The Upconversion Luminescence of Ca3Sc2Si3O12:Yb3+,Er3+ and Its Application in Thermometry

To develop novel luminescent materials for optical temperature measurement, a series of Yb3+- and Er3+-doped Ca3Sc2Si3O12 (CSS) upconversion (UC) phosphors were synthesized by the sol–gel combustion method. The crystal structure, phase purity, and element distribution of the samples were characterized by powder X-ray diffraction and a transmission electron microscope (TEM). The detailed study of the photoluminescence emission spectra of the samples shows that the addition of Yb3+ can greatly enhance the emission of Er3+ by effective energy transfer. The prepared Yb3+ and Er3+ co-doped CSS phosphors exhibit green emission bands near 522 and 555 nm and red emission bands near 658 nm, which correspond to the 2H11/2→4I15/2, 4S3/2→4I15/2, and 4F9/2→4I15/2 transitions of Er3+, respectively. The temperature-dependent behavior of the CSS:0.2Yb3+,0.02Er3+ sample was carefully studied by the fluorescence intensity ratio (FIR) technique. The results indicate the excellent sensitivity of the sample, with a maximum absolute sensitivity of 0.67% K−1 at 500 K and a relative sensitivity of 1.34% K−1 at 300 K. We demonstrate here that the temperature measurement performance of FIR technology using the CSS:Yb3+,Er3+ phosphor is not inferior to that of infrared thermal imaging thermometers. Therefore, CSS:Yb3+,Er3+ phosphors have great potential applications in the field of optical thermometry.


Introduction
As a promising remote temperature measurement method, fluorescence intensity ratio (FIR) technology has attracted extensive attention because of its wide temperature response range, high sensitivity, fast response, and submicron measurement scale compared with traditional contact temperature measurement technology [1][2][3][4][5][6][7][8]. In addition, it can be used in highly corrosive, high-pressure, and internal biological tissues [9]. For instance, Vetrone et al. [10] devised a novel nanothermometer, capable of accurately determining the temperature of biological systems such as HeLa cancer cells. The nanothermometer is based on the temperature-sensitive fluorescence of NaYF 4 :Er 3+ ,Yb 3+ nanoparticles, where the intensity ratio of the green fluorescence bands of Er 3+ ions changes with temperature. Following incubation of the nanoparticles with HeLa cervical cancer cells and their subsequent uptake, the fluorescent nanothermometer measured the internal temperature of the living cells from 25 • C to 45 • C under the excitation of a 920 nm laser. Optical thermometry based on FIR technology has been widely studied in rare-earth-doped upconversion (UC) phosphors. Generally, FIR technology is realized through the temperature dependence of thermally coupled energy levels (TCL) such as the 2 H 11/2 and 4 S 3/2 levels of Er 3+ , the 3 F 2, 3 and 3 H 4 levels of Tm 3+ , and the 5 F 4 and 5 S 2 levels of Ho 3+ [11][12][13]. Among them, Er 3+ is usually used as a luminescence center to detect temperature due to the typical was improved by using Ce 3+ as a sensitizer. Under 350 mA current driving, the NIR LED prepared with CSS:0.06Ce 3+ ,0.03Cr 3+ and a 450 nm blue chip achieved an output power of 21.65 mW and demonstrated excellent human tissue penetration ability. However, the temperature sensing performance of a Yb 3+ -and Er 3+ -doped Ca 3 Sc 2 Si 3 O 12 phosphor has not been reported and brought to attention so far.
