Room-temperature synthesis, optimized photoluminescence and warm-white LED application of a highly efficient non-rare-earth red phosphor

Accepted Manuscript Room-temperature synthesis, optimized photoluminescence and warm-white LED application of a highly efficient non-rare-earth red phosphor Feng Hong, Haiming Cheng, Yan Song, Dan Li, Guixia Liu, Wensheng Yu, Jinxian Wang, Xiangting Dong PII:

S0925-8388(18)33183-9

DOI:

10.1016/j.jallcom.2018.08.288

Reference:

JALCOM 47376

To appear in:

Journal of Alloys and Compounds

Received Date: 5 May 2018 Revised Date:

27 August 2018

Accepted Date: 28 August 2018

Please cite this article as: F. Hong, H. Cheng, Y. Song, D. Li, G. Liu, W. Yu, J. Wang, X. Dong, Room-temperature synthesis, optimized photoluminescence and warm-white LED application of a highly efficient non-rare-earth red phosphor, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.08.288. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

ACCEPTED MANUSCRIPT Room-temperature synthesis, optimized photoluminescence and warm-

white LED application of a highly efficient non-rare-earth red phosphor Feng Hong, Haiming Cheng, Yan Song, Dan Li, Guixia Liu*, Wensheng Yu, Jinxian Wang,

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Xiangting Dong*

Science and Technology, Changchun 130022. P. R. China *Corresponding author. Tel.: +86-431-85582574. Fax: +86-431-85383815

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E-mail address: [email protected]; [email protected].

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Key Laboratory of Applied Chemistry and Nanotechnology at Universities of Jilin Province, Changchun University of

1

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ABSTRACT

Non-rare-earth red phosphors as alternative to commercial (oxy) nitride phosphors have attracted considerable attention for energy-efficient warm white light-emitting diodes (WLEDs). An excellent narrow-band red emission K2GeF6:Mn4+ product was successfully obtained through co-precipitation method. The crystal structure, morphology, composition and optical

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properties of K2GeF6:Mn4+ samples were investigated in details via X-ray powder diffraction (XRD), field emission scanning electron microscope (FESEM), energy dispersive spectrometry (EDS) and photoluminescence spectra, respectively. Mn4+ activated K2GeF6 red phosphor

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possesses two strong and wide absorption bands in the region of 300-500 nm, which can be

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applied to UV and blue LED chips and bright intense red light (635 nm) with high color purity. Simultaneously, mechanisms of concentration and temperature quenching were systematically elucidated. After a systematic investigation, the luminescence properties of K2GeF6:Mn4+ have been optimized by changing synthetic conditions, such as the doping concentration, the concentration of HF, the potassium source, the amount of KF and the addition of surfactant.

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The red phosphor K2GeF6:Mn4+ was packaged via mixing commercial YAG:Ce yellow phosphor, an outstanding warm WLED with low CCT (3882 K), high CRI (Ra = 90.4) and high

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luminous efficacy (LE) of 125.84 lm/W has been obtained, implying that K2GeF6:Mn4+ is the potential red phosphor for warm WLEDs. Warm

WLEDs,

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Keywords:

K2GeF6:Mn4+

Photoluminescence properties

2

red

phosphor,

Co-precipitation

method,

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1. Introduction

White light-emitting diodes (WLEDs) have been nowadays expected to be a class of promising lighting device due to their superior properties of high energy efficiency, high stability, long operational lifetime and environmental friendliness, which have attracted more and more attention [1-3]. The photoluminescence properties of phosphors are very important to

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determine the optical parameters of warm WLEDs including correlated color temperature (CCT), color rendering index (CRI) and luminous efficiency [4]. Currently, the well-known commercial yellow-emitting phosphors (YAG:Ce3+) are employed to merge with GaN blue

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chip to achieve white light emission [5, 6]. This method is the most mature way to fabricate

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commercial WLEDs. Nevertheless, owing to the deficiency of a red light emission in the spectra of this method, it is difficult to obtain warm WLEDs with low correlated color temperature (CCT < 4,000K) and high color rendering index (CRI, Ra > 80), both of which are important parameters for practical applications such as back lighting source for display and indoor lighting [7, 8] Therefore, the present challenge is to improve the color rendering and

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decrease the correlated color temperature of LEDs, which can be effectively resolved through adding red phosphor into this system [9]. Numerous researchers have devoted their efforts to exploit novel red-emitting phosphors, including rare earth ions activated sulfides [10, 11],

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nitrides [12, 13], oxides [14, 15] and transition metal ions activated fluorides [16, 17]. They can

