Restricted Phonon Relaxation and Anomalous Thermalization of Rare Earth Ions in Nanocrystals

Abstract

An anomalous thermalization effect induced by optical excitation of Er 3+ ions in nanocrystals of Y 2 O 2 S is investigated. Due to the absence of low-energy phonon modes in the 20−40 nm crystals we studied, phonon relaxation is restricted, and as a result, the intensity of hot bands that originate from the upper crystal field levels in the 4 I 15/2 ground state increases rapidly as temperature decreases below 8 K. This unusual increase in hot band intensity is interpreted satisfactorily by calculations of temperature dependent multiphonon relaxation rates in nanoparticles. Our theoretical analysis provides a fundamental understanding of confinement effects on the spectroscopic properties of rare earth ions in nanocrystals. This analysis applies also to the previously observed unusual hot bands in 4−6 nm particles of Eu 2 O 3 .