Shijie Qiu,
Hengwei Wei,
Mengmeng Wang,
Shuai Zhang,
Yang Zhou,
Ling Xu,
Xiaoming Wang* and
Huan Jiao*
Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi'an 710062, Shaanxi Province, P. R. China. E-mail: xmwang@snnu.edu.cn; jiaohuan@snnu.edu.cn
First published on 30th October 2017
A novel ternary-alkaline red emitting fluoride phosphor K2NaGaF6:Mn4+ was successfully synthesized through co-precipitation method. The crystal structure, morphology, electronic band structure and luminescence properties of K2NaGaF6:Mn4+ phosphors were investigated in details by using Rietveld refinement of X-ray diffraction data, scanning electron microscopy (SEM), density functional theory (DFT) calculation and different reaction parameters. The K2NaGaF6 host has a cubic unit cell with the space group Fmm and lattice parameters of a = 8.2577 (4) Å, Z = 4, Vcell = 563.08 (8) Å3. Under blue light excitation, Mn4+ activated K2NaGaF6 exhibits a bright narrow-band red emission. The PL properties of the K2NaGaF6:Mn4+ red phosphors were optimized with different Mn4+ concentrations and aging times. A warm-white LED device was fabricated using a blue LED chip combined with commercial yellow YAG:Ce3+ phosphor and synthesized K2NaGaF6:Mn4+ red phosphor. The color rendering index (CRI, Ra = 81.6) and corresponding color temperature (CCT = 3643 K) easily reached the commercial warm white light LED standards (Ra > 80 and CCT < 4000 K). All these results indicate that K2NaGaF6:Mn4+ phosphor would be a suitable red phosphor candidate for warm-white LED applications.
Recently, Mn4+ doped fluoride phosphors have been widely investigated due to their simple synthesis strategy and excellent luminescence properties.4,6,7 They have a strong broad absorption in the near UV and blue region which match well with the UV and blue LED chips and emit a series of narrow red emissions from 600 nm to 650 nm. Compared to other W-LEDs red phosphors, Mn4+ doped fluoride shows better chemical stability than sulfide phosphors, and simpler synthesis conditions and lower cost than nitride phosphors. Many series of fluoride phosphors such as A2MF6:Mn4+ (A = Na, K, Rb, Cs, M = Si, Ge, Sn, Zr, Ti),8–15 BMF6:Mn4+ (B = Mg, Ca, Ba, Zn, M = Si, Ge, Ti)16,17 and A3MF6:Mn4+ (A = Na, K, M = Al, Ga)18–22 have been reported. All of these Mn4+ activated fluorides phosphors exhibit intense broadband excitation and sharp red emission, which can be efficiently excited by near-UV or blue light and emit red light urgently required in warm W-LEDs.
Tuning phosphors emission bands is an important approach to improve the luminescence performance of W-LEDs. A usual way is to replace neighbour cations, changing the activator ions structure environment and making the luminescence properties change continuously. This strategy has been applied in many fluorides systems, for example, cubic elpasolite phosphors A2BLF6:Mn4+ (A: larger alkali ion (K), B: smaller alkali ion (Na, Li), L: trivalent cation (Al, Sc, Ga)),23–28 Mn4+ ions occupy the positions of Al3+, Sc3+, or Ga3+. Recently, Wang group reported two novel Mn4+-doped fluoride phosphors K2LiAlF6:Mn4+ and K2NaAlF6:Mn4+.24,29 These substitutions are mainly focused on the A site and B site. In order to improve the luminescent properties of fluoride phosphors, for example luminous intensity, thermal quenching effect, quantum efficiency and so on, more attempts could be tried on L site or the 3 cations combinations. With diverse cation radius, different structure and luminescent properties can be expected.
In this paper, we successfully synthesized a series of ternary-alkaline gallate fluoride red K2NaGaF6:Mn4+ phosphors through co-precipitation method at room-temperature. This phosphor shows pale yellow in daylight and emits a narrow-band red light under blue light excitation. The crystal structure, morphology, composition, electronic band structure and luminescence properties were investigated in details. In addition, the reaction parameters such as Mn4+ doping concentration and aging time have been investigated to optimize the photoluminescence properties. Finally, we fabricated a warm W-LED using a blue LED chip combined with a yellow YAG:Ce3+ phosphor and a K2NaGaF6:Mn4+ red phosphor, and the luminescence properties of this warm W-LED was obtained.
For optimization of the luminescence performance, different concentrations of Mn4+ doped K2NaGaF6 samples were synthesized using different mole ratios of Ga to Mn and the real doped concentrations has been measured, as shown in Table 1.
