Thesis presented December 17, 2019

Abstract:
Two-dimensional transition metal dichalcogenides (TMDCs) are promising materials for a variety of applications, especially in optoelectronics. However, the lack of understanding of their epitaxy - i.e. growth mechanism, microscopic structure, nature of the 2D layer-substrate interaction, etc. - is still a crucial issue to address. In this PhD thesis we explored a series of epitaxial growths of monolayer and thin film TMDCs grown by molecular beam epitaxy (MBE) on a variety of substrates. We studied their atomic structures and we attempted the modifications of some of them with various in situ methods. Several systems and processes have been investigated: (i) transition metal ditellurides, ZrTe₂ , MoTe₂ and TiTe₂ on InAs(111) substrate, (ii) the intercalation of alkali metal species between single layer MoS₂ and its Au(111) substrate, (iii) the growth and the thermal treatments in H₂S atmosphere of monolayer PtSe₂ on Pt(111). Our work relies on both phenomenological and quantitative methods based on surface X-ray diffraction, often complemented by parallel analysis performed with other probes, e.g. STM, TEM, XPS, ARPES. Most notably, we found that: (i) a metastable orthorhombic phase and a charge density wave phase can be stabilized at room temperature in MoTe₂ and TiTe₂ owing to the epitaxial strain in the materials; (ii) the intercalation of Cs atoms under MoS₂ induces structural and electronic decoupling of the 2D MoS₂ layer from its Au(111) substrate; (iii) the sulfurization of PtSe₂ promotes the Se-by-S substitution in one (or both) of its two chalcogen layers, leading either to the full conversion of the selenide into a sulfide or even to an ordered Janus alloy.

Keywords:
Synchrotron radiation, 2D materials, van der Waals epitaxy, charge density waves, Intercalation, Janus