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meet the requirements for various applications. Among the advanced red phosphors, Eu2+ ions doped nitrides are widely investigated, such as CaAlSiN3:Eu2+ [18], SrLiAl3N4:Eu2+ [19] and SrAlSi4N7:Eu2+ [20]. However, the synthesis conditions are rigorous for the whole preparation process. For example, ammonia is used under high pressure and temperature. In addition, serious re-absorption can be found when these phosphors are combined with some yellow or green phosphors [21]. To overcome the above issues, efficient red-emitting phosphor with broadband absorption, narrow emission bands and relatively mild preparation conditions is highly expected for warm WLEDs. 3

The transition metal tetravalent manganese ions (Mn4+) activated oxide or fluoride red ACCEPTED MANUSCRIPT phosphors, which can be obtained under milder environment, have extracted extensive research interest because of their low price and excellent optical properties. At present, most of previous researches have been made to explore the excellent Mn4+ doped oxide series phosphors which can enhance the chromaticity performance of WLEDs [22-32], including Ba2GdSbO6:Mn4+,

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Gd2ZnTiO6:Mn4+, La(MgTi)1/2O3:Mn4+, Na2MgAl10O17:Mn4+ and Sr4Al14O25:Mn4+. However, for Mn4+-activated oxide red phosphors, the wavelength of some emission peaks exceeds 650 nm, which is insensitive to human eyes. Additionally, the near ultraviolet absorption peak is

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higher than those of blue light, which restricts the use of blue light chips. Hence, extensive efforts are required for developing Mn4+ doped fluoride phosphors, such as NaHF2:Mn4+ [33],

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KTeF5:Mn4+ [34], A2MF6 (A = K+, Cs+ and NH4+; M = Si4+, Ge4+ and Ti4+)[35-39], BMF6 (B = Ba2+ and Zn2+; M = Si4+, Sn4+, Ge4+ and Ti4+)[40-43], A3X(Ⅲ)F6 (A = Na+, K+ and Li+; X = Al3+ and Ga3+)[44-47], K2XF7:Mn4+ (X = Ta and Nb) [48] and Li3Na3X2F12:Mn4+ (X = Ga3+ and Al3+) [49, 50]. Due to a unique 3d3 outer electron configuration, Mn4+ emission of spin-

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forbidden transition (2Eg → 4A2) is highly sensitive to its crystal field environment. Mn4+activated non-hexa-coordination red phosphors, K2XF7:Mn4+ (X = Ta and Nb) and KTeF5:Mn4+, inevitably induce structure defects or even deviate from octahedral coordination as the unusual

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substitution, which undoubtedly affects the local environment of Mn4+ and may reduce the thermal stability. Furthermore, for the Mn4+ activated hexa-fluoride phosphors, Ge4+ ion is the

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best central ion owing to the same ionic radius of Ge4+ (0.53 Å) and Mn4+ (0.53 Å). There are a large number of octahedral sites in the K2GeF6 crystal, demonstrating that it is an outstanding host for Mn4+ ion doping due to the favorable coordination environment. For example, Adachi and Wei had synthesized Mn4+activated K2GeF6 red phosphors [36, 51]. However, Adachi et al reported the luminescent properties of K2GeF6:Mn4+ synthesized through wet chemical etching of Ge shots in the mixture of KMnO4/HF. Although Wei et al prepared K2GeF6:Mn4+ product via coprecipitation method, they did not systematically study the relationship between luminescent properties and the synthesis conditions. 4

In this work, we successfullyACCEPTED synthesized efficient red phosphors K2GeF6:Mn4+ through coMANUSCRIPT precipitation method at room temperature. These red phosphors possess narrow emission peaks and broad absorption band in the region of blue light, which can make up for the lack of red light from the current commercial WLEDs. The effect of synthesis conditions, including the doping amount of Mn4+, the concentration of HF, the potassium source and the amount of KF,

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have been studied in detail to improve the luminescent properties of K2GeF6:Mn4+. We tried to modify the sample with different surfactants, and successfully found that the introduction of anionic surfactants can enhance the luminescence of the samples. Moreover, the optimized

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mechanism of anionic surfactants has been studied. The mechanisms for the concentration quenching and thermal quenching of K2GeF6:Mn4+ have been systematically investigated. By

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using K2GeF6:Mn4+ as red component, we realize an excellent warm WLED with low CCT (3882K), high CRI (Ra = 90.4) and luminous efficacy (125.84 lm/W), which demonstrates K2GeF6:Mn4+ red phosphors as good red supplements for warm WLED.

