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Article

Up-Conversion Luminescence and Optical Temperature Sensing Behaviour of Y2O3:Ho3+, Yb3+ Phosphors

1
Department of Physics, University of the Free State, Bloemfontein ZA-9300, South Africa
2
The State Key Laboratory of Luminescent Materials and Devices, School of Physics and Optoelectronic, South China University of Technology, Guangzhou 510641, China
*
Authors to whom correspondence should be addressed.
Crystals 2023, 13(8), 1288; https://doi.org/10.3390/cryst13081288
Submission received: 15 July 2023 / Revised: 12 August 2023 / Accepted: 15 August 2023 / Published: 21 August 2023

Abstract

:
The up-conversion (UC) and temperature sensing behaviours of Y2O3:Ho3+, Yb3+ phosphors were investigated. A series of Y2O3:Ho3+, Yb3+ phosphors were synthesized using a solution combustion method. The cubic structure of the Y2O3 with an Ia 3 ¯ space group was analysed by using X-ray powder diffraction. Scanning electron microscopy was conducted to study the surface morphologies of the UC phosphors. Under 980 nm excitation, the UC emissions of Ho3+ from the 5S25I8, 5F55I8 and 5S25I7 transitions were observed, which occurred through UC energy transfer (ET) processes. The Yb3+ ion concentration severely affected the UC emission. The sensing behaviour of the phosphor was investigated through the green (5F4, 5S25I8) to red (5F55I8) fluorescence intensity ratio (FIR). The maximum absolute and relative sensitivity values of SA = 0.08 K−1 and SR = 0.64% K−1 were obtained. The results revealed that the prepared Y2O3:Ho3+, Yb3+ phosphor is suitable for optical sensing at high temperatures.

1. Introduction

Lately, up-conversion (UC) materials based on lanthanide ions have been investigated extensively because of their potential application in colour displays, solar cells, optical communication, optical temperature sensors, and the lamp industry [1,2,3,4,5,6]. For efficient UC emission, Yb3+ has been extensively utilized as a sensitizer for trivalent lanthanide ions (Ln3+) doped into various hosts [7,8,9,10]. Yb3+ is an interesting ion that possesses only a 2F7/2 ground state and a 2F5/2 excited state that is set apart by approximately 10,000 cm−1. Yb3+ is usually excited with near-infrared (NIR) and transfers energy to an activator resulting in visible emission through the energy transfer up-conversion (ETU) process. The energy level matching between Ho3+ and Yb3+ ions makes the couple to be a great choice for UC investigations [11]. Dwivedi et al. [7] investigated the UC luminescence of Gd2O3:Ho3+, Yb3+ phosphor and obtained a strong green UC emission. The visible UC luminescence of Y2O2S:Er3+, Yb3+ phosphor has been reported to be 2.2 times less bright than that of Y2O2S:Ho3+, Yb3+ phosphor [8]. The Ho3+, Yb3+ co-doping has also been proven to be a highly efficient UC system by other researchers [12].
The host material is of great importance in UC emission intensity. For efficient UC output, the host should have low energy phonons to minimize the probability of multiphonon relaxations of the lanthanide ions. The cubic phase of Y2O3 host possesses low phonon energy (~550 cm−1), a wide bandgap, high melting point, and thermal stability that make it suitable for various phosphor applications [13,14]. Therefore, due to the matching ionic radii of Y3+ (0.900 Å), Ho3+ (0.901 Å), and Yb3+ (0.868 Å), Y2O3 may be considered as an ideal host for Ho3+-Yb3+ couple [15]. Pandey et al. [16] reported that the Yb3+ co-doping in Y2O3:Ho3+ enhanced the green emission intensity by nearly ~290 times. Wei et al. [17] investigated the UC luminescence strong dependence on concentration doping in Ho3+, Yb3+ co-doped Y2O3 and obtained a strong green emission that is applicable in biomedical fluorescent labels.
Research interest in optical temperature sensing with UC phosphors based on lanthanide ions is increasing in recent years. The fluorescence intensity ratio (FIR) between two emission bands, which calls for either thermally coupled or non-thermally coupled energy levels of the luminescent ions, is used in this. [18,19]. In the literature, it is reported that the FIR of green to red emission of Ho3+ ion can be identified as an optical temperature sensing indicator [20]. The FIR technique offered a high detection resolution and good sensitivity as it is typically independent of excitation-power fluctuation and spectrum loss. The FIR of thermally coupled energy levels (TCLs) (Ho3+:5F3/3K8) can be explained by a Boltzmann distribution of electrons [21,22]. According to the Boltzmann distribution, the temperature sensing sensitivity proportional to ΔE, can barely be improved further to a higher level since the energy separation ΔE of such levels is typically limited to 200–2000 cm−1 to avoid an intense overlap between the two emission bands [23]. For this reason, using non-thermally coupled energy levels (non-TCLs) (Ho3+:5F5/5F4, 5S2) is regarded as a useful addition to improve FIR sensitivity [19,21,23]. However, studies on the temperature sensing performance with the non-TCLs for Ho3+/Yb3+:Y2O3 phosphor are currently few. Wang et al. [24] investigated the performance of the optical temperature sensing based on non-TCLs of Ho3+ ion ((5F4/5S25I8)/(5F55I8)) and found a maximum absolute sensitivity of 0.1603 K−1. More deep investigations on the optical temperature sensing with non-TCLs of Ho3+ ion for Ho3+/Yb3+:Y2O3 phosphor are required.
In this work, Ho3+, Yb3+:Y2O3 up-conversion phosphors with various concentrations of Yb3+ were synthesized by the solution combustion method. The structural and optical properties were studied by X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), and UV-vis-NIR spectra measurements. The UC emission and temperature sensing behaviour of the Ho3+, Yb3+:Y2O3 were investigated. Power dependence measurements and an energy level diagram confirmed the phenomenon involved in the UC process. The green (5F4, 5S25I8) to red (5F55I8) FIR was used to analyse the behaviour of temperature sensing of the developed phosphor.

