The sensitive dependence of monolayer materials on their environment often gives rise to unexpected properties. It was recently demonstrated that monolayer FeSe on a ${\mathrm{SrTiO}}_3$ substrate exhibits a much higher superconducting critical temperature $T_c$ than the bulk material. Here, we examine the interfacial structure of FeSe/${\mathrm{SrTiO}}_3$ and the effect of an interfacial ${\mathrm{Ti}}_{1+x}{\mathrm{O}}_2$ layer on the increased $T_c$ using a combination of scanning transmission electron microscopy and density functional theory. We find ${\mathrm{Ti}}_{1+x}{\mathrm{O}}_2$ forms its own quasi-two-dimensional layer, bonding to both the substrate and the FeSe film by van der Waals interactions. The excess Ti in this layer can reconstruct the FeSe Fermi surface in a manner consistent with experimental observations. Moreover, the interfacial layer introduces symmetry-breaking distortions in the FeSe film that may favor a $T_c$ increase. These results suggest that this common substrate may be functionalized to modify the electronic structure of a variety of thin films and monolayers.

• Received 2 May 2018
• Revised 27 May 2019

DOI:https://doi.org/10.1103/PhysRevB.100.144103