Dy3+-, Tb3+-, and Eu3+-activated NaCa4(BO3)3 phosphors for lighting based on near ultraviolet light emitting diodes

Accepted Manuscript 3+ 3+ 3+ Dy -, Tb -, and Eu -activated NaCa4(BO3)3 phosphors for lighting based on near ultraviolet light emitting diodes Meiling Shi, Chaofeng Zhu, Mingzhi Wei, Zhigang He, Meng Lu PII:

S0042-207X(17)31750-5

DOI:

10.1016/j.vacuum.2018.01.014

Reference:

VAC 7763

To appear in:

Vacuum

Received Date: 5 December 2017 Revised Date:

4 January 2018

Accepted Date: 8 January 2018

3+ 3+ 3+ Please cite this article as: Shi M, Zhu C, Wei M, He Z, Lu M, Dy -, Tb -, and Eu -activated NaCa4(BO3)3 phosphors for lighting based on near ultraviolet light emitting diodes, Vacuum (2018), doi: 10.1016/j.vacuum.2018.01.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Dy3+-, Tb3+-, and Eu3+-activated NaCa4(BO3)3 phosphors for lighting based on near ultraviolet light emitting diodes Meiling Shi, Chaofeng Zhu*, Mingzhi Wei, Zhigang He, Meng Lu

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Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, School of Materials Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, P.R. China

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ABSTRACT

A variety of Dy3+-, Tb3+- and Eu3+-singly or doubly or triply activated NaCa4(BO3)3 phosphors for light

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emitting diodes applications were synthesized via a high temperature solid state reaction technique under an ambient atmosphere. These phosphors were studied by X-ray diffraction, scanning electron microscopy, photoluminescence excitation and emission spectra, Commission International de I’Eclairage chromaticity coordinates, and correlated color temperatures. The emission colors of Dy3+-, Tb3+-, and Eu3+-doped phosphors can be successfully tuned from green to yellow and then to white by

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appropriately changing the Tb3+ content and the excitation wavelengths. The correlated color temperatures can also be tailored from cold color to warm color. Furthermore, the energy transfer process from Dy3+ to Tb3+ in the Dy3+/Tb3+ codoped phosphors exists, which is discussed based on the luminescence spectra and energy level diagram analysis. The NaCa4(BO3)3:Dy3+, Tb3+, Eu3+ phosphors

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reported here demonstrate promising applications in the fields of near ultraviolet based light emitting

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diodes.

Keywords: Phosphor; Light emitting diode; Luminescence; Energy transfer

1. Introduction

Nowadays, white light emitting diodes (W-LEDs) as green illumination technology have aroused increasing attentions due to their superior advantages of environmental friendliness, long *

Corresponding author. E-mail address: [email protected] (C.F. Zhu); Tel: +86-531-89631227; Fax: +86-531-89631226. 1

ACCEPTED MANUSCRIPT lifetime, and low energy consumption compared with conventional incandescent and fluorescence lamps [1-5]. The generally employed approach to fabricate W-LED device is the combination of a blue or near ultraviolet (NUV) semiconductor chip with down-converting

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phosphors [6]. The previous studies on NUV-pumped W-LEDs were performed using two or three different phosphors, resulting in disadvantages of complicated phosphor blending operation in device packaging procedure and efficiency loss caused by the self-absorption among respective phosphors [7]. Alternatively, a single component full-color emitting phosphor for

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NUV-pumped W-LEDs can greatly simplify the packaging procedure and improve device

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performances [8]. Therefore, the investigation of efficient single component white light emission phosphors is of significance to develop NUV-pumped W-LEDs.

The phosphors for LEDs have been investigated for many kinds of hosts such as silicate, aluminate, sulphide, nitride, phosphate, and borate, etc [9-16]. Rare earth doped borate phosphors have gained much research attention since they own the advantages of superior stability, low cost,

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and low synthetic temperature [17-19]. Dy3+ doped phosphors generally display two intense emission bands in the blue and yellow regions, which provide the feasibility of obtaining near white light emission via the mixing of blue and yellow light. On the other hand, Tb3+ ions with

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green emission can also be introduced to the Dy3+-doped phosphors to enrich the emission

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spectra and realize the white light emission. Eu3+ doped phosphors generally show the emission of red light. H.X. Guan et al. reported the BaGdF5: Dy3+, Tb3+, Eu3+ phosphors prepared by hydrothermal method [20]. The luminescence, energy transfer, and paramagnetic properties were investigated. The BaGdF5: Dy3+, Tb3+, Eu3+ materials may have potential applications in fullcolor displays, biological labels and bio-separation. In this work, we investigate a series of new single component white light emitting NaCa4(BO3)3:Dy3+, Tb3+, Eu3+ phosphors. The crystal structure, the luminescence property, and the energy transfer from Dy3+ to Tb3+ in the phosphors 2

