A red-emitting phosphor K2[MoO2F4]•H2O:Mn4+ for warm white light-emitting diodes with a high color rendering index

Journal Pre-proof A red-emitting phosphor K2 [MoO2 F4 ]•H2 O:Mn4+ for warm white light-emitting diodes with a high color rendering index Yan Liu, Hong Li, Shu Tang, Qiang Zhou, Kaimin Wang, Huaijun Tang, Zhengliang Wang

PII:

S0025-5408(19)32187-7

DOI:

https://doi.org/10.1016/j.materresbull.2019.110675

Reference:

MRB 110675

To appear in:

Materials Research Bulletin

Received Date:

23 August 2019

Revised Date:

10 October 2019

Accepted Date:

18 October 2019

Please cite this article as: Liu Y, Li H, Tang S, Zhou Q, Wang K, Tang H, Wang Z, A red-emitting phosphor K2 [MoO2 F4 ]•H2 O:Mn4+ for warm white light-emitting diodes with a high color rendering index, Materials Research Bulletin (2019), doi: https://doi.org/10.1016/j.materresbull.2019.110675

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A red-emitting phosphor K2[MoO2F4]•H2O:Mn4+ for warm white light-emitting diodes with a high color rendering index Yan Liu, Hong Li, Shu Tang, Qiang Zhou *, Kaimin Wang, Huaijun Tang, Zhengliang Wang*

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Key Laboratory of Green-Chemistry Materials in University of Yunnan Province, National and Local Joint Engineering Research Center for Green Preparation Technology of Biobased

Materials, School of Chemistry & Environment, Yunnan Minzu University, Kunming, 650500,

Corresponding author

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*

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P. R. China.

Fax: +86-871-65910017;

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Tel: +86-871-65910017;

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E-mails: [email protected] (W.L. Wang);

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[email protected] (Q. Zhou)

Graphical Abstract

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A novel red-emitting phosphor K2[MoO2F4]•H2O:Mn4+ prepared by a simple

precipitation method emits the intense red light with the high internal quantum efficiency of 86.5 % and the high thermal stability of luminescence. A w-LED fabricated with this red phosphor shows intense warm white-light with a high color rendering index of 94, a low correlated color temperature of 3945 K. Therefore, 1

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K2[MoO2F4]•H2O:Mn4+ maybe find application on w-LED lighting.

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K2[MoO2F4]•H2O:Mn4+ was designed and synthesized by a simple precipitation

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method.

This phosphor exhibits intense red-emission and high internal quantum

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

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Highlights:

efficiency.



This phosphor has the high thermal stability of luminescence.

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A w-LED coating this sample has excellent photoelectric performances.

Abstract 2

A red-emitting phosphor K2[MoO2F4]•H2O:Mn4+ (denoted as KMOF:Mn4+) was designed and synthesized by a simple precipitation method, and its structure and luminescence properties were studied in details. The as-prepared KMOF:Mn4+ phosphors show broad excitation band in the blue-light region. The as-optimized phosphor KMOF:Mn4+ (3.28 mol%) has the strongest red-emission with high internal quantum efficiency of 86.5 % among these samples with different contents of

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Mn4+. Meanwhile, this sample is of the high thermal stability of luminescence. One white light-emitting diode (w-LED) was fabricated by coating KMOF:Mn4+ (3.28

mol%) and Y3 Al5O12:Ce3+ on a GaN chip. This w-LED can emit intense warm

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white-light and show excellent photoelectric performances with a high color rendering

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index of 94, a low correlated color temperature of 3945 K, and the high luminous

red phosphor for w-LEDs.

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efficiency of 150.1 lm/W. Hence, KMOF:Mn4+ (3.28 mol%) could be a promising

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Phosphors

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Keywords: A. Optical materials; B. Fluorides; C. Luminescent properties; D.

1. Introduction

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Recently, more and more attentions were paid on the fluoride phosphors doped with

Mn4+, due to their excellent photo-luminescent (PL) properties, such as extensive absorption in blue region and intense emission with high color purity [1-5]. These unique PL properties of Mn4+ in fluorides well meet the requirements of white light-emitting diodes (w-LEDs) based on blue GaN chips. For example, A2XF6:Mn4+ 3

(A = K, Cs, Rb; X = Ti, Si and Ge) exhibit a couple of red emitting peaks under blue light excitation [6-11]. But, no obvious zero phonon line (ZPL) of Mn4+ can be observed in their emission spectra. It’s well known that the intensity of ZPL emission is related to the distortion of the Mn4+ octahedron local symmetry [12-14]. When Mn4+ is located at the disordered octahedral field, its ZPL emission can be easily observed. Such as, intense emission of ZPL can be found in some fluoride phosphors

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with the non-equivalent doping of Mn4+ [15-19]. It is obvious that such ZPL emission

is beneficial to improving the color purity of the red-emitting phosphor for warm w-LEDs.

