Enhanced red luminescence in Gd2O3:Eu3+,Sm3+ and its dependence on temperature

Optics Communications 328 (2014) 23–29

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Enhanced red luminescence in Gd2O3:Eu3 þ ,Sm3 þ and its dependence on temperature Xiaofeng Wu a, Shigang Hu a, Congbing Tan b, Yunxin Liu b,n a b

School of Information and Electrical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China Department of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan 411201, China

art ic l e i nf o

a b s t r a c t

Article history: Received 19 February 2014 Received in revised form 4 April 2014 Accepted 22 April 2014 Available online 5 May 2014

Here, we show that co-activation in the Gd3 þ –Eu3 þ –Sm3 þ system can generate the intense red emission from Eu3 þ ion. Gd2O3:Sm3 þ ,Eu3 þ colloidal spheres were synthesized by combining a low temperature hydrothermal process and a higher temperature of pyrolysis. Under the excitation of 380 nm near UV light, Gd2O3:Sm3 þ ,Eu3 þ emits intense red light while the reference Gd2O3:Eu3 þ emits μ very weak one. The calculated polarizability αb based on the shift (16 nm) of Eu3 þ –O2  charge transfer band and the crystallography analysis revealed that Sm3 þ ions codoping in Gd2O3:Eu3 þ has generated a coordinating environment of Eu3 þ ions with high polarizability and low symmetry which leads to high energy transfer rate not only between Sm3 þ and Eu3 þ pairs, but also between Eu3 þ (S6) and Eu3 þ (C2) pairs, ensuring the efficient excitation of Gd2O3:Eu3 þ ,Sm3 þ . The energy migration from Sm3 þ to Eu3 þ and a corresponding feedback were also experimentally evidenced by the fluorescent decay, which is in well agreement with the theoretical calculation. In addition, the thermal stability of the photoluminescence in Gd2O3:Eu3 þ ,Sm3 þ colloidal spheres was evaluated. & 2014 Elsevier B.V. All rights reserved.

Keywords: Rare earth Gd2O3 Near UV excitation Photoluminescence Eu3 þ Sm3 þ

1. Introduction Lanthanide Eu3 þ ion doped luminescent materials have been extensively used in the fields of display and illumination, since Eu3 þ ion can emit intense red light ( 610 nm) by the 4f n electric dipolar transition of 5D0–7F2 under the vacuum (140–180 nm) and deep (180–300 nm) ultraviolet (UV) excitation [1–4]. Recently, much effort has been taken focusing on the photoluminescence of Eu3 þ ion doped materials under the excitation of ultraviolet light emitting diodes (UV LEDs), due to the energy saving of the luminescent devices based on LEDs relatively to the conventional display and illumination devices [5–8]. But, it is noted that most Eu3 þ ion doped red light materials can only be efficiently excited by the vacuum and deep UV instead of the near UV. On the other hand, the longer the emission wavelength of the UV LEDs is, the higher the emission efficiency is [8], e.g., the UV LEDs with emission wavelength of 380 nm have higher output efficiency than the one with  280 nm. For solving this problem, one way is to improve the luminescent efficiency of Eu3 þ doped red light materials under the near UV LEDs excitation by codoping [9–13]. e.g., Yan et al. have reported that the strongly enhanced red emission of Eu3 þ originating from 5D0–7F2 transition can be

n

Corresponding author. Tel.: þ 86 731 8291 433. E-mail address: [email protected] (Y. Liu).

http://dx.doi.org/10.1016/j.optcom.2014.04.051 0030-4018/& 2014 Elsevier B.V. All rights reserved.

