White-Light-Emitting Polymer Composite Film Based on Carbon Dots

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White-Light-Emitting Polymer Composite Film Based on Carbon Dots and Lanthanide Complexes Bin Chen and Jiachun Feng*

J. Phys. Chem. C 2015.119:7865-7872. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 08/02/18. For personal use only.

State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Polymers and Polymer Composite Materials, Department of Macromolecular Science, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: A white-light-emitting polymer composite film was designed and synthesized by using carbon dots (CDs) and lanthanide complexes as primary light emitters and skillfully embedding them into a poly(methyl methacrylate) (PMMA) matrix. The hydrophilic CDs used as blue light source were prepared and functionalized by copolymerizing with methacrylate to prevent their aggregate in the hydrophobic matrix. The lanthanide complexes Eu(DBM)3 and Tb(DBM)3 (DBM: dibenzoylmethide), in which the rare earth ions have not been fully coordinated, were fabricated and used as red and green emitters. The coordinatively unsaturated lanthanide ions could further coordinate with the oxygen atoms in the PMMA chains, which makes the complexes homogeneously dispersed in matrix as well as benefits to the energy transfer process. By adjusting the ratio of CDs, Eu(DBM)3 and Tb(DBM)3 in the matrix, the high transparent film with improved thermal stability, which prepared by a simple solution cast method, could emit pure white light (CIE coordinate located at (0.31, 0.32)) under 400 nm laser with a quantum efficiency of 16.6%. The energy transfer mechanism in the white-light-emitting material was also discussed.

1. INTRODUCTION White-light-emitting materials have attracted broad interest from scientific and industrial community because of their diverse applications in displays and lighting.1,2 Basically, whitelight-emitting systems are composed of components that emit either the two complementary (e.g., blue and yellow) or three primary colors (blue, green, and red).3 Because of the good color rendering properties and high luminous efficiency, the trichromatic systems are typically superior to the dichromatic ones, and thus have attracted even more attention.4 Generally, the trichromatic white-light-emitting systems are constructed by choosing effective primary color sources and incorporating them together properly. For example, Xu et al. used blue fluorescent polyfluorene as a host material, when it was doped with green phosphorescent fac-tris[2-(4′-ter-butyl)phenylpyridine]iridium(III) and red bis(1-phenylisoquinolyl)iridium(III)(1trifluoro)acetylacetonate, pure white-light emission could be achieved through adjusting the ratio of the components.5 Wang et al. embedded the red chromophore in the polymer’s main chain which itself could emit blue light, and connected the green chromophore in the polymer’s side chain. By adjusting the ratio of the chromophores, they got highly efficient white light.6 Although great progress has been achieved, considering the importance of the white-light-emitting materials, the search for more new facile strategies to fabricate trichromatic whitelight systems is still highly desired. Lanthanide ions, as important light emitters, have been largely used in designing trichromatic white-light-emitting systems due © 2015 American Chemical Society

to their fascinating properties such as intense and sharp luminescence, long lifetime, and high efficiency.7−13 Duan et al. obtained white light from a Eu3+ complex with organic ligands that contained blue and green emitting fluorophores.14 Zheng et al. reported some white-light-emitting metal−organic frameworks synthesized through the self-assembly of various lanthanide ions and 1,1′,1″-(benzene-1,3,5-triyl)tripiperidine4-carboxylic acid ligands.15 Among all lanthanide ions, Eu3+ and Tb3+ are the best for luminescence.16 They can respectively emit pure red and green light in almost any environment and free of the influence of external factors due to their light emission is originated from the electron redistribution of 4f orbitals, which are effectively shielded by the overlying 5s2 and 5p6 orbitals. It is expected that, white light could be easily achieved by codoping Eu3+, Tb3+ ions with a certain high efficient blue light emission source. Recently, carbon nanodots (CDs), a new class of carbon nanomaterials with sizes below 10 nm, have been extensively spotlighted for their excellent photoluminescent properties and broad applications.17 Compared with traditional semiconductor quantum dots and organic dyes, CDs are superior in terms of tunable photoluminescence, robust chemical inertness, easy functionalization, high resistance to photobleaching, low toxicity and good biocompatibility.18 It has been widely reported that Received: January 8, 2015 Revised: March 11, 2015 Published: March 18, 2015 7865

DOI: 10.1021/acs.jpcc.5b00208 J. Phys. Chem. C 2015, 119, 7865−7872

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Scheme 1. Strategy in Constructing the White Emission System: (a) Functionalize the CDs with PMMA and (b) Integrate the Emitters into the PMMA Matrix by Solution Mixing

