Optical properties of eu3+ ions in laf3 nanocrystals - Hoang Manh Ha

Journal of Science and Technology 54 (1A) (2016) 88-95 OPTICAL PROPERTIES OF Eu 3+ IONS IN LaF3 NANOCRYSTALS Hoang Manh Ha 1,* , Tran Thi Quynh Hoa 2 , Nguyen Ngoc Long 3 , Le Van Vu 3 , Phan Van Do 4 1Hanoi Architectural University, Km 10 Nguyen Trai, Thanh Xuan, Hanoi 2National University of Civil Engineering, 55 Giai Phong, Hai Ba Trung, Hanoi 3Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi 4Thuy Loi University, 175 Tay Son, Dong Da, Hanoi * Email: hahm1982@gmail.com Received: 30 August 2015; Accepted for publication: 25 October 2015 ABSTRACT LaF3 nanocrystals doped with different concentrations of Eu 3+ ions were synthesized by hydrothermal method. The XRD, luminescence and excitation spectra have been studied at room temperature. The ratio of the 5 D0→ 7 F2 electric dipole transition intensity to the 5 D0→ 7 F1 magnetic dipole transition intensity (R) was used to evaluate the asymmetry of Eu 3+ site. Judd- Ofelt parameters were calculated from emission spectra and were used to predict the radiative properties of 5 D0→ 7 FJ transitions also the lifetime of 5 D0 level. The small values of Ω2 parameter and R ratio relate to high degree of symmetry of ligand around Eu 3+ site in this material. Keywords: LaF3:Eu 3+ nanocrystals, luminescence, Judd-Ofelt analysis. 1. INTRODUCTION Rare earth (RE) doped crystals have attracted the attention of scientists due to their wide applications in many optical devices like lasers, light converters, sensors, high-density memories and optical amplifiers [1 - 5]. Compared to the oxide crystals, the fluoride crystals show more featured advantages such as long lifetime, weak nephelauxetic effect and low phonon energy [6 - 8], so these crystals are promising candidates for producing the laser and upconversion devices [4 - 6, 8]. Lanthanum fluoride nanocrystals (LaF3) are known with the maximal phonon mode frequency about 380 cm -1 [9], so they are an ideal material for RE dopant because they minimize multiphonon decay processes between energy levels which are very close of the RE 3+ ions, this increases the quantum efficiency of the fluorescent transitions. In practice, LaF3 nanocrystals doped with RE 3+ have been used in fibre optics, electrodes, fluorescent lamps, optical amplifiers and radiation applications [9 - 11]. Among the RE 3+ ions used to optically activate materials, the Eu 3+ ions are mostly chosen due to Eu 3+ ions emit narrow-band, almost monochromatic light and have long lifetime of the optically active states. Further, Eu 3+ ions have often been used as probes for estimation of local Optical properties of Eu 3+ ions in LaF3 nanocrystals 89 environment around the Ln 3+ ions in different matrices [11 - 13]. In this paper, Eu 3+ ions are used as probe to study the ligand field around RE 3+ in LaF3 nanocrystals. In addition, optical properties of LaF3:Eu 3+ are analyzed using Judd–Ofelt theory. 2. EXPERIMENTAL LaF3 nanocrystals doped with 0; 0.05; 0.1; 0.5 and 1.0 mol% of Eu 3+ ions were prepared by hydrothermal method. All the chemicals used in our experiment, including lanthanum oxide (La2O3), europium oxide (Eu2O3), ammonium fluoride (NH4F) and glycine (NH2CH2COOH) are of analytic grade without further purification. Annealing process was carried out for 12 hours at temperature of 425 K. The final product was dried in air at 330 K for 12 hours. The synthesized samples were characterized for their structure by an X-ray diffractometer (SIMENS D5005, Bruker, Germany) with Cu–Kα1 irradiation (λ = 1.54056 Å). The morphology of the samples was observed by using a scanning electron microscopy (JEOL-JSM 5410 LV). The optical absorption spectra were recorded in the range of wavelength from 200 to 3000 nm using a spectrometer (Cary-5000). Room temperature photoluminescence (PL) and photoluminescence excitation (PLE) of the samples were measured on a spectrofluorometer (FL 3-22 Jobin Yvon Spex) using 450 W Xe arc lamp as the excitation source. 