Strain Release Induced Novel Fluorescence Variation in CVD-Grown Monolayer WS₂ Crystals

Tensile strain is intrinsic to monolayer crystals of transition metal disulfides such as Mo(W)S₂ grown on oxidized silicon substrates by chemical vapor deposition (CVD) owing to the much larger thermal expansion coefficient of Mo(W)S₂ than that of silica. Here we report fascinating fluorescent variation in intensity with aging time in CVD-grown triangular monolayer WS₂ crystals on SiO₂ (300 nm)/Si substrates and formation of interesting concentric triangular fluorescence patterns in monolayer crystals of large size. The novel fluorescence aging behavior is recognized to be induced by the partial release of intrinsic tensile strain after CVD growth and the induced localized variations or gradients of strain in the monolayer crystals. The results demonstrate that strain has a dramatic impact on the fluorescence and photoluminescence of monolayer WS₂ crystals and thus could potentially be utilized to tune electronic and optoelectronic properties of monolayer transition metal disulfides.