A promising blue-emitting phosphor CaYGaO₄:Bi³⁺ for near-ultraviolet (NUV) pumped white LED application and the emission improvement by Li⁺ ions

Abstract

In this paper, a blue-emitting phosphor CaYGaO₄:Bi³⁺ is reported, which has a broad emission band peaked at 435 nm covering the 375 nm to 575 nm spectral region. The CIE coordinates of CaYGaO₄:Bi³⁺ is (0.1536, 0.0885) with color purity of 85.2%. Importantly, when Bi³⁺ ions are introduced into the CaYGaO₄ host, the excitation peak of Bi³⁺ is located at 370 nm, which matches well with the efficient near-ultraviolet (NUV) light emitting diode (LED) chips. Furthermore, the emission intensity of the CaYGaO₄:Bi³⁺ phosphor increased ∼1.47 times by adding Li₂CO₃ as a fluxing agent. The quantum efficiency of the prepared CaYGaO₄:Bi³⁺,Li⁺ phosphor reaches 45.7% under 370 nm excitation, and the PL intensity remains 76.3% of that at room temperature at 423 K. Using the CaYGaO₄:Bi³⁺,Li⁺ phosphor, coupled only with an orange phosphor (Sr,Ba)₃SiO₅:Eu²⁺ and a 370 nm NUV chip driven by 20 mA current, a NUV-based WLED, with chromaticity coordinates of (0.3610, 0.3360), correlated color temperature (CCT) = 4304 K and color rendering index (CRI) = 86.9, has been achieved. The white light emitting diodes employing CaYGaO₄:Bi³⁺ as a blue-emitting component exhibit superior electroluminescence properties. All results demonstrate that CaYGaO₄:Bi³⁺ is a promising blue component for application in NUV-based WLEDs.

---

1. Introduction

Solid-state lighting devices based on white-light-emitting diodes (WLEDs) have shown various advantages of energy saving, environmental protection, small size, light weight, fast response and long service life, and they have already replaced conventional incandescent, and are fluorescent and widely used in lighting display and decoration fields.^1–8 WLEDs are most commonly fabricated by combining a blue LED chip with yellow or yellow/red phosphors.^9–11 WLEDs where a blue LED chip is combined with only a yellow phosphor often manifest high efficiency but suffer from poor color rendering properties; the addition of a second red phosphor improves the color rendering but usually with a sacrifice in efficiency. Furthermore, the sharp peak of the blue LED chip has two serious effects: the first is that if the WLED is a bright light source or the viewer is looking at it directly or even indirectly for some time it can cause retinal injury, and the second is that the blue light affects the human body clock.¹² One approach to avoid the problems associated with using blue LEDs is to use near-ultraviolet (NUV)-driven WLEDs that employ NUV-LED chips and blue/green/red-emitting phosphors. The use of a variety of monochromatic phosphors can effectively increase the color rendering index of the system. Moreover, this excitation mode can provide higher excitation energy and ultraviolet light does not affect white light emission. In this scheme, high efficiency phosphors, which can be well matched with commercial NUV LED chips, are key components. At present, most scientists mainly focus on materials that usually take rare earth ions such as Ce³⁺ or Eu²⁺ as activators.^13–17 However, these phosphors exhibit strong absorption bands in the visible region due to the nature of 4f to 5d transitions. This will unavoidably lead to reabsorption once they are utilized for WLEDs. In recent years, studies on trivalent bismuth ion doped phosphors have received more extensive attentions.^18–24 Bi³⁺ has broad absorption bands in the UV region and no reabsorption in the visible region, indicating that Bi³⁺ doped phosphors can solve the problems existing in traditional rare earth ions activated phosphors.²⁵ But actually, among the Bi³⁺ doped phosphors reported so far, most of the peak positions of the excitation spectra are concentrated in the ultraviolet region, especially in the relatively short wavelength region, such as KGaGeO₄:Bi³⁺ (240, 320 nm), CaSnO₃:Bi³⁺ (260, 308 nm), Ca₃Al₂O₆:Bi³⁺ (285, 305 nm), LiBaPO₄:Bi³⁺ (260 nm), and CaGa₂O₄:Bi³⁺ (280, 337 nm),^26–30 which cannot match with the NUV chips, thus greatly restricting the application of Bi³⁺ doped phosphors in the field of WLEDs. Therefore, it is highly desirable to develop a novel Bi³⁺ doped phosphor with strong NUV absorption.

In this work, a novel blue-emitting phosphor CaYGaO₄:Bi³⁺ was prepared, which exhibits an intense blue emission in the NUV excitation range of 300–400 nm. To further explore the potential application of the CaYGaO₄:Bi³⁺ phosphor in WLEDs, the optical properties (photoluminescence and photoluminescence excitation) and the optimal Bi³⁺ concentration have been investigated. In addition, the luminescence intensity and near ultraviolet absorption of the CaYGaO₄:Bi³⁺ phosphor can be further improved by adding Li₂CO₃. Based on our phosphor and the commercial phosphors ((Sr,Ba)₃SiO₅:Eu²⁺, (Ba,Sr)₂SiO₄:Eu²⁺, CaAlSiN₃:Eu²⁺) we fabricated a series of NUV-based WLED devices, which exhibit superior electroluminescence properties. All results indicate that the CaYGaO₄:Bi³⁺ phosphor would be a potential blue emitting phosphor for NUV-based WLEDs.

