Superior white light emission and color tunability of tri-doped YBO₃:Tb³⁺, Eu³⁺ and Dy³⁺ for white light emitting diodes

Abstract

A tri-doped YBO₃:Tb³⁺, Eu³⁺ and Dy³⁺ phosphor, which is capable of producing white light by combining blue, green, yellow, orange and red emissions when excited at 365 nm ultraviolet (UV) light, was developed using a general hydrothermal method. The samples showed strong photoluminescence spectra at 485, 541, 578, 591, 611, and 627 nm under the excitation wavelength of 365 nm (ultraviolet light) indicating the presence of blue, green, yellow, orange and red light due to the transitions ⁴F_9/2 → ⁶H_15/2 (Dy³⁺), ⁵D₄ → ⁷F₅ (Tb³⁺), ⁴F_9/2 → ⁶H_13/2 (Dy³⁺), ⁵D₀ → ⁷F₁ (Eu³⁺) and ⁵D₀ → ⁷F₂ (Eu³⁺), respectively. Tri-doping is seemed to be more successful in the creation of white light. The effect of the variations in doping percentages on color tunability was also evident. Evidence of efficient energy transfers from the host excitations to activators Tb³⁺ and Eu³⁺, and from Dy³⁺ to Tb³⁺ to Eu³⁺ existed. Although there is a weak energy transfer from host to Dy³⁺ occurred, strong photoluminescence excitation bands existed in Dy³⁺.

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1. Introduction

Orthoborates (commonly denoted as REBO₃, where RE = Rare Earths = Y, La, Gd, and Lu), doped with lanthanide ions (Eu³⁺, Ce³⁺, Dy³⁺, or Tb³⁺), have taken a prominent position among the phosphors and have a wide range of applications. These phosphors have strong photoluminescence (PL) intensity and exceptional optical damage threshold, and are very stable.^1–7 One of the important applications of these phosphors are the (LEDs). YBO₃ is seemingly more popular among the REBO₃s due to its excellent ultraviolet absorption and high chemical stability.^7–13

The development of solid-state lighting has shifted the focus to create high-efficiency and high-quality white light using light-emitting diodes (WLEDs). Solid-state LEDs have always been in the limelight due to emission efficiencies as high as 95% which are much higher than incandescent and fluorescent lights.^14,15 The greatest disadvantage of these LEDs is that the light emission ranges within a few nanometers. So additional materials, in most cases the inorganic phosphors are used to expand the emission spectrum. The phosphors are mostly doped with rare earth (RE) ions as this process causes highly stable f–f energy level transitions and a broad range of emission wavelengths. High quality and efficient white light are produced by, doping complex inorganic hosts with dopants. The methods to synthesize these inorganic hosts are complicated and or expensive. First commercially available WLED was prepared by combining Gallium Nitride (GaN) chip with yttrium aluminum garnet (YAG) doped with cerium which is the prominent practice still today.¹⁶ White color produced from this method suffers from poor color rendering index as the YAG doped cerium doesn't have sufficient red light emission. The condition is improved by adding red phosphor. Another method is combining UV LED chip with a white light-emitting single-phosphor. Again this method also suffers the same problem as described in the previous method. Some dopants like Tb³⁺, Eu³⁺, Dy³⁺ or Ce³⁺ can absorb UV radiation and their emissions are between green to red band.

In our recent works, we have reported a general hydrothermal method successfully used without any additional organic solvent or surfactant for the synthesis of YBO₃:Eu³⁺ with the highest chromaticity (red/orange photoluminescence emission) value.⁷ For the first time, synthesis of YBO₃:Tb³⁺, Eu³⁺ phosphors with white and tunable luminescence under UV excitation and by varying the relative ratio of Tb³⁺ and Eu³⁺ doping was also reported by our group.⁹ Due to incorporation of Tb³⁺ and Eu³⁺ ions into YBO₃, efficient energy transfers from the host excitations to activators, as well as from Tb³⁺ to Eu³⁺, occurred. In addition, our group has reported structural and optical studies of YBO₃ when tri-doped with Eu³⁺, Ce³⁺, and Tb³⁺, focusing on the role of terbium concentration. It has been shown that the PL excitation bands related to both Ce³⁺ and Tb³⁺ increase in intensity for red emission from the Eu³⁺ with increasing terbium concentration. Results were consistent with a Ce³⁺ → (Tb³⁺)_n → Eu³⁺ energy transfer scheme.¹⁰

