Energy Transfer and Tunable Luminescence of NaLa(PO3)4:Tb3+

Jan 17, 2014 - ... Tunable Luminescence of NaLa(PO3)4:Tb3+/Eu3+ under VUV and Low-Voltage Electron Beam Excitation. Chunmeng Liu,. †. Dejian Hou,. â...
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Energy Transfer and Tunable Luminescence of NaLa(PO3)4:Tb3+/Eu3+ under VUV and Low-Voltage Electron Beam Excitation Chunmeng Liu,† Dejian Hou,† Jing Yan,† Lei Zhou,† Xiaojun Kuang,† Hongbin Liang,†,* Yan Huang,‡ Bingbing Zhang,‡ and Ye Tao‡ †

MOE Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China ‡ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China ABSTRACT: Series of NaLa(PO3)4:Tb3+/Eu3+ phosphors were prepared by a hightemperature solid-state reaction technique. Structure refinements were performed based on powder X-ray diffraction (XRD) data. VUV−UV−vis photoluminescence (PL), fluorescence decays, time-resolved emission spectra (TRES), and low-voltage cathodoluminescence (CL) spectra were utilized to investigate the luminescence and energy transfer processes. Under VUV−UV light and low-voltage electron beam excitation, NaLa(PO3)4:Tb3+ and NaLa(PO3)4:Eu3+ exhibit characteristic emissions of Tb3+ (5D4 → 7FJ) and Eu3+ (5D0 → 7FJ), respectively. By adjusting the doping concentration of Eu3+ ions in NaTb0.70La(0.30‑x)Eux(PO3)4, tunable emission colors are realized in a large color gamut, in which energy transfer from Tb3+ to Eu3+ was observed and discussed in detail. On the basis of the good VUV−vis PL and CL properties, NaLa(PO3)4:Tb3+/Eu3+ phosphors might be promising for applications in plasma display panels (PDPs) and field emission displays (FEDs).

1. INTRODUCTION 3+

NaLa(PO3)4 (NLPO) is an excellent host compound for luminescence of lanthanide ions.16,17 To get efficient phosphor for FED/PDP, in this paper we focus on luminescence of Tb3+, Eu3+ doped NLPO samples under VUV−UV and low-voltage electron beam excitation, investigate the energy transfer in detail with fluorescence decays and time-resolved emission spectra, and demonstrate the tunable luminescence of Tb3+− Eu3+ codoped samples due to partial energy transfer from Tb3+ to Eu3+.

3+

Phosphors doped with Eu or Tb ions have wide actual applications in lightings and displays. For example, (La, Ce)PO4:Tb3+ and CeMgAl11O19:Tb3+ provide green components in tricolor fluorescent lamps, Y(V,P)O4:Eu3+, Y2O3:Eu3+, and Y2O2S:Eu3+ are red-emitting phosphors in 3D-PDP (threedimension plasma display panel) and FED (field emission display), respectively. PDP and FED are two types of selfemissive flat panel displays (FPDs), in which red (R), green (G) and blue (B) tricolor emission is generated respectively from the phosphors under 147/172 nm vacuum ultraviolet (VUV) light or low-voltage (1−5 kV) cathode ray excitation.1−4 In addition, liquid crystal display (LCD) has now been the most popular FPD in commercial market. Different from PDP and FED, LCD belongs to a nonemissive display device, so the backlight unit becomes a necessary and important component. Light emitting diodes (LED) are generally used as backlight units in LCD at present.5 To enlarge the color gamut of LED backlight units and also to improve the color-rendering index (CRI) of LED-based white-emitting illumination units, luminescence of Tb3+, Eu3+ and energy transfer of Tb3+−Eu3+ in different host compounds under UV−vis excitation have attracted much attention in recent years.6−12 However, to our knowledge, research on the energy transfer and sensitization effect of Tb3+−Eu3+ under VUV−UV and low-voltage electron beam excitation is limited, although it is a key issue for efficient phosphor of FED/PDP.13−15 © 2014 American Chemical Society

2. EXPERIMENTAL SECTION Series of polycrystalline NaLa(1‑y)Tby(PO3)4 (y = 0.01−1.0), NaLa(1‑z)Euz(PO3)4 (z = 0.01−1.0), and NaTb0.70La(0.30‑x)Eux(PO3)4 (x = 0−0.30) samples were prepared by a solid-state reaction method at high temperature. Stoichiometric amount of analytical reagents Na 2CO 3 , NH4H2PO4, and 99.99% pure rare-earth oxides (La2O3, Eu2O3, and Tb4O7) were ground in an agate mortar and then reacted at 873 K (600 °C) for 20 h in thermal-carbon reducing atmosphere. Subsequently, the products were gradually cooled to room temperature by switching off the muffle furnace and ground into powder for subsequent analysis. The phase purity of the products was examined by X-ray diffraction (XRD) using a D8 ADVANCE powder diffractometer with Cu Kα radiation at room temperature. HighReceived: October 12, 2013 Revised: January 16, 2014 Published: January 17, 2014 3220

