  Near-field optical microscopy can be used as a viable route to understand the
nanoscale material properties below the diffraction limit. On the other hand,
atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs)
are the materials of recent interest to study the spatial confinement of charge
carriers, photon, and phonons. Heterostructures based on Mo or W based
monolayer TMDs form type-II band alignment, and hence the optically excited
carriers can be easily separated for applications pertaining to photonics and
electronics. Mapping these spatially confined carriers or photons in a
heterostructure with nanoscale resolution as well as their recombination
behavior at the interfaces are necessary for the effective use of these
materials in future high performance optoelectronics. We performed tip-enhanced
photoluminescence (TEPL) imaging to increase the spatial resolution on
multi-junction monolayer MoSe2-WSe2 lateral heterostructures grown by chemical
vapor deposition (CVD) method. The near-field nano-PL emission map was used to
distinguish the presence of distinct crystalline boundaries and the
heterogeneities across the interfaces. This method significantly improves the
nanoscale resolution of 2D materials, especially for understanding the PL
emission properties at the vicinity of hetero-interfaces.
