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

Abstract

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.

Introduction

Temperature is an important physical parameter which governs many fields of science, such as medicine, biology, microelectronics, physics, and chemistry^1. Therefore, many methods of temperature measurements, both contact and non-contact, have been developed based on the mechanical, electrical or optical characteristics^2,3,4. However, to determine the temperature of small, moving or inaccessible objects, the non-contact luminescent thermometry which exploits thermal changes of spectral or temporal properties of the phosphor attached to the object is a preferred technique^4,5. One of the most extensively investigated group of lanthanide-based luminescent thermometers are those which for temperature sensing, take advantage of thermally coupled energy levels, like in the case of Er³⁺, Tm³⁺, Ho³⁺, Pr³⁺ or Eu^3+6,7,8,9,10. In recent years, the upconversion luminescence of rare-earth ions (RE³⁺) doped fluoride nanomaterials has received much attention due to their potential applications in various fields such as bioimaging, biolabeling, photodynamic therapy, as drug carriers, optical temperature sensors, etc.^{11,12,13,14,15,16,17,18}. In the case of the optical temperature sensing applications, researchers focused especially on the sodium yttrium fluoride nanoparticles doped or co-doped with rare earth ions (NaYF₄:RE³⁺ nanoparticles)^{19,20,21,22,23,24}, among which the NaYF₄:Er³⁺, Yb³⁺ nanoparticles were regarded as one of the most efficient upconversion materials for temperature sensing due to their unique luminescence properties and the large energy gap between two thermally coupled ²H_11/2 and ⁴S_3/2 energy levels^25,26,27. One of the most important parameter of temperature sensor is its relative sensitivity to temperature changes which verify its quality as well as range of their applications. The improvement of the thermometer sensitivity can be influenced by many factors, e.g. the effect of the phase of β-NaYF₄:Er³⁺,Yb³⁺ and α-NaYF₄:Er³⁺,Yb³⁺ nanoparticles on the relative sensitivity has been compared. Under the same experimental conditions, i.e. under 980 nm excitation and in a wide temperature range from 328 to 550 K, the β-NaYF₄:Er³⁺,Yb³⁺ nanoparticles revealed significantly higher sensitivities of 0.42% K⁻¹ and 0.466% K⁻¹ at 328 K and 550 K²⁶, respectively than α-phase counterpart²⁷. This confirms the importance of conscious synthesis process for the intentional obtaining of highly temperature sensitive nanomaterials and explains the reason for using beta phase nanomaterials in our research.

In our previous research the upconversion (UC) emission properties of uncoated β-NaYF₄:Er³⁺,Yb³⁺ nanoparticles under 976 nm excitation have been investigated. In that case, emission spectra consisted mainly of red emission band at 650 nm, while the green part of spectrum covering the bands at 520 nm and 545 nm was very weak²⁸. In order to enhance the green emission intensities used for temperature sensing applications, in this study a β-NaYF₄:Er³⁺,Yb³⁺ nanoparticles coated with NaYF₄ shell layer are presented. These core–shell nanoparticles were synthesized through the slightly modified soft template method reported previously²⁸. The detailed comparison of UC emission properties under 976 nm excitation with power density changed from 0.73 to 9.95 W/cm², the lifetimes and CIE chromaticity coordinates as well as emission mechanisms of uncoated β-NaYF₄:Er³⁺,Yb³⁺ nanoparticles and NaYF₄:Er³⁺, Yb³⁺@NaYF₄ core–shell nanoparticles was also made. Moreover, the luminescence intensity ratio (LIR) of thermally coupled ²H_11/2 and ⁴S_3/2 excited states as a function of temperature ranging from 158 to 320 K was investigated. Based on the values of LIR, both the absolute and relative thermal sensitivities were determined. The maximal value of relative sensitivity reached 4.25% K⁻¹ (at 158 K), confirming great suitability of these NaYF₄:Er³⁺, Yb³⁺@NaYF₄ core–shell nanoparticles for optical thermometry.

Experimental

Materials

The rare earth acetate hydrate salt precursors (RE(CH₃COO)₃.xH₂O, 99.9%) including yttrium (III) acetate hydrate (Y(CH₃COO)₃.xH₂O, 99.9%), ytterbium(III) acetate hydrate (Yb(CH₃COO)₃.xH₂O, 99.9%), erbium(III) acetate hydrate (Er(CH₃COO)₃.xH₂O, 99.9%) were purchased from Sigma-Aldrich. The oleic acid (OA, CH₃(CH₂)₇CH = CH(CH₂)₇COOH, 99%), ammonium fluoride (NH₄F, 98%), sodium acetate (CH₃COONa, 99%), isopropanol (IPA, (CH₃)₂CHOH, 99%), cyclohexane (C₆H₁₂, 99%) and methanol (CH₃OH, 99%) were purchased from Merck.

