J. Phys. Chem. C 2007, 111, 8161-8165
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Up-Conversion in Yb3+-Tm3+ Co-Doped Lutetium Fluoride Particles Prepared by a Combustion-Fluorization Method Siguo Xiao,*,† Xiaoliang Yang,† J. W. Ding,† and X. H. Yan‡ Institute of Modern Physics, Faculty of Materials and Photoelectronic Physics, and Key Laboratory for AdVanced Materials and Rheological Properties of Ministry of Education of China, Xiangtan UniVersity, Hunan, 411105 China, and College of Science, Nanjing UniVersity of Aeronautics and Austronautics, Nanjing, 210016 China ReceiVed: NoVember 6, 2006; In Final Form: February 23, 2007
A Yb3+ and Tm3+ co-doped LuF3 sample has been successfully prepared with a combustion-fluorization method. Five up-converted emission bands attributing to eight transitions have been observed under a 980 nm diode laser excitation. The possibility of a cooperative sensitization mechanism and a three-step energytransfer mechanism performing the 1G4 level population of Tm3+ is discussed on the basis of rate equation theory. It is believed that the Yb3+-Yb3+ cooperative sensitization is the main process for populating the 1G4 level of Tm3+ in Yb3+ and Tm3+ co-doped LuF3 particles, which leads to the strong blue-green emission at 481 nm.
1. Introduction Since the end of the 20th century and especially at the beginning of the new millenium, there has been a renaissance in the study of rare earth doped powder phosphors.1-4 It is believed that a very broad range of luminescence properties such as radiative transition, nonradiative emission, and energy-transfer are determined by the host matrix and the doping concentration.5 Thus, with the selection of a suitable doping matrix, excellent luminescent properties might be observed. In Lu-containing crystals, it has been found that the top of the valence band is composed mainly of Lu 4f orbitals.6,7 Therefore, the oscillator strength of doped rare earth ions in these crystals would increase according to the intensity-borrowing mechanism proposed by Guillot-Noel et al.8 It has been reported that, when comparing the Y-containing oxide and fluoride crystals, the Lu-containing crystal has stronger luminescence intensity,9,10 which indicates that Lu could be a more favorable cation for trivalent lanthanide dopant emission.11,12 The current interest in up-conversion is fueled by its potential application on optical-disk technology, information technology, color displays, and fluorescent labels for the sensitive detection of biomolecules. The luminescence properties of up-conversion11 in an Er3+ doped Lu2O3 host have been investigated. However, studies of the luminescence properties in Tm3+ doped materials are of greater interest because Tm3+ has two stable levels, 1G4 and 3F4. Through the 1G4 f 3H6 emission, blue up-conversion luminescence near 480 nm can be obtained that might be applied in biomedical diagnostics, high-density optical data storage and reading, photoprinting, etc.5 Through the 3H4 f 3H6 emission, 795 nm near-infrared (NIR) luminescence can be obtained for use in the amplifier of the first communication window of a quartz optical fiber. In this paper a Yb3+ and Tm3+ co-doped LuF3 powder has been successfully prepared with a combustion-fluorization method. Up-conversion under excitation from * Corresponding author. E-mail address:
[email protected]. † Xiangtan University. ‡ Nanjing University of Aeronautics and Austronautics.
Figure 1. X-ray diffraction pattern of the Yb3+-Tm3+ co-doped LuF3 particles.
a 980 nm diode laser has been investigated. Intense blue-green as well as relatively weak violet, ultraviolet, red, and NIR upconversion emissions have been observed, and their upconversion mechanisms have also been discussed. 2. Experimental Procedures Yb3+ and Tm3+ co-doped LuF3 with a composition of Lu0.938Tm0.002Yb0.06F3 was prepared by using a solution combustion (propellant) synthesis procedure followed by a NH4HF2 fluorization process. An aqueous solution containing glycine, Lu(NO3)3, Tm(NO3)3, and Yb(NO3)3 was used to synthesize the Yb3+ and Tm3+ co-doped Lu2O3 powder. The combustion process is similar to what has been previously reported.13,14 The molar ratio of glycine to metal nitrate was 1.2:1. After the combustion, the obtained Yb3+ and Tm3+ co-doped Lu2O3 was mixed with NH4HF2 and then fired in a closed corundum crucible at 200 and 600 °C for 3 h, respectively. Then the powder was washed with distilled water to remove residual NH4HF2 or NH4F. After the sample was dried at 80 °C for several hours, the final product was obtained.
