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Abstract
Achieving single-band upconversion (UC) is a challenging but rewarding approach to attain optimal performance in diverse applications. In this paper, we successfully achieved single-band red UC luminescence in Yb2O3: Er transparent ceramics (TCs) through the utilization of a sensitizer-rich design. The Yb2O3 host, which has a maximum host lattice occupancy by Yb3+ sensitizers, facilitates the utilization of excitation light and enhances energy transfer to activators, resulting in improved UC luminescence. Specifically, by shortening the ionic spacing between sensitizer and activator, the energy back transfer and the cross-relaxation process are promoted, resulting in weakening of green energy level 4S3/2 and 2H11/2 emission and enhancement of red energy level 4F9/2 emission. The prepared Yb2O3: Er TCs exhibited superior optical properties with in-line transmittance over 80% at 600 nm. Notably, in the 980nm-excited UC spectrum, green emission does not appear, thus Yb2O3: Er TCs exhibit ultra-pure single band red emission, with CIE coordinates of (0.72, 0.28) and color purity exceeding 99.9%. To the best of our knowledge, this is the first demonstration of pure red UC luminescence in TCs. Furthermore, the luminescent intensity ratio (LIR) technique was utilized to apply this pure red-emitting TCs for temperature sensing. The absolute sensitivity of Yb2O3: Er TCs was calculated to be 0.319% K-1 at 304 K, which is the highest level of optical thermometry based on 4F9/2 levels splitting of Er3+ known so far. The integration between pure red UC luminescence and temperature sensing performance opens up new possibilities for the development of multi-functional smart windows.
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1. Introduction
Since discovered by Auzel in 1966, upconversion luminescence (UCL) which is capable of converting near-infrared (NIR) photons into visible emission, has gained considerable attention [1–4]. Due to superior properties such as high chemical stability, excellent photostability, sharp emission band, and weak background autofluorescence, UCL materials in diverse forms have been extensively investigated for various applications, including solid state laser, optical sensors, lighting, solar cell, high-resolution displays, and more [5–9]. In the past decades, Yb3+ co-doped with Er3+, Tm3+, and Ho3+ etc. systems remain the key constituents for the majority of red, green, and blue UCL material [2,10]. Typically, Yb3+ act as the sensitizer in UC process by virtue of the large absorption cross-sections around 980 nm, which match well with commercial GaAs lasers. Er3+ is commonly selected as the activator due to its high UC efficiency and emission tuning properties. Due to the abundant excitation energy levels of Er3+, Er3+/Yb3+-doped UC materials generally exhibit multiple emission bands with green emission at ∼520-550 nm and red emission at ∼660 nm. While some applications benefit from this multi-band UC emission characteristics, such as the realization of white UC light and the acquisition of multicolor UC, other applications, like color display [5], security coding [11], and multiplexed molecular imaging [12], require the utilization of single-band pure UC light for the refinement or determination of the emitted color [13,14]. Additionally, considering that multi-band emissions result in the allocation of available energy into multiple frequencies, single-band emission is preferred when a high conversion efficiency is required in photonic devices, such as in nonlinear photonic devices [5]. Specifically, the NIR region of 650-1000 nm is typically considered the first biological window, as it exhibits weak tissue absorption, light scattering, and autofluorescence [15]. For example, in bio-imaging applications, short wavelength emission bands outside the biological window (λ < 650 nm) can interfere with the bio-imaging signals, resulting in reduced spatial resolution and sensitivity [16]. The red UC emission of Yb/Er system, located in the first biological window (BW-I), has a stronger penetration ability compared to green UC emission, which effectively minimizes background interference and improves detection efficiency [17]. Hence, tuning both emission and excitation bands of UC into the biological window will improve the anti-interference capabilities of UCL devices to the external environment [18,19].
To attain pure red UCL in Er3+-doped materials, many efforts have been made to design single-band emission UC materials of high purity. One common approach is doping ions with suitable energy levels, such as Mn2+. For example, Tian et al. obtained single-band red UC from NaYF4:Er3+/Yb3+ nanoparticles via Mn2+ doping with effective energy transfer (ET) occurring between Mn2+ and Er3+ ions [20]. Similar results have been achieved using Mn2+-containing hosts such as NaMnF3, KMnF3, and MnF2 [21–23]. However, owing to the considerable changes in valence state and ionic radius between the Ln3+ and Mn2+ ions, undesirable modifications to the size and phase of NaLnF4: Yb/Er were also introduced. Moreover, it was found that Mn2+ co-doping weakened the UC intensity in NaYF4 [24,25]. Another approach is to select a specific matrix, such as KMgF3, KSc2F7, NaxScF3 + x, ScOF, Na3ZrF7, or Na3HfF7, which allows for the attainment of single-band UCL [26–33]. However, the specific factors, whether structural or electronic, responsible for the single-band emission in these matrices are still uncertain. Other methods, like coating with organic dyes to extinguish green emission, are employed to generate single-band UC, but eliminating green emission entirely is challenging [13,16]. Therefore, to obtain single-band emission without reducing the UC efficiency, modulating the spectral emission by effective ET process such as EBT and CR is a more promising approach [16].
