Self-activated multicolor emissions in Ca2NaZn2(VO4)3:Eu3+ phosphors for simultaneous warm white light-emitting diodes and safety sign

Accepted Manuscript 3+ Self-activated multicolor emissions in Ca2NaZn2(VO4)3:Eu phosphors for simultaneous warm white light-emitting diodes and safety sign Peng Du, Jae Su Yu PII:

S0143-7208(17)31251-2

DOI:

10.1016/j.dyepig.2017.07.065

Reference:

DYPI 6154

To appear in:

Dyes and Pigments

Received Date: 31 May 2017 Revised Date:

17 July 2017

Accepted Date: 25 July 2017

Please cite this article as: Du P, Yu JS, Self-activated multicolor emissions in Ca2NaZn2(VO4)3:Eu phosphors for simultaneous warm white light-emitting diodes and safety sign, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.07.065.

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Graphical abstract

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Excitation and emission spectra of Na2CaZn2(VO4)3:Eu3+ phosphors. Luminescent images of the

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fabricated WLED devices at different forward bias currents

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Self-activated multicolor emissions in Ca2NaZn2(VO4)3:Eu3+ phosphors for simultaneous warm white light-emitting diodes and safety sign Peng Du and Jae Su Yu*

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Department of Electronic Engineering, Kyung Hee University, Yongin-si 446-701, Republic of Korea

Abstract

Self-activated Eu3+-doped Ca2NaZn2(VO4)3 multicolor-emitting phosphors were prepared by a

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facile critic-assisted sol-gel technique. Both the excitation and three-dimensional emission spectra suggest that the synthesized samples exhibit an intense absorption band in the range of

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250-370 nm. The emission intensity of the phosphors is greatly dependent on the doping concentration and the optimal value is found to be 7 mol%. The concentration quenching mechanism is attributed to the dipole-dipole interaction and the critical distance is 18.8 Å. With the addition of Eu3+ ions, multicolor emissions are observed in the resultant phosphors when excited at 290 nm. The thermal stability of phosphors is characterized by temperature-dependent emission spectra. By manipulating the temperature, the emitting color of the prepared products is

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tuned from yellowish red to orange, and ultimately to pure red. Additionally, the near-ultraviolet (NUV) chip-based light-emitting diode device, which is fabricated by coating the blended resultant phosphors, commercial blue-emitting and green-emitting phosphors on the NUV chip, emit glaring warm white light with superior color rendering index and impressive correlated

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color temperature. These results illustrate that the self-activated Eu3+-doped Ca2NaZn2(VO4)3 multicolor-emitting phosphors are suitable for safety sign in high-temperature circumstance and

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indoor lighting.

Keywords: Self-activated, WLEDs, Luminescence, Phosphors, Vanadates *Corresponding author:

E-mail: [email protected] (J. S. Yu) Tel: 82 31 201 3820 Fax: 82 31 206 2820

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1. Introduction Recently, rare-earth (RE) ions-based phosphors with multicolor emissions have received considerable attention on account of their vivid applications ranging color displays, white light-

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emitting diodes (WLEDs), medical diagnosis, non-invasion thermometry, solar cells, optical heaters to plant growth [1-7]. Up to date, enormous methods, such as using different excitation wavelengths, manipulating the energy transfer between the sensitizers and activators, regulating

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temperature, tailoring the local crystal field around the dopants, modulating band gap and employing various synthetic routes, have been developed to achieve the multicolor emissions in

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the phosphors [8-13]. Meanwhile, Xia et al. proposed another two novel approaches, namely, cation nanosegragation and chemical unit consubstitution, to tune the emitting color of Eu2+ ions doped inorganic materials [14,15]. Compared to other techniques, the modulation of energy transfer between the sensitizers and activators is widely applied to pump the controllable

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emissions in RE ions doped luminescent materials and some remarkable achievements have been obtained by utilizing this strategy [16-18]. It was revealed that the color-tunable NaSrBO3:Ce3+/Sm3+/Tb3+ phosphors with high thermal stability can serve as promising

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candidates for WLEDs [19]. However, these aforementioned strategies usually suffer from complicated synthetic process, multiple excitation wavelengths, the different responses of

