Spectral investigations on undoped and Cu2+ doped ZnO–CdS composite nanopowders

•SEM and TEM micrographs reveal spherical like structures in nanoscale.•Cu2+ entered into host lattice as tetragonally distorted octahedral symmetry.•PL spectra of ZnO–CdS composite nanopowders exhibited a broad green emission.•Luminescence of nanocomposite in visible region is enhanced with CdS sensitization.

Undoped and Cu2+ doped ZnO–CdS composite nanopowders were synthesized by simple chemical precipitation method. Structural and spectroscopic properties of the prepared samples have been characterized by XRD, SEM with EDS, TEM, FT-IR, UV–Vis, EPR and Photoluminescence studies. X-ray diffraction pattern contains a series of peaks corresponds to hexagonal phase of ZnO and CdS. The average crystallite sizes of undoped and Cu2+ doped samples are determined and are in the range of 25–30 nm. SEM and TEM micrographs reveal that the samples show spherical like structures with little agglomeration. FT-IR spectra show the fundamental mode of vibrations of ZnO at 515 cm−1, CdS at 621 cm−1 and other functional groups. Optical absorption spectrum of Cu2+ doped sample consists of three bands at 665, 823 and 1192 nm attributed to the transitions 2B1g → 2Eg, 2B2g and 2A1g respectively. Crystal field and tetragonal field parameters are evaluated as Dq = 1214, Ds = 1610 and Dt = 389 cm−1. From EPR, spin-Hamiltonian and hyperfine splitting parameters are evaluated for Cu2+ doped sample as g‖ = 2.3391, g⊥ = 2.0550 and A‖ = 130 × 10−4 cm−1, A⊥ = 36 × 10−4 cm−1. The optical and EPR data suggests that Cu2+ entered into host lattice as tetragonally distorted octahedral site symmetry. PL spectra consists two emission bands at 367, 380 nm in UV region. A sharp blue emission peak at 425 nm and a broad green emission peak in the range of 450–570 nm are observed. The enhanced visible emission is observed after doping.

Graphical abstractPL spectra show four characteristic emission peaks in both UV and visible regions. UV emission peaks centered at 367, 380 nm are attributed to near-band-edge emission transition arising from energy loss due to strong electron–phonon interactions at room temperature. The blue emission peak at 425 nm can be assigned to surface vacancies like oxygen vacancies and zinc vacancies, i.e. to the recombination of electron–hole pairs at the valance band. The strong green deep level emission (DLE) band in the range of 450–570 nm is assigned to hole–electron recombination at surface traps of CdS. On the other hand, comparison of the PL spectra of both undoped and Cu2+ doped sample indicates that the emission intensity of the PL spectra increases in presence of Cu2+. This is because the electronic transitions in the PL emission spectra occur due to the defect states and surface vacancies.Figure optionsDownload full-size imageDownload as PowerPoint slide