PMMA (Mn = 50000 g/mol, Mw/Mn = 1.80) used in this study were received from Zhenjiang Chimei Petrochemical Co., Ltd. (Jiangsu, China). All the other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used without further purification except methyl methacrylate which was purified to remove polymerization inhibitor. Synthesis of CDs, PCDs, and FCDs. The CDs with amino groups on their surface were synthesized by a simple microwaveassisted pyrolysis process using acrylic acid and 1,2-ethanediamine as the carbon source and surface passivation agent, respectively.23 Typically, 5.35 mL of 1,2-ethanediamine (80 mmol) and 5.49 mL of acrylic acid (80 mmol) were dissolved in a common 100 mL beaker with 40 mL of water under stirring. The solution was put into a 700 W domestic microwave oven, holding this condition for 7 min until a light smoke appeared. After the reaction was allowed to cool to room temperature, 20 mL of water was added to dissolve the resulting CDs. Dry CDs were obtained by lyophilization of the remaining water solution. To synthesize the polymerizable carbon nanodots (PCDs), GMA was used as a vinylation agent to react with amine groups on the CDs’ surface (step 1, Scheme 1). Typically, about 1.5 g of dry CDs were dissolved in 20 mL of water, then 10 mL of GMA was added into the above CDs solution, and stirred for 24 h at 30 °C. Subsequently, the oil phase of the solution was removed and the water phase was further washed by n-hexane to remove the unreacted GMA molecules. Finally, the dry PCDs were obtained by lyophilization of the remaining water solution. The functionalized CDs (FCDs) was synthesized by radical polymerization. 9.8 g methacrylate monomers and 0.2 g of PCDs were dissolved in 50 mL N, N-dimethylformamide under magnetic stirring, with N2 bubbling for 15 min. Then the reaction system was heated to 80 °C, followed by the addition of 100 mg azodiisobutyronitrile (AIBN) to initiate polymerization. The reaction was kept for 6 h, and then the polymers were precipitated by 1000 mL water, and washed by water for several times. Finally, FCDs were dried under vacuum at 50 °C for 20 h.

most CDs emit blue light, and the quantum yield can be up to 80%.19 Therefore, whether white emission can be achieved by taking CDs as blue emitter and incorporating with Eu3+ and Tb3+ complexes is an interesting topic and is worth to be studied. Considering the lanthanide complexes and CDs both are brittle and hard to process, a proper matrix is usually needed to endow the system with good mechanical toughness and processability. Polymers are ideal matrices for light emitting materials due to several attractive features including high transparency, excellent mechanical strength, flexibility, controllable cost, and ease of processing.20 Unfortunately, most polymers used in optical materials are hydrophobic while the unmodified CDs are hydrophilic and may aggregate in the polymer matrix. Actually, though a lot of efforts have been devoted,21 the uniform dispersion of lanthanide in the polymer matrix is still a practical problem. It is reported that the aggregate of the emitters will seriously quench the luminescence through the charge- or energy-transfer process,22 thus making homogeneous dispersion of both CDs and lanthanides in the polymer matrix and fully unleashing the potential of the emitters is most important in constructing such a white emission system. Here, we synthesized a novel white-light-emitting film by incorporating CDs and lanthanide complexes (Eu(DBM)3 and Tb(DBM)3, (DBM: dibenzoylmethide)) into poly(methyl methacrylate) (PMMA) matrix (Scheme 1). The CDs were functionalized by copolymerizing them with methacrylate to improve the compatibility between CDs and PMMA. The lanthanide complexes, Eu(DBM)3 and Tb(DBM)3, are well-dispersed in the matrix due to the possible interaction between the lanthanide ions and the oxygen atoms in the PMMA chains. Intense white light emission could be observed by carefully adjusting the ratio of the components and excitation wavelength. The PL properties and the thermal stability of the film was studied and the energy transfer process in the system was also discussed.

2. EXPERIMENTAL SECTIONS Materials. Glycidyl methacrylate (GMA, 97%) was obtained from Aladdin Industrial Corporation (Shanghai, China). 7866