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3.38%, Rp = 2.47%, and RB = 0.87%. The inset of Figure 1 presents the crystal structure of NaTb0.7La0.2Eu0.1(PO3)4 as viewed along a-axis. Three-dimensional framework of the NaTb0.7La0.2Eu0.1(PO3)4 structure is made up of (PO3)44− wavy chains running along a-axis as well as irregular oxygen polyhedra of Tb/La/Eu atoms. Notably, the Tb/La/Eu occupies one crystal site with C1 symmetry surrounded by PO4 tetrahedra and coordinated by eight O2−. F i g u r e 2 a s h o w s t he X R D p a t t e r n s o f Na T b0.7La0.3‑xEux(PO3)4 (x = 0, 0.05, 0.10, 0.20, 0.30) and NaLa0.3Eu0.7(PO3)4. The samples all maintain the feature of the monoclinic structure, no other phase was detected. However, the diffraction peaks shift slightly to the higher angle side with concentration increase of Eu3+ in NaTb0.7La0.3‑xEux(PO3)4, as shown in Figure 2b. This observation indicates that substitution of smaller Eu3+ (106.6 pm, 8-fold coordinated) for La3+ (116 pm, 8-fold coordinated)21 changes the lattice constants, which can be calculated by structure refinement based on the powder XRD data. As given in Figure 2c, the unit cell volume V clearly decreases when more Eu3+ ions are doped in NaTb0.7La0.3‑xEux(PO3)4 (x = 0−0.30) samples, which is due to the decrease of the lattice constants a, b, and c. Herein, when we linearly fit the shrink rates of the lattice constants a, b, and c, it can be found that c-axis has evident contraction with a slop about −0.225, a-axis shows the smallest shrink only with slop −0.046. Obviously, the contraction rates (slopes) of the lattice constants a, b, and c are different. Meanwhile, the angle β seems to slowly increase. These observations suggest that Eu3+ ions were effectively incorporated into the La3+ site along with contraction and distortion of the unit cell. 3.2. Photoluminescence of NaLa(PO3)4:Ln3+ (Ln = Tb, Eu). The emission and excitation spectra of NaLa(PO3)4:Ln3+ (Ln = Tb, Eu) in the 125−750 nm range are shown in Figure 3. The emission intensities of Tb3+ and Eu3+ as a function of the doping concentrations [y and z values in NaLa(1‑y)Tby(PO3)4 and NaLa(1‑z)Euz(PO3)4, respectively] and decay curves of Tb3+ and Eu3+ in NaLa(1‑y)Tby(PO3)4 and NaLa(1‑z)Euz(PO3)4 are given in Figure 4. It can be seen that luminescence intensities of Tb3+ and Eu3+ are enhanced with the increase of their concentrations (a and c in Figure 4), respectively. The decay curves of Tb3+ and Eu3+ are nearly overlapped one another at different Tb3+ and Eu3+ concentrations as shown in Figure 4b,d. Each curve is an exponential decay. The decay time for all NaLa(1‑y)Tby(PO3)4 (y = 0.1−1.0) and NaLa(1‑z)Euz(PO3)4 (y = 0.1−1.0) samples at room temperature were measured to be ∼3.5 and ∼4.4 ms, respectively. These observations confirm that the concentration quenching of Tb3+ or Eu3+ does not occur in NaLa(PO3)4 host, so highly doping concentration samples are performed here. For clarity, each curve in Figure 3 is normalized to its highest intensity. The VUV−UV (using synchrotron radiation) and UV−vis (using a xenon lamp) excitation spectra of NaLa0.3Tb0.7(PO3)4 of the Tb3+ emission (5D4 → 7F5) at 544 nm are exhibited in curves A and B of Figure 3a, respectively. Sharp absorption lines in 270−500 nm correspond to the 4f8−4f8 transitions of Tb3+ from the 7F6 ground state to the different excited states of Tb3+. That is, 272 nm (5I7), 285 nm (5H3), 304 nm (5H6), 319 nm (5D0), 327 nm (5D1), 341 nm (5G2), 351 nm (5D2), 358 nm (5G5), 368 nm (5L10), 377 nm (5G6), 381 nm (5D3), and 489 nm (5D4), respectively.22 Broad bands in the range of 130−270 nm, correspond to the host-related absorption, spin-allowed

quality XRD data for Rietveld refinement were collected over a 2θ range from 10° to 100° at an interval of 0.02°. Structure refinements of XRD data were performed using Topas Academic software.18 The UV−visible luminescence spectra and luminescence decay curves were recorded on an Edinburgh FLS 920 combined fluorescence lifetime and steady state spectrometer, which was equipped with a time-correlated single-photon counting (TCSPC) card. A 450 W xenon lamp was used as the excitation source for the UV−visible spectra recording, the excitation photons for the luminescence decay curves and timeresolved emission spectra (TRES) determining were provided by a 60 W μF flash lamp with a pulse width of 1.5−3.0 μs. The vacuum ultraviolet (VUV) excitation and corresponding luminescence spectra were measured at the VUV spectroscopy experimental station on beamline 4B8 of the Beijing Synchrotron Radiation Facility (BSRF).19 The cathodoluminescence (CL) measurements were carried out in a vacuum chamber (∼10−4 Pa), where the phosphors were excited by an electron beam in the voltage range of 1−5 kV and at different filament currents. The cathodoluminescent spectra were recorded by a spectrometer (Ocean Optics QEB0388) with a charge-coupled device camera through an optical fiber.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization. Figure 1 shows results of the Rietveld refinement on the XRD pattern of

Figure 1. Experimental (crosses) and calculated (red solid line) XRD patterns and their difference (blue solid line) for NaTb0.7La0.2Eu0.1(PO3)4. The red ticks mark the Bragg reflection positions. The inset shows the projection of the NaTb0.7La0.2Eu0.1(PO3)4 along the a-axis (the sodium atoms are omitted for clarity). Blue and red spherical balls represent Tb/La/Eu and O atoms, respectively.

