Eu3+-Doped ZnB2O4 (B = Al3+, Ga3+) Nanospinels: An Efficient Red

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Eu Doped ZnMO (M = Al , Ga ) Nanospinels, an efficient Red Phosphor Megan E. Foley, Robert W Meulenberg, James R. McBride, and Geoffrey F. Strouse Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03789 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 25, 2015

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Chemistry of Materials

Eu3+ Doped ZnM2O4 (M = Al3+, Ga3+) Nanospinels, an efficient Red Phosphor Megan E. Foley†, Robert W. Meulenberg§, James R. McBride‡, Geoffrey F. Strouse†* † Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306-4390 § Laboratory for Surface Science and Technology and Department of Physics and Astronomy, University of Maine, Orono, ME 04469 ‡

Department of Chemistry, Vanderbilt University, Nashville, TN 37235

Abstract. This paper describes the synthesis of Eu(III) doped ZnB2O4 (B = Al(III) or Ga(III)) nanospinels with Eu(III) concentrations varying between 1 and 15.6%. The synthesis was achieved through a microwave (MW) synthetic methodology producing 3 nm particles by the thermal decomposition of zinc undecylenate (UND) and a metal 2,4pentanedionate (B(acac)3, B = Al3+ or Ga3+) in oleylamine (OAm). The nanospinels were then ligand exchanged with the βdiketonate, 2-thenoyltrifluoroacetone (tta). Using tta as a ligand on the surface of the particles resulted in soluble materials that could be embedded in lens mimics, such as polymethylmethacrylate (PMMA). Through a Dexter energy transfer mechanism, tta efficiently sensitized the Eu(III) doped within the nanospinels, resulting in red phosphors with intrinsic quantum efficiencies (QEs) and QEs in PMMA as high as 50% when excited in the UV. Optical measurements on the out of batch and tta passivated nanospinels were done to obtain absorption, emission, and lifetime data. The structural properties of the nanospinels were evaluated by ICP-MS, pXRD, TEM, FT-IR, EXAFS, and XANES.

Solid state lighting (SSL) technology based upon LED pumped down-shifting phosphors is rapidly replacing incandescent and compact fluorescent bulb technology due to their significant energy efficiency.1 Down-shifting phosphors2, which are often referred to inaccurately as down conversion phosphors in the literature,3 rely on the excitation of the phosphor at higher energy using a commercial LED (InGaN 390 nm or 445 nm) to produce white light either through broad band emission from a single white phosphor or through the combination of individual red (R), green (G), and blue (B) phosphors. The process of down-shifting requires efficient absorption of the excitation source and energy transfer to the emissive states without losses from non-radiative pathways or scattering. A classic down-shifting single component white light phosphor is YAG:Ce(0.03%) available as a bulk powder or prepared at the nanoscale is commonly employed in SSLs. While highly emissive when pumped at 445 nm by a (Ga,In)N LED, the phosphor produces poor color rendering.4 In lighting applications, nanoscale RGB phosphors have been targeted over the broad band emitters due to the ability to color tune the white light by adjusting the RGB ratios to reflect the needs within various markets. The common emitters in RGB phosphors are lanthanides incorporated into a host lattice and require 390 nm (In,Ga)N LED pump sources. For the lanthanide emission to be observed, either the host has to absorb the energy from the excitation source or a molecular sensitizer must be employed to populate the lanthanide emitter sites. While relying on the host to absorb the excitation source

results in high quantum efficiencies for lanthanide emission, when individual RGB phosphors are employed UV-excitation is often required, which is more costly. Near UV sources can be employed when molecular photosensitizers used for red phosphors where Eu(III) is doped into metal oxide hosts.5,6 The research on molecular photosensitizers appended to nanocrystal phosphors has generated excitement due to the reduced scattering of the light, improving performance of the device, and miscibility of the nanocrystal phosphor in the LED lens material. To realize the proposed energy efficiency gains that may be achieved in SSL, while ensuring cost effectiveness, nanoscale down-shifting red phosphors based upon earth abundant host materials are still a priority.7,8 To date, the most efficient reported nanocrystalline red phosphors have relied on Eu:Y2O3,5 Eu:YPO4,6 Eu:NaYF49 nanocrystals. While highly efficient in energy conversion the use of yttrium based host lattices is not cost effective due to the limited supply of rare earths. A host material of interest is the AB2O4 spinel lattice that exhibits high chemical, thermal, and radiation stability, making them viable for use in optoelectronic devices. In the spinel lattice the Aion is a divalent metal occupying a tetrahedral site and the B-ion is a trivalent metal occupying an octahedral site. Of the spinel lattices, ZnAl2O4 represents an earth abundant host lattice, which has recently been reported by sol-gel methods to incorporate Eu ions. 10, 11 The use of the ZnAl2O4 as a host lattice for the lanthanide emitter could lower the cost of the material as the host accounts for approximately

