Single Component Lanthanide Hybrids Based on Metal–Organic

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Single Component Lanthanide Hybrids Based on Metal−Organic Framework for Near-Ultraviolet White Light LED Yan-Wu Zhao,† Fu-Qiang Zhang,† and Xian-Ming Zhang*,†,‡ †

School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, People’s Republic of China Institute of Crystalline Materials, Shanxi University, Taiyuan 030006, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Near-UV single-phase white-light phosphor (Eu0.045Tb0.955CPOMBA/La0.6Eu0.1Tb0.3CPOMBA) based on metal−organic framework was prepared by in situ doping isostructural lanthanide MOF with Eu3+ and Tb3+, and it is found that the energy can effectively transfer from organic ligand to lanthanides, which can overcome weak absorption under direct excitation of lanthanide ions due to the forbidden f−f transitions. The photoluminescence and thermostability of the new MOF phosphor are investigated, and effective whitelight emission is achieved under 365 and 380 nm excitations. By employing Eu0.045Tb0.955CPOMBA as phosphor, we fabricated a near-ultraviolet white-light-emitting diode (n-UV WLED) (365 nm) with low CCT (5733 K), high CRI (Ra = 73.4), and CIE chromaticity coordinate (0.3264, 0.3427). This approach may open new perspectives for developing single-phase UV phosphors. KEYWORDS: single-phase, phosphors, lanthanide, metal−organic framework, UV white-light-emitting diode

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INTRODUCTION Since the 1880s, incandescent lights have been invented and are still used as a civilian lighting source in some places today, but its energy consumption is very severe because 98% of the input power emits in the form of heat instead of light.1 Fluorescent light as a higher efficiency lighting source can convert 10−15% of energy input into light. However, short lifespan, poor color rendering, and toxicity are obviously deficient. Moreover, disposal problem is nerve-wracking at the end of the lamp’s life because of mercury in every lamp.1−3 In view of ever-increasing energy demands and the environment concerns, it is imperative for the development of high-efficiency light sources to reduce energy consumption and environmental pollution. White-lightemitting diodes (WLEDs), which are considered as the new generation of lighting systems, can conquer the aforementioned shortcomings because of their advantages such as environmental friendliness, low power consumption, high efficiency, and long lifetime.4−9 At present, three widespread approaches to generate white light based on LEDs are as follows: (a) the mixing of red, green, and blue LEDs (RGB WLEDs), (b) the blue lighting (450−470 nm) that excites a yellow-emitting phosphor (blue WLEDs), and (c) the near-ultraviolet (360−420 nm) LED to excite RGB phosphors or white phosphors (n-UV WLEDs).10−13 The achievement of RGB WLEDs is challenging by balancing the luminous intensity and even color of RGB light.11−13 Nowadays, the most efficient WLEDs, phosphor-converted white LED (pc-WLED), are a combination of LED chips (blue or UV chip) and phosphors, as a medium converting the radiation from the primary source into visible light.9,14−16 For blue WLEDs, the main disadvantages are poor color rendering © XXXX American Chemical Society

index (CRI) and low stability of correlated color temperature (CCT),17,18 which are important parameters for application.19 Furthermore, few phosphors have an absorption band around 450 nm.20 Different from the RGB WLEDs and blue WLEDs, UV-WLEDs fabricated by UV-LED chips coated with white light-emitting single-phased phosphors or RGB tricolor phosphors possess most remarkable advantages, high CRI, high chromatic stability under different driving currents, and tunable CCT due to the invisible emission of the UV-LED chip.11,20 So, UV-WLEDs are considered to be the future direction of solid-state light (SSL) development.11 Phosphors in UV-WLEDs, as critical components, play a key role in controlling the quality of the white light, including photoconversion efficiency, luminous efficiency, CCT, CRI and CIE coordinates, and so on.15,19,21 RGB phosphors obtained by mixing three phosphors with colors of red, green, and blue are recognized as one of most effective phosphors. Yet high costs, the low luminescent efficiency, the luminescence reabsorption, and uncontrollable color balance of RGB phosphors have been insurmountable shortcomings of such phosphors.7,19,22−24 In recent years, single host emission color-tunable phosphors based on n-UV LED or UV LED have been widely investigated because the single-phase phosphors with white emission have higher CRI, tunable CCT, pure CIE chromaticity coordinates, and so on, as compared to blue chips coated with YAG:Ce3+ phosphor (the first WLED was fabricated by blue chip and YAG:Ce3+, yellow-emitting phosphor, whose weakness is poor Received: June 27, 2016 Accepted: August 25, 2016

