Tb3+ co-doped Ba3Sc(PO4)3 phosphors

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Photoluminescence properties and energy transfer behavior of Eu2 þ /Tb3 þ co-doped Ba3Sc(PO4)3 phosphors Yuanyuan Zhang, Lefu Mei, Haikun Liu, Xiaoxue Ma, Zhaohui Huang, Libing Liao

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Cite this article as: Yuanyuan Zhang, Lefu Mei, Haikun Liu, Xiaoxue Ma, Zhaohui Huang, Libing Liao, Photoluminescence properties and energy transfer behavior of Eu2 þ /Tb3 þ co-doped Ba3Sc(PO4)3 phosphors, Ceramics International, http://dx.doi.org/ 10.1016/j.ceramint.2015.07.193 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.

Photoluminescence properties and energy transfer behavior of Eu2+/Tb3+ co-doped Ba3Sc(PO4)3 phosphors Yuanyuan Zhang, Lefu Mei*, Haikun Liu, Xiaoxue Ma, Zhaohui Huang*, Libing Liao* School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, China *

Corresponding author:

Libing Liao, Lefu Mei E-mail: [email protected], [email protected] School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, China. Tel.: +86-10-8233-1701; Fax.: +86-10-8233-1701

Abstract: Ba3Sc(PO4)3:Eu2+,Tb3+ (BSP:Eu2+,Tb3+) phosphors were successfully prepared via conventional high-temperature solid-state reaction, their luminescence properties as well as energy transfer mechanism have been investigated in detail. The energy transfer mechanism was proposed to be the dipole-quadrupole mechanism, and the energy transfer efficiency was also estimated. BSP:Eu2+,Tb3+ phosphors showed a broad emitting band centered at 518 nm and several sharp emission peaks, which was assigned to the 5d →4f allowed transitions of Eu2+ ions and characteristic optical 5D4 – 7FJ (J = 6, 5, 4, and 3) transitions of Tb3+ ions, respectively. The emission color of BSP:Eu2+,yTb3+ exhibited a controlled luminescence evolution in the green area owing to the Eu2+-Tb3+ energy transfer process.

1. Introduction: Recently, white light-emitting diodes (w-LEDs) have been considered that they have a high potential for replacement of conventional light sources such as incandescent and fluorescent lamp owing to longer operational lifetime, low power consumption, thermal resistance and compactness.(1-3) Presently, the commercial w-LEDs

are

fabricated

by

combining

a

blue

InGaN

chip

and

yellow-emittingY3Al5O12:Ce3+ (YAG:Ce3+) phosphor. However, this kind of w-LEDs has some disadvantages such as low color rendering index (CRI) and a high correlated color temperature (CCT) due to the lack of sufficient red emission. Accordingly, an alternative approach on fabricating w-LEDs is realized by the near-ultraviolet (n-UV) chip LED assigned with tricolor (red, green, and blue) phosphors.(4, 5) This method is believed to offer the greatest potential in application due to excellent CRI, high efficiency and low CCT. Consequently, there are many efforts have been focused on developing emission-tunable phosphors by designing energy transfer in single-phase matrix.(6-8) It is well known that the rare earth (RE) ions have been playing an important role in novel luminescence materials due to the abundant emission colors based on their 4f–4f or 5d–4f transitions.(9) Among these of RE ions, Ce3+ and Eu2+ ions have been widely investigated in several compounds assigned with their intense and broad excitation and emission bands owing to the 4f–5d parity allowed electric dipole transition. Additional, rare earth Tb3+ ion could also be a kind of important activator with weak absorption peaks at about 300–400 nm due to the 4f

→4f absorption

transitions. In view of the Eu2+ and Ce3+ ions are always acted as the efficient sensitizer through transferring a part of its excitation energy to co-activators. Considering that the 4f–4f transition of Tb3+ ions cannot be effectively excited by blue or near-UV LEDs.(10) Therefore, it is expected from the viewpoint of energy transfer between the strong broad-band excitation of 4f–5d transition from Ce3+ or Eu2+ ions to the sharp line emission of 4f–4f transition from Tb3+ ions.(11-14) Up to now, many researchers have studied the energy-transfer mechanism between Eu2+/Ce3+ and Tb3+ in some proper single host lattice, which were reviewed by Lin et al. (15-16) In this work we will use the energy transfer strategy between Eu2+ and Tb3+ to obtain controlled luminescence evolution in the green area. Additionally, the energy transfer from Eu2+ to Tb3+ in the phosphors occurred and was systematically investigated by photoluminescence excitation (PLE) and emission (PL) spectra and lifetime values. The luminescence properties of Ba3Sc(PO4)3:Eu2+,Tb3+ phosphors are optimized via utilizing the energy transfer mechanism from Eu2+ to Tb3+ ions, which enable them to have potential applications as near UV-convertible phosphors for w-LEDs.

