Low temperature insulator–semiconductor transition in NiCo2S4/Co9S8 nanocomposites

This study reports on the magnetoresistance, transport, and optical properties of NiCo2S4/Co9S8 nanocomposites synthesized via a microwave-assisted hydrothermal method. The sample shows positive magnetoresistance at 5, 10, and 80 K, with resistivity (ρ) vs temperature plots exhibiting semiconducting behavior under applied magnetic fields of 0, 0.05, 1, 5, and 8 T. The resistivity decreases rapidly with increasing temperature, and the first derivative of resistivity reveals an insulator-to-semiconductor transition near 11.14 K under all applied fields. At low temperatures, the material shows semiconducting behavior, with ρ(T) data fitting well to the weak localization effect and power law of resistivity. In addition to these electrical and magnetic properties, PL spectra exhibit emission peaks at 686 and 795 nm, indicating the presence of multiple defect states such as sulfur vacancies and lattice distortions. The energy bandgap of the sample is extracted from the Tauc plot and is found to be 2.25 eV. X-ray diffraction and Raman spectra confirm the crystalline structure, and transmission electron microscopy shows the formation of nanocomposites.