Size-dependent structural and electronic properties of stoichiometric II-VI quantum dots and gas sensing ability of CdSe quantum dots: a DFT study

The structural, electronic, and quantum confinement effects observed in II-VI quantum dots have been described using density functional theory. Various properties like binding energy, Fermi energy, charge distribution, and band gap of various clusters have been determined as a function of cluster size in order to find out the most stable of all the clusters considered. The binding energies are found to be a function of the cluster size but converge to a maximum. Cadmium is observed to possess a larger tendency to form clusters with higher coordination numbers compared to zinc and mercury. In mercury sulfide (HgS)(n), the clusters with n = 6 and 13 get dissociated into two graphene-like parallel layers. The adsorptions of single gas molecules on the (CdSe)(13) quantum dots are exothermic, indicating that most of the gas molecules adsorb spontaneously on the CdSe quantum dots. Among the various gases, O-2 and NO2 are the gas molecules that get most strongly chemisorbed. The CdSe quantum dot acts as an electron donor when it interacts with the oxidizing gases, O-2, CO, NO2, and SO2 gases. The vibrational analysis of the combined systems indicates that the intensities of the peaks due to CdSe reduce after adsorption. Extra peaks appear at higher frequencies due to the adsorbed gas molecules. The present work shows insights into the gas sensing properties of the quantum dots under study.