DOI: 10.1021/acs.jpcc.5b00208 J. Phys. Chem. C 2015, 119, 7865−7872

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The Journal of Physical Chemistry C Synthesis of Lanthanide Complexes. The unsaturated lanthanide complexes Eu(DBM)3 and Tb(DBM)3 were synthesized as follows. DBM (0.674 g; 3 mmol) was dissolved in 15 mL of ethanol. The pH of the solution were adjusted into 5 with 1 M NaOH, then a solution of 0.7 mmol hydrated lanthanide chloride (0.257 g of EuCl3·6H2O, or 0.267 g of TbCl3· 6H2O in 10 mL of water, respectively) was added to the solution of DBM ligand. After another 50 mL of water was added and the mixture was stirred for 2 h at room temperature (25 °C), the precipitated complexes were filtered and then washed with water for several times. Finally, the product was dried in a vacuum oven at 60 °C for 20 h. Preparation of the White-Light-Emitting PMMA Films. The primary color sources were incorporated together with the PMMA matrix through solution mixing. The tetrahydrofuran (THF), in which all these constituents have excellent solubility, was chosen as the solvent. Typically, the PMMA powder was dissolved in 3 mL of THF, followed by addition of the required amount FCDs and complexes solutions (because of the contents of Eu(DBM)3 and Tb(DBM)3 were too small to weigh directly, their THF solutions with concentrations of 0.25 and 2.5 mg/mL were respectively prepared and added). In order to get white emission, we prepared a series of films in which the ratio of the emitters were adjusted by fixing the contents of the CDs and Tb(DBM)3 while varying the Eu(DBM)3 contents, as shown in Table 1. The resulting solution was holding at 50 °C

The thermal gravimetric analysis (TGA) was conducted on a PerkinElmer Pyris Thermal Analyzer (PerkinElmer) under N2 atmosphere. The FTIR spectra of the samples were measured on a Nicolet 6700. 1H NMR spectras of the samples were recorded with Varian Mercury plus 400 spectrometer with D2O as solvent. The UV absorption of the samples was measured with UV−vis spectrophotometer Lambda 750 from PerkinElmer. The amounts of C and H of the complexes were determined with an Elementar Vario El III Elemental analyzer (EA).

3. RESULTS AND DISCUSSION 3.1. Preparation and Functionalization of CDs. The successful synthesis of CDs can be confirmed by the TEM images (Figure 1a, and Figure S1 in Supporting Information), which revealed that the as-prepared CDs with a spherical shape are well-dispersed with a narrow size distribution and the diameters of the CDs are about 3−5 nm. Furthermore, HRTEM image (Figure 1b) shows that the CDs have certain crystallinity with average lattices 0.33 nm, coordinating to the typical interlayer distance of graphite (002, 0.34 nm).24 The photoluminescence performance is crucial for CDs application. Figure 1c is the PL spectra for an aqueous solution of CDs (1 mg/mL). The systematic PL study with different excitation wavelengths ranging from 320 to 420 nm revealed that the CDs have typical wavelength-dependent PL behavior: the maximum emission shifted to long wavelength when the excitation wavelength increases. This is most probably due to the different surface states and size dispersion of the carbon nanoparticles.25 The strongest fluorescence emission band, located at 420 nm, is observed under 340 nm excitation, which gave blue fluorescence under UV light. Since neither acrylic acid nor 1,2-ethanediamine emit in the visible or near-UV, the fluorescence is attributed to the C-dots. The quantum yield of the CDs at 340 nm excitation is 51.08%, indicating that they have excellent PL performance. Figure 2a is the 1H NMR spectra of the CDs and PCDs samples. The newly appeared 1H NMR peaks at δ 5.99 and δ 5.56 (−CCH2) indicate the successful linkage of the GMA molecule onto the nanodots’ surface. The CDs and PCDs were also characterized by FTIR spectra. The amide I (CO stretching) and amide II (N−H bending/C−N stretching) bands were observed at 1643 and 1558 cm−1, respectively, which could be attribute to the formation of the amide bond between acrylic acid and 1,2-ethanediamine in both CDs and PCDs. The newly appeared FTIR peak at 1716 cm−1 (−COOR) in the spectra of PCDs could further confirm that there are GMA molecules on PCDs’ surface. As mentioned above, the diameters of the CDs are only about 3−5 nm, such small sizes could effectively minimize the

Table 1. Prescription for Preparation of Composite Films with Different Eu(DBM)3 Contents samples film film film film film film

1 2 3 4 5 6

PMMA (g)

FCDs (g)

Tb(DBM)3 (mg)

Eu(DBM)3 (mg)

0.9 0.9 0.9 0.9 0.9 0.9

0.1 0.1 0.1 0.1 0.1 0.1

2.5 2.5 2.5 2.5 2.5 2.5

0.0125 0.025 0.0375 0.05 0.0875 0.125

for 30 min under rigorous stirring and then poured into a Teflon mold (length, 5 cm; width, 4 cm). The composite film was obtained after the complete evaporation of the solvent at 30 °C. Characterizations. Photoluminescence (PL) emission measurements were performed using FLS920 (Edinburgh Instruments, Great Britain). The overall quantum yields are based on the absolute method using a calibrated integrating sphere in a QM40 Fluorescence Lifetime Spectrometers. The morphology and microstructure of the CDs were examined by highresolution transmission electron microscopy (HRTEM) on Tecnai G2 F20 S-Twin with an accelerating voltage of 200 kV.