2. Experimental

2.1 Sample preparation

The CaY_1−xGaO₄:xBi³⁺ (x = 0, 0.002, 0.005, 0.007, 0.010, and 0.020) samples were synthesized by the conventional solid state reaction at high temperatures. CaCO₃ (99.95%), Y₂O₃ (99.99%), Ga₂O₃ (99.99%), and Bi₂O₃ (99.999%) were used as starting materials. The starting materials were weighed accurately according to the stoichiometric ratio and mixed in an agate mortar, and then the precursors were heated at 1523 K for 6 h in air and were cooled down to room temperature naturally. The faint white products were obtained, and were ground once again for subsequent measurements.

2.2 WLED fabrication

WLEDs were fabricated by combining the representative CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ phosphors and 370 nm InGaN NUV chips (Shenzhen Looking Long Technology Co., Shenzhen, China). The CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ phosphor was weighed out in a total amount of 0.15 g, and were evenly blended with the epoxy resins at a ratio of 1 : 5 (that is, 0.15 g CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ and 0.75 g epoxy resins). The acquired mixture was coated on the surface of the 370 nm InGaN NUV chips and dried at 393 K to fabricate WLEDs.

2.3 Characterization

The crystal structure and crystallinity of all samples were confirmed by X-ray diffraction (XRD) using a Bruker D8 advance powder diffractometer with Cu Kα radiation (λ = 1.54059 Å) at room temperature (RT). Static and dynamic PL spectra at room temperature were recorded using a high-resolution PL spectrometer FLS920 (Edinburgh Instruments) in a Peltier air-cooled house in the single photon counting mode. For high temperature PL spectra, the sample was first loaded into a holder with a circular copper substrate, the temperature of which could be controlled by a high temperature setup (Tianjin Orient-KOJI). The unit is removable and it can be inserted into the FLS920 spectrometer for the high temperature PL. The PL scans were performed at a step of 30 K between 300 and 480 K. A 450 W Xenon lamp was used as the excitation light source. Excitation curves were corrected over the lamp intensity with a silicon photodiode, and all emission spectra were corrected over the PMT spectral response. The WLED parameters were measured using a UV-VIS-NIR spectrophotometer (PMS-50 PLUS, Everfine, Hangzhou, China).

3. Results and discussion

3.1 Crystal structure and phase purity

In the last few years gallate phosphors have been the subject of several studies because of their chemical stability and remarkable optical properties.^31–35 Recently, Derakhshan et al. reported a new olivine type gallate, CaYGaO₄, which crystallizes in a different space group compared to all the previously reported gallates.³⁶ The work by Derakhshan et al. showed that this Eu³⁺/Tb³⁺-doped systems show red green luminescence, which would be ideal for solid-state white lighting purposes. Since then, this gallate has been selected as the host, and the Y³⁺ lattice sites in its structure can be effectively replaced by doped ions.³⁷Fig. 1 shows the unit cell structure of CaYGaO₄ and the coordination environments of Ca²⁺, Y³⁺ and Ga³⁺. According to a previous report, CaYGaO₄ crystallizes in orthorhombic crystal system with the Pnma space group and the cell parameters are a = 11.3484(1) Å, b = 6.5712(6) Å, c = 5.2818(8) Å and V = 393.86(8) ų. It is clear that there are three independent cationic positions. The two octahedral coordination environments are occupied by Ca²⁺ and Y³⁺, while Ga³⁺ resides in tetrahedral voids. Calcium ions occupying the 4a Wyckoff position form the CaO₆ octahedral units that share their edges along the b crystallographic axis forming one-dimensional chains. Yttrium ions occupying the 4c Wyckoff position form YO₆ octahedral units that share their corners and create puckered layers perpendicular to the a crystallographic axis.³⁶ The unique cationic ordering in this system is different from both previously studied classes of silicate and germanate olivines.

Fig. 1 (a) The unit cell structure of CaYGaO₄, Ca²⁺, Y³⁺, Ga³⁺ and O²⁻ ions are represented by yellow, green, blue, and red spheres, respectively. (b) The coordination environments of the Ca²⁺, Y³⁺ and Ga³⁺.

