Synthesis and Fingerprint of a Fully Coalescent, High-Quality Epitaxial Co₉S₈–CoS₂Thin Film

Cobalt sulfide (Co–S) phases are of significant interest for energy conversion and sensing technologies due to their combined catalytic activity and electronic conductivity. In particular, Co₉S₈ is a promising precatalyst for oxygen evolution reaction, but its synthesis as high-quality thin films has been scarcely reported. Here, we demonstrate the synthesis of highly crystalline, coalescent, mixed-phase Co–S thin films using plasma-assisted molecular beam epitaxy (PAMBE). The epitaxial film exhibits substantially improved crystallinity of Co₉S₈ compared to conventional deposition techniques and reveals the formation of epitaxial, tetragonal CoS₂, which has not been previously observed in this nonmagnetic form. Thus, this work provides crucial diagnostic tools supporting the fabrication of phase-pure, single-crystal Co–S thin films and, with the discovery of the nonmagnetic CoS₂ phase, a new platform for investigating Co–S catalysts in energy and sensing applications.