A new single-component KCaY(PO₄)₂:Dy³⁺, Eu³⁺ nanosized phosphor with high color-rendering index and excellent thermal resistance for warm-white NUV-LED

Abstract

In this study, novel KCaY(PO₄)₂:Dy³⁺, Eu³⁺ (KCYP:Dy³⁺, Eu³⁺) nanophosphor has been successfully prepared by a facile solution-combustion route. The crystal structure, morphological and microstructural features, and photoluminescence (PL) properties of KCYP:Dy³⁺, Eu³⁺ nanophosphor, as well as its thermal properties, have been studied. Investigations based on XRD, FTIR and Raman verify the formation of single-phased and well-crystallized KCYP:Dy³⁺, Eu³⁺. SEM and TEM observations reveal that the KCYP:Dy³⁺, Eu³⁺ nanophosphor displays uniform spherical morphology with an average particle size of about 47 nm. The optimal concentrations of Dy³⁺ and Eu³⁺ ions in KCYP were determined to be 3 mol% and 7 mol%, respectively. Under near-ultraviolet (NUV) excitation, individual Dy³⁺- or Eu³⁺-doped samples exhibit characteristic PL emissions in their respective regions. As for the Dy³⁺, Eu³⁺ co-doped KCYP, energy transfer has been confirmed happening from Dy³⁺ to Eu³⁺ via an electric dipole–dipole interaction, and the critical distance (R_c) was calculated to be 17.07 Å. Our experiments reveal that it is easy to produce warm white or tunable emissions by regulating the Eu³⁺ content. Moreover, the single-component KCYP:3%Dy³⁺, 7%Eu³⁺ nanosized phosphor has a very low correlated color temperature (2766 K) and a high color-rendering index (R_a = 81.6). The quenching temperature (T_0.5) of the KCYP:Dy³⁺, Eu³⁺ nanophosphor was found to be higher than 220 °C, indicating its superior thermal resistance. Consequent three rounds of the heating–cooling cycle experiments demonstrate no occurrence of thermal degradation. These findings suggest that this nanophosphor is a promising candidate for application in NUV-pumped glareless warm-white LED.

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1. Introduction

Nowadays, white-light-emitting diodes (WLEDs) are a research hotspot in solid-state lighting because of their special superiorities of saving energy, long lifetime, reliability, environmental protection, and so on.^1–6 There are several ways to assemble WLEDs. The most prevalent strategy is generated by combining a blue InGaN chip with yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) phosphors. Nevertheless, because of the innate scarcity of red composition in the emission of YAG:Ce, it is notoriously difficult to create warm WLEDs with high color-rendering index (CRI, R_a > 80) and low correlated color temperature (CCT < 4500 K), both of which significantly restrict it to provide sunlight-like illumination.^7–12 In order to overcome these drawbacks, single-phased white emitting phosphor is much desired for WLEDs because it possesses many surprisingly favorable properties, such as tunable CIE chromaticity coordinates, excellent R_a values and color stability, etc.^13–17 The perception of it is more comfortable to human eyes. One of the approaches for producing white light from single phase phosphors is by co-doping sensitizer and activator into the same host based on the energy transfer.^18–20 Very recently, KCaY(PO₄)₂ (KCYP), a member of double phosphate family, is considered as a superior luminescent host for the doping of rare-earth (RE) ions due to its excellent thermal and chemical stability and strong absorption in the near-ultraviolet (NUV) region.^21,22 Besides, the tetrahedral rigid three-dimensional matrix of phosphate compounds is thought to be ideal for charge stabilization.²³ In consequences, RE-doped (RE = Ce³⁺, Tb³⁺, Eu³⁺, Eu²⁺, and Mn²⁺) KCYP-based phosphors have been vigorously studied.^{19,21,24–27} However, their works have solely focused on spectroscopic characterization, the CCT, CRI and quantum yield (QY), as well as the thermal properties of the prepared phosphors are seldom given. And these parameters are very crucial for their real application. Furthermore, to the best of our knowledge, the photoluminescence properties of Dy³⁺, Eu³⁺ co-doped KCYP and the energy transfer mechanism have also not been reported in the literatures.

To investigate the luminescence and energy transfer properties of KCYP:Dy³⁺, Eu³⁺, approach for the synthesis of single-phase KCYP:Dy³⁺, Eu³⁺ should be considered first. In the previous KCYP systems, the preparation routes are mainly focused on solid-state reaction, sol–gel method, hydrothermal synthesis, and chemical co-precipitation.^{19,21,24–27} However, these methods have many shortcomings, including high synthesis temperature, long preparation period, complex synthesis procedure, and difficult in precise control the stoichiometric amount of the precipitate. Additionally, the resulting products have a large particle size and exhibit inhomogeneous morphology. Due to the luminescence properties largely depend on grain size, attractive applications become possible when the grain size is decreased.²⁸ On this account, facile combustion approach is a good choice because it is rapid and versatile, and requires extremely low temperature, besides, the products are in the nanometer range with narrow size distribution and have uniform morphology.^28–31 Up to now, almost no work has come to the notice of author regarding co-doping of Dy³⁺ and Eu³⁺ in KCYP by combustion synthesis.

Herein, we have discovered and first report a facile synthesis of novel single phase KCYP:Dy³⁺, Eu³⁺ nanosized phosphor prepared through a low-cost combustion method. In order to demonstrate the superior properties of the synthesized KCYP:Dy³⁺, Eu³⁺, the crystal structural, morphological and microstructural features are characterized first. Then, the photoluminescence properties between Dy³⁺ and Eu³⁺ in KCYP host are investigated. The mechanisms are thoroughly discussed for the energy transfer behavior. Moreover, the energy transfer parameters (η and P), CRI and CCT as well as QY are also given. Finally, the thermal quenching properties of KCYP:Dy³⁺, Eu³⁺ nanophosphor are studied in detail.