2. Experimental section A series of Ba3Sc(PO4)3:Eu2+,Tb3+ phosphors were synthesized by a solid-state technique. The raw materials were selected from the following materials, BaCO3 (A.R.), (NH4)2HPO4 (A.R.), Eu2O3 (99.99%), Sc2O3(99.99%) and Tb4O7 (99.99%). After mixing and thoroughly grinding, the stoichiometric mixture was placed into an alumina crucible and annealed at 1250°C in a CO reducing atmosphere for 5 h with highly pure carbon as a reducing agent. Then all of the samples were furnace-cooled to room temperature naturally. Finally, the products were ground again into powder for measurement. Phase structures of the as-prepared samples were measured by the X-ray diffractometer (XD-3, PGENERAL, China) with Cu-Kα radiation (λ = 0.15406 nm) operated at 40 kV and 30mA. The continuous scanning XRD data were collected in a 2θ ranging from 10o to 70o. The excitation and emission spectra of the samples were measured on a JOBIN YVON FluoroMax-3 fluorescence spectrophotometer with a photomultiplier tube operating at 550 V, and a 150 W Xe lamp as the excitation lamp. A 400 nm cutoff filter was used in the measurement to eliminate the second-order emission of source radiation. The elemental analysis was carried out by energy dispersive spectroscopy (EDS) using an X-ray detector attached to the SEM instrument. The lifetimes were recorded on a spectro-fluorometer (HORIBA JOBIN YVON FL3-21), and the 370 nm pulse laser radiation (370-nm Nano LED, model number 08254) was used as excitation source. All the measurements were performed at room temperature.

3. Results and discussion 3.1 Phase structure and morphology The phase purity of as-prepared samples was checked by XRD patterns. Fig. 1 shows the XRD patterns of the BSP:0.05Eu2+,yTb3+ (y = 0, 0.05, and 0.15) phosphors. It exhibits that all the positions and relative intensity agree well with the standard JCPDS card no.33-0175 of Ba3Sc(PO4)3 crystals, and no second phase was detected. This indicates that the Ba3Sc(PO4)3 single phase has formed, Eu2+ and Tb3+ ions have

2+

3+

127

235 026 145

233 224 015 125

BSP:0.05Eu ,0.15Tb 123

112

022

Intensity (a.u.)

013

been successfully incorporated in the host structure.

2+

3+

BSP:0.05Eu ,0.05Tb

2+

BSP 0.05Eu

JCPDS No.33-0175 Ba3Sc(PO4)3

10

20

30

40

50

60

70

2 Theta (Degree) Fig. 1 XRD patterns of as-prepared BSP:0.05Eu2+,yTb3+ (y = 0, 0.05, and 0.15) phosphors. The standard data for Ba3Sc(PO4)3 (JCPDS card No. 33-0175) is shown as a reference.

The EDS analysis was carried out to determine the composition of the as-prepared Ba3Sc(PO4)3:Eu2+ product as shown in Fig. 2. It can be seen that the elements of Ba, O, Sc, P, Cu, C and Eu were detected. Among them, the C and Cu peaks in the spectra were attributed to the electric latex of SEM sample holder.(17) That is to say, the as-prepared Ba3Sc(PO4)3:Eu2+ phosphor is composed of Ba, O, Sc, P, Cu, C and Eu, and not any extra impurity element can be found. Furthermore, the

inset of Fig. 2 demonstrates the SEM image of Ba3Sc(PO4)3:Eu2+ phosphor. The as-obtained micrograph shows that the particles are agglomerated with irregular morphology with an average diameter of about 2 µm.

Fig. 2 EDS spectrum of Ba3Sc(PO4)3:Eu2+ powder phosphor. The inset gives the SEM image of the as-prepared phosphors.

