Tetrahedral Transition Metal Chalcogenides as Functional Inorganic Materials

We provide a perspective on a series of materials that we have termed tetrahedral transition metal chalcogenides (TTMCs), which have a common layered structural motif that could carry novel functionalities on account of the d-orbital filling. While strong covalent bonding predominates within the TTMC layers, the layers themselves can be held together by van der Waals interactions, Coulombic forces, or even hydrogen bonding. Although similar to transition metal dichalcogenides (TMDs) in some respects, TTMCs have been less explored in their synthesis and materials properties. Unlike TMDs where the transition metal is typically tetravalent and in a 6-coordinate environment, TTMCs contain the transition metal in a tetrahedral environment and in a low valent state of I or II. Structurally, TTMCs crystallize in tetragonal or orthorhombic structures on account of the square lattice formed by the transition metal centers. We present their electronic structure and resulting properties, including superconductivity, metallic conductivity, and itinerant ferromagnetism. We briefly discuss their synthesis and the intercalation chemistry that can be performed to form new phases. Like TMDs, they also offer the tantalizing opportunity to be manipulated toward the formation of two-dimensional (2D) structures. Finally, we provide a future outlook on their development and the possibility that they could be integrated with other 2D materials to form novel heterostructures.