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Article

Multi-Band Emission of Pr3+-Doped Ca3Al2O6 and the Effects of Charge Compensator Ions on Luminescence Properties

1
School of Materials Science and Engineering, Hanshan Normal University, Chaozhou 521041, China
2
School of Chemical and Environmental Engineering, Hanshan Normal University, Chaozhou 521041, China
*
Authors to whom correspondence should be addressed.
Nanomaterials 2024, 14(1), 2; https://doi.org/10.3390/nano14010002
Submission received: 29 October 2023 / Revised: 6 December 2023 / Accepted: 7 December 2023 / Published: 19 December 2023

Abstract

:
Multi-band emission luminescence materials are of great significance owing to their extensive application in diverse fields. In this research, we successfully prepared a series of Pr3+-doped Ca3Al2O6 multi-band emission phosphors via a high-temperature solid-state method. The phase structure, morphology, luminescence spectra and decay curves were investigated in detail. The Ca3Al2O6:Pr3+ phosphors can absorb blue lights and emit lights in the 450–750 nm region, and typical emission bands are located at 488 nm (blue), 525–550 nm (green), 611–614 nm (red), 648 nm (red) and 733 nm (deep red). The influence of the Pr3+ doping concentration was discussed, and the optimal Pr3+ doping concentration was determined. The impacts of charge compensator ions (Li+, Na+, and K+) on the luminescence of Pr3+ were also investigated, and it was found that all the charge compensator ions contributed positively to the emission intensity. More importantly, the emission intensity of the as-prepared phosphors at 423 K can still maintain 65–70% of that at room temperature, and the potential application for pc-LED was investigated. The interesting results indicate that the prepared phosphors may serve multifunctional and advanced applications.

1. Introduction

In recent years, luminescence materials have attracted great attention owing to their extensive applications, such as daily lighting, backlight displays, biological imaging and medical diagnosis [1,2,3,4,5]. Nowadays, the demand for high-performance materials is growing rapidly with the development of technology. Thus, an investigation on novel luminescence materials is of great significance for both application and basic research. Luminescence center ions play a vital role for phosphors, which determine the luminescence performance of a phosphor to a large extent. Over the past decades, rare earth ions have been widely investigated, as luminescence centers in phosphors, such as Ce3+, Eu3+, Tb3+, Pr3+, and Er3+ [6,7,8,9,10]. Pr3+ is one of the important luminescence activators, which is rich in energy levels. More importantly, the emission spectra of Pr3+ in different host lattices can cover ultraviolet to infrared regions [11,12].
Up to now, a large number of Pr3+-doped phosphors have been reported. For example, the 5d-4f transition emissions of Pr3+ was found to be located in 220–300 nm for the Sr2P2O7:Pr3+ phosphor, and continuous ultraviolet-C persistent luminescence could be achieved after X-ray irradiation [13]. Y3Si6N11:Pr3+ exhibits a red broad-band emission, which may be potentially used for w-LEDs and temperature sensing [14]. Thanks to color-tunable persistent luminescence, the Ca2Sb2O7:Pr3+ phosphor is considered to have a bright application prospect in the anti-counterfeiting field [15]. A Ba3MgSi2O8:Pr3+ ceramic can change color from white to pink rapidly when irradiated by a UV lamp, and the responses are reversible. These rapid high-contrast reversible properties make Ba3MgSi2O8:Pr3+ a prospective material for rewritable paper [16]. Furthermore, the luminescence properties of Pr3+-doped LiLuSiO4, BaLu2Al2Ga2SiO12, CaSc2O4, BaY2Si3O10, Ca2LaTaO6, YNbO4, etc., have been reported [17,18,19,20,21,22]. Owing to the superior luminescence performance of Pr3+-doped materials, Pr3+ has been extensively used in lasers, scintillators, w-LEDs and optical temperature sensing [23].
The Ca3Al2O6 compound may be a good host matrix due to its excellent physical and chemical stability [24]. To our knowledge, the luminescence of Ce3+, Eu2+, Dy3+, Bi3+, Sm3+, Tb3+, and Mn2+ in Ca3Al2O6 has hitherto been investigated [24,25,26,27,28,29]. In this research, considering the advantages of the Ca3Al2O6 host and the Pr3+ ion mentioned above, we performed a study on the luminescence properties of Pr3+ in the Ca3Al2O6 host. As reported, charge compensator ions may play a vital role to achieve a balance of charges, which could further have great impacts on the luminescence properties of a phosphor [30,31]. Herein, considering the charge imbalance between Ca2+ and Pr3+, alkali metal R+ (R = Li, Na, K) ions were selected as charge compensator ions. The effects of charge compensator ions (Li+, Na+, and K+) were also discussed in detail.

