Impact of Quantum Confinement on the Optical and Magnetic Properties of Cobalt-Doped CdS Quantum Dots

The cobalt-doped CdS quantum dots that were synthesized using the co-precipitation process is discussed in this article. The x-ray diffraction analysis revealed a reduction in the crystallite size from 5.9 to 2.9 nm upon increasing the cobalt content from 0 to 10%. The energy gap of CdS and Co-doped CdS quantum dots was assessed from Tauc’s plot and it infers that Co-doping widens the bandgap of CdS quantum dots from 3.12 to 3.45 eV. The photoluminescence peak was observed to have a maximum intensity for 10% Co doping. The substitution of Co and the formation of CdS were confirmed by fourier transform infrared spectroscopy through the peaks at 654 cm⁻¹ and 426 cm⁻¹, respectively. The Raman spectra of undoped and Co-doped CdS quantum dots revealed the first longitudinal optical phonon mode at 300.8 cm⁻¹ and the second longitudinal optical phonon mode at 599.3 cm⁻¹ modes, with a decreasing intensity ratio. The presence of nearly spherical particles with a typical size of 7.2–3.9 nm was confirmed by transmission electron microscopy, which is a good attribute for several applications. Room temperature magnetization measurement revealed a drop in saturation magnetization as particle size decreases, indicating that the ferromagnetism is caused by defects. Electron paramagnetic resonance analysis confirmed the ferromagnetic property of the CdS sample. Spin-spin relaxation time and spin-lattice relaxation time were calculated. The current study emphasizes the possibility of 10% Co doping in the CdS matrix in a simple co-precipitation method. The developed nearly spherical quantum dots will be studied for biomedical applications including biosensors and bioimaging.