Article pubs.acs.org/cm
Monodisperse Fluorescent Organic/Inorganic Composite Nanoparticles: Tuning Full Color Spectrum Zaicheng Sun,†,‡ Feng Bai,‡,⊥ Huimeng Wu,∥ Daniel M. Boye,# and Hongyou Fan*,‡,∥ †
State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 East Nanhu Road, Changchun 130033, People’s Republic of China ⊥ Key Laboratory for Special Functional Materials of the Ministry of Education, Henan University, Kaifeng 475004, People’s Republic of China ‡ Department of Chemical and Nuclear Engineering, The University of New Mexico/NSF Center for Micro-Engineered Materials, Albuquerque, New Mexico 87131, United States ∥ Advanced Materials Laboratory, Sandia National Laboratories, 1001 University Boulevard SE, Albuquerque, New Mexico 87106, United States # Physics Department, Davidson College, Davidson, North Carolina 28035, United States S Supporting Information *
ABSTRACT: Monodisperse fluorescent organic/inorganic composite nanoparticles are synthesized through the spontaneous self-assembly of block copolymer polystyrene-blockpoly(vinylpyridine) and rare-earth ions (europium, terbium, thulium, etc.). Depending on the rare-earth ions selected, tunable light-emission colors, including the primary red, green, and blue, are accomplished. Further, by stoichiometric mixing of the nanoparticles that emit different colors, the full color spectrum can be accessed. Both electron microscopy and spectroscopic characterizations confirm specific interactions of rare-earth and block copolymers. The resulting nanoparticles are monodisperse as characterized by dynamic light scattering. They are very stable and can be dispersed in common solvents, and together with homopolymers, they form ordered arrays and thin films (both supported and free-standing) upon solvent evaporation. The resulting nanoparticle thin films exhibit mechanical flexibility for ease of processing or device integration. KEYWORDS: monodisperse nanoparticles, self-assembly, quantum dots, white light, nanoparticle array
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inorganic silica,12,13 etc., to form continuous nanoparticle thin films and to increase process flexibility. Rare-earth (RE) complexes often show excellent luminescent characteristics such as high luminous intensity, long lifetime, and sharp emission bands from the f−f electron transition of RE ions.14−16 However, pure RE complexes usually do not have good thermal and mechanical stability and process flexability, which restricts their practical applications. To overcome these problems, the RE complexes have been incorporated into an organic, an inorganic, or a hybrid matrix, such as polymers,17−19 sol−gel precursors,20,21 etc., to form composite materials. One of the problems associated with the preparation of composites of RE complexes in polymer materials is that RE complexes usually exhibit serious emission concentration quenching because of their aggregation in polymers.17,22,23 Additionally, the solubility of RE complexes in a polymer matrix appears to
INTRODUCTION
Semiconductor quantum dots with controlled optical emissions (or emitting colors) such as CdSe, ZnO, and nanoparticles containing fluorescent rare earths are materials of great interest for applications in display, sensor, and nanoelectronics.1−4 In order to achieve reliable charge transport, it is desirable to form continuous thin films or arrays of these nanoparticles for the above applications. However, these nanoparticles are often functionalized on their surface with organic ligands to stabilize them and to avoid aggregations.5−10 Consequently, formation of nanoparticle thin films or arrays often relies on the van der Waals interactions between interdigitated organic ligands surrounding the nanoparticles. The thermally defined, interdigitated ligand interactions result in weak mechanical stability, causing cracking of the nanoparticle thin film.11 To date, it has been shown that organic/inorganic composite systems may provide much more efficient transport.4 Methods have been developed through the encapsulation of these inorganic nanoparticles into another matrix such as organic polymers,4 © 2012 American Chemical Society
Received: May 25, 2012 Revised: August 21, 2012 Published: August 21, 2012 3415
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be poor, which makes it difficult to prepare a uniformly distributed RE composite thin film.24 Previously, we developed sol−gel self-assembly methods that allow the synthesis of continuous and highly ordered metal nanoparticle arrays25 and thin films.12 We found that it is essential for the nanoparticles to have miscible surface chemistry so that there can be homogeneous mixing between the nanoparticles and the matrix. Such miscible surface chemistry ensures the formation of continuous films for charge transport.26 From a practical standpoint, it is important to provide compatibility with standard microelectronic processing/patterning. Herein we report a simple method to prepare monodisperse, light-emitting inorganic/organic composite nanoparticles through the spontaneous self-assembly of block copolymer polystyrene-block-poly(vinylpyridine) and RE ions. Depending on the choice of RE ions, tunable light-emission colors are accomplished including the primary red, green, and blue colors. Further, by stoichiometric mixing of nanoparticles that emit different colors, the full-color spectrum can be accessed. The resulting nanoparticles are very stable and can be dispersed in common solvents, together with homopolymers, to form homogeneous solutions. Ordered arrays and thin films (both supported and free-standing) with mechanical flexibility are formed upon solvent evaporation.
