DFT/TD-DFT study on the electronic and spectroscopic properties of hollow cubic and hollow spherical (ZnO)m quantum dots interacting with CO, NO2 and SO3 molecules

Hollow spherical (HS) and hollow cubic (HC) (ZnO)m quantum dots (QDs) were constructed and optimized using density functional theory (DFT) method. CO, NO2 and SO3 molecules were used to interact with the HC and HS (ZnO)m QDs at the centre and on the surface of the QDs. The changes in the electronic energy levels of HC and HS (ZnO)m QDs due to the interactions of CO, NO2 and SO3 molecules have been studied. The electronic and spectroscopic properties, such as density of states, HOMO–LUMO energy gap, absorption spectra, IR and Raman spectra of HC and HS (ZnO)m QDs have been studied using DFT and Time dependent-DFT (TD-DFT) methods. The interaction energy values show that the SO3 molecule has strongly interacted with HC and HS (ZnO)m QDs than the CO and NO2 molecules. The results of the density of states show that the HC QDs have peaks that are very close to each other, whereas the same is found to be broad in the HS QDs. The HOMO–LUMO energy gap is more for the HS QDs than the HC QDs, and also it gets decreased, when the NO2 and SO3 molecules interact at the centre of the HC and HS (ZnO)m QDs. The blue and red shifts were observed in the absorption spectra of HS and HC QDs. The natural transition orbital (NTO) plot reveals that the interaction of the molecules on the surface of the QDs reduce the chance of electron–hole recombination; hence the energy gap increases for NO2 and SO3 molecular interactions on the surface of the HC and HS (ZnO)m QDs. The vibrational assignments have been made for HC and HS QDs interacting with CO, NO2 and SO3 molecules.