2. Experimental

2.1. Synthesis

The phosphor powder samples of Y2−x−yO3: Hox=0.005, Yby (y = 0, 0.002, 0.006, 0.01, 0.05, 0.1, 0.2) were prepared by a solution combustion method. Y(NO3)3·4H2O (Sigma Aldrich, Darmstadt, Germany, 99.99%), Ho(NO3)3·5H2O (Sigma Aldrich, 99.99%), Yb(NO3)3·5H2O (Sigma Aldrich, 99.99%), and CH4N2O (Sigma Aldrich, 99.5%) were starting materials. As an example, for y = 0.05, 1.6026 g of Y(NO3)3·4H2O, 0.0098 g of Ho(NO3)3·5H2O, 0.0994 g of Yb(NO3)3·5H2O, and 0.6690 g of CH4N2O were dissolved in 50 mL distilled water. The mixture was stirred using a magnetic stirrer in a 100 mL beaker for 30 min until a homogeneous solution was obtained. The solution was then heated at 500 ± 10 °C in a muffle furnace. The foam-like product was crushed using a pestle and mortar and was then annealed at 1100 °C in air for 2 h.

2.2. Characterization

The crystallinity and structure of the prepared samples were analysed by X-ray powder diffraction (XRPD) (Bruker AXS GmbH, Karlsruhe, Germany) using a Bruker D8 Advance diffractometer (40 mA, 40 kV) with Cu radiation (0.154 nm). The morphology of the samples was analysed by a scanning electron microscope (JEOL JSM-7800F) equipped with an energy dispersed X-ray spectroscopy (EDS) device (JEOL, Tokyo, Japan). The diffuse reflectance measurements were acquired using a Lambda 950 UV-vis-NIR spectrophotometer (PerkinElmer Ltd, Beaconsfield, United Kingdom) in the range of 200–1200 nm. The UC emissions and decay curves were recorded using an FLS980 fluorescence spectrometer with 980 nm emitting diode lasers as the excitation source and power of 96 mW.

