Micrometer-size Crystalline Monolayer MoS2 Domains Obtained by Sulfurization of Molybdenum Oxide Ultrathin Films

In this work, the formation of micrometer-size crystalline monolayer (1L) MoS2 flakes with triangular shape and a central multilayer core is obtained by the sulfurization at 800 degrees C of pre-deposited ultrathin MoOx films (1.2-1.8 nm) on c-sapphire substrates. The thickness uniformity, crystalline quality, doping and strain distri-bution in the MoS2 flakes as a function of the initial MoOx thickness was evaluated by micro-Raman (mu R) mapping. The excellent crystalline quality of the triangular 1L-MoS2 flakes was confirmed by micro -photoluminescence (mu PL) maps, showing a very intense peak at-1.85 eV, that decreases in the central part, as expected for multilayers MoS2. A biaxial strain of-0.38-0.4% was deduced from the mu PL data, in perfect agreement with mu R results. Our results show how the sulfurization of pre-deposited MoOx films on c-sapphire allows, under proper conditions, to obtain 1L-MoS2 flakes with quality comparable to the one typically reported by the conventional chemical vapour deposition, with important implications for device applications.