Oriented silver chloride microcrystals and nanocrystals embedded in a crystalline KCl matrix, as studied by means of electron paramagnetic resonance and optically detected magnetic resonance

Self-trapped holes (STH), self-trapped excitons (STE) and shallow electron centres in small AgCl crystals embedded in a KCl crystalline matrix have been observed by means of optically detected magnetic resonance (ODMR). The existence of the impurity clusters in heavily doped KCl:AgCl single crystals, ranging from single and paired Ag ions to AgCl nanometre- and micrometresize crystals (nanocrystals and microcrystals) retaining the orientation of the matrix, was confirmed. ODMR spectra were used as a fingerprint of the embedded AgCl microcrystals and a signature of their crystallinity. For AgCl nanocrystals the anisotropy of the g-factor both for isolated STH and for STH forming STE was found to be substantially reduced compared with those of bulk AgCl crystals and AgCl microcrystals embedded in KCl. This implies a considerable suppression of the Jahn-Teller effect in nanoparticles. A rather general mechanism of the suppression of the Jahn-Teller effect in nanocrystals is developed, taking into account the additional deformation field appearing because of the strong vibronic interaction at the interface. It allows evaluation of the critical size of the embedded AgCl nanocrystals, at which the suppression of the Jahn-Teller effect has its onset; the value obtained is approximate to 10 nm, in agreement with experiment.