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Eu3+-codoped NaLa(MoO4)2 phosphors
Journal of Alloys and Compounds 653 (2015) 468e473
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Energy transfer mechanism and color controllable luminescence in Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors Peng Du, Jae Su Yu* Department of Electronics and Radio Engineering, Kyung Hee University, Yongin 446-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2015 Received in revised form 28 August 2015 Accepted 30 August 2015 Available online 4 September 2015
Dy3þ/Eu3þ-codoped as well as Dy3þ-doped NaLa(MoO4)2 phosphors were synthesized via a facile solidstate reaction method. The X-ray diffraction patterns, field-emission scanning electron microscope images, photoluminescence (PL) spectra and decay curves were employed to characterize the synthesized samples. Under the excitation of 387 nm, all the Dy3þ-doped NaLa(MoO4)2 phosphors exhibited the characteristic emissions of Dy3þ ions (blue emission at 486 nm: 4F9/2 / 6H15/2 and yellow emission at 573 nm: 4F9/2 / 6H13/2). Furthermore, the PL emission intensity increased with Dy3þ ion concentration and the concentration quenching effect occurred when the Dy3þ ion concentration was over 5 mol%. Meanwhile, color-tunable emissions were observed in Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors owing to the efficient energy transfer (ET) from Dy3þ to Eu3þ ions. Through theoretical calculation, it was evident that the ET efficiency increased gradually with the increment of Eu3þ ion concentration, reaching ~ 70% of its maximum value at 9 mol% of Eu3þ ion concentration. Additionally, the ET mechanism between Dy3þ and Eu3þ ions was found to be electric dipoleedipole interaction. © 2015 Elsevier B.V. All rights reserved.
Keywords: Rare-earth doping Luminescence NaLa(MoO4)2 phosphors Optical materials
1. Introduction Trivalent rare-earth (RE) ions doped luminescent materials, which exhibit promising applications in white light-emitting diodes, solar cells, optical temperature sensors and bio-imaging, have drawn extensive attention [1e4]. To meet the requirement of these applications, luminescent materials are expected to show highefficiency photoluminescence (PL) emissions, color controllable emissions, high physical-chemical stability and so on. As is known, the PL properties of RE ions doped materials can be significantly influenced by crystal field around the RE ions in the host lattices. Thus, choosing an appropriate host is very important. Nowadays, molybdates have been widely investigated owing to their excellent optical properties as well as their potential applications in photocatalysts, phosphors and lasers [5e7]. Among these molybdates, double molybdate compound NaLa(MoO4)2 with high density, thermal and physical stability is thought to be good luminescent host material. In the NaLa(MoO4)2 compound, the Mo6þ ions are coordinated by four oxygen atoms, which makes the MoO2 4 group more stable, while both Naþ and La3þ ions are eight coordinated [8].
* Corresponding author. E-mail address: [email protected] (J.S. Yu). http://dx.doi.org/10.1016/j.jallcom.2015.08.256 0925-8388/© 2015 Elsevier B.V. All rights reserved.
Furthermore, the MoO2 4 group has a broad absorption band in the ultraviolet (UV) region and the energy can be transferred from MoO2 4 group to RE ions, resulting in the enhancement of external quantum efficiency of RE ions doped materials [9,10]. Therefore, strong PL emissions are expected to be obtained in RE ions doped NaLa(MoO4)2 phosphors under UV light excitation. In recent years, as a member of trivalent RE ions, dysprosium (Dy3þ) ions have been intensively studied because of their unique luminescence properties [11,12]. Generally, Dy3þ ions exhibit two dominant emissions in blue (470e500 nm) and yellow (560e600 nm) regions, which are attributed to the 4F9/2 / 6H15/2 and 4F9/2 / 6H13/2 transitions, respectively [13]. Monika et al. [14] reported that strong PL emissions were obtained in Dy3þ-doped Sr2CeO4 nanophosphors and the PL emission intensities were enhanced with Liþ codoping. Moreover, europium (Eu3þ) ions doped compounds are considered to be alternative candidates for red-emitting phosphors owing to their high efficiency characteristic red emission at about 615 nm corresponding to the transition of 5D0 / 7F2 [15,16]. It was revealed that strong red emissions were observed in Eu3þ-doped phosphors, such as Y2O3:Eu3þ, CaMoO4:Eu3þ and NaBaBO3:Eu3þ [17e19]. In addition, it was also demonstrated that the energy transfer (ET) from Dy3þ to Eu3þ ions was efficient, so color controllable emissions are expected to be obtained in Dy3þ/Eu3þ-codoped compounds [20,21]. Das et al. [22]
P. Du, J.S. Yu / Journal of Alloys and Compounds 653 (2015) 468e473
revealed that the emission color of ZrO2:Dy3þ/Eu3þ phosphors was changed from yellow to near white light and eventually to pure white light by adjusting the ratio of Dy3þ and Eu3þ ion concentrations. Similar results were also observed in Dy3þ/Eu3þ-codoped Ca2La8(GeO4)6O2 phosphors [23]. Although some impressive achievements in Dy3þ/Eu3þ-codoped phosphors have been obtained, to the best of our knowledge, there is no investigation on the ET mechanism and PL properties of Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors. In this work, a series of Dy3þ-doped and Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors were synthesized by a high-temperature solid-state reaction technique. The chromatic properties as well as their phase structure, morphology, decay behaviors and PL spectra were analyzed. Furthermore, the ET mechanism between the luminescence centers of Dy3þ and Eu3þ ions was also investigated in detail. 2. Experimental The Dy3þ-doped NaLa(MoO4)2 (abbreviated as NaLa(MoO4)2:xDy3þ; x ¼ 0.01, 0.03, 0.05, 0.07, 0.09, and 0.11, where x is the molar mass) and Dy3þ/Eu3þ-codoped NaLa(MoO4)2 (abbreviated as NaLa(MoO4)2:0.05Dy3þ/yEu3þ; y ¼ 0.01, 0.03, 0.05, 0.07, and 0.09, where y is the molar mass) phosphors were prepared by a conventional solid-state reaction method. The stoichiometric amounts of raw materials, i.e., sodium carbonate (Na2CO3, 99%), lanthanum oxide (La2O3, 99.9%), molybdenum oxide (MoO3, 99.5%), dysprosium oxide (Dy2O3, 99.9%) and europium oxide (Eu2O3, 99.5%), were weighted and grinded thoroughly for 30 min in an agate mortar to achieve uniformity. After that, the mixtures were transferred to the alumina crucibles followed by the calcination in a furnace at 900 C for 6 h in air with a heating ratio of 3 C/min. Finally, these mixtures were cooled down naturally to room temperature. The X-ray diffractometer (XRD) (Mac Science, M18XHF-SRA) with Cu Ka (l ¼ 1.5406 Å) radiation was used to check the phase structure properties of the obtained samples. The morphology of the samples was measured by using a field-emission scanning electron microscope (FE-SEM) (LEO SUPPA 55, Carl Zeiss). The room-temperature PL emission and PL excitation (PLE) spectra were measured by using a fluorescence spectrophotometer (Photon Technology International (PTI) fluorimeter) equipped with a Xe arc lamp (60 W power), and the lifetime was determined by using a phosphormeter attached to the main system with a Xe flash lamp (25 W power).
