Preparation and Characterization of Upconversion Luminescent

Nov 19, 2009 - E-mail: [email protected]. Cite this:J. Phys. ... Upconversion Luminescent Materials: Advances and Applications. Jing Zhou , Qian .... Ta...
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J. Phys. Chem. C 2010, 114, 610–616

Preparation and Characterization of Upconversion Luminescent NaYF4:Yb3+, Er3+ (Tm3+)/ PMMA Bulk Transparent Nanocomposites Through In Situ Photopolymerization Ruitao Chai, Hongzhou Lian, Zhiyao Hou, Cuimiao Zhang, Chong Peng, and Jun Lin* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P. R. China ReceiVed: September 24, 2009; ReVised Manuscript ReceiVed: October 29, 2009

The in situ photopolymerization method was applied to synthesize bulk nanocomposites consisting of hydrophobic NaYF4:Yb3+, Er3+ (Tm3+) nanoparticles as the filler and poly(methyl methacrylate) (PMMA) as the host material. The oleic acid stabilized NaYF4:Yb3+, Er3+ (Tm3+) nanoparticles and NaYF4:Yb3+, Er3+ (Tm3+)/PMMA nanocomposites have been well characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), the thermogravimetric analysis (TGA), flexural tests, UV/vis transmission spectra, upconversion photoluminescence spectra, and luminescence decays. The well-crystallized NaYF4:Yb3+, Er3+ (Tm3+) nanoparticles are spherical with a mean diameter of 40 nm. The obtained solid NaYF4:Yb3+, Er3+ (Tm3+)/PMMA nanocomposites have similar mechanical properties to that of pure polymer. NaYF4:Yb3+, Er3+/PMMA and NaYF4:Yb3+, Tm3+/PMMA nanocomposites are transparent in the visible spectral region and exhibit strong green and blue upconversion photoluminescence upon 980 nm laser excitation due to the integration of luminescent NaYF4:Yb3+, Er3+ and NaYF4:Yb3+, Tm3+ nanoparticles, respectively. 1. Introduction “Nanocomposite polymers” are polymers that have dispersed phase in them with ultrafine dimension, typically on the nanometer scale.1 The introduction of inorganic nanoparticles into a polymer matrix has proved to be an effective method to improve the performance of polymer materials and bring about novel properties in them.2 The choice of the polymer and the dispersed phase is determined by the properties required for the end product. Among the broad variety of available polymers, poly(methyl methacrylate) (PMMA) has been one of the most widely studied in the past decades because of its outstanding and promising mechanical and chemicophysical properties and has been applied in many sectors such as aircraft glazing, signs, lighting, architecture, and transportation. Moreover, since PMMA is nontoxic, it could be also useful in dentures, medicine dispensers, food handling equipment, throat lamps, and lenses. So far, a lot of inorganic or organically modified inorganic particles have been embedded in PMMA films for introducing down-conversion photoluminescent properties.3 Compared with the commonly used down-conversion fluorescent materials, near-infrared (NIR)-to-visible up-conversion fluorescent nanoparticles have advantages such as minimum photodamage to living organisms, weak background fluorescence, high detection sensitivity, and high light-penetration depth in tissues.4 The ability to synthesize transparent upconverting polymer nanocomposites can facilitate the manufacture of a three-dimensional (3D) volumetric optical display.5 Usually, when the polymer matrix is incorporated by nanoparticles, the transparency of bulk composites is lost to some extent, which results from the agglomeration of nanoparticles and scattering of final turbid products. So the key issue is the development of synthesis methods allowing for particle generation in solution * Author to whom any correspondence should be addressed. E-mail: [email protected].

