Narrow Red Emission Band Fluoride Phosphor KNaSiF6:Mn4+ for

Subscriber access provided by Loyola University Libraries

Letter

Narrow Red Emission Band Fluoride Phosphor KNaSiF:Mn for Warm White Light-Emitting Diodes 6

4+

Ye Jin, Mu-Huai Fang, Marek Grinberg, Sebastian Mahlik, Tadeusz Lesniewski, Mikhail G. Brik, Guan-Yu Luo, Jauyn Grace Lin, and Ru-Shi Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01905 • Publication Date (Web): 22 Apr 2016 Downloaded from http://pubs.acs.org on April 24, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Narrow Red Emission Band Fluoride Phosphor KNaSiF6:Mn4+ for Warm White Light-Emitting Diodes Ye Jin†‡, Mu-Huai Fang†, Marek Grinberg§, Sebastian Mahlik§, Tadeusz Lesniewski§, M.G. Brik∇ ┴ £¤¥ , Guan-Yu Luo #, Jauyn Grace Lin#, and Ru-Shi Liu†П* † Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. ‡ School of Optoelectronic Information, Chongqing University of Technology, Chongqing 400054, China. § Institute of Experimental Physics, University of Gdańsk, 80-952 Gdańsk, Poland.

∇ College of Sciences, Chongqing University of Posts and Telecommunications, Chongqing 400065, China. £ Institute of Physics, University of Tartu, Tartu 50411, Estonia.

¤ Institute of Physics, Jan Dlugosz University, Czestochowa PL-42200, Poland. ¥ Institute of Physics, Polish Academy of Sciences, Warsaw 02-668, Poland. ┴ Department of Physics, Nation Taiwan University, Taipei 10617, Taiwan. # Center for Condensed Matter Sciences, Nation Taiwan University, Taipei 10617, Taiwan. П Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan.

4+

KEYWORDS: KNaSiF6:Mn , red phosphor, ZPL, high pressure, warm white LED ABSTRACT: Red phosphors AMF6:Mn4+ (A=Na, K, Cs, Ba, Rb; M=Si, Ti, Ge) have been widely studied due to the narrow red emission bands around 630 nm. The different emission of the zero-phonon line (ZPL) may affect the color rendering index of White Light-Emitting Diodes (WLED). The primary reason behind the emergence and intensity of ZPL, taking KNaSiF6:Mn4+ as an example, was investigated here. The effects of pressure on crystal structure and luminescence were determined experimentally and theoretically. The increase of band gap, red shift of emission spectrum and blue shift of excitation spectrum were observed with higher applied pressure. The angles of ∠FMnF and ∠FMF(M = Si, Ti, Ge) were found clearly distorted from 180° in MF62octahedron with strong ZPL intensity. The larger distorted SiF62- octahedron, the stronger ZPL intensity. This research provides a new perspective to address the ZPL intensity problem of the hexafluorosilicate phosphors caused by crystal distortion and pressuredependence of the luminescence. The efficacy of the device featuring from Y3Al5O12:Ce3+ (YAG) and KNaSiF6:Mn4+ phosphor was 118 lm/W with the color temperature of 3455 K. These results reveal that KNaSiF6:Mn4+ presents good luminescent properties and could be a potential candidate material for application in back-lighting systems.

White light-emitting diodes (WLEDs) are used extensively in backlights, flashlights, and automobile headlamps as replacements for conventional incandescent and fluorescent lamps because of their high energy efficiency, durability, reliability, and capacity for use in products with various sizes and more eco-friendly components.1-2 Phosphors have been widely investigated for use in phosphor-converted WLEDs. The most widely used WLED combines the yellow phosphor Y3Al5O12:Ce3+ (YAG) with a blue InGaN LED chip. However, this LED presents a low color rendering index (CRI) (Ra < 80) and high correlated color temperature (CCT > 6000 K) because of the absence of red emission in its spectrum.3 Therefore, red phosphors, such as Sr2Si5N8:Eu2+ and (Sr,Ca)AlSiN3:Eu2+ or Mn4+-doped red phosphors, have been added to the devices in order to improve their color-rendering properties.4–15 The broad emission band of nitride phosphors is often longer than 650 nm, and serious reabsorption renders them unsuitable for improving the efficiency of a device.

Therefore, Mn4+-doped red phosphors, especially AMF6:Mn4+ (A = Na, K, Cs, Ba, Rb; M = Si, Ti, and Ge) with narrow band emissions at around 630 nm, have recently attracted increased research attention.5–15 Team of Adachi et al.7-12 synthesized a series of fluorides, including KNaSiF6, using wet chemical etching. The authors demonstrated the luminescence properties of the phosphor at normal pressure and showed that the KNaSiF6:Mn4+ system is an intermediate of homodialkaline K2SiF6:Mn4+ and Na2SiF6:Mn4+ narrow emission band red phosphors.11 While Zhu et al.13 successfully synthesized K2TiF6:Mn4+ through a simple cation exchange reaction. The emission wavelength of the d-d transition of Mn4+ ions is easily influenced by various environmental factors, contrary to the parity-forbidden f-f transitions of rare-earth ions. The AMF6:Mn4+ (A = Na, K, Cs, Ba, Rb; M = Si, Ti, and Ge) have different zero-phonon lines (ZPLs) because of their

