Percolation Threshold and Luminescence in Films of Binary Mixtures of Spherical Particles Covered with Quantum Dots
The results of experimental studies of films fabricated on the basis of binary mixtures consisting of bare submicron silica particles (SPs) and silica particles covered with CdS quantum dots (NPs) are reported. The performed analysis concerns various coverage degrees and various SP-to-NP concentration ratios. By analyzing the sedimentation time and the absorption and luminescence spectra of the NP and SP aqueous suspensions and their mixtures, two exciton percolation thresholds are revealed: quasi-two-dimensional (2D) and bulk (3D) ones. The former arises in an ensemble of CdS quantum dots on the SiO2 surface at a critical coverage value, when the wave function of excitons spans over the whole surface of NPs. The latter occurs in the binary-film plane at the critical concentration of NPs in the binary mixture. The phase transition is found to take place only if the system is above the both thresholds, which is confirmed by the optical spectra of the specimens.
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