Electron paramagnetic resonance (EPR) spectroscopy can be fruitfully
applied to study the interplay of localized and itinerant spins for carbon nanomaterials,
including carbon nanotubes (CNTs), and thus provides a unique spectroscopic probe of
their electronic properties upon integration as active components in composite
materials. In this chapter, EPR spectroscopy is exploited to investigate the magnetic
properties of double-walled carbon nanotubes (DWCNTs) and composites of oxidized
multi-walled carbon nanotubes (MWCNTs) embedded in an elastomeric poly(etherester)
block copolymer. In the case of DWCNTs, an asymmetric resonance line was
observed that could be accurately analyzed in terms of two independent metallic
lineshapes with similar g-factors, a narrow and a broad one, related to the distinct
contributions of defect spins located on the inner and outer DWCNTs layers,
respectively. Analysis of the spin susceptibilities indicated a ferromagnetic phase
transition at low temperatures, alike metallic single wall CNTs. Interlayer coupling
between the DWCNT layers is accordingly suggested to enhance exchange interactions
between localized spins via conduction electrons. Conversely, in the case of
MWCNTs-polymer composites, EPR spectra in combination with static magnetization
measurements revealed a drastic reduction of orbital diamagnetism and g-anisotropy
along with a marked enhancement of spin susceptibility, with respect to the anisotropic
EPR spectrum of pristine MWCNTs. These effects indicate considerable hole doping
by oxygen functional groups on the MWCNTs’ surface and an excessive increase of
the density of paramagnetic defects, which are sensitive to the polymer relaxation and
to the underlying MWCNT-polymer interfacial coupling.
Keywords: Carbon nanotubes (CNTs), Curie-Weiss law, EPR, EPR linewidth,
EPR spectra deconvolution, Exchange interaction, Ferromagnetic phase transition, Ferromagnetic resonance, Spin dynamics, Double wall carbon nanotubes (DWCNTs),
Lorentzian lineshape, Magnetic properties, Multiwall carbon nanotube (MWCNTs)
polymer composites, Raman spectra, Single wall carbon nanotubes (SWCNTs), Single
metallic lineshape, Skin effect, Spin susceptibility, Static magnetization, Temperature
dependence.
