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Lanthanum Silicate and Lanthanum Zirconate Nanoparticles Co-Doped with Ho3+ and Yb3+: Matrix-Dependent Red and Green Upconversion Emissions Neralagatta M. Sangeetha and Frank C. J. M. van Veggel* Department of Chemistry, UniVersity of Victoria, P.O. Box 3065, Victoria, British Columbia, Canada V8W 3V6 ReceiVed: May 14, 2009; ReVised Manuscript ReceiVed: June 23, 2009
Excitation of Ho3+ and Yb3+ co-doped lanthanum silicate and lanthanum zirconate nanoparticles with 980 nm diode laser light gave a red and green glow, respectively, observable by naked eye. Spectroscopic investigations on these materials revealed green (540 nm), red (660 nm), and near-infrared (750 nm) upconversion emissions with the green to red ratio varying with the matrix type and the dopant ion (Yb3+) concentration. The emission was predominantly red for Ho3+:Yb3+ (1:3) in lanthanum silicate nanoparticles, while it was predominantly green for Ho3+:Yb3+ (1:7) in lanthanum zirconate nanoparticles. A mechanism involving cross-relaxations and energy back transfer has been proposed to explain the observed behavior. The predominance of red emission has been attributed to a strong quenching of Ho3+ green emitting level mainly by energy back transfer from Ho3+ to Yb3+ on the basis of the near-infrared (NIR) emission spectral analysis. Introduction Upconversion is a phenomenon that converts two or more low-energy photons to a higher energy photon and occurs in materials that are less susceptible to multiphonon relaxation of the excited states, such as the lanthanides doped in low-phonon energy matrixes. Lanthanide ions involve intra-4f transitions which are barely affected by external influence because of the shielding of the 4f orbitals by the filled outer 5s and 5p orbitals. When these ions are placed in a low-phonon energy environment, multiple long-lived metastable excited states result, which are essential for the upconversion process because the quenching of these excited states by multiphonon relaxation is reduced considerably. Upconversion can occur in several ways:1 (1) ground-state absorption followed by excited-state absorption, (2) sequential energy transfers, (3) a combination of the former and the latter, (4) photon avalanche,2 and (5) cooperative upconversion.3 Upconversion from lanthanide-doped materials is increasingly studied these days owing to their application potential in solidstate lasers, color displays, optical data storage, biolabeling, and other applications.4 Of the various lanthanides studied for their upconversion emissions, Ho3+ has attracted considerable attention because lasing action has been demonstrated for the green upconverted emission of Ho3+ doped materials.5 Ho3+, Yb3+ codoped materials are particularly interesting in view of the availability of cheap pump source (InGaAs diode laser) for Yb3+ excitation. Consequently, mechanistic aspects of Ho3+ upconversion in Ho3+, Yb3+ codoped systems are being studied extensively.6,7 In general, Ho3+ upconversion yields emissions in the blue, green, red, and near-infrared (NIR) corresponding to 5F2(5F3) f 5I8, 5S2(5F4) f 5I8, 5F5 f 5I8, and 5S2(5F4) f 5I7 transitions, and of these, green and red are readily observed. A predominantly green upconversion luminescence has been observed for Ho3+ when doped in many of the low-phonon energy inorganic hosts,8 several of which are fluoride glasses.9 Instances of a predominant red upconversion emission are fewer in number.10 * To whom correspondence should be addressed. E-mail:
[email protected].
