Restricted phonon relaxation and anomalous thermalization of rare earth ions in nanocrystals

An anomalous thermalization effect induced by optical excitation of Er3+ ions in nanocrystals of Y2O2S 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 I-4(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 Eu2O3.