In most cases, altering the sensitizer to activator ratio can adjust red/green emission ratio in the Yb3+/Er3+ system. The EBT between the sensitizer and the activator inhibits green emission, which can be augmented by increasing the sensitizer concentration. Based on this principle, single-band red UCL can be easily realized in high sensitizer to activator ratio material [34]. Actually, single-band red emissions have been obtained from sensitizer-rich Yb-based materials such as SrYbInO4: Er, YbOF: Er, YbOCl: Er, and Yb3Al5O12: Er [35–39]. Therefore, a feasible strategy is to improve the probability of EBT process by shortening the distance between sensitizer and activator, reducing the green emission and thus obtaining a pure red UCL [40]. Here, Yb2O3 was chosen as the host, which is distinctive as all cations act as sensitizers. Due to the single excitation energy level of Yb3+, it still can mediate effective long-range energy transfer/migration to stimulate activators even at high concentrations when used as a sensitizer [41]. The reduced ionic spacing is predicted to generate strong Yb-Er interactions, potentially resulting in highly efficient UC performance [42]. Additionally, Yb2O3 can fully utilize the 980 nm energy flux due to its large absorption cross-section. Moreover, oxides tend to possess greater chemical, thermal, and mechanical stability compared to fluorides, making them appropriate hosts for UCL applications [2,13].
For transparent bulk materials, smart windows are a promising application scenario. Recently, numerous novel types of smart windows have been developed for various applications, such as temperature sensing, switchable sunroof, and optical data recording and storage [43–45]. Conventional transparent materials such as glass or plastics have been considered as multifunctional window materials in cooperation with RE3+ ions, but they may be limited by low thermal and mechanical properties in harsh environments [46–48]. A better alternative is transparent ceramic (TC) due to its intriguing geometric versatility, doping flexibility, and manufacturing scalability compared to single crystals [49–51]. Typical transparent ceramics such as YAG, AlON etc. have been widely investigated as solid-state laser, armor, and phosphor materials [52–54]. Yb2O3 belongs to the cubic crystal system and does not undergo the birefringence effect, so it has the prerequisite to becoming a transparent ceramic [55]. Therefore, we aim to make Yb2O3 powder into fully-dense bulk ceramics with desirable transparency, high thermal conductivity and thermal stability, and excellent mechanical properties [56]. These Yb2O3 TCs are expected to exhibit superior spectroscopic properties compared to those of powders due to the reduction of adverse light scattering.
In this work, we prepared a series of highly transparent Er-doped Yb2O3 ceramics. The highest transmittance of Yb2O3: Er TC reach 82% at 600 nm, which is the highest value of Yb/Er doped transparent ceramics reported so far. The realization of 980 nm-excited single-band red UCL in TCs was demonstrated. The color purity of red UCL is calculated to be >99.9% and the single-band characteristic is independent of the doping concentration and pump power. The temperature sensing performance of the pure red emission was evaluated using the luminescent intensity ratio (LIR) technique. The Yb2O3: Er TCs possesses the advantages of red UCL's high penetration and anti-interference resistance, as well as the outstanding physical and chemical characteristics of transparent ceramics. Our work provides a new idea for the preparation and application of multi-functional window materials with both lighting and sensing capabilities.
2. Experimental
2.1 Sample preparation
Commercial Yb2O3 and Er2O3 (99.99%, Xingda, China) powders were chosen as the starting materials. Based on our previous work, ZrO2 (99.99%, Dahua, China) was employed as sintering additives [55]. On the basis of the ratio of (Yb0.97−xErxZr0.03)2O3 (x = 0.01, 0.03, 0.06, 0.1, 0.2), powders were stoichiometric weighted and mixed with ethanol in a 500 mL nylon jar. The weight ratio of zirconia balls (Φ5 mm) to raw material was set at 18:1, and the ball milling speed was 250 rpm for 20 h. After mixing, the slurries were dried at 70 °C for 24 hours, then the powder was sieved through a 200-mesh sieve. After calcined at 1200°C for 4 h, these pretreated powders were dry-pressed into 20 mm diameter and 2 mm thick disks under uniaxial pressure of 20 MPa. Then, the pellets were cold isostatically-pressed at 200 MPa for 10 min. The sintering process was carried out in a vacuum carbon canister furnace at 1800 °C for 10 h under a vacuum of 10−3 Pa. Then the samples were annealed at 1450 °C for 20 hours in air to remove oxygen vacancy. Finally, these sintered Yb2O3 TCs disks were machined to a thickness of 1.5 mm and polished on both sides for further characterization. The preparation flow chart of Yb2O3: Er TCs is shown in Supplement 1, Fig. S1.