Synthesis methods

The β-NaYF₄ co-doped with 2% mol of Er³⁺ and 19% mol of Yb³⁺ core nanoparticles (β-NaYF₄:Er³⁺,Yb³⁺ CNP) and β-NaYF₄@NaYF₄ co-doped 2% mol of Er³⁺ and 19% mol of Yb³⁺ core–shell nanoparticles (β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP) were prepared by soft-template method as shown in Fig. 1.

Figure 1

Schematic diagram of preparation of β-NaYF₄:Er³⁺,Yb³⁺ CNP and β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP using soft-template method.

Synthesis of β-NaYF₄:Er³⁺,Yb³⁺ CNP

The β-NaYF₄:Er³⁺,Yb³⁺ CNP were prepared via the slightly modified soft template method reported previously²⁸. Yttrium acetate hydrate (0.79 mmol), erbium acetate hydrate (0.02 mmol) and ytterbium acetate tetrahydrate (0.19 mmol) were dispersed in 50 ml solution containing 10 ml IPA and 15 ml OA by ultrasonic cleaner for 30 min. To form a homogeneous solution, the mixture were stirred at 60 °C for 60 min under argon atmosphere. Then the mixed solution containing 1.1 mmol sodium acetate, 4.0 mmol ammonium fluoride and 10 ml of methanol was slowly added to the solution under magnetic stirring for another 30 min. Finally, the reaction mixture was heated up to 160 °C and stirred for 2 h under argon atmosphere, then cooled down to room temperature in the ice water bath to obtain the stock solution of β-NaYF₄:Er³⁺,Yb³⁺ CNP.

Preparation of the shell precursor solution of NaYF₄

The shell precursor solution of NaYF₄ was prepared analogously to the β-NaYF₄:Er³⁺,Yb³⁺ CNP as described above however using 1.0 mmol yttrium acetate hydrate, without addition of erbium and ytterbium acetates.

Synthesis of β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP

Dispersion of 30 ml stock solution of prepared β-NaYF₄:Er³⁺,Yb³⁺ CNP (described above) in mixed solution of 10 ml oleic acid and 15 ml methanol was carried out using ultrasonic bath for 30 min. Then, the homogeneous mixture of β-NaYF₄:Er³⁺,Yb³⁺ CNP with oleate ligands capping the surface was achieved via stirring at 60 °C for 30 min under argon atmosphere. Finally, during another 30 min lasting stirring at 60 °C under argon atmosphere the 30 ml of the shell precursor solution of NaYF₄ was slowly added to the mixture and then heated up to 160 °C for 4 h, then cooled down to room temperature in the ice water bath to obtain the solution of β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP.

The precipitated β-NaYF₄:Er³⁺,Yb³⁺ CNP and β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP were centrifuged and washed three times with methanol, then dried at 60 °C and dispersed in cyclohexane.

Characterization

The morphology of synthesized β-NaYF₄:Er³⁺,Yb³⁺ CNP and β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP were carried out by Field Emission Scanning Electron Microscope (FESEM, Hitachi S-4800). Transmission Electron Microscopy images were recorded using Tecnai 20 D2094 X-Twin Microscope operating at 200 kV, in which, the samples were prepared by dispersion in ethanol with an ultrasonic bath for 20 min, then put on the copper grids. Selected Area Electron Diffraction (SAED) patterns and Energy Dispersive X-ray Spectroscopy (EDX) elementals mapping were carried out using Tecnai 20 D2094 X-Twin Microscope operating at 200 kV. Structural analysis of samples was determined by X-Ray Diffraction (PANalytical X’Pert Pro Powder Diffractometer) using Cu K_α radiation (λ = 1.54060 Å) in the 2θ range from 10° to 80°.

The Fourier Transform Infrared Spectroscopy (FTIR) analysis was recorded using a Thermo Nicolet NEXUS 670 FTIR (USA) in a KBr suspension in the region of 4000 to 400 cm⁻¹.

The upconversion emission spectra were measured with a Jobin–Yvon HR1000 monochromator, equipped with a charge-coupled device (CCD) camera at temperature in range from 158 to 320 K by the use of THMS600 heating–cooling stage from Linkam and using a 976 nm laser diode of power density changed in a range from 0.73 to 9.95 W/cm². The upconversion luminescence decay curves were measured with a LeCroy WaveSurfer 400 oscilloscope.