During the course of spectroscopic investigations on Ho3+, Yb3+ codoped lanthanum silicate and lanthanum zirconate nanoparticles for near-infrared emissions,11 we observed a red glow from the former and a green glow for the latter nanoparticles under 980 nm laser excitation. Visible spectra recorded on these materials revealed that the lanthanum silicate particles with Ho3+:Yb3+ (1:3) yield a dominant red emission peak while lanthanum zirconate particles Ho3+:Yb3+ (1:7) yield a dominant green emission. Evidently, the ratio of red to green emissions could be tuned by varying the Yb3+ concentration. These interesting observations called for a detailed investigation on these systems. Results pertaining to this investigation along with the possible mechanism that resulted in the observed behavior are entailed in this report. Experimental Section Synthesis. Synthesis of all the nanoparticle samples used for the current study were prepared and characterized according to procedures described in a full paper submitted for publication.11 Luminescence Studies. All photoluminescence spectra were recorded with an Edinburgh Instruments FLS 920 luminescence spectrometer equipped with a Peltier-cooled Hamamatsu R955 photomultiplier tube (PMT) for photon detection in the visible and with a nitrogen-gas-cooled Hamamatsu R5509 NIR PMT for photon detection in the near-infrared. A 10 Hz Q-Switched Quantel Brilliant in which the third harmonic of the Nd:YAG laser pumps the optical parametric oscillator (OPO) with a tunable optical range from 410 to 2400 nm (full width at halfmaximum (fwhm) ) 5 ns) was used as the excitation source for all fluorescence spectra, and continuous wave 980 nm laser diode was used as the pump source for all upconversion emission spectra. All spectra were recorded with 1 nm resolution and were corrected for the instrument response. Samples suitable for luminescence analysis were prepared by mixing 1 part of sample with 10 parts of KBr and were pressed into translucent discs. The disc was placed in a solid sample holder, and the excitation light was shone at an angle of 45° with respect to the surface normal. The emission from the sample was collected from the rear side of the disc to minimize the amount of scattered
10.1021/jp904516s CCC: $40.75 2009 American Chemical Society Published on Web 07/15/2009
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TABLE 1: Lifetimes of Some Important Emissions Obtained from the Upconverting Nanoparticlesa emission lifetimes in µs
La3+:Ho3+:Yb3+ (relative atom % from EDX)
Ho :Yb ratio
Lanthanum Silicate sample 1 sample 2 sample 3 sample 4
98.2:1.8:0 97.0:1.5:1.5 90.0:2.5:7.5 66.0:4.0:30.0
1:0 1:1 1:3 1:7.5
Lanthanum Zirconate sample 5 sample 6 sample 7 sample 8
98.2:1.8:0 98.0:1.0:1.0 92.5:2.5:5.0 84.0:2.0:14.0
1:0 1:1 1:2 1:7
a
3+
3+
1190 nm 5 6 10 1210 nm 68 60 220
540 nm
1000 nm (ex. 940 nm)
590 380 410
770 200 45
22 22 58
730 370 125
Excitation wavelength ) 450 nm, and lifetimes are given in µs. Error on the measured lifetimes: (10%.
excitation light. Edinburgh Instruments F900 deconvolution software package was used to fit the lifetime decays using a mono-, di-, or triexponential function to obtain the reduced χ2 values in the range of 1.0-1.3. Multiple lifetime values obtained were then averaged using the following equation:12
τav )
ΣAiτi2 ΣAiτi
where τav is the average lifetime and τi and Ai are the lifetimes and the corresponding pre-exponential factors obtained from the fit. Signal intensities down to 1% of the maximum intensity were included for the lifetime analyses. The errors in the calculated lifetimes measured on samples of different batches and on several spots of the same samples are within 10%. Transmission Electron Microscopic (TEM) Measurements. TEM images were obtained from a Hitachi H-7000 transmission electron microscope operating at 75 kV. The suspensions of nanoparticles in ethanol were sonicated overnight, and TEM specimens were prepared by drop-casting the ethanol suspensions of the nanoparticles on copper grids (600 mesh) coated with an amorphous carbon film and by drying at ambient temperature. Energy-Dispersive X-ray (EDX). Energy-dispersive X-ray spectroscopic (EDX) analysis was done on a Hitachi S-3500N scanning electron microscope operating at 20 kV and a resolution of 102 eV. Dry powdered samples were stuck to the substrate using a double-sided carbon tape. Concentrations of the doped lanthanide ions in the prepared NPs are given as atom % with respect to the total lanthanide ion concentration in the NP. A relative error of 20-30% was estimated from three to six measurements on different spots of the sample. Results and Discussion Lanthanum silicate and zirconate nanoparticles were obtained from the same starting material, namely, Ho3+/Yb3+ doped LaF3 nanoparticles. Lanthanum silicate nanoparticles were obtained by coating a shell of silica on Ho3+/Yb3+ doped LaF3 nanoparticles of sizes 5-7 nm by ammonia-catalyzed sol-gel polycondensation of TEOS in EtOH/water and annealing at 800 °C for 24 h, while lanthanum zirconate nanoparticles were obtained by coating a shell of zirconia on Ho3+/Yb3+ doped LaF3 nanoparticles by acid-catalyzed sol-gel polycondensation of zirconium tetra-isopropoxide in EtOH/water and annealing at 800 °C for 24 h. X-ray diffraction (XRD) analysis of these nanoparticles indicated the presence of a lanthanum silicate
