Luminescent Nanocomposites Made of Finely Dispersed Y3Ga5O12

Aug 24, 2012 - This paper reports the initial results of an original and simple method to elaborate flexible, self-standing, and thick luminescent fil...
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Luminescent Nanocomposites Made of Finely Dispersed Y3Ga5O12:Tb Powder in a Polymer Matrix: Promising Candidates for Optical Devices Audrey Potdevin,*,†,‡ Geneviève Chadeyron,†,‡ Sandrine Thérias,‡,§ and Rachid Mahiou‡,§ †

Clermont Université, ENSCCF, Institut de Chimie de Clermont-Ferrand, BP10448, F-63000 CLERMONT-FERRAND CNRS, UMR 6296, ICCF, BP 80026, F-63171 AUBIERE § Clermont Université, Université Blaise Pascal, Institut de Chimie de Clermont-Ferrand, BP 10448, F-63000 CLERMONT-FERRAND ‡

S Supporting Information *

ABSTRACT: This paper reports the initial results of an original and simple method to elaborate flexible, self-standing, and thick luminescent films suitable for optical devices. PVP/Y3Ga5O12:Tb3+ nanocomposite films have been successfully achieved from a sol− gel derived Y3Ga5O12:Tb3+ powder and an alcoholic solution of poly-N-vinylpyrrolidone (PVP). The structural, morphological, and optical properties of these nanocomposite films have been studied and compared to those of a pristine PVP film and Y3Ga5O12:Tb3+ powder. The nanocomposite films were characterized by infrared and Raman spectroscopies as well as scanning and transmission electron microscopies (SEM and TEM) and demonstrated good dispersion of the phosphor particles within the polymer matrix via an alveolar mesostructure. The optical properties of these nanocomposites were fully characterized, and both their excitation and emission spectra and decay curves were recorded. Furthermore, photostability of the nanocomposite films and of the luminescent raw powder has been studied after exposure to an accelerated artificial photoageing at wavelengths higher than 300 nm. The elaboration process used is both tunable and applicable to a large variety of powders and polymers because it does not require any additive to form homogeneous and easily shapeable phosphor/polymer nanocomposites applicable in a large variety of optical devices such as solid-statelighting.

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INTRODUCTION Over the past few years, phosphor films have attracted much attention because of their potential applications to a large variety of optoelectronic devices, such as medical imaging, white light generation via Hg-free lamps (solid-state lighting), or photovoltaic applications.1,2 Various methods can be used to shape thin films: pulsed-laser deposition, ion beam sputtering, liquid phase epitaxy, and sol−gel.3,4 Of the different techniques, only the sol−gel process allows for the elaboration of transparent thin films (thicknesses of several nanometers to approximately 5 μm) on both simply and complexly shaped substrates from cheap precursors, such as chlorides.5 However, due to the difficulty in obtaining thick coatings (thicknesses over 5 μm) of optical quality from metal alkoxides, such films cannot presently compete with powder-based devices. Consequently, new strategies to develop crack-free thick luminescent films have been developed based on the use of a polymer to encapsulate functional nanoparticles.6,7 This polymer both improves the film processability and protects the particles from environmental variations. Furthermore, the incorporation of these particles can improve the polymer properties with the primary objective of obtaining a © 2012 American Chemical Society

combination of properties not available in any of the individual components.7−9 Different methods have been used to form luminescent polymer nanocomposites: the direct blending of inorganic particles with either a polymer or monomer solution followed by in situ polymerization, a melt processing, and the in situ synthesis of nanoparticles within a polymer medium.7,8,10−16 Among these methods, direct blending appears to be the simplest, but it usually results in inhomogeneous composites in which inorganic particles tend to agglomerate.17 The resultant films are characterized by weak emission intensities because of luminescence quenching.7 The particles can be functionalized before mixing to counterbalance these disadvantages and ensure their compatibility with the polymers.8,10,18 The luminescent nanocomposite films primarily produced so far involve metallic nanoparticles, luminescent organic complexes, or semiconductors, such as quantum dots, dispersed in a polymer matrix. These nanocomposite films are typically cast Received: July 12, 2012 Revised: August 9, 2012 Published: August 24, 2012 13526

