Nano-optical imaging of monolayer MoSe2-WSe2 lateral heterostructure with subwavelength domains

Abstract

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are the materials of recent interest to study the spatial confinement of charge carriers, photons, and phonons. Heterostructures based on TMD monolayers, especially composed of Mo and W, form type-II band alignment, and hence, the optically excited carriers can be easily separated for applications pertaining to optoelectronics. Mapping the spatially confined carriers or photons in lateral heterostructures with nanoscale resolution as well as their recombination behavior at the heterointerfaces is necessary for the effective use of 2D materials in optoelectronic devices. Near-field (NF) optical microscopy has been used as a viable route to understand the nanoscale material properties below the diffraction limit. The authors performed tip-enhanced photoluminescence (TEPL) imaging with a spatial resolution of 40 nm of multijunction monolayer MoSe2-WSe2 lateral heterostructures with subwavelength domains grown by chemical vapor deposition. Monolayer MoSe2 and WSe2 domains were identified by atomic force microscopy (AFM) through the topography and phase mapping. Far-field (FF) and NF techniques were used for the optical imaging of the WSe2 ↔ MoSe2 multijunction heterostructure correlated with AFM phase imaging. Near-field TEPL imaging was able to successfully distinguish the presence of distinct crystalline boundaries across the WSe2 ↔ MoSe2 interfaces in 2D lateral heterostructures with a higher spatial resolution, as compared to the far-field imaging, which failed to resolve the interfaces on one of the crystal sides due to the asymmetric FF excitation.