2.2 Measurements and characterization
X-ray Diffraction (XRD, DX-2700, Dandong Fangyuan, China) was used to determine the compositions and phase structures of powders and annealed TCs using nickel-filtered Cu-Kα radiation (λ = 1.5406 Å), and the diffraction patterns were recorded in the range of 2θ from 20 to 70°. The scanning electron microscope (SEM, JSM-IT500HR, JEOL Japan) was used to obtain the morphology of as-prepared powders and microstructures of sintered ceramics. At least 100 grains were measured using the linear intercept method on Nanomeasure software to determine the mean disc size. An intensification factor of 1.56 was used to obtain the equivalent mean grain size [57]. The distribution of elements in Yb2O3: Er TCs was obtained with a Quantax EDS (Energy Dispersive X-ray Spectrometer) detector (Aztec Energy X-Max 20, Oxford, UK). The density of TCs was measured by Archimedes method. The Raman spectrum were used to determine the phonon energy of the Yb2O3 matrix using a Raman spectrometer (HR800; Horiba, France) with a 532 nm excitation source. The in-line transmittance of TCs was measured with UV/Vis/NIR spectrophotometer (Lambda-750, Perkin-Elmer, USA). UC fluorescence spectra and lifetime were measured on a Fluorolog-3 spectrofluorometer (Horiba JobinYvon, France) with a 980 nm laser (MDL-III-980) as the excitation source and a picosecond photon detection module (PPD-850) as the detector. The heating apparatus was constructed using an alumina disk regulated by a custom-made control unit. To minimize temperature inaccuracies, two channels of a thermometer were employed for both calibration and temperature measurement.
3. Results and discussion
3.1 Structural and optical properties
XRD patterns of Yb2O3: Er TCs and precursor powders are shown in Fig. 1(a) and Supplement 1, Fig. S2, respectively. Clearly, the crystalline phases of all samples are identical to the standard cubic Yb2O3 phase (PDF #43-1037). No secondary phase was observed, regardless of the doping concentration, indicating that Er3+ ions were successfully incorporated into the Yb2O3 lattice, substituting for Yb3+. The lattice parameters of the formed Yb2O3: Er were refined from XRD patterns to evaluate the impact of the concentration of Er3+ on its incorporation into the matrix. The estimated lattice constants from the Bragg equation are 10.434 Å, 10.438 Å, 10.439 Å, 10.444 Å, and 10.451 Å for the specimens 1%, 3%, 6%, 10%, and 20% respectively. The lattice constant linearly increases with the increase of Er3+ concentration. This is due to the slightly larger ionic radius of Er3+ (0.89 Å, coordination number (CN) = 6) compared to that of Yb3+ (0.868 Å, CN = 6).
Fig. 1. (a) XRD pattern (b) Raman spectra (c) EDS mapping (d) photograph (e) In-line transmittance of Yb2O3: Er TCs.
Figure 1(b) shows the Raman spectrum of undoped Yb2O3 TCs. The most prominent band is observed at 362 cm−1, which is the characteristic peak of cubic Yb2O3 and it is ascribed to Ag + Fg vibrations [55]. In addition, the maximum phonon energy of Yb2O3 was observed to be 614 cm-1, which is lower than commonly used oxide UC hosts such as YAG (850 cm-1) and YPO4 (1080 cm-1). The low phonon energy host helps to reduce non-radiative processes that can lead to energy loss, which is beneficial for photoluminescence and UC applications. The crystal structure of Yb2O3 is depicted in the inset of Fig. 1(b). Yb2O3 crystallizes in the cubic space group Ia3(206) with two six-coordinate sites (C2, C3i). 24 Yb3+ are on the sites with point symmetry C2 and 8 Yb3+ ions are on the sites with symmetry C3i. Due to the inversion symmetry of the C3i sites, the optical characteristics are almost exclusively attributed to the ions on C2 sites [58].
Generally, the density is required to be greater than 99% of the theoretical density for the ceramic to become transparent. Thus, the density of this ceramic is expected to be high because both Yb and Er have large atomic numbers. The density of 1%, 3%, 6%, 10% and 20% Er3+ doped Yb2O3 transparent ceramic measured by Archimedes method was 8.971 g/cm3, 8.968 g/cm3, 8.927 g/cm3, 8.922 g/cm3, and 8.870 g/cm3 respectively. Since the relative atomic mass of Yb (173.04) is larger than that of Er (167.26), the density of the Yb2O3 TCs sample decreases slightly with increasing Er content. It is well known that scintillator and X-ray imaging applications usually require high atomic numbers and densities [59,60]. This means that the Yb2O3: Er in the form of transparent ceramic may also have potential applications in X-ray CT imaging and will be reported in our future work.