The phase purity of the CaYGaO₄:Bi³⁺ phosphor was characterized by XRD at room temperature. XRD patterns of all samples are listed in Fig. 2(a) together with simulated XRD pattern of CaYGaO₄ (based on the parameters of the crystal structure).³⁶ The obtained XRD patterns were demonstrated to agree well with the calculated XRD of CaYGaO₄, suggesting that no impurity phase is caused by small amounts of the Bi³⁺ dopant. On the basis of the effective ionic radii and charge balance of cations with different coordination numbers, we proposed that both Y³⁺ and Ca²⁺ sites in this host may be replaced by Bi³⁺.³⁷ According to the Bragg equation 2d sin θ = nλ (where d is the interplanar spacing, θ is the diffraction angle, n is the integer, and λ is the wavelength of X-ray) the interplanar spacing of CaYGaO₄ increases with the introduction of bigger ions. The ionic radius of Bi³⁺ (1.03 Å for coordinate number (CN) of 6) is bigger than that of Ca²⁺ (1.00 Å for CN of 6) and Y³⁺ (0.90 Å for (CN) of 6).^38–40 However, the XRD peaks shift towards larger angles when Bi³⁺ is doped into the CaYGaO₄ host, and the shift extent is proportional to the dopant quantity, as depicted in Fig. 2(b). Table 1 shows the crystal lattice parameters and volumes of CaY_1−xGaO₄:xBi³⁺ (x = 0–0.020) samples from indexing the experimental powder X-ray diffraction patterns. These results indicate that the doping of Bi³⁺ causes the lattice to shrink slightly.

Fig. 2 (a) XRD patterns of CaY_1−xGaO₄:xBi³⁺ (x = 0, 0.002, 0.005, 0.007, 0.010, 0.015, 0.020) and the simulated XRD pattern of CaYGaO₄. (b) Magnified 2θ region of 31–34° in the XRD patterns of the CaY_1−xGaO₄:xBi³⁺ samples.

Table 1 Crystal lattice parameters and volumes of CaY_1−xGaO₄:xBi³⁺ (x = 0, 0.002, 0.010, 0.015, and 0.020) samples

Sample	a (Å)	b (Å)	c (Å)	V (Å^3)
x = 0.000	11.3546	6.5768	5.2802	394.31
x = 0.002	11.3575	6.5714	5.2825	394.26
x = 0.010	11.3576	6.5727	5.2797	394.13
x = 0.015	11.3459	6.5736	5.2851	394.18
x = 0.020	11.3529	6.5727	5.2799	393.98

---

3.2 Photoluminescence properties of CaYGaO₄:Bi³⁺ phosphor at room temperature

Fig. 3(a) presents the PL spectra (λ_ex = 330 nm) and PLE spectra (λ_em = 430 nm) of CaY_1−xGaO₄:xBi³⁺ (x = 0.000–0.020), the inset shows integrated emission intensities with various Bi³⁺ concentrations. It can be noticed that the excitation and emission spectra of blank CaYGaO₄ (black line) are peaked at 330 nm and 430 nm, respectively.

Fig. 3 (a) The PL/PLE spectra of CaY_1−xGaO₄:xBi³⁺ (x = 0, 0.002, 0.005, 0.007, 0.010, 0.015, 0.020) at room temperature (λ_ex = 330 nm, λ_em = 430 nm); inset shows the integrated emission intensity as a function of Bi³⁺ concentration; and (b) the PL spectra of CaY_1−xGaO₄:xBi³⁺ (x = 0.002, 0.005, 0.007, 0.010, 0.015, 0.020) at room temperature (λ_ex = 370 nm), the inset shows the integrated emission intensity as a function of Bi³⁺ concentration (x) in CaY_1−xGaO₄:xBi³⁺ samples.

Compared with the excitation spectrum of the blank sample, an obvious excitation peak appears at 370 nm, which is attributed to ¹S₀ → ³P₁ electronic transitions of Bi³⁺. The emission intensity increases to the maximum at x = 0.005, and then it begins to decrease rapidly with further increase of x, due to the concentration quenching effect.^41–44 In addition, with the increases of the Bi³⁺ content, the emission peaks redshift from 430 nm to 440 nm (λ_ex = 330 nm), which means that there may be energy transfer between the host to Bi³⁺. On the other hand, the emission wavelengths of Bi³⁺ are slightly longer than the host emission, and overlap it. Fig. 3(b) shows the PL spectra (λ_ex = 370 nm) of CaY_1−xGaO₄:xBi³⁺ as a function of Bi³⁺ concentration x from 0.002 to 0.020. It is obvious that the emission intensity increases to the maximum at x = 0.005 and the peaks show a slight redshift. The reason for the observed redshift is possibly because Bi³⁺ replaces the smaller Y³⁺ lattice at a higher bismuth doping concentration. The small unit cell with a higher crystal field strength leads to emission redshift.²⁷

Phosphorescence decay data for CaY_1−xGaO₄:xBi³⁺ (x = 0.002–0.020) are presented in Fig. 4(a). The excitation wavelength and the monitored emission wavelength are 370 and 435 nm, corresponding to the absorption and emission peak of Bi³⁺, respectively.

Fig. 4 (a) Decay curves (λ_ex = 370 nm, λ_em = 435 nm) of CaY_1−xGaO₄:xBi³⁺ (x = 0.002–0.020) samples at room temperature; (b) measured and fitted decay curves of CaY_0.995GaO₄:0.005Bi³⁺, the excitation and monitored wavelengths are shown in the panel; (c) Decay lifetimes of CaY_1−xGaO₄:xBi³⁺ (x = 0.002–0.020) samples as a function of Bi³⁺ concentration.

