Site selectively excited luminescence and energy transfer of X 1 -Y 2 SiO 5 :Eu at nanometric scale

JOURNAL OF APPLIED PHYSICS VOLUME 86, NUMBER 7 1 OCTOBER 1999 Site selectively excited luminescence and energy transfer of X 1 -Y 2 SiO 5 :Eu at nanometric scale M. Yin a) Groupe de Radiochimie, Institut de Physique Nucléaire, B.P. 1, 91406 Orsay Cedex, France C. Duan, W. Zhang, L. Lou, and S. Xia Department of Physics, University of Science and Technology of China, 230026 Hefei, People s Republic of China J.-C. Krupa Groupe de Radiochimie, Institut de Physique Nucléaire, B.P. 1, 91406 Orsay Cedex, France Received 2 November 1998; accepted for publication 30 June 1999 Two different types of structure called X 1 and X 2 are existing in Y 2 SiO 5 at normal conditions. In X 1 type, Y 3 ions occupy two sites where they are surrounded respectively by nine and seven oxygen ions, while in X 2 structure, only six and seven oxygen ions are involved. Nanometric scale X 1 -Y 2 SiO 5 crystals were prepared by sol-gel method with particle size around 50 nm. Site selective excitation at low temperature has shown four different luminescent centers. Two of them belong to Eu 3 embedded in X 1 -Y 2 SiO 5, the other two are attributed to Y 2 O 3 :Eu phase and to a particular site on the surface. The occurrence of the latter site may be related to the nanometric size of the sample. A pronounced excitation energy transfer from site 2 to site 1 was also observed on excitation spectra. The energy transfer rate increases rapidly with increasing Eu 3 concentration and for x 0.7 in Y 2 x SiO 5 :Eu x, no fluorescence takes place in site 2 at 15 K. The lifetimes of the 5 D 0 levels of Eu 3 in the two sites were measured as a function of Eu 3 concentration. The results have shown that the lifetime of the 5 D 0 level of Eu 3 in site 2 decreases with increasing Eu 3 concentration. The energy transfer rate dependence upon temperature was studied in detail and compared to a theoretical simulation. 1999 American Institute of Physics. S0021-8979 99 06319-7 I. INTRODUCTION Recently, Shen and Kachru introduced a new process for parallel data storage using coherent time-domain optical memory and proposed Y 2 SiO 5 :Eu for practical developments. 1 The usual preparation method for Y 2 SiO 5 :Eu in powder form is the direct solid state reaction between rare-earth oxides and amorphous silicon dioxide 1:1 molar in air. Depending upon the synthetic temperature, Y 2 SiO 5 can crystallize in the two different structures: X 1 type, space group P2 1 /c and X 2 type, space group B2/b. 2,3 In both structures, there are two crystallographically nonequivalent RE RE rare-earth sites of the low symmetry C 1, which differ in coordination to oxygen: 7 and 9 for X 1 type; 6 and 7 for X 2 type. 4 The study of the luminescent properties on Y 2 SiO 5 :Eu was almost restricted to X 2 phase and there are only very few spectroscopic results on X 1 -Y 2 SiO 5 :Eu. This is essentially due to the fact that the pure X 1 phase using the above-mentioned preparation method is difficult to obtain. In this article, we present site selectively excited luminescence and energy transfer in nanometric scale X 1 -Y 2 SiO 5 :Eu prepared by sol-gel technique. The origins of the four different luminescent centers and the mechanism of energy transfer between the two sites are discussed. a On leave from Department