J. Phys. Chem. C 2007, 111, 2539-2544
2539
Photoluminescent Rare-Earth Based Biphenolate Lamellar Nanostructures Mohamed Karmaoui,†,§ Luı´s Mafra,† Rute A. Sa´ Ferreira,‡ Joa˜ o Rocha,† Luı´s D. Carlos,‡ and Nicola Pinna*,†,§ Department of Chemistry and Department of Physics, CICECO, UniVersity of AVeiro, 3810-193 AVeiro, Portugal and Martin-Luther-UniVersita¨t Halle-Wittenberg, Institut fu¨r Anorganische Chemie, Kurt-Mothes-Str. 2, 06120 Halle (Saale), Germany ReceiVed: NoVember 3, 2006; In Final Form: December 11, 2006
A general nonaqueous route has been applied for the preparation of rare-earth ordered nanocrystalline hybrid structures. In a simple one-pot reaction process, Y3+ and Gd3+ isopropoxides were reacted in an autoclave with 4-biphenylmethanol at 275 °C. This approach leads to crystalline rare-earth oxide thin layers (∼0.6 nm) regularly separated from each other by organic layers of biphenolate molecules. The optical properties of these hybrid materials doped with Eu3+ and Nd3+, which are optical active lanthanides ions emitting in the red and near infrared, respectively, are presented. They show significant advantages compared to standard phosphors, such as higher radiance and luminance values, the possibility to tune the emission chromaticity by varying the excitation wavelength, and a much larger excitation range (250-350 nm and 300-500 nm for the Eu3+ and Nd3+ doped nanohybrids, respectively) shifted toward the red.
Introduction The present requirements for phosphors demand for highemission efficiency and for lower energy excitation compared to standard materials used in luminescent lamps, flat screen television, and biological labeling.1 However, to comply with such requirements new materials are needed. In fact, standard phosphors are based on the absorption due to a charge-transfer mechanism related to the excitation of an electron from the oxygen 2p orbital to the 4f orbitals of a lanthanide ion (e.g., Eu3+), which in oxides generally takes place at around 250 nm (i.e., ∼5 eV).2 To decrease the required excitation energy, materials based on different absorption physical processes are needed. A good approach is using the larger absorption crosssections and low energy absorptions of organic molecules to absorb the incident radiation, which will be further transferred to the emitting lanthanide ions (“antenna effect”).3 However, lanthanide complexes are know to be unstable under UV excitation.4,5 Ordered organic-inorganic hybrids are multifunctional materials offering a large variety of physical properties that not only depend on the inorganic and organic components but also on the interface between the two phases. Furthermore, the organic component can be easily modified to precisely tune the global properties of the final material.6,7 In the past few years, it has been shown that nonaqueous sol-gel reactions of benzyl alcohol with different metal oxides precursors (alkoxides, chlorides, and acetylacetonates) allow the controlled and facile synthesis of various crystalline metal-oxide nanoparticles.8-13 In particular, rare-earth based organicinorganic lamellar nanohybrids have been synthesized by the “benzyl alcohol route”. They are based on alternating thin (0.6 * Corresponding author. Fax: +351 234370004. E-mail: pinna@ ciceco.ua.pt. † Department of Chemistry. ‡ Department of Physics. § Martin-Luther-Universita ¨ t Halle-Wittenberg, Institut fu¨r Anorganische Chemie.
