Comprehensive Analysis of Band Gap and Nanotwinning in Cd1-xMgxS QDs

Planar defects, viz. stacking faults, grain boundaries, and twin boundaries, are observed in chemically synthesized Cd1-xMgxS quantum dots (QDs), with x ranging from 0 to 0.05 with an interval of 0.01. The density of nanotwins is augmented at the cost of stacking faults in Cd1-xMgxS QDs for x = 0.03, as revealed from high-resolution transmission electron microscopy (HRTEM). The interchange in defect density reflects an alteration in emission spectra. Astonishingly, we have observed above-bandgap emission (ABE) centered at similar to 430 nm along with the usual below-band-gap emission centered at similar to 526 nm. Emission above the band gap is perhaps exemplified for the first time for II-VI compound semiconductors and explicitly in Cd1-xMgxS QDs. The incorporation of Mg results in the band-gap tunability of CdS QDs as a consequence of synergetic effects of size variation and generation of new electronic states. For insight into the electronic states, density functional theory (DFT) based calculations were carried out. DFT calculations revealed that the substitution of Cd ions by Mg ions in the CdS creates electronic states of Mg just below the electronic state of Cd. Therefore, we assume that the modification in the band gap mainly results as a consequence of quantum confinement. The theoretical results qualitatively agree with the experimental outcomes of the present study.