Figure 1(c) and Supplement 1, Figs. S3-S5 show SEM images and EDS elemental distributions of the (Yb0.97-xErxZr0.03)2O3 powders and thermally etched ceramics. All samples present homogeneous microstructure without abnormal grain growth. Furthermore, all the grains are firmly bonded together with virtually no pores or impurities present. The average grain size of ceramics obtained from SEM images is approximately 3.5 µm, regardless of the concentration of Er3+ doping. This may be attributed to the similarities in the crystal structure of Er2O3 and Yb2O3. According to the Rayleigh approximation, the intensity of scattered light is to the sixth power of grain size, which means that a smaller average size distribution leads to higher transmittance [61]. The elemental distribution on the surface confirms that the Yb, Er as well as Zr elements are uniformly distributed in the powder and ceramic. The dense and uniform microstructure of the ceramics contributes to its high level of transparency.
To better understand the optical properties, photographs and in-line transmittance of the Yb2O3: Er TCs are depicted in Fig. 1(d) and Fig. 1(e). The letters under the TCs are clearly visible. These TCs samples appear pink to the naked eye because of the large absorption of red and green light in visible region, and the pink deepens with increasing Er3+ content. These absorption bands in the transmission spectra originate from the electron transitions within the 4f12 subshell of Er3+. All samples show high transparency in part of visible and infrared regions, with the transmittance of ∼82% at ∼600 nm. The difference in transmittance is primarily attributed to the presence of porosity and impurities introduced during the preparation process, as there is no absorption of Yb and Er ions at 600 nm. Supplement 1, Table S1 stats the transmittance of Yb/Er doped TCs reported so far. It is worth mentioning that the transmittance of the Yb2O3: Er TCs in this work is the highest level of Yb-Er doped TCs that have been reported. This indicates that even with such high doping concentrations, the prepared samples still have high optical quality. The absorption bands in the range from 900 to 1000 nm are ascribed to 2F7/2→2F5/2 electronic transitions of Yb3+ ions [62]. Transmittance of all samples was almost 0 in this range owing to the high Yb3+ concentration. The absorption spectra can be derived from transmittance spectra by the formula: α=1/d·ln(1/T), where α is the absorption coefficient, d is the thickness of samples, and T is the transmittance. The calculated absorption spectra were shown in Supplement 1, Fig. S6. These absorption peaks correspond to Yb3+:2F5/2 and Er3+:4S3/2, 4F9/2, 4I9/2, 4I11/2, 4I13/2, respectively, and detailed schematic diagram of the energy level transitions can be found in Supplement 1, Fig. S7 [63]. The intensity of absorption peaks from Er3+ increases with the increase of Er3+ concentration. The strong and broad absorption band in the 900-1000 nm range is mainly attributed to the Yb2O3 matrix, corresponding to 2F7/2 → 2F5/2 transitions of Yb3+ as well as 4I15/2 → 4I11/2 transitions of Er3+ ions. As a result, it has been proven that the sample is of good quality and has a relatively high transmittance, making it suitable for luminescence and sensing tests.
3.2 UCL properties
The UCL spectra of Yb2O3 TCs doped with different Er3+ concentrations under 980 nm-laser excitation were measured as demonstrated in Fig. 2(a). Several emission peaks are observed in the red region with maximum intensity at 662 nm, originating from the 4F9/2 → 4I15/2 transition of Er3+. Generally, the 2H11/2 and 4S3/2 → 4I15/2 transitions of Er3+ also generate green emission. It is surprising to find that green UC emission does not appear at all in the spectrum, leaving a very pure red UC emission. Inset of Fig. 2(a) shows the relationship between the integrated red emission intensity and the concentration of Er3+ doping. The UCL intensity gradually becomes stronger at first and decreases when the Er3+ concentration exceeds 6%. There are two possible mechanisms to explain this phenomenon. Firstly, this phenomenon could be attributed to non-radiative relaxation processes, such as cross-relaxation, which become more prominent with an increase in the concentration of Er3+ ions [64]. Secondly, the notable decrease in peak intensity may result from the self-absorption of emitted photons by the Er3+ ions themselves. This effect is also evident in the absorption spectra (Fig. S6), where an increase in Er3+ concentration leads to higher absorbance. Boccolini et al. found that quantum yield values are constrained by self-absorption losses [6]. Their findings indicate the presence of an optimal thickness and doping concentration of ions involved in the up-conversion/down-conversion process to minimize self-absorption and enhance the photoluminescence quantum yield (PLQY) of the material [65]. The similar phenomenon appears in the downshifting emission spectrum excited by 980 nm as shown in Supplement 1, Fig. S7. The board emission band around 1530 nm originates from the 4I13/2 → 4I15/2 transition of Er3+.