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90% of the material. The reported sol-gel prepared Eu:ZnAl2O4 nanomaterials were insoluble and had low down shifting efficiencies. While the reported materials had poor quality at the nanoscale due to a lack of a stable surface passivation layer and the Eu ion occupying both the A- and B-sites, the observation of line emission from the Eu doped ZnAl2O4 following thermal annealing suggest these material could be important phosphors if they can be prepared efficiently at the nanoscale by lyothermal techniques allowing controlled surface passivation. In this manuscript, we report a microwave (MW) synthetic methodology to produce a set of 2thenoyltrifluoroacetone (tta) passivated Eu:ZnB2O4 (B = Ga or Al) nanospinel phosphors that are organically soluble and do not require a post annealing step to produce red line emission from Eu(III) centers with high quantum efficiency for down-shifting following near UV excitation (ex < 390 nm). Unlike the early report on sol-gel prepared Eu:ZnAl2O4 the isolated spinels are smaller, are nanocrystals prior to annealing, are soluble in the LED matrix and organic solvents, have the Eu centers occupying the B-site, and exhibit Eu(III) line emission in all samples without an annealing step required. Furthermore, the lyothermal methods developed herein extend the host to the Ga based lattice to demonstrate the flexibility of the synthetic protocol. The nanospinel phosphors exhibit line emission from the Eu(III) centers following excitation into the tta passivant 11* absorption feature. The strongest emission line is for the hypersensitive 5D0  7F2 transition with a high red to orange (R/O) ratio indicative of a asymmetric Eu(III) site. The crystallinity of the nanospinel is evidenced by the narrow full width at half maximum height for the 5D0  7F0 line (1 nm) and the resolving of the stark splitting for each of the 7Fj lines in the 11K emission spectra. The appearance of the 5D0  7F0 (J=0 forbidden transition) in the emission spectra for the Eu(III) sites is a strong indicator of loss of site symmetry for the Eu ion in both the ZnAl2O4 and ZnGa2O4 lattices. The ratio of the electronic (5D0  7F2 line) to the magnetic (5D0  7F1 line) dipole allowed transition, referred to as the red/orange (R/O) ratio in Eu doped materials, max out at 12.1 (9.7% Eu:ZnAl2O4) and 11.5 (15.6% Eu:ZnGa2O4) for the nanospinel samples, as compared to a bulk Eu:Y2O3 phosphor standard which has a value of ~6.12 The CIE coordinates for the samples are tunable from blue to red dependent on the concentration of Eu(III) and the spinel B-ion. The results demonstrate that the spinel (AB2O4) oxide host represents an ideal host lattice for incorporating the Eu(III) centers, effectively eliminating the requirement for the yttrium component. For the nanospinels, high quantum efficiencies are the result of a Dexter energy transfer process. As evidenced by a Perrin model analysis of non-diffusive energy transfer, the observed efficient energy transport from the passivant layer to the doped Eu(III) ions reflects the energy overlap integral () between the singlet and triplet * energy levels for the tta ligand and the 5D0 levels for Eu(III), defined by the Dexter mechanism. Since the Dexter mechanism for Ln(III) materials is typically 100% energy transfer) leading to ~70% of the total particle volume containing Eu(III) sites being directly accessible. Hopping within the lattice or less efficient transfer to deeper sites can also occur in the system, leading to the improved phosphor performance. Phosphor Characteristics. An important criteria for a phosphor is its CIE coordinates, which for Eu(III) is related to the intensity of the magnetic to electronic allowed transitions, which is often termed the red/ orange (R/O) ratio (I (5D0  7F2) / I (5D0  7F1)). In Figure 6C the R/O ratio (I (5D0  7F2) / I (5D0  7F1)) for the samples is plotted, with values tabulated in Table 3. The ratio of the magnetic (5Do  7F1) to electronic (5Do  7F1) dipole allowed transition is observed to change with dopant level and the B ion in the nanospinel samples The R/O values exhibits a non-linear response with increasing concentration of Eu(III). The change in R/O ratios reflects the influence of site symmetry which results in increased intensity from the electronically allowed transition if the Eu(III) occupies an asymmetric site.46,47,48 In addition to the R/O values, a critical performance parameter for a down-shifting phosphor is the position in the CIE coordinate. The CIE xy values equate to the actual