NaTb0.7La0.2Eu0.1(PO3)4 sample using the P21/n structure model reported by Zhu et al.20 All of the observed peaks satisfy the reflection conditions, confirming the formation of a single phase with no impurities. The remarkable good fit between the experimental data and calculated line supports the structural parameters listed in Table 1. For NaTb0.7La0.2Eu0.1(PO3)4, the lattice constants were determined to be a = 7.1892(3) Å, b = 13.0710(6) Å, c = 9.8260(4) Å, β = 90.507(2)°, and the cell volume was obtained as 923.32(4) Å3. All atom positions, background factors and temperature factors were refined convergence with goodness of fit parameters Rwp = 3221

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Table 1. Final Refined Structural Parameters for NaTb0.7La0.2Eu0.1(PO3)4a atom

site

x

y

z

occ.

Biso (Å2)

Tb La Eu P1 P2 P3 P4 Na1 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12

4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e

−0.0071(6) −0.0071(6) −0.0071(6) 0.122(2) −0.267(2) 0.367(2) −0.242(2) −0.007(3) 0.193(4) −0.083(2) −0.284(3) −0.455(3) −0.296(4) 0.396(4) −0.076(3) −0.424(2) 0.205(4) 0.124(4) 0.305(3) −0.201(4)

0.7775(3) 0.7775(3) 0.7775(3) 0.611(1) 0.593 (1) 0.6249(7) 0.604(1) 0.713(2) 0.510(2) 0.588(2) 0.475(1) 0.613(2) 0.833(2) 0.650(1) 0.652(2) 0.603(2) 0.881(2) 0.685(2) 0.716(1) 0.665(2)

0.0247(5) 0.0247(5) 0.0247(5) −0.277(1) −0.204(1) 0.200(1) 0.253(1) 0.449(2) −0.248(3) −0.288(2) −0.166(2) −0.273(3) 0.091(3) 0.354(2) 0.192(3) 0.162(2) −0.103(2) −0.145(2) 0.099(2) −0.100(2)

0.7 0.2 0.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

1.08 (4) 1.08(4) 1.08(4) 1.0(1) 1.0(1) 1.0(1) 1.0(1) 1.4(3) 0.8(1) 0.8(1) 0.8(1) 0.8(1) 0.8(1) 0.8(1) 0.8(1) 0.8(1) 0.8(1) 0.8(1) 0.8(1) 0.8(1)

Symmetry, monoclinic; space group, P21/n; Z = 4; a = 7.1892(3) Å, b = 13.0710(6) Å, c = 9.8260(4) Å, β = 90.507(2)°, V = 923.32(4) Å3; ρ = 3.55 g/cm3; weighted-profile agreement index Rwp = 3.38%, profile residual agreement index Rp = 2.47%, Bragg-intensity agreement index RB = 0.87%. The occupancy (occ.) factors for Tb/La/Eu were fixed as 0.7/0.2/0.1 according to the nominal composition in the refinement. a

Figure 2. (a) XRD patterns of phosphors NaTb0.7La0.3‑xEux(PO3)4 (x = 0, 0.05, 0.10, 0.20, 0.30) and NaLa0.3Eu0.7(PO3)4 ; (b) Magnified XRD patterns in the region from 22 to 23.5 deg for NaTb0.7La0.3‑xEux(PO3)4 (x = 0, 0.05, 0.10, 0.20, 0.30) phosphors; (c) Unit cell parameters of NaTb0.7La0.3‑xEux(PO3)4 (x = 0, 0.05, 0.10, 0.15, 0.20, 0.30) show a contraction in the lattice constants a, b, c, and V and a expansion in β with Eu3+ concentration increases.

(SA) 4f8 → 4f75d1 transitions, and spin-forbidden (SF) 4f8 → 4f75d1 transitions of Tb3+ ion, respectively.23,24 The main features of the excitation spectra of Tb3+ in NaLa(PO3)4 can be explained using a shift model developed by Dorenbos.25 The model allows estimating the position of the 4fn‑15d energy levels of a lanthanide ion using that observed for

Ce3+ in the same site of a certain matrix. The energies of spinallowed 4f8 → 4f75d transitions of Tb3+ are constructed by adding a constant value ΔE =1.66 ± 0.12 eV to the energies of 4f → 5d transitions of Ce3+. The energies of SF transitions can be generated by applying a shift of −1 eV to the energy of SA transitions of Tb3+. Using the main excitation bands of Ce3+ in 3222

