  Substrate engineering provides an opportunity to modulate the physical
properties of quantum materials in thin film form. Here we report that TiSe$_2$
thin films grown on TiO$_2$ have unexpectedly large electron doping that
suppresses the charge density wave (CDW) order. This is dramatically different
from either bulk single crystal TiSe$_2$ or TiSe$_2$ thin films on graphene.
The epitaxial TiSe$_2$ thin films can be prepared on TiO$_2$ via molecular beam
epitaxy (MBE) in two ways: by conventional co-deposition using selenium and
titanium sources, and by evaporating only selenium on reconstructed TiO$_2$
surfaces. Both growth methods yield atomically flat thin films with similar
physical properties. The electron doping and subsequent suppression of CDW
order can be explained by selenium vacancies in the TiSe$_2$ film, which
naturally occur when TiO$_2$ substrates are used. This is due to the stronger
interfacial bonding that changes the ideal growth conditions. Our finding
provides a way to tune the chemical potential of chalcogenide thin films via
substrate selection and engineering.