2. Materials and Methods

In this research, a series of Pr3+-activated multi-band emission phosphors, Ca3−xPrxAl2O6 and Ca2.97−xPr0.03RxAl2O6 (R+ = Li+, Na+, K+), were synthesized through a solid-state reaction technique. The raw starting reactants CaCO3 (99.99%, Aladdin, Wallingford, CT, USA), Al2O3 (99.99%, Aladdin), Pr2O3 (99.9%, Aladdin), Li2CO3 (99.9%, Aladdin), Na2CO3 (99.5%, Aladdin) and K2CO3 (99.5%, Aladdin) were used for chemical reactions. As is typical for the process of synthesis, the raw reactants were accurately weighed according to the stoichiometric ratio, and then the mixed reactants were ground for about 30 min in an agate mortar. Afterwards, the well-mixed reactants were fully transferred into alumina crucibles and sintered at 1623 K for 4 h in air. Lastly, the samples were cooled down to room temperature naturally and thoroughly ground to obtain fine white powders.
The X-ray diffraction (XRD) patterns were measured using a D8 Advance diffractometer (Cu Kα, λ = 1.5406 Å) for phase analysis. Structure refinements were performed using the TOPAS 5.0-Academic software. Scanning electron micrographs (SEM) were conducted on a field emission scanning electron microscopy (Hitachi SU5000, Tokyo, Japan) for micro-morphology analysis. Diffuse reflectance spectra were measured by a UV3600 spectrofluorometer (Shimadzu, Kyoto, Japan). Luminescence spectra and decay curves at different temperatures were all collected on an Edinburgh FLS1000 spectrofluorometer, and the excitation sources were a 450 W xenon lamp and a μF900 lamp, respectively. The electroluminescence spectra of a pc-LED device were measured on an OHSP-350M LED fast-scan spectrophotometer (Hangzhou Hopoo Light and Color Technology Co., Ltd., Hangzhou, China).

3. Results and Discussion

3.1. Phase Structure and Morphology

To confirm the phase structure of the as-prepared phosphors, X-ray Rietveld refinements were performed for two typical samples, Ca3Al2O6 and Ca2.97Pr0.03Al2O6. Figure 1a,b depict the refinement results. All the calculated diffraction patterns accord well with the observed ones, which indicates that the as-prepared samples are of a single pure phase. The detailed cell parameters for the X-ray Rietveld refinements are illustrated in Table 1. The undoped and Pr3+-doped Ca3Al2O6 phosphors crystallize in a cubic system with a P a 3 ¯ space group. Due to the similar ionic radii for Ca2+ and Pr3+ (e.g., r(Ca2+) = 1.00 Å, CN = 6; r(Pr3+) = 0.99 Å, CN = 6), the cell parameters a, b, c, and cell volumes remain nearly unchanged for Ca3Al2O6 and Ca2.97Pr0.03Al2O6. Herein, a small expansion is most probably ascribed to the experimental errors. The crystal structure of the Ca3Al2O6 host is displayed in Figure 1c. The frame structure consists of [AlO4] tetrahedrons and [CaO6/CaO7/CaO8/CaO9] polyhedrons. There are six different Ca2+ sites in this structure. Ca2+(1), Ca2+(2) and Ca2+(3) are coordinated with six oxygen atoms, and the average Ca2+-O2− bond lengths are 2.338 Å, 2.391 Å and 2.354 Å, respectively. Ca2+(4), Ca2+(5), Ca2+(6) are coordinated with nine, eight and seven oxygen atoms, and the average Ca2+-O2− bond lengths are 2.693 Å, 2.625 Å and 2.525 Å, respectively. The coordination environments of Ca2+ are also shown in Figure 1c. In the present case, Pr3+ ions may enter all six Ca2+ sites, and the luminescence properties we observed should be the whole contribution of Pr3+ in Ca2+ sites.
Figure 2a shows the XRD patterns of Pr3+-doped phosphors Ca3−xPrxAl2O6. The observed diffraction peaks are similar in the investigated doping concentration range, and all the diffractions are consistent with the standard card PDF 38-1429[Ca3Al2O6], demonstrating that Pr3+ ions were successfully introduced into the Ca3Al2O6 host. The doping of Pr3+ did not have a significant impact on the host structure. In addition, a series of Ca2.97−xPr0.03RxAl2O6 (R = Li+, Na+, K+) samples were also prepared, and the XRD results are shown in Figure 2b. As can be seen, the diffraction patterns also accord well with the standard card. All the samples are of a single pure phase. Figure 2c displays the representative SEM image of the Ca2.97Pr0.03Al2O6 sample, and the as-prepared sample shows irregular morphology with several micrometers in size. The EDS (energy-dispersive spectroscopy) images were obtained from one particle [marked by a red square] selected from the SEM image in Figure 2c. The elements Ca, Al, Pr and O were successfully detected, as shown in Figure 2d. The elemental mapping results indicate all the elements Ca, Al, O, Pr have been uniformly distributed over the whole particle, and there is no obvious element aggregation in the particles.

