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Citation: Luo Z L, Bao J, Ding J J, et al. Applications of combinatorial approach to the investigation of optical functional materials. Chinese Sci Bull, 2009, 54: 1836- 1844, doi: 10.1007/s11434-009-0339-4

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Applications of combinatorial approach to the investi-gation of optical functional materials

LUO ZhenLin1, BAO Jun1, DING JianJun1, CHEN Lei1*, LI XueFei1, JIU HongFang2, ZHANG QiJin2, CHAN TingShan3, LIU RuXi3 & GAO Chen1† 1 National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China; 2 Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China; 3 Department of Chemistry, Taiwan University, Taipei 106, China

Recent applications of combinatorial methodology to the investigation of optical functional materials are reviewed on the basis of the authors’ work. The content includes: basic concepts of combinatorial materials science, combinatorial investigations of UV/VUV excited photoluminescence, luminescent rear earth complex doped polymer and magneto-optical materials. The fundamental synthesis tech-niques and characterization methods in combinatorial methodology are also illustrated in the text.

combinatorial materials science, luminescent materials, magneto-optical materials

1 Introduction of combinatorial materials science

Functional materials, as an important material founda-tion, dramatically promote the development of science and technology, and improve people’s living level. With the rapid development of modern science and technol-ogy, it is urgent to discover new advanced functional materials.

Although researchers have understood to some degree the relationship between composition, structure, prop-erty and performance of materials, there is no reliable theory to guide the search of advanced functional mate-rials yet. Discovery of new materials is still highly em-pirical and often conducted through conventional one-at-a-time synthesis and characterization, which sig-nificantly limit the efficiency of development.

As the conjunction of combinatorial methodology (combinatorial strategy) and materials science, combi-natorial materials science revived in the middle of 1990s[1,2] and aims to dramatically enhance the effi-ciency of materials discovery by adopting parallel syn-thesis techniques (forming materials library) and high-throughput characterization methods. Here, “com-

binatorial strategy” origins from the combination con-cept in statistics and refers to a parallel technique, which covers multi-variable space through limited steps and is often used for parallel synthesis of materials library.

Combinatorial materials libraries, composed of spa-tially addressable tiny samples with diverse composition, are often synthesized by combination of sequentially depositing thin-film precursors and motion of precisely positioned shadow masks. Here, we show the effective-ness of combinatorial strategies by illustrating the qua-ternary combinatorial mask scheme. A series of quater-nary physical masks are presented in Figure 1 and the usage is shown in Figure 2. The scheme involves n dif-ferent masks, which successively subdivide the substrate into a series of self-similar patterns of quadrants. The rth (1≤r≤n) mask contains 4r−1 windows and each win-dow exposes one-quarter of the area deposited with the previous mask. Each mask is used in up to four sequen-

Received November 19, 2008; accepted March 2, 2009 doi: 10.1007/s11434-009-0339-4 †Corresponding author (email: cgao@ustc.edu.cn) * Current address: School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China Supported by the National Natural Science Foundation of China (Grant Nos. 50721061, 50772106)

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Figure 1 Quaternary physical masks[3].

Figure 2 The usage of quaternary physical masks.

tial depositions and rotated by 90° each time. This proc-ess produces 4n different compositions with 4n deposi-tion steps.

It is an obvious advantage of combinatorial method that arithmetic growth of the workload creates a geo-metric increase of the specimen’s quantity. Thus, it is a big challenge to rapidly explore the numerous small samples in the materials libraries, which requires char-acterization techniques with features of high sensitivity, good spatial resolution and high throughput. It is clear that most of the traditional characterization techniques can not meet this requirement. In addition, since the measurement methods of various properties are quite different, it is also not easy to construct specific devices for characterization of different properties. Therefore, at this stage, high-throughput characterization is the bot-tleneck in the combinatorial investigation process.