A

DOI: 10.1021/acsami.6b07724 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces CRI and low stability of CCT),4,8,20,21,25,26 and, simultaneously, also can avoid the defect of RGB phosphor.27 However, the reported single-phase phosphors based on ultraviolet-pumped WLEDs mainly focus on organic molecules, metal complexes, hybrid inorganic materials, and polymers.4,13,18,23,28−30 Organic−inorganic hybrid materials with diversified structures and affluent spectra31 are seldom investigated as to the practical effect of the new white-light phosphors on ultraviolet converted WLEDs.32 The recent observation of white-light materials from luminescent metal−organic frameworks (MOFs) opens a new hybrid route toward white phosphor, which significantly differs from the conventional hybrid inorganic materials. First, as compared to current hybrid inorganic phosphors prepared by power-wasting solid-state reaction at high (or reducing) pressures and high temperature (1000 °C or so),11,33 hydrothermal and solvothermal syntheses of MOFs materials at low temperature are remarkably advantageous. Second, tunable structures, wide materials, and abundant spectra of MOFs make them superior to single hybrid inorganic phosphors, because d- and f-block metal centers, numerous organic ligands of MOFs, and even encapsulating dye molecules within porous MOFs can all act as luminescent emissive sources,26 and plenty of combinations of metal centers with organic linkers have resulted in a series of luminescent MOFs with various topologies and tunable emissive wavelengths ranging from 300 to 1550 nm,34−42 which lay the foundation for constructing white-light MOFs materials. Third, lanthanide ions are subjected to low light absorption due to the forbidden f−f transitions, making the direct excitation inefficient. This problem can be overcome by coupling species that can participate in energy transfer processes, known as “luminescence sensitization” or “antenna effect”.36 The mechanism of antenna sensitization within MOFs is comprised of three steps: light is absorbed by the organic ligands around lanthanide ions, energy is transferred to lanthanide ions from organic ligands, and then luminescence is generated from lanthanide ions. So, as compared to conventional inorganic phosphor, simple synthesis, controllable structure, abundant spectra, and effective energy transfer of lanthanide MOF materials construct unique opportunities for developing new MOF phosphor. Currently, several research groups have made LEDs by coating MOF materials on commercially available UV LED,26,32,43,44 which can demonstrate feasibility of white-light MOF materials but does not embody the practical performance of WLEDs due to the absence of effective parameters such as CRI, CCT, CIE coordinate, etc. We attempted to develop a practical application of MOF as white-light phosphors and tried to fabricate a WLED with doping LnMOF (lanthanide MOF) materials by combining n-UV LED chip with single-phase white emitting phosphors. Our strategy to single-phase white emitting phosphor is tuning the relative fraction of blue light from organic ligand, green light from Tb3+, and red light from Eu3+ in a single-phase LnMOF materials. According to the Dexter energy transfer theory, the energy-level match between the triple state energy of the ligands and the resonance emission energy level of green emitting Tb3+ and red emitting Eu3+ ions will determine the luminescence of the lanthanide MOFs.45 Theoretically, the blue emitting organic linker as a sensitizer should have suitable triplet excited-state energy (22 000−27 000 cm−1) to match the energy of Eu3+ (5D0, 17 250 cm−1) and Tb3+ (5D4, 20 430 cm−1) as shown in Scheme 1.36,46,47 This is supported by our

Scheme 1. Schematic Representation of Energy Absorption, Migration, Emission, and Processes in Luminescent MixedLanthanide MOFa

a

Abbreviations: S, singlet; T, triplet, A, absorption probability; F, fluorescence; P, phosphorescence; ISC, intersystem crossing, ET, energy transfer.