without aggregation and subsequent functionalization and transfer into the hydrophobic matrix without isolation of the particles. Some groups have investigated the incorporation of upconverting nanoparticles in thin films of PMMA.6 In these cases, the small thickness lead to the transparency of these thin films. Hexagonal (β) phase NaYF4:Yb3+, Er3+ (Tm3+) are known as the most efficient 980 nm NIR-to-visible up-conversion phosphors, and they have attracted more and more attention because of their potential applications, including in solid-state lasers,7 3D flat-panel displays,8 low-intensity IR imaging,9 and sensitive bioprobes.10 Recently, Boyer and co-workers reported the thermal polymerization method for preparation of transparent lanthanide-doped NaYF4/PMMA bulk composite.5 Oleic acidstabilized NaYF4 nanoparticles could be dispersed in MMA for a couple of minutes, but thermal polymerization would take a longer time, so poly(ethylene glycol)-monooleate was utilized to help stabilize the nanoparticles in monomer mixture. Photopolymerization has advantages over thermal polymerization, including rapid polymerization rate, low polymerization temperature, low energy consumption, and easy process control.11 Photopolymerization is composed of ionic polymerization and radical polymerization. In a typical radical polymerization, multifunctional acrylate monomers were polymerized to generate 3D networks through the photoinduced decomposition of free radical initiators. Numerous acrylate monomers and photoinitiators used for specific purposes have been developed and commercialized by chemical companies like Sartomer and the Dow Chemical Company.12 In this article, we describe a radical photopolymerization method to prepare oleic acid stabilized β-NaYF4:Yb3+, Er3+ (Tm3+)/PMMA bulk nanocomposites. As a result of short reaction time of photopolymerization, the nanocomposites could be polymerized and cured completely before nanoparticles aggregate. In our experiments, it took just 3 min for the

10.1021/jp909180s  2010 American Chemical Society Published on Web 11/19/2009

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formation of the nanocomposites. The as-prepared nanocomposites without introducing stabilizer have an advantage in that the mechanical properties of the nanocomposites are not influenced by interfacial interaction between the PMMA and the stabilizer. The morphology, structure, thermal stability, mechanical, luminescent, and energy transfer properties of these nanocomposites were investigated in detail. The obtained composite materials show strong green or blue photoluminescence under 980 nm excitation and are transparent in the visible region. 2. Experimental Section Materials. Y2O3 (99.999%), Yb2O3 (99.999%), Er2O3 (99.999%), and Tm2O3 (99.999%) were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry. Methyl methacrylate (MMA, 99%), oleic acid, and trifluoroacetic acid (99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. 1-Octadecene was obtained from Aldrich. Sodium trifluoroacetate, butanediol monoacrylate (BDMA), trimethylolpropane triacrylate (TMPTA), and photoinitiator 2,4,6-trimethylbenzoyldiphenylphosphine oxide (TPO) were obtained from TCI (Tokyo, Japan) and used as received. Rare earth trifluoroacetates were prepared by dissolving the corresponding rare earth oxides into a mixed solution of water and trifluoroacetic acid (50:50 by volume) and then evaporating the solvent. Preparation of β-NaYF4:17%Yb3+, 3% Er3+ and β-NaYF4: 25% Yb3+, 0.3% Tm3+ Nanoparticles. All of the doping ratios of Ln3+ are molar in our experiments. The β-NaYF4:17% Yb3+, 3% Er3+ and β-NaYF4:25% Yb3+, 0.3%Tm3+ nanoparticles employed in this study were synthesized according to the literature method.13 For the synthesis of β-NaYF4:17% Yb3+, 3% Er3+, 2.06 mmol CF3COONa, 0.75 mmol Y(CF3COO)3, 0.16 mmol Yb(CF3COO)3, and 0.03 mmol Er(CF3COO)3 were mixed with 60 mmol oleic acid and 60 mmol 1-octadecene in a 250 mL three-necked flask. The solution was heated to 100 °C to remove water and oxygen, with vigorous magnetic stirring under vacuum for 30 min, and then heated to 325 °C and maintained for 1.5 h under Argon protection. After the solution was cooled naturally, nanocrystals were centrifugally separated from the solution with ethanol and washed with ethanol three times. Similarly, β-NaYF4:25% Yb3+, 0.3% Tm3+ nanocrystals were synthesized using Y(CF3COO)3, Yb(CF3COO)3 and Tm(CF3COO)3 with a molar ratio of 0.747:0.25:0.003. Preparation of β-NaYF4:17% Yb3+, 3% Er3+/PMMA and β-NaYF4:25% Yb3+, 0.3% Tm3+/PMMA Nanocomposites. MMA (2.3 g) was mixed with 0.8 g of BDMA and 0.8 g of TMPTA. To this solution, 0.1 wt % (0.0039 g) of NaYF4:Yb3+, Er3+ (Tm3+) nanoparticles were added. The solution was sonicated for 10 min to disperse the nanoparticles. After photoinitiator TPO was added to the solutions, the particle dispersions in monomers were transferred into a glass mold formed by separating two glass plates with thick glass seals, and the glass templates were heated to 50 °C before use. Polymerization was carried out in a UV inert cube using UV radiation (200 W cm-1, LAMP, PHILIPS) for 5 min. Different doping concentrations (0.5 and 1 wt %) of NaYF4:Yb3+, Er3+ (Tm3+) nanoparticles in nanocomposites were selected to investigate the effect of the factor on the transparence and optical properties. Characterization. X-ray diffractions (XRD) of the powder samples were examined on a D8 Focus diffractometer (Bruker) using Cu KR radiation (λ ) 0.15405 nm). Scanning electron microscopy (SEM) micrographs and energy-dispersive X-ray