nm) Re2O3 (Re ) rare earth) oxide sheets separated by a double layer of benzoate molecules.14,15 They combine high-temperature stability and good emission properties in the visible and infrared spectrum.16,17 Here, we extend the benzyl alcohol route to a new alcoholic system, and we apply it to the synthesis of very thin Re2O3 lamellae separated by a double layer of biphenolate molecules. The idea to replace benzoate species with biphenolate ones is based on the larger absorption cross section and excitation range of the latter. In fact, from an optical point of view the biphenolate molecules between the layers give to the hybrid material the following advantages: (i) higher radiance and luminance values, (ii) larger excitation range with the maximum shifted toward the red, (iii) UV photostability, and (iv) the possibility to tune the emission chromaticity simply by varying the excitation wavelength. Moreover, the local structure of these nanohybrids also is studied using inedited solid-state NMR techniques. Especially, we present the first high-resolution 1H-89Y heteronuclear correlation spectrum, recorded using homonuclear (FrequencySwitched Lee-Goldburg) 1H decoupling. To the best of our knowledge, this is the first example of such a spectrum of a low-γ nucleus. Experimental Methods The synthesis procedures were carried out in a glovebox (O2 and H2O < 0.1 ppm). In a typical synthesis of the nanohybrids, yttrium(III) isopropoxide (Strem; Y5O(OC3H7)13; 500 mg, 98%), or gadolinium(III) isopropoxide (synthesized following published method;18 Gd(OC3H7)3; 500 mg, 99.9%) was added to anhydrous 4-biphenylmethanol (Alfa; 10 g, 98%). In Eu(III) and Nd(III) doped nanocomposites, 5% molar lanthanide(III) isopropoxide was replaced by anhydrous europium(III) chloride (Strem 99.9%) or neodymium(III) isopropoxide (Strem 99.9%). The reaction mixture was transferred into a 45 mL steel autoclave and carefully sealed. The autoclave was taken out of the glovebox and was heated in a furnace at 275 °C for 2 days.
10.1021/jp0672609 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/25/2007
2540 J. Phys. Chem. C, Vol. 111, No. 6, 2007 The resulting precipitates were washed thoroughly with ethanol and dichloromethane and subsequently were dried in air at 80 °C. For each synthesis, the final yield is around 70%. Carbon, hydrogen, and nitrogen elemental analysis (CHN) and atomic absorption elemental analysis were performed to determine the stoichiometry of the different samples. The X-ray powder diffraction (XRD) diagrams of all samples were measured in transmission mode (Co KR radiation) on a Stoe Stadi MP equipped with IP-PSD image plate detector. For transmission electron microscopy (TEM) measurements, one or more drops of the solution of nanohybrids in ethanol are deposited on the amorphous carbon film. A Philips CM20 microscope operating at 200 kV was used. Fourier transform infrared spectroscopy (FT-IR, Mattson 5000) was carried out in the range of 4000-450 cm-1 in transmission mode. The pellets were prepared by adding 1-2 mg of the nanohybrid powder to 200 mg of KBr. The mixture was then carefully mixed and compressed at a pressure of 10 Kpa to form transparent pellets. The thermal behavior of the nanopowders was investigated with a thermoanalyser (Netzsch Sta 409C/CD) apparatus. All samples were recorded at a scan rate of 10 °C min-1 from room temperature to 800 °C under air and under an argon atmosphere. 1H and 13C NMR spectra were recorded at 9.4 T on a Bruker Avance 400 WB spectrometer (DSX model) on a 4 mm BL Cross polarization magic angle spinning (CPMAS) VTN probe at, respectively, 400.11 and 100.62 MHz. 89Y NMR were recorded at 19.6 MHz using a dedicated low-γ 4 mm BL CPMAS DVT probe. The sample was packed into 4 mm ZrO2 rotors. CPMAS experiments: the Hartmann-Hahn “sideband” matching condition νS1 - ν11H ) νr (n ) (1, n ) (2) was carefully matched by calibrating the 1H and the S (13C or 89Y) rf fields. For 13C and 89Y, glycine and Y(NO3)3 6H2O were used, respectively, to adjust the S rf field strength employed during the contact time (CT). 