We report intercalation of charged polyiodide chains into the interstitial channels in a singlewall carbon nanotube (SWNT) rope lattice, suggesting a new carbon chemistry for nanotubes, distinctly different from that of graphite and C 60 . This structural model is supported by results from Raman spectroscopy, x-ray diffraction, Z-contrast electron microscopy, and electrical transport data. Iodine-doped SWNTs are found to be air stable, permitting the use of a variety of techniques to explore the effect of charge transfer on the physical properties of these novel quantum wires.[S0031-9007(98)
Electrochemical doping of bisulfate ions into single wall carbon nanotube (SWNT) bundles has been studied
using coulometry, cyclic voltammetry, mass-uptake measurements, and Raman scattering experiments. A
spontaneous charge-transfer reaction is observed prior to the application of an electrochemical driving force,
in sharp contrast to previous observations in the graphite−H2SO4 system. A mass increase of the SWNT
sample and a concomitant upshift of the Raman-active tangential mode frequency indicate oxidation (i.e.,
removal of electrons) of the SWNT bundles. In fact, using Raman scattering, we were able to separate the
spontaneous and electrochemical contributions to the overall charge transfer, resulting in the value of an
upshift of 320 cm-1 per hole, per C-atom introduced into the carbon π-band by the bisulfate (HSO4
-) dopant.
This value may prove to be a universal measure of charge transfer in acceptor-type SWNT compounds. At
a critical electrochemical doping, the SWNT bundles are driven into an “overoxidation” regime, where they
are irreversibly oxidized with the formation of C−O covalent bonds, analogous to electrochemical formation
of graphite oxides.
The upconversion mechanism of Er3+ ions has been studied for lead-germanate glasses containing Er2O3 concentrations from 0.1 mol % to 2 mol %. Intense green emission was observed at room temperature due to 4S3/2→4I15/2 transition excited by a cw near-infrared laser beam at 797 nm. This green emission shows a similar intensity for samples with different Er3+ ion concentrations. A weak blue emission of 410 nm originating from the 2H9/2→4I15/2 transition was also observed. This blue emission and a red emission from the 4F9/2 level increase with the increase in Er3+ ion concentration. The bright green emission is attributed to the excited level absorption while the blue emission is due to a third step excitation where energy transfer between excited ions owing to their Coulomb interaction plays a key role.
The electrical transport properties of single-wall carbon nanotubes are shown to be strongly influenced by the presence of transition-metal impurities derived from the catalyst introduced to stimulate their growth. Data on thermoelectric power and electrical resistance in the temperature range 10-400 K were obtained on a series of samples prepared using MY catalysts ͑M ϭCr, Mn, Co, Fe, Ni͒. The unusual transport behavior observed is tentatively assigned to an interaction between the magnetic moment of the M atom and the spin of the conduction electrons of the nanotubes, i.e., the Kondo effect. ͓S0163-1829͑99͒50240-3͔ RAPID COMMUNICATIONS R11 312 PRB 60 L. GRIGORIAN et al.
We report in situ measurements of four-probe dc resistance ͑R͒ and thermopower ͑S͒ of Cs-and K-doped single-wall carbon nanotube ͑SWNT͒ mats as a function of a doping and temperature ͑T͒. With increasing dopant exposure, the mat resistance has been found to first decrease and then increase, exhibiting a minimum for optimal Cs doping. In contrast, for K doping, the mat resistance decreased monotonically and saturated. This unexpected result suggests that the diameter of the alkali-metal ion plays a role in the transport properties of the tube bundles. A doping-induced decrease in R by factors of ϳ120 and ϳ40 were observed for Cs-and K-doped SWNT mats, respectively. The low-temperature upturn of R(T) observed in all pristine SWNT samples was progressively suppressed with increased K doping. The optimally Cs-doped sample exhibited a positive dR/dT over the entire range of measurement ͑80 KϽTϽ300 K͒. In contrast to the anomalously large positive S 300 K ϳϩ40-ϩ50 V/K observed in pristine SWNT at room temperature, the Cs-doped samples exhibited a small negative SϳϪ7 V/K as expected for an ordinary metal.
Raman spectra of iodine-doped single-walled carbon nanotube ͑I-SWNT͒ bundles excited with 514.5 nm were studied at room temperature and elevated pressure up to 7 GPa. The low frequency modes in I-SWNT exhibit very small pressure-induced frequency shifts, in contrast to the pressure shift of ϳ7 cm Ϫ1 /GPa ͑or larger͒ reported for the radial modes in undoped SWNT samples. This weak pressure dependence and the resonance with 514.5 nm excitation, corroborate the previous assignment of the Raman bands at ϳ110 and 175 cm Ϫ1 in I-SWNT to polyiodide chains. Furthermore, based on a comparison between the pressure behavior of the I-SWNT and undoped SWNT samples, we suggest that the I n Ϫ molecules might reside both in the interstitial channels and inside the pores of the tubes in SWNT bundles.
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