Combinatorial investigations can be classified into several types according to the core object of characteri-zation: optical, electric, magnetic, catalytic and so on. Fortunately, characterization of the optical properties is natural parallel, i.e., either excitation/emission or irra-diation/reflection is easy to achieve high spatial resolu-tion under parallel analysis. So, applications of combi-

natorial method to the investigation of functional optical materials such as luminescent, electro-optical, mag-neto-optical materials have been carried out exten-sively[3–11]. Below, the progress in this area will be in-troduced by using the recent studies conducted by the authors as samples.

In fact, the revival of combinatorial methodology has promoted the development of optical functional materi-als, and combinatorial methodology has been widely applied to catalysis research in which a lot of studies have been conducted in Dalian Institute of Chemical Physics of CAS[12,13]. In addition, researchers from Nan-jing University, Shanghai Institute of Ceramics of CAS and other institutions have applied combinatorial ap-proach to the materials research of ferroelectrics[14], piezoelectrics[15] and alloy[16]. Due to space limitation, only the progress in optical materials is included in this paper.

2 Combinatorial investigations of lumi-nescent materials

The use of combinatorial approach has effectively pro-moted the development of luminescent materials. Some typical examples are: Sun et al. fabricated a materials library containing 100 powder specimens using ink-jet delivery technique[4]; the quaternary physical masks was used by Wang et al. in conjunction with photolithogra-phy to generate phosphor libraries consisted of 1024 isolated thin-film specimens and an efficient blue photoluminescent composite Gd3Ga5O12/SiO2 was iden-tified[3]; binary and multi-composition gradient tech-niques were used by Mordkovich et al. to prepare mate-rials library and four ZnO-based phosphors with effi-cient low-voltage cathodoluminescence were discovered and optimized[6]. Following are combinatorial investiga-tions of luminescent materials conducted by our group.

2.1 Luminescent materials for advanced plate dis-play technology

High-performance UV/VUV (vacuum ultraviolet) light excited luminescent materials have attracted consider-able attention because of their potential applications to white LED, mercury-free fluorescent lamp and plasma plate display. Here, in cooperation with Prof. Liu of Taiwan University, we apply the combinatorial method to the investigation of these materials[17–22].

(i) Synthesis of the materials libraries

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Since luminescent materials are often used in powder form, a home-made drop-on-demand inkjet delivery system[17,18] as showed in Figure 3, is adopted here for combinatorial synthesis of the powder-material libraries.

Figure 3 Schematic diagram of the drop-on-demand inkjet deliv-ery system.

In the device, there are eight inkjet heads and each

head consists of a sapphire nozzle, a stainless steel dia-phragm, a piezoelectric disk and is connected to a solu-tion reservoir through a tube. When a high-voltage elec-tric pulse is applied to the piezoelectric disk, it produces a mechanical vibration and thus one drop of precursor is ejected out of the nozzle. The average droplet volume is ~10 nL with a standard deviation of ~10%. The ejection repeatability is 0.5―2 kHz. Beneath the inkjet heads is one substrate with a micro-reactor array fixed on the X-Y stage. The eight independent piezoelectric inkjet heads and X-Y stage are controlled by a computer so as to automatically coordinate the ejection of inkjet heads and the position of stage according to the concentrations of solutions, interval between reactors, substrate size and the composition map, etc. After the whole ejection is completed, a library with different solution-mixing pre-cursors is generated. Then, the precursors are evaporated and reacted in the micro-reactors to form a library of powder materials.

However, a significant limitation of the inkjet deliv-ery technique mentioned above was its applicability: only soluble compounds could be used as the precursors. Based on the fact that a lot of suspensions could be used with inkjet technique, Chen et al. developed a universal preparation method for stable suspensions of ul-trafine/nano insoluble oxide using a wet ball-milling technique to make insoluble oxides be used as precur-sors in inkjet combinatorial synthesis.

Herein, to explore luminescent materials suitable for advanced plate display, a series of luminescent materials libraries were fabricated on Al2O3 substrate using com-binatorial inkjet of solution or suspension precursors. These libraries included:

(1) Eu3+ doped Y2O3-B2O3-P2O5-SiO2, (2) Pr3+/Tb3+/Sm3+/Bi3+

doped (Y,Gd)BO3:Eu3+, (3) (Y1 zGdz)1xyBiyEuxBO3, (4) (Y2 xEuxBiy)O3. (ii) Characterization of the materials libraries Photography and spectrum scanning technique were

used in our study to characterize the luminescence mate-rials libraries.