3.2. Luminescence Properties of Pr3+ in Ca3Al2O6

Figure 3a illustrates the diffuse reflectance spectra of Ca3Al2O6 and Ca2.97Pr0.03Al2O6. A very weak absorption band before 400 nm can be observed, which was ascribed to host-related absorption. The diffuse reflectance spectrum of the Ca3Al2O6 host was in good accordance with the reported one [25]. For the Ca2.97Pr0.03Al2O6 sample, a series of sharp absorption lines in the 400–650 nm range also can be observed. In comparison with the pure Ca3Al2O6 host, the absorptions in 400–650 nm are assigned to the 4f-4f transition absorptions of Pr3+ in the host. To further characterize the luminescence properties of Ca2.97Pr0.03Al2O6, the excitation spectrum and the corresponding emission spectrum are shown in Figure 3b,c, respectively. After monitoring the emission at 612 nm, a series of excitation bands were detected. The sharp excitation bands in the 425–500 nm wavelength range are ascribed to the 3H43P0,1,2 transition absorptions of Pr3+. A very weak band at around 300 nm may relate to the essential absorption of the Ca3Al2O6 host. The excitation spectrum agrees with the diffuse reflectance spectra in Figure 3a. This result indicates that a weak energy transfer from the host lattice to Pr3+ may occur. Based on the excitation spectrum, the corresponding emission spectrum was measured, as shown in Figure 3c. Upon 446 nm excitation, a series of sharp emission bands from the blue to deep red region were achieved, which are mainly related to the 3P03H4, 3P03H5, 3P03H6/1D23H4, 3P03F2, 3P03F4 transitions of Pr3+ [32,33]. In order to the reveal luminescence process, the energy levels of Pr3+ in the 0–25,000 cm−1 range are shown in the inset of Figure 3c. Upon 446 nm excitation, the exaction energy was absorbed by Pr3+ through 3H43P2 transitions. Then, electrons returned to 3P0 and 1D2 levels via non-radiative relaxation processes, and the emission bands in 450–750 nm were finally observed. Herein, a multi-band emission can be obtained under blue light (446 nm) excitation for samples singly doped with Pr3+, which indicates that the phosphor may have potential applications, such as LED application.
When the Pr3+ doping concentration increases from 0.002 to 0.10, all the emission spectra are similar [see Figure 3d], demonstrating that the Pr3+ doping concentration has little effect on the spectral shape of Ca3−xPrxAl2O6. However, the integrated emission intensity changes greatly with the doping concentration. As displayed in Figure 3e, the emission intensity greatly increases with increasing Pr3+ concentrations at first and reaches a maximum at x = 0.03, then it decreases gradually with x value owing to the concentration quenching and the non-radiative energy transferring to quenching centers. Normally, the optimal doping concentration is associated with the crucial energy transfer distance (Rc). Herein, the Rc value between Pr3+ (activator ions) in the Ca3Al2O6 host could be estimated through the following equation [3]:
R c 2 ( 3 V 4 π x c N ) 1 3
where V represents the unit cell volume, N is the number of Ca2+ ions in a unit cell, and xc refers to the optimal doping concentration. For Pr3+-doped Ca3Al2O6, V = 3558.45 Å3 and N = 24. As a consequence, the Rc value is estimated to be 26.63 Å. In general, exchange interaction should be responsible for forbidden transitions with an Rc value less than 5 Å. Clearly, the Rc value is much larger than 5 Å in the present case. Therefore, multipolar interactions should be the dominant factor for the concentration quenching of Pr3+.