Fig. 1. (a) XRD patterns of the NaLa(MoO4)2, NaLa(MoO4)2:0.05Dy3þ NaLa(MoO4)2:0.05Dy3þ/0.09Eu3þ phosphors. FE-SEM images of NaLa(MoO4)2:0.05Dy3þ and (c) NaLa(MoO4)2:0.05Dy3þ/0.09Eu3þ phosphors.
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and (b)
the O2 / Mo6þ transition [12]. The other is composed of a series of narrow peaks located at approximately 325, 351, 366, 387, 426, 453, and 472 nm corresponding to the 6H15/2 / 6P3/2, 6H15/2 / 6P7/ 6 6 6 4 6 4 6 4 2, H15/2 / P5/2, H15/2 / I13/2, H15/2 / G11/2, H15/2 / H11/2, 6 4 3þ and H15/2 / F9/2 transitions of Dy ions, respectively [10,20]. Fig. 2(b) depicts the PL spectra of the NaLa(MoO4)2:xDy3þ phosphors at different Dy3þ ion concentrations under the excitation of 387 nm. As shown in Fig. 2(b), all the samples exhibited strong yellow emissions at ~573 nm due to the 4F9/2 / 6H13/2 transition of Dy3þ ions. Furthermore, two relatively weak emissions were also observed at 486 and 663 nm which are ascribed to the 4F9/2 / 6H15/
3. Results and discussion Fig. 1(a) shows the XRD patterns of the NaLa(MoO4)2, NaLa(MoO4)2:0.05Dy3þ and NaLa(MoO4)2:0.05Dy3þ/0.09Eu3þ phosphors. All the diffraction peaks were in good agreement with the standard JCPDS card No. 24-1103 and no other impurity phases were observed, indicating that all the as-prepared samples possessed pure tetragonal phase and the dopants (Dy3þ and Eu3þ ions) were incorporated into the NaLa(MoO4)2 host lattices. From the FE-SEM images (Fig. 1(b)e(c)), it is observable that all the obtained samples were composed of aggregated and irregular particles with sizes ranging from ~2 to 6 mm. Note that, with doping of RE (Dy3þ and Eu3þ) ions, the shape and size of the particles changed a little. The PLE spectra of the NaLa(MoO4)2:xDy3þ phosphors while monitoring at 573 nm corresponding to the 4F9/2 / 6H13/2 transition of Dy3þ ions are displayed in Fig. 2(a). It is clear that the PLE spectrum consisted of two parts: one is the weak broad charge transfer (CT) band ranging from 240 to 340 nm, which is assigned to
Fig. 2. (a) PLE spectra of the NaLa(MoO4)2:xDy3þ phosphors monitored at 573 nm. (b) PL spectra of the NaLa(MoO4)2:xDy3þ phosphors excited at 387 nm. The inset of (b) shows the PL emission intensity as a function of Dy3þ ion concentration.
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4 6 2 and F9/2 / H11/2 transitions, respectively [13,21]. It is known that the blue (4F9/2 / 6H15/2) and yellow (4F9/2 / 6H11/2) emissions are two characteristic emissions of Dy3þ ions. Among them, the blue emission is the magnetic dipole transition which is insensitive to the local environment, while the yellow emission belongs to the hypersensitive electric dipole transition and its intensity can be significantly affected by the crystal field around the Dy3þ ions [14,24]. Generally, the yellow emission takes the domination when the Dy3þ ions occupy the non-inversion center, while the blue emission will be stronger if the Dy3þ ions are located at a high symmetry sites. The intense yellow emission at 573 nm demonstrates that the Dy3þ ions occupied the low symmetry sites without inversion center in NaLa(MoO4)2 host lattices. Also, the PL emission intensity increased gradually with the increment of Dy3þ ion concentration and reached its optimum value when x ¼ 0.05, and then it started to decrease with further increasing the Dy3þ ion concentration due to the concentration quenching effect (see the inset of Fig. 2(b)). The cross relaxation (CR) mechanism, which originates from the resonance ET between the adjacent Dy3þ ions, can be used to explain the concentration quenching phenomenon. In terms of the CR mechanism, several possible channels (CR1, CR2, and CR3 as shown in Fig. 6(a)) are involved [9,25]: 4
F9/2 þ 6H15/2 / 6H9/2/6F11/2 þ 6F5/2
4
F9/2 þ 6H15/2 / 6H7/2/6F9/2 þ 6F3/2
4
F9/2 þ 6H15/2 / 6H9/2/6F11/2 þ 6F1/2.
Owing to these effective CR processes, the population of the 4F9/2 level was decreased, resulting in the quenched luminescence of the 4 F9/2 level (see Fig. 2(b)). According to Dexter's ET formula for multipolar interaction and Reisfeld's approximation, the relation between the PL emission intensity (I) and activator ion concentration (x) can be expressed by Ref. [26]:
logðI=xÞ ¼ c ðq=3ÞlogðxÞ;
(1)
where x is the Dy3þ ion concentration, c is constant and q ¼ 6, 8, and 10 corresponding to electric dipoleedipole, dipoleequadrupole, and quadrupolee-quadrupole interactions, respectively. As presented in Fig. 3, the dependence of log(I/x) on log(x) was linear and its slope was 1.58. Hence, the q was calculated to be 4.74, which is
Fig. 3. Linear fitting of log(I/x) versus log(x) for the 4F9/2 / 6H13/2 transition of Dy3þ ions in NaLa(MoO4)2:xDy3þ phosphors.