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Scheme 1. Schematic Illustration of the Self-Assembly and Formation of Monodispserse RE Composite Nanoparticles
was then added to a 1,4-dioxane solution containing PS−PVP. Dioxane is a selective solvent for PS−PVP. The PS block can dissolve in dioxane very well, but the PVP block only partly dissolves in it. The dioxane solution of PS−PVP is semitransparent and exhibits a milky white color, which suggests the aggregation and formation of PS−PVP micelles (Figure S1 in the Supporting Information). After RE(NO3)3·nH2O was added to the micelle solution, the RE ions were sequestered into the core of the micelles because of interaction between the RE ion and PVP block. Insight into the coordination interaction between PVP and lanthanide ions was gained from FTIR spectroscopic studies of the PS−PVP/lanthanide composite nanoparticles. Figure 1
EXPERIMENTAL SECTION
1. Materials. Rare-earth nitrate salts [RE(NO3)3·nH2O, RE = Eu, Tb, Tm, Ce, Er, Pr, Gd, Sm] and solvents like 1,4-dioxane and toluene were purchased from Aldrich-Sigma Chemical without further purification. Polystyrene-block-poly(4-vinylpyridine) (PS−PVP) was purchased from Polymer Source, Inc. Its molecular weights and polydispersity index were MWPS = 20000 g/mol and MWPVP = 19000 g/mol and PDI = 1.09, respectively. 2. Syntheses of PS−PVP/RE Nanoparticles and Films. RE(NO3)3·5H2O (1−50 mg) was dissolved in 1 mL of 1,4-dioxane. The above solution was added to 1 mL of 5 wt % PS−PVP in a 1,4dioxane solution and stirred overnight at 70 °C. The nanoparticles were collected by centrifugation at 100000 rpm for 1 h. The nanoparticles were cleaned through at least three cycles of centrifugation to remove free ions. The nanoparticles were redispersed into different normal solvents, such as toluene, 1,4-dioxane, tetrahydrofuran (THF), and chloroform for the formation of ordered arrays and characterization. To form films of composite nanoparticles, 100 mg of homopolymer PS with a molecular weight of 350K was added to 1 mL of toluene. A 0.2 mL nanoparticle solution was added into the above solution. The final solution was then cast-coated on silicon wafers or glass slides to form supported films. Free-standing films were peeled from the substrate. 3. Characterization. Scanning electron microscopy (SEM) images were taken using a Hitachi 5200 FEG microscope. Transmission electron microscopy (TEM) was performed on a JEOL 2010 microscope with 200 kV acceleration voltage, equipped with a Gatan slow-scan CCD camera. The PS−PVP/RE solution was drop-cast onto the carbon-coated copper grid without any stain. Fluorescent spectra were obtained from a Jobin Yvon FluoroMax-4 spectrofluorimeter. For fluorescent lifetime measurement, Eu(NO3)3·5H2O was dissolved in dioxane. Fourier transform infrared (FTIR) spectra were carried on a Thermo Scientific Nicolet 6700 FTIRspectrometer. Particles' size distributions were obtained from a Malvern Nano-ZS instrument.