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transition lines within 4f8 configuration of Tb3+. That is, 5D4 → 7 F6 (484 nm), 5D4 → 7F5 (544 nm), 5D4 → 7F4 (585 nm), and 5 D4 → 7F3 (620 nm). The strongest one is located at 544 nm corresponding to 5D4 → 7F5 transition of Tb3+. Curves F and G in Figure 3b show the VUV−UV and UV− vis excitation spectra of NaLa0.3Eu0.7(PO3)4 sample of Eu3+ 611 nm emission, respectively. The sharp peaks in the range of 250−550 nm are associated with the f - f intraconfiguration transitions of Eu3+ (for example: 393 nm, 7F0 → 5L6; 415 nm, 7 F0 → 5D3; 464 nm, 7F0 → 5D2). The broad band between 130 and 260 nm in curve F of Figure 3b is mainly related to the host adsorption (short wavelength side), and to the charge transfer (CT) transitions of ligand O2‑ atoms to Eu3+ (long wavelength side) since the position of the O2‑ → Eu3+ charge transfer band (CTB) usually locates in the range of 170−260 nm in most phosphates.26−28 The curves H and K in Figure 3b represent the emissions after 172 nm excitation into the host-related absorption band and 219 nm excitation into O2‑ → Eu3+ charge transfer band, respectively. They exhibit the same features of four main bands with maxima at about 589, 611, 650, and 698 nm, due to the 5D0 → 7FJ (J = 1, 2, 3, 4) transitions. In addition, it can be found that the intensity of 5D0 → 7F1 magnetic-dipole transition (590 nm) is near that of 5D0 → 7F2 electric-dipole transition (611 nm). However, the Eu3+ ions form EuO8 dodecahedra and occupy a single crystallographic position with C1 symmetry as shown in the inset of Figure 1, which is not an inversion symmetry site, the electric-dipole transition is no longer strictly forbidden and hypersensitive emission of 5D0 → 7F2 should be the dominant one. So there are some discrepancies between our results and most cases. This phenomenon was also observed and discussed in previous reports,26,29,30 which suggests that the intensity of the hypersensitive transition (5D0 → 7F2) is low when the covalency of the Eu−O bond is weak, and high CTB energy often leads to a low percentage of electric dipole transitions. 3.3. VUV−UV Sensitization of Eu3+ Emission by Tb3+ in Codoped Samples. To study the sensitization effect between Eu3+ and Tb3+, the excitation and emission spectra of sample NaTb0.7La0.2Eu0.1(PO3)4 in VUV−UV−vis range were measured at room temperature and shown in Figure 5. Curves A1 and A2 in Figure 5a recorded the VUV−UV and UV−vis excitation spectra of sample NaLa0.2Tb0.7Eu0.1(PO3)4 by monitoring the emission of Tb3+ at 544 nm, which are almost identical to those of NaLa0.3Tb0.7(PO3)4 (curves A and B in Figure 3a) within experimental error, respectively. As previously mentioned, the spectra are in connection with the host-related absorption, the f−d and f−f transitions of Tb3+ in NLPO host lattice. Eu3+-related excitation band/line is undetectable in the spectra of NaLa0.2Tb0.7Eu0.1(PO3)4 when Tb3+ emission is monitored, implying that Eu3+ cannot transfer energy to Tb3+. The VUV−UV and UV−vis excitation spectra recorded at the 611 nm of Eu3+ emission (curves B1 and B2 in Figure 5a) are dominated by Tb3+ bands/lines, which are similar to that of monitoring the Tb3+-emission, but shows large difference with that of Eu3+ (curves F and G in Figure 3b). Only several f−f transition lines of Eu3+ are evidently observed (marked by stars in Figure 5a), the CTB of O2‑ → Eu3+ around 220 nm for Eu3+ in metaphosphate host lattice,26,27 is strongly overwhelmed by the f−d transitions of Tb3+. The presence of the excitation bands and lines of Tb3+ in the excitation spectra of Eu3+

Figure 3. Normalized excitation and emission spectra of NaLa0.3Tb0.7(PO3)4 (a) and NaLa0.3Eu0.7(PO3)4 (b) in VUV−UV−vis range at room temperature. The vertical bars show the calculated positions of SA (red, above) and SF (blue, below) 4f8 → 4f75d transitions of Tb3+.

NaLa(PO3)4 (294, 253, 222, 210, and 199 nm),17 we estimate that the SA transition bands of Tb3+ in NaLa0.3Tb0.7(PO3)4 may be at ∼211, ∼189, ∼171, ∼164, and ∼157 nm, and the SF transition bands at ∼254, ∼223, ∼199, ∼188, and ∼180 nm. Reassuringly, this result is supported by the experimental data: at ∼216 and ∼257 nm, we observed the first SA and the first SF f−d transition bands of Tb3+, respectively. It is also worthwhile to remark that the short-wavelength band of Tb3+ overlaps with the host-related absorption due to PO3− groups, which usually presents in the range of 150−185 nm.17,26,27 Such spectral overlap may be a reason for the energy transfer from the host lattice to Tb3+ ions. Curves C, D, and E in Figure 3a show the normalized emission spectra of NaLa0.3Tb0.7(PO3)4 under 172, 193, and 216 nm excitations at room temperature. These three excitation wavelengths locate in the range where distribute the hostrelated absorption, SA and SF 4f8 → 4f75d1 transitions of Tb3+. Three normalized emission spectra nearly overlapped one another, suggesting that the different excitation wavelengths almost have no influence on the shape of emission spectra. This provides confidence that no bands from possible impurity species are present. The emission spectra consist of f → f 3223

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Figure 4. Emission intensities of Tb3+ (a) and Eu3+ (c) as a function of the doping concentration in NaLa(1‑y)Tby(PO3)4 and NaLa(1‑z)Euz(PO3)4; Decay curves of Tb3+ (b) and Eu3+ (d) in NaLa(1‑y)Tby(PO3)4 (y = 0.1−1.0) and NaLa(1‑z)Euz(PO3)4 (z = 0.1−1.0).

