Emission and percolation of excitons in dense sensembles of quantum dots on the spherical surface

We study core/shell heterostructures in which a core (SiO₂) is overcoated with a shell of ZnO quantum dots, randomly distributed on the sphere surface with the surface filling factor P∼0.45. Due to the high surface energy of SiO₂ spheres, ZnO quantum dots have the shape of disks in which, in spite of the large radii, quantum size effects of excitons are retained. The height of ZnO disks estimated by the effective mass approximation is comparable with the exciton diameter in bulk ZnO. Analysis of optical spectra has shown that, at the given density of ZnO quantum dots, the exciton system is above the percolation threshold. The quantitative parameters characterizing such phenomenon are obtained using the elements of percolation theory and the topology of the samples.