obtained by adding Bi3 þ instead of increasing the Eu3 þ concentration under the 395 nm near UV excitation [13]. In order to achieve the luminescence enhancement of Eu3 þ ion by codoping, the key is to select a proper codoped ion for cooperating with Eu3 þ ion. Gd3 þ or Sm3 þ ions codoping with Eu3 þ ions were confirmed to be efficient for enhancing the photoluminescence of Eu3 þ ions, due to the energy transfer and cooperative activation between Eu3 þ and Sm3 þ /Gd3 þ [13]. In particular, it was revealed that the energy migration process of Gd3 þ –(Gd3 þ )n–Sm3 þ might occur and lead to the shift of excitation band of Sm3 þ ion [14], which implied the possibility of tuning the excitation and emission bands of Eu3 þ ion by cooperating with Sm3 þ and Gd3 þ ions. However, to our knowledge, there is rarely report on the investigation of Eu3 þ – Sm3 þ –Gd3 þ system. Inspired by these report, we try to tune the excitation and emission bands of Eu3 þ ion by co-activating with Gd3 þ and Sm3 þ ions so that the Eu3 þ ion can emit intense red light under the near UV excitation. For ensuring the high energy migration of Gd3 þ –(Gd3 þ )n–Sm3 þ [14], we select Gd2O3 as the host materials for Eu3 þ ions. Actually, many reports have confirmed the excellent optical properties of Gd2O3 matrix for rare earth doped luminescent materials [11,14]. Herein, we show that the emission and excitation bands of Eu3 þ can be efficiently tuned in the Eu3 þ and Sm3 þ doped Gd2O3 system for generating intense red light under near UV (380 nm) excitation. The activating and cooperating mechanisms in this Gd2O3:Sm3 þ ,Eu3 þ system were properly analyzed and suggested based on the excitation and emission spectra, and the

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X. Wu et al. / Optics Communications 328 (2014) 23–29

calculated polarizability. Importantly, the energy migration from Sm3 þ to Eu3 þ and a corresponding feedback were directly evidenced by the fluorescent decay.

2. Experimental section

were dissolved in distilled water with cation ions of 1 mol/l. then, 4 ml of rare earth nitrate aqueous solution were mixed with 15 ml of ethanol, 10 ml of oleic acid, and 0.2 g of sodium oleate and stirred for 30 min. Finally, the mixed solution was transferred into an autoclave, sealed, and reacted at the temperature of 180 1C for 24 h. when the reaction finished, the products at the bottom was collected, washed with ethanol for two times, and dried overnight.

2.1. Synthesis 2.2. Characterization Samples with the composition of Table 1 were synthesized as follows: rare earth (RE) oxides Gd2O3, Eu2O3, and Sm2O3 were dissolved in HNO3 to produce RE nitrate. The RE(NO3)3 were redispersed in distilled water and stirred for 30 min at room temperature. Subsequently, the RE(NO3)3 aqueous solution was heated to 60 1C with continuous stirring, and the urea was added. Then, the mixed solution was stirred for 20 min at temperature of 60 1C. Finally, the prepared solution was transferred into a 50 ml autoclave, sealed, and reacted at the temperature of 100 1C for 12 h. After the reaction finished, the products were washed with distilled water and ethanol several times and dried at the temperature of 60 1C for 24 h. The as prepared samples (RE(OH)CO3) were subsequently taken by a pyrolysis process at different temperatures to obtain the final oxide products. The sample without the doping of Sm3 þ was synthesized with the same procedure. NaGdF4:Sm3 þ ,Eu3 þ nanocrystals were synthesized as follows: First, Gd(NO3)3, Sm(NO3)3, and Eu(NO3)3 with stoichiometric ratio Table 1 Compositions (mol%) and heat-treatment condition of synthesized samples. The content of urea is kept to 1600 mol% while reaction temperature and time is set to 500 1C/1.5 h for all samples. Referred to

