Crystal structure, photoluminescence and afterglow properties of red-emitting phosphors Ca₄Nb₂O₉:Pr³⁺ for AC-LEDs

Abstract

Alternating current light-emitting diodes (AC-LEDs) set up a new era of economic semiconductor-based lighting. However, the flicker of AC-LEDs in the AC cycles is unhealthy for human eyes. In this work, the red-emitting afterglow phosphor Ca₄Nb₂O₉:Pr³⁺ was successfully synthesized via a traditional high temperature solid-state reaction method. The phase composition, photoluminescence spectra, afterglow curve, and thermoluminescence were investigated. It was revealed that the red emission (617 nm) is ascribed to the ³P₀ → ³H₆ transition of Pr³⁺ ions, and the afterglow time was in the second range after removal of the excitation source. In order to gain a deeper understanding of the origin of afterglow, a subsequent thermoluminescence analysis was performed and a relatively shallow trap within the host was found. These results indicated that Ca₄Nb₂O₉:Pr³⁺ phosphors could compensate for the flickering behavior of AC-LEDs and may be a candidate for application in AC-LEDs.

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1. Introduction

Due to their advantages such as convenience and high efficiency, white light-emitting diodes (LEDs) are in the stage of vigorous development, and are considered as alternatives to fluorescent lamps and incandescent lamps.^1–3 At present, the leading products in the industrial market are direct current (DC) LEDs. It is worth noting that there is 30% power loss during the process of alternating current (AC) to DC transfer. In addition, a large amount of heat will be generated during the AC to DC transfer process, which will cause the packaging resin to turn yellow and the phosphor to be degraded.⁴ Recently, AC driven LED (AC-LED) systems have attracted much attention from the industry because of the possibility to overcome these disadvantages. Besides, AC-LEDs will produce a dark interval of 5–20 ms,^5,6 and the dark duration phenomenon leads to an undesirable flicker effect that may cause visual fatigue and other symptoms in the human body.⁷ Therefore, an effective scheme is proposed to develop phosphors with short afterglow to compensate for the dark time and eliminate flicker.

Afterglow luminescence is a phenomenon of light persistent emission after the excitation source is stopped, which has attracted large-scale research attention because of its wide application in anti-counterfeiting, night displays, in vivo bioimaging, optical storage and so on.^8,9 However, the afterglow time of these phosphors is required to be much longer in some applications mentioned above. In contrast, little attention has been paid to short afterglow phosphors applied to AC-LEDs. There are two processes that need attention in the application of AC-LEDs: firstly, the optimal excitation wavelength should be adjusted to about 450 nm to match the commercial InGaN chip; secondly, the depth of the modulated trap should be shallow in order to detrap easily; thirdly, the carrier density in the shallow trap should not be too high, otherwise the afterglow time will be too long. In 2012, Yeh et al. first reported the afterglow phosphor SrSi₂O₂N₂:Eu²⁺,Mn²⁺ and proposed to reduce flicker for AC-LEDs.⁶ Over the same year, a blue-light activated Gd_2.98Al₂Ga₃O₁₂:0.02Ce³⁺ yellow–green persistent phosphor was reported by Lin et al., with a decreased flicker rate from 100% to 69%.¹⁰ Particularly, Chen et al. reported a green phosphor SrAl₂O₄:Eu²⁺,Y³⁺ for AC-LEDs.¹¹ However, SrAl₂O₄:Eu²⁺,Y³⁺ and most of the garnet phosphors emitted yellow or green light under blue light excitation and a white-colour-emitting afterglow had not yet been vigorously developed because of the difficulties associated with red-emitting afterglow phosphors.^12–14 As far as we know, red-emitting afterglow phosphors, which have a millisecond afterglow time, used in AC-LEDs have not been much reported so far. Therefore, in this study, a novel AC-LED phosphor Ca₄Nb₂O₉:Pr³⁺ was developed based on red-emitting afterglow phosphors. As an important host-material, niobates have been extensively explored due to their diverse crystal structure and multifunctional properties. As for the selection of doped ions, the Pr³⁺ ion is a common element to explore rare earth doped multifunctional luminescent materials. In general, the luminescence spectra of Pr³⁺ can exhibit intense ³P₀–³H₆ emission at about 610 nm, and consequently the Pr³⁺ ion is widely used in the red emission region. For example, a bright red-emitting perovskite phosphor (Ca, Zn)TiO₃:Pr³⁺ in long afterglow applications was reported by Haranayh et al. In 2006, Liu et al. developed a highly stable red oxynitride β-SiAlON:Pr³⁺ for white LED applications.^15,16 Additionally, a variety of Pr³⁺ doped phosphors has been studied so far, such as long afterglow phosphors CdSiO₃:Pr³⁺,¹⁷ X-ray storage phosphors YNbO₄:Pr^3+ 18 and magnetic materials Bi–FeO₃:Pr³⁺.¹⁹ Owing to the properties of Pr³⁺-doped functional materials, they are widely used in night displays, in vivo bioimaging and temperature sensors, which have been explored by many researchers.²⁰