Fig. 4. PLE spectra of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors monitored at (a) 573 nm and (b) 615 nm.
near to 6, demonstrating that the dominant concentration quenching mechanism of Dy3þ ions in NaLa(MoO4)2:xDy3þ phosphors was electric dipoleedipole interaction. To investigate the ET mechanism and luminescence properties of Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors, a series of NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors were prepared. Fig. 4 shows the PLE spectra of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors. As illustrated in Fig. 4(a), the PLE spectrum monitored at
Fig. 5. PL spectra of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors excited at 362 nm. Inset presents the PL emission intensity as a function of Eu3þ ion concentration.
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362 nm, electrons are excited to the 6P5/2 level from the ground state. After that, the nonradiative (NR) transition occurs and the 4F9/ 2 level is populated. Finally, the blue, yellow and red emissions of Dy3þ ions from 4F9/2 to 6HJ (J ¼ 15/2, 13/2, and 11/2) are observed. Furthermore, the 5D0 and 5D1 levels of Eu3þ ions can be populated through the ET from Dy3þ to Eu3þ ions (see Fig. 6(b)). Eventually, the characteristic emissions of Eu3þ ions corresponding to the 5 D1 / 7F1, 5D0 / 7F0, 5D0 / 7F1, and 5D0 / 7F2 transitions are observed. To further identify the ET behavior between the Dy3þ and Eu3þ ions, the PL decay curves of NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors as a function of Eu3þ ion concentration were measured. From Fig. 7, one obtains that the decay curves can be well fitted by a second-order exponential expression [29]:
IðtÞ ¼ A1 expðt=t1 Þ þ A2 expðt=t2 Þ þ I0 ;
Fig. 6. Energy level diagram of Dy3þ and Eu3þ ions as well as the proposed ET mechanism.
573 nm emission (4F9/2 / 6H13/2) of Dy3þ ions consisted of a weak board CT band from 240 to 340 nm and some sharp peaks at 325, 351, 366, 387, 426, 453, and 472 nm owing to the transitions of Dy3þ ions from the ground state of 6H15 to the 6P3/2, 6P7/2, 6P5/2, 4I13/2, 4 G11/2, 4H11/2, and 4F9/2 excited levels, respectively. Moreover, the PLE peak intensity showed a downward trend with increasing the Eu3þ ion concentration. In comparison, a strong board CT band centered around 300 nm, which is attributed to the overlap of the Eu3þ / O2 and Mo6þ / O2 transitions [27], was observed in the PLE spectra when monitored at 615 nm corresponding to the 5 D0 / 7F2 transition of Eu3þ ions (see Fig. 4(b)). Furthermore, the PLE spectra also revealed the several narrow peaks centered at 361 nm (7F0 / 5D4), 375 nm (7F0 / 5G2), 380 nm (7F0 / 5G3), 393 nm (7F0 / 5L6), 415 nm (7F0 / 5D3), and 464 nm (7F0 / 5D2) which are assigned to the characteristic fef transitions of Eu3þ ions [6,28]. Additionally, several weak excitation bands centered at 351, 366, 426, and 453 nm corresponding to the transitions of Dy3þ ions of 6H15/2 / 6P7/2, 6H15/2 / 6P5/2, 6H15/2 / 4G11/2, and 6H15/ 2 / 4H11/2, respectively, were also observed, demonstrating that the energy can be transferred from Dy3þ to Eu3þ ions in NaLa(MoO4)2:0.05Dy3þ/yEu3þ material system. Compared with that monitored at 573 nm, the PLE peak intensity monitored at 615 nm exhibited an opposite trend, namely, it increased monotonously with the increment of Eu3þ ion concentration as shown in Fig. 4(b). Fig. 5 depicts the PL spectra of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors under the excitation of 362 nm. From Fig. 5, one knows that the emissions at about 486, 573, and 663 nm were attributed to the transitions of 4F9/2 / 6H15/2, 4F9/2 / 6H13/2, and 4F9/2 / 6H11/2 of Dy3þ ions, respectively. Furthermore, some other emission peaks located at approximately 534, 590, 595, and 615 nm were corresponding to the transitions of 5D1 / 7F1, 5D0 / 7F0, 5D0 / 7F1, and 5 D0 / 7F2 of Eu3þ ions, respectively [6]. As shown in the inset of Fig. 5, it is obvious that the PL emission intensity of Eu3þ ions (energy acceptor) increased gradually, while that of Dy3þ ions (energy donor) was found to decrease with the increment of Eu3þ ion concentration. The variations in the PL emission intensities of Dy3þ and Eu3þ ions suggested that the occurrence of ET from Dy3þ to Eu3þ ions in NaLa(MoO4)2:0.05Dy3þ/yEu3þ material system and color-tunable emissions are expected to be obtained by properly adjusting the ratio of Dy3þ and Eu3þ ion concentrations. The energy level diagram of Dy3þ and Eu3þ ions as well as the possible ET processes is shown in Fig. 6(b). Briefly, under the excitation of
(2)
where I0 and I(t) are the emission intensities at time 0 and t, A1 and A2 are the constants, t1 and t2 are the rapid and slow components of decay time, respectively. The average lifetime (tav) can be estimated by the following equation:
. tav ¼ A1 t21 þ A2 t22 ðA1 t1 þ A2 t2 Þ:
(3)
On the basis of Eq. (3) and the decay curves, the average lifetime values for y ¼ 0, 0.01, 0.03, 0.05, 0.07 and 0.09 in NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors were determined to be approximately 135.78, 128.46, 121.63, 109.93, 102.16 and 90.31 ms (see the inset of Fig. 7), respectively. It is obvious that the luminescent lifetime of Dy3þ ions decreased gradually with the increment of Eu3þ ion concentration which further proved that there existed a significant ET from Dy3þ to Eu3þ ions. Furthermore, based on the lifetime, the ET rate (PDy/Eu) from Dy3þ to Eu3þ ions is defined as [20]:
PDy/Eu ¼ 1=t 1=t0 ;
(4)
where t and t0 are the lifetimes of Dy3þ ions with and without Eu3þ ions. Therefore, the ET rate was calculated to be 5.4, 10.4, 19, 24.8, and 33.5% for y ¼ 0, 0.01, 0.03, 0.05, 0.07 and 0.09 in NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors, respectively. On the other hand, the ET efficiency from Dy3þ to Eu3þ ions can be roughly evaluated using the fluorescence emission intensity,
Fig. 7. PL decay curves of NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors (excited at 362 nm and monitored at 573 nm). Inset depicts the dependence of lifetime of Dy3þ ions as a function of Eu3þ ion concentration.
