Photoluminescence and energy transfer properties of Eu2+ and Tb3+ co-doped gamma aluminum oxynitride powders

Optical Materials 58 (2016) 290e295

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

Photoluminescence and energy transfer properties of Eu2þ and Tb3þ co-doped gamma aluminum oxynitride powders Jiantao Zhang, Chaoyang Ma, Zicheng Wen, Miaomiao Du, Jiaqi Long, Ran Ma, Xuanyi Yuan, Junting Li, Yongge Cao* Beijing Key Laboratory of Opto-electronic Functional Materials & Micro-nano Devices, Department of Physics, Renmin University of China, Beijing 100872, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 March 2016 Received in revised form 13 May 2016 Accepted 25 May 2016 Available online 6 June 2016

Gamma-AlON: 0.2Eu2þ, Tb3þ phosphors were firstly synthesized via a high temperature solid-state reaction. For the phosphors, luminescence properties as well as energy transfer mechanism were investigated in detail. The energy transfer from Eu2þ to Tb3þ in the g-AlON host was ascribed to the dipoledipole mechanism, and the efficiency and critical distance in the energy transfer process were also estimated. g-AlON: 0.2Eu2þ, Tb3þ phosphors showed a broad-band emission centered at about 405 nm and other several emission peaks, which were assigned to the 5de4f transition of Eu2þ ions and the 5D4 e7FJ (J ¼ 6, 5, 4, and 3) characteristic transitions of Tb3þ ions, respectively. The results indicated that gAlON: 0.2Eu2þ, Tb3þ phosphors have great potential application in white light-emitting diodes due to its broad-band excitation in the ultraviolet range and the high-efficient green light emission. © 2016 Elsevier B.V. All rights reserved.

Keywords: Fluorescent and luminescent materials Energy transfer Spectroscopy Fluorescence and luminescence Rare-earth-doped materials

1. Introduction Compared to the traditional incandescent and currently implemented fluorescent lamps, white light-emitting diodes (w-LEDs) have received lots of attention in solid-state lighting area, because of their long lifetime, high luminous efficiency, low power consumption, and environmentally friendly characteristic [1e3]. Currently, the commercially available w-LEDs can be fabricated by the combination of a blue InGaN chip and yellow-emitting Y3Al5O12 (YAG: Ce) phosphors [4]. However, this kind of w-LEDs faces some deficiencies such as low color rendering index (CRI) and high correlated color temperature (CCT) due to the lack of sufficient red emission [5,6]. Accordingly, w-LEDs employed with tricolor phosphors and near ultraviolet lighting diodes was introduced. This method was considered to offer the greatest potential in application due to excellent CRI, low CCT, and high efficiency [7,8]. Consequently, the exploration of novel phosphors materials plays an important role in the development of w-LEDs. Up to now, the Tb3þ ion was frequently used as an activator of green luminescent materials owing to its predominant 5D4-7F5 transition peak at

* Corresponding author. E-mail address: [email protected] (Y. Cao). http://dx.doi.org/10.1016/j.optmat.2016.05.048 0925-3467/© 2016 Elsevier B.V. All rights reserved.

~542 nm [9,10]. However, the intensities of the Tb3þ absorption in the UV region were very weak due to the forbidden 4f-4f absorption transitions [9]. The energy transfer from sensitizer to activator by rare earth ions have been investigated in many inorganic host, such as fluorides, silicates, phosphates, and borates [11,12]. The Eu2þ as a promising sensitizer for Tb3þ has been used since the 4f-5d transition of the Eu2þ ion shows superior absorption in the UV region and the efficient energy transfer between Eu2þ and Tb3þ [12,15,16]. Recently, many researchers have studied the energy-transfer mechanism between Eu2þ and Tb3þ in some proper single host lattice especially in nitride system [13,14]. As an important family of nitrides, AlON based phosphors have great potential applications as novel luminescent materials due to their environmental benignity, high chemical and physical stability. Till now, many phosphors of gAlON have been thoroughly investigated by several research groups, e.g. phosphors of g-AlON doped with Eu2þ,Mn2þ or codoped with Eu2þ/Mn2þ or Mg2þ/Mn2þ [15e17]. In this paper, using the energy transfer mechanism, enhanced green luminescence of Eu2þand Tb3þco-doped g-AlON phosphors were firstly synthesized. Additionally, the energy transfer from Eu2þ and Tb3þ in the phosphors was systematically investigated by photoluminescence spectra (PLE, PL) and lifetime. The energy transfer mechanism from Eu2þ to Tb3þ in the g-AlON host was ascribed to the dipole-dipole mechanism. The critical energy-