ACCEPTED MANUSCRIPT are examined. The Dy3+/Tb3+ and Dy3+/Tb3+/Eu3+ doped NaCa4(BO3)3 phosphors as a kind of fullcolor luminescent material for NUV W-LEDs application are rarely investigated up to now. The results of the present work are scientifically intriguing and might also be technically significant to

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the fabrication of LED devices.

2. Experimental

NaCa4(BO3)3:Dy3+, Tb3+, Eu3+ phosphors were synthesized via a high temperature solid state

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reaction technique. Analytical purity Na2CO3, CaCO3, H3BO3, and high purity (99.99%) Dy2O3, Tb4O7 and Eu2O3were used as raw materials. The appropriate amounts of raw materials were

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weighed in the requisite proportions and subsequently mixed and ground for 30 min in an agate mortar. The mixed powders were then transferred into an alumina crucible, and pre-sintered in a furnace at 600 oC for 2 h in air to eliminate the water and decompose the carbonate. The presintered samples were subsequently cooled to room temperature and fully ground to form a homogeneous mixture. Then the mixture was re-sintered at a high temperature of 800 oC for 4 h.

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After sintering, the samples were allowed to cool down naturally to room temperature in the furnace. The obtained products were pulverized for further measurements. The nominal molar compositions of the phosphors are as follows: NaCa3.98(BO3)3: 0.02Dy3+, NaCa3.99(BO3)3:

nTb3+ (n = 0, 0.005, 0.01, 0.015, 0.025).

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3+ n(BO3)3:0.02Dy ,

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0.01Tb3+, NaCa3.98(BO3)3: 0.02Eu3+, NaCa3.95(BO3)3: 0.02Dy3+, 0.01Tb3+, 0.02Eu3+ and NaCa3.98-

The crystal phase identification of the as-prepared samples was examined by the X-ray diffractometer (XRD, Shimadzu, LabX XRD-6100, Kyoto, Japan ) with Cu Kα radiation (λ = 0.154 nm). The morphology of the phosphors was characterized using a field emission scanning electron microscope (FE-SEM, HITACHI, S-4800, Tokyo, Japan). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were obtained by a fluorescent spectrometer (Hitachi, F-4600, Tokyo, Japan) equipped with a Xe lamp as an excitation source. To eliminate 3

ACCEPTED MANUSCRIPT the second order emission from the light source radiation, a cutoff filter was used during the measurements. All measurements were performed at room temperature with the same instrumental parameters.

Fig.

1

shows

the

XRD

patterns

of

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3. Results and discussion phosphors

NaCa3.98(BO3)3:0.02Dy3+,

NaCa3.99(BO3)3:0.01Tb3+, NaCa3.97(BO3)3:0.02Dy3+, 0.01Tb3+, and NaCa3.95(BO3)3:0.02Dy3+, 0.01Tb3+, 0.02Eu3+.The XRD profile of the host compound NaCa4(BO3)3 is also included in Fig.

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1 for comparison. Some sharp diffraction peaks can be observed in the XRD patterns, indicating

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that the crystalline phases exist. Compared to the XRD profiles of crystals Ca3B2O6 (JCPDS No. 26-0347) and NaCa4(BO3)3 (JCPDS No. 75-3604), as also shown in Fig. 1, all the XRD patterns of samples display the diffraction peaks of these two crystalline phases. So the as-prepared samples contain Ca3B2O6 and NaCa4(BO3)3 crystals. The fundamental building units of NaCa4(BO3)3 crystal are isolated planar [BO3]3- anionic groups, which are distributed along six

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directions [17]. The Na atoms are surrounded by eight oxygen atoms. The NaO8 polyhedra are connected via a bridging oxygen to form infinite long chains, and they share four edges and two corners with the BO3 triangles. The Ca atoms locate in three different microenvironments. On the

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other hand, the basic units of the Ca3B2O6 structure are columns of Ca polyhedrons, with the coordination number with respect to oxygen of eight. All oxygen atoms coordinating Ca atoms

