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Combinatorial Search for New Red Phosphors of High Efficiency at VUV Excitation Based on the YRO4 (R ) As, Nb, P, V) System Kee-Sun Sohn,*,† Il Woon Zeon,‡ Hyunju Chang,‡ Seung Kwon Lee,‡ and Hee Dong Park‡ Display Phosphor Group, Korea Research Institute of Chemical Technology, 305-600 Taejon, Korea, and Department of Material Science & Metallurgical Engineering, Sunchon National University, 540-742 Sunchon, Chonam, Korea Received October 2, 2001. Revised Manuscript Received February 11, 2002
On the basis of the possibility that there might exist a mixed composition of the Y(As,Nb,P,V)O4 system of better luminescent performance than the single-compound phosphors at the VUV excitation, a solution combinatorial chemistry synthesis and characterization was employed in the present investigation. Quaternary and ternary combinatorial libraries were designed to implement an efficient screening process. In parallel, the first-principle calculations were also carried out on the basis of the density functional theory to give a reasonable interpretation to the results from the combinatorial chemistry screening process. It was found that a new phosphor, Y0.9(P0.92V0.03Nb0.05)O4:Eu3+, which was obtained through the three-step combinatorial screening, shows promising luminance and CIE color chromaticity that could be comparable to the commercially available red phosphor used for the PDP application. As a result of the luminescent mechanism study in association with the calculated density of states (DOS), the entire energy transfer route under the 147nm excitation was revealed for the newly found phosphor, Y0.9(P0.92V0.03Nb0.05)O4:Eu3+.
1. Intoduction To search for a promising red phosphor for a plasma display panel (PDP), a combinatorial chemistry method was employed in the present investigation. Combinatorial chemistry has been known as an efficient methodology enabling the systematic screening of various functional materials. In particular, the applicability of combinatorial chemistry has been recently widened spanning from drugs to inorganic luminescent materials. With regard to inorganic luminescent materials, conventional solid-state combinatorial chemistry in which phosphors are dealt with based on thin film technology has been carried out on a very large scale;1-7 namely, a huge number of compounds are deposited on the small substrate. It is, however, noted that the amount of each compound is too small to be characterized properly and that the luminescent properties in the thin film state may be different from the powder * To whom correspondence should be addressed. E-mail: kssohn@ sunchon.ac.kr. † Sunchon National University. ‡ Korea Research Institute of Chemical Technology. (1) Bunin, B. A.; Plunkett, M. J.; Ellman, J. A. Proc. Natl. Acad. Sci. USA 1994, 91, 4708. (2) Xiang, X.-D. Science 1995, 268, 1738. (3) Briceno, G.; Chang, H.; Sun, X.; Schultz, P. G.; Xiang, X.-D. Science 1995, 270, 273. (4) Senkan, S. M. Nature 1998, 394, 350. (5) Danielson, E.; Golden, J. H.; McFarland, E. W.; Reaves, C. M.; Weinberg, W. H.; Wu, X. D. Nature 1997, 389, 944. (6) Danielson, E.; Devenney, M.; Giaquinta, D. M.; Golden, J. H.; Haushalter, R. C.; Mcfarland, E. W.; Poojary, D. M.; Reaves, C. M.; Weinberg, W. H.; Wu, X. D. Science 1998, 279, 837. (7) Sun, X.-D.; Wang, K.-A.; Yoo, Y.; Wallace-Freedman, W. G.; Gao, C.; Xiang, X.-D.; Schultz, P. G. Adv. Mater. 1997, 9, 1046.
properties. Solution-based combinatorial chemistry is applied to the synthesis of phosphor powders in the present investigation. A large number of methods have been developed in the pharmaceutical field to generate solution-based libraries. But in solution-based combinatorial chemistry, reagents are typically delivered individually to each sample site, and so the creation of these libraries is relatively inefficient with respect to process speed and accuracy. To improve these shortcomings, we have developed a scanning multi-injection delivery system to deliver several hundred microliter volumes of precursor solutions to sample sites rapidly and accurately. Several Eu3+-doped borates such as YBO3:Eu3+, LuBO3:Eu3+, ScBO3:Eu3+, and (YGd)BO3:Eu3+ have been considered as red phosphors for the PDP application. The starting point of the present investigation lies in the possibility that some other oxide compounds could also be applied to the PDP. In this regard, yttrium-based YRO4 compounds such as YAsO4, YNbO4, YPO4, and YVO4 could be potential candidates and worth investigating. These compounds were developed long ago and the luminescent properties have been studied extensively.8,9 But the former investigations dealing with these materials were focused either on the photoluminescence under 254-nm excitation or on the cathodoluminescence. One has no doubt that the development of new phosphors to be used for the PDP application is of primary concern in the present investigation so that the (8) Blasse, G. Philips Res. Rep. 1969, 24, 131. (9) Blasse, G.; Bril, A. J. Chem. Phys. 1969, 50, 2974.
