Absorption and photoluminescence study of CdS quantum dots : The role of host matrix and nanocrystal size and density

In this paper we present results of the absorption and photoluminescence (PL) of CdS-doped SiO₂ films fabricated by RF co-sputtering (semiconductor volume fraction f=1-15%, nano-crystallite's mean size 5-7nm) and matrix-free films of close-packed CdS nano-crystallites (f-30%, size 2-5nm) produced by an original chemical method. The absorption spectra have been modelled using the modified Maxwell-Gamett model. This gives the e-h pair state energies and evidence of a strong absorption in the glass matrix containing CdS. The temperature dependence of the spectral position and broadening of the PL peak is analysed. It is concluded that a photo-generated hole is captured on an acceptor-type trap before the radiative recombination with a confined electron. The excitation of this 'band-edge' PL occurs through some states in the matrix and directly in the CdS crystallites for the two kinds of samples, respectively. The temperature coefficients of the optical transition energies for the nearly matrix-free films are similar to those of bulk CdS, while for the CdS/glass films they are smaller. This may be because of the different boundary conditions for the thermal expansion of CdS crystallites. © 2000 Materials Research Society.