Up-Converting Luminescence and Temperature Sensing of Er3+/Tm3+/Yb3+ Co-Doped NaYF4 Phosphors Operating in Visible and the First Biological Window Range

Accurate and reliable non-contact temperature sensors are imperative for industrial production and scientific research. Here, Er3+/Tm3+/Yb3+ co-doped NaYF4 phosphors were studied as an optical thermometry material. The typical hydrothermal method was used to synthesize hexagonal Er3+/Tm3+/Yb3+ co-doped NaYF4 phosphors and the morphology was approximately rod-like. The up-conversion emissions of the samples were located at 475, 520, 550, 650, 692 and 800 nm. Thermo-responsive emissions from the samples were monitored to evaluate the relative sensing sensitivity. The thermal coupled energy level- and non-thermal coupled energy level-based luminescence intensity ratio thermometry of the sample demonstrated that these two methods can be used to test temperature. Two green emissions (520 and 550 nm), radiated from 2H11/2/4S3/2 levels, were monitored, and the maximum relative sensing sensitivities reached to 0.013 K−1 at 297 K. The emissions located in the first biological window (650, 692 and 800 nm) were monitored and the maximum relative sensing sensitivities reached to 0.027 (R692/650) and 0.028 K−1 (R692/800) at 297 K, respectively. These results indicate that Er3+/Tm3+/Yb3+ co-doped NaYF4 phosphors have potential applications for temperature determination in the visible and the first biological window ranges.


Introduction
Temperature (T) is an important physical parameter in many fields, like scientific research, industrial production and biotherapy. Accurate T can usually be detected via contacting the temperature sensors, such as with thermal resistance, thermocouples and semiconductor temperature sensors. However, these temperature sensors limit their applications in temperature exploration when the measured objects are displayed in electromagnetic noise environments or beings. Thus, it is crucial to explore non-contact temperature sensors, such as IR thermography, Raman spectroscopy, and luminescence [1][2][3][4][5][6]. The non-contact temperature sensor based on temperature-dependent luminescence properties has drawn a lot of attention for its high resolution, stability and repeatability [7].
For lanthanide ion-doped materials, their luminescence intensity, peak position, emission band width, emission lifetime and luminescence intensity ratio (LIR) have been extensive researched for non-contact optical thermometry [7][8][9][10][11]. One of the most interesting developments is LIR-based temperature sensing as it is not influenced by pressure, light source and/or atmosphere [12]. Er 3+ doped nanomaterials are promising in LIR-based temperature sensing for their evident green emissions from 2 H 11/2 / 4 S 3/2 and excellent thermal coupling properties [13][14][15][16]. Thus, we choose Er 3+ as one of the doped rare earth ions and the green emissions can be used as the detected signal in the visible range for 2 of 9 thermometry. However, the green emissions have obvious absorption and limited penetration depth in biological tissues [17,18]. Therefore, the selected emissions shall be located in biological windows when the object is located in biological tissues [18]. Under 980 nm laser excitation, Tm 3+ can emit red (650 nm) and near-infrared emissions (692 and 800 nm) [19], which can be used as the detected signal in the first biological window (650-1000 nm).
The non-thermal coupled levels have also been used in LIR thermometry because of their high sensing sensitivity [20][21][22]. For example, high relative sensing sensitivity (0.0034 K −1 ) was obtained in NaLuF 4 :Yb/Er/Ho nano-rods at 503 K, which is based on the emissions at 659 and 547 nm [23]. Therefore, non-thermal coupled level-based LIR thermometry is an excellent method for temperature measuring, which can promote relative sensing sensitivity and select suitable wavebands.
Herein, Er 3+ /Tm 3+ /Yb 3+ were selected as the doped ions, with which Er 3+ /Tm 3+ acted as the emitting centers and Yb 3+ acted as the sensitizer. In this study, we selected hexagonal phase NaYF 4 as the host matrix due to its relatively excellent chemical and thermal stabilities and its low phonon energy (~370 cm −1 ) [24]. The rod-like NaYF 4 : Er 3+ , Tm 3+ , and Yb 3+ phosphors were prepared through the hydrothermal method. The emissions (450-850 nm) from Er 3+ /Tm 3+ /Yb 3+ co-doped NaYF 4 phosphors were systemically investigated. High relative temperature sensitivity was achieved via choosing suitable LIR of green emissions and the emissions located in the first biological window. We can take advantages of this multi-band noninvasive thermometry in harsh environments or biological tissues. The Er/Yb co-doped NaYF 4 phosphors were prepared using the hydrothermal method. The preparation processes are described below. First, calculated amounts of sodium hydroxide were dissolved into 2 mL deionized water. Second, 10 mL absolute ethyl alcohol and 18 mL oleic acid were added to the nitrate solution and then stirred for 5 min at room temperature to form a faint yellow solution. Third, 5 mL aqueous solution which contained calculated amount of Y(NO 3 ) 3 ·6H 2 O, Yb(NO 3 ) 3 ·6H 2 O and Er(NO 3 ) 3 ·6H 2 O was added. Then 5 mL ammonium fluoride aqueous was immediately added. After stirring for 30 min at room temperature, the mixed solution was transferred into a 50 mL autoclave and heated at 180 • C for 12 h in a vacuum drying oven. After cooling down to room temperature and adding a certain percentage of ethanol and cyclohexane, the khaki suspension was centrifuged (8000 rpm, 2 min) for collection and washed three times with ethanol and deionized water. Finally, the phosphors were obtained after drying at 60 • C for 10 h. The Er/Tm/Yb co-doped NaYF 4 phosphors were prepared using the same method, except for the amount of Y(NO 3