10.1021/cm0109701 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/30/2002
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Figure 1. Schematic diagrams showing the whole combinatorial chemistry process. Table 1. Details about the Solutions Used in the Precursor Delivery solution Y2O3 + 60%HNO3 Eu2O3 + 60%HNO3 Nb2O5 + 50%HF As2O3 + 37%HCl V2O5 + C2H2O4 + H2O (NH4)2HPO4 + H2O metal concn 0.5 M 0.5 M 0.5 M 0.5 M 0.5 M 1M
photoluminescence at the 147-nm excitation, which is adopted as an excitation source in the PDP application, should be taken into consideration. In fact, each single compound has been well-developed in terms of luminescence efficiency at the 254-nm excitation (PL) and at the excitation by the electron bombardment (CL). But there has been little consideration with regard to the luminescent property of mixed compounds. A tetrahedron-type composition array ()quaternary library) was developed in terms of the luminance and color chromaticity at 147-nm excitation in the present investigation so that it could be possible to screen all of the composition that the four above-mentioned compounds constitute. A finer screening using a ternary library was also carried out around the optimum composition obtained from the quaternary library. As a result of the screening process, a final composition was obtained, which shows almost tantamount luminescent efficiency to the commercially available red phosphor. In parallel, we have performed first-principles calculations based on the density functional theory to give a reasonable interpretation to the results from the combinatorial chemistry screening. Thus, the nature of optical excitations near the band gap of YPO4, YVO4, and YNbO4 was clarified using the first-principles calculations. It was shown that the absorption near the band edges in YRO4 (R ) P, V, and Nb) involves excitations from the oxygen 2p-like states near the top of the valence band (VB) to the cations nd-like states near the bottom of the conduction band (CB). 2. Experimental Procedures Combinatorial chemistry has been widely applied to the synthesis of a variety of materials such as drugs or various inorganic functional materials, etc.1-7 With regard to the inorganic materials, most investigations are focused on solidstate combinatorial chemistry based on thin film2-6 or liquidstate printing7 technology. Unlike the solid-state combinatorial chemistry method based on thin film technology, the methodology taken by the authors adopted a solution method so that it could rather mimic the combinatorial chemistry used for the pharmaceutical industry. As a result, a certain amount of powders can be obtained, which could be sufficient for any kind of conventional characterization process without any
further special instrumentation. By considering that the thin film property may be different from the powder properties and that most phosphors are applied to an actual system as a powder form, the present approach could be more promising for the phosphor research. The composition table and array used for the combinatorial screening, so-called library, was designed on the basis of the quaternary and ternary systems by programming tools (MS Visual basic linked to AutoCAD R14). The quaternary combinatorial library including 121 different compositions is a tetrahedron-type representation, and the ternary system includes 66 compositions. The quaternary combinatorial library consists of 5 tetrahedrons, each of which will be called the first, second, third, fourth, and fifth shells from the outermost surface to the core. The compounds located at the apex of the tetrahedrons will be called apex compounds. The overall process of the combinatorial chemistry adopted in the present investigation is shown in Figure 1. More details are also presented in our previous works.10,11 All the raw powders, excepting diammonium hydrogen phosphate ((NH4)2HPO4), were in their oxide form such as Y2O3, As2O3, Nb2O5, V2O5, and Eu2O3. They were dissolved into weak acids and then the correct amount of each solution was collected in the specially designed ceramic container according to the composition table with assistance of a computer-programmed injection system. The details about the solutions used in the precursor delivery are summarized in Table 1. The containers containing solutions were dried at 100 °C for 4 h and further dried at 500 °C for 2 h. The dried samples were pulverized and successively fired. The firing temperatures were 1100, 1200, and 1300 °C for 2 h in an ambient atmosphere. Some of the samples chosen among those in the library were taken out of the containers and then examined by X-ray diffraction (XRD) analysis (Rigaku:DMAX-33, Cu KR1, scan speed 4°/min, scan range 10-80°). The emission intensity of the fired samples at the UV excitation (254 nm) was monitored using the plate reader accessory attached to the Perkin-Elmer LS50B spectrometer with a xenon flash lamp. The PL efficiency of phosphors at the VUV excitation was measured using a photoluminescence (PL) measuring system consisting of a Kr2 excimer lamp producing a broad 147-nm emission band as an excitation source, vacuum chamber, CCD detector, automatically maneuverable X-Y stage on which the sample plates can be mounted, and computer-controlling unit. To examine the (10) Sohn, K.-S.; Park, E. S.; Kim, C. H.; Park, H. D. J. Electrochem. Soc. 2000, 147, 4368. (11) Sohn, K.-S.; Seo, S. Y.; Park, H. D. Electrochem. Solid State Lett. 2001, 4, H26.
