Efficient red-emitting phosphor ScVO₄ doped with Bi³⁺ and Eu³⁺ for near-ultraviolet-activated solid-state lighting

Abstract

In this paper, Bi³⁺- and Eu³⁺-co-doped ScVO₄ had been investigated as an alternative red-emitting phosphor under near-ultraviolet excitation. Structural and spectroscopic characterizations were carried out. The results show that the samples can efficiently absorb near-ultraviolet light and produce very bright red emission. In particular, the optimized sample shows much stronger emission intensity than the commercial phosphor Y₂O₂S: 5% Eu³⁺ in the excitation range of 365–395 nm, and can reach a strength 5.4 times of the latter under 385 nm excitation. It also has better CIE chromaticity coordinates. These suggest ScVO₄: Bi³⁺, Eu³⁺ as a promising red-emitting phosphor for white light-emitting diodes based on near-ultraviolet light-emitting diode chips.

Introduction

Solid-state white light-emitting diodes (LEDs) developments have been blooming since the landmark achievement of bright-blue LEDs came out in the mid-1990s. A typical white LED, covering the broadband yellow-emitting phosphor YAG: Ce³⁺ on a blue LED chip, has been widely applied in many fields, owing to its superior properties such as high efficiency, energy-saving, friendly to the environment and long lifetime. Unfortunately, the major drawback of lacking red light component limits such white-lighting fixture to have a poor color rendering index (CRI, usually <75) and high correlated color temperature (CCT, 4000–8000 K) [1,2,3,4,5,6,7]. To generate a warm white light (CRI >80; CCT <4000 K), there is much room to improve. Many strategies have been proposed and that coating red–green–blue (RGB) tricolor phosphors on near-ultraviolet (NUV) LED chips (365–395 nm) is considered as one of the optimal [8].

In contrast to blue and green components, red-emitting phosphors still demand great attentions to enhance their performance [9]. Divalent Eu ion (Eu²⁺)-activated nitrides can give off very bright red light, which, however, is always quite far into deep red region where human eye is insensitive. Moreover, they could absorb visible light covering as far as the red spectral scope thus may suffer from color change and lower luminous efficiency (efficiency of light conversion subject to the human eye sensitivity) [6, 10]. Trivalent Eu ion (Eu³⁺) is considered as an ideal source of red light with excitation lines at NUV region. Traditional high-efficient red-emitting phosphors activated by Eu³⁺ ions such as Y₂O₂S: Eu³⁺ and YVO₄: Eu³⁺, however, show a dramatic decrease in excitation efficiency in the NUV region and a tiny, narrow absorption around 395 nm, not suitable for NUV-activated white light applications. Therefore, host materials that can efficiently absorb NUV light and then transfer to red-emitting center is imperative.

For orthovanadates, the polarization of the oxygen ions resulting from the introduction of foreign ions in the host lattice influences the position of charge-transfer bands. By replacing Y³⁺ with trivalent cation with a smaller radius, such as Sc³⁺ and Lu³⁺, the absorption band of LnVO₄ (Ln = Lu or Sc) doped with Eu³⁺ had a large red shift, which would be more significant upon introduction of Bi³⁺ ion [11, 12]. A sketch (Fig. 1) presents the energy absorption and transfer mechanism in LnVO₄: Bi³⁺, Eu³⁺. Besides the famous charge-transfer (CT) transition between O²⁻ and V⁵⁺ (O–V), the metal–metal CT transition between Bi³⁺ and V⁵⁺ is also taken into account in view of the 6s level of Bi³⁺ that lies above the highest filled 2p level of O²⁻ and the electron in the 6s level is apt to transfer to the empty 3d level of V⁵⁺. The absorption band is accordingly extended to violet range upon introduction of Bi³⁺. Moreover, the high energy levels of Eu³⁺ are embedded into the conduction band (V-3d states), so the energy transfer to Eu³⁺ could be highly efficient.

