Franckeite: a naturally occurring van der Waals heterostructure

TL;DR

This paper introduces franckeite, a naturally occurring van der Waals heterostructure, as a new material for 2D heterostructures, with detailed characterization and potential applications in photodetectors and p-n junctions.

Contribution

It demonstrates a novel, simpler method to obtain ultrathin heterostructures from natural materials, avoiding complex stacking procedures.

Findings

Franckeite can be exfoliated to few-layer thicknesses.

It exhibits a narrow bandgap (<0.7 eV) and p-type doping.

Potential for near-infrared photodetectors and p-n junction applications.

Abstract

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)