Photoluminescence and Ce3+→Tb3+→Eu3+ Energy Transfer Processes of the Ce3+/Tb3+/Eu3+-doped β-NaYF4 Phosphors with Broadened Excitation Spectrum

HUANG Jianhua, CHEN Nan, WANG Xiaojing, DU Guoping, ZHANG Aisheng

(1. School of Materials Science and Engineering, Nanchang University, Nanchang 330031, China; 2. Hunan Engineering Laboratory for Control and Optimization of PV Systems, Hunan Vocational Institute of Technology, Xiangtan 411104, China)

Abstract: Ce3+/Tb3+ co-doped and Ce3+/Tb3+/Eu3+ tri-doped β-NaYF4 photoluminescent microcrystals using oleic acid as surfactant were synthesized using the solvothermal method. Their microstructural characteristics and photoluminescence properties were investigated in detail. They have the shape of hexagonal prism bipyramids with uniform particle size, which decreases with the concentrations of Tb3+ and Eu3+. The energy transfer processes of both the Ce3+→Tb3+ and the Ce3+→Tb3+→Eu3+ were systematically studied. Compared with Eu3+ or Tb3+ single-doped β-NaYF4 microcrystals, the sensitization by Ce3+ for the photoluminescence of Tb3+ and Eu3+ leads to a broad excitation spectral bandwidth in the ultraviolet (UV)range. Meanwhile, the corresponding optical absorption efficiency is greatly enhanced. High energy transfer efficiencies have been observed from Ce3+ to Tb3+ and from Tb3+ to Eu3+.

Key words: lanthanide doped β-NaYF4; photoluminescence; energy transfer; broad excitation spectrum;solvothermal method

1 Introduction

Recently, lanthanide ion (Ln3+) doped inorganic photoluminescence (PL) materials have attracted considerable attention due to their potential applications in various fields such as white light-emitting diodes(WLEDs)[1], solar cells, solid state lasers, display devices, biological labels and high pressure mercury lamps[2-8],etc. In particular, WLEDs have broad applications in solid state lighting areas because of their characteristic merits of energy saving, high energy efficiency and long life time[9]. The host lattices have significant effects on the photoluminescence properties of Ln3+ions. Sulfides[10-12], oxides[13-16], nitrides[2]and fluorides[17-19]are candidates for host materials of phosphors. However, sulfide phosphors have the fatal weaknesses of undesirable physical, chemical and thermal stability[20]. Besides, their synthesis process is relatively complicated[11]. Nitride phosphors have a harsh and expensive synthesis process[20,21]. Oxides phosphors have higher stability and can be prepared relatively more easily, but their high phonon energy is not beneficial to their photoluminescence properties. However, Ln3+doped fluorides have received more and more attention because of their high stability and strong photoluminescence properties, which are ascribed to the advantages of their low phonon energy, narrow emission band and excellent optical transparency and so on[22]. Among the fluorides, sodium yttrium fluoride (NaYF4) has been one of the most ideal host matrix of photoluminescence materials for its outstanding characteristics[23,24].

NaYF4has two different phases: cubic (α-NaYF4)and hexagonal (β-NaYF4) crystal structures. Theβ-NaYF4lattice has higher symmetry than theα-NaYF4,so the former has higher energy transfer efficiency[25].β-NaYF4has been widely investigated as an excellent host matrix of Ln3+based photoluminescence materials in recent years. Up to now, research work on Ln3+dopedβ-NaYF4phosphors has been mainly focused on visible light and near infrared-to-visible up-conversion photoluminescence. But research on the near ultraviolet/blue-to-visible down-conversion photoluminescence has been paid less attention[26]. It is well known that some LEDs and all of mercury lamps are excited in the ultraviolet spectral range. The excitation spectrum of most Ln3+ions is narrow because of the forbidden nature of theirf-ftransitions. In addition, their optical absorption cross section is small. One exception is the trivalent cerium (Ce3+), which is the most common Ln3+used as the sensitizer of down-conversion photoluminescence materials. This is because the 4f→5dtransition of Ce3+ions is allowed and there is a broad strong absorption band around the ultraviolet region.Furthermore, Ce3+ions have broad emission spectra in the ultraviolet and blue regions, and it overlaps with the absorption peaks of Tb3+and Eu3+ions for allowing efficient energy transfer from the former to the latter. These merits make Ce3+an ideal sensitizer. Tb3+and Eu3+ions possess the typical green color purity of5D4→7FJtransitions of Tb3+and excellent red color purity of5D0→7FJtransitions of Eu3+, respectively. Thus,Ce3+, Tb3+and Eu3+co-doped β-NaYF4is expected to have desirable photoluminescence properties.