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which can be expressed as below [20,30]:
hT ¼ 1
Is ; I
(5)
where hT is the ET efficiency, Is and I are the PL emission intensities of Dy3þ ions with presence and absence of Eu3þ ions. It is evident that the ET efficiency showed an upward trend with the increment of Eu3þ ion concentration, achieving ~ 70% of its maximum value when y ¼ 0.09 (see Fig. 8), indicating that the ET from Dy3þ to Eu3þ ions in NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors was efficient. In terms of the ET between the sensitizer (Dy3þ) and activator (Eu3þ), two different mechanisms are involved: exchange interaction and multipoleemultipole interaction. The type of interaction mechanism can be identified by knowing the critical distance between the sensitizer and activator. If the critical distance is larger than 5 Å, the multipoleemultipole interaction dominates. Otherwise, the exchange interaction prevails. According to Blasses's report, the critical distance can be written by Ref. [31]:
3V 1=3 Rc z2 : 4pxc Z
(6)
Here, Rc is the critical distance, V is volume of the unite cell, xc is the critical concentration, at which the PL emission intensity of Dy3þ ions is half of that in the sample without Eu3þ ions. For the NaLa(MoO4)2 host lattice, V ¼ 335.24 Å3, Z ¼ 2, and xc ¼ 0.054 (obtained from the inset of Fig. 8). Therefore, the Rc was calculated to be about 18.1 Å which is larger than 5 Å, demonstrating that ET mechanism between the Dy3þ and Eu3þ ions in NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors was multipoleemultipole interaction. Additionally, on the basis of Dexter's ET formula of multipolar interaction and Reisfeld's approximation, the following relation can be obtained [32,33]:
I fC n=3 ; Is
(7)
Fig. 9. Dependence of I/Is of Dy3þ ions on (a) C6/3, (b) C8/3 and (c) C10/3.
Dy3þ to Eu3þ ions was electric dipoleedipole interaction mechanism. The Commission International de I'Eclairage (CIE) chromaticity diagram of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors is depicted in Fig. 10. The color coordinate as well as the color correlated temperature (CCT) are summarized in Table 1. The CCT can be calculated by the following expression [9,34]:
CCT ¼ 437n3 þ 3601n2 6861n þ 5514:31; and n ¼ ðx xe Þ=ðy ye Þ;
(8)
where x and y are the color coordinates, (xe,ye) is the coordinate of the epicenter and its value is xe ¼ 0.3320 and ye ¼ 0.1858. As shown in the inset of Fig. 10, the emission color of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors was changed from greenyellow to yellow and finally to orange-red with increasing the Eu3þ
where C is the total concentration of Dy3þ and Eu3þ ions, n ¼ 6, 8, and 10 correspond to the electric dipoleedipole, dipoleequadrupole, and quadrupoleequadrupole interactions, respectively. The I/ Is versus Cn/3 plots are shown in Fig. 9. It is obvious that, when n ¼ 6, the linear fitting result was the best, suggesting that the ET from
Fig. 8. Dependence of ET efficiency, hT, on Eu3þ ion concentration in NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors. Inset shows the normalized PL emission intensity of Dy3þ ions in NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors as a function of Eu3þ ion concentration.
Fig. 10. CIE chromaticity diagram of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors (lex ¼ 362 nm). Inset shows the typical phosphor images under the excitation of 362 nm.
P. Du, J.S. Yu / Journal of Alloys and Compounds 653 (2015) 468e473 Table 1 CIE parameters of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors excited at 362 nm. Points
Sample
CIE coordinate (x,y)
CCT (K)
1 2 3 4 5 6
NaLa(MoO4)2:0.05Dy3þ NaLa(MoO4)2:0.05Dy3þ/0.01Eu3þ NaLa(MoO4)2:0.05Dy3þ/0.03Eu3þ NaLa(MoO4)2:0.05Dy3þ/0.05Eu3þ NaLa(MoO4)2:0.05Dy3þ/0.07Eu3þ NaLa(MoO4)2:0.05Dy3þ/0.09Eu3þ
(0.360,0.383) (0.384,0.361) (0.426,0.348) (0.454,0.340) (0.474,0.326) (0.486,0.325)
4611 3784 2663 2124 1805 1739
ion concentration, suggesting that color tunable emissions can be obtained by the modulation of the ratio of Dy3þ and Eu3þ ion concentrations and these color-tunable phosphors may have promising applications in color displays and lighting fields. Furthermore, the CCT value varied from 4661 K to 1739 K with the increment of Eu3þ ion concentration (see Table 1). 4. Conclusions In summary, a series of Dy3þ-doped and Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors were synthesized by a simple hightemperature solid-state reaction method. Under 387 nm light excitation, the characteristic emissions of Dy3þ ions were observed in NaLa(MoO4)2:xDy3þ phosphors. The optimum concentration of Dy3þ ions was 5 mol% and the concentration quenching mechanism of Dy3þ ions in NaLa(MoO4)2 host lattice was electric dipoleedipole interaction. Furthermore, the emission color of Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors could be tuned from green-yellow to yellow and eventually to orange-red by adjusting the ratio of Dy3þ and Eu3þ ion concentrations. From the PL spectra, one obtains that there existed an efficient ET from Dy3þ to Eu3þ ions and the efficiency of this transfer was as high as 70% when the Eu3þ ion concentration was 9 mol%. In addition, the ET mechanism between the Dy3þ and Eu3þ ions was demonstrated to be electric dipoleedipole interaction and the critical distance was estimated to be 18.1 Å. Ultimately, the CCT value decreased from 4661 to 1739 K with the increase of Eu3þ ion concentration. From the industrial point of view, the color-tunable Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors may have potential applications in color displays and lighting fields. Acknowledgments This work was supported by the National Research Foundation
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of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014- 069441).
