  Macroscopic arrays of highly crystalline nanocarbons offer the possibility of
modifying the electronic structure of their low dimensional constituents, for
example through doping, and studying the resulting collective bulk behaviour.
Insertion of electron donors or acceptors between graphitic layers is an
attractive method to reversibly increase charge carrier concentra-tion without
disruption of the sp$2$-conjugated system. This work demonstrates FeCl$_{3}$
intercalation into fibres made up of collapsed (flattened) carbon nanotubes.
The bundles of collapsed CNTs, similar to crystallites of graphitic
nanoribbons, host elongated layered FeCl$_{3}$ crystals of hundreds of $nm$
long, much longer than previous reports on graphitic materials and directly
observable by transmission electron microscopy and X-ray diffraction.
Intercalated CNT fibres remain stable after months of exposure to ambient
conditions, partly due to the spontaneous formation of passivating monolayers
of FeClO at the crystal edge, preventing both desorption of intercalant and
further hydrolysis. Raman spectroscopy shows substantial electron transfer from
the CNTs to FeCl$_{3}$, a well-known acceptor, as observed by G band upshifts
as large as $25 cm^{-1}$. After resolving Raman features for the inner and
outer layers of the collapsed CNTs, strain and dynamic effect contributions of
charge transfer to the Raman upshift could be decoupled, giving a Fermi level
downshift of $- 0.72 eV$ and a large average free carrier concentration of
$5.3X10^{13}$ $cm^{-2}$ ($0.014$ electrons per carbon atom) in the intercalated
system. Four-probe resistivity measurements show an increase in conductivity by
a factor of six upon intercalation