10.1021/jp067323n CCC: $37.00 © 2007 American Chemical Society Published on Web 05/18/2007
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Figure 4. Emission spectra of Yb3+-Tm3+ co-doped LuF3 particles under 355 nm excitation. Figure 2. Scanning electron microscopy (SEM) pattern of the Yb3+Tm3+ co-doped LuF3 particles.
Figure 5. Up-converted emission spectra of Yb3+-Tm3+ co-doped LuF3 powders under 980 nm excitation.
Figure 3. Medium infrared transmission spectrum of the Yb3+-Tm3+ co-doped LuF3 particles.
The X-ray diffraction (XRD) pattern of the sample was measured by using a D/max-IIA X-ray diffractometer system with a monochromatized Cu-KR irradiation (1.5418 Å). The scanning electron micrograph (SEM) was recorded on an LEO 1525 field emission scanning electron microscope. The transmission spectrum in the medium infrared (MIR) region was measured at room temperature with an AVATAR 370 FTIR spectrometer. A photomultiplier tube coupled spectrometer was used to record the luminescence spectra at room temperature. 3. Results and Discussion The X-ray diffraction pattern in Figure 1 shows that the sample crystallized in the orthorhombic LuF3 structure with space group Pnma (No. 62). The lattice constants calculated in terms of X-ray diffraction data are a ) 6.14, b ) 6.79, and c ) 4.53 Å. Figure 2 shows an SEM image of the LuF3 powder. Coherent sphere-like particles are observed, and the mean diameter of the particles is about 150 nm. The MIR transmission spectrum in Figure 3 shows an intense band at 669 cm-1, which might be assigned to the deformation vibration of absorbed FN2.15 Because the fluorization process was performed in a hermetical container, the FN2 group might form during the reaction. The bands peaking around 3437 and 1626 cm-1 in Figure 3 are assigned to stretching and bending vibrations of the absorbed water.16-18 Its presence appears to be unavoidable,
Figure 6. Dependence of up-conversion intensity as a function of excitation power at 980 nm in a double logarithmic plot.
as the NH4HF2 reagent used for fluorization inevitably contains a small amount of water. Figure 4 shows the emission spectra of Yb3+-Tm3+ co-doped LuF3 powders under 355 nm (xenon lamp) excitation. Only one emission band at 451 nm has been observed. The 355 nm light can excite the Tm3+ ion to the 1D2 state, and then the1D2 f 3F4 transition leads to the luminescence emission at 451 nm. Figure 5 presents the up-converted emission spectra of Yb3+Tm3+ co-doped LuF3 powders under excitation of a 980 nm diode laser. An intense blue-green emission centered at about 481 nm is observed and is ascribed to the 1G4 f 3H6 transition of Tm3+. Other relatively weak emissions in the UV and visible range, approximately centered at 290, 348, 361, 451, and 649 nm, have also been observed. They may be assigned to the 1I6 f 3H6, 1I6 f 3F4, 1D2 f 3H6, 1D2 f 3F4, and 1G4 f 3F4 transitions,19 respectively. In the NIR a broad emission band
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Figure 7. Energy level diagram for the Yb3+-Tm3+ co-doped system, and the transition pathways excited by a 980 nm laser. Solid lines represent transitions of absorption or radiation. Dotted lines represent muti-phonon relaxations. Dashed lines represent energy-transfers. The energy-transfer pathways are also marked by dashed-dotted lines.