Figure 1

Sketch of probable energy absorption, transfer and relaxation processes in ScVO₄: Bi³⁺, Eu³⁺

In this work, through a conventional high-temperature solid-state reaction, we synthesize samples of ScVO₄: Bi³⁺, Eu³⁺ and optimize the doping proportion to achieve the best luminous efficiency. Under NUV light excitation, the obtained products show an excellent performance which will be discussed as follows.

Experimental

The samples of ScVO₄: Bi³⁺, Eu³⁺ were synthesized through a conventional high-temperature solid-state reaction in air. The started materials were Sc₂O₃ (99.99%), Bi₂O₃ (AR), V₂O₅ (99.99%), Eu₂O₃ (99.99%). First, the raw materials were mixed according to a stoichiometric ratio and ground in an agate mortar. Then, the mixture was transferred into a ceramic crucible. The presintering step was performed at 750 °C for 4 h in a muffle furnace. After that, the samples were ground again and sintered at 1000 °C for 5 h. Finally, the obtained samples were ground to be powders for the subsequent measurements.

The XRD data were recorded via X-ray diffractometer (MAC Science Co. Ltd MXP18AHF), using nickel-filtered Cu K_α radiation (λ = 1.5418 Å) in the range of 2θ = 10–70º. The excitation and emission spectra were recorded with a Hitachi 850 fluorescence spectrophotometer using a 150-W xenon lamp as the excitation source. The decay curves were measured with a Tektronix TDS2024 digital storage oscilloscope under the excitation of 355 nm pulse laser yielded in a Q-switched flash-pumped Nd: YAG laser with the pulse duration of 10 ns.

Results and discussion

XRD characterization

The XRD patterns of the samples are shown in Fig. 2, along with the standard data of ScVO₄ (PDF-6-260). All the strong diffraction peaks agree well with the standard data, which belong to the zircon-type structure of space group I4₁/amd. Bi³⁺ and Eu³⁺ ions are supposed to occupy the Sc³⁺ sites, and because of larger ionic radii of Bi³⁺ and Eu³⁺ than Sc³⁺, the peaks shift slightly to small angle with increasing the doping proportion of Bi³⁺ or Eu³⁺. It is noteworthy that two tiny impurity peaks at 29° and 25° emerge when the doping increases to a relatively heavy level, which are, respectively, ascribed to the main diffraction peak of BiVO₄ and EuVO₄. Such impurity will shrink the expected concentration of the concerned ion in the final product, suggesting heavy doping in this host is unsuitable.

Figure 2

X-ray powder diffraction patterns of Eu³⁺- and Bi³⁺-co-doped ScVO₄ samples. For the patterns in the upper part, the concentration of Eu³⁺ is fixed at 4%, while for those in the lower part, the concentration of Bi³⁺ is fixed at 2%. PDF-6-260 is the standard data for ScVO₄

Luminescent properties

Figure 3 shows the luminescent properties of ScVO₄ doped with Eu³⁺ and Bi³⁺ samples, composed of excitation spectra (a) monitored the emission at 616 nm (Eu³⁺, ⁵D₀ → ⁷F₂) and emission spectra under 395 nm excitation for samples with different doping concentrations of Bi³⁺ (b) and Eu³⁺ (c). In the excitation spectra, besides the weak narrow peaks from 450 to 550 nm corresponding to the Eu³⁺ 4f–4f transitions, there are intense broadbands covering from 200 to 430 nm which are attributed to the overlap of O–Eu, O–V and Bi–V CT transitions, as proposed earlier [11]. It is of significance that upon introduction of Bi³⁺ ions in the host a striking shoulder bursts from 350 to 430 nm attaching to the dominant O–V CT transition centered at 326 nm. This shoulder has been identified as the Bi–V metal–metal CT transition, ascribed to electron transfer from Bi-6s state to V-3d state [11]. It should be noted that the charge-transfer transition is allowed by the spin and parity selection rules, which is responsible for the efficient absorption of NUV light, and here overwhelmingly outshines the parity-forbidden intra-4f transition of Eu³⁺. It should also be worthy of mention that for many Eu³⁺-activated red-emitting materials, the near-ultraviolet absorption around 395 nm is usually attributed to the transition of ⁷F₀ → ⁵L₆, which is always weak unless the doping is heavy. Moreover, shielded by the filled 5s ² and 5p ⁶ shell, the intra-4f transition is generally narrow and sharp, leading the excitation efficiency to be vulnerable to the tiny emission wavelength shift of the NUV LED chip at various temperatures. Fortunately, owing to the sensitization of Bi³⁺, the current material ScVO₄: Bi³⁺, Eu³⁺ has shown a broad excitation band covering NUV, indicating it a promising candidate for excellent NUV-excited red-emitting phosphor.