Gd3 þ

Eu3 þ

Sm3 þ

Heat-treatment

GE5 GES5 GE58 GES58 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11

96 94.5 96 94.5 98.5 98 97.5 96.5 92.5 88.5 95.5 95 93 91 88

4 4 4 4 0 0.5 1 2 6 10 4 4 4 4 4

0 1.5 0 1.5 1.5 1.5 1.5 1.5 1.5 1.5 0.5 1 3 5 8

No No 800 1C 800 1C 800 1C 800 1C 800 1C 800 1C 800 1C 800 1C 800 1C 800 1C 800 1C 800 1C 800 1C

for for for for for for for for for for for for for

1.5 h 1.5 h 1.5 h 1.5 h 1.5 h 1.5 h 1.5 h 1.5 h 1.5 h 1.5 h 1.5 h 1.5 h 1.5 h

Scanning Electron Microscopy (SEM) images were recorded using a FE-SEM JEOL JSM-6700F microscope. X-ray diffraction (XRD) data were recorded on a D/max-3c X-ray diffractometer system with graphite monochromatized Cu Kα irradiation (λ ¼0.15418 nm). Fluorescence spectra were measured with a Jobin Yvon U1000 spectrophotometer under the excitation of a 254 or 380 nm UV lamp. Excitation spectra were recorded at room temperature using a Hitachi F-4500 spectrophotometer. For fluorescent and excitation spectra, all the experimental conditions were kept constant during the measurement. The thermal fluorescent spectra of Gd2O3:Eu3 þ , Sm3 þ were measured after the sample being kept at some temperature for 5 min. e.g., For measuring the fluorescent spectra at 500 K, the sample was quickly heated to 500 K and kept at this temperature for 5 min, then, the fluorescent spectra were detected.

3. Results and discussion Fig. 1 shows the SEM images of GES5, of which the sphere like morphology can be clearly observed. A single Gd2O3:Eu3 þ ,Sm3 þ micro sphere was enlarged in Fig. 1(b) showing the nano-scale roughness of the surface. Fig. 2 shows the XRD spectra of GES5, in which the cubic phase structure of Gd2O3 can be easily assigned according to the corresponding standard XRD spectra (JCPDS no. 86-2477) [15]. The X-ray diffraction peaks arise mainly from crystal planes (2 1 1), (2 2 2), (4 0 0), (4 4 0), and (6 2 2). Although the doping of Eu3 þ or Sm3 þ ions could not be addressed by XRD measurement due to the low concentration, it can be seen from the inserts in Fig. 2 that the Eu3 þ or Sm3 þ ions can occupy two different crystallographic sites in Gd2O3 host with site symmetry S6 and C2 [16]. The emission spectra of GES5 and GE5 micro spheres were recorded under the 254 and 380 nm excitation, respectively, and shown in Fig. 3(a). The emission peak centered at 612 nm originates from electric dipole transition of 5D0–7F2 of Eu3 þ ion while the one centered at 589 nm corresponds a magnetic dipole transition of 5D0–7F1. It is noted that the emission intensity of

Fig. 1. SEM images of GES5: (a) low magnification and (b) a single Gd2O3:Eu3 þ ,Sm3 þ micro sphere showing the nano-scale roughness of the surface.

X. Wu et al. / Optics Communications 328 (2014) 23–29

Fig. 2. XRD spectra of GES5.

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(site symmetry: C2) pairs will be correspondingly enhanced [16]. Of course, it should be taken into account that the Sm3 þ ions can transfer efficiently their energy to Eu3 þ for improving the red emission from 5D0–7F2 transition [18]. Moreover, the 598 nm emission of Sm3 þ ion was observed for GES5, which originates from the 4G5/2–6H7/2 transition of Sm3 þ ion [14,19]. Differentially, the emission intensities of both GE5 and GES5 are very weak under the 380 nm excitation, implying their poor excitation efficiency by the 380 nm UV light. The crystallinity of both GE5 and GES5 was improved by further heat-treatment at temperature of 800 1C for 1.5 h. Fig. 3(b) shows the emission spectra of GE58 and GES58. The emission intensity of GES58 under the 380 nm excitation is significantly enhanced to be almost equal to that under the 254 nm excitation, while the emission intensity of GE58 under the 380 nm excitation is still much lower than that under the 254 nm excitation. For revealing the unique change of the optical properties from GES5 to GES58, excitation spectra of GES5 to GES58 were measured by activating the samples with a Xe lamp and detecting the emission intensity as the activating wavelength changing from 200 to 500 nm, and shown in Fig. 4. Comparing Fig. 4(a) and (b), it was clearly observed that the excitation peak centered at 380 nm was significantly improved for GES58, meanwhile, the red shift of the 256 nm excitation band was detected in GES58. Undoubtedly, the significant enhancement of the 380 nm excitation peak has led to the intense luminescence of GES58 under the 380 nm excitation. It can be easily understood that the poor excitation of GE5 and GE58 by 380 nm UV light is due to the lack of energy transfer from Sm3 þ to Eu3 þ ions [19]. But, why GES58 can be efficiently excited by 380 nm UV light, rather than GES5? Firstly, it should be noted that the 256 nm excitation band of GES5 shifts to longer wavelength (272 nm) after fired at temperature of 800 1C for 1.5 h. The 256 nm broad band of GES5 or the 272 nm one of GES58 can be assigned to the Eu3 þ –O2  charge transfer (CT) [15,16]. The CT energy (ECT ) of Eu3 þ ion in the compounds can be expressed with the environment factor (he ) as the following equation [20,21] ECT ¼ A þ B  expð  k  he Þ

