Ba2B2O5:Eu3+: A novel red emitting phosphor for white LEDs

Ba2B2O5:Eu3+: A novel red emitting phosphor for white LEDs

Optical Materials xxx (2014) xxx–xxx Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat S...

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Optical Materials xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Short Communication

Ba2B2O5:Eu3+: A novel red emitting phosphor for white LEDs Panlai Li ⇑, Zhijun Wang ⇑, Zhiping Yang, Qinglin Guo College of Physics Science & Technology, Hebei University, Baoding 071002, China

a r t i c l e

i n f o

Article history: Received 22 January 2014 Received in revised form 2 June 2014 Accepted 23 October 2014 Available online xxxx Keywords: Luminescence Phosphors Ba2B2O5:Eu3+

a b s t r a c t A novel red emitting phosphor Ba2B2O5:Eu3+ is synthesized by a high temperature solid state method, and its luminescent properties are investigated. Ba2B2O5:Eu3+ can be excited by the 393 nm radiation excitation, and produce the red emission. For the dominating 618 nm emission, the excitation spectrum has one broad band at 295 nm and several narrow bands at 318, 362, 382, 393, 416, 465 and 526 nm, respectively. The emission intensity of Ba2B2O5:Eu3+ is influenced by the Eu3+ doping content, and the concentration quenching effect of Eu3+ in Ba2B2O5 is also observed. The CIE color coordinates of Ba2B2O5:0.06Eu3+ are about (0.636, 0.362), and locate in the red region. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Recently, the new lighting and display technology such as lightemitting diodes (LEDs), plasma display panels (PDPs), and field emission displays (FEDs) have been proposed or developed [1–3]. At present, the commercial white LEDs are achieved by combining a blue LED chip with a yellow emitting phosphor YAG:Ce, however, they have a less satisfactory color rendering in the red region [4]. White LEDs fabricated with tri-color (red, green and blue) phosphors excited by an ultraviolet (UV) or near-UV LED are potential approach to solve the problem [5–7]. However, the commercial red phosphor Y2O2S:Eu3+ has some defects, such as the low absorption in the 380–410 nm region, the shorter working lifetime under UV irradiation, and the instability due to releasing of sulfide gas [8]. Therefore, it is urgent to search for novel and highly efficient red phosphors that can be excited by n-UV LED. For the high efficiency and proper CIE chromaticity coordinates of red-emitting phosphors, Eu3+ doped materials are primarily considered because they can absorb the UV or n-UV light and create an obvious red emission corresponding to the 5D0 ? 7FJ(J=1, 2, 3, 4) transitions of Eu3+ [9–12]. Generally, as luminescent materials, the compounds must have the excellent physical and chemical stability [13–15]. Therefore, in order to achieve the newly efficient red emitting phosphor, some researches are focused on the borate materials which have the higher physical and chemical stability, and the lower harm to the environment and health [16–20]. However, there has no report on the red emitting phosphor Ba2B2O5:Eu3+, therefore, the luminescent properties of Ba2B2O5:Eu3+ are investi-

gated in this work. The results may be of benefit to the development of red phosphors. 2. Experimental A series of Ba2xB2O5:xEu3+ (x:mole concentration) are synthesized by a high temperature solid state reaction method. The initial materials, including BaCO3 (A.R.), H3BO3 (A.R.) and Eu2O3 (99.99%) are weighted in stoichiometric proportion, thoroughly mixed and ground by an agate mortar and pestle for more than 30 min till they are uniformly distributed. The obtained mixtures are heated at 500 °C for 2 h in crucibles along with an atmosphere. After that, the samples are thoroughly ground and sintered at 900 °C for 3 h, then slowly cooled down to room temperature. In order to measure the characteristics of phosphor, the samples are ground into powder. The phase formation is determined by X-ray diffraction (XRD) in a Bruker AXS D8 advanced automatic diffractometer (Bruker Co., German) with Ni-filtered Cu Ka1 radiation (k = 0.15405 nm), and a scan rate of 0.02°/s is applied to record the patterns in the 2h range from 10° to 50°. The excitation and emission spectra are detected by a fluorescence spectrophotometer (Hitachi F-4600), and the exciting source is a 450 W Xe lamp. The Commission International de I’Eclairage (CIE) chromaticity coordinates of the phosphors are measured by a PMS-80 spectra analysis system. All measurements are carried out at room temperature. 3. Results and discussion 3.1. Phase formation

⇑ Corresponding authors. E-mail addresses: [email protected] (P. Li), [email protected] (Z. Wang).

The phase formation of Ba2xB2O5:xEu3+ is determined by the X-ray diffraction pattern, and a similar diffraction patterns are

http://dx.doi.org/10.1016/j.optmat.2014.10.062 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.