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by either molding or spin-coating onto a substrate.7,19−22 Only a few papers address polymer encapsulation of either luminescent oxides or fluorides such as SrAl2O4:Eu2+, Dy3+, Y3Al5O12:Ce3+ or CeF3:Yb3+, and Er3+.11,12,17,23−26 These functional materials were typically developed for either electronic and optoelectronic devices or biological labeling. Since phosphor materials, such as yttrium garnet particles, usually possess poor reactivity, their functionalization to avoid aggregation and the resultant luminescence quenching is not a simple process.12 Saladino et al. reported the use of methacrylic acid to improve the stability of YAG:Ce3+ nanoparticle dispersions in methyl methacrylate, whereas Nyman and coworkers preferred to use a solvent-exchange process to efficiently disperse the YAG:Ce3+ nanoparticles in an epoxy resin.12,23 In this work, a nanostructured Tb3+-doped Y3Ga5O12 (YGG) powder has been incorporated into poly-N-vinylpyrrolidone (PVP) through an alcoholic solution. This YGG:Tb3+ powder was obtained via a nonaqueous sol−gel process developed by our group25 and has been proven to represent a promising green phosphor for solid-state lighting. As a consequence, nanocomposite films involving YGG:Tb3+ nanoparticles scattered in a polymer matrix are of great interest for both fundamental and applied research. PVP was chosen as the polymer because of several of its characteristics; for example, it is soluble in both aqueous and alcoholic solutions, does not absorb in the visible range, and can even enhance the luminescence efficiency via an effective energy transfer between PVP and the luminescent centers.19,26,27 PVP has already been employed as a capping agent to enhance nanoparticle dispersion in water or to modify the morphology of the nanoparticles. However, the use of PVP as a polymer matrix for phosphor nanoparticles to obtain flexible functional films has been scarcely reported to the best of our knowledge.28−31 Moreover, the photochemical behavior of PVP films has already been characterized and can be considered to be well-known.32 PVP was used as a model polymer to study the influence of phosphor nanofillers on polymers under irradiation and as a proof of concept for optical applications such as solid-state lighting or displays. The approach developed in this work possesses the advantage of being an easy process that is readily applicable to a large variety of luminescent powders (oxides, fluorides), which can be dispersed into soluble polymers up to high concentrations (more than 30 wt %) without an additive. The powder was directly poured into the polymer alcoholic solution and dispersed by the crushing and shearing forces. Then, the polymer/phosphor nanocomposite can be shaped via several coating methods, such as bar-coating, both dip- and spincoating, and spray coating. Furthermore, this strategy uses ecofriendly solvents such as alcohols and water and is, therefore, environmentally sound and economically feasible in addition to being versatile. Self-standing flexible luminescent YGG:Tb/PVP nanocomposite films were cast onto a Teflon surface using a coat-master. The structural, morphological, and optical properties of these YGG:Tb/PVP films were studied and compared to those for both a pristine PVP film and the initial YGG:Tb powder. It is noteworthy that the present paper develops a different approach from the one reported in a previous study.33 This latter concerned a process involving amorphous luminescent materials (acac-modified isopropoxides) mixed as a solution (in isopropanol) in PVP alcoholic solution. The present work deals

with crystallized submicronic luminescent powders directly dispersed in a PVP viscous alcoholic solution. The novelty resides in the high homogeneity of dispersion of the crystallized aggregates into the polymer matrix, resulting in flexible and free-standing luminescent thick films designed for a wide range of applications (biolabeling, solid-state lighting, and solar cells, among others). Moreover, the photostability of the luminescent nanocomposites under irradiation at λ ≥ 300 nm and 60 °C has been investigated. To our knowledge, this kind of study led on crystallized phosphor dispersed in a polymer matrix has not yet been reported in the literature. It can be found rare investigations about the photostability of luminescent composites: the study of Zou et al.,27 for example, deals with ZnO nanoparticles embedded in polyfluorene, a π-conjugated polymer able to emit light by itself, as well as with the study of the photodegradation of these composites.