from 760 to 840 nm, which is assigned to the 1G4 f 3H5/3H4 f 3H6 transitions, has also been observed. To discuss the up-conversion mechanism, the power dependent up-conversion behavior has also been investigated. Figure 6 shows a logarithmic plot of the up-converted luminescence intensity as a function of the pumping laser intensity. The experimental data were obtained at low power excitation (90200 mW/cm2). The up-conversion luminescence intensity (I) is related to the pump intensity (Φ) via the formula I ∝ Φm, where m is the number of pump photons required to populate the emitting level. As shown in Figure 7, multiple step energy-transfers from Yb3+ to Tm3+ might be responsible for the up-converted emissions. These energy-transfer processes are as follows:
F5/2 (Yb3+) + 3H6 (Tm3+) f 3H5 (Tm3+) + 2F7/2 (Yb3+)
2
rate equations needed to describe the up-conversion processes are the following:
dn0 ) W50n5 + W40n4 + W30n3 + W20n2 + dt W10n1 - γ0n0N1 - δN21n0 (1) dn1 ) γ0n0N1 + W51n5 + W41n4 + W31n3 + dt W21n2 - γ1n1N1 - W10n1 (2)
dn2 ) γ1n1N1 + W52n5 - (W21 + W20)n2 - γ2n2N1 (3) dt
F5/2 (Yb3+) + 3F4 (Tm3+) f 3F2 (Tm3+) + 2F7/2 (Yb3+)
2
F5/2 (Yb ) + H4 (Tm ) f G4 (Tm ) + F7/2 (Yb )
2
3+
3
3+
1
3+
2
3+
F5/2 (Yb3+) + 1G4 (Tm3+) f 1D2 (Tm3+) + 2F7/2 (Yb3+)
dn3 ) γ2n2N1 - (W30 + W31)n3 - γ3n3N1 + δN21n0 (4) dt
2
F5/2 (Yb ) + D2 (Tm ) f P2 (Tm ) + F7/2 (Yb )
2
3+
1
3+
3
3+
2
3+
On the other hand, considering that the energy of the 1G4 level of Tm3+ is almost equal to the sum of two 980 nm photons, a cooperative sensitization process might also be responsible for the population of the 1G4 level of Tm3+ ions.20 This mechanism is described in the following equation:
dn4 ) γ3n3N1 - γ4n4N1 - (W41 + W40)n4 dt
(5)
dn5 ) γ4n4N1 - (W51 + W50)n5 dt
(6)
G4 (Tm3+) + 2 2F7/2 (Yb3+)
dN1 ) σFΦN0 - A10N1 - γ0n0N1 - γ1n1N1 - γ2n2N1 dt γ3n3N1 - γ4n4N1 - δN21n0 (7)
As mentioned above, the up-conversion intensity (I) is proportional to some power m of the excitation intensity (Φ),21 i.e., I ∝ Φm, and it can be deduced theoretically from the rate equations.22,23 To understand the up-conversion properties, it is helpful to compare them with the experimental result. The
dN0 ) -σFΦN0 + A10N1 + γ0n0N1 + γ1n1N1 + γ2n2N1 + dt γ3n3N1 + γ4n4N1 + δN21n0 (8)
2 2F5/2 (Yb3+) + 3H6 (Tm3+) f 1
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n ) n1 + n2 + n3 + n4 + n5
(9)
N ) N1 + N0
(10)
where σ is the cross-section of the ground state absorption of Yb3+ ions, Φ is the incident pumping power, Ni is the population density of the ith level of Yb3+, ni is the population density of the ith level of Tm3+ involved in the up-conversion process (as shown in Figure 7), n and N are the total concentration of Tm3+ and Yb3+ ions, respectively, the terms of Wij and Wnij represent the radiative transition rates and nonradiative decay rates between the levels i and j of Tm3+, A10 is the radiative transition rate of Yb3+, γi is the probability of energy-transfer from Yb3+ to Tm3+, and δ is rate of the cooperative sensitizing process from Yb3+ to Tm3+. In a simplified model, here we label the 3H level of the Tm3+ ion as level 0; the 3H and 3F levels as 6 5 4 level 1; the 3F2, 3F3, and 3H4 levels as level 2; the 1G4 level as level 3; the 1D2 level as level 4; and the 3P2 and 1I6 levels as level 5. The pump constant is24 given in eq 11:
F ) λp/hcπS2p
(11)