Figure 1. XRD patterns of NaYF4:17% Yb3+, 3% Er3+ and NaYF4: 25% Yb3+, 0.3% Tm3+ nanoparticles. The line spectrum corresponds to the standard data of β-NaYF4 (JCPDS No. 16-0334).

spectroscopy (EDS) were measured on a field emission scanning electron microscope (FE-SEM, XL30, Philips). The nanocomposite samples were cryogenically broken after immersion in liquid nitrogen, and the fractured surfaces were coated with Pt and observed by an SEM. Transmission electron microscopy (TEM) were performed on a JEOL-2000 TEM operating at 200 kV. Samples for TEM were prepared by depositing a drop of a colloidal ethanol solution of the powder sample onto a carbon coated copper grid. The excess liquid was wicked away with filter paper, and the grids were dried in air. The thermogravimetric analysis (TGA) data were recorded with a Thermal Analysis Instrument (Pyris Diamond, PerkinElmer) with a heating rate of 10 °C/min in air. Flexural tests were carried out at room temperature by using an Instron mechanical testing instrument (INSTRON-1121). The ultraviolet-visible (UV-vis) transmission spectra were measured on a TU-1901 spectrophotometer, and the as-prepared bulk nanocomposites (35 × 15 × 2 mm) were measured without any processing. The upconversion emission spectra were obtained using a 980 nm laser from an optical parametric oscillator (OPO, Continuum Surelite, USA) as the excitation source and detected by an R955 (HAMAMATSU) from 400 to 850 nm (the power density used was about 6 W cm-2). The photoluminescence lifetimes of the samples were measured with a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) using a tunable laser (pulse width ) 4 ns, gate ) 50 nm) as the excitation source (Contimuum Sunlite OPO). All the measurements were performed at room temperature. 3. Results and Discussion 3.1. β-NaYF4:17% Yb3+, 3% Er3+ and β-NaYF4:25% Yb3+, 0.3% Tm3+ Nanoparticles. The synthesis of NaYF4: Yb3+, Er3+ (Tm3+) nanoparticles was adopted from Mai et al.13 with a little change. The NaYF4 reported in the literature is hexagonal nanoplates with size of 200 nm. The bigger particles are difficult to disperse well in the solvent. We lowered the reaction temperature (325 °C for our experiment and 330 °C for the literature) and extended the reaction time (1.5 h for our experiment and 30 min for the literature). As a result, we still got pure-phase β-NaYF4 (Figure 1) whose particle size decreased to around 40 nm. The phase structures of the nanoparticles were investigated by XRD. Figure 1 shows XRD patterns of NaYF4: 17% Yb3+, 3% Er3+ and NaYF4:25% Yb3+, 0.3% Tm3+ nanoparticles and the standard data for β-NaYF4 as well. The results of the XRD indicate that the as-formed NaYF4:17%Yb3+, 3% Er3+ and NaYF4:25% Yb3+, 0.3% Tm3+ samples are well