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transfer distance between Eu2þ and Tb3þ and the energy-transfer efficiency were also calculated. 2. Experimental procedures 2.1. Synthesis of g-AlON: Eu2þ, Tb3þ The Eu2þ/Tb3þ co-doped g-AlON (Al22.5O26.8N4.7) phosphors were synthesized by a high temperature solid-state reaction. The staring materials used for the studied phosphors were powders of Al (Alfa Aesar, 99.95%), Al2O3 (Aladdin, 99.99%), Eu2O3 (Aladdin 99.995%), and Tb4O7 (Aladdin 99.995%). The powders were weighted precisely according to the chemical composition of gAlON: xEu2þ, yTb3þ and mixed with ethanol (1:1.5). The mixed slurry was then ball milled using a planetary milling machine for 8 h. After thorough balling grinding, the mixture were dried at 80  C for 8 h and then screened through 200 mesh sieve. The resulting fine powders were placed in a BN crucible and then heated under flowing N2 (0.5 MPa) for 2 h at 1700  C in a carbon furnace. The heating and the cooling rates were 300 K/h.

Fig. 1. XRD patterns of (a) g-AlON (b) g-AlON: 0.2%Eu2þ (c) g-AlON: 0.2%Eu2þ, 0.5% Tb3þphosphors and the standard data for g-AlON (JCPDS card no. 480686) is shown as a reference.

2.2. Characterization and measurement The composition and phase purity of the samples were checked by X-ray diffraction (XRD) using a Bruker D2 PHASER diffractometer with Cu Ka (l ¼ 1.5418 Å) operating at 40Kv and 40 mA. The continuous scanning XRD data were collected in a 2q ranging from 10 to 80 . Morphology of the phosphors was characterized by the image analysis of photographs obtained by a field scanning electron microscopy (SEM, FEI, NOVA, NANOSEM, 450). Photoluminescence (PL) spectra were recorded by a photoluminescence measurement system, which is consisted of a UV-LED chips as the excitation source, a miniature fiber optic spectrometer as the detector, and a cylindrical cell (with an active area of 1.5  1.5 cm2), which contained the pressed powder samples. Photoluminescence excitation (PLE) spectra of the powders were recorded by the F-7000 fluorescence spectrophotometer equipped with a 200w Xe tube as the excitation source in the fluorescence mode. The decay curves were recorded on the instrument (FLSP920), and a mF900 fiash lamp was used as the excitation resource. The elemental analysis was carried out by energy dispersive spectroscopy (EDS) using an X-ray detector attached to the SEM instrument. 3. Results and discussion 3.1. Phase structure and morphology Fig. 1 presents the XRD patterns of (a) g-AlON, (b) g-AlON: 0.2Eu, and (c) g-AlON: 0.2Eu, 0.5 Tb samples. The XRD patterns of as-prepared g-AlON compounds associated with cubic structure (space group Fd3m) and are agree well with the reported g-AlON phase (JCPDS card no. 480686). No other phases or impurities were detected, indicating that Eu2þ and Tb3þ ions are completely doped into the g-AlON host lattice without significant change in the crystal structure. However, the diffraction peaks shift slightly to the higher angle, as shown in Fig. 1a. It is ascribed to the deficiency of nitrogen in the g-AlON system, because of the Al-N (1.79 Å) is shorter than the Al-O (1.86 Å). The EDS analysis was carried out to determine the composition of the as-prepared g-AlON products, as shown in Fig. 2. It can be seen that the elements of Al, O, N, C, and Pt were detected. Among them, the C and Pt peaks in the spectra were attributed to the electric latex of SEM sample holder [18]. The element of Eu and Tb cannot be detected, as their content is too low. That is to say, the asprepared g-AlON phosphors is composed of Al, O, and N, and no