Photography using either film cameras or digital charge-coupled devices (CCD) has the advantages of parallel analysis and excellent spatial resolution (~10 μm). This simple technique can be used to characterize the luminescent intensities and color coordinates of a phosphors library by taking the photo-luminescent pho-tograph of the library under UV or other high-energy excitations. In most cases, human eyes are sensitive enough to pick up the phosphor lead of interest directly under UV-lamp excitation. By the way, specific photog-raphy as infrared thermography could also be used to detect the catalytic activity in combinatorial library of catalyst[23].

For more quantitative analysis, an automatic scanning spectrometer system was usually used to measure the emission spectra of luminescent samples in the library (Figure 4). This system consists of a mercury lamp, a portable optical fiber spectrometer (Ocean Optics, Inc., Model SD2000) and an X-Y stage. The spectrometer is equipped with a 25/200-μm slit, a 600-grooves/mm grating and a 2048-element linear silicon CCD-array detector, so as to cover a spectral range of 200―850 nm with a spectral resolution of 1.34 nm/7.68 nm. During

Figure 4 Schematic diagram of the scanning spectrometer[18].

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the operation, the materials library is fixed on the mov-ing table of the X-Y stage and the emission spectrum from each sample is obtained in turn by moving the sample to the focus of the fiber-optic probe.

To implement high throughput screening of the PDP phosphor libraries, a specific photographic device for VUV photoluminescence was constructed (Figure 5). The light from the VUV light source is projected on the library and the photoluminescence from samples in the library is imaged in a parallel way with a scientific grade CCD outside the chamber. Detailed spectrum informa-tion can be obtained by placing a set of band-pass filters between the window and the camera in-turn.

Figure 5 High-throughput screening system for VUV phosphors library[19].

(iii) Results and discussion The VUV light excitated luminescence of Eu3+ doped

Y2O3-B2O3-P2O5-SiO2 was systematically studied in the form of quaternary combinatorial materials library[20]. Figure 6(a) shows the photoluminescent photographs of the quaternary library annealed at 1150℃ and the green frames indicate the most effective host for the VUV ex-cited luminescence of Eu3+ is YBO3.

VUV (147 nm) excited photoluminescence of Pr3+/ Tb3+/Sm3+/Bi3+ doped (Y,Gd)BO3:Eu3+ are shown in Figure 6(b). The experiment results demonstrate that Bi3+ can sensitized Eu3+ luminescence under VUV exci-tation, and trace of Pr3+/Tb3+ is helpful to improve Eu3+ luminescence intensity, while Sm3+ is unhelpful. How-ever, when the content of Pr3+/Tb3+ increases, the quench effect of Pr3+/Tb3+ on Eu3+ is more serious than Bi3+ and Sm3+.

Figure 6 VUV photoluminescence of (a) Eu3+ doped quaternary Y2O3-B2O3-P2O5-SiO2 materials libraries and (b) Pr3+/Tb3+/Sm3+/Bi3+ doped (Y,Gd)BO3:Eu3+ materials library[19,21].

Based on the above results, the VUV photolumines-

cence of (Y1−zGdz)1−x−yBiyEuxBO3 is optimized with the combinatorial approach and a new high efficient red phosphor with nominal composition of (Y1−zGdz)1−x−yBiy- EuxBO3 (0.02≤x≤0.05, y≤1×10−3, 0.1≤z≤0.2) is obtained. In this phosphor, the dominant energy transfer from Bi3+ to Eu3+ is Bi3+→Gd3+…Gd3+→Eu3+[21].