Figure 1. FTIR spectra of PS−PVP and PS−PVP/RE composite nanoparticles.
shows FTIR spectra of pure PS−PVP and PS−PVP/lanthanide composite nanoparticles. For pure polymer PS−PVP, the peaks at 1597, 1416, and 994 cm−1 are assigned to the characteristic vibrational modes of pyridine. In comparison with the spectra of pure PS−PVP, these PVP characteristic peaks have shifted to higher wavenumbers for all lanthanide ions. PVP blocks show a rich basic Lewis character: the nitrogen atom incorporated within the aromatic ring tends to share a free-electron pair with the available f electron orbitals from lanthanide ions, causing changes in the electron distributions of the pyridine ring. The result is a shift in energy for those peaks related to stretching modes of the pyridine ring. Such spectral shifts have also been observed in the hydrogen-bonding coordination behavior of other precursors and polymer systems.27−29 In addition to FTIR studies, we have observed that the key photo-
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RESULTS AND DISCUSSION Scheme 1 describes the synthesis process of the organic/ inorganic composite nanoparticles through preferential coordination between the PVP chains and lanthanide ions. In a typical preparation, the lanthanide RE precursor was first dissolved in 1,4-dioxane, forming a homogeneous solution. The solution 3416
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luminescence peak from PS−PVP/RE composites (Figure S2 in the Supporting Information) also blue shifts because of interactions between RE ions and block copolymers. Electron microscopic analysis further confirms the existence of RE elements within micellar nanoparticles (see Figure 2).
Figure 3. Optical properties of composite nanoparticles: (A) optical pictures of nanoparticle solutions of PS−PVP/Tm (blue), PS−PVP/ Tb (green), PS−PVP/Eu (red) and a mixture of the above three solutions (white, with a volume ratio of PS−PVP/Tm:PS−PVP/ Tb:PS−PVP/Eu = 8:1:1) excited with a 300 nm UV light. (B) PL spectra of nanoparticle solutions of PS−PVP/Tm, PS−PVP/Tb, and PS−PVP/Eu. (C) Luminescence spectra of polymer nanoparticle solutions plotted on the CIE diagram and composition trajectory to tune color through stoichiometric mixing. (D) PL spectra of the nanoparticle solution in B (emits white color). Inset shows an optical picture of the nanoparticle solution excited with a 300 nm UV light.
Figure 2. Monodisperse composite PS−PVP/lanthanide nanoparticles and size distribution: (A-C) TEM image of the PS−PVP/Tb, PS− PVP/Eu, and PS−PVP/Tm nanoparticles, respectively; insets show the corresponding EDS. (D) DLS results of these nanoparticles.
properties of the corresponding lanthanide ions. Figure 2B shows photographs of three PS−PVP/RE solutions excited by 300 nm UV light. It shows, respectively, the three primary colors of blue, green, and red and white light emitted from PS− PVP/Tm, PS−PVP/Tb, PS−PVP/Eu, and a mixture of the above three solutions with a volume ratio of 8:1:1. The mixture emits white light with the CIE coordinates x = 0.33 and y = 0.34. Coordination interaction between the PVP chains and lanthanide ions essentially sequesters the lanthanide ions completely within the polymer nanoparticles, leaving PS as the outer shell of the nanoparticles. Thus, the composite PS− PVP/RE nanoparticles can independently emit the corresponding color of the ions that are encapsulated within the nanoparticles. In addition to the three primary colors (red, green, and blue), variations of the color can be tuned through the mixture of nanoparticles of independent color. Therefore, a large area of the color map can be accessed by simply mixing two or three primary polymer nanoparticle solutions and following the trajectory of the CIE coordinates through compositional changes of the nanoparticles. Using such a procedure, we have fabricated solutions with a variety of emission colors using these composite nanoparticles (Figures S2−S5 in the Supporting Information). We have measured the fluorescent lifetime of the composite nanoparticle (PS−PVP/Eu) in comparison with Eu(NO3)3·5H2O. The luminescence decays for both PS−PVP/ Eu and Eu(NO3)3·5H2O can be fit by single exponentials as I = I0 exp(−t/τ), where τ is the 1/e lifetime of RE ions. This indicates that all of the Eu3+ ions have similar environments within the composite nanoparticles. The lifetime of the excited state for Eu3+ can be determined by the fits shown in Figure 4. It is shown that the luminescence lifetime of PS−PVP/Eu