Fig. 2. (a) The UCL spectra of Yb2O3: Er TCs with different Er3+ concentrations. The inset displays the changes in the UCL intensities in relation to the Er3+ concentration. (b) CIE chromaticity diagram of the Yb2O3:Er3+ TCs. The inset presents the photograph of the TCs excited by 980 nm laser diode. (c) fluorescence decay curves of emission at 662 nm.
Based on the recorded UCL emission spectra, the Commission International de I’Eclairage (CIE) chromaticity of the samples was evaluated, as shown in Fig. 2(b). Color purity is an important indicator to evaluate chromatic characteristics of luminescent materials, which could be calculated using the following formula:[66]
Table 1. CIE Chromaticity Coordinates of Yb2O3: Er transparent ceramics
To elucidate the luminescence dynamic involved, luminescence decay curves at room temperature of the ceramics were investigated. Figure 2(c) depicts the fluorescence lifetime curves of the Yb2O3: Er3+ TCs with different Er3+ ion concentrations (λex = 980 nm, λem = 662 nm). The detected decay curves can be well fitted with the single exponential decay model as follows:
where I(t) and ${I_0}$ represent the emission intensity at time t and t = 0, respectively, τ represents the lifetime, and A is constant. The lifetimes of Yb2O3: Er TCs were calculated to be 5.94, 5.56, 5.31, 4.96, and 4.11 µs corresponding to 1%, 3%, 6%, 10%, and 20% Er3+ concentrations, respectively. The value of lifetime decreases with the increasing Er3+ concentration, which further verifies the existence of concentration quenching in Yb2O3: Er TCs.3.3 Excitation mechanism of UCL
To understand the UCL mechanism of the red emission, the dependence of UCL intensity on the pump power of the 980 nm laser was measured as shown in Fig. 3(a). For the unsaturated UCL process, the intensity I and pump laser power P generally abide by the following equation:[68]
where n denotes the number of infrared photons absorbed for each visible photon emitted, which is equivalent to the slope of the line obtained from fitting the log I versus log P. Clearly, the intensity of UCL increases with the increase of pumping power. The slopes n of 1%, 3%, 6%, 10%, and 20% for the red emission are calculated to be 0.38, 0.6, 1.53, 1.62, and 1.96, respectively. The relationship between n and Er3+ concentration is shown in Fig. 3(b). Surprisingly, the slopes of 1% and 3% Er3+ doped sample is 0.38 and 0.6 respectively, which is far deviated from 2 for the two-photon process. The reason may be as follows: The low doping level induces a long distance between the participating activator ions in Yb2O3 host material, which is adverse to the ET process but facilitate the EBT to another Yb3+ [69]. In this case, fewer photons participate in UC process, resulting in a lower n. The same phenomenon happens in low concentration Ho3+ doped Yb2O3 nanoparticles [70,71]. When the concentration of Er3+ increases, ET from Yb3+ to Er3+ will result in a more efficient UC from the intermediate energy level to the UCL energy level, leading to an increase in the proportion of UC processes. This could explain the increase of slope n with increasing Er3+ concentration.Fig. 3. (a) The pump power dependence of the UCL intensities in the Yb2O3: Er TCs. (b) Relationship between n and Er3+ concentration (c) Energy level diagrams of Yb3+, Er3+ and possible transition pathways and ET processes upon 980 nm-excitation.