1H-13C CPMAS parameters: ν113C, 1 55 kHz; ν1H, ramped from 60 to 40 kHz; recycle delay (RD) ) 5 s; CT ) 2 ms; number of scans (NS), 1258; shining at 12 kHz. 1H-89Y CPMAS parameters: ν189Y, 45 kHz (measured from 90o pulse on 39K); ν11H, ramped from 40 to 20 kHz; RD ) 5 s; CT ) 4-28 ms; NS, 3300 (for the yttrium-benzoate compound) and 21600 (for yttrium-biphenolate compound); spinning at 5 kHz. Two-dimensional (2D) 1HFS-LG-89Y heteronuclear correlation (HETCOR) experiment: the pulse sequence used in this experiment is described in our previous works19,20 and consists of a 1H-89Y CPMAS experiment with 1H time evolution (t1) between the preparation (1H 90o pulse) and mixing (CT) pulses applied to the 1H spins. During this period, a 1H1H homonuclear decoupling scheme (FS-LG) is used to enhance the 1H resolution.21 The 1H rf field amplitude employed during the FS-LG was set ca. 83 kHz. The phase switching between each Lee-Goldburg (LG) pulse was optimized on the multiplet structure of adamantane. Before each LG pulse, the frequency offset was alternated between a calculated positive and negative 1 1H offset ((∆LG ) ((ν H/x2 + ν )) corresponding to 1g 1 +58925 + ν1g Hz and -58925 + ν1g Hz. Best results were obtained by use of asymmetric offsets during FS-LG decoupling of ν1g ) 3 kHz. The LG pulse length (τLG) was set to 9.8 µs and, consequently one FS-LG unit equals two successive LG units of duration each (τFS-LG ) 2 × τLG). Quadrature detection in t1 was achieved by the States-TPPI method.22 The correct 1H chemical shifts and the scaling factor, λ, of the FS-LG dimension (F1) of 1H-89Y HETCOR spectrum were determined by comparison with the 1H MAS spectrum. A scaling factor of
Karmaoui et al. 0.45-0.58 was obtained. Specific experimental conditions are given in the figure caption. Chemical shifts are quoted in ppm from tetramethylsilane for the 1H and 13C nuclei and Y(NO3)3‚ 6H2O for the 89Y nucleus. The visible and infrared emission and excitation spectra were recorded between 14 and 300 K on a modular double grating excitation spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Jobin Yvon-Spex) coupled to a R928 (visible) and H9170-75 (infrared) Hamamatsu photomultipliers. The excitation source was a 450 W Xe arc lamp. All the spectra, corrected for optics and detection spectral response, were measured in the front face acquisition mode at room temperature. The photostability was investigated by monitoring the 5D0 f 7F0-4 integrated emission intensity using a Jobin Yvon-Spex spectrometer (HR 460) coupled to a R928 Hamamatsu photomultiplier under continuous excitation of a Xe arc lamp (150 W) coupled to a Jobin Yvon monochromator (TRIAX 180). The maximum exposure time was 10 h. The radiance measurements and the Commission Internationale d’Eclairage (CIE; x,y) emission color coordinates for the 2o standard observer were estimated using a telescope optical probe (TOP 100 DTS140111, Instrument Systems). The excitation source was a Xe arc lamp (150 W) coupled to a Jobin Yvon-Spex monochromator (TRIAX 180). The width of the rectangular excitation spot was set to 2 mm, and the diameter used to collect the emission intensity was set to 0.5 mm. As the radiance values depend on the surface density of the emitting centers, care also was taken in preparation of the samples, whereby pellets containing the same amount (50 mg) and with the same compaction degree were used. The radiance values were corrected for the spectral distribution of the excitation source intensity. Results and Discussion TEM images of the as-synthesized nanohybrids are shown in Figure 1 a,c. Similar to the case of the rare-earth nanohybrids synthesized in benzyl alcohol, these materials show a typical lamellar structure, composed of (i) oxidic thin layers that scatter strongly the incident electrons and hence seen as dark layers (ii) and an organic part that is practically invisible between those layers. However, in this case the interlamellar distance, compared to the benzyl alcohol case, increases from 1.8 to 2.6 nm. As demonstrated below, this is due to the larger intercalated molecules. The Fourier transform of these images (Figure 1b,d) gives rise to pairs of spots, which can be attributed to the reflections of the lamellar mesostructure. Such reflections also are present in the small-angle powder X-ray diffraction patterns in which three or more reflections due to the lamellar order are present (Supporting Information), proving that such an order is constant over the whole sample. CHN elemental analysis showed that a large amount of the samples is made of carbon (Table 1). The carbon/hydrogen ratio is in good agreement with the assumption that only biphenolate species are present between the layers. The slightly larger quantity of hydrogen detected, compared to the expected value, is certainly due to some water adsorbed at the surface as was previously observed for the nanohybrids containing benzoate species.14 In fact, oxidic nanopowders obtained by the benzyl alcohol route are rather hygroscopic and adsorb up to 5 wt % of water upon contact with air within a short period of time.23 The temperature stability of the nanohybrids was studied by thermogravimetric analysis (TGA) coupled with differential thermal analysis (DTA) under air. They show the same global behavior (Supporting Information): before 400 °C the weight
Biphenolate Lamellar Nanostructures
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2541
Figure 2. 13C CPMAS NMR spectrum of the yttrium-based biphenolate nanohybrid.