It has been reported that upon 273 nm UV excitation, trivalent dysprosium (Dy³⁺) exhibits blue (∼470–500 nm), yellow (∼570–600 nm) and red (∼650–680 nm) emissions through ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2, ⁴F_9/2 → ⁶H_11/2 transitions, respectively when YAl₃(BO₃)₄ (YAB) phosphors are co-doped with Dy³⁺.¹⁷ Also, Bi³⁺ and Dy³⁺ co-doped YBO₃ exhibits blue emission at 481 nm and yellow emission at 578 nm.¹⁸ To our knowledge, luminescence study of tri-doped YBO₃:Tb³⁺, Eu³⁺ and Dy³⁺ has not been reported.

In this paper, we report a superior white light generation and color tunability by tri-doping YBO₃:Tb³⁺, Eu³⁺ and Dy³⁺ for the production of white LEDs with an effective absorption in the ultraviolet window range. On being excited under 365 nm single doping Tb³⁺ mainly emits green (at 540 nm), Eu³⁺ emits red (at 611 and 627 nm) and Dy³⁺ emits yellow (at 578 nm) and blue (at 481 nm) light, respectively.

2. Experimental

2.1 Synthesis of single-doped, co-doped and tri-doped YBO₃

All chemicals were analytical grade and used directly without any treatment. First YBO₃ was synthesized hydrothermally by the chemical reactions of K₂B₁₀O₁₆·8H₂O (Alfa Aesar, 99.9%) dissolved in the solvent ethylene glycol (EG) and Y(NO₃)₃·6H₂O (Alfa Aesar, 99.9%). During the synthesis process at first K₂B₁₀O₁₆·8H₂O was dissolved in ethylene glycol (EG) by stirring vigorously using a magnetic stirrer. 1 M of Y(NO₃)₃·6H₂O was added to 0.2 M of the former solution and stirred vigorously. Both the solutions were mixed in various proportions. Then the dopants were added. For example, for tri-doping, Tb(NO₃)₃·6H₂O (terbium nitrate hexahydrate, Sigma Aldrich, 99.9%), Eu(NO₃)₃·6H₂O (europium nitrate hexahydrate, Sigma Aldrich, 99.9%) and Dy(NO₃)₃·6H₂O (dysprosium nitrate hexahydrate, Sigma Aldrich, 99.9%) were added to the solution in various proportions as required and stirred vigorously. After stirring the liquid for some time a white precipitate was formed on the liquid surface. 10% ammonia water was added to the solution until the pH value turned to 9. The solution was then poured into a 60 mL autoclave and the volume of the prepared solution was adjusted to fill 80–85% of the autoclave. The system was sealed tightly, heated in the oven at 180 °C for 12 hours and then cooled to room temperature. The final crystal product was then washed with deionized water and ethanol for several times, dried in vacuum at 60 °C for four hours and collected for characterization.² The percentages of Dy³⁺ doping for single doped samples were varied from 1 to 12%. Various doping percentage combinations were tried for both co-doped and tri-doped YBO₃. Table 1 shows percentages of doping for co-doped and tri-doped samples reported in this paper as they met the objectives of this research. For co-doping, both Dy³⁺ and Eu³⁺ doping percentages were varied between 2 to 18% with total being maintained as 20%. For tri-doping after various trials, Dy³⁺ percentage was kept at 2%, Tb³⁺ percentage was kept at 5 or 6%, and Eu³⁺ percentage was varied to obtain points in the white region in the chromaticity diagram. All the dopant concentrations were verified using energy dispersive X-ray spectroscopy (EDS) in an SEM. One representative EDS spectra and data are shown for the sample YBO₃:Dy³⁺ 2%, Tb³⁺ 5%, Eu³⁺ 4% in Fig. S1 in the ESI.†