2. Experimental section

2.1. Materials and preparation

A series of single-phased KCaY(PO₄)₂:Dy³⁺, Eu³⁺ nanophosphors were synthesized via a facile combustion approach. The raw materials were KNO₃ (A.R.), Ca(NO₃)₂ (A.R.), Y₂O₃, Dy₂O₃ and Eu₂O₃ (99.5%; Sinopharm Chemical Reagent Co. Ltd., Beijing, China), H₃PO₄ (A.R.) and citric acid (A.R.). All of the chemical reagents were used as received without further purification. Typically, Y₂O₃, Dy₂O₃ and Eu₂O₃ were dissolved in HNO₃ (G.R.) to obtain Y(NO₃)₃, Dy(NO₃)₃ and Eu(NO₃)₃, respectively. Then the nitrates with a molar ratio of K : Ca : Y : Dy : Eu = 1 : 1 : 1 : m : n (both m and n = 0, 0.5, 1, 3, 5, 7, and 9, respectively) were mixed under magnetic stirring at room temperature. Meanwhile, the citric acid as a combustible fuel was added to the above mixed solution and stirred for 10 min, followed by adding moderate to the H₃PO₄ solution. As the mixture was completely dissolved, the solution was continuously stirred for 40 min until a homogenous solution formed. Subsequently, the homogeneous solution was put into a porcelain crucible to start combustion reaction in a preheated furnace maintained at 400 °C. After the rapid reaction process (only a few seconds), foamy white powders with porous structures were obtained, as reaction products because of the exothermic reaction that occurred between mixture and fuel. Finally, these resulting powders were ground for further characterizations.

2.2. Measurements and characterization

The crystalline nature and phase purity were determined by an X-ray diffractometer (XRD; Rigaku D/max 2500, Tokyo, Japan) with CuK_α (λ = 1.54178 Å) radiation (200 mA, 40 kV) at a step size of 0.02° and a scanning rate of 1° per min over a 2θ range of 10–60°. The raw XRD data were further analyzed using the Rietveld method in order to confirm the formation of the title phase. The chemical structures of samples were characterized by Fourier-transform infrared spectroscopy (FTIR; Nicolet 6700, US) with KBr pellets and by Raman spectroscopy (DXR, Thermo scientific, US) with a laser beam of 514 nm. The particle size, powder morphology and microstructural characteristics were studied by a laser particle size analyzer (Mastersizer 3000, Malvern, UK), a field emission-scanning electron microscope (FE-SEM; Hitachi S-4700, Tokyo, Japan), and a 200 kV Tecnai G² F20 high resolution transmission electron microscope (HRTEM; FEI, the Netherlands). The diffuse reflection (DR) spectra were recorded on a Hitachi F-7000 ultraviolet-visible spectrometer. White BaSO₄ (reflection 100%) was used as the standard reference for DR measurements. The steady-state photoluminescence excitation (PLE) and emission (PL) spectra and the time-resolved decay curves were conducted on an Edinburgh Instrument FLS980 spectrofluorometer using a 450 W Xe lamp as the excitation source (excitation slit width = 0.5 nm, emission slit width = 0.5 nm). In order to eliminate the second-order emission of the source radiation, a cutoff filter was used in the measurements. Quantum yield (QY) value was measured using an integrating sphere whose inner face was coated with BaSO₄ assembly fitted to a FLS980 spectrofluorometer. The CRI and CCT as well as the Commission International de I'Eclairage 1931 chromaticity (CIE) were evaluated for the lamp at the driving current of 20 mA on a V2.00 LED photoelectricity test system. And the temperature-dependent (30–210 °C) PL spectra were also carried out on FLS980 spectrofluorometer with an external heater. All of the measurements were performed at room temperature unless mentioned specially.

3. Results and discussion

3.1. Crystal structure, phase formation, and microstructural features

The crystal structure of KCYP belongs to a hexagonal system with the space group of P6₂22, and the lattice parameters are a = b = 6.903 Å, c = 6.331 Å, V = 261.3 ų, and Z = 1. Fig. 1a represents the crystal structure of KCYP of the 1 × 1 × 1 unit cell. The divalent ions occupied the large empty tunnels of the lattice and the trivalent position is statistically occupied by both Y³⁺ and Ca²⁺ ions, which results in chains of edge-sharing (Ca,Y)O₈ polyhedron interconnected by corner sharing.^19,27 The structures of constructive polyhedrons are also exhibited in Fig. 1b for a clear insight into the crystal structure. The effective ion radii (IR) of K⁺ = 1.51 Å (CN = 8), IR of Ca²⁺ = 1.12 (CN = 8) and IR of Y³⁺ = 1.02 Å (CN = 8), while IR of Dy³⁺ = 1.03 Å (CN = 8) and IR of Eu³⁺ = 1.07 Å (CN = 8) are known according to literature.³² Therefore, for the consideration of IR matching, these Dy³⁺ and Eu³⁺ ions should substitute for the Ca²⁺ or Y³⁺ sites in the KCYP host.

Fig. 1 (a) Crystal structure of the 1 × 1 × 1 unit cell of KCYP. (b) Typical constructive polyhedrons in KCYP. (c) Powder XRD patterns of the as-synthesized pure KCYP host, KCYP:3%Dy³⁺, KCYP:7%Eu³⁺, and KCYP:3%Dy³⁺, 7%Eu³⁺ samples. The standard data for KCYP (PDF 51-1632) is shown as a reference.

In order to verify of the composition and phase purity of as-prepared products, the samples were examined by XRD. Fig. 1c illustrates the XRD patterns of pure KCYP host and representative KCYP:3%Dy³⁺, KCYP:7%Eu³⁺ and KCYP:3%Dy³⁺, 7%Eu³⁺ samples. It can be obviously seen that all the diffraction peaks agree well with that of the JCPDS standard pattern (PDF #51-1632). This indicates that the single phase KCYP powder was obtained in our study, and the Dy³⁺ and Eu³⁺ ions can be completely dissolved in the host lattice without generating any impurity phase. Then, we refine the crystal structure of the as-synthesized KCYP:3%Dy³⁺, 7%Eu³⁺ using the validate XRD data. The refined lattice constants were examined to be a = b = 6.899 Å, c = 6.328 Å, and the cell volume was determined as 260.47 ų. The parameters of the KCYP:3%Dy³⁺, 7%Eu³⁺ lattice were found to decrease after doping Dy³⁺/Eu³⁺ because of the smaller ionic radii of dysprosium and europium ions. Fig. S1 (ESI†) shows the XRD patterns of prepared KCYP:3%Dy³⁺, n%Eu³⁺ with various Eu³⁺ contents (n). It is clear that all samples are single-component in our experimental doping-content ranges.

