Novel bluish white-emitting CdBaP2O7:Eu2+ phosphor for near-UV white-emitting diodes

Author’s Accepted Manuscript Novel bluish white -emitting CdBaP2O7: Eu 2+phosphors for near-UV white-emitting diodes Mona Derbel, Aïcha Mbarek, Geneviève Chadeyron, Mohieddine Fourati, Daniel Zambon www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(15)30561-5 http://dx.doi.org/10.1016/j.jlumin.2016.03.003 LUMIN13881

To appear in: Journal of Luminescence Received date: 30 September 2015 Accepted date: 4 March 2016 Cite this article as: Mona Derbel, Aïcha Mbarek, Geneviève Chadeyron, Mohieddine Fourati and Daniel Zambon, Novel bluish white -emitting CdBaP2O7: Eu 2+phosphors for near-UV white-emitting diodes, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.03.003 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 galley proof before it is published in its final citable 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.

Novel bluish white -emitting CdBaP2O7: Eu 2+ phosphors for near-UV white-emitting diodes

Mona Derbela, Aïcha Mbarek*a, Geneviève Chadeyronb,c, Mohieddine Fouratia, Daniel Zambonc

a

Laboratory of Industrial Chemistry, National School of Engineers of Sfax, University of

Sfax, BP W 3038, Sfax, Tunisie. b

Clermont Université, ENSCCF, Institut de Chimie de Clermont-Ferrand, BP 10448, F-63000

CLERMONT-FERRAND, France. c

Clermont Université, Université Blaise Pascal, Institut de Chimie de Clermont-Ferrand, BP

10448, F-63000, CLERMONT-FERRAND, France.

* Corresponding author: Aïcha Mbarek, Laboratory of Industrial Chemistry, National School of Engineers of Sfax, University of Sfax, BP W 3038, Sfax, Tunisie. Tel.: (+216) 25902073. Fax: (+216) 74676908. E-mail : [email protected]

ABSTRACT

A new bluish white -emitting phosphor based on a phosphate host matrix, CdBaP2O7: Eu

2+

, was prepared by a conventional solid-state reaction method. The

photoluminescence properties were investigated in both ultraviolet (UV) and vacuum ultraviolet (VUV) regions. The Eu2+-doped CdBaP2O7 phosphor was efficiently excited at wavelengths of 250-400 nm, which is suitable for the blue emission band for near-UV lightemitting-diode (LED) chips (360-400 nm) and red emission peaks up to 700 nm. CdBaP2O7: Eu

2+

displays two different luminescence centers, which were suggested to Ba2+ and Cd2+

sites in the host. The dependence of luminescence intensity on temperatures was measured. The chromaticity coordinates and activation energy for thermal quenching were reported. The phosphor shows a good thermal stability on temperature quenching.

Keywords: Phosphors; UV-VUV; Photoluminescence; Eu2+; Light-emitting diodes

1. Introduction

In recent years, several researchers paid their attention to improve the luminous efficiency and color-rendering properties of white LEDs and especially phosphors, using phosphates, borates, silicates, etc. [1–7]. Among them, phosphates are better luminescence materials because of their excellent properties, e.g., large band gap and high absorption in VUV–UV region, the moderate phonon energy, high chemical stability and exceptional optical damage threshold [8–11]. However, there are few reports on the research of pyrophosphate system as host for a white phosphor. Recently, a yellow–red emitting phosphor SrMgP2O7:Eu2+/Mn2+ and a blueemitting phosphor Sr2−xCaxP2O7:Eu2+ used for NUV-pumped WLEDs have been reported [12–13]. The doped Eu2+ ions occupy the Sr2+ sites in the SrMgP2O7 lattice, while Mn2+ ions substitute not only the Mg2+ ions but also the Sr2+ ones with quite different ionic radius. As a result, eye-visible emissions tunable from red to yellow are easily generated in the as-prepared SrMgP2O7:Eu2+/Mn2+ phosphors by simply adjusting the Mn2+ content under 350 nm excitation corresponding to the Eu2+: 4f–5d transition [12]. In the present work, the bluish-white emitting phosphor, Eu2+- doped CdBaP2O7 was synthesized by the solid-state reaction method under reductive atmosphere. The VUV-UV photoluminescence spectra, the Eu2+ site occupation and CIE chromaticity coordinates have been presented and discussed. In addition, temperature-dependent luminescence of CdBaP2O7:1% Eu2+ shall be investigated. 2. Experimental 2. 1. Powder synthesis

