Silver halide micro- and nanocrystals embedded in an alkali halide matrix: suppression of the Jahn-Teller effect in nanoparticles

Self-organized microcrystals and nanocrystals (quantum dots) of AgCl and AgBr embedded in KCl and KBr crystalline matrices and retaining the orientation of the host lattice were studied by optically detected magnetic resonance. It was unambiguously shown that self-organized microcrystalline silver halides can be grown inside alkali halide crystals with the properties of bulk crystals since the optically detected magnetic resonance spectra of the embedded microcrystals were practically the same or close to those in bulk AgCl and AgBr and could be used as a `fingerprint' for AgCl and AgBr. For AgCl nanocrystals in a KCl matrix the anisotropy of the g-factor both for isolated self-trapped holes and for self-trapped holes forming self-trapped excitons was found to be substantially reduced compared with those of bulk AgCl crystals. This implies a considerable suppression of the Jahn-Teller (JT) effect in nanoparticles. A rather general mechanism of the suppression of the JT effect in nanocrystals is proposed, taking into account the additional deformation field appearing because of the strong vibronic interaction at the interface. It was concluded that the distribution of exchange interactions for electron-hole pairs and triplet excitons in the KBr:AgBr system is due to a distribution of AgBr crystal sizes. The holes seem to be self-trapped in the AgBr because of the dynamical JT effect. The exchange splitting increases for distant electron-hole pairs with a decrease of AgBr size. The spectra with exchange splitting larger than that in bulk AgBr (1.9 cm^-1) seem to belong to AgBr nanocrystals. In contrast to AgCl the wavelength of the luminescence in AgBr micro- and nanocrystals embedded in a KBr matrix decreases with the decrease of AgBr crystal size.