It is worth noting that the energy transfer from Ce3+to Tb3+ions and that from Tb3+to Eu3+ions is workable, but it is a pity that Eu3+cannot be sensitized directly by Ce3+due to the metal-metal charge transfer(MMCT)[27]. The photoluminescence of Ce3+and Eu3+ions would be seriously quenched as the consequence of MMCT. Fortunately, Tb3+ions can be used as a bridge between Ce3+and Eu3+ions to minimize the effect of the MMCT[28]. The energy transfer strategy of Ce3+→Tb3+→Eu3+have been successfully induced in Y2SiO5:Ce3+,Tb3+,Eu3+powders[9], YASON:Ce3+,Tb3+,Eu3+powders[2], CaF2:Ce3+,Tb3+,Eu3+phosphors with highly uniform shapes[29], Na2Y2B2O7:Ce3+,Tb3+,Eu3+phosphors[1],etc.Researchers have studied the energy transfer processes of Ce3+→(Tb3+)n→Eu3+,the energy transfer efficiency among the Ln3+ions and their photoluminescence properties[30,31]. However, little research has been reported on such energy transfer process in Ln3+co-doped fluorides such as theβ-NaYF4and others.

Herein, we synthesized uniform hexagonal prism bipyramidsβ-NaYF4:Ce3+, Tb3+, Eu3+microcrystals using the solvothermal method. Similar to the hydrothermal method[32], the solvothermal method is also usually used to synthesize nano- and micron-sized particles.Oleic acid was used as surfactant to control the morphology of theβ-NaYF4: Ce3+, Tb3+, Eu3+microcrystals.It was found that their microstructural characteristics were strongly influenced by the doping ratios. Mechanism for the microstructure evolution of theβ-NaYF4:Ce3+, Tb3+, Eu3+hexagonal prism bipyramidal microcrystals was proposed. The photoluminescence properties of theβ-NaYF4: Ce3+, Tb3+, Eu3+microcrystals was investigated systematically. As expected, broadband excitation spectrum and high efficiency of energy transfer process were achieved successfully for theβ-NaYF4:Ce3+, Tb3+, Eu3+microcrystals. The energy transfer processes and related mechanisms were studied in detail.

2 Experimental

2.1 Materials

In this work, the lanthanide nitrates Ln(NO3)3·6H2O (99.99%) (Ln = Y, Ce, Tb, Eu)and sodium nitrate (NaNO3) and ammonium fluoride (NH4F) were used as the raw materials for synthesizing β-NaYF4: Ce3+, Tb3+, Eu3+. Terbium nitrate (Tb(NO3)3·6H2O) and europium nitrate (Eu-(NO3)3·6H2O) were used as activators while cerium nitrate (Ce(NO3)3·6H2O) was sensitizer. Oleic acid(OA) was used as surfactant, and ethylene glycol (EG,C2H6O2) was used as solvent. All of the raw materials were used as received. Deionized water was used throughout this work.

