Selective synthesis and upconversion luminescence of GdF₃ and NaGdF₄ nano/submicro-crystals doped with Yb³⁺/Ho³⁺

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In this work, GdF₃ and NaGdF₄ nano/submicro-crystals are selectively synthesized via a simple hydrothermal method. By merely adjusting the amounts of sodium citrate added in the precursor solutions, GdF₃ and NaGdF₄ nano/submicro-particles with different upconversion emission colors and luminescent decay lifetimes can be controllably synthesized, which is confirmed by XRD, SEM and spectroscopic analyses. The particle sizes of NaGdF₄ nano/submicro-spheres decrease gradually from 230 to 100 nm with increasing Na₃Cit dosages from 0.4 to 1.0 mmol. Under 980 nm laser excitation, GdF₃:Yb³⁺/Ho³⁺ and NaGdF₄:Yb³⁺/Ho³⁺ nano/submicro-crystals exhibit yellow-green and orange upconversion luminescence with four intense emission bands. Due to surface quenching effects, the emission intensities of NaGdF₄:Yb³⁺/Ho³⁺ nanocrystals decrease with decreasing particle sizes. NaGdF₄:Yb³⁺/Ho³⁺ nanocrystals possess shorter decay times than GdF₃:Yb³⁺/Ho³⁺. The electronic transition processes for upconversion photoluminescence of Yb³⁺/Ho³⁺ are further identified.