ð1Þ 3þ

ion. These where A¼2.804, B¼ 6.924, and k ¼1.256 for the Eu constants are only dependent on the type of rare earth ion. The environment factor (he ) represents the effect of four chemical bond parameters, i.e., the covalency, the bond volume polarization, the presented charge of the ligand in the chemical bond, and the coordination number of the central ion, on the centre ions Eu3 þ , which was calculated to be 0.972 for GES5 from the Eu3 þ –O2  CT band centered at 256 nm (4.84 eV), and 1.093 for GES58 from the

Fig. 3. Emission spectra of (a) GE5 and GES5 and (b) GE58 and GES58, under the excitation of 254 and 380 nm UV light, respectively. All the experimental conditions were kept constant during the measurement.

GES5 is much higher than that of GE5 under the 254 nm UV excitation, demonstrating the remarkable influence of Sm3 þ ion on the emission of Eu3 þ ion. Ananias et al. have revealed that slight distortions of Eu coordination sphere deviating from the center of inversion were sufficient to produce non-vanishing intensity parameters Ωλ [17]. Sm3 þ ion codoping in Gd2O3:Eu3 þ will induce considerable distortion of Eu3 þ coordination environment since the ionic radius (0.958 Å) of Sm3 þ is obviously larger than that (0.937 Å) of Gd3 þ ion [18]. Then, it can be easily inferred that the energy transfer between Eu3 þ (site symmetry: S6)–Eu3 þ

Fig. 4. Excitation spectra of (a) GE58 and (b) GES58 monitoring the 5D0-7F2 emission at 610 nm. The insert shows the digital photographs of red luminescence of GE58 (a) and GES58 (b) under 380 nm UV excitation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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X. Wu et al. / Optics Communications 328 (2014) 23–29

Eu3 þ –O2  CT band centered at 272 nm (4.56 eV). The calculated environment factor (he ) of GES5 and GES58 implied that the Sm3 þ ions were more efficiently incorporated into the host in GES58 than in GES5, which led to higher environment variation of Eu3 þ ion in GES58 than in GES5 [22]. Moreover, the environment factor (he ) can be obtained using the relative equation [20,21] !1=2 he ¼

μ μ

μ2

∑ f c αb Q B μ

ð2Þ

μ

where Q B is for the presented charge of the coordinates in the μ chemical bond of the type-μ, f c is the covalency for any individual μ bond μ surrounding the center ion in a multibond crystal, and αb stands for the polarizability of the coordinating bond volume in μ μ the type-μ bond. Q B equals to 2 for GES5 or GES58, while f c has almost the same value for GES5 and GES58 due to the fact that GES5 and GES58 have the same composition and all cations have the same valency (þ 3). Then, the relationship of the polarizability αμb between GES5 and GES58 can be addressed [20]