In this work, a red-emitting afterglow phosphor Ca₄Nb₂O₉:Pr³⁺ was developed successfully, and the crystal structure, morphology, photoluminescence properties and afterglow phenomenon were deeply discussed, and all the results indicated that the Ca₄Nb₂O₉:Pr³⁺ phosphor is suitable for the application of AC-LEDs.

2. Experimental details

2.1 Synthesis of samples

CaCO₃ (AR, Sinopharm Chemical Reagent Co., Ltd), Nb₂O₅ (99.9%, Shanghai Aladdin Biochemical Technology Co., Ltd), and Pr₆O₁₁ (99.99, Shanghai Aladdin Biochemical Technology Co., Ltd) were used by raw materials. Ca₄Nb₂O₉:xPr³⁺ powders with various Pr³⁺-doped concentrations were synthesized by the traditional solid-state reaction technique. The stoichiometric amounts of CaCO₃, Nb₂O₅, Pr₆O₁₁ and ethanol were added into an agate mortar to mix and grind for 30 min. Then, the mixture was dried and sintered at 1450 °C for 5 h under 95% N₂ + 5% H₂ reductive atmosphere in a tube furnace.

2.2 Synthesis and characterization

The X-ray diffraction (XRD) patterns were recorded on a Germany Brucker D2 PHASER diffractometer in the 2θ range of 10–80° equipped with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA. The crystal structure was refined by the Rietveld method using the General Structure Analysis System (GSAS) program. The photoluminescence (PL), photoluminescence excitation (PLE), persistent luminescence (PersL), and decay curves were measured using an FLS980 spectrometer (Edinburgh Instruments, the United Kingdom). The diffuse reflection spectra (DRS) of the samples were measured using a UV-Vis-NIR spectrophotometer (PerkinElmer, Lambda 950), using BaSO₄ as a standard reference. For the thermoluminescence (2D-TL/3D-TL) measurements, the samples were mounted on a thermal stage (298–770 K, LTTL-3DS, Radi Tec), after irradiation with 254 nm UV light for 5 min, and the heating rate was 1 K s⁻¹ and reached 770 K. The persistent luminance (mcd m⁻²) was measured by using a PR 305 persistent luminance instrument (Zhejiang University Sensing Instrument Co. Ltd, Hangzhou, China). The samples were irradiated for 5 min, and then measured and the duration of the luminance 15 s was calculated after the excitation was stopped, until the intensity reached 0.00032 cd m⁻². The sample morphology was measured by using a scanning electron microscope TESCAN VEGA 3 SEM (Tescan China, Ltd). Energy-dispersive spectroscopy (EDS) and elemental mapping (SEM) were performed by using a table-top electron microscope.