where λp is the pump wavelength, h is Planck’s constant, c is the vacuum speed of light, and Sp is the pump radius. To understand the up-conversion mechanism, two extreme cases might be considered for weak power excitation condition. If the cooperative sensitizing process is sufficiently weak, multiple step sensitizing energy-transfers from Yb3+ should be the dominant courses performing the population of upper levels of Tm3+. Under this condition, the term δ in the rate eqs 1, 4, 7, and 8 might be set to zero. Thus, the population densities of the 1G4, 1D2, and 1I6 levels of Tm3+ can be respectively written as shown in eqs 12-14:
n3 )
n4 )
γ0γ1γ2nN3F3σ3Φ3 (W20 + W21)W10(W30 + W31)(A10)3 γ0γ1γ2γ3nN4F4σ4Φ4
W10(W30 + W31)(W20 + W21)(W40 + W41)(A10)4
(12)
(13)
γ0γ1γ3γ4γ5nN5σ5F5Φ5 (W20 + W21)W10(W50 + W51)(W40 + W41)(W30 + W31)(A10)5 (14) On the contrary, if the cooperative sensitizing process is intense enough, the population of the 1G4 level would be mainly performed through these processes, and the multiple step sensitizing energy-transfers that occur at the 1G4 level is negligible. If the terms γ0, γ1, and γ2 in eqs 1, 2, 3, and 4 are set to zero, then the population densities of the 1G4, 1D2, and 1I levels of Tm3+ can be respectively rewritten as follows: 6
n4 )
δσ2F2Φ2nN2 (W30 + W31)(A10)2 δγ3nN3σ3F3Φ3
W10(W30 + W31)(W40 + W41)(A10)3
δγ3γ4nN4σ4F4Φ4 (W20 + W21)W10(W50 + W51)(W40 + W41)(W30 + W31)(A10)4 (17) According to eqs 12-17 and the equation Ii ) nihViWi,21 the relation Ii ∝ Φm is obtained in the above two extreme cases. The deduced m values for the emissions of Tm3+ in the two extreme cases as well as the experimental results are given in Table 1. One finds that the result of the experimental investigation on the power dependent up-conversion behaviors is more consistent with the cooperative sensitization mechanism. In principle, both the cooperative sensitization mechanism and three-step energy-transfers are likely to realize the population of the 1G4 level. However, the cooperative sensitization mechanism might be the dominant course performing the population of the 1G4 level when the Yb3+ and the Tm3+ ions are doped into a suitable environment. Combining eqs 12 and 15, the ratio of the direct cooperative sensitization probability to the three-step energy-transfers probability can be expressed in the following equation:
f)
δ(W20 + W21)W10A10 γ0γ1γ2σFNΦ
(15)
(16)
(18)
If f . 1, then it means that the population of the 1G4 level will be performed by the cooperative sensitization mechanism. If f , 1, then the three-step energy-transfer will be the main way of performing the population of the 1G4 level. Many factors will determine the value of f, among which the probabilities of the energy-transfers involved in the up-conversion might be very important. Although the theory on energy-transfer has been developed since the 1950s, it is still difficult to accurately estimate the probabilities of phonon-assisted energy-transfers, especially that of the cooperative sensitization that concerns three ions.25 However, it is known that the energy mismatch will largely influence the energy-transfer probability, which includes the cooperative sensitization probability. The phononassisted energy-transfer probability (γ) can be expressed as26 eq 19:
γ ) Pe-R∆E
n5 )
n3 )
n5 )
(19)
where P is the probability of the energy-transfer process without energy mismatch, ∆E is the mismatch for the energy-transfer, and R is the electron-lattice coupling coefficient. Equation 19 shows that the larger energy mismatch is, the smaller the energytransfer probability will be. When considering the three-step energy-transfers that populate the 1G4 level, one may find that each step needs phonons to assist. Notably, the mismatch for the processes 2F5/2 (Yb3+) + 3H6 (Tm3+) f 3H5 (Tm3+) + 2F7/2 (Yb3+) and 2F5/2 (Yb3+) + 3H4 (Tm3+) f 1G4 (Tm3+) + 2F7/2 (Yb3+) are about 1900 and 1100 cm-1, respectively. According to