Instruments
X-ray diffraction (XRD) patterns of the sample were tested using an X-ray diffractometer (D8-02, BrukerAXS, Karlsruhe, Germany). The morphology was tested using a transmission electron microscope (TEM: Tecnai G2 F20, FEI, Hillsboro, OR, USA). The spectra of the samples were tested through the iHR550 grating spectrograph (iHR550, Horiba, Paris, France). The 980 nm laser used to excite the sample was purchased from the Beijing For the thermometry experiments, we introduced a Linkam THMS 600 heating stage to heat the sample. Then the temperature of the sample was measured by thermocouple. The spectra of the sample at certain temperatures were acquired using the iHR550 grating spectrometer (iHR550, Horiba, Paris, France).

XRD Analysis
The XRD patterns of NaYF 4 : Er/Yb and NaYF 4 : Er/Tm/Yb phosphors are presented in Figure 1a. The XRD patterns of the samples can be indexed to hexagonal NaYF 4 crystal (the JCPDS standard card no. , indicating that the dopants (Er, Tm and Yb ions) are successfully incorporated into the host lattice and do not cause significant changes to the crystal structure. Figure 1b,c show the TEM images of the samples. Two samples' morphologies are approximately rod-like. The lengths of the rods are~890 nm and the length-diameter ratios are~3.3.

Instruments
X-ray diffraction (XRD) patterns of the sample were tested using an X-ray diffractometer (D8-02, BrukerAXS, Karlsruhe, Germany). The morphology was tested using a transmission electron microscope (TEM: Tecnai G2 F20, FEI, Hillsboro, USA). The spectra of the samples were tested through the iHR550 grating spectrograph (iHR550, Horiba, Paris, France). The 980 nm laser used to excite the sample was purchased from the Beijing Kipling Photoelectric technology Co., Ltd, Beijing, China. (model: K980F14CC-10.00 W). For the thermometry experiments, we introduced a Linkam THMS 600 heating stage to heat the sample. Then the temperature of the sample was measured by thermocouple. The spectra of the sample at certain temperatures were acquired using the iHR550 grating spectrometer (iHR550, Horiba, Paris, France).

XRD Analysis
The XRD patterns of NaYF4: Er/Yb and NaYF4: Er/Tm/Yb phosphors are presented in Figure 1a. The XRD patterns of the samples can be indexed to hexagonal NaYF4 crystal (the JCPDS standard card no. , indicating that the dopants (Er, Tm and Yb ions) are successfully incorporated into the host lattice and do not cause significant changes to the crystal structure. Figure 1b

