Coexistence of crystalline rhombohedral stacking and hexagonal moiré superlattices in exfoliated highly oriented pyrolytic graphite

Abstract

The recent observation of unusual persistent currents at the stacking faults of highly-oriented-pyrolytic-graphite (HOPG) has attracted significant attention.In this work, we investigate the structural properties of these systems and especially the possible magnetic effects arising from coexistence of both rhombohedral-stacking and hexagonal moiré -superlattices, within exfoliated samples. Raman spectroscopy and X-ray diffraction (XRD) allowed for the identification of hexagonal and rhombohedral graphitic structural-components coexisting within manually-exfoliated thick HOPG-samples (thickness of ~ 300–400 µm). Comparative characterization of thin (thickness of ~ 0.1–10 µm) samples revealed a significant weakening of the rhombohedral-component, highlighting a rapid ABC to AB relaxation. This interpretation was further confirmed by TEM analyses performed on thin-samples (e-beam perpendicular to the layers). Noticeably, the unusual presence of large-area hexagonal-moiré -superlattices in coexistence with rhombohedral domains could be identified in one of the analysed lamellae. The unusual grain-boundary between the two phases was analysed in relation to the possible variation of the θ-twist parameter. Disappearance of the superlattices was found in those disclination-rich regions created by the exfoliation process. Magnetic characterization of both thick and thin samples, allowed for the identification of an unusual overlay between zero field cooled (ZFC) and field cooled (FC) magnetic signals, with a T-dependent enhancement of the diamagnetic susceptibility from T~ 300 K to T~2 K, possibly resulting from a disclination-free structural configuration. A spin-glass-like behaviour was instead found in those samples exhibiting a broader and disorder-rich non-monotonic X-ray rocking-curve distribution.