For further investigating the structure of synthesized samples, the FTIR and Raman spectra were measured and are given in Fig. 2a and b, respectively. Previous work by Jastrzebski et al.³³ has confirmed that [PO₄] group can be characterized by four types of vibration modes, as depicted in Fig. 2c: symmetrical stretching vibration of P–O bond (ν₁), asymmetrical stretching vibration of P–O bond (ν₂), deformational bending vibration of O–P–O fragment (ν₃), and deformational scissors vibration of O–P–O fragment (ν₄). In the FTIR spectra (Fig. 2a), the characteristic peak positions between the pure KCYP and KCYP:3%Dy³⁺, 7%Eu³⁺ are identical. The absorption peaks at 528 and 612 cm⁻¹ are attributed to the ν₃ mode, while the broader band at 1004–1088 cm⁻¹ is ascribed to the ν₂ mode.³⁴ In the Raman spectra (Fig. 2b), both pure KCYP and KCYP:3%Dy³⁺, 7%Eu³⁺ samples have the same spectra shape. In accordance with those previous reports,^33,34 the Raman spectra of both contain four main bands originating from the ν₄ mode at 400, 436 and 472 cm⁻¹, the ν₃ mode at 613 and 635 cm⁻¹, the ν₁ mode at 999 cm⁻¹, and the ν₂ mode at 1101 cm⁻¹, respectively. The samples from different doping concentrations have similar FTIR and Raman spectra to the KCYP:3%Dy³⁺, 7%Eu³⁺ sample and are not shown here for brevity. All these results further prove the successful formation of single-component KCYP:Dy³⁺, Eu³⁺ fabricated by solution-combustion method.

Fig. 2 FTIR (a) and Raman (b) spectra of KCYP and KCYP:3%Dy³⁺, 7%Eu³⁺ samples. Schematic diagram of vibration modes for [PO₄] group (c).

Powder morphology and particle size of a phosphor material plays an important role in deciding the luminescence quality of the material. Uniform particle-size distribution and fine particles are some of the requirements of good luminescent materials.^35,36 Fig. 3 shows the typical morphology and microstructural characteristics of combustion-derived KCYP:3%Dy³⁺, 7%Eu³⁺. SEM photograph (Fig. 3a) reveals that the as-prepared samples are well-crystallized and exhibit spherical-like morphology with smooth surfaces. Even though the agglomeration exists to some extent owing to absorbing water in the air easily, a multitude of KCYP:3%Dy³⁺, 7%Eu³⁺ particles have a uniform dispersion with a mean particle size of less than 50 nm, which is interesting for a number of applications. In Fig. 3b, the size distribution of KCYP:3%Dy³⁺, 7%Eu³⁺ is in a narrow-range of 20–90 nm and the average value is approximately 47 nm, which concurs with the SEM result. The high crystallinity of the as-synthesized nanophosphor can be further certified by HRTEM. The overview image (Fig. 3c) shows that voluminous and delicate porous nanometer particles are formed due to the evolution of gaseous products during combustion. The estimated dimension of the KCYP:3%Dy³⁺, 7%Eu³⁺ nanocrystalline from Fig. 3c is ca. 25–55 nm, which is well consistent with the broad peaks in XRD patterns (Fig. 1c). Fig. 3d shows several sets of lattice fringes oriented in different directions, which indicates that there are several crystallites in a particle and these single-phase samples are highly crystalline and no significant structural defects appear. The d spacings between the lattice fringes of 0.299 and 0.281 nm can be assigned to the (200) and (102) planes of KCYP, respectively.

Fig. 3 Microstructures of the combustion-derived KCYP:3%Dy³⁺, 7%Eu³⁺ nanophosphor: SEM image of typical morphology (a), particle-size distribution (b), TEM micrographs of an overview observed at low magnification (c), and an enlarged image illustrates several grains in a particle (d).

3.2. Photoluminescence properties of KCYP:Dy³⁺, Eu³⁺ nanophosphor

The PL an PLE spectra of the KCYP:3%Dy³⁺ sample are shown in Fig. 4a. The PLE spectrum monitored with 580 nm emission (⁴F_9/2 → ⁶H_13/2) of Dy³⁺ ion is composed of several bands centered at 322, 346, 361, 382, 424, (444 and 452), and 472 nm, which are assigned to the transitions of Dy³⁺ from the ground state ⁶H_15/2 to the excited states ⁴D_7/2, ⁶P_7/2, ⁶P_5/2, ⁴I_13/2, ⁴G_11/2, ⁴I_15/2, ⁴F_9/2, respectively.^37,38 These excitation bands indicate that the phosphor can forcefully adsorb purple and blue light to obtain Dy³⁺ ion emission. Under excitation of 346 nm, the PL spectrum of KCYP:3%Dy³⁺ consists of three major emission peaks at 475, 580 and 667 nm, belonging to the Dy³⁺ transitions of ⁴F_9/2 → ⁶H_15/2 (blue), ⁴F_9/2 → ⁶H_13/2 (yellow) and ⁴F_9/2 → ⁶H_11/2 (red), respectively.³⁹ It can be seen that the 475 nm lines due to ⁴F_9/2 → ⁶H_15/2 transition splits up into two components at 475 nm and 491 nm, with maximum intensity at 475 nm. Also, the 580 nm peaks due to ⁴F_9/2 → ⁶H_13/2 transition splits up into two components at 569 nm and 580 nm, with highest intensity at 580 nm. The ⁴F_9/2 → ⁶H_15/2 (475 nm) is from a magnetic–dipole transition and ⁴F_9/2 → ⁶H_13/2 (580 nm) is from an electric–dipole transition, respectively.³⁷ The emission at 580 nm lines is more intense than that at 475 nm lines, which suggests that the site of Dy³⁺ has low symmetry.²² Fig. 4b describes the emission intensity of the ⁴F_9/2 → ⁶H_13/2 (580 nm) and ⁴F_9/2 → ⁶H_15/2 (475 nm) transitions with various Dy³⁺ concentrations under 346 nm excitation, from which one can find that all PL spectra display similar profiles with different relative intensities. With the increase of Dy³⁺ concentration, the intensity of the phosphor increases initially, reaching a maximum at 3 mol% concentration, and then gradually decreases when the Dy³⁺-doped content is larger than 3 mol% (inset of Fig. 4b). This is because of concentration quenching effect due to mutual Dy³⁺–Dy³⁺ interactions at higher concentrations.^22,38 Therefore, the optimal concentration of Dy³⁺ ions in KCYP is determined to be 3 mol%.