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

MF62- octahedral structures.12 In this study, for example, whereas K2SiF6:Mn4+ shows no ZPL intensity,13,14 an intense ZPL is observed for Na2SiF6:Mn4+.15 K2GeF6:Mn4+ exhibits two phases, one with an intense ZPL and another without ZPL.5 In this work, KNaSiF6:Mn4+ was synthesized via a simple co-precipitation method. The intermediate phase of KNaSiF6:Mn4+ showed different structures and luminescent properties, especially in terms of ZPL intensity, from K2SiF6: Mn4+ and Na2SiF6: Mn4+. The mechanism of the ZPL emergence, as well as its intensity and position, has not been previously reported yet. This study aims to elucidate the mechanism of the ZPL intensity in AMF6:Mn4+ (A = Na, K, Cs, Ba, Rb; M = Si, Ti, and Ge). We clearly found variations in the angles ∠ = ∠( = , , ) of the phosphors AMF6:Mn4+ (A = Na, K, Cs, Ba, Rb; M = Si, Ti, and Ge). Distortion of the MnF62- octahedron is the key factor influencing ZPL intensity. The properties of the resultant phosphors were examined by photoluminescence (PL) under both ambient pressure and high pressure and are verified by the calculation method. Moreover, the LED obtained from mixing of the narrow-band red emission phosphor KNaSiF6:Mn4+ with conventional YAG:Ce3+ yellow phosphor can be excited by blue LED chip. This LED shows an enhanced CRI for lighting and backlighting applications. The fluoride phosphor compounds, namely, KNaSiF6:0.06 Mn4+, K2SiF6:0.06 Mn4+ and Na2SiF6:0.06 Mn4+, were prepared from high-purity NaF, KF, SiO2, HF, KMnO4, and H2O2 via two-step co-precipitation.14,15 K2MnF6 was prepared first A mixture consisting of 6.7 g of KF and 0.45 g of KMnO4 was dissolved in a 30 mL solution of 48% HF. Then, 0.3 mL of H2O2 was added to the solution drop by drop. The solution gradually turned from deep purple to yellow, and a yellow precipitate was obtained. The powder was washed several times using alcohol and acetone and then dried at 70 °C. The prepared K2MnF6 powder (0.30 g) was dissolved in silicon fluoride solution (1.2 g of SiO2 in 35 mL of 48% HF) with stirring. Meanwhile, 1.26 g of NaF and 1.75 g of KF were dissolved in 15 mL of 48% HF. Then, the NaF and KF solution was added to the K2MnF6 solution with stirring for 5 min. A yellow powder was obtained; this powder was washed with ethanol (99.9%) and dried at 70 °C. The samples were subsequently prepared for measurement. K2SiF6:0.06 Mn4+ and Na2SiF6:Mn6+ were prepared using the same method. In order to prevent any injure form hydrogen fluoride, all the experimental processes are conducted under well protective cloth, glove and goggle. The samples were analyzed by X-ray diffraction (XRD) using a D2 Phaser (Bruker) diffractometer operating with Cu Kα radiation (λ = 1.5418 Å) to identify their phase purity. The synchrotron XRD pattern of KNaSiF6:Mn4+ was also examined for refinement at the BL01C2 beamline of the National Synchrotron Radiation Research Center (Taiwan) at a wavelength of 0.688808 Å. Total Pattern Analysis Solution (TOPAS) software was used to perform XRD Rietveld refinement. PL spectra were measured using a FluoroMax-3 spectrophotometer. Temperature-dependent luminescence (25300 °C) was determined using a THMS-600 apparatus combined with PL equipment. All solid-state nuclear magnetic resonance (NMR) spectra were acquired on a 14.1 T widebore Bruker Avance III spectrometer equipped with a 4 mm

