In situ Raman scattering studies of alkali-doped single wall carbon nanotubes

Abstract

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

Ionic Liquid for in situ Vis/NIR and Raman Spectroelectrochemistry: Doping of Carbon Nanostructures

“…The exciting photons (energies 2.41 eV or 2.54 eV) resonate with the transition (v s 3 3c s 3 ) in tubes of diameters 1.2 ± 1.5 nm. [17,29,30] The overall intensities of the RBM and TM modes decrease as a result of both cathodic and anodic charging in ionic liquid (Figure 4 and (5)), which matches the behavior in aqueous [20,39,40] and aprotic [8,12,21] electrolyte solutions. By comparing Figure 4 with Figure 6 and Figure 5 with Figure 7, the tube-related modes in peapods show a similar response to electrochemical charging as those of empty tubes.…”

“…[8,20] This also causes reversible quenching of resonance Raman scattering of both the radial breathing mode (RBM) and the tangential displacement mode (TM) of SWCNT. [8,11,20,21] Analogous effects can be traced in the tube-related parts of optical and Raman spectra of C 60 @SWCNT (peapods). [12] The C 60 -related Raman modes in peapods show a peculiar asymmetric increase of Raman intensities at larger anodic potentials.…”

“…[13] Similar red shifts were reported for SWCNT in LiClO 4 acetonitrile (200 AE 30 cm À1 ) [8] or in LiPF 6 ethylene carbonate/dimethylcarbonate (10 ± 100 cm À1 ). [21] However, in ionic liquids the behavior is just the opposite: we trace a blue shift of the TM band for n-doped SWCNT (Figures 4 and 5) and peapods (Figures 6 and 7). This again illustrates the complexity of n-doping [10,13,16,21,42] and the influence of the compensating counter-ion.…”

“…This again favors ionic liquids over acetonitrile solutions, in which, for example, the dC-H band overlaps the bands of tubes or peapods. [12] In contrast to p-doping, which always causes a blue shift of the TM band (see above), [8,12,13,20,39] the n-doping, carried out electrochemically [8,10,21] or chemically [13,16,42] may cause both blue and red shifts depending on the counter-ion (Li , K , Rb ), doping level and the excitation wavelength. The interpretation is far from being clear and consistent.…”

“…The interpretation is far from being clear and consistent. [8,10,13,16,21,42] In the simplest model, the n-doping causes softening of graphene modes, since electrons are introduced into the p* band. [13] For graphite, this softening causes a red shift of 140 cm À1 per electron, per carbon atom.…”