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Blue emitting hybrid organic–inorganic materials
Optical Materials 18 (2001) 309±320
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Blue emitting hybrid organic±inorganic materials E. Cordoncillo a, F.J. Guaita a, P. Escribano a, C. Philippe b, B. Viana b,*, C. Sanchez b a
Departamento de Quimica Inorganica y Organica, Campus Riu Sec, Universitat Jaume I. Castellon, Spain b Laboratoire de Chimie de la Mati e re Condens ee, Universit e Pierre et Marie Curie, UMR CNRS 7574, 4 place Jussieu, Paris 75252, France Received 20 July 2000; accepted 15 May 2001
Abstract Undoped and Eu2 - or Ce3 -doped hybrid organic±inorganic materials were prepared at room temperature from hydrolysis and condensation of organohydrosilanes HSi CH3 OCH2 CH3 2 , HSi OCH2 CH3 3 , alkoxysilanes Si CH3 OCH2 CH3 3 and zirconium propoxide in the presence of EuCl3 or Ce NO3 4 , 2NH4 NO3 . The transition metal alkoxide catalyzed cleavage of the SiAH bonds was used to reduce in situ at room temperature the rare earth cations. Depending on the chemical strategy, the resulting hybrid materials can be processed as transparent bulks or coatings, which exhibit a good transparency in the UV±visible domain (cut-o at 250±280 nm). Both the undoped matrices and the rare earth-doped matrices exhibit a strong blue emission. The nature of dierent emitting species is clearly assigned by using their very dierent kinetics of ¯uorescence (s 50 ns for Ce3 , s < 1 ls for Eu2 s 3 and 4 ls for the undoped matrices) and through excitation experiments. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Luminescence; Hybrid; Sol±gel; Eu2 ; Ce3
1. Introduction Sol±gel technology is widely used to prepare and shape oxide-based materials [1±3]. Moreover, the mild characteristics oered by the sol±gel process allow introducing organic molecules inside an inorganic network [4±8]. Inorganic and organic components can then be mixed at the nanometric scale, in virtually any ratio leading to the so-called hybrid organic±inorganic nanocomposites which are extremely versatile in their composition, processing and properties [5,9,10]. In this sense, the
*
Corresponding author. E-mail address: [email protected] (B. Viana).
synthesis and characterization of organic±inorganic hybrid materials is a rapidly growing ®eld of research opening a land of opportunity for the design of new materials for photonics [10], mechanics [11] and biology [12,13]. Mixed siloxane± metal oxide materials, from which transparent coatings can be easily prepared, are good hybrid matrices for optics [10,14]. They can be synthesized by using alkoxysilane precursors SiR0x OR4 x , (R0 being any organic group; CH3 , H, or any organic dye) which lead through hydrolysis and condensation reactions to the formation of a siloxane type network. The siloxane-based network can be eventually cross-linked or mixed at the nanosize level with metal oxo polymers obtained via hydrolysis±condensation of metal
0925-3467/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 1 ) 0 0 1 7 0 - 7
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alkoxides M(OR)n , (M Si, Ti, Zr and Al) [14± 18]. The metal oxo polymeric network can be used to implement another property (modify the refractive index of the hybrid matrix, induce magnetic properties, etc.) or simply as a host for guest cations that need to accommodate a speci®c coordinance. Due to their advantages, such as low temperature processing, easy shaping of thick ®lms, higher sample homogeneity and purity, and the opportunity to prepare non-crystalline solids, siloxane-based hybrid matrices are currently investigated as potential matrices for rare earth luminescence [10,19±22]. The optical properties of rare earth ions present many interests because they are used as visible and near-IR radiation sources, some of them being particularly important for lasers and optical communication devices. Rare earth ions incorporated in metal oxides or ¯uorides are usually stabilized in trivalent states. Furthermore, some rare earth cations incorporated in crystalline or glassy matrices can exhibit lower valence states (divalent ions as Eu2 , Sm2 ). The emission of the divalent lanthanide ions originates either from an intracon®gurational 4f n transition or from an intercon®gurational 4f n 1 5d ! 4f n transition: line or band emission, respectively. Among divalent lanthanides, Eu2 ion may present some stability because of its 4f 7 con®guration. This Eu2 ion is particularly unique because its broad and intense band luminescence 4f 6 5d1 ! 4f 7 is strongly host dependent with emission wavelength extending from the UV to the red range of the electromagnetic spectrum [23±25]. Previously reported studies have demonstrated the possibility to use Eu2 doped compounds as phosphors, or for medical applications and skin tanning [23±25]. Some tetravalent rare earth ions (Ce4 , Pr4 , Tb4 ) can also be stabilized and in that case, charge transfer absorption bands could also be observed in the UV± visible range. There is also a lot of interest in Ce3 -doped crystalline or glassy hosts for applications in the scintillator ®eld and in the research of tunable lasers in the UV and visible ranges [26±28]. Recently, transparent inorganic±organic hybrid monoliths doped with Ce3 ions have been prepared by sol± gel process [21,29]. Such inorganic hydrogenous
materials can be used as neutron detectors [30]. In comparison to the organic scintillators, they present higher hydrogen density and very high scintillation eciency. Of course, due to their low density, these materials could not be used as high energy detectors. Dierent anities of the silane functional groups for Ce3 were observed. Optical properties such as absorption spectra, emission spectra and ¯uorescence quantum yield were strongly aected by the Ce3 ion environment. The precursors used in this study, HSiCH3 OCH2 CH3 2 MDES, Si CH3 OCH2 CH3 3 MTEOS and H±Si OCH2 CH3 3 TREOS are particularly versatile for the synthesis of new hybrid materials [31±33]. Indeed, hybrid xerogels containing SiAH bonds show high hydrophobicity [31,32] and are very ecient host matrices for spirooxazine photochromic dyes [34]. Moreover, dehydrocondensation of organic hydrosilanes with silanols for the synthesis of the siloxane linkage [35] occurs with the evolvement of hydrogen gas. In this sense, alkoxide precursors containing SiAH groups have shown the possibility of using the SiAH groups as an in situ reducing agent, which allows the formation of metal/silica nanocomposites [36]. Moreover, at room temperature the transition metal alkoxide catalyzed the cleavage of the SiAH bonds and can be used to reduce, during the ®rst step of hydrolysis and condensation reactions, europium (III) into its divalent state [20]. The present paper addresses the synthesis at room temperature of several hybrid organic±inorganic materials made from hydrolysis and condensation of organo hydrosilanes, alkoxysilanes and zirconium propoxide. The undoped matrices present some remarkable blue emission properties under UV excitation. In the presence of EuCl3 or Ce NO3 4 , 2NH4 NO3 Eu2 - or Ce3 -doped hybrid materials are obtained through a reduction process which occurs in situ at room temperature via the transition metal alkoxide catalyzed cleavage of the SiAH bonds. These rare earth-doped hybrid materials also exhibit an intense blue emission. The absorption and emission properties of the doped and undoped matrices are presented, discussed and completed by using kinetics of ¯uorescence, excitation and ESR measurements.