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enter the composition of nearly planar [BO3]3- groups isolated in the structure [21]. The Dy/Tb/Eu ions prefer to occupy the site of Ca2+ ions in the host lattice due to the ion radii matching of Dy/Tb/Eu with Ca2+ ions. We can see that the XRD patterns of Dy3+- , Tb3+-, and Eu3+-doped phosphors are nearly identical to that of the host compound NaCa4(BO3)3, indicating that the concentrations of the dopants (Dy3+ , Tb3+, and Eu3+) are too low to induce any significant changes to the host crystal structure. 4

ACCEPTED MANUSCRIPT SEM can be employed as a powerful tool to illustrate the morphologies of the phosphors. Fig. 2 shows the SEM images of the as-prepared samples. We find that the introduction of rare earth ions does not induce noticeable variations of the particle shape and size. Some phosphor particles

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are not uniform/ regular in shape, probably due to the agglomeration among the particles during the sintering process. The particle size and particle size distribution of the phosphors are important parameters, which affect the properties of the LED device. It is reported that the phosphor particles in the micron range are suitable for fabrication of LED devices [8]. Also based

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on the technology of the LED device manufactures, the phosphor particles in the range of 4-18

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µm are preferable. Herein, the size of particles in these phosphors is observed in the micron range around 4-15 µm, which can meet the demand for practical applications. The emission and excitation spectra of the Tb3+-single activated NaCa4(BO3)3 phosphor are displayed in Fig. 3(a). The excitation spectrum includes three bands at ~ 352, 368, and 485 nm, which result from the electronic transitions from ground state 7F6 to the excited states 5L9, 5G5,

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and 5D4 respectively [22-27]. The corresponding electronic transitions are also indicated in the energy level diagrams (Fig. 4). The emission spectrum of Tb3+ single doped NaCa4(BO3)3 phosphor exhibits characteristic emission peaks at ~ 490, 543, 583, and 620 nm, which are

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ascribed to the electronic transitions of 5D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4, and 5D4 → 7F3 of Tb3+

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ions, respectively (see Fig. 4).

The emission and excitation spectra of the Dy3+ single-doped NaCa4(BO3)3 phosphor are shown in Fig. 3(b). The excitation spectrum monitored at the dominant emission at 571 nm exhibits several excitation bands peaking at ~ 325, 351, 365, and 388 nm , which correspond to the electronic transitions from ground state 6H15/2 to the excited states 6P3/2, [4I11/2,4P7/2], [4P3/2,6P5/2], and [4M21/2,4I13/2, 4K17/2,4F7/2] [28-33], respectively (see Fig. 4). The emission spectrum excited at 365 nm contains dominant emissions at blue (480 and 492 nm), yellow (571 5

ACCEPTED MANUSCRIPT nm), and red (658 nm) regions. The blue emissions at ~ 480 and 492 nm are attributed to the magnetic dipole transition 4F9/2 → 6H15/2 of Dy3+ ions, and another yellow emission located at 571 nm is related to the electric dipole transition 4F9/2 → 6H13/2 of Dy3+ ions. The 4F9/2 → 6H13/2 is a

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hypersensitive electric dipole transition, which is influenced strongly by the local surroundings. Generally, the 4F9/2 → 6H13/2 transition is dominant when Dy3+ ions are located at low-symmetry sites with no inversion center, whereas the 4F9/2 → 6H15/2 transition is stronger when Dy3+ ions are

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located at high symmetry with an inversion center [34]. Herein, the yellow emission is found to be predominant over the blue emission for NaCa3.98(BO3)3:0.02Dy3+ phosphor, implying that the

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Dy3+ ions are situated away from inversion symmetry. Furthermore, we can see that the emission band splitting occurs for this phosphor. The blue emission band displays two emission peaks (480 and 492 nm), and the yellow emission band also shows a shoulder at ~581 nm. Energy level splitting can occur in the 4F9/2, 6H15/2, and 6H13/2 energy levels of Dy3+ ions, which are affected by the crystal field strength of the phosphors. The number of emission peaks corresponding to the F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transition is dependent on the energy level splitting of the 4F9/2,

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H15/2, and 6H13/2 states. Comparing Fig. 3 (a) with (b), it is worth pointing out that the dominant

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4

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absorption band at ~ 485 nm of phosphor NaCa3.99(BO3)3:0.01Tb3+ can overlap with the blue emission of NaCa3.98(BO3)3:0.02Dy3+. Thus, the energy transfer from Dy3+ to Tb3+ is expected to