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Figure 2. TEM images of NaYF4:17% Yb3+, 3% Er3+ (a) and NaYF4:25% Yb3+, 0.3% Tm3+ (b) nanoparticles.

Figure 3. NIR-to-visible upconversion emission spectra of β-NaYF4: 17% Yb3+, 3% Er3+ (a), β-NaYF4:25% Yb3+, 0.3% Tm3+ (b), and β-NaYF4:20% Yb3+, 2% Tm3+ (c) nanoparticles under 980 nm laser excitation.

crystallized, and the patterns could be readily indexed to the hexagonal β-NaYF4 phase (JCPDS: 16-0334) with a space group of P63/m. Additionally, no other peaks can be found in the patterns, revealing that a pure phase of β-NaYF4 can be obtained via the current method. The general morphologies and the structures of the asprepared nanoparticles are independent of the doped Ln3+ ions in β-NaYF4. Figure 2 shows the TEM images of β-NaYF4: 17%Yb3+, 3% Er3+ (a) and β-NaYF4:25% Yb3+, 0.3% Tm3+ (b) nanoparticles. The TEM images illustrate that these nanoparticles have similar morphology (spherical) and size (average: 40 nm). According to the literature,14q 17% Yb3+/3% Er3+ are the optimal doping levels for β-NaYF4 crystals, which has the highest relative emission intensity. So we chose these doping

Figure 4. Decay curve of Er3+ luminescence in β-NaYF4:17% Yb3+, 3% Er3+ (a) and Tm3+ luminescence in β-NaYF4:25% Yb3+, 0.3% Tm3+ (b) particles.

levels of Yb3+ and Er3+ in the β-NaYF4 sample to investigate its upconversion luminescence properties. Under 980 nm laser excitation, the strong green luminescence can be observed in the 17% Yb3+/3% Er3+ ion-pair codoped β-NaYF4 nanoparticles. Figure 3a gives its upconversion spectra. The emission of β-NaYF4:17% Yb3+, 3% Er3+ nanoparticles (Figure 3a) are attributed to the transitions 2H11/2-4I15/2 (521 nm), 4S3/2-4I15/2 (539 nm), and 4F9/2-4I15/2(654 nm) for the Er3+ ions.14 In Figure 3b,c for the β-NaYF4:25% Yb3+, 0.3% Tm3+ and β-NaYF4: 20% Yb3+, 2% Tm3+nanoparticles, the emission bands centered at 450, 477, 646, and 695 nm correspond to the 1D2-3F4, 1 G4-3H6, 1G4-3F4, and 3F3-3H6 transitions of Tm3+ ions, respectively.14 An NIR emission at 800 nm is attributed to the 3 H4-3H6 transition, which is invisible to both the human eye and the camera. From Figure 3b,c, we can conclude that the blue emission (1G4-3H6 and 1D2-3F4) for β-NaYF4:25% Yb3+, 0.3% Tm3+ nanoparticles is more obvious than that of β-NaYF4:

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Figure 5. Photographs of the as-prepared β-NaYF4:17% Yb3+, 3% Er3+/PMMA (a-f) and β-NaYF4:25% Yb3+, 0.3% Tm3+/PMMA (g-l) nanocomposites. The concentrations of NaYF4 in nanocomposites for (a,b,g,h), (c,d,i,j), and (e,f,k,l) are 0.1, 0.5, and 1 wt %, respectively. (a,c,e,g,i,k) under daylight; (b,d,f,h,j,l) under 980 nm laser excitation in dark.