Figure 1. TEM image of Gd-based (a) and Y-based (c) biphenolate nanohybrids and their respective Fourier transform (b) and (d) showing pairs of spots, organized in pseudo circles, which are due to the mesostructural order.
TABLE 1: CHN Elemental Analysis (weight %) and Experimental and Calculated Carbon/Hydrogen Ratios sample Gd2O3-based Y2O3-based
carbon(%) hydrogen(%) nitrogen(%) 26.09 44.31
1.75 3.43
0.0 0.0
C/H exp
C/H calc
14.9 17.3 12.9 17.3
loss is principally due to molecules adsorbed at the surface like benzyl alcohol that it is known to start to desorb at around 150 °C.24 In fact, no DTA peak is observed between 150 and 400 °C. The main weight loss takes place between 400 and 500 °C and is accompanied by a sharp exothermic DTA peak attributed to the combustion of the intercalated organic molecules. To prove that only biphenolate species are responsible for the formation of the lamellar structure, solid-state NMR and FT-IR studies were performed. 13C CPMAS NMR analysis (Figure 2) shows three groups of resonances. The less shielded carbonyl resonances appearing in the range of 171-176 ppm belong to the carboxylate groups. Additionally, there are two other groups of aromatic 13C resonances, which appear above 100 ppm. As shown in Figure 2, one group refers to the aromatic 13C peaks resonating at ca. 140.5 and 142.6 ppm, and the other group resonating at 126-133 ppm corresponds to the two carbons bridging the phenyl rings and the remaining aromatic carbons. The absence of the methylene group from the starting material (biphenyl methanol), which resonates at ca. 64 ppm,
indicates its complete conversion to a biphenolate species. 1H MAS NMR analysis (Supporting Information) presents a typical broad line spectrum. The main resonance centered at ca. 7 ppm corresponds to aromatic protons. However, a shoulder is present between 1 and 3 ppm, suggesting the presence of OH groups (e.g., from water) adsorbed at the surface. No proton resonances are observed above 8 ppm, indicating that no acidic protons are present. The vibrational spectra of metal-organic complexes and salts of carboxylic acids have been studied extensively in the literature.25 To gain more information about the organic moieties between the oxide layers and their binding to the metal centers, FT-IR spectra were recorded (Supporting Information). The comparison of vibrational data with X-ray crystal structures provides information about the kind of bond that exists between the carboxylic group and the metal center by calculating the difference (∆) of the asymmetric (νa(CO2)) and symmetric (νs(CO2)) stretching frequencies.26 These stretching frequencies are found at 1569 and 1391 cm-1 for the gadolinium and at 1553 and 1408 cm-1 for the ytrrium-based nanohybrids, respectively, pointing to ∆ ) 178 and 145 cm-1 thus corresponding to a bridged-type bond, as in the case of the previously reported benzoate-based hybrids.14,15 Yttrium observation by 89Y solid-state NMR (SSNMR) is usually a difficult task, owing to its long longitudinal relaxation time, low sensitivity, and low gyromagnetic constant (γ). However 89Y SSNMR spectra can be obtained by crosspolarization (CP) using the pool of 1H spins.27 In this work, we profit from this technique (89Y CPMAS) to probe the yttrium chemical environment of two yttria hybrids: the one containing benzoate species, which was previously reported,14 and the one containing biphenolate species. The 