  Carbon nanotube fibers are materials with an exceptional combination of
properties, including higher toughness than carbon fibers, electrical
conductivity above metals, large specific surface area (250 $\frac{m^2}{g}$)
and high electrochemical stability. As such, they are a key component in
various multifunctional structures combining augmented mechanical properties
with efficient interfacial energy storage/transfer processes. This work
presents a thorough XPS study of CNT fibers subjected to different purification
and chemical treatments, including spatially-resolved micro XPS synchrotron
measurements. The dominant feature is an inherently high degree of $sp^2$
conjugation, leading to a strong plasmonic band and a semi-metallic valence
band lineshape. This high degree of CNT perfection in terms of longitudinal
graphitization helps to explain reported bulk properties including the high
electrical and thermal conductivity, and accessible quantum capacitance. There
is also presence of organic impurities, mostly heavy carbonaceous molecules
formed as by-products during fiber synthesis and which are adsorbed on the
CNTs. Sulfur, a promoter used in the CNT growth reaction, is found both in
these surface impurities and associated with the Fe catalyst. The observation
of strongly-adsorbed surface impurities is consistent with the high ductility
of CNT fibers, attributed to interfacial lubricity.
