tube-tube interactions in SWNT bundles, which can affect electronic properties of SWNTs such as g(EF) [5], remain to be investigated. Nuclear magnetic resonance (NMR) provides a direct measure of local g(EF) in bulk samples. Thus, NMR characterizations of SWNTs could play an important role in understanding the electronic structures of SWNTs. Here, we report the observation of two types of tubes in bulk SWNT samples and the quantitative determination of g(EF) for the metallic SWNTs using "C NMR.
143 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
EXPERIMENT The SWNT bundles used in this study are synthesized by ablating a graphite target, containing Ni/Co metal catalysts of 0.6 at% each, using a 1064 nm Nd:YAG laser (1000 mJ/pulse) in an Ar-filled (200 sccm and 700 torr) furnace at 1150TC [6]. For the NMR study, enrichment of ' 3C isotope is accomplished by mixing the starting target with 10 wt% of amorphous carbon which contains 99 at% ' 3C. The raw materials of SWNTs are purified by reflux in 20% H 20 2 solution at I00°C for 12 hours. Amorphous carbon impurity is preferentially oxidized by H 20 2 yielding CO 2 gas. The remaining materials are rinsed in CS 2 to remove C60 and then in methanol. They are subsequently filtered through a membrane with 2 im pores. A significant fraction of the carbon and magnetic Ni/Co nanoparticles are removed by repeating the filtration process several times until the liquid passin6g through the filter is clear. After filtration, the sample is annealed at I 100'C for 1 hour in 5x10" torr vacuum and is sealed under vacuum in Pyrex glass tubing for NMR measurements. Although the precise purity of the SWNT sample can not be determined, transmission electron microscopy (TEM) (Fig. 1) and powder x-ray diffraction (XRD) measurements (Fig. 2) suggest that materials purified by the above process contain over 90% SWNT bundles. A
FIG. 1. A representative TEM micrograph of the SWNT material after purification. sma
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