89Y CPMAS NMR spectra (Figure 3a,b) of these materials are very similar, exhibiting two 89Y resonances almost at the same isotropic chemical shift (δ iso ∼ 195 and 215 ppm). The presence of two distinct 89Y resonances at the same frequency for both materials indicates that the yttrium local environment and the crystalline structures are analogous. The 89Y NMR signals usually fall into different regions depending upon the nature of the ligands. NMR studies are available on yttrium compounds containing carbon-based ligands, and chemical shift ranges of +570 to +100 ppm and +300 to 20 ppm have been reported for 89Y in solution28-30 and in solid state,31 respectively. The peaks observed in Figure 3a, b fall into the latter range, indicating that the yttrium atoms
2542 J. Phys. Chem. C, Vol. 111, No. 6, 2007
Karmaoui et al.
Figure 3. 13C CPMAS spectra of (a) yttrium-biphenolate and (b) yttrium-benzoate. (c) 2D 1H FS-LG-89Y CP HETCOR NMR spectra of yttrium-benzoate recorded with 128 t1 increments with 128 transients each were collected. The F1 increments were synchronized with an integer number of FS-LG units (n × 2 × τLG) with n ) 3.
Figure 4. Excitation (Ex) and emission (Em) spectra of the yttrium (λem ) 612 nm, λex ) 316 nm) (a) and gadolinium (λem ) 612 nm, λex ) 320 nm) (b) based nanohybrids doped with Eu3+. In (b), the excitation spectra of Eu3+-doped Gd2O3 (dashed line) and Eu3+-doped gadoliniumbased benzoate nanohybrid16 (dotted line) are given as reference.
are complexed to the organic moieties through the carboxylate groups, establishing Y-O-C type bonds. Additionally, we have noticed that by performing various 89Y CPMAS experiments at different contact times the relative intensity of both peaks does not change significantly. This fact suggests that the 89Y resonance at ca. 195 ppm corresponds to a site more populated than that giving the peak at 215 ppm. The deconvolution of the 89Y spectrum of the yttrium-benzoate compound yields a 2:1 ratio. This fact supports the hypothesis that the inorganic sheet is made up of three layers of yttrium oxide: one inner layer (which is not in contact with the organic molecules) and two outer layers. These findings are supported by TEM and XRD measurements. In fact, a thickness of 0.6 nm of Y2O3 would correspond to three yttrium layers. Thus, following this assumption, the 89Y environments corresponding to the outer and inner layers should resonate at ca. 195 and ca. 215 ppm, respectively. A 2D 1H-89Y CPMAS (HETCOR) NMR experiment (Figure 3c), using 1H FS-LG decoupling, could be recorded and shows a correlation between the 89Y and 1H resonances. The more intense 1H-89Y correlation signals correspond to the cross-peaks labeled as “A” (Figure 3c), which are assigned to the stronger correlation involving the two 89Y sites, and the 1H resonance centered at ca. 6.9 ppm, which is characteristic of aromatic rings (see right projection of Figure 3c). Because the different aromatic 1H environments have very similar chemical shifts, it was not possible to resolve them, even by applying FS-LG decoupling to enhance the 1H resolution in solid samples.19,21 In addition, these two strong cross-peaks in region “A”, show that the main source of 1H magnetization to polarize 89Y nuclei comes from the 1H nuclear spins of the aromatic rings. This indicates close proximity between the organic moieties and the yttrium centers. It is worth noting that as the yttrium oxide layers are relatively thin, even the 89Y nuclei located in the inner layer can be easily cross-polarized by the surrounding 1H nuclei. Additional faint cross-peaks, labeled as “B” are observed at chemical shifts between 0 and 4 ppm, which almost fall into