3. Results and Discussion

3.1. Structural and Surface Morphology Analysis

XRPD patterns of the Y2−x−yO3:Hox=0.005, Yby phosphors are shown in Figure 1. The XRPD patterns agree with the standard reflection peaks reported in JCPDS# 71-0099 [25], and indicate the cubic structure with an Ia 3 ¯ space group of the Y2O3 crystal. The XRPD patterns of the Y2−x−yO3:Hox=0.005, Yby phosphors are shown in Figure 1. The introduction of Ho3+ and Yb3+ ions as co-dopants into Y2O3 did not change the structure type, but a small shift towards high angles in the diffraction reflection peaks was observed with increasing Yb concentration (Figure 1b). The shift is attributed to the ionic radius difference between the Yb3+ (0.868 Å) and Y3+ (0.900 Å) ions. Also, the peak shift verifies that Yb3+ are effectively incorporated into the Y3+ site [15].
The morphologies and elemental analyses of the prepared samples were studied using SEM and EDS, respectively. Figure 2 displays the SEM images of the prepared phosphors. All the images exhibited agglomerated spherical nanoparticles. The addition of Ho3+, Yb3+ did not change the morphology of the Y2O3 host. The EDS spectra of the Y2−x−yO3:Hox=0.005, Yby=0.2 are displayed in Figure 3. All the Y, O, Yb, and Ho elements expected in the sample were observed, which confirms the successful incorporation of Ho3+ and Yb3+ into the Y2O3 host. In addition, carbon was also detected in the spectrum, which could be from the carbon tape used when mounting the samples. The extra peak detected around 0.03 keV is due to electronic noise within the system. No impurities were detected in the spectra, which agrees with the XRD results.

3.2. Optical Properties

The reflectance spectra for the UC phosphors are presented in Figure 4. The absorption peak around 220 nm is associated with band-to-band transitions in the Y2O3 material [26]. The Ho3+ single doped Y2O3 exhibited sharp peaks centred at 361, 448, and 1008 nm due to 5I83H6, 3D2, 5I85G6 and 3S2, 5F45I6 transitions of the Ho3+ ion, respectively [27,28]. The Ho3+ 4f-4f absorptions in the UV-vis region remained unchanged when Yb3+ was added, which confirms that the Ho3+ ion concentration remained uniform. An additional absorption peak observed at 976 nm was observed with Yb3+ ion, which is associated with the 2F5/22F7/2 transition of the Yb3+ ion [28].
The optical bandgap of the Y2O3 was estimated from the Tauc plot [29], which is given by:
F R h v 1 n = A ( h v E g )
where A is a constant, h v is the photon energy, E g is the energy bandgap and n = 1 2 for the allowed direct bandgap of Y2O3. The diffuse reflectance, R , of the sample was used to calculate the Kubelka–Munk function F(R) [29]
F R = ( 1 R ) 2 2 R
Figure 5 shows the Tauc plot and estimated optical bandgap of pure Y2O3. The linear fit extrapolation of the curve to zero absorption gives the optical bandgap [30,31,32]. The bandgap of the Y2O3 was estimated to be 5.74 eV, which agrees with the reported value of 5.8 eV reported by Jones et al. [33]. The optical bandgaps of the Y2O3 singly and co-doped with Ho3+ and Yb3+ ions were calculated using the same procedure and are tabulated in Table 1. An alteration in the optical bandgap of the doped Y2O3 material was observed. It is known that introducing dopants into the Y2O3 lattice creates energy levels in the energy gap between the valence and conduction bands, which would decrease the optical band gap [34]. It is also known that the defects of the material, which are introduced during synthesis, can influence the bandgap value [35].