covalency of Tb−O/Eu−O may be slightly stronger than that of La−O in the same site of a certain host. Meanwhile, the effective substitution of smaller Eu3+/Tb3+ ions for La3+ induced the contraction and distortion of the host lattice, which also increased the covalency of Ln−O due to the decrease of the Ln−O bond-lengths. Therefore, much more intense hypersensitive transition of 5D0 → 7F2 was observed in NaLa0.2Tb0.7Eu0.1(PO3)4 while the magnetic-dipole transition (5D0 → 7F1) is more intense in NaLa0.3Eu0.7(PO3)4. Under 351 nm excitation (5D2 of Tb3+), emissions from both Eu3+ and Tb3+ can be detected. There is no absorption for Eu3+ ion at ∼351 nm (see curve G in Figure 3b), confirming that Tb3+ can partially transfer excitation energy to Eu3+ via its absorption of 4f state. It can be found that the intensity of Eu3+ emission under 393 nm excitation is higher than that under 351 nm excitation, which is in line with the excitation spectrum of the Eu3+ emission at 611 nm (curve B2 in Figure 5a). These observations indicate that a direct excitation into the Eu3+5L6 energy level gives much more luminescence intensity of Eu3+ than an indirect excitation through Tb3+ → Eu3+ energy transfer. In the last case, we choose 172 nm as excitation wavelength (curve E in Figure 5), which is a main excitation wavelength in PDPs or Hg-free lamps and is near the hostrelated absorption as shown in Figure 3. After we normalized the curves D (under 351 nm excitation) and E (under 172 nm excitation) to the intensity of Tb3+5D4-7F5 transition, it can be seen that the intensity of Eu3+ emission in curve E is slightly lower than that in curve D. This may relate to the high energy transfer efficiency from host to Tb3+, but we cannot give a sound viewpoint at present and further measurements will be conducted in detail. On the basis of these results, the schematic energy-level diagram and the energy transfer processes are shown in Figure 6. Upon VUV−UV excitation, the energy is first absorbed by (SA and SF) 4f8 → 4f75d transitions of Tb3+ and CT transition of Eu3+−O2‑ as shown in the color lines of Figure 6. Herein, the host may also absorb excitation energy and then transfer to Eu3+/Tb3+, due to the occurrence of host-

Figure 5. (a) Excitation (A1, A2, λem = 544 nm; B1, B2, λem = 611 nm) and (b) emission spectra [λex = 393 (C), 351 (D), and 172 (E) nm] of sample NaLa0.2Tb0.7Eu0.1(PO3)4 at room temperature. The peaks marked with stars are excitation peaks of Eu3+.

emission clearly indicates that energy transfer occurs from Tb3+ to Eu3+. Figure 5b exhibits the emission spectra of NaLa0.2Tb0.7Eu0.1(PO3)4 under different excitations at room temperature. Upon 393 nm excitation (5L6 of Eu3+), only emission from Eu3+ is observed, and the positions of all emission peaks are identical to those in Figure 3b (H, K) of NaLa0.3Eu0.7(PO3)4, while the relative intensity is changed, the intensity of electric-dipole transition (5D0 → 7F2) is higher than that of magnetic-dipole transition (5D0 → 7F1). As we all know, for lanthanide ions, the electronegativity is somewhat increased along with the increase of the atomic number,31 so the 3224

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where It and I0 are the luminescence intensities at time t and t = 0, respectively; τ is the decay time. The values of τ were calculated to be about 3.50, 3.30, 2.96, 1.83, 1.27, 0.93, 0.75, 0.59, and 0.51 ms for NaTb0.7La(0.3‑x)Eux(PO3)4 with x = 0, 0.005, 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30, respectively, as shown in Figure 7b. It can be observed that the decay rate of the 5D4 state within Tb3+ ion under 351 nm UV excitation become more and more rapid with the increase of Eu3+ concentration, due to the energy transfer from Tb3+ to Eu3+. A simple experimental formula can be used to estimate the Tb3+ → Eu3+ energy-transfer probability (PTb→Eu).33 PTb → Eu =

(2)

where τ0 and τ are the corresponding lifetimes of the donor (Tb3+) in the absence and presence of the acceptor (Eu3+) for the same donor concentration, respectively. Furthermore, the energy transfer efficiency (ηTb→Eu) can be calculated according to the general eq 3:

Figure 6. Schematic energy-level diagram of Tb3+ and Eu3+ in NaLa(PO3)4 and the energy transfer from Tb3+ to Eu3+.

⎛τ⎞ ηTb → Eu = 1 − ⎜ ⎟ ⎝ τ0 ⎠

related absorption in VUV range. After multistep relaxations, the electrons in different excited states finally reach the 5DJ states of Eu3+/Tb3+, respectively. Subsequently, the populated 5 DJ states of Eu3+/Tb3+ ions give emissions via 5DJ→7FJ transitions. Meanwhile, Tb3+ ions at 5DJ emitting states may also transfer energy to the excited energy levels of Eu3+ through phonon-assisted cross relaxation as shown in the dash lines of Figure 6.6,32 3.4. Tb3+→ Eu3+ Energy Transfer. The efficient energy transfer from Tb3+ to Eu3+ has been observed in the VUV− UV−vis spectra. Herein, luminescence decays and timeresolved emission spectra (TRES) were measured to investigate the energy transfer between Tb3+ and Eu3+. Fluorescence decays of Tb3+5D4 → 7F5 transitions under 351 nm excitation as a function of the Eu3+ doping concentrations in NaTb0.7La(0.3‑x)Eux(PO3)4 (x = 0−0.3) are exhibited in Figure 7a. A single-exponential decay process was observed with different x values in NaTb0.7La(0.3‑x)Eux(PO3)4 (x = 0− 0.3), the curves were well fitted by eq 1: It = I0 exp( −t /τ )

