Power dependent variation in relative intensities of blue doublets in Tm doped NaYF4 nanocrystalline system

Abstract

Nanocrystalline β-NaYF₄ with an average crystallite size of ∼90 nm and doped with Tm³⁺ and Yb³⁺, were synthesized using the polyol method. Significant variations in intensity ratio of the blue doublets (emission peaks at 476 nm and 449 nm) and their growth rates ($\frac{d{I}_{\lambda }}{d{P}_D}$) as a function of power density confirm that the cross-relaxation process becomes dominant over the three-photon absorption process as power density increases. This power-induced enhancement of the cross-relaxation process is attributed to the increased steady-state population in the intermediate excited states (³H₄ and ³F_2,3 levels), thereby reducing the effective distance between excited Tm³⁺ ions in NaYF₄:Yb,Tm particles. The variation in the fractional intensity of upconversion luminescence at different wavelengths (I_λ/I_tot), combined with photoluminescence (PL) intensities in the NIR region under 808 nm excitation, further confirms existence of back energy transfer (BET) from Tm³⁺ to Yb³⁺ ions. Additionally, the measured lifetimes of the ¹D₂, ¹G₄, and ³H₄ states support the occurrence of the BET process. Temperature-dependent PL studies of β-NaYF₄:Yb,Tm(19:1) particles show variations in the intensity ratio associated with Stark sub-level transitions (¹G_4(1,2) → ³H₆), resulting in absolute (S_a) and relative (Sᵣ) temperature sensitivities of 0.81 % K⁻¹ and 1.3 % K⁻¹ respectively.