The possible population processes and feasible transitions were schematically given in the energy level diagram of Er3+ and Yb3+ ions, as depicted in Fig. 3(c). The major UC mechanisms can be depicted as following:
4I15/2(Er3+) + 2F5/2(Yb3+) →4I11/2(Er3+) + 2F7/2(Yb3+)[ET1]
4I11/2(Er3+) + 2F5/2(Yb3+) →4F7/2(Er3+) + 2F7/2(Yb3+)[ET2]
4I13/2(Er3+) + 2F5/2(Yb3+) →4F9/2(Er3+) + 2F7/2(Yb3+)[ET3]
2F7/2(Yb3+) + a photon (980 nm) → 2F5/2 (Yb3+)[GSA]
4I15/2(Er3+) + a photon (980 nm) → 4I11/2 (Er3+)[GSA]
4I13/2(Er3+) + a photon (980 nm) → 4F9/2 (Er3+)[ESA]
4I11/2(Er3+) + a photon (980 nm) → 4F7/2 (Er3+)[ESA]
4F7/2 (Er3+) + 4I11/2 (Er3+) → 4F9/2 (Er3+) + 4F9/2 (Er3+)[CR]
4S3/2 (Er3+) + 2F7/2 (Er3+) → 4I13/2 (Er3+) + 2F5/2 (Er3+)[EBT]
UC excitation processes involve ground-state absorption (GSA), excited-state absorption (ESA), and energy transfer (ET), additional process such as cross relaxation (CR) between two activator ions may occur as well. The GSA or ESA process generally involves in a single ion and it is the dominant UC excitation process when doping level is low, while the ET process involves two adjacent ions and it is dominant in high-doping concentration materials [72]. The ET process from Yb3+ to Er3+ makes prominent contributions to the population of the emission energy levels because Yb3+ has a higher absorption cross section than Er3+ at the infrared region and a high doping concentration. Under the 980 nm excitation, the GSA of Yb3+ take places via absorbing an infrared photon: 2F7/2 + a (980 nm) photon → 2F5/2 transition. Subsequently, since 4I11/2 of the Er3+ and 2F5/2 of the Yb3+ are syntonic, the ET1 process occurs: 4I15/2(Er3+) + 2F5/2(Yb3+) →4I11/2(Er3+) + 2F7/2(Yb3+). For populating the 4F7/2 level of Er3+, the ESA takes place from the 4I11/2 level to the 4F7/2 level by absorbing another 980 nm photon. Another route for populating the 4F7/2 level is through the ET2 process: 4I11/2(Er3+) + 2F5/2(Yb3+) →4F7/2(Er3+) + 2F7/2(Yb3+). There are three routes for the red emission originating in the energy level 4F9/2 [66]. The first route is the electrons at upper level 4S3/2 and 2H11/2 non-radiatively relax to 4F9/2 level. The second is the CR process, in which 4F9/2 is populated through 4F7/2 → 4F9/2 and 4I11/2 → 4F9/2 through CR transfer. As for the third route, the electrons at 4I11/2 (Er3+) non-radiatively relax to the 4I13/2 (Er3+), followed by the ESA process: 4I13/2(Er3+) + photon → 4F9/2 (Er3+) or the ET2 process: 4I13/2 (Er3+) + 2F5/2 (Yb3+) → 4F9/2(Er3+) + 2F7/2(Yb3+) . According to Eqs. 4 and 6, the probability of cross-relaxation is not high in samples with low Er concentrations, but the samples still obtain red UCL, indicating that the contribution of this process is not significant. As for the NR process and ET3, Wu. et al. compares the PL spectra excited at 488 nm and the UCL spectra excited at 980 nm of the materials Y2O3, BGZO, β-NaYF4, and Ba3Y4O9 doped with 5% Er3+,20% Yb3+ and discusses the contribution of the NR and ET3 processes for the red UCL [58,73]. In the PL spectrum, the Er3+ 4F7/2 state will be directly populated, followed by relaxation to the 2H11/2 and 4S3/2 levels under 488 nm excitation. Thus, the red emitting level 4F9/2 will be populated exclusively from the green level via NR. The results indicate that the R/G ratio of these matrices in the UCL spectrum is greater than that in the PL spectrum, which means that the contribution of NR to the red UCL of all samples is not significant. Therefore, the ET3 process is the dominant process of the red UCL in Yb2O3: Er. Further, there exists the energy back transfer (EBT) process from Er3+ to Yb3+ (2F7/2 + 4S3/2- 2F5/2 + 4I13/2) at high Yb3+ concentration, leading to a decrease in the green energy level [74]. The processes discussed above could cause an increase at the 4F9/2 and 4I13/2 level and a reduction of the population at the 4S3/2 and 4F7/2 level. Therefore, the red emission intensity is enhanced, while the green emission is extremely weak.
