Environmentally Friendly Pathway to Kesterite Nanoparticles with Controlled Sn Content: An In-Depth Study of Magnetic and Optical Properties

Copper-poor Cu2ZnSnS4 (copper zinc tin sulfide, CZTS) inorganic semiconducting nanoparticles were synthesized by an environmentally friendly low-temperature (100 °C) synthetic path, which allows precise control of the Sn content without any relevant presence of Zn- and Sn-related secondary phases. The resulting nanoparticles are polycrystalline and quasi-spherical, with an average diameter of 10 nm. The shape and composition were assessed using a multitechnique approach based on X-ray photoelectron spectroscopy (XPS), energy-dispersive fluorescence X-ray spectroscopy (EDXRF), inductively coupled plasma atomic emission spectrometry (ICP-AES), and high-resolution transmission electron microscopy (HR-TEM). The presence of paramagnetic species associated with Cu2+ cations was highlighted by electron paramagnetic resonance (EPR) spectroscopy, pinpointing the presence of significant exchange interactions between Cu2+ ions. The mixed oxidation state of Cu induces the generation of free holes, which are confined in the nanoparticles, giving rise to a plasmonic resonance. The plasmonic properties were investigated as a function of Sn doping through vis–NIR absorption spectroscopy combined with magnetic circular dichroism (MCD). This approach enabled the extraction of charge carriers’ density and mass, a key step for further optimization of CZTS-based photovoltaic devices.