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various dopants to the excitation wavelength and reabsorption among the dopants. In view of these characteristics, developing a single excitation wavelength-pumped phosphors with multicolor emissions and high luminescent efficiency is urgent. As is known, to achieve superior luminescent performance in phosphors, a proper luminescent host material should be selected. Currently, large amounts of inorganic materials including borates, vanadates, tungstates, chlorides, silicates and phosphates have been developed

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and investigated as luminescent host [20-25]. In comparison, the interest in vanadates is increasing as a result of their intrinsic emission band ranging from 400-700 nm as well as their promising applications in photocatalysis, electrochemistry and displays [26-28]. Moreover, the

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vanadates also exhibit intense charge transfer band in the near-ultraviolet (NUV) region arising from the VO4 group and can transfer the captured energy to the luminescent centers (dopants) by means of nonradiative transition, resulting in the glaring luminescence in RE ions doped

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vanadates [27-30]. By integrating the intrinsic emission of vanadates with that of the dopants, color-tunable emissions are expected to be achieved. Until now, some vanadates doped with RE

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ions, such as K3Gd(VO4)2:Eu3+, GdVO4:Dy3+ and Sr3(VO4)2:Eu3+, were revealed to emit multicolor emissions at single wavelength excitation and they show potential applications in solid-state lighting [31-33]. On the other hand, the Eu3+ ion is extensively studied as the redemitting activator because of its narrow emission at around 610 nm derived from the intra-4f

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transition within the 4f6 configuration and the Eu3+ ions-based phosphors are already successfully used to improve the performance of the phosphors-converted WLEDs [34,35]. Considering these inspiring metrics, it would be very interesting to investigate the luminescent

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behaviors of Eu3+ ions doped vanadates and their promising applications in solid-stated lighting. In this work, the Ca2NaZn2(VO4)3, which possesses garnet structure and superior intrinsic

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luminescent property [36,37], is chosen as the luminescent host material and the self-activated Eu3+-doped Ca2NaZn2(VO4)3 phosphors were synthesized via a conventional sol-gel method. Herein, the phase compositions, microstructure, lifetime and optical properties of the final products were investigated in detail. The temperature-dependent photoluminescence (PL) emission spectra are employed to characterize the thermal stability of the synthesized compounds. Furthermore, the temperature-dependent chromatic behaviors of the studied samples are also

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analyzed to explore their potential applications in high-temperature environment as safety sign. Ultimately, a WLED device consisting of NUV chip, resultant self-activated phosphors, commercial blue-emitting and green-emitting phosphors was fabricated to verify the feasibility

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of the Eu3+-doped Ca2NaZn2(VO4)3 phosphors for solid-state lighting. 2. Experimental Procedure

The self-activated Ca2-2xNaZn2(VO4)3:2xEu3+ (Ca2NaZn2(VO4)3:2xEu3+; x = 0.01, 0.03, 0.05,

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0.07, 0.09 and 0.11) phosphors were prepared by a simple citrate-assisted sol-gel method. The raw materials including Ca(NO3)2·4H2O, NaNO3, Zn(NO3)2·6H2O, NH4VO3, Eu(NO3)·5H2O and

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citric acid were purchased from Sigma-Aldrich Co. Based on the stoichiometric ratio, the proper amount of Ca(NO3)2·4H2O, NaNO3, Zn(NO3)2·6H2O, NH4VO3 and Eu(NO3)3·5H2O were weighted and dissolved into 200 ml of de-ionized water to form a homogenous mixture. After that, the citric acid was added into the above solution. The molar concentration ratio of the citric

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acid to the total metal ions is 2:1. Subsequently, the solution was closed with a polyethylene lid and heated at 80 °C for 30 min with drastic mechanical stirring. When the solution color was finally changed from yellow to blue (see the inset of Fig. 1(a)), the lid was removed from the

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beaker and the solution was made to evaporate, resulting in the gray wet-gel. Then, the xerogel was obtained after 12 h of heat treatment in oven at 120 °C. Ultimately, the xerogel was kept in

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an alumina crucible and calcined at 900 °C for 5 h to synthesize the self-activated phosphors. The crystal structure and phase compositions of the final products were analyzed by utilizing a Bruker D8 Advance diffractometer. The field-emission scanning electron microscope (FESEM) (LEO SUPRA 55, Carl Zeiss) equipped with an energy-dispersive X-ray (EDX) spectrometer was applied to examine the microstructure and chemical component of the prepared phosphors. The PL emission and excitation spectra of the phosphors were collected by utilizing a