Figure 1. TEM (a) and HRTEM (b) images of CDs. (c) PL emission spectrum of CDs (1 mg/mL, the inset is the digital photos of CDs under sunlight (left) and 365 nm UV lamp (right)). 7867

DOI: 10.1021/acs.jpcc.5b00208 J. Phys. Chem. C 2015, 119, 7865−7872

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Figure 2. 1H NMR (a) and FTIR (b) spectra of CDs and PCDs. (c) Digital photos of FCDs-doped (left) and CDs-doped (right) PMMA films (CDs contents of both samples are 1 wt %).

Figure 3. TGA curves (a) and FTIR spectra (b) of Eu(DBM)3 and Tb(DBM)3.

Considering all above, six-coordinated Eu(DBM)3 and Tb(DBM)3 was synthesized. TGA curves shows no significant mass loss in the temperature region of 150−220 °C (Figure 3a), and no absorption band in the 3100−3600 cm−1 region (corresponding to the −OH stretching vibration of inner coordinate water molecules22) on the FTIR spectra (Figure 3b), which means that DBM is the only ligand coordinated with lanthanide ions and there are no innercoordinate water molecules in the complexes. The result is further supported by the elemental analysis (%) for Eu(DBM)3 (Calcd: C, 65.53; H, 4.36. Found: C, 64.09; H, 4.26) and Tb(DBM)3 (Calcd: C, 64.98; H, 4.33. Found: C, 64.22; H, 4.29). To explore the optical properties of the lanthanide complexes, UV−vis absorption and PL spectra were studied. Figure 4a is the UV−vis spectra of lanthanide complexes and DBM. The DBM shows a strong absorption from 290 to 390 nm (peak is centered at 343 nm) due to the π−π* electronic transition of carbonyl group and aromatic ring. Eu(DBM)3 and Tb(DBM)3 also displayed a strong wide absorption in the same range, which means the fluorescence emission of the metal complexes was mainly sensitized by the absorption of the DBM. Besides, The absorption peak of Eu(DBM)3 and Tb(DBM)3 red-shifted to 349 nm, implying that the metal ions was coordinated with the oxygen atoms of DBM. Figure 4b shows the PL emission spectrum of Eu(DBM)3. Maximum peak intensities at 579, 592, 612, 651, and 703 nm were observed for the J = 0, 1, 2, 3, and 4 transitions, respectively, and the J = 2 so-called “hypersensitive” transition is intense. As to Tb(DBM)3, its emission is too weak to recgonize. The quantum yields of Eu(DBM)3 and Tb(DBM)3 are 6.0% and 1.6%, respectively. 3.3. Preparation and Properties of Composite Films. At first we prepared a Eu(DBM)3/PMMA composite film in which the content of Eu(DBM)3 is 0.01 wt %. Figure 5 is the UV−vis spectrum of PMMA and Eu(DBM)3/PMMA composite films. PMMA shows a strong wide absorption in the range of from 320 to 380 nm due to the π−π* electronic transition of

steric hindrance effect and make them ideal to polymerize. Figure 2c is the digital photos of CDs-doped PMMA films. The left one which was prepared by solution mixing of FCDs and PMMA is totally transparency, meaning the molecular level dispersion of CDs. The right one prepared by directly doping the same amount of CDs is opaque and light yellow, which means that the CDs have got together and the macroscopic phase separation happens. This obvious difference in transmittance of the two samples sugests that functionalization of the CDs by copolymerizing them with methacrylate could effectively improve the compatibility between CDs and PMMA matrix and prevent the aggregatation. 3.2. Synthesize and Characterization of Lanthanide Complexes. Luminescent complexes usually consist of a central lanthanide ion and chelating organic ligands. The ligands act as a photosensitizer which can efficiently absorb and transfer energy to the central lanthanide ion. This sensitization process is much more effective than the direct excitation of the Ln3+ ions. Typically, the Ln3+ ions is eight-coordinated with three β-diketone ligands and another coligand such as 1,10phenanthroline or solvent molecular (water, ethanol and so on). It is reported that in some complex-polymer systems in which the polymer chain contains carbonyl oxygen atoms, the chains can act as coligand,26−28 which could make the homogeneous dispersion of lanthanides because of the improved interactions between the polymer matrix and the lanthanide complex. Therefore, we try to synthesize lanthanide complexes which were not fully coordinated so that the unsaturated center ions could further coordinate with the oxygen atoms in the PMMA chains. DBM could be a suitable ligand not only because it was widely used and studied in synthesizing lanthanide complexes,29,30 but more importantly is that, compared with other common β-diketone ligands such as thenoyltrifluoroacetonate or acetylacetonate, DBM is more hydrophobic, thereby it may obstruct the water molecule to coordinate with lanthanide ions. 7868

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Figure 4. (a) UV−vis spectra of Eu(DBM)3, Tb(DBM)3, and DBM. (b) PL emission spectrum of Eu(DBM)3 (10−5 mol/L in chloroform).