Polycrystalline phosphors with compositions of Cd(Ba1-xEux)P2O7 (x = 0.01 ; 0.05) described were prepared by a solid state reaction method at high temperature. The raw materials BaCO3 (Aldrich, 99.9%), CdCO3 (Aldrich, 99.9%), (NH4)H2PO4 (Acrõs Organics,

99+%) and Eu2O3 (Aldrich, 99.99%) were weighted in stoichiometric amount and ground homougenously in an agate mortar. The materials were pre-sintered under air atmosphere at 400°C for 4 h to remove volatile gas and then sintered under a 5%H2/95%Ar reducing atmosphere at 900°C for 5 h. The as-obtained were pulverized for further characterizations.

2.2. Characterization techniques

The X-ray powder diffraction (XRD) patterns were measured with a Philips X-Pert Pro diffractometer operating with Cu-Kα radiation (λ = 1.5406Ǻ). The data were collected in a 2θ range from 10° to 70°. The infrared spectra were recorded with a Nicolet 5700-FTIR spectrometer equipped with an attenuated total reflection (ATR) accessory. Spectra were obtained using a summation of 32 scans and a resolution of 8 cm-1. The photoluminescence spectra in the range UV-visible and temperature-depended emission spectra were measured using a Jobin-Yvon set-up consisting of a Xe lamp operating at 400 W and two monochromators (Triax 550 and Triax 180) combined with a cryogenically cold charge coupled device (CCD) camera (Jobin-Yvon Symphony LN2 series) for emission spectra and Hamamatsu 980 photomultiplicator for excitation ones. For the temperature measurement, the setup was equipped with a homemade heating cell connected to a temperature controller. The VUV excitation spectra and corresponding emission spectra were recorded at the Deutsches Elektronen Synchrotron (DESY, Hamburg, Germany) using synchrotron radiation from DORIS III storage ring and SUPERLUMI station facilities at HASY Laboratory [14]. Emission spectra were measured range using ARC “Spectra Pro 308” 30 cm monochromatorspectrograph in Czerny-Turner mounting equipped with Princeton Instruments CCD detector and HAMAMATSU R6358P PMT detector (200-800 nm range). Luminescence excitation

spectra were scanned with the primary 2 m monochromator in 15 McPherson mounting (resolution of 3.2 Å) and have been corrected on the incident photon flux. The commission International de l’Eclairage (CIE) chromaticity coordinate values are measured by a PMS-80 Plus-UV–Vis-near-IR Spectrorphoto colorimeter. The luminescence quantum efficiency (QE) was measured by an Absolute Photoluminescence Quantum Yield Measurement System (C9920-02, Hamamatsu) with an integrating sphere, which allows obtaining the absolute QE value. All of the measurements were conducted at room temperature.

3. Results and discussion

3.1. X-Ray diffraction CdBaP2O7 crystallizes in the triclinic crystal system with the space group of P ̅ with the lattice parameters a = 5.641 Ǻ, b = 7.038 Ǻ, c = 7.624 Ǻ and V (cell volume) = 296.18 Ǻ3. The crystal structure of CdBaP2O7 is characterized by infinite layers anions of (CdP2O7)2separated by Ba atoms (as shown in Fig. 1), with only one crystallographic site exists for nine-fold coordination Ba2+ with Ba–O distances ranging from 2.64(1) - 3.07(1) Ǻ [15]. Moreover, cadmium has a severe distorted octahedral environment in CdBaP2O7 with four oxygen atoms at average distance 2.25(1) Ǻ and two O atoms at distances 2.46(1) Ǻ and 2.58(1) Ǻ. According to the electric charge and ionic radii, the doped Eu2+ ions (1.25 Ǻ) should prefer to substitute the Ba sites (1.43 Ǻ). XRD patterns of CdBaP2O7 host and Cd(Ba1-xEux)P2O7 (x = 0.01 ; 0.05) phosphors are shown in Fig. 2. All diffraction peaks were found to be in good agreement with the corresponding pattern simulated from ICSD data base (N° 72673) [15], indicating that all samples have single phase and the doping of Eu2+ ion in host matrix do not cause any significant structural change.

