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

Wide range optical studies on transparent SWNT films

We present transmission spectra from the far infrared through the ultraviolet region on freestanding SWNT films at temperatures between 40 and 300 K. Several interesting features are observed in the low-frequency part of the spectrum: the Drude-like frequency dependence of the metallic tubes as well as a (sample-dependent) peak in the conductivity around 0.01 eV. We also studied the accidental nitrate doping of the SWNT samples during purification by nitric acid. Asprepared purified samples exhibit increased metallic absorption and decreased interband transitions; these features disappear on heating in vacuum

Chemical doping of individual semiconducting carbon-nanotube ropes

We report the effects of potassium doping on the conductance of individual semiconducting single-walled carbon nanotube ropes. We are able to control the level of doping by reversibly intercalating and de-intercalating potassium. Potassium doping changes the carriers in the ropes from holes to electrons. Typical values for the carrier density are found to be ∼100–1000 electrons/μm. The effective mobility for the electrons is μeff∼20–60 cm2 V-1 s-1, a value similar to that reported for the hole effective mobility in nanotubes [R. Martel et al., Appl. Phys. Lett. 73, 2447 (1998)]

Study of charge dynamics in transparent single-walled carbon nanotube films

We report the transmission over a wide frequency range (far infrared - visible) of pristine and hole-doped, free-standing carbon nanotube films at temperatures between 50 K and 300 K. Optical constants are estimated by Kramers-Kronig analysis of transmittance. We see evidence in the far infrared for a gap below 10 meV. Hole doping causes a shift of spectral weight from the first interband transition into the far infrared. Temperature dependence in both the doped and undoped samples is restricted to the far-infrared region.Comment: 6 pages, 4 figures, submitted to Phys. Rev. B v3: Fig. 2 replaced, changes in caption of Table II, minor changes in tex

Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes

Hydrogen adsorption on crystalline ropes of carbon single-walled nanotubes (SWNT) was found to exceed 8 wt.%, which is the highest capacity of any carbon material. Hydrogen is first adsorbed on the outer surfaces of the crystalline ropes. At pressures higher than about 40 bar at 80 K, however, a phase transition occurs where there is a separation of the individual SWNTs, and hydrogen is physisorbed on their exposed surfaces. The pressure of this phase transition provides a tube-tube cohesive energy for much of the material of 5 meV/C atom. This small cohesive energy is affected strongly by the quality of crystalline order in the ropes

Polarized spectroscopy of aligned single-wall carbon nanotubes

Polarized resonant Raman and optical spectroscopy of aligned single-wall carbon nanotubes show that the optical transitions are strongly polarized along the nanotubes axis. This behavior is consistent with recent electronic structure calculations

Localized and Delocalized Charge Transport in Single-Wall Carbon-Nanotube Mats

We measured the complex dielectric constant in mats of single-wall carbon-nanotubes between 2.7 K and 300 K up to 0.5 THz. The data are well understood in a Drude approach with a negligible temperature dependence of the plasma frequency (omega_p) and scattering time (tau) with an additional contribution of localized charges. The dielectric properties resemble those of the best ''metallic'' polypyrroles and polyanilines. The absence of metallic islands makes the mats a relevant piece in the puzzle of the interpretation of tau and omega_p in these polymers.Comment: 4 pages including 4 figure

Size Effects in Carbon Nanotubes

The inter-shell spacing of multi-walled carbon nanotubes was determined by analyzing the high resolution transmission electron microscopy images of these nanotubes. For the nanotubes that were studied, the inter-shell spacing ${\hat{d}_{002}}$ is found to range from 0.34 to 0.39 nm, increasing with decreasing tube diameter. A model based on the results from real space image analysis is used to explain the variation in inter-shell spacings obtained from reciprocal space periodicity analysis. The increase in inter-shell spacing with decreased nanotube diameter is attributed to the high curvature, resulting in an increased repulsive force, associated with the decreased diameter of the nanotube shells.Comment: 4 pages. RevTeX. 4 figure