Information). This could be explained by the so-called “innerfilter effect” that mainly occurs in condensed matter: when short-wavelength light emitted from one point of the film, it could be absorbed by surrounding CDs, from which red-shifted long-wavelength light is re-emitted. This process repeated until the emitted light completely passes through the film and gives rise to the “cut-off” of short-wavelength emission followed by enhancement of long-wavelength emission.35,36 The peak at 490 nm is caused both by CDs and 5D4 → 7F6 of Tb3+. The peak centered at 550 nm comes from 5D4 → 7F5 of Tb3+. The character emission peak of Tb3+ almost cannot be observed in solution while it can be seen clearly in the PMMA matrix, which means that the PMMA could effectively improve the energy transfer efficiency. The peak at 612 nm comes from 5 D0 → 7F2 of Eu3+. The light-transmission quality and thermal stability are also important for the light emitting materials. The prepared composite film shows high transparency in sun light (Figure 6d), which means that there are little Rayleigh scattering from the nanomaterials in the matrix and could confirm the molecular level dispersion and completely preventable aggregation of the emitters.37 The TGA under dynamic N2 atmosphere shows that the major mass loss of the films occurs at 350−450 °C (Figure 7). After doping, the thermal decomposition comes up on higher temperature, suggesting that the incorporation of CDs and lanthanide complexes into the PMMA matrix increases the overall thermal stability. Compared with PMMA−CDs samples, the PMMA−CDs−complex samples are even more thermal stable, indicating that the coordination between PMMA chains and lanthanide ions may have some positive effect in improving the thermal stability. The TGA curves of the Film 1 to Film 6 are almost overlapped with each other, which reveals that the addition of small amount of Eu(DBM)3 have no obvious effect on the thermal stability of the film. 3.4. Energy Transfer Mechanisms in the White Emission Material. It is interesting to note that when white light is achieved, the contents of Eu(DBM)3 are much lower than that of Tb(DBM)3. One reason is that the triplet exited state of DBM and the 5D0 energy state of Eu3+ ion could well match the energy required for the transition from DBM to Eu3+ ion. The energy differences between the triplet exited state of DBM and the resonant energy state of Tb3+ ion were too small, which might cause the inverse energy transition in PMMA− Tb3+−DBM system by thermal deactivation mechanism.26 The other reason is that the Tb(DBM)3 could effectively enhance the fluorescence intensity of the Eu(DBM)3 in PMMA matrix. We prepared a series of films in which the contents of CDs and Eu(DBM)3 were fixed at 0.2 and 0.00375 wt %, respectively,

Figure 5. UV−vis spectra of PMMA and Eu(DBM)3/PMMA composite films.

carbonyl group, and the absorption peak is centered at 333 nm. Eu(DBM)3/PMMA composite also displayed a strong wide absorption in the wavelength range of 300−390 nm, but the peak was red-shifted to 351 nm. Compared with the spectrum of PMMA, the red-shift of about 18 nm was observed for that of Eu(DBM)3/PMMA composite because of the decreased electron density of carbonyl group after its coordination of oxygen atom with Eu3+ ion. The results reveal that the PMMA could indeed act as a macro-ligand coordinating with the lanthanide complexes, resulting in the homogeneously dispersion of the lanthanide complex in the PMMA matrix. Then a series of composite films in which the contents of CDs and Tb(DBM)3 were fixed at 0.2 and 0.25 wt %, respectively, and the Eu(DBM)3 contents varied from 0.00125 to 0.0125 wt % (films 1−6) were prepared. As manifested by Figure 6a, with the Eu(DBM)3 contents increasing, the CIE (Commission Internationale d’Éclairage) coordinate shifting to the red region gradually (λex = 400 nm). When the Eu(DBM)3 contents is 0.00375 wt % (film 3), strong white light (CIE coordinate located at (0.31, 0.32)) could be observed (Figure 6b). The quantum efficiency of the white light emission was found to be 16.6% (Table 2), which is superior to lots of reported values of white-light-emitting materials.31−34 The emission spectrum of the film takes on three major peaks (Figure 6c). The peak in the range of 420−520 nm originated from the CDs. It is interesting that the emission band of CDs in the white emission composite film is centered at about 470 nm, while in solution phase it centered at 440 nm when excited with same 400 nm laser. Moreover, further experiments shows that the emission of the PMMA/FCD composite films shift to more long-wavelength region with the increase of the concentration of the in-film CDs (Figure S2 in Supporting 7869

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Figure 6. (a) Chromaticity coordinates of the prepared films on the CIE 1931 chromaticity chart. (b) Photograph of film 3 excited with 400 nm laser. (c) Emission spectrum of the primary color emitters and the white-light-emitting film (λex = 400 nm). (d) Photograph of film 3 in sunlight.