3. RESULTS AND DISCUSSION 3.1. Structure characterization and morphology X-ray diffraction patterns of pure LaF3 and LaF3:Eu 3+ (1 mol% of Eu 3+ ions) nanocrystals are presented in Fig. 1. First, quite strong diffraction lines in XRD patterns indicate that LaF3 and LaF3:Eu 3+ nanoparticles have been crystallized well. Second, all the XRD peaks are unambiguously indexed to hexagonal phase with P3c1 space group of LaF3 structure (JCPDS card no. 32-0483 for LaF3) with the following diffraction peaks: (002), (110), (111), (112), (300), (113), (004), (302), (221), (114), (222), (223), (304) and (410). The lattice parameters were calculated to be a = 7.174 Å and c = 7.344 Å. Next, the obtained results showed that under given experimental conditions no critical change in XRD pattern of Eu 3+ ion doped (1 mol%) LaF3 was observed, compared to that of pure LaF3, indicating that there was no significant interfacial interaction between Eu and LaF3. Thus, the fact that crystal structure of Eu doped LaF3 was consistent with LaF3 for the investigated concentration range of Eu without presence of Eu own phase or shift in d-spacing, this allows to conclude that metallic Eu ions are homogeneously distributed in LaF3 matrix. 20 30 40 50 60 70 (0 0 4 ) 2 theta (degrees) In te n s it y ( a .u .) LaF 3 :Eu 3+ LaF 3 (0 0 2 ) (1 1 0 ) (1 1 1 ) (1 1 2 ) (3 0 0 ) (1 1 3 ) (3 0 2 ) (2 2 1 ) (2 2 3 ) (4 1 0 ) (3 0 4 ) (2 2 2 ) (1 1 4 ) Figure 1. XRD patterns of LaF3 and LaF3:Eu 3+ samples. Hoang Manh Ha, Tran ThiQuynhHoa, Nguyen Ngoc Long, Le Van Vu, Phan Van Do 90 The crystallite sizes of LaF3 and LaF3:Eu 3+ were estimated by using Scherrer’s formula [4] depending on selected peaks. Average sizes of LaF3 and LaF3:Eu 3+ nanocrystals are about 23 and 25 nm, respectively. Figure 2 shows FE-SEM image at efficiently high magnification which can provide a rough evaluation about particle size and morphology. From FE-SEM images it can be seen that the average diameter of the particles is approximately 50 nm that is larger than value calculated from Scherrer’s formula. This perhaps is because the crystallites have aggregated, forming bigger particles. 3.2. Photoluminescence excitation spectrum and sideband phonon energy The excitation spectrum of LaF3:Eu 3+ (1.0 mol %) nanocrystal was recorded in the spectral region 330-560 nm by monitoring the emission at 617 nm ( 5 D0- 7 F2 transition) and shown in Fig. 3. The excitation spectrum consists of the sharp bands due to the f-f transitions from 6 F0,1,2 of ions Eu 3+ to the excited levels. The most intense excited band at wavelength of 397 nm corresponds to the 7 F0→ 5 L6 transition, which is often used in fluorescence excitation for Eu 3+ . The shoulder appears at wavelength around 458 nm can be related to the phonon sideband (PSB), which is used to understand the vibrational modes around the Eu 3+ [13, 14]. The PSB of Eu 3+ in LaF3 is associated with the 7 F0 → 5 D2 transition and shown in inset of Fig. 3, in which the 7 F0→ 5 D2 excited transition is the pure electronic transition (PET). The PET is set as zero energy shift, the sideband phonon energy in LaF3 can be calculated to be 290 cm -1 .This phonon energy is related to the Eg vibration group in LaF3 [9]. The electron phonon coupling constant (g) have been calculated by [13]: dI dI g PET PSB )( )( (1) where IPSB is the intensity of the phonon sideband and IPET is the intensity of the pure electric transition. In LaF3:Eu 3+ (1.0 mol%), the g value is found to be 0.059. This value is equal to the corresponding values in lead fluoroborate (LFB) glasses [13], but is much lower than that in borotellurite glasses [14]. Figure 2. FE-SEM image of LaF3:Eu 3+ sample. 