Figure 3f depicts the luminescence decay curves of 3P03H4 transition emission (488 nm). The decay processes nearly follow a first-order exponential form at a low Pr3+ concentration, and then exhibit certain deviations for a high Pr3+ concentration. First, some defects will exist in the phosphors due to the charge imbalance between Ca2+ and Pr3+, and the high temperature sintering process may also generate some defects as well. The complex defects could affect the excited state relaxation process of Pr3+ in the host. Second, the inner energy transfer or interactions between adjacent Pr3+ ions increase gradually with the Pr3+ doping concentration. Third, the multi-site luminescence of Pr3+ exists in Ca3Al2O6, and the luminescence decay for Pr3+ in each Ca2+ site may also show some differences. Therefore, the decay curves of Pr3+ exhibit bi-exponential or even multi-exponential decay behaviors with increasing the Pr3+ doping concentration. Because of the deviations, the average decay constants can be estimated using Equation (2) [34]:
τ = 0 I ( t ) t d t 0 I ( t ) d t
The estimated decay constants are also shown in Figure 3f. The decay constants shortened from 127.84 μs (x = 0.002) to 118.22 μs (x = 0.10). For luminescence materials singly doped with Pr3+, the average lifetime τ is the reciprocal sum of all the radiative transition (WR) and non-radiative transition (WNR), as can be described by Equation (3) [35]:
τ = 1 w R + w N R
Herein, the decrease in τ confirms the increasing non-radiative energy transfer with x value. The influence of temperature is a key factor for further applications. Figure 4a shows the emission spectra of Ca2.97Pr0.03Al2O6 at various temperatures. The 3P03H4, 3P03H5, 3P03H6/1D23H4, 3P03F2, 3P03F4 transition emission lines of Pr3+ can be observed, All the emission spectra are similar, but the emission intensity changes remarkably. Figure 4b displays the emission intensity dependent on temperature. The relative emission intensity decreases gradually with increasing temperature owing to the temperature-involved thermal quenching. The emission intensity at 423 K maintains about 66.8% of that at 298 K (room temperature). In general, the ΔEa value (activation energy) can be used to evaluate the thermal quenching properties of a phosphor, and the relevant equation is described as following [36]:
I T = I 0 1 + A e x p ( Δ E a / k T )
where I0 and IT refer to the initial emission intensity and the intensity at a given temperature T, respectively. k represents the Boltzmann constant, and A can be treated as constant in specific cases. The Equation (4) could also be expressed as [37]
ln I 0 I 1 = l n A Δ E a k T
As a consequence, the activation energy ΔEa can be obtained according to the relationship between ln[(I0/I) − 1] and 1/(kT). As depicted in the inset of Figure 4b, a well-fitted straight line with a slope of −0.149 was achieved. Thus, the ΔEa value is 0.149 eV for Ca2.97Pr0.03Al2O6. As reported, the ΔEa value for the Ca2ZnSi2O7:0.005Pr3+ phosphor is 0.2255 eV [38], and the values are 0.22 eV, 0.18 eV for Pr3+-doped SrLaMgTaO6:Pr3+ and BaLaMgTaO6:Pr3+, respectively [35]. The ΔEa value in this research is similar to that of Ca9MgLi(PO4)7:Pr3+Ea = 0.15 eV), which is slightly larger than that of CaLaB7O13:Pr3+Ea = 0.116 eV) [39,40].
Figure 4c shows the CIE (Commission International de I′Eclairage 1931) chromaticity coordinates for the emission of Ca2.97Pr0.03Al2O6 at various temperatures. Although all the emission lines can be observed in Figure 4a, the chromaticity coordinates also show some differences, which move from (0.431, 0.379) at 298 K to (0.416, 0.417) at 573 K due to the thermal population of electrons between the 3P0 and 1D2 levels. The emission colors are located at the orange–yellow region in all the temperature ranges.
Upon 446 nm excitation and detecting the emission at 488 nm, temperature-dependent luminescence decay curves were collected and illustrated in Figure 4d. The luminescence decay times become shorter and shorter with increasing temperature, which also demonstrates the increasing non-radiative energy transfer processes. These results are consistent with the temperature-dependent emission spectra in Figure 4a.