20% Yb3+, 2% Tm3+ nanoparticles. This phenomenon results from concentration quenching of Tm3+. The excess of Tm3+ ions causes the self-interaction between adjoining Tm3+ ions and results in a smaller population on 1G4 and 1D2 levels. Eventually, emission intensities of 1G4-3H6 and 1D2-3F4 transitions are diminished. We chose NaYF4:25% Yb3+, 0.3% Tm3+ nanoparticles with the stronger blue luminescence for the composites. These emission spectra in our experiments are similar to those for Yb3+/Er3+- and Yb3+/Tm3+-activated β-NaYF4 nanocrystals and bulks reported previously.4,14 Figure 4 shows the luminescence decay curves of Er3+ in β-NaYF4:17%Yb3+, 3%Er3+ nanoparticles (a) and Tm3+ in NaYF4:25% Yb3+, 0.3% Tm3+ nanoparticles (b). Both of the curves can be well-fitted into a single-exponential function as I ) I0 exp (-t/τ) (I0 is the initial emission intensity at t ) 0, and τ is 1/e lifetime of the Er3+ ion). The lifetime of Er3+ is determined to be 0.45 ms in β-NaYF4:17%Yb3+, 3% Er3+ nanoparticles. The lifetime of Tm3+ was determined to be 1.11 ms in NaYF4:25% Yb3+, 0.3% Tm3+ nanoparticles. 3.2. β-NaYF4:17% Yb3+, 3% Er3+/PMMA and β-NaYF4: 25% Yb3+, 0.3%Tm3+/PMMA Nanocomposites. The liquid mixtures including various concentrations of NaYF4 were transformed into solid transparent polymer materials by in situ photopolymerizations. TMPTA and TPO were added for the photopolymerization process serving as the cross-linker and radical initiator, respectively. The addition of BDMA makes

the liquid mixtures remain transparent during polymerization. Then these liquid mixtures were transferred into glass molds and polymerized subsequently under UV irradiation. The glass templates were heated to 50 °C before use to decrease cured time. Later, the solid and transparent polymers were obtained. Figure 5 gives the photographs of β-NaYF4:17% Yb3+, 3% Er3+/ PMMA (a,c,e) and β-NaYF4:25% Yb3+, 0.3% Tm3+/PMMA (g,i,k) nanocomposites under daylight. It is found that the transparency of these nanocomposites decreases gradually with the increase of incorporating concentration of NaYF4 nanoparticles. From these photos we can also see that even the 1 wt % nanocomposites have high degree of transparency. To establish the degree of nanoparticles dispersion within the polymeric matrix, nanocomposite samples were cryogenically broken after immersion in liquid nitrogen, and the fractured surfaces were coated with Pt and observed by SEM. Figure 6 shows the SEM micrographs of the fractured surfaces of β-NaYF4:17% Yb3+, 3% Er3+/PMMA (a-c) and β-NaYF4: 25%Yb3+, 0.3% Tm3+/PMMA (e-g) nanocomposites containing 0.1 (a,e), 0.5 (b,f), and 1 wt % (c,g) of inorganic nanoparticles, respectively. In the figures, fine nanoparticles dispersed in the polymeric matrix can be seen clearly, and a little agglomeration of nanopowder could be noted in the case of high content (1 wt %, see Figure 6c, g). The average size of NaYF4 in nanocomposites is 40 nm, which is in agreement with that of TEM images (Figure 2). Figure 6d, h show the EDS patterns

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Chai et al. TABLE 1: Flexural Properties of Pure Polymer and NaYF4:17% Yb3+, 3% Er3+/PMMA Nanocompositea sample

flexural modulus (MPa)

ultimate strength (MPa)

ultimate strain (MPa)

pure polymer nanocomposite

3370.05 3215.05

129.35 120.85

5.1843 6.3501

a

Figure 6. SEM images (a-c and e-g) and EDS spectra (d and h) of the fractured surface of NaYF4:17% Yb3+, 3% Er3+/PMMA (a-d) and NaYF4:25% Yb3+, 0.3% Tm3+/PMMA (e-h) nanocomposites. The content of inorganic phase for (a,e), (b,f), and (c,d,g,h) are 0.1, 0.5, and 1 wt %, respectively.