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observed for each sample. As a representative, Fig. 1 shows the XRD patterns of Ba1.94B2O5:0.06Eu3+. Compared the diffraction data with the standard JCPDS card (No.24-0087), the results indicate that there has no difference between Ba2xB2O5:xEu3+ and the pure Ba2B2O5. It means that the phase formation of Ba2B2O5 is not influenced by a little amounts of Eu3+. Ba2B2O5 has a monoclinic crystal structure with a space group P2/m(10), the cell parameters a = 1.1014 nm, b = 1.2684 nm and c = 1.6586 nm. 3.2. Luminescent properties

λex=393 nm 5D -7F 0

Relative intensity/(a.u.)

2

Ba1.94B2O5:0.06Eu3+

2

(618 nm) 5

D0-7F1 (592 nm)

5 5

D0-7F4

7

D0- F0

5

D0-7F3

3+

650

700

750

Wavelength/(nm) Fig. 2. Emission spectrum of Ba1.94B2O5:0.06Eu3+ (kex = 393 nm).

7

200

300

5

F0- D2

400

λem=592 nm

200

7

7

5

5

4

CTB

F0- D3

F0- G2-4

7

5

Relative intensity/(a.u.)

5

F0- L6

λem=618 nm

7

5

F 0- D1

500 Ba1.94B2O5:0.06Eu

300

400

Wavelength/(nm)

3+

500

550

600

Intensity/(a.u.)

x=0.01 x=0.02 x=0.04 x=0.06 x=0.08 x=0.1 x=0.15

0.00 3+0.05 0.10 0.15 Eu content/(mol)

650

700

Relative intensity/(a.u.)

Fig. 3. Excitation spectra of Ba1.94B2O5:0.06Eu3+ (kem = 618 nm; kem = 592 nm).

ð1Þ

where I0 and I are intensities at zero time and time t, respectively, and s is the lifetime for transition. The life times for the 5D0 ? 7F2 transition of Eu3+ are calculated with the different Eu3+ doping content, and the results are also shown in Fig. 5. With the lower Eu3+ doping content (x < 0.06), the decay times of Ba2xB2O5:xEu3+ have only a little shorter with increase the Eu3+ concentration. With further increase the Eu3+ concentration (x > 0.06), the decay times of Ba2xB2O5:xEu3+ have an obvious decrease trend. The above results

600

-D F0 5 H 3 7 F 0

I ¼ I0 expðt=sÞ

550

7

Fig. 2 depicts the emission spectra of Ba1.94B2O5:0.06Eu . A series of emission peaks locate at 580, 592, 618, 657 and 707 nm which are assigned to the typical 5D0–7FJ(J=0, 1, 2, 3, 4) transitions of Eu3+, respectively. And the dominating emission locates at 618 nm, therefore, Ba2B2O5:Eu3+ can emit red light. Moreover, the emission bands show a splitting pattern, such as the 5D0 ? 7F2 transition of Eu3+ (612 and 618 nm). For the 592 and 618 nm emissions, Fig. 3 shows the excitation spectra have the same spectral profile. For example, there has a broad absorption in the region of 200–300 nm (with a maximum at 290 nm), which is assigned to the charge transfer state (CTB), moreover, some excitation bands locate at 318 nm (7F0–5H3), 362 nm (7F0–5D4), 382 nm (7F0–5L7), 393 nm (7F0–5L6), 416 nm (7F1–5D3) 465 nm (7F0–5D2) and 526 nm (7F0–5D1), respectively [9–12]. Fig. 4 shows Ba2xB2O5:xEu3+ (x = 0.01–0.15) have the same spectral profile with the different Eu3+ concentration, however, the emission intensities are influenced by the Eu3+ concentration (x). As shown in the inset, the emission intensities increase with increase Eu3+ concentration and reach a maximum value at x = 0.06, then the intensities decrease when the Eu3+ doping ratio is higher than 0.06. In other words, there has the concentration quenching effect of Eu3+ in Ba2B2O5:Eu3+ [21,22]. To further validate the concentration quenching effect of Eu3+ in Ba2B2O5:Eu3+, we investigated the lifetimes of Eu3+, the decay curves of Ba2xB2O5:xEu3+ samples (x = 0–0.1) excited at 393 nm and monitored at 618 nm are shown in Fig. 5. For single exponential decay, it can be expressed as [23]

750

Wavelength/(nm) Fig. 4. Emission spectra of Ba2xB2O5:xEu3+ with the different Eu3+ doping content (x) (kex = 393 nm). Inset: the emission intensities of Ba2xB2O5:xEu3+ as function of the Eu3+ doping content (x) (kex = 393 nm).

Relative intensity/(a.u.)

