  Strain engineering is an efficient tool to tune and tailor the electrical and
optical properties of 2D materials. The built-in strain can be tuned during the
synthesis process of a two dimensional semiconductor, as molybdenum disulfide,
by employing different growth substrate with peculiar thermal properties. In
this work we demonstrate that the built-in strain of MoS2 monolayers, grown on
SiO2/Si substrate using liquid precursors chemical vapor deposition, is mainly
dominated by the size of the monolayer. In fact, we identify a critical size
equal to 20 um, from which the built-in strain increases drastically. The
built-in strain is maximized for 60 um sized monolayer, leading to 1.2% tensile
strain with a partial release of strain close to the monolayer triangular
vertexes due to formation of nanocracks. These findings also imply that the
standard method for evaluation of the number of layers based on the Raman modes
separation becomes unreliable for monolayer with a lateral size above 20 um.
