New insights into size effects in luminescent oxide nanocrystals

We here investigate the emission properties of rare-earth-doped oxide nanoparticles with the aim to understand the commonly observed altered properties of nanoparticles as compared to the bulk counterparts. This is usually attributed to the detrimental effect of surface states that quench the excited states involved in the emission process. We study the influence of crystalline defects that are present due to the low temperature of the synthesis of 30 nm sized YVO₄/Eu nanoparticles. Annealing treatments up to 1000 °C in a porous silica matrix allow the recovery of perfectly crystalline particles as colloidal suspensions and compare their properties to those of the pristine particles obtained by conventional colloid chemistry. Emission properties of pristine and annealed particles are compared with those of the bulk material. A simple model of the emission process allows an accurate fit of the luminescence decay and of the dependence of the quantum yield on europium content. Our results show that pristine particles exhibit altered emission properties mainly due to quenching from defects, among which are surface OH groups, and altered energy transfers within the particle. Annealed particles exhibit properties that are almost the same as those of the bulk material, except that the emission yield for the optimum Eu content is limited to 40 instead of 70% for the bulk material. We show that the difference may be simply explained by the difference of the radiative lifetime that results from the lower effective refractive index in the case of the particles. This effect then seems to be the ultimate limitation for the emission properties of perfectly well-crystallized nanoparticles as compared to those of the bulk material. This work provides an example of a general strategy toward the investigation of the physical properties of nanocrystals which may be altered by crystalline defects.