Confinement of electron-phonon interaction on luminescence dynamics in nanophosphors of Er3+ :Y2O2S

Abstract Observation of an anomalous thermalization effect induced by optical excitation of Er3+ in nanocrystals of Y2O2S was recently reported (Liu et al. Nano Lett. 2 (2002) 535). Due to the absence of low-energy phonon modes in nanocrystals, the nonradiative relaxation between crystal field levels of Er3+ is significantly diminished, whereas this confinement effect on high-energy phonon relaxation and thermalization is negligible. It is also shown that absorption line broadening for Er3+ ions in the surface layer of the nanoparticles enables coincident excitation of Er3+ ions at defect and intrinsic sites. As a result of the combined excitation and relaxation processes, Er3+ population accumulates in the upper crystal field levels of the 4 I 15/2 ground state; and the intensity of hot bands originating from these levels increases abruptly as temperature decreases below 7 K. This anomalous thermalization effect is interpreted satisfactorily based on calculations of temperature-dependent multiphonon relaxation rates in nanocrystals of confined phonon modes.