Phosphors

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Functional Inorganic Materials and Devices 3

6

4+

Highly Regular, Uniform KScF:Mn Phosphors: Facile Synthesis, Microstructures, Photo-luminescence Properties, and Application in LED Devices Hong Ming, Shuifu Liu, Lili Liu, Jiaqing Peng, Junxiang Fu, Fu Du, and Xinyu Ye ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01885 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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ACS Applied Materials & Interfaces

Highly Regular, Uniform K3ScF6:Mn4+ Phosphors: Facile Synthesis, Microstructures, Photo-luminescence Properties, and Application in LED Devices Hong Ming,†,§ Shuifu Liu,†,§ Lili Liu,†,§ Jiaqing Peng,†,§ Junxiang Fu,†,§ Fu Du,†,§ Xinyu Ye*,†,‡,§ †

School of Metallurgy and Chemistry Engineering, Jiangxi University of Science and

Technology, Ganzhou 341000, P.R.China ‡

National Engineering Research Center for Ionic Rare Earth, Ganzhou 341000,

P.R.China §

Key Laboratory of Rare Earth Luminescence Materials and Devices of Jiangxi

Province, Ganzhou 341000, P.R.China *

Corresponding author E-mail: [email protected]

KEYWORDS: red phosphor, morphology, K3ScF6:Mn4+, luminescence, warm white LEDs

ABSTRACT: A new generation of red phosphors of complex fluoride matrices activated with Mn4+, has gained a broad interest in getting high color quality, low color temperature of solid-state white light-emitting diodes(WLEDs). However, besides their instability towards moisture, the extremely irregular and nonuniform morphologies of these phosphors have limited their practical industry applications. In the present study, a novel type of K3ScF6:Mn4+ red phosphor with highly regular, uniform and high color purity was obtained successfully through a facile co-precipitation route under mild conditions. The crystal structure was identified with aids of the powder X-ray diffraction (XRD), Rietveld refinement, density functional theory (DFT) calculations. The prototype crystallizes in space group Fm3m with cubic structure and the lattice parameters are fitted well to be a = b = c = 8.4859(8) Å

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and V = 611.074(2) Å3. The Mn4+ ions occupy Sc3+ sites and locate at the centers of the distorted ScF6 octahedrons. A wide band gap of approximately 6.15 eV can provide sufficient space to accommodate impurity energy levels. Unlike most other Mn4+ ion activated fluoride phosphors, the as-prepared K3ScF6:Mn4+ phosphors demonstrate highly uniform and regular morphologies with shapes transforming from cube to octahedron with increasing Mn4+ ion concentration. Under blue light excitation, the as-prepared K3ScF6:Mn4+ sample exhibits intense sharp line red fluorescence (the strongest peak located at 631 nm) with high color purity. An excellent recovery in luminescence upon heating and cooling processes implies high stability of K3ScF6:Mn4+. Furthermore, a warm WLED fabricated with blue GaN chips merged with the mixture of K3ScF6:Mn4+ and the well-known commercial YAG:Ce3+ yellow phosphors exhibit wonderful color quality with lower CCT (3250 K) and higher CRI (Ra = 86.4). These results suggest that K3ScF6:Mn4+ phosphor possesses stupendous potentiality for practical applications.

INTRODUCTION Solid-state white light emitting diodes (WLEDs) is deemed to be the next-generation lighting source owing to their admirable properties, such as energy saving, high efficiency, long service life, and environmental friendliness, etc.1-4 Currently, the most popular WLED in the market is manufactured using an InGaN-based blue LED chip with the well-known commercial YAG:Ce3+ yellow phosphor.5-6 However, due to the innate absence of red light, this type of WLED


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suffers from a high correlated color temperature (CCT > 4500 K), a low color-rendering index (CRI, Ra < 80) which seriously limit its application in indoor lighting and panel display.7-8 To improve the optical performance, the red phosphor are suitable for compensating the red component of the WLEDs.9 Currently, the most successful red phosphors are Eu2+ doped nitride compounds, such as (Sr, Ca)2Si5N8:Eu2+ and CaAlSiN3:Eu2+.10-11 Eu2+ doped nitrides possessing brilliant luminescence properties and thermal stabilities are commercially available for warm WLED applications. However, the synthesis of rare-earth (RE)-doped nitrides requires expensive raw materials and rigorous synthetic conditions, such as high pressure and high temperature.12-14 Besides, serious reabsorption and too broad wide-band emission (FWHM of ~ 90 nm) make them unfavorable candidates for high quality products.15 Therefore, it is desirable to explore other favorable red phosphors with narrow-band red emission, dominant broadband excitation, and low re-absorption prepared via milder synthesis routes. Recently, the transition metal Mn4+ activated red phosphors, especially fluorides and oxides,16-17 has become a hot topic in the phosphor community because of the special 3d3 electron structure of Mn4+, which brings out the broad blue light absorption and sharp red line emissions. Noteworthily, the emission position of Mn4+ ion depends on the surrounding crystalline field to great extent.18 For instance, when the Mn4+ ions are situated in octahedral crystallographic sites in oxides, they usually exhibit a dominant broad emission peak at wavelengths over 650 nm, which is located outside the visual sensitivity range of human eye on account of the strong nephelauxetic effect and high covalency of the hosts.19 Comparison with the oxides, due to the weak nephelauxetic effect and high ionicity, Mn4+ activated fluoride phosphors present several narrow sharp emission lines ranging from 600 to 650 nm, demonstrating great prospect for improving the performance of WLEDs.20-21 To date, a series of Mn4+ doped fluoride phosphors composed of A2X(IV)F6:Mn4+ (A = K, Na, Rb, Cs or NH4; X = Si, Ge, Zr, Ti, Sn or Hf; A2 = Ba or Zn) and B2BX(III)F6:Mn4+ (B = Li, Na or K; X = Al or Ga) have been reported. For example, Adachi’s team successfully synthesized a variety of A2X(IV)F6:Mn4+ (A = K, Na, Cs or NH4; X = Si, ACS Paragon Plus Environment


