1. Introduction

Solid state lighting based on high-brightness white light emitting diodes (LEDs) has emerged as a new potentially revolutionary technology that could save up to half of energy used for lighting applications, and it has been regarded as the next generation lighting source [1,2]. So far, two main strategies have been developed to produce white light [2]. The first one is the combination of blue InGaN chip with a yellow-emitting phosphor such as the well-known Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce). However, the lamps fabricated in this manner give poor color rendering index (CRI, Ra < 80) and high correlated color temperature (CCT > 4500 K) [2,3]. Additionally the color of the white light will change with input power [4]. Due to these drawbacks, the second method by combining a near ultraviolet (NUV) LED chip with Red/Green/Blue tri-color phosphors has been suggested. White LEDs fabricated in this way can exhibit surprisingly favorable properties, including tunable CCTs, excellent CRI values, and high color tolerance [5]. On the other hand, it is known that the luminescence properties of phosphors strongly affect the performance of white LEDs. However, the currently used phosphors for NUV LEDs, especially red phosphor, suffer some problems such as low luminous efficiency and bad matching with LED chips. In addition, nitride-based red phosphors have been also developed recently to overcome these shortcomings, but it is known that the synthesis of nitride phosphors requires harsh conditions such as high temperature and pressure [6], which cause lots of energy consumption. Based on the above reasons, it is urgent to explore new long-wavelength-emitting phosphors with broad and intense excitation bands in NUV region by some facile approaches at present.

Mostly, Eu²⁺-Mn²⁺ ions are doped in single-phased hosts to generate white light due to their energy transfer (ET). But in this work, the main object using Eu²⁺-Mn²⁺ ions pairs is to obtain tunable and red emissions by ET from Eu²⁺ to Mn²⁺. On the other hand, phosphate materials are widely used as the matrixes of phosphors owing to the high physical and chemical stabilities, facile synthesis, and environmental-friendly characteristics. The Ca₈MgLa(PO₄)₇ compound was reported to have an iso-structure with β-Ca₃(PO₄)₂ which has served as the base for the crystallochemical design of compounds with ferroelectric, nonlinear-optical and ion-conductive properties [7]. Based on these points, to explore novel long-wavelength-emitting phosphors for NUV LEDs, we prepared a series of Ca₈MgLa(PO₄)₇:Eu²⁺, Mn²⁺ samples and investigated their luminescence properties in detail.

2. Experimental

Powder samples of Ca_8(1-x)Mg_1-yLa(PO₄)₇:xEu²⁺, yMn²⁺ (CMLP:xEu²⁺, yMn²⁺, 0 ≤ x ≤ 0.02, 0 ≤ y ≤ 0.9) were prepared by solid-state reaction method from CaCO₃ (AR), (MgCO₃)₄·Mg(OH)₂·5H₂O (AR), (NH₄)₂HPO₄ (AR), La₂O₃ (99.99%), Eu₂O₃ (99.99%) and MnCO₃ (99%). An excess (5%) of (NH₄)₂HPO₄ was used to compensate for the evaporation at a high temperature. The power reagents were ground together and pre-fired in air at 600 °C for 3 h, reground, then calcined in a reduction atmosphere (N₂: H₂ = 95: 5) at 1110 °C for 7 h. The phase purity was analyzed by using an ARL X'TRA powder X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 35 mA. Diffuse reflection spectra (DRS) were obtained by a UV/visible spectrophotometer (UV-3600, SHIMADZU) using BaSO₄ as a reference in the range of 200-700 nm. The luminescence spectra, decay curves and quantum efficiency (QE) were measured with the HORIBA Jobin Yvon Fluorlog-3 spectrofluorometer system. All the spectra measurements were performed at room temperature.

3. Results and discussion

Figure 1 shows the XRD patterns of CMLP:xEu²⁺, yMn²⁺ (0 ≤ x ≤ 0.02, 0 ≤ y ≤ 0.9). It can be seen that all the diffraction peaks are identified to the CMLP host lattice (JCPDS # 46-0801), indicating the single-phase samples have been obtained when Eu²⁺ and Mn²⁺ ions of various concentrations are doped.

 

Fig. 1 XRD patterns of CMLP:xEu²⁺, yMn²⁺.