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2. Experimental section 2.1 Materials

All starting materials were used as purchased without any further purification in the whole

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process of experiment, including Hexadecyl trimethyl ammonium Bromide (CTAB, > 99.0 %),

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potassium permanganate (KMnO4, > 99.5 %), sodium citrate (Na3Cit, > 99.0 %), sodium dodecyl benzene sulfonate (SDBS, > 90.0 %), potassium fluoride (KF, > 98%), silicon dioxide (SiO2,

>

99.0

%),

potassium

formate

(CHO2·K,

>

98

%),

tetrabutyl

titanate

(Ti(OC4H9)4 , >98.0 %), potassium sulphate (K2SO4, > 99.0 %), germanium oxide (GeO2, 99.99 %), hydrofluoric acid solution (HF, 40wt %), potassium carbonate, crystals (2K2CO3·3H2O, > 98.0 %) and hydrogen peroxide aqueous solution (H2O2, 30.0 %) were purchased from the Sinopharm Chemical Reagent Co. Ltd. 2.2 Synthesis of K2GeF6:Mn4+ samples 5

The co-precipitation methodACCEPTED was employedMANUSCRIPT to obtain red phosphor K2GeF6:Mn4+ by using K2MnF6 as the Mn4+ source, which was schematically illustrated in Fig. 1. The K2MnF6 powders were synthesized according to the reference [52]. In a typical procedure of preparing representative K2GeF6: 9%Mn4+, GeO2 (0.4761 g) (SiO2 and Ti(OC4H9)4 were used as raw materials for preparing K2SiF6:Mn4+ and K2TiF6:Mn4+, respectively.) and K2MnF6 (0.1111 g)

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were mixed with 10 mL 40wt % HF solution under magnetic stirring. Then, 30 mL distilled water mixture containing 1.1627 g of KF, which was added dropwise into the above solution with magnetic stirring for 30 minutes. After that, the mixed solution was deposited for 20

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minutes. Finally, the solid powders obtained were separated by centrifugation and washed five times using ethanol. Other samples were obtained using a similar process, apart from changing

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HF concentrations, amount of KF and K2MnF6, potassium source and surfactants (surfactant and KF were simultaneously added in 30 mL distilled water). 2.3 Characterization

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The powder X-ray diffraction (XRD) patterns of the phosphor samples were investigated on a RigakuD/max-RA x-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å) in the region of 2θ from 10 ° to 90 °. A JEOL JSM-7610F field emission scanning electron microscope (SEM)

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with an attached energy-dispersive X-ray spectrometer (EDS) was used to perform the morphological analysis and elemental composition analysis of the as-synthesized samples.

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Excitation spectra, emission spectra and luminescence decay curves of phosphors in the natural environment were measured and both the data interval of excitation and emission spectra were set up to be 1.0 nm using a HITACHI F-7000 fluorescence spectrophotometer equipped with the Xenon lamp (150 W) as the excitation source. An integrating sphere whose inner face was coated with the barium sulfate equipped with a spectrophotometer (C9920-02, Hamamatsu Photonics K. K., Japan) measured the PL quantum efficiency of the red phosphor. The thermal stability of red phosphor was examined by a JobinYvon fluoro Max-4 equipped with a 150 W xenon lamp as the excitation source. 6

3. Results and discussion

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3.1 Structure, morphology and element composition of K2GeF6:Mn4+ samples The X-ray diffraction (XRD) pattern of the as-prepared K2GeF6:9%Mn4+ product is displayed in Fig. 2a. All diffraction peak positions are coincided well with the standard JCPDS

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cards (PDF No. 73-1531) of hexagonal K2GeF6 (a = b = 5.620 Å and c = 4.650 Å, space group P-3m1 (164)). But the diffraction intensities are different from the JCPDS data. Although the relative intensity does not match the standard pattern, we think that the as-obtained product is

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still a pure phase of K2GeF6, and has different preferred orientation comparing with the standard phase. This result is agreement with the references [36, 53]. Additionally, the

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introduction of Mn4+ ions in K2GeF6 host does not cause any visible change of the diffraction peak positions, evidencing the Mn4+ ion is successfully introduced into the host without significantly influencing the K2GeF6 crystal structure. Fig. 2b illustrates the crystal structure of the K2GeF6 unit cell and cation coordination environment of octahedral. The Mn4+ ions prefer

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to substitute for Ge4+ ions in octahedral sites, owing to the fact that Mn4+ ions (0.53 Å, CN = 6) possess the identical ionic radius and the same coordination number with Ge4+ ions (0.53 Å, CN = 6).