of Physics, University of Science and Technology of China, 230026 Hefei, People s Republic of China. II. EXPERIMENT The X 1 -Y 2 x SiO 5 :Eu x with x 0.01, 0.1, 0.2, 0.4, and 0.7 were synthesized as powders by sol-gel method. The starting materials Y 2 O 3 and Eu 2 O 3 were dissolved in diluted HNO 3, and the precursor Si OCH 3 4,inC 2 H 5 OH. The mixing of both solutions gives rise to a gel which, after drying and heating sequentially for several hours at 70 80 and at 1100 C during 3 h, gives a white crystallized powder. X-ray experiments Cu K radiation showed that the samples have crystallized in the X 1 type of Y 2 SiO 5 :Eu. The average particle size measured by transmission electron microscope TEM is around 50 nm. This nanometric size can induce some mesoscopic-related features. Fluorescence and excitation spectra were recorded at 10 300 K and analyzed by a 1 m Jobin-Yvon monochromator with a dispersion of 0.8 nm/mm. The samples were excited with either radiation from a Hg lamp or pulsed radiation pulse duration 10 ns at a repetition rate of 30 Hz from a rhodamine 6 G dye laser pumped by the second harmonic of a YAG:Nd laser 20 W Quantel. The photons were detected by a R374 Hamamatsu photomultiplier and the output signals were fed into a Stanford SR510 lock-in amplifier. The whole experimental setup was controlled by a personal computer. The luminescence decay curves and the fluorescent lifetime of the 5 D 0 level were measured by using a Lecroy 9410 oscilloscope interfaced with a computer. 0021-8979/99/86(7)/3751/7/$15.00 3751 1999 American Institute of Physics

3752 J. Appl. Phys., Vol. 86, No. 7, 1 October 1999 Yin et al. FIG. 1. Emission spectrum of X 1 -Y 2 SiO 5 :Eu 0.01 under 254 nm excitation at room temperature. III. RESULTS A. Site selective luminescence The X 1 -Y 2 SiO 5 :Eu sample exhibits an intense orangered fluorescence under 254 nm UV excitation using a Hg lamp. The five groups of sharp lines between 565 and 725 nm Fig. 1 can easily be attributed to 5 D 0 7 F 0 4 transitions in Eu 3 ions. Owing to the absence of an inversion center in the RE 3 site symmetries, strong electric dipole ED transitions 5 D 0 7 F 2,4 are recorded around 620 and 700 nm with the strongest emission peaking at 614.7 nm. The parity allowed 5 D 0 7 F 1 transitions induced by magnetic dipole MD interactions, located around 590 nm, have intensities almost equal to those of electric dipole transitions and the strongest peak is located at 587.4 nm. The strictly forbidden 5 D 0 7 F 0 transition also has a significant intensity. The strength of this transition depends on the amount of mixing with the other 7 F JM wave functions. The weak 5 D 0 7 F 3 transitions originate from a mixture of 7 F 1,2,4 wave functions with the 7 F 3 one, leading to a mixed electricmagnetic dipole character for this transition. The nondegenerate 5 D 0 7 F 0 transition is usually used to deduce the number of RE sites in luminescent crystalline samples. Three sharp 5 D 0 7 F 0 transitions located at 577.7, 579.7, and 580.9 nm, respectively, appear in Fig. 1. They indicate the presence of at least three Eu 3 sites of low symmetry. Site selectively excited luminescence spectra recorded under pumping at 579.7, 577.7, and 580.9 nm are shown in Fig. 2. Figure 2 c is known as the typical emission spectrum of Y 2 O 3 :Eu, 5 therefore, the 580.9 nm peak is assigned as 5 