Typical TEM images of as-made composite PS−PVP/ lanthanide nanoparticles including PS−PVP/Tb, PS−PVP/Eu, and PS−PVP/Tm nanoparticles are illustrated in Figure 2. The images show that all nanoparticles are very monodisperse, demonstrating the high-quality production of composite nanoparticles. The average size of these composite nanoparticles is 25 nm in diameter with a standard size deviation of ∼5%. TEM energy-dispersion spectra (EDS) confirmed the existence of RE elements within the nanoparticles (Figure 2, A−C insets). TEM images further show that the nanoparticles form ordered hexagonal arrays. This is possibly due to the monodispersity and van der Waals interaction of the PS shell of individual nanoparticles. Figure 2D shows the size distribution of the composite nanoparticles in solutions measured by using dynamic light scattering (DLS) techniques. DLS results further demonstrate the monodispersity and narrow size distribution of PS−PVP/RE nanoparticles. We observe a difference in size between the DLS and TEM results. In solutions, the nanoparticles show much larger size than those measured by TEM. This is likely caused by a drying effect. In the solution, the composite polymer nanoparticles are solvated and the polymer chains (especially the PS block) are extended, therefore showing larger dynamic diameters. However, in the solid state (TEM sample), the nanoparticles are dried and shrunken, which results in a smaller size. Figure 3A shows the photoluminescent (PL) spectra of PS− PVP/Tm (414 nm, blue), PS−PVP/Tb (545 nm, green), and PS−PVP/Eu (617 nm, red) nanoparticles. The PL spectra confirm that the composite nanoparticles retain the optical 3417
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emit light with tunable colors depending on the use of different lanthanide ions. The nanoparticles are easily redispersed into common solvents like toluene, THF, CHCl3, etc. By adding homopolystyrene as the support matrix, uniform films can be spin-coated or casted that exhibit high transparency and mechanical flexibility.
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ASSOCIATED CONTENT
S Supporting Information *
Optical characterization. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
Figure 4. Fluorescent lifetime of PS−PVP/Eu nanoparticles in comparison with precursor Eu(NO3)3·5H2O.
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ACKNOWLEDGMENTS We thank Dr. Dongmei Ye for her valuable discussions and help on the paper. This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, Sandia National Laboratories’ LDRD program, National Science Foundation (Grant DMI-0625897), and the National Natural Science Foundation of China (Grants 61176016, 21171049, and 50828302). TEM studies were performed in the Department of Earth and Planetary Sciences at the University of New Mexico. We acknowledge use of the SEM facility supported by NSF EPSCOR and NNIN grants. Sandia is a multiprogram laboratory operated by Sandia Corp., a wholly owned subsidiary of Lockheed Martin Corp. for the U.S. Department of Energy’s National Nuclear Security Administration under Contract DEAC04-94AL85000. Z.S. thanks the support from CAS Hundred Talent Program.
(∼0.5 ms) is longer than those of Eu3+ in the dioxane solution (∼0.25 ms). A likely cause is that water molecules complexed with Eu3+ have been replaced by pyridine molecules in PS− PVP. Water molecules promote nonradiative relaxation of excited Eu3+. Nonradiation relaxation is depressed by the replacement of pyridine.1,30 The resulting composite nanoparticles exhibit several advantageous properties with regard to processing. These nanoparticles are dispersible in common solvents. The PS shell is compatible with homopolymer PS. We have added homopolystyrene as a primary matrix to the solution. Because the nanoparticles have PS blocks forming the outer layer,the nanoparticles are completely compatible with the PS matrix. No macrophase separation is observed in the solution or film. Through spin coating or casting of these nanoparticle solutions, highly transparent and flexible films can be readily formed. Figure 5 shows optical pictures of the three films. Under UV
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Figure 5. PS films of PS−PVP/RE nanoparticles under UV light (A− C) and normal light (D). The bottom images show the films in the above images after rolling into a tube: (A) PS−PVP/Tm; (B) PS− PVP/Tb; (C) PS−PVP/Eu.
excitation, the films emit uniform blue, green, and red corresponding to the PS−PVP/Tm, PS−PVP/Tb, and PS− PVP/Eu nanoparticles, respectively. The fact that these films can be rolled without cracking and a loss of color emission indicates the great mechanical flexibility of the films.
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SUMMARY We have developed a simple self-assembly process to prepare monodisperse, light-emitting inorganic/organic composite nanoparticles. Through a coordination interaction with PVP chains, lanthanide ions were encapsulated within PS−PVP nanoparticles with PS as the outer shell. The resulting nanoparticles maintain their luminescence properties and 3418
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