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fluorescence spectrometer (Scinco FluroMate FS-2) attached with a thermocouple (NOVA ST540) in the temperature range of 303-483 K. The Fourier transform infrared (FTIR) and diffuse reflectance spectra were collected by means of Thermo Nicolet-570 FTIR

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spectrophotometer and V-670 (JASCO) UV-vis spectrophotometer, respectively. The decay curves of the studied compounds were measured by the Photon Technology International fluorimeter attached to a phosphorimeter with a Xe-flash lamp. The electroluminescence (EL)

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emission spectrum of the packaged LED device was monitored by multi-channel spectroradiometer (OL 770).

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3. Results and discussion

The crystalline structure and phase compositions of the final products were characterized by X-ray diffraction (XRD) and the corresponding diffraction patterns are illustrated in Fig. 1(a). As depicted in Fig. 1(a), the compounds with x ≤ 0.01 exhibit a pure cubic phase of

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Ca2NaZn2(VO4)3 (JCPDS# 24-1044), while a small amount of impurity diffraction peaks, which are assigned to the EuVO4 phase (JCPDS# 15-0809), are formed when the doping concentration is higher than 3 mol%. Furthermore, the diffraction peak intensities of EuVO4 phase increase

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gradually with elevating the Eu3+ ion concentration from 3 to 11 mol%. Owing to the inconsistent ionic radii between the Ca2+ (1.12 Å, when coordinate number (CN) is 8) and Eu3+

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(1.06 Å, when CN is 8) ions, the diffraction peaks slightly shift to larger angles with the increase of Eu3+ ion concentration, and the shift extent is strongly dependent on the doping concentration (see Fig. 1(b)). With the help of Diamond software, the spatial structure of the Ca2NaZn2(VO4)3 was drawn as displayed in Fig. 1(c). Clearly, the Zn2+ ions are surrounded by six oxygen anions, forming the six-fold octahedron of ZnO6. In comparison, Ca2+/Na+ and V5+ ions are coordinated

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by eight and four oxygen anions, respectively, resulting in the eight-fold dodecahedra of (Ca/Na)O8 and four-fold tetrahedron of VO4. From the FE-SEM image of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor (see the inset of Fig.

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2(a)), one obtains that the resultant samples consist of aggregated microparticles with irregular shape and the particle size ranges from about 2 to 7 µm. The EDX spectrum presented in Fig. 2(a) indicates that the main chemical composition of the studied samples is Ca, Na, Zn, V, Eu and O,

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further demonstrating the synthesis of Eu3+-doped Ca2NaZn2(VO4)3 compounds. Meanwhile, the detection of Pt peak in the EDX spectrum is associated to the platinum electrode for FE-SEM

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measurement. The elemental mapping results shown in Fig. 2(b) confirm that all the constituent elements are homogeneously distributed in the range of the microparticles. The FTIR spectrum was recorded to analyze the surface functionalization of the prepared samples. As presented in Fig. 2(c), the absorption peaks centered at approximately 3433 and 1637 cm-1 are related to the

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stretching (O-H) and blending (H-O-H) vibrations of the absorbed moisture from air, respectively [38]. Furthermore, the strong absorption band located at around 810 cm-1 is ascribed to the symmetric V-O stretching mode [39].

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The diffuse reflectance spectrum of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor in the wavelength range of 200-700 nm is illustrated in Fig. 2(d). It is evidence that the

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Ca2NaZn2(VO4)3:0.14Eu3+ phosphors exhibit a broad absorption from 200 to 400 nm corresponding to the host absorption, demonstrating that the synthesized samples can be efficiently pumped by NUV chip. In order to estimate the optical band gap of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphors, the following expression is employed [38,40]:  hvF ( R∞ )  = A ( hv − Eg ) . 2

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(1)

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In this expression, hv denotes the phonon energy, A is constant, Eg refers to the band gap and F(R∞) is so-called Kubella-Munk function that can be defined as [41]:

2R

2

=

k , s

(2)

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F ( R∞ ) =

(1 − R )

where R, k and s represent the reflectance, absorption and scattering parameters, respectively. According to plot of [hvF(R∞)]2 vs. hv (see the inset of Fig. 2(d)), the energy band gap is

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determined to be 2.64 eV by extrapolating the linear line to [hvF(R∞)]2 = 0.