Page 2 of 11

magic-angle-spinning (MAS) probe head. The Larmor frequency for 29Si was 119.2 MHz, the spinning rate for the 29 Si MAS NMR spectra was 10 kHz, and the recycle delay was 60 s. PL excitation (PLE) spectra were recorded using a system consisting of a 150 W Xe lamp, two monochromators SPM2 and two Hamamatsu R928 photomultipliers (the first for luminescence and the second for reference signal detection). Steady-state luminescence measurements were performed with an Andor SR-750-D1 spectrometer equipped with a CCD camera (DU420A-OE). The excitation source was a He–Cd laser at 442 nm. We used the system consisting of a PL 2143 A/SS laser and a PG 401/SH parametric optical generator to obtain luminescence decays. Emission signals were analyzed using a Bruker Optics 2501S spectrograph and a Hamamatsu Streak camera (Model C4334-01). High hydrostatic pressure was applied to a Merrill Bassett-type diamond anvil cell. Polydimethylsiloxane oil was used as the pressure-transmitting medium, and pressure was measured in terms of shifts in the R1 luminescence line of ruby. The composition, crystal structure, and phase purity of the as-synthesized samples were investigated by powder XRD. The synchrotron XRD with synchrotron light source and TOPAS fitting software were also used to obtain more detailed structural information of KNaSiF6:Mn4+ phosphor. Figure 1a displays the Rietveld structure refinement results of KNaSiF6:0.06Mn4+. The crystallographic data of KNaSiF6 are used as the initial structure model during refinement, and the actual structure of KNaSiF6:0.06Mn4+ is constructed. The diffraction peaks of KNaSiF6:Mn4+ could be indexed to an orthorhombic Pnma structure (ICSD No. 07-1334). The lattice parameters, including a = 9.33208(21) Å, b = 5.50742(11) Å, c = 9.80288(22) Å, α = β = c = 90°, and cell volume = 503.825(19) Å3, are also accurately determined. The χ2 and RBragg values are 1.44 and 6.01 respectively, which indicates the validity of the refinement process. The bond angle of the central atom was examined to study the relationship between the phosphor stucture and its PL spectrum. The refinement data show that there are two differenet bond angles. The angle of the axial bond pair is approximately 179.29°, while the angle of two other bond pairs in the equatorial part is 174.11°. When Mn4+ ions are introduced to the host, these ions substitute Si4+ sites to form the MnF62- group, comparing with the ionic radii and valence states (rSi4+ = 0.040 nm and rMn4+ = 0.053 nm), and the unit cell crystal structure and coordination environment of KNaSiF6 are shown in Figure 1b. A KNaSiF6 unit cell clearly contains four formula units. Only one type of SiF62- octahedral group is observed. The Si4+ ions are surrounded by six F− ions, with two different angles: ∠ = 174.11° and ∠ ! = 179.29° (Figure 1c). The 29Si NMR spectrum of KNaSiF6:0.06Mn4+ is shown in Figure 1d. The spectrum of KNaSiF6 contains only one resonance at -188 ppm because this phosphor presents only one Si six-coordinate site.

2


Page 3 of 11

a

Comparative results from Table 1 show that the experimental and theoretical structural data (including the lattice constants and fractional atomic positions) present good agreement among all data sets. The largest differences between experimental and optimized lattice constants were +4% for constant c (LDA calculations) and –3% for a constant (GGA calculations). An overall agreement with the experimental data is beneficial for the GGA calculations.

Intensity (a.u.)

Experimental Fitting curve Difference Background Standard

Table 1. Experimental and calculated structural data of KNaSiF6 Exp.

10

15

20

25

30

35

40

2θ (degree)

d

-188

-500

-400

-300

-200

-100

0

100

200

300

29

Si chemical shift (ppm)

Figure 1. (a) Rietveld refinement results of the powder X-ray diffraction profile of KNaSiF6:0.06Mn4+ phosphor (Rwp = 10.48%, Rp = 8.18% and χ2 = 1.44); (b) crystal structure of KNaSiF6:Mn4+; (c) crystal structure of the SiF62- octahedron; (d) 29 Si NMR spectrum of KNaSiF6:0.06Mn4+ In order to investigate the structure well, two sets of density functional theory (DFT)-based calculations were performed. These calculations are based on generalized gradient approximation (GGA) with the Perdew-BurkeErnzerhof (PBE) functional16 and local density approximation (LDA) with the Ceperley-Alder-Perdew-Zunger (CA-PZ) functional.17,18 The calculations are performed with the CASTEP module19 of the Materials Studio package, and a summary of the experimental and calculated structural data for KNaSiF6 is presented in Table 1. The electron configurations are considered as follows: Na, 2s22p63s1; K, 3s23p64s1; Si, 3s23p2; and F, 2s22p5. The cut-off energy is chosen as 370 eV. The k-points mesh is set to 5×8×5, corresponding to a separation between k-points of 0.02 1/Å. The convergence criteria are as follows: energy, 10-5 eV/atom; maximal force, 0.03 eV/Å; maximal stress, 0.05 GPa; and maximal displacement, 0.001 Å.

LDA GGA Lattice constants (Å) a 9.3246 9.102674 (+2.4%) 9.606752 (-3.0%) b 5.4992 5.293121 (+3.7%) 5.583111 (-1.5%) c 9.7892 9.388901 (+4.1%) 9.840516 (-0.5%) Fractional atomic positions x y z x y z x y z K 0.5136 0.2500 0.1798 0.5154 0.2500 0.1747 0.5117 0.2500 0.1753 Na 0.3718 0.2500 0.5596 0.3789 0.2500 0.5501 0.3736 0.2500 0.5506 Si 0.2290 0.2500 0.9206 0.2300 0.2500 0.9226 0.2288 0.2500 0.9214 F1 0.3274 0.4660 0.9941 0.3316 0.4757 0.9998 0.3288 0.4684 0.9952 F2 0.1254 0.0367 0.8548 0.1221 0.0262 0.8525 0.1241 0.0322 0.8542 F3 0.1244 0.2500 0.0601 0.1191 0.2500 0.0679 0.1230 0.2500 0.0642 F4 0.3313 0.2500 0.7843 0.7806 0.2500 0.7806 0.3323 0.2500 0.7813

