Tensile strain creates trion: Excitonic photoluminescence distribution over bilayer MoS₂ grown by CVD

Raman and photoluminescence (PL) hyperspectral mapping of a bilayer MoS2 flake grown on SiO2/Si substrate by chemical vapor deposition is studied focusing on the surface variation of the exciton and trion PL charac-teristics. The Raman modes and the excitonic PL in the flake interior are redshifted compared to those at the perimeter. This is attributed to tensile strain present after the post-growth cooling so that the flake perimeter is less strained. At the perimeter, the PL blueshift is also enhanced due to a reduction of the trion (a negatively charged exciton) peak, being attributed to an outflow of electrons (p-doping) induced by a weakening of tensile strain towards the flake edge. This occurs because tensile strain bends down the conduction band of MoS2, causing a drift of electrons towards the strained interior region and, in turn, creating there an increased electron density which fosters the creation of trion.