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EXPERIMENTAL SECTION

Materials. Yttrium chloride, terbium chloride, gallium chloride and dry isopropanol were obtained from Aldrich (Milwaukee, WI). Poly-Nvinylpyrrolidone, PVP (average molar weight 360 000 g·mol−1), was obtained from Scientific Polymer Products Inc. (New York, NY). Synthesis of YGG:Tb Powder. The Y3Ga5O12:Tb3+(20 mol %) (YGG:Tb) powder was synthesized via a nonaqueous sol−gel process according to the reported procedure.28 A heat treatment at 1100 °C for 4 h was required to efficiently form the nanoscale phosphor as a powder. Elaboration of PVP/YGG:Tb Nanocomposites. A transparent alcoholic PVP solution (83.3 g·L−1) was obtained by dissolving PVP in dry isopropanol at 60 °C and stirring for several minutes. Prior to use, the solution was degassed for 20 min using an ultrasonic bath. The PVP/YGG:Tb nanocomposites were formed by mixing the phosphor powder with the PVP solution in isopropanol. The YGG:Tb powder was then poured into the PVP solution with stirring and dispersed by the crushing and shearing forces so as to attain 33 wt % of fillers. Film Preparation. PVP and PVP/YGG:Tb nanocomposite films were cast onto a Teflon surface from the PVP and nanocomposite solutions, respectively, using a Coatmaster 509MC-Erichsen (Dr. Blade). The height of the Dr. Blade knife was 350 μm, and the casting speed was 25 mm·s−1. The films were allowed to dry on the Dr. Blade plate for 10 min at 40 °C and then at room temperature for 1 day. Irradiation. Irradiations were performed under artificial aging conditions at wavelengths higher than 300 nm. The aging device was a SEPAP 12/24 unit from Atlas equipped with four medium pressure mercury lamps (Novalamp RVC 400 W) located in a vertical position at each corner of the chamber. Wavelengths below 295 nm were filtered by the glass envelopes of the sources. The samples were placed on a rotating carousel positioned at the center, and the temperature at their surface was regulated at 60 °C, controlled by a Pt thermocouple.29 For comparison with composite films, YGG:Tb crystallized powder was cold-pressed into 12 mm diameter pellets under the load of about 6 tons. Characterization. X-ray Diffraction. The XRD measurements were performed using a Philips Xpert Pro diffractometer with Cu Kα radiation. The XRD patterns can be found in the Supporting Information. Infrared Spectroscopy. The FTIR transmission spectra were recorded using a Nicolet 760 Fourier transform infrared spectrophotometer with OMNIC software. The spectra were obtained using 32 scans and a 4 cm−1 resolution. Raman Spectroscopy. The Raman spectra were recorded using a T64000 Jobin-Yvon confocal micro-Raman spectrograph. The confocal configuration of the micro-Raman instrument allowed for the depth profiling of the samples and permitted Raman detection from volumes as little as 1 μm3 by focusing at different sample depths. The excitation source used was a 514.5 nm wavelength line from a 13527

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Figure 1. Photographs of the PVP/YGG:Tb nanocomposite film under (a) daylight and (b) 365 nm.

approximately 25 μm (±2 μm) as characterized by a CADAR-MI20 μm. It should be noted that the luminescent films were not completely transparent but were slightly milky and translucent, as shown in Figure 1. The decreased transparency of the polymer/phosphor nanocomposites has already been reported with regards to CeF3 nanoparticles dispersed in PMMA.17 It was also noted that the luminescence appears to be uniform over the entire film surface, which indicates a homogeneous dispersion of the phosphor agglomerates throughout the polymer matrix. Both the particle dispersion within the polymer matrix and the type of interactions that could exist between the inorganic phosphor and the polymer matrix were characterized. To develop efficient luminescent composites suitable for optical devices, it is of prime importance that the structural, morphological and optical properties of the fillers be retained after their incorporation into the polymer matrix. The UV−visible spectra for both neat PVP and the nanocomposite films were compared to that recorded for the YGG:Tb powder in Figure 2. PVP does not exhibit any