phase (identified as La10Si6O27) and some amount of lowcristobalite silica in the annealed silica-coated particles and lanthanum zirconate and baddeleyite zirconia in the annealed zirconia-coated particles (Figure S1, Supporting Information). The highest phonon energies of lanthanum silicate and lanthanum zirconate were determined to be 915 and 510 cm-1, respectively, from infrared (IR) spectral analysis (Figure S2, Supporting Information), and this is consistent with the phonon energies reported for the lanthanum silicate13 and lanthanum zirconate14 bulk phases. TEM images of these particles are shown in Figure S3, Supporting Information. The average concentrations of the doped ions, Ho3+/Yb3+ in the nanoparticles studied here, were determined by EDX and are tabulated in Table 1. Ho3+/Yb3+ co-doped lanthanum silicate and lanthanum zirconate nanoparticles gave three upconverted emissions, namely, green (540 nm), red (660 nm), and NIR (750 nm) when excited with 980 nm diode laser. There was a progressive increase in the intensities of all the upconversion emissions with an increase in Yb3+ concentration. This suggests an efficient transfer of energy from the excited Yb3+ ions to the Ho3+ ions. Both lanthanum silicate and lanthanum zirconate nanoparticles singly doped with Ho3+ did not show any upconverted emission for excitation at 980 nm with the CW diode laser. Therefore, the presence of a certain amount of Yb3+ ions is a prerequisite for upconversion in these materials. This is roughly a ratio of Yb3+/ Ho3+ > 0.4 in lanthanum silicate and a ratio of Yb3+/Ho3+ > 1 in lanthanum zirconate nanoparticles (980 nm excitation, power density ) 350 W/cm2). The ratio of red (640 nm) to green (540 nm) varied considerably between the lanthanum silicate and the lanthanum zirconate nanoparticles. Lanthanum silicate nanoparticles gave a predominant red upconverted light, and the ratio of red to green emission was found to vary with Yb3+ ion concentration (Figure 1a). At 1:1 Ho3+, Yb3+ ratio, a predominantly red emission was obtained, and as the ratio was increased to 1:3, the emission was almost exclusively red. Upon a further increase in Ho3+:Yb3+ ratio to 1:7.5, the green emission increased considerably to a value comparable to red (Figure 1a). For lanthanum zirconate particles, there was no upconversion for 1:1 Ho3+, Yb3+, while comparable intensities of red and green for 1:2 Ho3+, Yb3+ ratio and a predominant green emission for Ho3+, Yb3+ ratio of 1:7 were observed (Figure 1b). To determine the number of photons involved in the upconversion process, the upconversion emission was studied as a function of excitation pump power. Theory predicts a power law relationship between the upconversion intensity (I) and the excitation pump power (P). For a sequential absorption of n
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Figure 1. Upconversion luminescence from Ho3+, Yb3+ co-doped lanthanum silicate (left) and lanthanum zirconate particles (right) normalized to the red emission from the Ho3+.
Figure 2. Variation of upconversion emission intensity with excitation power density for lanthanum silicate (left) and lanthanum zirconate (right) doped with a Ho3+, Yb3+ ratio of 1:3 and 1:2, respectively.
TABLE 2: Power Law Exponents of Green and Red Upconversion Emissions from Lanthanum Silicate and Lanthanum Zirconate Nanoparticles lanthanum silicate
lanthanum zirconate
Ho3+:Yb3+
green
red
green
red
1:1 1:3 1:2 1:7.5 1:7
1.85 1.93
1.83 1.79
NA
NA
1.85
1.82
1.45
1.55 1.86
2.00
photons leading to upconversion, I R Pn, (n g 2).15 Representative power law plots for lanthanum silicate and lanthanum zirconate are shown in Figure 2 with a Ho3+ to Yb3+ ratio of 1:3 and 1:2, respectively. The power law exponents obtained for the red and green emission in the two nanoparticles were all close to 2 (Table 2, see also Figure S4, Supporting Information), an indication that both green and red emissions are two-photon processes. As none of these plots show a threshold for upconversion emission (i.e., the curve is not sigmoidal), a photon avalanche mechanism for the upconversion emission can be ruled out and the upconversion emission should therefore be arising from sequential transfer of absorption energy from excited Yb3+ ions to Ho3+ ions. For both lanthanum silicate and lanthanum zirconate nanoparticles, the ratio of red to green intensity did not vary with pump power indicating that the upconversion mechanism does not change with respect to pump power. An explanation for the observed upconversion emissions from the two matrixes can be provided by considering the effect of the matrix and that of the doped ion (Yb3+) on the Ho3+
Figure 3. NIR emission from Ho3+ doped lanthanum silicate and lanthanum zirconate particles for excitation at 450 nm (OPO).