All decay curves could be fitted by the double exponential decay equation:⁴⁵

The value of the average lifetime τ* could be obtained using the expression:^46,47where t is time, τ₁ and τ₂ are the short- and long-decay components, respectively, and A₁ and A₂ are constants. As depicted in Fig. 4(c) and Table 2, the lifetimes of CaY_1−xGaO₄:xBi³⁺ as the Bi³⁺ concentration increases reduce gradually from 392 to 279 ns, which is caused by the increase of nonradiative relaxation. It has been reported that the energy of an excited ion is likely to be consumed by another neighboring ions or the traps formed by the defects if the distances between these are close enough via a nonradiative way, resulting in a lifetime reduction.⁴⁸ On the other hand, these lifetimes are similar to the fluorescence lifetimes of Bi³⁺ (hundreds nanosecond).⁴⁹

Table 2 Decay lifetimes (λ_ex = 370 nm, λ_em = 435 nm) of CaY_1−xGaO₄:xBi³⁺ (x = 0.002–0.020) samples

Sample	τ 1 (ns)	A 1	τ 2 (ns)	A 2	γ ^2	τ ave (ns)
x = 0.002	136	335.81	407	1940.48	0.9988	392
x = 0.005	75	673.46	382	1836.30	0.9988	362
x = 0.007	79	779.99	388	1699.27	0.9985	361
x = 0.010	71	968.64	373	1604.03	0.9986	341
x = 0.015	58	1338.18	357	1529.14	0.9981	320
x = 0.020	34	2612.48	326	1434.90	0.9976	279

---

The PL and PLE spectra at various wavelengths for CaY_0.995GaO₄:0.005Bi³⁺ at room temperature were recorded. As shown in Fig. 5, these excitation bands are unchanged obviously. However the emission bands redshift when the excitation wavelength increases from 300 to 370 nm, which is due to the emission overlap that makes the emission change from the host to bismuth luminescence centre.⁵⁰

Fig. 5 PL and PLE spectra of CaY_0.995GaO₄:0.005Bi³⁺ at various excitation and emission wavelengths measured at room temperature.

3.3 The emission improvement of the CaYGaO₄:Bi³⁺ phosphor by Li⁺ ion

Codoping with Li⁺ is a prevalent strategy to improve the optical efficiency of luminescent materials, and this strategy has been successfully used in several systems such as: SrAl₂O₄:Ce³⁺, CeO₂:Eu³⁺ and Lu₂MoO₆:Eu³⁺ phosphors.^51–53 Therefore, we used Li₂CO₃ as flux materials to try to improve the emission intensity of CaY_0.995GaO₄:0.005Bi³⁺. Fig. 6(a) presents the emission spectra of CaY_0.995GaO₄:0.005Bi³⁺ and CaY_0.995GaO₄:0.005Bi³⁺,x%(wt)Li₂CO₃ (x = 0.2–1.5). It is observed that the emission intensity of CaY_0.995GaO₄:0.005Bi³⁺ increases ∼1.47 times upon adding increasing concentrations of Li₂CO₃ to 0.3%(wt). In addition, as illustrated in Fig. 6(b), the excitation peak at 370 nm is enhanced, indicating that the addition of Li₂CO₃ could make Bi³⁺ more easy to be incorporated into the CaYGaO₄ lattice. It can be seen from Fig. 6(c) that no impurity phase is observed upon the addition of small amounts of Li₂CO₃. The major role of Li⁺ ions is to serve as flux materials, resulting in higher crystallinity and larger crystallites for improved luminescence. With the crystallite enlargement, more and more activator ions locate in the interior of the crystals, rather than near the surface where energy can rapidly transfer to surface defects and then be consumed by high vibrational energies.^54–56 Another fundamental reason is that when the Bi³⁺ substitutes the Ca²⁺ in the host material, Li⁺ ions can provide charge compensation to reduce the number of cation vacancies and eventually promote more Bi³⁺ ions to go into the host lattice.^57,58

Fig. 6 (a) PL spectra and (b) PLE spectra of CaY_0.995GO₄:0.005Bi³⁺ and CaY_0.995GaO₄:0.005Bi³⁺,x%(wt)Li₂CO₃ (x = 0.2–1.5) phosphors (λ_ex = 370 nm λ_em = 435 nm) at room temperature; (c) XRD patterns of CaY_0.995GO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ and the simulated XRD pattern of CaYGaO₄; (d) excitation line of the BaSO₄ reference and the emission spectrum of CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ recorded using an integrating sphere. Inset shows a magnification of the emission spectrum from 395 nm to 500 nm; (e) emission spectra of sample CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ at different temperatures of 300–480 K (λ_ex = 370 nm); (f) activation plots for thermal quenching of CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ phosphor using the Arrhenius equation.

3.4 Luminescence quantum efficiency and thermal quenching properties of the CaYGaO₄:Bi³⁺,Li⁺ phosphor

The quantum efficiency (QE) and thermal stability of phosphors are two key critical factors for evaluating their applicability for WLEDs. The QE of CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ is measured using an integrating sphere coupled spectrofluorometer and calculated by the following equation:^59,60where P_c is the photon flux of the sample in the emission wavelength region detected by the equipment. L_a and L_c are the photon flux without and with the sample in the sphere detected in the wavelength region of the incident irradiation, respectively. Under 370 nm excitation, the internal QE value of the explored CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ phosphor reaches 45.7%, as illustrated in Fig. 6(d), which is higher than the QE (18.5%) of the CaY_0.995GaO₄:0.005Bi³⁺ sample. This result can be beneficial for improving its performance in WLEDs application.