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Figure 2. Excitation and emission spectra of each constituent compound: (a) YAsO4:Eu3+; (b) YNbO4:Eu3+; (c) YPO4:Eu3+; (d) YVO4:Eu3+. The excitation spectra were detected with the emission probe fixed at 613 nm and emission spectra were measured under both 254 (upper) and 147 (lower) nm excitations. excitation spectra in the wide range spanning from VUV to UV, another system was used, which includes a deuterium lamp, vacuum chamber, vacuum monochromator to monochromatize excitation light, emission monochromator, photomultiplier tubes, and computer-controlling unit. The excitation spectra were measured in the range from 140 to 350 nm using sodium salicylate powder as a reference. The measurement of excitation spectra was carried out in the same way as Koike et al.12 The luminance was calculated by integrating the product of the emission spectrum and the standard visual spectral efficiency curve by obeying the CIE regulation.13 The introduction of the standard visual spectral efficiency curve is essential in this instance where the shape of emission spectra vary significantly with the composition. The color chromaticity was also calculated according to the CIE regulation.13 Single-point DFT calculations were carried out as implemented in the DMol3 program package14 of Molecular Simulation Inc. We took the nonlocal density approximation, so-called the generalized gradient approximation (GGA) functional suggested by Perdew and Wang.15 As the basis sets, we have used double numerical basis functions with polarization functions (DNP) for all the calculations.16 The effective core potential (ECP) approximation was used for the core electrons of Y, V, and Nb. All calculations were carried out in the spinrestricted model. The partial density of states (PDOS) was calculated using Loewdin’s population analysis with Gaussian broadening.14 (12) Koike, J.; Kojima, T.; Toyonaga, R.; Kagami, A.; Hase, T.; Inaho, S. J. Electrochem. Soc. 1979, 126, 1008. (13) Shionoya, S.; Yen, W. M. Phosphor Handbook; CRC Press: Boca Raton, FL, 2000. (14) DMol3, User’s Reference; Molecular Simulation Inc.: San Diego, 1998. (15) Perdew, J. P.; Wang, Y. Phys. Rev. 1992, B45, 13244. (16) He´bant, P.; Picard, G. S. J. Mol. Struct. (Theochem.) 1995, 358, 39.
3. Results and Discussions Spectral Analysis. Prior to the combinatorial approach, spectral analysis was performed for the constituent compounds ()apex compounds of the first shell). Figure 2 shows the emission and excitation spectra of YAsO4:Eu3+, YNbO4:Eu3+, YPO4:Eu3+, and YVO4:Eu3+. The excitation spectra of 5D0-7F2 emission were measured in the range from 140 to 350 nm. The results shows that only the YPO4:Eu3+ compound exhibits considerable emission at VUV excitation. On the other hand, the excitation spectra of all the other compounds show a dramatic drop in the VUV range below 200 nm. If 147 nm was adopted as an excitation light wavelength, it would be quite reasonable to assume that the excitation energy is absorbed first by the host lattice, which involves the transition between 4d-like states of Y and 2p-like states of O. The absorbed energy may then be transferred to RO4 groups and last transferred to the Eu3+ center. Otherwise, the excitation energy is absorbed by the host lattice and transferred directly from the host lattice to the Eu3+ center. Among the four YRO4s, only the YPO4 host pertains to the latter case, while all the others belong to the former case; namely, the YPO4 host never undergoes the energy transfer from the host to PO4 group en route because the 2p- and 3dlike states of P are located far above the 4d-like states of Y. The electronic structure of the YPO4 host is discriminated from the other compounds, which is associated with the information taken from the excitation spectra, wherein only the YPO4 host shows strong absorption in the VUV range. More detailed discussions
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Table 2. Branching Ratio of 5D0 f 7Fj Transitions (Eu Concentration ) 0.1 mol)
Table 3. Summarization of DOS Calculations crystallographic band gap band gap valence conduction data calc. exp. band band
host materials YAsO4 5D
f 7F1 7 0 f F2 5D f 7F 0 4 0
5D
5D
0
5D
0f
f 7F1 7F 2 5D f 7F 0 4
YPO4
YVO4
100 100 20
λex ) 254 nm 28 100 3.3
YNbO4
100 100
