Doping of C₆₀ Fullerene Peapods with Lithium Vapor: Raman Spectroscopic and Spectroelectrochemical Studies

Abstract

Raman spectroscopy and in situ Raman spectroelectrochemistry have been applied to the study of the lithium vapor doping of C60@SWCNTs (peapods; SWCNT=single‐walled carbon nanotube). A strong degree of doping was proven by the disappearance of the radial breathing mode (RBM) of the SWCNTs and by the attenuation of the tangential (TG) band intensity by two orders of magnitude. The lithium doping causes a downshift of the Ag(2) mode of the intratubular C60 by 27 cm−1 and changes the resonance condition of the encapsulated fullerene. In contrast to potassium vapor doping, the strong downshift of the TG band was not observed for lithium doping. The peapods treated with lithium vapor remained partially doped even when they were exposed to humid air. This was reflected by a reduction in the intensity of the nanotube and the fullerene modes and by the change in the shape of the RBM band compared with that of the undoped sample. The Ag(2) mode of the intratubular fullerene was not resolved after contact of the lithium‐doped sample with water. Lithium insertion into the interior of a peapod and its strong interaction with the intratubular fullerene is suggested to be responsible for the air‐insensitive residual doping. This residual doping was confirmed by in situ spectroelectrochemical measurements. The TG band of the lithium‐doped peapods did not undergo an upshift during the anodic doping, which points to the formation of a stable exohedral metallofullerene peapod.

Spectroelectrochemistry of Carbon Nanotubes

“…[118,119] The penetration is more pronounced in the case of Li-vapor-treated C 60 @SWNT, when the intratubular species was particularly stable. [54] Interestingly, thin SWNTs react rapidly with Li vapor to give Li 2 C 2 . [64,66] There are only a few works on other peapods.…”

Self-Assembled Hierarchical Structure of Fullerene Building Blocks; Single-Walled Carbon Nanotubes and C60

“…The first step to recognize the full potential of such a nanostructured class of matter, however, is to investigate the thermodynamically stable phases possible by the assembly of their building blocks. Many theoretical and experimental studies have addressed the interaction between single-walled carbon nanotubes and C 60 fullerene on a processing level directed at the formation mechanism and properties of “peapods” − as the C 60 nanospheres enter into the nanotube; and “nanobuds − as the C 60 molecule is covalently attached to the surface of SWCNT. Unfortunately, investigations of the interaction between SWCNT and C 60 , on an assembly level, to address the nature of resulting meso-structures have not been sufficiently addressed.…”

An Anomalous Enhancement of the Ag(2) Mode in the Resonance Raman Spectra of C60 Embedded in Single-Walled Carbon Nanotubes during Anodic Charging

“…An efficient approach to modify the electronic structure of molecules inside carbon nanotubes is doping. , This can be realized either by treatment of carbon nanostructures with redox active molecules or by electrochemical charging. However, in several cases the chemical dopant can penetrate into the SWCNT and react directly with the encapsulated species. − A chemical doping is highly irreversible, and in some cases the chemical dopant cannot be removed even by extraction with solvent. − The doped intratubular species can even assist the coalescence of the C 60 molecules inside SWCNTs . Electrochemical charging in electrolyte solutions, on the other hand, mimics a double layer capacitor, where the charge carriers are injected into the nanotube from the electrode support and the electrolyte ions compensate the injected charge .…”

“…However, in several cases the chemical dopant can penetrate into the SWCNT and react directly with the encapsulated species. [9][10][11][12][13] A chemical doping is highly irreversible, and in some cases the chemical dopant cannot be removed even by extraction with solvent. [10][11][12] The doped intratubular species can even assist the coalescence of the C 60 molecules inside SWCNTs.…”

“…This has been shown on the example of doping of fullerene peapods with alkali metals. 9,10 Despite the fact that numerous studies have dealt with the doping of inner species inside SWCNTs, open questions still persist. One of the least understood issues is the enhancement of the Raman signal of C 60 fullerene modes in peapods at positive potentials (the so-called "anodic Raman enhancement") 8 which is not reproduced for C 70 peapods 8 nor for inner tubes of DWCNTs.…”