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The representative morphology of the as-obtained K2GeF6:Mn4+ is displayed in Fig. 2c. The

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K2GeF6:Mn4+ sample is composed of some irregularly-shaped micrometer-sized particles with size of 30 - 70 µm and smooth surface. The element compositions of the as-prepared samples are given by EDS, as seen in Fig. 2d. The corresponding EDS spectrum verifies that the main chemical elements existence of K, Ge, F and Mn. 3.2 Optical properties of K2GeF6:Mn4+ samples The photoluminescence excitation (PLE) and photoluminescence (PL) spectra of red phosphor K2GeF6:9%Mn4+ are displayed in Fig. 3a. The excitation spectrum consists of two wide bands, centered at 363 nm and 468 nm, which correspond to the spin-allowed 4A2g→4T1g 7

4+ and 4A2g→4T2g transitions of Mn ion, respectively. The result indicates that the K2GeF6:Mn4+ ACCEPTED MANUSCRIPT

product can be efficiently applied to commercial blue GaN chips (460 nm). Under 468 nm blue light excitation, the PL spectrum is composed of six fundamental sharp peaks located at 613, 617, 623, 635, 639 and 651 nm with a maximum at 635 nm, because of the spin-forbidden nature 2Eg→4A2g transition of Mn4+ ions. The quantum efficiency of K2GeF6:Mn4+ product is

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calculated to be 70% under excitation at 468 nm. The Commission Internationale de L'Eclairage (CIE) chromaticity coordinate of the K2GeF6:Mn4+ red phosphor is marked at the position (x = 0.6950, y = 0.3047) under 468 nm blue light excitation, as shown in Fig. 3b. The

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point of K2GeF6:Mn4+ is near to the point of pure red (x = 0.67, y = 0.33) (the standard values of National Television Standard Committee standard). Under blue light irradiation, bright red

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light is observed by naked eyes, as displayed in the inset photograph of K2GeF6:Mn4+ in Fig. 3b. The characteristics indicate that the red phosphor possesses good optical properties and is very suitable for practical applications.

conditions

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3.3 Structure and optical properties of K2GeF6:Mn4+ phosphors at different reaction

To optimize the doping amount of Mn4+ ions, some K2GeF6:xMn4+ red phosphors are

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synthesized with different concentrations (x = 1%, 5%, 9%, 13%, and 17%). As given in Fig. 4a, the identification of the phase purity of the as-obtained K2GeF6:Mn4+ red phosphors with

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various Mn4+ ions contents is conducted using XRD. All the characteristic diffraction peak positions of the samples agree well with the corresponding standard card data of K2GeF6 (PDF#73-1531). Without additional diffraction peaks are observed, revealing that the Mn4+ ions are introduced into the K2GeF6 crystal structure and do not significantly influence the phase purity of the K2GeF6. The PL spectra of the K2GeF6:Mn4+ samples obtained with various amount of Mn4+ under excitation of 468 nm are displayed in Fig. 4b. With increasing the content of Mn4+ ions, the PL intensity has an evident rising tendency first and then decreases gradually when further increasing Mn4+ ions content beyond 9%. Therefore, Mn4+ has an 8

optimum doping concentration of 9% in KMANUSCRIPT 2GeF6 host. This behavior is related to the ACCEPTED concentration quenching among Mn4+ ions. In order to ascertain the concentration quenching mechanism of Mn4+ in the K2GeF6:Mn4+ product. The critical distance (Rc) between Mn4+ ions in K2GeF6 is calculated with the following equation [54]: 1ൗ 3

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3ܸ ܴܿ ≈ 2 × ൬ ൰ 4ߨ‫ܰ ܿݔ‬

Here, V represents the K2GeF6 unit-cell volume (V = 127.92 Å), Xc stands for the Mn4+ ions

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critical dopant concentration (Xc = 0.09), N represents the number of total Mn4+ sites in the K2GeF6 unit cell (N = 1). Thus, the calculated value of Rc is ~13.95 Å, which is much higher

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than 5 Å. The result implies that the exchange interaction of the energy transfer mechanism is eliminated for among Mn4+ in the K2GeF6. Thus, the principal reason for the concentration quenching is the electric multipolar interactions. Based on the theory of Dexter, the related type of electric multipolar interaction between two nearest Mn4+ centers in the K2GeF6 host is

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further investigated, and the formula below proposed can be used [50, 55]:

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ߠ −1 ‫ܫ‬ = ݇ ൤1 + ߚሺ‫ݔ‬ሻ3 ൨ ‫ݔ‬

where I represents the luminescence intensity, and x stands for the amount of Mn4+ ion, the

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expressions K and β are constants for the same excitation conditions and θ is an electric multipolar feature with the values of θ = 10, 8, and 6 corresponding to the quadrupolequadrupole (q-q), dipole-quadrupole (d-q) and dipole-dipole (d-d) interaction, respectively. The transformation equation is as follows:

‫ܫ‬ ߠ log ൬ ൰ = − logሺ‫ݔ‬ሻ + ‫ܣ‬ ‫ݔ‬ 3 The dependence of log (I/x) on log (x) is plotted in Fig. 4c. A relative linear fitted is gained and the fitting result appears to be linear with a slope θ/3 = 1.67. Therefore, the calculated θ 9

value is nearest to 6, which demonstrates thatMANUSCRIPT the dipole-dipole interaction may be the main ACCEPTED interaction mechanism for the concentration quenching in the host of K2GeF6. As performed in Fig. 4d, a series of luminescent decay curves of Mn4+ in K2GeF6:x%Mn4+ (x = 1, 5, 9, 13 and 17) red phosphors monitored at 635 nm under 468 nm excitation are tested at

which is given by −‫ݐ‬ൗ ൯ ߬

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‫ܫ‬ሺ‫ݐ‬ሻ = ‫ܫ‬0 + ‫ ݁ܣ‬൫

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room temperature. The PL decay time measured is related to the single-exponential function,

where I0 and I(t) stand for the fluorescence intensity of K2GeF6:xMn4+ at time t0 and t1, τ

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represents the corresponding luminescence decay time, which is calculated by fitting the decay curve. With increasing Mn4+ concentration from x = 1% to 17%, the decay lifetimes continuously reduce from 7.75 ms to 3.05 ms, which ascribes to the increase of the nonradiation transition process among the Mn4+ ions with increasing the Mn4+ doped concentration

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[46, 50].

HF not only plays a significant fluorine source, but also inhibits the hydrolysis of K2MnF6. Therefore, the amount of HF has a significant influence on the structure and luminescence of

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K2GeF6:9%Mn4+ sample. Fig. 5a displays the XRD patterns of K2GeF6:9%Mn4+ red phosphors

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with different amounts of HF. It can be seen that all the diffraction peak positions are consistent with the corresponding JCPDS card of K2GeF6 (No. 73-1531), which suggests the formation of pure hexagonal phase K2GeF6:Mn4+. The influences of HF concentration for the luminescence intensity of K2GeF6:Mn4+ are given in Fig. 5b. We observed that the luminescence intensity of K2GeF6:9%Mn4+ initially increases with increasing HF concentration, then reaches the maximum for the sample with HF (30 wt%) concentration and finally reduces gradually as HF concentration further increases. The improvement of luminescence intensity is due to the higher concentration of HF, which facilitates the entry of Mn4+ into the K2GeF6 host lattice. However, When the HF concentration reaches 40 wt%, energy-transfer probability among the Mn4+ ions 10

is enhanced and the sample surface may be etched by excess HF. These two possible reasons ACCEPTED MANUSCRIPT lead to a decrease in luminescence intensity [56, 57]. In order to prove that the higher HF concentration is conducive to Mn4+ ions entering the host and affecting the luminescence, we have measured the EDS spectra of all as-prepared phosphors with difference HF concentration. The result shows that the doping content of Mn4+ ions in the EDS spectra exhibits an increasing

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trend, and the corresponding mole percentages of Mn4+ ions are 8.9 %, 9.1 %, 9.5 % and 9.6 % when HF concentration at 10%, 20%, 30% and 40%, respectively, suggesting that Mn4+ ions doped concentration in the phosphor increases when the higher HF concentration is applied.

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For the sake of discussing the influence of the different amount of KF for the phase and

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photoluminescence properties of K2GeF6:9%Mn4+ phosphors, a series of K2GeF6:9%Mn4+ samples are synthesized by using different amounts of KF, Fig. 6a presents their XRD patterns. All the diffraction peak positions match well with pure hexagonal structure of K2GeF6:Mn4+ (PDF#73-1531), and without other impurity peaks are observed. The peak positions have not

structure of K2GeF6.

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shifted, which indicates the excessive KF has no influence for the phase purity and crystal

Fig. 6b exhibits the evolution of luminescent intensities with different concentrations of KF.

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It is obviously observed that appropriate amount of KF truly improves the luminescence intensity. The reason is that the small amount K+ doping can decrease the non-radiative

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transition, which may be attributed to reduction in the number of surface vacancies. However, when excess amount of KF is added, the emission intensity shows a downward trend, owing to that the excessive K+ ions aggravate the imbalance of the charge in the solution. It is also proved by the experiment that the optimum amount of KF is 30 mmol [58, 59]. The luminescence intensity of the red phosphor is strongly dependent on starting materials. Therefore, we investigate the influence of different potassium source on crystal structure and luminescence properties. A series of 9% Mn4+-activated K2GeF6 samples have been prepared using different potassium sources (KF, K2SO4, CHO2·K and 2K2CO3·3H2O). As displayed in 11

Fig. 7a, the XRD patterns of the red phosphorsMANUSCRIPT K2GeF6:Mn4+ are chosen to prove the purity of ACCEPTED the sample. According to the K2GeF6 standard card (No. 73-1531) as reference, all the diffraction peak positions in these red phosphors are exactly allotted to pure K2GeF6. No other diffraction peaks in virtue of impure phase are found, which indicates that the phase purity and crystal structure of products have no obvious shift when varying the potassium sources.

