In this work, we report the impact of the substrate type on the morphological and structural properties of molybdenum disulfide (MoS₂) grown by chemical vapor deposition (CVD). MoS₂ synthesized on a 3D substrate, that is SiO₂, in response to the change of the thermodynamic conditions yielded different grain morphologies, including triangles, truncated triangles, and circles. Simultaneously, MoS₂ on graphene is highly immune to the modifications of the growth conditions, forming triangular crystals only. We explain the differences between MoS₂ on SiO₂ and graphene by the different surface diffusion mechanisms, namely hopping and gas-molecule-collision-like, respectively. As a result, we observe the formation of thermodynamically favorable nuclei shapes on graphene, while on SiO₂ a full spectrum of domain shapes can be achieved. Additionally, graphene withstands the growth process well, with only slight changes in strain and doping. Furthermore, by the application of graphene as a growth substrate we realize van der Waals epitaxy, and achieve strain-free growth, as suggested by the photoluminescence (PL) studies. We indicate that PL, contrary to Raman spectroscopy, enables us to arbitrarily determine the strain levels in MoS₂.

Oxygen breaks up Mg–H complexes and induces high diffusion of Mg in GaN/AlGaN heterostructures.

Van der Waals heterostructures (vdWHSs) provide a unique playground to study fundamental physics and practical applications of two-dimensional (2D) materials. However, most 2D heterostructures are prepared by transfer, hindering their technological implementation. Here, we report the first chemical vapour deposition of monolayered MoS₂/WS₂/graphene vertical vdWHS without transfer step. By atomic force microscopy, photoluminescence, Raman spectroscopy, and secondary ion mass spectroscopy, we confirmed the vertical stacking of three different 2D materials. The use of WS₂, graphene, and sapphire as growth substrates allowed us to describe the 2D materials growth process better. We determined that for the synthesis of 2D materials, only the chemical potential of the crystal formation and the substrate-layer adhesion energy are relevant factors. In addition, we used MoS₂/WS₂/graphene vdWHS to fabricate a photoresponsive memory device, showing the application potential of such heterostacks. Our results clarify the growth mechanisms of 2D materials and pave the way for the growth of more complex vdWHSs.