350 400 450 500 550 -200 -100 0 100 200 300 400 7 F 0 -5 D 2 x3 P S B ( 2 9 0 c m -1 ) P E T In te n s it y Energy shift 7 F 0 -5 D 4 7 F 2 -5 G 6 7 F 1 -5 G 6 7 F 0 -5 G 6 7 F 2 -5 L 6 7 F 1 -5 L 6 7 F 0 -5 L 6 7 F 0 -5 D 2 7 F 1 -5 D 2 7 F 2 -5 D 2 7 F 0 -5 D 1 7 F 1 -5 D 1 7 F 2 -5 D 1 P L i n te n s it y ( a .u ) Wavelength (nm) Figure 3. The excitation spectrum of Eu 3+ in LaF3 nanocrystals. Optical properties of Eu 3+ ions in LaF3 nanocrystals 91 This behavior shows that the electron-phonon coupling in LaF3 crystal and LFB glass is weaker than that in borotellurite glasses. Thus, the ionic of Eu 3+ -F - bond in LaF3 nanocrystal is higher than that Eu 3+ -O 2- bond in borotellurite glasses [13]. 3.3. Emission spectra Figure 4 illustrates the emission spectra of LaF3:Eu 3+ nanocrystals using the 397 nm excitation wavelength of xenon lamp source. The luminescence lines are interpreted according to Carnall’s paper [15]. Owing to very low phonon energy, the multiphonon relaxation rates from 5 D2 and 5 D1 levels to 5 D0 level are very small, so luminescence from higher excited states of 5 D2 and 5 D1 were also observed. The emission spectra consist of 12 observed emission bands in Vis and a band in NIR region. There are nine quite strong emission peaks at 488, 509, 525, 534, 553, 582, 590, 617 and 681 nm, which are attributed to 5 D2→ 7 F2, 5 D2→ 7 F3, 5 D1→ 7 F0, 5 D1→ 7 F1, 5 D1→ 7 F2, 5 D0→ 7 F0, 5 D0→ 7 F1, 5 D0→ 7 F2, 5 D0→ 7 F4 transitions, respectively, and three weak emission peaks at 648, 749 and 815 nm corresponding to 5 D0→ 7 F3, 5 D0→ 7 F5 and 5 D0→ 7 F6 transitions, respectively. The 5 D0→ 7 F0,3,5 transitions are forbidden under selection rules, so their intensity are often very weak, however in our case the 5 D0→ 7 F0 transition is quite strong. The 5 D0→ 7 F2 transition is induced electric dipole (ED), so its intensity depends strongly on the local symmetry around Eu 3+ ion, polarizability of ligand and energy separation between initial level and final level. The 5 D0→ 7 F1 transition is the most intense of all, this is allowed magnetic dipole (MD), the intensity of this transition depends on the host but it is independent local symmetry and reduced matrix elements. So this transition is often used as an internal standard to evaluate the asymmetry of ligand and fluorescent efficiency of 5D0→ 7 F2 transition. The fluorescence intensity ratio (R) of 5 D0→ 7 F2 to 5 D0→ 7 F1 transitions of Eu 3+ ions allows one to estimate the deviation from the site symmetries of Eu 3+ ions. For LaF3 doped with 0.05, 0.1, 0.5 and 1.0 mol % of Eu 3+ ions, the R values are equivalent as shown in table 1. The values of R are less than unity, i.e. the fluorescent efficiency of 5D0→ 7 F2 transition is lower than those of 5D0→ 7 F1 transition. Moreover, these values are smaller than those of many different hosts [13-15]. The lower R values are attributed to the lower asymmetry and covalency around the Eu 3+ ions in LaF3 than other hosts. In LaF3 crystal, Eu 3+ ion is positioned in a La 3+ site with C2 point group symmetry [9], with this symmetry the 5 D0→ 7 F0,1,2 transitions split into 1, 3 and 5 components [12], respectively, like in Fig. 4. In principle, the electricant magnetic dipoles are allowed in C2 symmetry, so both the 5 D0→ 7 F1 and 5 D0→ 7 F2 transitions have strong intensity [9]. However, the 5 D0→ 7 F2 transition is only very intense in the case of highly polarizable, whereas fluoride ligands have low polarizability. This also is the reason for the reduction of R values in the fluoride compounds. Moreover, the luminescence intensity of the 5 D0→ 7 F1 transition is stronger than that of 5 D0→ 7 F2 transition, this suggests that Eu 3+ ions take a site with inversion symmetry in LaF3 crystal [13]. 