Results and discussion

Comparison of morphological and structural properties of the uncoated nanoparticles (NaYF₄:Er³⁺,Yb³⁺ CNP) and core–shell structures (NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP) is shown in Figs. 2, 3 and 4. As can be seen in the FESEM image (Fig. 2a), the synthesized NaYF₄:Er³⁺,Yb³⁺ CNP were roughly spherical in shape with sizes of around 35 ± 5 nm. The HR-TEM image in Fig. 2b presents a representative nanoparticle with size of 36 nm and lattice distance of 0.3 nm corresponding to d-spacing for the (110) lattice plane of hexagonal β phase of NaYF₄ structures, suggesting that the [110] is the preferred growth direction of the NaYF₄:Er³⁺,Yb³⁺ CNP. The clear difference between core and shell part of the nanoparticle observed in TEM images may results from the difference in the porosity of the particular parts of the nanocrystal. In addition, the obtained SEAD pattern in Fig. 2c indicates the diffraction rings which correspond to the (100), (110), (101), (200), (111), (201), (210) and (300) planes of the synthesized β-NaYF₄ nanoparticles. The EDX spectrum analysis shows the major peaks at energy around 0.68, 1.04, 1.91 and 8.04 keV corresponding to F, Na, Y of synthesized NaYF₄:Er³⁺,Yb³⁺ CNP and Cu elements of copper grid (Fig. 2d). The minor intensity peaks observed at energy around 1.54, 7.39, 8.42, 14.9 and 16.73 keV have proven the presence of Yb³⁺ and Y³⁺ ions in the samples. The presence of Er³⁺ ions at energy around 6.9 keV is almost invisible which is due to the low concentration of Er³⁺ ions in the material.

Figure 2

Characterization of synthesized NaYF₄:Er³⁺,Yb³⁺ CNP: (a) FESEM and (b) HR-TEM images, (c) SAED pattern and (d) EDX spectrum corresponding to the isometric nanoparticles in (c).

Figure 3

Characterization of NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP: (a) FE-SEM, (b) TEM, (c, d) HR-TEM images, (e) SAED pattern and (f) EDX spectrum corresponding to the isometric nanoparticles (e).

Figure 4

The normalized XRD patterns of synthesized NaYF₄:Er³⁺,Yb³⁺CNP (a) and NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP (b) compared to standard data of hexagonal β-NaYF₄ phase (JCPDS card No. 028-1192).

The characteristic properties of NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP are presented in Fig. 3. As can be observed in Fig. 3a–c, after coating the surface of NaYF₄:Er³⁺,Yb³⁺ CNP with a shell layer of NaYF₄, the synthesized NaYF₄:Er³⁺,Yb³⁺@NaYF₄ samples revealed also roughly spherical shape however, with obviously larger average size than that of the above-mentioned NaYF₄:Er³⁺,Yb³⁺ CNP precursors. The diameter of the core–shell nanoparticles was on average 100 ± 10 nm. It can be clearly seen that the NaYF₄ shell layer with thickness about 40 nm covered the entire surface of NaYF₄:Er³⁺,Yb³⁺ CNP, indicating the formation of core–shell structure in the synthesized NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP. In particular, the HR-TEM image (Fig. 3d) has shown that the lattice distances were equal to 0.3 nm as in the case of uncovered NPs and 0.5 nm corresponding to d-spacing for the (110) and (100) lattice planes of β-NaYF₄ nanoparticles. It suggests that the [110] growth direction is preferred both in the case of uncovered NaYF₄:Er³⁺,Yb³⁺ CNP as well as in the case of core–shell NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP. Figure 3e displays the diffraction rings corresponding to the (100), (110), (101), (200), (111), (201), (210) and (220) planes of the β-NaYF₄ nanoparticles thus confirming the formation of hexagonal β phase of synthesized NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP. Furthermore, the results of EDX spectrum analysis confirmed the presence of F, Na, Y host elements which were related to the major peaks at energy around 0.67, 1.04, 1.92 and 8.04 keV (Fig. 3f). However, the intensities of peaks corresponding to Yb element at energy around 1.54, 7.39 and 8.42 keV in the NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP were much lower compared with the NaYF₄:Er³⁺,Yb³⁺ CNP, suggesting as expected a decrease of Yb³⁺/NaYF₄ ratio in the NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP since the ratio of Yb³⁺ ions to the total amount of ions is lower in the case of CSNP in respect to the CNP counterpart due to the additional shell layer with no Yb³⁺ ions. The presence of Er³⁺ ions was not observed in the NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP due to the Er³⁺ concentration below the detection limit of the measurement.