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occur in the Dy3+/Tb3+ codoped NaCa4(BO3)3 phosphor. Fig. 3(c) presents the emission and excitation spectra of NaCa3.97(BO3)3:0.02Dy3+, 0.01Tb3+ phosphor. The emission spectrum is obtained upon the excitation at 365 nm and the excitation spectra are measured at the dominant emissions of 543 and 571 nm, where a series of excitation peaks at ~ 351, 365, 368, 388, and 485 nm are observed. Efficient excitation of the Dy3+/Tb3+ codoped NaCa4(BO3)3 phosphor with NUV radiation is confirmed here and well meets the requirements of LED, that is, well absorbing the NUV light of GaInN-based LED chip and down6

ACCEPTED MANUSCRIPT converting it into visible light. It can be also seen in Fig. 3(c) that a significant spectral overlap between the blue emission band of Dy3+ (at ~ 480 and 492 nm) and the excitation band of Tb3+ (at ~485 nm) can be observed. According to the Dexter’s theory, the probability of energy

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transfer (ET) from Dy3+ to Tb3+ ions exists. To further investigate the ET process from Dy3+ to Tb3+ in the NaCa3.97(BO3)3:0.02Dy3+, 0.01Tb3+ phosphor, we analyze the energy level diagrams of Dy3+ and Tb3+ ions. Fig. 4 illustrates the related electronic transitions and the possible energy transfer routes from Dy3+ to Tb3+. It demonstrates that the Dy3+ ions can be populated under the

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365 nm irradiation. Since the energy gap between the 4F9/2 level of Dy3+ and the 5D4 level of Tb3+

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is small, Dy3+ ions can serve as effective sensitizers to transfer part of their excitation energy to Tb3+ ions. Thus, the emission of Tb3+ can be well enhanced by Dy3+ via an energy transfer process.

We can see from Fig. 3(c) that the locations of the excitation bands monitored at Tb3+ and Dy3+ emission peaks are not same, which might lead to the variation in the emission spectra of

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NaCa3.97(BO3)3:0.02Dy3+, 0.01Tb3+ phosphor under various excitation wavelengths. Fig. 5 exhibits the emission spectra of the Dy3+/Tb3+ codoped NaCa4(BO3)3 phosphor excited at different wavelengths, where the emissions from Dy3+ and Tb3+ can be clearly observed.

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Moreover, the correlation of the emission spectra with the excitation wavelength can be seen, e.g.,

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the relative emission peak intensity depends drastically on the excitation wavelength (see the inset of Fig. 5). The difference of the excitation spectra corresponding to the Dy3+ and Tb3+ emissions (shown in Fig. 3(c)) can determine the variation in emission behavior of the Dy3+/Tb3+ codoped NaCa4(BO3)3 phosphor upon various excitation conditions. Fig. 6(a) and (b) presents the emission spectra of Dy3+ and Tb3+-coactivated NaCa3.983+ n(BO3)3:0.02Dy ,

nTb3+ phosphors with different Tb3+ contents upon excitations at 365 and 371

nm, respectively. The relative emission intensities of Tb3+ are gradually enhanced with increasing 7

ACCEPTED MANUSCRIPT Tb3+ concentrations. Fig. 6(c) shows the emission spectra of Dy3+ and Tb3+ coactivated NaCa3.955(BO3)3:0.02Dy3+, 0.025Tb3+ phosphor under excitations at 365 and 371 nm. The emission peak intensity varies with the excitation wavelength, as shown in the inset of Fig. 6 (c).

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In the present work, we also investigated the Eu3+ doped NaCa4(BO3)3 phosphor. As shown in Fig. 7(a), the emission spectra of NaCa3.98(BO3)3:0.02Eu3+ phosphor exited at 362, 382, and 394 nm consist of the emission bands centered at around 580, 586/597, 616/626, 656, and 703/710 nm, which arise from the5D0 → 7FJ (J = 0, 1, 2, 3, and 4) electronic transitions of Eu3+. We can

F1, 5D0 → 7F2, and 5D0 → 7F4 display two emission peaks, respectively. Fig. 7(b) presents the

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7

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see that the emission band splitting occurs for this phosphor. The electronic transitions of 5D0 →

excitation spectrum of NaCa3.98(BO3)3: 0.02Eu3+ phosphor which contains some peaks between 355 to 410 nm (7F0 → 5D4 at 362 nm,7F0 → 5G4 at 382 nm, and 7F0 → 5L6 at 394 nm, see Fig. 4) [35-36]. In order to further improve the performance of white light emission of Dy3+/Tb3+ codoped NaCa4(BO3)3 phosphors for W-LEDs application, the red emission component is

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desirable. Herein, we examined the NaCa3.95(BO3)3:0.02Dy3+, 0.01Tb3+, 0.02Eu3+ phosphor. Its emission spectra excited at various wavelengths are shown in Fig. 8, where the emissions from

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Dy3+, Tb3+ and Eu3+ can be clearly observed. The difference of the excitation spectra corresponding to the Dy3+, Tb3+ and Eu3+ emissions can determine the variation in emission

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behavior of the Dy3+/Tb3+/Eu3+ codoped NaCa4(BO3)3 phosphor upon various excitation conditions.

