Owing to its reduced dimensionality, monolayer NbSe2 hosts various strongly correlated phenomena, including superconductivity. However, exposure to air rapidly degrades its superconducting properties. Moreover, NbSe2 flakes obtained by existing growth methods are often too small or insufficiently uniform or have poor crystal quality for scalable applications. Zheng and colleagues overcame these limitations by developing an encapsulation-epitaxy strategy, in which a two-dimensional layer such as graphene or hexagonal boron nitride acts both as a growth template and as a protective cap.
Specifically, they transferred the encapsulation layer onto a SiO2/Si substrate. They then placed the substrate and the niobium oxide film face to face at the centre of a furnace, with selenium powder and NaCl positioned upstream. As a result of the strong interaction between the adatom and SiO2, as well as the low diffusion barrier on the surface of the encapsulation layer, gaseous precursors diffused into the van der Waals gap between the encapsulation layer and the substrate, nucleated and grew into a continuous monolayer NbSe2 film.