eq 19, these processes might be inefficient. On the other hand, the cooperative sensitization mechanism might be very effective if the photon energy of cooperative emission closely matches the 1G4 level of Tm3+. In a Yb3+-Tm3+ co-doped ZrO2 matrix, an additional Yb3+Yb3+ cooperative up-conversion emission band has been directly observed by Patra et al.5 under 980 nm excitation, and it is also been found that the 1G4 f 3H6 transition is a three-photon process, i.e., the measured value of m is approximately equal to 3. In a Yb3+-Tm3+ co-doped Gd3Ga5O12 matrix, as pointed out by Pandozzi et al.,27 no separate Yb3+-Yb3+ cooperative
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TABLE 1: Expected Number of Photons (m) for the Multiple-Step Energy-Transfers Mechanism and Cooperative Sensitization Mechanism and the Experimentally Measured m Value
radiation
multiple step energy-transfer mechanism
cooperative sensitization mechanism
experiment result
1I f 3H 6 6 1 I6 f 3F4 1D f 3H 2 6 1 D2 f 3F4 1G f 3H 4 6 1G f 3H 4 4
5 5 4 4 3 3
4 4 3 3 2 2
3.50 ( 0.08 3.53 ( 0.05 2.59 ( 0.06 2.63 ( 0.06 1.88 ( 0.03 1.90 ( 0.08
up-conversion emission band has been observed under 980 nm excitation. The measured value of m for the 1G4 f 3H6 transition in this matrix equals 2.09, which indicates that a two-photon process is responsible for this emission. In the ZrO2 matrix, the appearance of a separate Yb3+-Yb3+ cooperative up-conversion emission band indicates that the cooperative up-conversion photon energy does not match the 1G level of Tm3+ well. The probability of the cooperative 4 sensitization from Yb3+ to Tm3+ might be small. The value of f in eq 18 might be much smaller than 1, and therefore the population of the 1G4 level of Tm3+ is achieved predominatingly by a three-step energy-transfer. In the Gd3Ga5O12 matrix, however, no additional Yb3+-Yb3+ cooperative up-conversion emission has been observed, which means that the cooperative up-conversion photon energy closely matches the 1G4 level of Tm3+. The probability of the cooperative sensitization from Yb3+ to Tm3+ might be very effective. Thus the value of f in eq 18 should be much larger than 1, and therefore the population of the 1G4 level of Tm3+ is performed mainly through a cooperative sensitization process. Presumably, the cooperative sensitization process, in which two excited Yb3+ ions transfer their energy to one Tm3+ ion at ground state, might be more strictly dependent upon the energy match condition. The band shape (G(V)) of the Yb3+-Yb3+ cooperative upconversion emission is a convolution of the infrared Yb3+ emission,28 as shown in eq 20:
G(V) )
∫ F(V)F(2V - V′) dV′
(20)
in which F(V) is the fluorescence line shape of the 2F5/2 Yb3+ infrared emission. The properties of stark splitting and emissions between stark levels of the Yb3+ ion will change from one host to another, which leads to the difference of Yb3+-Yb3+ cooperative emission band shape in different hosts. As a result, the photon energy of cooperative emission will closely match the 1G4 level of Tm3+ in some hosts and not in the other ones. In Yb3+/Tm3+ co-doped LuF3 materials a separate Yb3+-Yb3+ cooperative emission was not observed. This indicates that the photon energy of cooperative emission closely matches the 1G4 level. Therefore, the f value should be much larger than 1, and the cooperative sensitization mechanism should be very effective. Hence, it is believed that the 1G4 level of Tm3+ is mainly populated by the cooperative sensitization process in the LuF3 host. 4. Conclusion In conclusion, a Yb3+ and Tm3+ co-doped LuF3 sample has been successfully fabricated with a combustion-fluorization
