Optical excitations in stoichiometric uncapped ZnS nanostructures

Abstract

We calculate the optical absorption spectra of low-energy uncapped zinc sulfide nanostructures found by global optimisation (basin-hopping/simulated annealing) using time-dependent density functional theory (TD-DFT) and compare the results with experimental spectra. We predict that for all nanostructures studied the lowest excited state found by TD-DFT corresponds to an exciton with an exciton binding energy that is much larger than that of excitons in bulk zinc sulfide. We further show that for the more symmetrical nanostructures some of the excitons are dark and that the absorption on-sets, the energy of the lowest exciton, for the different nanostructures show no clear evidence of quantum confinement. We propose that this apparent lack of quantum confinement finds its origin in the fact that the lowest exciton is not evenly spread over the whole nanostructure but shows large contributions for specific groups of atoms. Finally, we show that the predicted optical absorption spectra fit with those reported experimentally.

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“…Indeed, computational global optimisation studies show that in contrast to materials that in the bulk crystallise with the zincblende or wurtzite structure, e.g. zinc or cadmium sulfide, [15][16][17][18][19] the lowest energy nanoparticles of materials that crystallise with the rocksalt structure, such as MgO, [20][21][22] are generally cuts from the that crystal structure. The fact that magnesium and oxygen are relatively light elements also means that relativistic effects including spin-orbit coupling will be small in MgO nanoparticles in contrast to other rocksalt nanoparticles, such as those made from PbS or PbSe.…”