2.2 Synthesis of β-NaYF4:Ce3+,Tb3+ microcrystals

Theβ-NaYF4:1%Ce3+,x%Tb3+(or in the formula ofβ-NaY0.99-xCe0.01TbxF4) microcrystals were synthesized using a solvothermal method[17]. In a typical procedure, Y(NO3)3·6H2O (1.98 – 2xmmol)，Tb(NO3)3·6H2O (2xmmol), Ce(NO3)3·6H2O (0.02 mmol) and NaNO3(0.1700 g, or 2 mmol) were mixed and dissolved adequately in a solution of 10 mL ethylene glycol (EG) to form a clear solution A. Similarly,48 mmol NH4F (1.7779 g, or 48 mmol) worked as the fluorine source was dissolved in 15 mL EG to form another clear solution B. Therefore, the molar ratio of Ln3+to F-ions was 1 : 24. Then, 5 mL OA was added to solution A, followed by mixing the two solutions under stirring for 1 h respectively to form a mixture solution. The mixture solution was poured into a 50-mL Teflon-lined stainless steel autoclave, which was heated at 180oC for 6 h to synthesizeβ-NaYF4:1%Ce3+,x%Tb3+microcrystals before being cooled down to room temperature. The resultantβ-NaYF4:1%Ce3+,x%Tb3+microcrystals were collected in a similar manner as described previously[17].

2.3 Synthesis of β-NaYF4:Ce3+, Tb3+, Eu3+microcrystals

The starting materials for synthesizing theβ-NaYF4:1%Ce3+,30%Tb3+,y%Eu3+(with the formulaβ-NaY0.69-yCe0.01Tb0.3EuyF4) microcrystals included Y(NO3)3·6H2O (1.38 - 2xmmol)，Tb(NO3)3·6H2O (0.6 mmol), Eu(NO3)3·6H2O (2y mmol), Ce(NO3)3·6H2O(0.02 mmol) and NaNO3(0.1700 g, or 2 mmol). The synthesis procedure was the same as the above procedure described in Section 2.2.

2.4 Characterization

X-ray diffraction technique (XRD, PNAalytical,Empyrean) and field emission scanning electron microscope (FESEM, FEI-Quanta200F) were used to study their phase structures and observe their microstructural characteristics. The X-ray diffractometer equipped with a graphite monochromatized Cu Ka radiation (λ =1.540 6 Å) was operated at a scanning step of 0.013 130 3oin the 2θrange from 10oto 90o. Energy dispersive spectrometry (EDS, Link-ISIS) was used to analyze their compositions. The photoluminescence excitation and emission spectra were recorded with an Hitachi F-4600 fluorescence spectrophotometer equipped with an xenon lamp source. All of the measurements in this work were conducted at room temperature.

3 Results and discussion

3.1 The β-NaYF4:Ce3+,Tb3+ microcrystals

3.1.1 Microstructural characteristics

Fig.1 XRD patterns of β-NaYF4:1%Ce3+, x%Tb3+ microcrystals (x= 1, 5, 10, 20, 40, 60, 80)

Fig.1 shows the XRD patterns ofβ-NaYF4:1%Ce3+,x%Tb3+(x= 1, 5, 10, 20, 40, 60 and 80) microcrystals with different concentrations of Tb3+ions. For comparison, the Joint Committee on Powder Diffraction Standards (JCPDS) data for the hexagonal crystal structure of NaYF4(PDF#16-0334) was also included in Fig. 1. All of the XRD diffraction peaks can be readily indexed to the pure hexagonal crystal structure ofβ-NaYF4phase, and no peaks of secondary phases were observed (Fig.1), implying that the Tb3+ions were completely incorporated into the host lattice. Hence,forβ-NaYF4, Tb3+can substitute Y3+to a large extent.The sharp diffraction peaks in Fig.1 suggest that theseβ-NaYF4:Ce3+, Tb3+particles were well crystallized.The EDS measurement (not shown) indicate that theβ-NaYF4:Ce3+and Fig.2 SEM images of theβ-NaYF4:1%Ce3+,x%Tb3+microcrystals (x= 1, 10, 20, 40, 60,80)-NaYF4: Ce3+, Tb3+microcrystals consisted of all of the intended compositional elements. This indirectly supports the incorporation of Ce3+and Tb3+ions into theβ-NaYF4host lattice.