D 0 7 F 0 transition in Eu 3 embedded in Y 2 O 3. This means that, a small amount of Y 2 O 3 :Eu phase still remains in the sample, showing that the phase mixing should be always considered for powder samples. The two other peaks of 577.7 and 579.7 nm are corresponding to 5 D 0 7 F 0 transition in Eu 3 embedded in the two sites of X 1 -Y 2 SiO 5. For convenience, lines at 579.7 and 577.7 nm are referred as sites 1 and 2, respectively. The photoluminescence PL spectra of Eu 3 in site 1 and site 2 are shown in Figs. 2 a and 2 b. The wavelength values of 5 D 0 7 F 0 4 transitions for these two sites are listed in Table I. FIG. 2. Emission spectra of X 1 -Y 2 SiO 5 :Eu 0.01 obtained at 15 K under excitation at a 579.7 nm, b 577.7 nm, c 580.9 nm, and d 578.7 nm. Eu 3 is a non-kramers ion and its electronic J-level degeneracy can be completely removed by a low symmetry crystal field interaction. Thus 2J 1 emission lines are expected for the transitions 5 D 0 7 F J. In some cases, the total number of lines exceeds the maximum 2J 1 see Table I, indicating the possibility of structural disorders due to, for example, different orientations of the SiO 4 grouping. 6,7 In our samples, the complete removing of the degeneracy is consistent with the low C 1 point symmetry for the sites occupied by the Eu 3 ions. B. Excitation spectra and nature of the fourth luminescent center Figure 3 shows the excitation spectra of emission lines A and B in Fig. 2 which correspond to the strongest emission peaks of site 1 and site 2, respectively. As seen before, three of the four peaks in Fig. 3 located at 577.7, 579.7, and 580.9 nm belong to Eu 3 in site 2, site 1, and in Y 2 O 3 phase, respectively. The fourth peak located at 578.7 nm, called the fourth center, gives a different emission spectrum shown on Fig. 2 d. In order to understand the origin of the fourth center, the luminescence dependence upon concentration was studied. The PL spectra of X 1 -Y 2 x SiO 5 :Eu x for different Eu 3 concentration (x 0.1, 0.2, 0.4, and 0.7 at room temperature Fig. 4 show that there is no noticeable difference among the spectra for different concentrations. The strongest peak is always located at 614.7 nm and corresponds to the 5 D 0 7 F 2 transition of Eu 3 in site 2. This behavior sug-

J. Appl. Phys., Vol. 86, No. 7, 1 October 1999 Yin et al. 3753 TABLE I. Wavelengths of the 5 D 0 7 F 0 4 transitions observed in site selective excited luminescence in X 1 Y 2 SiO 5 :Eu 0.01 at 15 K. Transitions Site 1 nm Site 2 nm 5 D 0 7 F 0 579.7 577.7 5 D 0 7 F 1 587.4 583.5 590.1 587.9 591.9 592.9 598.7 600.4 603.5 602.1 5 D 0 7 F 2 611.5 605.3 612.6 606.3 615.9 614.7 619.4 621.4 628.5 624.8 632.7 5 D 0 7 F 3 650.1 642.7 650.8 643.8 654.8 659.8 664.0 5 D 0 7 F 4 687.8 682.9 688.6 692.1 693.9 695.4 698.1 699.0 701.7 704.1 706.4 706.0 709.2 709.5 712.8 FIG. 3. Excitation spectra of a line A 587.4 nm and b B 614.7 nm in Fig.2at15K. gests that the fourth center has no relation with Eu 3 embedded in the bulk of the grain but should be related to Eu 3 located in surface sites. C. Energy transfer between the two sites The luminescence spectrum of X 1 -Y 2 x SiO 5 :Eu x at low temperature 15 K changes rapidly with Eu 3 concentration Fig. 4 b. For x 0.1, the emission spectrum remains nearly the same as at room temperature and the strongest emission peak belonging to site 2 is located at 