Size effects in the conduction electron spin resonance of anthracite and higher anthraxolite

Abstract

Electron paramagnetic resonance spectroscopy of conduction electrons, i.e. Conduction Electron Spin Resonance (CESR), is a powerful tool for studies of carbon samples. Conductive samples cause additional effects in CESR spectra that influence the shape and intensity of the signals. In cases where conduction electrons play a dominant role, whilst the influence of localized paramagnetic centres is small or negligible, the effects because of the spins on conduction electrons will dominate the spectra. It has been shown that for some ratios of the bulk sample sizes (d) to the skin depth (δ), which depend on the electrical conductivity, additional size effects become visible in the line asymmetry parameter A/|B|, which is the ratio of the maximum to the absolute, minimum value of the resonance signal. To study these effects the electrical direct current-conductivity and CESR measurements are carried out for two amorphous bulk coal samples of anthracite and a higher anthraxolite. The observed effects are described and discussed in terms of the Dyson theory. Copyright © 2015 John Wiley & Sons, Ltd.

Unraveling Origins of EPR Spectrum in Graphene Oxide Quantum Dots

“…A similar statement can be found in the article written by Shames et al [53] for 40 nm multi-shell onion structures, where one of the Lorentzian lines was assumed to belong to conduction electrons [53]. Therefore, according to this theory, the conduction electrons in this case would also exhibit a Lorentzian line shape [38]. Figure 5d shows metal contamination, which appears clearly in the lowest temperature measured at 5 K but is also visible at 300 K with a strong amplification and large modulation amplitude used only for the magnification of the background signals.…”

“…Previous experimental attempts included: Creating vacancies by irradiation [27,28], creating sp 3 defects by surface doping with H and F ions [29,30], and adatoms such as C-OH groups [31][32][33][34]. Previously, we studied larger GO flakes interconnected into the form of aerogels [35], their spin relaxation processes [36,37], as well as other carbon materials, in which magnetic properties depended on particle size [38,39], and showed ferroand anti-ferromagnetic properties [40,41].…”

Preparation and characterization of partially reduced graphene oxide aerogels doped with transition metal ions

“…The EPR signals shape, intensity, g factor, linewidth, spin concentration is source of large number of information about local site symmetry, local dynamic and electron relaxation [50]. EPR study of graphene and graphene-related materials gives an important information about interactions and magnetism sources like defects, not passivated magnetic moments on edges, surface adatoms with unpaired magnetic moments, conduction electrons, and also remaining metal ion contamination [28,[51][52][53][54][55][56][57][58][59]. Unpaired electrons located on edges, on/in the graphene surface, are extremely sensitive to external conditions like atmosphere (oxygen, helium and vacuum) or moisture influencing the localization of Figure 7 Raman spectra of the reference and TMi-doped prGO aerogels.…”

“…EPR was used here as a method of confirming the purity of the GO and for the spin system characterization. The presence of impurities in the form of Mn or Na ions can influence the magnetic properties through the spin-orbit coupling [51,55], as well as electron conductivity by influencing the density of electron states at Fermi level [63]. Such impurities can influence the EPR spectra of prGO and rGO aerogels as well.…”

Tuning Properties of Partially Reduced Graphene Oxide Fibers upon Calcium Doping

“…Graphene-based samples can show complicated magnetic behavior due to zigzag edges [ 22 , 23 ], which can give rise to the edge states magnetism and basal-plane sp 3 defects formed by -OH groups (~1–1.2 µ B ) and adatom-induced magnetism [ 24 , 25 ]. We will discuss the Ca 2+ -doped fibers and compare the results to different carbon samples previously reported in the literature: graphene [ 21 , 26 ], GO in the form of aerogels [ 13 , 20 ], as well as with other carbon materials [ 27 , 28 ] which magnetic properties depend on particle size [ 29 ]. We will finally show the competitive mechanical and electrical response of the fibers, as well as the suitability of Ca 2+ -doping for tuning physico-chemical properties in reduced graphene oxide (rGO) fibers.…”