Investigation of the intrinsic defects role on optical and optoelectronic properties of tran-sition metal dichalcogenides monolayers and lateral heterostructures prepared by their tailored synthesis

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Layered materials such as graphite and MoS2 provide an inspiring platform to combine atomically thin semiconductors with metals and insulators for fascinating physics and applications. The current design concept is to use individual flakes and combine them by stacking thin sheets in van der Waals structures, to access new functionalities. Here we propose a very different approach to extend the repertoire of 2D materials beyond the naturally occurring crystals by exploring new avenues in Chemical Vapour Deposition (CVD) of transition metal dichalcogenides (TMDs). The project will combine 3 very recent breakthroughs pioneered by the 3 partners: •Jena: has recently significantly improved the reproducibility and crystal quality of CVD grown TMD monolayers by introducing a novel growth procedure based on Knudsen effusion cells and will extend this growth approach to lateral heterostructures and Janus monolayers such as SMoSe. •Ulm: developed sub-Angstrom low-voltage electron microscopy (SALVE), enabling access to the intrinsic defect structure and density in clean monolayer materials as well as to the electronic properties of the defects – this is a key issue for monitoring the CVD quality progress from Jena and uncover the atomic structure of the lateral interfaces as well as the defect’s properties. •Toulouse: has shown that the CVD grown samples (Jena), with confirmed low defect densities (Ulm) show optical quality comparable to the highest quality exfoliated material. Accessing the intrinsic optical properties is achieved by eliminating dielectric disorder (i.e., extrinsic effects), which dominates so far the optical studies reported of CVD samples in the literature. The main project objectives are: • CVD growth of large area TMD monolayers everywhere on the substrate (large yield for device fabrication) with controlled defect density through optimised growth and feedback from a combination of atomic-resolution low-voltage transmission electron microscopy techniques (TEM). Establishing high quality growth with controlled defect types and densities is solid starting point before adapting growth for heterostructures and alloys. Accessing individual defects and understanding their origin is relevant for quantum technologies. • Growth of lateral heterostructures such as MoSe2-WSe2 without equivalent in exfoliated samples; we avoid problem of interface contamination during stacking; investigation of atomic sharpness of boundaries versus gradual alloying in TEM; investigation of exciton diffusion along bandgap gradient; towards creation of exciton “traps” for efficient, localized light emission or future exciton condensation experiments.• Growth of Janus monolayers with different top and bottom chalcogen such as SMoSe; layers with great potential in chemistry (catalysis) and optoelectronics; TEM investigation of Janus monolayers; optics on Janus monolayers: second harmonic generation, electric field tuning of main optical transitions.