αμbðGES5Þ =αμbðGES58Þ  ðheðGES5Þ =heðGES58Þ Þ2

ð3Þ

Combining the previous calculated values of 0.972 (heðGES5Þ ) and 1.093 (heðGES58Þ ), it can be inferred from Eq. (3) that the polarizμ ability αb of the Eu3 þ coordinating bond volume in GES58 is higher than that in GES5. Consequently, considering the dependence of energy transfer rate on the polarizability of Eu3 þ and Sm3 þ , the energy transfers from the electric dipole transition of Sm3 þ to that of Eu3 þ ions and between Eu3 þ (S6)–Eu3 þ (C2) pairs were both considerably improved in GES58 that led to the significant enhancement of the 380 nm excitation peak of GES58. Secondly, from the crystallography point of view, Eu3 þ and Sm3 þ ions occupy two different crystallographic sites in Gd2O3 host with site symmetry S6 and C2. Energy transfer may occur between Eu3 þ (S6)/(C2), Sm3 þ (S6)/(C2), or Eu3 þ /Sm3 þ pairs with a direct one-phonon assisted or resonant two-phonon assisted process [18]. The energy of the 4f n transitions of Eu3 þ depends on the crystal field of the Eu3 þ lattice sites, resulting in inhomogeneous broadening of this transition if the sample contains Eu3 þ ions in different lattice sites [23]. The observed line width of the 5 D0–7F2 transition in GES5 is at least 10 nm, much wider than that (6 nm) in GES58, which in fact indicates Eu3 þ occupying more types of lattice sites in the Gd2O3 host in GES5 than in GES58 [24]. However, comparing the emission spectra of GES5 and GES58 under the 254 nm excitation, it was found that the emission intensity ratio of 5D0–7F2 to 5D0–7F1 transition was remarkably enhanced in GES58 (12.3) relatively to that (7.4) in GES5, demonstrating the lower crystal field symmetry of Eu3 þ ions in GES58, since the electronic dipole transition 5D0–7F2 is centrosymmetric forbidden, while the magnetic dipole transition 5D0–7F1 is independent on the crystal field symmetry [25,26]. Then, it is reasonable for GES58 to suggest that Sm3 þ ions have homogeneously distributed around of Eu3 þ ions, bringing about the lowering of the crystal field symmetry of Eu3 þ ions. As a result, the efficient energy transfer occurred not only between Sm3 þ and Eu3 þ pairs, but also between Eu3 þ (S6)/(C2) pairs and Sm3 þ (S6)/(C2) pairs that led to the corresponding enhancement of the 380 nm excitation peak originating from the 5G2–7F0 transition. This is in well agreement with the above calculated results. Moreover, it is noted in Fig. 4(b) that an excitation peak centered at 242 nm arises in GES58 which is assigned to the strong absorption of Gd2O3 host lattices [14]. Jin et al. have reported that the Sm3 þ ion codoping will generate a new excitation peak centered at 405 nm for Eu3 þ ion in MMoO4 (M ¼Ca, Ba, and Sr) [19]. This excitation peak was not observed in GES5 or

GES58 which might be ascribed to the crystal lattice differences between Gd2O3 and MmoO4 (M ¼Ca, Ba, and Sr). From the diffuse reflectance spectra in Fig. 5, the efficient optical tuning of Sm3 þ doping on the Eu3 þ ion emission could also be observed that the absorbance edge of Gd2O3:Eu3 þ was extended from  435 to 490 nm. On the other hand, the absorbance edge is depended on the optical band gaps of absorption ions. After the codoping of Sm3 þ ions, it can be calculated from Fig. 5 that the optical band gap was lowered. But, previous work reported that the optical band gap of Sm3 þ is higher than that of Eu3 þ [27]. Thus, it can be inferred that the extending of the optical band gap is not the direct absorbance of the codoped Sm3 þ ions, but the tuned band gaps of Eu3 þ or Sm3 þ ions. Fig. 6 shows the fluorescence decay of 5D0(Eu) and 4G5/2(Sm) under 380 nm pulse excitation. It should be noted that luminescence decay curves from 5D0(Eu) and 4G5/2(Sm) states were almost overlapped due to a fact that the position of 5D0 state of Eu and 4 G5/2 state of Sm on the energy level scheme is nearly the same. However, according to the comparison among the decay times of 5 D0(Eu), 4G5/2(Sm), and 5D0(Eu) þ 4G5/2(Sm), it is possible to deduce whether the energy transfer from 4G5/2(Sm) to 5D0(Eu) has occurred. The luminescence decay curves (Fig. 6) were fitted to the double-exponential fitting functions with a long-decay and a short-decay. The luminescence intensity IðtÞ could be described by the sum of two exponential decay components using following relation     t t IðtÞ ¼ A1 exp  þ A2 exp  ð4Þ

τ1

τ2

where τ1 and τ2 were short- and long-decay components, parameters A1 and A2 were fitting constants, respectively. Furthermore,

Fig. 5. Diffuse reflectance spectra of GE58 and GES58.