Ge, Sn, or Ti) phosphors through etching the corresponding elementary substance X or oxide XO2 in AMnO4/AF/HF mixed solution.22-26 In 2014, Chen and Liu firstly prepared a red-emitting phosphor K2TiF6:Mn4+ with a high quantum efficiency up to 98%, through a cation exchange method in HF solution.27 Recently, Song et al. synthesized some novel red phosphors B2BAlF6:Mn4+ (B = Na, K) with good thermal stability and efficient luminescence, which was proved to enables their potential applications in warm WLED devices.28-29 In addition, using K2MnF6 instead of KMnO4 as a manganese source, Zhu et al. synthesized an K2LiGaF6:Mn4+ red nanophosphor with superior thermal stability and color stability by a facile cation-exchange method.30 Hence, it is reasonable to conclude that the Mn4+-doped ternary-alkaline fluorides may possess huge potential as red converters for WLEDs. Unfortunately, besides their instability towards moisture31-32, the extremely irregular, nonuniform morphologies of these phosphors have limited their practical industry applications.33 Therefore, it still remains a formidable challenge to obtain Mn4+ activated fluoride red emitting phosphor with regular and uniform morphology without extra post-treatment, which is highly desirable in industrial applications. In the present study, we give an insightful and meaningful investigation on the luminescence of K3ScF6:Mn4+ red phosphors. The novel K3ScF6:Mn4+ red phosphors were synthesized via a controllable and mild room-temperature co-precipitation method. The crystal structure, morphologies, elemental analysis and luminescent properties of the title K3ScF6:Mn4+ phosphors were totally investigated. The K3ScF6:Mn4+ phosphors demonstrate highly regular, uniform morphologies and strong sharp line red fluorescence (the strongest peak located at 631 nm) with high color purity. Meanwhile, to get intense red emission, the doping concentration of Mn4+ ions, reaction time as well as the reaction temperature, were totally optimized. A warm WLED was produced with the as-prepared K3ScF6:Mn4+ samples and the commercialized YAG:Ce3+ yellow phosphor. The experiment results indicate that K3ScF6:Mn4+ can be a promising red phosphor candidate for the application in LED devices.


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EXPERIMETAL SECTION 2.1 Raw materials All materials and reagents including KMnO4 (A.R.), H2O2 (30 wt.%), KHF2 (99.5%), Sc2O3 (99.9%), HF (40 wt.%), acetone (A.R.) and ethylalcohol (A.R.) were used as purchased without further purification. KHF2 and Sc2O3 were purchased from Shanghai Macklin Biochemical Technology Co., Ltd (China) and Shanghai Aladdin Biochemical Technology Co., Ltd (China), respectively, while other materials were supplied by Guangdong Xilong Scientific Co., Ltd (China). To ensure safety, it is necessary to equip some safeguard requirements (such as patch, glove, fume hoods, etc.) during the whole synthesis process. 2.2 Synthesis 2.2.1 Preparation of precursor K2MnF6 2.25 g KMnO4 (A.R.) and 45.0 g KHF2 (99.5%) were weighted and dissolved into 150 mL HF (40 wt.%) in a 250 mL Teflon beaker to form a deep purple mixture solution. After stirring for 30 min, 2 mL H2O2 (30 wt.%) solution was added drop by drop into the above mixture solution. Subsequently, the purple solution gradually turned brown, meanwhile, a yellow precipitate generated. The precipitate was collected by using a centrifuge, washed three times with acetone, and the K2MnF6 powder was finally obtained after oven-dried at 70 °C for 2 h. 2.2.2 Preparation of K3ScF6:Mn4+ In a typical procedure, the synthesis of K3ScF6:Mn4+ was achieved through using a co-precipitation method in a HF solution. 0.5516 g of Sc2O3 was weighed and poured into 15 ml HF solution with magnetic stirring for 30 min. And then, the precursor K2MnF6 (0.0198 g) was put into the solution under the vigorous magnetic stirring. After K2MnF6 completely dissolved, 18.744 g of KHF2 was put into the above mixture solution and the solution was kept to further stirred for 30 min. The red emitting fluoride K3ScF6:Mn4+ phosphor was gradually produced. Finally, the precipitate was also collected by using the centrifuge and washed several times with HF (10 wt.%) solution, and finally displaced by washing with ethanol, subsequently



dried at 70 °C for 3 h under vacuum to acquire the final product. The whole schematically illustrated experimental process of K3ScF6:Mn4+ is displayed in Figure 1. As shown in Figure 1a and 1b, the final obtained product is pale yellow and it emits a red emission light under the irradiation of UV light or blue light. For comparison and optimization of the luminescence, a series of samples were obtained by the same procedure and all the synthetic parameters were kept constant except for the examined variables.

Figure 1. Synthesis schematic diagram of K3ScF6:Mn4+ through a co-precipitation method, and digital images of the samples under (a) the natural light and (b) the 365 nm UV light irradiation, respectively.