The normalized XRD patterns of NaYF₄:Er³⁺,Yb³⁺ CNP and NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP are shown in Fig. 4. The diffraction peaks of samples perfectly matched with the standard reference pattern of hexagonal β phase of NaYF₄ (JCPDS card, No. 028-1192) which confirmed the high purity and crystallization of synthesized NaYF₄:Er³⁺,Yb³⁺ CNP and NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP.

The average crystallite size D of samples was calculated using Scherrer formula²⁹:

$${\text{D}} = \frac{k\lambda}{\beta\cos\theta }$$

(1)

where λ is the X-ray wavelength, β is the full width at half maximum, θ is the diffraction angle and k is a constant. The determined D values were equal 36 nm and 85 nm for NaYF₄:Er³⁺,Yb³⁺ CNP (pattern a) and β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP (pattern b), respectively²⁹. The results were entirely consistent with the SEM and TEM images presented above in Figs. 2 and 3.

The ligand binding on the surface of β-NaYF₄:Er³⁺,Yb³⁺ CNP and β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP were investigated by FTIR spectrum in the wavenumber range from 4000 to 400 cm⁻¹ (Fig. 5). As can be seen, in both core and core–shell samples, a broad absorption peak of the O–H stretching vibrations due to intermolecular hydrogen bonding at around 3448 cm⁻¹ was observed³⁰. The weak absorption peaks at 2929 cm⁻¹, 2647 cm⁻¹ corresponding to the asymmetric and symmetric stretching vibrations of the long alkyl chains (= CH(CH₂)_n groups) of oleic acid can be observed confirming its residual presence on the surface of β-NaYF₄:Er³⁺,Yb³⁺ CNP and β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP. The broad bands at about 1635 cm⁻¹ and 1122 cm⁻¹ have been assigned to the presence of asymmetric and symmetric C = O stretching vibration of carboxylic acid salts on the surface of β-NaYF₄:Er³⁺,Yb³⁺ CNP and β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP³¹ and the peaks at 547 cm⁻¹ and 515 cm⁻¹ might be a result of a presence of the RE³⁺–F stretching vibrations. Exceptionally, a strong band at 1430 cm⁻¹ related to O–H bending vibration of carboxyl group was observed only in the β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP^17,30. Therefore, the presence of carboxylic acid groups in the synthesized β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP is suspected³⁰.

Figure 5

The FTIR spectra of: (a) β-NaYF₄:Er³⁺,Yb³⁺ CNP and (b) β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP in the wavenumber range of 4000 to 400 cm⁻¹.

In order to study the impact of Er³⁺ and Yb³⁺ ions as well as the energy transfer mechanisms in synthesized β-NaYF₄:Er³⁺,Yb³⁺ CNP and β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP on their spectroscopic properties, the dependence of UC luminescence intensity on the 976 nm excitation (power density changed in the range from 0.73 to 9.95 W/cm²) have been measured at room temperature. As it is well known, in the low excitation density regime, the UC emission intensity (I), is dependent on the excitation laser power (P) as follows³²:

$$I = AP^{n}$$

(2)

where A is a proportional parameter, n is the number of pumping photons involved in the UC process.

Figure 6 shows the UC emission properties of β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP as a function of pump power of 976 nm laser diode. As can be observed, the β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP revealed strong red emission band at 650 nm corresponding to ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ ions and minor green emission bands at 520 nm and 545 nm corresponding to ²H_11/2, ⁴S_3/2 → ⁴I_15/2 transitions, respectively (Fig. 6a). The UC emission intensity increased with increasing the pump power and the intensity ratio of green to red emission decreased from 0.3 to 0.1 in response to increased excitation power density from 0.73 to 9.95 W/cm². These results suggest that at high excitation densities the red emission is even more dominant. High excitation density provides heating of the nanocrystals, therefore observed changes of green to red emission intensities could be explained in terms of higher probability of nonradiative depopulation of ²H_11/2 and ⁴S_3/2 states to ⁴F_9/2 at higher temperatures. The calculated CIE chromaticity coordinates of β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP under 976 nm excitation confirmed that the CIE (x, y) values increased from (0.337, 0.331) to (0.409, 0.356) with the excitation power (Fig. 6b). The obtained CIE (x, y) values proved that the emission color of the β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP can be tuned from yellowish to red by excitation power changes (Table 1). The calculated slopes of the fitting lines of the first 6 points for green (²H_11/2, ⁴S_3/2 → ⁴I_15/2) and red (⁴F_9/2 → ⁴I_15/2) emission bands were equal to 1.8 and 1.3, indicating that two photons process was involved to populate excited levels resulting in both green and red emission bands in the synthesized β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP (Fig. 6c).