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2. Materials synthesis The raw materials used in samples preparation were the following: [methyldiethoxysilane HSi CH3 OCH2 CH3 2 , MDES DH , 100% ABCR], triethoxysilane [HSi OCH2 CH3 3 , TREOS TH , >90% Fluka], methyltriethoxysilane [Si CH3 OCH2 CH3 3 , MTEOS TMe , >98% Fluka], zirconium tetrapropoxide [Zr OPrn 4 , (ZrP), solution 70% in propanol, Fluka], acetylacetone [CH3 COCH2 COCH3 , also named acac in the paper, 99.5% Fluka], ethanol [CH3 CH2 OH, 99.5% Normasolv], EuCl3 (99.9% Strem) and Ce NO3 4 , 2NH4 NO3 (99.9% Merck). The reagents were used without further puri®cation. 2.1. Preparation of undoped and rare earth-doped hybrid materials The ®rst system (labeled A±acac or DH =TMe / Zr±acac) is obtained through the hydrolysis± condensation of diethoxymethylsilane HSi CH3 OCH2 CH3 2 , MDES DH and methyltriethoxysilane CH3 Si OCH2 CH3 3 , MTEOS TMe precursors in the presence of acetylacetone (acac) complexed zirconium propoxide. Eu-doped materials were synthesized by adding trivalent europium (EuCl3 ) to the zirconium propoxide solution and the cerium compounds using a tetravalent cerium salt. The undoped compounds are obtained according to the same procedure but without the use of EuCl3 or Ce NO3 4 ; 2NH4 NO3 . The typical synthetic procedure is the following: DH and TMe precursors are ®rst hydrolyzed with neutral water in ethanol. The resulting siloxanebased sol is then slowly added to an ethanolic solution of ZrP in the presence of acetylacetone. The second and third systems (labeled system A or DH =TMe =Zr and system B or DH =TH =Zr) were synthesized as follows: an ethanol±water solution containing europium chloride was ®rst mixed with siloxane precursors. The resulting solution was then added to a commercial propanolic solution of zirconium propoxide. For every matrix two concentrations of rare earth ions have been achieved. Considering that hybrid materials are homogeneous amorphous matrices with density close to 1.2 [18], evaluations of the rare earth concentra-
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tions in ions=cm3 are given in Table 2. Many hybrid networks made from hydrolysis and condensation of alkoxy silane precursors and zirconium or titanium metal alkoxides have been extensively characterized by solid state multinuclear NMR, scattering and X-ray absorption techniques [5,14±18]. They generally can be described as nanocomposites built from siloxanebased polymer cross-link by metal oxo species [5,14±18]. In the present study the metal oxo species are made of zirconium oxo species. For the cerium compounds, acetylacetone is preferably not used even if the sol±gel synthesis is more dicult, to avoid additional absorption in the UV range. The molar ratios MDES/MTEOS/H2 O/ethanol/ Zr/acac (system A±acac), MDES/MTEOS/H2 O/ ethanol/Zr (system A) and MDES/TREOS/H2 O/ ethanol/Zr (system B), the ®nal concentrations (ions=cm3 ) and the RE/Si and RE/Zr molar ratios are reported in Tables 1 and 2. For all the dierent hybrid systems (labeled A, A±acac, B, EuA, EuA±acac, EuB, CeA, CeB), the mixing between zirconium propoxide solutions and siloxane sol leads to an evolving of hydrogen gas which was used as a reducing agent to decrease the valence of europium cations from Eu3 to Eu2 and from Ce4 to Ce3 . The resulting clear sols were magnetically stirred for 30 min in argon atmosphere. In order to obtain transparent monolithic xerogels and coatings obtained by spin or dip coating with a thickness around 10 lm (measured by optical step focusing), an appropriate amount of the sols were poured into plastic cuvettes or deposited onto previously cleaned glass sheets.
Table 1 Labeling of the undoped hybrid matrices and the corresponding synthesis molecular ratio of precursors Reference
Matrix
Molar ratio
A±acac
DH =TMe =Zr±acac
A
DH =TMe =Zr
B
DH =TH =Zr
EtOH/MDES/MTEOS/ H2 O/ZrP/acac 7:82=1=1=2=0:22=0:22 EtOH/MDES/MTEOS/ H2 O/ZrP/ 7:82=1=1=2=0:22 EtOH/MDES/TREOS/ H2 O/ZrP/ 1=1:4=0:6=2=0:4
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Table 2 Labeling of the Eu- and Ce-doped hybrid matrices and an estimation of the corresponding europium and cerium concentrations Eu-doped matrices
Eu/Zr
Eu/Si
Ions=cm3
EuA±acac (10%) EuA±acac (1%) EuA (10%) EuA (1%) EuB (5%) EuB (0.5%)
0.1 0.01 0.1 0.01 0.05 0.005
0.01 0.001 0.01 0.001 0.01 0.001
8 1019 9 1018 8 1019 9 1018 4 1019 4 1018
Ce-doped matrices
Ce/Zr
Ce/Si
Ions=cm3
CeA (2.5%) CeA (0.6%) CeB (6%) CeB (0.2%)
0.025 0.006 0.06 0.002
0.003 0.0006 0.01 0.0003
2 1019 5 1018 1 1020 3 1018
Gelation time ranges between one week for systems A and A±acac down to three days for system B. In all cases, an argon atmosphere has been used for preparing the samples in order to favor the stability of the reduced oxidation states. In all the gels, the optical quality is very good and it is therefore possible to investigate their optical properties. 3. Experimental The optical absorption spectra have been recorded at room temperature for hybrid xerogels (systems A, A±acac, B, Eu±A, Eu±A±acac, Eu±B, Ce±A, Ce±B) with a Cary 5 (Varian) spectrophotometer using as reference undoped sols and xerogels. Emission measurements in the visible range have been carried out at room temperature after excitation at 355 nm (Eu and Ce) and 266 nm (Ce) with a frequency-tripled and quadrupled Q-switched Nd3 :YAG laser (BM Industries). The emission wavelength was analyzed using a HR250 monochromator (Jobin-Yvon) and then detected by an optical multichannel analyzer (OMA-EG & G). Lifetime measurements have been performed at dierent wavelengths. The Eu3 emission was characterized with an excitation in the emitting levels (5 D0 , and 5 D1 levels) coming from an optical parametric oscillator (OPO) pumped by the third harmonic of the Nd:YAG laser. Excitation measurements were recorded in the UV range with a Kontron-SFM15 spectrophotometer.
The ESR spectra were recorded at 77 K in the X-band on a Bruker ESP300E Spectrometer. The g values were determined by using DPPH (g 2:0037) as a reference. The undoped hybrid matrices were irradiated a few minutes in a quartz tube with UV lamp (VL 206 BLB, 365 nm tube) power 24 W. 4. Characterization of the undoped hybrid matrices 4.1. Optical properties of undoped hybrid matrices A and B systems described in Tables 1 and 2 are transparent in the visible and UV range until 250 nm (for A) and 280 nm (for B), respectively. The A±acac system is transparent in the visible range but presents a strong absorption below 400 nm due to the charge transfer acac±Zr, corresponding to a p ! p transition [37]. This absorption which expend slightly in the visible range is responsible for the yellow color observed in these gels (see Fig. 1). A very intense emission centered at 450 nm (22 200 cm 1 , 2.76 eV) is obtained in the A±acac system under UV excitation (see Fig. 2). The emission band is composed of several components in the visible range with a weak shoulder around 530 nm (18 900 cm 1 , 2.33 eV). For the acac-free systems named A and B, the emission bands present the same shape but are centered at higher energy 425 nm (23 500 cm 1 ) and 410 nm (24 390 cm 1 ) for A and B systems, respectively.
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Fig. 1. Room temperature absorption of the undoped matrices: (a) A±acac system (DH =TMe =Zr± acac); (b) A system DH =TMe =Zr; (c) B system (DH =TH =Zr). Absorption spectra have been recorded on 1 mm thick gels.
As from the absorption spectra, A and B matrices present no absorption bands similar to the ones observed in the excitation spectra, very low emission was at ®rst expected under 355 nm laser and xenon lamp excitation in the UV range. Excitation spectra have been recorded and reveal some intense bands in the UV spectral range (see Fig. 2). Then, the Stokes shift value of the transition could be estimated from both the emission and excitation measurements presented in Fig. 2. Stokes shift values are around 4000 cm 1 for the three systems. More surprising, excitation spectra corresponding to the dierent maxima on the emission bands are slightly dierent. When the excitation photon energy increases, i.e., when the excitation wavelength decreases, the emission band also shifts towards the higher energy from 440 to 425 nm for an excitation energy varying between 400 and 350 nm. This eect was assigned in the literature to intrinsic defects created by irradiation [38]. Fluorescence decay pro®les of these emissions under laser excitation at 355 nm reveal an exponential behavior with time constant values, respectively, around 3.7 and 3 ls for the B and A systems.
Fig. 2. Room temperature emission (plain) under 350 nm excitation and excitation (dashed) spectra of undoped hybrid matrices: (a) A±acac, (b) A and (c) B matrices. The intensities are normalized for each sample. For the excitation spectra the detector is set at the wavelength corresponding to the maximum of the emission.
