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Cite this: Nanoscale, 2014, 6, 3526
Received 17th October 2013 Accepted 16th January 2014
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Dielectric properties of barium titanate supramolecular nanocomposites Keun Hyung Lee,†a Joseph Kao,a Saman Salemizadeh Parizi,b Gabriel Caruntub and Ting Xu*acd
DOI: 10.1039/c3nr05535c www.rsc.org/nanoscale
Nanostructured dielectric composites can be obtained by dispersing high permittivity fillers, barium titanate (BTO) nanocubes, within a supramolecular framework. Thin films of BTO supramolecular nanocomposites exhibit a dielectric permittivity (3r) as high as 15 and a relatively low dielectric loss of 0.1 at 1 kHz. These results demonstrate a new route to control the dispersion of high permittivity fillers toward high permittivity dielectric nanocomposites with low loss. Furthermore, the present study shows that the size distribution of nanofillers plays a key role in their spatial distribution and local ordering and alignment within supramolecular nanostructures.
Dielectric materials with high permittivity, low loss, light weight and good processability are highly desirable for a broad range of applications including electromechanical actuators, gate dielectrics, and energy storage devices.1–4 Polymers oen have excellent insulating nature (i.e. low loss), solution processability, mechanical exibility, and lightweight.5 However, they usually exhibit low permittivity, typically below 10. To overcome the limitation, ferroelectric metal oxide nano- or micro-particles such as barium titanate (BTO), lead zirconium titanate (PZT), and calcium copper titanate (CCTO) have been incorporated into a host polymer matrix as llers to enhance the dielectric permittivity.6–11 To effectively increase the permittivity of the composites, the volume fraction of the high dielectric constant ller should be large enough to be close to a percolation limit. Using an effective medium approximation, theoretical studies suggested that the well-dispersed nanollers shi the percolation threshold
a
Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA. E-mail: [email protected]; Fax: +1 510-643-5792; Tel: +1 510-6421632
b
Department of Chemistry and the Science of Advanced Materials Program, Central Michigan University, Mount Pleasant, MI, 48858, USA
c
Department of Chemistry, University of California, Berkeley, CA 94720, USA
d
Materials Sciences Division, Lawrence Berkeley National Laboratory, CA 94720, USA
† Current address: Department of Chemical Engineering, Inha University, Incheon 402-751, Korea.
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toward lower volume fraction of the nanollers than that based on macroscale particles. This can be attributed to the high surface area to volume ratio of the nanoscopic llers.12 Thus, it is requisite to achieve good dispersion of high dielectric nanollers in the polymer matrix to obtain nanocomposites with desirable dielectric properties. A range of homopolymers or random copolymers have been used as host matrices to fabricate dielectric materials. Simply mixing nanollers with the host polymers usually yields nonuniform distribution of the nanollers and poor lm quality due to the chemical incompatibility between the nanollers and the polymers and the entropic penalty for the polymer chains to deform upon nanoller incorporation.13–15 Unfunctionalized silica nanoparticles at a weight concentration of 10 wt% were shown to aggregate into large clusters in the size range of 1100 mm upon blending with a polystyrene homopolymer.16 These aggregated nanollers increase the leakage current and dielectric loss of the nanocomposites. Homogeneous dispersion of nanollers in a polymer matrix can be achieved by attaching ligand molecules on the surface of the particles to engineer favorable ller/polymer interactions. The surface chemistry of the nanollers has been modied either by directly attaching molecular ligands onto or growing polymers from the surface of the nanoller.9,17–19 Kumar et al. showed both theoretically and experimentally that the molecular weight of graed polystyrene on each nanoparticle should be large enough to achieve spatially uniform particle dispersion in a polystyrene homopolymer matrix.20 Phosphoric acid ligands were used to passivate BTO nanollers to prevent the particle agglomeration in nanocomposites.9,11 However, surface modication of nanollers, especially for metal oxide nanoparticles, is not trivial and the surface modication process can lead to the aggregation of oxide nanoparticles. Nanoscopic distribution of nanollers with macroscopic alignment of the ller assembly can be advantageous to further optimize the macroscopic dielectric properties of nanocomposites. Upon blending with homopolymers, nanollers at best are randomly distributed. Functional nanocomposites were
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also prepared by blending nanollers with block copolymers (BCPs) or BCP-based supramolecules.21–23 The BCP macrophase separates to form well-ordered arrays of nanostructures, providing nanostructural frameworks to disperse nanollers.24,25 BCP-based supramolecules were used to achieve controlled dispersion of a wide range of nanoparticles without modifying the chemistry of neither constituent by employing small molecule linkers that favorably interact with both nanollers and one of the polymer blocks.26–28 Pre-synthesized nanollers with common alkane-based ligands, like alkyl acid or amine, can be readily dispersed to obtain supramolecular nanocomposites. We have successfully achieved ordered assemblies of spherical semiconducting and metallic nanoparticles in BCP-based supramolecules.26–28 However, there are challenges to uniformly disperse metal oxide nanollers in polymer hosts due to the surface charge and the faceted sides of the oxide nanollers that make them unstable and aggregate even in colloidal solutions. Furthermore, most of the studies are based on small nanoparticles with very low polydispersity in size. There has been limited investigation on how the large nanoparticles with high polydispersity in size can be assembled within the supramolecular framework. The particle size distribution may also contribute to the local organization of nanoparticles. In addition, the cube-shaped oxide nanollers may have stronger interparticle interactions than those of small spherical particles due to the entropic contribution originated from the particle shape. Here, functional dielectric composites using oleic acid-capped BTO nanocubes as high permittivity llers were fabricated using the supramolecular approach. A schematic illustration of nanocomposite formation and chemical structures of the supramolecule are shown in Fig. 1. The supramolecule is composed of BCP polystyrene-b-poly(4-vinylpyridine) PS-b-P4VP and 3-pentadecylphenol (PDP). The alkyl tail of the PDP interacts favorably with the oleic acid ligand on BTO nanocubes.27–29 In bulk and thin lms of supramolecular nanocomposites, BTO nanocubes were dispersed and incorporated reasonably well and remain preferentially within the P4VP(PDP) microdomain.