measurement on excitation spectra. In addition, as compared to nonrare earth fluorescent materials, the rare earth involved fluorescent materials generally have better performance on luminous efficiency and light color.36,47 In the search of organic ligands, H3CPOMBA (4,4′-(((5carboxy-1,3-phenylene)bis(oxy))bis(methylene)) dibenzoic acid) attracted our attention. On the basis of DFT calculations (Energy Levels of Calculation and Figure S1), the H3CPOMBA has the triplet excited-state energy of 23 177 cm−1, and the energy gap ΔE (1ππ*−3ππ*) of H3CPOMBA is 7088 cm−1 (Energy Levels of Calculation and Figure S2). In the light of Reinhoudt’s empirical rule,35 the intersystem crossing (ISC) process will be effective when the ΔE (1ππ*−3ππ*) is greater than 5000 cm−1. Thus, we expect that the ISC process in H3CPOMBA is effective, and Eu3+ and Tb3+ emission could be simultaneously sensitized. Herein, we report the syntheses and spectral properties of single-phase white-light MOF materials Eu0.045Tb0.955CPOMBA (λex = 365 nm) and La0.6Eu0.1Tb0.3CPOMBA (λex = 380 nm) prepared by self-assembly of codoping RE ions and organic ligands. By using Eu0.045Tb0.955CPOMBA, we fabricated an nUV WLED (365 nm), which has low CCT (5733 K), high CRI (Ra = 73.4) and CIE chromaticity coordinate (0.3264, 0.3427), and comparable commercially available blue WLED with YAG:Ce3+ phosphor. More importantly, UV WLED not only expands the new application of MOF materials but also enriches the family of phosphor, which opens a new perspective for developing single-phase UV phosphors.

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals for the syntheses were commercially available reagents of analytical grade and were used without further purification. The FT-IR spectra were recorded from KBr pellets in the range 400−4000 cm−1 on a Perkin Elmer Spectrum BX FT-IR spectrometer. UV absorption spectra eres recorded (in DMF solution) with a U-3310 spectrophotometer. Elemental analysis was performed on a Vario EL-II elemental analyzer. Scanning electron microscopic (SEM) images were obtained with a JSM-7500F operated at a beam energy of 10.0 kV. The composition was analyzed using energy-dispersive X-ray spectroscopy (EDS) in the SEM. Powder Xray diffraction (PXRD) data were recorded on a Bruker D8 ADVANCE powder X-ray diffractometer. The thermogravimetric analyses (TGA) were carried out in an air atmosphere using SETARAM LABSYS equipment at a heating rate of 10 °C/min. B

