Influence of Inert Shell Dielectric Effect on Luminescent Properties of Core-shell Upconversion Nanocrystals

Abstract A series of core/shell upconversion nanocrystals (UCNCs) featuring -NaYF 4 Yb 3+ /A 3+ (A=Er Ho Tm) cores and distinct inert shells (NaLuF 4 NaYF 4 NaGdF 4 ) were synthesized through the co-precipitation method combined with a successive layer-by-layer strategy.The influence of shell composition on luminescent properties was investigated and the un derlying mechanism was analyzed through upconversion emission spectrum and luminescent lifetime measurements.Experi mental results indicate that both the upconversion luminescence intensity and the Yb 3+ luminescent lifetime of core/shell UCNCs depends on the composition of inert shell exhibiting a gradual decrease in the order of NaLuF 4 >NaYF 4 >NaGdF 4 .This trend suggests progressively reduced capabilities of three kinds of inert shell in suppressing surface quenching.The influ ences of three kinds of inert shell become more pronounced in aqueous environments with high quenching probabilities.Theo retical calculations reveal that the variation in dielectric constant among the inert shell materials may play a critical role with NaLuF 4 showing the lowest relative dielectric constant.The shell layer with low dielectric constant can mitigate the electric di pole interaction between Yb 3+ ions in the core and surface-adsorbed ligands thereby suppressing non-radiative energy losses.This study reveals the possible regulatory mechanism of the dielectric effect of inert shell on the surface quenching effect pro viding valuable guidance for the compositional design of high-brightness core/shell UCNCs. Key words