500 550 600 650 700 750 800 7 F 6 7 F 5 7 F 4 7 F 3 7 F 2 7 F 1 7 F 0 5 D 1 -7 F 0 5 D 2 -7 F 3 605 610 615 620 625 630 635 S 5 S 4 S 3 S 2 S 1 5 D 0 - 7 F 2 584 586 588 590 592 594 S 3 S 2S 1 5 D 0 - 7 F 1 d c b a 5 D 2 -7 F 2 5 D 1 -7 F 1 5 D 1 -7 F 2 5 D 0 P L i n te n s it y ( a .u ) Wavelength (nm) Figure 4. The emission spectra of LaF3 nanocrystals doped with Eu 3+ : a) 0.05;b) 0.1; c) 0.5 and d) 1.0 mol%.. Hoang Manh Ha, Tran ThiQuynhHoa, Nguyen Ngoc Long, Le Van Vu, Phan Van Do 92 3.4. Judd-Ofelt analysis The Judd-Ofelt (JO) theory was shown to be useful to characterize radiative transitions for RE 3+ -doped solids, as well as aqueous solutions, and to estimate the intensities of the transitions for RE 3+ ions. This theory defines a set of three intensity parameters, Ωλ (λ = 2,4,6), that are sensitive to the environment of the rare-earth ions. Commonly, JO intensity parameters are usually derived from absorption spectrum. However, owing to the special energy level structure of Eu 3+ ion, these JO parameters could be estimated from the emission spectra. Four main emission peaks 5 D0→ 7 F1,2,3,4 are used to calculate JO parameters. The 5 D0→ 7 F1 is a MD transition and its spontaneous emission probability Amd is given by [2,8,9]: )12(3 64 334 Jh Sn A mdmd (2) where h is the Planck constant, is the wave number of the transition in interest, J is the total angular momentum of the excited state, and n is the refractive index. Smd is the MD line strength, which is a constant and independent from the host material. The value of Amd can be estimated using the reference value of A’md published somewhere, and using the relationship Amd = (n/n’) 3 A’md [2], where A’md and n’ are spontaneous emission probability and refractive index of the reference material, respectively. The 5 D0 → 7 F2,4,6 transitions are an ED partially allowed. The spontaneous emission probabilities Aed of ED transition is given using the following expression: 2 6,4,2 )( 2234 9 2 123 64 U nn Jh A Jed (3) where J is the wave number of transition 5 D0 → 7 FJ, e is the electron charge, 2 )(U are the squared doubly reduced matrix elements of the unit tensor operator of the rank λ = 2, 4, 6, and are calculated from intermediate coupling approximation for a transition ' 'J J . These reduced matrix elements did not nearly depend on host matrix as noticed from earlier studies. Thus the parameters could be evaluated simply by the ratio of the intensity of the 5 D0 7 FJ=2,4,6 transitions to the intensity of 5 D0 7 F1 transition as follows: )( )( 1 7 0 5 6,4,2 7 0 5 1 FDA FDA dI dI J 2 6,4,2 )( 223 11 2 9 2 U nn S e J md (4) For 5 D0 7 F2 transition, U (2) = 0.033; U (4) = U (6) = 0, 5 D0 7 F2 transition, U (2) = 0; U (4) = 0.023; U (6) = 0 and 5 D0 7 F2 transition,U (2) = U (4) , U (6) = 0.003. Using equation (4) and the reduced matrix elements, the JO parameters were calculated for different concentrations. The results are shown in Table 1 in comparison with different matrix. The Ωλ parameters are important to study the symmetry of local structure around RE 3+ ions and nature of RE–X (X = F, O) bonding. The Ω4 and Ω6 are related to the bulk properties such as viscosity and rigidity whereas the Ω2 is more sensitive to the local environment of the RE 3+ ions and is often related with the asymmetry of the local crystal field. The Ω2 and Ω6 parameters in LaF3 nanocrystal are smaller than those of other hosts. The smaller value of Ω2 can be attributed to higher symmetry of the ligand field and lower covalency in Eu 3+ -F - bond than other hosts, Optical properties of Eu 3+ ions in LaF3 nanocrystals 93 whereas the smaller of Ω6 parameter shows that the rigidity of the media in which rare earth ions is put into is higher in other hosts. In addition, with the low concentration of Eu 3+ in LaF3 nanocrystal, the values of Ω2 and R are almost independent of the concentration of Eu 3+ . That is, small changes in the concentration do not alter the asymmetry of the ligand as well as covalency in Eu 3+ -F - bond. Table 1. The Ωλ (×10 -20 cm 2 ) JO parameters for Eu 3+ doped various hosts. Host matrix R Ω2 Ω4 