First-Principles DFT Insights into the Stabilization of Zinc Diphosphide (ZnP₂) Nanocrystals via Surface Functionalization by 4-Aminothiophenol for Photovoltaic Applications

The resurgence of interest in zinc phosphide compounds as efficient solar absorbers has initiated increasing efforts to improve their stability under humid and oxygen-rich conditions. Although organic functionalization has been suggested as a promising strategy to passivate zinc phosphide nanoparticles, fundamental atomic-level insights into the adsorption processes and structures at zinc diphosphide (ZnP₂) surfaces are still lacking. In this study, the interactions between 4-aminothiophenol and the low-Miller index surfaces of monoclinic ZnP₂ have been investigated by means of density functional theory calculations. A bidentate adsorption mode, in which 4-aminothiophenol binds through both its functional groups via Zn-N and Zn-S bonds, was predicted to be the strongest form of interaction, and monolayer-functionalized ZnP₂ surfaces were found to be highly stable under adsorbate-rich conditions. Changes in the equilibrium morphology of ZnP₂ nanocrystallites upon functionalization and effects of humidity are also discussed. The results are expected to contribute toward the rational design of ZnP₂-based materials for photovoltaic (PV) devices.