In this study, a CSS:Yb 3+ ,Er 3+ phosphor was prepared by the sol-gel combustion method. The lattice structure, UC emission characteristics, and temperature sensing performance under the 980 nm excitation of the samples were studied in detail. By monitoring the temperature-dependent luminescence behaviors of the 2 H 11/2 and 4 S 3/2 energy levels of Er 3+ , it was found that the maximum absolute sensitivity of the CSS:Yb 3+ ,Er 3+ phosphor is 0.67% K −1 at 500 K and the maximum relative sensitivity is 1.34% K −1 at 300 K based on FIR technology, which can be compared with the reported Yb 3+ -and Er 3+ -doped phosphors. The results show that the novel CSS:Yb 3+ ,Er 3+ phosphor has great potential for application in optical temperature sensors. , and Na 2 CO 3 (A.R.), were added to an evaporating dish. Appropriate amounts of urea and ethanol were added to the reaction solution and heated to 65 • C on a heating agitator, evaporated overnight to remove excess water until a transparent sol was obtained. After that, the temperature was increased to 95 • C and maintained for several hours to obtain a dry gel. The obtained gel was heated from room temperature to 700 • C in the air in a muffle furnace at a heating rate of 5 • C/min and kept for 3 h at 700 • C and then naturally cooled to room temperature to obtain the precursor. After being ground, the precursor was transferred to a corundum crucible. It was heated from room temperature to 1400 • C in the air at a heating rate of 5 • C/min and kept for 6 h at 1400 • C. After naturally cooling to room temperature, the synthesized product was ground into powder and collected for further measurements. SYLGARD 184 (Dow Corning, including the base components and a curing agent with a weight ratio of 10:1) was stirred with the 5 wt% CSS:0.2Yb 3+ ,0.02Er 3+ phosphor at room temperature for 30 min and then heated in an oven at 80 • C for 1 h to encapsulate the CSS:0.2Yb 3+ ,0.02Er 3+ phosphor in solidified poly(dimethylsiloxane) (PDMS).

Materials and Methods
Powder X-ray diffraction (XRD) measurements were performed using a Rigaku D-max 2200 X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation at 40 kV and 26 mA. Diffuse reflectance spectroscopy (DRS) was measured by a Cary 5000 UV Vis NIR spectrophotometer (Agilent, Santa Clara, CA, USA) produced by the Varian company in the United States, and BaSO 4 was used as the standard.
The morphology of the as-prepared sample was observed by scanning electron microscopy (SEM, Quanta 400 F, FEI Company, Hillsboro, OR, USA). Transmission electron microscopy (TEM) and element mapping analysis were carried out on a Tecnai G2 F30 instrument (FEI Company, Hillsboro, OR, USA). The photoluminescence emission (PL) spectrum was measured by a FLS 980 time-resolved and steady-state fluorescence spectrometer (Edinburgh Instruments, Livingston, UK). The light source was a 980 nm laser, and the temperature controller was an Oxford OptistatDN liquid nitrogen temperature control system (Oxford Instruments, Oxford, UK). The temperature measurement of the CSS:0.2Yb 3+ ,0.02Er 3+ /PDMS composite was carried out by an infrared thermal imager (FLIR ONE Pro, Teledyne FLIR, Wilsonville, OR, USA) and fiber spectrophotometer (QE pro, Ocean Insight, Orlando, FL, USA).

Results and Discussion
As shown in Figure 1a, the crystal structure of Ca 3 Sc 2 Si 3 O 12 belongs to a cubic crystal system with the space group Ia−3d (No. 230). The lattice parameters are a = 12. 25

Results and Discussion
As shown in Figure 1a, the crystal structure of Ca3Sc2Si3O12 belongs to a cubic crystal system with the space group Ia−3d (No. 230 The powder XRD patterns of the host, Yb 3+, and Er 3+ singly doped and co-doped CSS samples are shown in Figure 1b. The diffraction patterns of all samples are in good agreement with the standard reference data of Ca3Sc2Si3O12 (ICDD #72−1969), and no obvious impurity phase is observed, indicating that Yb 3+ and Er 3+ are successfully doped into the lattice of Ca3Sc2Si3O12 without obvious changes in the crystal structure. According to the SEM image (Figure S1), the particle size of the material is approximately 20 µm. The high-resolution TEM image of the sample is shown in Figure 1c, and the clear lattice fringes indicate the high crystallinity of the sample. The measured d-spacing value of the (321) plane is 0.3202 nm, which is very close to the theoretical value of 0.3274 nm. The element mapping images of the CSS:0.2Yb 3+ ,0.02Er 3+ sample are shown in The powder XRD patterns of the host, Yb 3+, and Er 3+ singly doped and co-doped CSS samples are shown in Figure 1b. The diffraction patterns of all samples are in good agreement with the standard reference data of Ca 3 Sc 2 Si 3 O 12 (ICDD #72−1969), and no obvious impurity phase is observed, indicating that Yb 3+ and Er 3+ are successfully doped into the lattice of Ca 3 Sc 2 Si 3 O 12 without obvious changes in the crystal structure.