3. Results and discussion

3.1 Crystal structure analysis

The phase purity of the sample was determined by XRD and further data analysis. Fig. 1(a) shows the XRD pattern of the samples with different concentrations of Pr³⁺ ion doped Ca₄Nb₂O₉, and it can be seen that all the diffraction peaks match well with standard card ICSD NO. 49-0911. This indicates that a small number of Pr³⁺ ions was successfully incorporated into the host. Fig. 1(b) depicts the results of Rietveld refinement for Ca₄Nb₂O₉:0.003Pr³⁺. It can be found that there is little difference between the experimental value and the calculated value. The refinement factors of χ², the weighted profile R-factor (R_wp), and the expected R factor (R_p) are determined to be ∼3.04, ∼11.56%, and ∼7.72%, respectively, indicating that the refined results are reliable. More crystallographic data of Ca₄Nb₂O₉:0.003Pr³⁺ are listed in Table 1. These results indicate that all the samples have a single phase with a monoclinic structure. The Ca₄Nb₂O₉ structure belongs to the space group P2₁/c with medium lattice parameters a = 9.813 Å, b = 5.535 Å, c = 17.344 Å and V = 767.048 ų. We have listed the lattice parameters of the samples doped with different concentrations of Pr³⁺ in Table 1 according to this suggestion, and the corresponding descriptions were added as follows: “owing to the similar valence and ionic radius of Pr³⁺ ions (CN = 6, R_Pr³⁺ = 0.99) and Ca²⁺ (r = 1.00 Å for CN = 6), the Pr³⁺ ions tend to replace the site of Ca²⁺ in the Ca₄Nb₂O₉ host. In order to prove that the Pr³⁺ ions occupy the Ca²⁺ crystallization site, lattice parameters of the samples doped with different concentrations of Pr³⁺ were calculated and are shown in Table 2. It can be seen that the lattice parameters gradually decrease with the increase of the Pr³⁺ doping concentration, confirming that the Pr³⁺ ions occupy the Ca²⁺ crystallization site.

Fig. 1 (a) XRD patterns of the as-prepared Ca₄Nb₂O₉:xPr³⁺ (x = 0.001, 0.003, 0.005, 0.007, and 0.01) samples and the standard data presented as a reference, (b) XRD Rietveld refinement of Ca₄Nb₂O₉:0.003Pr³⁺, (c) the unit cell structure of Ca₄Nb₂O₉, and (d) the coordination environment of Ca.

Table 1 Final refined structural parameters for Ca₄Nb₂O₉:0.003Pr³⁺

Formula	Ca4Nb2O9
Crystal system	Monoclinic
Space group	P21/c
Vol (Å^3)	767.048
Unit cell dimension (Å)	a = 9.813, b = 5.535, c = 17.344
Reliability factors	R wp = 8.83%, Rp = 7.72%
Program	GSAS
Profile range	5–80°
Step	0.006°
Radiation	Cu Kα

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Table 2 Final refined structural parameters for Ca₄Nb₂O₉:x%Pr³⁺ (x = 0, 0.1, 0.3, 0.5, 0.7, and 1)

Ca4Nb2O9:xPr^3+	Crystal system	Space group	a/(Å)	b/(Å)	c/(Å)	V/(Å^3)
x = 0	Monoclinic	P21/c	9.832	5.542	17.361	769.229
x = 0.001	Monoclinic	P21/c	9.827	5.539	17.352	768.485
x = 0.003	Monoclinic	P21/c	9.813	5.535	17.344	767.048
x = 0.005	Monoclinic	P21/c	9.805	5.531	17.336	766.862
x = 0.007	Monoclinic	P21/c	9.796	5.529	17.329	766.134
x = 0.01	Monoclinic	P21/c	9.788	5.525	17.315	765.683

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The crystal structure of Ca₄Nb₂O₉ was drawn using Diamond software and is shown in Fig. 1(c and d). It is composed of [NbO₆] and [CaO₆] octahedra, where Nb and Ca are located at the center of the octahedra and the oxygen atoms are located on six peaks, in which the oxygen between the layers is shared by Nb and Ca. There are two different Ca sites in the host lattice. Ca1, Ca2, and Ca3, Ca4, Ca5 are octahedral arrangements and pentahedron arrangements, respectively.