Tb(DBM)3, which means that the fluorescence of Eu(DBM)3 can be effectly enhanced by Tb(DBM)3 addition in our system. Considering the resonance energy level of Eu3+ (5D0, 17500 cm−1) is lower than that of Tb3+ (5D4, 20400 cm−1) and the triplet state of DBM (20520 cm−1), it is reasonable to conclude that both the Tb3+ and the triplet state of DBM in Tb(DBM)3 could transfer energy to 5D0 of Eu3+.30 As mentioned above, the PMMA matrix could effectively improve the lanthanides’ PL intensity. You et al.26 reported that the polymers which have carbonyl oxygen could act as second ligand and directly transfers energy to the lanthanide ions. However, the UV−vis spectra of PMMA (Figure 5) shows that, though the PMMA could coordinate with the lanthanide ions, it have scarcely any absorbance in 400 nm. The result means that in our system, the PMMA cannot absorb energy and transfer it to the lanthanide ions. The enhancement of the PL intensity of the lanthanide by PMMA could be explained that the PMMA chains could enwrap the lanthanide complex and keeps the acceptor and donor close. In such a case, energy can be transferred efficiently from ligand to lanthanide ions, resulting in the enhancement of PL emission. The possible energy transfer mechanisms in the white emission material is illustrated in Scheme 2.

Table 2. Luminescence Properties of the Primary Light Emitters and the White-Light-Emitting Composite Film (Film 3) Eu(DBM)3 Tb(DBM)3 CDs film 3

colors

CIE (x, y)

red green blue white

(0.67, (0.26, (0.19, (0.31,

0.33) 0.51) 0.18) 0.32)

Φ (%) 6.0 1.6 51.1 16.6

Figure 7. TGA curves of pure PMMA film, PMMA film contains CDs, and PMMA film contains both CDs and lanthanide complexes.

4. CONCLUSIONS In summary, we successfully functionalized the carbon dots with PMMA and take it as a blue light source to incorporate with lanthanide complexes to produce a white-light-emitting material. Both the carbon dots and the lanthanide complexes were immobilized via chemical and physical interactions within

and varied the Tb(DBM)3 contents from 0 to 0.25 wt %. PL spectra (Figure S4 in Supporting Infomation) show that the intensity of the character emission peak of Eu(DBM)3 at 612 nm is improved remarkably with the increase of the contents of 7870