3.2. IR Spectroscopy

The infrared spectra of pyrophosphates give information regarding the internal and external modes of P2O74- group and metal–oxygen vibrations. Fig. 3 displays the FTIR/ATR spectra of CdBaP2O7, CdBa0.99Eu0.01P2O7 and CdBa0.95Eu0.05P2O7 in the range 1600-400 cm-1. All these spectra are similar to each other and show prominent multiple absorption bands in the regions 1020-1240, 670-1000 and 610-400 cm-1. The sharp bands observed at 732 cm-1 and 917 cm-1 are due to the presence of symmetric and asymmetric stretching vibrations of the P–O–P group [16]. The bands observed in the range 1020-1240 cm-1 are due to the symmetric and asymmetric vibration modes of terminal PO3 group, while the bands observed in the low frequency regions are assigned to the deformation and rocking modes of PO3 [17-18]. 3.3.

Luminescence Spectroscopy

3.3.1. UV- vis Spectroscopy

The excitation spectra of CdBa1-xP2O7: Eux (x = 0.01; 0.05) phosphors are shown in Fig.4. The excitation spectra consist of the broad absorption bands from 250 to 450 nm attributed to 4f–5d transition of Eu2+ ions. This indicates that the phosphor can well match with the light of UV-LED chips (360–400 nm) but also by the blue LED chips, which is essential for improving the efficiency and quality of white light-emitting diodes (W-LEDs). Fig. 5 displays the normalized emission spectra of CdBaP2O7: Eu2+ (1%, 5%) excited by 360 nm. The emission spectra display very broad band from 400 to 700 nm with a maximum at about 500 nm, which is ascribed to 4f65d1  4f7 (8S7/2) transition of Eu2+ ions. However, the asymmetric emission bands indicate that there are maybe different coordination environments of the Eu2+ ion within the lattice [19]. However, it can be suggest that there are two kinds of cation sites, i.e. Ba and Cd in the host lattices for Eu occupying. The emission

intensity begins decrease for doped-Eu2+ concentration increase from 1 to 5 mol% and then decreases because of conventional concentration quenching process. Under 392 nm excitation, red-orange emission peaks as well as broad bleu emission band are observed in Eu2+-doped CdBaP2O7 phosphor (as shown in Fig.6). The former is attributed to 4f→ 4f transitions of Eu3+ and the latter is due to 5d→ 4f allowed transitions of Eu2+ ions. The appearance of narrow line emission from Eu3+, indicating that Eu3+ ions in the matrix have not been reduced to Eu2+ completely under the reducing 5%H2/95%Ar atmosphere. The inserted figure shows the emission spectra of CdBa1-xP2O7: Eux (x = 0.01; 0.05) in the range 580-700 nm. For both samples, the spectra are composed of lines corresponding to 5

D0  7FJ (J = 24) transitions of the Eu3+ ions. The intense line at 700 nm corresponds to the

electric dipolar 5D07F4 transition. The spectral distribution of the Eu3+ doped materials results in an orange-red emission. As well known, the dopant trivalent Eu3+ ion replaces Cd2+ in the host compounds, the substituted Cd2+ ion has a similar radius to that Eu3+ (e.g. rCd = 0.95 Ǻ, rEu = 0.947 Ǻ in the six-coordination sites [20]). Consequently, more electrons on negative defects were created in the mentioned compounds. Fig. 7 shows the emission spectra of CdBaP2O7: Eu2+ (1%, 5%) upon 313 nm excitation using a deuterium lamp as the lighting source. The emission spectra display very broad band from 400 to 600 nm, which is ascribed to 4f65d1 → 4f7(8S7/2) transition of Eu2+ ions. The spectra show two distinct emission bands centered around 425 nm (bleu emission band) and 519 nm (green emission band), indicating that Eu2+ ions probably occupy two distinguishable kinds of cation sites and form two different luminescent centers. The Eu2+shows broad emission bands which strongly depends on the chemical nature of the host lattice surrounding the Eu2+ ions present in host lattice [19]. The 5d orbital of Eu2+