Coherent model 70C5 Ar+ laser operating at a power of 200 mW. A 50-fold objective lens was used such that only a 1 μm3 volume was required for sampling. The data were collected twice over 60 and 800 s for the powders and films, respectively. The resolution was approximately 1 cm−1. UV−Visible Absorption. The UV−visible spectra were recorded using a Shimadzu UV-2101 PC spectrometer equipped with an integrating sphere. Transmission Electron Microscopy (TEM). TEM was conducted using a Hitachi H7650 120 kV microscope with an 80 kV acceleration and combined with a Hamamatsu AMT HR 1K×1K CCD camera placed in a side position. Scanning Electron Microscopy (SEM). The SEM micrographs were generally recorded using a ZEISS Supra 55VP scanning electron microscope operating under high vacuum at 3 kV and using a secondary electron detector (Everhart-Thornley detector). The specimens were metalized using an Au coating and then attached to either a piece of film or an adhesive carbon film to observe the film surface or installed between two metallic folds to observe the film cross sections. Luminescence. All luminescence properties were studied at room temperature. Both the emission spectra upon UV excitation and the excitation spectra recorded on nonirradiated samples were collected using a Jobin-Yvon setup that consisted of a Xe lamp operating at 400 W and two monochromators (Triax 550 and Triax 180) combined with a cryogenically chilled charge coupled device (CCD) camera (Jobin-Yvon Symphony LN2 series), which collected the emission spectra, and a Hamamatsu 980 photomultiplier for the excitation spectra. Both the emission and excitation spectra have been corrected for the instrument response and Xe lamp intensity. Luminescence spectra upon blue excitation (Supporting Information) were recorded using a Jobin-Yvon HR 1000 monochromator with a dye laser (continuum ND62) pumped by a frequency-doubled pulsed YAG:Nd3+ laser (Continuum Surelite I). The dye solution was prepared by mixing Rhodamines 610 and 640. To achieve a resonant pumping in the blue wavelength range, the output of the dye laser was up-shifted to 4155 cm−1 via stimulated Raman scattering in a highpressure gaseous H2 cell. The fluorescence decays were measured using a LeCroy 400 MHz digital oscilloscope. Finally, emission spectra recorded for comparing non irradiated and irradiated samples were measured using C9920-02G PL-QY measurement system from Hamamatsu. The setup comprises a 150 W monochromatized Xe lamp, an integrating sphere (Spectralon Coating, Ø = 3.3 in.) and a high sensitivity CCD spectrometer for detecting the whole spectral luminescence. The automatically controlled excitation wavelength range spread from 250 to 950 nm with a resolution bandwidth better than 5 nm.

Figure 2. UV−visible spectra of the PVP and PVP/YGG:Tb nanocomposite films.

RESULTS AND DISCUSSION The nanocomposite films obtained after solvent evaporation are 10 cm wide and 20 cm long, remarkably flexible, handy, robust and, most importantly, free-standing, as shown in Figure 1. These crack-free films were easily peeled from the Teflon surface (see the Experimental Section) even when highly loaded (33 wt % phosphor), as shown in Figure 1. The obtained films possessed a homogeneous thickness of

absorption band above 250 nm, which is important for luminescent nanocomposites incorporating phosphors such as garnets. Indeed, as shown by Mishra et al., the photoluminescent properties of the phosphor/polymer composite depends on the nature of the latter. Polymers containing chromophors must be avoided for application in UV and visible excitation-based devices because there would be competition

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Figure 3. (a) IR spectra of the PVP and PVP/YGG:Tb nanocomposite films with (b) a comparison to the YGG:Tb powder.