2.3 Fabrication of LED devices The WLED devices were packaged based on the purchased commercialized yellow phosphor YAG:Ce3+ or the mixture of red phosphor K3ScF6:Mn4+ and yellow phosphor YAG:Ce3+ on the commercial 450 nm GaN chips. The phosphors were completely mixed with silicone and then coated on the surface of the InGaN-based light emitting diode chips. The solidification of devices was conducted at 135 °C for 2 h to generate solid-state WLEDs. 2.4 Characterization The X-ray powder diffraction (XRD) measurements were operated on a powder X-ray diffractometer (BRUKER D8 ADVANCED type) with Cu Kα radiation (λ =


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1.5418 Å) at a scanning rate of 10° min-1 in the 10-90° 2θ range. The Rietveld refinement was conducted on the General Structure Analysis System (GSAS) program, with the split Pseudo-Voigt profile function.34 The morphologies and elemental compositions of the as-prepared products were analyzed by MIRA3 LMH (TESCAN) scanning electron microscope (SEM) and a JEOL-2010 transmission electron microscope (TEM) equipped with the energy dispersive X-ray spectrum (EDS) as well as the selected area electron diffraction (SEAD). The doping amount of Mn4+ ions in samples were verified by ICP-AES (Optima 8300, PerkinElmer). The photoluminescence excitation (PLE) and photoluminescence (PL) spectra were collected using a Hitachi F-7000 fluorescence spectrometer equipped a 450 W xenon lamp as the light source. The luminescence decay curves were performed with an FLS980 (Edinburgh) spectrometer. The quantum efficiency (QE) of the sample was measured using an integrating sphere coated with barium sulfate, which is attached to FLS980 spectrometer. The temperature dependent luminescence properties were monitored by an exciting spectra and thermal quenching analyzed system (EX-1000). The luminescent spectrum, CCT, Ra, and CIE value of the LEDs, were measured by a light and radiation measuring instrument (HAAS-2000, Everfine, China).

RESULTS AND DISCUSSION 3.1 Phase identification and structure determination By increasing the molar ratio of KHF2/Sc2O3 (6, 12, 18, 24, 30, 36, 42), we achieved the crystalline phase transformations from ScF3 to KSc2F7 to K3ScF6. Figure 2a shows the XRD patterns of Mn4+ activated samples that were prepared with various molar ratios of KHF2 to Sc2O3. All the samples are prepared in 15 ml HF (40 wt.%) and all the synthetic parameters were kept identical except for the molar ratio of KHF2 : Sc2O3. When the molar ratio of KHF2 : Sc2O3 is small (6 : 1), all diffraction peaks can be well indexed to the normal XRD pattern of ScF3 (JCPDS 85-1078) and no other peaks can be found. Afterwards, a mixture composed of dominant ScF3 (JCPDS 85-1078) and a small quantity of KSc2F7 (JCPDS 77-1321) phase is obtained when the molar ratio of KHF2 : Sc2O3 increases to 12 : 1. With the proportion of



KHF2 further increasing, the phase of KSc2F7 increases in the mixture. When the molar ratio of KHF2 : Sc2O3 increased to a higher ratio of 24 : 1, the diffraction peaks from ScF3 fully disappear and some extra peaks from the K3ScF6 structure can be detected besides the characteristic diffraction peaks of KSc2F7. Due to the lack of standard pattern of K3ScF6 compound, the existing XRD pattern of K2NaScF6 (JCPDS 79-0770) was used in consideration of the characteristics of measured patterns are in accordance with cubic structure. With the mole ratio further increasing from 30 : 1 to 42 : 1, all the diffraction peaks can be well indexed into K3ScF6, suggesting that a pure-phase K3ScF6:Mn4+ was achieved. The enlarged XRD patterns in the vicinity of 2θ = 30° are shown in Figure 2b. Through zooming in the XRD pattern, it is found that the (220) diffraction peak shifts towards smaller angle. In accordance with the Bragg’s law 2dsinθ = nλ (where d, θ and λ represent the interplanar distance, diffraction angle, and X-ray wavelength, respectively), the lattice expands and interplanar distance increases, which results in the smaller diffraction angle θ when Na+ ions (r = 1.02 Å) are replaced by K+ ions (r = 1.38 Å). This phenomenon confirms the formation of K3ScF6 phase.

Figure 2. (a) XRD patterns of samples prepared with various molar ratios of KHF2/Sc2O3, (b) The corresponding XRD patterns of zoomed (220) diffraction peaks.

To investigate the subtle differences in the structure of K3ScF6:Mn4+, the Rietveld refinement of X-ray powder diffraction data was conducted. The Rietveld


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refinement was carried out on the GSAS program. The crystallographic data and initial structural model were originated from K2NaScF6 (JCPDS 79-0770) compound, which is similar to the real structure of K3ScF6. The pattern and refinement results are shown in Figure 3, and the corresponding crystal structure parameters as a result of refinement are shown in Table 1. The crosses, red lines, green lines, blue lines and purple bars in Figure 3 represent raw data, calculated values, background, differences, and Bragg positions, respectively. The observed XRD patterns of K3ScF6:Mn4+ match well with the calculated one, confirming the phase purity. The results of refinement demonstrate that the R factors finally converged to RWP = 7.15% and RP = 5.45% for K3ScF6:Mn4+, indicating that the real structure agrees well with the initial structural. The lattice parameters are fitted well to be a = b = c = 8.4859(8) Å and V = 611.074(2) Å3. The lattice parameters of K3ScF6:Mn4+ and K2NaScF6 are listed in Table 2. Obviously, the lattice parameters of K3ScF6 are a little bigger than those of K2NaScF6. The results further indicate that K3ScF6 phase has been synthesized successfully. When K+ (r = 1.38 Å) replace Na+ (r = 1.02 Å), the cell lattice expands, resulting in the lattice parameters and unit cell volume getting larger. This is consistent with the phenomenon that the (220) diffraction peak is shifted to a small angle.