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Chemistry of Materials site in the spinel lattice, evidenced in the XAS and optical data optimizes the red/orange ratio in these materials at higher Eu content leading to better overall performance. The QE for the phosphor reaches a value of ~50% in ZnGa2O4 with a red/orange ratio of 11. Similar results are observed for the ZnAl2O4 sample. Additionally, embedding the tta coated nanocrystals inside a lens mimic such as PMMA, has shown to greatly stabilize the nanophosphors. Excitation at 390 nm, a commercial LED wavelength, shows the potential of these new phosphors for rapid integration into SSL devices. Continued improvement of SSL devices require high QE, a narrow emission linewidth (5-25 nm), tunability, thermal stability, and low cost. MATERIALS AND METHODS

A

B C D E F G H I J K L

Figure 8. CIE diagram above for all the doped spinel samples with the solids excited at 364 nm embedded in a PMMA below. Eu doped ZnAl2O4 are represented by letters A-F, where A is 1% Eu:ZnAl2O4 and F is 12.7% Eu:ZnAl2O4. Eu:ZnGa2O4 are represented by G-L where G is 1.2% Eu:ZnGa2O4, and L is 15.6% Eu:ZnGa2O4.

coordinates of the perceived color on the CIE diagram with x and y axes. A white phosphor will have a CIE coordinate of 0.33, 0.33. In Figure 8, the Eu(III) concentration dependent optical coordinates are plotted on the CIE diagram and emissivity for simulated LED lenses for all Eu(III) concentrations in both ZnAl2O4 and ZnGa2O4 is shown. The CIE coordinates are listed in Table SF1. Increasing Eu(III) content results in a clear shift to red in the CIE diagram. CONCLUSION The preparation of Eu(III) doped nanospinels passivated by tta and grown near the critical nucleation size (2-4 nm) lead to enhanced molecular sensitization of the Eu(III) centers. Further ligand optimization may allow higher QEs to be realized. The synthesis of a 3 nm nanospinel leads to efficient energy transfer from the tta ligand shell excited states to the Eu(III) centers via Dexter energy transfer process resulting in nearly 70% efficient pumping of the incorporated centers. Incorporating Eu(III) onto the octahedral B-site for the nanospinel ZnB2O4 (B= Al(III) or Ga(III)) passivated by tta produces a highly efficient red down-shifting phosphor. The spinel represents an earth abundant host lattice that is thermally and oxidatively stable and allows the Eu ion to be incorporated to higher concentration without self-quenching. The distorted Oh

Materials. All commercially available reagents and solvents, with the exception of dimethylforamide (DMF) were used without further purification. DMF was dried over molecular sieves. 2-thenoyltrifluoroacetone (Matrix Scientific, 99%), Zinc (II) Undecylenate (Spectrum, 98%), Aluminum (III) 2,4-pentanedionate (Strem Chemicals, 99%), Gallium (III) 2,4-pentanedionate (Aldrich, 99.99%), Europium (III) 2,4-pentanedionate hydrate (Alfa Aesar, 99.9%), Oleylamine (Aldrich, 70%), Nitrosium tetrafluoroborate (Alfar Aesar. 98%), N,NDimethylformamide (Sigma Aldrich, ≥99.8%). Synthesis of Eu:ZnB2O4 (B = Al3+ or Ga3+, 1%, 2.5%, 5%, 7.5%, 10%, 12.5% Eu) Nanocrystals. Nanocrystals passivated by 2,4-pentanedione (acac) were prepared by microwave induced decomposition of zinc undecylenate (Zn(UND)2) with aluminum (III) 2,4-pentanedionate (Al(acac)3) or gallium (III) 2,4-pentanedionate (Ga(acac)3) and europium (III) 2,4-pentanedionate (Eu(acac)3) dissolved in oleylamine (OAm) using a CEM Discover Microwave (MW) operating under single mode, continuous power at 2.45 GHz. The CEM MW cavity was commercially modified by CEM with a Teflon insert to allow sustainable heating at 300oC. To synthesize Eu:ZnAl2O4, a glass microwave reactor vessel loaded with Zn(UND)2 (0.108 g, 0.25 mmol), Eu(acac)3 (1% (0.0024 g), 2.5% (0.0061 g), 5% (0.0121 g), 7.5% (0.0182 g), 10% (0.0243 g), 12.5% (0.0304 g)), Al(acac)3 (1% Eu (0.1605 g), 2.5% Eu (0.1581 g), 5% Eu (0.154 g), 7.5% Eu (0.150 g), 10% Eu (0.1459 g), 12.5% Eu (0.1419 g)), and OAm (4mL). The MW vessel was sealed, and the reactants were sonicated until the solid dissolved, approximately 30 min. The MW vessel was then inserted into the MW cavity and heated at 300 W to 280°C then held at that temperature for 5 min. The solution was cooled to 50oC and the Eu:ZnAl2O4 nanocrystals were precipitated by addition of MeOH (~5 mL). The product was isolated by centrifugation, washed four times using toluene/EtOH and then dried under vacuum overnight. The isolated sample is white in color and can be dispersed in toluene or chloroform. Eu:ZnGa2O4 was synthesized in the same fashion, though using Ga(acac)3 (1.2% Eu (0.1814 g), 2.1% Eu (0.1787 g), 5.4% Eu (0.174 g), 8.7% Eu (0.1695 g), 12.4% Eu (0.1649 g), 15.6% Eu (0.1603 g)). Elemental composition analysis was performed using ICP-MS for Zn, Al, Ga, and Eu. Ligand Exchange Protocol. Ligand exchange of the OAm by 2-thenoyltrifluoroacetone (tta) is accomplished