After optimization of the KNaSiF6 structure, the electronic and optical properties of the phosphor were calculated at ambient and elevated hydrostatic pressure. Figure 2 shows the calculated band structure at ambient pressure. The band gap is direct, and the band gap values are 7.417 eV (LDA) and 6.98 eV (GGA). The real band gap is approximately 8-9 eV because DFT-calculated band gaps are always underestimated. The valence band of KNaSiF6 is remarkably flat, which reveals the very low mobility of the holes. By contrast, the dispersion of electronic states forming the conduction band is highly pronounced. Well-resolved sub-bands separated by narrow gaps are found in the valence band. The nature of these gaps, as well as the origin and composition of all calculated bands, can be understood with the help of the density of states (DOS) diagrams (Figure 3). 12 10 8

Energy (eV)

5

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

GGA, 6.980 eV LDA, 7.417 eV

4 2 0 -2 -4 -6 -8

G

Z

T

Y

S

X

U

R

Figure 2. Calculated band structure (GGA-calculated: solid line; LDA-calculated: dashed line) of KNaSiF6 The conduction band is formed by the 3s and 4s states of Na and Ka, respectively. The valence band is formed by the 2p states of F and 3p states of Si. The 2p states of F ions at nonequivalent crystallographic sites are separated from each other and form the unusual structure of the valence band. The 3p states of Si sharply peak at the bottom of the valence band at approximately -4 eV, whereas the F 2p states are spread 3


ACS Applied Materials & Interfaces over the whole valence band. The 3p states of K produce a sharp peak at approximately -9 eV, whereas the 2p states of Na, 2s states of F are spread from around -18 eV to -22.5 eV. Density of states, electrons/(cell eV)

50

K

25 0 50

s states p states

%$Na

25 0 6

Si

0 80

F

40 0 80

Total

40

-25

-20

-15

-10

-5

0

5

10

15

Energy (eV)

Figure 3. Calculated density of states diagrams of KNaSiF6 The calculated effective Mulliken charges are as follows (LDA/GGA data are given): K +1.04/+1.02, Na +0.74/+0.76, Si +2.10/+2.16, and F -0.65/-0.65. Nearly all of the ions (except for K) show charges that are very different from those expected from their chemical formula. These findings indicate a considerable degree of covalency between the Si–F and Na– F bonds; K ions are bound to their neighbors in the crystal lattice predominantly by ionic bonds. 10.00

b

LDA GGA

b, b,

c=9.758 9-0.0

9.25

c=9.32510.0

ÅÅÅÅ

B=24.25+/-0.58 GPa B' = 6.18 +/-0.25

0.85

B=22.83+/-0.32 GPa B' = 4.13 +/-0.12

0

2

4

8.0

a=9.0

8.50 8.25

6

8

5.6 5.4

10

0

d

Eg, LDA Eg, GGA

2

4

6

8

(1)

where V and V0 are the volumes at elevated pressure P and ambient pressure, respectively, B is the bulk modulus, and B′ is its pressure derivative. Figure 4a shows the results of the relative volume of a unit cell change in the LDA/GGA runs. The fit of the calculated data (symbols) to the Murnaghan equation (solid line) yields the following values: for LDA, B = 24.25±0.58 GPa, B′ = 6.18±0.25; and for GGA, B = 22.83±0.32 GPa, B′ = 4.13±0.12. The lattice constants a, b, and c also decrease with pressure according to the linear (for a and c) and quadratic (for b) fits shown in Figure 4b. Furthermore, the band gap changes with pressure (Figure 4c). Figure 4d shows the cross-section of the electron density difference. In this figure, the loss of electrons is described by a blue coloration, and electron enrichment is indicated by a red coloration. The white regions correspond to a very small difference in electron density compared with that of a free atom. These characteristics confirm our conclusion obtained from the analysis of effective Mulliken charges that the chemical bonds between Si and F ions are strongly covalent because of the overlap of red and blue colors, whereas Na and K ions are bound to the crystal lattice by ionic bonds because of the scarce overlap of colors. KNaSiF6: Mn4+ 4 A2g→4T2g

a 4

a=9.05 790-0 .0925P 627-0 .0823P

8.75

b=5.57 64-0.0 701P+ 0.0031 2 b=5.28 P 20-0.0 528P + 0.002 2 5.0 6P

Prressure P (GPa)

8.2

673 P

479 P

5.2

0.80

8.4

Lattice constants ( )

9.00

0.90

c, LDA c, GGA

9.50

0.95

c

a, a,

9.75

,

) + . *

= (1 + '( )

A2g→4T1g

2

Eg→4A2g

λex =460 nm

Intensity (a.u.)