614.7 nm. But for x 0.2, the strongest band is peaking at 592.9 nm and corresponds to 5 D 0 7 F 1 transitions attributed to Eu 3 in site 1 see Table I. At the same time, the 5 D 0 7 F 4 transitions become stronger as the concentration increases. In addition, the fluorescence of Eu 3 in site 1 becomes more and more dominant and for x 0.7 sample, all the peaks characteristic of site 2 have totally disappeared. These results show that under UV excitation, at low temperature only the fluorescence of site 1 occurs for high Eu 3 concentration. The site selectively excited luminescence of Eu 3 in site 1 and site 2 at low temperature are shown in Fig. 5. Excitation in site 1 gives almost the same emission spectrum for the different Eu 3 concentrations Fig. 5 a, but when exciting Eu 3 in site 2, different results were obtained Fig. 5 b. For x 0.1, all the strong Eu 3 emission lines belong to site 2 and some weak peaks can be attributed to site 1. When Eu 3 concentration increases, the emission peaks attributed to site 1 become more and more intense and for x 0.4, these emissions have their intensity equal to or even larger than those of site 2. For x 0.7, almost no lines from site 2 are recorded. The above results show that at low temperature and high concentration, only fluorescence from site 1 can be recorded, no matter whether excitation occurs directly either in Eu 3 in site 1 or site 2 or even through pumping in the charge transfer O(2p) Eu 3 (4f ) state at 254 nm. This special behavior indicates that there is a very fast energy transfer process from site 2 to site 1 when the Eu 3 concentration increases. When the fourth center is excited selectively, a similar behavior is recorded Fig. 6. From this observation, together with the conclusion drawn in Sec. B, it is reasonable to attribute the fourth luminescent center to Eu 3 ions in sites located at the surface of the nanocrystals. The ratio: surface area versus bulk volume, increases when the size of the crystalline grains decreases. The most straightforward and efficient method to investigate excitation transfer is to measure the intensity decay of the related emissions and determine the lifetimes of the corresponding radiative levels. Some results are given in Fig. 7 where curves a and b are the decay profiles of the 5 D 0 7 F 1 emission of Eu 3 in site 1 after Eu 3 excitation in site 1 and site 2 for x 0.2 sample. When site 1 is excited only, the intensity decay is a single exponential with 5 D 0 lifetime equal to 2.90 ms, while the excitation in site 2 induces a rising stage before the decay, showing clearly an excitation transfer from site 2 1.

3754 J. Appl. Phys., Vol. 86, No. 7, 1 October 1999 Yin et al. FIG. 4. Emission spectra of X 1 -Y 2 SiO 5 :Eu x with x 0.1, 0.2, 0.4, and 0.7 under 254 nm excitation at a RT and b 15 K. FIG. 5. Emission spectra of X 1 -Y 2 SiO 5 :Eu x with x 0.1, 0.2, 0.4, and 0.7 obtained at 15 K by exciting a site 1 and b site 2. IV. DISCUSSION A. Energy transfer rate dependence upon concentration As expected, the energy transfer between Eu 3 ions embedded in the two sites of X 1 -Y 2 SiO 5 becomes more efficient for higher Eu 3 concentration. It results in a stronger interaction and a larger transfer rate, 8 and as a consequence, the lifetime of the levels attributed to site 2 decreases with increasing Eu 3 concentration, as can be noticed in Table II.