Figure 3. The Rietveld refinement of the observed XRD pattern for K3ScF6:Mn4+.



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Table 1. The Refined structure parameters of K3ScF6:Mn4+ sample. Atom

Site

x

y

z

Occ.

Uiso (Å2)

K1

8c

0.25

0.25

0.25

1

0.0395(6)

K2

4b

0.50

0.50

0.50

1

0.0251(11)

Sc

4a

0

0

0

0.9902(8)

0.0169(6)

Mn

4a

0

0

0

0.0097(5)

0.0466(7)

F

24e

0.2324(2)

0

0

1

0.0208(5)

Table 2. Crystallographic data of K3ScF6:Mn4+ (derived from the Rietveld structure analysis of XRD pattern) and K2NaScF6. Formula

K3ScF6:Mn4+

K2NaScF6

Crystal system

Cubic

Cubic

Space group

Fm3m

Fm3m

a, Å

8.4859(8)

8.47

b, Å

8.4859(8)

8.47

c, Å

8.4859(8)

8.47

Vcell (Å3)

611.074(2)

608.01

Z

4

4

RP

7.15%

RWP

5.45%

χ2

2.46

The structure scheme of K3ScF6, and coordination environments of the Sc, K(1) and K(2) cations are presented in Figure 4. The crystal structure is plotted from CIF file, which was generated by the GSAS Rietveld refinement, and read by Diamond software (Version 3.2). In K3ScF6 crystal structure, the K+ cations can occupy two different crystallographic sites named K(1) and K(2), respectively. Each Sc3+ ion or K(1) ion is situated in the center of the regular octahedron with 6-fold coordination by


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F- ions to form two different octahedra types. The common corners link ScF6 and KF6 two octahedra into a network, wherein the K(2) ions occupy 12-coordinated cavities to forming a [KF12] decahedral cage. Due to that the effective ionic radius of Mn4+ (r = 0.53 Å) at CN = 6 approaches to that of Sc3+ (r = 0.745 Å), the Mn4+ ions will replace Sc3+ and occupy the centers of the ScF6 octahedrons.

Figure 4. The crystal structure diagram of K3ScF6 as well as the coordination environments of the Sc, K(1) and K(2) cations.

The first principle calculation based on density functional theory (DFT) was carried out using the VASP codes35 to investigate the lattice structure and electronic structure of K3ScF6. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof

(PBE)

functional36

was

employed.

The

electronic

configurations were chosen as follows: 2s22p5 for F, 3s23p64s1 for K and 3p64s23d1 for Sc. The plane-wave basis energy cutoff was set as 550 eV, an d the k-points mesh generated by the Monkhorst-Pack scheme37 was set as 5×5×5. The convergence criteria for ionic and electronic relaxation were 1×10-3 eV and 1×10-5 eV, respectively. The DFT calculation shows that the optimized crystal structure is the same as the Rietveld refinement result. However, the calculated lattice constants were 9.10 Å, about +7% larger than the Rietveld refined structure of K3ScF6:Mn4+. The difference may be explained by the calculation error and the larger Sc3+ ionic radius (r = 0.745 Å) compared with that of Mn4+ (r = 0.53 Å).



The calculated band structure of the relaxed K3ScF6 unit cell is shown in Figure 5a. It is obvious that K3ScF6 exhibits a direct band gap of approximately 6.15 eV at Gamma center. The actual band gap can reach above 8 eV since the band gaps are always underestimated by DFT method. This wide band gap can provide sufficient space to accommodate impurity energy levels, indicating that K3ScF6 is a good luminescence host. The valence band is narrow and highly flat, while the conduction band is remarkably dispersed. Figure 5b shows the total and partial density of states of K3ScF6, which can help understand the electronic band composition. The conduction band mainly consists of 3d states of Sc and a little 2p states of F. The narrow and flat valence band is formed by F 2p states, spreading from -2 to 0 eV. Other deeply located bands come from K 3p states (around -10 eV), F 2s states (-19 eV), Sc 3p states (-25 eV), and K s states (-27 ~ -25 eV).

Figure 5. (a) Calculated band structure and (b) calculated total and partial density of states of K3ScF6.

3.2 Morphology and composition analysis The scanning electron microscope (SEM) images in Figure 6a-6h show K3ScF6:Mn4+ with the various Mn4+ doping concentrations obtained at 80 °C for 0.25 h. The morphologies of the as-prepared samples exhibit well dispersed particles with smooth surfaces, corners, and clear edges. It is also observed that the morphologies and sizes of K3ScF6:Mn4+ particles are strongly dependent on the Mn4+ doping concentration. As the nominal molar doping concentration of Mn4+ increases from 0.3


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to 6%, the size of K3ScF6:Mn4+ particles gradually decreased from 1µm to 500 nm. Closely viewing their particle morphologies from Figure 6a-6h, a morphology evolution from cube to octahedron can be observed in Figure 7. When the Mn4+ doping concentration is lower than or equal to 1%, the morphologies of the as-prepared samples are cube with exposed 6 {100} facets (gray areas). As the Mn4+ doping concentration is increased to 1.5%, a truncated octahedron with exposed 6 large {100} facets (gray areas) and 8 small {111} facets (blue areas) is observed. The void spaces of some truncated octahedron particles are attributed to the rapid growth of the outer crystal planes. Upon a further increase of the Mn4+ doping concentration, the area of {100} facets gradually decrease while the area of {111} facets gradually expands. When the Mn4+ doping concentration gradually rises to 4% or higher than 4%, the {100} facets fully disappear and a clear-cut octahedron with exposed 8 {111} facets (blue areas) can be observed. In this case, it seems that Mn4+ is significant for the control of seeds formation and the growth rates of different crystallographic planes to form the K3ScF6:Mn4+ particles from cube to octahedron. The crystal shape-evolution is basically driven by the continuous decrease of the total surface energy, and eventually ceases at the point of minimum surface energy under the given growth

condition.

