  The fabrication of van der Waals heterostructures, artificial materials
assembled by individually stacking atomically thin (2D) materials, is one of
the most promising directions in 2D materials research. Until now, the most
widespread approach to stack 2D layers relies on deterministic placement
methods which are cumbersome when fabricating multilayered stacks. Moreover,
they tend to suffer from poor control over the lattice orientations and the
presence of unwanted adsorbates between the stacked layers. Here, we present a
different approach to fabricate ultrathin heterostructures by exfoliation of
bulk franckeite which is a naturally occurring and air stable van der Waals
heterostructure (composed of alternating SnS2-like and PbS-like layers stacked
on top of each other). Presenting both an attractive narrow bandgap (<0.7 eV)
and p-type doping, we find that the material can be exfoliated both
mechanically and chemically down to few-layer thicknesses. We present extensive
theoretical and experimental characterizations of the material's electronic
properties and crystal structure, and explore applications for near-infrared
photodetectors (exploiting its narrow bandgap) and for p-n junctions based on
the stacking of MoS2 (n-doped) and franckeite (p-doped)