The EBT and CR processes significantly contribute to enhancing red emission through depopulating the 2H11/2 and 4S3/2 levels and increasing the population at the 4F9/2 level [75]. The probability of energy transfer, as outlined by Dexter's energy transfer theory, is dependent on the distance 1/Rθ between two ions. Consequently, the probability of ${\mathbf C}{{\mathbf R}_{{\mathbf{Er}} - {\mathbf{Er}}}}$ and ${\mathbf EB}{{\mathbf T}_{{\mathbf{Er}} - {\mathbf{Yb}}}}$ can be mathematically represented as:
where, ${{\mathbf R}_{{\mathbf{Er}} - {\mathbf{Er}}}}$ and ${{\mathbf R}_{{\mathbf{Er}} - {\mathbf{Yb}}}}$ represent the average distance between Er3+ - Er3+ and Er3+ - Yb3+, and θ represent a positive integer taking the values of 6, 8, and 10, which correspond to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The ${{\mathbf R}_{{\mathbf{Er}} - {\mathbf{Er}}}}$ and ${{\mathbf R}_{{\mathbf{Er}} - {\mathbf{Yb}}}}$ can be expressed as: where V represents the unit cell volume, N is the number of substitutable sites for RE3+ in each unit cell, ${{\boldsymbol C}_{{\boldsymbol Er}}}$ and ${{\boldsymbol C}_{{\boldsymbol Er} + {\boldsymbol Yb}}}$ are Er3+ doping concentration and total doping concentration of Yb3+ and Er3+, respectively. For cubic Yb2O3, the value of N is 32 and the unit cell volume is 1135.93 Å3. Furthermore, in the Yb2O3 matrix, Yb3+ ions not only contribute to the crystal structure, but also serve as sensitizers in the UC process. As a result, the total concentration of Yb3+ and Er3+ ions are fixed at 0.97, excluding the 3at% of ZrO2 used as a sintering aid. Table 2 summarizes the ion distance of Yb2O3: Er and other Yb based materials. It is obvious that the increasing Er3+ concentration will decrease the ${{\mathbf R}_{{\mathbf{Er}} - {\mathbf{Er}}}}$ and the probability of CR occurrence will increase according to the Eq. (4). Clearly, compared to Yb3Al5O12 and Yb2Ti2O7, Yb2O3 has a smaller Yb ion spacing because its cation composition is only Yb3+. Additionally, the REr-Yb of Yb2O3 is considerably shorter than in Gd2O3 and Y2O3 hosts, which possess similar Ia3 structures. This suggests that a high concentration of Yb3+ would result in a reduction of the REr-Yb, and thus increase the probability of EBT as predicted by Eq. (5). The EBT process increases the population in 4I13/2 level by decreasing the population of the 4S3/2 level, and the the ET3 process further populates the 4F9/2 level [76]. The process described above could cause an increase in the population at the red-emitting 4F9/2 level and a decrease at the green-emitting 4S3/2and 2H11/2 level. Specifically, the strong red UC and negligible green UC.Table 2. Volume of a unit cell (V), number of formula units in a unit cell (N), doping concentration of RE3+ (CEr-Er and CEr-Yb), and distance between RE3+ (REr-Er and REr-Yb) of Yb2O3, Y2O3, Gd2O3 and other Yb-based matrices.
3.4 Optical thermometry behavior
The LIR technique is based on the temperature-dependent UCL of RE ions at two thermally-coupled energy levels, which makes it independent of excitation intensity fluctuations and spectral losses, resulting in high resolution and accuracy [77]. The autofluorescence of biological tissues can be minimized in an optical window ranging from the red region (600-700 nm) and the near-infrared region (700-1100 nm), thus it would be effective if pure red emission is studied as an application for optical thermometry. In most previous reports on optical thermometry based on LIR, the thermally-coupled 2H11/2 and 4S3/2 levels of Er3+ were the most commonly employed due to appropriate energy gap and the excellent sensing properties [78]. However, in this work, this pair of thermally-coupled levels could not be utilized for temperature sensing due to the extremely weak green UC emission. Fortunately, the red-emitting Stark sublevels of 4F9/2 of Er3+ can also be employed to optical sensing with perfect sensitivity [44,76,79,80]. For example, Suo et al. reported on the use of two Stark sublevels of the 4F9/2 manifold in Er3+ for optical thermometry [81]. In Yb2O3: Er system, the splitting in UCL spectrum originates from the splitting of the 4F9/2 level into Stark sublevels, designated as 4F9/2 (0), 4F9/2 (1) and 4F9/2 (2).To distinguish the transitions between Stark sub-levels of and 4I15/2 and 4F9/2 states, Wu. et al. has measured the 4I15/2-4F9/2 absorption spectrum and the 4F9/2-4I15/2 emission spectrum at 77 K [73]. And the transitions from the populated 4F9/2 level to the 4I15/2 level give rise to a red emission from 648 to 685 nm with a split peak at 655, 662, and 685 nm. According to the peaks in the UCL spectrum measured at room temperature, the energy levels of 4F9/2 in Yb2O3: Er TCs were assigned as shown in Supplement 1, Fig. S10. Here, we focus on the 655 nm and 662 nm emission peaks because they are derived from the thermally coupled Stark level (2) and (1) of the 4F9/2 manifold. Temperature-dependent UCL spectra of the Yb2O3: Er TCs was shown in Fig. 4(a) and Supplement 1, Fig. S9. Apparently, the red emission peak did not shift and the red UCL intensity gradually decreased with the increase of temperature. For a more intuitive presentation, Fig. 4(b) displays bar graphs depicting the intensity changes at 662 nm and 655 nm with varying temperatures. In addition, Fig. 4(c) exhibits the temperature-dependent UC spectra, which have been normalized by the intensity at 662 nm. Certainly, as the temperature increases, the emission intensity at both 662 nm and 655 nm will decrease. However, the intensity at 662 nm experiences a more rapid decrease in comparison to that at 655 nm. The color coordinates in CIE chromaticity diagram of UCL (shown in Fig. 4(d)) slightly towards the green direction as the temperature rises, which may be attributed to the decrease of UCL intensity leads to the reduction of signal-to-noise ratio.
