Synthesis and photoluminescence enhancement of Ca₃Sr₃(VO₄)₄:Eu³⁺ red phosphors by Sm³⁺ doping for white LEDs

Abstract

A new type of red-emitting Eu³⁺ and Sm³⁺ co-doped Ca₃Sr₃(VO₄)₄ phosphor was synthesized by the combustion method. The photoluminescence properties and microstructure were investigated by photoluminescence spectroscopy, X-ray powder diffraction, and scanning electron microscopy. All samples were found to match perfectly with the rhombohedral structure and belong to the R3c space group. The photoluminescence emission intensity of the optimal phosphor Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ at 619 nm was significantly enhanced compared with those of Ca₃Sr₃(VO₄)₄:Eu³⁺ samples and commercial Y₂O₃:Eu³⁺ at an excitation wavelength of 393 nm, as a result of the energy transfer from Sm³⁺ to Eu³⁺. The energy transfer process between Sm³⁺ and Eu³⁺ is discussed and interpreted by considering the energy level diagram. Furthermore, the CIE chromaticity coordinate of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ was closer to the standard red-emitting point (x = 0.67, y = 0.33) than Y₂O₃:Eu³⁺. The luminescence performance of Ca₃Sr₃(VO₄)₄:Eu³⁺, Sm³⁺ upon excitation by near-UV radiation makes it a promising red phosphor for manufacturing white-light-emitting diodes.

1 Introduction

Rare-earth ions have been extensively used in various devices, such as scintillators [1], fluorescent lamps [2], and white LEDs [3]. Among these applications, white LEDs and Hg-free fluorescent lamps are considered to be eco-friendly. White LEDs have many merits, such as high efficiency, long operational lifetime, and compactness [4]. One conventional method to fabricate white light is using a yellow phosphor (typically YAG:Ce³⁺ [5, 6]) in combination with a blue-emitting InGaN LED chip. However, this approach leads to a high color temperature and is an imperfect match in terms of the color rendering in the red region, and it does not satisfy the requirements for low-color-temperature illumination [7]. Another approach is combining semiconductor chips of tri-color phosphors to fabricate white LEDs excited by one UV or near-UV (NUV) chip [8, 9]. The main advantages of this method are avoiding light loss in the process of white light emitting and controlling the emission intensity of different colors to obtain the desired output color [10,11,12]. However, the most common commercially available red-emitting phosphors, such as Y₂O₂S:Eu³⁺ and Y₂O₃:Eu³⁺, are inefficient and chemically unstable. Therefore, new inorganic red-emitting phosphors need to be developed to combine with an appropriate NUV LED for signaling or illumination applications. In addition, the majority of commercial red-emitting phosphors at present use rare-earth elements as the positive ion in the host lattices (e.g., Y³⁺, La³⁺, Ga³⁺), which means that the cost of red-emitting phosphors is much higher than those of yellow-emitting and blue-emitting phosphors.

Recently, solid-state phosphors based on vanadates doped with rare-earth ions have attracted increasing attention because they can be excited by a broad range of wavelengths and have stable chemical characteristics [13, 14]. In the vanadate group (VO₄ ³⁻), four oxygen ions are coordinated to a V⁵⁺ ion in a structure with tetrahedral symmetry, which is considered to be an effective luminescent center [15]. Because of the occurrence of energy transfer, the photoluminescence intensity of vanadate-based phosphors could be improved by doping trivalent rare-earth ions into the vanadate hosts. In particular, vanadate-based phosphors with doped Eu³⁺ have been investigated because they can be applied extensively in the solid-state luminescence industry [13]. Choi et al. [16] reported the solid-state synthesis of Ca₃Sr₃(VO₄)₄:Eu³⁺ and Ca₃Sr₃(VO₄)₄:Eu³⁺, M⁺ (M = Li, Na, K) red-emitting phosphors and studied the effects of Eu³⁺ concentration, the type of charge compensation, and their concentration on emission intensity. Sun et al. [17] reported the use of another solid-state reaction to synthesize Ca₃Sr₃(VO₄)₄:Sm³⁺, Na⁺ red-emitting phosphors. Comparing to conventional solid state reaction, the combustion method can be reacted at relatively low temperature [18]. It also has many advantages such as short reaction time, high efficiency of energy conservation and homogeneous grain size. To the best of our knowledge, there have been no reports of Ca₃Sr₃(VO₄)₄:Eu³⁺ red-emitting phosphors using Sm³⁺ co-doping for white LEDs. Sm³⁺ can be applied as the sensitizer in Eu³⁺-doped phosphors and the excitation bands of Eu³⁺-doped samples can be strengthened and broadened by Sm³⁺ doping in the range of ultraviolet–near ultraviolet (UV–NUV) [19]. Moreover, it has been reported that the energies of the ⁴G_5/2 energy level of Sm³⁺ and the ⁵D₀ energy level of Eu³⁺ are similar [20]. Thus, it seemed plausible that Sm³⁺ could transfer absorbed energy to Eu³⁺ in Ca₃Sr₃(VO₄)₄ phosphors and that the emission intensity could be improved significantly.

