Selenides

Selenides are a class of chemical compounds that contain selenium in the anion form Se2− or selenide bonded to metals or nonmetals. They encompass binary metal selenides with the general formula MxSe, and ternary and complex selenides such as MSe2, M2Se3, or layered transition-metal dichalcogenides like MX2 (where M is a transition metal such as Mo or W and X is Se).

Chemistry and structure: Many selenides are ionic or covalent; metal selenides often have semiconducting or insulating properties; transition-metal selenides like MoSe2 and WSe2 crystallize in layered structures with weak van der Waals interlayer bonding, enabling exfoliation to two-dimensional sheets. Some selenides, such as NbSe2, NbSe3, or TaSe3, show metallic conductivity or superconductivity and charge-density waves.

Preparation: synthesized by direct combination of elements at high temperature, by reaction of chalcogen precursors with metal sources, or by solid-state reactions in evacuated tubes; chemical vapor transport and electrodeposition are used for thin films and nanostructures. Natural selenides occur as minerals, but many important compounds are prepared synthetically, e.g., CdSe, ZnSe, and CuInSe2 used in optoelectronics and photovoltaics.

Applications and properties: widely used in optoelectronics (CdSe quantum dots, ZnSe), photovoltaics (Cu(In,Ga)Se2), and as lubricants (molybdenum diselenide MoSe2). They are also studied for electronics, catalysis, and as superconductors (NbSe2). Safety: many selenides pose inhalation or ingestion risks; proper handling required; selenium compounds can be toxic.