method. Five up-converted emission bands, including a strong blue emission attributed to eight transitions, have been observed. Up-conversion luminescence intensities against the excitation power are both theoretically and experimentally investigated. The result indicates that the Yb3+-Yb3+ cooperative sensitization should be a key process for population of the 1G4 level of Tm3+ in Yb3+ and Tm3+ co-doped LuF3 materials, which results in the strong blue-green emission at about 481 nm. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 10674113), the Program for New Century Excellent Talents in University (NCET-06-0707), Hunan Provincial Natural Science Foundation of China (No. 06JJ50006), and partially by the Scientific Research Fund of Hunan Provincial Education Department (No. 06A071). References and Notes (1) Maciel, G. S.; de Araujo, C. B.; Messaddeq, Y.; Aegerter, M. A. Phys. ReV. B 1997, 55, 6335. (2) Hebert, T.; Wannemacher, R.; Lenth, W.; Macfarlane, R. M. Appl. Phys. Lett. 1990, 57, 1727. (3) Dowing, E.; Hesselink, L.; Ralston, J.; Macfarlane, R. Science 1996, 273, 1185. (4) Patra, A.; Friend, C. S.; Kapoor, R.; Prasad, P. N. Appl. Phys. Lett. 2003, 83, 284. (5) Patra, A.; Saha, S.; Alencar, M. A. R. C.; Rakov, N.; Maciel, G. S. Chem. Phys. Lett. 2005, 407, 477. (6) Maunier; C.; Doualan, J. L.; Moncorge, R.; Speghini, A.; Bettinelli, M.; Cavalli, E. J. Opt. Soc. Am. B 2002, 19, 1794. (7) Moine, B.; Dujardin, C.; Lautesse, H.; Pedrini, C.; Combes, C. M.; Belski, A.; Martin, P.; Gesland, J. Y. Mater. Sci. Forum 1997, 245, 239241, (8) Guillot-Noel, O.; Bellamy, B.; Viana, B.; Gourier, D. Phys. ReV. B 1999, 66, 1668. (9) Rambaldi, P.; Moncorge´, R.; Wolf, J. P.; Pe´drini, C.; Gesland, J. Y.; Opt. Commun. 1998, 146, 163. (10) Maunier; C.; Doualan, J. L.; Moncorge´, R.; Speghini, A.; Bettinelli, M.; Cavalli, E. In Proceedings of ASSL 2001, Topical Meeting and Tabletop Exhibit 2001; Westin Hotel, Seattle, Washington, 2001; p 28-31. (11) Capobianco, J. A.; Vetrone, F.; Boyer, J. C.; Speghini, A.; Bettinelli, M. Opt. Mater. 2002, 19, 259. (12) Boyer, J. C.; Vetrone, F.; Capobianco, J. A.; Speghini, A.; Bettinelli, M. J. Phys. Chem. B 2004, 108, 20137. (13) Tao, Y.; Zhao, G.; Zhang, W.; Xia, S. Mater. Res. Bull. 1997, 32, 501. (14) Tessari, G.; Bettinelli, M.; Speghini, A.; Ajo`, D.; Pozza, G.; Depero, L. E.; Allieri, B.; Sangaletti, L. Appl. Surf. Sci. 1999, 144-145, 686. (15) Shen, S.; Durig, J. R. J. Mol. Struct. 2003, 661-662, 49. (16) Hair, M. L. J. Non-Cryst. Solids 1975, 19, 299. (17) Bartholomew, R .F.; Butler; B. L.; Hoover, H. L.; Wu, C. K. J. Am. Ceram. Soc. 1980, 63, 481. (18) Capobianco, J. A.; Vetrone, F.; Boyer, J. C.; Speghini, A.; Bettinelli, M. J. Phys. Chem. B 2002, 106, 1181. (19) Guinhos, F. C.; Nobrega, P. C.; Santa-Cruz, P. A. J. Alloys Compd. 2001, 323-324, 358. (20) Jacquier, B.; Linare`s, C.; Mahiou, R.; Adam, J. L.; De´noue, E.; Lucas, J. J. Lumin. 1994, 175, 60-61. (21) Chamarro, A.; Cases, R. J. Lumin. 1990, 46, 59. (22) Zou, X.; Toratani, H.; J. Non-Cryst Solids 1995, 181, 87. (23) Suyver, J. F.; Aebischer, A.; Garcı´a-Revilla, S.; Gerner, P.; Gu¨del, H. U. Phys. ReV. B 2005, 71, 125123. (24) Pollnau, M.; Gamelin, D. R.; Lu¨thi, S. R.; Gu¨del, H. U.; Hehlen, M. P. Phys. ReV. B 2000, 61, 3337. (25) Miyakawa, T.; Dexter, D. L. Phys. ReV. B 1970, 1, 70. (26) Auzel, F. Phys. ReV. B 1976, 13, 2809. (27) Pandozzi, F.; Vetrone, F.; Boyer, J. C.; Naccache, R.; Capobianco, J. A.; Speghini, A.; Bettinelli, M. J. Phys. Chem. B 2005, 109, 17400. (28) Guyot, Y.; Moncorge´, R.; Merkle, L. D.; Pinto, A.; McIntosh, B.; Verdun, H. Opt. Mater. 1996, 5, 127.