For WLED applications, the increasing working time leads to more heat generation at the p–n junction, resulting in the high temperature of the phosphor up to 423 K, which has considerable influence on the light output and CRI.⁶¹ Thus, the excellent thermal stability of the luminescence intensity is an important parameter for phosphor application in WLEDs. Fig. 6(e) illustrates emission spectra of CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ excited by 370 nm in temperature ranging from 300–480 K. It is obvious that the increased temperature from 300–480 K leads to the decrease of the emission intensity while the emission band remains unchanged. As temperature increases from 300–423 K, the PL intensity decreases from 100% to ∼76.3%. Generally speaking, with increasing temperature, electron–phonon interaction is intensified. The excited electron could obtain the phonon energy to overcome the energy barrier, which leads to thermal quenching.⁶² On the other hand, the emission band broadening is obvious with increasing temperature, resulting from the excited electrons spreading to higher vibration levels and the radiative transitions from these different levels.⁶¹ To better understand the thermal quenching, the activation energy (ΔE) has been calculated by the Arrhenius formula:^63,64

where I_T and I₀ are the integrated emission intensity at the testing temperature and room temperature, respectively, k represents the Boltzmann constant (8.629 × 10⁻⁵ Ev K⁻¹), and c denotes a constant for a certain host. Fig. 6(f) depicts a plot of ln[(I₀/I_T) − 1] versus 1/kT. Through the best fit using the Arrhenius equation, the activation energy (E) was obtained as 0.2855 eV for CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃.

3.5 Application of a blue-emitting CaYGaO₄:Bi³⁺,Li⁺ phosphor in LED devices

As a proof of concept, we fabricated a series of LED devices by combining the present and commercial phosphors with NUV LED chips (with emission peaked at 370 nm, 20 mA, 3 V), as shown in Fig. 7. The emission of Bi³⁺ ions could be excited by 370 nm UV light according to previous discussions. The EL spectrum of the CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ phosphor with CIE color coordinates (0.1536, 0.0885) is illustrated in Fig. 7(a). To study the impact of driving current on the EL of the CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ coated LED, we measured the relative emission intensity as a function of direct current, which changed from 10 to 200 mA, as illustrated in Fig. 7(b). It can be seen that the emission intensity initially increases with increasing current, reaching a maximum at 180 mA, which is dominated by the increase of excitation light intensity (Fig. 7(b) red curve). When the current increases further, the emission intensity decreases, that is, thermal quenching dominates (the temperature of LED chip increases with increasing drive current).⁶¹

Fig. 7 (a) EL spectrum of a LED lamp based on 370 nm GaN LED chip, CIE chromaticity coordinates and an image of the lamp are inserted beside the EL curve; (b) the dependence of relative emission intensity of CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ coated LED device (blue curve) and 370 nm chip (red curve) on the direct current in the range of 10–200 mA; (c) EL spectrum of fabricated NUV-pumped WLED using the blue-emitting CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ phosphor and a commercial orange phosphor ((Sr,Ba)₃SiO₅:Eu²⁺); (d) EL spectrum of white LED lamp combining a 370 nm NUV chip, CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ and commercial (Ba,Sr)₂SiO₄:Eu²⁺ (green), CaAlSiN₃:Eu²⁺ (red) phosphors. (All EL spectra were measured under 20 mA and 3 V).

The WLED coupled only with the commercial orange phosphor ((Sr,Ba)₃SiO₅:Eu²⁺) was fabricated (the mass ratio of CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ : (Sr,Ba)₃SiO₅:Eu²⁺ = 4 : 1, that is, 0.1 g CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ and 0.025 g (Sr,Ba)₃SiO₅:Eu²⁺). The EL spectrum and the photograph of the WLED are obtained and demonstrated in Fig. 7(c). The WLED manifests white light with a CCT of 4304 K and CIE color coordinates of (x, y) = (0.3610, 0.3360), CRI = 86.9. Besides, a white LED lamp combining commercial (Ba,Sr)₂SiO₄:Eu²⁺ (green) and CaAlSiN₃:Eu²⁺ (red) phosphors was fabricated (the mass ratio of CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ : (Ba,Sr)₂SiO₄:Eu²⁺ : CaAlSiN₃:Eu²⁺ = 3 : 1 : 2, that is, 0.075 g CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃, 0.025 g (Ba,Sr)₂SiO₄:Eu²⁺ and 0.05 g CaAlSiN₃:Eu²⁺). As shown in Fig. 7(d), white light has a CCT of 4170 K and CIE color coordinates of (x, y) = (0.3692, 0.3552), CRI = 92.5, which is higher than the NUV-pumped WLED coupled with blue-emitting phosphor Gd₂ZnTiO₆:Bi³⁺ (CRI = 89.4).⁶⁵