between the polymer and phosphor absorptions that could prevent phosphor to be excited.24 In the UV−visible spectrum of the luminescent nanocomposite film (Figure 2), a shoulder corresponding to the most intense absorption band of the YGG:Tb powder appears at 260 nm and is ascribed to the 4f−4f5d transition of the Tb3+ ions in the garnet matrix.30 The spectrum of the nanocomposite film was also characterized by a baseline drift relative to pure PVP. This decrease in the transmitted light intensity can be ascribed to light scattering caused by the presence of 33 wt % of the fillers11,31 and is consistent with the milky appearance of the loaded film shown in Figure 1b. Structural Properties. X-ray diffraction (XRD) was used to determine the crystallinity of the nanocomposite materials. The XRD patterns are gathered in Figure S1. The PVP matrix was found to be completely amorphous (only a single broad band at approximately 2θ = 24°, Figure S1c), whereas both the nanocomposite film (Figure S1b) and the initial YGG:Tb powder (Figure S1a) were characterized by diffraction peaks of the Y3Ga5O12 matrix (JCPDS file 83-1036). Both IR and Raman spectroscopy were used to detect any interaction between the phosphor and the polymer matrix and to evidence the presence of the M−O (M = Y or Ga) bonds. Figure 3 presents the IR spectra for PVP and the PVP/YGG:Tb nanocomposite films and compares them to that of the YGG:Tb powder. The 1900−400 cm−1 range of the spectra in Figure 3a shows that the presence of the inorganic fillers neither shifts the vibration bands relative to the PVP matrix nor produces any new absorption bands from PVP degradation. The absorption bands relative to the PVP matrix are consistent with those observed in earlier studies.32,33 The only difference between the two spectra appears in the 900−400 cm−1 domain (see Figure 3b) and is caused by the vibrational bands of the M−O bonds in YGG powder, which were ascribed to the stretching mode of the tetrahedra in the garnet structure.34,35 Figure 4 only shows the Raman spectra between 200 and 500 cm−1 for both the nanocomposite and PVP films, as this domain corresponds to the vibrational bands inherent to the inorganic particles. The primary Raman signals in the other energy domains (up to 2000 cm−1) correspond to the PVP matrix, as shown in Figures S2 and S3.33,36 In all cases, the spectra of the films were compared to that of the powder. The data have been normalized to simplify the comparison.

Figure 4. Comparison of the Raman spectra from the PVP and PVP/ YGG:Tb nanocomposite films to that of the YGG:Tb powder.

The spectrum of the nanocomposite film in Figure 4 is dominated by the two signals located at approximately 244 and 357 cm−1, whereas the PVP matrix does not exhibit any significant vibrational band. The two Raman lines observed for the nanocomposite were assigned to the ν2 and ν4 vibrations of the tetrahedral MO4 in the garnet YGG (M = Ga), respectively.37 The signals observed at approximately 532, 597, and 753 cm−1 in Figure S2 are assigned to the ν3 and ν1 vibrations of the same tetrahedron, respectively. The presence of the inorganic fillers was only noticeable over the range from 900 to 1200 cm−1 (Figure S3) because of the broadening of the Raman signals situated at approximately 1025 and 1075 cm−1 relative to the PVP matrix. Morphological Properties. TEM images of the initial YGG:Tb powder and the PVP/YGG:Tb nanocomposite containing 33 wt % YGG:Tb are shown in Figure 5. The image of the powder (Figure 5a) displays the primary particles, which are between 60 and 80 nm in size and seem to be connected to each other via bridges to form submicronic aggregates. These aggregates were consistent with the network aggregation process previously evidenced by the SAXS and SEM analyses on sol−gel based Y3Al5O12:Tb powders.38 The presence of these clusters results in the milky aspect of the films 13529

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Figure 5. TEM images of (a) the YGG:Tb powder and (b,c) PVP/YGG:Tb nanocomposite film.

Figure 6. SEM images of the PVP/YGG:Tb nanocomposite film.

Figure 7. Excitation spectra of the PVP and PVP/YGG:Tb nanocomposite (b) films with a comparison to the YGG:Tb powder (a).

These 20 μm bulky films were characterized by a specific alveolar structure that permeated the entire sample thickness (Figure 6a). The alveoli possessed different dimensions and depths, but almost all of them contained phosphor nanoparticles gathered into bunches up to 500 nm in size (Figure 6b). These SEM results are in good agreement with the TEM images, as both indicate that the phosphors were well dispersed within the polymer matrix beneath the submicronic clusters of interconnected luminescent nanoparticles. For comparison, the only other reported flexible luminescent films were produced via a bar-coat from powder dispersed in a soluble polymer that exhibited less particle dispersion and later displayed very heterogeneous diameters.26 Optical Properties. The potential applications of YGG:Tb powders in solid-state lighting have been previously demon-

