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 materials by design with unique electronic properties. Franckeite (Fr) is a naturally occurring vdWH comprised of two distinct alternately stacked semiconducting layers. Because it is naturally heterostructured, it enables the study of a complex layered system without the presence of fabrication defects, and where the crystal orientation between layers has been preserved. Unlike other layered sulfide-based 2D materials, the exfoliation of thin flakes of Fr is especially challenging, leading to a knowledge gap of its fundamental properties. Raman spectroscopy is a powerful method to characterize and study the fundamental properties of 2D crystals. All the power this technique provides in characterizing 2D materials stems from well-established literature on the signature of these crystals, which is lacking for Fr. In this work, we performed an extensive micro-Raman spectroscopy study of Fr. We show resonance-enhanced Raman signal, the evolution of the Raman peaks for increasing temperature, and the presence of breathing and shearing modes. Moreover, we performed theoretical calculations to fully characterize the vibrational modes of Fr.