As shown in Fig. 2(a–f), SEM-EDS and elemental mapping analysis of Ca₄Nb₂O₉ and Ca₄Nb₂O₉:0.003Pr³⁺ were performed to determine whether the dopants result in element aggregation. The SEM images (Fig. 2(a–f)) with different magnifications show partial aggregation of the elements and exhibit relatively homogeneous sizes, which range from a few microns to tens of microns. The elemental map displays that the elements of Ca, Nb, O and Pr are uniformly distributed as shown in Fig. 2(h), and the energy-dispersive X-ray spectroscopy (EDS) analysis as shown in Fig. 2(j) indicated that each component element is distributed in the selected region. All the results indicated that Pr³⁺ has been completely mixed into this phosphor.

Fig. 2 SEM images at different magnifications of the host (a–c) and Ca₄Nb₂O₉:0.003Pr³⁺ (d–f), (j) EDS analysis of Ca₄Nb₂O₉:0.003Pr³⁺, and (h) EDS mapping of Ca, O, Nb and Pr in the selected area, respectively.

3.2 Photoluminescence properties

Fig. 3 displays the diffuse reflectance spectra of the un-doped and Pr³⁺-doped Ca₄Nb₂O₉ and the two spectra are basically the same except for many sharp absorption peaks in the range of 450–500 nm. In addition, it can be seen that the Pr³⁺-doped Ca₄Nb₂O₉ phosphor has a strong absorption band of 225–400 nm and a high reflection band of 400–500 nm. Therein, the reason for the increase in the intensity at 200–275 nm and 275–400 nm is due to host absorption and 4f–5d transition. By comparing the PLE spectra detected at 617 nm of the sample Ca₄Nb₂O₉:0.003Pr³⁺, extensive absorption was found at ∼260 nm and ∼299 nm, among which the 260 nm band belongs to the absorption band of the Ca₄Nb₂O₉ host material and the 299 nm band was due to the 4f → 5d transitions of Pr³⁺. However, the broadband in the range of 225–350 nm may be considered as the overlap of the Nb⁵⁺–O²⁻ charge transfer band and the 4f → 5d transition of Pr³⁺. Additionally, another three distinguishable absorption peaks located at ∼450 nm, ∼476 nm and ∼482 nm are assigned to strong ³H₄ → ³P₂, ³H₄ → ³P₁ and ³H₄ → ³P₀ spin-allowed transitions of Pr³⁺ ions. A satisfactory result is that the DRS of Pr³⁺-doped Ca₄Nb₂O₉ is consistent with the PLE spectrum as shown in Fig. 3. The band gap (E_g) is calculated as eqn (1):^21–23

[αhν]² = A(hν − E_g) (1)

where hν was the incident photon energy, α was the absorption coefficient, and A was a constant.²⁴ The absorption coefficient can be obtained by using eqn (2):where, R is the observed reflectance in the DRS. The relationship between [αhν]² and incident energy hν is plotted in the inset of Fig. 3. Based on equations, the intersection of the linear part and the X-axis can be calculated to estimate the value of E_g as 4.132 eV. More importantly, it was found that there is a strong absorption peak located at 450 nm in the DRS and PLE spectra of Ca₄Nb₂O₉:Pr³⁺, indicating that it can match well with the blue light chip, so it might be used as a blue light excited phosphor for LEDs.

Fig. 3 Diffuse reflectance spectra of un-doped Ca₄Nb₂O₉ and Ca₄Nb₂O₉:0.003Pr³⁺; the inset shows the relationship of [F(R)hν]²versus photon energy hν, and the bottom colored spectrum shows the normalized PLE spectrum of Pr³⁺ ions in the Ca₄Nb₂O₉:0.003Pr³⁺sample.