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Energy transfer mechanism and color controllable luminescence in Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors Peng Du, Jae Su Yu* Department of Electronics and Radio Engineering, Kyung Hee University, Yongin 446-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2015 Received in revised form 28 August 2015 Accepted 30 August 2015 Available online 4 September 2015
Dy3þ/Eu3þ-codoped as well as Dy3þ-doped NaLa(MoO4)2 phosphors were synthesized via a facile solidstate reaction method. The X-ray diffraction patterns, field-emission scanning electron microscope images, photoluminescence (PL) spectra and decay curves were employed to characterize the synthesized samples. Under the excitation of 387 nm, all the Dy3þ-doped NaLa(MoO4)2 phosphors exhibited the characteristic emissions of Dy3þ ions (blue emission at 486 nm: 4F9/2 / 6H15/2 and yellow emission at 573 nm: 4F9/2 / 6H13/2). Furthermore, the PL emission intensity increased with Dy3þ ion concentration and the concentration quenching effect occurred when the Dy3þ ion concentration was over 5 mol%. Meanwhile, color-tunable emissions were observed in Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors owing to the efficient energy transfer (ET) from Dy3þ to Eu3þ ions. Through theoretical calculation, it was evident that the ET efficiency increased gradually with the increment of Eu3þ ion concentration, reaching ~ 70% of its maximum value at 9 mol% of Eu3þ ion concentration. Additionally, the ET mechanism between Dy3þ and Eu3þ ions was found to be electric dipoleedipole interaction. © 2015 Elsevier B.V. All rights reserved.
Keywords: Rare-earth doping Luminescence NaLa(MoO4)2 phosphors Optical materials
1. Introduction Trivalent rare-earth (RE) ions doped luminescent materials, which exhibit promising applications in white light-emitting diodes, solar cells, optical temperature sensors and bio-imaging, have drawn extensive attention [1e4]. To meet the requirement of these applications, luminescent materials are expected to show highefficiency photoluminescence (PL) emissions, color controllable emissions, high physical-chemical stability and so on. As is known, the PL properties of RE ions doped materials can be significantly influenced by crystal field around the RE ions in the host lattices. Thus, choosing an appropriate host is very important. Nowadays, molybdates have been widely investigated owing to their excellent optical properties as well as their potential applications in photocatalysts, phosphors and lasers [5e7]. Among these molybdates, double molybdate compound NaLa(MoO4)2 with high density, thermal and physical stability is thought to be good luminescent host material. In the NaLa(MoO4)2 compound, the Mo6þ ions are coordinated by four oxygen atoms, which makes the MoO2 4 group more stable, while both Naþ and La3þ ions are eight coordinated [8].
* Corresponding author. E-mail address: [email protected] (J.S. Yu). http://dx.doi.org/10.1016/j.jallcom.2015.08.256 0925-8388/© 2015 Elsevier B.V. All rights reserved.
Furthermore, the MoO2 4 group has a broad absorption band in the ultraviolet (UV) region and the energy can be transferred from MoO2 4 group to RE ions, resulting in the enhancement of external quantum efficiency of RE ions doped materials [9,10]. Therefore, strong PL emissions are expected to be obtained in RE ions doped NaLa(MoO4)2 phosphors under UV light excitation. In recent years, as a member of trivalent RE ions, dysprosium (Dy3þ) ions have been intensively studied because of their unique luminescence properties [11,12]. Generally, Dy3þ ions exhibit two dominant emissions in blue (470e500 nm) and yellow (560e600 nm) regions, which are attributed to the 4F9/2 / 6H15/2 and 4F9/2 / 6H13/2 transitions, respectively [13]. Monika et al. [14] reported that strong PL emissions were obtained in Dy3þ-doped Sr2CeO4 nanophosphors and the PL emission intensities were enhanced with Liþ codoping. Moreover, europium (Eu3þ) ions doped compounds are considered to be alternative candidates for red-emitting phosphors owing to their high efficiency characteristic red emission at about 615 nm corresponding to the transition of 5D0 / 7F2 [15,16]. It was revealed that strong red emissions were observed in Eu3þ-doped phosphors, such as Y2O3:Eu3þ, CaMoO4:Eu3þ and NaBaBO3:Eu3þ [17e19]. In addition, it was also demonstrated that the energy transfer (ET) from Dy3þ to Eu3þ ions was efficient, so color controllable emissions are expected to be obtained in Dy3þ/Eu3þ-codoped compounds [20,21]. Das et al. [22]
P. Du, J.S. Yu / Journal of Alloys and Compounds 653 (2015) 468e473
revealed that the emission color of ZrO2:Dy3þ/Eu3þ phosphors was changed from yellow to near white light and eventually to pure white light by adjusting the ratio of Dy3þ and Eu3þ ion concentrations. Similar results were also observed in Dy3þ/Eu3þ-codoped Ca2La8(GeO4)6O2 phosphors [23]. Although some impressive achievements in Dy3þ/Eu3þ-codoped phosphors have been obtained, to the best of our knowledge, there is no investigation on the ET mechanism and PL properties of Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors. In this work, a series of Dy3þ-doped and Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors were synthesized by a high-temperature solid-state reaction technique. The chromatic properties as well as their phase structure, morphology, decay behaviors and PL spectra were analyzed. Furthermore, the ET mechanism between the luminescence centers of Dy3þ and Eu3þ ions was also investigated in detail. 2. Experimental The Dy3þ-doped NaLa(MoO4)2 (abbreviated as NaLa(MoO4)2:xDy3þ; x ¼ 0.01, 0.03, 0.05, 0.07, 0.09, and 0.11, where x is the molar mass) and Dy3þ/Eu3þ-codoped NaLa(MoO4)2 (abbreviated as NaLa(MoO4)2:0.05Dy3þ/yEu3þ; y ¼ 0.01, 0.03, 0.05, 0.07, and 0.09, where y is the molar mass) phosphors were prepared by a conventional solid-state reaction method. The stoichiometric amounts of raw materials, i.e., sodium carbonate (Na2CO3, 99%), lanthanum oxide (La2O3, 99.9%), molybdenum oxide (MoO3, 99.5%), dysprosium oxide (Dy2O3, 99.9%) and europium oxide (Eu2O3, 99.5%), were weighted and grinded thoroughly for 30 min in an agate mortar to achieve uniformity. After that, the mixtures were transferred to the alumina crucibles followed by the calcination in a furnace at 900 C for 6 h in air with a heating ratio of 3 C/min. Finally, these mixtures were cooled down naturally to room temperature. The X-ray diffractometer (XRD) (Mac Science, M18XHF-SRA) with Cu Ka (l ¼ 1.5406 Å) radiation was used to check the phase structure properties of the obtained samples. The morphology of the samples was measured by using a field-emission scanning electron microscope (FE-SEM) (LEO SUPPA 55, Carl Zeiss). The room-temperature PL emission and PL excitation (PLE) spectra were measured by using a fluorescence spectrophotometer (Photon Technology International (PTI) fluorimeter) equipped with a Xe arc lamp (60 W power), and the lifetime was determined by using a phosphormeter attached to the main system with a Xe flash lamp (25 W power).