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At present, some oxyfluoride phosphors doped with Mn4+ have been developed

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[20-25]. Such as, Cai et al. studied the PL properties of Cs2 WO2F4:Mn4+, which presents intense emission at 632 nm with broad excitation band in the blue-light

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region [20]. Na2WO2F4:Mn4+ exhibits an ultra-intense zero phonon line Mn4+ under

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the blue light excitation, which provides a great potential application in w-LEDs lighting and displays [21]. As we known, the Mo (VI) and W (VI) ions are belonged

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to the same family and have the same charge and similar ionic radii. It is expected that the oxyfluorides based on Mo (VI) are excellent hosts for doping of Mn4+. For

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example, K3MoOF7:Mn4+ exhibits excellent PL properties under the blue light excitation [25].

So it is interesting to develop new Mn4+-activated red-emitting

phosphors based on such oxyfluoride hosts. In

this

work,

we

successfully

prepared

a

red-emitting

phosphor

K2[MoO2F4]•H2O:Mn4+ (denoted as KMOF:Mn4+) via one precipitation method. As 4

expected, KMOF:Mn4+ presents excellent PL properties with an extremely intense ZPL at ~ 620 nm upon the blue light excitation. Meanwhile, this red-emitting has one high thermal stability of luminescence. At last, a warm w-LED with excellent performances was obtained by coating this red phosphor and commercial Y3Al5O12:Ce3+ (YAG) on a blue LED chip. 2. Experimental

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2.1 Synthesis

All reagents including hydrofluoric acid, ammonium molybdate, potassium

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fluoride, glacial acetic acid and ethanol are all of analytical grades and used

without further purification. The commercial yellow phosphor YAG was

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purchased from Quanjing Photon Co. Ltd, (Shenzhen, China). The synthetic

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process of K2MnF6 has already been depicted in our previous work [26]. A group of red-emitting phosphors KMOF:Mn 4+ were prepared with K 2MnF6

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as the Mn4+ source. Firstly, 0.179 mmol (NH4)6Mo7O24•4H2O and 0.125 mmol K2MnF6 were dissolved into 5 ml HF (40 wt%) with magnetic stirring. Then

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0.006 mol KF were added into the above solution. After stirring for 3 hours, the

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precipitates were washed for 3 times with acetic acid and ethanol, respectively. Finally, the precipitates were dried at 333 K for 8 h. The other KMOF:Mn4+ samples were prepared by changing the amount of K2MnF6 using the same steps.

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The LED devices were fabricated by combining blue LED chips with different amount of KMOF:Mn 4+ and YAG phosphor. The fabrication process is according to our previous work [26].

2.2 Characterizations XRD pattern of the sample was received on a powder X-ray diffraction (D8

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Advance, Bruker, Germany). SEM image was obtained on a scanning electron microscopy (SEM, FEI Quanta 200 Thermal FE Environment scanning electron

microscopy) with an attached energy-dispersive X-ray spectrometer (EDS). The

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contents of Mn4+ in KMOF:Mn4+ were measured on a Shimadzu EDX-8000 energy

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dispersive X-ray fluorescence spectrometer (XRF). The decay curves and the quantum efficiency were collected by a Hitachi (F-7000) spectrophotometer used an Xe lamp

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as the light resource. The diffuse reflectance ultraviolet-visible spectra (DRS) were obtained on a Cary 5000UV-Vis-NIR spectrophotometer. The photo-luminescent

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spectra were tested by a Cary Eclipse FL1011M003 (Varian) spectrofluorometer. The optical performances of these LED devices were characterized on a high accurate

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array spectrometer (HSP6000).

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3. Results and discussion 3.1 Composition, morphology analysis A series of KMOF:Mn 4+ samples were prepared with different initial mole ratios between (NH4)6Mo7O24•4H2O and K2MnF6 (40:7, 30:7, 20:7, 10:7 and 5:7). The detailed contents of Mn4+ in these samples have been confirmed by the XRF 6

measurement, as listed in Tab. 1. Obvious, the doping content of Mn4+ increases with the increasing amount of K2MnF6. Fig. 1a is the XRD pattern of as-obtained KMOF:Mn4+