Small colour fluctuations under varied excitation powers is crucial for the long-term service of WLEDs. To study the chromatic stability of WLEDs, we measured the variations of CIE color coordinates, CCT, CRI, luminous flux (LF) and luminous efficiency (LE) under direct driving currents in a range of 20–120 mA, and are illustrated in Fig. 8(a), (b) and Tables 3, 4 respectively. We found that the CIE color coordinates of WLEDs change slightly under different drive currents, indicative of very small colour fluctuation. The slight variation of the chromaticity coordinates are observed, merely 0.0058 for CIE-x, 0.0075 for CIE-y in Table 3 and 0.0034 for CIE-x, 0.0046 for CIE-y in Table 4. These results demonstrate that the blue-emitting phosphor CaY_0.995GaO₄:0.005Bi³⁺,0.3%(wt)Li₂CO₃ matches well with the NUV LED chips and commercial phosphors, implying its potential application in practical WLED devices.

Fig. 8 (a) and (b) CIE chromaticity coordinate diagrams of Fig. 7(c) and (d) under direct current in the range of 20–120 mA.

Table 3 CIE chromaticity coordinates (x, y), CCT, CRI, luminous flux (LF) and luminous efficiency (LE) of WLED (Fig. 7(c)) under direct current in the range of 20–120 mA

Current (mA)	Color coordinates	CTT (K)	CRI	LF (lm)	LE (lm W^−1)
20	(0.3610, 0.3360)	4304	86.9	0.95	14.32
40	(0.3580, 0.3350)	4412	86.8	2.06	14.71
60	(0.3572, 0.3323)	4420	86.6	3.07	14.02
80	(0.3596, 0.3310)	4317	85.8	3.93	12.93
100	(0.3608, 0.3293)	4255	85.2	4.74	11.28
120	(0.3630, 0.3285)	4161	84.3	5.34	10.29

---

Table 4 CIE chromaticity coordinates (x, y), CCT, CRI, luminous flux (LF) and luminous efficiency (LE) of WLED (Fig. 7(d)) under direct current in the range of 20–120 mA

Current (mA)	Color coordinates	CTT (K)	CRI	LF (lm)	LE (lm W^−1)
20	(0.3692, 0.3552)	4170	92.5	0.98	15.25
40	(0.3682, 0.3546)	4197	92.6	2.07	15.78
60	(0.3685, 0.3551)	4193	93.0	3.10	15.53
80	(0.3692, 0.3562)	4176	93.3	4.02	15.00
100	(0.3707, 0.3580)	4143	93.6	4.77	14.16
120	(0.3716, 0.3592)	4122	93.8	5.53	13.57

---

4. Conclusions

A novel blue-emitting CaYGaO₄:Bi³⁺ phosphor was synthesized by the conventional high temperature solid state reaction route, and its luminescence properties were studied. The results show that the obtained CaYGaO₄:Bi³⁺ phosphors have a broad emission band ranging from 375 nm to 575 nm, which peaked at 435 nm. The CIE coordinates of CaYGaO₄:Bi³⁺ is (0.1536, 0.0885) with color purity of 85.2%. The PL spectra of CaYGaO₄:Bi³⁺ phosphors exhibit an intense blue emission under NUV excitation range from 300 to 400 nm. Moreover, the emission intensity of the CaYGaO₄:Bi³⁺ phosphor increased ∼1.47 times by adding Li₂CO₃ as a fluxing agent. The quantum efficiency of the prepared CaYGaO₄:Bi³⁺,Li⁺ phosphor reaches 45.7% under 370 nm excitation, and the emission intensity remains 76.3% of that at room temperature at 423 K and ΔE was estimated to be 0.2855 eV. By combining our discovered phosphor with the commercial orange phosphor ((Sr,Ba)₃SiO₅:Eu²⁺), we have fabricated a NUV-based WLED with a CCT of 4304 K and CIE color coordinates of (x, y) = (0.3610, 0.3360), CRI = 86.9. Besides, a NUV-based WLED employing blue (CaYGaO₄:Bi³⁺,Li⁺)/green ((Ba,Sr)₂SiO₄:Eu²⁺)/red (CaAlSiN₃:Eu²⁺) phosphors was achieved, which has a CCT of 4170 K and CIE color coordinates of (x, y) = (0.3692, 0.3552), CRI = 92.5. These WLEDs also show excellent chromaticity coordinate stability with the drive current varying from 20 to 120 mA. All results indicate that CaYGaO₄:Bi³⁺ would be a promising blue phosphor for NUV-based WLEDs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge financial support from the Program for Jiangmen Innovative Research Team (No. (2017)385), the National Natural Science Foundation of China (Grant No. 51672085), the Program for Innovative Research Team in University of Ministry of Education of China (Grant No. IRT_17R38), The Joint Fund of Ministry of Education of China, Major Basic Research Cultivation Project of Natural Science Foundation of Guangdong Province (Grant No. 2018B03038009), and the Local Innovative Research Team Project of ‘‘Pearl River Talent Plan’’ (Grant No. 2017BT01X137).