Figure 3

a Excitation spectra of ScVO₄: x% Bi³⁺, 4% Eu³⁺ (x = 0, 1, 2, 4, 6, 8, 10) by monitoring 616 nm from ⁵D₀ → ⁷F₂ transition of Eu³⁺. b Corresponding emission spectra excited at 395 nm. c Emission spectra of ScVO₄: 2% Bi³⁺, x% Eu³⁺ (x = 1, 2, 4, 8, 16) under 395 nm excitation

On the other hand, the presence of energy transfer from CT band to Eu³⁺ ions can be verified by the observation of the charge-transfer band in the excitation spectra of Eu³⁺ ions. When the doping concentration of Bi³⁺ increases, the O–V CT band decreases monotonously, while the Bi–V CT band rises to the maximum until the doping concentration of Bi³⁺ reaches 2%. As elucidated in Fig. 1, two relaxation paths are considered for the excitation energy in this material: One is quenching in Bi³⁺ ions, and the other is being emitted by Eu³⁺ ions. The decline of O–V CT band in the Eu³⁺ excitation spectra is ascribed to the increase in the energy transfer from O–V CT band to Bi³⁺, and part of the excitation energy would subsequently be quenched in Bi³⁺ ions. The excitation energy transferred to Eu³⁺ ions is thus reduced, resulting in a decline of O–V CT band in the excitation spectra.

The rising of Bi–V band is owing to the increase in the absorption of Bi–V CT band and the energy transfer possibility to Eu³⁺ ions, resulting in a stronger Eu³⁺ emission, i.e., a higher Bi–V CT band. However, when the doping concentration of Bi³⁺ exceeded 2%, energy transfer from Bi–V CT band to Eu³⁺ ions would be reduced due to the fast increase in nonradiative relaxation in Bi³⁺ ions, leading to the suppression of Bi–V CT band in the excitation spectra. So, it is conclusive that the excitation energy prefers to quench in Bi³⁺ ions rather than transfer to Eu³⁺ in high doping concentration of Bi³⁺. In other words, the doping of Bi³⁺ should not be high in this host material for good luminescence. A similar phenomenon was also reported in Ref. 13 for Bi³⁺ singly doped ScVO₄ samples. Only in the case of very low Bi³⁺ doping concentration (usually lower than 2 or 1%), exciting into O–V CT band could give off the Bi³⁺ broadband ³P₁ → ¹S₀ emission centered at 635 nm. This emission drops quickly with increasing the doping concentration of Bi³⁺ because the nonradiative relaxation would dominate [13].

Under 395 nm light excitation, the samples doped with Eu³⁺ emit very bright red light, corresponding to ⁵D₀ → ⁷F₂ transition of Eu³⁺. As seen in Fig. 3b, the Eu³⁺ electric dipole transition ⁵D₀ → ⁷F₂ is much stronger than magnetic dipole transition ⁵D₀ → ⁷F₁. It is because the Sc³⁺ site in ScVO₄ has very large deviation from centrosymmetry. Such site surrounding is desired for Eu³⁺-activated red-emitting phosphor to gain a suitable color rendering index. The samples doped with 2 and 4% Bi³⁺ yield apparently the stronger red emission, submerged by a very week broadband. In view of the V–O CT band emission has a quenching temperature far below room temperature and always locates around 465 nm, the week bands mentioned above are accordingly considered to originate from Bi^{3+ 3}P₁ → ¹S₀ transition centered at 635 nm [13], and here is too weak to significantly alter the energy distribution of emission. So, it can be concluded that the excitation energy could be efficiently transferred to Eu³⁺ when the doping concentration of Bi³⁺ is below 2%. For those with higher doping concentration, the quenching in Bi³⁺ ions grows rapidly to dominate the majority of the relaxation, resulting in much weaker Eu³⁺ emission. This is in accordance with the situation in excitation spectra.

