MoSe2/graphene/6H-SiC heterojunctions: energy band diagram and photodegradation

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 MoSe₂/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 MoSe₂ and its dependence on the number of layers were determined by Kelvin probe microscopy. A nonuniform distribution of the photoluminescence intensity from the MoSe₂ monolayer was observed because of bilayer graphene (BLG) inclusions in the monolayer graphene (MLG) covering the 6H-SiC substrate. The measured values of the MoSe₂ work function allow the construction of band energy diagrams for MoSe₂/MLG and MoSe₂/BLG heterobarriers, which show an increased barrier height for MLG with respect to BLG. This relatively high potential barrier at MoSe₂/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 MoSe₂ layers on MLG due to photooxidation during light illumination was observed. The photooxidation was promoted by accumulation of excess holes at the MoSe₂/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 MoSe₂. The dependence of the photoluminescence and photodegradation of MoSe₂ on the graphene layers shows the importance of the substrate selection for good device stability based on van der Waals heterostructures with MoSe₂.