Table 1 Doping concentrations of Dy³⁺, Eu³⁺ and Tb³⁺ in YBO₃

Sample no.	Conc. of Dy^3+	Conc. of Eu^3+	Conc. of Tb^3+
1	18%	2%
2	15%	5%
3	12%	8%
4	8%	12%
5	5%	15%
6	2%	18%
7	2%	4%	5%
8	2%	7%	5%
9	2%	10%	5%
10	2%	5%	6%
11	2%	8%	6%
12	2%	11%	6%

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2.2 Characterization

A Rigaku Ultima III diffractometer with Cu-kα radiation and operating in parallel-beam geometry was used for the powder X-ray diffraction (XRD). The lattice parameters were obtained by using the software of MDI Jade version 9.1 (Material Data Inc., Livermore, CA, USA; http://www.materialsdata.com). Scanning electron microscopy (SEM) (Hitachi, S-570) was used to characterize the morphology and particle sizes. For the high-resolution transmission electron microscopy (HRTEM) observation, the powders were separated by diluting them in ethyl alcohol. It was then sonicated for 30 minutes and dropped on the carbon film supported on 200-mesh Cu grids. JEOL 2100 TEM was operated at 200 kV. The PL measurement was performed on a Jobin Yvon fluorescence spectrometer (Fluorolog-3-TAU) using a xenon lamp as an excitation source.

3. Results and discussion

3.1 Crystal structure and morphology

X-ray diffraction analysis of one of the YBO₃ samples is shown in Fig. 1. The analysis shows that the YBO₃ samples have pure crystal quality and a single phase. It can be concluded that pure YBO₃ have been formed which indicates that no individual Y₂O₃ and B₂O₃ were formed. There is a very small shift in the peak position which indicates the effect due to doping, and doping level is strong enough to make this shift.⁹

Fig. 1 X-ray diffraction pattern of one representative YBO₃ sample.

Fig. 2 shows the morphology of YBO₃ under Scanning Electron Microscope (SEM). The image shows that microflower structure of size approximately 16 μm were formed. Each of these microflowers was formed from nanoflakes.

Fig. 2 Scanning electron microscopy (SEM) image of a representative YBO₃ sample showing microflower structure of size approximately 16 μm.

HRTEM analysis was performed to probe the crystalline type of grown YBO₃ samples. Fig. 3 shows a HRTEM image from a single nanosheet of one of the doped YBO₃ samples with the flower like microstructure. The Fast Fourier Transform (FFT) pattern showed that the HRTEM imaging is along the [0001] zone axis. The FFT pattern of the HRTEM image indicates that YBO₃ crystal grown has a hexagonal structure which is also confirmed by the XRD result. The interplanar distance of 0.324 nm in the HRTEM image can be indexed to the d-spacing of (101̄0) planes.

Fig. 3 HRTEM image of a single YBO₃ nanosheet of the flower like microstructure. The insert shows the corresponding fast Fourier transform (FFT).

3.2 Photoluminescence spectra and CIE diagram of single doped YBO₃:Dy³⁺, co-doped YBO₃:Dy³⁺and Eu³⁺, and tri-doped YBO₃:Dy³⁺, Eu³⁺and Tb³⁺

