Two-Dimensional Sheets Derived from Non-Van der Waals Crystals: Mechanism of the Mechanochemical Synthesis

Recent studies of mechanically-synthesised two-dimensional (2D) sheets from otherwise three-dimensional non-van der Waals (vdW) crystals are harbingers of a new approach for materials engineering. Yet, the nature in which 2D sheets can be derived from a non-layered, isotropic crystal remains unknown. In the case of boron, the complex crystal structure of the bulk material and its abundance of phases, as well as the influence of crystal orientation and thickness on structure interpretation, make it challenging to determine the nature of the exfoliated 2D sheets. Here we demonstrate, using aberration-corrected scanning transmission electron microscopy (STEM) and extensive structural modelling, that liquid phase exfoliated boron sheets consist of a planar arrangement of icosahedral subunits, formed from the cleaving of the {001} planes of β-rhombohedral boron. Our experimental results supported by density functional theory (DFT) calculations show that planar defects in the form of stacking faults — which form parallel layers of faulted planes in the same orientation — play a significant role in the exfoliation of 2D sheets from bulk boron, reducing the energy required to cleave the bulk {001} surface by ~ 36%. We demonstrate that planar defects are key to an engineerable pathway for the exfoliation of 2D sheets from boron, and more broadly of other covalently-bonded materials. This provides opportunities for the design of new 2D materials from non-vdW crystals.