Point defects in [Ca$_2$Nb$_3$O$_{10}$]$^−$ monolayers and intralayer excitons in [Ca$_2$Nb$_3$O$_{10}$]$^−$ /WSe$_2$ heterostructures

We report the fabrication and atomic-scale characterization of a novel van der Waals heterostructure composed of monolayer WSe$_2$ and monolayer [Ca$_2$Nb$_3$O$_{10}$]$^-$ (CNO). Using 60kV Cc/Cs-corrected high-resolution transmission electron microscopy, we obtain atomic-resolution images of freestanding CNO monolayers, revealing their crystalline structure and beam-induced defect formation. Structure modeling, image simulations, and first-principles calculations point at the preferential oxygen and calcium vacancy formation under electron irradiation. Spatially resolved monochromated STEM-EELS analysis reveals a small but reproducible redshift (∼20 meV) of the D exciton of WSe$_2$ when covered by CNO, consistent with a weak interfacial modification
of the local dielectric/electronic environment. Additional first-principles calculations indicate weak interfacial coupling and show that the electronic structure is highly sensitive to the charge-compensating environment of the CNO layer. Our findings demonstrate that functional oxide nanosheets can be integrated with transition metal dichalcogenide (TMD) monolayers to form oxide/TMD van der Waals heterostructures with locally modified excitonic properties, offering new opportunities for engineered 2D material systems.