as shown in Figure 1. This specific phosphor morphology was maintained within the polymer matrix (Figure 5b and Figure S4), and a rather homogeneous dispersion of the aggregates in the polymer was observed (Figure 5c). This feature is very important since segregation of the phosphor clusters would lead to greater light scattering and significantly decrease the luminescence.18,21 The nanocomposite film morphology was also analyzed using SEM; both the film surface and cross-section were studied. The images of the film side (Figure S5) showed that the nanoparticles were well embedded into the polymer matrix, with some agglomerates overflowing onto the surface. These agglomerates were composed of interconnected primary nanoparticles with diameters between 60 and 80 nm. The cross section of the nanocomposite films is shown in Figure 6. 13530

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Figure 8. Emission spectra for the PVP and PVP/YGG:Tb nanocomposite films with a comparison to that of the YGG:Tb powder upon both (a) 265 nm and (b) 370 nm excitation. The inset shows a photograph of the PVP/YGG:Tb nanocomposite film deposited on a semirigid PC substrate.

Figure 9. Decay curves for the (a) YGG:Tb powder and (b) PVP/YGG:Tb nanocomposite film collected by monitoring the 5D4−7F5 transition upon 484.4 nm excitation at 300 K.

strated.28 To confirm that the phosphors retain their peculiar optical features even when dispersed within a flexible polymer film, the excitation and emission spectra were collected for both the films and powder as shown in Figures 7 and 8, respectively. Figure 7a exhibits the excitation spectrum of the initial YGG:Tb(20 mol %) crystallized powder that monitored the Tb3+5D4→7F5 fluorescence (λem = 544 nm). Two groups of excitation bands were observed, the first of which consisted of a very broad band located at 265 nm and was associated with another less intense absorption signal at approximately 303 nm. These bands can be attributed to the intershell electronic transitions between the ground state 7F6 (4f) and the split 5d energy levels.28,30,39 The 5d Tb3+ excited states are characterized by both low-spin and high-spin states identified by the 7 DJ (J = 0...5) and 9DJ energy levels, respectively.40 According to Mayolet’s assumptions,30 the bands centered at 265 and 303 nm may be attributed to the 7F6→7D5 and 7F6→9D2 transitions, respectively. A comparison to the previous results obtained for YAG:Tb powders indicates that the broad band at 265 nm covers a weak excitation signal usually located at approximately 230 nm in YAG:Tb and corresponding to 7F6→7D5.28 In addition, there

exists a second group of absorption bands that involves two weak signals located at approximately 350 and 398 nm and a very intense signal at 372 nm, which were related to the intrashell 4f−4f transitions 7F6→5D2, 7F6→5G6 and 7F6→5D3, respectively.28,39,41 It is noteworthy that the signals were slightly shifted relative to the 4f−5d transitions in the YAG matrix toward higher energies because of the crystal-field changes caused by incorporating the larger Ga3+ ions instead of the Al3+ ions.42,43The nanocomposite films exhibited the same excitation bands as the YGG:Tb powder over the wavelength range of 200−400 nm, and the PVP did not produce any absorption signal (see Figure 7b). A baseline drift can be observed above 400 nm for both films and could be related to the measurements conditions. Figure 8a shows the emission spectra for the nanocomposite film, neat PVP, and YGG:Tb powder recorded after excitation at 265 nm. Both the powder and the nanocomposite exhibited the characteristic 5D4→7FJ (J = 6 to 0) transitions of Tb3+ ions in a crystallized YGG matrix with sharp multiplets because of the discrete Stark components.44 The primary emission band was located at 544 nm and gave rise to the well-known green luminescence for Tb3+.43 Emission spectra were also collected 13531

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Figure 10. FTIR spectra of composite PVP/YGG:Tb nanocomposite film photooxidized at λ≥300 nm and 60 °C (a) in the domain 1900−1500 cm−1 (b) in the domain 1100−700 cm−1.