DOI: 10.1021/acsami.6b07724 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Luminescence spectra for solid samples were recorded, and lifetime measurements were measured by a single-photon counting spectrometer using an Edinburgh FLS920 spectrometer equipped with a continuous Xe900 xenon lamp, a mF900 μs flash lamp, a red-sensitive Peltier-cooled Hamamatsu R928P photomultiplier tube (PMT), and a closed Janis CCS-350 optical refrigerator system. The corrections of excitation and emission for the detector response were performed from 200 to 900 nm. Lifetime data were fitted with two-exponentialdecay functions. The quantum yields were measured by use of an integrating sphere with an Edinburgh Instrument FLS920 spectrometer. Inductively coupled plasma spectroscopy (ICP) was performed on a Perkin Elmer Optima 8000 DV ICP. The Eu/Tb ratios of Eu0.045Tb0.955CPOMBA and La/Eu/Tb ratios of La0.6Eu0.1Tb0.3CPOMBA were determined by ICP analysis (Table S1). The photoelectric parameters of fabricated LED were measured by an integrating sphere spectroradiometer system (HP-8000, Hopoo, China). Syntheses of [Ln(CPOMBA)] Materials. All compounds including codoping lanthanide materials were synthesized in a similar method except for the different starting lanthanide salts. The synthesis of Gd(CPOMBA)(H2O)2·nH2O is detailedly introduced as a representative: a mixture of Gd(NO 3 ) 3 ·6H 2 O (0.1 mmol), H3CPOMBA (0.1 mmol), and 5 mL of the mixed solvent of deionized water and ethanol (3:2) was placed in a Teflon reactor (15 mL), and then heated at 140 °C for 4 h. The mixture was gradually cooled to room temperature at a rate of 5 °C h−1, and crystals suitable for single crystal X-ray structure determination were obtained. The product was isolated as colorless crystals upon cooling of the reaction mixture, collected by filtration, washed with 95% EtOH, and dried under atmosphere (yield 54.5 mg, 89%). Anal. Calcd for C23H19GdO10 (%): C, 45.05%; H, 3.11%. Found: C, 45.12%; H, 3.02%. IR (KBr pellet, cm−1): 3426 (s), 2921 (s), 2863 (s), 1594 (s), 1535 (s), 1420 (s), 1370 (s), 1140 (s), 1044 (s), 861 (s), 834 (s), 780 (s), 635 (s), 426 (s). With respect to isostructural Ln analogies, Eu3+/Tb3+ or La3+/Eu3+/ Tb3+ codoped compounds employ the same methods as those mentioned above just by adding the corresponding Ln(NO3)3·6H2O as the starting material in stoichiometric ratios. Their structure and purity were confirmed by single-crystal XRD (cell parameters, Table S2), powder XRD (Figure S5), IR spectra (Figure S7), and elemental analyses (ICP) (Table S1). X-ray Crystallography. X-ray single-crystal diffraction data were collected on an Agilent Technologies Gemini EOS diffractometer at room temperature with Mo Kα radiation (λ = 0.71073 Å). The program SAINT was used for integration of diffraction profiles, and the program SADABS was used for absorption correction. All of the structures were solved with the XS structure solution program by direct methods and refined by the full-matrix least-squares technique using Olex2. All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms of organic cations were generated theoretically onto the specific carbon atoms, and refined isotropically with fixed thermal factors. Further details for structural data are summarized in Tables S2 and S3.

space group C2/m, and show a 2D layer structure, which is isostructure with the Ce/Nd MOF crystal.48 The crystal structure of GdCPOMBA, as a representative of these isostructural compounds, has been described in detail (Figures S3 and S4). All of the Ln(III) ions have the same coordination environment surrounded by eight O atoms from six oxygen atoms of four different H3CPOMBA ligands and two oxygen atoms of two water molecules, forming a two-dimensional grid structure. PXRD analysis of these compounds (Figure S5) shows close diffraction peaks with the simulated ones of the corresponding single crystals, indicating the purity of the crystal. For codoping lanthanide MOFs, the molar ratio of the lanthanide elements is basically identical to that of primitive lanthanide nitrate, which were completely demonstrated by ICP, distinctly revealing that in situ doping of the lanthanide ions was successful. Actually, the cell parameters of their single crystals (Table S2) and their powder XRD (Figure S5) suggest that the codoping lanthanide MOFs are also isostructural to single lanthanide ones. As shown in Figure 1a, the crystal of codoping La0.6Eu0.1Tb0.3CPOMBA/Eu0.045Tb0.955CPOMBA mainly con-

Figure 1. Morphologies of as-synthesized phosphors. (a) Optical photographs of representative Eu0.045Tb0.955CPOMBA crystal sample in natural light. (b and c) Representative scanning electron microscope images of Eu0.045Tb0.955CPOMBA. (d) Amplified SEM image.

sists of an approximate sphere and irregular piece. We respectively choose a representative roundish crystal and irregular piece as carefully observed by SEM (Figure 1b−d). The SEM indicates these crystals (particle size in the range of 20−30 μm) stick together by a regular strip (breadth about 1 μm), which may be relative to the 2D layer structure. EDS composition analyses reveal that codoping MOFs are respectively composed of Eu/Tb or La/Eu/Tb and H3CPOMBA49 (Figure S8a,b), which again confirm the doping of MOFs is successful. Thermogravimetic data were collected for Gd(CPOMBA)(H2O)2·nH2O as a representative to explore their thermal stability and to see if the water molecules of lanthanide MOF could be removed without destroying the framework. The first weight loss at about 80−150 °C shows a rapid decrease, which is assigned to removal of crystal lattice water molecules. A gradual weight loss of coordinated water occurs in the temperature range of 150−350 °C, and the collapse of the framework only starts at 450 °C, which demonstrates that the codoping phosphors will be stable and feasible to meet the temperature of the operating temperature of LED50 (Figure S9).