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through the use of nitrosium tetrafluoroborate (NOBF4) to assist ligand place exchange reactions, as described previously.15 The exchange is accomplished by treatment of 25 mg of the nanospinel dispersed in 5 mL toluene with 5 mL of 0.01 M NOBF4 in dry DMF under N2. The dispersion is allowed to react for 5 min, and the precipitate is collected by centrifugation and washed with acetone (2x). The resultant precipitate is added to a stirring mixture containing 1 ml of dry DMF, 2 mL toluene, and 50 mg of tta. The dispersion was heated to 60°C until the solution is clear (~ 30 min). The tta passivated nanospinel is isolated from solution by addition of acetone and collection by centrifugation. The resultant nanospinels are soluble in toluene. Ligand passivation was analyzed by FT-IR measured on powdered samples using a PerkinElmer Spectrum 100 FTIR Spectrometer equipped with an attenuated total reflectance (ATR) sample chamber consisting of a ZnSe crystal over coated with diamond using a singlebounce configuration. X-ray Powder Diffraction. Structural characterization was performed using powder X-ray diffraction (pXRD) recorded on a Rigaku DMAX 300 Ultima III Powder X-ray diffractometer (using Cu Kα λ = 1.5418 Å radiation). The pXRD was patterned matched using JADE and the nanospinel size fit using the Scherrer broadening expression L=

Kλ βcosθ

(4)

where L is the crystallite size, K is the shape factor and assumed to be 1.0 corresponding to spherical nanocrystals, λ is the X-ray wavelength measured in nm, β is the peak width at half height at a given 2θ value, and θ is the Bragg angle. The size was evaluated by inspection of the (220) and (311) reflections. Transmission Electron Microscopy. Size, size dispersity, morphology, and structure were analyzed by Transmission electron microscopy (TEM). TEM measurements were carried on a Tecnai Osiris operating at 200 kV using either carbon type A (300 mesh) with removable Formvar on Cu grids or graphene films deposited on lacey carbon grids (300 mesh). Image analysis of >100 nanocrystals were used to generate the size distribution curves. X-ray absorption near edge (XANES) and X-ray absorption fine structure (XAFS). Characterization of the local electronic and atomic structure of the materials was performed by synchrotron based x-ray absorption fine structure spectroscopy (XAFS) measurements. The XAFS spectra were acquired at the bend magnet beamline X19A at the National Synchrotron Radiation Light Source, Brookhaven National Laboratory. A double crystal Si(111) monochromator was used and detuned by 40-50% for reduction of harmonics. Three ion gas chambers were used to record in transmission mode the incident (filled with N2), transmitted (filled with Ar/He), and reference (filled with Ar/He) beam. A metal foil was placed after the second ion chamber for energy calibration purposes. All measurements were performed in transmission mode.