Fig. 2. EDS spectra of the obtained powder phosphor. The inset shows the SEM image of the prepared phosphors.

extra impurity element can be found. It confirmed that we have successfully prepared the single-phased g-AlON. Furthermore, the inset demonstrated the SEM image of single-phased g-AlON: 0.2 at % Eu2þ, yTb3þ phosphors. The micrograph showed that the particles are agglomerated with irregular morphology with an average diameter of about 5 mm. 3.2. Luminescence properties of g-AlON: 0.2 at% Eu2þ, yTb3þ Fig. 3a shows the PL and PLE spectra of g-AlON: 0.2 at% Eu2þ phosphor. The PLE spectrum monitored at 409 nm shows a broad absorption from 200 to 380, which can be attributed to 5de4f transition of Eu2þ. It can be seen that the emission spectrum displays a broad asymmetrical band from 350 to 550 nm and exhibits a blue band with a peak wavelength at about 409 nm under the excitation wavelength of 298 nm. The PLE and PL spectra of Tb3þ individually doped g-AlON are illustrated in Fig. 3b. Upon 225 nm excitation, the emission spectrum exhibits two groups of emission of Tb3þ.One group is the shorter transition emission of 5D3-7FJ (J ¼ 5.4.3) located at about 420, 432, 470 nm [19,20], and the other consist of several bands located at 488,542,584, and 619 nm [21],

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phosphor for WLEDs. In order to further investigate the energy transfer process between the Eu2þ and Tb3þ ions in the g-AlON host lattice, a series of samples were prepared. The concentration of Eu2þ was fixed at optimal value x ¼ 0.2 and the content of Tb3þ was varied in the range of 0.05e0.5. Fig. 4 shows the emission spectra of g-AlON: 0.2Eu2þ, yTb3þ phosphors. Except for g-AlON: 0.2Eu2þ, the emission spectra (lex ¼ 330 nm) contain a blue emission band of the Eu2þ ions and a series of strong emission lines at 488, 542, 584, and 619 nm, due to the 5D4-7FJ (J ¼ 6, 5, 4, and 3) characteristic transitions of Tb3þ ions, and the green emission line at 542 nm from 5 D4-7FJ transitions dominates the whole spectra. Fig. 5 illuminates the dependence of the intensities of the5d-4f transitions of Eu2þ at 409 nm and 5D4-7FJ transition of Tb3þ at 542 nm as a function of Tb3þ concentrations, respectively. With the increasing of Tb3þ concentration, it is found that the emission intensities of Eu2þ decreases gradually, while the emission intensity of Tb3þ increases systematically and no concentration quenching occurs until Tb3þ content reached 0.5, which further confirms the fact that the energy transfer takes place from Eu2þ to Tb3þ.

3.3. Energy transfer mechanism in g-AlON: 0.2 at% Eu2þ, yTb3þ To investigate the dynamic luminescence process between Eu2þ and Tb3þ, the PL decay curves of Eu2þ are measured. Fig. 6 presents the decay of Eu2þ in g-AlON: 0.2 at%, xat% 3þ Tb (lex ¼ 330 nm,lem ¼ 542 nm). All the decay curves of Eu2þ are well fitted with a single exponential equation:

IðtÞ ¼ I0 expðt=tÞ

Fig. 3. PLE and PL spectra of samples (a) g-AlON: 0.2 at%Eu (c) g-AlON: 0.2 at%Eu2þ, 0.5 at% Tb3.

