Infrared study of plasmon-phonon coupling in pure and Nd-doped CeO<inf>2-y</inf> nanocrystals

Abstract

Plasmon-longitudinal-optical (LO) phonon interaction in pure and Nd-doped CeO2-y nanocrystals was investigated by measuring far-infrared reflectivity spectra in the 100-700 cm-1 spectral range at room temperature. Analysis of the obtained results revealed that the presence of free charge carriers becomes significant with the particle size decrease to nanometer range and increase of lattice defects. The free charge carriers were found to be responsible for a plasmon mode which coupled strongly with two LO phonon modes of ceria. The presence of more pronounced low-frequency Drude tail and the screening of the phonon modes in Nd-doped CeO2-y nanocrystals implied that the Plasmon-LO phonon interaction increased with doping. Factorized and additive dielectric function models were applied to deduce about the coupled and decoupled LO phonon frequencies and the structure of the decoupled plasmon mode in pure and Nd-doped CeO2-y nanocrystals. These models were combined with Bruggeman effective medium approximation in order to properly describe the influence of porosity on the infrared reflectivity spectra. With increasing dopant content, the decoupled plasmon mode exhibited redshift and damping decrease implying that doping induced semiconductor-to-metalic state transition took place.