the spectral noise. These signals are tentatively assigned to adsorbed water residues in different chemical environments. No further peaks are seen above the aromatic 1H resonances, which supports the idea that the organic residues exists solely in their carboxylate form. In conclusion, structural studies show that the composites consist of thin layers (0.6 nm, i.e., 3 monolayers) of crystalline yttrium and gadolinium oxide alternated with a double layer of biphenolate species bridged to the oxidic nanosheets. The doping of yttrium and gadolinium oxidic matrixes with Eu3+ and Nd3+ luminescent ions gives them interesting emission properties.2 For an excitation wavelength within the UV (typically around 250 nm) a charge-transfer process related to the excitation of an electron from the oxygen 2p orbital to the 4f6 (Eu3+) configuration takes place.2 A subsequent radiative deexcitation leads to sharp emission in the red. Figure 4a,b show the room-temperature excitation spectra monitored within the 7F manifold of the yttrium and gadolinium nanohybrids doped 2 with 5% of europium. The spectra consist of a broad band extending from 240 to 350 nm, peaking at around 320 nm. A series of low-intensity intra-4f6 lines also are present at 395, 465, and 535 nm (the two latter are not shown) ascribed to the 7F f 5L ,5D ,5D transitions, respectively. The main excitation 0 6 2 1 band is much broader than the one expected for the chargetransfer process related to the excitation of an electron from the 2p oxygen orbital to the 4f6 orbital (250 nm)2 (Figure 4b, dashed line) and the one observed from similar hybrid materials containing benzoate molecules (Figure 4b, dotted line). Such a band is probably due to the superposition of two contributions: the excitation of the phenyl rings of the biphenolate molecules in between the layers that is further transferred to the Eu3+ centers (maximum at around 320 nm) and the charge transfer related to the excitation of an electron from the 2p oxygen orbital to the 4f6 orbital (peak at 254 nm).16 The presence of such a low-energy band in the excitation spectrum demonstrates that the biphenolate complex plays an important role on the emission properties of the nanohybrids. The emission spectra measured
Biphenolate Lamellar Nanostructures
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2543 TABLE 2: (x,y) CIE Emission Color Coordinates, Radiance, and Luminance Values for the Yttrium and Gadolinium-Based Nanohybrids Doped with Eu3+ and the Red Standard Phosphor Measured for Selected Excitation Wavelengths (λex)a sample Y2O3/Eu (standard) Gd2O3/Eu (hybrid) Y2O3/Eu (hybrid)
luminance λex radiance (nm) (µW cm-2 sr-1) (cdm- 1) 254 325 325 325
2.24 3.07 2.31 0.52b
4.89 6.92 2.28 1.24b
CIE (x,y) (0.66,0.33) (0.68,0.32) (0.28,0.16) (0.66,0.34)c
a The experimental errors are within 5%. b Eu3+ intra-4f6 5D0 f 7F0-4 transitions contribution to the overall radiance and luminance values. c Eu3+ intra-4f6 5D0 f 7F0-4 transitions emission color coordinates.
Figure 5. Excitation (Ex) and emission (Em) spectra of yttrium (λem ) 1064 nm, λex ) 321 nm) (a) and gadolinium (λem ) 1064 nm, λex ) 321 nm) (b) based nanohybrids doped with Nd3+.
