Role of Cobalt Doping in CdS Quantum Dots for Potential Application in Thin Film Optoelectronic Devices

Colloidal quantum dots (QDs) are promising materials for optoelectronic devices. In this paper, monodispersed and environment stable cobalt (Co)-doped CdS QDs have been synthesized and characterized for potential application in thin film optoelectronic devices. The Rietveld refinement profiles of X-ray diffraction data reveal that both undoped and Co-doped CdS QDs exhibit a zinc blende structure without any impurity phase. X-ray photoemission spectroscopy has been used for electronic structure and valence state analysis. The detailed information about the doping, coordination number, and local geometry has been studied with XANES and EXAFS measurements. Analysis of Raman spectra reveals that the intensity of longitudinal optical (LO) modes varies considerably due to short-range structural disorder. Absorption spectra also show the creation of a new doping band (DB) near the NIR region in Co-doped CdS QDs which is not observed for doping of many other transition metals. The width of this DB increases with an increase in the doping concentration, and enhancement of photoconductivity of the thin film heterojunction of the samples has been obtained. Evolution of the new DB and enhancement of the photocurrent upon Co doping make the prepared quantum dots very promising materials to exploit for fabricating UV–vis/NIR thin film optoelectronic devices.