Fig. 4 (a) Excitation (λ_em = 580 nm) and emission (λ_ex = 346 nm) spectra of Dy³⁺-doped KCYP. (b) Emission spectra (λ_ex = 346 nm) of KCYP:m%Dy³⁺ samples with varied m values (0.5–9), the inset exhibits the emission intensity as a function of Dy³⁺ doping content.

Fig. 5a shows the PL and PLE spectra of KCYP:7%Eu³⁺. In the excitation spectrum monitored at 706 nm, the excitation peaks from 300 nm to 500 nm are due to the Eu³⁺ ion transitions of ⁴F₀ → ⁵H₄ (316 nm), ⁴F₀ → ⁵D₄ (359 nm), ⁴F₀ → ⁵L₇ (378 nm), ⁴F₀ → ⁵L₆ (391 nm), ⁴F₀ → ⁵D₃ (413 nm) and ⁴F₀ → ⁵D₂ (462 nm), respectively.^37,38 In the emission spectrum of KCYP:7%Eu³⁺, there are many sharp peaks ranging from 550 nm to 750 nm corresponding to the transitions from the excited ⁵D₀ level to the ⁷F_J (J = 1, 2, 3 and 4) levels of Eu³⁺.⁴⁰ It is noteworthy that the ⁵D₀ → ⁷F₄ transition becomes strongest among all these transitions owing to the lack of inversion symmetry,²² which is beneficial for obtaining pure red colors with good CIE chromaticity. In order to analyze the influence of Eu³⁺ concentration on the luminescence properties, the concentration of Dy³⁺ ions is fixed for 3 mol%, a optimized concentration which is chosen based on the results of the PL intensity of Dy³⁺-doped nanophosphors, a series of KCYP:3%Dy³⁺, n%Eu³⁺ (n = 0.5–9) samples are prepared and their PL spectra are compared in Fig. 5b. It is apparent that the shape of the PL spectra is similar in all the phosphors except in the intensity. The inset of Fig. 5b gives the variation of the emission intensity as a function of Dy³⁺ concentrations under excitation at 391 nm. The emission intensity increases and achieves a saturation at n = 7, and then decreases because of the phenomena of internal concentration quenching. Thus, the optimal concentration of Eu³⁺ ions in KCYP should be 7 mol%. The reason for concentration quenching is that the distance between Eu³⁺ ions becomes smaller with the concentration of Eu³⁺ increasing, and collision or a resonant energy transfer from one luminescence center to another quenching center occurs.^37,41 Fig. S2 (ESI†) displays the relative DR spectra of the undoped and the 3 mol% Dy³⁺ and 7 mol% Eu³⁺-doped KCYP sample, respectively. The spectrum for KCYP host exhibits an absorption band from 200 nm to 240 nm, which is ascribed to the host absorption.¹⁹ While doping 3 mol% Dy³⁺ and 7 mol% Eu³⁺ in the host, owing to the 4f-intraconfigurational transitions of Dy³⁺ and Eu³⁺ ions, a strong and broad absorption band in the NUV region (270–500 nm) is observed, which is analogous to some of popular NUV chip pumped phosphors, such as silicates,⁴² borates,⁴³ vanadates,⁴⁴ etc., meaning that the present nanophosphor could be excited by commercial NUV LEDs.

Fig. 5 (a) Excitation (λ_em = 706 nm) and emission (λ_ex = 391 nm) spectra of Eu³⁺-doped KCYP. (b) Emission spectra (λ_ex = 391 nm) of KCYP:3%Dy³⁺, n%Eu³⁺ samples with changed n values (0.5–9), the inset presents the emission intensity as a function of Eu³⁺ doping content.

3.3. Decay lifetime, quantum yield, and energy transfer mechanism

By investigating the PL behaviors of the KCYP:Dy³⁺/Eu³⁺ phosphors, we can find that energy transfer (ET) may take place from Dy³⁺ to Eu³⁺. To prove this conjecture, the first criterion for ET process is the spectra overlap of the acceptor (Eu³⁺) absorption and donor (Dy³⁺) emission. As shown in Fig. 6a, the comparison of the PLE and PL spectra of the KCYP:Eu³⁺ (λ_em = 706 nm) and KCYP:Dy³⁺ (λ_ex = 346 nm) phosphors exhibits a conspicuous partly spectral overlap, which means that the presence of ET from Dy³⁺ to Eu³⁺ is possible. This conclusion can be further elucidated in terms of the PLE spectra of KCYP:3%Dy³⁺, KCYP:7%Eu³⁺, and KCYP:3%Dy³⁺, 7%Eu³⁺ phosphors measured under same experimental conditions. As shown in Fig. 6b, when monitored with the ⁵D₀ → ⁷F₄ emission of Eu³⁺ (706 nm), a noteworthy ⁶H_15/2 → ⁶P_7/2 transition of Dy³⁺ (346 nm) can be observed in the PLE spectrum of KCYP:3%Dy³⁺, 7%Eu³⁺ in comparison with that of KCYP:7%Eu³⁺. However, the excitation peak at 391 nm is much more intense than that of 346 nm (blue line in Fig. 6b), which might indicate the Dy³⁺–Eu³⁺ energy transfer is not nonradiative relaxation, but the radiative type due to spectral overlap.⁴⁵ Moreover, the PLE spectra of KCYP:3%Dy³⁺, 7%Eu³⁺ excited at 706 nm and 580 nm are investigated. By comparison, it is noticed that the shape of PLE spectra by using different monitoring wavelength have similar spectral profiles (Fig. S3 (ESI†)), which also give an evidence of energy transfer from Dy³⁺ to Eu³⁺.⁴⁶ Additionally, from Fig. S4 (ESI†) one can see that the emission intensity of Eu³⁺ at 706 nm is increased whereas that of Dy³⁺ at 580 nm is simultaneously found to be decreased monotonically with increasing Eu³⁺ concentration. These facts effectively prove the existence of ET between Dy³⁺ and Eu³⁺.

