EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Within layered (two-dimensional) materials, the stacking sequences (polytypes) and physical properties of the materials are deeply intertwined. Tailoring specific stacking sequences thus provides a powerful handle in the design of nanostructures based on layered materials exhibiting novel physical properties. In this context, Transition-Metal Dichalcogenides (TMDs) based nanostructures crystallizing within in either the 2H or 3R phases have been extensively studied, but unfortunately our knowledge on TMD nanomaterials based on mixed polytypes is far more limited. In this work, we present an exhaustive characterization of CVD-grown free-standing flower-like WS2 nanostructures, displaying a mixed 2H/3R polytype, by means of state-of-the-art transmission electron microscopy. Their rich variety of shape-morphology configurations is correlated with relevant local electronic properties such as edge, surface, and bulk plasmons. Furthermore, machine learning techniques are deployed to determine that the band gap of the WS2 2H/3R polytype is indirect in nature and has a value of $E_{BG}$ = 1.6 eV. High resolution electron energy-loss spectroscopy reveals another striking property of the WS2 2H/3R polytype, namely the presence of energy-gain peaks exhibiting a gain-to-loss ratio greater than unity. This remarkable property could be exploited to design new cooling strategies for atomically-thin TMD nanostructures and devices built upon them. We investigate systematically the differences in the locations of the energy-gain-to-loss ratios, arising between the different shape-morphologies, including with measurements taken on exfoliated WS2 flakes and with theoretical calculations based on Density functional Theory methods. The latter provide unique insights on the underlying mechanism responsible for the appearance and properties of energy-gain features in WS2 2H/3R polytype. Our findings represent a stepping stone towards an improved understanding of TMD nanomaterials based on mixed crystalline phases.