  Weak interlayer van der Waals interactions in bulk crystals facilitate their
mechanical exfoliation to monolayer and few-layer two-dimensional (2D)
materials, which exhibit striking physical phenomena absent in their bulk form.
Here we study a 2D form of a mineral franckeite and show that phase segregation
into discrete layers at the sub-nanometre scale facilitates its layered
structure and basal cleavage. This behaviour is likely to be common in a wider
family of complex crystals and could be exploited for a single-step synthesis
of van der Waals heterostructures, as an alternative to stacking of 2D
materials. Mechanical exfoliation allowed us to produce crystals down to a
single unit cell thickness and rationalise its basal cleavage by
atomic-resolution scanning transmission electron microscopy (STEM). We
demonstrate p-type electrical conductivity and remarkable electrochemical
properties in exfoliated crystals, which shows promise for energy storage
applications.