1.00

Relative volume change V/V0

a

considered material at elevated pressures. The pressure range was set from 0 GPa (ambient pressure) to 10 GPa, with a step size of 2 GPa. Decreases in the volume of a crystal with pressure can be fitted to the Murnaghan equation of state, as follows:20 $

0

Band gap Eg (eV)

10

Pressure P (GPa)

λem =629 nm

Eg=7.479 + 0.082P

350

7.8

400

450

500

550

600

650

700

Wavelength (nm)

7.6

4+

: Mn b λKNaSiF =460 nm

7.4

6

Eg=7.061 + 0.094 P 7.2

ν6

ex

7.0 6.8 0

2

4

6

8

10

Pressure P (GPa)

Figure 4. (a) Calculated dependence of the relative volume V/V0 change (filled/open symbols, LDA/GGA) of KNaSiF6 and fitting to the Murnaghan equation (solid lines); (b) calculated dependence of the lattice constants (filled/open symbols, LDA/GGA) of KNaSiF6 and fitting as a function of pressure P (solid lines); (c) calculated dependence of the band gap (filled/open symbols, LDA/GGA) of KNaSiF6 and fitting as a function of pressure (solid lines); (d) Cross-section of the electron density difference in KNaSiF6 in the plane perpendicular to the b axis. The effects of pressure on the physical properties of a solid can be modeled by optimizing the structure of a

ZPL

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 11

ν4

ν6 ν4

ν3

580 590 600 610 620 630 640 650 660 670 680

Wavelength (nm)

Figure 5. (a) Photoluminescence (PL) and PLE spectra of KNaSiF6:Mn4+; (b) PL spectra of KNaSiF6:Mn4+ phosphor. 4


PL and PLE spectra of the phosphors measured at room temperature are shown in Figure 5a. The excitation spectrum contains two broad bands around 355 nm and 460 nm (matching with the LED chips), corresponding with the spinallow transitions 4A2g→4T1g and 4A2g→4T2g of Mn4+, respectively. The phosphor emits red emissions originating from the spin-forbidden d-d transition 2Eg→4A2g with several narrow bands at 610-650 nm under 460 nm excitation. The strongest emission is found at 629 nm. According to group theory, there are six fundamental internal vibronic modes, namely, ν1, ν2, ν3, ν4, ν5, and ν6, of the octahedral group with Oh symmetry.13 Three peaks (longer than 620 nm) belong to Stokes ν6, ν4, and ν3 peaks, and two other peaks (shorter than 620 nm) are anti-Stokes ν6 and ν4 peaks. The peak at approximately 620 nm is the ZPL (Figure 5b), which is the electronic dipole forbidden in the octahedral MnF62-. The MnF62- octahedron is distorted because of ∠ = ∠ = 174.11° and ∠ ! = ∠ ! = 179.29° (Figure 1c). Intense ZPL could be observed at 620 nm. The higher distortion of MnF62- octahedron gives rise to the stronger the ZPL intensity. The distortion of a coordination octahedron around Mn4+ ion removes the center of inversion and then the parity selection rule is lifted (there are no odd/even states). The following examples and the PL spectra of KNaSiF6:Mn4+ under high pressure are in good agreement with that fact or conclusion. ZPL intensities are different among K2SiF6:Mn4+, KNaSiF6:Mn4+, and Na2SiF6:Mn4+ (at 616 nm) (shown in Figure S1) due to their distinct distortions. The crystal structures of K2SiF6 and Na2SiF6 are shown in Figure S2 and Figure S3. Only one type of MnF62- octahedron without distortion (∠ = ∠ = 180°) is found in K2SiF6. Thus, no ZPL or very weak is found in the PL spectrum of this phosphor.8,9,13,14 By contrast two types of MnF62- octahedrons are found for Na2SiF6. One octahedron is without distortion ( ∠ = ∠ = 180° ), but the other is seriously distorted (∠ = ∠ = 171.48°). Thus, a strong ZPL is observed in KNaSiF6:Mn4+ phosphor. The position of the ZPL depends on the perturbation of the cation ion. The full-width at half maximum of the PLE provides more evidence of distortion. The orbital overlaps or hybridizes because of the distortion. Moreover, the PLE spectrum is broadened. As the angles of K2SiF6, KNaSiF6 and Na2SiF6 are 180°, 174.11° and 171.48°, respectively (just considering only larger distortions), the full width at half maximum of the PLE should follow the order: K2SiF6 (E, \) = ]O Ê, _ Ò> (\) + ] a Ê, _ bc+d ∑k

h (i) j fg %h

(8)

(UWkℏl)

In Eq. (8), ψn ^q, _ and ] a Ê, _ are the electronic wave functions describing the 2Eg and 4T2g states, r is the electronic coordinate, and Ò> (\), àk (\) are the respective vibronic wave functions of the 2Eg and 4T2g electronic manifolds. VS-OVp-q is the spin orbit coupling constant, which can be determined as follows: bc+d = ? ]O∗ Ê, _ sc+d ] a Ê, _ DE (9) where HS-O is the spin-orbit coupling Hamiltonian and S0n is the vibronic overlap integral, which calculated as follows: h t j5