fluorescence emission. In general, direct excitation of Ho3+ to 5 G6 level with 450 nm light (OPO) in samples singly doped with Ho3+ yields a strong green emission at 540 nm (5S2 f 5I8) and weaker emissions at 660 nm (5F5 f 5I8), 750 nm (5S2 f 5 I7), 1020 nm (5S2 f 5I6), 1200 nm (5I6 f 5I8), and 1385 nm (5S2 f 5I5). A comparison of the NIR emission spectrum from the two matrixes indicates that the emission from the zirconate sample is red-shifted by about 15 nm (Figure 3). Differences in the relative intensities of the NIR emissions from the two matrixes are also noteworthy. For samples co-doped with Yb3+, an additional emission at 978 nm (2F5/2 f 2F7/2) from the Yb3+ ions was observed (Figure 4).11 This spectral pattern was observed for all samples studied except for the 1:3 Ho3+, Yb3+ co-doped lanthanum silicate. For this particular sample, the green emission was weak and the NIR region of the spectrum
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Figure 4. NIR emission from Ho3+ and Yb3+ co-doped lanthanum silicate particles for excitation at 450 nm (OPO).
was dominated by a strong, broad emission at around 980 nm. The NIR emission at 1200 nm emission was weak as well (Figure 4b). The measured lifetimes of the Ho3+ excited states achieved by 450 nm excitation were consistently lower for lanthanum silicate matrix as compared to lanthanum zirconate matrix (Table 1).11 For example, the measured lifetime of 5I6 level was ∼6 µs (1190 nm emission) in lanthanum silicate and 60 µs (1210 nm emission) in lanthanum zirconate particles for 1:1 Ho3+, Yb3+ doped samples. This was expected as lanthanum zirconate has a lower phonon energy than lanthanum silicate. However, the lifetime of the green emission (5S2 level) at 540 nm for lanthanum silicate was much higher than that observed in lanthanum zirconate matrix. For example, the lifetime of the green emission (5S2 level) at 540 nm for lanthanum silicate doped with 1:3 Ho3+, Yb3+ was 380 µs, while it was 22 µs for lanthanum zirconate with similar Ln3+ dopant ratio, that is, 1:2. Considering this along with the NIR emission spectrum of lanthanum silicate doped with 1:3 Ho3+, Yb3+ as shown in Figure 4b in which the only dominant emission is from Yb3+ at 980 nm, the following can be concluded. The energy gap between Yb3+(2F7/2, 2F5/2) states is resonant with that of Ho3+(5S2, 5 I6) states in lanthanum silicate, and this energy match leads to a rapid energy transfer between Ho3+ and Yb3+ ions. This results in the measurement of an artificially lengthened emission lifetime for green emission of Ho3+ (Table 1), and this measured lifetime is a function of the lifetimes of Ho3+ and Yb3+ energy levels involved in the energy-transfer process. A similar phenomenon of such an artificially lengthened emission lifetime due to energy transfer has been documented for the green upconversion emission obtained for a LaF3:Er3+, Yb3+ system16 and also for YVO4 crystal doped with Ho3+, Yb3+.17 Evidently, this type of energy transfer is not as efficient in lanthanum zirconate sample where the overlap between the Ho3(5S2, 5I6)
Figure 5. Scheme for the upconversion mechanism.
