Correlation between the Covalency and the Thermometric Properties of Yb3+/Er3+ Codoped Nanocrystalline Orthophosphates

Abstract

Lanthanide-doped NaYF₄ nanoparticles are most frequently studied host materials for numerous biomedical applications. Although efficient upconversion can be obtained in fluoride nanomaterials and good homogeneity of size and morphology is achieved, they are not very predestined for extensive material optimization toward enhanced features and functions. Here, we study the impact of rare-earth metals RE = Y, Lu, La, and Gd ions within Yb³⁺/Er³⁺ codoped nanocrystalline REPO₄ orthophosphates. The enhanced luminescent thermometry features were found to be in relation to the covalency of RE³⁺-O^2- bonds being modulated by these optically inactive rare-earth ion substitutes. Up to 30% relative sensitivity enhancement was found (from ca. 3.0 to ca. 3.8%/K at -150 °C) by purposefully increasing the covalence of the RE³⁺-O^2- bond. These studies form the basis for intentional optimization thermal couple-based luminescent thermometers such as Yb³⁺-Er³⁺ upconverting ratiometric thermometer.

Figure 1

Schematic illustration of the impact of the metal-oxygen bond length on thermal response in luminescent thermometers based on thermally coupled levels .

Figure 2

Structure and morphology of REPO₄:Yb³⁺, Er³⁺ (RE; Y, Lu, La, and Gd) materials. The visualization of the unit cells of the YPO₄ and LaPO₄ structures (a); XRD patterns of REPO₄:Yb³⁺, Er³⁺ (RE; Y, Lu, La, and Gd) nanocrystals (b); Raman spectra of REPO₄:Yb³⁺, Er³⁺ (RE; Y, Lu, La, and Gd) (c); representative TEM image of the LuPO₄ (d) and YPO₄ (e) nanocrystals.

Figure 3

Comparison of the luminescent properties of REPO₄:Yb³⁺, Er³⁺ (RE; Y, Lu, La, and Gd) NPs. Schematic energy level diagram and energy transitions in the Yb³⁺ and Er³⁺ ions (a). Emission (b) and excitation (c) spectra as well as luminescence decay profiles monitored at 550 nm (⁴S_3/2 → ⁴I_15/2) (d), 670 nm (⁴F_9/2 → ⁴I_15/2) (e), and 1550 nm (⁴I_13/2 → ⁴I_15/2) (f). Respective average luminescence decay times are presented in the insets.

Figure 4

Narrow-range excitation spectra measured at 10 K for REPO₄: Yb³⁺,Er³⁺ (λ_em = 1550 nm) (a); the influence of the RE³⁺ substitute ion radius on the splitting of the respective ²H_11/2 (b, circles) and ⁴S_3/2 (c, rectangles) multiplets of the Er³⁺ ions; the impact of the RE³⁺–O^2– bond length on the barycenters of ²H_11/2 (circles) and ⁴S_3/2 (rectangles) multiplets (d); and the energy difference between these bands barycenters, ΔE (e) for the REPO₄: Yb³⁺,Er³⁺ nanocrystals.

Figure 5

Representative thermal evolution of the upconverted emission spectra of the YPO₄:Yb³⁺,Er³⁺ nanocrystals at λ_exc = 976 nm excitation (a); thermal dependence of LIR (b) and relative sensitivity (c) for different REPO₄: Yb³⁺,Er³⁺ nanocrystals; the RE³⁺–O^2– distance (host variable)-dependent S_R at −150 °C (d), 300 °C (e), and 500 °C (f). Color coding of the samples is unified for the (b-f) graphs.