The content of NaYF4:Yb3+, Er3+ is 1 wt %.

heat absorption, is ascribed to the degradation of the polymer’s unsaturated groups; in contrast, the second reaction is due to unzipping initiated by the random scission produced by monomer volatilization. Beyond 530 °C PMMA is destroyed completely.15 The decomposition steps and the causation of weight loss of NaYF4:17% Yb3+, 3% Er3+/PMMA nanocomposites are similar to those of pure polymer. It is noteworthy that the temperature for nanocomposite degradation increases with respect to that of pure polymer during 320-460 °C, which indicates that the interaction between the NaYF4 nanoparticles and the PMMA polymer matrix are so strong that the thermal degradation of the PMMA polymer becomes more difficult.15 Mechanical parameters of pure polymer and NaYF4: 17%Yb3+, 3%Er3+/PMMA were evaluated by flexural tests, and the data are summarized in Table 1. The specimens were beamsized 35 × 15 × 2 mm. The rate of loading was 10 mm/min. Pure polymer and the as-formed nanocomposites are very similar to the flexural properties. The flexural modulus and ultimate strength for NaYF4:17%Yb3+, 3%Er3+/PMMA (the content of NaYF4:Yb3+, Er3+ is 1 wt %) only decrease by 4.6% and 6.6%, respectively, compared with that of pure polymer. The ultimate strength of pure polymer and nanocomposites are 2.19% and 3.59%, respectively. The UV-vis transmittance spectra of pure polymer and nanocomposites containing different NaYF4:17% Yb3+, 3% Er3+ concentrations are shown in Figure 8. Pure polymer has a transmittance of 90%. The loss of 10% is caused by reflection on the two surfaces of the block (2 mm thickness). According to Fresnel equations, when light moves from a medium of a given refractive index n1 into a second medium with refractive index n2, both reflection and refraction of the light may occur. When the light is at near-normal incidence to the interface, the reflection (R) and transmission (T) coefficient are given by

R)

(

n1 - n2 n1+n2

T)1-R

Figure 7. TGA diagrams of pure polymer and NaYF4:17% Yb3+, 3% Er3+/PMMA nanocomposites.

for the β-NaYF4:17% Yb3+, 3% Er3+/PMMA (d) and β-NaYF4: 25% Yb3+, 0.3% Tm3+/PMMA (h) nanocomposites, which confirm the presence of Na, Y, F, and Yb in both of them. The Pt peak and Si peak in the figures come from the coating for the measurement and the glass template, respectively. The thermal stabilities of NaYF4:17%Yb3+, 3%Er3+/PMMA nanocomposites were investigated by TGA with pure PMMA polymer as a comparison, and the corresponding TGA plots are illustrated in Figure 7. For the pure polymer, there are three degradation steps in the curve: (1) 280-450 °C, (2) 450-530 °C, and (3) 530-600 °C. The first one, which takes place by

)

2

(1) (2)

Note that reflection by a window is from the front side as well as the back side, and that some of the light bounces back and forth a number of times between the two sides. The combined reflection coefficient for this case is

2R/(1 + R)

(3)

when interference can be neglected (Wikipedia, The Free Encyclopedia). The values of n1 (refractive index of air) and n2 (refractive index of PMMA) are 1 and 1.49, respectively. Applying eqs 1, 2, and 3, the transmission coefficient (T) of PMMA can be determined to be 92%, which is close to the experimental value. The abrupt absorption at 400 nm is due to excess of the photoinitiator TPO in the material.16 The transmittance of NaYF4:17% Yb3+, 3% Er3+/PMMA nanocomposites (0.1 wt % NaYF4:Yb3+, Er3+) reduces a little but is still as high as 80% (800 nm). The nanocomposites containing 0.5 wt % and 1 wt % NaYF4:Yb3+, Er3+ appears optically transparent even though the transmittances are only 67% and 55% at 800 nm.

