Direct observation of Mirror-Twin-Boundaries in WS2 quantum dots via HRTEM imaging

• Direct observation of mirror twin boundaries in WS 2 quantum dots. • Low-voltage 60 kV high-resolution electron microscopy of WS 2 nanoparticles. • Structural analysis separates ∼ 5 nm single-domain from ∼ 10 nm twin-boundary particles. • Mirror-related 2H domains separated by boundaries are evidenced. • ∼110° angle with the basal lattice directions is deduced. Quantum Dots (QDs) are a promising platform to develop quantum information processing and quantum light sources. Among the many compounds which are candidates for QDs fabrication, WS 2 nanoparticles have gathered attention because of their scalability and manufacturability. However, up today, structural characterization of grain-to-grain interfaces in WS 2 nanoparticles lacks direct observation. One of these features is the formation of Mirror-Twin-Boundaries (MTBs), which are expected to host metallic states and to affect Fermi-level pinning of Transition Metal Dichalcogenides (TMDCs), thus tunning the electronic band structure in electronic devices −according to literature models-. Our structural data provide the nanoscale geometrical parameters needed to feed such models. In this manuscript, WS 2 MTBs are evidenced, delivering a direct characterization which determines their crystallographic configuration and geometrical parameters. These structural descriptors constitute the necessary input for subsequent studies on formation mechanisms and on the electronic and optical consequences of MTBs in confined WS 2 systems.