Van Hove singularities, moiré-superlattices and rhombohedral stacking in exfoliated sublattices of highly oriented pyrolytic graphite

Abstract

The presence of twisted sublattices in exfoliated lamellae of highly oriented-pyrolytic-graphite (HOPG) has been recently shown to induce a significant enhancement of percolative-ferromagnetic ordering at low temperature, opening new directions towards possible applications of graphitic lamellae in spintronics. Despite the significant progress, additional work is needed to elucidate possible magnetic effects arising from rhombohedral phases. In this work, we report a novel structural, electronic, and magnetic investigation. TEM allowed for the observation of multiple hexagonal moiré-superlattices and unusual staircase-like defects in those sublattices exposed by the exfoliation process. STM and STS further evidenced the appearance of multiple singularities in LDOS spectra acquired from certain sublattice-areas comprised within comparable staircase-defect features. Unusual effects involving a sharp enhancement in the density of states near the Fermi level are reported. These observations possibly imply the coexistence of both moiré superlattices and rhombohedral domains in certain twisted-sublattice regions. Further, Raman spectroscopy, through area mapping of the D, G, and D′ bands, evidenced additional signals in the proximity of ∼1,800–2,100 cm−1, attributable to electronic Raman-scattering (ERS). These effects arise locally from emerging rhombohedral domains possibly created by (1) slip-induced transformation of stacking order during tape exfoliation and (2) twisted sublattices, with θrot ∼0.1°. T-EPR measurements also evidenced an unusual enhanced broadening and peak shift in the differential π-electron feature acquired from some of the lamellae exfoliated from the inner regions of the main HOPG.