3.3. UC Luminescence Studies

Figure 6a presents UC emission spectra of the Y2−x−yO3: Hox=0.005, Yby nanophosphors excited using 980 nm excitation wavelength. The emission spectra are composed of four bands; the intense green, red, and near-infrared emission bands centred at 536, 550, 668, and 756 nm ascribed to the 5F45I8, 5S25I8, 5F55I8, and 5S25I7 transitions of the Ho3+ ions, respectively [36,37,38]. The increase in the Yb3+ concentration strongly enhanced the green emission. The maximal enhancement of green (5F45I8 and 5S25I8) emission was reached at y = 0.05 Yb3+ concentration, while, for higher concentrations, the emission intensity was then quenched, whereas the maximal enhancement of the red (5F55I8) and NIR emission (5S25I7) occurred at y = 0.1 Yb3+ concentration, as shown in Figure 6b. The reduction in UC intensity is attributed to a self-quenching effect [39,40]. It is known that the Yb3+ ion serves as a sensitizer of the Ho3+ ion in the UC emission, and thus, the Yb3+ ion efficiently transfers its energy to the Ho3+ ion [33]. According to the up-conversion process, the optimal concentrations of green (5S25I8) and NIR (5S25I7) luminescence should be the same as these two transitions have the same initial level 5S2. The reason behind the different optimal concentrations between these transitions is still unclear, therefore, further analysis is needed to obtain reliable conclusions. Pandey et al. [16] reported that Yb3+ co-doping enhances the intensity of UC emission. It was reported that green emission was enhanced by nearly ~290 times and red by approximately ~150 times. The Gd2O3:Ho3+, Yb3+ phosphor exhibited a strong green emission alongside additional UC emission bands [7].
The UC emission normally occurs through UC ET from the Yb3+ to the Ho3+ ones. The decay curves were acquired to further confirm the UC between the Yb3+–Ho3+ pair. Figure 7 displays the luminescence decay curves of the 5S25I8 transition (situated at 550 nm) recorded under the 980 nm excitation, where the intensity is plotted versus time. All the luminescence decay curves were fitted with a double exponential function [28]:
I = A 1 exp t τ 1 + A 2 exp ( t τ 2 )
where I is the intensity at time t , A 1 and A 2 are fitting constants, and τ 1 and τ 2 are the lifetimes. The average lifetimes ( τ a v e ) were determined using τ a v e = ( A 1 τ 1 2 + A 2 τ 2 2 ) / ( A 1 τ 1 + A 2 τ 2 ). The 5S2 level’s lifetime decreased as the Yb3+ concentration increased up to y = 0.05, but did not decrease further for higher doping concentrations, as presented in Figure 7b and Table 2. The decrease in the lifetime corresponds to the UC luminescence intensity improvement, which appears to indicate that it might be related to an efficient ET from Yb3+ to Ho3+. The trend in the lifetime for concentrations above y = 0.05 may be related to a self-quenching effect, as shown in Figure 6b.
The relation of UC emission intensity to the laser power for the co-doped phosphor was investigated to determine the number of excitation photons needed for each emission photon. Figure 8a represents the emission spectra of Y2−x−yO3:Hox=0.005, Yby=0.05 as a function of the laser power. The UC intensity continuously increased with laser power. The increase in laser power did not affect the peak position but enhanced the UC emission intensity for each of the observed bands (Figure 8b). The UC emission intensity relationship to laser power is modelled using the I α Pn [41] relation, where I, P, and n are the UC emission intensity, pump power, and the number of pump photons involved in the UC emission process, respectively. Figure 8c represents the ln(power) versus ln(intensity). These plots give straight lines with slopes (n values) of 1.69, 1.88, and 1.92 for green (536 and 550 nm) and red (668 nm) emissions, respectively. According to the energy level diagram (see Figure 9), 536 and 550 nm peaks (5F4, 5S25I8) and 756 nm peak (5S25I7) result from the same populated levels (5F4, 5S2) of Ho3+, and therefore, they have nearly same slopes. Pandey et al., [16] found n values of 2.28 and 1.91 for the green (550 nm) and red (668 nm) emissions. All the slope values are close to two, indicating the involvement of two NIR photons in all of the emission bands.
The energy transfer (ET) mechanism of the UC in the Ho3+, Yb3+:Y2O3 powders is illustrated in Figure 9. The Yb3+ ion is excited from the ground state 2F7/2 to the excited state 2F5/2 through the 980 nm excitation. The absorbed energy is then transferred via a non-radiative resonance ET to the nearby Ho3+ ion, exciting an electron from the 5I8 ground state to the 5I6 excited level. The excited ions in the 5I6 level are further excited to the 5S2, 5F4 level of the Ho3+ ion through energy transfer up-conversion (ETU). The populated 5S2, 5F4 levels relax to the 5I8 and 5I7 states radiatively and result in green emissions at 536 and 550 nm through the 5F45I8, 5S25I8 transitions, and NIR emission at 756 nm through the 5S25I7 transitions. Moreover, the relaxation process from the 5S2, 5F4 level may populate the 5F5 level via non-radiative multiphonon relaxation and result in a red emission at 668 nm through the radiative 5F55I8 transition [42,43]. The UC emission can also take place through cooperative energy transfer (CET) between two Yb3+ ions and one Ho3+ ion. The 5F4, 5S2 excited states of Ho3+ ions are situated twice of that Yb3+:2F5/2 excited state. In this case, two Yb3+ ions can transfer their energy cooperatively to one Ho3+ ion in the ground state in which the Ho3+ ion is promoted to the 5F4 and 5S2 excited states. All these emissions are two-photon processes.