Notes and references

1. P. Dai, X. Zhang, L. Bian, S. Lu, Y. Liu and X. Wang, J. Mater. Chem. C, 2013, 1, 4570–4576.
2. P. P. Han, A. Sewaiwar, S. V. Tiwari and Y.-H. Chung, J. Opt. Soc. Korea, 2015, 19, 74–79.
3. P. Xiong and M. Peng, J. Mater. Chem. C, 2019, 7, 6301–6307.
4. W. Lu, X. Zhang, Y. Wang, Z. Hao, Y. Liu, Y. Luo, X. Wang and J. Zhang, J. Alloys Compd., 2012, 513, 430–435.
5. M. Peng, X. Yin, P. A. Tanner, M. Brik and P. Li, Chem. Mater., 2015, 27, 2938–2945.
6. X. Yang, H. Song, L. Yang and X. Xu, J. Am. Ceram. Soc., 2011, 94, 164–171.
7. P. Xiong and M. Peng, Opt. Mater.: X, 2019, 2, 100022.
8. L. Wang, R.-J. Xie, T. Suehiro, T. Takeda and N. Hirosaki, Chem. Rev., 2018, 118, 1951–2009.
9. A. A. Setlur, Electrochem. Soc. Interface, 2009, 18, 32–36.
10. H. Zhu, C. C. Lin, W. Luo, S. Shu, Z. Liu, Y. Liu, J. Kong, E. Ma, Y. Cao and R. S. Liu, Nat. Commun., 2014, 5, 4312.
11. H. Ju, W. Deng, B. Wang, J. Liu, X. Tao and S. Xu, J. Alloys Compd., 2012, 516, 153–156.
12. Y. Liu, J. Silver, R.-J. Xie, J. Zhang, H. Xu, H. Shao, J. Jiang and H. Jiang, J. Mater. Chem. C, 2017, 5, 12365–12377.
13. B. Lei, B. Li, H. Zhang and W. Li, Opt. Mater., 2007, 29, 1491–1494.
14. G. Li, Z. Wang, Z. Quan, C. Li and J. Lin, Cryst. Growth Des., 2007, 7, 1797–1802.
15. T.-C. Liu, B.-M. Cheng, S.-F. Hu and R.-S. Liu, Chem. Mater., 2011, 23, 3698–3705.
16. R. K. Tamrakar, D. P. Bisen, I. P. Sahu and N. Brahme, J. Radiat. Res. Appl. Sci., 2014, 7, 417–429.
17. M. Yu, J. Lin, Z. Wang, J. Fu, S. Wang, A. Zhang and Y. Han, Chem. Mater., 2002, 14, 2224–2231.
18. P. Xiong, Y. Li and M. Peng, iScience, 2020, 23(10), 101578.
19. H. Li, R. Pang, Y. Luo, H. Wu, S. Zhang, L. Jiang, D. Li, C. Li and H. Zhang, ACS Appl. Electron. Mater., 2019, 1, 229–237.
20. Z. Zhou, P. Xiong, H. Liu and M. Peng, Inorg. Chem., 2020, 59, 12920–12927.
21. Y. Fu, X. Wang and M. Peng, J. Mater. Chem. C, 2020, 8, 6079–6085.
22. P. Xiong, C. Zheng, M. Peng, Z. Zhou, F. Xu, K. Qin, Y. Hong and Z. Ma, J. Am. Ceram. Soc., 2020, 124, 063101.
23. J. Xue, H. M. Noh, B. C. Choi, S. H. Park, J. H. Kim, J. H. Jeong and P. Du, Chem. Eng. J., 2020, 382, 122861.
24. C. Zheng, P. Xiong, M. Peng and H. Liu, J. Mater. Chem. C, 2020, 8(39), 13668–13675.
25. F. Kang, H. Zhang, L. Wondraczek, X. Yang, Y. Zhang, D. Y. Lei and M. Peng, Chem. Mater., 2016, 28, 2692–2703.
26. W. Sun, R. Pang, H. Li, D. Li, L. Jiang, S. Zhang, J. Fu and C. Li, J. Mater. Chem. C, 2017, 5, 1346–1355.
27. R. Cao, J. Zhang, W. Wang, Q. Hu, W. Li, W. Ruan and H. Ao, Luminescence, 2017, 32, 908–912.
28. H. Ju, W. Deng, B. Wang, J. Liu, X. Tao and S. Xu, J. Alloys Compd., 2012, 516, 153–156.
29. R. Cao, G. Quan, Z. Shi, Q. Gou, T. Chen, Z. Hu and Z. Luo, J. Mater. Sci.: Mater. Electron., 2018, 29, 5287–5292.
30. S. Wang, W. Chen, D. Zhou, J. Qiu, X. Xu and X. Yu, J. Am. Ceram. Soc., 2017, 100, 3514–3521.
31. X. Fu, S. Zheng, J. Shi, Y. Li and H. Zhang, J. Lumin., 2017, 184, 199–204.
32. Y. Li, H. Yi, J. Xu and X. Kuang, J. Alloys Compd., 2018, 740, 143–147.
33. X. Wang, P. Boutinaud, L. Li, J. Cao, P. Xiong, X. Li, H. Luo and M. Peng, J. Mater. Chem. C, 2018, 6, 10367–10375.