⎛1⎞ ⎛ 1 ⎞ ⎜ ⎟ − ⎜ ⎟ ⎝ τ ⎠ ⎝ τ0 ⎠

(3)

According to the above formulas 2 and 3, all the calculated results of PTb→Eu and ηTb→Eu are shown in parts a and b of Figure 8, respectively. We can observe that the energy-transfer

(1)

Figure 8. Dependence of the energy transfer probability PTb→Eu (a) and efficiency ηTb→Eu (b) in NaTb0.7La(0.3‑x)Eux(PO3)4 with different Eu3+ concentration (x = 0.005, 0.01, 0.03, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30).

probabilities and efficiencies from Tb3+ to Eu3+ increase with the increasing Eu3+ concentration, indicating a much more efficient energy-transfer process occurred in the sample with high Eu3+ ion concentration. Figure 9 shows the luminescence decay curves for samples NaLa0.3Eu0.7(PO3)4 and NaTb0.7La(0.3‑x)Eux(PO3)4 (x = 0.1, 0.2, 0.3) when Eu3+ ions were directly excited by 393 nm violet light through 7F0 → 5L6 transition within Eu3+ ions at room temperature. All the decay curves also can be fitted with the single-exponential decay eq 1, and they have nearly the same decay time value (∼4.4 ms) for Eu3+ transient emission at 612 nm, indicating the lifetime values are almost not influenced by Eu3+ concentrations. The luminescence decays also verify further the inexistence of the energy transfer from Eu3+ to Tb3+.

Figure 7. Decay curves (a) and fluorescence lifetimes (b) of Tb3+ under 351 nm excitation as a function of x values in NaTb0.7La(0.3‑x)Eux(PO3)4 (x = 0−0.3). 3225

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When the NaTb0.7La(0.3‑x)Eux(PO3)4 samples were excited by 351 nm, the rate equations for the population densities in the 5 D4 level of Tb3+ ion and 5D0 level of Eu3+ ion can be written as follows:27 dNTb N = − Tb − KTb − EuNTb τTb dt dNEu N = − Eu + KTb − EuNTb τEu dt

(4)

(5) 5

where the NTb and NEu are the population densities in the D4 level of Tb3+ and 5D0 level of Eu3+, respectively. KTb−Eu is the nonradiative energy transfer rate from 5D4 state of Tb3+ to 5D0 state of Eu3+ in NaTb0.7La(0.3‑x)Eux(PO3)4 samples. Hence the fluorescence intensity I(t) of Eu3+ ions at 612 nm under 351 nm excitation can be given as eq 6: I(t ) = NEu(t ) =

Figure 9. Luminescence decay curves of NaLa0.3Eu0.7(PO3)4 and NaTb0.7La(0.3‑x)Eux(PO3)4 (x = 0.1, 0.2, 0.3) samples at room temperature (λex = 393 nm, λem = 612 nm).

⎛ t ⎞⎤ KTb − EuNTb ⎡ ⎛ t ⎞ ⎢exp⎜ − ⎟ − exp⎜ − ⎟⎥ 1 1 ⎢⎣ ⎝ τTb ⎠ ⎝ τEu ⎠⎥⎦ −

(

τEu

τTb

)

(6)

here τTb data are presented in Figure 7, and τEu data are from Figure 9. The theoretical curves in Figure 10b show two processes for Eu3+ emission as in Figure 10a, and exhibit a similar trend to that of the experimental results, which gives a proof to our analysis of energy transfer in the system. To study the energy transfer process from Tb3+ to Eu3+ in more detail, time-resolved emission spectra (TRES) of NaTb0.70La0.20Eu0.10(PO3)4 were measured by excited into the Tb3+ peak with a 351 nm UV light. The emission spectra collected at different delay times (td) are shown in Figure 11a.

Figure 10 depicts the decay curves of samples NaTb0.7La(0.3‑x)Eux(PO3)4 with different doping concentration (x

Figure 10. Eu3+ ion concentration dependence of the luminescence decay curves of NaTb0.7La(0.3‑x)Eux(PO3)4 (x = 0.05, 0.10, 0.15, 0.20, 0.30) samples at room temperature (λex = 351 nm, λem = 612 nm).

= 0.05, 0.10, 0.15, 0.20, 0.30) at room temperature (λex = 351 nm, λem = 612 nm). When Tb3+ ions were excited by 351 nm UV light, there are two processes in the decay curves for Eu3+ emission: build-up process and decay process. In build-up process, the energies absorbed by the 7F6 → 5D2 transition within Tb3+ ions are transferred to Eu3+ ions through energy migration. The build-up process is significantly influenced by the Eu3+ concentration, which become faster and faster with the increase of Eu3+ ions as shown in the magnified figure of Figure 10a, indicating Tb3+ → Eu3+ energy-transfer process is more efficient with the increase of Eu3+ doping concentration. The decay processes are nearly the same for all samples with decay constant about 4.4 ms. This value is identical to the decay time value when Eu3+ ions were directly excited by 393 nm UV light (Figure 9).