3.4. Optical Temperature Sensing

To study the properties of optical temperature sensing of the sample, the temperature-dependent UC was analysed using the FIR technique. Figure 10a shows the temperature-dependent UC spectra of the Y2−x−yO3: Hox=0.005, Yby=0.05 phosphor, with the most intense green emission centred at 550 nm associated with the 5S25I8 transition of the Ho3+ ion. For all the UC emission bands, no significant shift in wavelength position was observed from room temperature up to 623 K. It is also evident that as the temperature rises, thermal quenching causes all the bands to progressively lose their intensity (Figure 10b).
Figure 11 presents the temperature dependence of the I536/I668, I550/I668 and (I536+I550)/I668 FIRs. Non-TCLs are often fitted by empirical functions due to the absence of quality physical models classifying temperature-dependences of the FIR parameters [44]. Therefore, FIR of the bands at 536, 550, and 668 nm can be fitted using the function:
F I R = I 2 I 1 = B 0 + B 1 T + B 2 T 2 + B 3 T 3
where I 1 and I 2 are the integrated emission intensities, B 0 ,   B 1 , B 2 ,   B 3 are fitting constants, and T is the absolute temperature, respectively. The fitting data are included in Figure 11.
For temperature sensing, the relative sensitivity (SR) and the absolute sensitivity (SA) are two indispensable parameters which were calculated using the equations [45]
S A = d F I R d T
S R = 1 F I R d F I R d T × 100 %
Figure 12a,b shows the absolute and relative sensitivities versus temperature. As the temperature increased, both the absolute sensitivity and the relative sensitivity decreased. All the values reached maxima at 303 K. The maximum SR values were determined to be 0.47 (I536/I668), 0.64 (I550/I668), and 0.59%K−1 ((I536 + I550)/I668), respectively, while the SA values were found to be 0.02 (I536/I668), 0.06 ((I536 + I550)/I668), and 0.08 K−1 ((I550/I668), respectively. The (I536 + I550)/I668 FIR had the largest SA, while I550/I668 had the highest SR values. The 5F5/5F4, 5S2 levels are non-thermally coupled energy levels, and thus, the temperature sensor sensitivity is substantially higher [21].
Table 3 compares the optical temperature sensing performance data of Ho, Yb co-doped various UC hosts. From Table 3, in this work Ho, Yb:Y2O3 noticeably show better sensitivity results than other UC host materials. These results differ because other reported ones were acquired at different measuring temperature ranges and the hosts show a temperature-dependent nature. Different host materials could also make a difference in sensitivity due to the different properties of the hosts. Moreover, some studies report on temperature sensors that are based on ratiometric measurements of the sensing of the 3K85I8 and 5F35I8 transitions (blue band emissions), which are due to three photon UC processes, and they typically need much higher power of excitation [46,47]. Additionally, other studies focus on TCLs that use the 5F45I8 and 5S25I8 transitions in the two green bands, which may not be the best method for temperature monitoring since the bands are spectrally overlapped [48,49,50]. The difference in sensitivity results could also result due to the different experimental conditions of the Ho3+, Yb3+ co-doped UC host materials. Therefore, the Ho, Yb:Y2O3 phosphor used in this study represents a better optical temperature sensing option.
To further study the efficiency of the optical high temperature sensor, the temperature resolution or uncertainty (δT) associated with FIRs used was calculated as described in [52]:
δ T = 1 S R δ F I R F I R
δ F I R represents the resolution/uncertainty of the FIR value. The temperature uncertainty ( δ T ) depends on the signal-to-noise ratio of the emission spectra and relative sensitivity (SR) [53]. The best temperature uncertainty determined at 303 K was for I536/I668 FIR, i.e., δ T = 0.71 K. For other FIRs, temperature uncertainty values were determined to be 0.51 K for I550/I668 and 0.57 K for (I536 + I550)/I668 at the same temperature, Figure 13. The temperature uncertainty at this temperature can be used for biological research [54]. As the temperature increased to 623 K, the temperature uncertainty reached maximum values at 7.61, 0.70, and 1.08 K for I536/I668, I550/I668 and (I536 + I550)/I668, respectively. The temperature uncertainty at 623 K results in sufficient optical temperature sensing. Moreover, such temperature uncertainty at high temperature could be potentially used in industrial applications, especially for processes which require high temperature conditions as well as other fields, i.e., submicron-scale resolution [44]. Moreover, stability and repeatability for optical temperature sensing are also important factors. Therefore, several heating-cooling cycling experiments were performed on the Y2−x−yO3:Hox=0.005, Yby=0.05 phosphor to verify the repeatability of the synthesized material, Figure 14a–c. This was achieved by studying the FIR changes in the I536/I668, I550/I668, and (I536 + I550)/I668 at 303 and 623 K, respectively. The deviation percentage of the FIR in the repeatability experiment was found to be 1.19%, 3.49%, and 3.25% for I536/I668, I550/I668, and (I536 + I550)/I668, respectively. After 10 heating-cooling repeated cycles, the FIR values showed no obvious difference and could be reversed and repeated. These results indicate that the FIR based on non-TCLs own good stability, indicating that Y2−x−yO3:Hox=0.005, Yby=0.05 phosphor has good signal reproducibility and is a capable candidate for optical temperature sensing.