3.3. The Influences of Charge Compensator Ions

In the above section, the phosphors were designed by nonequivalent substitution, that is, one Pr3+ substitutes one Ca2+ in the Ca3Al2O6 host. Therefore, charge defects will exist due to the nonequivalent substitution, which may have impacts on the luminescence of Pr3+. Figure 5a shows the emission spectra of Ca2.97−xPr0.03LixAl2O6 (x = 0, 0.01, 0.02, 0.03) samples. The introduction of compensator ions Li+ does not significantly influence the emission spectral shape. The inset depicts the integrated emission intensity at various Li+ concentrations. The co-doping of Li+ contributes positively to the emission intensity of Pr3+. The emission intensity is about 3.4 (0.01 Li+), 2.3 (0.02 Li+), 2.6 (0.03 Li+) times that of Ca2.97Pr0.03Al2O6, respectively. The emission spectra of Ca2.97−xPr0.03NaxAl2O6 and Ca2.97−xPr0.03KxAl2O6 samples are displayed in Figure 5b,c. The incorporation of Na+ and K+ can also improve the emission intensity of Pr3+. The relative emission intensity is about 2.8 (0.01 Na+), 2.3 (0.02 Na+), 2.7 (0.03 Na+), 1.2 (0.01 K+), 1.6 (0.02 K+), 1.3 (0.03 K+) times that of Ca2.97Pr0.03Al2O6, respectively. Among all the samples, the optimal emission intensity can be achieved for Ca2.96Pr0.03Li0.01Al2O6. For the Ca2.97Pr0.03Al2O6 sample, defects may be caused via several paths [31,41,42]: (1) Three Ca2+ replaced by two Pr3+ ions generates a Ca2+ vacancy at the same time, which can be described by 3CaCa→2 P r C a · + V C a . (2) Two Ca2+ ions replaced by two Pr3+ ions may cause an interstitial O i defect. (3) Two Ca2+ ions replaced by two Pr3+ ions may cause an oxygen vacancy V O according to the possible process 2CaCa→2 P r C a · + V O . In fact, some defects could act as killers of luminescence centers, resulting in the quenching of luminescence intensity. For Ca2.97−xPr0.03RxAl2O6 samples, two Ca2+ ions would be substituted by one Pr3+ ion and one R+ ion according to 2CaCa P r C a · + R C a . Some vacancies or defects were reduced. Therefore, the observed luminescence intensity can be improved. Furthermore, the ionic radii of K+ and Na+ are larger than that of Li+. Li+ ions more easily fill the vacancy defects, which may also further promote the effective entrance of the Pr3+ into Ca2+ sites in the host [31,43,44]. As a consequence, the emission intensity can be significantly enhanced by the introduction of Li+ ions into the host lattice. The influence of charge compensator ions on some phosphors have been reported, such as BaZrGe3O9:Cr3+, Ca2GdTaO6:Mn4+, M (M = Li+, Na+, K+, and Mg2+), Ca2ZnSi2O7:Pr3+ and α-Sr2P2O7:Dy3+ [38,42,45,46].
Luminescence decay curves of Ca2.97−xPr0.03LixAl2O6, Ca2.97−xPr0.03NaxAl2O6 and Ca2.97−xPr0.03KxAl2O6 were collected at room temperature to confirm the influence of charge compensator ions, as illustrated in Figure 5d–f. As can be observed, several luminescence decay curves show notable increase in comparison with Ca2.97Pr0.03Al2O6, especially for Ca2.96Pr0.03Li0.01Al2O6 and Ca2.96Pr0.03Na0.01Al2O6. When charge compensator ions were introduced into the host lattice, the defects and interactions between adjacent Pr3+ ions were be changed. Luminescence decay curves further demonstrate that non-radiative energy transfer processes have been reduced, which leads to the increases of emission intensity in Figure 5a–c.
To evaluate the influence of charge compensator ions on thermal quenching properties, temperature dependent emission spectra and luminescence decay curves were measured for Ca2.96Pr0.03Li0.01Al2O6, Ca2.96Pr0.03Na0.01Al2O6 and Ca2.95Pr0.03K0.02Al2O6. Emission intensity declines with increasing temperature for Li+, Na+ and K+ co-doped samples, and all the emission profiles are similar as shown in Figure 6a–c. Normalized integrated emission intensity dependent on different temperatures are listed in Figure 6d. The observed emission intensities at 423 K are all about 65–70% of those at 298 K, which are similar to that of Ca2.97Pr0.03Al2O6.
The CIE chromaticity coordinates of Ca2.96Pr0.03Li0.01Al2O6, Ca2.96Pr0.03Na0.01Al2O6 and Ca2.95Pr0.03K0.02Al2O6 at different temperatures are shown in Figure 7a–c. The variation tendencies are the same and accord with the Ca2.97Pr0.03Al2O6 sample in Figure 4c. Luminescence decay curves and decay times at various temperatures are illustrated in Figure 7d–i. Luminescence decay processes become faster and gradually deviate from the first-order exponential, owing to the heat-involved energy transfer. The average decay times were also estimated by Equation (2), and the results are shown in Figure 7g–i. The decay times decrease from 122.37 μs (298 K) to 120.16 μs (573 K) for Ca2.96Pr0.03Li0.01Al2O6. The values are 122.33 μs (298 K, Na+ doped), 119.81 μs (573 K, Na+ doped), 122.16 us (298 K, K+ doped), 119.93 μs (573 K, K+ doped). The decreases in decay times are also similar for the three samples, which are very consistent with the observed temperature-dependent emission spectra. Based on the above discussions, it is can be found that certain amounts of compensator ions will enhance the emission intensity of Pr3+, especially for Li+ co-doped ones.

