Optical identification of grain boundaries of monolayer MoS₂

Monolayer molybdenium disulfide (MoS₂), one of promising 2 dimensional transition metal dichalcogenide materials, is mono-atomic semiconducting layer with visible wavelength emission, being actively studied for novel device application.¹ However the CVD process where grains are separately grown from multiple scattered seeds is inevitably subjected to grain boundaries (GBs) between micro-size MoS₂ grains, which was shown to affect the electrical and optical properties of MoS₂ devices. Optical method of GB visualization of large area MoS₂ would be convenient for the large-area and non-invasive inspection capability and also contribute to the understanding of the physical nature of GB in MoS₂. photoluminescence (PL) variation on GBs were also reported to occur on the GBs of MoS₂ where PL reduction or enhancement were observed.² While the PL variation along the GBs could provide a convenient optical visualization methodology for GB identification, it is still unclear whether the GB defined as the line discontinuity of atomic orientation of monolayer MoS₂, can really cause the PL variation that is spatially extensive enough to be detected by routine micro-PL imaging having a few hundreds nanometer spatial resolution. To answer this question, not only PL imaging also other characterization process in a space-correlated manner that can fully describe the physical conditions that may cause the PL variation on the GBs must be carried out.