UV (365 nm) excited photoluminescence of (Y2−xEux- Biy)O3 is screened using the combinatorial method[22]. When the content of Bi3+/Eu3+ increases, the intensity of 610 nm emission increases while the luminescent color varies in a trend of blue→violet→pink→red (Figure 7). The optimized composition was found to be (Y2−xEuxBiy)O3 (x = 0.16―0.18, y = 0.08―0.10) (Figure 8).

2.2 Luminescence of rare-earth complex doped polymer

Luminescent rare-earth complexes have been widely applied to illumination, optical communication and so on. The luminescence of rare-earth complexes doped polymer origins from the forbidden intra-4f transitions of rare-earth ion, which leads to poor absorptivity cross sections and relative low emission efficiency. An effec-tive approach to increase the luminescent efficiency is to modify the complexes with different kinds of ligands that have broad and intense absorption bands, or mix the complexes with different sensitization ions. The kinds and contents of rare earth ions and sensitization ions, the

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Figure 7 luminescent photograph of (Y2−xEuxBiy)O3 materials library under 365 nm UV excitation[22].

Figure 8 Emission spectra of (a) (Y2−xEuxBiy)O3 (y = 0.04) and (b) (Y2−xEuxBiy)O3 (x = 0.16) under 365 nm UV excitation[22]. wavelength of the excited light source, the conglomera-tion structure of polymers may be the important factors for the photoluminescent performances of rare-earth complexes. So, such a systematic search involves a lot of time-consuming synthesis and characterization efforts using the traditional one-at-a-time approach. We carried out combinatorial research on these materials in coop-eration with Prof. Zhang Qijin at USTC.

(i) Synthesis of the materials libraries Because of the easy volatilization of organic solvent

and high viscosity of polymer solution, it is hard to syn-thesize the library with inkjet technique. In order to mix the rare-earth complexes and polymer (poly-methyl methacrylate, PMMA) uniformly, cyclopentanone was

selected as the solvent to control the viscosity and vola-tilization speed of the solutions. The libraries were syn-thesized by dispensing the precursor solutions into the microwells drilled on a fluoropholgopite substrate with different dose using a microliter pipet (0.1―2.5 μL) according to the library design.

Herein, a series of libraries were synthesized for the research of the luminescence enhancement effect of europium and samarium complexes:

(1) The sensitization luminescence of RE(DBM)3phen (RE = Tb3+, Ce3+, La3+, Dy3+, Gd3+, Sm3+, Y3+; DBM = dibenzoylmethane; phen = 1,2-phenanthroline) sensitized Eu(DBM)3phen doped in PMMA matrix.

(2) In PMMA-Sm(DBM)3phen system, the sensitiza-tion luminescence of RE(DBM)3phen (RE = Tb3+, La3+, Gd3+, Y3+) sensitized Sm(DBM)3phen.

(ii) Characterization of the materials libraries The photoluminescence of the libraries was charac-

terized by photography and a scanning spectrometer system under 365 nm UV illumination, as introduced in 2.1.

(iii) Results and discussion Figure 9(a) shows the photoluminescent photograph

of the libraries with different kinds and contents of sensi-tization ion complexes that sensitized the Eu(DBM)3phen in the PMMA matrix (all of the subscripts in the figure are defined as the weight ratio of the rare earth com-plexes to PMMA). The results show that all the sensiti-zation ions except Ce3+ have sensitization effect, and the

Figure 9 (a) Luminescent photograph of the libraries of various rare earth complexes sensitized Eu(DBM)3phen in a PMMA ma-trix[24,25]; (b) Luminescent photograph of the library with different contents of La(DBM)3phen and Eu(DBM)3phen in the PMMA ma-trix[24]; (c) Luminescent photograph of the library with different con-tents of Tb(DBM)3phen and Eu(DBM)3phen in the PMMA matrix[25].

