Influence of Co-doping on the optical and magnetic properties of CdS nanoparticles

Undoped and Cobalt (Co) doped Cadmium sulfide (CdS) nanoparticles with Co in proportion of 10, 15 and 20 mol% are synthesized by chemical method at room temperature. UV-Vis-NIR absorption spectra shows blue shift in the absorption edge as compared to bulk material. The five characteristic absorption peaks at 475, 523 (513), 685, 729 and 741 nm are observable for Co doped CdS samples due to d-d transitions of Co2+ (d(7)) in tetrahedral symmetry and these peaks are assigned as O2--Co2+ charge transfer process, (4)A(2) (F)-E-2 (G), (4)A(2) (F)-3/2 U', (4)A(2) (F)-EaEuro(3) and (4)A(2) (F)-5/2 U' transitions respectively. The band gap values of undoped CdS nanoparticles is found to be 3.23 eV and the decrement in band gap from 3.23 to 3.18 eV due to Co concentration may be due to direct energy transfer between the semiconductor excited state and the 3d levels of the Co2+ ions. Interaction between polyvinylpyrrolidone (PVP) with CdS nanoparticles as well as conformation of the Cd-S and Co-S stretching band in Co doped CdS nanoparticles is studied by Fourier transform infrared spectroscopy. The photoluminescence (PL) spectra recorded at room temperature with 283 nm excitation wavelength show small shift in emission peak with Co doping which attribute to indirect recombination of the free electrons moving from the trap levels formed by the Co-atoms to the holes in the valence band. The luminescence intensity increases with Co2+ concentration because composite luminescence centre of Co2+ ions are formed, as a result more electrons are easily excited and enhance radiative recombination of luminescence process. The diamagnetic and paramagnetic behavior of undoped and Co doped CdS particles is confirmed by M-H curves and Gouy method. No branching is observed in both field cooled and zero field cooled curves of M-T loop, thereby removing the probability of superparamagnetism behavior of the material.