Epitaxial growth of crystalline CaF$_2$ on silicene

Silicene is one of the most promising 2D materials for the realization of next-generation electronic devices, owing to its high carrier mobility and bandgap tunability through the imposition of an external electric field. To exploit this fundamental characteristic, it is necessary to engineer an insulating layer that can be interfaced directly to silicene without perturbing its bidimensional nature. At the same time, this insulating layer should exhibit low leakage currents even when highly scaled, to fully exploit the advantages of using a 2D material at the core of the device. CaF$_2$ is known to form a quasi van der Waals interface with 2D materials, as well as to maintain its insulating properties even at ultrathin scales. Here we investigate the growth of CaF$_2$ layers on silicene by molecular beam epitaxy: diffraction images show that CaF$_2$ grows epitaxially on silicene/Ag(111), with its domains fully aligned to the 2D silicon lattice. In-situ XPS analysis evidences that no changes in the chemical state of the silicon atoms can be detected upon CaF$_2$ deposition, excluding the formation of covalent bonds between Ca, F and Si. Polarized Raman analysis shows that silicene undergoes a structural change upon interaction with CaF$_2$, however retaining a bidimensional character and without transitioning to a sp3-hybridized, bulk-like silicon.