The emission color of the phosphors is determined by the spectral energy distribution of the emitted light. To further give insight into the trace of color variation, the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates are calculated from the emission spectra, as shown in Fig. 9. It can be seen that the CIE color coordinates depend greatly on the excitation wavelengths and the concentration of Tb3+ ions. Some color coordinates locate within 8

ACCEPTED MANUSCRIPT the white light region of CIE chromaticity diagram. We find that the emission color of Dy3+/Tb3+ co-doped NaCa4(BO3)3 phosphors can be adjusted easily by changing the excitation wavelengths and Tb3+ content. The emission colors can be varied from green, yellow to white, which gives us

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more selections of application. Moreover, the correlated color temperatures (CCTs) of the phosphors are calculated from CIE chromaticity coordinates x and y by McCamy equation [37]: CCT = an3 + bn2 + cn + d,

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where the parameters are introduced with a = -499, b = 3525, c = -6823.3, d = 5520.33, and n =

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(x-xe)/(y-ye) with xe = 0.3320 and ye = 0.1858. Fig. 10 demonstrates the CCTs for the phosphors. We find that the CCTs of these phosphors can be successfully tuned from cold color to warm color by changing excitation wavelengths and Tb3+ content to meet special requirements. Based on Figs. 9 and 10, we find that both the luminescence color coordinates and CCTs of the Dy3+-, Tb3+-, and Eu3+-doped NaCa4(BO3)3 phosphors can be tailored effectively by

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modulating the excitation wavelengths and Tb3+ content, which is interesting for LED applications because the light hue can be conveniently controlled. These results indicate that the as-prepared Dy3+-, Tb3+-, and Eu3+-doped NaCa4(BO3)3 phosphors are promising candidates to

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develop white lighting devices under the excitation of NUV-LEDs, and a smart lighting system

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based on the Dy3+-, Tb3+-, and Eu3+-doped NaCa4(BO3)3 phosphors is a potential illumination source offering controllability of color hue to match specific environments and requirements.

4. Conclusion

In summary, a variety of novel color tunable Dy3+-, Tb3+-, and Eu3+-doped NaCa4(BO3)3 phosphors for LEDs are developed. The Ca3B2O6 and NaCa4(BO3)3 crystals exist in the phosphors. The tunable emission colors and CCTs of the Dy3+/Tb3+ co-doped phosphors are realized by appropriately tuning the Tb3+ content and excitation wavelengths. The CIE 9

ACCEPTED MANUSCRIPT chromaticity coordinates of some phosphors are located in the white light region. Furthermore, the energy transfer process from Dy3+ to Tb3+ in the Dy3+/Tb3+ co-doped phosphor is analyzed via the luminescence spectra and energy level diagrams. The NaCa4(BO3)3: Dy3+, Tb3+, Eu3+

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phosphors demonstrate promising applications in the fields of NUV excited LEDs.

Acknowledgments

This research is financially supported by the Natural Science Foundation of Shandong

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Province (Grant No. ZR2015EM049), Jinan Youth Science and Technology Star Project (Grant No. 201406004), Science and Technology Plan Program of Shandong College (Grant No.

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J15LA01), and Open Foundation of State Key Laboratory of Silicate Materials for Architectures (Wuhan University of Technology) under Project No. SYSJJ2016-12. We also thank the support from the Incubation Program of Universities' Preponderant Discipline of Shandong Province

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[24] F. Wang, D.C. Liu, B. Yang, Y.N. Dai, Characteristics and synthesis mechanism of Gd2O2S:Tb phosphors prepared by vacuum firing method, Vacuum 87 (2013) 55-59. [25] C. Zhang, H. Liang, S. Zhang, C. Liu, D. Hou, L. Zhou, G. Zhang, J. Shi, Efficient sensitization of Eu3+ emission by Tb3+ in Ba3La(PO4)3 under VUV-UV excitation: energy transfer and tunable emission, J. Phys. Chem. C 116 (2012) 15932–15937. [26] E. Cavalli, P. Boutinaud, R. Mahiou, M. Bettinelli, P. Dorenbos, Luminescence dynamics in Tb3+-doped CaWO4 and CaMoO4 crystals, Inorg. Chem. 49 (2010) 4916–4921. 12

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a color tunable phosphor: Dy3+, Tm3+, and Eu3+-coactivated KSr4(BO3)3 for warm white UV LEDs, J. Mater. Chem. 22 (2012) 6463-6470.