Fig. 4a and b shows the PL emission spectra of single doped YBO₃:Dy³⁺ with doping percentages of 1, 2, and 3%, and 4, 8 and 12%, respectively. Upon excitation by 365 nm, the emission spectra include two characteristics peaks located at 481 (blue) and 578 nm (yellow) of Dy³⁺ due to the transitions of ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2, respectively.¹⁹ In addition, a broad UV emission band with maxima around 420 nm of YBO₃ host exists in the PL spectra. This broad band is attributed to the recombination of self-trapped excitations (STEs) that may be associated with band-gap excitations or molecular transitions within the BO₃³⁻ group.^9,20 Since the intensity of ⁴F_9/2 → ⁶H_13/2 (yellow) transition is greater than the intensity of ⁴F_9/2 → ⁶H_15/2 (blue) transition and the B/Y intensity ratio (i.e. integrated intensity under the respective peaks) is approximately 1.1 or greater than unity, the Dy³⁺ ions occupy the higher symmetry local site with non-inversion centre in the YBO₃ lattice.²¹ When the Dy³⁺ concentration was increased from 1% to 12%, the emission intensity first increased and then decreased concluding that the optimum doping concentration of Dy³⁺ in YBO₃ is 2%. Beyond this optimum doping concentration, the PL emission intensity decreases due to the concentration quenching phenomena resulting from a cross relaxation between neighbouring Dy³⁺ ions which are in resonance of their energy levels due to the Dy³⁺ (⁴F_9/2) + Dy³⁺ (⁶H_15/2) → Dy³⁺ (⁶F_3/2) + Dy³⁺ (⁶H_11/2) transitions.²²

Fig. 4 PL spectra of single doped YBO₃:Dy³⁺ with doping percentages of (a) 1, 2, 3% and (b) 4, 8 and 12%, respectively.

Fig. 5 shows the CIE chromaticity diagram in which the CIE index point for the single doped YBO₃:Dy³⁺ 2% with the coordinates (0.198, 0.162) are well located within the blue region. Thus this phosphor emits blue color which could be partly due to the contribution from the broad UV band at 420 nm in addition to blue emission being more intense than the yellow emission.

Fig. 5 CIE chromaticity diagram for the single doped YBO₃:Dy³⁺ 2%.

Fig. 6a and b show the PL spectra of co-doped YBO₃:Dy³⁺ and Eu³⁺ samples for various doping concentrations as show in Table 1. It can be seen that the PL spectra contain emissions peaks at 481 (blue) and 578 nm (yellow) from Dy³⁺ as well as at 591 (orange), and 611 and 627 nm (red) from Eu³⁺ due to the transitions ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂, respectively.²³ With the increase of Eu³⁺ concentration the emission peaks at 591, 611 and 627 nm become more intense and also the chromaticity (R/O) ratio increases.

Fig. 6 (a) and (b) PL spectra of co-doped YBO₃:Dy³⁺ and Eu³⁺ with various doping percentages.

Fig. 7 shows CIE index points in the CIE chromaticity diagram for all six co-doped samples and Table 2 gives respective x and y coordinate points. As shown in Fig. 7, the color can be tuned from blue/violet to white by changing the doping concentration of the activators Dy³⁺ and Eu³⁺.

Fig. 7 CIE chromaticity diagram for the co-doped YBO₃:Dy³⁺ and Eu³⁺ with various doping percentages.

Table 2 Sample number, doping percentages, points in the CIE diagrams and the corresponding coordinates

Sample no.	Dy^3+ conc.	Eu^3+ conc.	Tb^3+ conc.	Points	Color coordinates (x, y)
1	18%	2%		a	(0.18, 0.18)
2	15%	5%		b	(0.18, 0.16)
3	12%	8%		c	(0.19, 0.16)
4	8%	12%		d	(0.21, 0.17)
5	5%	15%		e	(0.24, 0.18)
6	2%	18%		f	(0.30, 0.21)
7	2%	4%	5%	g	(0.30, 0.28)
8	2%	7%	5%	h	(0.28, 0.20)
9	2%	10%	5%	i	(0.29, 0.21)
10	2%	5%	6%	j	(0.29, 0.24)
11	2%	8%	6%	k	(0.30, 0.23)
12	2%	11%	6%	l	(0.27, 0.22)