In this paper, the combustion method has been applied to synthesize Ca₃Sr₃(VO₄)₄:Eu³⁺, Sm³⁺ phosphors for the first time, to the best of our knowledge. In addition, Ca₃Sr₃(VO₄)₄:Eu³⁺ and commercial Y₂O₃:Eu³⁺ phosphors have also been synthesized by the same method for comparison. The photoluminescence properties of Sm³⁺ and Eu³⁺ single-doped and co-doped samples were investigated and the relevant mechanism is analyzed.

2 Experimental

2.1 Sample preparation

The Ca₃Sr₃(VO₄)₄:Eu³⁺, Ca₃Sr₃(VO₄)₄:Sm³⁺, and Ca₃Sr₃(VO₄)₄:Eu³⁺, Sm³⁺ phosphors were synthesized by the combustion method. The starting materials were high-purity europium oxide (Eu₂O₃), high-purity samarium oxide (Sm₂O₃), analytical reagent (AR) grade calcium carbonate (CaCO₃), AR grade strontium carbonate (SrCO₃), AR grade ammonium metavanadate (NH₄VO₃), nitric acid (HNO₃), and citric acid (C₆H₈O₇·H₂O). The appropriate stoichiometric ratio of Eu₂O₃ and Sm₂O₃ was dissolved in 1 mL HNO₃ with 15 mL distilled water to afford a homogeneous solution. Then, CaCO₃, SrCO₃, C₆H₈O₇·H₂O, and NH₄VO₃ were added to the solution with heating and stirring at a temperature of 70–80 °C. After stirring this solution for around 30 min, we obtained the blue sol precursor. This was placed into a furnace at a defined temperature for 1 h under an air atmosphere and then allowed to cool to room temperature.

2.2 Characterization

The crystal structures of the samples were determined by X-ray diffraction with Cu Kα radiation (Philips X’Pert Pro MPD, Cu Kα, 40 kV, 40 mA, λ = 1.5418 Å). The sample morphologies and particle sizes were investigated using a JEX-100CX scanning electron microscope with a running voltage of 10 kV. Excitation and emission spectra of all of the synthesized samples were recorded using a Hitachi F-7000 photoluminescence spectrometer. The fluorescence lifetime of the obtained samples was investigated using a fluorescence spectrophotometer (Fluorolog-3-Tau, Jobin Yvon Inc., USA). The absolute PL quantum yields (QYs) of powder samples were determined on a FLS980 (Eidinburgh) equipped with an integrating sphere. The fluorimeter have been corrected for the wavelength dependence of the sensitivity of the detectors and throughput of the monochromators. All samples were measured at room temperature.