Samples | Moar ratio of Ga to Mn | Dopant amount of Mn (mol%) in K2NaGaF6 |
---|---|---|
S1 | 100:1.0 | 0.82 |
S2 | 100:1.5 | 1.33 |
S3 | 100:1.8 | 1.62 |
S4 | 100:2.0 | 1.76 |
S5 | 100:2.5 | 2.12 |
S6 | 100:3.0 | 2.67 |
S7 | 100:3.5 | 3.30 |
Density functional theory (DFT) calculation for K2NaGaF6 was performed with the Cambridge Serial Total Energy Package (CASTEP) code, in which a plane wave basis set was chosen for expansion of valence-electron wave functions at the local density approximation (LDA) level.30 There were two steps of calculations to get the band structure of K2NaGaF6. The first step was to optimize its crystal structure using the Broyden–Fletcher–Goldfarb–Shannon (BFGS) method. The second step was to calculate the band structure and density of states (DOS). In this calculation, the energy cutoff was set as 450 eV. Criterion for the self-consistent field (SCF) was eigenenergy convergence within 1.0 × 10−7 eV per atom.
The morphology and EDX mapping of samples was investigated by scanning electron microscopy (SEM, Philips-FEI Quanta 200, America) with an attached energy-dispersive X-ray spectrometer (EDS, INCA-Oxford, High Wycombe, UK).
The Mn4+ content in samples was measured on a flame atomic absorption spectrophotometer (FAAS) TAS-990F (Beijing Purkinje General Instrument Co., Ltd., China). PL spectra were acquired with a Hitachi F-4600 fluorescence spectrophotometer with the excitation and emission slits set to 2.5 nm, and the xenon lamp was used as excitation source. The diffuse reflectance ultraviolet-visible spectra (DRS) were collected on a UV-vis-NIR spectrophotometer UV-lambda 950 (PerkinElmer General Instrument Co., Ltd., America). The luminescence quantum efficiencies of obtained samples were measured using a quantum efficiencies measurement system C9920-02G (Hamamatsu, Japan).
Fig. 2(b) shows XRD patterns of K2NaGaF6, K2NaGaF6:Mn4+ as well as the simulated card of K2NaGaF6. With 1.76 mol% Mn4+ doping in K2NaGaF6, no impurity peaks were observed, which indicated the doped phosphor has a pure phase as the non-doped samples. Using K2NaAlF6 as a structure model, the Rietveld refinements based on powder XRD data indicated that the prepared K2NaGaF6 has the cubic unit cell with the space group Fmm and lattice parameters of a = 8.2577 (4) Å, Z = 4 and Vcell = 563.08 (8) Å3 (simulated) (in Table 2). The observed and calculated XRD patterns for K2NaGaF6, as well as difference profile are illustrated in Fig. 3. The refinement on XRD data shows a good fitting of residual factors Rp = 6.57%, Rwp = 8.54%, Rexp = 6.17%, and the GOF = 1.38 (Table 2).
Formula | K2NaGaF6 |
Crystal system | Cubic |
Space group | Fmm |
Lattice parameter a (Å) | 8.2577 (4) |
Vcell (Å3) | 563.08 (8) |
Z | 4 |
Temp. (K) | 293 |
Profile range (°) | 10 ≤ 2θ ≤ 80 |
Profile function | PVII |
No. of data points | 3501 |
Rp (%) | 6.57 |
Rwp (%) | 8.54 |
Rexp (%) | 6.17 |
GOF | 1.38 |
Fig. 3 Observed (blue dots) and calculated (red line) powder XRD patterns as well as difference profile (grey line) for Rietveld refinements of K2NaGaF6. |
The photoluminescence excitation (PLE) and photoluminescence (PL) spectra of red phosphor K2NaGaF6:Mn4+ (1.76 mol% Mn4+) are shown in Fig. 4(a). There are two excitation bands in PLE spectra, centred at 355 nm and 466 nm when monitored at 630 nm. The broad excitation bands are attributed to the spin-allowed and parity-forbidden transitions 4A2g → 4T1g and 4A2g → 4T2g of Mn4+.7,31
Under the excitation of 466 nm, the K2NaGaF6:Mn4+ phosphors emit a series of narrow red emission lines located at 598 nm, 607 nm, 613 nm, 622 nm, 630 nm, 633 nm and 646 nm, which can be assigned to the transitions of anti-Stokes v3(t1u), v4(t1u), and v6(t2u), zero phonon line (ZPL), and Stokes v6(t2u), v4(t1u) and v3(t1u) vibronic modes respectively.25,26 In Fig. 4(a), it is obvious that the ZPL of K2NaGaF6:Mn4+ appears significantly stronger compared to K2SiF6:Mn4+ or K2GeF6:Mn4+ phosphor. Normally, the emission intensity of the ZPL is highly dependent on the local symmetry of Mn4+ surrounding. But, in this work it is attributed to the damage of symmetry of Mn4+ ion in K2NaGaF6 because of non-equivalent doping between Ga3+ and Mn4+.21 The phosphor colour is pale yellow and it emits an intense red light under blue light and UV light, as shown in Fig. 4(b) and (c).
The X-ray photoelectron spectroscopy (XPS) spectrum of K2NaGaF6:Mn4+ is shown in Fig. 4(d). The XPS spectrum depicts the potassium (K), sodium (Na), gallium (Ga), fluoride (F), and manganese (Mn) elements. There were little oxygen (O) and carbon (C) observed due to absorption of CO2 or H2O. Since the low concentration of Mn4+, its peak is not so obvious. Hence, a magnification of Mn part of the spectrum is provided in the inset to show Mn element clearly.
