1. Introduction

In these years, researchers extensively integrated the technology of phosphor-converted white light-emitting diodes (pc-WLEDs) due to its long lifetime, robustness, and environmentally friendly nature [1–3]. Currently the most common method for commerce pc-WLEDs is based on blue emitting In:GaN coupled with yellow emitting Ce doped YAG phosphors. Due to the lack of red light, the commercial pc-WLEDs shows a low CRI (~70-80) value, which hinders the practical applications [4–6]. In order to eliminate these obstacles for pc-WLEDs, combination with red, green and blue light-emitting phosphors, with a ultraviolet excitation source could be another choice, which obtain a high CRI and suitable correlated colour temperature (CCT). However, this method not only requires controlled synthesis of their individual phosphor with uniform particle size in order to avoided agglomeration, which is itself a cumbersome task [7–10], but also involves complex interactions among the emission and excitation lights. Therefore, white light emission via combine with the intrinsic luminescence from a single host under the particular excitation is more preferred, which due to the significant advantages including lower manufacturing costs, an easier fabrication process and better reproducibility [11–13].

In order to realize white light in a single host, the most current available methods are: (i) doping a single rare earth ion, such as Eu²⁺ and/or Ce³⁺, (ii) co-doping rare earth ions with transition metal ions, based on the energy transfer mechanism [14–22]. However, these approaches mentioned above either shows a broad emission spectra which cause the luminescence cannot be adjusted effectively, or require a complex synthesis technique which further prevent their easy deployment as a single-phase white light emitting phosphor [23–25]. Hence, a design strategy with a simple synthesis technique and tunable white light defines a major milestone for the easy realization of single phase white emitting phosphor.

Inorganic materials doped with Pr³⁺ are well-known for red phosphors by the promising emission properties. The three red emissions are from different origins of the 4f²-4f² transitions, i.e.¹D₂-³H₄, ³P₀-³H₆ and ³P₀-³F₂ [26–28]. In addition to the red emissions, there are two green emissions (³P₁-³H₅ (530 nm) and ³P₀-³H₅ (547 nm)) and one blue emission (³P₀-³H₄ (491 nm)) exist in Pr³⁺-doped materials [28–30]. However, due to the rather low emission intensity at room temperature, the blue and green emissions are all usually ignored for phosphors application. Through the aforementioned studies, we can found the emissions of Pr³⁺-doped inorganic materials contain the three primary colors of white light, if one can regulate the combination of the blue to red emissions in a controlled manner, white emission from a single host may be realized from Pr³⁺-doped materials. But until now, few researchers care about this research.

In this work, we have synthesized β-NaYF₄:Pr³⁺ microprisms with a Pr³⁺ concentration in the range of 0-1% by a hydrothermal method, and investigated the photoluminescence (PL) properties. Different from the mostly Pr³⁺-doped inorganic materials, particularly significant and dominated blue emission (483 nm) together with minor green and red emissions have been achieved. Varied doping concentration has been employed to modify the absolute and relative intensities of these emissions, aiming on a proper combination of the blue to red emissions. Fortunately, adequate white emission successfully accomplished from β-NaYF₄:Pr³⁺ products with CIE chromaticity coordinates of (0.323, 0.338) and a correlated color temperature of 5951 K. Meanwhile, by variable temperature experiment, we demonstrated NaYF₄:Pr³⁺ has a high-color-stability. Integrate with the excellent white emission and high-color-stability properties, the β-NaYF₄:Pr³⁺ indicates its remarkable potentials in white-light-emitting diodes, which may open up a distinct and fresh route in the field of single phase white emitting phosphor by a simple synthesis technique.

2. Experimental details

A series of β-NaYF₄:mPr³⁺ (m = 0-1%) micro-crystals were synthesized by a hydrothermal method. First, the aqueous solutions of Y(NO₃)₃, Pr(NO₃)₃ (lanthanide ion molar ratio, Y/Pr = (1-m):m were mixed with the aqueous solutions of ethylenediaminetetraacetic acid disodium salt (EDTA) vigorous stirring resulting in a white complex. Then, aqueous solutions of NaF were added into the complexes and stirred for 1 h. The newly-formed complex precursor solution were then transferred into an 80 ml autoclave and heated to 180°C for 24 h. After cooling down to room temperature, the precipitates of β-NaYF₄:Pr³⁺ in the autoclave were separated from the reaction media by centrifugation (10000 rpm, 10 min) and rinsed several times with deionized water and ethanol, eventually dried overnight in the air.

