Novel Fluoride Matrix for Dual‐Range Optical Sensors and Visualization

1. Introduction

Fluorescent thermometry is an effective way to display the temperature in the analyzed objects. The main advantage of luminescent thermometry is possibility of contactless temperature measurement [1,2,3,4,5]. This approach efficiently apply in medicine, biology, detecting catalytic reactions and another areas, where contact measurement is inconvenient or inapplicable [6,7,8,9].

Temperature measuring using REE luminescence [10] based on two main approaches such as luminescence intensity from the one type of ion [11,12] or two or more ions doping one matrix [13,14,15]. The first approach is based on the dependence of the ratio of populations and, consequently, luminescence intensities, thermally related levels on temperature. The energy gap between the thermal bound levels is so small that, according to Boltzmann's law, an increase in temperature can lead to an increase in the population of the overlying state. In the luminescence spectrum, an increase in the shorter-wavelength peak will be observed. The second approach was proposed by Sekiyama et al. [16] and is based on a matrix doped with three ions of rare earth elements. One of them (Yb) acts as a donor, absorbs exciting radiation and transmits it to two other acceptor ions. Acceptor ions are selected in such a way that one of the energy transfers is resonant (for example, Yb → Er), its probability does not depend on temperature and this signal is used as a reference. The second transfer is non-resonant (for example, Yb → Ho) and with increasing temperature its probability increases (phonon assisted Energy transfer). This approach allows thermometry in the short-wave IR region.

The most express approach is to determine the ratio of the intensities of two luminescence bands, one of which is thermally bound, and the second is thermally independent. Such luminescence bands are known for Ho³⁺ and Er³⁺, which are realized when the matrix is so-doped with Yb³⁺ ions, due to resonant and phonon-assisted energy transfers. When measuring the thermal response in the near IR range (SWIR), pumping is carried out into the absorption band Yb³⁺ (⁷F_7/2→⁵F_5/2), followed by energy redistribution between Ho³⁺ and Er³⁺ ions as a result of non-radiative energy transfer. In this case, the energy transfer of Yb³⁺ → Ho³⁺ is non-resonant, as a result of which the luminescence intensity of Ho³⁺ ions at the transition ⁵I₆→⁵I₈ will depend on temperature, and the energy transfer of Yb³⁺ → Er³⁺ is resonant, as a result of which the luminescence of Er³⁺ ions at the transition ⁴I_13/2→⁴I_15/2 depends on temperature weakly [17]. This makes it possible to measure the relative coefficient of temperature sensitivity by the ratio of the integral intensities of the luminescence bands. For the visible range, measurements are carried out by the ratio of the red and green bands of erbium luminescence.

Using thermally coupled levels, taking into account resonant and non-resonant energy transfer channels from the sensitizer, it is possible to expand the range of wavelengths in which thermometry can be performed.

Fluoride matrices such as NaYF₄ [18], NaGdF₄ [15], MF₂, where M = Ca, Sr, Ba [19], have proven to be effective photonics materials [20]. It is known that weighting of matrix leads to a decrease in multi-phonon relaxation and increasing the luminescence energy yield. In this connection, the purpose of the study is search for new "heavy" matrices based on fluorides and recognize their spectral and luminescent characteristics for luminescent nanothermometers. As part of the study, a method was developed for the synthesis of a new "heavy" matrix Ba_0.5Gd_0.5-xNa_xF_2.5-x (x = 0.05) and phosphors based on it for testing as a dual-band luminescent thermometer.

2. Materials and Methods

2.1. Synthesis of Luminophores

We synthesized the samples by crystallization from a solution in a sodium nitrate melt according to the method [21]. Sodium fluoride (Reagent grade, Chimmed) was used as a fluorinating agent, and sodium nitrate (Reagent grade, Chimmed) was used as a solvent. The reagents were used without additional purification steps.

The synthesis is described by the following reaction equation:

(1-x)Ba(NO₃)₂ + xLn(NO₃)₃*nH₂O + (2+x)NaF → Ba_1-xLn_xF_2+x↓ + (2+x)NaNO₃ + nxH₂O↑,

(1)

there x = 0.33, 0.40. The samples of barium nitrate (specialty grade 10-2, Reachem) and nitrate hydrates of rare earth elements (99.99% purity in terms of cation impurity, Lanhit) were homogenized in an agate mortar, and then added to the samples of fluoride and sodium nitrate. After homogenization, the mixture was transferred to a porcelain glazed crucible and annealed at a temperature of 500 °C for 1 hour. After natural cooling, the sinter was removed from the crucible and washed with bidistilled water until a negative reaction of a solution of diphenylamine in sulfuric acid to nitrate anions. The samples were dried under an IR lamp at a temperature of 40-60 °C for 6 hours.

2.2. Characterizasion of Luminophores

The samples were characterized by X-ray phase analysis (XRD) (Bruker, D8 Advance with CuKa-irradiation), scanning electron microscopy (SEM) (Carl Zeiss, NVision 40), energy dispersion analysis (EDX) (Oxford Instruments, X-Max 80 mm2), differential scanning calorimetry (DSC) (Netzsch, STA 449 F3 Jupiter).