In

accordance

with

Gibbs-Wulff’s

theorem,

in

cubic

crystallographic system, the crystal facets possessing higher surface energies would finally be subjected to reduction or disappearance from the final appearance, particularly for the high-index facets.38 Consequently, both the low-index facets {100} and {111} maintain in final appearance while the high-index facets will not keep under the conditions of equilibrium growth without capping agent.39 During the process of crystal growth, the inorganic additives including ions and molecules will make the relative order of surface energies change as they selectively adsorb and stabilize onto a certain crystallographic facet.40 Therefore, the selective adsorption will decrease surface energy of the corresponding bound facet and impede the growth of crystal along the normal directions, thereby forming a non-equilibrium Wulff construction.41 In the current solution-phase synthesis route of K3ScF6 crystal, Mn4+ ions serve as both activators and impurity additives. The Mn4+ ions near crystal ACS Paragon Plus Environment


surface are consumed by the nucleation and growth of K3ScF6 crystal. When the Mn4+ doping concentration is high in solution, the residual Mn4+ ions selectively adsorb onto the {111} facets of the grown K3ScF6 crystal, which would reduce the surface energy of the {111} bound facets and hinder K3ScF6 crystal growth along its normal directions, leading to the formation of octahedron. In addition, considering the unequal valence state and different ion radius in Sc3+ and Mn4+, the total surface energy of the lattice surface atoms may be increased after doping. To minimized the anisotropic surface free energy of the K3ScF6 crystal, the {111} crystal facets with lower surface energies eventually maintained.

Figure 6. SEM images for samples with varying Mn4+ doping amount: (a) 0.3%, (b) 0.5%, (c) 0.7%, (d) 1%, (e) 1.5%, (f) 2%, (g) 4%, (h) 6%.


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Figure 7. The morphological evolution of K3ScF6:Mn4+ samples from cubic to octahedral form by varying Mn4+ doping concentrations. Gray surfaces represent {100} crystal faces, blue surfaces represent {111} crystal faces.

Figure 8a shows the corresponding EDS spectrum of the as-synthesized K3ScF6:Mn4+ sample. The detected C, O and Au elements of the sample are mainly from the conducting resin and metal spraying. The obtained red phosphor K3ScF6:Mn4+ is composed of F, Sc, K, and Mn elements. In addition, the atom ratios of K, Sc, F are about 27.17%, 8.24%, 54.19% respectively, which is close to 3 : 1 : 6 ratio of K3ScF6. These results confirm that Mn4+ has been truly doped into the Sc lattice site. Morphological observation by the TEM is also shown in Figure 8b. The TEM image of the sample indicates the particle is octahedron, which accords well with the SEM result of K3ScF6:6%Mn4+. The SAED pattern from K3ScF6:Mn4+ sample, as shown in Figure 8c, exhibited the single crystalline nature, in which the visible diffraction dots could be well indexed to the cubic phase of K3ScF6. In combination with high resolution TEM image (HRTEM) (see Figure 8d), it can be clearly seen that the lattice fringes show the imaging characteristics in which the cubic structure K3ScF6 crystal where the interplanar spacing of 0.299 nm corresponds to the distance of the (220) planes. The above results indicated that the pure and ultrafine K3ScF6:Mn4+ particles were successfully obtained.



Figure 8. (a) EDS spectrum, (b) TEM images, (c) SAED pattern and (d) HRTEM image of K3ScF6:Mn4+ sample.

3.3 Optical properties of K3ScF6:Mn4+ To investigate the luminescence properties of Mn4+ in K3ScF6, we choose the K3ScF6:0.3%Mn4+ samples by co-precipitation at 40 °C for 0.25 h as an example. Figure 9a shows the excitation and emission spectra of K3ScF6:Mn4+ monitored at 631 nm. There are three broad excitation bands in its excitation spectrum at ~ 247, ~ 367 and ~ 470 nm, respectively. The first one can be ascribed to the charge transfer band (CTB), the other two bands (wavelength ranges: 320-410 nm and 420-520 nm) peaked at 367 nm and 470 nm are originated from the 4A2g → 4T1g and 4A2g → 4T2g spin-allowed transitions of Mn4+, respectively.22, 42, 43 The excitation band intensity peaked at 470 nm is far higher than that of 367 nm, indicating the excitation of K3ScF6:Mn4+ fits well with the commercialized blue light emitting LED chip and the as-prepared phosphors possess a promising application for WLEDs. Under 470 nm light excitation, the emission spectrum of the K3ScF6:Mn4+ phosphor is composed of a series of narrow-band emissions from 590 nm to 660 nm with the strongest peak at ~631 nm, which are attributed to the spin forbidden transitions 2Eg → 4A2g of Mn4+.44 As shown in Figure 9b, all of the emission peaks centered at ∼599, ∼610, ∼614,