The dierences in emission (intensity and maxima location) observed between A±acac and A or B matrices are probably due to a rigidity variation of the sol±gel hosts depending on the synthesis procedure. For instance the presence of chelating acac±Zr groups in matrix A±acac has been evidenced by infrared spectroscopy. Indeed, the IR spectrum of this matrix exhibits the characteristic vibrations mCO 1598 cm 1 and mC@C 1527 cm 1 ) of the enolic form of acetylacetonate ligand bonded to a transition metal [39]. Then, to go further into the understanding of the optical properties of the undoped matrix, electron spin
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resonance (ESR) measurements have been performed. 4.2. ESR results The ESR spectra of the three hybrid matrices (A, A±acac or B) recorded at 77 K were ESR silent. After a few minutes of irradiation with a UV lamp emitting at 365 nm, a wavelength very close to the laser wavelength used for the optical measurements, the typical ESR spectrum recorded at 77 K is presented in Fig. 3. This spectrum shows the presence of paramagnetic species characterized by magnetic g values close to those expected for inorganic radicals (g 2). After about 15 min this photo-induced paramagnetic signal vanished. The g values measured for the paramagnetic species created upon irradiation range in the expected domain for paramagnetic silicon-based species [40] or adsorbed paramagnetic oxygen [41,42]. The former species have been reported in sol±gel derived hydrosiloxane materials HSiO1:5 cured under inert atmosphere at 600°C [43] or in irradiated silicon oxide-based materials [38,40]. Three types of luminescent intrinsic defects induced by irradiation in silica have been identi®ed. They are E0 centers, the peroxy radical (SiAOAO ) or adsorbed oxygen radical and non-bridging oxygen hole
Fig. 3. Typical ESR spectra recorded at 77 K for a UV irradiated (365 nm) A or B hybrid matrices.
center (SiAO ) [38,44]. The E0 center usually denoted by BSi corresponds to a dangling electron in an sp3 orbital. It may be formed upon the synthesis of the hybrid matrices because the cleavage of SiAH bonds can yield transitory reductive species such as H0 or BSi . A careful analysis of the shape of the ESR signal yields the following ®tted g values: gx 2:033, gy 2:009, gz 2:0033. In view of the g values determined from our ESR study, paramagnetic E0 type defects have not been identi®ed. On the other hand the peroxy radical (SiAOAO ) was identi®ed in silica matrices and characterized by a g tensor of 2.067, 2.0074 and 2.0014 [45] which values are quite dierent from those measured in the present study. Finally, the ESR data do not correspond to those reported for adsorbed O or SiAO species [41,44]. The observed resonance is similar to the characteristic orthorhombic signal reported for superoxide diatomic O2 ions adsorbed on oxide surfaces [41,42]. Moreover, the g values are close to those reported for O2 ions adsorbed at the surfaces of silica (gx 2:03, gy 2:009, gz 2:0031) [46] or titania (gx 2:03, gy 2:008, gz 2:004 or gx 2:025, gy 2:009, gz 2:003) [47]. The largest measured g value (g 2:033) is in good agreement with prediction of the ionic model for a cation of charge 4 (Si or Zr) adsorbed at the surface. It is therefore possible to correlate this photoinduced paramagnetic signal with the blue luminescence observed for the undoped hybrid matrices because the observed emission maxima and line widths are close to those reported for luminescence of defects (2:93 0:28 eV) associated with BSiAOAOASiB or O2 species. Then, the undoped hybrid matrices present the main emission in the visible range under UV irradiation which likely involves O2 adsorbed species, whose precursors may be, peroxy links as BSiAOAOASiB, BZrAOAOASiB or BZrAOAOAZrB. The location of these species in the siloxane or in the zirconium oxo-based domains cannot be determined on the basis of our experimental data. This stable emission, observed under irradiation several months after the gel preparation, is particularly intense in the A±acac system while the three gel
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systems remain stable under relatively high laser 2 irradiation energy (up to 70 mJ=cm ). 5. Optical properties of Eu- and Ce-doped hybrid matrices In the following part of the paper, cerium and europium ions' optical properties in the sol±gel matrices are considered and, in particular, trivalent cerium should present a very intense and fast emission (in the range of a few tens of nanoseconds) in the UV-blue spectral range. Fast decays, of the order of 1 ls, are also expected with the divalent europium. 5.1. Absorption of the rare earth ions The room temperature absorption spectra of Eu-doped hybrid coatings are very similar in the sol and xerogels states indicating that during the geli®cation process the surrounding around the rare earth cations remains similar. Absorption spectra are constituted of a broad absorption in the UV range attributed to the 4f 7 5d0 ! 4f 6 5d1 transition (Eu2 ). This parity-allowed electric dipole transition between the 4f and 5d states features intense and broad absorption and ¯uorescence bands. The intensity is related to the high probability of the transition while the broadness is linked to the different ligand positions for 4f and 5d electronic states. The absorption spectrum of the Eu-doped gels shows an asymmetric broad band in the UV range. The composite shape observed for the absorption bands (Fig. 4(A)) can result from the fact that the 5d orbitals are splitted by the crystal ®eld interaction into two e orbitals separated from the t2 orbitals. It is however dicult to assign in more detail the e and t2 orbitals in these systems, as some other absorption bands lying at higher energy can be blurred by the intrinsic matrix absorption. The absorption maxima are located at 355 nm (28 200 cm 1 ), 305 nm (32 800 cm 1 ), and 325 nm (30 800 cm 1 ), for the matrices Eu±A±acac, Eu± A, and Eu±B, respectively. For these systems,very small peaks, not visible in Fig. 4, attributed to Eu3 , can also be detected in the visible range corresponding to transitions
Fig. 4. (A) Absorption spectra of europium in: (a) Eu±A (1% Eu at=cm3 ) and (b) Eu±B matrices (0.5%), (B) Absorption spectra of cerium in: (a) Ce±A (0.6%), (b) Ce±B (0.2%) matrices. Absorption has been recorded on around 1 mm thick samples.
from the fundamental 7 F0 to the excited 5 D0;1;2;3 and 5 L6 levels. This clearly indicates that not all the Eu3 have been reduced to divalent europium. A previous work based on the relative intensity of the Eu2 and Eu3 absorption bands and lifetime values [20] leads to a ratio of the divalent species about ®ve times the value of the trivalent species. In the present paper,the presence of Eu3 will only be shortly described in the luminescence part. Eu±acac absorption is not presented in Fig. 4(A) as this material presents an absorption spectrum comparable to the absorption band presented in Fig. 1(c), i.e., a broad and intensive band below 400 nm.
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The room temperature absorption spectra of Ce3 ions are presented in Fig. 4(B). In the A and B Ce-doped systems, a saturation of the absorption transition is observed for the high cerium content. This indicates that in situ reduction occurs leading to a Ce4 ! Ce3 conversion during the gel synthesis. However the proportion of the cerium ions in the two oxidation states 3+ and 4+ is dicult to determine, as Ce4 ions are optically inactive. The room temperature absorption spectra of Ce-doped hybrid coatings exhibit a broad band centered at 305 nm (32 800 cm 1 ). This band assigned to the broad 4f ! 5d transition is located at the same energy for the two hybrid matrices (Ce±A and Ce±B). In the Ce3 absorption spectra, only one band is clearly observed, but the 5d orbitals could also be split by the crystal ®eld interaction into e orbitals separated from the t2 orbitals and this should therefore lead to the observation of several bands in the UV spectral range. This is not possible in our case, because A and B matrices absorption bands are below 280 nm. 5.2. Rare earth emissions and kinetic measurements Emission features of divalent and trivalent europium and trivalent cerium have been observed in the rare earth-doped siloxane-oxide coatings. Eu2 and Eu3 emission spectra present very dierent characteristics. The emission bands of the divalent europium should appear in the blue range corresponding to the broad inter-con®gurational 5d ! 4f transition while narrow lines around 600 nm are expected for the intra-con®gurational 4f ! 4f 5 D0 ! 7 FJ , transitions of the trivalent europium. The emission spectra reported in Fig. 5 for Bsystem show a broad emission in the blue range under UV excitation at 355 nm (in the edge of the Eu2 absorption bands) and peaks emission in the red part of the spectrum. The broad emission could be attributed to the Eu2 species but also to a host emission as previously described. Indeed, the excitation spectra at dierent emission wavelengths indicate that the emission is mainly due to the matrix emission, with a contribution to the excitation spectrum of the Eu2 species at the
Fig. 5. Emission spectra of Eu-doped B-hybrid matrix, under 355 nm excitation, recorded with dierent time delays after the laser pulses. The Eu3 emissions from the 5 D0 and 5 D1 emitting levels are also presented at longer wavelengths.