(3.28

mol%)

corresponding

with

the

JCPDS

card

of

K2[MoO2F4]•H2O (marked as KMOF) matrix (No.73-2350, P21/c, a = 6.214 Å, b = 6.192 Å, a = 18.079 Å, V = 691.47 Å3, Z = 4). The most diffraction peaks of KMOF:Mn4+ can be indexed for the JCPDS card of KMOF, but the peaks of a

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little unknown phase can be found in this pattern. In the crystal structure of

KMOF (Fig. 1b), each Mo6+ was coordinated by four F- and two O2- with four kinds of Mo-F bonds and two kinds of Mo-O bonds. So Mo6+ occupies the

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center of a seriously distorted octahedron of [MoO2F4]2-. Due to the same charge and

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similar volume between [MoO2F4]2- and [MnF6]2-, [MnF6]2- will replace [MoO2F4]2in KMOF:Mn4+. This replacement will produce the structural defects with oxygen

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vacancies and decrease the local symmetry of Mn4+ in KMOF:Mn4+.

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SEM image and EDS spectrum of KMOF:Mn 4+ (3.28 mol%) are shown in Fig. 2. This sample is well crystallized. The particles are of rod shapes with

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length of 10-15μm and width of 1-2 μm. The EDS spectrum proves the existence of F, Mo, K, O and Mn elements, indicating that Mn4+ ions have been

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doped into KMOF:Mn 4+. 3.2 Optical properties and application in w-LEDs Fig. 3a is the excitation and diffuse reflection spectra of the as-prepared KMOF:Mn4+ (3.28 mol%), accompanied with the diffuse reflection spectra of KMOF. Two broad excitation bands around 373 nm and 471 nm can be found 7

from the excitation spectrum. The excitation bands are attributed to the spin-allowed transitions 4A2–4T1g,2g of Mn4+. Correspondingly, we can observe one intense absorption band at the blue-light region from the diffuse reflection spectrum of KMOF:Mn4+. In contrast, the pure host of KMOF exhibits little absorption in the blue-light region. Fig. 3b presents the emission spectra of KMOF:Mn4+ (3.28 mol%) at different temperatures under 471 nm excitation.

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The emission peaks of Mn 4+ at room temperature are located at 597 nm, 605 nm, 611 nm, 619 nm, 627 nm, 630 nm, 643 nm. They are due to the anti-Stokes transitions with the v3, v4 and v6 vibronic modes, the ZPL, and the Stokes

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transitions with the v6, v4 and v3 vibronic modes, respectively. Obviously, the

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intense ZPL emission can be found at 619 nm, this means that Mn 4+ ions in KMOF:Mn4+ host occupy the low symmetry sites. When the temperature drops

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down to 77 K, the anti-Stokes transitions with the v3, v4 and v6 vibronic modes

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disappear in the emission spectra. Meanwhile, the relative intensity of the ZPL emission turns stronger, due to the stronger electron-phonon coupling. The

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luminescent behaviours of Mn4+ ions in KMOF:Mn4+ can be expressed by the Tanabe−Sugano energy-level diagram (Fig. 3c). The corresponding crystal-field

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strength Dq as well as Racah parameters B and C for Mn4+ in KMOF:Mn4+ can be calculated according to its PL spectra [27]. The calculated values of Dq, B, C and Dq /B are, 2123 cm-1, 510 cm-1, 4049 cm-1 and 4.16, respectively. In order to obtain the efficient phosphors, we investigated the influence of the doping amount of Mn 4+ in KMOF:Mn4+ on their PL properties. Fig. 3d 8

shows the PL spectra of KMOF:Mn 4+ with different contents of Mn 4+. All these samples exhibit similar PL spectra except for their intensities. When the doping content is up to 3.28 mol%, the as-obtained sample is of the strongest excitation and emission intensity. With the further increasing of K 2MnF6, the concentration quenching effect for Mn 4+ can be observed. It’s well known that the concentration quenching in many phosphors is due to energy transfer

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between the luminescent centers [28-30]. So, the critical energy transfer distance (Rc) between Mn4+ and Mn4+ ions was calculated using the following equation: 3𝑉

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Rc = 2(4𝜋𝑥 𝑁 )3 𝑐

(1)

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where V = 691.47 Å 3, xc =0.0328, and N = 4 in this case. Then the calculated Rc value is ~21.6 Å. In general, the distance for the electric multipole interaction is

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larger than 5-7 Å [28,31]. Thus, it is presumed that the electric multipole

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interaction dominates in the case of the KMOF:Mn4+phosphor. Additionally, we have investigated the decay curves of the 2Eg→4 A2g transitions of

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Mn4+ in KMOF:Mn4+ samples, as shown in Fig. 4a. These decay curves are well fitted into a single-exponential function and their decay times are calculated to be 3.78, 3.77,

