Temperature-dependent Raman studies of a natural van der Waals heterostructure*

Van der Waals heterostructures (vdWH) comprised of two-dimensional (2D) materials offer a platform to obtain designed materials with unique electronic properties. Research on 2D vdWH has so far been focused on fabricating such heterostructures by stacking individual 2D crystals, which leads to stacks with the presence of fabrication defects. Franckeite (Fr) is a naturally occurring vdWH comprised of two different alternately stacking semiconducting layers, that enables the study of a complex layered system where the crystal orientation between layers has been preserved. Unlike other layered sulfide-based materials, Fr is a grained-textured rock composed of few-millimeter flakes in random orientation. For this reason, exfoliation of thin flakes with a uniform and large surface area for optical characterization and transfer techniques is especially challenging. By precise manipulation of the starting flake in combination with different substrates we were able to increase the quality of exfoliation and obtain few-layer (< 5) Franckeite flakes with uniform surface areas of approximately 5 μm. Here we present results on the developed exfoliation technique and temperature-dependent Raman spectroscopy of Franckeite.