In this work, the formation of micrometer-size crystalline monolayer (1 L) MoS2 flakes with triangular shape and a central multilayer core is obtained by the sulfurization at 800 °C of pre-deposited ultrathin MoOx films (1.2–1.8 nm) on c-sapphire substrates. The thickness uniformity, crystalline quality, doping and strain distribution in the MoS2 flakes as a function of the initial MoOx thickness was evaluated by micro-Raman (μR) mapping. The excellent crystalline quality of the triangular 1 L-MoS2 flakes was confirmed by micro-photoluminescence (μPL) 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 μPL data, in perfect agreement with μR results. Our results show how the sulfurization of pre-deposited MoOx films on c-sapphire allows, under proper conditions, to obtain 1 L-MoS2 flakes with …