Fig. 3: Definition of vdW interaction, vdW distance and vdW gap.

a, Energies of various molecular interactions. vdW interaction is the weakest intermolecular interaction (also termed physical interaction), much smaller than typical intramolecular interactions (also termed chemical interactions). b, Potential energy versus distance for an Ar dimer system, with a vdW distance of about 0.38 nm (lowest potential point) and a vdW gap of about 0.23 nm (difference between vdW distance and covalent radius). Image adapted from ref. ²⁴ with the permission of AIP Publishing. c, Schematic illustration of the vdW gap and the vdW distance in a covalent-bonded system and a vdW system. d, e, Comparison between layer spacing and calculated vdW gap in various layered 2D materials. The calculated vdW gap is around 0.2 nm, comparable to that of the Ar dimer.