Table 1 List of a, c, c/a, V, 2θ and d of (110) for the as-synthesized β-NaYF4 and β-NaYF4: 1%Ce3+, x%Tb3+ microcrystals

The lattice constantsaandc,c/aratio, cell volumeV, 2θand the (110) crystal planar spacingd(110)of the as-synthesizedβ-NaYF4andβ-NaYF4: 1%Ce3+,x%Tb3+microcrystals are listed in Table 1. From the PDF#16-0334 data, for theβ-NaYF4crystal,a= 0.596 0 nm,c= 0.353 0 nm and space groupP63/mmc[33]. It can be seen from Table 1 that thec,c/aratio andVof theβ-NaYF4: 1%Ce3+,x%Tb3+microcrystals steadily increased with the Tb3+concentrationx, and the 2θangle of (110) shifts to lower angles suggesting an increasingd(110)withx. This can be expected because both Ce3+(rCe= 0.103 4 nm) and Tb3+(rTb= 0.092 3 nm) ions have larger ionic radius than Y3+(rY= 0.088 nm)[12,34,35]. The lattice constanta, however, increased withxto reach a maximum ofa= 0.599 8 nm atx= 20, and then decreased for higherx(Table 1). The above results imply that Tb3+was successfully doped.

Fig.2 SEM images of the β-NaYF4: 1%Ce3+, x%Tb3+ microcrystals (x = 1, 10, 20, 40, 60, 80)

The morphologies of theβ-NaYF4: 1%Ce3+,x%Tb3+(x= 1, 10, 20, 40, 60, 80) microcrystals are shown in Fig. 2. These particles have the shape of hexagonal prism bipyramids. With the increasing of Tb3+concentrationx, the particle size quickly decreased from the micron scale atx= 1 to the submicron or nanometer scale atx≥ 10. Meanwhile, the length (L)to the diameter (D) ratio (L/D) increases withx. Atx= 1 (Fig.2(a)),Dis about 1.2 μm, whileLis about 1.3 μm, soL/Dis about 1.1. Some smallβ-NaYF4: 1%Ce3+,1%Tb3+particles are present on the surfaces of larger ones (Fig.2(a)), and this should be ascribed to the Ostwald ripening. Atx= 10 (Fig.2(b)),DandLdecrease to about 700-800 nm and about 1 μm, respectively, and theL/Dis about 1.4. Forx= 20, 40 and 60, theL/Dis about 1.6, 1.8 and 2.3, respectively. Atx= 80 (Fig.2(f)),DandLare about 80 and 200 nm, respectively, soL/Dis about 2.5. Forβ-NaYF4: Ln3+nanocrystals, it has been reported that doping Ln3+with larger ionic radius than Y3+usually results in smaller particle size[34-37].The smaller particle size of theβ-NaYF4: Ce3+, Tb3+microcrystals for higherxcan be partly attributed to the larger ionic radius of Tb3+. Another contribution for the smaller size could be the surface charge modification as a density functional theory (DFT) calculation indicates that the electron charge density of the crystal surface increased after larger radii ions substituted the Y3+ions in the crystal lattice[35]. The increased electron charge density on the particle surfaces can substantially slow down the diffusion of negatively charged F-ions to the surface, and this will decrease theβ-NaYF4particle size. However, it is relatively easier for the F-ions to diffuse to the conical poles of theβ-NaYF4: 1%Ce3+,x%Tb3+hexagonal prism bipyramids, and this explains why theL/Dincreases with x.

3.1.2 Photoluminescence properties

He enquired18: And who is the bridegroom? Then someone pointed19 out to him the golden lad, who was still wrapped up in the bear s skin, and the father exclaimed angrily: Never shall a mere20 bear-keeper have my daughter, and tried to rush at him and kill him

Fig.3 The PL excitation (left lines) and emission (right lines)spectra of β-NaYF4:1%Ce3+ (a), β-NaYF4: 10%Tb3+ (b), and β-NaYF4: 1%Ce3+, 20%Tb3+ (c) (The schematic diagrams of corresponding energy transfer processes are also included)