In order to further verify the luminescence properties of Ca₄Nb₂O₉:Pr³⁺ upon excitation by blue light, the PL spectra were measured and are shown in Fig. 4(a). The PL spectra show that the Pr³⁺ ion doped samples have the strongest emission band located at 613 nm for the red emission region. The dependence of the Pr³⁺ doping concenstration on the emission intensity is illustrated in the inset of Fig. 4(a). It can be seen that the relative emission intensity of the series of samples first enhanced with the increase of the Pr³⁺ doping concenstration and then decreased immediately. The enhancement of the emission intensity is caused by the increasing luminescence centers when the Pr³⁺ concentration was less than 0.3%. And the concentration quenching effect occurred due to the decrease of the Pr³⁺–Pr³⁺ distance and the nonradiative transition increased in the Ca₄Nb₂O₉ host when the Pr³⁺ concentration reached 0.3%. These results show that the optimal Pr³⁺ doping concentration in Ca₄Nb₂O₉:xPr³⁺ is 0.3%. Under 450 nm excitation, intense red emission peaks located at 613, 617 and 660 nm were observed in the PL spectra of Ca₄Nb₂O₉:xPr³⁺, which could be ascribed to the ¹D₂ → ³H₄, ³P₀ → ³H₆ and ³P₀ → ³F₂ transitions of Pr³⁺ ions, respectively. Due to the electron–phonon coupling effect, the transitions of ¹D₂ → ³H₄ and ³P₀ → ³H₆ coincide approximately. Based on the Judd–Ofelt theory, the strong emission intensity of 613 and 617 nm is caused by the host lattice distortion or non-radiative relaxation of Pr³⁺ ions.^25,26 The peaks centered around 480 nm and 750 nm are attributed to ³P₀ → ³H₄ and ³P₀ → ³F₄ of Pr³⁺, respectively. In order to further appreciate the mechanism of concentration quenching, the critical distance (R_c) between the neighboring Pr³⁺ ions is obtained as shown in eqn (3):²⁷

where, x_c is the critical concentration of the Pr³⁺ ion, V represents the volume of unit cell, and N is the value of cation in the unit cell. For the Ca₄Nb₂O₉ host, the crystallographic parameters are V = 767.048 ų, N = 4, and x_c = 0.3%, respectively. Generally, there are generally two mechanisms that cause concentration quenching: one is multipole interaction when R_c > 5 Å, and another is exchange interaction when R_c < 5 Å.²⁸ According to eqn (3), the critical distance of the Pr³⁺ doped Ca₄Nb₂O₉ host is 49.61 Å much greater than 5 Å. Therefore, concentration quenching is achieved through multipole interactions rather than exchange interaction mechanisms. Further discussion of the types of multipole interactions between Pr³⁺ can be made by using eqn (4):²⁹

I/x = K[1 + β(x)^θ/3]⁻¹ (4)

where I represents the integrated emission intensity, x is the concentration of the activator, and K and β are constants. θ is the index of the electric multipole and θ = 6, 8 or 10 for dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively. eqn (4) can be approximately simplified log(I/x) = A − (θ/3)log x, and the curve of lg(I/x) vs. lg(x) is shown in Fig. 4(b) to obtain the value of θ considering β(x)^θ/3 ≫ 1 when x is more than the critical concentration.³⁰ A nearly linear fitting can be obtained and the slope of the fitting line is about ∼1.5308 (−θ/3). So, the value of θ can be calculated to be 4.6, which is approximately equal to 6. The result indicates that the d–d interaction is the major mechanism for the luminescence quenching of Pr³⁺ in the Ca₄Nb₂O₉ host.

Fig. 4 (a) PL spectra of Ca₄Nb₂O₉:xPr³⁺ (x = 0.001, 0.003, 0.005, 0.007, and 0.01) upon excitation at 450 nm. The inset shows the PL intensity versus Pr³⁺ concentration of Ca₄Nb₂O₉:xPr³⁺ (x = 0.001, 0.003, 0.005, 0.007, and 0.01) and (b) the relationship between lg(I/x) and lg(x).

