Effect of Cu concentration on the photoconductivity properties of ZnS nanoparticles synthesized by co-precipitation method

In the present work, solution based co-precipitation method has been used to prepare Cu-doped ZnS nanoparticles and characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), UV-vis absorption spectroscopy and photoconductivity (PC). XRD analysis shows the cubical zinc blende structure while for higher doping concentration extra peaks of CuS have been observed. The estimated crystallite sizes of Cu-doped ZnS lies in the range of 2-4 nm. SEM micrographs show spherical shape of nanoparticles. UV-vis absorption study shows blue shift of absorption shoulder for un-doped and Cu-doped ZnS NPs as compared to the bulk counterpart. In photoconductivity study, the enhancement of photosensitivity has been observed in 4% Cu-doped sample while the maximum current is found in 1% Cu-doped sample. The substitution of Cu is found to affect the photoconductivity properties owing to the CuS phase. The variation of darkcurrent and photocurrent with applied voltages exhibits super-linear nature (r>1). Negative photoconductivity (NPC) is observed in all the samples in which the photocurrent decreases during steady illumination.