The PL excitation and emission spectra of theβ-NaYF4: 1%Ce3+,β-NaYF4:10%Tb3+andβ-NaYF4:1%Ce3+, 30%Tb3+are illustrated in Fig.3, and the schematic diagrams of corresponding energy transfer processes are also included in Fig.3. For theβ-NaYF4:1%Ce3+(Fig.3(a)), there is a broad and strong absorption band peaking at 255 nm in the PL excitation spectrum (left of Fig.3(a)), which is assigned to the electron transition from the 4fto 5denergy level of Ce3+. Under excitation at 255 nm, the emission spectrum consists of a broad emission band peaking at about 311 nm (right of Fig.3(a)), which originates in the electric-dipole-allowed 4f-5dtransition of Ce3+[36].For theβ-NaYF4:Tb3+microcrystals, as shown at the left of Fig.3(b), all of the excitation peaks monitored at 549 nm are from the characteristic excitations of Tb3+ions ranging from 200 to 400 nm, where the broad but weak emissions are from thef→dtransition of Tb3+,7F6→5H7or5D0,1(318 nm),7F6→5G2or5L6(343 nm),7F6→5L9or5G4(354 nm),7F6→5L10(370 nm) and7F6→5G6or5D3(380 nm)[19]. Similarly, the emission peaks of theβ-NaYF4:Tb3+microcrystals excited at 380 nm are the characteristic emissions of Tb3+, including5D4→7F6,5D4→7F5,5D4→7F4and5D4→7F3transitions(right of Fig.3(b)), and the green emission5D4→7F5at 549 nm is the strongest. However, emission from the5D3level is not present in Fig.3(b) due to the cross-relaxation effect[19].

It can be seen from Fig.3(a) and 3(b) that the broad emission band originated from 4f→5dtransition of Ce3+overlaps with some excitation peaks of Tb3+. This therefore allows for Ce3+to be used as a sensitizer for Tb3+through the energy transfer process of Ce3+→Tb3+.In this way, the excitation spectral bandwidth of Tb3+can be effectively extended. This strategy is realized in this work by co-doping Ce3+and Tb3+ions in theβ-NaYF4host lattice. Fig.3(c) shows the PL excitation and emission spectra of theβ-NaYF4:1%Ce3+,30%Tb3+microcrystals. When excited at 255 nm through the 4f-5dtransition of Ce3+(right of Fig.3(c)), the emission spectrum exhibits the strong emissions of Tb3+(5D4→7FJ(J= 6, 5, 4, 3)), and a very weak emission peak from the electric-dipole-allowed 4f–5dtransitions of Ce3+is also present. This, in fact, provides an evidence for the efficient energy transfer process from Ce3+to Tb3+in theβ-NaYF4:1%Ce3+,30%Tb3+microcrystals. When monitored at 549 nm through the5D4→7F5transition of Tb3+(left of Fig.3(c)), the excitation contains both the broad excitation band of Ce3+peaking at 255 nm and the excitation peaks of Tb3+ranging from 300-390 nm.Thus, with the aid of the energy transfer of Ce3+→Tb3+,the excitation spectrum of Tb3+in theβ-NaYF4is effectively broadened in NUV region.

In order to investigate the energy transfer processes from Ce3+to Tb3+ions in detail, a series of Ce3+/Tb3+co-dopedβ-NaYF4phosphors were synthesized further. The PL excitation spectra forβ-NaYF4: 1%Ce3+,x%Tb3+(x= 0, 1, 3, 5, 10, 15, 20, 30, 40, 50, 60, 80)samples and the reference sampleβ-NaYF4: 10%Tb3+are shown in Fig. 4, and they were monitored at 549 nm of Tb3+. All of the excitation spectra consist of the broad excitation band of Ce3+and the characteristic excitation peaks of Tb3+(Fig.4). This again supports the energy transfer processes of Ce3+→Tb3+as stated in the previous paragraph. It is noted in Fig.4 that the excitation intensity increases withxwhenx≤ 20, and then decreases for higherxdue to the effect of concentration quenching.

