The Effect of Particle Size on the Optical and Electronic Properties of MgO Nanoparticles

evGW-BSE is used to predict the (band) edge states, fundamental gap, optical gap, exciton binding energy and UV-Vis absorption spectra for a series of cuboidal MgO rocksalt nanoparticles, the largest of with has 216 atoms and edges of 1 nm. The evolution of the electronic and optical properties with particle size was studied, where it was found that while the edge states and fundamental gap change with particle size, the optical gap remains essentially fixed for cuboid nanoparticles containing 48 atoms or more. The explanation for that observation is that while the optical gap is associated with an exciton that is well localised on the magnesium corner atoms and the oxygen atoms directly surrounding it, the edge states, while primarily localised on the magnesium corner atoms (electron) and oxygen corner atoms (hole), show significant delocalisation along the edges away from the corner atoms. The BSE/evGW optical gap for the smallest particles, the (MgO)₄ cube, matches with that obtained independently from coupled cluster theory, while for the (MgO)₃₂, a cube with edges of 0.6 nm, the BSE/evGW predicted excitation spectrum agrees well with the experimentally reported reflection spectra of MgO nanoparticles.