A First-Principles Study of n-Type Defects and Cobalt Doping on Magnetic and Optical Properties of ZnS Nanowires: Implications for Spintronic and Photovoltaic Applications

Diluted magnetic semiconductors (DMSs) are promising candidates for advanced spintronic and photovoltaic applications due to their induced ferromagnetism, spin-dependent interactions, and elevated Curie temperatures. However, the underlying mechanisms of ferromagnetism and the dynamics of optical emission in these materials remain incompletely understood due to their complex microstructural and compositional properties. In this study, we employed density functional theory (DFT) calculations to explore the optoelectronic and magnetic properties of Co-doped ZnS nanowires, with and without structural defects such as Zn interstitial doping (I_Zn) and sulfur vacancy (V_S) or iodine codoping. Our results show that in defect-free ZnS nanowires, Co ions, whether substitutional or interstitial, exhibit antiferromagnetic (AFM) coupling. However, the presence of structural defects or iodine codoping introduces additional electron carriers that interact with the d-states of Co ions, leading to the formation of bound magnetic polarons (BMPs) and, consequently, strong ferromagnetic (FM) coupling between Co ions. Notably, the defective Co-doped ZnS nanowires exhibit a Curie temperature exceeding room temperature, which is crucial for practical device applications. Optical analysis reveals that substitutional Co-doped ZnS has a d–d transition peak at 1.92 eV and a fundamental band-gap peak at 3.56 eV, while interstitial Co doping results in a d–d transition peak at 1.78 eV and a fundamental band-gap peak at 3.47 eV. Interstitial Co doping reduces the band gap from 3.5 to 3.47 eV, whereas substitutional Co doping increases it to 3.56 eV. Structural defects or iodine codoping in the substitutional Co-doped ZnS nanowires introduce optical bands in the infrared, visible, and ultraviolet regions, enhancing optical absorption efficiency. The study indicates that in the FM state, the d–d transition peaks of the Co ions and the fundamental band-gap transition are lower in energy compared to the AFM state. These findings underscore the potential of Co-doped ZnS nanowires with tailored structural modifications for next-generation spintronic devices and high-performance photovoltaic systems, where enhanced magnetic and optical properties are critical for device efficiency and reliability.