strongly interacts with neighborhood ligand ions, and the position of the degenerate 5d band depends on the crystal field strength [21]. In some compounds, Eu2+ ions were reported to occupy two different cation sites and form two luminescent centers [22-25]. Based on the structural information, the doping Eu2+ ions can substitute Ba2+ and Cd2+ ions simultaneously in the CdBaP2O7 host matrix, thus producing two luminescent centers, i.e. EuBa and EuCd in the phosphors for Eu2+ occupying. According to the crystal field theory [26], the crystal field strength is increased when the bond length decreases. When Eu2+ is located at a strong crystal field, the emission position of Eu2+ lies in the long wavelength range. In the structure of CdBaP2O7 the bond-length of Cd2+–O is shorter than that of Ba2+–O. Thus, the high-energy emission at 425 nm should originate from the Eu2+ ions, which occupy the loose crystal circumstance with larger Ba–O bond length; and the low-energy emission at 519 nm can be ascribed to the Eu2+ ions occupying the compact crystal circumstance with shorter Cd2+–O bond length.

3.3.2. VUV Spectroscopy In Fig. 8, the VUV excitation spectra of BaCdP2O7: Eu2+ (1%, 5%) are plotted by monitoring the 425 and 519 nm emission in 100-400 nm region. Five broad bands around 140, 163, 191, 228 and 322 nm are observed.  The bands at 140 and 163 nm could be related to the absorption of (P2O7)4- groups. A similar VUV excitation bans were also observed in other rare earth doped phosphate phosphors, such as Y(PO3)3 [9], GdP5O14 [8] and Na3La(PO4)2 [27]. According of the structure, there are two different (P2O7)4- anionic groups in the CdBaP2O7 host [15], which indicates that two absorption bands are shown in Fig.6.

 The band centered at 191 nm could be assigned to the charge transfer (CT) band of Ba–O [28]. in the case of the emission monitored at 425 nm (Fig. 6(a)) or Cd–O under monitoring the emission intensity at 519 nm (Fig. 6(b)).  The excitation band around 227 nm could be correlated to the charge transfer (CT) band of Eu–O. The positions of charge transfer bands were investigated in phosphates by Tuan et al. for different host lattices [29].  The

broad

excitation

bands

between

250-400

nm,

is

ascribed

to

the

4f7(8S7/2) → 4f65d(7FJ) transitions in Eu2+ electronic levels [28]. The VUV emission spectra of CdBaP2O7: xEu2+ (x = 0, 01; 0, 05) under the excitation of 163 nm and are present in Fig. 9. Two blue bands at 345 and 405 nm could be seen, which were ascribed to 4f55d → 4f7(8S7/2) transition emission of Eu2+ ions. The two emission bands testify two distinct Eu2+ centers in CdBaP2O7 lattices, referred to as EuBa around 345 nm and EuCd at 405 nm. It is worthy to be noted that there are discrepancies between the UVemission spectra (Fig. 7) and the VUV emission spectra (Fig.8) over the range 400-600 nm. This is because VUV photons have much higher energy than UV photons and are more

sensitive to crystal structure, then resulted in the energy change and the wavelength shift in the VUV-excitation spectra when compared with the UV-excitation spectra. It can be clearly show that the high luminescence intensity of EuCd can be explained by its characteristics of layered crystal structure [30]. Then, the distortion of octahedral CdO6 layer (as shown in Fig.2) gives an intense influence on the excited energy states of Eu2+ [31].