10

>k = ? Ò>∗ (\) àk (\)D\ = +5 (10) √k! 2 4 Eg→ A2g luminescence lifetime can be calculated from following relation:

Time (ms)

w

=

wg

|bc+d | ∑k

j%h 5

(11)

(UWkℏl)5

where 1/τy is the probability of 4T2g→4A2g radiative transition. When both Sℏω and ℏω are smaller than the separation energy ∆, the following relation can be obtained:

13 12

decay time (ms)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11

10

w

9

≈

|${|} |5 wg

(12)

(∆Wjℏl)5

The 2Eg →4A2g luminescence lifetime can be calculated using Eq.(12), and the results are presented by the solid curve

8 7

in Figure 9b. Fitting was achieved by assuming that

6

9

5 4 0

5

10

15

20

25

30

Pressure (GPa) Figure 9. (a) 2Eg→4A2g decay profiles of KNaSiF6:Mn4+; (b) decay times of 2Eg→4A2g emission in KNaSiF6:Mn4+ as a

-2 -1

|${|} |5 wg

is

equal to (5.3±0.5)×10 cm s and is independent of pressure. For pressures greater than 15 GPa the lifetime τ increase slower, then it is predicted by Eq. (12). This result can be explained by the non-hydroelasticity of pressure, which generates shear-strength in the pressure transmitting medium and sample. The 4T2g→4A2g transition is parity forbidden and can be allowed due to interactions with optical phonons and 7


ACS Applied Materials & Interfaces odd parity lattice distortions. The effects of shear-strength at high pressure generate odd parity distortion that diminishes the value of τT and this effect is responsible for additional diminishing of the Mn4+ luminescence decay time at high pressure, not considered in relation (12). It is consistent with the effect of the increase of the intensity of the ZPL with respect to phonon repetitions with increasing pressure. Considering possible distortions one can related this effect to odd parity static distortion of the MnF6 system induced by pressure above 15 GPa. Figure 10 shows the temperature-dependent luminescent spectrum of KNaSiF6:Mn4+ under 460-nm excitation. No peak shifts while the intensity decreases as the temperature increasing. The integrated area, rather than the intensity, is considered in this study because of the narrow-band emissions of Mn4+. Figure 10b shows the integrated area of KNaSiF6:Mn4+ in the temperature range of 298-573 K. At 350 K, the relative intensity remains over 100% compared with that at 298 K and then decrease to 79% at 400 K. This result indicates excellent thermal stability for LEDs because the chip temperature of LED is approximately 400 K. The KNaSiF6:Mn4+ red-emitting phosphor is a prospective candidate material for LEDs.

a Intensity (a.u.)

25 oC 50 oC 75 oC 100 oC 125 oC 150 oC 175 oC 200 oC 225 oC 250 oC 275 oC 300 oC

575

600

625

650

where LS is the emission spectrum of the studied sample, ES is the spectrum of the light used to excite the sample, and ER is the spectrum of the excitation light without the sample in the integrating sphere. The internal efficiency QE of KNaSiF6:0.06Mn4+ at 460 nm excitation is 0.90, and its external QE is 0.41. All of the results indicate that KNaSiF6:Mn4+ phosphor may be a candidate material for LED applications. Package experiments were performed to evaluate the potential application ability for KNaSiF6:Mn4+ (Figure 11). Table 3 compares two different LED devices. The Ra and R9 of the device combining a blue LED chip with YAG phosphor are only 69 and 0, respectively, because of the absence of red phosphor. Moreover, the CCT is 5318 K; thus, the device emits cool white light. The CCT should be lower than 4000 K and Ra should be higher than 80 to obtain warm white light. For comparison, the device consistent with the blue chip, YAG, and KNaSiF6:Mn4+ shows warm white light with Ra = 90 and R9 = 93. The CCT of this device is much lower than that of the previous one. Normally, adding red phosphor into LED devices will decrease their luminescent efficacy because of Stokes shifting. However, the efficacy of the device we fabricated from YAG and KNaSiF6:Mn4+ phosphor was 118 lm/W as the color temperature is 3455 K. These results reveal that KNaSiF6:Mn4+ presents good luminescent properties and could be a potential candidate material for use in for backlighting systems.

a

675

Wavelength (nm)

b

1.0

0.8

Intensity (a.u.)

b b

0.6

c

0.4

0.2

d 0.0

Blue Chip + YAG + NaKSiF6:Mn4+

300

350

400

450

500

550

600

Temperature (K)

Figure 10. (a) Temperature-dependent thermal luminescent spectrum of KNaSiF6:Mn4+ and (b) relative intensity as integrated emission spectra

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 11

450

Quantum efficiency (QE), another important parameter for practical applications, can be calculated using the following expression: ηiO =

? {

? O +? O{

(14)

500

550 600 Wavelength (nm)

650

700

Figure 11. (a) Chromaticity coordinates of the devices of the Commission Internationale de l’Eclairage 1931 color spaces; (b) light-emitting diode (LED) device with blue chip and YAG phosphor; (c) LED device with blue chip, YAG, and 8


