Electron Spin Resonance Study on the Magnetic Properties of Graphene and Its Derivative

Abstract

The magnetic properties of graphene are related to the presence of localized and conduction electrons and their interplay. A variety of graphene-based materials have been prepared and investigated using electron paramagnetic resonance (EPR) and Raman spectroscopy in order to understand the relationship between defects and electron-electron interaction. The graphene samples were prepared by using sonication-assisted liquid-phase exfoliation (LPE), electrochemical exfoliation (EC), reduced graphene oxide (rGO) and fluorinated graphene (FGn) produced from sonication-assisted LPE of fluorinated graphite (FG). The graphene flakes produced were further characterised using an atomic force microscope (AFM). The EPR samples analysed in the form of laminates in order to strengthen the EPR signal. Continuous-wave (CW) EPR experiments on the LPE graphene laminates revealed multicomponent, anisotropic, spectra showing the presence of narrow and broad components. A temperature-dependent study of the g value, line shape, signal intensity and Curie-Weiss fit of the magnetic susceptibility found that the narrow component could be attributed to localized electrons (vacancy defects) and the broad was attributed to the interplay of electrons between graphene layers. Several different thicknesses of laminates were prepared and further comparisons were made to graphite. It was found that an increase of disorder could be associated with an increase in laminate thickness/graphene stacking and further related to the interlayer electron-electron interaction of the defective and disordered graphene. The EPR and Raman spectroscopic analysis on the anode and cathode graphite foils produced through electrochemical exfoliation showed the presence of defects and expansion. The spectral analysis was consistent with the current mechanistic understanding of electrochemically prepared graphene. The graphene laminates prepared using electrochemical exfoliation and reduced graphene oxide showed similar spectral characteristics and the contribution of localized and conduction electrons for each type of graphene laminate were identified and characterized. There was evidence to suggest that the coupled and decoupled states of localized and itinerant conduction electrons were also influenced by defects and functionalization. The paramagnetic stability and defects of graphene laminate samples induced by ageing and action of a nanosecond pulsed laser irradiation were investigated. Ageing of graphene laminates showed a reduction in the EPR intensity with time in both atmospheric and argon atmospheres indicating passivation. Laser irradiation of the aged sample caused an increase in the numbers of spins whereas a reduction was observed for unaged samples. It was shown that the defects created by the laser could break the 〖sp〗^2 carbon-carbon bonds and create new spin centres. EPR spectroscopy of FGn revealed an isotropic line shape indicative of a homogeneously broadened EPR resonance arising from electron-electron interactions. The Curie-Weiss fit of the magnetic susceptibility behaviour showed two temperature regions, which show the magnetic moments to couple both ferromagnetically and antiferromagnetically. Hyperfine sublevel correlation (HYSCORE) spectroscopy was able to measure the fluorine hyperfine interaction.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Eric Mcinnes (Supervisor), Cinzia Casiraghi (Supervisor) & Sarah Haigh (Supervisor)