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(7) Zhang, H.; Shan, X.; Zhou, L.; Lin, P.; Li, R.; Ma, E.; Guo, X.; Du, S. Full-Colour Fluorescent Materials Based on Mixed-Lanthanide (iii) Metal-Organic Complexes with High-Efficiency White Light Emission. J. Mater. Chem. C 2013, 1, 888−891. (8) Xu, H.; Zhu, R.; Zhao, P.; Huang, W. Monochromic RedEmitting Nonconjugated Copolymers Containing Double-CarrierTrapping Phosphine Oxide Eu3+ Segments: Toward Bright and Efficient Electroluminescence. J. Phys. Chem. C 2011, 115, 15627− 15638. (9) Sun, Z.; Bai, F.; Wu, H.; Boye, D. M.; Fan, H. Monodisperse Fluorescent Organic/Inorganic Composite Nanoparticles: Tuning Full Color Spectrum. Chem. Mater. 2012, 24, 3415−3419. (10) Khanna, A.; Dutta, P. Narrow Spectral Emission CaMoO4: Eu3+, Dy3+, Tb3+ Phosphor Crystals for White Light Emitting Diodes. J. Solid State Chem. 2013, 198, 93−100. (11) Ramya, A.; Varughese, S.; Reddy, M. Tunable White-Light Emission from Mixed Lanthanide (Eu3+, Gd3+, Tb3+) Coordination Polymers Derived from 4-(Dipyridin-2-yl) Aminobenzoate. Dalton Trans. 2014, 43, 10940−10946. (12) Xu, H.; Zhu, R.; Zhao, P.; Xie, L.-H.; Huang, W. Photophysical and Electroluminescent Properties of a Series of Monochromatic RedEmitting Europium-Complexed Nonconjugated Copolymers Based on Diphenylphosphine Oxide Modified Polyvinylcarbazole. Polymer 2011, 52, 804−813. (13) Xu, H.; Wang, J.; Wei, Y.; Xie, G.; Xue, Q.; Deng, Z.; Huang, W. A Unique White Electroluminescent One-Dimensional Europium(III) Coordination Polymer. J. Mater. Chem. C 2015, 3, 1893−1903. (14) He, G.; Guo, D.; He, C.; Zhang, X.; Zhao, X.; Duan, C. A ColorTunable Europium Complex Emitting Three Primary Colors and White Light. Angew. Chem., Int. Ed. 2009, 48, 6132−6135. (15) Tang, Q.; Liu, S.; Liu, Y.; He, D.; Miao, J.; Wang, X.; Ji, Y.; Zheng, Z. Color Tuning and White Light Emission via In Situ Doping of Luminescent Lanthanide Metal-Organic Frameworks. Inorg. Chem. 2013, 53, 289−293. (16) Bünzli, J.-C. G.; Piguet, C. Taking Advantage of Luminescent Lanthanide Ions. Chem. Soc. Rev. 2005, 34, 1048−1077. (17) Xie, Z.; Wang, F.; Liu, C. Y. Organic-Inorganic Hybrid Functional Carbon Dot Gel Glasses. Adv. Mater. 2012, 24, 1716− 1721. (18) Yang, S.-T.; Wang, X.; Wang, H.; Lu, F.; Luo, P. G.; Cao, L.; Meziani, M. J.; Liu, J.-H.; Liu, Y.; Chen, M. Carbon Dots as Nontoxic and High-Performance Fluorescence Imaging Agents. J. Phys. Chem. C 2009, 113, 18110−18114. (19) Zhu, S.; Meng, Q.; Wang, L.; Zhang, J.; Song, Y.; Jin, H.; Zhang, K.; Sun, H.; Wang, H.; Yang, B. Highly Photoluminescent Carbon Dots for Multicolor Patterning, Sensors, and Bioimaging. Angew. Chem., Int. Ed. 2013, 125, 4045−4049. (20) Binnemans, K. Lanthanide-based Luminescent Hybrid Materials. Chem. Rev. 2009, 109, 4283−4374. (21) Wang, D.; Zhang, J.; Lin, Q.; Fu, L.; Zhang, H.; Yang, B. Lanthanide Complex/Polymer Composite Optical Resin with Intense Narrow Band Emission, High Transparency and Good Mechanical Performance. J. Mater. Chem. 2003, 13, 2279−2284. (22) Zhao, H. X.; Liu, L. Q.; De Liu, Z.; Wang, Y.; Zhao, X. J.; Huang, C. Z. Highly Selective Detection of Phosphate in Very Complicated Matrixes with an Off-On Fluorescent Probe of Europium-Adjusted Carbon Dots. Chem. Commun. 2011, 47, 2604− 2606. (23) Zhang, P.; Li, W.; Zhai, X.; Liu, C.; Dai, L.; Liu, W. A Facile and Versatile Approach to Biocompatible “Fluorescent Polymers” from Polymerizable Carbon Nanodots. Chem. Commun. 2012, 48, 10431− 10433. (24) Liu, H.; Ye, T.; Mao, C. Fluorescent Carbon Nanoparticles Derived from Candle Soot. Angew. Chem., Int. Ed. 2007, 46, 6473− 6475. (25) Sun, Y.-P.; Zhou, B.; Lin, Y.; Wang, W.; Fernando, K. S.; Pathak, P.; Meziani, M. J.; Harruff, B. A.; Wang, X.; Wang, H. Quantum-sized Carbon Dots for Bright and Colorful Photoluminescence. J. Am. Chem. Soc. 2006, 128, 7756−7757.

Scheme 2. Energy Transfer Mechanisms in the White Emission Material

the PMMA polymer matrix. Intense white light (CIE coordinate located at (0.31, 0.32)) can be observed when exciting with 400 nm laser. The quantum efficiency of the white light emission was measured to be 16.6%, which is superior to lots of reported white-light-emitting materials. The as-prepared whitelight-emitting PMMA film shows good transparency in sun light and the overall thermal stability increased in comparison to the pure PMMA film. The results indicate that taking CDs and lanthanide complexes as primary light emitters and integrating them into proper matrix can be a promising solution for white light sources.

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ASSOCIATED CONTENT

S Supporting Information *

HRTEM image of CDs, PL emission spectrum of FCDs, PMMA/FCDs composites, and composite films with different Tb(DBM)3 contents, and TGA curves of the films (Film 1 to Film 6). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86 21 6564 3735. Fax: +86 21 6564 0293. E-mail: [email protected] (J.F). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (51373042 and 21174032). REFERENCES