3.4. Potential Applications

A phosphor-converted light-emitting diode (pc-LED) was fabricated with a blue LED chip, Y3Al5−xGaxO12:Ce3+ (YAGG:Ce, yellow–green component) and Ca2.96Pr0.03Li0.01Al2O6. Multi-emission bands from ~425 nm to 750 nm were detected at 20–320 mA driven currents, as shown in Figure 8a. Herein, differences appear in comparisons with the above emission spectra. Several factors may contribute to this: ① The responses of the relative emission intensity some exhibit are different for different spectrophotometers. ② The filters used in measurement setup. ③ Most importantly, the absorption of YAGG:Ce phosphor in blue region. The emission intensity of the pc-LED device increases gradually with driven currents. We did not observe light saturation in the 20–320 mA current range. The inset of Figure 8a displays the photographs of the pc-LED, and bright white light can be observed clearly with the driven power on. CIE chromaticity coordinates of the working pc-LED are (0.3682, 0.3598), and the CRI (color rendering index) and CCT (correlated color temperature) are 81.9 and 4236 K driven by 160 mA current, respectively. The output optical power also increases with a driven current, as shown in Figure 8b. The luminous efficiency in this situation is around 8.99 lm/W, and the maximum photoelectric efficiency is about 3.5%.

4. Conclusions

In summary, a series of Pr3+-doped Ca3Al2O6 with multi-band emission were successfully designed and prepared. All the samples crystallize in cubic system, but the emission intensity is strongly dependent on the Pr3+ doping concentration. The optimal Pr3+ concentration is 0.03, and the crucial energy transfer distance Rc was determined to be 26.63 Å. Pr3+-doped phosphors exhibit good thermal quenching properties. The emission intensity at 423 K can maintain 65–70% of that at room temperature, and the estimated activation energy ΔEa is 0.149 eV for Ca2.97Pr0.03Al2O6. The introduction of charge compensator ions can greatly enhance the emission intensity of Pr3+ due to a possible decrease in charge defects, especially for the Li+ co-doped ones. The luminescence intensity of Ca2.96Pr0.03Li0.01Al2O6 can be increased by 340% in comparison to that of Ca2.97Pr0.03Al2O6. A white light emission pc-LED was created using Y3Al5−xGaxO12:Ce3+ and Ca2.96Pr0.03Li0.01Al2O6 as color converters. The CIE coordinates of the working pc-LED are (0.3682, 0.3598), and the CRI and CCT are 81.9 and 4236 K under 160 mA current. Thanks to good multi-band emission properties, the designed phosphors may have potential applications.

Author Contributions

D.H.: conceptualization, writing—review and editing, investigation, formal analysis. R.H.: investigation, formal analysis. Y.Z.: investigation, formal analysis. H.L.: investigation. W.Z.: investigation. Z.L. (Zhisen Lin): investigation. Y.G.: investigation. Z.L. (Zewen Lin): investigation. J.D.: investigation. J.-Y.L.: writing—review and editing, formal analysis. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 12104117), the Scientific Research Project of Education Department of Guangdong Province (2021KTSCX075, 2022ZDJS067), the Hanshan Normal University Start-up Fund for Doctoral Scientific Research (QD202320), and the Advanced Materials and Devices Laboratory (623012).