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RESULTS AND DISCUSSION Synthesis and Structural Description. A series of isostructural lanthanide MOFs including codoping lanthanide complex, Ln(CPOMBA)(H2O)2·nH2O (Ln = La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Ho3+, Er3+, and Tm3+), were hydrothermally synthesized by the reaction of H3CPOMBA with Ln(NO3)3 at 140 °C for 4 h. Through continually optimizing experiment conditions such as different solvents and temperature, we are fortunate to find an optimal experiment scheme, in which we can acquire well-repeated vast crystals in almost single phase. These materials are air-stable and insoluble in water or any common organic solvents. Single-crystal X-ray diffraction analysis reveals that all of the compounds are isomorphous, crystallizing in the monoclinic C

DOI: 10.1021/acsami.6b07724 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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which exhibit characteristic fluorescence peaks at 488, 543, 591, 614, and 699 nm, are shown in Figure 3a. In the course of

To further confirm the thermostability of MOF as phosphor, the selected three Eu0.045Tb0.955CPOMBA crystal samples are heated for 24 h at 100 °C, for 24 h in boiling water, and for 48 h at 150 °C, respectively. As shown in Figure S6, the PXRD of three samples is basically consistent with the PXRD at room temperature and displays intact crystal order, which also indicates the phosphor is a highly stable solid-state material. Photoluminescence Properties. The generation of white light in UV WLEDs substantially is the photoluminescence of corresponding phosphors excited by different wavelengths of UV chip. Because of insensitivities to ultraviolet light for the human eyes, the chromaticity of WLED is mainly dependent on the properties of phosphors. So, it is vital to study the photoluminescence properties of MOF materials. Under UV radiation, LnCPOMBA (Ln = La3+, Eu3+, and Tb3+) compounds emit blue, red, and green light, respectively. The emission spectrum of LaCPOMBA consists of two emission peaks at about 433 and 466 nm (Figure S10 and the corresponding excitation spectra in Figure S12). As to the emission of the pure ligand (Figure S11), the small red-shifted emission of LaCPOMBA may be the result of a slight change in intramolecular or intermolecular interactions among organic ligands.51 For example, there is some change in the HOMO− LUMO gap between free ligand and coordinated ligand. Another reason is a slight conformational change in coordinated ligand as compared to free ligand. Besides, as compared to that in solution, ligands in solid MOF are close together, which enable charge transfer among the organic ligands, resulting in a shift of the spectra. The red and green emissions of EuCPOMBA and TbCPOMBA are characteristic of Eu3+ and Tb3+ ions (Figure 2a and b, corresponding excitation spectra in Figures S13 and

Figure 3. (a) Emission spectra of the EuxTb1−x-CPOMBA (x = 0−20 mol %) solid samples under 324 nm excitation. (b) CIE chromaticity diagram for EuxTb1−x-CPOMBA (x = 0 for A, 0.5% for B, 4.5% for C, 7.5% for D, 10% for E, 15% for F, and 20% for G).

doped Eu3+, we find it impossible to acquire white-lighting materials from the changes of CIE coordinates (Figure 3b and CIE coordinates are listed in Table S4). However, in Figure 2, the most interesting is the columinescence of Tb3+ (or Eu3+) and ligand under 370 nm (380 nm) excitation, which indicates that with the increase of exciting wavelength, ligand with blue emission also participates in luminescence of the coordination polymer. As far as trichromatic theory is concerned, if ligand (blue), Tb3+ (green), and Eu3+ (red) are incorporated into one MOF structure by carefully adjusting their mole fraction, production of white light should be achievable under certain wavelength excitation. We choose Eu0.045Tb0.955CPOMBA, which is excited under different wavelengths from 330 to 370 nm (Figure 4 and

Figure 2. (a) Emission spectra of solid-state Eu(CPOMBA)·2H2O at room temperature (λex = 313 and 380 nm). (b) Emission spectra of solid-state Tb(CPOMBA)·2H2O at room temperature (λex = 313 and 370 nm).

