Color tuning of Ba2ZnSi2O7:Ce3+, Tb3+ phosphor via energy transfer

Journal of Luminescence 153 (2014) 412–416

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Color tuning of Ba2ZnSi2O7:Ce3 þ , Tb3 þ phosphor via energy transfer Zhongfu Yang, Yihua Hu n, Li Chen, Xiaojuan Wang School of Physics & Optoelectronic Engineering, Guangdong University of Technology, Waihuan Xi Road, No. 100, Guangzhou 510006, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 31 December 2013 Received in revised form 6 March 2014 Accepted 28 March 2014 Available online 5 April 2014

A series of Ce3 þ or Tb3 þ doped and Ce3 þ /Tb3 þ co-doped Ba2ZnSi2O7 phosphors were prepared via the conventional high temperature solid state reaction method. The photoluminescence and energy transfer properties of samples were studied in detail. The optimal proportion of Ce3 þ single doping is 2 mol% with maximal fluorescence intensity. Ba1.98  xZnSi2O7:0.02Ce3 þ , xTb3 þ shows both a blue emission (428 nm) from Ce3 þ and a yellowish-green emission (542 nm) from Tb3 þ with considerable intensity under ultraviolet (UV) excitation (352 nm). The emission chromaticity coordinates can be adjusted from blue to green region by tuning the concentration of Tb3 þ ions from 0.00 to 0.06 through an energy transfer process. The energy transfer mechanism from Ce3 þ to Tb3 þ ions was proved to be dipole–dipole interaction. The Ce3 þ and Tb3 þ co-doped Ba2ZnSi2O7 phosphors are potential UV-convertible candidates with green light emitting in UV-LEDs for the high efficient energy transfer from Ce3 þ to Tb3 þ ions. & 2014 Elsevier B.V. All rights reserved.

Keywords: Ba2ZnSi2O7 Energy transfer Ce3 þ /Tb3 þ Emitting color tuning Dipole–dipole interaction

1. Introduction Inorganic luminescent materials have attracted considerable attention for applications in field emission displays, high definition television, photo-electric devices, security devices, etc. [1,2]. Alkaline earth silicates have been extensively studied and become an interesting topic in the field of luminescent materials because of their waterresistant property and high chemical stability, by comparing with sulfide phosphors and strontium aluminate phosphors [3–5]. Recently, silicate compounds have been extensively studied as host lattices for phosphors activated by Ce3þ and Tb3þ [6–8]. It is well known that Ce3 þ is a high efficiency emission center because the 4f–5d transitions of Ce3 þ are allowed by the Laporte parity selection rules [9]. The 4f–5d transitions of the Ce3 þ ion have been widely investigated, and Ce3 þ ions are known to act as highly efficient activators as a result of this transition. The 5d–4f emissions of the Ce3þ ion can vary from longwavelength UV to red emission depending on the host composition, the crystal structure or the lattice symmetry. Because the emission spectrum can shift so easily, the optical properties of this phosphor have poor reproducibility. The Tb3þ ion is a significant luminescent activator used in advanced lighting and displays that exhibits the characteristic 4f–4f transitions [10,11]. In comparison to the 5d–4f emissions of the Ce3 þ ion, the Tb3þ ion exhibits excellent reproducibility in its optical properties in the green spectral region, while the Tb3þ ion exhibits weak absorption in the 300–410 nm UV region as the result of the 4f–4f absorption transitions which are forbidden by

n

Corresponding author. Tel.: þ 86 20 39322262; fax: þ86 20 39322265. E-mail address: [email protected] (Y. Hu).

http://dx.doi.org/10.1016/j.jlumin.2014.03.066 0022-2313/& 2014 Elsevier B.V. All rights reserved.