Fig.4 Excitation spectra of the β-NaYF4: 1%Ce3+, x%Tb3+ (x = 0, 1, 3, 5, 10, 15, 20, 30, 40, 50, 60 and 80) and the β-NaYF4: 10%Tb3+ for comparison

Fig.5 Emission spectra (λ ex = 255 nm) of the β-NaYF4:1%Ce3+, x% Tb3+ microcrystals (a); the corresponding luminescent photographs observed under 254 nm UV lamp (b); the energy transfer efficiency ηCe-Tb (c); and Tb3+ emission intensity (d) in the as-synthesized samples as a function of Tb3+ concentration, respectively

Fig.5(a) shows the PL emission spectra ofβ-NaYF4: 1%Ce3+,x%Tb3+microcrystals when excited at 255 nm of Ce3+, and the luminescent photographs of the corresponding samples are shown in Fig.5(b). As shown in Fig.5(a), with the increase of the Tb3+concentrationx, the emission intensity of Ce3+at 311 nm decreased monotonously, while the emission intensity of Tb3+increased initially to reach a maximum atx=20 and then began to decrease as a result of the concentration quenching effect of Tb3+. Fig.5(c) and 5(d )show the energy transfer efficiency hCe-Tband Tb3+emission intensity in the as-synthesized samples as a function of Tb3+concentrationx. The energy transfer efficiency of Ce3+→Tb3+can be calculated by the following equation[9,29,38]:

whereIandI0represent the emission intensities of Ce3+ions in the presence and absence of Tb3+ions,respectively. As shown in Fig.5(c), the energy transfer efficiency of Ce3+→Tb3+increased monotonously with increasing the Tb3+concentration to 80%. The efficiency reached to about 99.51% when the concentration up to 80%. The distance between sensitizer Ce3+and activator Tb3+becomes closer with the increment ofx,which is beneficial for the energy transfer, leading to the decrease of Ce3+emission and enhancement of Tb3+emission. However, whenxbecomes larger and larger,the distance between Tb3+ions gets closer and closer so that the Tb3+-Tb3+internal concentration quenching will occur.

3.2 The β-NaYF4: Ce3+, Tb3+, Eu3+ microcrystals

3.2.1 Microstructural characteristics

As stated in the introduction part (Section 1),Tb3+ions can act as an energy transfer bridge for the energy transfer process between Ce3+and Eu3+ions by minimizing the MMCT effect and maximizing the Ce3+→(Tb3+)n→Eu3+energy transfer rate[41]. In this section, theβ-NaYF4: 1%Ce3+, 30%Tb3+,y%Eu3+microcrystals were synthesized to investigate the energy transfer process.

The XRD patterns of as-preparedβ-NaYF4:1%Ce3+, 30%Tb3+andβ-NaYF4: 1%Ce3+, 30%Tb3+,y%Eu3+(y= 1, 10, 30, 50) microcrystals are shown in Fig.6. All of the XRD patterns are identical to that of the pure hexagonal NaYF4phase (PDF#32-0483) and no peaks of secondary phases are observed. The sharp and strong XRD peaks indicate that they had good crystallinity. The EDS measurement (not shown) of theβ-NaYF4: Eu3+andβ-NaYF4:Ce3+,Tb3+,Eu3+indicates that all of the compositional elements Na, Y, F, Eu, Ce and Tb are present, indicating the successful doping of the lanthanide ions.

Fig.6 XRD patterns of the β-NaYF4:1%Ce3+, 30%Tb3+, y%Eu3+microcrystals (y = 1, 10, 30, 50)

Table 2 List of a, c, c/a, V, 2θ and d of (110) for the as-synthesized β-NaYF4, β-NaYF4: 5%Eu3+ and β-NaYF4:1%Ce3+, 30%Tb3+, y%Eu3+ microcrystals

Fig.7 shows the morphologies of theβ-NaYF4:1%Ce3+, 30%Tb3+,y%Eu3+(y= 1, 5, 10, 20) microcrystals. As expected, theβ-NaYF4:1%Ce3+, 30%Tb3+,y%Eu3+microcrystals also have the shape of hexagonal prism bipyramids. It is noted in Fig.7 that Eu3+doping rapidly decreased the particle size ofβ-NaYF4: 1%Ce3+,30%Tb3+,y%Eu3+, and the particle diameter decreased from about 200 nm fory= 1 to about 80 nm for y ≥10. The reason of such decreasing particle size withyis discussed in Section 3.1.