2.3. Temperature-Dependent Luminescence Spectra Measurements

Luminescent spectroscopy of the samples was performed on an installation consisting of a laser with a wavelength of 974 nm, a spectrometer (StellarNet, DWARF-Star) in the SWIR range of 1000-1700 nm, a spectrometer (BIOSPEC, LESA-01-BIOSPEC) in the visible range of 300-900 nm and a thermal infrared camera (CEDIP, JADE MWIR SC7300M) for temperature measurements in the range from 24 to 44 °C [20]. Heating was carried out using a Primelab PL-R-basic H heated magnetic stirrer (Primelab, Russia) with 0.1 °C temperature heating sampling. Luminescence intensity ratio (LIR) was used to quantify the dependence of luminescence spectra on temperature in the SWIR range:

LIR = I_Ho/I_Er

(2)

The relative coefficient of temperature sensitivity (S_r) was calculated using the following formula:

$$S_r=\frac{{LIR}_{T1}-{LIR}_{T2}}{T_2-T_1}\ast 100\%$$

(3)

3. Results

In the course of our work, we synthesized samples in the BaF₂-GdF₃ system doped with Yb, Ho, Er. The nominal composition and composition determined according to the EDX data of the samples obtained, as well as the practical yield of the synthesis reaction are shown in Table 1. EDX results show that the crystal lattice of synthesized samples contains sodium in an amount of up to 8 mol.%, regardless of the content of the rare earth element. Apparently, this value corresponds to the limit of solubility of sodium in this solid solution. The content of rare earth elements was overestimated compared to the initial charge, and the content of barium was underestimated.

Figure 1 shows X-ray diffraction patterns of prepared samples.

According to X-ray diffraction patterns, Samples 1-5 are single phase, and have a structure derived from the fluorite structure in which BaF₂ crystallizes. The X-ray pattern of Sample 1 is characterized by a fine splitting of the main X-ray diffraction reflexes, and is indicated in tetragonal crystallographic system, space symmetry group (SSG) I4. X-ray pattern of Samples 2-5 also indexed in SSG I4. It is worth noting that X-ray diffraction patters of Samples 2-5 contains superstructural peaks, which are well described when the crystal lattice parameter "c" is increased three times. The Supplementary Materials, Tables S1–S5, shows the results of indexing of X-ray patterns. For all samples, the estimated values of the coherent scattering regions D were more than 100 nm. The lattice parameters of Samples 1-5 and the calculated X-ray density (Dx) values are given in Table 2.

SEM micrographs were obtained for all samples (Figure 2).

All powders have submicron sizes with a bimodal particle size distribution. The first maximum is about 200 nm, the second about 700 nm. Histograms of particle size distribution one can find in Supplementary Materials, Figure S1. Detection of backscattered electrons showed that the particles in Sample 3 are homogeneous in composition, there is no contrast in microphotographs.

The results of thermal analysis are shown in Figure 3. Peaks with maximums at 986 °C and 1010 °C correspond to phase reactions, the effect with a maximum at 1074 °C corresponds to the end of melting of the sample (liquidus point). DSC data show that Sample 3 stable in flow of argon until 962.5 °C.

Figure 4 shows the luminescence spectra in the visible range of the spectrum for sample 5 at different temperatures (a) and in SWIR range for samples 3 and 5 at a temperature of 296 K (b).

Peak in SWIR region at 1150 nm wavelength corresponds to Ho³⁺ (⁵I₆→⁵I₈), and peak at 1550 nm corresponds to Er³⁺ (⁴I_13/2→⁴I_15/2).

The temperature dependences of the ratio of the intensities of the red and green erbium luminescence bands for the visible range of the spectrum are shown in Figure 5a, and the intensity ratios of the erbium and holmium luminescence bands for the SWIR range are shown in Figure 5b.

From the dependences presented in Figure 5, the relative coefficients of temperature sensitivity (Sr) in the temperature range from 24 to 44 °C are determined: Sr = 5.55% * K^-1 in the visible range and Sr = 0.34% * K^-1 in the SWIR range.

4. Discussion

According to the XRD data, the synthesized samples crystallize in tetragonal crystallographic system with the spatial symmetry group (SSG) I4. Comparing the data obtained with the well-known phase diagram of the BaF₂-GdF₃ system, presented in Figure 6, makes it obvious that the samples prepared by us correspond to a high-temperature T phase having a similar composition [22]. This phase with a fluorite-derived structure is characterized by a tetragonal cell, SSG I4, with the parameter "c" increased three times. The cause of tetragonal distortion is the ordered arrangement of clusters of defects in a fluorite-type lattice. The synthesis of the phase of such a structure at a low synthesis temperature of 500 °C is apparently explained by the entry of sodium into the lattice of synthesized samples, leading to stabilization of the high-temperature phase.

As the data in Figure 5 show, the luminescent thermometer synthesized by us can be used to detect temperature in both visible and SWIR ranges. At the same time, the sensitivity in the visible range is high and significantly higher than in the IR range. However, the obtained characteristic of our phosphor is consistent with the analogues described in the literature (Table 3).

5. Conclusions

The synthesis of a heavy matrix based on barium-gadolinium fluoride with a density of more than 6 g/cm³ was developed as a result of the study. The entry of sodium into the composition of the samples was recorded by EDX method. The presence of sodium stabilizes the high-temperature phase of the tetragonal crystallographic system with the general formula Ba_0.5Gd_0.5-xNa_xF_2.5-x.

The relative coefficients of temperature sensitivity (Sr) in the temperature range 296-316 K are equal to Sr = 5.55% * K^-1 in the visible range and Sr = 0.34% * K^-1 in the SWIR range. Thus, a novel dual-range luminescent thermometer of the composition Ba_0.5Gd_0.45Na_0.05F_2.45: Yb, Ho, Er has been developed.