J. Appl. Phys., Vol. 86, No. 7, 1 October 1999 Yin et al. 3755 FIG. 6. Emission spectra of X 1 -Y 2 SiO 5 :Eu x with x 0.1, 0.2, 0.4, and 0.7 obtained at 15 K by exciting at 578.7 nm. FIG. 7. Decay curves for the 5 D 0 7 F 1 transition intensity of site 1 at 15 K by exciting a site 1 and b site 2. The lifetime of the 5 D 0 level of Eu 3 in site 1 decreases slightly with increasing Eu 3 concentration, while that belonging to site 2 decreases noticeably, from 2.18 ms for x 0.01 to 1.34 ms for x 0.4. Moreover, when Eu 3 in site 2 is excited selectively, for the more diluted sample (x 0.01), only the fluorescence from site 2 is observed Fig. 2 b, while for a higher concentration, for example, x 0.2, luminescence from both sites is recorded Fig. 5 b. B. Energy transfer rate dependence upon temperature The above experimental results Figs. 4, 5, and 7 tell us that the energy transfer rate between Eu 3 in site 2 and 1 is depending on the temperature. Figure 8 shows more straightforward the effect of the temperature on emission intensity of Eu 3 transitions in site 2 for the sample x 0.4. In the following discussion, we will limit our analysis x 0.4 which corresponds to a large Eu 3 concentration. In this case, the resonant energy migration among the same site is characterized by a transfer rate D. 9,10 The temperature dependence can be modelized by using rate equations. Let us consider the rate P i for Eu 3 ions in site i(i 1,2), we have dp 1 dt 1 P 1 x 1 2 P 1 x 2 1 P 2 g 1, 1 dp 2 dt 2 P 2 x 2 1 P 2 x 1 2 P 1 g 2, 2 where 1, 2 are the decaying rates of site 1 and site 2, respectively, g 1,g 2 are their populating rate, and x (1 2) is the average transfer rate from site 1 2. Under continuous excitation, dp 1 /dt dp 2 /dt 0 and Eqs. 1 and 2 can be solved as P 1 x 2 1 g 1 g 2 2 g 1 1 2 2 x 1 2 1 x 2 1, P 2 x 1 2 g 1 g 2 1 g 2 1 2 2 x 1 2 1 x 2 1. Setting g 2 /(g 1 g 2 ) gives TABLE II. Lifetimes of the 5 D 0 level of the two sites in X 1 -Y 2 SiO 5 :Eu x at 15 K. x Lifetime ms Site 1 Site 2 0.01 3.00 2.18 0.1 2.94 1.82 0.2 2.90 1.59 0.4 2.80 1.34 0.7 2.80 3 4

3756 J. Appl. Phys., Vol. 86, No. 7, 1 October 1999 Yin et al. FIG. 8. Emission spectra of X 1 -Y 2 SiO 5 :Eu 0.4 under 254 nm excitation at different temperatures. FIG. 9. Emission intensity ratio of sites 2 and 1 at different temperatures. Dot: experimental result; line: fitting result (A 250 S 1, C 2.0). P 2 x 1 2 1 P 1 x 2 1 1 2 An E 12 1. A 1 n E 12 1 2 In the last step, the emitting level in site 2 is located higher in energy than that of site 1 with an energy difference E 12 59 cm 1 and therefore J 2 f g 2 E 12 x 2 1 h/2 4 s 5 s s n E 12 1 A n E 12 1, x 1 2 An E12 can be used. 11 Here, J is the matrix element of the site-site coupling Hamiltonian, f and g are the ground-state and excited-state coupling strength for the ion at the site, is the mass density, s is of order of unity and n( E 12 ) exp( E 12 /kt) 1 1. When the x 0.4 sample is excited by 254 nm or other high energy radiation, 1/2 but for direct selective excitation of Eu 3 in site 2, 1. For the x 0.01 sample, where no energy transfer can be involved in the relaxation stage, 1 and 2 are the reciprocal of lifetimes (1/ ) of 5 D 0 level of Eu 3 in site 1 and site 2. 5 6 7 Because of the emission intensity is the product of the transition rate by the number of ions in excited state, an other parameter related to the ratio of the decaying rate in the two sites must be introduced for the simulation of the experimental results reported in Fig. 8. Using A and C as these parameters for the two sites, the ratio between the emission intensities in the two sites was fitted by the following equation: R I 2 C P 2. 