Fig. 6. Fluorescent decay of 5D0(Eu) and 4G5/2(Sm) states in the samples doped with 0%Eu3 þ þ1.5%Sm3 þ , 0%Eu3 þ þ 1.5%Sm3 þ , and 0%Eu3 þ þ 1.5%Sm3 þ , respectively, under the 380 nm pulse excitation.

X. Wu et al. / Optics Communications 328 (2014) 23–29

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the effective lifetime value, τef f , was also calculated by [28]

τef f ¼

Z 0

1

 IðtÞdt =Ið0Þ

ð5Þ

The main decay components, τ1 , from fitting procedure and the effective lifetime value τ ef f are given in Table 2, from which it is observed that the effective lifetime of Eu3 þ and Sm3 þ codoped sample is shortened relatively to that of single Eu3 þ doped one indicating the occurring of the energy transfer from Sm3 þ to Eu3 þ ions [29]. The fluorescence decay of 5D0(Eu) and 4G5/2(Sm) under 365 nm pulse excitation (Fig. 7) is completely different from that under 380 nm pulse excitation. Surprisingly, under 365 nm pulse excitation, the electrons from 5D0(Eu) and 4G5/2(Sm) states decay fast to the lower states. The calculated effective lifetime values are equal to 12, 9, and 7 ms for GE58, GES58, and S1, respectively. This phenomenon indicates that the energy migration from Sm3 þ to Eu3 þ ion is neglected under 365 nm excitation, since the efficient energy migration from Sm3 þ to Eu3 þ ion generates high population probability of 5D0(Eu) and 4G5/2(Sm) states and leads to long lifetimes of 5D0(Eu) and 4G5/2(Sm) states, e.g., the case under 380 nm excitation. This elucidation is also evidenced by the excitation spectra in Fig. 4. It can be seen from the excitation spectra that the excitation peak of 365 nm is obviously weaker than 380 nm one. This means that the 365 nm excitation is less efficient than the 380 nm excitation and leads to low population probability of 5D0(Eu) and 4G5/2(Sm) states. On the other hand, the efficient energy migration between Eu3 þ and Sm3 þ ions depends on the population probability of 5D0(Eu) and 4G5/2(Sm) states. Then, it can inferred that the energy migration between Eu3 þ and Sm3 þ ions is almost negligible under the 365 nm excitation. Furthermore, the fluorescent decay is investigated under the excitation of 254 nm UV light. It can be seen from Fig. 8 that the fluorescent lifetime of 5D0(Eu) and 4G5/2(Sm) states in the sample doped with 4%Eu3 þ and 1.5%Sm3 þ ions is shorter than that in the sample doped 4%Eu3 þ or the sample doped with 1.5%Sm3 þ . This indicates the occurrence of the energy migration between Eu3 þ and Sm3 þ ions which is similar to the condition under the 380 nm

Fig. 8. Fluorescent decay of 5D0(Eu) and 4G5/2(Sm) states in the samples doped with 0%Eu3 þ þ1.5%Sm3 þ , 0%Eu3 þ þ 1.5%Sm3 þ , and 0%Eu3 þ þ 1.5%Sm3 þ , respectively, under the 254 nm pulse excitation.

Fig. 9. Simplified energy diagram of Eu3 þ and Sm3 þ ions and possible energy migration processes.

Table 2 The average lifetime (τ) obtained from fitting procedure. Sample

A1

τ1 [μs]

A2

τ2 [μs]

τef f [μs]

GE58 GES58 S1

230 2n104 800

46 18 32

8.5 2.7 0.8

98 260 520

61 17 31

Fig. 7. Fluorescent decay of 5D0(Eu) and 4G5/2(Sm) states in the samples doped with 0%Eu3 þ þ 1.5%Sm3 þ , 0%Eu3 þ þ 1.5%Sm3 þ , and 0%Eu3 þ þ1.5%Sm3 þ , respectively, under the 365 nm pulse excitation.

