Electronic and structural properties of alkali doped SWNT

Comprehensive experiments on structural and transport properties of alkali intercalated single walled carbon nanotubes (SWNT) are presented. The increasing electron density was measured as a shift of the Drude-edge in optical reflectivity in-situ with progressive doping. In saturation-doped samples the Drude-edge shifts into the visible (to 25,000 30,000 cm— 1 for potassium and rubidium doped samples) and the samples have a golden-brown color, similar to stage I graphite. X-ray diffraction reveals a crystalline rope structure with expanded lattice constant, similar to results of Duclaux et al. The change in the low temperature divergence of the resistivity after degassing at high temperature and high vacuum and after K-doping is studied in-situ. Alkali doped single walled carbon nanotubes (SWNT) have been the focus of recent research. The main picture that emerges is that the alkali dopants are located between single SWNTs within the bundles or ropes and each donates an electron to the SWNT host as is well established for graphite, polyacetylene or C60. Many important experiments have been published on alkali doped SWNT, including resistivity [1, 2], Raman spectroscopy[3,4, 5, 6], EELS[7], structural data from x-ray diffraction (XRD)[8, 9] IR and visible optical spectroscopy[10, 11,12]. Our main finding in this report is that careful doping with alkali vapour yields SWNT with colors similar to graphite intercalation compounds (GIC). Surprisingly, this has not been reported before. There has also been a controversy over whether the 2D crystallinity of the rope lattice is destroyed upon alkali doping. We reported a loss of crystallinity in electrochemically K or Li doped samples[13], similar to reports on alkali-vapour doped SWNT structure[8], while the Orleans group reported an expanded rope lattice observed with XRD[9] and Pichler et al observed a shift in the zero-loss peaks in EELS [7]. We repeated the XRD experiments on vapour-doped SWNT samples and find that the rope lattice expands as shown by the shift of the XRD peaks. For the in-situ reflectivity measurements we used carefully purified, well annealed and well characterized 'buckypapers' [14]. Small pieces were placed in 4mm EPR quartz tubes and degassed at 600-800 °C at a base-pressure of « 10~ torr for one day or longer, a procedure which is reported to remove adsorbed oxygen[15,16]. Then alkali metal (K, Rb or Cs) was distilled into the tubes without exposing the sample to ambient. For doping, the SWNT were kept at 200 °C and carefully controlled temperature gradients were successively established, always keeping the buckypapers hotter than the alkali, ensurCP633, Structural and Electronic Properties of Molecular Nanostructures, edited by H. Kuzmany et al. © 2002 American Institute of Physics 0-7354-0088-l/02/$19.00