Fig. 6 (a) Spectral overlap of Dy³⁺ (donor) emission and Eu³⁺ (acceptor) absorption. (b) Excitation spectra of KCYP:3%Dy³⁺, KCYP:7%Eu³⁺, and KCYP:3%Dy³⁺, 7%Eu³⁺. (c) Decay curves (λ_ex = 346 nm, λ_em = 580 nm) of KCYP:3%Dy³⁺, n%Eu³⁺ (n = 0, 0.5, 1, 3, 5 and 7) measured at room temperature. (d) Dependence of the fluorescence lifetime and energy transfer (ET) efficiency on doped Eu³⁺ concentration.

To shed light on the ET process from Dy³⁺ to Eu³⁺ in KCYP host, the decay curves of the KCYP:3%Dy³⁺, n%Eu³⁺ (n = 0, 0.5, 1, 3, 5 and 7) samples were measured, the results are plotted in Fig. 6c and listed in Table S1 (ESI†). The excitation and monitored emission wavelengths are marked in the figure. All decay curves were found following the double exponential decay equation as below:^47,48

where τ₁ and τ₂ are short- and long-decay components, respectively, and parameters A₁ and A₂ are the fitting constants, respectively. Using the eqn (1), the average lifetimes (τ_ave) can be further evaluated by the equation:

The decay lifetimes against a function of Eu³⁺ concentration are determined in the order of 708, 646, 584, 543, 511 and 485 μs for n = 0, 0.5, 1, 3, 5 and 7, respectively. Fig. 6d shows that the fluorescence lifetime of Dy³⁺ decreases with increasing Eu³⁺ concentration because the energy absorbed by Dy³⁺ transfers to Eu³⁺, which is strong evidence for the ET process from Dy³⁺ to Eu³⁺. On account of the above results, Dy³⁺ acts a sensitizer to produce sensitized luminescence from Eu³⁺ in KCYP. The energy transfer efficiency can be defined by:

where τ₀ and τ are the luminescence lifetimes of the sensitizer (Dy³⁺) in the absence and presence of the activator (Eu³⁺), respectively. Thus, the relationship between the energy transfer efficiency and the activator concentration of Eu³⁺ ion can be obtained (Fig. 6d). As shown in Fig. 6d, the ET efficiency significantly increases with the increase of Eu³⁺ concentration, and the maximum value is approximately 31.5% at n = 7.

From all the above results, we can conclude that the ET process from Dy³⁺ to Eu³⁺ in the KCYP host indeed takes place. For the sake of better clarifying this process, the ET mechanism has been studied. Usually, the ET from a sensitizer to an activator may occur via two aspects: (1) exchange coupling and (2) electric multipolar interactions, which both belong to the resonant-type energy transfer. For the first model, the critical distance (R_c) between the sensitizer and activator should be smaller than 3–4 Å.^25,49 The R_c for ET from Dy³⁺ to Eu³⁺ ions can be calculated according to Blasse's equation:⁵⁰

where V is the volume of the unit cell, and Z is the number of cation sites in the unit cell. X_c is the total critical concentration of Dy³⁺ and Eu³⁺ ions. By using eqn (4), the R_c for Dy³⁺ and Eu³⁺ in KCYP is determined to be 17.07 Å. This value is much bigger than 3–4 Å, implying little possibility for ET through exchange coupling mechanism. Thus, we use Dexter's ET formula of multipolar and Reisfeld's approximation as expressed by:⁵¹

I_S0/I_S ∝ C^θ/3 (5)

where I_S0 and I_S represent the luminescence intensity of Dy³⁺ in the absence and presence of Eu³⁺, C represents the sum of the concentration of Dy³⁺ and Eu³⁺ ions, θ = 6, 8 or 10, corresponding to dipole–dipole (d–d), dipole–quadrupole (d–q), or quadrupole–quadrupole (q–q) interactions, respectively. The I_S0/I_S ∝ C^θ/3 plots are depicted in Fig. 7a–c. It is obvious that the commendably linear relationship between I_S0/I_S and C^6/3 is attained by comparing the fitting factors of R² values, which implies that the ET mechanism of Dy³⁺ → Eu³⁺ in KCYP arises from the electric d–d interactions. For a d–d ET process, rate of energy transfer probability (P) by means of lifetime is expressed as:³⁷where τ₀ is the intrinsic decay time of donor (Dy³⁺) in the absence of activator and τ is the lifetime of the donor in the presence of the acceptor (Eu³⁺). The P value is computed as a function of Eu³⁺ concentrations and is plotted in Fig. 7d. With increasing Eu³⁺ contents, the ET probability from Dy³⁺ to Eu³⁺ is found to enhance and reaches 65%.

Fig. 7 Dependence of I_S0/I_S of Dy³⁺ on C^6/3 (a), C^8/3 (b) and C^10/3 (c). The probability of energy transfer from Dy³⁺ to Eu³⁺ as a function of Eu³⁺ doping content (d). Quantum yield of KCYP:3%Dy³⁺, n%Eu³⁺ (n = 0.5, 1, 3, 5, 7, 9) excited at 346 nm (e). CIE 1931 chromaticity diagram of KCYP:Dy³⁺, Eu³⁺ nanophosphor upon 346 nm excitation (f).

For modern white LED applications, the quantum yield (QY) is a crucial parameter for LED phosphors, we therefore measure the QY values of KCYP:3%Dy³⁺, n%Eu³⁺ (n = 0.5, 1, 3, 5, 7 and 9) by integrating sphere principle. The QY is defined as the ratio of the number of emitted photons (I_em) to the number of absorbed photons (I_abs), and can be calculated by:^52–54

where E_S, E_R are the spectra of the excitation light with and without the sample in the integrating sphere, respectively, L_S is the luminescence emission spectrum of the sample in the integrating sphere. The excitation wavelength used for QY testing in all specimens is 346 nm. All of the recorded spectroscopic data are corrected with the correction files supplied by the manufacturer. Fig. 7e exhibits the dependences of QYs on Eu³⁺ dosage, from which one can find that the QY value is firstly strengthened with the increased Eu³⁺ concentration, reaching a maximum QY value of 29.8% at n = 7. Once the concentration of Eu³⁺ exceeds 7, a rapid decline in QY can be observed because of emission quenching appearance.^3,52 Hence, the optimal concentration of Eu³⁺ ion is 7 mol%, which is fairly in agreement with the aforementioned PL intensity analysis. We anticipate that the QY value of the as-prepared KCYP:Dy³⁺, Eu³⁺ nanophosphor can be further improved by synthetic process optimization.