UV excitation. From the excitation spectra in Fig. 4, it is clear that the 254 nm UV excitation wavelength is located at the intense Eu3 þ –O2  absorption band centered at 256 nm. As a result, it is addressed that the energy migration from Eu3 þ to Sm3 þ has taken place under the 254 nm excitation. Combining with the energy migration from Sm3 þ to Eu3 þ ions evidenced by the 380 nm excitation, it is concluded that the energy migration occurs simultaneously from Eu3 þ to Sm3 þ and from Sm3 þ to Eu3 þ under the UV excitations, but the one from Sm3 þ to Eu3 þ dominates the 380 nm excitation process and the one from Eu3 þ to Sm3 þ dominates the 254 nm excitation process. The energy migration between Eu3 þ and Sm3 þ ions can be intuitively illustrated by the simplified energy diagrams in Fig. 9. Under the 380 nm UV excitation, the excitation photons can be absorbed by the 4FJ band of Sm3 þ ions after taking a multiphonon relaxation process. Then, the electrons of the 4FJ (Sm3 þ ) states directly transfer their energy to 5D3 state of Eu3 þ or depopulate the 4G5/2 (Sm3 þ ) state. The 4G5/2 (Sm3 þ ) state can directly transfer their energy to the 5D0 state of Eu3 þ ions. In addition, the 5D3 (Eu3 þ ) state will depopulate the 5D0 state by a multiphonon relaxation process. As a result, the intense red emission from the transition 5D0-7F1,2 was observed under the 380 nm excitation. However, the sample undoped with Sm3 þ ion has weak

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X. Wu et al. / Optics Communications 328 (2014) 23–29

absorption to the 380 nm UV light due to the mismatching between band gaps of Eu3 þ and the excitation photons, which lead to the low population density of 5D0 state and thus a weak red emission from the transition 5D0-7F1,2. Under the 254 nm excitation, the Eu3 þ –O2  band absorbs the excitation photons and depopulate the 5DJ band by multiphonon relaxations. The 5D3 state can depopulate 5D0 state or transfers their energy to 4F7/2 level of Sm3 þ ion. Comparing the integral emission intensity of the sample doped with 4%Eu3 þ and the one doped with 4%Eu3 þ and 1.5Sm3 þ , it can be inferred that the energy feedback has taken place from Sm3 þ to Eu3 þ ion by the energy transfer processes 4 F7/2-5D3 and 4G5/2-5D0. We also changed the concentration of Eu3 þ and Sm3 þ ions to investigate the emission characteristics excited with 380 near UV light. Doping with 0 mol% Sm3 þ ions, the samples with various Eu3 þ content emit negligible light under the 380 nm near UV excitation (see Fig. 10a). But, intense red emission was observed for the samples codoped with 1.5 mol% Sm3 þ ions. This means that the doping of Sm3 þ ion plays an indispensable role for obtaining intense emission of Gd2O3:Eu3 þ ,Sm3 þ under the 380 nm near UV excitation. Furthermore, it was seen from Fig. 10b that both of the emissions under 254 and 380 nm excitations are highly depended on the concentration of Sm3 þ ion. Under the excitation of 380 nm UV light, the optimal concentration of Sm3 þ is equal to 1.5 mol% if the concentration of Eu3 þ is set as 4 mol%. Interestingly, under the 254 nm deep UV

Fig. 11. (a) TEM image and (b) fluorescent spectra of NaGdF4:Eu3 þ ,Sm3 þ nanocrystals. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 10. The emission intensity dependence of a) Gd2O3:x%Eu3 þ ,0%Sm3 þ and Gd2O3:x%Eu3 þ ,1.5%Sm3 þ on the concentration (x¼ 0.5,1,2,4,6,10) of Eu3 þ and b) Gd2O3:4%Eu3 þ ,x%Sm3 þ on the concentration (x¼ 0,0.5,1,1.5,3,5,8) of Sm3 þ under 380 nm UV excitation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