Page 9 of 11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


KNaSiF6:Mn4+; (d) electroluminescent spectra of blue chips with YAG and KNaSiF6:Mn4+ Table 3. Photoelectric parameters of the LEDs Phosphor blend

YAG+KNaSiF6:Mn4+ YAG

CCT Ra R9 Chromaticity Efficacy (K) coordinate (lm W-1) x y 3455 90 93 0.3977 0.3654 118

5318 69 0 0.3374 0.3614

164

In summary, a simple two-step co-precipitation method was used to synthesize the hexafluorosilicate phosphor compounds, namely, Na2SiF6:Mn4+, K2SiF6:Mn4+, and KNaSiF6:Mn4+. We clearly demonstrated the luminescence properties and ZPL intensity of the phosphors based on their PL spectra under ambient pressure and high pressures. Distortion of the MnF62- octahedron was demonstrated based on the crystal structure of the phosphors. Analytical results from the PL spectra under ambient pressure and high pressure, XRD measurements, and corresponding calculations of lattice parameters showed that the ZPL intensity of the phosphors behaves as follows: the ZPL intensity appears when the MnF62- octahedron is distorted and the more distorted the angle ∠ = ∠( = , , ) is from 180°, the more intense the ZPL intensity. This study provides a new perspective of the ZPL intensity of AMF6:Mn4+ phosphors. The LED device was fabricated from YAG and KNaSiF6:Mn4+ phosphor and its efficacy was 118 lm/W with the color temperature of 3455 K. These results reveal that KNaSiF6:Mn4+ presents good luminescent properties and could be a candidate material for use in back-lighting systems.

Commission (No. KJ1500913). MB appreciates the support from the Programme for the Foreign Experts offered by Chongqing University of Posts and Telecommunications, European Regional Development Fund (Center of Excellence “Mesosystems: Theory and Applications,” TK114), Ministry of Education and Research of Estonia, Project PUT430, the Recruitment Program of High-end Foreign Experts (No. GDW20145200225). MB also thanks the Institute of Physics, Polish Academy of Sciences for the Visiting Professorship position. We thank Dr. G. A. Kumar (University of Texas at San Antonio) for the Materials Studio package.

References (1)

(2) (3)

(4)

(5)

Associated content Supporting Information. Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Crystal data and Rietveld refinement results of the 4+ KNaSiF6:0.06Mn phosphor; PL and PLE spectra of 4+ 4+ 4+ and Na2SiF6:Mn ; Crystal Na2SiF6:Mn , KNaSiF6:Mn 4+ 2structures of K2SiF6:Mn and the SiF6 octahedral; Crystal 4+ 22structures of Na2SiF6:Mn , Si(1)F6 octahedral, and Si(2)F6 octahedral; Dependence of IZPL/Iν6 ratio on pressure

Author information

(6)

(7)

(8)

(9)

Corresponding Author *E-mail: [email protected]

(10)

Notes The authors declare no competing financial interest.

(11)

Acknowledgment The authors are grateful to the Ministry of Science and Technology of Taiwan (Contract Nos. of MOST 104-2119-M002-027-MY3 and MOST 104-2923-M-002-007-MY3) and National Center for Research and Development Poland Grant (No. SL-TW2/8/2015). YJ thanks the National Nature Science Foundation of China (No. 11104366), Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2014jcyjA50018), and the Scientific and Technological Research Program of Chongqing Municipal Education

(12)

(13)

Nakamura, S.; Mukai, T.; Senoh, M., Candela-class High-brightness InGaN/AlGaN Double-heterostructure Blue-light-emitting Diodes. Appl. Phys. Lett. 1994, 64, 1687-1689. Schubert, E. F.; Kim, J.K., Solid-State Light Sources Getting Smart. Science 2005, 308, 1274-1278. Jang, H. S.; Im, W. B.; Lee, D. C.; Jeon, D. Y.; Kim, S. S., Enhancement of Red Spectral Emission Intensity of Y3Al5O12:Ce3+ Phosphor via Pr Co-doping and Tb Substitution for the Application to White LEDs. J. Lumin. 2007, 126, 371-377. Tsai, Y. T.; Chiang, C. Y.; Zhou, W.; Lee, J. F.; Sheu, H. S.; Liu, R. S., Structural Ordering and Charge Variation Induced by Cation Substitution in (Sr,Ca)AlSiN3:Eu Phosphor. J. Am. Chem Soc. 2015, 137, 8936-8939. Wei, L. L.; Lin, C. C.; Wang, Y. Y.; Fang, M. H.; Jiao, H.; Liu, R. S., Photoluminescent Evolution Induced by Structural Transformation Through Thermal Treating in the Red Narrow-Band Phosphor K2GeF6:Mn4+. ACS Appl. Mater. Interfaces, 2015, 7, 10656-10659. Kim, M.; Park, W. B.; Bang, B.; Kim, C. H.; Sohn, K. S., Radiative and non-radiative decay rate of K2SiF6:Mn4+ phosphors. J. Mater. Chem. C 2015, 3, 5484-5489. Xu, Y.; Adachi, S., Properties of Na2SiF6:Mn4+ and Na2GeF6:Mn4+ Red Phosphors Synthesized by Wet Chemical Etching. J. Appl. Phys. 2009, 105, 013525-1013525-6. Takahashi, T.; Adachi, S., Mn4+-Activated Red Photoluminescence in K2SiF6 Phosphor. J. Electrochem. Soc. 2008, 155, E183-E188. Arai, T.; Arai, Y.; Takahashi, T.; Adachi, S., A Yellow Phosphor K2SiF6 Activated by Mn2+ Ions. J. Appl. Phys. 2010, 108, 063506-1-063506-7. Sekiguchi, D.; Adachi, S., Synthesis and Optical Properties of BaTiF6:Mn4+ Red Phosphor. ECS J. Solid State Sci. Technol. 2014, 3, R60-R64. Adachi, S.; Abe, H.; Kasa, R.; Arai, T., Synthesis and Properties of Hetero-Dialkaline Hexafluorosilicate Red Phosphor KNaSiF6:Mn4+. J. Electrochem. Soc. 2012, 159, J34-J37. Hoshino, R.; Adachi, S., Photo-induced degradation and thermal decomposition in ZnSnF6·6H2O:Mn4+ redemitting phosphor, Opt. Mater. 2015, 48, 36-43 Zhu, H. M.; Lin, C. C.; Luo, W. Q.; Shu, S. T.; Liu, Z. G.; Liu, Y. S.; Kong, J. T.; Ma, E.; Cao, Y. G.; Liu, R. S.; Chen, X. Y., Highly Efficient Non-rare-earth Red 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