4. Conclusions

The UC intensity and optical thermometry characteristics of Y2O3:Ho3+, Yb3+ were investigated. The XRPD results confirmed the cubic structure of Y2O3 in space group Ia 3 ¯ . The Yb3+ concentration did not change the agglomerated spherical nanoparticles morphology of the samples. From the diffuse reflectance spectra, the band gap decreased with increasing Yb3+ concentration. Under the 980 nm excitation wavelength, the UC luminescence showed four emission bands at 536, 550, 668, and 756 nm, assigned to the 5F45I8, 5S25I8, 5F55I8, and 5S25I7 transitions of the Ho3+ ions, respectively. The UC intensity had increased to a maximum value at y = 0.05 Yb3+ concentration. The UC process is ruled by a two-photon absorption process. The UC lifetime of the green emission (5S25I8) confirmed that the ET between the Yb3+ and Ho3+ plays a major role in the UC process. The optical temperature sensing behaviour was studied at a temperatures range of 303 K–623 K. The maximum absolute and relative sensitivity values were SA = 0.08 K−1 and SR = 0.64% K−1, respectively. The results reveal that Y2−x−yO3:Hox=0.005, Yby=0.05 is useful for optical high temperature sensors.

Author Contributions

Conceptualization, M.Y.A.Y. and V.M.; Validation, V.M. and R.E.K.; Formal analysis, V.M.; Investigation, M.Y.A.Y., Z.X. and H.C.S.; Writing—original draft, V.M.; Writing—review & editing, M.Y.A.Y., Z.X., H.C.S. and R.E.K.; Visualization, M.Y.A.Y. and R.E.K.; Supervision, M.Y.A.Y., H.C.S. and R.E.K.; Project administration, H.C.S.; Funding acquisition, H.C.S. All authors have read and agreed to the published version of the manuscript.

Funding

The South African Research Chairs Initiative of the Department of Science and Technology (84415), and the National Research Foundation of South Africa. The financial assistance from the University of the Free State is also highly recognized.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Experimental data are available upon reasonable request from the corresponding authors.