3.2 Luminescence properties of BSP:0.05Eu2+,yTb3+ Fig. 3 shows the PLE and PL spectrum of BSP:0.05Eu2+,0.15Tb3+ phosphor. Upon the 365 nm excitation, the PL spectrum exhibits both sharp emission peaks as well as one broad emission band, which is attributed to the typical characteristic optical 5D4 – 7FJ (J = 6, 5, 4, and 3) transitions of Tb3+ ions and 5d →4f 4f allowed transitions of Eu2+ ions, respectively. Additional, the PLE spectrum monitored at 54 543 nm which is the typical emission of Tb3+ ions consists of two excitation bands of both Eu2+ and Tb3+ ions, which indicates the existence of energy transfer from Eu2+ to Tb3+. In view of the PLE spectrum of BSP:0.05Eu2+,0.15Tb3+ phosphor monitored at the emission of Eu2+ (518 18 nm) is not inconsistent with that monitored at the emission of

Tb3+ (543 nm) for the difference of the relative intensity, this phenomenon may be related with the observed low energy transfer efficiency. 2+

3+

Intensity (a.u.)

BSP:0.05Eu ,0.15Tb

λem = 543 nm λem = 518 nm

λex = 365 nm

200 250 300 350 400 450 500 550 600 650 700 750

Waveleng (nm) Fig. 3 PLE and PL spectra of the BSP:0.05Eu2+, 0.15Tb3+ phosphor

In order to further confirm the energy transfer process involved in BSP:0.05Eu2+,Tb3+, a series of BSP:0.05Eu2+,yTb3+ phosphors with fixed Eu2+ contents were prepared and the selected dependence of emission spectra for BSP:0.05Eu2+,yTb3+ (y = 0, 0.01, 0.05, 0.15, and 0.25) are exhibited in Fig. 4. A series of phosphors with fixed Tb3+ content are synthesized to discuss the effect of Eu2+-doping concentration on the luminescence properties of phosphors. The PL spectra of the BSP:xEu2+,0.15Tb3+ phosphors are shown in Fig. 5. Because of the weaker absorption at 365 nm of Tb3+ single-doped phosphor, we hardly see the emission of Tb3+ ion. With increasing of the Eu2+ content and fixed the concentration of Tb3+, the emission intensity of Tb3+ dramatically increases to a maximum at x = 0.05, further confirming an efficient energy transfer from the Eu2+ to Tb3+ ions.(18-21)

Intensity (a.u.)

2+

3+

BSP:0.05Eu ,yTb

y=0 y = 0.01 y = 0.05 y = 0.15 y = 0.25

λex = 365 nm

400

450

500

550

600

650

700

Waveleng (nm) Fig. 4 PL spectra of BSP:0.05Eu2+,yTb3+ phosphors as a function of Tb3+ doping content (y).

2+

x=0 x = 0.03 x = 0.05 x = 0.10 x = 0.20

3+

Intensity (a.u.)

BSP:xEu ,0.15Tb

400

450

500

550

600

650

700

Wavelength (nm) Fig. 5 PL spectra of BSP:xEu2+,0.15Tb3+ phosphors as a function of Eu2+ doping content (x).

Variation of the Tb3+ concentration for the Tb3+ emission, Eu2+ emission and energy transfer efficiency of Eu2+-Tb3+ in BSP:0.05Eu2+,yTb3+ phosphors are shown in Fig. 6. It is found that the emission intensities of Eu2+ decreases gradually with the increasing of Tb3+ concentration, while the emission intensity of Tb3+ increases systematically and no concentration quenching occurs until Tb3+ content reached 0.25, which further confirms the energy transfer takes place from Eu2+ to Tb3+ ions.(18-19)

Relative intensity (a.u.)

25

15 2+

Eu 4f-5d transition 3+ 5 7 Tb D4- F5 transition

10 5

ET efficiency η (%)

20

0 0.00

0.05

3+

0.10

0.15

0.20

0.25

Tb concentration (mol) Fig. 6 Variation of the Tb3+ concentration for the Tb3+ emission, Eu2+ emission and energy transfer efficiency of Eu2+-Tb3+ in BSP:0.05Eu2+,yTb3+ phosphors.

3.3 Energy transfer mechanism in BSP:0.05Eu2+,yTb3+

Notwithstanding the low energy transfer efficiency as mentioned above, the energy transfer mechanism between Eu2+ and Tb3+ ions also can be confirmed by the PL decay times. Fig. 7 exhibits the room temperature PL decay curves and lifetime of Eu2+ in BSP:0.05Eu2+,yTb3+ (y = 0, 0.15, and 0.25) samples. All the decay curves can be well fitted based on a non-exponential which can be expressed by:(22) ∞

τ =

∫ ∫

0 ∞ 0

I(t)tdt

(1)