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The influence of potassium source for the luminescence intensity of K2GeF6:Mn4+ products is given in Fig. 7b. The K2GeF6:Mn4+ phosphors possess a series of red-light emission peaks extending from 550 nm to 700 nm. It can be clearly observed that, using different potassium

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source, the emission peak position does not shift. This experimental phenomenon shows that

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the K2GeF6:Mn4+ red phosphor synthesized from CHO2·K possesses the strongest luminescence intensity compared with the as-prepared red phosphors with difference potassium sources. The significant change in emission intensity definitely resulted from the change in crystallinity and the quantity of the defects [60]. The crystallinity of the as-prepared samples can be estimated from the line broadening of XRD patterns. The samples’ full width at half

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maximum (FWHM) can be obtained by fitting the XRD diffraction data. Using CHO2·K, 2K2CO3·3H2O, K2SO4 and KF as the potassium sources, the FWHM of the strongest diffraction

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peaks are 0.113°, 0.136°, 0.160° and 0.172°, respectively. The smaller FWHM means that the crystalline grain is large and the crystallinity is well. Moreover, different crystallinity will lead

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to different number of defects. So, the better the crystallinity is, the stronger the luminescence intensity is.

It is well known that surfactants can be adsorbed on the surface of the sample by combining with metal cations. Therefore, they can significantly change the growth behavior of nanomaterials. In addition, surface modification is also an effective method, which can be used to improve the luminescent properties and to expand the practical applications of luminescence materials [61]. Here, hexadecyl trimethyl ammonium bromide (CTAB), sodium dodecyl benzene sulfonate (SDBS) and sodium citrate (Na3Cit) are collected as surfactants to modify 12

phosphor and optimize optical properties. MANUSCRIPT Fig. 8a presents the XRD patterns of the ACCEPTED K2GeF6:9%Mn4+ products synthesized by different surfactants. All the diffraction peak positions of the samples prepared by the modification of different surfactants are well consistent with the corresponding JCPDS card no. 73-1531 of K2GeF6, indicating that single hexagonal phase K2GeF6:9%Mn4+ can be obtained and the participation of the surfactant has no significant

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influence on the K2GeF6 host crystal structure. From the emission spectra of Fig. 8b, it is distinctly observed that the fluorescence intensity of the phosphors is effectively improved after modification of anionic surfactants. However, cationic surfactants have no significant effect on

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luminescence performance. The reason of this result is due to different crystal growth processes. The formation process is shown in scheme 1. In the experiments, the anionic surfactants

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combine with K+ ions and Ge4+ ions to form two kinds of stable metal complexes by the chelation function. Thus, the process of nucleation and growth of K2GeF6 crystals will be prolonged, and the crystal growth is changed [62-64].

It is essential to evaluate the thermal stability of luminescence for phosphors above room

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temperature, which are frequently used as important parameters in WLEDs. Owing to that the operating temperature of the LED chip is above room temperature. The thermal stability of the

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K2GeF6:Mn4+ red phosphor is evaluated in the temperature extending from 98 K to 498 K by temperature-dependent PL spectra, as shown in Fig. 9a. It is noticeable that the shape of PL

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spectra is independent of measurement temperature. Nevertheless, there is a significant difference in the luminescence intensity of the strongest emission peaks. The relative luminescence intensity as a function of temperature is depicted in the inset of Fig. 9b. The luminous intensity of K2GeF6:Mn4+ increases primitively as increasing test temperature and achieves a maximum value when the test temperature is 198K. The luminescence intensity reduces continuously when the heating process is above 198 K, which is originated from the thermal quenching. Upon heating the product to 448 K, at that temperature the white LEDs usually work, the luminescence intensity maintains about 57% of that under room temperature. 13

In order to better confirm the temperature quenching behavior of the as-obtained K2GeF6:Mn4+. ACCEPTED MANUSCRIPT For the activation energy (∆E) of thermal quenching, the following expression can be described.

‫= ܶܫ‬

‫ܫ‬0

1 + ‫ ݌ݔ݁ × ܥ‬ቀ−

∆‫ܧ‬ ቁ ݇ܶ

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Where I0 and IT are the PL intensity at room temperature and testing temperature T (T = 98498K), respectively, and K expresses the Boltzmann constant (8.629×10-5 eV/K). The activation energy of the phosphor K2GeF6:Mn4+ is calculated to be 0.21 eV, which indicates the

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excellent thermal stability behavior of the K2GeF6:Mn4+ phosphors for application in warm WLEDs. The cross process in Fig. 9c illustrates the mechanism of thermal quenching. Upon

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UV or blue-light excitation, the electrons of ground state 4A2g level will directly jump to the excited state 4T1g and 4T2g. Subsequently, the electrons of 2Eg state will move back the ground state 4A2g through the form of radiative transitions and a deep red emission can be observed. Under sufficient test temperature, nevertheless, a portion of electrons can reach the intersection

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of 4A2g and 2Eg states by absorbing activation energy, which leads to nonradiative transition. Hence, the deterioration of luminescence intensity occurs with increasing test temperatures. The large activation energy means that a large amount of energy is needed for electrons to

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stability.