Fig. 1. (a) XRD patterns of the NaLa(MoO4)2, NaLa(MoO4)2:0.05Dy3þ NaLa(MoO4)2:0.05Dy3þ/0.09Eu3þ phosphors. FE-SEM images of NaLa(MoO4)2:0.05Dy3þ and (c) NaLa(MoO4)2:0.05Dy3þ/0.09Eu3þ phosphors.
469
and (b)
the O2 / Mo6þ transition [12]. The other is composed of a series of narrow peaks located at approximately 325, 351, 366, 387, 426, 453, and 472 nm corresponding to the 6H15/2 / 6P3/2, 6H15/2 / 6P7/ 6 6 6 4 6 4 6 4 2, H15/2 / P5/2, H15/2 / I13/2, H15/2 / G11/2, H15/2 / H11/2, 6 4 3þ and H15/2 / F9/2 transitions of Dy ions, respectively [10,20]. Fig. 2(b) depicts the PL spectra of the NaLa(MoO4)2:xDy3þ phosphors at different Dy3þ ion concentrations under the excitation of 387 nm. As shown in Fig. 2(b), all the samples exhibited strong yellow emissions at ~573 nm due to the 4F9/2 / 6H13/2 transition of Dy3þ ions. Furthermore, two relatively weak emissions were also observed at 486 and 663 nm which are ascribed to the 4F9/2 / 6H15/
3. Results and discussion Fig. 1(a) shows the XRD patterns of the NaLa(MoO4)2, NaLa(MoO4)2:0.05Dy3þ and NaLa(MoO4)2:0.05Dy3þ/0.09Eu3þ phosphors. All the diffraction peaks were in good agreement with the standard JCPDS card No. 24-1103 and no other impurity phases were observed, indicating that all the as-prepared samples possessed pure tetragonal phase and the dopants (Dy3þ and Eu3þ ions) were incorporated into the NaLa(MoO4)2 host lattices. From the FE-SEM images (Fig. 1(b)e(c)), it is observable that all the obtained samples were composed of aggregated and irregular particles with sizes ranging from ~2 to 6 mm. Note that, with doping of RE (Dy3þ and Eu3þ) ions, the shape and size of the particles changed a little. The PLE spectra of the NaLa(MoO4)2:xDy3þ phosphors while monitoring at 573 nm corresponding to the 4F9/2 / 6H13/2 transition of Dy3þ ions are displayed in Fig. 2(a). It is clear that the PLE spectrum consisted of two parts: one is the weak broad charge transfer (CT) band ranging from 240 to 340 nm, which is assigned to
Fig. 2. (a) PLE spectra of the NaLa(MoO4)2:xDy3þ phosphors monitored at 573 nm. (b) PL spectra of the NaLa(MoO4)2:xDy3þ phosphors excited at 387 nm. The inset of (b) shows the PL emission intensity as a function of Dy3þ ion concentration.
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4 6 2 and F9/2 / H11/2 transitions, respectively [13,21]. It is known that the blue (4F9/2 / 6H15/2) and yellow (4F9/2 / 6H11/2) emissions are two characteristic emissions of Dy3þ ions. Among them, the blue emission is the magnetic dipole transition which is insensitive to the local environment, while the yellow emission belongs to the hypersensitive electric dipole transition and its intensity can be significantly affected by the crystal field around the Dy3þ ions [14,24]. Generally, the yellow emission takes the domination when the Dy3þ ions occupy the non-inversion center, while the blue emission will be stronger if the Dy3þ ions are located at a high symmetry sites. The intense yellow emission at 573 nm demonstrates that the Dy3þ ions occupied the low symmetry sites without inversion center in NaLa(MoO4)2 host lattices. Also, the PL emission intensity increased gradually with the increment of Dy3þ ion concentration and reached its optimum value when x ¼ 0.05, and then it started to decrease with further increasing the Dy3þ ion concentration due to the concentration quenching effect (see the inset of Fig. 2(b)). The cross relaxation (CR) mechanism, which originates from the resonance ET between the adjacent Dy3þ ions, can be used to explain the concentration quenching phenomenon. In terms of the CR mechanism, several possible channels (CR1, CR2, and CR3 as shown in Fig. 6(a)) are involved [9,25]: 4
F9/2 þ 6H15/2 / 6H9/2/6F11/2 þ 6F5/2
4
F9/2 þ 6H15/2 / 6H7/2/6F9/2 þ 6F3/2
4
F9/2 þ 6H15/2 / 6H9/2/6F11/2 þ 6F1/2.
Owing to these effective CR processes, the population of the 4F9/2 level was decreased, resulting in the quenched luminescence of the 4 F9/2 level (see Fig. 2(b)). According to Dexter's ET formula for multipolar interaction and Reisfeld's approximation, the relation between the PL emission intensity (I) and activator ion concentration (x) can be expressed by Ref. [26]:
logðI=xÞ ¼ c ðq=3ÞlogðxÞ;
(1)
where x is the Dy3þ ion concentration, c is constant and q ¼ 6, 8, and 10 corresponding to electric dipoleedipole, dipoleequadrupole, and quadrupolee-quadrupole interactions, respectively. As presented in Fig. 3, the dependence of log(I/x) on log(x) was linear and its slope was 1.58. Hence, the q was calculated to be 4.74, which is
Fig. 3. Linear fitting of log(I/x) versus log(x) for the 4F9/2 / 6H13/2 transition of Dy3þ ions in NaLa(MoO4)2:xDy3þ phosphors.
