Effect of Mn2+ on Upconversion Emission, Thermal Sensing and Optical Heater Behavior of Yb3+ - Er3+ Codoped NaGdF4 Nanophosphors

Abstract

In thiswork, we investigate the influence of Mn²⁺ on the emission color, thermal sensing and optical heater behavior of NaGdF₄: Yb/Er nanophosphors, which the nanoparticles were synthesized by a hydrothermal method using oleic acid as both a stabilizing and a chelating agent. The morphology and crystal size of upconversion nano particles (UCNPs) can be effectively controlled through the addition of Mn²⁺ dopant contents in NaGdF₄: Yb/Er system. Moreover, an enhancement in overall UCL spectra of Mn²⁺ doped UCNPs for NaGdF₄ host compared to the UCNPs is observed, which results from a closed back-energy transfer between Er³⁺ and Mn²⁺ ions (⁴S_3/2 (Er³⁺) → ⁴T₁ (Mn²⁺) → ⁴F_9/2 (Er³⁺)). The temperature sensitivity of NaGdF₄:Yb³⁺/Er³⁺ doping with Mn²⁺ based on thermally coupled levels (²H_11/2 and ⁴S_3/2) of Er³⁺ is similar to that particles without Mn²⁺ in the 303-548 K range. And the maximum sensitivity is 0.0043 K^-1 at 523 K for NaGdF₄:Yb³⁺/Er³⁺/Mn²⁺. Interestingly, the NaGdF₄:Yb³⁺/Er³⁺/Mn²⁺ shows preferable optical heating behavior, which is reaching a large value of 50 K. These results indicate that inducing of Mn²⁺ ions in NaGdF₄:Yb³⁺/Er³⁺ nanophosphors has potential in colorful display, temperature sensor.

Keywords: Mn2+; NaGdF4:Yb3+/Er3+; optical heater; temperature sensing; upconversion luminescence.

Figure 1

(A) XRD patterns of NaGdF₄: Yb/Er/x Mn NCs (0 ≤ x ≤ 40); (B) EDX spectrum of the corresponding sample, all the signals are normalized to Gd one, and Cu signals come from copper grid; (C), (D) schematic illustrations of the crystal structures for pure and Mn²⁺ doped NaGdF₄, respectively.

Figure 2

(a, c) Low-resolution and High-magnification TEM images of the as-synthesized NaGdF₄:Yb/Er nanocrystals; (b) The corresponding High-resolution TEM (HRTEM) image of a single nanocrystal; (d) The selected-area electron diffraction (SAED) patterns of the TEM image shown in (f); (e) High-magnification TEM image of the as-synthesized NaGdF₄: Yb/Er/5 Mn nanocrystals; (f) Low-resolution TEM image of the as-synthesized NaGdF₄: Yb/Er/5 Mn nanocrystals; (g) Histograms of particle size distributions for the NaGdF₄: Yb/Er NCs; (h) and (i) show histograms of length and width distributions of the NaGdF₄: Yb/Er/5 Mn NCs, respectively.

Figure 3

(A) Room-temperature UC emission spectra of NaGdF₄: Yb/Er/x Mn (0 mol% ≤ x ≤ 40 mol%) nanocrystals under an excitation irradiance of 980 nm laser; (B) Simplified energy level diagrams of Er³⁺, Yb³⁺, and Mn²⁺ ions and proposed energy transfer mechanism in NaGdF₄.

Figure 4

Pump power dependence of the UC emissions in (A) NaGdF₄: Yb/Er UCNPs; (B) NaGdF₄: Yb/Er/5 Mn UCNPs; (C), (D) Corresponding decay curves of ²H_11/2 → ⁴I_15/2 transition (@522 nm), ⁴S_3/2 → ⁴I_15/2 transition (@541 nm), and ⁴F_9/2 → ⁴I_15/2 transition (@654 nm) of the UCNPs, respectively.

Figure 5

The corresponding integrated UC intensities of green (²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2) and red (⁴F_9/2 → ⁴I_15/2) emissions vs. power. (A) NaGdF₄: Yb/Er UCNPs; (B) NaGdF₄: Yb/Er/5 Mn UCNPs; (C) Red-to-green ratio of NaGdF₄ UCNPs as a function of power.

Figure 6

Temperature dependent (303–548 K) normalized UC emission spectra of the (A) 0 mol% Mn; (E) 5 mol% Mn doped NaGdF₄: Yb/Er samples in the wavelength range of 500–590 nm; (B,F) Monolog plots of FIR as a function of inverse absolute temperature; (C,G) The FIR of NaGdF₄: Yb/Er and NaGdF₄: Yb/Er/5 Mn phosphors as a function of temperature based on ²H_11/2/⁴S_3/2 levels; (D,H) The relative sensitivity (SR) vs. absolute temperature.

Figure 7

Temperature variation of xMn²⁺, Er³⁺, Yb³⁺ codoped NaGdF₄ (x = 0, 5 mol%) powder as a function of pump power density.