Passivation of lanthanide surface sites in sub-10 nm NaYF4:Eu3+ nanocrystals

We examined in detail the optical properties of NaYF4:Eu3+ nanocrystals of similar to 9 nm in diameter. For such small nanocrystals roughly 17 % of Y3+ ions occupy surface sites and can be efficiently substituted by optically active Eu3+ ions. In order to determine the influence of surface Eu3+ on the optical properties of the whole nanocrystal, small beta-NaYF4: Eu3+ nanocrystals with homogenous size distribution were prepared using trioctylphosphine oxide as a coordinating solvent. In order to passivate the surface sites, a thin beta-NaYF4 shell was further deposited on nanocrystals core and the optical properties were investigated. For this purpose absorption, photoluminescence, photoluminescence excitation, and photoluminescence decays were recorded and analyzed. The optical characteristics of surface Eu3+ significantly diminish for surface passivated nanocrystals. We calculated the increase of quantum yield to the value of 64 % when NaYF4:Eu3+ core was capped by undoped shell. The optical spectroscopy techniques were shown to be sufficient in determination of surface passivation of nanocrystals with high surface to volume ratio.