According to the SEM image ( Figure S1), the particle size of the material is approximately 20 µm. The high-resolution TEM image of the sample is shown in Figure 1c, and the clear lattice fringes indicate the high crystallinity of the sample. The measured d-spacing value of the (321) plane is 0.3202 nm, which is very close to the theoretical value of 0.3274 nm. The element mapping images of the CSS:0.2Yb 3+ ,0.02Er 3+ sample are shown in Figure 1d-j. It can be observed that all the elements are evenly distributed in the sample particle, which further confirms the successful doping of Yb 3+ and Er 3+ in the CSS host. Figure 2a shows the UV-vis diffuse reflectance spectra of CSS:0.2Yb 3+ ,0.02Er 3+ . There is a strong absorption band at about 980 nm, which is mainly attributed to the 2 F 7/2 → 2 F 5/2 transition of Yb 3+ . The absorption peaks at 380, 449, 486, 523, 647, and 1523 nm are caused by the electronic transitions from 4 I 15/2 to 4 G 11/2 , 4 F 3/2 , 4 F 7/2 , 2 H 11/2 , 4 F 9/2 , and 2 I 13/2 of Er 3+ , respectively. Nanomaterials 2023, 13, x FOR PEER REVIEW 5 of 10 Figure 1d-j. It can be observed that all the elements are evenly distributed in the sample particle, which further confirms the successful doping of Yb 3+ and Er 3+ in the CSS host. Figure 2a shows the UV-vis diffuse reflectance spectra of CSS:0.2Yb 3+ ,0.02Er 3+ . There is a strong absorption band at about 980 nm, which is mainly attributed to the 2 F7/2→ 2 F5/2 transition of Yb 3+ . The absorption peaks at 380, 449, 486, 523, 647, and 1523 nm are caused by the electronic transitions from 4 I15/2 to 4 G11/2, 4 F3/2, 4 F7/2, 2 H11/2, 4 F9/2, and 2 I13/2 of Er 3+ , respectively. The UC emission spectra of the CSS:xYb 3+ ,0.02Er 3+ (x = 0, 0.01, 0.02, 0.05, 0.1, and 0.2) samples excited by the 980 nm laser are shown in Figure 2b. The emission peaks at 522, 555, and 658 nm are attributed to the characteristic transitions of 2 H11/2, 4 S3/2, and 4 F9/2→ 4 I15/2 of Er 3+ , respectively. With the increase in the Yb 3+ concentration, the UC emission of the sample is obviously enhanced. This is because the absorption cross-section of Yb 3+ is larger than that of Er 3+ in the near-infrared region around 980 nm, which can absorb more excitation energy and transfer it to Er 3+ . In addition, there is no concentration quenching in the concentration range of this study.
In order to determine the mechanism of UC luminescence, the UC emission spectra of the CSS:0.2Yb 3+ ,0.02Er 3+ sample at different pump power values were characterized, as shown in Figure 2d. Obviously, the UC emission intensity of the sample increases with the increase in the pump power. As a nonlinear process, the emission intensity (I) of the The UC emission spectra of the CSS:xYb 3+ ,0.02Er 3+ (x = 0, 0.01, 0.02, 0.05, 0.1, and 0.2) samples excited by the 980 nm laser are shown in Figure 2b. The emission peaks at 522, 555, and 658 nm are attributed to the characteristic transitions of 2 H 11/2 , 4 S 3/2, and 4 F 9/2 → 4 I 15/2 of Er 3+ , respectively. With the increase in the Yb 3+ concentration, the UC emission of the sample is obviously enhanced. This is because the absorption cross-section of Yb 3+ is larger than that of Er 3+ in the near-infrared region around 980 nm, which can absorb more excitation energy and transfer it to Er 3+ . In addition, there is no concentration quenching in the concentration range of this study.