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luminescent intensity increases with the content of sen-sitization ion complexes and reaches a maximum, then further increase of the content decreases the luminescent intensity. Among these sensitization ion complexes, La(DBM)3phen exhibits the highest sensitization effi-ciency, and the sensitization efficiency increases re-markably with the decreasing of the Eu(DBM)3phen content in PMMA matrix. At the Eu(DBM)3phen content of 5 wt%, the maximum sensitization efficiency of La(DBM)3phen is more than 20 times (Figure 9(b)). For Tb(DBM)3phen, the maximum sensitization efficiency is about 27 times at the Tb(DBM)3phen content of 8 wt% (Figure 9(c)). To confirm these results, scale-up samples were fabricated. The tendency of the intensity as the function of La(DBM)3phen or Tb(DBM)3phen content is fairly consistent with that from the small samples in the library.

Similarly, the luminescence enhancement of RE(DBM)3phen (RE = Tb3+, La3+, Gd3+, Y3+) sensitized Sm(DBM)3phen in PMMA matrix was studied with the combinatorial method. The result showed that only Y(DBM)3phen did not display sensitization effect. Among these sensitization ion complexes, Tb(DBM)3phen exhibits the highest sensitization efficiency. At the Sm(DBM)3phen content of 20 wt%, the maximum sen-sitization efficiency of Tb(DBM)3phen is more than 10 times (Figure 10).

Figure 10 Fluorescence enhancement factor of the 648 nm emis-sion as a function of the contents of Tb(DBM)3Phen and Sm(DBM)3Phen in the PMMA matrix[26].

3 Combinatorial investigations of mag-neto-optical materials

Magneto-optical Kerr effect (MOKE) has been widely applied to modern information technologies, especially to the design and manufacture of high-density mag-

neto-optical storage devices. In the search for novel MO storage materials, the Kerr rotation angle is an important parameter since higher Kerr rotation leads to a better signal-to-noise ratio. Tsui et al.[7–9] adopted the combi-natorial methodology in the research on novel MO ma-terials. However, the lack of proper characterization tools limited the efficiency gain. Although the MOKE imaging systems developed by Zhao et al.[10,11] recently could work in a parallel mode, the limited brightness of the light sources made it difficult to obtain high effi-ciency and high signal to noise ratio at the same time. Here, we developed a MOKE imaging device using laser as light source and with which we systematically ex-plored the (BixDyyYb3−x−y)Fe5O12 material system using the combinatorial approach[27,28].

(i) Synthesis of the materials libraries The ternary (BixDyyYb3−x−y)Fe5O12 thin film materials

libraries were fabricated using magnetic co-sputtering technique as shown in Figure 11(a).

Figure 11 (a) Schematic diagram of the co-sputtering system; (b) sputtering rate distributions of the Dy3Fe5O12 target.

Three targets were placed at the corners of the equi-

lateral triangle and were tilt to the substrate. The deposi-tion rate gradient naturally occurring from the off-axi- ally sputtering was utilized to generate the composition gradient in fabricating the spreads (materials libraries). The sputtering rate distribution of Dy3Fe5O12 target on the corresponding vertical central intersection plane at the 50W RF power is shown in Figure 11(b). The inset shows two typical sputtering rate distribution curves along the horizontal direction at 40 mm and 100 mm

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substrate-target distances. It is obvious that the gradient of distribution decreases with the increasing of the sub-strate-target distance. In this study, the primary spread was fabricated 40 mm beneath the target plane to cover the major composition space of the ternary material sys-tem. To further zoom-in the high Kerr rotation region found in the primary spread, a second spread was fabri-cated at a substrate-target distance of 100 mm. Since the garnet film is normally transparent, the thickness-inter- ference effect on Kerr rotation angle is not neglectable. Here, opaque substrate, large and uniform film thickness were adopted to eliminate the influence.

(ii) Characterization of magneto-optical Kerr rotation Quantitative parallel Kerr rotation characterization

was carried out using a homemade MO imaging system that is schematically shown in Figure 12. The laser beam from a HeNe laser first passes through a beam ex-pander/spatial frequency filter (consisting of microob-jective lens, pinhole and double-convex lens) and a po-larizer to increase the uniformity and polarizability. Then, the laser beam is incident on the materials library in a near normal direction. The reflected beam passes through a polarization analyzer and projects on the sen-sitive elements of a CCD camera and is recorded by the camera.

