Fabrication and characterization of up-converting β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ core-shell nanoparticles for temperature...

This paper presents the use of soft template method to synthesize core and core-shell up-converting nanoparticles usefull for temperature sensing applications. Based on the stock solutions of core β-NaYF₄:Er³⁺,Yb³⁺ nanoparticles and involving soft template method without any additional process of surface functionalization, it is possible to directly design the core-shell β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ nanoparticles, which can be perfectly dispersed in cyclohexane and surfactants like oleic acid (OA), triethanolamine (TEA) or Cetyltrimethylammonium bromide (CTAB). The morphological, crystalline and elemental characteristics of samples were investigated by Field Emission Scanning Electron Microscopy, X-Ray Diffraction, High Resolution Transmission Electron Microscopy, Selected Area Electron Diffraction patterns and Energy-Dispersive X-Ray Spectroscopy (EDX) measurements. The results showed that the synthesized NaYF₄:Er³⁺,Yb³⁺@NaYF₄ core-shell nanoparticles have roughly spherical shape, pure hexagonal β phase with core size of about 35 ± 5 nm and shell thickness of about 40 ± 5 nm. It has been shown that the coating of the β-NaYF₄:Er³⁺,Yb³⁺ core with NaYF₄ shell layer enables to enhance the green upconversion (UC) emission intensities in respect to red one. Under 976 nm excitation, the synthesized β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ core-shell nanoparticles revealed three strong emission bands at 520 nm, 545 nm and 650 nm corresponding to ²H_11/2, ⁴S_3/2 and ⁴F_9/2 to ⁴I_15/2 transitions of Er³⁺ ions with the lifetimes of 215, 193 and 474 µs, respectively. The calculated CIE chromaticity coordinates proved that the emission colour of core-shell nanoparticles was changed from red into yellowish green upon increasing the power density of the 976 nm laser from 0.73 to 9.95 W/cm². The calculated slopes indicated that in the β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ core-shell nanoparticles, two-photon and three-photon UC processes took place simultaneously. Although the former one is similar as in the case of β-NaYF₄:Er³⁺,Yb³⁺ bare core nanoparticles, the latter one, three-photon UC process for green emission occurs, due to cross relaxation processes of two Er³⁺ ions only within nanoparticles with core-shell architecture. Moreover, the energy difference between the ²H_11/2 and ⁴S_3/2 levels and associated constant of NaYF₄@NaYF₄ host lattice were determined and they reached ~ 813 cm^-1 and 14.27 (r² = 0.998), respectively. In order to investigate the suitability of nanoparticles for optical temperature sensing, the emission spectra were measured in a wide temperature range from 158 to 298 K. An exceptionally high value of relative sensitivity was obtained at 158 K and it amounted to 4.25% K^-1. Further temperature increase resulted in gradual decrease of relative sensitivity, however, it maintained a high value > 1% K^-1 in the entire analyzed temperature range.