Fig. 3(a) illustrates the PL excitation (λem = 610 nm) and emission (λex = 290 nm) spectra of

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the Ca2NaZn2(VO4)3:0.14Eu3+ sample. The excitation spectrum consists of two overlapped strong broad absorption bands located at around 290 and 330 nm and a relatively weak sharp peak centered at 392 nm (see Fig. 3(a)). These two broad excitation bands are assigned to the VO4 group absorption originating from the group sate of 1Al to the excited sates of 1T1 and 1T2,

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respectively, while the narrow excitation peak at 392 nm is ascribed to the 7F0 → 5L6 transition of Eu3+ ions.21,29 Clearly, the excitation band for the Eu3+-doped Ca2NaZn2(VO4)3 phosphors ranges from 220 to 400 nm, which is matched well with previous literatures, indicating that the

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NUV chips are suitable excitation sources for the resultant phosphors [42,43]. Under the excitation of 290 nm, the emission spectrum of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor was

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recorded, as displayed in Fig. 3(a) (red line). It is evident that the emission spectrum contains a broad emission band and several narrow peaks. The broad emission band, which is so-called charge transfer band derived from the oxygen 2p orbital to vacant 3d orbital, can be divided into two emission bands centered at 477 and 518 nm originating from the 3T1 → 1A1 and 3T2 → 1A1 transitions of VO4 group, respectively (see the inset of Fig. 3(b)) [21,29]. In comparison, the sharp emission bands centered at approximately 591, 610, 652 and 705 nm are associated to the 5

D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3 and 5D0 → 7F4 intra-4f transitions within the 4f6 configuration 7

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of Eu3+ ions, respectively.30,35 As for the Eu3+ ion, it has two characteristic emissions in the yellow (5D0 → 7F1) and red (5D0 → 7F2) regions. Specially, the 5D0 → 7F1 transition is assigned to the magnetic dipole (MD) transition, which is insensitive to the crystal field and it usually

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prevails in the emission spectrum when the Eu3+ ions occupy the high symmetry sites, whereas the 5D0 → 7F2 transition is related to the hypersensitive electric dipole (ED) transition which is largely dependent on the chemical environment surrounding the Eu3+ ions and it is dominated in

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the emission spectrum when the Eu3+ ions are located at low symmetry sites without inversion symmetry [34,44]. The intense red emission in the emission spectrum reveals that the Eu3+ ions

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possess the noninversion symmetry sites in the Ca2NaZn2(VO4)3 host lattices. Furthermore, the three-dimensional (3D) PL emission spectra and contour line of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor were measured to verify the resultant products which can be efficiently excited by NUV light, as depicted in Fig. 4(b) and 4(c), respectively. As demonstrated in Fig. 4(b), the

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studied samples exhibit the emissions of VO4 group and Eu3+ ions under different excitation wavelengths. In addition, both the 3D PL emission spectra and contour line spectra show the strongest emission intensities in the excitation wavelength of 250-370 nm, further implying that

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the Eu3+-doped Ca2NaZn2(VO4)3 phosphors can be pumped by NUV light. To portray the excitation and emission mechanism in the Ca2NaZn2(VO4)3:2xEu3+ system, the schematic energy

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level diagram as well as the possible luminescent processes is shown in the inset of Fig. 3(b). For the sake of exploring the optimum doping concentration of Eu3+ ions in the Ca2NaZn2(VO4)3, a series of Ca2NaZn2(VO4)3:2xEu3+ phosphors were successfully synthesized by means of the conventional citric-assisted sol-gel method. The emission spectra of the Ca2NaZn2(VO4)3:xEu3+ phosphors, which are pumped at 290 nm light, are depicted in Fig. 3(b). As demonstrated, all the samples exhibit the similar emission profile, whereas the emission

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intensity varies with increasing the dopant concentration. It can be seen that the emission intensity increases sharply with the increase of Eu3+ ion concentration and a maximum value is achieved when x = 0.07 (see Fig. 3(c)). However, the emission intensity starts to decrease when

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the doping concentration is over 7 mol% which is attributed to the concentration quenching effect caused by the nonradiative energy transfer among the Eu3+ ions. To realize the concentration quenching, two possible routes should be taken into account [45,46]. Obviously,