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Since co-doping resulted only one point in the white region in the CIE diagram, tri-doping of YBO₃ was attempted by adding Tb³⁺ in addition to Dy³⁺ and Eu³⁺. Fig. 8a and b show the PL emission spectra from tri-doped YBO₃:Dy³⁺, Eu³⁺ and Tb³⁺ samples with various doping concentrations (Table 1). In addition to the strong emission peaks at 481 (blue) and 578 nm (yellow) from Dy³⁺ as well as at 591 (orange), and 611 and 627 nm (red) from Eu³⁺, the PL spectra contain a strong emission peak at 540 nm (green) due to the transition ⁵D₄ → ⁷F_5.²⁴

Fig. 8 (a) and (b) PL spectra of tri-doped YBO₃:Dy³⁺, Eu³⁺ and Tb³⁺ with various doping percentages.

The CIE index points in the CIE chromaticity diagram for all six tri-doped samples are shown in Fig. 9 and Table 2 gives respective x and y coordinate points. As shown in Fig. 9, the color can be tuned from violet to white by changing the doping concentration of the activators Dy³⁺, Eu³⁺ and Tb³⁺. White light was obtained for all five out of six tri-doped YBO₃:Dy³⁺, Eu³⁺ and Tb³⁺ samples by successful blending of blue, green, orange and red emissions which is an excellent achievement.

Fig. 9 CIE chromaticity diagram for the tri-doped YBO₃:Dy³⁺, Eu³⁺ and Tb³⁺ with various doping percentages.

3.3 Spectroscopic investigation of energy transfer mechanism in co-doped YBO₃:Dy³⁺and Eu³⁺, and tri-doped YBO₃:Dy³⁺, Eu³⁺and Tb³⁺

Co-doped YBO₃:Dy³⁺ 2% and Eu³⁺ 18% was selected to further investigate various energy transfer mechanisms and their efficiencies (Fig. S2a–f in the ESI†). Fig. S2a† shows the PL emission spectra from undoped YBO₃ and YBO₃:Dy³⁺ 2% and Eu³⁺ 18%. The STE emission intensity of YBO₃ was reduced when YBO₃ was co-doped which indicates the energy transfer from host to activators. The intrinsic UV emission peak of undoped YBO₃ hardly overlaps with PL excitation bands of Dy³⁺ ions indicating very little host to activator energy transfer (Fig. S2b and c†). On the other hand, the emission peak of undoped YBO₃ partially overlaps with the Eu³⁺ 4f–4f absorptions, assuring the Förster–Dexter energy transfer condition between the host and Eu³⁺ ions (Fig. S2d–f†). According to the Förster–Dexter theory, a nonradiative energy transfer can occur between a host and an activator in case of a spectral overlap between host emission and absorption of the activator.²⁵

Fig. S2b and c† display the PL excitation spectra of co-doped YBO₃:Dy³⁺ 2% and Eu³⁺ 18% monitored at 481 and 578 nm, respectively, together with the emission spectra from the co-doped sample and undoped YBO₃. Fig. 10b shows that the excitation spectrum contains four bands with peaks at 295, 325, 352 and 365 nm corresponding to electron transitions of Dy³⁺ from the ground state ⁶H_15/2 to higher levels ⁴D_7/2, ⁶P_3/2, ⁶P_7/2 and ⁶P_5/2, respectively. The PL excitation spectrum in Fig. 10c exhibits bands with prominent peaks at 325, 352, 365, 387 and 453 corresponding to electron transitions of Dy³⁺ from the ground state ⁶H_15/2 to higher levels ⁶P_3/2, ⁶P_7/2, ⁶P_5/2, ⁴F_7/2 and ⁴I_15/2, respectively.²⁶ This indicates that near-UV LEDS (i.e. 365 nm excitation wavelength) based on the Dy³⁺ doped YBO₃ phosphor can be used as pumping sources for efficient emission. Fig. S2d–f† reveal a significant spectral overlap between the excitation band of Eu³⁺ and the Dy³⁺ emission transitions. Therefore, an effective energy transfers from Dy³⁺ to Eu³⁺ is expected. This type of energy transfer has been observed in several Dy³⁺ and Eu³⁺ co-activated phosphors.^27–29 Comparing Fig. 4 and 6 it can be observed that the PL emission intensity of the peaks from Dy³⁺ are quite strong in single doped YBO₃:Dy³⁺. Whereas, for the co-doped YBO₃:Dy³⁺ and Eu³⁺ the PL emission intensity of the Dy³⁺ peaks decrease quite a bit indicating a clear energy transfer phenomenon from Dy³⁺ ion to Eu³⁺ ion. Here, Eu³⁺ acts as an activator and Dy³⁺ acts as a sensitizer.