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In order to obtain the optical bandgap and investigate the influence of Eu²⁺/Mn²⁺ to the optical behavior of CMLP, Fig. 2(a)presents the DRS of the CMLP:xEu²⁺ samples. The undoped CMLP (x = 0, y = 0) phase exhibits two remarkable drops in reflectance in the UV range, starting at about 350 and 250 nm. The latter one corresponds to the optical bandgap in the CMLP compound. To better localize the threshold for this transition, the corresponding absorption spectrum was obtained from its reflection spectrum (see inset) using the Kubelka-Munk (K-M) function $F(R)={(1-R)}^2/2R=K/S$ [8], where R, K, and S are the reflection, absorption and scattering coefficients, respectively. By extrapolating the K-M function to K/S = 0, the value of the bandgap was calculated to be ~5.34 eV. As regards the former reflectance drop, we predict that it could be connected with the defects energy levels. And the photoluminescence of CMLP was detected at room temperature as shown in Fig. 2(b). Upon 365 nm excitation, a weak emission band around 432 nm is found; by monitoring 432 nm, an obvious excitation band from 250 to 400 nm is observed which could correspond to the absorption from 250 to 350 nm in Fig. 2(a). Since no activators are doped in the CMLP host, we attribute this luminescence to the transitions related to the defects energy levels at present. Similar phenomenon can be also found in the SrY₂O₄:Eu phosphor [9]. In Fig. 2(a), it can be seen the Eu²⁺/Mn²⁺ substitution causes an immediate change in the optical behaviour of CMLP, as indicated by the extension of absorption band to about 500 nm which is mainly owing to the 4f⁷-4f⁶5d transition of Eu²⁺.

 

Fig. 2 (a) DRS of CMLP:xEu²⁺, yMn²⁺, inset shows the absorption spectrum of CMLP as calculated by the K-M formula; (b) normalized excitation and emission spectra of CMLP.

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Figure 3 shows the emission spectra of CMLP:xEu²⁺ (0.001 ≤ x ≤ 0.020) upon 365 nm excitation. Very broad emission bands peaking at about 610 nm are found, and obvious shoulder peaks around 515 nm appear. As mentioned above, the CMLP compound is isostructural with β-Ca₃(PO₄)₂, in which five kinds of Ca²⁺ sites can take in Eu²⁺ ions, Ca(1)-Ca(5) [10,11]. The Ca(1)-Ca(3) sites are in the general position (18b) with coordination numbers of 7, 6, and 8, respectively. The Ca(4) is a half-filled site and Ca(5) is a distorted site with coordination number of 6. Thus, one can predict that more than one Eu²⁺ emission center exists in the CMLP:Eu²⁺ phosphors, and the broad emission bands in Fig. 3 are due to the emission-spectra overlap for various Eu²⁺ centers. In addition, it can be seen the CMLP:0.005Eu²⁺ sample exhibits the strongest emission intensity. Beyond this Eu²⁺ content, the emission decays gradually owing to concentration quenching.

 

Fig. 3 Emission spectra of CMLP:xEu²⁺.

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To further confirm the various Eu²⁺ centers in the CMLP:Eu²⁺ phosphor, the normalized excitation spectra of CMLP:0.005Eu²⁺ monitored by 475, 515, 590, and 700 nm are shown in Fig. 4(a). Broad and intense absorptions are found in NUV region (350-420 nm). Thus, they could well match with the emission wavelengths of the NUV LED chip indicating the potential applications in white LEDs. With the monitoring wavelength increased, the excitation band shows a continuous red-shift. This could be owing to the contribution of various Eu²⁺ emission centers in the CMLP:Eu²⁺, and the long-wavelength excitation between 420 and 500 nm mainly corresponds to the long-wavelength emission since it has been enhanced obviously by monitoring at 590 and 700 nm. To further verify this conclusion, Fig. 4(b) presents the normalized emission spectra of CMLP:0.005Eu²⁺ upon 335, 365, and 435 nm excitation. As expected, the emission below 600 nm decays gradually and a red-shift beyond 600 nm is observed with increasing excitation wavelength. This result implies the excitation band from 420 to 500 nm in Fig. 4(a) is mainly produced by Eu²⁺ centers with long-wavelength emissions (beyond 500 nm).

 

Fig. 4 Normalized excitation (a) and emission (b) spectra of CMLP:0.005Eu²⁺.