Acknowledgments

This work is based on research funded by the National Research Foundation of South Africa and the Department of Science and Technology’s South African Research Chairs Initiative (grant 84415). This acknowledges the financial support for this research provided by the National Research Foundation (NRF), CSIR-HCD IBS and the University of the Free State.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Figure 1. (a) XRPD patterns of the Y2O3 and Y2−x−yO3:Hox=0.005, Yby (y = 0, 0.006, 0.2) phosphors (b) magnified view of the 222 diffraction peak.
Figure 1. (a) XRPD patterns of the Y2O3 and Y2−x−yO3:Hox=0.005, Yby (y = 0, 0.006, 0.2) phosphors (b) magnified view of the 222 diffraction peak.
Crystals 13 01288 g001
Figure 2. SEM images of (a) the Y2O3, (b) Y2−x−yO3:Hox=0.005, Yby=0, (c) Y2−x−yO3:Hox=0.005, Yby=0.006 and (d) Y2−x−yO3:Hox=0.005, Yby=0.2.
Figure 2. SEM images of (a) the Y2O3, (b) Y2−x−yO3:Hox=0.005, Yby=0, (c) Y2−x−yO3:Hox=0.005, Yby=0.006 and (d) Y2−x−yO3:Hox=0.005, Yby=0.2.
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Figure 3. EDS spectra of the Y2−x−yO3:Hox=0.005, Yby=0.2.
Figure 3. EDS spectra of the Y2−x−yO3:Hox=0.005, Yby=0.2.
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Figure 4. Diffuse reflectance spectra of the Y2−x−yO3:Hox=0.005, Yby UC phosphors.
Figure 4. Diffuse reflectance spectra of the Y2−x−yO3:Hox=0.005, Yby UC phosphors.
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Figure 5. Tauc plot of pure Y2O3.
Figure 5. Tauc plot of pure Y2O3.
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Figure 6. (a) UC emission spectra of Y2−x−yO3: Hox=0.005, Yby powders and (b) green (5S25I8), red (5F55I8,), and NIR (5S25I7) emission intensity peak as a function of y.
Figure 6. (a) UC emission spectra of Y2−x−yO3: Hox=0.005, Yby powders and (b) green (5S25I8), red (5F55I8,), and NIR (5S25I7) emission intensity peak as a function of y.
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Figure 7. (a) The fitted decay curves of Ho3+, Yb3+ co-doped Y2O3 phosphors for different concentrations of the Yb3+ ions under the 980 nm excitation and (b) lifetime as a function of y.
Figure 7. (a) The fitted decay curves of Ho3+, Yb3+ co-doped Y2O3 phosphors for different concentrations of the Yb3+ ions under the 980 nm excitation and (b) lifetime as a function of y.
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Figure 8. (a) Variation in the UC emission intensity of the Y2−x−yO3:Hox=0.005, Yby=0.05 as a function of laser power, (b) power dependences of the UC intensities for various bands, and (c) dependences of peak intensity of the green and red UC emissions as a function of pump power.
Figure 8. (a) Variation in the UC emission intensity of the Y2−x−yO3:Hox=0.005, Yby=0.05 as a function of laser power, (b) power dependences of the UC intensities for various bands, and (c) dependences of peak intensity of the green and red UC emissions as a function of pump power.
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Figure 9. UC energy level mechanism in the Ho3+, Yb3+: Y2O3 nanophosphor powders.
Figure 9. UC energy level mechanism in the Ho3+, Yb3+: Y2O3 nanophosphor powders.
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Figure 10. (a) UC emission of Y2−x−yO3:Hox=0.005, Yby=0.05 at different temperatures, (b) UC intensity versus temperature.
Figure 10. (a) UC emission of Y2−x−yO3:Hox=0.005, Yby=0.05 at different temperatures, (b) UC intensity versus temperature.