The CIE 1931 chromaticity coordinates for KCYP:3%Dy³⁺, n%Eu³⁺ specimens were measured at 346 nm, the results are shown in Fig. 7f and summarized in Table S2 (ESI†). As illustrated in Fig. 7f, with the increasing Eu³⁺ concentration, the CIE coordinates of the specimens are varied from (0.354, 0.377) to (0.464, 0.366) because of the different emission components of the Dy³⁺ and Eu³⁺ resulting from the ET from Dy³⁺ to Eu³⁺ ions. Besides, from the CIE diagram of KCYP:Dy³⁺, Eu³⁺ phosphors, it is found that the trend of their color tones can tune from the edge of the white area to warm-white by regulating the content of Eu³⁺. Furthermore, it is well-known that the low color temperature is more popular in solid-state lighting. Thereby, the correlated color temperature (CCT) as one of the characteristics of phosphors are evaluated, the corresponding data are also tabulated in Table S2 (ESI†). It is seen that the CCTs of all the KCYP:Dy³⁺, Eu³⁺ nanophosphors are very small, and the CCT values pronouncedly decrease with the Eu³⁺ concentration changing from 0.5% to 9%, owing to the incremental red component from Eu³⁺ emission. Additionally, the color rendering index (R_a) is widely employed in lighting community to evaluate the lighting performance including white LEDs.^14,20 For the KCYP:3%Dy³⁺, 7%Eu³⁺ sample, its R_a value decided from the full set of eight CRIs (Table S3 (ESI†)) is obtained to be 81.6. These fascinating results suggest that single-component KCYP:Dy³⁺, Eu³⁺ nanoscale phosphor with tunable luminescence, low CCT and high CRI can display warmer white color when excited by a single excitation wavelength, which is positive to promote its potential warm-white LED applications.

3.4. Temperature-dependent PL and potential application for WLEDs

For application in high-powered WLEDs, thermal stability of a phosphor is another key issue to be considered since the luminescence intensity for most phosphors reduces if the operating temperature exceeds a certain value because of the temperature quenching effect.^55,56 Therefore, the thermal quenching properties of KCYP:Dy³⁺/Eu³⁺ upon 391 nm excitation were investigated. For the single Eu³⁺-doped KCYP sample (Fig. S5 (ESI†)), it is obvious that increasing the temperature from 30 to 210 °C leads to the decrease of the emission intensity while it does not change the emission positions. Similarly, the profiles of the PL spectra for the KCYP:3%Dy³⁺, 7%Eu³⁺ sample (Fig. 8a) are almost unchanged with the increase of temperature, suggesting that the thermal stability does not any affect by co-doping. Usually, phosphor chosen for LEDs must sustain long-term stable emission efficiency at temperature of about 150 °C.^57,58 Fig. 8b (black line) plots the dependence of the integrated PL intensity on the temperature. The thermal quenching temperature (T_0.5, the temperature at which the PL intensity decreases to 50% of T = 25 °C) is visibly higher than 220 °C. Furthermore, three rounds of the heating–cooling cycle experiments approve that the integrated PL intensity can be restored basically to its initial state, as depicted in Fig. 8b (red line and blue line), which in turn implies the potential application in WLEDs.

Fig. 8 (a) PL spectra (λ_ex = 391 nm) of KCYP:3%Dy³⁺, 7%Eu³⁺ at temperature between 30 and 210 °C. (b) The temperature dependence of the integrated emission intensity of KCYP:3%Dy³⁺, 7%Eu³⁺ in three cycles of heating–cooling processes. (c) Relationship of ln[(I₀/I(T)) − 1] versus 1/(k_BT) for the single-phased KCYP:3%Dy³⁺, 7%Eu³⁺ nanoscale phosphor, and the fitted activation energy for thermal quenching using the Arrhenius equation.

In order to better understand the temperature dependence of photoluminescence, the activation energy (E_a) for the thermal quenching of luminescence was calculated by the Arrhenius equation given as:^59,60

where I₀ is the initial emission intensity, T is the temperature in Kelvin, I(T) is the integral emission intensity at temperature T, A is a constant parameter, and k_B is the Boltzmann constant (8.617 × 10⁻⁵ eV K⁻¹). Fig. 8c depicts a plot of ln[(I₀/I(T)) − 1] versus 1/(k_BT). Through the best fit using the Arrhenius equation, E_a was obtained to be 0.174 eV for KCYP:3%Dy³⁺, 7%Eu³⁺. The activation energy is much higher than (or comparable to) some currently reported MCaLn(PO₄)₂ (M⁺ = Na, K; Ln³⁺ = Y, Gd)-based phosphors with excellent performance in Table S4 (ESI†), indicating that this material has high thermal stability. For the probability of a nonradiative per unit time (α), it can be expressed as:^5,42

α = se^{(−E_a/k_BT)} (9)

where s is frequency factor (s⁻¹). Since s, k_B, and T are constant, it is clear that the smaller E_a means larger α. These results further suggest that the KCYP:Dy³⁺, Eu³⁺ nanophosphor has better thermal quenching and degradation resistance, which is an essential condition for practical application.