3.4. Dependence of luminescence on temperature

Thermal quenching is one of the important technological parameters for phosphors used in the WLEDs. At temperature ranges of 300 to 500 K, the emission spectra of the 1mol% Eu2+doped CdBaP2O7 phosphor under excitation at 320 nm were measured. The

measurement results were shown in Fig. 10. The intense broad band is originated from 5d-4f transition of Eu2+. As can be seen in the inset, relative emission intensity decreases with an increase in heating temperature. With temperature increasing up to 423 K, the normalized emission intensity of CdBaP2O7: 0.01Eu2+ decreased to 79% of the initial value (303 K). The thermal stability for white LEDs in the temperature region 300–500 K has been investigated in Eu2+-activated inorganic phosphors such as phosphates, silicates and oxynitrides. Chiu et al. [32] reported that the Ca3Si2O4N2:Eu2+ phosphor has higher thermal stability (70%) than the Ba2SiO4:Eu2+(49%). The relative intensity of Na2CaPO4F:Eu2+ was reported to be 66% of its initial intensity at 473 K [33]. In a comparison, CdBaP2O7:1% Eu2+ phosphor exhibits better thermal stability on temperature quenching. The activation energy (Ea) for the thermal quenching of the Eu2+ emission was determined by a modified Arrhenius equation as follows [34]:

(

)

Where I0 is the initial intensity, IT the intensity at a given temperature T, c a constant and kB Boltzmann’s constant (8.617× 10–5 eV/K). The activation energy Ea is the energy required to raise the electron from the relaxed excited level into the host lattice conduction band. Fig. 11 plots of ln[(I0/IT)-1] againt 1/kBT, where a straight slope equals –Ea. The thermal quenching mechanism can be illustrated by schematic configuration coordinate diagram for the 4f65d→4f7 transition of Eu2+ ion as shown in inset in Fig. 10. The activation

energy was found to be 0.3679 eV for CdBaP2O7:0.01Eu2+ phosphor. The relatively high activation energy indicates that the phosphor could be applied for high-powered LED applications.

3.5. QE and CIE chromaticity The quantum efficiency (QE) of phosphors is an important parameter for their potential application in solid-state lighting. So the QE of the obtained phosphors was also measured. For CdBa0.99P2O7:0.01Eu2+ and CdBa0.95P2O7:0.05Eu2+ phosphors, their QE were determined to be 34.3% and 30.7% upon excitation of 320 nm at 300 K. It is noted that the QE value of CdBaP2O7:Eu2+ is not so high and lower than the reported commercial phosphors. However, this could be further enhanced by improving the synthesis conditions to reduce the number of defects and interstices. Fig. 12 shows the CIE chromaticity coordinates of CdBaP2O7: Eu2+ phosphors measured under excitation of 320 nm. The CIE (x, y) coordinates are (0.214, 0.259) for and CdBaP2O7: 0.01Eu2+ and (0.222, 0.281) for CdBaP2O7: 0.05Eu2+, which means the phosphors can be used as a bluish-white emitting phosphor for w-LEDs application.

4. Conclusions The bluish-white phosphors CdBaP2O7: Eu2+ were synthesized by convenient solidstate reaction in a reducing atmosphere. The luminescence properties under VUV and UV excitation are investigated for potential applications in white LEDs. Eu2+ ions occupy two different cation sites in CdBaP2O7 lattice and form two kinds of luminescent centers (ninecoordinated EuBa and six-coordinated EuCd) emit blue light under VUV-excitation. A red-shift in the emission band is observed before UV irradiation due to the residual Eu3+ ions. The temperature dependence of luminescence shows has a good thermal stability on the temperature quenching, which is related to the special layered crystal structure. The

activation energy Ea was calculated to be 0.3679 eV. The results indicate the produced Eu2+- doped CdBaP2O7 phosphors have been the potential application for fabricating white LED devices.

