Quantum-Size Effect and Exciton Percolation in Porous and Disordered Films on the Basis of Spherical “Core/Shell” Elements

Abstract

The results of spectroscopic researches of porous and disordered films fabricated on the basis of spherical elements of the “core/shell” type, SiO2/CdS nanoparticles (SiO2 spheres covered with CdS quantum dots) are reported. The quantum-size effect of excitons in the quantum dots located on the surface of spheres is found to depend on the sphere size and to weakly depend on the quantum dot radius, which results from the quantization of the exciton motion normally to the sphere surface. The quantization survives, even if the sphere coverage exceeds the exciton percolation threshold. To evaluate the latter in the film plane and to determine the critical concentration of SiO2/CdS nanoparticles, a number of specimens on the basis of the mixtures 20–80% SiO2 : 80–20% SiO2/CdS are studied. The exciton percolation threshold is registered in those structures for the first time at a fraction of SiO2/CdS nanoparticles in the mixture of about 60%, which is twice as high as the value predicted by the hard sphere model. A qualitative explanation of this phenomenon is proposed.