The crystallization nature and morphology of the samples were characterized using powder X-ray diffraction (XRD; Bruker, Advanced D8, with Cu Kα radiation) and scanning electron microscopy (SEM; JEOL 6700F). Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the samples were measured using a spectrofluorometer (HORIBA, Fluoromax-4). The internal and external quantum yields under a 443 nm excitation were tested by Edinburgh Instruments (FLS920) equipped with 450 W xenon lamps.

3. Results and discussion

3.1 Material characterization

In order to optimize the emission properties and regulate the combination of the blue to red emissions, we tuned the concentration of Pr³⁺ and synthesized a series of β-NaYF₄:mPr³⁺ samples, where m = 0, 0.1%, 0.3%, 0.5%, 0.7%, 0.8% and 1%. Typical XRD patterns of β-NaYF₄ with variable concentrations of Pr³⁺ from 0 to 1 mol% are shown in Fig. 1. All of the strong and sharp diffraction peaks in the patterns can be readily indexed as pure hexagonal-phase NaYF₄, which is in good agreement with the standard XRD pattern (JCPDS 16-0334). No other impurity peaks can be identified, which indicates formation of the pure hexagonal-phase NaYF₄ structure with a highly crystalline nature. The typical SEM as shown in Fig. 2, showing the β-NaYF₄:0.5%Pr³⁺ sample consists of regular-shaped hexagonal prisms with perfect uniformity, monodispersity, and smooth surfaces. The average diameter of the hexagonal micro-prisms is 1.73 μm and the average length is 4.58 μm.

 

Fig. 1 XRD patterns of β-NaYF₄:mPr³⁺ (m = 0, 0.1%, 0.3%, 0.5%, 0.7%, 0.8% and 1%) microprisms.

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Fig. 2 SEM image of β-NaYF₄:0.5%Pr³⁺ microprisms. (Inset) Corresponding high-magnification SEM images.

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3.2 Optical property

The excitation spectra and emission spectra of β-NaYF₄:0.5%Pr³⁺ at room temperature was depicted in Fig. 3. The excitation spectra were monitored by emission at 483 nm (³P₀-³H₄) and 604 nm (³P₀-³H₆). It is note that, different from the conventional Pr³⁺-doped oxide phosphors reported in the literature, there are no obvious host absorption band or 4f–5d transition band of Pr³⁺ ions could be detected in the excitation spectra [31]. Only the typical 4f²-4f² intra-configuration forbidden transitions of Pr³⁺ have been achieved, which are respectively attributed to ³H₄-³P₂, ³H₄-³P₁ and ³H₄-³P₀ transitions [26]. This phenomenon illustrates that the host-dependent intervalence charge transfer (IVCT) state does not exist in β-NaYF₄:0.5%Pr³⁺, which provides a convenient channel for electrons in ³P₀ level transferred to ¹D₂ level [32–34]. Figure 3 also shows the emission spectrum of the β-NaYF₄:0.5%Pr³⁺ sample under the excitation of 443 nm, which is dominated by a very strong blue emission at 483 nm together with two weak green emissions (524 nm and 540 nm) and three red emissions (604 nm, 617 nm and 643 nm), all emissions are attributed to the transitions from ³P₀ or ³P₁ to ³H_J (J = 4, 5 and 6) or ³F₂. Additionally, as the traditional dominant transition in red phosphor, emission from ¹D₂-³H₄ was more difficult to obtain. The primary reason is the phonon energy of NaYF₄ relative to the energy gap of about 4000 cm⁻¹ between the ³P₀ and ¹D₂ level is really low, which result in the electrons in ³P₀ level transfer to ¹D₂ level by multi-phonon relaxation is not very efficient [35–37]. Moreover, the electrons in ³P₀ level cannot transfer to ¹D₂ level by IVCT state is an essential factor.

 

Fig. 3 PL spectrum of the as-grown β-NaYF₄:0.5%Pr³⁺ microprisms with an excitation of 443 nm and its PLE spectra monitored at 483 nm and 604 nm.