Fig. 4. PLE spectra of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors monitored at (a) 573 nm and (b) 615 nm.
near to 6, demonstrating that the dominant concentration quenching mechanism of Dy3þ ions in NaLa(MoO4)2:xDy3þ phosphors was electric dipoleedipole interaction. To investigate the ET mechanism and luminescence properties of Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors, a series of NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors were prepared. Fig. 4 shows the PLE spectra of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors. As illustrated in Fig. 4(a), the PLE spectrum monitored at
Fig. 5. PL spectra of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors excited at 362 nm. Inset presents the PL emission intensity as a function of Eu3þ ion concentration.
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471
362 nm, electrons are excited to the 6P5/2 level from the ground state. After that, the nonradiative (NR) transition occurs and the 4F9/ 2 level is populated. Finally, the blue, yellow and red emissions of Dy3þ ions from 4F9/2 to 6HJ (J ¼ 15/2, 13/2, and 11/2) are observed. Furthermore, the 5D0 and 5D1 levels of Eu3þ ions can be populated through the ET from Dy3þ to Eu3þ ions (see Fig. 6(b)). Eventually, the characteristic emissions of Eu3þ ions corresponding to the 5 D1 / 7F1, 5D0 / 7F0, 5D0 / 7F1, and 5D0 / 7F2 transitions are observed. To further identify the ET behavior between the Dy3þ and Eu3þ ions, the PL decay curves of NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors as a function of Eu3þ ion concentration were measured. From Fig. 7, one obtains that the decay curves can be well fitted by a second-order exponential expression [29]:
IðtÞ ¼ A1 expðt=t1 Þ þ A2 expðt=t2 Þ þ I0 ;
Fig. 6. Energy level diagram of Dy3þ and Eu3þ ions as well as the proposed ET mechanism.
573 nm emission (4F9/2 / 6H13/2) of Dy3þ ions consisted of a weak board CT band from 240 to 340 nm and some sharp peaks at 325, 351, 366, 387, 426, 453, and 472 nm owing to the transitions of Dy3þ ions from the ground state of 6H15 to the 6P3/2, 6P7/2, 6P5/2, 4I13/2, 4 G11/2, 4H11/2, and 4F9/2 excited levels, respectively. Moreover, the PLE peak intensity showed a downward trend with increasing the Eu3þ ion concentration. In comparison, a strong board CT band centered around 300 nm, which is attributed to the overlap of the Eu3þ / O2 and Mo6þ / O2 transitions [27], was observed in the PLE spectra when monitored at 615 nm corresponding to the 5 D0 / 7F2 transition of Eu3þ ions (see Fig. 4(b)). Furthermore, the PLE spectra also revealed the several narrow peaks centered at 361 nm (7F0 / 5D4), 375 nm (7F0 / 5G2), 380 nm (7F0 / 5G3), 393 nm (7F0 / 5L6), 415 nm (7F0 / 5D3), and 464 nm (7F0 / 5D2) which are assigned to the characteristic fef transitions of Eu3þ ions [6,28]. Additionally, several weak excitation bands centered at 351, 366, 426, and 453 nm corresponding to the transitions of Dy3þ ions of 6H15/2 / 6P7/2, 6H15/2 / 6P5/2, 6H15/2 / 4G11/2, and 6H15/ 2 / 4H11/2, respectively, were also observed, demonstrating that the energy can be transferred from Dy3þ to Eu3þ ions in NaLa(MoO4)2:0.05Dy3þ/yEu3þ material system. Compared with that monitored at 573 nm, the PLE peak intensity monitored at 615 nm exhibited an opposite trend, namely, it increased monotonously with the increment of Eu3þ ion concentration as shown in Fig. 4(b). Fig. 5 depicts the PL spectra of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors under the excitation of 362 nm. From Fig. 5, one knows that the emissions at about 486, 573, and 663 nm were attributed to the transitions of 4F9/2 / 6H15/2, 4F9/2 / 6H13/2, and 4F9/2 / 6H11/2 of Dy3þ ions, respectively. Furthermore, some other emission peaks located at approximately 534, 590, 595, and 615 nm were corresponding to the transitions of 5D1 / 7F1, 5D0 / 7F0, 5D0 / 7F1, and 5 D0 / 7F2 of Eu3þ ions, respectively [6]. As shown in the inset of Fig. 5, it is obvious that the PL emission intensity of Eu3þ ions (energy acceptor) increased gradually, while that of Dy3þ ions (energy donor) was found to decrease with the increment of Eu3þ ion concentration. The variations in the PL emission intensities of Dy3þ and Eu3þ ions suggested that the occurrence of ET from Dy3þ to Eu3þ ions in NaLa(MoO4)2:0.05Dy3þ/yEu3þ material system and color-tunable emissions are expected to be obtained by properly adjusting the ratio of Dy3þ and Eu3þ ion concentrations. The energy level diagram of Dy3þ and Eu3þ ions as well as the possible ET processes is shown in Fig. 6(b). Briefly, under the excitation of
(2)
where I0 and I(t) are the emission intensities at time 0 and t, A1 and A2 are the constants, t1 and t2 are the rapid and slow components of decay time, respectively. The average lifetime (tav) can be estimated by the following equation:
. tav ¼ A1 t21 þ A2 t22 ðA1 t1 þ A2 t2 Þ:
(3)
On the basis of Eq. (3) and the decay curves, the average lifetime values for y ¼ 0, 0.01, 0.03, 0.05, 0.07 and 0.09 in NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors were determined to be approximately 135.78, 128.46, 121.63, 109.93, 102.16 and 90.31 ms (see the inset of Fig. 7), respectively. It is obvious that the luminescent lifetime of Dy3þ ions decreased gradually with the increment of Eu3þ ion concentration which further proved that there existed a significant ET from Dy3þ to Eu3þ ions. Furthermore, based on the lifetime, the ET rate (PDy/Eu) from Dy3þ to Eu3þ ions is defined as [20]:
PDy/Eu ¼ 1=t 1=t0 ;
(4)
where t and t0 are the lifetimes of Dy3þ ions with and without Eu3þ ions. Therefore, the ET rate was calculated to be 5.4, 10.4, 19, 24.8, and 33.5% for y ¼ 0, 0.01, 0.03, 0.05, 0.07 and 0.09 in NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors, respectively. On the other hand, the ET efficiency from Dy3þ to Eu3þ ions can be roughly evaluated using the fluorescence emission intensity,
Fig. 7. PL decay curves of NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors (excited at 362 nm and monitored at 573 nm). Inset depicts the dependence of lifetime of Dy3þ ions as a function of Eu3þ ion concentration.
