Bromine Doped Single-walled Carbon Nanotubes
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Bromine Doped Single-walled Carbon Nanotubes Bingbing Liu, Jan Carlsten, and Bertil Sundqvist Department of Experimental Physics, Umeå University, S-901 87 Umeå, SWEDEN ABSTRACT Raman spectra for three SWNT samples doped with Br2 were studied using different excitation energies, 2.41 eV and 1.58 eV. One sample was also studied under high pressure up to 2 GPa. The vibrations in the low frequency range behave similarly for all samples. With an excitation energy of 1.58 eV, additional peaks at ca 240 cm-1 and 209 cm-1 are observed. With increasing pressure, the peak at 209 cm-1 gradually softens by 2~3 cm-1 and becomes very weak at 1.7 GPa, while a new broader peak at ca 250 cm-1 remains constant up to 2 GPa. A number of high intensity overtone modes are observed under high pressure. These results indicate that the vibrations are attributed to bromine, not to the breathing modes of SWNTs, and that bromine resides in the bundles, possibly also inside tubes as a form of polymer. INTRODUCTION The electronic and vibrational properties of single-walled carbon nanotubes doped with halogens and alkali metals exhibit large differences with respect to the undoped pristine material. An enhancement of electrical conductivity has been obeserved for Br2 and K doped SWNTs. Recently, Raman and Z-contrast STEM studies on I2-doped SWNTS show that iodine resides in the tubes as a linear polyiodide chain[1-3]. Although bromine has properties similar to those of iodine, the structure of Br2 doped materials is still not clear. In the Raman spectra for samples doped with K, Rb, or I, the breathing modes of SWNTs are hardly visible. For Br-doped sample, Kataura et al. [4] report a mode near 240cm-1, attributed to the breathing mode of partially Br2–doped nanotube bundles, but disappearing for excitation energies in the near infrared region, below 1.8eV. The effect of hydrostatic pressure (HP), which significantly changes the breathing modes in pristine SWNT samples [5], gives us a further possibility to understand the characteristic properties. We present here Raman results for three SWNT samples doped with Br2. Measurement were made using excitation energies of 1.58 eV and 2.41eV, and in one case under high pressure. The study indicates that the strongest vibrations observed at low frequency are from bromine in the form of a polymer. EXPERIMENTAL SWNTs were prepared by both arc evaporation and laser ablation methods. Sample A was produced by the arc method using Ce/Ni as catalyst in He gas. It contains small diameter and chiral tubes [6]. Two other samples were made by laser ablation using a second harmonic Nd:YAG laser pulse. Sample B was produced in Umeå using 2.6 at% Ni/Co as catalyst in Ar gas while sample C was obtained from Rice Univ. (TUBES@RICE Inc). Samples A and B were as-grown mats while sample C was a mat made by drying a SWNT suspension, in which the purity of nanotubes is greater than 90%. A13.36.1
The three kinds of samples were first heated at 1000 °C for 1h under an Ar atmosphere and inserted in a quartz tube for doping.
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