The corresponding decay curves of ³P₀ → ³H₆ transitions of Pr³⁺ (excited at 299 nm and 450 nm) have been measured at room temperature and are shown in Fig. 5(a and b). The decay curves can be well fitted by a double exponential fitting and the average lifetimes can be further calculated by using eqn (5):³¹

I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (5)

where I(t) corresponds to the luminescence intensities at time t, respectively. Both A₁ and A₂ are constants, and τ₁ and τ₂ represent the luminescence lifetimes of the fast and slow decay components, respectively. The average decay times (τ*) are determined viaeqn (6) using the parameters after fitting the decay curves:^32,33

τ* = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (6)

accordingly, excited at 299 nm, and the decay times were 0.0691, 0.0624, 0.0540, 0.0538 and 0.0461 ms corresponding to x = 0.001, 0.003, 0.005, 0.007, and 0.01 in Ca₄Nb₂O₉:xPr³⁺. On excitation at 450 nm, the decay times were 0.0709, 0.0635, 0.0550, 0.0547 and 0.0480 ms corresponding to x = 0.001, 0.003, 0.005, 0.007, and 0.01 in Ca₄Nb₂O₉:xPr³⁺ as represented in Fig. 5(b). Therefore, the luminescence life decreases monotonously with the increase of the Pr³⁺ concentration. When the concentration of Pr³⁺ increased from 0.1 mol% to 1 mol%, the life span was reduced by ∼0.0230 ms and ∼0.0229, respectively. This is because with the increase of the Pr³⁺ ion doping concentration, the critical distance between the Pr³⁺ ion and the Pr³⁺ ion is shortened, which makes the energy transfer more obvious, and the lifetime decreases. This shows that weak Pr³⁺–Pr³⁺ interactions among the Pr³⁺ ions in this system and the Pr³⁺ ions tend to distribute in the host lattice Ca₄Nb₂O₉ evenly.

Fig. 5 (a and b) Luminescence decay curves of Pr³⁺ under 299 nm and 450 nm excitation in Ca₄Nb₂O₉:xPr³⁺ (x = 0.001, 0.003, 0.005, 0.007, and 0.01).

It is essential to investigate the thermo-stability of phosphors used for LEDs. Fig. 6(a) illustrates the PL spectra of the typical Ca₄Nb₂O₉:0.003Pr³⁺ sample measured at various temperatures. It is found that with the increasing temperature from 298 K to 523 K, all the PL spectra show the same shapes and peak positions, but the PL intensity decreases gradually due to the influence of thermal quenching. It is worth noting that the PL intensity at 348 K is more potent than that at all temperatures. We speculate that the PL intensity increased abnormally at 348 K due to the contribution of shallow trap electrons discharged in the heating process. The relationship between the maximum strength integral value and the temperature is illustrated in Fig. 6(a). It can be seen that the strength decreases with the increase of temperature due to thermal quenching. The emission intensity of the Ca₄Nb₂O₉:0.003Pr³⁺ phosphors at 423 K remained about 92% compared with that at 298 K, indicating that the Ca₄Nb₂O₉:0.003Pr³⁺ phosphors had excellent thermal stability. To further explore the thermal quenching characteristics, the activation energy (E_a) is studied by using the Arrhenius formula:^34,35

Fig. 6 (a) Temperature-dependent PL spectra (λ_ex = 450 nm) of Ca₄Nb₂O₉:0.003Pr³⁺; the inset shows the relationship between the emission intensity and the temperature of Ca₄Nb₂O₉:0.003Pr³⁺ and (b) the relationship of 1/kT and ln[(I₀/I_T) − 1].

The above equation can be written as:

In(I₀/I − 1) = In A − E_a/KT (8)

where I₀ is the intensity at 298 K, I denotes the intensity of the corresponding temperature, k represents the Boltzmann constant, and A indicates a constant. According to the PL intensity as shown in Fig. 6(a), the relationship between ln(I₀/I − 1) and 1/kT is plotted in Fig. 6(b). The activation energy of Ca₄Nb₂O₉:0.003Pr³⁺ was obtained from the fitting result in Fig. 6(b). Thus, this suggests that the activation energy (E_a) for the thermal quenching is ∼0.2587 eV.

