Helical superstructure of continuum graphene cone uncovered by TEM analysis of herringbone-striped pattern in graphitic whiskers

•Periodic herringbone-striped pattern appears on lattice images of graphitic whiskers.•Electron microscopy (TEM) reveals helical superstructure of continuum graphene cone.•Stepwise overlap by conical coiling of continuous layers causes the periodicity.

Cone-shaped graphitic whiskers (CGWs) are a form of pyrolytic carbon, consisting of conically stacked hexagonal carbon layers with an apex angle of ~135–140°. Under transmission electron microscopy (TEM), CGWs often exhibit herringbone-striped patterns. Bright-field (BF) and dark-field (DF) TEM images indicated that the stripes are due to periodical appearance of a strong inter-planar reflection, which is consistent with helical rotation of layers with stepwise “layer overlap”. High-resolution TEM revealed that the period was ~14–15 layers. The relationship between apex angle and stripe periodicity of CGWs could be consistently explained in terms of a helical superstructure of tightly coiled continuous graphene cone.