Figure 12 Schematic diagram of the combinatorial mag-neto-optical characterization system.

This MOKE imaging system possesses the following

merits. First, utilization of laser projecting technique to parallelly measure the Kerr rotation of all the numerous small samples in the materials library promises both considerable spatial resolution and high signal-to-noise ratio. Second, the polarization angle of the analyzer is set at a small angle from the perpendicular direction of the polarizer. Thus, the pure MO images can be obtained through digital processing of the two images collected

with and without magnetic field. Third, to estimate the laser diffraction effect which would spoil the spatial resolution, a fully coherent illuminated Fresnel diffrac-tion model was constructed and the resolution was cal-culated as a function of sample-screen distance.

(iii) Results and discussion As mentioned above, the primary spread prepared at a

substrate-target distance of 40 mm covers almost the whole composition space of the ternary system. Its Kerr rotation image under 4000 Gauss perpendicular mag-netic field is shown in Figure 12. From the image, a high Kerr rotation region can be easily disclosed. To further zoom-in this high Kerr composition region, a second spread was fabricated at 100 mm beneath the target plane. The MOKE image of the second spread is shown in Figure 14. and the highest MO composition is identi-fied to be Dy0.6Yb0.5Bi1.9Fe5O12 with a saturation Kerr rotation angle of 1.32°, which is much larger than the 0.3°―0.5° rotation angles of magnetic alloys used in commercial MO disks.

To study the thickness dependence of the extrinsic Kerr rotation, the authors deposited a thickness gradient Dy0.6Yb0.5Bi1.9Fe5O12 film and imaged the MO rotation angle as the function of the film thickness, which was shown in Figure 14. As predicted, the MO rotation angle oscillation is becoming weak when the film thickness exceeds 1000 nm. However, obvious interferential maxima and minima can be identified when the film thickness ranges from 147 to 1000 nm and the maximum 3.27° rotation appears at film thickness of 510 nm, which implies that magneto-optical Kerr rotation could be greatly enhanced by thickness interference.

Finally, the authors found that the Kerr rotation hys-teresis loop of sample annealed at higher temperature

Figure 13 Kerr rotation image of the primary (BixDyyYb3−x−y)Fe5O12 spread. Lighter region corresponds to larger Kerr rotation angle as indicated by the gray bar.

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Figure 14 Kerr rotation image of the second (BixDyyYb3x y)Fe5O12 spread. Lighter region corresponds to larger Kerr rotation angle as indicated by the gray bar.

Figure 15 Kerr rotation image of the thickness gradient Dy0.6Yb0.5Bi1.9Fe5O12 film. (590℃ vs 570℃) seemed to be more like rectangle and relate to a bigger remnant magnetization value. This phenomenon is ascribed to the temperature-dependent domain wall motion.

4 Outlook

In the past 100 years, only less than 100 useful commer-

cial phosphor materials have been discovered through the conventional one-at-a-time synthesis and characteri-zation. Similar situations also exist in the development of other materials, such as electric or magnetic materials. On the other hand, almost all 3-element compounds have been studied and people will have to explore a much bigger compositional space of compounds with 4 or more elements for advanced materials in the future. Unfortunately, there is no reliable theory to guide the search of advanced materials yet. Thus, it is inevitable to use parallel or high-throughput experimental techniques to accelerate the search for advanced materials for vari-ous applications. After several years of hard work, our group has developed systemic combinatorial synthesis and high-throughout characterization technologies for optical functional materials. Through these technologies, several advanced luminescent materials and mag-neto-optical materials were discovered successfully. The construction of research platforms for combinatorial materials science requires considerable investment, and their applications will also produce massive specimen and data. To improve the use efficiency of research platforms, cooperative research and resources sharing are inexorable development trends of combinatorial ma-terials science. This article surveys some of our current work on the combinatorial luminescent materials and magneto-optical materials. The purpose is to disseminate the basic concepts of combinatorial materials science and introduce the usage of fundamental libraries synthe-sis and characterization techniques to people who are interested in combinatorial materials science.

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