As shown in Figure 2c, with the increase in Yb 3+ concentration, the green and red emissions of the CSS:xYb 3+ ,0.02Er 3+ samples monotonically increase, while the red/green (R/G) ratio also shows an upward trend, which is similar to the reports of other Yb 3+ /Er 3+ codoped phosphors [24][25][26]. Due to the cross-relaxation (CR) of 2 H 11/2 / 4 S 3/2 (Er 3+ ) + 2 F 7/2 (Yb 3+ )→ 4 I 13/2 (Er 3+ ) + 2 F 5/2 (Yb 3+ ) ( Figure S2), the green UC emission is inhibited. Conversely, the population at the 4 I 13/2 state is increased by this CR process and the red emission is then enhanced through energy transfer, as discussed in detail in Figure 2f. Therefore, the R/G ratio rose with the increase in the Yb 3+ concentration.
In order to determine the mechanism of UC luminescence, the UC emission spectra of the CSS:0.2Yb 3+ ,0.02Er 3+ sample at different pump power values were characterized, as shown in Figure 2d. Obviously, the UC emission intensity of the sample increases with the increase in the pump power. As a nonlinear process, the emission intensity (I) of the UC emission should increase in proportion to the pump power (P) of the excitation source, which can be expressed as [27][28][29]: where n is the number of pump photons required to excite the luminescence center from the ground state to the excited state. Figure 2e shows the natural logarithm curves of the integrated emission intensity of the green emission (510-575 nm) and the red emission (630-695 nm) of the CSS:0.2Yb 3+ ,0.02Er 3+ sample at different pump power values. The curves were linearly fitted, and the n values were 1.68 and 1.42, respectively, indicating that the UC emissions of the phosphor are all derived from the two-photon process.
To explore the temperature sensing characteristics of the prepared sample, the temperaturedependent UC emission spectra normalized at 555 nm of CSS:0.2Yb 3+ ,0.02Er 3+ in the range of 300-500 K under 980 nm excitation was measured, as shown in Figure 3a. The corresponding contour map is illustrated in Figure 3b. Obviously, the position of the emission peaks of CSS:0.2Yb 3+ ,0.02Er 3+ does not change with the increase in temperature. However, the emission intensity at 522 nm increases significantly with increasing temperature.  Absolute sensitivity (Sa) and relative sensitivity (Sr) are two important parameters that characterize the temperature measurement capability of a material, which are defined as follows [35,36]: Therefore, according to the Boltzmann distribution law, the FIR of the two thermally coupled levels 2 H 11/2 and 4 S 3/2 can be expressed as [32][33][34]: where B = g H ω H A H /g s ω s A s , the subscripts H and S represent the thermally coupled 2 H 11/2 and 4 S 3/2 energy levels, respectively, and I, N, ω, g, and A are defined as the UC emission intensity, population number, frequency, degeneracy, and spontaneous radiation transition rate, respectively. k is the Boltzmann constant. ∆E is the energy gap between the 2 H 11/2 and 4 S 3/2 states, and T is the absolute temperature. Equation (2) can also be simplified to the form of a linear equation, as shown below: (3) Figure 3c shows the relationship between the Ln(FIR) and 1/T of the CSS:0.2Yb 3+ ,0.02Er 3+ in the temperature range of 300-500 K. The fitting degree of the experimental data is 0.9993, indicating a good linear fitting result. The slope of the fitted line is −∆E/k = −1209.3. Therefore, the calculated ∆E = 840 cm −1 , which is in line with the theoretical energy gap between the 2 H 11/2 and 4 S 3/2 energy levels of Er 3+ . Figure 3d shows the change in FIR with different temperatures. The coefficient B value obtained by fitting the experimental data is 15.63.