Figure 11. (a) Time-resolved emission spectra of the NaTb0.7La0.2Eu0.1(PO3)4 sample (λex = 351 nm). (b) Decay time dependence of the emission intensity at 544 nm (Tb3+, curve A) and 612 nm (Eu3+, curve B).

When td = 0.4 ms, the characteristic emission of Tb3+ is dominated, a little Eu3+ emission also appears, because only a small part of the excitation energy of Tb3+ has been transferred to Eu3+ within this short time. As the extension of delay time, the emissions of Tb3+ decrease accompanied by the increase of Eu3+ emissions due to the energy transfer from Tb3+ to Eu3+, and when td = 2.0 ms, the emission intensity of Eu3+ is 3226

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Figure 12. Emission spectra (a) and CIE chromaticity diagram (b) for NaTb0.7La(0.3‑x)Eux(PO3)4 (x = 0, 0.01, 0.05, 0.10, 0.15, 0.20, 0.30) under 172 nm excitation.

strongest. Then, the emission intensities of Tb3+ and Eu3+ decrease gradually with further increase in the delay time, and disappear finally due to the depopulation of the excited states. Because of the Tb3+ → Eu3+ energy transfer and the luminescence decay of Tb3+, the emission intensity of Tb3+ always decreases with increasing delay time, as shown in curve A of Figure 11b. The dependence of the Eu3+ luminescence intensity on the decay time shows a maximum at ∼2.0 ms, as shown in curve B of Figure 11b. This observation relates to the strengthening of Eu3+ luminescence through Tb3+ → Eu3+ energy transfer and the weakening of Eu3+ luminescence due to its decay. Although the fluorescence decay of Eu3+ (∼4.4 ms) is slightly slower than that of Tb3+ (∼3.5 ms), when td = 8.0 ms, the Tb3+ emission is so weak that it nearly disappeared, Eu3+ emission is still strong and clearly observed (Figure 11a). This is the result of the efficient energy transfer from Tb3+ to Eu3+. 3.5. Tunable Emission under VUV and Low-Voltage Electron Beam Excitation. Figure 12a shows the emission spectra of samples NaTb0.7La(0.3‑x)Eux(PO3)4 with different Eu3+ contents (x value) under 172 nm excitation. In Eu3+undoped NaTb0.7La(0.3‑x)Eux(PO3)4 (x = 0) sample, the characteristic emissions of Tb3+ are observed. With the doping of Eu3+ (x = 0.01), besides Tb3+ emissions, we can also observe the Eu3+ emissions. With the increase of Eu3+ concentration, the luminescence of Tb3+ begins to decrease, and that of Eu3+ increases, due to the enhancing probability of energy transfer from Tb3+ to Eu3+. This effect can be confirmed by the fluorescent dynamics of Tb3+ and Eu3+, as shown in Figures 7 and 9. Therefore, samples NaTb0.7La(0.3‑x)Eux(PO3)4 (x = 0− 0.30) show a tunable emission from yellowish-green to reddishorange, depending on the Eu3+ content, as shown in the CIE chromaticity diagram of Figure 12b, in which the CIE chromaticity coordinate position of different samples are indicated. Under 172 nm excitation, the integrated emission intensity of the yellow-emitting NaTb0.7La0.2 Eu0.1 (PO3 )4 phosphor in the visible region is calculated to be about 77% of that of the PDP commercial phosphor (Y, Gd)BO3:Eu3+ (red-emitting).34 We know that the comparison of yellowemitting and red-emitting may be unsuitable. However, the PDPs are only with RGB tricolor phosphors and without yellow-emitting phosphor. So we selected the red-emitting

PDPs commercial phosphor (Y, Gd)BO3:Eu3+ as a reference, just to show the relative intensity of our sample. To check the potential application of the as-synthesized NaLa(PO3)4:Tb3+/Eu3+ phosphors in FEDs and explore their stability under low-voltage electron beam excitation, their CL properties were studied. The CL spectra as well as their corresponding luminescence photographs of NaTb0.7La(0.3‑x)Eux(PO3)4 (x = 0, 0.01, 0.05, 0.10, 0.30) and NaLa0.3Eu0.7(PO3)4 (NLPO-Eu3+) were obtained under electron beam excitation (51 μA·cm−2 and 2 kV) as shown in Figure 13. Under low-voltage electron beam bombardment, the

Figure 13. CL spectra and luminescence photographs of NaTb 0.7 La (0.3‑x) Eu x (PO 3 ) 4 (x = 0, 0.01, 0.05, 0.10, 0.03) and NaLa0.3Eu0.7(PO3)4 (NLPO-Eu3+) under cathode ray excitation (excitation voltage = 2 kV, current density = 51 μA·cm−2).

energy transfer processes from Tb3+ to Eu3+ also were observed in the NLPO host. As shown in Figure 13, maintaining the Tb3+ concentration at 0.7 and changing the Eu3+ concentration from 0 to 0.3 in the NLPO host, the emission intensity of Tb3+ gradually decreases while the intensity of Eu3+ gradually increases. The insets of Figure 13 exhibit the corresponding luminescence photographs of NaTb0.7La(0.3‑x)Eux(PO3)4 (x = 0−0.30), demonstrating that the phosphors show intensive 3227