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Figure 8. Transmission spectra of pure polymer and NaYF4:17% Yb3+, 3% Er3+/PMMA nanocomposites.

Figure 10. Upconversion emission spectra of nanocomposites with various NaYF4:25% Yb3+, 0.3% Tm3+ concentrations: (a) 0.1 wt %; (b) 0.5 wt %; (c) 1 wt %. (d) Decay curve for the luminescence of Tm3+ in NaYF4:25% Yb3+, 0.3% Tm3+/PMMA nanocomposites with NaYF4 content of 1 wt %.

Figure 9. Upconversion emission spectra of nanocomposites with various NaYF4:17% Yb3+, 3% Er3+ concentrations (a) and decay curve for the luminescence of Er3+ in NaYF4:17% Yb3+, 3% Er3+/PMMA nanocomposites with NaYF4 content of 1 wt % (b).

With increasing NaYF4:17% Yb3+, 3% Er3+ content, the transparency of the nanocomposites decreases. This result is consistent with that of photographs (Figure 5). Under 980 nm laser excitation, NaYF4:17% Yb3+, 3% Er3+/ PMMA and NaYF4:25% Yb3+, 0.3% Tm3+/PMMA nanocomposites show green and blue emission, respectively, and their photographs under 980 nm laser excitation in the dark are shown in Figure 5 (b, d, and f for NaYF4:Yb3+, Er3+/PMMA and h, j, and l for NaYF4:Yb3+, Tm3+/PMMA). From the photographs, we can see that, even at the lowest concentrations of NaYF4 (0.1 wt %), the bulk nanocomposites exhibit excellent luminescent properties. A very strong green and blue emission could be observed respectively in NaYF4:17%Yb3+, 3%Er3+/PMMA and NaYF4:25% Yb3+, 0.3% Tm3+/PMMA nanocomposites when the contents of NaYF4 are 0.5 and 1 wt %. The strong fluorescence is probably due to the high crystallinity of the nanoparticles, which dispersed uniformly in bulk composites. It is important that the upconverting nanoparticles retain their

original optical properties when dispersed in the nanocomposites.5 Room-temperature upconversion fluorescence spectra of NaYF4:17%Yb3+, 3%Er3+/PMMA and NaYF4:25% Yb3+, 0.3% Tm3+/PMMA nanocomposites are shown in Figure 9a and Figure 10, respectively. The upconversion fluorescence spectra of nanocomposites have similar profiles to those of the corresponding nanoparticles, indicative of the fact that they have the same kind of emission centers. For NaYF4:17% Yb3+, 3% Er3+/ PMMA nanocomposites, the most intense green emission line is located at 539 nm (4S3/2-4I15/2). The main emission of blue luminescence (477 nm) arises from the 1G4-3H6 energy transition of the Tm3+ ion in NaYF4:25% Yb3+, 0.3% Tm3+/PMMA nanocomposites. From Figures 9 and 10, it is realized that with the increase of NaYF4:Yb3+, Er3+ (Tm3+) content, the photoluminescence intensity of nanocomposites is enhanced gradually, which is in good agreement with the results of their photographs (Figure 5). In view of their unique upconversion properties, these nanocomposites could perhaps be used in photovoltaic devices, which may increase solar cell conversion efficiency by making better use of the solar spectrum. In conventional solar cells, only a part of solar spectrum are absorbed in the solar cell semiconductor, and all the photons of the solar spectrum with energy less than the solar cell energy gap are lost. The transmission of sub-band gap light is one of the major loss mechanisms in conventional solar cells. The lost part, lowenergy NIR photons from the solar spectrum, can be converted to high-energy photons using the processes of upconversion in rare earth-doped low-phonon crystals, which can then be utilized by the solar cell.14c,17 Yb3+/Er3+ and Yb3+/Tm3+ codoped NaYF4 embedded in PMMA could be directly applied to the surface of the solar cell.18 The nanocomposites containing 0.1, 0.5, and 1 wt % NaYF4: 17% Yb, 3% Er show similar luminescence decay behaviors

