Structural and optical studies of strong red emitting NaxScF3+x:Yb3+,Er3+ upconversion nanoparticles

In this work, NaxScF3+x:Yb,Er upconversion nanoparticles (UCNPs) were synthesized by co-precipitation method. The pH dependent effect on phase formation of the UCNPs and its upconversion emission is investigated. XRD analysis confirmed NaScF4:Yb,Er nanoparticles are crystallized in mixed phases of monoclinic and hexagonal structure. The nanoparticles are formed in 80-120 nm size with spherical, rectangular and thin flake morphologies that depend upon neutral and alkali precipitation conditions. Under the 980nm diode laser excitation, the hexagonal NaScF4:Yb,Er nanoflake shaped nanoparticles yielded intense red upconversion emission. A four-fold improvement in red upconversion is obtained in lower symmetric hexagonal NaScF4:Yb,Er than high symmetric cubic NaScF4:Yb,Er nanoparticles. The intense red emitting NaScF4:Yb,Er is preferable than green emitting NaYF4:Yb,Er UCNPs for bioimaging applications.