shortest wavelengths. According to the excitation data, the emission of the matrix is around one order of magnitude more intense than the divalent europium emission.The ratio of the divalent and trivalent europium species is about ®ve while this ratio is about one for the A-system, due to a more ecient reductive medium provided by the initial mixture of the europium trichloride with the MDES and TREOS silane precursors in case of the B-system [20]. Therefore, at ®rst, for the Bsystem, a relatively intense Eu2 emission was expected. But, as the energy positions of the divalent europium and host bands are very close, energy transfer phenomena could also occur. Lifetime measurements have been performed at dierent emission wavelengths in the xerogel systems. The lifetime value measured at the maximum of the broad emission band for the Eu±A matrix, around 420 nm, is estimated to be about 0.5 ls. This value is smaller than those of the undoped system (see Part 4.1). At 610 nm, only the emission occurring from the 5 D0 Eu3 level is observed and presents an exponential decay pro®le with a lifetime value around 670 ls. It is also possible to observe the 5 D1 emission around 550 nm with a much shorter lifetime. The dierent kinetics of the
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emission can be visualized in Fig. 5. The 5 D1 emitting level presents a short lifetime value of 2 ls as this emission decay is shortened by a non-radiative process. Two non-radiative deexcitation processes are in competition: ®rst a cross-relaxation process involving the 5 D1 =5 D0 and 7 F0 =7 F2 and a non-radiative deexcitation related to the presence of hydroxyl groups in the gels. The latter hypothesis is the more likely process as the 5 D1 ! 5 D0 multiphonon relaxation mechanism involved energy levels separated by only 2000 cm 1 . Similar results have been obtained with Nd3 -doped hybrid gels (see Ref. [19]), where the lifetime values were strongly aected by the presence of the remaining hydroxyl groups. Finally for the europium species, the Eu2 luminescence decreases in comparison with the matrix emission when the xerogels were kept in air during a few weeks, showing that the stability of the Eu2 species still need to be improved. For Ce3 ions, an emission band at 370 nm is observed for both Ce-doped hybrid matrices. It corresponds to the parity-allowed electric dipole transition 5d ! 4f (see Fig. 6 for the Ce±B system). In addition to this emission, a slight emission of the host (one order of magnitude smaller, see Fig. 6) is also observed at longer wavelengths
Fig. 6. Emission (plain) and excitation (dashed) spectra of Ce3 -doped hydrid Ce±B (0.2%) matrix kexcit 300 nm for the emission).
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(ranging between 400 and 550 nm) as previously described in this paper. Excitation measurements are also used to extract the rare earth luminescence from the host emission (see Fig. 6). The excitation spectrum of the Ce±B emission is characterized by two bands peaking at about 270 and 302 nm in very good agreement with the absorption data (see Fig. 4). For the Ce3 ions in the siloxane-oxide matrix, a very fast emission, with a decay time of 50 ns, is observed in the UV-blue range (see Fig. 7). The Stokes shift of the cerium emission is around 5400 and 5000 cm 1 for the Ce±A and Ce±B compounds, respectively, corresponding to relatively large values for this ion and therefore high destabilization of the CeAO bonding [49]. In the case of high Stokes shift, non-radiative deexcitation could limit the ¯uorescence and should lead to relatively low quantum yields. Indeed similar values were recently obtained by Iwasaki [21] where the emission peak positions are reported between 352 and 381 nm while the corresponding Stokes shift varies between 4050 and 5800 cm 1 . For these hybrid sol±gels the reported quantum yield is of the order of 10%. The shift between the absorption and emission energies of Ce3 located in an oxygen ligand ®eld has been assigned to a combination of crystal ®eld
Fig. 7. Intensity decay pro®le of Ce±A sol±gel hybrid at two dierent emission wavelengths: (a) 423.1 nm corresponding to Ce3 emission and (b) 524 nm corresponding to the matrix emission.
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and nephelauxetic eects [23]. Structure containing oxygen atoms in higher coordination number environments (highly coordinated by metal atoms) should produce Ce3 emission at longer wavelengths, and distortion of the oxygen polyhedra from ideal coordination geometry results in a larger Stokes shift. Large Stokes shift has also been associated with a more asymmetric dopant geometry [24,25]. The intensity of the cerium luminescence seems not to be aected when the xerogels (A and B systems) are kept in air during several months, showing that this rare earth cation is ef®ciently trapped inside the hybrid matrix. Energy transfer processes between the sol±gel host and the rare earth ions could be considered even if this aspect is not completed in the present work. In the gels, the extension of the ligand orbitals is favorable to the energy transfer even if the 4f orbitals are in general considered as insensitive to the local surrounding. The decay time of the emission in the visible range is aected by the presence of europium cations. A decrease of the initial decay time values from about 3 ls to about 1 ls is observed in the Eu-doped hybrids. As the emission in the visible range in the Eu-doped hybrids is composed mainly of the matrix emission, and that selective lifetime measurements were not possible, only general considerations could be expressed. Spectral overlap is observed between the emission bands of the Eu2 species and the broad absorption of the photo-induced emitted centers in the sol±gel matrices below 400 nm. This could indicate an energy transfer from the rare earth dopants to the hybrid matrix host. In that case, as the divalent europium should present a very fast decay, the kinetics of the emission band around 440 nm should not be aected. The opposite transfer, from the matrix to the rare earth ions, which can aect the kinetic value, is not likely to occur because of the very bad resonance [28,48±51] between the absorption of the rare earth cations and the matrix emission. Then this process is only possible assisted by the phonon of the matrix and in that case host local phonons have to be considered. Another criterion corresponds to the distance between the species, but this distance is quite dicult to establish at the present state of our study.
Then energy transfer processes are not evidenced at the present time and this part requires further work due to the particular structure of these sol±gel hosts. This can be performed for instance with one selected matrix doped with various rare earth concentrations, but the stability of the reduction ± stabilization of the Eu2 cations ± needs to be improved. 6. Conclusions Sol±gel derived transparent hybrid matrices (the cut-o can be at 250 or 280 nm) which exhibit at room temperature intense blue luminescence (maximum of emission located at about 400±450 nm) have been synthesized. The luminescence arises either from matrix-peroxide defects promoted by the reductive medium of the synthesis or from Eu2 or Ce3 phosphors which have been generated in situ upon the zirconium alkoxide catalyzed cleavage of the SiAH bonds. For the cerium ions, the intensity of the luminescence did not vary when the matrices are kept in air during several months, but on the contrary, the stability of the divalent europium needed to be improved. But the ®rst goal of this work was to demonstrate the reduction capability of these sol±gels. The emitting species exhibit very dierent kinetics of ¯uorescence ranging between 50 ns and few microseconds. Such dierences can be used either to assign the dierent emitting species (Ce3 , Eu2 , Eu3 ) or for the tuning of properties corresponding to dierent ®elds of potential applications. The very strong emission of the undoped or Ce3 -doped hybrid matrices could be very useful in the ®eld of scintillators and new neutron detector systems. Eu3 also observed in the Eu-doped matrices indicates that the in situ reduction is not complete and that the divalent species are not stable. This optical analysis also shows that the energy dierence between the absorption and the emission corresponding to the rare earth ions depends on the chemical protocol and thus on the resulting matrix (A±acac, A, B). In particular, for the cerium ion, relatively high Stokes shift values around 5000 cm 1 are measured and in that case this
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should lead to a relatively small quantum eciency. Then, this study opens the possibility to embed preferentially the rare earth in zirconium oxo or siloxane-based domains and to analyze the corresponding optical properties. The next step of the work will be to determine the quantum eciency and the scintillating potential of the dierent undoped and rare earth doped hosts. However, to be able to improve eciently the promising properties of these hybrid materials, future work will also be devoted to the improvement of our knowledge of the relation between the optical properties of the luminescent centers and the nanostructure and chemical composition of the hybrid hosts. One other part should be devoted to the investigation of the gel host to the rare earth energy transfers, as well as to the energy transfer between the dopant ions.
Acknowledgements P. Aschehoug is gratefully acknowledged for his precious help during optical measurements. The authors would also like to thank ``Accion Integrada Hispano-Francesa Picasso'' no. HF19980.196 and DGCYT (project PB 98/1042) for the ®nancial support. B. Morin is strongly acknowledged for his ecient help during ESR measurements.
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[38] M.A. Stevens Kalce, M.R. Phillips, Phys. Rev. B 52 (5) (1995) 3122. [39] J. Garcõa, M.A. Mondrag on, O. Maya, A. Campero, J. Alloys Comp. 275 (1998) 273. [40] E. Holzenkampfer, F.W. Richter, J. Stuke, U. VogetGrote, J. Non-Cryst. Solids 32 (1979) 327. [41] M. Che, Advances in Catalysis, vol. 32, Academic Press, New York. [42] J.H. Lunsford, Copyright by Marcel Dekker, New York, 1973. [43] V. Belot, R. Corriu, D. Leclercq, P. Lefevre, P.H. Mutin, A. Vioux, A.M. Flanck, Chemical Processing of Advanced Materials (1992) 143. [44] D.L. Griscom, J. Non-Cryst. Solids 73 (1985) 51.