3 Results and discussion

3.1 Synthesis temperature

To determine the optimum synthesis temperature, this parameter was varied while other experimental conditions were kept the same. The phase, degree of crystallinity, and photoluminescence properties of the samples obtained using various synthesis temperatures were investigated. The powder XRD patterns of the Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphors obtained at four different synthesis temperatures (700, 800, 900, 1000 °C) are shown in Fig. 1. It can be seen that the diffraction peaks for all samples matched perfectly with the standard diffraction peaks for Ca₃Sr₃(VO₄)₄ (PDF card: JCPDS 52-0468), which has a rhombohedral structure. These results confirmed that all of the Ca₃Sr₃(VO₄)₄ samples synthesized here after firing at the various temperatures for 1 h were highly pure and belonged to the R3c space group, without the occurrence of other obvious mixed phases, and that Eu³⁺ entered into the crystal lattice of Ca₃Sr₃(VO₄)₄ without affecting the host structure. Taking the ion sizes of Sr³⁺ (0.118 nm), Ca³⁺ (0.1 nm), and Eu³⁺ (0.094 nm) and valence states into consideration, Eu³⁺ ions can easily substitute for Ca³⁺ ions in the Ca₃Sr₃(VO₄)₄ phosphor due to the similar ion radii. A small shifting of the predominant diffraction peaks around 29.6° (2${\theta }$) in the XRD patterns of the Ca₃Sr₃(VO₄)₄:Eu³⁺ samples was observed as shown in Fig. 1 right side. This further confirmed that the Eu³⁺ ions successfully replaced the Ca³⁺ ions in the host lattice. The shifting in the XRD patterns was due to the ionic radii difference between the Eu³⁺ ion and Ca³⁺ ion. With the increase of temperature from 700 to 900 °C, the intensity of diffraction peaks enhanced orderly and increasingly sharp and symmetrical, which indicates the enhancement of crystalline. It could be also observed from the magnified XRD predominant diffraction peaks in Fig. 1 that the XRD peak width declined orderly. Meanwhile, with the increase of the synthesis temperature from 700 to 1000 °C, the FWHM of the intense diffraction peaks (0, 2, 10) decreased orderly (0.190 at 700 °C, 0.169 at 800 °C, 0.155 at 900 °C, and 0.149 at 1000 °C), which can be calculated by the software of Jade. The FWHM results mean that the crystallinity and the grain size of samples were enhanced successively. The FWHM of the intense diffraction peaks were used to calculate the average crystallite sizes obtained at different synthesis temperatures by using the Scherrer equation:

$${\text{D}}=\frac{{{{\text K{\uplambda}}}}}{{{{{\upbeta}}}\cos {{{\uptheta}}}}}$$

(1)

where D is the average crystallite size, K (K = 0.89) is the Scherrer constant, $\lambda$is the wavelength of the X-ray, $\beta$is the FWHM of the intense diffraction peaks (0, 2, 10) and $\theta$ is the diffraction angle of the intense diffraction peaks. According to this equation, the average crystallite sizes of Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphors synthesized at 700, 800, 900, 1000 °C were 47.55, 53.49, 58.26 and 60.64 nm, respectively. Compared with the solid-state reaction method, highly purified and crystalline Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphors can be obtained by the citric acid assisted solution combustion method at relatively lower synthetic temperatures [18]; the reason for this is that the precursor of the citric acid assisted sol combustion method is the sol system, and it is possible that the strong exothermic action can occur quickly and easily at synthesis temperatures of 700–900 °C [21].

Fig. 1

X-ray diffraction patterns of the Ca₃Sr₃(VO₄)₄:Eu³⁺ samples obtained at different synthesis temperatures

The synthesis temperature affects the grain size and this in turn affects the photoluminescence properties of phosphors [22, 23]. Figure 2 shows the SEM images of the samples synthesized at the various temperatures. The sample synthesized at 700 °C (Fig. 2a) did not exhibit a homogenous size distribution and some particles gathered together, because the synthesis temperature was too low and the combustion reaction was insufficient. The samples synthesized at 800–1000 °C (Fig. 2b–d) exhibited homogenous size distributions and the particle size grew quickly with the increasing synthesis temperature. The grain diameters of the samples were approximately 200–300 nm at 700 °C, 400–500 nm at 800 °C, 600–800 nm at 900 °C, and over 1000 nm at 1000 °C.

Fig. 2

SEM images of the samples synthesized at different temperatures: a 700 °C, b 800 °C, c 900 °C, d 1000 °C

Figure 3 shows the emission spectra of the Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphors synthesized at the various temperatures, revealing that the synthesis temperature had little influence on the overall peak positions and shapes of the spectra of the four samples, although the relative peak intensities showed some differences. In particular, it can be seen that for the samples obtained at different synthesis temperatures, the intensities of the main emission peak at 619 nm were different. From 700 to 900 °C, higher synthesis temperatures led to stronger emission spectra, and the red emission intensity was strongest at a synthesis temperature of 900 °C. Upon further increasing the synthesis temperature to 1000 °C, the red emission intensity decreased.