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reach the intersection point, which indicates red phosphor K2GeF6:Mn4+ has excellent thermal

In order to understand the characteristics of red phosphor and its application in the devices of warm WLEDs. we chose K2SiF6:Mn4+, K2TiF6:Mn4+ and K2GeF6:Mn4+ as compared red phosphors to prepare the warm WLED. Fig. 10a shows the representative XRD pattern of the prepared K2SiF6:Mn4+ and K2TiF6:Mn4+ phosphors, along with the standard JCPDS cards of their corresponding hosts, all the diffraction peaks are perfectly consistent with the space group P-3m1 (164) of hexagonal K2TiF6 (JCPDS No. 08-0488, a = b = 5.715 Å and c = 4.656 Å) and the space group Fm-3m (225) of cubic K2SiF6 (JCPDS No. 85-1382, a = b = c = 8.142 Å), no secondary phase is identified, indicating the cation exchange result of Mn4+ substituting for Ti4+ 14

or Si4+ has not alter the phase purity of the corresponding matrix. The morphology and ACCEPTED MANUSCRIPT composition of the synthesized K2SiF6:Mn4+ and K2TiF6:Mn4+ products are characterized using SEM and EDS, and the test results are displayed in Fig. 10. In Fig. 10(b, c), it is noted that the as-prepared K2SiF6:Mn4+ and K2TiF6:Mn4+ products show irregular morphology. The diameters of the particles varied from 3 to 8 µm and 29 to 86 µm, respectively. In the corresponding EDS

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spectra (Fig. 10d, e). We can clearly observe that the main chemical elements existence of F, Si or Ti, K, and Mn. A series of warm WLEDs are fabricated from the blue light chip-YAG:Ce3+ (a), and YAG:Ce3+-K2GeF6:Mn4+ (b), K2TiF6:Mn4+ (c) and K2SiF6:Mn4+ (d). the

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electroluminescence spectra are shown in Fig. 11. From Fig. 11A, we can clearly see that the luminescence spectra have three typical parts: blue emission peak (460 nm) comes from the

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blue chip, yellow emission peak (546 nm) comes from YAG:Ce3+ yellow phosphor and red emission peak (633 nm) comes from three different red phosphors, respectively. In addition, the three red phosphors have different effects on LED, the most effective phosphor is K2GeF6:Mn4+. Using K2GeF6:Mn4+ as a red phosphor, the CIR of the LED is improved from 70.8 to 90.4 and

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the CCT is reduced from 5117 K to 3882 K. Nevertheless, the LE of the LED is reduced from 158.17 lm/W to 125.84 lm/W. The Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of all white LEDs are given in Fig. 11B. The corresponding CIE

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coordinates of two WLEDs with composition of blue chip + YAG:Ce3+ (point a) and blue chip + YAG:Ce3+ + K2GeF6:Mn4+ (point b) are (0.3438, 0.3759) and (0.3723, 0.3344), respectively.

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The chromaticity coordinates verify the change of emission color from cool white light to warm white light. The powder exposed to blue light (460 nm) emits a bright red light unlike the phosphor powder under sunlight, which is given in the inset photographs of Fig. 11B. The relative performance of the WLEDs is listed in Table 1. These experimental parameters reveal that the addition of the K2GeF6:Mn4+ product can effectively improve performance of WLED and make it an outstanding red phosphor for warm WLEDs. 4. Conclusions 15

In conclusion, a class of ACCEPTED highly efficient K2GeF6:Mn4+ red phosphor with improved MANUSCRIPT luminescence intensity has been obtained through co-precipitation method at room-temperature. The as-obtained phosphors can absorb blue light at 468 nm and emit an outstanding narrow redlight emission peaks at 635 nm. The external quantum efficiency value of the phosphor K2GeF6:Mn4+ achieves 70%. The influence of synthesis conditions, including the Mn4+ doping

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amount, the concentration of HF, the potassium source and the amount of KF on crystal structure and optical performances of K2GeF6:Mn4+ products have been studied in detail. The optimum doping amount of Mn4+ ion is experimentally estimated to be 0.09. The concentration

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quenching mechanism is determined to the d-d interaction. Moreover, it is surprising to find that the anion surfactant can significantly improve the luminescence intensity of the phosphors.

