Direct Observation of Intercalant and Catalyst Particle in Single Wall Carbon Nanotubes
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2Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY,
40506 3 Department of Physics and Astronomy, University of Kentucky, Lexington, KY, 40506 "4Departmentof Physics and Astronomy, Vanderbilt University, Nashville. TN 37235, * Current address: 104 Davey Laboratory, The Pennsylvania State University, University Park. PA 16802-6300 ","Current address: Fundamental Research Lab, Honda R&D Americas, Inc., 100 Town Center, Suite 2050, Southfield, MI 48075 "t"Permanent address: Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland ABSTRACT The Z-contrast scanning transmission electron microscopy (STEM) imaging technique has been used to study dopant atoms and catalyst particles in single wall carbon nanotubes (SWNT). Iodine and bromine have been doped respectively in arc-grown SWNTs. We have directly observed the dopant sites and distributions. Both dopants appear to be incorporated linearly within the SWNT bundles. SWNT were also grown by pulsed laser ablation with mixed Ni and Co catalyst, and the size and distribution of catalytic particles was studied. By using Z-contrast imaging, we found that the size distribution of the catalyst particles varied over a large range, but even the smallest were larger than the diameter of an individual SWNT. Furthermore, electron energy loss spectroscopy (EELS) is used to determine the composition of individual nanocatalyst particles, and were found to consist of a uniform alloy of Co and Ni. INTRODUCTION Since the discovery of carbon nanotubes[1], their remarkable electronic and structural properties have promised to revolutionize application ranging from nanoscale electronics to ultralightweight structural materials.[2] Transmission electron microscopy (TEM) has played a very important role in the discovery and early stage of development. As many other techniques have been developed to study the numerous properties of carbon nanotubes, TEM has become an important technique to check the quality of new samples created by the ever-increasing new growth techniques. Furthermore, electron microscopy can provide critical insights for the study of carbon nanotubes where no other technique can substitute. Here we show that by using the Z-contrast imaging technique in a scanning transmission electron microscope (STEM), we can distinguish high-Z elements incorporated inside carbon nanotubes with high sensitivity, including dopant atoms/molecules and catalyst particles. Single wall carbon nanotubes (SWNTs) have been shown to be amphoteric in character, i.e., they can exchange electrons with a dopant atom (or molecule) to form the corresponding positively or negatively charged counter ion [3,4]. Using the SWNT grown by the arc-discharge
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method[5], tubes were doped either by bromine in its vapor phase[4], or iodine by immersion in molten iodine[6], as described previously. In both cases, resistivity and Raman scattering studies indicated charge transfer and signifi
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