Figure 4. Emission spectra of solid-state complex Eu0.045Tb0.955CPOMBA with excitation wavelengths from 330 to 370 nm (left). CIE-1931 chromaticity diagram of complex Eu0.045Tb0.955CPOMBA from 330 to 370 nm excitation (right).

S14), respectively. Obviously, their ligand-based emissions of 422 and 462 nm were not observed under 313 nm excitation, which clearly demonstrate the ligand acting as an effective antenna can perfectly sensitize to lanthanide ions. This further supports Scheme 1 of energy absorption, transfer, and emission. For lanthanide MOF, the tunable color of emission can be achieved by in situ doping of different RE ions. Indeed, Eu3+ was doped into a host of TbCPOMBA to afford EuxTb1−xCPOMBA, a mixed-lanthanide MOF. In this complex, by gradually increasing the doped Eu3+, the emission of mixed lanthanide MOF changed from the pure green emission of TbCPOMBA to basically red (x = 20%); meanwhile, when x = 4.5%, yellow lies between green and red. The aforementioned things may be attributable to the enhanced probability of Tb3+to-Eu3+ energy transfer38,49,52,53 (as shown in Scheme 1) with the increase of Eu3+ concentration. The emission spectra at various concentrations of doped Eu3+ in EuxTb1−xCPOMBA,

CIE coordinates are listed in Table S5). When excited at 330 nm, blue emission from ligand hardly appears. Nevertheless, with increase of the excitation wavelength, the ligand-based blue emission intensity significantly increases. Upon increase the excitation wavelength to 355 nm, the emission intensities from ligand, Eu3+ ion, and Tb3+ ion are comparable, which result in visible white light. By continuously increasing the excitation wavelength, nearly pure white light emission was obtained for its CIE coordinates of (0.333, 0.320) under 365 nm excitation (Figure 5 and corresponding excitation spectra in Figure S15). Meanwhile, we also investigate the lifetime and quantum yield of the white-light materials. The average values of Eu0.045Tb0.955CPOMBA under 365 nm excitation are τTb3+ = 0.176 ms and τEu3+ = 0.656 ms (Figure S17). The overall D

DOI: 10.1021/acsami.6b07724 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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energy transfer from Tb3+ to Eu3+. Such energy transfer behavior shows that the as-synthesized LaxEuyTb1−x−yCPOMBA (or previous EuxTb1−xCPOMBA) is not a mixture of LaL EuL and TbL (or the latter two), but a single-phase coordination polymer, in which Eu3+ and Tb3+ have successfully been incorporated into LaL lattice (or Eu3+ into TbL lattice). Otherwise, the Tb3+ → Eu3+ energy transfer in the separated phases can not occur. Furthermore, obviously in Figure 3 and Figure 6, the photoluminescence colors of the complex EuxTb1−xCPOMBA and LaxEuyTb1−x−yCPOMBA can be tuned from green to green-yellow, yellow, orange, redorange by doping the different Ln3+ concentrations. These MOF materials can be used as emitting green, yellow, orange, and red light materials by selecting the appropriate doping concentrations of Ln3+. Likewise, as shown in Figure 7,

Figure 5. Left: Emission spectra of complex Eu0.045Tb0.955CPOMBA under 365 nm excitation. Inset: Optical image of a sample of complex under the excitation of 324 nm (top left) and 365 nm (top right), respectively. Right: CIE-1931 chromaticity diagram of complex Eu0.045Tb0.955CPOMBA under 365 nm excitation.