the parity selection rule, suggesting that it does not match well with a UV chip. Ce3þ ions that are co-doped in the hosts could not only increase the intensity of the Tb3 þ ions emission, but could also cause excitation in the 300–410 nm UV region, thus overcoming the drawbacks of individually doped Ce3þ or Tb3 þ ions. Generally, the Ce3þ ions act as sensitizers in most hosts as the energy transfer occurs. Ce3þ and Tb3þ co-doped phosphors have been investigated in some hosts [12,13]. The phase transition of Ba2ZnSi2O7 was reported by Segnit and Holland in 1970 [14]; Yao and Xue [15] synthesized Ba2ZnSi2O7: Eu2 þ with a considerable emission (522 nm) from Eu2 þ under ultraviolet excitation (370 nm), finding that it is a potential green phosphor for UV-LEDs. In our early work, the result shows that the optimal proportion of Ce3 þ single doping is 2 mol% with maximal fluorescence intensity. Then, Ba2ZnSi2O7:Ce3 þ , Ba2ZnSi2O7:Tb3 þ , and Ba2ZnSi2O7:Ce3 þ , Tb3 þ phosphors were synthesized by a solid state reaction method. The structure analysis and the photoluminescence analysis of all these samples have been carried out. The energy transfer from Ce3 þ to Tb3 þ has been elucidated.

2. Experimental procedure Ba1.98ZnSi2O7:0.02Ce3 þ , Ba1.98ZnSi2O7:0.02Tb3 þ , and Ba1.98 x ZnSi2O7:0.02Ce3 þ , xTb3 þ (x¼0.001, 0.003, 0.005, 0.01, 0.02, 0.04, and 0.06) were synthesized by a solid-state reaction method. The raw materials were high-purity BaCO3, ZnO, SiO2, CeO2 (99.99%) and Tb4O7 (99.99%), 20 mol% H3BO3 was added as a flux, and 20% excess of ZnO was added to offset the losses in reducing atmosphere. Stoichiometric mixtures of starting materials were homogeneously

Z. Yang et al. / Journal of Luminescence 153 (2014) 412–416

413

mixed in an agate mortar and milled thoroughly for 1 h, then sintered at 1150 1C for 2 h under a reducing atmosphere (N2:H2 ¼ 9:1) in a tube furnace. The samples were cooled to room temperature after annealing in the furnace, and milled again into powder for subsequent use. The phases of prepared phosphor samples were identified by powder X-ray diffraction (XRD) analysis working with Cu Kα irradiation (λ ¼ 1.5406 Å) at 36 kV tube voltage and 20 mA tube current. The photoluminescence (PL) spectra were obtained using a Hitachi F-7000 fluorescence spectrophotometer. The fluorescence lifetimes were measured using a FLS920 fluorescence spectrophotometer. All the measurements were performed at room temperature.

3. Result and discussion The XRD phase analysis was carried out. Fig. 1 shows the XRD patterns of the samples. All the peaks can be indexed to the phase of Ba2ZnSi2O7 (JCPDS#23-0842) crystallized in the monoclinic crystal system in space group C2/c with cell parameters a ¼8.4340 Å, b¼ 10.7220 Å, and c¼ 8.4360 Å [15]. No impurity phase has been observed in any samples, clearly implying that the obtained samples are single phase and the doping of Ce3 þ or Tb3 þ does not cause any significant change to the detection limit of the technique in the host structure. In order to study the photoluminescence properties of the phosphors, the excitation spectra and the emission spectra of Ba1.98ZnSi2O7:0.02Ce3 þ were measured. A Xe lamp was utilized to excite the phosphors when measuring. All the spectra are exhibited in Fig. 2. As we can see from the figure, the excitation spectrum consists of broad absorption bands from 250 to

Fig. 1. XRD patterns of all samples.

Fig. 2. Excitation and emission spectra of Ba1.98ZnSi2O7:0.02Ce3 þ .

Fig. 3. Double-peak fitting of Ce3 þ emission with 352 nm excitation.

