Introducing Excess Ni into the Van der Waals Gap of NiTe ₂ by Topotactic Reaction

NiTe2 is a layered van der Waals transition metal dichalcogenide (TMD). We demonstrate that Ni-enriched compositional phases can be achieved in ultrathin NiTe2 films by increasing the Ni content while maintaining the layered TMD backbone structure. The excess Ni atoms are being inserted between the NiTe2 layers in ordered superstructures. An increase in the Ni content of thin films has been accomplished either by thermal desorption of tellurium in a vacuum at temperatures above 280°C or by postgrowth topotactical reaction of vapor-deposited Ni with the NiTe2 film. In both cases, the same (√3 × √3) R30° superstructures are observed in low-energy electron diffraction, while X-ray photoemission spectroscopy indicates a maximum Ni concentration consistent with a Ni5Te6 compound. Density functional theory and scanning tunneling microscopy suggest that the excess Ni atoms are inserted in between TMD layers with octahedral coordination and forming two phases with either 1/3 or 2/3 of the octahedral sites occupied. Insertion of excess transition metals between TMD layers by topotactical reactions opens opportunities to modify NiTe2 not just by excess Ni incorporation but also by other suitable transition metals and thus provides a potential avenue for materials engineering of layered compounds.