Phase segregation facilitates exfoliation of franckeite crystals to a single unit cell thickness

TL;DR

This study reveals that phase segregation in franckeite crystals enhances their exfoliation into single-unit-cell layers, enabling new 2D material applications with notable electrical and electrochemical properties.

Contribution

It demonstrates that phase segregation facilitates exfoliation of franckeite to monolayer thickness, offering a new pathway for synthesizing van der Waals heterostructures.

Findings

Crystals exfoliated down to a single unit cell.

Exfoliated crystals exhibit p-type conductivity.

Remarkable electrochemical properties observed.

Abstract

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.