∼623, ∼631, ∼635 and ∼648 nm, which can be ascribed to the anti-Stokes ν3(t1u),


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ν4(t1u), ν6(t2u), zero phonon line (ZPL) and Stokes ν6(t2u), ν4(t1u) and ν3(t1u) vibronic modes, respectively.28 Additionally, K3ScF6:Mn4+ phosphor emits an strong ZPL at room temperature, which is similar to those of the Mn4+ non-equivalent doped fluoride phosphors.29, 45 As a matter of fact, the emitted intensity of ZPL is greatly determined by the local symmetry of Mn4+ surroundings,46 and the ZPL would be stronger as the substituted sites in host lattice was lower symmetry.47 In this work, due to the unequal valence state and ion radius difference between Sc3+ and Mn4+, the symmetry of Mn4+ ion in K3ScF6 were damaged, which may be responsible for the strong zero-phonon emission phenomenon.48 As shown in Figure 9c, the fluorescence decay behavior of K3ScF6:0.3%Mn4+ red phosphor was monitored at 631 nm by 470 nm excitation. The decay curve could be fitted well with a single exponential function as below:49

= + ∕ 1 Where I0 and I(t) stand for the luminescence intensities at time t0 and t, respectively, A is constant, τ is the lifetime for exponential components, which could be easily calculated by fitting the decay curves. The fluorescence lifetime value is about 3.46 ms. The CIE chromaticity coordinates of K3ScF6:0.3%Mn4+ are (0.6938, 0.3061) as indicated in Figure 9d. Furthermore, the strongest peak located at 631 nm is very close to the red (~ 630 nm) proposed by the International Telecommunication Union for ultra-high definition television.50 In addition, the FWHM of the emission band of this phosphor was determined to be ~ 26 nm, which is much narrower than that of the popular commercial CaAlSiN3:Eu2+ red phosphor (~ 90 nm).51 Therefore, this phosphor can produce a very high red purity. The QE is another crucial parameter to assess the performance of a luminescent material. We measured the K3ScF6:Mn4+ sample using an integrating sphere coated with barium sulfate at 25 °C. The internal QE is defined by using the following equation:48

=

2 −



Where LS represents the emission spectrum of the studied K3ScF6:Mn4+ product, and ES and ER are the excitation spectra of the excitation light used for exciting the product and without the product in the integrating sphere, respectively. Based on the above equation, the internal QE value of K3ScF6:Mn4+ under excitation at 470 nm was determined to be 67.18%.

Figure 9. (a) PL excitation (blue) and emission (red) spectra, (b) emission spectra of 590 nm to 660 nm, (c) decay curve and (d) CIE chromaticity coordinate of K3ScF6:0.3%Mn4+.

As we know, the synthesis conditions play a considerable role on the luminescent properties of phosphors. To obtain better luminescence, we systematically studied the effects of doped concentration of Mn4+ ions (Table 3), reaction time and reaction temperature on the luminescent properties. Figure 10a presents the XRD patterns of K3ScF6:Mn4+ phosphors doped with different concentrations of Mn4+. All diffraction peaks agree well with the standard card (JCPDS 79-0770). The results indicate that the Mn4+ ions entered in the host lattice of K3ScF6 without altering the crystal


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structure, which is due to low concentration of dopant. Figure 10b demonstrates the emission spectra of all samples with various concentrations of K2MnF6. It is observed that all of them have the same spectral features of the Mn4+ ion except for the fluorescence intensity, as shown Figure 9b. The inset in Figure 10b shows the relationship between the integrated PL intensity and Mn4+ concentration. The PL intensity monotonically increases with the increasing of doping amount of Mn4+ and reaches the maximum at 0.97%. Then, the PL intensity decrease with the increase of actual doping amount of Mn4+ owing to the effect of concentration quenching.

Table 3. ICP results of K3ScF6:Mn4+ phosphors synthesized with various mole ratios of Sc2O3 to K2MnF6. Samples

Molar ratio of Sc2O3 to K2MnF6

Actual doping amount of Mn4+ (mol%)

1

100 : 0.3

0.34

2

100 : 0.5

0.53

3

100 : 0.7

0.68

4

100 : 1.0

0.97

5

100 : 2.0

1.88

6

100 : 4.0

3.46

7

100 : 6.0

5.17

Generally, concentration quenching might be because of the occurrence of energy transfer within the neighboring Mn4+, which eventually transfer to quenching sites or traps. According to the literatures,52 the energy transfer mechanism for Mn4+ ions has three modes, namely, radiation re-absorption, exchange interaction and multipolar interactions. It is clear that the mechanism of energy transfer is not caused by radiation re-absorption owing to no overlap between the PLE and PL spectra of K3ScF6:Mn4+. Therefore, it may be in connection with the dipole-dipole interaction or the exchange interaction. To distinguish the point, the critical distance Rc between the adjacent Mn4+ ions was calculated based on the K3ScF6:Mn4+ following formula:53



&( '

3 = 2 × % 4"# $

3

Where V means the unit cell volume of K3ScF6, xc means the critical concentration of Mn4+, N means the number of available sites for the dopant in a host lattice. For the K3ScF6 host, the value of V and N are 611.07 Å3 and 4 in the crystal lattice. Due to 0.97% is the quenching concentration in K3ScF6:Mn4+ phosphor, we supposed that the critical concentration xc is 0.97%, the critical distance of Rc is calculated to be 31.09 Å. As we all know, only when Rc is less than 5 Å, the exchange interaction could happen.54 The above calculated results of Rc are much larger than 5 Å, hence the exchange interaction is not very important in the energy transfer among the Mn4+ ions. Therefore, the mechanism of non-radiative energy transfer for Mn4+ ions seems to be ascribed to a dominant multipolar interaction. According to the Dexter theory, the interaction type between Mn4+ ions in K3ScF6 can be estimated via the following equation:55 , & = ) *1 + +# ' - 4 # Where I and x represent the integrated emission intensity and activator concentration,

respectively. θ represents the interaction type, and K and β represent two constants. If θ equals to 6, 8 and 10, the values are in correspondence with dipole-dipole (d-d), dipole-quadrupole (d-q), and quadrupole-quadrupole (q-q) interactions, respectively. The Eqn (4) can be approximately predigested to be the following:56

1 ./0 % = − ./0 # + 5 # 3 As shown in Figure 10c, the values of log(I/x) as a function on the log(x) with a slope of (-θ/3) are plotted. A liner fitting could be obtained and the slope is about 1.75. The corresponding θ is 5.25 and close to 6. Therefore, the concentration quenching of Mn4+ in K3ScF6 host is basically owing to the d-d interaction.