I(t)dt

where I(t) stands for the intensity at time t. On the basis of Eq. (1), The average decay times (τ) were estimated to be 0.83, 0.64 and 0.49 ms for BSP:0.05Eu2+,yTb3+ with y = 0, 0.15, and 0.25, respectively. It can be seen that the decay time begins to decrease gradually with the increasing of Eu2+ concentration. Such behavior also strongly demonstrates the energy transfer behavior occurs between Eu2+ and Tb3+ in the Ba3Sc(PO4)3 matrix. The interaction type between sensitizers or between sensitizer and activator can be calculated by Reisfeld's approximation as shown in following equation:(23, 24)

η0 / η ∝ C n / 3

(2)

where C is the concentration of Tb3+; where IS0 and IS are the luminescence intensities of the sensitizer Eu2+ in the absence and presence of the activator Tb3+. In(η0/ηs) can be estimated approximately by the logarithmic value of relative luminescence intensity ratio (In(IS0/Is)). The n = 6, 8 or 10 are dipole-dipole, dipole-quadrupole or quadrupole-quadrupole interactions, respectively.

Fig. 7 Decay curves for the luminescence of Eu2+ ions in BSP:0.05Eu2+,yTb3+ samples.

The linear fits to the relationship between IS0/IS and Cn/3 based on the above equation are illustrated in Fig. 8. It is observed clearly that the relationship between IS0/IS and Cn/3, revealing a linear behavior only when n = 8. Therefore, the dominant interaction mechanism for Ba3Sc(PO4)3:Eu2+,Tb3+ is based on the dipole–quadrupole mechanism.

3.5

R2 = 0.826

R2 = 0.915

R2 = 0.838

Iso/Is of Eu

2+

3.0 2.5 2.0 1.5 1.0

(a) 0.00

6/3

3+

(c)

0.02 0.000 0.025 0.050 0.000 0.004 0.008

0.01

EuTb

(b)

3

×10

EuTb

8/3

3+

4

×10

EuTb

10/3

3+

5

×10

Fig. 8 Dependence of Iso/Is of Eu2+ on (a) CTb6/3 (b) CTb8/3 and (c) CTb10/3.

3.4 Variation of the CIE values of BSP:0.05Eu2+,yTb3+

The variation of the Commission International de L'Eclairage (CIE) chromaticity coordinates of the BSP:0.05Eu2+,yTb3+ phosphors with different doping contents of Tb3+ are determined on the corresponding PL spectrum upon 365 nm excitation are shown in Table. 1 and Fig. 9. On basis of the Fig. 9, the emission color of BSP:0.05Eu2+,yTb3+ all locate at green area. Increasingly, the CIE color coordinate of BSP:0.05Eu2+,yTb3+ was modulated from (0.265, 0.393) to (0.320, 0.451) with the increasing doping content of the Tb3+ ions, which is due to the variation of the emission intensity of Eu2+ and Tb3+ through the energy transfer from Eu2+ to Tb3+ ions. These results imply this kind of phosphor has great potential application as promising green-emitting phosphor to meet the application requirements for n-UV w-LEDs.

Table. 1 CIE chromaticity coordinates (x, ( y) for BSP:0.05Eu2+,yTb3+ (y = 0, 0.0 0.01, 0.05,

0.15, and 0.25) samples (λex = 365 nm). Sample NO.

Sample composition (y)

CIE coordinates ((x, y)

1

y=0

(0.265, 0.393 393)

2

y = 0.01

(0.298, 298, 0.419) 0.419

3

y = 0.05

(0.300, 300, 0.418) 0.418

4

y = 0.15

(0.314, 314, 0.446) 0.446

5

y = 0.25

(0.320, 0.451 451)

Fig. 9 CIE chromaticity diagram for BaSP:0.05Eu2+,yTb3+ (y = 0, 0.01, 0.05, 0.15 15 and 0.25) 0.25 phosphors excited at 365 nm.

4. Conclusion Wee have successfully synthesized a series of BSP:Eu2+,Tb3+ phosphors with by

traditional high-temperature solid-state reaction. The luminescence properties and energy transfer behavior have been investigated and the energy transfer interaction mechanism was determined to be dipole–quadrupole mechanism. In addition, the energy transfer efficiency as well as PL decay curves was also estimated. The energy transfer efficiency increases with increasing Tb3+ doping content. By changing the doping contents of the Tb3+ ions with fixed Eu2+ ions content, the emission color of the BSP:Eu2+,Tb3+ phosphor is maintained green color.

Acknowledgement This present work was supported by the National Natural Science Foundations of China (Grantnos.41172053, 51202226 and 51172216), the Fundamental Research Funds for the Central Universities (Grant no. 2652013043 and 2-9-2015-307), and Science and Technology Innovation Fund of the China University of Geosciences (Beijing).

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