Green upconversion emission dependence on size and surface residual contaminants in nanocrystalline ZrO₂:Er³⁺

Abstract

The luminescence lifetime of the green upconversion emission Er³⁺ ions in ZrO₂ nanocrystals was found to be sensitive to the particle size and crystalline phase, as well as to the residual surface contaminants. Erbium doped (0.2 mol% Er₂O₃) ZrO₂ nanocrystals ranging from 54 to 120 nm in size were prepared by a sol–gel process with the presence of nonionic PLURONIC P127 surfactant, and the upconversion emission was characterized. PLURONIC P127 at a molar ratio of 0.0082 promoted both an enhancement in the green upconversion emission as well as a strong reduction of surface contaminants such as CO, CH₂, and OH. A fluorescence decay analysis via a simple microscopic rate equation model suggests that crystallite size and nonradiative relaxation mechanisms to different surface contaminants have to be taken into account to explain the observed green luminescence quenching. XRD, FTIR and luminescence lifetime measurements allow the quantification of the nonradiative processes that lead to the green luminescence quenching; and prove the relevance of using nonionic surfactants in the synthesis to reduce residual surface contaminants.