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www.elsevier.com/locate/optmat
Blue emitting hybrid organic±inorganic materials E. Cordoncillo a, F.J. Guaita a, P. Escribano a, C. Philippe b, B. Viana b,*, C. Sanchez b a
Departamento de Quimica Inorganica y Organica, Campus Riu Sec, Universitat Jaume I. Castellon, Spain b Laboratoire de Chimie de la Mati e re Condens ee, Universit e Pierre et Marie Curie, UMR CNRS 7574, 4 place Jussieu, Paris 75252, France Received 20 July 2000; accepted 15 May 2001
Abstract Undoped and Eu2 - or Ce3 -doped hybrid organic±inorganic materials were prepared at room temperature from hydrolysis and condensation of organohydrosilanes HSi CH3 OCH2 CH3 2 , HSi OCH2 CH3 3 , alkoxysilanes Si CH3 OCH2 CH3 3 and zirconium propoxide in the presence of EuCl3 or Ce NO3 4 , 2NH4 NO3 . The transition metal alkoxide catalyzed cleavage of the SiAH bonds was used to reduce in situ at room temperature the rare earth cations. Depending on the chemical strategy, the resulting hybrid materials can be processed as transparent bulks or coatings, which exhibit a good transparency in the UV±visible domain (cut-o at 250±280 nm). Both the undoped matrices and the rare earth-doped matrices exhibit a strong blue emission. The nature of dierent emitting species is clearly assigned by using their very dierent kinetics of ¯uorescence (s 50 ns for Ce3 , s < 1 ls for Eu2 s 3 and 4 ls for the undoped matrices) and through excitation experiments. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Luminescence; Hybrid; Sol±gel; Eu2 ; Ce3
1. Introduction Sol±gel technology is widely used to prepare and shape oxide-based materials [1±3]. Moreover, the mild characteristics oered by the sol±gel process allow introducing organic molecules inside an inorganic network [4±8]. Inorganic and organic components can then be mixed at the nanometric scale, in virtually any ratio leading to the so-called hybrid organic±inorganic nanocomposites which are extremely versatile in their composition, processing and properties [5,9,10]. In this sense, the
*
Corresponding author. E-mail address: [email protected] (B. Viana).
synthesis and characterization of organic±inorganic hybrid materials is a rapidly growing ®eld of research opening a land of opportunity for the design of new materials for photonics [10], mechanics [11] and biology [12,13]. Mixed siloxane± metal oxide materials, from which transparent coatings can be easily prepared, are good hybrid matrices for optics [10,14]. They can be synthesized by using alkoxysilane precursors SiR0x OR4 x , (R0 being any organic group; CH3 , H, or any organic dye) which lead through hydrolysis and condensation reactions to the formation of a siloxane type network. The siloxane-based network can be eventually cross-linked or mixed at the nanosize level with metal oxo polymers obtained via hydrolysis±condensation of metal
0925-3467/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 1 ) 0 0 1 7 0 - 7
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alkoxides M(OR)n , (M Si, Ti, Zr and Al) [14± 18]. The metal oxo polymeric network can be used to implement another property (modify the refractive index of the hybrid matrix, induce magnetic properties, etc.) or simply as a host for guest cations that need to accommodate a speci®c coordinance. Due to their advantages, such as low temperature processing, easy shaping of thick ®lms, higher sample homogeneity and purity, and the opportunity to prepare non-crystalline solids, siloxane-based hybrid matrices are currently investigated as potential matrices for rare earth luminescence [10,19±22]. The optical properties of rare earth ions present many interests because they are used as visible and near-IR radiation sources, some of them being particularly important for lasers and optical communication devices. Rare earth ions incorporated in metal oxides or ¯uorides are usually stabilized in trivalent states. Furthermore, some rare earth cations incorporated in crystalline or glassy matrices can exhibit lower valence states (divalent ions as Eu2 , Sm2 ). The emission of the divalent lanthanide ions originates either from an intracon®gurational 4f n transition or from an intercon®gurational 4f n 1 5d ! 4f n transition: line or band emission, respectively. Among divalent lanthanides, Eu2 ion may present some stability because of its 4f 7 con®guration. This Eu2 ion is particularly unique because its broad and intense band luminescence 4f 6 5d1 ! 4f 7 is strongly host dependent with emission wavelength extending from the UV to the red range of the electromagnetic spectrum [23±25]. Previously reported studies have demonstrated the possibility to use Eu2 doped compounds as phosphors, or for medical applications and skin tanning [23±25]. Some tetravalent rare earth ions (Ce4 , Pr4 , Tb4 ) can also be stabilized and in that case, charge transfer absorption bands could also be observed in the UV± visible range. There is also a lot of interest in Ce3 -doped crystalline or glassy hosts for applications in the scintillator ®eld and in the research of tunable lasers in the UV and visible ranges [26±28]. Recently, transparent inorganic±organic hybrid monoliths doped with Ce3 ions have been prepared by sol± gel process [21,29]. Such inorganic hydrogenous
materials can be used as neutron detectors [30]. In comparison to the organic scintillators, they present higher hydrogen density and very high scintillation eciency. Of course, due to their low density, these materials could not be used as high energy detectors. Dierent anities of the silane functional groups for Ce3 were observed. Optical properties such as absorption spectra, emission spectra and ¯uorescence quantum yield were strongly aected by the Ce3 ion environment. The precursors used in this study, HSiCH3 OCH2 CH3 2 MDES, Si CH3 OCH2 CH3 3 MTEOS and H±Si OCH2 CH3 3 TREOS are particularly versatile for the synthesis of new hybrid materials [31±33]. Indeed, hybrid xerogels containing SiAH bonds show high hydrophobicity [31,32] and are very ecient host matrices for spirooxazine photochromic dyes [34]. Moreover, dehydrocondensation of organic hydrosilanes with silanols for the synthesis of the siloxane linkage [35] occurs with the evolvement of hydrogen gas. In this sense, alkoxide precursors containing SiAH groups have shown the possibility of using the SiAH groups as an in situ reducing agent, which allows the formation of metal/silica nanocomposites [36]. Moreover, at room temperature the transition metal alkoxide catalyzed the cleavage of the SiAH bonds and can be used to reduce, during the ®rst step of hydrolysis and condensation reactions, europium (III) into its divalent state [20]. The present paper addresses the synthesis at room temperature of several hybrid organic±inorganic materials made from hydrolysis and condensation of organo hydrosilanes, alkoxysilanes and zirconium propoxide. The undoped matrices present some remarkable blue emission properties under UV excitation. In the presence of EuCl3 or Ce NO3 4 , 2NH4 NO3 Eu2 - or Ce3 -doped hybrid materials are obtained through a reduction process which occurs in situ at room temperature via the transition metal alkoxide catalyzed cleavage of the SiAH bonds. These rare earth-doped hybrid materials also exhibit an intense blue emission. The absorption and emission properties of the doped and undoped matrices are presented, discussed and completed by using kinetics of ¯uorescence, excitation and ESR measurements.