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Figure 4 shows the PL spectra of β-NaYF₄:mPr³⁺ (m = 0, 0.1%, 0.3%, 0.5%, 0.7%, 0.8% and 1%) as a function of Pr³⁺ concentration excited at 443 nm. Dependence of the 483 nm, 594 nm and 604 nm integrated intensities (I₄₈₃, I₅₉₄ and I₆₀₄) on m are shown in the inset. We can found the emission from ³P₀ intensity increases systematically up to 0.5 mol% and then decreases monotonously when m is >0.5 mol%, this phenomenon is the typical quenching effect of concentration. To further research the mechanism for fluorescence concentration quenching of Pr³⁺ emission, we studied the equation according to the Van Uitert model [38]:

 

Fig. 4 PL of different as-grown β-NaYF₄:mPr³⁺ (m = 0.1%-1%) microprisms under 443 nm excitation. The inset shows the Pr³⁺ concentration dependence of the emission intensity of β-NaYF₄:mPr³⁺.

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where m is the activator concentration, I is the fluorescence emission intensity, K and β are constants for host crystal, and Q is index of electric multipole. According to Eq. (1), the log (I/m) vs log m plot for the ³P₀-³H₄ transition of Pr³⁺ ions at 483 nm with m ≥ 0.5% has been studied in Fig. 5. It can be found the ratio of log (I/m) vs log m decreases monotonically as the m increases. By use linearly fitting the experimental data, the slope parameter is obtained to be 1.81, which is approximately equal to 2, so the index of electric multipole energy transfer is 6. As we know, Q is 6, 8, and 10 for electric dipole-dipole, electric dipole-quadrupole, and electric quadrupole-quadrupole interaction, respectively. The result indicates that the dipole-dipole interaction is the major mechanism for fluorescence concentration quenching of Pr³⁺ emission in β-NaYF₄. Meanwhile, consistent with previous results, the quenching effect has occurred when m≥0.001 for the I₅₉₄ [39]. Interestingly, I₅₉₄ also shows a small incensement at m = 0.5%, similar to I₄₈₃ and I₆₀₄, which due to the nonradiative transition from ³P₀ level to ¹D₂ level [40]. The external quantum yields, internal quantum yields and absorption efficiency of β-NaYF₄:0.5%Pr³⁺ are 5.13%, 56.49% and 9.08% respectively. Since the parity of each state in the 4f^N configuration is the same, and the value of the matrix element of the electric dipole transition between them is zero, the 4f-4f transition is parity forbidden. The narrow absorption cross section results in low absorption efficiency. The relatively low quantum yields hinder the further development of β-NaYF₄:Pr³⁺, how to improve the luminescence efficiency of Pr³⁺ doped a single host will be the next research direction for our group.

 

Fig. 5 The relationship between log (I/m) and log m of β-NaYF₄:0.5%Pr³⁺.

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The varied emission properties, especially the inconsistencies variation tendency of different emissions dependent on the Pr³⁺ concentration indicate directly that the emission color of β-NaYF₄:Pr³⁺ products is tunable in a controlled manner. In order to evaluate the colorimetric performance of the obtained samples, the Commission Internationale de I^,Eclairage (CIE) 1931 chromaticity coordinates are calculated using the intensity emission spectra. As shown in Fig. 6, the CIE coordinates (x, y) for as-grown β-NaYF₄:mPr³⁺ phosphors were measured, obtaining (0.354, 0.339), (0.334, 0.339), (0.323, 0.338), (0.316, 0.338), (0.313, 0.339), and (0.307, 0.335) for m = 0.1%,0.3%, 0.5%, 0.7%, 0.8% and 1%, respectively. It is obvious that these coordinates change significantly as a function of Pr³⁺ contents from the warm white light (0.354, 0.339) CCT = 4563K to standard white light (0.323, 0.338) CCT = 5951K to cool light white CCT = 6767K. These results clearly indicate that the white emission obtained by NaYF₄:mPr³⁺ phosphors not only be used for indoor illumination, but also could be used for outdoor illumination, by adjusting the doping ions concentration under 443 nm excitation. More important, as the highest emission efficiency of β-NaYF₄:m%Pr³⁺, the CIE (0.323, 0.338) of β-NaYF₄:0.5%Pr³⁺ is revealed to be the nearest to the standard CIE (0.33, 0.33) coordinates of white light (see in Fig. 6) with a calculated correlated color temperature (CCT) of 5951 K, within the range of 5500-6000 K for typical daylight.