The dependence of integrated emission intensity on the doping concentrations of Eu³⁺ or Bi³⁺ is plotted in Fig. 4a, b, respectively. The ideal doping level of Eu³⁺ is 4%. As shown in Fig. 4c, the decay lifetimes of ⁵D₀ → ⁷F₂ emission of Eu³⁺ at 616 nm for 2% Bi³⁺-doped ScVO₄ samples are almost invariant for different Eu³⁺ concentrations. The above results indicate that there is no back energy transferring from Eu³⁺ to Bi³⁺. Therefore, the Eu³⁺ luminescence quenching in high doping concentration may be due to the formation of slight EuVO₄, as shown in the XRD patterns.

Figure 4

Dependence of integrated emission intensity on concentrations of Eu³⁺ (Bi³⁺ concentration being 2%) (a) and Bi³⁺ (Eu³⁺ concentration being 4%) (b) under the excitation of 395 nm. Decay curves of Eu^{3+ 5}D₀ → ⁷F₂ transition at 616 nm of ScVO₄ doped with 2% Bi³⁺ and a series of Eu³⁺ concentrations (c)

A comparison on luminescence performance between the optimized ScVO₄: Bi³⁺, Eu³⁺ and the commercial red-emitting phosphor Y₂O₂S: 5% Eu³⁺ is shown in Fig. 5. Distinct characteristics can be seen in both excitation and emission spectra. In the scope from 360 to 410 nm in the excitation spectra, obviously the optimized sample ScVO₄: 2% Bi³⁺, 4% Eu³⁺ with the contribution of Bi–V CT band shows more efficient excitation than Y₂O₂S: 5% Eu³⁺. Accordingly, with NUV light excitation, ScVO₄: 2% Bi³⁺, 4% Eu³⁺ shows more bright red emission than Y₂O₂S: 5%Eu³⁺ does. Concretely speaking, the former yields red emissions 4.6, 5.4, 3.1 and 1.7 times of the latter, respectively, under 395, 385, 375 and 365 nm excitations. Moreover, the main emission peak of ScVO₄: 2%Bi³⁺, 4%Eu³⁺ is located at 616 nm, which is an ideal peak position to provide good color rendering [7], whereas Y₂O₂S: 5% Eu³⁺ gives off a relatively deep red emission mainly at 627 nm. Besides, the CIE chromaticity coordinates for ScVO₄: 2% Bi³⁺, 4% Eu³⁺ are calculated to be (0.65, 0.35), closer to the National Television System Committee standard value (0.67, 0.33) for red-emitting phosphor, compared with those of Y₂O₂S: 5% Eu³⁺ (0.64, 0.36). These results propose ScVO₄: 2% Bi³⁺, 4% Eu³⁺ to be a very competitive candidate for red component in tricolor-based white LEDs.

Figure 5

Comparison of a excitation and b emission spectra between ScVO₄: 2% Bi³⁺, 4% Eu³⁺ (black curve) and Y₂O₂S: 5% Eu³⁺ (red curve)

Conclusion

In summary, a series of ScVO₄: Bi³⁺, Eu³⁺ samples were successfully synthesized, and their luminescent properties were investigated in detail to achieve the best luminous efficiency. The experimental results show that the samples exhibit very efficient excitation in NUV range and can be excited even at 430 nm. All samples show the characteristic red emission of Eu³⁺ at 616 nm corresponding to ⁵D₀ → ⁷F₂ transition. And the optimized one produces an emission intensity 5.4 times of the commercial phosphor Y₂O₂S: 5%Eu³⁺ under 385 nm excitation. Moreover, it also has a better CIE chromaticity coordinates than the latter, closer to the National Television System Committee standard value for red-emitting phosphor. These results suggest that ScVO₄: Bi³⁺, Eu³⁺ is promising for NUV-excited white LEDs in illumination applications.