E. Cordoncillo et al. / Optical Materials 18 (2001) 309±320
2. Materials synthesis The raw materials used in samples preparation were the following: [methyldiethoxysilane HSi CH3 OCH2 CH3 2 , MDES DH , 100% ABCR], triethoxysilane [HSi OCH2 CH3 3 , TREOS TH , >90% Fluka], methyltriethoxysilane [Si CH3 OCH2 CH3 3 , MTEOS TMe , >98% Fluka], zirconium tetrapropoxide [Zr OPrn 4 , (ZrP), solution 70% in propanol, Fluka], acetylacetone [CH3 COCH2 COCH3 , also named acac in the paper, 99.5% Fluka], ethanol [CH3 CH2 OH, 99.5% Normasolv], EuCl3 (99.9% Strem) and Ce NO3 4 , 2NH4 NO3 (99.9% Merck). The reagents were used without further puri®cation. 2.1. Preparation of undoped and rare earth-doped hybrid materials The ®rst system (labeled A±acac or DH =TMe / Zr±acac) is obtained through the hydrolysis± condensation of diethoxymethylsilane HSi CH3 OCH2 CH3 2 , MDES DH and methyltriethoxysilane CH3 Si OCH2 CH3 3 , MTEOS TMe precursors in the presence of acetylacetone (acac) complexed zirconium propoxide. Eu-doped materials were synthesized by adding trivalent europium (EuCl3 ) to the zirconium propoxide solution and the cerium compounds using a tetravalent cerium salt. The undoped compounds are obtained according to the same procedure but without the use of EuCl3 or Ce NO3 4 ; 2NH4 NO3 . The typical synthetic procedure is the following: DH and TMe precursors are ®rst hydrolyzed with neutral water in ethanol. The resulting siloxanebased sol is then slowly added to an ethanolic solution of ZrP in the presence of acetylacetone. The second and third systems (labeled system A or DH =TMe =Zr and system B or DH =TH =Zr) were synthesized as follows: an ethanol±water solution containing europium chloride was ®rst mixed with siloxane precursors. The resulting solution was then added to a commercial propanolic solution of zirconium propoxide. For every matrix two concentrations of rare earth ions have been achieved. Considering that hybrid materials are homogeneous amorphous matrices with density close to 1.2 [18], evaluations of the rare earth concentra-
311
tions in ions=cm3 are given in Table 2. Many hybrid networks made from hydrolysis and condensation of alkoxy silane precursors and zirconium or titanium metal alkoxides have been extensively characterized by solid state multinuclear NMR, scattering and X-ray absorption techniques [5,14±18]. They generally can be described as nanocomposites built from siloxanebased polymer cross-link by metal oxo species [5,14±18]. In the present study the metal oxo species are made of zirconium oxo species. For the cerium compounds, acetylacetone is preferably not used even if the sol±gel synthesis is more dicult, to avoid additional absorption in the UV range. The molar ratios MDES/MTEOS/H2 O/ethanol/ Zr/acac (system A±acac), MDES/MTEOS/H2 O/ ethanol/Zr (system A) and MDES/TREOS/H2 O/ ethanol/Zr (system B), the ®nal concentrations (ions=cm3 ) and the RE/Si and RE/Zr molar ratios are reported in Tables 1 and 2. For all the dierent hybrid systems (labeled A, A±acac, B, EuA, EuA±acac, EuB, CeA, CeB), the mixing between zirconium propoxide solutions and siloxane sol leads to an evolving of hydrogen gas which was used as a reducing agent to decrease the valence of europium cations from Eu3 to Eu2 and from Ce4 to Ce3 . The resulting clear sols were magnetically stirred for 30 min in argon atmosphere. In order to obtain transparent monolithic xerogels and coatings obtained by spin or dip coating with a thickness around 10 lm (measured by optical step focusing), an appropriate amount of the sols were poured into plastic cuvettes or deposited onto previously cleaned glass sheets.
Table 1 Labeling of the undoped hybrid matrices and the corresponding synthesis molecular ratio of precursors Reference
Matrix
Molar ratio
A±acac
DH =TMe =Zr±acac
A
DH =TMe =Zr
B
DH =TH =Zr
EtOH/MDES/MTEOS/ H2 O/ZrP/acac 7:82=1=1=2=0:22=0:22 EtOH/MDES/MTEOS/ H2 O/ZrP/ 7:82=1=1=2=0:22 EtOH/MDES/TREOS/ H2 O/ZrP/ 1=1:4=0:6=2=0:4
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Table 2 Labeling of the Eu- and Ce-doped hybrid matrices and an estimation of the corresponding europium and cerium concentrations Eu-doped matrices
Eu/Zr
Eu/Si
Ions=cm3
EuA±acac (10%) EuA±acac (1%) EuA (10%) EuA (1%) EuB (5%) EuB (0.5%)
0.1 0.01 0.1 0.01 0.05 0.005
0.01 0.001 0.01 0.001 0.01 0.001
8 1019 9 1018 8 1019 9 1018 4 1019 4 1018
Ce-doped matrices
Ce/Zr
Ce/Si
Ions=cm3
CeA (2.5%) CeA (0.6%) CeB (6%) CeB (0.2%)
0.025 0.006 0.06 0.002
0.003 0.0006 0.01 0.0003
2 1019 5 1018 1 1020 3 1018
Gelation time ranges between one week for systems A and A±acac down to three days for system B. In all cases, an argon atmosphere has been used for preparing the samples in order to favor the stability of the reduced oxidation states. In all the gels, the optical quality is very good and it is therefore possible to investigate their optical properties. 3. Experimental The optical absorption spectra have been recorded at room temperature for hybrid xerogels (systems A, A±acac, B, Eu±A, Eu±A±acac, Eu±B, Ce±A, Ce±B) with a Cary 5 (Varian) spectrophotometer using as reference undoped sols and xerogels. Emission measurements in the visible range have been carried out at room temperature after excitation at 355 nm (Eu and Ce) and 266 nm (Ce) with a frequency-tripled and quadrupled Q-switched Nd3 :YAG laser (BM Industries). The emission wavelength was analyzed using a HR250 monochromator (Jobin-Yvon) and then detected by an optical multichannel analyzer (OMA-EG & G). Lifetime measurements have been performed at dierent wavelengths. The Eu3 emission was characterized with an excitation in the emitting levels (5 D0 , and 5 D1 levels) coming from an optical parametric oscillator (OPO) pumped by the third harmonic of the Nd:YAG laser. Excitation measurements were recorded in the UV range with a Kontron-SFM15 spectrophotometer.
The ESR spectra were recorded at 77 K in the X-band on a Bruker ESP300E Spectrometer. The g values were determined by using DPPH (g 2:0037) as a reference. The undoped hybrid matrices were irradiated a few minutes in a quartz tube with UV lamp (VL 206 BLB, 365 nm tube) power 24 W. 4. Characterization of the undoped hybrid matrices 4.1. Optical properties of undoped hybrid matrices A and B systems described in Tables 1 and 2 are transparent in the visible and UV range until 250 nm (for A) and 280 nm (for B), respectively. The A±acac system is transparent in the visible range but presents a strong absorption below 400 nm due to the charge transfer acac±Zr, corresponding to a p ! p transition [37]. This absorption which expend slightly in the visible range is responsible for the yellow color observed in these gels (see Fig. 1). A very intense emission centered at 450 nm (22 200 cm 1 , 2.76 eV) is obtained in the A±acac system under UV excitation (see Fig. 2). The emission band is composed of several components in the visible range with a weak shoulder around 530 nm (18 900 cm 1 , 2.33 eV). For the acac-free systems named A and B, the emission bands present the same shape but are centered at higher energy 425 nm (23 500 cm 1 ) and 410 nm (24 390 cm 1 ) for A and B systems, respectively.
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313
Fig. 1. Room temperature absorption of the undoped matrices: (a) A±acac system (DH =TMe =Zr± acac); (b) A system DH =TMe =Zr; (c) B system (DH =TH =Zr). Absorption spectra have been recorded on 1 mm thick gels.
As from the absorption spectra, A and B matrices present no absorption bands similar to the ones observed in the excitation spectra, very low emission was at ®rst expected under 355 nm laser and xenon lamp excitation in the UV range. Excitation spectra have been recorded and reveal some intense bands in the UV spectral range (see Fig. 2). Then, the Stokes shift value of the transition could be estimated from both the emission and excitation measurements presented in Fig. 2. Stokes shift values are around 4000 cm 1 for the three systems. More surprising, excitation spectra corresponding to the dierent maxima on the emission bands are slightly dierent. When the excitation photon energy increases, i.e., when the excitation wavelength decreases, the emission band also shifts towards the higher energy from 440 to 425 nm for an excitation energy varying between 400 and 350 nm. This eect was assigned in the literature to intrinsic defects created by irradiation [38]. Fluorescence decay pro®les of these emissions under laser excitation at 355 nm reveal an exponential behavior with time constant values, respectively, around 3.7 and 3 ls for the B and A systems.
Fig. 2. Room temperature emission (plain) under 350 nm excitation and excitation (dashed) spectra of undoped hybrid matrices: (a) A±acac, (b) A and (c) B matrices. The intensities are normalized for each sample. For the excitation spectra the detector is set at the wavelength corresponding to the maximum of the emission.