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3.76, 3.72 and 3.64 ms, respectively. As the doping concentration of Mn4+ increases, the decay times decrease, meaning the increase of the non-radiative transition process between the Mn4+-Mn4+ pair. Besides, the quantum efficiencies of these samples are measured and the results are shown in Fig. 4b. The dependent curves of the internal quantum efficiency (IQE) and external quantum efficiency (EQE) of KMOF:Mn 4+ on 9

contents of Mn4+ are in consistence with the concentration quenching curve for Mn4+. Among these samples, KMOF:Mn4+ (3.28 mol%) has the highest IQE of 86.5 % and EQE of 35.2 %. The thermal stability of luminescence is an important parameter of a phosphor for LED applications, so we further investigated the dependence of the PL properties on the temperature. Fig. 5 shows the emission spectra of KMOF:Mn4+ (3.28 mol%) at

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different temperatures. As the temperature increases, these emission spectra are of similar shapes and the red-emitting peaks of Mn4+ are located at the same positions

without obvious shift, indicating that KMOF:Mn4+ (3.28 mol%) has high color

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stability. However, the emission intensity shows a rising trend with the increasing

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temperature (the inserted figure in Fig. 5). It touches the maximum value at the temperature of 333 K. The emission intensity at 373 K is about 77 % of the initial

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intensity at room temperature, indicating that this red-emitting phosphor is of the

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excellent thermal stability of luminescence.

Since the as-prepared KMOF:Mn4+ (3.28 mol%) exhibits excellent PL

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properties, we studied its application on w-LEDs. Fig. 6 shows the

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luminescence spectra of some LED devices under 20 mA current excitation. The blue-light peak in the curve a is the emission of the GaN chip. With the introduction of KMOF:Mn4+ (3.28 mol%), intense red-emitting peaks appear and the emission of GaN turns weaker (curve b). The bright red light can be clearly observed by the naked eyes (the inset of Fig. 6), indicating that this red-emitting phosphor can efficiently convert the blue light into the red light. 10

Curves c and d are the spectra of w-LEDs fabricated without and with KMOF:Mn4+ (3.28 mol%), respectively. The

phosphor

KMOF:Mn4+

(3.28

mol%) can obviously enhance the red emission of w-LED based on YAG yellow phosphor. The related photoelectric parameters are listed in Tab. 2. The w-LED coated by the mixture of YAG and KMOF:Mn4+ (3.28 mol%) exhibit excellent performances with a very high color rendering index (Ra) of 94, a low

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correlated color temperature (Tc) of 3945 K, and the luminous efficiency (LE) of 150.1 lm/W. So, the red phosphor KMOF:Mn4+ (3.28 mol%) is a promising red

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phosphor for warm w-LEDs.

Conclusions

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In summary, a new red-emitting phosphor KMOF:Mn4+ has been successfully synthesized using one precipitation method. The as-obtained sample KMOF:Mn4+

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(3.28 mol%) exhibits broad excitation band in the blue region and sharp emission in

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red emission. Moreover, this red phosphor also has excellent thermal stability of luminescence. A w-LED fabricated with the mixture of YAG and KMOF:Mn4+ (3.28

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mol%) exhibit perfectly optical properties (Ra = 94, Tc = 3945 K, LE = 150.1 lm/W). Therefore, the KMOF:Mn4+ could find application on warm w-LEDs. Conflict of Interest The authors declare no conflict of interest. Acknowledgements 11

This work was financially supported by the National Natural Science Foundation of China (21661033), the Joint Funds of the National Natural Science Foundation of China (NSFC)-Yunnan Provinces (No. U1702254), the Applied Basic Research Project of Yunnan Province (2017FB017), Program for Innovative Research Team (in Science and Technology) in University of Yunnan Province.

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Fig. 1 XRD pattern (a) of red phosphor KMOF:Mn4+ (3.28 mol%) and crystal structure of K2[MoO2F4]•H2O (b) (

Belong to the secondary phase ).

Fig. 2 SEM image (a) and EDS spectrum (b) of KMOF:Mn4+ (3.28 mol%). Fig. 3 Excitation spectrum of KMOF:Mn4+ (3.28 mol%) and DRS of KMOF and 16

KMOF:Mn4+ (a), emission spectra of KMOF:Mn4+ (3.28 mol%) at 77 K and 293 K(b), Tanabe−Sugano energy-level diagram of Mn4+ in an octahedral crystal field (c), PL spectra of KMOF:Mn4+ with different contents of Mn4+ (λem=627 nm, λex=471 nm) (d). The inserted image in fig. 3b is the luminescent photograph of KMOF:Mn4+ (3.28 mol%) under blue light irradiation. Fig. 4 Decay curves (a), and quantum efficiencies of KMOF:Mn4+ with different

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contents of Mn4+ (b) (λem=627 nm, λex=471 nm).