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with the increase of the Eu3+ ion concentration in Ca2NaZn2(VO4)3 host lattices, the distance among the adjacent Eu3+ ions decreases, and thus the excitation migration possibility between the

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activators will increase, leading to the shifting of the activation energy to the quenching centers. Moreover, when the dopant concentration is over a certain value, the Eu3+ ions will be coagulated or paired and moved to the quenching centers. Owing to these aforementioned channels, the concentration quenching occurs and the optimal doping concentration of Eu3+ ions

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in the Ca2NaZn2(VO4)3 host lattices is found to be 7 mol%. As is known, either the exchange interaction or the electric multipolar interaction contributes to the nonradiative energy transfer among the dopants and it can be verified by analyzing the critical distance. According to Blasse’s

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report, the critical distance (Rc) can be estimated by using the following equation [47]: 1/3

 3V  Rc = 2   ,  4π xc Z 

(3)

where V, xc and Z refer to the volume of the unit cell, critical concentration and the number of Eu3+ sites in the unit cell, respectively. Herein, the values of V, xc and Z are 1953.1 Å3, 0.07 and 8, respectively. As a consequence, the Rc is calculated to be around 18.8 Å. Generally, when the critical distance is no larger than 5 Å, the exchange interaction contributes to the concentration quenching mechanism. Otherwise, the electric multipolar interaction prevails. Since the

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estimated critical distance is much larger than 5 Å, the concentration quenching of Eu3+ ions in the Ca2NaZn2(VO4)3:2xEu3+ phosphors is dominated by electric multipolar interaction. As Dexter pointed out, the relation between the emission intensity and activator concentration can

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be expressed as [48]:

I k = . x 1 + β xθ /3

(4)

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Here, I and x denote the emission intensity and dopant concentration, respectively. k and β are coefficients. θ = 6, 8 and 10 is associated to the dipole-dipole, dipole-quadrupole, quadrupole-

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quadrupole interaction, respectively. From the plot of log(I/x) versus log(x) (see Fig. 3(d)), the experimental data can be linearly fitted with a slope of -1.45, and thus, the θ value is determined to be 4.35 approaching to 6, revealing that the dipole-dipole interaction is responsible for concentration quenching in the Ca2NaZn2(VO4)3:2xEu3+ phosphors.

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On the basis of the recorded emission spectra, the Commission International I’Eclairage (CIE) chromaticity coordinates of the Ca2NaZn2(VO4)3:2xEu3+ phosphors were calculated as presented in Fig. 3(e). Clearly, with the increment of Eu3+ ions, the CIE coordinates are varied from

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(0.375,0.406) to (0.446,0.390) and the emitting color is changed from greenish yellow to pure yellow, and finally to yellowish red. This result suggests that the emitting color of the

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synthesized phosphors can be tuned by properly adjusting the doping concentration which makes them promising candidates for color displays and solid-state lighting. The decay curves of the samples excited at 290 nm and monitored at 610 nm were measured, as plotted in Fig. 4(a), to analyze aforementioned the concentration quenching phenomenon. Obviously, these decay curves can be perfectly fitted by means of double exponential equation, as defined below: I = I 0 + A1 exp ( − t τ 1 ) + A2 exp ( − t τ 2 ) ,

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(5)

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where I and I0 denote the emission intensities when time is 0 and t, respectively, A1 and A2 are fitting constants, and τ1 and τ2 present the fast and slow component of decay time, respectively. Moreover, the average lifetime (τavg) is elevated by utilizing the following equation: A1τ 12 + A2τ 22 . A1τ1 + A2τ 2

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τ avg =

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From the fitting results (see Fig. 4(a)), the average decay times of Ca2NaZn2(VO4)3:2xEu3+

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phosphors are found to be 71, 65, 66, 49, 46 and 39 µs, respectively, when the dopant concentration is 1, 3, 5, 7, 9 and 11 mol%. It is clear that the average lifetime slightly decreases

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from 71 to 66 µs when the Eu3+ ion concentration increases from 1 to 5 mol%, and then it rapidly declines with further raising the dopant concentration which is associated to the concentration quenching. Similar results are also reported in CaWO4:Dy3+, Ca6La4(SiO4)2(PO4)4O2:Eu2+ and Ca2Gd8Si6O26:Sm3+ phosphors [8,49,50]. The declined lifetime further confirms that the optimal