 

Fig. 6 The Commission CIE diagram for all the samples upon 443 nm excitation.

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The color-stability of phosphors applied in WLEDs is one of the most important technological parameters because it has considerable influence on the light output and color rendering index. The temperature dependent emission spectra for β-NaYF₄:0.5%Pr³⁺ under 443 nm excitation were measured and illustrated in Fig. 7. The inset displays a comparison of the thermal luminescence quenching of 483 nm (³P₀-³H₄) and 604nm (³P₀-³H₆). As compare with the Pr³⁺ doped oxides [41], the intensity ratio of ³P₀ blue-green emission and the ¹D₂ red emission in β-NaYF₄:0.5%Pr³⁺ didn^’t increased obviously with the temperature increased. The most important cause is there is no IVCT state in β-NaYF₄:0.5%Pr³⁺, what lead the electrons in ³P₀ state cannot transfer to ¹D₂ state by absorb thermal energy. Moreover, the emissions of 483 nm and 604 nm play the dominant role in white light, which derived from the same ³P₀ state. That means, with the temperature increases, the spectrum of β-NaYF₄:0.5%Pr³⁺ is more stability than some other co-doped and triply doped single-phase white lights [11,15,18], which is due to the thermostability of the most doped ions are inconformity, or the energy easily transfer between the different doped ions. It is important to note that in order to maintain good color quality, the CIE chromaticity coordinates are best not to change or move slightly as the temperature increases. As shown in Fig. 9 in the Appendix, the chromaticity coordinates changed little from 297K to 413K, indicating that the sample maintained good color quality at higher temperature.

 

Fig. 7 Temperature-dependent PL spectra of β-NaYF₄:0.5%Pr³⁺ phosphor and the inset shows the thermal quenching of 483 and 604 emissions.

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Moreover, the Arrhenius equation is fitted to the thermal quenching data following the below equation [42]:

where I₀ is the initial intensity, I is the intensity at a given temperature T, E_a is activation energy for thermal quenching, A is a constant, and k is the Boltzmann constant (8.629 × 10⁻⁵ eV). The Fig. 8 displays the relation between ln(I₀/I) and 1/T. From the slope value, the E_a was obtained to be 0.069 eV. The relatively low activation energy indicates that β-NaYF₄:0.5%Pr³⁺ has not so good thermal stability. For the application in high power LEDs, thermal quenching property has to be enhanced.

 

Fig. 8 Arrhenius fitting of emission intensity of β-NaYF₄:0.5%Pr³⁺.

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4. Conclusion

In summary, trivalent Pr³⁺-doped β-NaYF₄ phosphors have been synthesized by a conventional hydrothermal method. Warm and cool white light can be obtained by adjusting Pr³⁺ concentration. The optimum doping concentration is around 0.5 mol% and electric dipole-dipole interaction is a dominant energy-transfer mechanism between Pr³⁺ ions. The dependence of spectroscopic properties on the temperatures displays that the β-NaYF₄:0.5%Pr³⁺ keep good color quality compare with co-doped and triply doped single-phase white phosphors. Meanwhile the β-NaYF₄:0.5%Pr³⁺ should improve thermal quenching property for the application in high power LEDs.

Appendix

 

Fig. 9 CIE chromaticity diagram of NaYF₄:0.5%Pr³⁺ from 297K to 413K and the inset shows Commission Internationale de I^,Eclairage chromaticity coordinates.

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Funding

Science and Technology Projects of Jiangmen (No.(2017)307 and (2017)149); Program for Innovative Research Team of Jiangmen (No.(2017)385); Cooperative education platform of Guangdong Province (No.(2016)31); Innovative Research Team in University of Guangdong (No. 2015KCXTD027); the Science and Technology Projects of Guangdong Province (No.2016A020225009); Key Laboratory of Optoelectronic materials and Applications in Guangdong Higher Education (No. 2017KSYS011); Program for Key Basic Research of Guangdong (No. 2017KZDXM083).

Acknowledgments

We acknowledge Senior Investment Officer Liang Ping of Evergrande Real Estate Group for Emotional Support.

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