The dierences in emission (intensity and maxima location) observed between A±acac and A or B matrices are probably due to a rigidity variation of the sol±gel hosts depending on the synthesis procedure. For instance the presence of chelating acac±Zr groups in matrix A±acac has been evidenced by infrared spectroscopy. Indeed, the IR spectrum of this matrix exhibits the characteristic vibrations mCO 1598 cm 1 and mC@C 1527 cm 1 ) of the enolic form of acetylacetonate ligand bonded to a transition metal [39]. Then, to go further into the understanding of the optical properties of the undoped matrix, electron spin
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resonance (ESR) measurements have been performed. 4.2. ESR results The ESR spectra of the three hybrid matrices (A, A±acac or B) recorded at 77 K were ESR silent. After a few minutes of irradiation with a UV lamp emitting at 365 nm, a wavelength very close to the laser wavelength used for the optical measurements, the typical ESR spectrum recorded at 77 K is presented in Fig. 3. This spectrum shows the presence of paramagnetic species characterized by magnetic g values close to those expected for inorganic radicals (g 2). After about 15 min this photo-induced paramagnetic signal vanished. The g values measured for the paramagnetic species created upon irradiation range in the expected domain for paramagnetic silicon-based species [40] or adsorbed paramagnetic oxygen [41,42]. The former species have been reported in sol±gel derived hydrosiloxane materials HSiO1:5 cured under inert atmosphere at 600°C [43] or in irradiated silicon oxide-based materials [38,40]. Three types of luminescent intrinsic defects induced by irradiation in silica have been identi®ed. They are E0 centers, the peroxy radical (SiAOAO ) or adsorbed oxygen radical and non-bridging oxygen hole
Fig. 3. Typical ESR spectra recorded at 77 K for a UV irradiated (365 nm) A or B hybrid matrices.
center (SiAO ) [38,44]. The E0 center usually denoted by BSi corresponds to a dangling electron in an sp3 orbital. It may be formed upon the synthesis of the hybrid matrices because the cleavage of SiAH bonds can yield transitory reductive species such as H0 or BSi . A careful analysis of the shape of the ESR signal yields the following ®tted g values: gx 2:033, gy 2:009, gz 2:0033. In view of the g values determined from our ESR study, paramagnetic E0 type defects have not been identi®ed. On the other hand the peroxy radical (SiAOAO ) was identi®ed in silica matrices and characterized by a g tensor of 2.067, 2.0074 and 2.0014 [45] which values are quite dierent from those measured in the present study. Finally, the ESR data do not correspond to those reported for adsorbed O or SiAO species [41,44]. The observed resonance is similar to the characteristic orthorhombic signal reported for superoxide diatomic O2 ions adsorbed on oxide surfaces [41,42]. Moreover, the g values are close to those reported for O2 ions adsorbed at the surfaces of silica (gx 2:03, gy 2:009, gz 2:0031) [46] or titania (gx 2:03, gy 2:008, gz 2:004 or gx 2:025, gy 2:009, gz 2:003) [47]. The largest measured g value (g 2:033) is in good agreement with prediction of the ionic model for a cation of charge 4 (Si or Zr) adsorbed at the surface. It is therefore possible to correlate this photoinduced paramagnetic signal with the blue luminescence observed for the undoped hybrid matrices because the observed emission maxima and line widths are close to those reported for luminescence of defects (2:93 0:28 eV) associated with BSiAOAOASiB or O2 species. Then, the undoped hybrid matrices present the main emission in the visible range under UV irradiation which likely involves O2 adsorbed species, whose precursors may be, peroxy links as BSiAOAOASiB, BZrAOAOASiB or BZrAOAOAZrB. The location of these species in the siloxane or in the zirconium oxo-based domains cannot be determined on the basis of our experimental data. This stable emission, observed under irradiation several months after the gel preparation, is particularly intense in the A±acac system while the three gel
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systems remain stable under relatively high laser 2 irradiation energy (up to 70 mJ=cm ). 5. Optical properties of Eu- and Ce-doped hybrid matrices In the following part of the paper, cerium and europium ions' optical properties in the sol±gel matrices are considered and, in particular, trivalent cerium should present a very intense and fast emission (in the range of a few tens of nanoseconds) in the UV-blue spectral range. Fast decays, of the order of 1 ls, are also expected with the divalent europium. 5.1. Absorption of the rare earth ions The room temperature absorption spectra of Eu-doped hybrid coatings are very similar in the sol and xerogels states indicating that during the geli®cation process the surrounding around the rare earth cations remains similar. Absorption spectra are constituted of a broad absorption in the UV range attributed to the 4f 7 5d0 ! 4f 6 5d1 transition (Eu2 ). This parity-allowed electric dipole transition between the 4f and 5d states features intense and broad absorption and ¯uorescence bands. The intensity is related to the high probability of the transition while the broadness is linked to the different ligand positions for 4f and 5d electronic states. The absorption spectrum of the Eu-doped gels shows an asymmetric broad band in the UV range. The composite shape observed for the absorption bands (Fig. 4(A)) can result from the fact that the 5d orbitals are splitted by the crystal ®eld interaction into two e orbitals separated from the t2 orbitals. It is however dicult to assign in more detail the e and t2 orbitals in these systems, as some other absorption bands lying at higher energy can be blurred by the intrinsic matrix absorption. The absorption maxima are located at 355 nm (28 200 cm 1 ), 305 nm (32 800 cm 1 ), and 325 nm (30 800 cm 1 ), for the matrices Eu±A±acac, Eu± A, and Eu±B, respectively. For these systems,very small peaks, not visible in Fig. 4, attributed to Eu3 , can also be detected in the visible range corresponding to transitions
Fig. 4. (A) Absorption spectra of europium in: (a) Eu±A (1% Eu at=cm3 ) and (b) Eu±B matrices (0.5%), (B) Absorption spectra of cerium in: (a) Ce±A (0.6%), (b) Ce±B (0.2%) matrices. Absorption has been recorded on around 1 mm thick samples.
from the fundamental 7 F0 to the excited 5 D0;1;2;3 and 5 L6 levels. This clearly indicates that not all the Eu3 have been reduced to divalent europium. A previous work based on the relative intensity of the Eu2 and Eu3 absorption bands and lifetime values [20] leads to a ratio of the divalent species about ®ve times the value of the trivalent species. In the present paper,the presence of Eu3 will only be shortly described in the luminescence part. Eu±acac absorption is not presented in Fig. 4(A) as this material presents an absorption spectrum comparable to the absorption band presented in Fig. 1(c), i.e., a broad and intensive band below 400 nm.
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The room temperature absorption spectra of Ce3 ions are presented in Fig. 4(B). In the A and B Ce-doped systems, a saturation of the absorption transition is observed for the high cerium content. This indicates that in situ reduction occurs leading to a Ce4 ! Ce3 conversion during the gel synthesis. However the proportion of the cerium ions in the two oxidation states 3+ and 4+ is dicult to determine, as Ce4 ions are optically inactive. The room temperature absorption spectra of Ce-doped hybrid coatings exhibit a broad band centered at 305 nm (32 800 cm 1 ). This band assigned to the broad 4f ! 5d transition is located at the same energy for the two hybrid matrices (Ce±A and Ce±B). In the Ce3 absorption spectra, only one band is clearly observed, but the 5d orbitals could also be split by the crystal ®eld interaction into e orbitals separated from the t2 orbitals and this should therefore lead to the observation of several bands in the UV spectral range. This is not possible in our case, because A and B matrices absorption bands are below 280 nm. 5.2. Rare earth emissions and kinetic measurements Emission features of divalent and trivalent europium and trivalent cerium have been observed in the rare earth-doped siloxane-oxide coatings. Eu2 and Eu3 emission spectra present very dierent characteristics. The emission bands of the divalent europium should appear in the blue range corresponding to the broad inter-con®gurational 5d ! 4f transition while narrow lines around 600 nm are expected for the intra-con®gurational 4f ! 4f 5 D0 ! 7 FJ , transitions of the trivalent europium. The emission spectra reported in Fig. 5 for Bsystem show a broad emission in the blue range under UV excitation at 355 nm (in the edge of the Eu2 absorption bands) and peaks emission in the red part of the spectrum. The broad emission could be attributed to the Eu2 species but also to a host emission as previously described. Indeed, the excitation spectra at dierent emission wavelengths indicate that the emission is mainly due to the matrix emission, with a contribution to the excitation spectrum of the Eu2 species at the
Fig. 5. Emission spectra of Eu-doped B-hybrid matrix, under 355 nm excitation, recorded with dierent time delays after the laser pulses. The Eu3 emissions from the 5 D0 and 5 D1 emitting levels are also presented at longer wavelengths.
