ОПТИКА И СПЕКТРОСКОПИЯ, 2014, том 116, № 1, с. 68-73
СПЕКТРОСКОПИЯ ^^^^^^^^^^
КОНДЕНСИРОВАННОГО СОСТОЯНИЯ
УДК 535.37
THE FLUORESCENCE PROPERTIES OF Yb3+ AND Er3+ CO-DOPED YAl3(BO3)4 POWDERS PREPARED BY SOL-GEL METHOD
© 2014 г. Weixiong You, Fengqin Lai, Xiaolin Liu, Honghui Jiang, Jinsheng Liao, Ping Wang, Bin Yang
School of Material Science and Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, PR China
E-mail: you_wx@126.com Received February 25, 2013
Yb3+ and Er3+ co-doped YAB powders were prepared by sol-gel method. The structure and fluorescence properties were investigated. XRD pattern indicated that the single phase was obtained at 1150°C and the structure belonged to rhombohedral. Under 379 nm excitation, two emissions around 983 nm and 1531 nm were observed and the effect of Yb3+ ion concentration on the emission intensity was discussed. The energy transfer was observed under 930 nm excitation and the energy transfer efficiencies for all samples were calculated. The lifetimes of 2F5//2 level of Yb3+ ion and 4/i3/2 level of Er3+ ion were measured and the effect of Yb3+ ion concentration on the lifetime was also discussed. The results indicated that there was an additional mechanism for the decay of 4/i3/2 level in powder samples. The Yb3+ and Er3+ co-doped YAB powders should be a potential candidate for ceramic laser materials. DOI: 10.7868/S0030403414010231
INTRODUCTION
YAl3(BO3)4 (YAB) presents a rhombohedral structure and belongs to space group R32. In YAB crystal structure, Y atoms on the corner of unit cell coordinate with six O atoms and form YO6 octahedra with a local symmetry of D3. Al atoms occupy another set of octahedral sites. Boron atoms are arranged in sheets of planar trigonal BO3 atomic groups [1]. The Y3+ ion occupies prismatic site and can be easily replaced by other trivalent rare earth ions [2]. YAB possesses large nonlinear coefficient, good mechanism strength and high thermal conductivity [3—5] and has been demonstrated as an excellent material for the laser application [6].
However, the YAB single crystal material has only been grown by flux method, which is very complicated and consumes a long time. In addition, the segregation is observed when yttrium atoms are substituted by a high concentration of rare earth ions [7]. Due to the high cost and the difficulty to prepare large crystals, the polycrystalline ceramic laser materials have attracted much attention because a ceramic material compared to a single crystal has several advantages such as ease of fabrication, less expensive, fabrication of large size and high additive concentration [8]. Furthermore, the highly efficient laser output can be obtained in ceramic laser materials, whose efficiencies are comparable or superior to those of single crystals [9, 10].
In this paper, the Yb3+ and Er3+ co-doped YAB polycrystalline powders are prepared by the sol-gel method. The fluorescence properties as well as mech-
anism for emissions are investigated and the effect of Yb3+ concentration on the fluorescence properties is discussed.
EXPERIMENT
The YAB powders were prepared by sol-gel method. Y2O3 (99.99%), Yb2O3 (99.99%), Er2O3 (99.99%), H3BO3 (AR) and Al(NO3)3 • 9H2O (AR) were used as starting materials. According to the formula (YbxEryYj _x_y)Al3(BO3)4 (y = 0.02 and x = 0.04, 0.06, 0.08, 0.1), stoichiometric amounts of starting materials, except for H3BO3, were dissolved in nitric acid and then citric acid was added into the above the solution as the chelating agent. The molar ratio of metal ions and citric acid was 1 : 2. Because of the evaporation behavior of H3BO3, 30% excess of H3BO3 was joined. The mixture was stirred at about 80°C for 4 h until a light yellow sol was formed. The sol was transformed into sticky gel by evaporating the sol for several hours. The gel was dried at about 130°C in air. After being fully grounded, the dried powders were sintered at 1150°C in a muffle furnace in air atmosphere for about 10h and white powder samples were obtained. In brief, the four samples were marked with #1, #2, #3 and #4, respectively.
X-ray diffraction patterns of the samples in the range of 10° < 29 < 90° were recorded on PANalytical X'Pert Pro X-ray diffractometer with CuZ"a = = 0.15406 nm. The emission spectra within 800— 1700 nm were detected using spectrophotometer (FL920, Edinburgh) when the exciting wavelength were 379 nm and 930 nm, corresponding to the 4/15/2 ^ 4^11/2
12000 12000 8000 4000 0
12000
8000
4000
* 0 * 12000
£ 8000
s 4000
u
13 0 ~ 12000
8000
4000
0
80
40
: 1 1 L ■ #4
: j . , » #3 1_1 A »1 " Vc • - ^ -
#2
, 1 „ 1_1 A >1 «■> >*_...-, ■- — - -
1, 1 ,1 PDF#72-1978 Il 1 il 1 II It J. .... 1 1 1 - L
10 20 30 40
50
60
70
80 90 29, deg
Fig. 1. The X-ray diffraction patterns of the (YbxEr0 02Y0 98 _ x)Al3(BO3)4 samples.
transition of Er3+ ion and 2F7/2 ^ 2F5/2 transition of Yb3+ ion, respectively. The fluorescence decay curves at the wavelength of983 nm and 1531 nm were also detected using spectrophotometer (FL920, Edinburgh) with xenon lamp as the pump source and the excitation wavelength was 930 nm. The signal was detected with a NIR PMT (R5509, Hamamatsu). All measurements were carried out at room temperature and the spectra had been corrected according to the response curves of instrument.