Data Availability Statement

The 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.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) X-ray Rietveld refinements of Ca3Al2O6 host; (b) X-ray Rietveld refinements of Ca2.97Pr0.03Al2O6 phosphor; (c) crystal structure of Ca3Al2O6 and the coordination environments of Ca2+ in the host.
Figure 1. (a) X-ray Rietveld refinements of Ca3Al2O6 host; (b) X-ray Rietveld refinements of Ca2.97Pr0.03Al2O6 phosphor; (c) crystal structure of Ca3Al2O6 and the coordination environments of Ca2+ in the host.
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Figure 2. (a) XRD patterns of Ca3−xPrxAl2O6 phosphors; (b) XRD patterns of Ca2.94Pr0.03R0.03Al2O6 (R = Li+, Na+, K+); (c) SEM image of Ca2.97Pr0.03Al2O6 sample; (d) EDS images of one Ca2.97Pr0.03Al2O6 particle.
Figure 2. (a) XRD patterns of Ca3−xPrxAl2O6 phosphors; (b) XRD patterns of Ca2.94Pr0.03R0.03Al2O6 (R = Li+, Na+, K+); (c) SEM image of Ca2.97Pr0.03Al2O6 sample; (d) EDS images of one Ca2.97Pr0.03Al2O6 particle.
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Figure 3. (a) Diffuse reflectance spectra of Ca3Al2O6 and Ca2.97Pr0.03Al2O6; (b) excitation spectrum of Ca2.97Pr0.03Al2O6, λem = 612 nm; (c) emission spectrum of Ca2.97Pr0.03Al2O6, λex = 446 nm; (d) emission spectra of Ca3−xPrxAl2O6 upon 446 nm excitation; (e) integrated emission intensity as a function of Pr3+ doping concentration upon 446 nm excitation; (f) luminescence decay curves of Ca3−xPrxAl2O6.
Figure 3. (a) Diffuse reflectance spectra of Ca3Al2O6 and Ca2.97Pr0.03Al2O6; (b) excitation spectrum of Ca2.97Pr0.03Al2O6, λem = 612 nm; (c) emission spectrum of Ca2.97Pr0.03Al2O6, λex = 446 nm; (d) emission spectra of Ca3−xPrxAl2O6 upon 446 nm excitation; (e) integrated emission intensity as a function of Pr3+ doping concentration upon 446 nm excitation; (f) luminescence decay curves of Ca3−xPrxAl2O6.
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Figure 4. (a) Emission spectra of Ca2.97Pr0.03Al2O6 at various temperatures; (b) normalized emission intensity dependent on temperature, and inset displays the relationship between ln[(I0/I) − 1] and 1/(kT); (c) CIE chromaticity coordinates of Ca2.97Pr0.03Al2O6 at different temperatures; (d) luminescence decay curves of Ca2.97Pr0.03Al2O6 at different temperatures.
Figure 4. (a) Emission spectra of Ca2.97Pr0.03Al2O6 at various temperatures; (b) normalized emission intensity dependent on temperature, and inset displays the relationship between ln[(I0/I) − 1] and 1/(kT); (c) CIE chromaticity coordinates of Ca2.97Pr0.03Al2O6 at different temperatures; (d) luminescence decay curves of Ca2.97Pr0.03Al2O6 at different temperatures.
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Figure 5. (a) Emission spectra of Ca2.97−xPr0.03LixAl2O6 (x = 0, 0.01, 0.02, 0.03) under 446 nm excitation; (b) emission spectra of Ca2.97−xPr0.03NaxAl2O6 (x = 0, 0.01, 0.02, 0.03) under 446 nm excitation; (c) emission spectra of Ca2.97−xPr0.03KxAl2O6 (x = 0, 0.01, 0.02, 0.03) under 446 nm excitation; (d) luminescence decay curves of Ca2.97−xPr0.03LixAl2O6 (x = 0, 0.01, 0.02, 0.03) at room temperature; (e) luminescence decay curves of Ca2.97−xPr0.03NaxAl2O6 (x = 0, 0.01, 0.02, 0.03) at room temperature; (f) luminescence decay curves of Ca2.97−xPr0.03KxAl2O6 (x = 0, 0.01, 0.02, 0.03) at room temperature.
Figure 5. (a) Emission spectra of Ca2.97−xPr0.03LixAl2O6 (x = 0, 0.01, 0.02, 0.03) under 446 nm excitation; (b) emission spectra of Ca2.97−xPr0.03NaxAl2O6 (x = 0, 0.01, 0.02, 0.03) under 446 nm excitation; (c) emission spectra of Ca2.97−xPr0.03KxAl2O6 (x = 0, 0.01, 0.02, 0.03) under 446 nm excitation; (d) luminescence decay curves of Ca2.97−xPr0.03LixAl2O6 (x = 0, 0.01, 0.02, 0.03) at room temperature; (e) luminescence decay curves of Ca2.97−xPr0.03NaxAl2O6 (x = 0, 0.01, 0.02, 0.03) at room temperature; (f) luminescence decay curves of Ca2.97−xPr0.03KxAl2O6 (x = 0, 0.01, 0.02, 0.03) at room temperature.
