Synthesis and Fingerprint of a Fully Coalescent, High-Quality Epitaxial Co ₉ S ₈ –CoS ₂ Thin 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.