Fig. 3

Emission spectra of the Ca₃Sr₃(VO₄)₄:0.05Eu³⁺ samples prepared at different temperatures

The reason for this phenomenon is that the crystallization of Ca₃Sr₃(VO₄)₄ structure is improved with the increasing temperature in the range of 700–900 °C and the grain boundary gradually decreased, and high temperature favors the doping Eu³⁺ ions into Ca₃Sr₃(VO₄)₄ lattice, which is consistent with the results shown in Figs. 1 and 2. What is more, as the particle size increases with the increasing temperature in the range of 700–900 °C, stable site for the active ion could be increased increasing emission intensity. However, upon further increasing the synthesis temperature to 1000 °C, agglomeration of active ions at the elevated temperature is the possible reason why the emission intensity decreased. Figure 4 shows the Decay curves of the Ca₃Sr₃(VO₄)₄:Eu³⁺ samples obtained at different synthesis temperatures. The monitoring wavelength is at 619 nm with a 393 nm excitation. The decay curves of Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphors are well fitted by the second order exponential function:

Fig. 4

Decay curves of the Ca₃Sr₃(VO₄)₄:Eu³⁺ samples obtained at different synthesis temperatures

$${\text{y}}={{\text{y}}_0}+{{\text{A}}_1}\exp \left( {\frac{{ - {\text{x}} - {{\text{x}}_0}}}{{{{{\varvec{\uptau}}}_1}}}} \right)+{{\text{A}}_2}{\text{exp}}\left( {\frac{{ - {\text{x}} - {{\text{x}}_0}}}{{{{{\varvec{\uptau}}}_2}}}} \right)$$

(2)

where τ_1, and τ₂ are the luminescence decay times. The value of decay time can be calculated by the following equation [24]:

$${{\varvec{\uptau}}}=\frac{{({{\text{A}}_1}{{\varvec{\uptau}}}_{1}^{2}+{{\text{A}}_2}{{\varvec{\uptau}}}_{2}^{2})}}{{({{\text{A}}_1}{{{\varvec{\uptau}}}_1}+{{\text{A}}_2}{{{\varvec{\uptau}}}_2})}}$$

(3)

The values of decay time of Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphors synthesized at 700, 800, 900, 1000 °C were 578, 555, 558, 576 μs, respectively. It can be seen that the sintering temperature only has a slight influence on the luminescence lifetime, which indicates that the sintering temperature can effectively improve the emission intensity of samples in the range of 700–900 °C, but not change their luminescence lifetime. Therefore, the best photoluminescence properties for the Eu³⁺-doped Ca₃Sr₃(VO₄)₄ phosphors were obtained at a synthesis temperature of 900 °C.

3.2 Photoluminescence spectra of Eu³⁺/Sm³⁺-doped Ca₃Sr₃(VO₄)₄

Figure 5 shows the photoluminescence (excitation and emission) spectra of Ca₃Sr₃(VO₄)₄:Eu³⁺. The emission spectrum of Ca₃Sr₃(VO₄)₄:Eu³⁺, using an excitation of 393 nm, exhibited several emission peaks centered at 594, 619, 651, and 705 nm, which correspond to the ⁵D₀→⁷F_J (J = 1, 2, 3, 4) transitions of Eu³⁺, respectively. Of these peaks, the strongest peak was that situated at 619 nm derived from the ⁵D₀→⁷F₂ transition, because Eu³⁺ occupies the D _2d site without inversion symmetry [25]. Upon monitoring the ⁵D₀→⁷F₂ transition of Eu³⁺, the region of excitation was found to extend from 200 to 500 nm, which included a broad charge transfer band (CTB) and a number of small sharp peaks. The CTB was centered at 285 nm, which is mainly from the allowed transition ¹A₁→¹T₂ with the charge transfer states (CTS) of VO³⁻ [26]. Multiple sharp peaks could be observed from 350 to 500 nm, which was due to the f–f inner-shell transitions of Eu³⁺, regardless of the concentration of Eu³⁺, and these sharp peaks located at 360, 380, 393, 415, and 463 nm were assigned to the ⁷F₀→⁵D₄, ⁵L₇, ⁵L₆, ⁵D₃, and ⁵D₂ transitions of Eu³⁺, respectively. The most intense peak in the UV–NUV region was located at 393 nm, which represents the ⁷F₀→⁵L₆ transition of Eu³⁺. Figure 6 shows the emission spectra of different concentrations of Eu³⁺ single-doped Ca₃Sr₃(VO₄)₄ phosphors. It is evident that the emission intensity of Eu³⁺ single-doped Ca₃Sr₃(VO₄)₄ phosphors consistently increased with increasing percentage of Eu³⁺ up to 5 mol%, and then declined when the concentration of Eu³⁺ was above 5 mol%. Consequently, Ca₃Sr₃(VO₄)₄:0.05Eu³⁺ exhibited the strongest emission intensity.