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Figure 11. FIR of I536/I668, I550/I668 and (I536+I550)/I668 versus temperature.
Figure 11. FIR of I536/I668, I550/I668 and (I536+I550)/I668 versus temperature.
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Figure 12. (a) Absolute sensitivity as a function of temperature and (b) relative sensitivity as a function of temperature.
Figure 12. (a) Absolute sensitivity as a function of temperature and (b) relative sensitivity as a function of temperature.
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Figure 13. Temperature uncertainty versus the temperature of the Y2−x−yO3:Hox=0.005, Yby=0.05 phosphor.
Figure 13. Temperature uncertainty versus the temperature of the Y2−x−yO3:Hox=0.005, Yby=0.05 phosphor.
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Figure 14. Repeatability test of (a) I536/I668, (b) I550/I668, and (c) (I536 + I550)/I668 measured by alternating the temperature between 303 and 623 K.
Figure 14. Repeatability test of (a) I536/I668, (b) I550/I668, and (c) (I536 + I550)/I668 measured by alternating the temperature between 303 and 623 K.
Crystals 13 01288 g014aCrystals 13 01288 g014b
Table 1. Optical bandgap of Y2−x−yO3:Hox=0.005, Yby UC phosphors.
Table 1. Optical bandgap of Y2−x−yO3:Hox=0.005, Yby UC phosphors.
SampleBandgap (eV)
Y2O35.74
Y2−x−yO3:Hox=0.0055.72
Y2−x−yO3:Hox=0.005, Yby=0.0065.68
Y2−x−yO3:Hox=0.005, Yby=0.25.61
Table 2. The lifetime of the 5S25I8 transition of the Ho3+ ion of the Y2−x−yO3:Hox=0.005, Yby at different Yb3+ concentration.
Table 2. The lifetime of the 5S25I8 transition of the Ho3+ ion of the Y2−x−yO3:Hox=0.005, Yby at different Yb3+ concentration.
SampleAverage Lifetime (µs)
Y2−xO3:Hox=0.005254
Y2−x−yO3:Hox=0.005, Yby=0.002241
Y2−x−yO3:Hox=0.005, Yby=0.006226
Y2−x−yO3:Hox=0.005, Yby=0.01223
Y2−x−yO3:Hox=0.005, Yby=0.0592
Y2−x−yO3:Hox=0.005, Yby=0.199
Y2−x−yO3:Hox=0.005, Yby=0.2121
Table 3. Maximal SA and SR values of different Ho3+, Yb3+ co-doped UC hosts.
Table 3. Maximal SA and SR values of different Ho3+, Yb3+ co-doped UC hosts.
SampleTransitionsTemp Range (K)Max SA (K−1)Max SR (%K−1)Ref.
Ho, Yb: Y2O35F4, 5S25I8, 5F55I8
5S25I8, 5F55I8
303–6230.08
-
-
0.64
This work
Ho, Yb: LuYO35F4, 5S25I8, 5F55I8298–5780.16030.0102[24]
Ho, Yb: NaLuF45F2,3,5K85I8, 5G6, 5F15I8390–7800.00080.83[46]
Ho, Yb: CaWO45F2,3,5K85I8, 5G6, 5F15I8303–9230.00500.28[47]
Ho, Yb: NaYF45F4, 5S25I8313–3930.00380.7230[48]
Ho, Yb: Y2O35F4, 5S25I8293–5630.0071-[49]
Ho, Yb: Y2O35F4, 5S25I8348–5980.0130.622[50]
Ho, Yb: Y2O35F4, 5S25I8, 5S2, 5F45I70–3000.00971.90[51]
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Makumbane, V.; Yagoub, M.Y.A.; Xia, Z.; Kroon, R.E.; Swart, H.C. Up-Conversion Luminescence and Optical Temperature Sensing Behaviour of Y2O3:Ho3+, Yb3+ Phosphors. Crystals 2023, 13, 1288. https://doi.org/10.3390/cryst13081288

AMA Style

Makumbane V, Yagoub MYA, Xia Z, Kroon RE, Swart HC. Up-Conversion Luminescence and Optical Temperature Sensing Behaviour of Y2O3:Ho3+, Yb3+ Phosphors. Crystals. 2023; 13(8):1288. https://doi.org/10.3390/cryst13081288

Chicago/Turabian Style

Makumbane, Vhahangwele, Mubarak Y. A. Yagoub, Zhiguo Xia, Robin E. Kroon, and Hendrik C. Swart. 2023. "Up-Conversion Luminescence and Optical Temperature Sensing Behaviour of Y2O3:Ho3+, Yb3+ Phosphors" Crystals 13, no. 8: 1288. https://doi.org/10.3390/cryst13081288

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