Ba1.94B 2O5:0.06Eu3+

10

20

30

40

50

JCPDS#24-0087

10

20

30

40

50

2θ /(Deg) Fig. 1. XRD pattern of Ba1.94B2O5:0.06Eu3+ with the standard data of Ba2B2O5 (JCPDS No.24-0087).

mean that there have the concentration quenching effect of Eu3+ in Ba2B2O5:Eu3+. Generally, the concentration quenching is mainly caused by the nonradiative energy transfer among Eu3+ ions, which usually occurs as a result of an exchange interaction, radiation reabsorption, or multipole–multipole interaction [24]. Therefore, it is necessary to obtain the critical distance (Rc) that is the critical separation between the donor (activator) and acceptor (quenching site). According to Ref. [25], if the activator is introduced solely on Z ion sites, where xc is the critical concentration of activator, N is the number of Z ions in the unit cell, and V is the volume of unit cell, then there is on the average one activator ion per V/xcN. The critical transfer distance (Rc) is approximately equal to twice the radius of a sphere with this volume

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100

3

3+

Relative intensity/(a.u.)

Decay curve of Ba2-xB2O5:xEu x=0.02 (τ=1.95 ms) x=0.04 (τ=1.94 ms) x=0.06 (τ=1.81 ms) x=0.08 (τ=1.52 ms) x=0.1 (τ=1.31 ms)

10

1

0.1

0

5

10

15

20

Time/(ms) Fig. 5. Decay curves of Eu3+ emission monitored at 618 nm for Ba2xB2O5:xEu3+ (kex = 393 nm).

Rc  2½3V=ð4pxc NÞ1=3

ð2Þ

For Ba2B2O5:Eu3+, where xc is the concentration of Eu3+, N is the number of Z ions in the unit cell (N = 19 for Ba2B2O5), and V is the volume of the unit cell (V  2.28314 nm3). The critical concentration of Eu3+ in Ba2B2O5 is 0.06. As a result, the Rc value of Eu3+ in Ba2B2O5 is about 1.564 nm. Therefore, the electric multipolar interaction is involved in energy transfer. And there has three type of interactions, such as dipole–dipole (d–d), dipole–quadrupole (d–q) and quadrupole–quadrupole (q–q) interactions. Thus, there is a need to elucidate which type of interaction is involved in energy transfer. According to Dexter’s theory, the relation between luminescent intensity (I) and activator concentration (x) can be expressed by an equation, the modified equation is as follows [26]

I / ½ð1 þ AÞ=c½3Cð1 þ s=3Þ=a1s  ða  1Þ

ð3Þ

where a = x[(1 + A)X0/c]3/sC(1  s/3) / x, x is the activator concentration, s is the series of electric multipolar (for exchange interaction, d–d, d–q and q–q interactions, the values of s are 3, 6, 8 and 10, respectively), c is the intrinsic transition probability of activator, A and X0 are the constants. As shown in Fig. 6, s can be achieved by the slope (s/3) of the plot lg(I/x) vs lg x (kex = 393 nm). The dependence of lg(I/x) on lg x is linear and the slope is 2.017. Thus, the value of s can be calculated as 6.051 (very close to the theoretical value 6 for d–d interaction), which means that the d–d interaction is the main mechanism for the concentration quenching of Eu3+ in Ba2B2O5. Fig. 7 presents that the CIE chromaticity coordinates of Ba2xB2 O5:xEu3+ (x = 0.01–0.15) locate in the red region (kex = 365 nm), and they are very near to the standard red chromaticity (0.670,

λex=393 nm

5.2

Fig. 7. CIE chromaticity coordinates (x, y) of Ba2xB2O5:xEu3+ (kex = 365 nm). The inset: the luminescence photographs of Ba1.94B2O5:0.06Eu3+ (kex = 393 nm) and Y2O2S:Eu3+ (kex = 254 nm).

0.330) for National Television Standard Committee system. Fig. 7 also shows the luminescence photographs of the commercial Y2O2S:Eu3+ (kex = 254 nm) and Ba2B2O5:0.06Eu3+ (kex = 393 nm). The results depict the luminescent intensity of Ba2B2O5:0.06Eu3+ is appreciably stronger than that of Y2O2S:Eu3+. 4. Conclusions A series of red phosphors Ba2B2O5:Eu3+ are synthesized by the traditional solid-state method. Ba2B2O5:Eu3+ can produce the red emission under the 393 nm radiation excitation, and its emission intensities can be stronger than that of Y2O2S:Eu3+ by properly tuning the Eu3+ doping content, and its CIE chromaticity coordinates locate in the red region. Moreover, the concentration quenching effect is also observed, and the quenching results from the dipole–dipole interaction in Ba2B2O5:Eu3+. The results indicate that Ba2B2O5:Eu3+ may be a potential red phosphor for white LEDs. Acknowledgments The work is supported by the National Natural Science Foundation of China (No. 50902042), the Natural Science Foundation of Hebei Province, China (Nos. A2014201035 and E2014201037), and the Education Office Research Foundation of Hebei Province, China (Nos. ZD2014036 and QN2014085).

lg ( I/xEu3+)

References 4.8

the slope is -2.017 4.4

4.0 -2.0

-1.6

-1.2

-0.8

lgxEu3+ Fig. 6. Plot of lg(I/x) as function of lg x in Ba2B2O5:Eu3+ (kex = 393 nm).

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