Fig. 4. (a) Temperature-dependence UC emission spectra in the range 304 K-523 K. (b) UC luminescence intensity at 662 nm and 655 nm for Yb2O3: 6% Er. (c) Temperature-dependence UC fluorescence spectra normalized by emission intensity at 662 nm. (d) CIE diagram of 6% Yb2O3: Er3+ TCs with increasing temperature. (e) Experimental and fitted I655/I662 LIR versus temperature plots for the red UC emission. (f) Sensitivity variation with temperature obtained from the red UC emission.
The LIR for transition from two thermally-coupled levels could be expressed using the Boltzmann distribution law as [76]:
with B=$\frac{{{g_U}{\delta _U}{\omega _U}}}{{{\textrm{g}_L}{\mathrm{\delta }_L}{\mathrm{\omega }_L}}}$. . I, δ, g and ω are the fluorescence intensity, radiative rate, degeneracy degree, and transitions angular frequency, respectively. The subscript U and L denote the thermally-coupled upper and lower level, respectively. T, ΔE, and ${k_B}$ define the absolute temperature, energy separation, and Boltzmann constant, respectively. Based on Eq. 8, the variation of LIR between 655/662 nmith temperature could be expressed as: (shown in Fig. 4(e)) since ΔE/${k_B}$=293.01, the calculated ΔE between peak 655 nm and peak 662 nm is approximately 159 cm-1. Compared with the ΔE obtained from the UCL spectrum (203 cm-1), the error of ΔE is 44 cm-1, attributed to the enhancement of the multi-phonon relaxation and ET at higher temperature [82]. The absolute sensitivity ${S_a}$ and relative sensitivity ${S_r}$, which are the important parameters to evaluate the temperature sensor, could be calculated using the following equation:The sensitivity of the optical thermometry was calculated as a function of temperature and presented in Fig. 4(f). The values of both ${S_a}$ and ${S_r}$ keep decreasing in the testing range from 304 K to 523 K, which achieve the maximum 0.199%K-1 and 0.319%K-1 respectively at 304 K. Table 3 shows the comparison of temperature measurement sensitivity between Yb2O3: Er TCs and previously reported Er3+/Yb3+ co-doped materials based on 4F9/2 energy levels. The temperature sensitivity of the sensor is strongly influenced by the ΔE between the thermally coupled levels, so the sensitivity at the same thermally coupled energy level does not differ much in different matrices. This shows that this Yb2O3: Er TCs with both observation and temperature detection properties, has good prospects for application in special scenarios.
Table 3. Optical thermometric parameters in Er3+ doped materials based on 4F9/2 level using the LIR technique
4. Conclusion
In this work, we fabricated a series of Er3+ doped highly transparent Yb2O3 ceramics with distinct Er3+ concentrations. The phase and structural evolution of ceramics was systematically determined. All samples exhibit high optical quality with in-line transmittance >80%. In the 980nm-excited UC spectrum, green emission does not appear, thus Yb2O3: Er TCs exhibit ultra-pure single band red emission, with CIE coordinates of (0.72, 0.28) and color purity exceeding 99.9%. This is the first demonstration of pure red UC luminescence in TCs to the best of our knowledge. The UC mechanism was discussed through the pump power dependence of UCL intensity, which was determined to be a two-phonon process. The generation of pure red UCL is attributed to the EBT and CR processes, as these processes result in a decrease of the 2H11/2 and 4S3/2 green UC levels and an increase of the 4F9/2 red UC level. The optical thermometry behaviors of this pure red emission based on the Stark sublevels of Er3+ 4F9/2 have been investigated in detail. The maximum Sa and Sr were calculated to be 0.199%K-1 and 0.319%K-1, respectively. Combined with the strong penetration and anti-interference capabilities of red UCL with the desirable transparency, high thermal conductivity and thermal stability, and excellent mechanical properties of TC, Yb2O3: Er TCs are extremely promising multi-functional optical materials.
Funding
National Natural Science Foundation of China (U21A20441).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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