Fig. 5

a Excitation and b emission spectra of the Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphor

Fig. 6

Emission spectra of Ca₃Sr₃(VO₄)₄:xEu³⁺ with various values of x

The luminescence characteristics of Ca₃Sr₃(VO₄)₄:Sm³⁺ were also investigated and the results are shown in Fig. 7. The emission spectrum show four emission peaks located at 565, 605, 651, and 712 nm upon excitation at 393 nm. The highest peak centered at approximately 605 nm is due to the ⁴G_5/2→⁶H_7/2 transition of Sm³⁺. Meanwhile, the three other peaks at 565, 651, and 712 nm were ascribed to the ⁴G_5/2→⁶H_5/2, ⁴G_5/2→⁶H_9/2, and ⁴G_5/2→⁶H_11/2 transitions of Sm³⁺, respectively. Upon monitoring the ⁴G_5/2→⁶H_7/2 transition of Sm³⁺, the PLE spectrum was made up of a CTB from 200 to 350 nm and some sharp peaks extending from 350 to 500 nm; these small peaks were attributed to the electronic transitions of ⁶H_5/2→⁴L_17/2 (361 nm), ⁶H_5/2→⁶P_5/2 (376 nm), ⁶H_5/2→⁴F_7/2 (405 nm), ⁶H_5/2→(⁶P,⁴P)_5/2 (416 nm), ⁶H_5/2→⁴G_9/2 (439 nm), and ⁶H_5/2→⁴I_11/2 (476 nm). The most intense peak was that located at 405 nm.

Fig. 7

a Excitation and b emission spectra of the Ca₃Sr₃(VO₄)₄:Sm³⁺ phosphor

3.3 Photoluminescence spectra of Eu³⁺ and Sm³⁺ co-doped Ca₃Sr₃(VO₄)₄

Based on the analysis of the photoluminescence properties of Eu³⁺ and Sm³⁺ single-doped Ca₃Sr₃(VO₄)₄, Sm³⁺ and Eu³⁺ co-doped Ca₃Sr₃(VO₄)₄ samples were synthesized and their photoluminescence properties were analyzed. The emission spectra of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, ySm³⁺ (y = 0, 0.01, 0.03, 0.05, 0.07, 0.09, 0.11) are shown in Fig. 8. The emissions from the ⁵D₀→⁷F₂ transition of Eu³⁺ and the ⁴G_5/2→⁶H_9/2 transition of Sm³⁺ are changed similar with Sm³⁺ increasing when excited at 393 nm. This supports the theory that Sm³⁺ can transfer the absorbed energy to Eu³⁺. Moreover, the PL intensity of the Sm³⁺ and Eu³⁺ co-doped phosphors at 619 nm was much higher than samples without Sm³⁺ doping, confirming that the 393 nm excitation energy can be absorbed by Sm³⁺ and then subsequently transferred to Eu³⁺. Upon fixing the Eu³⁺ concentration at 5 mol% and increasing the Sm³⁺ concentration to 9 mol%, the luminescence intensity reached its maximum, and decreased at higher or lower concentrations of Sm³⁺. This phenomenon can be explained by the concentration quenching effect because Sm³⁺ can transfer energy to different sites of Sm³⁺ in the host lattice [18].