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doping concentration of Eu3+ ions in Ca2NaZn2(VO4)3 host lattices is 7 mol%. For the purpose of exploring the applicability of the resultant compounds for solid-state lighting, their thermal stability should be taken into account because it has considerable effect on

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the performance of the fabricated LED devices, such as light output, CRI value, chromaticity and lifetime. Upon 290 nm light excitation, the PL emission spectra of the Ca2NaZn2(VO4)3:0.14Eu3+

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phosphor at different temperatures increasing from 303 to 483 K were recorded to investigate its thermal stability, as illustrated in Fig. 5(a). As disclosed, the emission bands arising from VO4 group and Eu3+ ions are barely varied with the increase of temperature, while the emission intensities are largely dependent on the temperature. It can be seen that the PL emission intensity shows a decline tendency with raising the temperature from 303 to 483 K due to the thermal quenching effect (see Fig. 5(b)). Especially, the PL emission intensity of Eu3+ ions at 423 K decreases to 42.4% of its initial value at 303 K, while the emission intensity of VO4 group at 423 11

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K only keeps around 4.8% of its initial value at 303 K, implying that the relative emission intensity between the VO4 group and Eu3+ ions can be adjusted by properly modifying the

expression is employed to calculate the activation energy [51,52]: I=

I0 . 1 + A exp ( −∆E / kT )

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temperature. To further analyze the involved thermal quenching phenomenon, the following

(7)

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In the expression, I0 and I refer to the emission intensities at initial temperature and measured temperature T, respectively, A is constant, ∆E denotes the activation energy and k is Boltzmann’s

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coefficient. The plots of ln(I0/I-1) versus 1/kT for the emission bands originating from the Eu3+ ions and VO4 group are depicted in Fig. 5(c) and 5(d), respectively. Evidently, these two measured experimental data can be linearly fitted with slopes of -0.272 and -0.396, respectively. Thus, the activation energies for the Eu3+ ions and VO4 group are found to be 0.272 and 0.396

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eV, respectively. Meanwhile, these calculated activation energies are comparable with previous reported rare-earth ions doped phosphors, such as CaW0.4Mo0.6O4:Eu3+ (0.239 eV), La0.5Na0.5TiO3:Eu3+ (0.27 eV) and LiGd(WO4)2:Eu3+ (0.318 eV) [52-54], suggesting that the

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Eu3+-doped Ca2NaZn2(VO4)3 phosphors possess good thermal stability which make them suitable for solid-state lighting applications.

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Since the emission intensities of VO4 group and Eu3+ ions exhibit different responses to temperature (see Fig. 5(b)), the emission intensity ratio of Eu3+ ions to VO4 group is enhanced with raising the temperature from 303 to 483 K, as demonstrated in Fig. 5(e), resulting in the multicolor emissions in the studied samples. On the basis of the temperature-dependent PL emission spectra, the CIE coordinates of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphors as function of temperature were calculated and the corresponding results are presented in Fig. 5(f). As demonstrated, it is obvious that the emitting color of the resultant compounds is gradually 12

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changed from yellowish red to orange, and ultimately to pure red with the controlling the temperature from 303 to 483 K. Meanwhile, the calculated CIE coordinates are also varied from (0.421,0.396) to (0.629,0.350), as displayed in the inset of Fig. 5(f). The temperature-induced

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color-controllable emissions in the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor implies that the synthesized compounds may have promising applications in high-temperature circumstance as a safety sign.

chip-based

WLED

device

was

developed

by

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To verify the practical applicability of the resultant phosphors for solid-state lighting, a NUV coating

the

mixture

of

the

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Ca2NaZn2(VO4)3:0.14Eu3+ phosphor, BAM:Eu2+ blue-emitting and (Ba,Sr)2SiO4:Eu2+ greenemitting phosphors onto the InGdN NUV chip (365 nm). The optimal weight ratio among the resultant phosphors, BAM:Eu2+ and Ba,Sr)2SiO4:Eu2+ is 0.2g:0.03g:0.02g. Under different forward basis currents ranging from 100 to 250 mA, the EL emission spectra of the packaged

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WLED device were measured, as shown in Fig. 6(a). The recorded EL emission spectra are composed of two broad emission bands and several sharp peaks. These broad emission bands with the central wavelength of 463 and 545 nm are assigned to the emissions of commercial