quantum yield is about 15% (Table S8), which is a larger quality yield in all doping lanthanide MOFs, but lower than that of the reported Eu0.0075Tb0.9925BTPCA (H3BTPCA = 1,1′,1″(benzene-1,3,5-triyl)tripiperidine-4-carboxylic acid).52 To further explain the organic linker as a sensitizer with suitable triplet excited-state energy to simultaneously and effectively sensitize Eu3+ or Tb3+, the decay curves of Eu0.045Tb0.955CPOMBA at different excitation wavelengths are also measured and described (Figure S19). We find with the increase of excitation wavelength, the lifetime of Tb3+ is gradually reducing, which may be caused by relative weak absorption under the excitation of 365 or 380 nm. Lifetime τTb3+ = 0.176 ms after excitation at 365 nm means the sensitization is effective. Generally, all phosphors have the therm quenching properties, which means their luminescence intensity would decrease with the increase of temperature. For Eu0.045Tb0.955CPOMBA, as shown in Figure S16, the luminescent intensity at 450 K is about 86% of that at room temperature. Meanwhile, we find color emission of the aforementioned phosphors after heating for 48 h at 150 °C does not change under the excitation of 365 nm. Considering that light rare earth element La has a higher relative content and lower relative price in contrast to Tb of heavy RE, we anticipate the synthesis of more cheap white light materials. Being similar to the synthesis of EuxTb1−xCPOMBA, LaxEuyTb1−x−yCPOMBA is acquired by different ratio doping of Eu3+ and Tb3+ into LaCPOMBA. As shown in Figure 6, with the increase of Eu 3+ concentration, we find that the luminescence intensity of Tb3+ decreases, while the luminescence intensity of Eu3+ increases (CIE coordinates are listed in Table S6). This is caused by enhancing the probability of the

Figure 7. Emission spectra of solid-state complex La0.6Eu0.1Tb0.3CPOMBA with excitation wavelengths varying from 330 to 390 nm (left). CIE-1931 chromaticity diagram of complex La0.6Eu0.1Tb0.3CPOMBA from 330 to 390 nm excitation (right).

La0.6Eu0.1Tb0.3CPOMBA, emitting yellow light under 324 nm excitation, displays a white light under 380 nm excitation (the average lifetime values are τTb3+ = 0.579 ms and τEu3+ = 0.556 ms, and the overall quantum yield is about 14.4%, Figure S18 and Table S8) and the corresponding CIE coordinate (0.323, 0.326) (Figure 8) by exciting under different wavelengths from 330 to 390 nm (Figure 8 and CIE coordinates are listed in Table S7).

Figure 8. Left: Emission spectra of complex La0.6Eu0.1Tb0.3-CPOMBA under 380 nm excitation. Inset: Optical image of a sample of complex under the excitation of 324 nm (top left) and 380 nm (top right), respectively. Right: CIE-1931 chromaticity diagram of complex La0.6Eu0.1Tb0.3CPOMBA under 380 nm excitation.

Fabrication and Photoelectric Parameters of White Light LED. As compared to currently active blue WLEDs, the technology of UV LED is relatively new and represents the future development prospect of LED. In current technology, the intensity of light (visible or UV) emitted by an LED relies strongly on the wavelength of LED chips. Blue LEDs are mature relative to UV LEDs, but the energy of UV light

Figure 6. Emission spectra of LaxEuyTb1−x−yCPOMBA under 324 nm excitation (left). CIE chromaticity diagram for LaxEuyTb1−x−yCPOMBA (right). E

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LED 1 can emit natural white light; in accordance with the decrease of CCT, a remarkable warmer light appears, perhaps because the increase of the concentration of phosphor on the chip makes the practical content of Eu enhance, and the red light constituent in the emission spectrum of LED augments. In addition, we also attempt to grind the crystal into tiny particles (Figure S21), and then fabricate LED with tiny particles and GaN chip; through measuring the characteristic parameters, we get a similar outcome with LED 1. The commercial LED with YAG:Ce3+ phosphor has a low CRI (Ra < 70) and a high CCT (>6000 K),55,56 which indicates the potential application of the synthesized MOF phosphors on WLED.

emitting by LED is higher than that of blue light. The intensity of light drops more quickly when the wavelength gets shorter, especially below 365 nm, in this sense, which means application on UV WLED of phosphor under the excitation of 365−400 nm is interesting, consistent with the current mainstream research direction.54 In this regard, phosphors herein emitting white light upon 365 nm/380 nm excitation will have potential application on UV WLED. We try the new phosphors to be coated on commercially available UV LED. As shown in Figure S20, bright white emission was observed when a thin layer (method 1, Supporting Information) of Eu0.045Tb0.955CPOMBA was applied onto the UV-LED. This experiment shows the feasibility of the MOF as an effective white-emitting material. To further explore the device performance of this singlephase phosphor, we fabricated a series of WLED by combining n-UV LED chip (GaN-based 365 nm) with Eu0.045Tb0.955CPOMBA phosphor (method 2, Supporting Information). Typical LEDs with different concentrations of phosphor show corresponding CCT, CRI, and CIE chromacity coordinate under a drive current of 350 mA (Table 1).

