Engineering covalently bonded 2D layered materials by self-intercalation.

Two-dimensional (2D) materials offer a unique platform from which to explore the physics of topology and many-body phenomena. New properties can be generated by filling the van der Waals gap of 2D materials with intercalants; however, post-growth intercalation has usually been limited to alkali metals. Here we show that the self-intercalation of native atoms into bilayer transition metal dichalcogenides during growth generates a class of ultrathin, covalently bonded materials, which we name ic-2D. The stoichiometry of these materials is defined by periodic occupancy patterns of the octahedral vacancy sites in the van der Waals gap, and their properties can be tuned by varying the coverage and the spatial arrangement of the filled sites. By performing growth under high metal chemical potential we can access a range of tantalum-intercalated TaS(Se), including 25% Ta-intercalated TaS, 33.3% Ta-intercalated TaS, 50% Ta-intercalated TaS, 66.7% Ta-intercalated TaSe (which forms a Kagome lattice) and 100% Ta-intercalated TaSe. Ferromagnetic order was detected in some of these intercalated phases. We also demonstrate that self-intercalated VS, InSe and FeTe can be grown under metal-rich conditions. Our work establishes self-intercalation as an approach through which to grow a new class of 2D materials with stoichiometry- or composition-dependent properties.