excitation, the emission intensity of the samples did not take a monotonous variety with increasing the Sm3 þ content. For this phenomenon, it is suggested that the energy transfer of 4 G5/2(Sm3 þ )þ 7F0(Eu3 þ )-6H5/2(Sm3 þ )þ 5D0(Eu3 þ ) firstly increase the emission intensity of Eu3 þ ion with increasing the concentration of Sm3 þ ion due to the energy migration from Sm3 þ to Eu3 þ [30], but decrease the emission intensity of Eu3 þ ion due to the increased inverse energy migration from Eu3 þ to Sm3 þ and concentration quenching [31] as the concentration of Sm3 þ ion exceeding a threshold (  1 mol%). The thermal stability of Gd2O3:Eu3 þ ,Sm3 þ was evaluated and compared with NaGdF4:Eu3 þ ,Sm3 þ nanocrystals. For comparison, NaGdF4:Eu3 þ ,Sm3 þ nanocrystals with an average diameter of 16 nm (see Fig. 11a) were synthesized, which also emit red light centered at  614 nm. Noticeably, it is observed from Fig. 11b that the relative intensity of the emission band centered at 589 nm of NaGdF4: Eu3 þ ,Sm3 þ is much stronger than that of Gd2O3:Eu3 þ , Sm3 þ (GES5). Fig. 12 shows the dependence of the integral luminescent intensity of both GES5 and NaGdF4:Eu3 þ ,Sm3 þ on the temperature. It is clear that the integral luminescent intensity of GES5 has no obvious change with increasing temperature from 300 to 600 K, demonstrating its well thermal stability at the temperature lower than 600 K. However, it was observed that the integral luminescent intensity of NaGdF4:Eu3 þ ,Sm3 þ decreased by more than 10% with increasing temperature from 300 to 600 K. On the other hand, the calculated crystallite size of GES5 and NaGdF4:Eu3 þ ,Sm3 þ is at the same level. Therefore, it is undoubted that the thermal stability of Gd2O3:Eu3 þ ,Sm3 þ is much better than NaGdF4:Eu3 þ ,Sm3 þ for the same crystallite size.

X. Wu et al. / Optics Communications 328 (2014) 23–29

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Acknowledgements This work is supported by Natural Scientific Foundation of Hunan Province (Grant no. 13JJ4080), and Scientific Research Fund of Hunan Provincial Education Department (Grant no. 10C0661).

References

Fig. 12. The dependence of integral intensity of red emission of NaGdF4:Eu3 þ ,Sm3 þ and Gd2O3:Eu3 þ ,Sm3 þ on the temperatures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4. Conclusion In conclusion, Gd2O3:Eu3 þ red phosphors, which can only be excited by deep UV light (140–300 nm), was codoped with Sm3 þ ion so that they could emit intense red light under 380 nm near UV excitation. Comparing the excitation spectra of Gd2O3:Eu3 þ with that of Gd2O3:Eu3 þ ,Sm3 þ , it was found that the  380 nm excitation peak of Gd2O3:Eu3 þ was remarkably enhanced due to the optical tuning induced by doping Sm3 þ ion. The energy transfer of 4G5/2(Sm3 þ )-5D0(Eu3 þ ) was confirmed based on the comparative analysis of the fluorescence decay, which directly indicated the sensitization of Sm3 þ to Eu3 þ ion. The calculated μ polarizability αb based on the shift (16 nm) of Eu3 þ –O2  charge transfer band and the crystallography analysis revealed that Sm3 þ ions codoping in Gd2O3:Eu3 þ has generated a coordinating environment of Eu3 þ ions with high polarizability and low symmetry which leads to high energy transfer rate not only between Sm3 þ and Eu3 þ pairs, but also between Eu3 þ (S6) and Eu3 þ (C2) pairs, ensuring the efficient excitation of Gd2O3:Eu3 þ ,Sm3 þ by the 380 UV light. The efficient lifetime of 5D0(Eu) and 4G5/2(Sm) under 380 nm pulse excitation was calculated to be 61 μs for 4mol% Eu3 þ doped sample, 31 μs for 4mol% Sm3 þ doped sample, and 17 μs for 4mol% Eu3 þ and Sm3 þ codoped sample, respectively, which directly indicate the energy migration between Eu3 þ and Sm3 þ ions. The intense red emission in Gd2O3:Eu3 þ ,Sm3 þ was also ascribed to the energy migration of Gd3 þ –(Gd3 þ )n–Sm3 þ . In addition, the thermal stability evaluation shows that Gd2O3: Eu3 þ ,Sm3 þ colloidal spheres are highly stable at the temperature lower than 600 K.

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