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which can be expressed as below [20,30]:
hT ¼ 1
Is ; I
(5)
where hT is the ET efficiency, Is and I are the PL emission intensities of Dy3þ ions with presence and absence of Eu3þ ions. It is evident that the ET efficiency showed an upward trend with the increment of Eu3þ ion concentration, achieving ~ 70% of its maximum value when y ¼ 0.09 (see Fig. 8), indicating that the ET from Dy3þ to Eu3þ ions in NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors was efficient. In terms of the ET between the sensitizer (Dy3þ) and activator (Eu3þ), two different mechanisms are involved: exchange interaction and multipoleemultipole interaction. The type of interaction mechanism can be identified by knowing the critical distance between the sensitizer and activator. If the critical distance is larger than 5 Å, the multipoleemultipole interaction dominates. Otherwise, the exchange interaction prevails. According to Blasses's report, the critical distance can be written by Ref. [31]:
3V 1=3 Rc z2 : 4pxc Z
(6)
Here, Rc is the critical distance, V is volume of the unite cell, xc is the critical concentration, at which the PL emission intensity of Dy3þ ions is half of that in the sample without Eu3þ ions. For the NaLa(MoO4)2 host lattice, V ¼ 335.24 Å3, Z ¼ 2, and xc ¼ 0.054 (obtained from the inset of Fig. 8). Therefore, the Rc was calculated to be about 18.1 Å which is larger than 5 Å, demonstrating that ET mechanism between the Dy3þ and Eu3þ ions in NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors was multipoleemultipole interaction. Additionally, on the basis of Dexter's ET formula of multipolar interaction and Reisfeld's approximation, the following relation can be obtained [32,33]:
I fC n=3 ; Is
(7)
Fig. 9. Dependence of I/Is of Dy3þ ions on (a) C6/3, (b) C8/3 and (c) C10/3.
Dy3þ to Eu3þ ions was electric dipoleedipole interaction mechanism. The Commission International de I'Eclairage (CIE) chromaticity diagram of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors is depicted in Fig. 10. The color coordinate as well as the color correlated temperature (CCT) are summarized in Table 1. The CCT can be calculated by the following expression [9,34]:
CCT ¼ 437n3 þ 3601n2 6861n þ 5514:31; and n ¼ ðx xe Þ=ðy ye Þ;
(8)
where x and y are the color coordinates, (xe,ye) is the coordinate of the epicenter and its value is xe ¼ 0.3320 and ye ¼ 0.1858. As shown in the inset of Fig. 10, the emission color of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors was changed from greenyellow to yellow and finally to orange-red with increasing the Eu3þ
where C is the total concentration of Dy3þ and Eu3þ ions, n ¼ 6, 8, and 10 correspond to the electric dipoleedipole, dipoleequadrupole, and quadrupoleequadrupole interactions, respectively. The I/ Is versus Cn/3 plots are shown in Fig. 9. It is obvious that, when n ¼ 6, the linear fitting result was the best, suggesting that the ET from
Fig. 8. Dependence of ET efficiency, hT, on Eu3þ ion concentration in NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors. Inset shows the normalized PL emission intensity of Dy3þ ions in NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors as a function of Eu3þ ion concentration.
Fig. 10. CIE chromaticity diagram of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors (lex ¼ 362 nm). Inset shows the typical phosphor images under the excitation of 362 nm.
P. Du, J.S. Yu / Journal of Alloys and Compounds 653 (2015) 468e473 Table 1 CIE parameters of the NaLa(MoO4)2:0.05Dy3þ/yEu3þ phosphors excited at 362 nm. Points
Sample
CIE coordinate (x,y)
CCT (K)
1 2 3 4 5 6
NaLa(MoO4)2:0.05Dy3þ NaLa(MoO4)2:0.05Dy3þ/0.01Eu3þ NaLa(MoO4)2:0.05Dy3þ/0.03Eu3þ NaLa(MoO4)2:0.05Dy3þ/0.05Eu3þ NaLa(MoO4)2:0.05Dy3þ/0.07Eu3þ NaLa(MoO4)2:0.05Dy3þ/0.09Eu3þ
(0.360,0.383) (0.384,0.361) (0.426,0.348) (0.454,0.340) (0.474,0.326) (0.486,0.325)
4611 3784 2663 2124 1805 1739
ion concentration, suggesting that color tunable emissions can be obtained by the modulation of the ratio of Dy3þ and Eu3þ ion concentrations and these color-tunable phosphors may have promising applications in color displays and lighting fields. Furthermore, the CCT value varied from 4661 K to 1739 K with the increment of Eu3þ ion concentration (see Table 1). 4. Conclusions In summary, a series of Dy3þ-doped and Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors were synthesized by a simple hightemperature solid-state reaction method. Under 387 nm light excitation, the characteristic emissions of Dy3þ ions were observed in NaLa(MoO4)2:xDy3þ phosphors. The optimum concentration of Dy3þ ions was 5 mol% and the concentration quenching mechanism of Dy3þ ions in NaLa(MoO4)2 host lattice was electric dipoleedipole interaction. Furthermore, the emission color of Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors could be tuned from green-yellow to yellow and eventually to orange-red by adjusting the ratio of Dy3þ and Eu3þ ion concentrations. From the PL spectra, one obtains that there existed an efficient ET from Dy3þ to Eu3þ ions and the efficiency of this transfer was as high as 70% when the Eu3þ ion concentration was 9 mol%. In addition, the ET mechanism between the Dy3þ and Eu3þ ions was demonstrated to be electric dipoleedipole interaction and the critical distance was estimated to be 18.1 Å. Ultimately, the CCT value decreased from 4661 to 1739 K with the increase of Eu3þ ion concentration. From the industrial point of view, the color-tunable Dy3þ/Eu3þ-codoped NaLa(MoO4)2 phosphors may have potential applications in color displays and lighting fields. Acknowledgments This work was supported by the National Research Foundation
473
of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014- 069441).
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