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XAFS data processing and analysis was done using the IFEFFIT suite of programs.49 Initial estimates of the threshold energy values (E0) were obtained via the inflection point in the normalized absorption edges. A Kaiser-Bessel window was applied to a selected k-range to obtain the Fourier transformed extended XAFS (EXAFS) data. The k-space data was weighted by k3 and Fourier transformed between 2-9 Å-1. In general, the quality of the EXAFS data precludes rigorous fitting of the data outside of the 1st shell, but inspection of the data is useful in understanding the structure of the doped materials. Optical Measurements. Solution absorption spectra were obtained using a Varian Cary 50 UV−vis spectrophotometer. Solution photoluminescence measurements were carried out on a Varian Cary Eclipse ﬂuorescence spectrophotometer. Relative solution QEs were obtained by comparing to a reference standard quinine standard (Eu(tta)3phen, Φ = 0.365 in EtOH) according to literature procedures.25, 50 Absolute QEs were measured on phosphors embedded in PMMA as a lens mimic for the LED phosphor using the De Mello method equipped with an integrating sphere incorporated into a spectrofluorimeter (FLS980, Edinburgh Instruments).51 A long pass 374 nm filter was used for emission measurements.52 The lens mimics were prepared in a silicon mold by dissolving 0.01g of Eu:ZnB2O4 into 0.1 g of PMMA in acetone and allowing the acetone to evaporate. Lifetimes measurements were measured using a Nd– YAG laser pumped at 366nm coupled to a photo-multiplier tube, and recorded through an oscilloscope interfaced with a computer. The decay data was fit to a single exponential decay using a non-linear regression analysis.

ASSOCIATED CONTENT Supporting Information. pXRD of ZnB2O4 (B = Al3+ or Ga3+) with different Eu(III) concentrations (Figure SF 1), high resolution TEM micrograph of 12.7% Eu:ZnAl2O4 with size distribution (Figure SF 2-A), high resolution TEM micrograph of 15.6% Eu:ZnGa2O4 with size distribution (Figure SF 2B), TEM-EDS of 12.7% Eu:ZnAl2O4 (Figure SF 3-A), TEM-EDS of 15.6% Eu:ZnGa2O4 (Figure SF 3-B), Zn and Ga K-edge XANES of Eu(III) doped ZnAl2O4 and ZnGa2O4 (Figure SF 4), Absorption spectra of tta exchanged ZnB2O4 (B = Al3+ or Ga3+) with different Eu(III) concentrations (Figure SF 5), PL spectra at 298K of tta exchanged ZnB2O4 (B = Al3+ or Ga3+) dissolved in toluene with different Eu(III) concentrations (Figure SF 6), PL spectra at 11K in of tta exchanged ZnB2O4 (B = Al3+ or Ga3+) embedded in PMMA with different Eu(III) concentrations (Figure SF 7), graph of the ratios of Ratio of the integrated intensities of the 5D1  7F1 line to the 5D0  7F3 (Figure SF 8), toluene solution emission decays of tta-EuxZnAl2-xO4 (Figure SF 9). The supporting information is available free of charge at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected].

Author Contributions


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

Funding Sources Grant No. CHE-0911080 Grant No. CHE-1213758 Grant No. EPS-1004083 Grant No. DMR-0654118 Grant No. DE-AC02-98CH10886

Grant No. DMR-1206940

ACKNOWLEDGMENT We wish to thank the NSF (Grant No. CHE-0911080 and Grant No. CHE-1213758) for financial support of the research. The TEM measurements were conducted at Vanderbilt University, which is supported by the NSF (Grant No. EPS-1004083). The authors would also like to thank S. Khalid and N. Marinkovic for beamline assistance. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0298CH10886. RWM would like acknowledge support from the National Science Foundation under Grant No. DMR-1206940. The authors would like to thank Angela Dial as well for conducting all ICP-MS measurements needed for this paper.

ABBREVIATIONS Acac, acetylacetonate; DOE, department of energy; DMF, dimethylforamide; EDS, energy dispersive spectroscopy; EtOH, ethanol; EXAFS, Extended X-Ray Absorption Fine Structure; FT-IR, Fourier transform infrared; ICP-MS, Inductively coupled plasma mass spectrometry LED, light emitting diode; MeOH, methanol; MW, microwave; NMR, nuclear magnetic resonance; NOBF4, nitrosium tetrafluoroborate; OAm, oleylamine; PMMA, polymethyl methacrylate; QE, quantum efficiency; TEM, transmission electron microscope; tta, thenoyltrifluoroacetone; UND, undecylenate; UV-VIS, ultraviolet-visible; XANES, X-ray Absorption Near Edge Structure.

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