Strain engineering is a powerful tool for tuning physical properties of 2D materials, including monolayer transition metal dichalcogenides (TMDs)—direct bandgap semiconductors with strong excitonic response. Deformation of TMD monolayers allows inducing modulation of exciton potential and, ultimately, creating single-photon emitters at desired positions. The performance of such systems is critically dependent on the exciton energy profile and maximum possible exciton energy shift that can be achieved under local impact until the monolayer rupture. Here, we study the evolution of two-dimensional exciton energy profile induced in a MoSe₂ monolayer under incremental local indentation until the rupture. We controllably stress the flake with an atomic force microscope tip and perform in situ spatiospectral mapping of the excitonic photoluminescence in the vicinity of the indentation point. In order to accurately fit the experimental data, we combine numerical simulations with a simple model of strain-induced modification of the local excitonic response and carefully account for the optical resolution of the setup. This allows us to extract deformation, strain, and exciton energy profiles obtained at each indentation depth. The maximum exciton energy shift induced by local deformation achieved at 300 nm indentation reaches the value of 36.5 meV and corresponds to 1.15% strain of the monolayer. Our approach is a powerful tool for in situ characterization of local optomechanical properties of 2D direct bandgap semiconductors with strong excitonic response.

The electronic and optoelectronic properties of van der Waals heterostructures are strongly affected by heterobarrier heights at the interfaces between monolayers forming the structure and the environment. In this work, properties of MoSe ₂ /graphene/6H-SiC heterostructures are studied by scanning probe microscopy and photoluminescence spectroscopy in various ambient conditions. To improve our understanding of the electronic processes, a work function of the monolayer MoSe ₂ and its dependence on the number of layers were determined by Kelvin probe microscopy. A nonuniform distribution of the photoluminescence intensity from the MoSe ₂ monolayer was observed because of bilayer graphene (BLG) inclusions in the monolayer graphene (MLG) covering the 6H-SiC substrate. The measured values of the MoSe ₂ work function allow the construction of band energy diagrams for MoSe ₂ /MLG and MoSe ₂ /BLG heterobarriers, which show an increased barrier height for MLG with respect to BLG. This relatively high potential barrier at MoSe ₂ /MLG leads to quenching of the photoluminescence because of separation of the electron–hole pairs at the barrier before the formation of the exciton. Moreover, selective photodegradation of the MoSe ₂ layers on MLG due to photooxidation during light illumination was observed. The photooxidation was promoted by accumulation of excess holes at the MoSe ₂ /MLG interface, which further participate in splitting of a surface water film. Generation of the oxygen at the heterojunction interface leads to the oxidation of the MoSe ₂ . The dependence of the photoluminescence and photodegradation of MoSe ₂ on the graphene layers shows the importance of the substrate selection for good device stability based on van der Waals heterostructures with MoSe ₂ .