Figure 6

The UC emission spectra (a), CIE1931 chromaticity diagram (b), and integrated intensities (c) of blue (²H_11/2, ⁴S_3/2 → ⁴I_15/2) and red (⁴F_9/2 → ⁴I_15/2) emission bands of core β-NaYF₄:2%Er³⁺,19%Yb³⁺ nanoparticles upon 976 nm laser diode excitation in range of 0.73 to 9.95 W/cm². The luminescence decay curves of synthesized β-NaYF₄:2%Er³⁺ CNP (d) and β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP (e) recorded at 520 nm (blue curves), 545 nm (dark cyan curves) and 650 nm (red curves) under 976 nm excitation. The corresponding energy levels diagram of NaYF₄:Er³⁺,Yb³⁺ nanoparticles (f).

Table 1 The integrated UC intensities of particular emission bands, their ratios and CIE chromaticity coordinates of synthesized β-NaYF₄:Er³⁺,Yb³⁺ CNP.

In addition, luminescence decay curves of ²H_11/2 (emission band at 520 nm), ⁴S_3/2 (emission band at 545 nm) and ⁴F_9/2 (emission band at 650 nm) excited levels of β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP and β-NaYF₄@NaYF₄:2%Er³⁺,19%Yb³⁺ CSNP compared with the β-NaYF₄:2%Er³⁺ CNP and β-NaYF₄@NaYF₄:2%Er³⁺ CSNP under 976 nm excitation were recorded. The average luminescence lifetimes were calculated by the following equation³³:

$$\tau = \frac{{\mathop \int \nolimits_{0}^{\infty } tI\left( t \right)dt}}{{\mathop \int \nolimits_{0}^{\infty } I\left( t \right)dt}}$$

(3)

where I(t) represents the luminescence intensity at time t.

The average decay times of β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP recorded for ²H_11/2 level (emission band at 520 nm), ⁴S_3/2 level (emission at 545 nm) and ⁴F_9/2 level (emission at 650 nm) were equal 112, 118 and 512 µs, respectively (Fig. 6e), while for the β-NaYF₄:2%Er³⁺ CNP counterpart they were 19, 18 and 26 µs, respectively (Fig. 6d). The longer lifetimes of Er³⁺ ions in the synthesized β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP compared with the NaYF₄:2%Er³⁺ CNP confirmed the occurrence of effective energy transfer from Yb³⁺ to Er³⁺ ions in β-NaYF₄ host lattice³⁴. What is more, the comparison of the intensities of the bands within the β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP themselves revealed that green emission intensity was much weaker than the red one and associated lifetimes recorded at 520 and 545 nm was about 4.34 times shorter than that recorded at 650 nm.

Basing on the obtained results, the UC mechanism of the β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP have been proposed as follows: upon a 976 nm laser diode excitation, the activator ions (Er³⁺) were excited from the ⁴I_15/2 ground state to the ⁴I_11/2 excited state through ground states absorption (GSA), or energy transfer (ET) from the ²F_5/2 excited state of Yb³⁺ ions (4). Then, the population of ⁴I_11/2 excited state of Er³⁺ ions was followed by excitation to the ⁴F_7/2 level due to another energy transfer from Yb³⁺ ions (5).

$$^{4} {\text{I}}_{15/2} \left( {{\text{Er}}^{3 + } } \right) + {^{2} {\text{F}}_{5/2}} \left( {{\text{Yb}}^{3 + } } \right) \to{^{4} {\text{I}}_{11/2}} \left( {{\text{Er}}^{3 + } } \right) +{^{2} {\text{F}}_{7/2}} \left( {{\text{Yb}}^{3 + } } \right)$$

(4)

$$^{4} {\text{I}}_{11/2} \left( {{\text{Er}}^{3 + } } \right) +{^{2} {\text{F}}_{5/2}} \left( {{\text{Yb}}^{3 + } } \right) \to{^{4} {\text{F}}_{7/2}} \left( {{\text{Er}}^{3 + } } \right) +{^{2} {\text{F}}_{7/2}} \left( {{\text{Yb}}^{3 + } } \right)$$

(5)

After the excitation of ⁴F_7/2 level of Er³⁺ ions the nonradiative relaxation to the ²H_11/2 and ⁴S_3/2 levels occurred. The radiative depopulation of those levels back to the ground state leads to appearance of green UC emission bands at 520 and 545 nm corresponding to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively (Fig. 6f)¹⁵. Additionally, the electrons from the ⁴S_3/2 level of Er³⁺ ions can be nonradiatively relaxed to the ⁴F_9/2 level causing the occurrence of the red emission band at 650 nm related to the ⁴F_9/2 → ⁴I_15/2 electronic transition of Er³⁺ ions. Furthermore, the nonradiative relaxation from ⁴I_11/2 excited state to the lower ⁴I_13/2 state of Er³⁺ ions can take place enabling another act of excitation to occur. Therefore, absorption of a second NIR photon can provoke excitation from ⁴I_11/2 to the ⁴F_9/2 state³⁴.

