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Electrochemical doping and in-situ Raman scattering were used to study charge transfer in K-and Li-doped single wall carbon nanotubes (SWNT) as a function of alkali concentration. An 8 cm -1 downshift was observed for the tangential phonon mode of SWNT doped to stoichiometries of KC 24 and Li 1.25 C 6 . The shift in both systems is reversible upon de-doping despite an irreversible loss of crystallinity. These results indicate that the tangential mode shifts result from electron transfer from alkali dopants to the SWNT, and that these modes are only weakly affected by long-range order within the ropes. Comments ABSTRACTElectrochemical doping and in-situ Raman scattering were used to study charge transfer in K-and Li-doped single wall carbon nanotubes (SWNT) as a function of alkali concentration. An 8 cm -1 downshift was observed for the tangential phonon mode of SWNT doped to stoichiometries of KC 24 and Li 1.25 C 6 . The shift in both systems is reversible upon de-doping despite an irreversible loss of crystallinity. These results indicate that the tangential mode shifts result from electron transfer from alkali dopants to the SWNT, and that these modes are only weakly affected by long-range order within the ropes. IntroductionSingle wall carbon nanotubes (SWNT) constitute the newest carbon system in which chemical doping strongly modifies the physical properties [1,2]. The weak Van der Waals bonding between individual nanotubes in a semicrystalline bundle, or "rope", presumably allows for the insertion of dopants in the host lattice, as in graphite intercalation compounds and doped phases of C 60 .Alkali doping decreases the resistivity of bulk samples by a factor of 30-100 at 300K [1,3]. A similar result was observed for an individual SWNT rope [4], proving that the enhanced electron transport in bulk samples is an intrinsic property of the ropes. This phenomenon can be explained by valence electron transfer from the alkali atoms into the C anti-bonding band, which moves the Fermi energy into a region of higher density of states and enhances the conductivity. This mechanism has long been known in graphite intercalation compounds, doped polyacetylene and fullerides. Charge transfer from the alkalis to the nanotubes was proven using Raman scattering, which showed a softening of the tangential vibrational modes for the C-C bond upon doping with K or Rb [2]. A stiffening of the tangential modes and a decrease in resistivity were observed upon doping with electron acceptors, confirming the amphoteric nature of SWNT [1,2]. At this point, however, little is known about the reversibility of these phenomena and their dependence on dopant concentration. One of the limiting factors in this regard is the difficulty of controlling the composition using the vapor phase doping technique. An alternative method is electrochemical doping, which offers precise control of guest stoichiometry and allows for in-situ measurements on the guest-host systems. In-situ Raman scattering and

This work relates to a rigorous study of the surface chemistry ͑Fourier transform infrared, X-ray photoelectron spectroscopy͒, crystal structure ͑X-ray diffraction͒, galvanostatic, cyclic voltammetric, and impedance behavior of lithiated carbon electrodes in commonly used liquid electrolyte solutions. Two different types of disordered carbons and graphite, as a reference system, were explored in a single study. All three types of carbons develop a similar surface chemistry in alkyl carbonate solutions, which are dominated by reduction of solvent molecules and anions from the electrolyte. The differences in the crystal structure of these carbons lead to pronounced differences in the mechanisms of Li insertion into them. Whereas Li-ion intercalation into graphite is a staged process, Li-ion insertion into the disordered carbons occurs in the form of adsorption on both sides of the elementary graphene flakes and on their edges. The electroanalytical behavior of the disordered carbons was found to correlate well with their unique structure described in terms of the butterfly model. Both types of the disordered carbons reveal exceptionally good cyclability in coin-type cells ͑vs. Li counter electrodes͒, with only moderate capacity fading. Highly resolved plots of the chemical diffusion coefficient of Li-ions, D vs. potential E, for the disordered carbon electrodes were obtained. Surprisingly, a maximum in D appears on these plots at intermediate levels of Li-ion insertion corresponding to ca. 0.4-0.5 V ͑vs. Li/Li ϩ ͒. We propose that these maxima may originate from a combination of two effects, ͑i͒ repulsive interactions between the inserted species, and ͑ii͒ pronounced heterogeneity of Li insertion sites in terms of carbon-Li interactions and Li-ion mobility.

The structure and electronic properties of potassium-doped single-wall carbon nanotubes have been studied by conduction electron spin resonance, conductivity (), and x-ray diffraction ͑XRD͒, using in situ electrochemical methods. The spin susceptibility P of the K-saturated phase is independent of temperature; a lower bound is 5ϫ10 Ϫ8 emu/g. At 300 K both and P increase monotonically and reversibly with K/C. The spin relaxation rate and g factor do not change with doping, and XRD reveals an irreversible loss of crystallinity upon doping. We propose an inhomogeneous doping model to explain these results. RAPID COMMUNICATIONS R4846PRB 62 CLAYE, NEMES, JÁ NOSSY, AND FISCHER RAPID COMMUNICATIONS R4848PRB 62 CLAYE, NEMES, JÁ NOSSY, AND FISCHER