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BAM:Eu2+ blue-emitting and (Ba,Sr)2SiO4:Eu2+ green-emitting phosphors, respectively. In comparison, these narrow emission bands are attributed to the 5D0 → 7FJ (J = 0, 1, 2, 3, 4)

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transitions of Eu3+ ions in the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor. Furthermore, with elevating the forward bias current from 100 to 250 mA, the EL emission intensity increases gradually (see Fig. 6(a)). Fig. 6(b) describes the fully packaged WLED device. Under various forward bias currents, the fabricated LED device emits bright white light and the emitting color is hardly changed with the increment of forward bias current, as displayed in Fig. 6(c)-6(e). When the operating current is 100 mA, the color rendering index (CRI) and correlated color temperature

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(CCT) values of the packaged WLED device are found to be 90.47 and 4017 K, respectively. In comparison, the CRI and CCT values of the prepared WLED device are increased to 91.45 and 4209 K, respectively, when the forward bias current is 250 mA (see inset of Fig. 6(a)). These

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results further reveal that the Eu3+-doped Ca2NaZn2(VO4)3 phosphors have potential applications in warm WLEDs. 4. Conclusion

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In summary, a series of self-activated Ca2NaZn2(VO4)3:2xEu3+ multicolor-emitting phosphors were prepared by a typical sol-gel route. Under the excitation of NUV light, the

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resultant compounds emit the characteristic emissions of VO4 group and Eu3+ ions. The optimum Eu3+ ion concentration in the Ca2NaZn2(VO4)3 host lattices was determined to be 7 mol% and the concentration quenching mechanism is prevailed by dipole-dipole interaction. By manipulating the dopant concentration, the emitting color of the studied samples was tuned from greenish

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yellow to pure yellow, and finally to yellowish red. The temperature-dependent PL emission spectra revealed that the synthesized samples exhibited good thermal stability and the activation energies for Eu3+ ions and VO4 group were found to be 0.272 and 0.396 eV, respectively. Since

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the PL emission intensities of Eu3+ ion and VO4 group in Ca2NaZn2(VO4)3:2xEu3+ phosphors possess different responses to temperature, the emitting color of the synthesized compounds is

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varied from yellowish red to orange, and ultimately to pure red with the raising the temperature from 303 to 483 K, suggesting their potential applications in high-temperature environment as a safety sign. In addition, the fabricated LED device, which consists of a NUV chip, synthesized phosphors, commercial blue-emitting and green-emitting phosphors, emitted warm white light with high CRI and appropriate CCT. These results confirm that the self-activated Eu3+-doped

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Ca2NaZn2(VO4)3 multicolor-emitting phosphors are promising bifunctional materials for warm WLEDs and safety sign. Acknowledgements

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the Korea government (MSIP) (No. 2017R1A2B4011998).

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This work was supported by the National Research Foundation of Korea (NRF) Grant funded by

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Figure Captions Fig. 1 (a) XRD patterns of Ca2NaZn2(VO4)3:2xEu3+ (x = 0.01, 0.03, 0.05, 0.07, 0.09 and 0.11) phosphors sintered at 900 °C. (b) Magnified XRD patterns between 52.5° and 56°. (c) Spatial

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crystal structure of the Ca2NaZn2(VO4)3. Inset depicts the changed solution color during the heating process.

Fig. 2 (a) EDX spectrum (inset shows the FE-SEM image), (b) elemental mapping and (c) FTIR

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spectrum of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor (inset illustrates the EDS layered image). (d) Diffuse reflectance spectrum of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor. Inset presents the band

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gap of the Ca2NaZn2(VO4)3:0.07Eu3+ phosphor.

Fig. 3 (a) PL excitation (λem = 610 nm) and emission (λex = 290 nm) spectra of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor. (b) Emission spectra of Ca2NaZn2(VO4)3:2xEu3+ phosphors at different doping concentrations. (c) Emission intensity as a function of Eu3+ ion concentration.

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(d) Relationship of log(I/x) versus log(x) for Ca2NaZn2(VO4)3:2xEu3+ phosphors. (e) CIE chromaticity diagram for the Ca2NaZn2(VO4)3:2xEu3+ phosphors. Inset displays the calculated CIE color coordinates.