2þ

(b) g-AlON: 0.5 at% Tb

3þ

corresponding to the 5D4-7FJ (J ¼ 6, 5, 4, 3) transitions, respectively. The PLE spectrum of Tb3þ singly doped phosphor monitored at 542 nm contains several lines locating in the range of UV centered at 225 nm, which is ascribing to absorption f-f transition of the Tb3þ ions. It could be seen from Fig. 3b that g-AlON: Tb3þ powders shows week emission under UV excitation due to forbidden f-f absorption transitions of Tb3þ ions. In order to enhance the absorption intensity in the UV regions of the Tb3þ, Eu2þ ions can be doped as sensitizers to transfer excitation energy to Tb3þ ions. Furthermore, there is a weak spectral overlap between the emission band of the Eu2þ and the excitation band of the Tb3þ ions, indicating an efficient possibility of energy transfer from Eu2þ to Tb3þ ions. This can be further confirmed by the PL and PLE spectra of g-AlON: 0.2Eu2þ, 0.5 Tb3þ, as presented in Fig. 1c. Under the excitation of 330 nm, both the emission of Eu2þ and Tb3þ can be observed in the PL spectrum of the co-doped sample. However, in the range of 380e470 nm, it demonstrates the emission spectrum of Eu2þ, as a result of the intensity of the emission spectrum of Eu2þ is stronger than of Tb3þ. It can be seen clearly that the PLE spectrum monitored at 542 nm of Tb3þ is similar to that monitored at 409 m of Eu2þ, demonstrating the existence of energy transfer from Eu2þ to Tb3þ in g-AlON. And it is another evidence for the energy transfer in gAlON: Eu2þ, Tb3þ that the emission intensity of Tb3þ is enhanced. The PLE spectra show a broad band range from 250 to 380 nm which means that the g-AlON: Eu2þ, Tb3þ phosphor is a potential

where I(t) and I0 are the luminescence intensities. Based on the above eq, the lifetime of Eu2þ ions were determined to be 0.60 ms, 0.50 ms, 0.48 ms, 0.46 ms and 0.42 ms for g-AlON: 0.2 at% Eu2þ,yat% Tb samples with y ¼ 0 at%,0.05 at%, 0.1 at%, 0.3 at% and 0.5 at% respectively. The decrease in the lifetimes of Eu2þ with increasing Tb3þ content strongly demonstrates an energy transfer from Eu2þ to Tb3þ. The energy transfer efficiency ht from the Eu2þ to Tb3þ ions can be calculated by the following expression according to Paulose et al. [22,23].

Fig. 4. PL spectra for g-AlON: 0.2Eu2þ, yTb phosphors depending on various Tb3þ doping concentrations.

J. Zhang et al. / Optical Materials 58 (2016) 290e295

Fig. 5. Dependence of the Eu2þ emission, Tb3þ emission.

293

Fig. 7. Dependence of the energy transfer efficiency (h) on the Tb3þ concentration.

sensitizer in the absence and presence of an activator, respectively. The n ¼ 6, 8, and 10 are dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions, respectively. The relationship 3þ between (Io/Is) and Cn/32þ Eu þ Tb are illustrated in Fig. 8, and best fitting is found 98.3896% only when n ¼ 6. It implies, therefore, that the d-d interaction dominates the energy transfer from Eu2þ to Tb3þ. The critical distance of the Eu2þ / Tb3þ energy transfer Rc was calculated by using the concentration quenching method, where 3þ 2þ the critical distance R 2þ and Tb3þ can be estiEu þ Tb between Eu mated by the following formula suggested by Basse [26].