(22) (23) (24)

(25)

(26) (27)

(28)

(29)

(30)

Page 10 of 11

Emitting Phosphor for Warm White Light-emitting Diodes. Nat. Commun. 2014, 5, 4312-1-4312-10. Nguyen H. D.; Lin C. C.; Liu R. S., Waterproof Alkyl Phosphate Coated Fluoride Phosphors for Optoelectronic Materials. Angew. Chem. Int. Ed. 2015, 54, 10862-10866. Nguyen H. D.; Lin C. C.; Fang, M. H.; Liu R. S., Synthesis of Na2SiF6:Mn4+ Red Phosphors for White LED Applications by Co-precipitation. J. Mater. Chem. C 2014, 2, 10268-10272. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. Ceperley, D. M.; Alder, B. J., Ground State of the Electron Gas by a Stochastic Method. Phys. Rev. Lett. 1980, 45, 566-569. Perdew, J. P.; Zunger, A., Self-interaction Correction to Density-functional Approximations for Many-electron Systems. Phys. Rev. B 1981, 23, 5048-5079. Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C., First Principles Methods Using CASTEP. Z. Kristallogr. 2005, 220, 567570. Murnaghan, F. D., The Compressibility of Media under Extreme pressures. P. Natl. Acad. Sci. USA 1944, 30, 244-247. Grinberg, M., Spectroscopic Characterisation of Disordered Materials Doped with Chromium. Opt. Mater. 2002, 19, 37-45. Tanabe, Y.; Sugano, S., On the Absorption Spectra of Complex Ions II. J. Phys. Soc. Jpn. 1954, 9, 766-779. Henderson, B.; Imbush, G. F., Optical spectroscopy of Inorganic Solids, Oxford University Press, 1989. Reisfeld, M. J.; Matwiyoff, N. A.; Asprey, L. B., The Electronic Spectrum of Cesium Hexafluoromanganese(IV). J. Mol. Spectrosc. 1971, 39, 8-20. Henderson, B.; Bartram, R. H., Crystal–Field Engineering of Solid State Laser Materials, Cambridge, University Press, Cambridge 2000. Yen, W. M.; Shionoya, S.; Yamamoto, H., Phosphor Handbook. CRC Press 2006, p. 169 Grinberg, M.; Suchocki, A., Pressure-induced Changes in the Energetic Structure of the 3d3 Ions in Solid Matrices. J. Lumin. 2007, 125, 97-103. Kaminska, A.; Kaczor, P.; Durygin, A.; Suchocki, A.; Grinberg, M., Low-temperature High-pressure Spectroscopy of Lanthanum Lutetium Gallium Garnet Crystals Doped with Cr3+ and Nd3+. Phys Rev B 2002, 65, 104106-1-104106-8. Galanciak, D.; Perlin, P.; Grinberg, M.; Suchocki, A., High Pressure Spectroscopy of LLGG Doped with Cr3+. J. Lumin. 1994, 60-61, 223-226. Galanciak, D.; Grinberg, M.; Gryk, W.; Kobyakov, S.; Suchocki, A.; Boulon, G., Influence of High Pressure on the Luminescence Transitions of Mn4+-doped Gadolinium Gallium Garnet. J. Phys-Condens Mat. 2005, 17, 7185-7197.

10


Page 11 of 11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents

11 ACS Paragon Plus Environment

Narrow Red Emission Band Fluoride Phosphor KNaSiF6:Mn4+ for

Recommend Documents