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Fig. 4 (a) Decay curves of Ca2NaZn2(VO4)3:2xEu3+ (λex = 290 nm, λem = 610 nm) phosphors. (b) 3D PL emission spectra and (c) contour line curves for the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor

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as a function of excitation wavelength in the range of 220-390 nm. Fig. 5 (a) Temperature-dependent PL emission spectra of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor. (b) Emission intensities of Eu3+ ions and VO4 as a function of temperature. (c) and (d) Plots of ln(I0/I-1) versus 1/kT for Eu3+ ions and VO4, respectively. (e) Dependence of emission intensity ratio of Eu3+ ions to VO4 on temperature (f) CIE chromaticity diagram of the

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Ca2NaZn2(VO4)3:0.14Eu3+ phosphor at different temperatures. Inset shows the calculated CIE color coordinates. Fig. 6 (a) EL emission spectra of the packaged WLED device consisting of NUV chip (365 nm),

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synthesized Ca2NaZn2(VO4)3:0.14Eu3+ phosphor, commercial blue-emitting and green-emitting phosphors. (b) Fully packaged LED device. (c)-(e) Digital image of the packaged WLED device driven at the different forward bias currents of 100, 150, 200 and 250 mA, respectively. Inset

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describes the CCT and CRI values of fabricated WLEDs at various forward bias currents.

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Fig. 1 (a) XRD patterns of Ca2NaZn2(VO4)3:2xEu3+ (x = 0.01, 0.03, 0.05, 0.07, 0.09 and 0.11) phosphors sintered at 900 °C. (b) Magnified XRD patterns between 52.5° and 56°. (c) Spatial crystal structure of the Ca2NaZn2(VO4)3. Inset depicts the changed solution color during the

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heating process.

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Fig. 2 (a) EDX spectrum (inset shows the FE-SEM image), (b) elemental mapping and (c) FTIR

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spectrum of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor (inset illustrates the EDS layered image). (d) Diffuse reflectance spectrum of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor. Inset presents the band

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Fig. 3 (a) PL excitation (λem = 610 nm) and emission (λex = 290 nm) spectra of the

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Ca2NaZn2(VO4)3:0.14Eu3+ phosphor. (b) Emission spectra of Ca2NaZn2(VO4)3:2xEu3+ phosphors at different doping concentrations. (c) Emission intensity as a function of Eu3+ ion concentration.

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(d) Relationship of log(I/x) versus log(x) for Ca2NaZn2(VO4)3:2xEu3+ phosphors. (e) CIE chromaticity diagram for the Ca2NaZn2(VO4)3:2xEu3+ phosphors. Inset displays the calculated CIE color coordinates.

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Fig. 4 (a) Decay curves of Ca2NaZn2(VO4)3:2xEu3+ (λex = 290 nm, λem = 610 nm) phosphors. (b) 3D PL emission spectra and (c) contour line curves for the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor

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as a function of excitation wavelength in the range of 220-390 nm.

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Fig. 5 (a) Temperature-dependent PL emission spectra of the Ca2NaZn2(VO4)3:0.14Eu3+

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phosphor. (b) Emission intensities of Eu3+ ions and VO4 as a function of temperature. (c) and (d) Plots of ln(I0/I-1) versus 1/kT for Eu3+ ions and VO4, respectively. (e) Dependence of emission

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intensity ratio of Eu3+ ions to VO4 on temperature (f) CIE chromaticity diagram of the Ca2NaZn2(VO4)3:0.14Eu3+ phosphor at different temperatures. Inset shows the calculated CIE color coordinates.

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Fig. 6 (a) EL emission spectra of the packaged WLED device consisting of NUV chip (365 nm), synthesized Ca2NaZn2(VO4)3:0.14Eu3+ phosphor, commercial blue-emitting and green-emitting

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phosphors. (b) Fully packaged LED device. (c)-(e) Digital image of the packaged WLED device driven at the different forward bias currents of 100, 150, 200 and 250 mA, respectively. Inset

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Highlight: •

The resultant samples have a broad absorption band in the NUV region.

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Optimal doping content is 7 mol% and dipole-dipole interaction results in

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concentration quenching.

By adjusting the dopant concentration, multicolor emissions are observed in the prepared samples.

With increasing the temperature, color-tunable emissions are achieved.

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By using the synthesized phosphors, high performance WLED is designed.

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