Fig.7 SEM images of the β-NaYF4:1%Ce3+, 30%Tb3+, y% Eu3+microcrystals (y = 1, 5, 10, 20)

3.2.2 Photoluminescence properties

To demonstrate the Ce3+→(Tb3+)n→Eu3+energy transfer process, theβ-NaYF4:10%Eu3+phosphor was also prepared for comparison. The PL excitation and emission spectra ofβ-NaYF4:10%Eu3+andβ-NaYF4:1%Ce3+, 30%Tb3+, 3%Eu3+are shown in Fig.8, where the insets illustrate the corresponding schematic diagrams of their energy transfer processes. Forβ-NaYF4:10%Eu3+(Fig.8(a)), all of the excitation and emission peaks are from the characteristics excitation and emission of Eu3+[17,37], with the7F0→5L6transition as the strongest excitation (396 nm) and the5D0→7F2as the strongest emission (622 nm). Fig.8(b) shows the PL excitation and emission spectra ofβ-NaYF4: 1%Ce3+,30%Tb3+, 3%Eu3+microcrystals. When excited by the characteristic absorption wavelength of Ce3+at 311 nm,the emission spectrum (right of Fig.8(b)) exhibits not only the 4f–5dtransitions of Ce3+and the5D4→7FJ(J= 6, 5, 4, 3) transitions of Tb3+, but also the characteristic emission peaks of Eu3+(5D0→7FJ(J= 1, 2, 3, 4)).This provides an evidence for the energy transfer from Ce3+to Tb3+and finally to Eu3+in theβ-NaYF4: Ce3+,Tb3+, Eu3+microcrystals. Moreover, when monitored at 622 nm (the5D0→7F2transition of Eu3+), the excitation spectrum (left of Fig.8(b)) consists of the broad absorption bands and peaks from Ce3+and Tb3+. This further supports the presence of the energy transfer of Ce3+→(Tb3+)n→Eu3+. Hence, the excitation spectral bandwidth in the NUV region of theβ-NaYF4: Ce3+,Tb3+, Eu3+was effectively broadened as a result of the Ce3+→(Tb3+)n→Eu3+energy transfer process.

Fig.8 Excitation (left lines) and emission (right lines) spectra of the β-NaYF4:10%Eu3+ (a) and the β-NaYF4:1%Ce3+, 30%Tb3+, 3% Eu3+ (b)(The insets show the schematic diagrams of energy transfer processes in the corresponding doping modes)

Fig.9 Excitation spectra of the β-NaYF4:1%Ce3+, 30%Tb3+, y%Eu3+ (y = 0, 1, 3, 5, ,10, 15, 20, 30, 40 and 50) and the β-NaYF4: 10%Eu3+ for comparison

In order to systematically investigate the Ce3+→(Tb3+)n→Eu3+energy transfer process, a series of Ce3+/Tb3+/Eu3+tri-dopedβ-NaYF4phosphors were synthesized. The PL excitation spectra (monitored at 622 nm of Eu3+) for theβ-NaYF4: 1%Ce3+, 30%Tb3+,y%Eu3+(y= 0, 1, 3, 5, 10, 15, 20, 30, 40, 50) samples and the comparable sampleβ-NaYF4:10% Eu3+are shown in Fig. 9. All of the excitation spectra consist of the broad band of Ce3+, some excitation peaks of Tb3+and several excitation peaks of Eu3+(Fig.9). This further supports the presence of the Ce3+→(Tb3+)n→Eu3+energy transfer process in theβ-NaYF4:1%Ce3+,30%Tb3+,y%Eu3+phosphor. It can be seen in Fig.9 that the excitation band of Ce3+decreases successively with the Eu3+concentrationyand disappears wheny≥ 20, while the excitation intensities of Tb3+and Eu3+ions increase withy. This probably is because the MMCT gradually comes back into effect whenygets higher such that the distance between Ce3+and Eu3+ions becomes closer. The existence of MMCT therefore weakens the Ce3+→(Tb3+)n→Eu3+energy transfer for highery. On the other hand, the Tb3+and Eu3+ions can still absorb the UV photons for the emission of Eu3+, and this explains the increase of the excitation peaks of Tb3+and Eu3+ions withy.