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CONCLUSIONS We have developed a new approach toward n-UV single-phase phosphors (Eu0.045Tb0.955CPOMBA and La 0.6Eu0.1Tb0.3CPOMBA) by doping RE ions in MOF materials. In single-phase LnMOF phosphor, the organic linker acts as a sensitizer with suitable triplet excited-state energy to simultaneously and effectively sensitize Eu3+ and Tb3+, which can solve the disturbing low absorption of lanthanide ions under direct excitation of lanthanide ions. Through an investigation of thermostability and luminescent properties of the MOF phosphor, we found it is appropriate to apply in UV LED. Different from commercially available UV WLEDs with RGB phosphor, we successfully fabricated an n-UV WLED (365 nm) with the synthesized single-phase MOF as white light phosphor, which shows a CCT of 5733 K and CRI (Ra) of 73.4, comparable to that of commercial LED with YAG:Ce3+ phosphor (Ra < 70 and CCT > 6000 K).55,56 This work originates an example of single-phase MOF phosphor in practical UV WLED. Because diversified methods to acquire white light MOF are feasible, that is, selecting organic linkers to match the appropriate energy level of d- and f-block ions,43,47,57 encapsulation of luminescent ion or dye in pores of MOF,26,32,49 and lanthanide heterometal doping in MOFs,58 the application prospect of the white light materials will be promising, which will open a new horizon for UV single-phase phosphor.

Table 1. Important Photoelectric Parameters for Three Typical LEDs with Different Concentrations CIE coordinate (x,y) LED LED LED LED LED a

1 2 3 4a

proportion of phosphor and silica gel

CCT (K)

Ra

x

y

0.5:1 0.08:1 0.03:1 0.5:1

5733 6993 8936 5612

73.4 73 71.9 73.2

0.3264 0.3026 0.2754 0.3256

0.3427 0.3361 0.3250 0.3467

Phosphor is a tiny particle.

Obviously, in Table 1, with the increase of phosphor, the CIE chromacity coordinate grows, and the change of CRI is very small, but CCT drastically decreases. As shown in Figure 9,

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Figure 9. Photograph of fabricated WLEDs with 365 nm GaN chip and Eu0.045Tb0.955CPOMBA phosphor. (a and b) Photographs of asfabricated LEDs (not turned on). (c and d) Photographs of asfabricated LED 3 and LED 1 under a drive current of 350 mA, respectively.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b07724. Energy levels of calculation, ICP analyses, structure details and cell parameters of LnCPOMBA, PXRD patterns and IR spectra, EDS spectrum of the doping LnMOF, TGA curves, luminescence spectra, CIE coordinates, PL decay curve, photographs of LEDs and optical photographs of crystal, and fabrication method of UV WLEDs (PDF) X-ray data for compound EuCPOMBA (CIF)

LED 3 emits white light with a little blue-green, which is relative to the structure and composition of MOF phosphor, differing from some commercially available inorganic doping phosphor such as YAG:Ce3+. Actually, the compact crystal structure (Figure 1b and c) and the mixture of silicone and dilute phosphor (low absolute content of Eu, in contrast to the high content of Tb in the crystal) on GaN chip may directly result in more green light from Tb in the emission spectrum. So, we can observe a little blue-green in white light from LED 3. On the basis of the thinking, we attempt to increase the proportion of phosphor in silicone. Through continuous trial, after viscous gel (proportion of phosphor and silicone reach 0.5:1) is coated on chip, we eventually fabricate a satisfactory LED as shown in Figure 9.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acsami.6b07724 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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ACKNOWLEDGMENTS Financial support from the 973 Program (2012CB821701), the Ministry of Education of China (Grant IRT1156), the National Science Fund for distinguished Young Scholars (NSFC 20925101), and the 10000 Talent Plan is greatly appreciated.

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