34. S. Wang, W. Chen, D. Zhou, J. Qiu, X. Xu and X. Yu, J. Am. Ceram. Soc., 2017, 100, 3514–3521.
35. P. Xiong, M. Peng, K. Qin, F. Xu and X. Xu, Adv. Opt. Mater., 2019, 7, 1901107.
36. R. Clark, S. J. Zhu, S.-T. Zheng, X. Bu and S. Derakhshan, J. Alloys Compd., 2014, 616, 340–344.
37. S. M. Araiza, K. Slowinska and S. Derakhshan, J. Lumin., 2019, 216, 116747.
38. A. Dobrowolska and E. Zych, J. Lumin., 2017, 192, 397–403.
39. S. Gomes, A. Kaur, J.-M. Grenèche, J.-M. Nedelec and G. Renaudin, Acta Biomater., 2017, 50, 78–88.
40. Z. Qiu, T. Luo, J. Zhang, W. Zhou, L. Yu and S. Lian, J. Lumin., 2015, 158, 130–135.
41. P. Xiong and M. Peng, J. Mater. Chem. C, 2019, 7, 8303–8309.
42. J. Zhong, Y. Peng, D. Chen, M. Liu, X. Li, Y. Zhu and Z. Ji, J. Mater. Chem. C, 2018, 6, 13305–13315.
43. H. Liu, X. Liu, X. Wang, M. Peng, P. Xiong, Y. Chen, Y. Wang, B. Lei and Q. Zeng, J. Am. Ceram. Soc., 2020, 103, 5758–5768.
44. P. Xiong, M. Peng, J. Cao and X. Li, J. Am. Ceram. Soc., 2019, 102, 5899–5909.
45. J. Fu, S. Zhang, T. Ma, Y. Jia, R. Pang, L. Jiang, D. Li, H. Li, W. Sun and C. Li, RSC Adv., 2015, 5, 93951–93956.
46. X. Ding, G. Zhu, W. Geng, Q. Wang and Y. Wang, Inorg. Chem., 2015, 55, 154–162.
47. J. Cao, J. Peng, L. Wang, H. Luo, X. Wang, P. Xiong, Y. Wang and M. Peng, J. Mater. Chem. C, 2019, 7, 2076–2084.
48. X. Zhang, J. Wang, L. Huang, F. Pan, Y. Chen, B. Lei, M. Peng and M. Wu, ACS Appl. Mater. Interfaces, 2015, 7, 10044–10054.
49. F. Kang, Y. Zhang and M. Peng, Inorg. Chem., 2015, 54, 1462–1473.
50. P. Dang, S. Liang, G. Li, Y. Wei, Z. Cheng, H. Lian, M. Shang, A. A. Al Kheraif and J. Lin, Inorg. Chem., 2018, 57, 9251–9259.
51. L. A. Diaz-Torres, J. Oliva, D. Chavez and C. R. Garcia, Ceram. Int., 2016, 42, 16235–16241.
52. W. Huang, Y. Tan, D. Li, H. Du, X. Hu, G. Li, Y. Kuang, M. Li and D. Guo, J. Lumin., 2019, 206, 432–439.
53. L. Li, J. Shen, Y. Pan, X. Zhou, H. Huang, W. Chang, Q. He and X. Wei, Mater. Res. Bull., 2016, 78, 26–30.
54. E. L. Cates, M. Cho and J. Kim, Environ. Sci. Technol., 2011, 45, 3680–3686.
55. Z. Sun, Y. Zhou and M. Li, J. Mater. Res., 2008, 23, 732–736.
56. M. Peng, X. Yin, P. A. Tanner, C. Liang, P. Li, Q. Zhang and J. Qiu, J. Am. Ceram. Soc., 2013, 96, 2870–2876.
57. S. Ho, H. Jang, S. Yang, K. Wook and Y. Ji, Adv. Mater., 2008, 20(14), 2696–2702.
58. X. Wang, Z. Xue, Y. Shi and X. Jing, Dalton Trans., 2013, 42(14), 5167–5173.
59. X. Ding, G. Zhu, W. Geng, Q. Wang and Y. Wang, Inorg. Chem., 2016, 55, 154–162.
60. S. Liang, M. Shang, H. Lian, K. Li, Y. Zhang and J. Lin, J. Mater. Chem. C, 2017, 5, 2927–2935.
61. X. Wang, J. Wang, X. Li, H. Luo and M. Peng, J. Mater. Chem. C, 2019, 7, 11227–11233.
62. S.-P. Lee, T.-S. Chan and T.-M. Chen, ACS Appl. Mater. Interfaces, 2014, 7, 40–44.
63. J. Wang, M. Zhang, Q. Zhang, W. Ding and Q. Su, Appl. Phys. B: Lasers Opt., 2007, 87, 249–254.
64. H. Zeng, T. Zhou, L. Wang and R.-J. Xie, Chem. Mater., 2019, 31, 5245–5253.
65. C. Ji, Z. Huang, J. Wen, J. Zhang, X. Tian, H. He, L. Zhang, T.-H. Huang, W. Xie and Y. Peng, J. Alloys Compd., 2019, 788, 1127–1136.

---