Figure 11. Photographs of the PVP/YGG:Tb nanocomposite film UV light before and after irradiation at λ ≥ 300 nm and 60 °C.

fitted to a monoexponential function, I = I0 exp(−t/τ) (I0 is the initial emission intensity, and τ is the lifetime of the emitting center, Tb3+), as shown in Figure 9. Using these fitted curves, the Tb3+ emission lifetimes were determined to be approximately 4.6 and 3.3 ms in the powder (Figure 9a) and nanocomposite (Figure 9b), respectively. The result obtained for the powder was in good agreement with the Tb3+ optical characteristics in garnets.43,45 The decreased phosphor lifetime when incorporated into the polymer cannot be associated with the clustering of particles because the SEM and TEM analyses indicated that the morphological features were similar for the two samples. Therefore, this reduction is only related to the abundance of organic groups in the PVP that can quench the 5D4 Tb3+ emission via a multiphonon relaxation process, which decreases the lifetime.43 This phenomenon has been previously observed by Chai et al. with regard to YAG:Ce particles encapsulated in PMMA.11 Nevertheless, the Tb3+ luminescence decay time of the nanocomposite was sufficient for its application in optical devices such as solid-state lighting.

using excitation wavelengths in the near-UV and blue ranges (λexc = 370 nm and λexc = 484 nm), and similar emission profiles were obtained. Furthermore, the high-resolution emission spectrum of the nanocomposite film shown in Figure 8a contained the exact same multiplets as the powder; therefore, no amorphization of the phosphor occurred, and the local environment of the Tb3+ remained unchanged. Almost no emission signals were observed relative to the PVP polymer matrix at either of the excitation wavelengths. The sharp signal at approximately 546 nm (marked using ⧫) can be attributed to the sample measurement conditions (sample holder). By accounting for its optical features, the PVP/YGG:Tb nanocomposite can be efficiently excited using recently developed UV and near-UV LEDs, leading to an intense green luminescence.28 The film can be cut and stuck on any substrate, whether suitable or not for optical devices. For example, the inset in Figure 8b represents a piece of film fixed on a semirigid polycarbonate substrate and excited under UV radiation. Finally, the photoluminescence decay curves for the Tb3+ ions in the YGG:Tb powder and nanocomposite film were both 13532

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Photostability. PVP was used as a model polymer as explained in the Introduction. The study of the photostability as well as the lifetime of such luminescent nanocomposites involving a polymer matrix represents a key parameter in their elaboration and potential fields of application. Consequently, a preliminary study of the photostability of PVP:YGG:Tb films was carried out by studying structural and optical properties of irradiated (λ > 300 nm) films and comparing them to the initial ones. Figure 10 gathers FTIR spectra recorded on the nanocomposite film submitted to irradiation at λ ≥ 300 nm and 60 °C. In the carbonyl region (Figure 10a) as well as in the domain between 1100 and 700 cm−1 (Figure 10b), the spectral profiles are similar to those obtained for pristine PVP.32 This testifies to the incorporation of the luminescent particles does not modify the degradation mechanisms of this polymer matrix which is very sensitive to photooxidation. Whereas the presence of phosphors scattered in the polymer matrix does not impact on its photooxidation mechanism, irradiation leads to a decrease in emission intensity of nanocomposite films upon UV excitation, as shown in Figures 11 and 12. The major part of the decline of the luminescence efficiency occurs during the 50 first hours of irradiation.

Figure 13. Emission spectra of YGG:Tb powder before and after irradiation.

dozens of hours of irradiation. This could be explained by the high thermal stability of the garnet matrix and the fact that Tb3+ ions oxide in Tb4+ with more difficulty than Ce3+ ions. On the basis of these results, it can be concluded that the decrease in luminescence efficiency of luminescent nanocomposites is probably due to the presence of photoproducts resulting from the degradation of the PVP matrix. Another cause of this decrease could be the evolution of the morphology of the film after irradiation. This point requires further investigations to be solved. Nevertheless, thanks to their optical and photostability features, these composites could be used as they stand in devices that require only several hours of service such as emergency or entertainment temporary lighting devices. In this case, they should be combined with an economical UV LED source. It can be foreseen that, using a polymer matrix less sensitive to photooxidation than PVP, the optical properties of the luminescent composite films should be stable for weeks and then these nanocomposites should be applicable in other optical devices such as indoor or outdoor lighting sources or displays based on blue or UV LEDs. In this case, several phosphors are generally required to produce white light.