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Figure 10. (a) XRD patterns, (b) emission spectra of K3ScF6:Mn4+ products with different concentrations of Mn4+ under 470 nm light excitation, the inset photograph represents the dependence integrated emission intensity on Mn4+ doping amount, (c) the relationship plot between log(I/x) versus log(x) of Mn4+ doping amount x, (d) decay curves of red emission at 631 nm of the K3ScF6:Mn4+ products doped with different amount of Mn4+.

To further prove the mechanism of concentration quenching, the luminescence decay curves of K3ScF6:Mn4+ samples excited by 470 nm depicted in Figure 10d. All of them could be fitted well with the single exponential function equation (Eqn 1). We can see that the decay lifetimes of Mn4+ gradually decrease from 3.46 ms to 1.26 ms when the doping amount of Mn4+ is increased from 0.34 mol% to 5.17 mol%. This phenomenon indicates an extra energy decay process is active. The increase of Mn4+ doping concentration would decrease the distance between Mn4+-Mn4+ and thereby result in the increase of both energy transfer rate between Mn4+-Mn4+ and the probability of energy transfer to luminescent killer sites. Consequently, the larger the



Mn4+ amount, the shorter the lifetime is. In addition to the doping concentration of Mn4+ ions, the influence of reaction temperature and reaction time on the luminescence of K3ScF6:Mn4+ was also studied. The XRD patterns of K3ScF6:Mn4+ samples at various reaction temperatures are shown in Figure 11a. All samples were confirmed to be pure K3ScF6 phase. Increasing the reaction temperature did not change the phase purity of K3ScF6:Mn4+ products. The emission spectra of K3ScF6:Mn4+ prepared at different reaction temperatures are shown in Figure 11b. It is observed that all the PL spectra of K3ScF6:Mn4+ are nearly identical except the luminescent intensity. As shown in Figure 11b (inset), the emission intensity of K3ScF6:Mn4+ elevates with increasing reaction temperature and reach a maximum value at 60 °C, indicating that the proper reaction temperature is beneficial for Mn4+ into the lattice to substitute Sc3+ sites. As the reaction temperature further increases, the emission intensity of K3ScF6:Mn4+ gradually decreases. In the K3ScF6:Mn4+ synthesis process, we found that the solution will change from yellow to baby pink when the reaction temperature exceeds 80 °C. This phenomenon shows the Mn4+ is not stable at high temperature which causes a decrease in emission intensity. Figure 11c displays the XRD patterns of the K3ScF6:Mn4+ products generated at different reaction times. All involved patterns match well with the standard data of K2NaScF6 (JCPDS 79-0770). The powders prepared in different times are pure crystalline phase without impurity. As shown in Figure 11d and the illustration in the upper left corner, the emission intensity of K3ScF6:Mn4+ increases firstly with increasing the reaction time and reach a maximum at t = 2 h, then the luminescent intensity decreases obviously when the reaction time further increases. The emission intensity descends with the extended reaction time from 2 to 8 h may have two reasons: (Ⅰ) the Mn4+ ion is not stable in solution for such a long time; (Ⅰ) fluorescence quenching.


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Figure 11. (a) XRD patterns, (b) emission spectra (λex = 470 nm) of K3ScF6:1%Mn4+ products obtained at 20 °C, 40 °C, 60 °C, 80 °C, the inset photograph represents the dependence of emission intensity of reaction temperature; (c) XRD patterns and (d) emission spectra (λex = 470 nm) of K3ScF6:1%Mn4+ products prepared at 60 °C for t = 0.25 h, 1 h, 2 h, 4 h, 6 h, 8 h, the inset photograph represents the dependence emission intensity of reaction time.

3.4 Temperature stability properties Thermal stability is a significant parameter of phosphor for WLEDs. In order to investigate the emission features of the K3ScF6:Mn4+ phosphor above room temperature, its thermal stability was discussed through a cyclic process of heating and cooling among 298 to 498 K (see Figure 12a). All emission peaks located at the same positions with maximum emission peak at 631 nm, there are not any apparent band shifts in the whole cycle process. In addition, it is clearly that the emission intensity gradually decreases as the measurement temperature increase because of the marked non-radiative transition in the course of heating. Figure 12b displays the



integrated emission intensity trend of K3ScF6:Mn4+ in heating and cooling process, respectively. The emission intensity decrease is probably due to the considerable ionic radii difference between Sc3+ (r = 0.745 Å) and Mn4+ (r = 0.53 Å).33 However, after cooling back to 298 K, the emission intensity of K3ScF6:Mn4+ nearly recovers to the initial level, therefore, the “quenching” is invertible excluding the random errors of measurement. As a result, the experiments reveal no obvious thermal degradation for K3ScF6:Mn4+ phosphors. The rationale of thermal quenching can be illustrated by the configuration coordinate diagram, as shown in Figure 12c. Under 470nm blue light and 367nm near ultraviolet light irradiation, the electrons of Mn4+ in the ground state 4

A2g firstly absorb energy and climb to the excited levels 4T2g and 4T1g, then they relax

to the lower intermediate state 2Eg through non-radiative processes. The electrons in 2

Eg intermediate level are populated through its hot bands. Subsequently, the red

emissions of Mn4+ are procured by its spin-forbidden 2Eg → 4A2g transitions. However, as the temperature increases, the thermal excitation from 2T1g occurs, a non-radiative energy migrating channel maybe produce via the crossover point of the states of 4T2g and 4A2g, which results in thermal quenching.