8 I 1 P 1 As shown in Fig. 9, the calculated rate is in good agreement with the experimental behavior, with parameter values: C 2.0 and A 250 S 1. The A value is lower but of the same order than those obtained by Buijs et al. in (Gd 0.95 Eu 0.05 2 O 3 (1800 S 1, where Eu 3 (S 6 ) Eu 3 (C 2 ) energy transfer study included superexchange interactions between Eu 3 in two different sites. 12 The difference may be accounted for several factors, among them, one is E 12 which is proportional to A. E 12 is larger in (Gd 0.95 Eu 0.05 2 O 3 than in X 1 -Y 2 SiO 5 :Eu. Further comparisons are difficult because no data are available. The energy transfer rate for site 2 1 at room temperature in X 1 -Y 2 0.4 SiO 5 :Eu 0.4 can be obtained by using the fitting equation: x (2 1) A(n 1) 1000 S 1. From the value of C, one can estimate the ratio of Eu 3 ions embedded in both sites. The parameter C can be expressed as C ( 2 / 1 ) (q 2 /q 1 ), where q 2 and q 1 are the occupation numbers of site 2 and site 1, respectively. In our

J. Appl. Phys., Vol. 86, No. 7, 1 October 1999 Yin et al. 3757 sample q 2 /q 1 1.45, indicating that Y 3 ions in site 2 are more substituted than in site 1, although we can estimate from the crystallographic data that both sites are equally distributed in X 1 -Y 2 SiO 5. This behavior can be explained by a larger Y 3 -O 2 average distance d in site 2 compared with site 1 Ref. 13 allowing more flexibility in the substitution. There is one more point related to d that we would like to discuss. In X 1 -Y 2 SiO 5, sites 1 are surrounded by seven oxygen ions, and site 2 by nine oxygen ions. 14 Although the two sites have the same C 1 symmetry, site 2 has a relatively lower symmetry. This at least can be seen by comparing d max d d, which is larger for site 2. 13 The lower symmetry produces a larger wave function mixing and accordingly a larger ED/MD transition intensity as shown in Fig. 2, as well as a shorter lifetime see Table II. V. CONCLUSION Using site selective excitation at low temperature, four luminescent centers in X 1 -Y 2 SiO 5 :Eu crystalline powders with particle size around 50 nm were assigned. Two of them belong to Eu 3 sites in X 1 -Y 2 SiO 5, one is connected to Eu 3 in Y 2 O 3 :Eu phase and the last one to Eu 3 sites in surface layers. The energy transfer rate dependence upon temperature and concentration were studied in detail for the two sites and simulated theoretically. This energy transfer rate from site 2 to site 1 at room temperature is equal to 1000 S 1 and the ratio of Eu 3 ions occupying site 2/site 1 were deduced equal to 1.45. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China, the Returned Persons Foundation of the State Education Committee of China, and the Chinese Academy of Sciences. 1 X. A. Shen and R. Kachru, J. Alloys Compd. 250, 435 1997. 2 JCPDS ASTM File No. 41-4. 3 JCPDS ASTM File No. 21-1458. 4 D. Meiss, W. Wischert, and S. Kemmler-Sack, Phys. Status Solidi A 133, 575 1992. 5 R. B. Hunt, Jr. and R. G. Pappalardo, J. Lumin. 34, 133 1985. 6 O. K. Moune-Minn and P. Caro, J. Cryst. Spectr. Res. 12, 157 1982. 7 M. Buijs and G. Blasse, J. Lumin. 34, 263 1986. 8 D. L. Huber, in Laser Spectroscopy of Solids, edited by W. M. Yen and P. M. Selzer, Topics in Applied Physics Springer, Berlin, Heidelberg, New York, 1981, Vol. 49, p. 103. 9 D. L. Dexter, J. Chem. Phys. 21, 836 1953. 10 T. Forster, Ann. Phys. Paris 2, 55 1948. 11 T. Holstein, S. K. Lyo, and R. Orbach, in Disordered Systems, in Laser Spectroscopy of Solids, edited by W. M. Yen and P. M. Selzer, Topics in Applied Physics Springer, Berlin, Heidelberg, New York, 1981, Vol. 49, p. 47. 12 M. Buijs, A. Meyerink, and G. Blasse, J. Lumin. 37, 9 1987. 13 J. Felsche, Struct. Bonding Berlin 13, 180 1973. 14 C. Duan, S. Xia, W. Zhang, M. Yin, and J.-C. Krupa, J. Alloys Compd. 275 277, 450 1998.