Figure 12. Emission spectra of nanocomposite films depending on the irradiation time.

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The rare previous studies46−48 led on the photostability of yttrium garnets concerned cerium-doped Y3Al5O12 submitted to blue excitation. Results obtained for nanoparticles synthesized by soft chemistry have shown a decrease in the emission intensity after only few minutes. This photobleaching was attributed in all cases to the photooxidation of Ce3+ to Ce4+ near the surface of nanoparticles with a high proportion of surface atoms. Besides, a commercial powder of YAG:Ce, probably constituted of microsized particles, was exposed to the same irradiation process;46 it did not undergo any decline of its emission intensity which is probably related to a much less significant number of surface cerium atoms. In order to determine the photostability of YGG:Tb powders under irradiation at λ ≥ 300 nm and 60 °C, pellets only made of YGG:Tb powder have been submitted to 80 h of irradiation in the same conditions as the nanocomposite films. Figure 13 shows emission spectra of a pellet before and after irradiation. Contrary to YAG:Ce, YGG:Tb powder did not experience a decrease in its emission intensity after several

CONCLUSION We successfully prepared luminescent, flexible, and thick polymer-phosphor films in a clever manner using a sol−gel derived powder embedded in a soluble polymer matrix. The preparation method of these nanocomposite films was based on a simple and versatile process that ensured a homogeneous dispersion of the luminescent particles as indicated by both TEM and SEM. The phosphor particles retained their original size and morphology without further aggregation even at high particle loading. Consequently, they maintained their optical characteristics with regard to their spectral distribution without concentration quenching. While the Tb 3+ lifetime had decreased slightly, it remained suitable for optical devices. Besides, photoageing tests have shown that the presence of the inorganic particles did not influence the photodegradation mechanism of the polymer matrix. Irradiation at λ ≥ 300 nm and 60 °C entailed a significant decrease in luminescence intensity of the composite films. However, this could be related 13533

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to the presence of photoproducts of the polymer and, the longterm behavior of the luminescent nanocomposites could be tuned with a photostable polymer. Because this elaboration process can be applied to many different sol−gel-derived phosphors, it represents a very promising means to create tunable luminescent objects. Both the polymer and filler can be tailored according to the desired device characteristics. For instance, a polymer matrix suitable for long-lasting optical devices should be applied to elaborate luminescent nanocomposite films with sustainable optical properties. Moreover, the polymer−phosphor nanocomposite created can be both shaped and tuned to match the application. The use of various deposition techniques allows for a large variety of substrates with geometries of varying complexity. Indeed, whatever the polymer and phosphors used, obtained nanocomposite films can be cut and either stuck on a lot of substrates or used as they stand. Then, they have to be associated with an excitation source, mainly UV or blue LEDs, to produce colored or white light (when several phosphors are combined). Further work should be dedicated to determining the best compromise between the mechanical and optical properties of the film. Therefore, both the determination of the optimal polymer to phosphor ratio and the photostability of such nanocomposites under a 365 nm irradiation (working conditions of UV-LEDs) are currently under investigation.

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ASSOCIATED CONTENT

* Supporting Information S

XRD patterns of PVP/YGG:Tb powder, YGG powder and pristine PVP; Raman spectra of YGG:Tb crystallized powder and PVP films in the ranges 500−800 and 900−1200 cm−1; supplementary TEM image of YGG:Tb powder; SEM images of the surface of the PVP/YGG:Tb composite film; room temperature emission spectra of YGG:Tb crystallized powder and PVP films upon a 484.4 nm excitation. This material is available free of charge via the Internet at the http://pubs.acs. org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank both Anne-Marie Gélinaud (Casimir, Aubière, France) and the Centre Imagerie Cellulaire Santé (CICS, Université d’Auvergne, France), most notably Christelle Blavignac and Claire Szczepaniak, for their technical assistance during the acquisition of the SEM and TEM images, respectively.

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REFERENCES

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