RESULTS AND DISCUSSION
Figure 1 shows the X-ray diffraction patterns of the four samples. It can be seen from the figures that the diffraction peaks of YAB presented are in agreement with that of the standard JCPDS card (PDF#72-1978, also shown in Fig. 1). No diffraction peaks from other phase were observed in the XRD patterns, indicating that pure single-phase YAB can be obtained at 1150°C, which is different with the results obtained by Maia [7]. The reason may be due to the different thermal treating processes in our work because the process of solid-state synthesis ofYAB is very complicated and sensitive to temperature and time of calcinations [11]. Furthermore, the peaks do not shift in all samples, which mean that Yb3+ and Er3+ ion doped into the lattice do not cause any crystal structure distortion.
Figure 2 shows the emission spectra of YAB powders in the range of 800—1700 nm excited at 379 nm, corresponding to 4I15/2 ^ 4GU/2 transition of Er3+ ion. There are two emission bands peaking at 983 nm and 1531 nm, respectively. The 983 nm band is attributed
to the transitions of 2F5/2 ^ 2F1/2 of Yb3+ ion and the 4^ii/2 ^ 4^i5/2 ofEr3+ ion, while the 1531 nm band is at-
tributed to the 4I13/2 ^ 4I15/2 transitions of Er3+ ion. When absorbing the pump energy, the Er3+ ion can be excited to the 4GU/2 level. The Er3+ ion on this level will decay nonradiatively to the lower 4F7/2, 2H11/2, 4^3/2, 4F9/2 and 4I11/2 levels, as shown in Fig. 3. The Er3+ ion on 4I11/2 level will relax to the ground 4I15/2 level and 983 nm emission can be derived. In addition, the 2F5/2 level of Yb3+ can be populated by the cross-relaxation (CR) process between Yb3+ and Er3+ ions: 4F7/2 (Er) + + 2F7/2 (Yb) ^ ^I 11/2 (Er) + F5/2 (Yb) [12]. The Yb3+ ion on 2F5/2 level will relax back to 2F7/2 level and 983 nm emission occurs. Therefore, the 983 nm band is attributed to the transitions of 2F5/2 ^ 2F7/2 of Yb3+ ion and 4I11/2 ^ 4I15/2 transition of Er3+ ion. On the other hand, the Er3+ ion on 4I11/2 level will rapidly decay either radiatively or non-radiatively to the lower 4I13/2 level [13] and emissions around 1531 nm can be observed due to the 4I13/2 ^ 4I15/2 radiative transition. This channel is dominant because of high phonon energy of YAB crystal [14], so the emission intensities around 1531 nm are stronger than those around 983 nm, as shown in Fig. 2. For the 983 nm emissions, the intensities increase with the increasing of Yb3+ concentration and then decrease when the Yb3+ concentration exceeds 8at%. The reason may be due to the competitive processes between CR process and energy transfer (ET) process (2F5/2(Yb) + 4I15/2(Er) ^ ^ 2F7/2(Yb) + 4I11/2(Er)). With the increase of Yb3+ concentration, the distance between Yb3+ and Er3+
12000
8000
4000
0
12000
8000
3
a. 4000
0
n te 8000
Int
4000
0
8000
4000
0
" #4 ■ ...../V
" A #3 - A. ik
#2 ■ . vu
- \ #! , . —J Nv ., .
800
1000
Fig. 2. The emission spectra of different Yb3+ concentration doped samples within 800—1700 nm at 379 nm excitation.
1200
1400 1600
Wavelength, nm
379 nm
1531 nm
4G
11/2
4F-
2H
7/2
4S
4f<
11/2 3/2
9/2
ET
983 nm
4I
2F
5/2
15/2 -
Er3+ Yb3+
Fig. 3. Schematic energy level diagram of Yb3+ and Er3+ ions.
2F
7/2
will be closer and the CR probability improved, which making the population on the 2F7/2 level of Yb3+ and 4I11/2 level of Er3+ increased. Therefore, the emission intensities around 983 nm increase with the increase of Yb3+ concentration. However, when the Yb3+ concentration exceeds 8at%, the energy transfer from 2F5/2 level of Yb3+ to 4I11/2 level of Er3+ becomes very efficient [15]. As mentioned above, the Er3+ ion on 4I11/2 level will rapidly decay to the lower 4I13/2 level. So the net population on the 2F5,2 and 4I11/2 levels decreases.
Consequently, the emission intensities around 983 nm increase and then decrease with the increasing of Yb3+
concentration, while those around 1531 nm always increase with the increasing of Yb3+ concentration, as shown in Fig. 2.
For Er3+ doped materials, 1550 nm emission from 4I13/2 ^ 4I15/2 transition has attracted much attention because of its potential use in optical communication system [16, 17]. Yb3+ is always co-doped as a sensitizer in the Er3+ doped materials due to the high absorption cross-section around 980 nm, the pump wavelength used in practical systems. When the pump energy absorbed by Yb3+ ion, the energy can be transferred to the Er3+ through resonant energy transfer (ET) process mentioned above.
4000 2000 0
4000
2000 0
4000 2000
2000
#4
* ■*L,H I liMfcf_T*^
#3
#2
#1
1400
15
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.