and (2F5/2, 2F7/2) state is not as good (Figure 4d), and the measured lifetime of green emission reflects the true Ho3+ (5S2) lifetime. In light of all these observations, the following mechanism can be proposed for the upconversion emission in these materials (Figure 5). Typically in the Ho3+, Yb3+ co-doped lanthanum silicate and lanthanum zirconate nanoparticles, Yb3+ absorbs the excitation energy and transfers it to Ho3+ ions, which can be represented as Ho3+(5I8), Yb3+(2F5/2) f Ho3+(5I6), Yb3+(2F7/2), with the excess energy being transferred to the surrounding matrix. Ho3+(5I6) can undergo phonon relaxation to Ho3+(5I7). Ho3+(5I6) and Ho3+(5I7) could populate Ho3+(5S2) and Ho3+(5F5), respectively, either by excited-state absorption or by energy transfer from another Yb3+ ion, that is,
Ho3+(5I6), Yb3+(2F5/2) f Ho3+(5S2), Yb3+(2F7/2)
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Ho3+(5I7), Yb3+(2F5/2) f Ho3+(5F5), Yb3+(2F7/2) The radiative transfer of the Ho3+(5S2) to the ground-state ( I8) and (5I7) level gives green and 750 nm emissions, respectively, while that of Ho3+(5F5) to the ground state yields red emission (Figure 5). To understand the variation of red to green upconversion emission ratio in different samples, the nonradiative deactivation of the Ho3+(5S2) should be considered. Nonradiative deactivation of Ho3+(5S2) could happen in three ways (Figure 6). Pathway 1 Phonon relaxation of Ho3+(5S2, 5F4) to Ho3+(5F5). Pathway 2 Cross-relaxation between two Ho3+ ions, Ho3+(5S2, 5F4), Ho3+(5I7) f Ho3+(5F5), Ho3+(5I6). Pathway 3 Energy back transfer from Ho3+ to Yb3+, Ho3+(5S2, 5 F4), Yb3+(2F7/2) f Ho3+(5I6), Yb3+(2F5/2). Direct excitation of Ho3+(5S2, 5F4) energy level with 540 nm light from OPO source did not result in any detectable red emission in either of the matrixes implying that the feeding of 5 F5 level by 5S2 by multiphonon relaxation (pathway 1) should be minimal. The occurrence of Ho3+ to Yb3+ energy back transfer in lanthanum silicate and lanthanum zirconate matrixes was obvious from the study of their NIR emission, and this strongly supports that pathway 3 is a key contributor to the deactivation of Ho3+(5S2). This essentially decreases the green emission. The Ho3+ to Yb3+ energy back transfer has been identified as a cause of green UC emission quenching in several matrixes.18 This is a major concern with respect to lasing of the Ho3+ green upconversion emission and has been a subject of intense study. A decrease in green emission should be accompanied by an increase in red emission as energy transfer indicated in pathway 3 continuously populates Ho3+(5I6), and this in turn can populate the metastable level Ho3+(5I7) by phonon relaxation. Buildup of this energy level would boost cross-relaxation between two Ho3+ ions shown in pathway 2 leading to the buildup of the population of Ho3+(5F5) level which yields red emission. Thus, deactivation of Ho3+(5S2, 5F4) by pathways 2 and 3 contributes to populating the Ho3+(5F5). In short, pathways 2 and 3 build up the population of red-emitting Ho3+(5F5) level at the expense of green-emitting level Ho3+(5S2, 5 F4). In light of this mechanism, the variation of red to green emission ratios with the co-dopant Yb3+ concentration (lanthanum silicate/lanthanum zirconate) may be explained in the following fashion. Lanthanum silicate matrix doped with 1:1 Ho3+:Yb3+ showed both red and green upconversion emission, and the predominance of red upconversion was because of Ho3+ to Yb3+ excitation energy back transfer (pathway 3). As the Ho3+:Yb3+ ratio is changed to 1:3, the Ho3+ to Yb3+ back transfer Ho3+(5S2, 5F4), Yb3+(2F7/2) f Ho3+(5I6), Yb3+(2F5/2) (pathway 3) is increased considerably as was observed in the NIR region of the emission spectrum as the energy match of the two transitions is quite good as seen in Figure 4b. Consequently, the Ho3+(5S2) level is quenched to a large extent resulting in a reduction of the green upconversion emission. When the Ho3+, Yb3+ ratio is changed to 1:7.5, the Ho3+ to Yb3+ energy back transfer was reduced, which is supported by the fact that the overlap between the emission from Yb3+ and Ho3+ ions in the NIR spectrum decreases (Figure 4c). This could be due to the change in the environment around Ho3+. Any change in the environment of Ho3+ in the nanoparticles should also affect Ho3+-Ho3+cross-relaxation (pathway 2) leading to the buildup of Ho3+(5F5) population. Hence, the green emission was increased in this sample. 5
Figure 6. Scheme depicting the nonradiative deactivation of Ho3+(5S2). (1) Phonon relaxation of Ho3+(5S2, 5F4). (2) Cross-relaxation between two Ho3+ ions. (3) Energy back transfer from Ho3+ to Yb3+.