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activated Ba3Bi(PO4)3, J. Alloys Compd. 515 (2012) 16–19.

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Figure captions Fig. 1. XRD patterns of the samples. The XRD patterns of crystals Ca3B2O6 (JCPDS No. 26-0347) and NaCa4(BO3)3 (JCPDS No. 75-3604) are also shown for comparison.

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Fig. 2. SEM images of samples NaCa4(BO3)3 (a), NaCa3.98(BO3)3:0.02Dy3+ (b), NaCa3.99(BO3)3: 0.01Tb3+ (c), and NaCa3.97(BO3)3:0.02Dy3+,0.01Tb3+ (d), respectively.

Fig. 3. The emission and excitation spectra of phosphors NaCa3.99(BO3)3:0.01Tb3+ (a),

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NaCa3.98(BO3)3: 0.02Dy3+ (b), and NaCa3.97(BO3)3: 0.02Dy3+, 0.01Tb3+ (c). The excitation (λex) and monitored (λem) wavelengths are indicated in the figure.

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Fig. 4. The schematic energy level diagrams of Dy3+, Tb3+, and Eu3+ ions showing the related electronic transitions and the possible energy transfer (ET) process from Dy3+ to Tb3+. Fig. 5. The emission spectra for NaCa3.97(BO3)3:0.02Dy3+, 0.01Tb3+ phosphor upon various excitation wavelengths. The excitation wavelengths, i.e., 351, 365, 371, and 388 nm, are indicated in the figure. The inset shows the emission peak intensity (at 480, 543, and 571 nm)

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versus various excitation wavelengths (Ex wavelength). Fig. 6. The emission spectra of NaCa3.98-n(BO3)3:0.02Dy3+, nTb3+ (n = 0.005, 0.01, 0.015, and

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0.025) phosphors upon excitations at 365 (a) and 371nm (b). The electronic transitions corresponding to the respective emission bands are indicated in the figure. The dependence

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of the emission spectra on the excitation wavelengths (365 and 371 nm) for NaCa3.955(BO3)3:0.02Dy3+, 0.025Tb3+ phosphor is shown in (c). The inset in (c) presents the variation of emission peak intensity (the symbols

and

correspond to λex = 365 nm and

λex = 371 nm, respectively). Fig. 7. (a) The emission spectra of NaCa3.98(BO3)3:0.02Eu3+ phosphor upon different excitation wavelengths. (b) The excitation spectrum monitored at the dominant emission of 616 nm. The corresponding electronic transitions are indicated in the figure. 14

ACCEPTED MANUSCRIPT Fig. 8. The emission spectra of NaCa3.95(BO3)3: 0.02Dy3+, 0.01Tb3+, 0.02Eu3+ phosphor under different excitation wavelengths. Fig. 9. CIE chromaticity diagram of the NaCa3.97(BO3)3:0.02Dy3+,0.01Tb3+ phosphor excited at

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various wavelengths (a). CIE chromaticity diagram of the NaCa3.98-n(BO3)3:0.02Dy3+, nTb3+ phosphors upon excitations at 365 nm (b) and 371 nm (c). CIE chromaticity diagram of the NaCa3.95(BO3)3:0.02Dy3+,

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wavelengths (d). The CIE chromaticity coordinates are indicated in the figure.

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Fig. 10. The correlated color temperatures of the NaCa3.97(BO3)3:0.02Dy3+,0.01Tb3+ phosphor

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excited at various wavelengths (a). The correlated color temperatures of NaCa3.98nTb3+ phosphors upon excitation at 365 nm (b) and 371 nm (c). The

correlated color temperatures of the NaCa3.95(BO3)3:0.02Dy3+, 0.01Tb3+, 0.02Eu3+ phosphors

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Highlights Dy3+/Tb3+/ Eu3+ doped NaCa4(BO3)3 phosphors for LEDs are studied. Tunable luminescence of the as-prepared phosphors is realized.

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The energy transfer from Dy3+ to Tb3+ in the Dy3+/Tb3+ codoped phosphors occurs.