4. Conclusions

To sum up, a series of new single-component KCYP:Dy³⁺, Eu³⁺ nanophosphors have been successfully synthesized by a low-cost rapid combustion reaction. The XRD, FTIR and Raman analyses indicate that Dy³⁺ and Eu³⁺ are totally incorporated into the KCYP host lattice. The as-synthesized phosphor shows spherical morphology with characteristic dimension of less than 50 nm. Investigations based on PLE and PL spectra reveal that the optimal doping contents of Dy³⁺ and Eu³⁺ ions are determined to be 3 mol% and 7 mol%, respectively. The emission color of KCYP:Dy³⁺, Eu³⁺ nanophosphor can be tunable from the edge of the white area to warmer white-light by modulation of Eu³⁺ concentration. The energy transfer process takes place from Dy³⁺ to Eu³⁺ via an electric dipole–dipole mechanism, and the transfer efficiency can be promoted by higher content of Eu³⁺. In addition, the KCYP:3%Dy³⁺, 7%Eu³⁺ nanosized phosphor possesses low CCT value of 2766 K and high R_a value of 81.6. The temperature dependence of the PL demonstrates that the quenching temperature (T_0.5) of KCYP:3%Dy³⁺, 7%Eu³⁺ nanophosphor is larger than 220 °C, and the activation energy is calculated to be 0.174 eV using the Arrhenius equation. And the experiments on heating and cooling cycles have approved the excellent resistance to thermal quenching and degradation. As discussed, it is believed that this new KCYP:Dy³⁺, Eu³⁺ can potentially act as a single-phased warm-white-light nanophosphor for application involving NUV excited warm-white LED industry.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No. 50902028, 51602077), Natural Science Foundation of Guangdong Province (Grant No. 2016A030313663, 2016A030311006), and Shenzhen Science and Technology Plan Supported Project (Grant No. JCYJ20140616172915497, JCYJ20150625142543479, JCYJ20160226201232552). We appreciate the help of Ms Ya-Nan Fan of Sun Yat-Sen University in performing the PL measurements.