400 nm attributed to 5d14f1–5d14f1 transition of Ce3 þ ions. A full width at half-maximum of the phosphors was measured to be  140 nm, indicating that the phosphor can well match with light of UV-LED chips (260–400 nm), which is essential for improving the efficiency of W-LEDs. There are three independent bands peaking at 283 nm, 315 nm and 352 nm. Due to the crystal field interaction, the Ce3 þ 5d state is expected to be 5-fold split [16]. The fact that only three bands are observed means that either some of the five bands are so close to each other that they cannot be distinguished at room temperature, or the two bands missing are located below 250 nm. The three excitation bands are respectively corresponding to the transitions from 4f level to three independent levels of 5d. The emission spectrum of the Ce3 þ -doped single sample exhibits broad band from 350 nm to 550 nm peaking at about 428 nm with 352 nm excitation, which slightly moves towards short wavelength with 315 nm or 283 nm excitation. It can be assigned to 5d–4f transition of Ce3 þ ions due to the strong coupling of the 5d electron with host lattice. Although the 4f electrons of Ce3 þ are not sensitive to their surroundings, the 5d electrons are split by the crystal field. When the crystal field is weak, the emission band of Ce3 þ will exist in the short wavelength. In addition, the emission band is asymmetric including two peaks centered at 420 nm and 458 nm as shown in Fig. 3 (the Gaussian profiles). Generally in the process of de-excitation to the ground level, two prominent bands are observed separated by 2000 cm  1 due to the transitions from the lowest 5d level to the spin-orbit split 2F5/2 and 2F7/2 states of the 4f1 configuration [17]. The energy difference between 420 nm and 458 nm is 2350 cm  1, basically agreeing with the ground state splitting of Ce3 þ ions as discussed above. The excitation spectra of Ba1.98ZnSi2O7:0.02Tb3 þ and Ba1.96 ZnSi2O7:0.02Ce3 þ , 0.02Tb3 þ are illustrated in Fig. 4. The excitation spectra were obtained by monitoring the green emission at 542 nm of Tb3 þ . As shown in the figure, the excitation spectrum of Ba1.98ZnSi2O7:0.02Tb3 þ consists of two proportions: the strong one in the UV region (200–300 nm) which is attributed to the 4f8– 4f75d1 transitions of Tb3 þ ions and the weak one in the longer wavelength region (300–400 nm) which is attributed to the 4f8– 4f8 transitions of Tb3 þ ions. The Tb3 þ ion has a 4f8 electron configuration and prefers to give away one electron forming a more stable half-filled 4f7 configuration. Generally speaking, there are two different f–d transitions: the spin-allowed one 7FJ–7DJ with strong excitation band centered at 240 nm and the spinforbidden one 7FJ–9DJ with weak excitation band centered at 280 nm. The two bands have been observed easily in the figure. In the longer wave length region as shown in the inset of the figure, the excitation peaks are attributed to f–f transitions from

414

Fig. 4. Excitation spectra of Ba1.98ZnSi2O7:0.02Tb3 þ Ce3 þ 0.02Tb3 þ monitoring at 542 nm.

Z. Yang et al. / Journal of Luminescence 153 (2014) 412–416

and Ba1.96ZnSi2O7:0.02

Fig. 6. Schematic diagram of energy transfer process from Ce3 þ to Tb3 þ .

Table 1 The energy transfer efficiencies of samples Ba1.98  xZnSi2O7:0.02Ce3 þ , xTb3 þ calculated from Eq. (1) (ηT) and Eq. (2) (ητ).

Fig. 5. Emission spectra of all samples with excitation wavelength 352 nm.