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excited by the plasma produced by the incident electrons. The deeper the electron penetration depth, the more plasma will be produced, which results in more Eu3+ and Tb3+ ions being excited and thus the CL intensity increases. Similarly, under a 2 kV electron beam excitation, the CL integrated intensities of NaLa 0 . 3 Tb 0 . 7 (PO 3 ) 4 , NaLa0.2Tb0.7Eu0.1(PO3)4, and NaLa0.3Eu0.7(PO3)4 phosphors also increase with the current density from 25.5 to 152.9 μA·cm−2 (Figure 14b). Because FEDs operate at lower voltage (≤5 kV), the electron beam impinging on the phosphors must have a higher current density to compensate for the decreased voltage. Therefore, for FEDs phosphors, higher saturation current is needed. As shown in Figure 14b, no saturation was observed up to 152.9 μA·cm−2 for NaTb0.7La0.2Eu0.1(PO3)4 phosphor, which is in favor of FEDs application.

emission under low-voltage electron-beam excitation, and the emission color can be tuned from yellow-green to red-orange due to partial Tb3+ → Eu3+ energy transfer and simultaneous emission. Under cathode ray (51 μA·cm−2 and 2 kV) excitation, the integrated intensity of NaTb0.7La0.2Eu0.1(PO3)4, which is near that of other samples as shown in Figure 13, is estimated to be about 37% of the FEDs commercial phosphor Y2O3:Eu3+.35 Moreover, for Tb3+ or Eu3+ single-doped sample, the emission spectrum of Tb3+/Eu3+ is consist of the characteristic emissions of Tb3+/Eu3+ (curves 1 or 6 in Figure 13), which is basically identical with the PL spectra (Figure 3). However, for Tb3+, Eu3+ codoped samples, the ratio of Tb3+/Eu3+ emission intensity in the CL spectrum (Figure 13) is bigger than that in the PL spectrum (Figure 12) for the same sample. This phenomenon can be attributed to the somewhat different mechanisms between CL and PL. Under different excitation conditions (low energetic VUV/UV/visible photons and high energetic cathode ray), photoluminescence and cathodoluminescence as well as their energy transfer mechanisms may show some differences.36 NaLa0.3Tb0.7(PO3)4, NaLa 0.2Tb0.7Eu0.1(PO3)4, and NaLa0.3Eu0.7(PO3)4 phosphors were investigated as a function of accelerating voltage as shown in Figure 14a. When the current

4. CONCLUSIONS With a high-temperature solid-state reaction method, we prepared series of NaLa(PO3)4:Tb3+/Eu3+ phosphors. Structure refinements of typical samples were performed based on the powder X-ray diffraction data. The results reveal that the doped Tb3+/Eu3+ ions were effectively built into the La3+ site along with contraction and distortion of the NaLa(PO3)4 host lattice. In view of luminescence intensity/decay − doping concentration relationships, it is demonstrated that the concentration quenching of Tb3+ or Eu3+ does not occur in NaLa(PO3)4. VUV−UV−vis excitation and emission spectra confirm that Tb3+ can efficiently sensitize Eu3+ emission under VUV−UV excitation and the schematic energy transfer processes are suggested. By means of fluorescence decays at different measurement conditions and time-resolved emission spectra (TRES), the viewpoints on the energy transfer processes are reinforced, showing that the Tb3+→Eu3+ energy transfer probability and transfer efficiency increase with the increase of Eu3+ doping content in NaTb0.7La(0.3‑x)Eux(PO3)4 samples. Because of partial energy transfer from Tb3+ to Eu3+, the bright emission of NaLa(PO3)4:Tb3+, Eu3+ is tunable in a large color gamut, this series of phosphors is expected to have potential applications in PDPs and FEDs.

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AUTHOR INFORMATION

Corresponding Author

*(H.L.) E-mail: [email protected]. Telephone: +86-2084113695. Fax: +86-20-84111038.

Figure 14. CL integrated intensities of phosphors NaLa0.3Tb0.7(PO3)4, NaLa0.2Tb0.7Eu0.1(PO3)4, and NaLa0.3Eu0.7(PO3)4 as a function of accelerating voltage (a) and current density of electron beam (b).

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work is financially supported by the National Natural Science Foundation of China (Grant Nos. 10979027, 21171176, and U1232108), the Science and Technology Project of Guangzhou (2013Y2-00118) and the Natural Science Foundation of Guangdong Province (S2013030012842).

density is fixed at 51 μA·cm−2, the CL intensity increases upon raising the acceleration voltage from 1 to 5 kV. The increase of CL intensity with increasing electron energy is attributed to the larger electron beam current intensity and the deeper penetration of the electron into the phosphor’s body. The electron penetration depth can be estimated by the empirical formula L [Å] = 250(A/ρ)(E/Z1/2)n, where n = 1.2/(1 − 0.29 log10 Z), A is the atomic or molecule weight of the material, ρ is the bulk density, Z is the atomic number or the number of the electrons per molecule in the compounds, and E is the accelerating voltage (kV).37 For NaTb0.7La0.2Eu0.1(PO3)4, A = 493.10, Z = 230.2, and ρ = 3.55 g/cm3. According to the above empirical formula, the estimated electron penetration depth at 1, 2, 3, 4, and 5 kV are 0.11, 1.54, 7.23, 21.62, and 50.59 nm, respectively. For cathodoluminescence, Eu3+ and Tb3+ ions are

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