In order to study the effect of the shell layer on the UC properties of nanoparticles, an analogous spectroscopic analysis was carried out for β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP. The UC emission properties of synthesized β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP upon 976 nm laser diode excitation are presented in Fig. 7. It can be clearly seen that these CSNPs revealed three strong emission bands at 520 nm, 545 nm and 650 nm assigned to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively (Fig. 7a). The ratio of green to red emission intensities increased from 0.31 to 1.36 with increasing excitation power density from 0.73 to 9.95 W/cm². The calculated CIE chromaticity coordinates of synthesized β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP proved that the x value decreased from 0.34 to 0.31, while y value increased from 0.33 to 0.53 (Table 2) therefore, the emission colors changed from red into yellowish green (Fig. 7b). The slopes of fitting lines for emission bands associated with the transitions from ⁴S_3/2 (blue line), ²H_11/2 (wine line) and ⁴F_9/2 (red line) levels to ⁴I_15/2 ground state were determined as 2.5, 2.1 and 1.7, respectively (Fig. 7c). These results indicate that in the case of synthesized β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP not only two-photon UC process, as in the above-mentioned β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP took place, but also three-photon UC process for green emissions was involved.

Figure 7

The UC emission spectra (a), and CIE chromaticity diagram (b), and integrated intensities (c) of green (²H_11/2, ⁴S_3/2 → ⁴I_15/2, dark green- and blue- lines) and red (⁴F_9/2 → ⁴I_15/2, red-line) emission bands of synthesized β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP upon 976 nm laser diode excitation in range of 0.73 to 9.95 W/cm². The luminescence decay curves of synthesized β-NaYF₄:2%Er³⁺@NaYF₄ (d) and β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP (e) recorded at 520 nm (blue curves), 545 nm (dark cyan curves) and 650 nm (red curves) under 976 nm excitation. The corresponding energy levels diagram of NaYF₄:Er³⁺,Yb³⁺@ NaYF₄ nanoparticles (f).

Table 2 The integrated UC intensities of particular emission bands, their ratios and CIE chromaticity coordinates of synthesized β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP.

The three-photon UC process causing the green emission was observed only in the β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP. Therefore it is postulated in this work, that an additional shell layer reduces undesirable surface losses, detain energy within the sample and enable cross-relaxation between Er³⁺ ions to occur. The cross-relaxation processes of two Er³⁺ ions taking place in the way as follows^34,35:

$$^{4} {\text{I}}_{15/2} \left( {{\text{Er}}^{3 + } } \right) +{^{4} {\text{G}}_{11/2}} \left( {{\text{Er}}^{3 + } } \right) \, \to{^{4} {\text{I}}_{13/2}} \left( {{\text{Er}}^{3 + } } \right) \, +{^{2} {\text{H}}_{11/2}} ,{^{4} {\text{S}}_{3/2}} \left( {{\text{Er}}^{3 + } } \right)$$

(6)

$$^{4} {\text{I}}_{9/2} \left( {{\text{Er}}^{3 + } } \right) +{^{4} {\text{G}}_{11/2}} \left( {{\text{Er}}^{3 + } } \right) \to{^{2} {\text{H}}_{11/2}} \left( {{\text{Er}}^{3 + } } \right) +{^{2} {\text{H}}_{11/2}} ,{^{4} {\text{S}}_{3/2}} \left( {{\text{Er}}^{3 + } } \right)$$

(7)