The PL emission spectra of theβ-NaYF4:1%Ce3+,30%Tb3+,y%Eu3+microcrystals are shown in Fig.10(a) (λex= 255 nm, 4f–5dof Ce3+) and Fig.10(b)(λex= 380 nm,7F6→5G6or5D3380 nm) (see Figs.3(c)and 8(b) for the excitation transitions). Fig.10(c) shows the luminescent photographs of theβ-NaYF4: 1%Ce3+,30%Tb3+,y%Eu3+microcrystals (y=1, 3, 5, 10, 15,20, 30, 40, 50) observed by a UV lamp under 365 nm.As shown in Fig.10(a), when excited at 255 nm, the Tb3+emission quickly decreased withy, while the Eu3+emission rapidly increased to reach a maximum aty= 3 and then decreased for highery. This is consistent with the results presented in the previous paragraphs.The decrease of the Eu3+emission for higheryis also because of the appearance of MMCT between Ce3+and Eu3+ions. The Tb3+to Eu3+energy transfer efficiency can be calculated from the following equation[29,38]:

whereI1andI2are the emission intensities of Tb3+with and without Eu3+ions. As shown in Fig.10(d), the energy transfer efficiency of Tb3+→Eu3+increased monotonously withy, and it reached above 97% aty= 50.

Fig.10 Emission spectra at λex = 255 nm (a) and λex = 380 nm (b) of the β-NaYF4:1%Ce3+, 30%Tb3+, y%Eu3+ microcrystals; their corresponding luminescent photograph observed under 365 nm UV lamp (c); their energy transfer efficiency ηTb-Eu (d); and the variation of Eu3+emission intensity (λ ex=380 nm) with y (e)

Fig.11 Illustration of the energy levels and the Ce3+→(Tb3+)n→Eu3+ energy transfer processes in the β-NaYF4: Ce3+, Tb3+, Eu3+

The possible energy levels and the energy transfer processes of Ce3+→(Tb3+)n→Eu3+in theβ-NaYF4:1%Ce3+, 20%Tb3+,y%Eu3+is illustrated in Fig. 11[30,31].After the Ce3+ions are excited, the excited electrons relax to the lower level of their excited states, and some will emit light (about 311 nm) following the 4f-5dtransition of Ce3+, while others transfer their excitation energy to the5D3of Tb3+followed by relaxing to5D4of Tb3+ions[1,29,42]. The Tb3+emission is a result of the transition from5D4to7FJ(J= 3, 4, 5, 6). After Eu3+doping, Tb3+ions transfer the energy from5D4to5D1of Eu3+via a cross-relaxation process. After being relaxed to5D0, the Eu3+ions emit lights following the5D0→7FJ(J=1, 2, 3, 4) transitions[1,29,38].

4 Conclusions

In summary, a series of Ce3+/Tb3+co-doped and Ce3+/Tb3+/Eu3+tri-dopedβ-NaYF4phosphors with a shape of well-defined hexagonal prism bipyramids were successfully synthesized with the solvothermal method using oleic acid as surfactant. The average particle size decreases with the concentrations of Tb3+and Eu3+ions. The energy transfer processes among Ce3+,Tb3+and Eu3+ions were systematically investigated.Tb3+is found to be a highly efficient bridge for transferring energy from Ce3+to Eu3+ions by minimizing the metal-metal charge transfer effect. Compared with Eu3+or Tb3+single-dopedβ-NaYF4microcrystals, the sensitization by Ce3+for the photoluminescence of Tb3+and Eu3+leads to a broad spectral bandwidth in the UV range. Meanwhile, the corresponding optical absorption efficiency is greatly enhanced. By properly controlling the concentrations of Tb3+and Eu3+ions inβ-NaYF4host, strong green emission of Tb3+ions inβ-NaYF4:Ce3+, Tb3+phosphor and red emission of Eu3+ions inβ-NaYF4: Ce3+, Tb3+, Eu3+phosphor upon the NUV excitation can be successfully obtained. By controlling the dopant concentrations, the color of theβ-NaYF4:Ce3+, Tb3+, Eu3+phosphor can be readily tuned. The lanthanide co-doped and tri-dopedβ-NaYF4phosphors with broadband excitation spectrum will have promising applications in optoelectronics.