Fig. 5 Temperature-dependent emission spectra of KMOF:Mn4+ (3.28 mol%) under 471 nm excitation, the inserted figure is the temperature dependence of emission

-p

intensity of KMOF:Mn4+ (3.28 mol%).

re

Fig. 6 Luminescent spectra of LED chip (a), pink-emitting LED based on KMOF:Mn4+ (b), w-LEDs based on YAG:Ce3+ (c), w-LEDs based on the mixture of

Jo

ur

na

lP

YAG:Ce3+-KMOF:Mn4+ (d) under 20 mA current excitation.

17

K2(MoO2F4)·H2O:Mn4+

Intensity (a.u.)

(a)

20

30

40

-p

ro of

JCPDS No.73-2350

Jo

Fig. 1

ur

na

lP

re

2-Theta (o)

50

18

60

ro of -p

3 (b) 5x10

3x103 2x103

re

lP

4x10

F kα1 Mo Lα

na

Intensity (a.u.)

KKα 3

Jo

ur

1x103 O kα1 Mo Lβ K Kβ1 K Lα Mn Lα 0 0 2 4

Mn Kα 6

Energy (keV)

Fig. 2

19

8

10

(a)

100

(b) 800

λem= 627 nm 293 K 77 K

4

A2g→4T2g 40

200 4

0 300

600

ZPL

400

200

A2g→ T1g

350

v3 500

450

400

0 580

0 550

Wavelength (nm)

(d) 800

4

T1g

70

2

4

Intensity (a.u.)

60

T1g

4

T2g

50

16.6 mol% 7.51 mol% 3.28 mol% 2.21 mol% 1.40 mol%

600

re

E/B

A1g

Dq/B = 4.16

-p

80

40

2

T2g

30

2

T1g Eg

20

lP

2

10

4

A2g

0 0

1

3

2

4

5

na

Dq / B

Jo

ur

Fig. 3

680

660

640

620

600

Wavelength (nm)

(c)

v3

v4

20

4

v4

v6

ro of

60 400

Reflectance (%) Intensity (a.u.)

Intensity (a.u.)

v6 80

600

20

400

200

B B B B B

0 300

400

500

600

Wavelength (nm)

700

1

0.13534 0.04979

100 IQE EQE

80

0.01832 0.00674 0.00248

60 40 20

0

4

8

12

16

20

1.4

Tims (ms)

ro of

0.36788

Intensity (a.u.)

(b)

3.78 ms 3.77 ms 3.76 ms 3.72 ms 3.64 ms

QE (%)

(a)

2.21

3.28

7.51

-p

Contents of Mn4+ (mol%)

Jo

ur

na

lP

re

Fig. 4

21

16.59

293 K 313 K 333 K 353 K 373 K 393 K

Relative Intensity

600

0.8

0.6

0.4

0.2

0.0 300

400

320

340

360

380

400

-p

Temperature (K)

0 550

700 650 600 Wavelength (nm)

Jo

ur

na

Fig. 5

re

200

lP

Intensity (a.u.)

800

1.0

ro of

1000

22

750

ro of

(c) YAG

(b) KMOF:Mn4+

-p

Intensity (a.u.)

(d) YAG+KMOF:Mn4+

re

(a) GaN

500 600 Wavelength (nm)

700

na

lP

400

Jo

ur

Fig. 6

Tab.1 Contents of Mn4+ in KMOF:Mn4+ samples prepared with different molar ratios between (NH4)6Mo7O24•4H2O and K2MnF6. Samples

Molar

of Doping amount of Mn4+ (mol%)

ratios

(NH4)6Mo7O24•4H2O:K2MnF6

23

i

40:7

1.40

ii

30:7

2.21

iii

20:7

3.28

iv

10:7

7.51

v

5:7

16.59

ro of

Tab. 2 Important photoelectric parameters for LED devices under 20 mA current Phosphor

Tc (K)

Ra

CIE (x, y)

LE (lm/W)

GaNa

/

/

/

0.14, 0.04

33.1

red LEDb

KMOF:Mn4+

/

/

w-LEDc

YAG:Ce3+

6070

w-LEDd

3945

4+

0.44, 0.18

re 94

Jo

ur

na

KMOF:Mn

77

lP

YAG:Ce3++

-p

Device

24

51.0

0.31, 0.34

172.4

0.37, 0.35

150.1