To confirm also the energy transfer between Yb³⁺ to Er³⁺ ions in the core–shell β-NaYF₄@NaYF₄ host lattice, the comparison of spectroscopic properties of β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP and β-NaYF₄:2%Er³⁺@NaYF₄ CSNP was made. As can be seen in Fig. 7d, the calculated emission lifetimes of single doped β-NaYF₄:2%Er³⁺@NaYF₄ CSNP monitored at 520 nm, 545 nm and 650 nm were 79, 93 and 249 µs, respectively, while in the case of co-doped β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP, an approximately three-fold increase in the decay times of green emissions and two-fold of the red one was observed (Fig. 7e) and their values were 215, 193 and 474 µs, respectively. What is more, it can be seen that the lifetime of red emission recorded at 650 nm in the case of synthesized core–shell β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP was slightly shorter than in uncoated β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP, however, clear increase in lifetime recorded at green emission (corresponding to ²H_11/2 and ⁴S_3/2 states) was noted. It matched well with the UC emission spectrum and the calculated slopes for UC green and red emission bands as mentioned above. Such outcomes confirmed the significant influence of NaYF₄ shell layer on the UC energy transfer mechanism appearing in those samples. The schematic model of the energy transfer mechanisms of β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP including cross- relaxation processes is presented in Fig. 7f. The excitation wavelength of 975 nm involved in the experiment is matched to the excitation of the sensitizer Yb³⁺ ions from which energy is transferred to Er³⁺ ions. The energy transfer (ET) from Yb³⁺ to Er³⁺ ions marked with a dashed line in the diagram increases the efficiency of population of ⁴I_11/2, ⁴F_9/2 and ⁴I_7/2 levels, which, as can be seen in the diagram, in our co-doped core–shell structure are involved in both 2-photon and 3-photon upconversion processes. The population of ⁴I_11/2 level can be followed by two processes, i.e. 1) ⁴I_11/2 → ⁴F_7/2 ESA process, followed by rapid relaxation to ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels, from which the green and red emissions, respectively occur and 2) nonradiative population of the lower ⁴I_13/2 level, from which the next upconversion excitation act to the ⁴F_9/2 level takes place. The population of the ⁴F_9/2 level allows direct radiative transition to the ground ⁴I_15/2 state to occur, resulting in the appearance of red emission at 650 nm. However two other events from this state can also supervene, i.e. nonradiative relaxation to ⁴I_9/2 and ⁴I_11/2 levels or a population of higher lying ⁴G_11/2 level. Both the filling of the ⁴I_11/2 level followed by excitation to the ⁴F_7/2 level, as well as the process of cross-relaxation occurring as a result of the filling of the ⁴I_9/2 and ⁴G_11/2 levels, lead to subsequent emission transitions thus increasing the efficiency of the observed green emission. What is more, both these processes, i.e. the occurrence of nonradiative relaxation and excitation to the ⁴G_11/2 level reduce the population of the ⁴F_9/2 level, which explains the lowering of red emission efficiency in favor of green one.

In this work, we have shown that the coating of the CNP by the additional shell layer reduces the efficiency of the surface-related nonradiative quenching processes resulting in the enhancement of the green emission intensity for CSNP in respect to the CNP. Therefore, the core-sell structure in general improved the intensity of bands and thus also extended the decay times, excluding the red emission in the case of β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP, however, such a structure weakened the efficiency of energy transfer.

In order to investigate the suitability of β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP for optical temperature sensing, the temperature dependence of UC emission spectra under the 976 nm excitation (power density 4.29 W/cm²) at temperatures ranging from 158 to 320 K was measured (Fig. 8a). As can be clearly seen in emission spectra normalized to the intensity of red band which is presented in the Fig. 8a, the intensity of UC green emission of these CSNP increased with increasing temperature. The luminescence intensity ratio (LIR) related to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of two thermally coupled excited states can be derived from Boltzmann equation as follow³⁶:

$$LIR = \frac{{I_{H} }}{{I_{S} }} = \frac{{g_{H}\ A_{H }\ h\nu_{H } }}{{g_{S}\ A_{S }\ h\nu_{S } }}\exp \left( { - \frac{\Delta E}{kT}} \right) = B\exp \left( { - \frac{\Delta E}{kT}} \right)$$

(8)

where, I_H, I_S, g_H, g_S, A_H, A_S, ν_H and ν_S are the integrated emission intensities, degeneracies, spontaneous emission rates and frequencies of the transitions from the ²H_11/2 (or ⁴S_3/2) state to the ground state, respectively; h is Planck’s constant; k is Boltzmann constant (k ~ 0.695035 cm⁻¹/K); T is the temperature, ΔE is energy gap between the ²H_11/2 and ⁴S_3/2 levels and B is the associated constant of host lattice.

Figure 8

Normalized UC emission spectra of synthesized β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP upon 976 nm laser diode excitation measured at temperatures ranging from 158 to 320 K with step of 20 K (a), the fluorescence intensity ratio (I_H/I_S) of green emission as a function of temperature where the experimental data is marked as points and the line corresponds to the fitting data (b), the absolute (gray line and stars) and relative (dark blue line and spherical points) sensitivity as a function of temperature (c).