Fig. 7(a) shows the afterglow decay curves in single and double logic diagrams of the typical Ca₄Nb₂O₉:xPr³⁺(x = 0.1%, 0.3%, 0.7%, 0.9%, and 1%) samples. All the samples were charged for 5 min with an ultraviolet (λ_ex = 254 nm) lamp before measurements. The result indicated that the afterglow of the sample was sufficient to compensate for the scintillation time of the AC-LED. The afterglow luminescence behavior originating in 10 ms is shown in Fig. 7(a) inset. It can be seen that the afterglow intensity has no attenuation trend, indicating that it can be used to compensate for the flicker of the AC-LED.

Fig. 7 (a) Afterglow decay curves of Ca₄Nb₂O₉:xPr³⁺ (x = 0.001, 0.003, 0.005, 0.007, and 0.01) measured after being pre-irradiated by a 254 nm UV lamp for 5 min, and the inset shows the origin of 10 ms persistence luminescence behaviors. (b) TL curves of Ca₄Nb₂O₉:xPr³⁺ phosphors with different Pr³⁺ concentrations. (c) Three-dimensional (3D) TL spectrum of Ca₄Nb₂O₉:0.003Pr³⁺. The samples were pre-irradiated by a 254 nm UV lamp for 5 min and recorded at 10 s after the stoppage of the irradiation. (d) Afterglow spectra of Ca₄Nb₂O₉:0.003Pr³⁺ measured after being pre-irradiated by a 450 nm lamp for 5 min.

As shown in Fig. 7(b), the trap properties of Ca₄Nb₂O₉:xPr³⁺(x = 0.1%, 0.3%, 0.7%, 0.9%, and 1%) were carefully studied by means of recording TL curves that monitor the whole visible range immediately after UV light irradiation for 5 min. All the samples showed similar TL spectral characteristic of a conspicuous peak around 350 K. The changing trend of the TL intensity is the same as that of the afterglow. It can be seen that the integral TL intensity of the Ca₄Nb₂O₉:0.003Pr³⁺ sample is the largest of all in appearance. Generally, the intensity of TL can represent the trapped carrier concentration, and the location of the TL peak value is relevant to the depth of the trap. They all rely on the concentration of introduced Pr³⁺ for Ca₄Nb₂O₉:xPr³⁺. Meanwhile, the excess Pr³⁺ (x > 0.003) ions play a negative role in promoting the afterglow intensity and the half width of the TL peak.

As shown in Fig. 7(b), there are both deep and shallow traps, and the shallow traps are asymmetric, so there may be three kinds of defects. One is the oxygen vacancy created by the reduction of hydrogen at high temperatures, and the other is because of nonequivalent substitutions; an excess of positive charge in the host must be compensated. The only possible way to fulfil the charge compensation is that two Pr³⁺ ions replace three Ca²⁺ ions to balance the charge of the phosphor, which will create two positive defects and one negative defect, which leads to the formation of a large number of traps along with the increased concentration of Tb³⁺ ions, based on the defect reaction of .

The reason for the sudden increase of the 348 K PL intensity (as shown in Fig. 6(a)) can be further explained from the TL curves. In the TL curve, it was indicated that there is a shallow trap at 348 K; when the temperature rises to 348 K, a large number of trap electrons are discharged, so the PL intensity of 348 K increases sharply. For illustrating the afterglow process intuitively, the 3D-TL spectrum is displayed in Fig. 7(c). The trap depths can be calculated using the Urbach method:^36–38

E = T_m/500 (9)

where T_m is the peak temperature (in Kelvin). It can be seen from Table 3 that the trap is shallower when the Pr³⁺ doping concentration is 0.3%. It can be seen that the stronger the TL peak and the lower the peak corresponding temperature, the higher the intensity of its afterglow spectrum.

