EDTA functionalization of SrF2:Yb,Er nanoparticles by hydrothermal synthesis: Intense red upconversion, NIR-to-NIR emission and luminescence nanothermometry characteristics

Abstract

SrF2 based upconversion nanoparticles (UCNPs) were synthesized by hydrothermal method using ethylenediaminetetraacetic acid (EDTA) as surfactant, by doping with single-lanthanide-activator Er ion and sensitizer-activator Yb3+−Er3+ ion-pair. The excited state absorption (ESA) and energy transfer upconversion (ETU) properties were investigated respectively by synthesizing SrF2:2%Er and SrF2:20%Yb,2%Er nanoparticles. EDTA aided hydrothermal synthesis resulted in nano-rod, nano-plate and nano-sphere morphological particles. The hydrothermally grown SrF2:Yb,Er UCNPs are formed as nano-spheres in size ~ 30 to 60 nm. It gives a high specific surface area of 47.17 to 23.58 m2 g−1. The nano-spherical SrF2:Yb,Er resulted an intense red upconversion emission at 660 nm, under the near-infrared (NIR) 980 nm excitation, due to its high specific surface area and ETU mechanism. The post-calcined SrF2:Yb3+,Er3+ gives better red-to-green emission intensity ratio compared to the as-prepared counterparts. SrF2:Er3+ gives very weak red emission due to ESA mechanism. The optical energy gap of SrF2:Yb,Er UCNP was calculated using the barycentre of green emission at 524 and 542 nm from the thermally coupled energy levels of 2H11/2 and 4S3/2. It gives an optical energy gap of 766 cm−1 and the estimated thermal sensitivity is 1.22% K−1 at 300 K. The result is in agreement with other published reports. We also demonstrated the NIR-to-NIR (980-to-850 nm) upconversion emission in the SrF2:Yb3+, Er3+ nanoparticle.