(1) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lüssem, B.; Leo, K. White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency. Nature 2009, 459, 234−238. (2) Chen, H.; Zhang, W.; Lin, Z.; Ling, Q. White-Light HydrotalciteLike Compound Emission from the Incorporation of Red-, Green-, and Blue-Emitting Metal Complexes. Opt. Mater. Express 2013, 3, 105−113. (3) Praveen, V. K.; Ranjith, C.; Armaroli, N. White-Light-Emitting Supramolecular Gels. Angew. Chem., Int. Ed. 2014, 53, 365−368. (4) Rao, X.; Huang, Q.; Yang, X.; Cui, Y.; Yang, Y.; Wu, C.; Chen, B.; Qian, G. Color Tunable and White Light Emitting Tb3+ and Eu3+ Doped Lanthanide Metal-Organic Framework Materials. J. Mater. Chem. 2012, 22, 3210−3214. (5) Xu, Y.; Peng, J.; Mo, Y.; Hou, Q.; Cao, Y. Efficient Polymer White-Light-Emitting Diodes. Appl. Phys. Lett. 2005, 86, 163502. (6) Liu, J.; Zhou, Q.; Cheng, Y.; Geng, Y.; Wang, L.; Ma, D.; Jing, X.; Wang, F. The First Single Polymer with Simultaneous Blue, Green, and Red Emission for White Electroluminescence. Adv. Mater. 2005, 17, 2974−2978. 7871

DOI: 10.1021/acs.jpcc.5b00208 J. Phys. Chem. C 2015, 119, 7865−7872

Article

The Journal of Physical Chemistry C (26) Liu, D.; Wang, Z.; Yu, H.; You, J. Fluorescence Properties of Novel Rare Earth Complexes Using Carboxyl-Containing Polyaryletherketones As Macromolecular Ligands. Eur. Polym. J. 2009, 45, 2260−2268. (27) Raj, D. A.; Francis, B.; Reddy, M.; Butorac, R. R.; Lynch, V. M.; Cowley, A. H. Highly Luminescent Poly (methyl methacrylate)Incorporated Europium Complex Supported by a Carbazole-based Fluorinated B-Diketonate Ligand and a 4, 5-Bis (Diphenylphosphino)9, 9-Dimethylxanthene Oxide Co-ligand. Inorg. Chem. 2010, 49, 9055− 9063. (28) Kai, J.; Felinto, M. C.; Nunes, L. A.; Malta, O. L.; Brito, H. F. Intermolecular Energy Transfer and Photostability of LuminescenceTuneable Multicolour PMMA Films Doped with Lanthanide-BDiketonate Complexes. J. Mater. Chem. 2011, 21, 3796−3802. (29) McGehee, M. D.; Bergstedt, T.; Zhang, C.; Saab, A. P.; O’Regan, M. B.; Bazan, G. C.; Srdanov, V. I.; Heeger, A. J. Narrow Bandwidth Luminescence from Blends with Energy Transfer from Semiconducting Conjugated Polymers to Europium Complexes. Adv. Mater. 1999, 11, 1349−1354. (30) Jiu, H.; Ding, J.; Sun, Y.; Bao, J.; Gao, C.; Zhang, Q. Fluorescence Enhancement of Europium Complex Co-doped with Terbium Complex in a Poly (methyl methacrylate) Matrix. J. NonCryst. Solids 2006, 352, 197−202. (31) Chen, Q.-L.; Wang, C.-F.; Chen, S. One-Step Synthesis Of Yellow-Emitting Carbogenic Dots Toward White Light-Emitting Diodes. J. Mater. Sci. 2013, 48, 2352−2357. (32) Zhang, B.; Liu, C. y.; Liu, Y. A Novel One-Step Approach to Synthesize Fluorescent Carbon Nanoparticles. Eur. J. Inorg. Chem. 2010, 2010, 4411−4414. (33) Singh, S.; Tripathi, A.; Rastogi, C. K.; Sivakumar, S. White Light from Dispersible Lanthanide-Doped LaVO4 Core-Shell Nanoparticles. RSC Adv. 2012, 2, 12231−12236. (34) Kar, A.; Kundu, S.; Patra, A. A Simple Approach to Generate Efficient White Light Emission from a ZnO-Ionic Liquid Complex. RSC Adv. 2012, 2, 4879−4885. (35) Kwon, W.; Do, S.; Lee, J.; Hwang, S.; Kim, J. K.; Rhee, S.-W. Freestanding Luminescent Films of Nitrogen-Rich Carbon Nanodots toward Large-Scale Phosphor-Based White-Light-Emitting Devices. Chem. Mater. 2013, 25, 1893−1899. (36) Tran, N. T.; Shi, F. G. Studies Of Phosphor Concentration and Thickness for Phosphor-based White Light-Emitting-Diodes. J. Lightwave Technol. 2008, 26, 3556−3559. (37) Shishino, Y.; Yonezawa, T.; Udagawa, S.; Hase, K.; Nishihara, H. Preparation of Optical Resins Containing Dispersed Gold Nanoparticles by the Matrix Sputtering Method. Angew. Chem., Int. Ed. 2011, 50, 703−705.

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DOI: 10.1021/acs.jpcc.5b00208 J. Phys. Chem. C 2015, 119, 7865−7872