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Figure captions Fig. 1. The schematic views of the CdBaP2O7 structure along a-direction. Fig. 2. XRD powder patterns of pure and (1%, 5%) Eu2+-doped CdBaP2O7. Fig. 3. IR/ATR spectra of pure and (1%, 5%) Eu2+-doped CdBaP2O7. Fig. 4. Excitation spectra of CdBaP2O7:Eu2+ (1%, 5%), recorded at 300 K by monitoring the emission wavelength at 613 nm. Fig. 5. Emission spectra of CdBaP2O7:Eu2+ (1%, 5%), recorded at 300 K upon UV excitation at 320 nm. Fig. 6. Emission spectra of CdBaP2O7:Eu2+ (1%, 5%), λexc = 392 nm. The section marked with dot line was amplified. Fig. 7. Emission spectra of CdBaP2O7:Eu2+ (1%, 5%), λexc = 313 nm. Fig. 8. The VUV excitation spectra of CdBaP2O7: xEu2+ (x = 0, 01; 0, 05) under excitation wavelengths (a) λem = 425 nm and (b) λexc = 519 nm. Fig. 9. The VUV emission spectra of CdBaP2O7: xEu2+ (x = 0, 01; 0, 05) under 163 nm excitation. Fig. 10. The emission spectra of CdBaP2O7: 0.01 Eu2+, at the selected temperatures from 303 to 483 K under excitation of 320 nm; the insert shows thermal quenching for the relative emission intensity of CdBaP2O7: 0.01 Eu2+ sample. Fig. 11. A ln[(I0/I)1] vs. 1/kBT activation energy graph for thermal quenching of CdBaP2O7: 0.01 Eu2+ phosphor. Inset: the schematic configuration coordinate diagram for the excited state 4f65d and the ground state 4f7of Eu2+ ions. R means the Eu2+-ligand distance; kr is radiative transition rate and knr non-radiative (i.e., thermal phonon-assisted) transition rate. Fig. 12. CIE chromaticity coordinates of (A) CdBaP2O7: 0.01 Eu2+ and (B) CdBaP2O7: 0.01 Eu2+ phosphors.

Ba

CdO6

Fig. 1

CdBaP2O7 (ICSD 72673)

2+

I (a.u.)

CdBaP2O7:5% Eu

2+

CdBaP2O7:1% Eu

CdBaP2O7

10

20

30

40

2(°)

Fig. 2

50

60

70

δ(OPO)

νas(OPO)

&

νs(OPO)

νs(POP)

Log (1/R)

δ(POP)

νas(POP)

2+

CdBaP2O7:5%Eu

2+

CdBaP2O7:1%Eu

CdBaP2O7 400

600

800

1000

-1

(cm )

Fig. 3

1200

1400

1600

em= 600 nm

I (a.u.)

T= 300 K

2+

1%Eu 2+ 5%Eu 250

275

300

325

350

375

 (nm)

Fig. 4

400

425

450

475

exc= 360 nm 6

1

4f

T= 300 K

7

I (a.u.)

4f 5d

2+

1%Eu 2+ 5%Eu 300

350

400

450

500

550

 (nm)

Fig. 5

600

650

700

750

800

exc= 392 nm 5d

T= 300 K

2+

4f (Eu ) 5

D0

7

F2

I (a.u.)

I (a.u.)

5 5

D0

D0

7

F3

7

F4 2+

1%Eu

2+

5%Eu 580

600

620

640

660

680

700

 (nm)

4f

495

540

585

630

 (nm)

Fig. 6

3+

4f (Eu )

675

2+

1%Eu2+ 5%Eu 720

765

exc= 313 nm 6

T= 300 K

4f 5d

1

4f

7

I (a.u.)

EuCd EuBa

2+

1%Eu 2+ 5%Eu 300

350

400

450

 (nm)

Fig. 7

500

550

600

(b)

em= 519 nm 2+

BaCdP2O7:1%Eu 2+ BaCdP2O7:5%Eu

163

320 191 227

I (a.u.)

140

(a)

em= 425 nm 2+

BaCdP2O7:1%Eu

2+

BaCdP2O7:5%Eu

100

150

200

250

 (nm)

Fig. 8

300

350

400

6

4f

exc= 163 nm

7

T = 300 K

I (a.u.)

4f 5d

1

2+

1%Eu 2+ 5%Eu 250

300

350

400

450

 (nm)

Fig. 9

500

550

600

650

I (a.u.)

exc= 320 nm

100

303 K 323 K 333 K 353 K 373 K 393 K 403 K 423 K 443 K 463 K 483 K

400

I(a.u.)

80

60

40

20

0 300

320

340

360

380

400

420

440

460

480

T(k)

450

500

550

 (nm)

Fig. 10

600

650

700

500

3

2

ln[(I0/I)-1]

1

0

-1

Linear fit Slope = - 0.3679 Ea = 0.3679 eV

-2

-3 22

24

26

28

30

1/kBT

Fig. 11

32

34

36

38

CdBaP2O7:0.05Eu2+

CdBaP2O7:0.01Eu2+

Fig. 13

Fig. 12