This mechanism can also explain the predominance of green upconversion over the red upconversion emission observed in the lanthanum zirconate nanoparticles. The multiphonon relaxation of Ho3+(5I6) to Ho3+(5I7) should be slower in this matrix owing to its lower phonon energy, and hence, the green emission intensity is consistently higher than that observed in lanthanum silicate matrix. Support for this comes from the fact that the measured lifetime of 5I6 level (1200 nm emission) was 10 times higher than that in silicate matrix. In addition, the Ho3+(5S2, 5 F4), Yb3+(2F7/2) f Ho3+(5I6), Yb3+(2F5/2) back transfer (pathway 3 which populates red-emitting Ho3+(5F5) level) in this matrix would not be as efficient as in silicate matrix as there is a mismatch between the resonating energy levels of the two ions (Figure 4d). The fact that a higher Yb3+/Ho3+ ratio of about 1 was required for upconversion to be observed in lanthanum zirconate as opposed to a lower ratio of ∼0.4 required for upconversion to be observed in lanthanum silicate may be explained by analyzing the emission intensity ratios in the NIR. It is clear from Figures 3 and 4 that the relative intensity of 1020 nm emission (5S2 f 5 I6) to that of 1385 nm emission (5S2 f 5I5) is consistently lower in the lanthanum zirconate matrix as compared to the lanthanum silicate matrix. This suggests that the probability of (5S2 f 5I6) transition is lower in lanthanum zirconate matrix than in lanthanum silicate matrix. It is probable that the reverse transition (5I6 f 5S2), which is key for upconversion, is also of a similar probability, and this may be the reason for the requirement of a higher Yb3+/Ho3+ ratio required for upconversion in lanthanum zirconate. Conclusions The dominance of red and green upconversion emissions from lanthanum silicate and lanthanum zirconate nanoparticles, respectively, could be attributed to the phonon energy difference between the two matrixes. The variation in the ratio of the green to red upconversion emissions with a change in Yb3+ ion concentration is due to a variation of excitation energy back transfer from Ho3+ to Yb3+. Acknowledgment. Natural Science and Engineering Research Council (NSERC), the Canada Foundation for Innovation (CFI), and the British Columbia Knowledge Development Fund (BCKDF) of Canada are gratefully acknowledged for financial support. Supporting Information Available: This includes XRD pattern, IR spectra, and TEM images of the nanoparticles and
Nanoparticles Co-Doped with Ho3+ and Yb3+ power dependence graphs for the green and red upconversion when pumped at 980 nm power (Figures S1-S4). This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Auzel, F. Chem. ReV 2004, 104, 139–173. (b) Daniel, R. G., D.R.; Gu¨del, H. U. Top. Curr. Chem. 2001, 214, 1–56. (2) Chivian, J. S.; Case, W. E.; Eden, D. D. Appl. Phys. Lett. 1979, 35, 124. (3) Nakazawa, C.; Shionoya, S. Phys. ReV. Lett. 1970, 25, 1710. (4) (a) Suyver, J. F.; Aebischer, A.; Biner, D.; Gerner, P.; Grimm, J.; Heer, S.; Kra¨mer, K. W.; Reinhard, C.; Gu¨del, H. U. Opt. Mater. 2005, 27, 1111–1130. (b) Egger, P.; Hulliger, J. Coord. Chem. ReV. 1999, 183, 101– 105. (c) Singh, A. K. Sens. Actuators, A 2007, 136, 173–177. (5) (a) Johnson, L. F.; Guggenheim, H. J. Appl. Phys. Lett. 1971, 19, 44. (b) Allain, J. Y.; Monerie, M.; Poignant, H. Electron. Lett. 1990, 26, 261. (c) Funk, D. S.; Eden, J. G.; Osinki, J. S.; Lu, B. Electron. Lett. 1997, 33, 1958. (6) Zhang, X. X.; Hong, P.; Bass, M.; Chai, B. H. T. Appl. Phys. Lett. 1993, 63, 2606. (7) Osiac, E.; Soko´lska, I.; Ku¨ck, S. Phys. ReV. B 2002, 65, 235119. (8) (a) De la Rosa, E.; Salas, P.; Desirena, H.; Angeles, C.; Rodriguez, R. A. Appl. Phys. Lett. 2005, 87, 241912. (b) Boyer, J. C.; Vetrone, F.; Capobianco, J. A.; Sphegini, A.; Bettinelli, A. Chem. Phys. Lett. 2004, 390, 403. (c) Osiac, E. J. Alloys Compd. 2002, 341, 263. (d) Luo, X.-X.; Cao, W. H. Mater. Lett. 2007, 61, 3696. (e) Capobianco, J. A.; Boyer, J. C.; Vetrone, F.; Speghini, A.; Bettinelli, M. Chem. Mater. 2002, 14, 2915.
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