Tuning the optical and magnetic properties of Mn₃O₄ quantum dots by annealing process

Mn₃O₄ quantum dots (QDs) are synthesized at room temperature, and the resulting samples are annealed at the temperature of 523 K and 773 K. The crystallinity, average particle size, and lattice parameters of the prepared manganese oxide QDs are analyzed using X-ray crystal diffraction (XRD) studies. Furthermore, X-ray photoelectron spectroscopy (XPS) is used to investigate the elements present on the surface of nanomaterials, the oxidation state, and the type of functional groups attached to the surface of QDs. The optical and magnetic properties of the produced QDs have also been studied with UV–Visible absorption spectra and vibrating sample magnetometer (VSM), respectively. Single-phase Mn₃O₄ QDs with the most stable tetragonal structure are detected by XRD. Due to grain growth, the optical band gap of Mn₃O₄ QDs has shifted from 4.12 to 3.96 eV, reaching the lowest value (3.96 eV) for the sample annealed at 773 K. All of the prepared samples of Mn₃O₄ QDs exhibit ferromagnetism at room temperature (300 K), with the sample annealed at 773 K having higher saturation magnetization (M_s =366 memu/gm) and remanence (M_r = 73memu/gm) when compared with the same QDs annealed at low temperature.