Fig. 10 The energy level diagram and energy transfer scheme between YBO₃, Dy³⁺, Tb³⁺ and Eu³⁺.

Tri-doped YBO₃:Dy³⁺ 2%, Eu³⁺ 4% and Tb³⁺ 5% was selected to further investigate various mechanisms of energy transfer and their efficiencies (Fig. S3a–g in the ESI†). Fig. S3a† indicates the energy transfer from host to activators. Fig. S3b and c† shows very little overlap between the YBO₃ host emission peak and PLE excitation bands of Dy³⁺ indicating a weak host to Dy³⁺ activator energy transfer. The emission peak of undoped YBO₃ partially overlaps with both the Tb³⁺ and Eu³⁺ 4f–4f absorptions, assuring the Förster–Dexter energy transfer condition between the host and Tb³⁺ and Eu³⁺ ions (Fig. S3c, e–g†). Fig. S3c† reveals a significant spectral overlap between the excitation bands of Tb³⁺ and Dy³⁺ emission transitions. Therefore, an effective energy transfers from Dy³⁺ to Tb³⁺ is evident. This type of energy transfer has been observed in several Dy³⁺ and Tb³⁺ co-activated phosphors.³⁰ A significant spectral overlap between the excitation band of Eu³⁺ and the Tb³⁺ emission transitions is also clear from Fig. S3e–g† indicating energy transfers from Tb³⁺ to Eu³⁺.⁷ Thus the efficient energy transfers from host YBO₃ to Tb³⁺ and Eu³⁺, as well as Dy³⁺ → Tb³⁺ → Eu³⁺ is clearly demonstrated from the overlapping of Pl emission and excitation spectra. The energy level scheme and energy transfer mechanism between YBO₃, Dy³⁺, Tb³⁺ and Eu³⁺ are shown in Fig. 10.

4. Conclusions

In summary, we report for the first time synthesis of YBO₃:Dy³⁺, Tb³⁺ and Eu³⁺ phosphors with white and tunable luminescence under UV excitation by using the optimum doping concentration of Dy³⁺ and varying the relative doping ratio of Tb³⁺ and Eu³⁺. A facile hydrothermal method was used to achieve uniform microflower-like structure of YBO₃:Dy³⁺, Tb³⁺, Eu³⁺ consisting of nanoflakes. Upon the UV excitation at 365 nm the emission spectrum of undoped YBO₃ consists of a broad UV emission centred at 425 nm which is attributed to the recombination of STE. Efficient energy transfers from the host excitations to activators Tb³⁺ and Eu³⁺, as well as from Dy³⁺ to Tb³⁺ to Eu³⁺ occurred. Although there is a weak energy transfer from host to Dy³⁺, strong PL excitation bands existed in Dy³⁺, suggesting that YBO₃:Dy³⁺ is suitable for phosphor converted LEDS. The facile method to make the thermally stable hierarchical structure has potential applications in LED devices. White light and color tenability can be successfully achieved by tri-doping and therefore these phosphors may be considered as ideal candidates for solid-state lighting applications.

Acknowledgements

We like to acknowledge the support of the National Science Foundation (NSF) Grant #MRI10922898. We also acknowledge the support from the Anhui Provincial Natural Science Foundation of China (1308085QA06) and the National Natural Science Foundation of China (Grant no. 11404320). We thank Dr Callum Hethirington and the Imaging Science Center at Texas Tech University for the XRD work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18217h

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