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Figure 12. (a) Temperature-dependent emission spectra of K3ScF6:Mn4+ from 298 K to 498 K, (b) temperature-dependent relative integrated emission intensity of K3ScF6:Mn4+ sample in heating (black curve) and cooling (red curve) processes under excitation of 470 nm, (c) the configurational coordinate diagram of Mn4+ in the K3ScF6 host.

3.5 Performance of WLEDs To evaluate the device performance of the as-synthesized K3ScF6:Mn4+ phosphor, the synthesized red-emitting K3ScF6:Mn4+ was mixed with the commercial YAG:Ce3+, and the mixtures were then coupled with a blue GaN chip (~ 450 nm) to construct a WLED. The mass ratio of YAG:Ce3+ : K3ScF6:Mn4+ of WLEDs Ⅰ-Ⅰ are 1 : 5, 1 : 9 while the WLED-Ⅰ was fabricated by only combining with YAG:Ce3+. Figure 13a, 13b and 13c present the normalized electroluminescence spectra and according photographs of the three LEDs (WLED-Ⅰ, WLED-Ⅰ and WLED-Ⅰ) under a drive current of 20 mA. Compared with the spectrum of the WLED-Ⅰ prepared by only combining with YAG:Ce3+, a group of peaks (600 ~ 650 nm) in the red region shown in the spectrum of the WLED-Ⅰ and WLED-Ⅰ are attributed to the spin-forbidden 2Eg → 4A2g radiative transitions of Mn4+ ions from K3ScF6:Mn4+, which indicates that the phosphor K3ScF6:Mn4+ can absorb the electroluminescence of the blue GaN chip and covert it to an intense red light. The prepared LEDs shows cold and warm white light in the absence and presence of K3ScF6:Mn4+ by the naked eye, respectively. As shown ACS Paragon Plus Environment


in Figure 13d, the CIE chromaticity coordinates of WLED-Ⅰ (0.3134, 0.3169), WLED-Ⅰ (0.3922, 0.3904) and WLED-Ⅰ (0.4221, 0.4022) are located closely to the black body radiation curve in CIE 1931 color spaces.31 Obviously, after adding the tittle red phosphor K3ScF6:Mn4+, the corresponding CIE coordinates of LED device shift from cool white light to warm white region. In addition, the CCT of the WLED decreases from 6565 K, 3801 K to 3250 K, and the CRI increases from 73.0, 81.7 to 86.4. These results indicate that the great promise of K3ScF6:Mn4+ as an efficient red phosphor to construct warm WLEDs.

Figure 13. (a, b, c) The electroluminescence spectra and photographs of WLED devices (WLED-Ⅰ, WLED-Ⅰ and WLED-Ⅰ), (d) The corresponding CIE 1931 color spaces chromaticity coordinates of the as-fabricated LED devices.

CONCLUSION In summary, a new type of red phosphor K3ScF6:Mn4+ was obtained successfully via a facile co-precipitation method. The K3ScF6:Mn4+ prototype crystallizes in space


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group Fm3m with cubic structure and its lattice parameters are fitted well to be a = b = c = 8.4859(8) Å, Z = 4, and Vcell = 611.074(2) Å3. According to structure and PL analysis, Mn4+ ions occupy the octahedral Sc3+ sites. Different from previous most Mn4+ ion activated fluoride phosphors, the shape-controlled K3ScF6:Mn4+ phosphors demonstrate highly uniform and regular morphology with shapes from cube to octahedron with increasing Mn4+ ion concentration. Besides, the as-prepared K3ScF6:Mn4+ phosphors match well with the commercial blue LED chip, and show a sharp PL emission peaked at 631 nm with a narrow emission band. The optimal Mn4+ doping concentration in the K3ScF6 host is 0.97% and the mechanism of concentration quenching is a dipole-dipole interaction for the Mn4+ center. The K3ScF6:Mn4+ phosphors show favorable resistance to thermal impact during thermal cycling between 298 K and 498 K. A warm WLED is obtained by combining a 450 nm blue emitting chip with the mixtures of K3ScF6:Mn4+ and YAG:Ce3+ phosphors. The CCT and Ra value of the optimal device reach 3250 K and 86.4, respectively. All results reveal that K3ScF6:Mn4+ is a promising phosphor to construct white LED for lighting or display applications.

AUTHOR INFORMATION Corresponding Author: Xinyu Ye E-mail: [email protected] Author Contributions: The manuscript was written through the contributions of all the authors. / all authors have approved the final version of the manuscript. Notes: The authors declare no competing financial interest.

ACKNOWLEDGMENTS This project is supported by the National Natural Science Foundation of China



(51304086), Foundation of Natural Science Funds for Distinguished Young Scholar of Jiangxi Province (20171BCB23064), Natural Science Foundation of Jiangxi Province (20171BAB216013), Educational Commission of Jiangxi Province (GJJ150633), Science and Technology Program of Ganzhou city [2017]179, and the Program of Qingjiang Excellent Young Talents of Jiangxi University of Science and Technology.

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Phosphors

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