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and identical lifetimes, so we just give the decay curve for the luminescence of Er3+ in NaYF4:17% Yb3+, 3% Er3+/PMMA nanocomposites with NaYF4:17% Yb3+, 3% Er3+ concentration of 1 wt % in Figure 9b. It can be well fitted into a singly exponential function as I ) I0 exp (-t/τ) (I0 is the initial emission intensity at t ) 0, and τ is the 1/e lifetime of the emission center). Through this fitting, the lifetime of Er3+ in NaYF4:17% Yb3+, 3% Er3+/PMMA nanocomposites is determined to be 0.45 ms. Note that the lifetime of Er3+ in NaYF4: 17% Yb3+, 3% Er3+/PMMA nanocomposites is identical with that in the pure NaYF4:17% Yb3+, 3% Er3+ nanoparticles. This is a good indication that the efficiency of upconversion process is hardly compromised by the quenching of excited states in the polymer matrix. Namely, the optical properties of the nanoparticles are not affected in the composite materials. But for the NaYF4:25% Yb3+, 0.3% Tm3+/PMMA nanocomposite, the lifetime for the excited state of Tm3+, 1G4 (detected at 477 nm), was 0.87 ms (Figure 10d), which is a little shorter than that for NaYF4:25% Yb3+, 0.3% Tm3+ nanoparticles. This is because abundant organic groups with high-energy C-H and C-C vibrational oscillators in PMMA are efficient luminescence quenchers for nearby lanthanide ions,19 especially for the 3H4 (800 nm) excited state of Tm3+. This leads the NIR emission at 800 nm of Tm3+ to be less intense in the composites than in the particles. Namely, the intensity ratio between the transitions at 477 and 800 nm in composite is higher than that in the particles. 4. Conclusions In summary, a series of transparent NaYF4:17% Yb3+, 3% Er /PMMA and NaYF4:25% Yb3+, 0.3% Tm3+/PMMA nanocomposites have been successfully prepared by in situ photopolymerization routes. In the NaYF4:Yb3+, Er3+ (Tm3+)/PMMA nanocomposites, NaYF4:Yb3+, Er3+ (Tm3+) are well dispersed in the polymer matrix. These nanocomposites are very similar to that of pure polymer on flexural properties. NaYF4:17% Yb3+, 3% Er3+/PMMA and NaYF4:25% Yb3+, 0.3% Tm3+/PMMA nanocomposites exhibit high transparency for visible light and show strong green and blue emission, respectively, upon 980 nm laser excitation. These nanocomposites can be potentially used as 3D display materials and up-converters for photovoltaic applications, and this method may be extended to prepare other functional nanocomposites. 3+

Acknowledgment. This project was financially supported by National Basic Research Program of China (2007CB935502, 2010CB327704), and the National Natural Science Foundation of China (NSFC 50702057, 50872131, 60977013, 20901074, 20921002). References and Notes (1) Qutubuddin, S.; Fu, X. In Nano Surface Chemistry; Rosoff, M., Ed.; Marcel Dekker, Inc.: New York, 2002; p 653. (2) (a) Okamoto, M.; Morita, S.; Taguchi, H.; Kim, Y. H.; Kotaka, S.; Tateyama, H. Polymer 2000, 41, 3887. (b) Avella, M; Errico, M. E.; Martuscelli, E. Nano Lett. 2001, 1, 213. (c) Mallick, K.; Witcomb, M. J.; Dinsmore, A.; Scurrell, M. S. Langmuir 2005, 21, 7964. (d) Khaled, S. M.; Sui, R.; Charpentier, P. A.; Rizkalla, A. S. Langmuir 2007, 23, 3988.

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