Exfoliation of natural van der Waals heterostructures to a single unit cell thickness

Abstract

Weak interlayer interactions in van der Waals crystals facilitate their mechanical exfoliation to monolayer and few-layer two-dimensional materials, which often exhibit striking physical phenomena absent in their bulk form. Here we utilize mechanical exfoliation to produce a two-dimensional form of a mineral franckeite and show that the phase segregation of chemical species into discrete layers at the sub-nanometre scale facilitates franckeite's layered structure and basal cleavage down to a single unit cell thickness. This behaviour is likely to be common in a wider family of complex minerals and could be exploited for a single-step synthesis of van der Waals heterostructures, as an alternative to artificial stacking of individual two-dimensional crystals. We demonstrate p-type electrical conductivity and remarkable electrochemical properties of the exfoliated crystals, showing promise for a range of applications, and use the density functional theory calculations of franckeite's electronic band structure to rationalize the experimental results.

Photoresponse of Natural van der Waals Heterostructures

“…The sample was further characterized by micro-Raman spectroscopy by pumping the sample with a 532 nm laser. We have observed peaks near 165, 210, 260 and 320 cm -1 that are consistent with the recent report by Velicky et al 8 . The two peaks around 260 cm -1 and 320 cm -1 are probably originating from the stibnite (Sb2S3) and berndtite (SnS2) in the H layer, respectively.…”

Symmetry Breakdown in Franckeite: Spontaneous Strain, Rippling, and Interlayer Moiré

“…Several peaks observed in our spectra are in agreement with previously reported data. (13,14,18) Peaks at 195 and 328 cm -1 correspond to vibrational modes of the hexagonal layer of SnS2. (27,28) The two peaks at 259 and 284 cm -1 were previously assigned to the vibrations of Sb2S3, however the energies of these modes are lower than in isolated stibnite.…”

Mechanical and liquid phase exfoliation of cylindrite: a natural van der Waals superlattice with intrinsic magnetic interactions

“…The sulfosalt mineral franckeite has been recently reported as the first example of exfoliated naturally occurring vdW superlattices. [27][28][29][30][31][32] Interestingly, the sulfosalt family has other examples of natural vdW superlattices that could be exfoliated as well.…”