Fig. 8

a Excitation spectrum of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ with monitoring at 619 nm, and b emission spectra of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, ySm³⁺ (0 ≤ y ≤ 0.11) upon excitation at 393 nm

In this research, all of the Eu³⁺ and Sm³⁺ co-doped Ca₃Sr₃(VO₄)₄ phosphors can transmit bright red light if using UV or NUV lights as the excitation source. As Fig. 8 shows, the PLE spectrum monitoring the emission at 619 nm consisted of many excitation peaks in the range of 350–500 nm, which were located at 380, 393, 405, and 464 nm, corresponding to the ⁷F₀→⁵L₇ transition of Eu³⁺, the ⁷F₀→⁵L₆ transition of Eu³⁺, the ⁶H_5/2→⁴F_7/2 transition of Sm³⁺, and the ⁷F₀→⁵D₂ transition of Eu³⁺, respectively. It can be observed that the excitation spectrum of the Sm³⁺ and Eu³⁺ co-doped Ca₃Sr₃(VO₄)₄ phosphor was remarkably different from the samples without Sm³⁺ doping, because there is a new excitation peak located at approximately 405 nm. Meanwhile, this co-doped phosphor also has a CTB from 200 to 350 nm that is centered at 285 nm, which is mainly due to the allowed transition ¹A₁→¹T₂ with the CTS of VO³⁻ [26]. Meanwhile, we cannot exclude the existence of CTB of Sm³⁺→O²⁻ and Eu³⁺→O²⁻ in the spectrum, as these CTBs are also situated in this region [27]. Upon excitation at 393 nm, the emission spectra consist of four main emission peaks at 565, 594, 619, 651, and 705 nm, which correspond to the ⁴G_5/2→⁶H_5/2 of Sm^{3+, 5}D₀→⁷F₁ of Eu^{3+, 5}D₀→⁷F₂ of Eu^{3+, 4}G_5/2→⁶H_9/2 of Sm³⁺, and ⁴G_5/2→⁶H_11/2 of Sm³⁺, respectively. The highest emission peak, which is located at 619 nm, is due to a transition of Eu³⁺. As shown in Fig. 9, by comparing Eu³⁺ and Sm³⁺ co-doped samples and single-doped samples with the same concentration of Eu³⁺ and under otherwise identical conditions, it is obvious that the Sm³⁺ single-doped sample had no emission intensity at 619 nm but exhibited its strongest emission peak at 609 nm under excitation at 393 nm, whereas the emission intensity of the Ca₃Sr₃(VO₄)₄:Eu³⁺, Sm³⁺ phosphor at 619 nm was much stronger than that of the Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphor, which proves that the luminescence mechanism of the Eu³⁺ and Sm³⁺ co-doped Ca₃Sr₃(VO₄)₄ phosphor consists of Eu³⁺ luminescence and Sm³⁺ passing energy to Eu³⁺. The excitation peak at 405 nm can be divided into two excitation peaks by using Gaussian fitting, owing to the peak of the ⁷F₀→⁵L₆ transition of Eu³⁺ partially overlapping with the peak of the ⁴G_5/2→⁶H_7/2 transition of Sm³⁺. That is, the excitation of Eu³⁺-doped samples around 393 nm can be broadened and strengthened by co-doping with Sm³⁺, which is beneficial for application to NUV LEDs.

Fig. 9

Photoluminescence spectra of Eu³⁺ and Sm³⁺ single-doped and co-doped Ca₃Sr₃(VO₄)₄ phosphors: a Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, b Ca₃Sr₃(VO₄)₄:0.05Sm³⁺, c Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.05Sm³⁺

The lifetime of Eu^3+:5D₀ and Sm^3+:4G_5/2 with varying Sm³⁺ content is shown in Fig. 10a, b respectively. The monitoring wavelength is at 619 and 609 with a 393 nm excitation. The decay curves of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, ySm³⁺ (0 ≤ y ≤ 0.11) phosphors are well fitted by the second order exponential function:

Fig. 10

a The decay curves of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, ySm³⁺ (0 ≤ y ≤ 0.11) phosphors (λ_ex = 393 nm and λ_em = 619 nm) and the relation between lifetime and Sm³⁺ ion doping concentration. b The decay curves of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, ySm³⁺ (0 ≤ y ≤ 0.11) phosphors (λ_ex = 393 nm and λ_em = 609 nm) and the relation between lifetime and Sm³⁺ ion doping concentration

$${\text{y}}={{\text{y}}_0}+{{\text{A}}_1}\exp \left( {\frac{{ - {\text{x}} - {{\text{x}}_0}}}{{{{{\varvec{\uptau}}}_1}}}} \right)+{{\text{A}}_2}{\text{exp}}\left( {\frac{{ - {\text{x}} - {{\text{x}}_0}}}{{{{{\varvec{\uptau}}}_2}}}} \right)$$

