Hexagram bi-layer MoS2 flake: The impact of polycrystallinity and strains on the exciton and trion photoluminescence

Atomically-thin MoS2 is a favorable material for flexible device and nano-optoelectronics application due to unique optical and photoelectrical properties related to the exciton and trion formation. These properties crucially depend on the flake structure, strains and surrounding environment. MoS2 flakes of hexagram (six-pointed star) shape grown by CVD on SiO2/Si are known to be polycrystalline, consisting of six rhomb-like single-crystals with grain boundaries and having uneven residual tensile strains. So, it is an attractive object for studying the influence of polycrystallinity and strains on the exciton and trion emission behavior. In this paper, the micro-photoluminescence (µ-PL) of a bi-layer MoS2 flake of hexagram shape is studied in comparison with a triangular flake of the same thickness and a similar size. The morphology, structural and optical properties are inspected, focusing on the impact of the flake polycrystallinity and residual tensile strains on the exciton and trion PL. It is shown that the hexagram flake possesses special spectral features: an increased exciton PL intensity and the band peak energy shifted towards higher energy at the boundaries between the constituent single-crystals. In contrast, the triangular flake demonstrates rather even spatial distribution of PL. The spectral differences between both the flakes and their parts are explained in terms of tensile strains appearing in MoS2 on SiO2/Si after the post-growth cooling. The tensile strains are shown to redshift all the spectral bands and reduce the A exciton contribution to the spectrum, thus increasing the spectral weight of the B exciton and trion bands.