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It is known the d-d transition of Mn²⁺ is forbidden and difficult to pump [12], so Eu²⁺ is usually codoped as sensitizer to enhance the emission intensity of Mn²⁺. On the other hand, Mn²⁺ can give green or red emission, depending on the crystal field condition, i.e., if Mn²⁺ lies in octahedral surrounding with large crystal field, the emission is usually red; if in tetrahedral surrounding with a much smaller crystal field, a green emission is usually obtained [13]. In the CMLP host, Mn²⁺ ions occupy the Mg sites in octahedral surrounding [7]. Thus, red emission of Mn²⁺ can be expected. The emission spectrum of CMLP:0.5Mn²⁺ is shown in Fig. 5(a).The typical Mn²⁺ red emission band around 648 nm is observed, ascribed to the ⁴T₁-⁶A₁ transition of Mn²⁺ [3]. But it has a weak intensity. To enhance the Mn²⁺ emission, a series of CMLP:xEu²⁺, yMn²⁺ samples were synthesized and their emission spectra under 365 nm excitation are shown in Fig. 5(a). With increasing Mn²⁺ concentration, the emission intensity of Eu²⁺ is decreased sharply, while the emission intensity of Mn²⁺ is increased obviously. This phenomenon indicates an extremely efficient ET from Eu²⁺ to Mn²⁺ occurs. To further confirm this ET and determine the ET efficiency, Fig. 5(b) provides the decay curves of CMLP:0.005Eu²⁺, yMn²⁺ (0 ≤ y ≤ 0.9) with 365 nm excitation and 500 nm emission. They exhibit double-exponential feature and the average lifetimes have been calculated by the equation $<\tau >=(A_1{\tau }_1^2+A_2{\tau }_2^2)/(A_1{\tau }_1+A_2{\tau }_2)$ [14], which are 848, 635, 354, 267, and 143 ns for y = 0, 0.1, 0.3, 0.5, and 0.9, respectively. The corresponding ET efficiency (η_T) were determined by the formula ${\eta }_T=1-{\tau }_s/{\tau }_{s0}$, where τ_s0 and τ_s are the decay lifetimes of the sensitizer (Eu²⁺) in the absence and presence of the activator (Mn²⁺), respectively [15]. The obtained η_T values are plotted as a function of Mn²⁺ concentration in the inset of Fig. 5(b), indicating an extremely efficient ET from Eu²⁺ to Mn²⁺. In addition, from the inset of Fig. 5(a) which shows the Mn²⁺ emission intensity as a function of Mn²⁺ concentration, it can be seen the optimal Mn²⁺ content is for y = 0.9. Beyond this concentration, the emission intensity of Mn²⁺ starts to decay due to the typical concentration quenching. The external QE of the CMLP:0.005Eu²⁺, 0.9Mn²⁺ sample by integrating the emission counts from 400 to 800 nm was measured to be about 22.1%.

 

Fig. 5 (a) Emission spectra of CMLP:xEu²⁺, yMn²⁺, inset shows the Mn²⁺ emission intensity as a function of Mn²⁺ concentration; (b) decay curves of CMLP:0.005Eu²⁺, yMn²⁺, inset shows the ET efficiency as a function of Mn²⁺ concentration.

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To understand the emitting light color of the samples more clearly, the Commission International del’Eclairage (CIE) chromaticity coordinates of CMLP:xEu²⁺, yMn²⁺ upon 365 nm excitation were calculated from the emission spectra in the range of 400-800 nm using the 1931 CIE system. Figure 6 describes the corresponding CIE chromaticity diagram, and the chromaticity coordinates values are (0.412, 0.427), (0.452, 0.400), (0.540, 0.359), (0.566, 0.339), (0.587, 0.322), (0.624, 0.315), (0.628, 0.311), and (0.661, 0.317) for Points 1-8, respectively. As a result, tunable emission (from yellow to red) of CMLP:xEu²⁺, yMn²⁺ phosphors are achieved. To observe the emission colors visually, the inset of Fig. 6 shows the digital photographs of CMLP:0.005Eu²⁺ and CMLP:0.005Eu²⁺, 0.9Mn²⁺ under 365 nm UV lamp irradiation. The two samples take on yellow and red color respectively, which are in agreement with their chromaticity coordinates values.

 

Fig. 6 CIE chromaticity diagram for CMLP:xEu²⁺, yMn²⁺ (Point 1 for x = 0.005, y = 0; Point 2 for x = 0.005, y = 0.1; Point 3 for x = 0.005, y = 0.3; Point 4 for x = 0.005, y = 0.5; Point 5 for x = 0.005, y = 0.7; Point 6 for x = 0.005, y = 0.9; Point 7 for x = 0.005, y = 1.0; Point 8 for x = 0, y = 0.5), inset shows the digital photographs of CMLP:0.005Eu²⁺ (a) and CMLP:0.005Eu²⁺, 0.9Mn²⁺ (b) under 365 nm UV lamp irradiation.

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4. Conclusions

In conclusion, emission-tunable CMLP:Eu²⁺, Mn²⁺ phosphors were synthesized by solid-state reaction method in this paper. The XRD patterns show the prepared samples are all single-phase. Through measuring the excitation spectra, different spectral characteristics are found by monitoring various emission-wavelengths, and the excitation bands could well match with the emission wavelengths of the NUV LED chip. The emission spectra demonstrate broad emission bands originating from various Eu²⁺ emission centers. For Eu²⁺-Mn²⁺ codoped CMLP, an extremely efficient ET from Eu²⁺ to Mn²⁺ occurs, resulting in the emission color changes from yellow to red. The above results indicate the CMLP:Eu²⁺, Mn²⁺ phosphors have potential applications in white LEDs.