the 7F6 ground state to the different excited states, i.e., 340 nm (5L7), 351 nm (5L8, 5G3, 5L9, 5G4, 5D2, and 5G5), 369 nm (5L10), and 378 nm (5D3). The f–f inner shell transition of Tb3 þ is spinforbidden, consequently the samples with Tb3 þ single doped are not suit for the UV (260–400 nm) excited WLED materials because of weak excitation bands. However, as shown in Figs. 2 and 4, the emission band of Ce3 þ and the excitation band of Tb3 þ in the host Ba2ZnSi2O7 are partly overlapped, which means that energy transfer from Ce3 þ to Tb3 þ is possible. The excitation spectrum of Ba1.98  xZnSi2O7:0.02Ce3 þ , xTb3 þ (x¼ 0.02) is shown in Fig. 4, and other excitation spectra with different x value are similar except of different intensities. All the spectra were obtained monitoring the 542 nm emission (5D4–7F5) of Tb3 þ ions, which consist of the excitation bands of Tb3 þ (210–260 nm) and Ce3 þ (260–400 nm). The result indicates that the Tb3 þ ions are effectively excited in UV region with Ce3 þ co-doping, and energy transfer from Ce3 þ to Tb3 þ exists in these samples. Fig. 5 shows the emission spectra of Ba1.98  xZnSi2O7:0.02Ce3 þ , xTb3 þ (x ¼0.0001, 0.003, 0.005, 0.01, 0.02, 0.04 and 0.06), and the excitation wavelength is 352 nm. As shown in the figure, the broad emission bands from 370 nm to 475 nm are attributed to the 5d–4f transitions of Ce3 þ ions. Some sharp emission bands exist due to the f–f inner shell transitions of Tb3 þ , such as 5D4–7F6 (489 nm), 5 D4–7F5 (542 nm), 5D4–7F4 (583 nm), and 5D4–7F3 (620 nm). Of them, bright green emission exists at 542 nm arising from the Laporte-forbidden 5D4–7F5 transition. In the emission spectra, the PL intensities alter with Tb3 þ concentration. With x value increasing from 0.001 to 0.06, intensities of Ce3 þ emissions peaking at 428 nm gradually decrease, at the same time, intensities of Tb3 þ emissions at 489 nm, 542 nm, 583 nm and 620 nm gradually

X (%)

0.001

0.003

0.005

0.01

0.02

0.04

0.06

ηT ητ

7.4 9.3

17.7 19.2

28.7 30.9

35.9 35.6

49.2 46.6

58.4 54.1

60.8 56.3

increase. We can see that Tb3 þ can be doped up to 6.0 mol% in host without fluorescence quenching, however, the Ce3 þ emission gets very weak. Therefore, the subsequent work with x4 0.06 was not carried. The emission line of Ba1.98ZnSi2O7:0.02Tb3 þ with excitation wavelength 352 nm is added as a comparison, which is very weak without Ce3 þ co-doping. The results above show that effective energy transfer from Ce3 þ to Tb3 þ exists in these samples. The energy transfer process from Ce3 þ to Tb3 þ is shown in Fig. 6. First, the Ce3 þ ions are excited to the 5d level from the ground state under the 352 nm UV radiating. Then, non-radiative transitions of excited Ce3 þ ions occur and the electrons relax to the underlying excited level, 2D3/2 level. Subsequently, deexcitation to the ground state of Ce3 þ occurs and fluorescenceemissions at 420 nm and 458 nm appear. On the other hand, some energy returning from 2D3/2 (Ce3 þ ) to the ground state is transferred to excited levels of Tb3 þ , 5D2, 5D3 and 5D4, and nonradiatively decays to the level 5D4. Then, Tb3 þ ions radiatively decay down to the various underlying levels of 7FJ (J ¼6, 5, 4, and 3) with emissions at 489 nm, 542 nm, 583 nm and 620 nm respectively. Obviously, increasing the concentration of Tb3 þ ions will enhance the energy transfer rate from the 2D3/2 level of Ce3 þ to the 5DJ (J ¼2, 3, and 4) levels of Tb3 þ , which causes the high luminescence intensity of Tb3 þ ions in the Tb3 þ /Ce3 þ co-doped systems. The energy transfer efficiency from Ce3 þ to Tb3 þ is ηT [18,19]

ηT ¼ 1 

I I0

ð1Þ

where I0 is the emission intensity of Ce3 þ at peak position with Ce3 þ single-doping, and ‘I’ is the emission intensity of Ce3 þ at peak position with Ce3 þ /Tb3 þ co-doping. The parameters of Ba1.98  xZnSi2O7:0.02Ce3 þ , xTb3 þ are listed in Table 1. As shown in the table, the transfer efficiency ηT gradually increases with x from 0.001 to 0.06, and the maximal value is 60.8% (x¼ 0.06).

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Fig. 7. Photoluminescence decay curves of Ce3 þ and Tb3 þ in Ba2ZnSi2O7:0.02Ce3 þ , xTb3 þ .

Fig. 8. Dependence of (I0/I) for Ce3 þ emission on Cn/3 (n¼6, 8, and 10).