Table 3 Trap depth of Ca₄Nb₂O₉:xPr³⁺

Concentration (x)	x = 0.001	x = 0.003	x = 0.005	x = 0.007	x = 0.01
Trap depth (eV)	0.728	0.664	0.718	0.738	0.706

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In order to prove that the Ca₄Nb₂O₉:Pr³⁺ phosphor is useful for blue-chip AC-LEDs, Fig. 7(d) shows the blue light pre-irradiated afterglow spectrum of Ca₄Nb₂O₉:0.003Pr³⁺, for example. The similarity of the afterglow spectrum to the PL spectrum indicates that afterglow also originates from the Pr³⁺ ions. And the highest afterglow peak is attributed to the ³P₀–³H₆ (617 nm) and ¹D₂–³H₄ transitions (613 nm) of Pr³⁺.

Based on the previous data, an AC-LED device based on the Ca₄Nb₂O₉:0.003Pr³⁺ phosphor was fabricated as shown in Fig. 8(b). It is pumped by a 450 nm blue-chip with 5 V, 100 mA, and it emitted a red color. When the entire circuit is electrically connected, the Ca₄Nb₂O₉:0.003Pr³⁺ phosphor is irradiated to produce red light and the phosphors excited by the blue light chip begin to store the excited energy in the trap. CIE = (0.353, 0.3629), R_a = 74.96 and CCT = 2986 K can be obtained from the W-LEDs encapsulated as shown in Fig. 8(a). Meanwhile, quantum efficiency is also important for both theoretical and practical views, and the quantum efficiency of the Ca₄Nb₂O₉:Pr³⁺ sample is 30.3%. These results indicate that the prepared Ca₄Nb₂O₉:Pr³⁺ phosphors can be used as red light components of AC-LEDs.

Fig. 8 (a) EL spectra of the monochromatic lamp composed of 450 nm blue-chip, YAG:Ce³⁺ and Ca₄Nb₂O₉:0.003Pr³⁺ phosphor. (b) EL spectra of the W-LED lamp composed of the 450 nm blue-chip and the Ca₄Nb₂O₉:0.003Pr³⁺ phosphor. The inset figures show the corresponding W-LEDs and monochromatic lamp images.

4. Conclusions

In summary, we successfully prepared a series of Ca₄Nb₂O₉:Pr³⁺ phosphors. Structural analysis confirmed the formation of the monoclinic phase and the homogeneous distribution of Pr³⁺ in Ca₄Nb₂O₉. PL and PLE spectral analysis revealed that the Ca₄Nb₂O₉:Pr³⁺ phosphors can be efficiently excited by blue light and generated bright red emission. Temperature dependent PL property measurements demonstrated that this phosphor has excellent thermal stability for applications in the AC-LED field. Moreover, the afterglow luminescence time of Ca₄Nb₂O₉:0.003Pr³⁺ matched well with the requirement for AC-LEDs. Thus, the obtained outcomes with the Ca₄Nb₂O₉:xPr³⁺ phosphor reveals that it is very suitable for AC-LEDs as a potential candidate for depreciation of dark duration and flickering.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (11804038 and 11804007), the Natural Science Foundation of Shaanxi Province of China (2020JQ-891 and 2019JQ-424), the Opening Foundation of Key Laboratory for Special Function Materials and Structural Design of the Ministry of the Education (lzujbky-2020-kb06), the Baoji University of Arts and Sciences Key Research (ZK16071, ZK2018053, and ZK2017033), the Key Research and Development Plan of Shaanxi Province (2018ZDXMGY119), the Key Scientific Research Plan of Shaanxi Provincial Department of Education (18JC001), the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJQN202001325 and KJQN201901349), the Yongchuan Natural Science Foundation (YCSTC, 2020nb0601), and the Foundation of Chongqing University of Arts and Sciences (R2016DQ10 and R2019SCL01).

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