  The discovery of two dimensional Van der Waals materials has opened up
several possibilities for designing novel devices. Yet a more promising way of
designing exotic heterostutures with improved physical properties is to grow a
monolayer of these materials on a substrate. For example, in the field of
superconductivity it has been demonstrated that the superconducting transition
temperature of a monolayer of FeSe grown on some oxide substrates e.g.,
strontium titanate (STO) is by far higher than its bulk counterpart. Although
the system has been considered as a model system for understanding the
phenomenon of high-temperature superconductivity, the physical mechanism
responsible for this high transition temperature is still highly under debate.
Here using momentum and energy resolved high-resolution electron energy-loss
spectroscopy we probe the dynamic charge response of the FeSe/STO(001) system
and demonstrate that the frequency- and momentum-dependent dynamic charge
response is not compatible with a simple film/substrate model. Our analysis
reveals the existence of a depletion region at the interface between this Van
der Waals monolayer and the substrate. The presence of the depletion layer,
accompanied with a considerably large charge transfer from STO into the FeSe
monolayer, leads to a strong renormalization of the STO energy bands and a
substantial band bending at the interface. Our results shed light on the
electronic complexities of the FeSe/oxide interfaces and pave the way of
designing novel low-dimensional high-temperature superconductors through
interface engineering. We anticipate that the observed phenomenon is rather
general and can take place in many two dimensional Van der Waals monolayers
brought in contact with dielectric oxides or semiconducting substrates.