Some evidences prove that energy transfer from Ce3 þ to Tb3 þ is through a non-radiative process: one is the decreasing of the cerium emission intensity in the codoped samples with terbium as shown in Fig. 5. Another is the decay time shortening of the cerium emission in presence of terbium as shown in Fig. 7. All the curves are fitted by I ¼I0 exp(  t/τ) (where τ is the lifetime).With the increase of Tb3 þ , there is obviously a decrease of Ce3 þ lifetime from 26.8 ns (x ¼0.00) to 11.7 ns (x ¼0.06). The energy transfer efficiencies from such cerium decay times are ητ

ητ ¼ 1 

τ τ0

ð2Þ

where τ0 is the Ce3 þ lifetime with x ¼0.00. The calculated results are listed in Table 1, which are similar to the results from Eq. (1) with some difference to the detection limit of the technique. The Tb3 þ lifetimes remain unchanged with the value 2.1–2.2 ms. On the basis of Dexter's energy-transfer theory [20], the energy transfer from Ce3 þ to Tb3 þ may take place via electron cloud exchange interaction and multi-polar interaction. Usually, the electron cloud exchange interaction needs small distance (o 0.5 nm) between Ce3 þ ion and Tb3 þ ion, so that their electron clouds overlap partly. However, it is impossible with low doping concentration of Ce3 þ and Tb3 þ , which will bring on large distance much more than 0.5 nm [18]. Formula (3) comes into existence when the multi-polar interaction works [21] I0 p C n=3 I

ð3Þ

C is the doping concentration of Tb3 þ ions; n is equal to 6, 8 or 10; different n values represent different electric multi-polar interaction mechanisms in the energy transfer process between

Fig. 9. CIE chromaticity diagram of Ba1.98  xZnSi2O7:0.02Ce3 þ , xTb3 þ .

Ce3 þ and Tb3 þ ions. The n values of 6, 8, and 10 are corresponding to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interaction mechanisms, respectively. The dependence of I0/I values on C6/3, C8/3 and C10/3 is schematically depicted in Fig. 8 to explore the energy transfer mechanism between Ce3 þ and Tb3 þ ions in detail. The fitting factor R2 in the three fitted lines are shown in the figure. It is

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Z. Yang et al. / Journal of Luminescence 153 (2014) 412–416

obviously the most commendable linear relationship between I0/I and C6/3. Thus, it infers that the energy transfer mechanism between Ce3 þ and Tb3 þ in Ba1.98  xZnSi2O7:0.02Ce3 þ , xTb3 þ matrix is mainly the dipole–dipole interaction. From the emission spectra of Ba1.98 xZnSi2O7:0.02Ce3þ , xTb3 þ , the emitting color of phosphor can be tuned by changing the ratio of blue emission intensity from Ce3þ to the green emission intensity from Tb3þ , which could be realized by appropriately adjusting the doping concentration of Tb3 þ . As Fig. 9 shows, the CIE chromaticity coordinate positions are plotted. The emitting color of Ce3þ single-doped sample is dark blue. The emitting color of samples Ba1.98 xZnSi2O7:0.02Ce3þ , xTb3þ changes from blue region to green region with the rising of Tb3þ doping concentration. 4. Conclusions In summary, a series of Ce3 þ or Tb3 þ doped and Ce3 þ /Tb3 þ codoped Ba2ZnSi2O7 phosphors were prepared via the conventional high temperature solid state reaction method. The emission and excitation spectra have shown that all these samples can be effectively excited by UV light. There is an efficient energy transfer from Ce3 þ to Tb3 þ through a non-radiative process in the codoped samples, which resulted in enhanced green emission from them. The emission color of them can change from blue to green, which was achieved with the increase of Tb3 þ contents from 0.00 to 0.06 under UV excitation (352 nm) when the concentration of Ce3 þ was fixed at 0.02. The energy transfer mechanism between Ce3 þ and Tb3þ ions was proved to be dipole–dipole interaction in the host. The Ce3 þ and Tb3 þ co-doped Ba2ZnSi2O7 phosphors are potential UV-

convertible candidates with green light emitting in UV-LEDs for the high efficient energy transfer from Ce3þ to Tb3þ ions.

Acknowledgments The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 21271049).

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