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Figure 6. (a) Temperature-dependent emission spectra of Ca2.96Pr0.03Li0.01Al2O6 under 446 nm excitation; (b) temperature-dependent emission spectra of Ca2.96Pr0.03Na0.01Al2O6 upon 446 nm excitation; (c) temperature-dependent emission spectra of Ca2.95Pr0.03K0.02Al2O6 under 446 nm excitation; (d) normalized emission intensity dependent of different temperatures.
Figure 6. (a) Temperature-dependent emission spectra of Ca2.96Pr0.03Li0.01Al2O6 under 446 nm excitation; (b) temperature-dependent emission spectra of Ca2.96Pr0.03Na0.01Al2O6 upon 446 nm excitation; (c) temperature-dependent emission spectra of Ca2.95Pr0.03K0.02Al2O6 under 446 nm excitation; (d) normalized emission intensity dependent of different temperatures.
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Figure 7. (a) CIE chromaticity coordinates of Ca2.96Pr0.03Li0.01Al2O6 at different temperatures; (b) CIE chromaticity coordinates of Ca2.96Pr0.03Na0.01Al2O6 at different temperatures; (c) CIE chromaticity coordinates of Ca2.95Pr0.03K0.02Al2O6 at different temperatures; (d) luminescence decay curves of Ca2.96Pr0.03Li0.01Al2O6 at different temperatures; (e) luminescence decay curves of Ca2.96Pr0.03Na0.01Al2O6 at different temperatures; (f) luminescence decay curves of Ca2.95Pr0.03K0.02Al2O6 at different temperatures; (g) luminescence decay times of Ca2.96Pr0.03Li0.01Al2O6 dependent on temperature; (h) luminescence decay times of Ca2.96Pr0.03Na0.01Al2O6 dependent on temperature; (i) luminescence decay times of Ca2.95Pr0.03K0.02Al2O6 dependent on temperature.
Figure 7. (a) CIE chromaticity coordinates of Ca2.96Pr0.03Li0.01Al2O6 at different temperatures; (b) CIE chromaticity coordinates of Ca2.96Pr0.03Na0.01Al2O6 at different temperatures; (c) CIE chromaticity coordinates of Ca2.95Pr0.03K0.02Al2O6 at different temperatures; (d) luminescence decay curves of Ca2.96Pr0.03Li0.01Al2O6 at different temperatures; (e) luminescence decay curves of Ca2.96Pr0.03Na0.01Al2O6 at different temperatures; (f) luminescence decay curves of Ca2.95Pr0.03K0.02Al2O6 at different temperatures; (g) luminescence decay times of Ca2.96Pr0.03Li0.01Al2O6 dependent on temperature; (h) luminescence decay times of Ca2.96Pr0.03Na0.01Al2O6 dependent on temperature; (i) luminescence decay times of Ca2.95Pr0.03K0.02Al2O6 dependent on temperature.
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Figure 8. (a) Emission spectra of pc-LED device driven at 20–320 mA, and inset shows the pictures of pc-LED prototype and working LED; (b) output optical power on dependent of driven current.
Figure 8. (a) Emission spectra of pc-LED device driven at 20–320 mA, and inset shows the pictures of pc-LED prototype and working LED; (b) output optical power on dependent of driven current.
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Table 1. Refined cell parameters of Ca3Al2O6 and Ca2.97Pr0.03Al2O6.
Table 1. Refined cell parameters of Ca3Al2O6 and Ca2.97Pr0.03Al2O6.
SamplesCa3Al2O6 HostCa2.97Pr0.03Al2O6
Space groupP a 3 ¯ P a 3 ¯
a = b = c (Å)15.26326 (6)15.26700 (4)
α = β = γ (°)9090
Cell volume (Å3)3555.86 (4)3558.45 (5)
Rp (%)9.247.36
Rwp (%)11.529.85
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Hou, D.; Huang, R.; Zhang, Y.; Li, H.; Zhang, W.; Lin, Z.; Guo, Y.; Lin, Z.; Dong, J.; Li, J.-Y. Multi-Band Emission of Pr3+-Doped Ca3Al2O6 and the Effects of Charge Compensator Ions on Luminescence Properties. Nanomaterials 2024, 14, 2. https://doi.org/10.3390/nano14010002

AMA Style

Hou D, Huang R, Zhang Y, Li H, Zhang W, Lin Z, Guo Y, Lin Z, Dong J, Li J-Y. Multi-Band Emission of Pr3+-Doped Ca3Al2O6 and the Effects of Charge Compensator Ions on Luminescence Properties. Nanomaterials. 2024; 14(1):2. https://doi.org/10.3390/nano14010002

Chicago/Turabian Style

Hou, Dejian, Rui Huang, Yi Zhang, Hongliang Li, Wenxing Zhang, Zhisen Lin, Yanqing Guo, Zewen Lin, Jianhong Dong, and Jin-Yan Li. 2024. "Multi-Band Emission of Pr3+-Doped Ca3Al2O6 and the Effects of Charge Compensator Ions on Luminescence Properties" Nanomaterials 14, no. 1: 2. https://doi.org/10.3390/nano14010002

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