The energy difference between ²H_11/2 and ⁴S_3/2 levels and the associated constant of β-NaYF₄ host lattice were determined by fitting the data by exponential function as presented in Fig. 8b. The obtained ΔE/k and B values for fit were equal to ~ 813 cm⁻¹ and 14.27 (r² = 0.998), respectively.

To evaluate the sensor performance to temperature sensing, the relative thermal sensitivity (S_R) of luminescent thermometer has to be determined. Basing on the experimental data of LIR, both the absolute thermal sensitivity (S_A) and the relative thermal sensitivity (S_R) of synthesized β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP was calculated by the following equations³⁶:

$$S_{A} = \frac{\Delta (LIR)}{{\Delta T}}$$

(9)

$$S_{R} = \frac{1}{LIR}\frac{\Delta (LIR)}{{\Delta T}}$$

(10)

The results are presented in the Fig. 8c and as can be seen, the relative thermal sensitivity (dark blue line and spherical points) of β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP was decreasing from unprecedented value S_R = 4.25% K⁻¹ at 158 K with increasing temperature, however maintaining high value > 1% K⁻¹ in the whole analyzed temperature range. On the other hand, the absolute thermal sensitivity (gray line and stars) revealed opposite thermal behavior and increased in this temperature range reaching values of 0.337% K⁻¹ at 158 K and 1.338% K⁻¹ at 298 K.

Conclusions

In conclusion, the β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP were successfully synthesized by soft template method. The size, shape, crystallinity and UC emission properties of this material were investigated and compared with uncoated β-NaYF₄:Er³⁺,Yb³⁺ CNP. The results obtained shown that the synthesized β-NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP were about 100 ± 10 nm in size, therefore, they were nearly 2.5 times bigger than the β-NaYF₄:Er³⁺,Yb³⁺ CNP. The UC emission spectra measured at room temperature under 976 nm excitation of power density in range from 0.73 to 9.95 W/cm² demonstrated that the synthesized NaYF₄:Er³⁺,Yb³⁺@NaYF₄ CSNP compared with β-NaYF₄:Er³⁺,Yb³⁺ CNP revealed both the stronger UC emission intensity and the 13 times greater the intensity ratio of green to red emission. The calculated CIE (x, y) values have proven the predominance of red color emission in the case of β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP and the shift from red color into yellowish green area in the case of NaYF₄:2%Er³⁺,19%Yb³⁺ @NaYF₄ CSNP. The calculated values of slopes of green and red emission intensities versus excitation power density also confirmed that in the NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP not only two-photon UC process as in the case of β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP occurred, but also three-photon UC process, due to the cross-relaxation existing between Er³⁺ ions took place, enabling green emission intensity enhancement. The calculated lifetimes of ²H_11/2, ⁴S_3/2 and ⁴F_9/2 excited levels of Er³⁺ ions within single doped β-NaYF₄:2%Er³⁺ CNP and β-NaYF₄@NaYF₄:2%Er³⁺ CSNP were shorter than that of co-doped β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP and β-NaYF₄@NaYF₄:2%Er³⁺,19%Yb³⁺ CSNP, which confirmed that energy transfer from Yb³⁺ to Er³⁺ ions in synthesized materials occurred. What is more, the comparison of β-NaYF₄:2%Er³⁺,19%Yb³⁺ CNP and β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNP showed that the lifetime of ⁴F_9/2 excited level associated with red emission band recorded at 650 nm in the case of synthesized core–shell structures was slightly shorter than in uncoated nanoparticles, however, clear increase in lifetime recorded at green emission (corresponding to ²H_11/2 and ⁴S_3/2 states) was noted which confirmed the significant influence of NaYF₄ shell on the UC energy transfer mechanisms occurring in core–shell nanomaterials.

The temperature dependent UC emission spectra of β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNPs under the 976 nm laser diode excitation of power density of 4.29 W/cm² have been investigated to evaluate their suitability for temperature sensing applications. Basing on the LIR values of emission bands of two thermally coupled excited states of Er³⁺ ions, the energy gap between ²H_11/2 and ⁴S_3/2 levels and associated constant of NaYF₄@NaYF₄ host lattice were determined and they were equal to ~ 813 cm⁻¹ and 14.27 (r² = 0.998), respectively. The relative thermal sensitivity of β-NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNPs determined on the basis of the LIR achieved unusually high value of S_R = 4.25% K⁻¹ at 158 K and was decreasing with increasing temperature, however, maintaining value > 1% K⁻¹ in the whole analyzed temperature range, up to 298 K. These outcomes confirmed that NaYF₄:2%Er³⁺,19%Yb³⁺@NaYF₄ CSNPs reveal very high potential for temperature sensing applications.