via excitation of the phenyl rings of the biphenolate molecules (320 nm) are shown in the right part of Figure 4a,b. The emission spectra present the typical Eu3+ intra-4f6 5D0 f 7F0-4 electronic transitions. A detailed study of the Eu3+ local cordination based on the local field splitting of the intra-4f lines will be presented elsewhere. For excitation wavelengths below 350 nm, no emission arising from the ligands was detected in the spectrum of the gadolinium-based hybrid and just a lowintensity band centered at 420 nm was observed for the yttriumbased one (Supporting Information). When the nanohybrids are excited from 350 to 420 nm, a large broad band between 400 and 600 nm appears (Supporting Information), superimposed to the intra-4 f6 5D0 f 7F0-4 transitions. This points to an efficient energy transfer between the biphenolate complex and the Eu3+ ions. Moreover, this gives the possibility of changing the emission color chromaticity just by changing the excitation wavelength.16 The energy transfer involving the excited states of the biphenolate species also enables the sensitization of lanthanide ions active in the infrared spectral region, such as Nd3+. Figure 5 a,b shows the near IR (NIR) emission spectrum of the yttriumand gadolinium-based nanohybrids doped with Nd3+ excited within the states of the biphenolate molecules. The emission lines are assigned to the 4F3/2 f 4I11/2,13/2 Nd3+ intra-4f3 transitions centered at 1072 and 1351 nm, respectively. Figure 5a,b also shows the excitation spectra monitored within the Nd3+ NIR being more intense transition. The spectra consist of two large broad bands peaking at 330 and 430 nm ascribed to the excited states of the biphenolate phenyl rings and to a low intensity intra- 4f3 lines. The presence of the biphenolate phenyl rings excited states indicates that the biphenolate molecules between the layers absorb energy and then transfer it to the Nd3+ ions, demonstrating that there is an active visible-to-NIR energy conversion channel at room temperature. It should be stressed that it is not trivial to observe room-temperature emission of Nd3+-doped hybrids containing numerous organic groups particularly if hydroxyl groups are present, which are known to prevent any Nd3+ radiative emission. CIE emission color coordinates, radiance, and luminance were measured for the yttrium- and gadolinium-based biphenolate
nanohybrids doped with Eu3+ (Table 2). For comparison, the emission color coordinates of a well-known standard phosphor from Phosphor Technology emitting in the red (Y2O3-Eu) are given. The radiance values of the Eu3+-doped hybrids are larger than those of the red standard phosphor (ca. 3 and 37% for the yttrium and gadolinium, respectively). The larger radiance of the Eu3+-doped nanohybrids can be explained by the larger absorption cross section of the biphenolate molecules compared to the one of the CTB observed on the pure inorganic samples. Furthermore, the emission stability of the samples was monitored under UV excitation, which maximize the Eu3+ intra4f6 emission (320 nm), for several hours. No detectable decrease of the emission efficiency was detected. Hence, optical studies prove that these phosphors meet the actual required standards and at the same time show better radiance. Conclusions In conclusion, the successful nonaqueous sol-gel reaction between rare-earth alkoxides and biphenyl alcohol showed to be a general route to the formation of inorganic-organic ordered hybrid structures. The synthesis leads to the formation of crystalline thin layers of rare-earth oxides equally spaced by an organic layer exclusively formed by biphenolate molecules and the periodic lamellar structure is kept together by simple π-π interactions between the phenyl rings. Surprisingly, the hybrid structures are stable at temperatures above 400 °C. By doping them with Eu3+ and Nd3+, it was possible to obtain phosphors showing a room-temperature emission in the red and infrared, respectively. The emitting ions were excited via the organic subphase that allows to substantially lower the required excitation energy and at the same time increase the radiance. The gadolinium-based nanohybrid combines radiance and luminance values around 40% larger than in the case of standard phosphors and extended UV stability, which make it a valuable competitor for advanced applications. Acknowledgment. We thank the Max-Planck-Institute of Microstructure Physics and Professor U. Goesele for the use of the TEM. This work was partially supported by the European Network of Excellence FAME. Supporting Information Available: Small-angle X-ray scattering, FT-IR spectra and TGA and DTA of the nanohybrids, 1H NMR spectrum of the yttrium nanohybrid, and emission spectra in function of the excitation wavelength. This material is available free of charge via the Internet at http:// pubs.acs.org.
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