Magnetic properties of nitrogen- doped carbon nanospheres

Abstract

Electron spin resonance (ESR) was used to characterize a suite of carbon nanospheres (CNS) samples with varying nitrogen concentrations at room temperature. The CNS were produced using two different reactors (vertical and horizontal) under different preparatory conditions. Resonance spectra of samples produced from the vertical reactor showed resonance lines- a narrow paramagnetic component, and broader component. They were attributed to nitrogen paramagnetic impurities and carrier spins, respectively. Samples produced in the horizontal reactor revealed stronger line spectra that were narrower and Dysonian in shape. The nitrogen content of the samples produced by the horizontal reactor was determined through ESR analysis which involves integration of the resonance peak, and normalizing to the mass of the sample. The relative g-shift was also measured by using a DPPH reference sample. Room temperature power saturation experiments were performed on samples produced from the horizontal reactor with the aim of estimating the spin relaxation times. Two samples from the horizontal reactor were further investigated at low temperatures (4 K- 320 K) at a constant microwave power. The resonance parameters investigated were linewidth, asymmetry ratio and amplitude, and possible spin-lattice relaxation mechanisms were investigated. The variation of the amplitude with temperature was investigated using two models: (1) a model based on lattice vibrations, and (2) a model based on nanographites assembly (considered interaction between carrier and localized spins). At low temperatures both models have amplitude that changes inversely with temperature in accordance with Curie law. At high temperatures (T > 200 K) a model based on nanographites assembly provide an alternative; it describes the rise in the signal amplitude in terms of thermally activated paramagnetic electrons from non-magnetic ground state to excited state at energy . Analysis of linewidth and asymmetry ratio data confirmed that the spin-lattice relaxation governed by thermal activated electrons is a dominant relaxation mechanism at high temperatures.