Fig. 5 (a) Calculated energy band structure, (b) the total and partial density of states of K2NaGaF6. |
The UV-vis DRS of the representative non-doped K2NaGaF6 and Mn4+-doped K2NaGaF6 are displayed in Fig. 6. The slightly decreasing reflectance of non-doped K2NaGaF6 (black solid line) from 200 to 320 nm is the same as other reported fluoride phosphor.32–34
Fig. 6 DRS of the non-doped and Mn4+-doped K2NaGaF6 and PLE of K2NaGaF6:Mn4+ (1.76 mol%). The inset shows the Tauc plot for the non-doped sample. |
The band gap is estimated according to eqn (1).
(αhν)n = A(hν − Eg) | (1) |
In contrast with the non-doped host K2NaGaF6, the red phosphor K2NaGaF6:Mn4+ (1.76 mol%) sample has two intense absorption bands at ∼355 nm and ∼466 nm, which are due to the spin-allowed transition of 4A2g → 4T1g and 4A2g → 4T2g of Mn4+, respectively, as has also been clearly observed in the PLE spectrum. This result means that this phosphor can be effectively excited by blue InGaN chip, which shows great potential applications in W-LEDs.
Fig. 8 PL spectra of K2NaGaF6:Mn4+ with different doping concentration (a) and aging time (b). The inset in (a) and (b) show the integrated emission intensity of K2NaGaF6:Mn4+. |
It is well known that for phosphor particles, luminescence can be greatly affected by the crystallinity, surface defects, and doping concentration.36–39 For fluoride phosphors synthesized by co-precipitation method, particles are formed in a very short time. Therefore, a period of time is needed to modify the atom position and decrease surface defects. Aging as an effective way to obtain high crystallinity and low surface defect crystals is an important approach to optimize PL properties of fluoride phosphor. The K2NaGaF6:Mn4+ phosphors in HF solution were aged in ice-water to improve the crystallinity of the particles. Fig. 8(b) shows the emission spectra of K2NaGaF6:Mn4+ red phosphors obtained after different aging times. The emission intensity of K2NaGaF6:Mn4+ reaches a maximum after 4 h reaction. Then the emission intensity invariant as the aging time increases further. From Fig. S2,† it is clearly that the crystallinity of K2NaGaF6:Mn4+ particles can be improved by increasing the aging time. With the prolonging of aging time, the particle size and shape of K2NaGaF6:Mn4+ changed gradually. The particle size increased from 0.7 μm to 2 μm and the morphology transformed from sphere to octahedron gradually. The crystal size of K2NaGaF6:Mn4+ particles with less than 1 h aging time are much small, and their octahedron sharps are also not clear. With more than 4 h aging time, particles show higher crystallinity and better luminescence properties. The K2NaGaF6:Mn4+ particles are composed of octahedral shaped crystals featured by clear edges and corners. It is believed that the optimal reaction conditions to obtain the red light K2NaGaF6:Mn4+ are about 4 h aging time with 1.76 mol% Mn4+ concentration.
Fig. 9 shows the concentration-dependent PL decay curves of Mn4+ in K2NaGaF6:Mn4+ red phosphors under 466 nm blue light excitation, the relationship between Mn4+ doping concentration and lifetime was shown in inset. The PL decay time was fitted based on a single-exponential function. As is shown in Fig. 9, the lifetimes of Mn4+ decay from 4.24 ms to 2.35 ms along with increasing of Mn4+ concentration from 0.82 mol% to 3.3 mol%. However, when the doping concentration of Mn4+ more than 2.1 mol%, the PL decay curves deviated the single-exponential decay trend. That can be explained by the serious non-radiative transition processes among the Mn4+ ions at a high Mn4+ concentration level.
Fig. 9 Room temperature PL decay curves of K2NaGaF6 doped with different Mn4+ concentration. The inset shows the lifetimes of K2NaGaF6:Mn4+. |
Fig. 10 (a) Electroluminescence spectra and (b) CIE chromaticity diagram of fabricated white LED under various drive currents. |
From Fig. 10(a), it is obviously that there is no remarkable change on band shapes and positions of emission peaks when the drive current increases from 20 to 350 mA. Moreover, the emission intensity increased with the increasing driven current. All these results indicate this warm W-LED has a good stability in CRI and CCT. Photographs of the W-LED device are shown in the inset image of Fig. 10(a). Owing to the red light in the emission spectrum, a noticeable warm light emission can be observed. The chromaticity coordinates value under different currents are labelled in Fig. S3† and CIE chromaticity diagram Fig. 10(b). The Ra and CCT values of this W-LED are 81.6 and 3643 K respectively. Moreover, the colour tolerance adjustment (CTA) of this W-LED is 6.4 SDCM, which is very close to the commercial standard. All these results indicate the potential of K2NaGaF6:Mn4+ phosphor as red component for W-LEDs application.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra10274g |
This journal is © The Royal Society of Chemistry 2017 |