(4)

where τ_1, and τ₂ are the luminescence decay times. The value of decay time can be calculated by the following equation [24]:

$${{\varvec{\uptau}}}=\frac{{({{\text{A}}_1}{{\varvec{\uptau}}}_{1}^{2}+{{\text{A}}_2}{{\varvec{\uptau}}}_{2}^{2})}}{{({{\text{A}}_1}{{{\varvec{\uptau}}}_1}+{{\text{A}}_2}{{{\varvec{\uptau}}}_2})}}$$

(5)

It can be seen from Fig. 10 that the lifetime of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, ySm³⁺ (0 ≤ y ≤ 0.11) phosphors decreases with increasing of Sm³⁺ from 0 to 11 mol%. The result indicates that energy transfer occurs between Eu³⁺ and Sm³⁺ ions.

Figure 11 shows the energy transfer process between Sm³⁺ and Eu³⁺. As discussed previously, when the excitation wavelength is 393 nm, Sm³⁺ can be excited from its ground state ⁶H_15/2 to its corresponding excited state ⁴F_7/2, and Eu³⁺ can be excited from its ground state ⁷F₀ to its corresponding excited state ⁵L₆. After the excited electrons of Eu³⁺ and Sm³⁺ return to their respective ground states, the Sm³⁺ can transfer its absorbed energy to the Eu³⁺. Furthermore, a channel that can transfer energy between the ⁴G_5/2 energy level of Sm³⁺ and the ⁵D₀ energy level of Eu³⁺ exists, because these two energy levels are very close. The ⁴F_7/2 energy level of Sm³⁺ can be reached upon excitation with 393 nm NUV light, and the excited electron of Sm³⁺ automatically returns to its lowest excited energy level ⁴G_5/2. Subsequently, energy transfer can occur between the ⁴G_5/2 level of Sm³⁺ and the ⁵D₀ level of Eu³⁺ by resonance. This explains why the emission is improved in the Eu³⁺ and Sm³⁺ co-doped Ca₃Sr₃(VO₄)₄ phosphor. One of the decay channels of Sm³⁺ can be described as follows [18]:

Fig. 11

Energy level diagram for the energy transfer process between Sm³⁺ and Eu³⁺ ions

$$\frac{1}{{{\tau _s}}}~=\frac{1}{{{\tau _s}_{0}}}+{P_{NR}}+{P_T}$$

(6)

where τ _s is the lifetime of the sensitizer in the presence of the activator, τ _s0 is the lifetime of sensitizer in the absence of the activator, P _NR is the probability of nonradiative transitions, and P _T is the probability that Sm³⁺ can transfer energy to Eu³⁺. Exchanging τ _s/τ _s0 for I _s/I _s0 and taking no account of the influence of P _NR, the efficiency of the energy transfer from Sm³⁺ to Eu³⁺ can be described as follows:

$${\eta _T}=1-\frac{{{I_s}}}{{{I_{s0}}}}$$

(7)

where I _s0 is the luminescence intensity of the sensitizer in the absence of the activator Eu³⁺, which can be measured from the emission intensity of Ca₃Sr₃(VO₄)₄:0.09Sm³⁺ at 651 nm, and I _s is the luminescence intensity of the sensitizer in the presence of the activator Eu³⁺, which can be measured from the emission intensity of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ at 651 nm. According to this approach, the energy transfer efficiency of the optimal sample is 37%. For the purpose of confirming the type of interaction between Sm³⁺ and Eu³⁺, we calculated the distance between Sm³⁺ and Eu³⁺ using the following equation:

$${R_{Sm - Eu}}=2{\left[ {\frac{{3V}}{{4\pi zN}}} \right]^{\frac{1}{3}~}}$$

(8)

where V is the volume of the unit cell (4085.8 Å³), which can be obtained from the refinement result, z is the sum concentration of Eu³⁺ and Sm³⁺, and N (N = 10.5) is the number of Ca₃Sr₃(VO₄)₄ ions in the unit cell. According to this equation, the distance between Eu³⁺ and Sm³⁺ in the various co-doped Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, ySm³⁺ phosphors (y = 0.01, 0.03, 0.05, 0.07, 0.09, 0.11) was estimated to be 23.14, 21.02, 19.52, 18.37, 17.45, and 16.68 nm, respectively. However, the typical critical distance of the exchange interaction between Eu³⁺ and Sm³⁺ is 0.5 nm, which is much smaller than the distance between Eu³⁺ and Sm³⁺ in the co-doped samples. Under normal conditions, the exchange interaction, multipole interaction, and radiation reabsorption may occur by transferring nonradiative energy among different ions [28]. To sum up, the exchange interaction hardly occurred during the process of energy transfer between Sm³⁺ and Eu³⁺ in the host lattice. In addition to this, a broad overlap of fluorescence activator and sensitizer can induce the radiation absorption. Considering the spectral characteristics of the co-doped samples, the type of energy transfer should be multipole interaction. What is more, based on the energy level difference, the phonon assisted energy transfer can be also occurred.