Temperature-Dependent Up-Conversion Luminescence
In order to analyze the variation law of up-conversion luminescence as the temperature changes, the emission spectra were obtained when the NaYF4: Er 3+ , Tm 3+ , Yb 3+ phosphors were heated by heating stage. The up-conversion spectra of the sample at different temperatures are displayed in Figure 5a and the temperature-dependent integrated intensity of the emissions located at different wavelengths are displayed in Figure  5b. As can be seen, most of the emissions radiated from the sample decrease with increasing temperature except for the emissions located at 520 and 692 nm. The reason for emission decreases is that the non-radiative transition increases with the increase of temperature. However, the emission increases at 520 and 692 nm are due to the thermal excitation from the adjacent lower energy levels ( 4 S3/2 → 2 H11/2 (Er 3+ )/ 3 H4 → 3 F2 (Tm 3+ )). To measure temperature via LIR, we chose the emissions located at 520 and 550 nm as the detected signals in the visible range and the emissions located at 650, 692 and 800 nm as the signals in the first biological window. For thermal coupled energy levels, the

Temperature-Dependent Up-Conversion Luminescence
In order to analyze the variation law of up-conversion luminescence as the temperature changes, the emission spectra were obtained when the NaYF 4 : Er 3+ , Tm 3+ , Yb 3+ phosphors were heated by heating stage. The up-conversion spectra of the sample at different temperatures are displayed in Figure 5a and the temperature-dependent integrated intensity of the emissions located at different wavelengths are displayed in Figure 5b. As can be seen, most of the emissions radiated from the sample decrease with increasing temperature except for the emissions located at 520 and 692 nm. The reason for emission decreases is that the non-radiative transition increases with the increase of temperature. However, the emission increases at 520 and 692 nm are due to the thermal excitation from the adjacent lower energy levels ( 4 S 3/2 → 2 H 11/2 (Er 3+ )/ 3 H 4 → 3 F 2 (Tm 3+ )). Figure 4. The slopes of the fitting line in the lnI-lnP plot represents the photons (n) participating in the up-conversion processes [25]. The values of n are 2.1, 1.9, 1.8, 1.5, 1.8, 1.5, 1.4 and 1.4 for 475, 520, 550, 650, 692 and 800 nm emissions, respectively. Therefore, the emission of blue comes from the three-photon process and the other emissions derive from the two-photon process. The participating photons, calculated from the slopes, are consistent with the up-conversion processes in Figure 3.