Ω6 Refs LaF3 (0,05 mol%) 0.68 1.08 0.73 0.66 Present LaF3 (0,10 mol%) 0.67 1.06 0.72 0.62 Present LaF3 (0,50 mol%) 0.68 1.07 0.70 0.60 Present LaF3 (1,0 mol%) 0.69 1.07 0.68 0.57 Present K2YF10 (crystal) - 1.22 1.26 8.55 [17] YAlO3 (crystal) - 2.66 6.33 0.80 [17] PbFBE (glass) 2.09 2.55 0.36 0 [13] L4BE (glass) 3.69 6.34 4.97 5.10 [16] B0TN (glass) 2.86 3.29 0.21 0 [14] B4TN (glass) 2.28 3.34 0.28 0 [14] 3.5. Radiative properties The JO parameters have been used to estimate the radiative properties such as the radiative transition rates (AR, s -1 ), branching ratios (βcal, %) and stimulated emission cross-section (σ(λP), 10 -22 cm 2 ) for 5 D0→ 7 FJ transitions and radiative lifetime (τR) of 5 D0 level of Eu 3+ in LaF3 by using Eqs. in Ref. [18]. In addition, the gain band width (σ(λP)×Δλeff, 10 -28 cm -3 ) and optical gain (σ(λP)×τR, 10 -25 cm 2 s -1 ) are also calculated for 5 D0→ 7 FJ transitions. The results are presented in Table 2. Table 2. The radiative properties of Eu 3+ ions doped (1.0 mol %) in LaF3. 5 D0→ 7 F0 7 F1 7 F2 7 F3 7 F4 7 F5 7 F6 AR 0 51 40 0 13.2 0 8.6 βcal 0 45.5 35.4 0 11.7 0 7.7 βmes 6.6 48.3 32 0.98 10.6 0.9 0.6 σ(λP) 0 5.6 4.2 0 2.0 0 1.6 σ(λP)×Δλeff 0 3.2 2.9 0 1.5 0 1.9 σ(λP)×τcal 0 49.7 35.5 0 17.7 0 14.2 The predicted branching ratio (βcal) of 5 D0 → 7 F1 transition get a maximum value and is 45.2 % whereas the measured ratio (βmes) is 48.3 %, thus there is a good agreement between experimental and calculated branching ratios. The branching ratio, gain band width and optical gain of this transition are larger than those of other transitions. These results suggest that the 5 D0 → 7F1 transition of Eu 3+ ions in LaF3 nanocrystal is found to be suitable for developing the fibre optics [19]. Hoang Manh Ha, Tran ThiQuynhHoa, Nguyen Ngoc Long, Le Van Vu, Phan Van Do 94 4. CONCLUSIONS LaF3:Eu 3+ nanocrystals were prepared by hydrothermal method. The XRD patterns indicate that LaF3 nanoparticles have been crystallized well. From the excitation spectrum, the PSB was found with the energy phonon about 290 nm, which related to the Eg vibration group in LaF3. The smaller value of g parameter than those in borotellurite glasses shows that the electron phonon coupling in LaF3:Eu 3+ nanocrystal is weaker than that of borotellurite glasses. The optical properties of Eu 3+ -doped LaF3 nanocrystals have been investigated. The small value of R and Ω2 parameter shows that the coordination structure surrounding the Eu 3+ ions has high symmetry and Eu 3+ -F - bond has low polarizability. The radiative parameters show that the 5 D5/2→ 7 F1 transition of Eu 3+ ions in LaF3 nanocrystals is very useful fiber optic amplifier. Acknowledgments. 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TÓM TẮT TÍNH CHẤT QUANG CỦA CÁC ION Eu3+ TRONG TINH THỂ NANO LaF3 Hoàng Mạnh Hà1, *, Trần Thị Quỳnh Hoa2, Lê Văn Vũ3, Nguyễn Ngọc Long3, Phan Văn Độ4 1 Đại học Kiến trúc Hà Nội, Km 10 Nguyễn Trãi, Thanh Xuân, Hà Nội 2 Đại học Xây dựng, 55 Giải Phóng, Hai Bà Trưng, Hà Nội 3 Đại học Khoa học Tự nhiên, 334 Nguyễn Trãi, Thanh Xuân, Hà Nội 4 Đại học Thủy lơi, 175 Tây Sơn, Đống Đa, Hà Nội * Email: hahm1982@gmail.com Các hạt nano LaF3 pha tạp ion Eu 3+ với các nồng độ khác nhau được chế tạo bằng phương pháp thủy nhiệt. Các phép đo XRD, huỳnh quang và kích thích huỳnh quang được thực hiện tại nhiệt độ phòng. Tỉ lệ giữa cường độ dải huỳnh quang 5D0→ 7 F2 (dipole điện) và 5 D0→ 7 F1 (dipole từ) (R) được sử dụng để đánh giá tính bất đối xứng của trường ligand xung quanh ion Eu3+. Các thông số Judd – Ofelt được tính toán từ phổ huỳnh quang đã được sử dụng để dự đoán các tính chất phát xạ của các chuyển dời 5D0→ 7 FJ cũng như thời gian sống của mức 5 D0. Giá trị nhỏ của thông số Ω2 và tỉ số R liên quan đến tính đối xứng cao của trường ligand xung quanh ion Eu 3+ trong vật liệu này. Từ khóa: tinh thể nano LaF3:Eu 3+, huỳnh quang, phân tích Judd-Ofelt.