The emission spectra of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺, and the commercial phosphor Y₂O₃:0.05Eu³⁺ at an excitation wavelength of 393 nm were compared, as shown in Fig. 12. It is clear that the PL intensity of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺ was much lower than that of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺, and the integrated PL intensity of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ ranging from 550 to 750 nm was improved by about 1.6 times compared to that of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺. The integrated emission intensities of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ and commercial Y₂O₃:0.05Eu³⁺ were also compared, which revealed that the integrated emission intensity of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ was enhanced by around 2.4 times compared to that of Y₂O₃:0.05Eu³⁺ in the 550–750 nm range. In addition, the quantum efficiency of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺, and the commercial phosphor Y₂O₃:0.05Eu³⁺ were also compared in Table 1, the results show that Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ has the highest quantum efficiency. This comparison leads to the conclusion that the Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ phosphor has excellent photoluminescence properties for application in white-light-emitting diodes.

Table 1 The quantum efficiency of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺, and the commercial phosphor Y₂O₃:0.05Eu³⁺

Fig. 12

Emission spectrum of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺, and commercial Y₂O₃:0.05Eu³⁺ phosphors

Figure 13 shows the CIE chromaticity coordinates of the National Television System Committee (NTSC) standard for red phosphors (x = 0.67, y = 0.33) compared with Ca₃Sr₃(VO₄)₄:0.05Eu³⁺ (0.6448, 0.3548), commercial Y₂O₃:0.05Eu³⁺ (0.649, 0.343), and Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ (0.6595, 0.3402). These values indicate that the CIE chromaticity coordinate of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ is closer to the NTSC standard for red phosphors than other commercial phosphors. The Eu³⁺ and Sm³⁺ co-doped Ca₃Sr₃(VO₄)₄ phosphor could be used as a potential red-emitting phosphor for application in near-UV or blue-based white LEDs.

Fig. 13

CIE chromaticity coordinates of a the NTSC standard for red phosphors (x = 0.67, y = 0.33), (b) Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, (c) commercial Y₂O₃:0.05Eu³⁺, and (d) Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺

4 Conclusion

A series of Ca₃Sr₃(VO₄)₄:Eu³⁺ and Ca₃Sr₃(VO₄)₄:Eu³⁺,Sm³⁺ phosphors were synthesized by a citric acid assisted sol combustion method. After firing for 1 h, all samples were matched perfectly to Ca₃Sr₃(VO₄)₄ (PDF card: JCPDS 52-0468), which has a rhombohedral structure and belongs to the space group of R3c. The photoluminescence properties of Eu³⁺ and Sm³⁺ single-doped and co-doped Ca₃Sr₃(VO₄)₄ phosphors in the near-UV region were investigated. As a sensitizer, Sm³⁺ absorbs the excitation energy and transfers it to the Eu³⁺ ions. The integrated intensity of the optimal sample Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ was enhanced to 1.6 times and 2.4 times compared to Ca₃Sr₃(VO₄)₄:0.05Eu³⁺ and Y₂O₃:0.05Eu³⁺, respectively, indicating that it possesses excellent photoluminescence properties for application to NUV LEDs. Furthermore, the CIE chromaticity coordinate of Ca₃Sr₃(VO₄)₄:0.05Eu³⁺, 0.09Sm³⁺ was found to be closer to standard for red light than other commercial phosphors. Moreover, this research proves that Sm³⁺ can be employed to broaden and strengthen the absorption around 393 nm for Eu³⁺-doped samples. Finally, the Sm³⁺ and Eu³⁺ co-doped Ca₃Sr₃(VO₄)₄ phosphors may be applied as potential red-emitting luminescent materials for white-lighting devices under UV or NUV excitation.