Franckeite is a naturally occurring mineral discovered in Bolivia at the end of the eighteenth century. It belongs to the sulfosalt family and is composed of an alternating sequence of weakly bound, incommensurately stacked PbS and SnS2 layers separated by van der Waals gaps. The strong compositional segregation for Pb and Sn atoms leads to markedly different electronic band structures for the different layers, whose overall properties may be further complicated by Pb/Sb and Sn/Fe partial substitutions.

Now, writing in Nature Communications, two independent research groups report the exfoliation of franckeite into few-layer-thick heterostructures that can be thought of as natural counterparts of an artificial van der Waals heterostructure. Both A. J. Molina-Mendoza et al. (Nat. Commun. 8, 14409; 2017 ) and M. Velický et al. (Nat. Commun. 8, 14410; 2017 ) exploit mechanical and liquid-phase exfoliation processes to obtain ultrathin nanoflakes. The cross-section of a typical sample is shown (from the study of Velický et al.; scale bar, 1 nm), visualized by means of high-angle annular dark-field scanning transmission electron microscopy. Remarkably, Velický et al. even succeed in exfoliating single-unit-cell-thick heterostructures.

Credit: Macmillan Publishers Ltd

In comparison with their artificial analogues, franckeite-based van der Waals heterostructures show advantageous properties because the mutual orientation of the different layers is preserved across the nanoflakes and, simultaneously, the occurrences of spurious interlayer adsorbates are kept at a minimal level. Also, the franckeite nanoflakes are stable even after exposure to air for several months — even though Velický et al. observe a marked degradation of the thinnest samples and, in general, surface deterioration effects. This stability is reflected in the preservation of the p-type, narrow-bandgap semiconducting behaviour also observed for the bulk material, as confirmed by means of transport measurements in back-gated devices and by scanning tunnelling spectroscopy. This behaviour is in stark contrast to phosphorene, also a two-dimensional p-type, narrow-gap semiconductor possessing comparable electronic properties — yet, not air-stable.

The two groups also demonstrate possible technological applications for the considered nanoflakes. Molina-Mendoza et al. focus on the moderate bandgap values (∼0.7 eV) to investigate the potential of few-layer franckeite as a near-infrared photodetector. Moreover, exploiting the p-type semiconducting character, they fabricate proof-of-principle p–n photodiodes made of few-layer franckeite and n-doped few-layer MoS2. Velický et al. focus instead on the electrochemical properties of exfoliated franckeite samples and, in particular, show good electric double-layer capacitance and high electron-transfer rate values. These aspects suggest the possible exploitation of few-layer franckeite for applications in energy-storage and conversion applications.