Electron paramagnetic resonance study of nanostructured graphite

We report on a systematic temperature-dependent x-band electron paramagnetic resonance (EPR) study of nanosized graphite particles prepared by ball milling. In as-prepared samples a very intense and sharp EPR resonance at $g=2.0035$ has been measured. The EPR line width shows a Korringa-like linear temperature dependence arising due to the coexistence and strong exchange coupling of itinerant and localized edge states. With a prolonged aging in inert atmosphere, changes in the EPR signal suggest gradual structural reconstruction where the localized edge states dominate the EPR signal. In this case the EPR spin susceptibility shows a maximum at $\approx 23\mathrm{K}$ indicating the development of antiferromagnetic correlations as expected for the graphene lattice with a bipartite symmetry.

Figure 1

(a) Comparison between the powder x-ray diffractograms of the starting pure graphite (bottom in black) and the 1 h ball-milled samples (top in red). (b) TEM images of the ball-milled graphite sample shows that the sample consists of larger graphite nanosized platelets and smaller graphite nanoribbons. (c) Nanoribbons, whose mean dimensions along the stacking $c$ axis is 6–7 nm, appear crystalline and (d) preserve the initial graphite stacking distance of $\sim$0.35 nm.

Figure 2

Comparison of x-band EPR spectra measured on ball-milled graphite samples collected at different stages of sample treatments, i.e., (a) as-prepared, (b) aged in an Ar atmosphere for several months, and (c) immediately after exposure to air. The spectra were collected at 100 K. In all three cases open circles represent the experimental data, solid red line is a fit, while dotted lines represent the individual components used to fit the spectra.

Figure 3

Temperature dependence of the x-band EPR spectra in ball-milled graphite aged in inert atmosphere for a prolonged time. Please note gradual broadening and decrease in intensity of the spectra below $\sim 23$ K.

Figure 4

Temperature dependence of the EPR spin susceptibilities for the as-prepared (circles), aged in inert atmosphere (squares), and air exposed (triangles). Solid lines represent fits to a sum of Pauli temperature-independent and Curie-Weiss spin susceptibilities (see text for details). Inset: Temperature dependence of the inverse EPR spin susceptibilities for the same samples.

Figure 5

Temperature dependence of the EPR line width for the as-prepared (circles), aged in inert atmosphere (squares), and air exposed (triangles) samples.