 Rcz2

Fig. 6. Decay curves for the luminescence of Eu2þ ions in g-AlON: 0.2Eu2þ, yTb phosphors.

h¼1

ts tSO

where h is the energy transfer efficiency and the tso and ts are the lifetime of a sensitizer in the absence and presence of an activator, respectively. Fig. 7 displays the energy transfer efficiency as a function of Tb3þ concentration. It is shown that the energy transfer efficiency of g-AlON: 0.2%Eu2þ,0.5%Tb3þ phosphor increases gradually with increasing Tb3þ concentration. The energy transfer, h, were calculated to be 0%, 10%, 13%, 17% and 24% for the g-AlON: 0.2Eu, yTb with y ¼ 0, 0.05, 0.10, 0.30 and 0.50, respectively. In general, the interaction type between sensitizer or between sensitizer and activator can be calculated by Reisfeid’s approximation as shown in following equation [24,25]:

hs n 3 ∝C ho Eu2þ þTb3þ =

where C is the concentration of the sum of Eu2þ and Tb3þ where ho and hs are the luminescence quantum efficiencies of Eu2þ in the absence and presence of Tb3þ, respectively; the values of hs/hocan be estimated approximately by the ratio of relative luminescence intensity ratio Is/Io, Io and Is are the luminescence intensity of a

3V 4pxc N

1 3

where V is the volume of the unit cell, and N is the number of host cations in the unit cell, xc is the critical concentration (the total concentration of sensitizer ions of Eu2þ and activator ions of Tb3þ). For the g-AlON host, V ¼ 502.4 Å, N ¼ 5, and x ¼ 0.7. Accordingly, the critical transfer distance for Eu2þ and Tb3þ in g-AlON materials is calculated to be 6.49 Å. A schematic diagram of energy transfer (Eu2þ/Tb3þ) in the gAlON host upon excitation with UV radiation is illustrated in Fig. 9. We consider that the energy transfer process of Eu2þ / Tb3þ may take place in this way: When the Eu2þ ions are irradiated by UV light, an electron is pumped to the higher 5d level, and then it relaxes to the lowest 5d crystal field splitting state. An energy transfer process occurs from the excited 5d state of Eu2þ to the 5D3 and 5D4 levels of Tb3þ by dipole-dipole interaction. Then the excited Tb3þ relaxes to the 5D4 levels non-radioactively and shows a strong emission of Tb3þ (5D4 e 7FJ) while the green emission centered at 542 nm resulting from 5D4-7F5 transitions dominates the whole spectra, as shown the Fig. 8. All in all, a green emission is observed in the g-AlON system by utilizing the principle of the energy transfer mechanism of the energy from Eu2þ to Tb3þ.

4. Conclusion We have successfully synthesized a series of green emitting single-phased g-AlON: 0.2Eu2þ, Tb3þ phosphors for UV w-LEDs by traditional high-temperature solid-state reaction. The luminescence properties and energy transfer behavior have been investigated in detail and the energy transfer from Eu2þ to Tb3þ in the gAlON host has been demonstrated to be the dipole-dipole. The

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Fig. 8. Dependence of Iso/Is of Eu2þ on (a) C2þ Eu

3þ þ Tb,

(b) C6/32þ Eu

3þ þ Tb,

(c) C8/32þ Eu

3þ þ Tb,

and (d) C10/32þ Eu

3þ þ Tb.

References

Fig. 9. The schematic diagram of energy transfer in the g-AlON: 0.2Eu, yTb.

energy transfer critical distance was calculated to be 6.49 Å on the basic of the concentration quenching method. In addition, the energy transfer efficiency as well as PL decay curves was also estimated. Our results indicate that the single-phased g-AlON: 0.2 Eu2þ, Tb3þ phosphors exhibits great potential to serve as a green emitting phosphor for UV w-LEDs as a result of its broad excitation in the near-ultraviolet range and the efficient green emission light.

Acknowledgement This work was financially supported by the programs of National Natural Science Foundation of China (No. 51272282 & No.51302311) and significant achievement transformation project of colleges and universities of the Central in Beijing (ZD20141000201), supported by Beijing Municipal Education Commission.

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