Absolute sensitivity (S a ) and relative sensitivity (S r ) are two important parameters that characterize the temperature measurement capability of a material, which are defined as follows [35,36]: As shown in Figure 3e, S a gradually increases with the increasing temperature, and the maximum value is determined to be 0.67% K −1 at 500 K. However, S r shows a monotonous downward trend with the increasing temperature, and the maximum value is determined to be 1.34% K −1 at 300 K. By comparing S a and S r with other temperature sensing materials doped with Yb 3+ and Er 3+ (Table 1), we can find that CSS:0.2Yb 3+ ,0.02Er 3+ phosphor has superior temperature sensing performance and potential application prospects. The minimum temperature resolution obtained with this experimental setup is 1.03 K at 300 K, as shown in Figure S3, indicating that it still needs further optimization. In addition, the temperature sensor must also have excellent repeatability, so that it can be used repeatedly without performance degradation. Figure 3f shows the temperature dependence of the FIR values of CSS:0.2Yb 3+ ,0.02Er 3+ for five heating-cooling cycles. It can be observed that the FIR values are almost completely reversible during repeated heating (300→500 K) and cooling (500→300 K) processes, indicating the excellent thermal repeatability of the CSS:Yb 3+ ,Er 3+ phosphor.
To verify the actual temperature measurement performance, the CSS:0.2Yb 3+ ,0.02Er 3+ phosphor was encapsulated in PDMS and heated and simultaneously measured by FIR technology and infrared thermal imaging (IRT) temperature measurement technology, as shown in Figure 4a. The results measured by IRT technology are shown in Figure 4b. The emission spectra of the phosphor under the corresponding temperature and 980 nm excitation are shown in Figure 4c. The FIR values were obtained from the spectra and the corresponding temperatures were calculated using Equation (3). The results measured by these two techniques at different temperature points were compared, as shown in Figure 4d. The temperature difference in the range of 300-350 K did not exceed 3.45 K, indicating that the CSS:0.2Yb 3+ ,0.02Er 3+ phosphor had an accurate temperature measurement capability. The temperature difference in the range of 350-425 K increased, probably because the heater heated up too quickly and the two techniques had different measurement speeds. In addition, the IRT technique might have a higher measurement error at high temperatures.

Conclusions
In summary, a series of CSS:xYb 3+ ,0.02Er 3+ phosphors with excellent UC luminescence properties were synthesized by the sol-gel combustion method. Under 980 nm laser excitation, the green emission bands at 522 and 555 nm and the red emission band at 658 nm in the emission spectrum of the CSS:xYb 3+ ,0.02Er 3+ phosphors can be observed. In addition, Yb 3+ can effectively sensitize Er 3+ to increase the emission intensity, and the energy transfer mechanism was revealed. The temperature sensing behavior of the CSS:0.2Yb 3+ ,0.02Er 3+ phosphor was studied by FIR technology. The results show that the maximum absolute sensitivity is 0.67% K −1 at 500 K and the relative sensitivity is 1.34% K −1 at 300 K. The temperature measurement performance of FIR technology using the CSS:Yb 3+ ,Er 3+ phosphor was comparable to that of IRT technology, indicating that this material is expected to be used in optical thermometry.

Conclusions
In summary, a series of CSS:xYb 3+ ,0.02Er 3+ phosphors with excellent UC luminescence properties were synthesized by the sol-gel combustion method. Under 980 nm laser excitation, the green emission bands at 522 and 555 nm and the red emission band at 658 nm in the emission spectrum of the CSS:xYb 3+ ,0.02Er 3+ phosphors can be observed. In addition, Yb 3+ can effectively sensitize Er 3+ to increase the emission intensity, and the energy transfer mechanism was revealed. The temperature sensing behavior of the CSS:0.2Yb 3+ ,0.02Er 3+ phosphor was studied by FIR technology. The results show that the maximum absolute sensitivity is 0.67% K −1 at 500 K and the relative sensitivity is 1.34% K −1 at 300 K. The temperature measurement performance of FIR technology using the CSS:Yb 3+ ,Er 3+ phosphor was comparable to that of IRT technology, indicating that this material is expected to be used in optical thermometry.