References

1. H. Daicho, T. Iwasaki, K. Enomoto, Y. Sasaki, Y. Maeno, Y. Shinomiya, S. Aoyagi, E. Nishibori, M. Sakata, H. Sawa, S. Matsuishi and H. Hosono, Nat. Commun., 2012, 3, 1132–1141.
2. X. F. Li, J. D. Budai, F. Liu, J. Y. Howe, J. H. Zhang, X. J. Wang, Z. J. Gu, C. J. Sun, R. S. Meltzer and Z. W. Pan, Light: Sci. Appl., 2013, 2, E50–E59.
3. F. W. Kang, M. Y. Peng, Q. Y. Zhang and J. R. Qiu, Chem.–Eur. J., 2014, 20, 11522–11530.
4. N. Chouhan, C. C. Lin, S. F. Hu and R. S. Liu, J. Mater. Chem. C, 2015, 3, 1473–1479.
5. W. Z. Lv, Y. C. Jia, Q. Zhao, M. M. Jiao, B. Q. Shao, W. Lü and H. P. You, Adv. Opt. Mater., 2014, 2, 183–188.
6. R. J. Xie, N. Hirosaki, M. Mitomo, K. Takahashi and K. Sakuma, Appl. Phys. Lett., 2006, 88, 101104.
7. F. W. Kang, X. B. Yang, M. Y. Peng, L. Wondraczek, Z. J. Ma, Q. Y. Zhang and J. R. Qiu, J. Phys. Chem. C, 2014, 118, 7515–7522.
8. Y. C. Wu, D. Y. Wang, T. M. Chen, C. H. Lee, K. J. Chen and H. C. Kuo, ACS Appl. Mater. Interfaces, 2011, 3, 3195–3199.
9. H. M. Zhu, C. C. Lin, W. Q. Luo, S. T. Shu, Z. G. Liu, Y. S. Liu, J. T. Kong, E. Ma, Y. G. Cao, R. S. Liu and C. X. Yuan, Nat. Commun., 2014, 5, 4312–4321.
10. M. Y. Peng, X. W. Yin, P. A. Tanner, M. G. Brik and P. F. Li, Chem. Mater., 2015, 27, 2938–2945.
11. J. Zhao, C. F. Guo, T. Li, X. Y. Su, N. M. Zhang and J. Y. Chen, Dyes Pigm., 2016, 132, 159–166.
12. R. Y. Mi, C. L. Zhao and Z. G. Xia, J. Am. Ceram. Soc., 2014, 97, 1802–1808.
13. M. M. Jiao, Y. C. Jia, W. Lü, W. Z. Lv, Q. Zhao, B. Q. Shao and H. P. You, J. Mater. Chem. C, 2014, 2, 90–97.
14. S. P. Lee, C. H. Huang, T. S. Chan and T. M. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 7260–7267.
15. M. M. Shang, C. X. Li and J. Lin, Chem. Soc. Rev., 2014, 43, 1372–1386.
16. F. W. Kang, H. S. Zhang, L. Wondraczek, X. B. Yang, Y. Zhang, D. Y. Lei and M. Y. Peng, Chem. Mater., 2016, 28, 2692–2703.
17. P. L. You, Adv. Mater. Res., 2014, 919–921, 2052–2056.
18. C. F. Guo, X. Ding and Y. Xu, J. Am. Ceram. Soc., 2010, 93, 1708–1713.
19. W. R. Liu, C. H. Huang, C. W. Yeh, J. C. Tsai, Y. C. Chiu, Y. T. Yeh and R. S. Liu, Inorg. Chem., 2012, 51, 9636–9641.
20. P. P. Dai, C. Li, X. T. Zhang, J. Xu, X. Chen, X. L. Wang, Y. Jia, X. J. Wang and Y. C. Liu, Light: Sci. Appl., 2016, 5, E16024–E16032.
21. Z. J. Zhang, S. Chen, J. Wang, H. H. Chen, X. X. Yang, J. T. Zhao, Y. Tao and Y. Huang, Opt. Mater., 2009, 32, 99–103.
22. S. P. Puppalwar and S. J. Dhoble, Luminescence, 2015, 30, 745–750.
23. W. W. Wu and Z. G. Xia, RSC Adv., 2013, 3, 6051–6057.
24. D. Wang and Y. Wang, Mater. Sci. Eng., B, 2006, 133, 218–221.
25. T. M. Jiang, X. Yu, X. H. Xu, H. L. Yu, D. C. Zhou and J. B. Qiu, Mater. Res. Bull., 2014, 51, 80–84.
26. X. Y. Wu, Y. J. Liang, M. F. Zhang, M. H. Tong, D. Y. Yu, Y. L. Zhu, S. Q. Liu and C. J. Yan, J. Mater. Sci.: Mater. Electron., 2015, 26, 7324–7330.
27. X. G. Zhang, L. Y. Zhou, J. X. Shi and M. L. Gong, Mater. Lett., 2014, 137, 32–35.
28. R. E. Rojas-Hernandez, M. A. Rodriguez, F. Rubio-Marcos, A. Serrano and J. F. Fernandez, J. Mater. Chem. C, 2015, 3, 1268–1276.
29. S. P. Khatkar, V. B. Taxak, S. D. Han, J. Y. Park and D. Kumar, Mater. Chem. Phys., 2006, 98, 528–531.
30. R. S. Palaspagar, A. B. Gawande, R. P. Sonekar and S. K. Omanwar, J. Lumin., 2014, 154, 58–61.
31. J. J. Wu, S. K. Shi, X. L. Wang, J. B. Li, R. L. Zong and W. Chen, J. Mater. Chem. C, 2014, 2, 2786–2792.
32. R. D. Shannon, Acta Crystallogr., 1976, 32, 751–767.
33. W. Jastrzebski, M. Sitarz, M. Rokita and K. Bulat, Spectrochim. Acta, Part A, 2011, 79, 722–727.
34. S. L. Tie, Q. Su and Y. Q. Yu, J. Alloys Compd., 1995, 227, 1–4.
35. S. Mishra, R. Rajeswari, N. Vijayan, V. Shanker, M. K. Dalai, C. K. Jayasankar, S. S. Babu and D. Haranath, J. Mater. Chem. C, 2013, 1, 5849–5855.
36. L. J. Q. Maia, F. M. F. Filho, V. Jerez, A. L. Moura and C. B. Araújo, J. Mater. Chem. C, 2015, 3, 11689–11696.
37. H. X. Guan, G. X. Liu, J. X. Wang, X. T. Dong and W. S. Yu, New J. Chem., 2014, 38, 4901–4907.
38. Z. H. Leng, L. L. Li, Y. L. Liu, N. N. Zhang and S. C. Gan, J. Lumin., 2016, 173, 171–176.
39. D. Rajesh, K. Brahmachary, Y. C. Ratnakaram, N. Kiran, A. P. Baker and G. G. Wang, J. Alloys Compd., 2015, 646, 1096–1103.
40. Y. M. Deng, S. P. Yi, J. Huang, J. Q. Xian and W. R. Zhao, Mater. Res. Bull., 2014, 57, 85–90.
41. V. R. Bandi, B. K. Grandhe, H. J. Woo, K. W. Jang, D. S. Shin, S. S. Yi and J. H. Jeong, J. Alloys Compd., 2012, 538, 85–90.
42. F. W. Kang, Y. Zhang and M. Y. Peng, Inorg. Chem., 2015, 54, 1462–1473.
43. C. F. Guo, J. Yu, X. Ding, M. Li, Z. Y. Ren and J. T. Bai, J. Electrochem. Soc., 2011, 158, J42–J46.
44. F. W. Kang, M. Y. Peng, X. B. Yang, G. P. Dong, G. C. Nie, W. J. Liang, S. H. Shu and J. R. Qiu, J. Mater. Chem. C, 2014, 2, 6068–6076.
45. W. N. Xu, Q. W. Peng, J. H. Dong, Y. Gao, X. L. Liang, S. F. Wang and G. R. Chen, J. Am. Ceram. Soc., 2010, 93, 3064–3067.
46. M. L. Li, Z. G. Xia and Z. Y. Wang, Opt. Mater., 2014, 37, 446–450.
47. F. W. Kang, M. Y. Peng, S. H. Xu, Z. J. Ma, G. P. Dong and J. R. Qiu, Eur. J. Inorg. Chem., 2014, 8, 1373–1380.
48. F. W. Kang and M. Y. Peng, Dalton Trans., 2014, 43, 277–285.
49. N. Guo, W. Lü, Y. C. Jia, W. Z. Lv, Q. Zhao and H. P. You, ChemPhysChem, 2013, 14, 192–197.
50. G. Blasse, J. Solid State Chem., 1986, 62, 207–211.
51. Y. Liu, G. X. Liu, J. X. Wang, X. T. Dong and W. S. Yu, Inorg. Chem., 2014, 53, 11457–11466.
52. S. Leyre, E. Coutino-Gonzalez, J. J. Joos, J. Ryckaert, Y. Meuret, D. Poelman, P. F. Smet, G. Durinck, J. Hofkens, G. Deconinck and P. Hanselaer, Rev. Sci. Instrum., 2014, 85, 123115.
53. D. Cervantes-Vásquez, O. E. Contreras and G. A. Hirata, J. Lumin., 2013, 143, 226–232.
54. X. J. Zhang, L. Huang, F. J. Pan, M. M. Wu, J. Wang, Y. Chen and Q. Su, ACS Appl. Mater. Interfaces, 2014, 6, 2709–2717.
55. L. Chen, A. Q. Guo, Y. Zhang, F. Y. Liu, Y. Jiang, Q. S. Xu, X. H. Chen, Q. Z. Hu, S. F. Chen, K. J. Chen and H. C. Kuo, ACS Comb. Sci., 2012, 14, 636–644.
56. F. Baur, F. Glocker and T. Jüstel, J. Mater. Chem. C, 2015, 3, 2054–2064.
57. X. Y. Mi, H. Shi, Z. Wang, L. J. Xie, H. Y. Zhou, J. G. Su and J. Lin, J. Mater. Sci., 2016, 51, 3545–3554.
58. C. W. Yeh, W. T. Chen, R. S. Liu, S. F. Hu, H. S. Sheu, J. M. Chen and H. T. Hintzen, J. Am. Chem. Soc., 2012, 134, 14108–14117.
59. W. G. Xiao, X. Zhang, Z. D. Hao, G. H. Pan, Y. S. Luo, L. G. Zhang and J. H. Zhang, Inorg. Chem., 2015, 54, 3189–3195.
60. S. J. Gwak, P. Arunkumar and W. B. Im, J. Phys. Chem. C, 2014, 118, 2686–2692.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20912b

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