shortest wavelengths. According to the excitation data, the emission of the matrix is around one order of magnitude more intense than the divalent europium emission.The ratio of the divalent and trivalent europium species is about ®ve while this ratio is about one for the A-system, due to a more ecient reductive medium provided by the initial mixture of the europium trichloride with the MDES and TREOS silane precursors in case of the B-system [20]. Therefore, at ®rst, for the Bsystem, a relatively intense Eu2 emission was expected. But, as the energy positions of the divalent europium and host bands are very close, energy transfer phenomena could also occur. Lifetime measurements have been performed at dierent emission wavelengths in the xerogel systems. The lifetime value measured at the maximum of the broad emission band for the Eu±A matrix, around 420 nm, is estimated to be about 0.5 ls. This value is smaller than those of the undoped system (see Part 4.1). At 610 nm, only the emission occurring from the 5 D0 Eu3 level is observed and presents an exponential decay pro®le with a lifetime value around 670 ls. It is also possible to observe the 5 D1 emission around 550 nm with a much shorter lifetime. The dierent kinetics of the
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emission can be visualized in Fig. 5. The 5 D1 emitting level presents a short lifetime value of 2 ls as this emission decay is shortened by a non-radiative process. Two non-radiative deexcitation processes are in competition: ®rst a cross-relaxation process involving the 5 D1 =5 D0 and 7 F0 =7 F2 and a non-radiative deexcitation related to the presence of hydroxyl groups in the gels. The latter hypothesis is the more likely process as the 5 D1 ! 5 D0 multiphonon relaxation mechanism involved energy levels separated by only 2000 cm 1 . Similar results have been obtained with Nd3 -doped hybrid gels (see Ref. [19]), where the lifetime values were strongly aected by the presence of the remaining hydroxyl groups. Finally for the europium species, the Eu2 luminescence decreases in comparison with the matrix emission when the xerogels were kept in air during a few weeks, showing that the stability of the Eu2 species still need to be improved. For Ce3 ions, an emission band at 370 nm is observed for both Ce-doped hybrid matrices. It corresponds to the parity-allowed electric dipole transition 5d ! 4f (see Fig. 6 for the Ce±B system). In addition to this emission, a slight emission of the host (one order of magnitude smaller, see Fig. 6) is also observed at longer wavelengths
Fig. 6. Emission (plain) and excitation (dashed) spectra of Ce3 -doped hydrid Ce±B (0.2%) matrix kexcit 300 nm for the emission).
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(ranging between 400 and 550 nm) as previously described in this paper. Excitation measurements are also used to extract the rare earth luminescence from the host emission (see Fig. 6). The excitation spectrum of the Ce±B emission is characterized by two bands peaking at about 270 and 302 nm in very good agreement with the absorption data (see Fig. 4). For the Ce3 ions in the siloxane-oxide matrix, a very fast emission, with a decay time of 50 ns, is observed in the UV-blue range (see Fig. 7). The Stokes shift of the cerium emission is around 5400 and 5000 cm 1 for the Ce±A and Ce±B compounds, respectively, corresponding to relatively large values for this ion and therefore high destabilization of the CeAO bonding [49]. In the case of high Stokes shift, non-radiative deexcitation could limit the ¯uorescence and should lead to relatively low quantum yields. Indeed similar values were recently obtained by Iwasaki [21] where the emission peak positions are reported between 352 and 381 nm while the corresponding Stokes shift varies between 4050 and 5800 cm 1 . For these hybrid sol±gels the reported quantum yield is of the order of 10%. The shift between the absorption and emission energies of Ce3 located in an oxygen ligand ®eld has been assigned to a combination of crystal ®eld
Fig. 7. Intensity decay pro®le of Ce±A sol±gel hybrid at two dierent emission wavelengths: (a) 423.1 nm corresponding to Ce3 emission and (b) 524 nm corresponding to the matrix emission.
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and nephelauxetic eects [23]. Structure containing oxygen atoms in higher coordination number environments (highly coordinated by metal atoms) should produce Ce3 emission at longer wavelengths, and distortion of the oxygen polyhedra from ideal coordination geometry results in a larger Stokes shift. Large Stokes shift has also been associated with a more asymmetric dopant geometry [24,25]. The intensity of the cerium luminescence seems not to be aected when the xerogels (A and B systems) are kept in air during several months, showing that this rare earth cation is ef®ciently trapped inside the hybrid matrix. Energy transfer processes between the sol±gel host and the rare earth ions could be considered even if this aspect is not completed in the present work. In the gels, the extension of the ligand orbitals is favorable to the energy transfer even if the 4f orbitals are in general considered as insensitive to the local surrounding. The decay time of the emission in the visible range is aected by the presence of europium cations. A decrease of the initial decay time values from about 3 ls to about 1 ls is observed in the Eu-doped hybrids. As the emission in the visible range in the Eu-doped hybrids is composed mainly of the matrix emission, and that selective lifetime measurements were not possible, only general considerations could be expressed. Spectral overlap is observed between the emission bands of the Eu2 species and the broad absorption of the photo-induced emitted centers in the sol±gel matrices below 400 nm. This could indicate an energy transfer from the rare earth dopants to the hybrid matrix host. In that case, as the divalent europium should present a very fast decay, the kinetics of the emission band around 440 nm should not be aected. The opposite transfer, from the matrix to the rare earth ions, which can aect the kinetic value, is not likely to occur because of the very bad resonance [28,48±51] between the absorption of the rare earth cations and the matrix emission. Then this process is only possible assisted by the phonon of the matrix and in that case host local phonons have to be considered. Another criterion corresponds to the distance between the species, but this distance is quite dicult to establish at the present state of our study.
Then energy transfer processes are not evidenced at the present time and this part requires further work due to the particular structure of these sol±gel hosts. This can be performed for instance with one selected matrix doped with various rare earth concentrations, but the stability of the reduction ± stabilization of the Eu2 cations ± needs to be improved. 6. Conclusions Sol±gel derived transparent hybrid matrices (the cut-o can be at 250 or 280 nm) which exhibit at room temperature intense blue luminescence (maximum of emission located at about 400±450 nm) have been synthesized. The luminescence arises either from matrix-peroxide defects promoted by the reductive medium of the synthesis or from Eu2 or Ce3 phosphors which have been generated in situ upon the zirconium alkoxide catalyzed cleavage of the SiAH bonds. For the cerium ions, the intensity of the luminescence did not vary when the matrices are kept in air during several months, but on the contrary, the stability of the divalent europium needed to be improved. But the ®rst goal of this work was to demonstrate the reduction capability of these sol±gels. The emitting species exhibit very dierent kinetics of ¯uorescence ranging between 50 ns and few microseconds. Such dierences can be used either to assign the dierent emitting species (Ce3 , Eu2 , Eu3 ) or for the tuning of properties corresponding to dierent ®elds of potential applications. The very strong emission of the undoped or Ce3 -doped hybrid matrices could be very useful in the ®eld of scintillators and new neutron detector systems. Eu3 also observed in the Eu-doped matrices indicates that the in situ reduction is not complete and that the divalent species are not stable. This optical analysis also shows that the energy dierence between the absorption and the emission corresponding to the rare earth ions depends on the chemical protocol and thus on the resulting matrix (A±acac, A, B). In particular, for the cerium ion, relatively high Stokes shift values around 5000 cm 1 are measured and in that case this
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should lead to a relatively small quantum eciency. Then, this study opens the possibility to embed preferentially the rare earth in zirconium oxo or siloxane-based domains and to analyze the corresponding optical properties. The next step of the work will be to determine the quantum eciency and the scintillating potential of the dierent undoped and rare earth doped hosts. However, to be able to improve eciently the promising properties of these hybrid materials, future work will also be devoted to the improvement of our knowledge of the relation between the optical properties of the luminescent centers and the nanostructure and chemical composition of the hybrid hosts. One other part should be devoted to the investigation of the gel host to the rare earth energy transfers, as well as to the energy transfer between the dopant ions.
Acknowledgements P. Aschehoug is gratefully acknowledged for his precious help during optical measurements. The authors would also like to thank ``Accion Integrada Hispano-Francesa Picasso'' no. HF19980.196 and DGCYT (project PB 98/1042) for the ®nancial support. B. Morin is strongly acknowledged for his ecient help during ESR measurements.
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