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Temperature dependent luminescence measurement indicates that K2GeF6:Mn4+ red phosphor possesses an excellent thermal quenching behavior. More importantly, using the as-prepared red phosphor with commercial YAG:Ce3+ yellow phosphor and a blue GaN chip, a highperformance warm WLED with LE ∼ 125.84 lm/W, Ra ∼ 90.4, and CCT ∼ 3882 K at a 20 mA

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drive current is obtained. The results suggest that K2GeF6:Mn4+ products are promising candidate and have potential applications in warm WLEDs.

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Acknowledgements

This work was financially supported by National Natural Science Foundation of China

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(51072026, 51573023, 50972020), Natural Science Foundation of Jilin Province of China (20170101185JC, 20170101101JC), Industrial Technology Research and Development Project of Jilin Province Development and Reform Commission (2017C052-4), Education Department of Jilin Province "13th Five-Year" Science and Technology Research Project (2016-382). References

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ultraviolet upconversion emissions, CrystEngComm 14 (2012) 2302-2307.

Scheme 1 Possible formation mechanism for K2GeF6 crystals modified by anionic surfactants.

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Fig. 1 Synthesis process diagram of the red phosphor K2GeF6:Mn4+.

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Fig. 2 XRD pattern (a), schematic illustration of the K2GeF6 crystal structure (b), SEM picture (c) and EDS elemental composition (d) of K2GeF6:9%Mn4+ red phosphor. Fig. 3 PLE (left, λem = 635 nm) and PL (right, λex = 468 nm) spectra (a) and coordinate location points of the K2GeF6:9%Mn4+ red phosphor in the CIE chromaticity diagram (b). The inset represents the luminescence photos of the K2GeF6:9%Mn4+ red phosphor under excitation with a 460 nm lamp. Fig. 4 XRD patterns (a), PL spectra of K2GeF6:x%Mn4+ (x = 1, 5, 9, 13, and 17) under 468 nm excitation (b), Dependence of log(x) on log(I/x) based on the luminescence intensity (I) and 24

4+ Mn4+ amount x (c) and decay curves of the K2GeF 6:Mn phosphors monitored at 635 nm under ACCEPTED MANUSCRIPT

468 nm excitation (d). Fig. 5 XRD patterns (a) and PL spectra (b) of K2GeF6:9%Mn4+ synthesized at different HF concentrations. The inset represents the relative PL intensity of K2GeF6:9%Mn4+ on HF

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concentration. Fig. 6 XRD patterns (a) and PL spectra (b) of K2GeF6:9%Mn4+ synthesized at different concentrations of KF. The inset represents the relative PL intensity of K2GeF6:9%Mn4+ on KF

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concentration.

Fig. 7 XRD patterns (a) and PL spectra (b) of K2GeF6:9%Mn4+ synthesized at different

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potassium sources. The inset represents the relative PL intensity of K2GeF6:9%Mn4+ on potassium sources.

Fig. 8 XRD patterns (a) and PL spectra (b) of K2GeF6:9%Mn4+ phosphors modified at different

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surfactants.

Fig. 9 Emission spectra (a) of K2GeF6:Mn4+ upon excitation of 468 nm with different test temperatures, temperature dependence of the relative intensity of Mn4+ in K2GeF6 (b), and the

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configurational coordinate diagram including the ground and excited states of Mn4+ and the

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possible thermal quenching procedure (c). Fig. 10 XRD patterns (a), SEM pictures (b, c), EDS elemental composition (d, e) of the K2SiF6:Mn4+ and K2TiF6:Mn4+. Fig. 11 EL spectra of LED, which assembling the blue light chip-YAG:Ce3+ (a), and YAG:Ce3+-K2GeF6:Mn4+ (b), K2TiF6:Mn4+ (c) and K2SiF6:Mn4+ (d) under 20 mA current excitation (A). The corresponding coordinate location points of the WLED in the CIE chromaticity diagram (B). The inset represents luminescence photos of the K2GeF6:Mn4+ products at sunlight and 460 nm blue light irradiation. 25

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Table 1 Photoelectric parameters for four LEDMANUSCRIPT assemble system ACCEPTED

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Fig. 9

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Fig. 10

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Fig. 11 Table 1

Sample

CIE coordinates (x, y)

CCT

Ra

Luminous efficiency

a

(0.3438 0.3759)

5117 K

70.8

158.17 lm/W

b

(0.3723 0.3344)

3882 K

90.4

125.84 lm/W

c

(0.3567 0.3555)

4592 K

85.8

137.46 lm/W

d

(0.3258 0.3269)

5830 K

82.3

132.90 lm/W

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Highlights ● A highly efficient Mn4+ activated K2GeF6 red phosphor has been synthesized. ● Using K2GeF6:Mn4+ as a red phosphor, a high-performance warm WLED were obtained.

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● The luminescence properties of K2GeF6:Mn4+ have been improved by optimizing synthetic

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conditions.