Comparison of experimental and first-principle results of band-gap narrowing of MgO nanostructures and their dependence on crystal structural parameters

From experimental investigations of the bandgaps of magnesium oxide (MgO) nanostructures, the results show that band-gap narrowing occurred as the physical dimension of the MgO crystallites decrease. This is in contrast to other metal oxides such as ZnO. To obtain insights on this observed phenomenon, the first-principle studies using density functional theory were carried out. The strategy used here is different from the normal theoretical studies, such that information of the structural characterization obtained from experimental X-ray diffraction (XRD) data via the Rietveld method was used in the calculations. This is important, because nanostructures do not possess the same crystal parameters as the bulk and accurate real structural parameters should be used in the calculations. Based on these values, the crystal structures were simulated and the electronic band structures were calculated within the density functional theory (DFT). Results from the density of state (DOS) studies shows that the band-gap narrowing is due to the shifting of the valence and conduction bands. From our theoretical results, we can conclude that the narrowing of the bandgaps of MgO nanostructures is a consequence of the increase of their lattice parameters. The calculated results exhibit this trend and are in good agreement with the experimental results.