Temperature-Dependent Up-Conversion Luminescence
In order to analyze the variation law of up-conversion luminescence as the temperature changes, the emission spectra were obtained when the NaYF4: Er 3+ , Tm 3+ , Yb 3+ phosphors were heated by heating stage. The up-conversion spectra of the sample at different temperatures are displayed in Figure 5a and the temperature-dependent integrated intensity of the emissions located at different wavelengths are displayed in Figure  5b. As can be seen, most of the emissions radiated from the sample decrease with increasing temperature except for the emissions located at 520 and 692 nm. The reason for emission decreases is that the non-radiative transition increases with the increase of temperature. However, the emission increases at 520 and 692 nm are due to the thermal excitation from the adjacent lower energy levels ( 4 S3/2 → 2 H11/2 (Er 3+ )/ 3 H4 → 3 F2 (Tm 3+ )). To measure temperature via LIR, we chose the emissions located at 520 and 550 nm as the detected signals in the visible range and the emissions located at 650, 692 and 800 nm as the signals in the first biological window. For thermal coupled energy levels, the To measure temperature via LIR, we chose the emissions located at 520 and 550 nm as the detected signals in the visible range and the emissions located at 650, 692 and 800 nm as the signals in the first biological window. For thermal coupled energy levels, the relationship between luminescence intensity ratio (520 and 550 nm) and temperature can be mathematically expressed as follows [26,27] where I high and I low are the integrated intensities of the green emissions corresponding to the transition of high energy level to ground state ( 2 H 11/2 → 4 I 15/2 ) and low energy level to ground state ( 4 S 3/2 → 4 I 15/2 ). ∆E is the energy gap between high and low energy levels. k is the Boltzmann constant. C is a parameter related to the degeneracy, the radiative probabilities of the transitions and the angular frequency [26,27]. Using the integrated areas under the 520 and 550 nm bands and applying Equation (1), a perfect fit (R > 0.99) to the determined band intensity ratio was obtained. The temperature-dependent LIR of 520 and 550 nm (R 520/550 ) is shown in Figure 6a, in which the fitting function is R = 13.9exp(−1141/T). In order to compare the thermometry ability with other research, we calculated the relative sensing sensitivity (S r ) using the expression that follows [28] S r = 1 R dR dT (2) where Ihigh and Ilow are the integrated intensities of the green emissions corresponding to the transition of high energy level to ground state ( 2 H11/2 → 4 I15/2) and low energy level to ground state ( 4 S3/2 → 4 I15/2). E  is the energy gap between high and low energy levels. k is the Boltzmann constant. C is a parameter related to the degeneracy, the radiative probabilities of the transitions and the angular frequency [26,27]. Using the integrated areas under the 520 and 550 nm bands and applying Equation (1), a perfect fit (R > 0.99) to the determined band intensity ratio was obtained. The temperature-dependent LIR of 520 and 550 nm (R520/550) is shown in Figure 6a, in which the fitting function is R = 13.9exp(−1141/T). In order to compare the thermometry ability with other research, we calculated the relative sensing sensitivity (Sr) using the expression that follows [28] dT dR R S 1 r  (2) The temperature dependent Sr of R520/550 is shown in Figure 6b and the value of relative sensing sensitivity decreases as the temperature increases from 297 to 560 K. The maximum value reaches to 0.013 K −1 (297 K). In order to explore its thermometry ability in biological tissues, we studied the LIR of the emissions located in the first biological window (650, 692 and 800 nm). The temperature-dependent ratios of R692/650 and R692/800 are shown in Figure 7a. For the non-thermal coupled energy levels, the temperature-dependent ratios are fitted via the cubic function [29] and the fitting parameters are displayed in Table 1. To evaluate the sensing capacity, the relative sensing sensitivities are calculated through Expression (2) and the sensitivity curves are displayed in Figure 7b. The relative sensing sensitivities of R692/650 and R692/800 decrease as the temperature increases from 297 to 560 K. The maximum values reach to 0.027 and 0.028 K −1 (297 K), respectively. To compare the thermometry capacity of NaYF4: Er/Tm/Yb phosphors, the relevant parameters from other research are listed in Table 2. As can be seen, the sensing sensitivities of our samples are relatively high among these works based on thermal coupled levels and non-thermal coupled levels. The temperature dependent S r of R 520/550 is shown in Figure 6b and the value of relative sensing sensitivity decreases as the temperature increases from 297 to 560 K. The maximum value reaches to 0.013 K −1 (297 K).
In order to explore its thermometry ability in biological tissues, we studied the LIR of the emissions located in the first biological window (650, 692 and 800 nm). The temperaturedependent ratios of R 692/650 and R 692/800 are shown in Figure 7a. For the non-thermal coupled energy levels, the temperature-dependent ratios are fitted via the cubic function [29] and the fitting parameters are displayed in Table 1. To evaluate the sensing capacity, the relative sensing sensitivities are calculated through Expression (2) and the sensitivity curves are displayed in Figure 7b. The relative sensing sensitivities of R 692/650 and R 692/800 decrease as the temperature increases from 297 to 560 K. The maximum values reach to 0.027 and 0.028 K −1 (297 K), respectively. To compare the thermometry capacity of NaYF 4 : Er/Tm/Yb phosphors, the relevant parameters from other research are listed in Table 2. As can be seen, the sensing sensitivities of our samples are relatively high among these works based on thermal coupled levels and non-thermal coupled levels.

Conclusions
In summary, NaYF 4 : Er 3+ , Tm 3+ , Yb 3+ phosphors were prepared through the typical hydrothermal method. The up-conversion luminescence and temperature-dependent emissions were studied under 980 nm laser excitation. The slopes in the lnI-lnP plot are 2.1, 1.9, 1.8, 1.5, 1.8, 1.5, 1.4 and 1.4 for 475, 520, 550, 650, 692 and 800 nm emissions, respectively. This implies that the 475, 520, 550, 650, 692 and 800 nm emissions are three-photon, twophoton, two-photon, two-photon, two-photon and two-photon processes, respectively. Moreover, the thermal coupled energy level-and non-thermal coupled energy level-based LIR thermometry of the sample demonstrates that these two methods can be used to test temperature. The maximum relative sensing sensitivities of R 520/550 , R 692/650 and R 692/800 reach to 0.013, 0.027 and 0.028 K −1 at 297 K, respectively. The results reveal that Er/Tm/Yb co-doped NaYF 4 phosphors have great potential in LIR-based temperature sensing at room temperature. Meanwhile, the emissions for thermometry can alter from the visible range to the first biological window based on the actual requirements.  Data Availability Statement: The data are available from the corresponding author upon reasonable request.