Fig. 1 Schematic illustration of nanocomposite formation (top) and a thin film capacitor (bottom). The supramolecular nanocomposite is a blend of cube-like barium titanate (BTO) particles capped with oleic acid and a common supramolecule, PS(242 kDa)-b-P4VP(43 kDa)(PDP)1.2, constructed by hydrogen bonding between 3-pentadecylphenol (PDP) and the 4VP units of a diblock copolymer, polystyrene-b-poly(4-vinylpyridine) PS-b-P4VP at a PDP : 4VP stoichiometry of 1.2.
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The dielectric permittivity and loss tangent of the nanocomposites were systematically studied by impedance spectroscopy as functions of the excitation frequency and the BTO volume fraction. Hybrid composites show a dielectric permittivity value as high as 15 at 1 kHz that is 5 times higher than that of the pure supramolecule, while maintaining a relatively low dielectric loss tangent of 0.1. These results reveal an attractive strategy to generate functional hybrid dielectrics based on metal oxide nanocubes and polymer hosts. Oleic acid-capped BTO nanoparticles were synthesized by a previously reported low temperature hydrothermal technique and then suspended in chloroform.30 The size distribution of cubic-like BTO particles was quite broad (10100 nm). A supramolecular solution was prepared by co-dissolving PS(252 kDa)-b-P4VP(43 kDa) and PDP in chloroform. The molar ratio between the 4VP unit and the PDP small molecule was kept at 1 : 1.2. To rst evaluate the morphology of supramolecular nanocomposites, PS-b-P4VP(PDP) and BTO solutions were mixed and cast in a Teon beaker. The solvent was then slowly evaporated overnight at room temperature. Bulk nanocomposites were characterized by transmission electron microscopy (TEM) to investigate the co-assembly of BTO nanollers and PS-bP4VP(PDP). Thin lms of nanocomposites were directly spin coated on SiO2 or glass substrates from chloroform solutions at spinning speeds ranging from 1000 to 3000 rpm and subsequently placed in a vacuum oven at 60 C for 2 h to remove the residual solvent. Thicknesses of the spin coated lms were measured by a Veeco Dektak 150 surface proler. The typical thickness of the composite thin lms was 100200 nm. For TEM analysis, bulk nanocomposite lms were embedded in an epoxy resin (Araldite 502, Electron Microscopy Sciences) and cured at 60 C overnight. A thin section of the nanocomposites was microtomed using an RMC MT-X Ultramicrotome (Boeckler Instruments) and then picked up on a copper TEM grid. For the cross-sectional samples, composite lms were rst oated off from the SiO2 substrate by using a 5% HF solution, and then transferred onto an epoxy block. The epoxy/ sample assemblies were subsequently cured at 60 C for 4 h to ensure a good contact between the epoxy and the nanocomposites and microtomed to obtain thin slices of the nanocomposites. The TEM images were collected using a FEI Tecnai 12 transmission electron microscope. For a nanocomposite lm with a BTO concentration of 0.5, a freeze-fractured cross-section was imaged by using a JEOL JSM-6340F scanning electron microscope (SEM). To investigate the dielectric response of the nanocomposite lms, parallel-plate capacitors were fabricated on indium tin oxide (ITO) coated glass slides as bottom electrodes. A schematic diagram of the capacitor is shown in Fig. 1 (bottom). ITO substrates were sequentially washed with copious amounts of de-ionized water, acetone, and isopropyl alcohol prior to the active layer coating. Composite lms were then directly spin coated on the bottom electrode from chloroform solutions. Two-terminal capacitors were designed by thermally depositing a top Al electrode (thickness: 100 nm, area: 1 cm2) on the nanocomposite lms by using a stainless steel shadow mask. Dielectric measurements were conducted on these capacitors by
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using a Solartron 1260 impedance gain-phase analyzer controlled by SMaRT soware over the frequency range of 1–106 Hz with an AC amplitude of 100 mV at room temperature. The impedance (Z*) was collected as Z* ¼ Z0 + iZ00 , where i is the imaginary unit, and Z0 and Z00 are the real and imaginary components of the impedance, respectively. Fig. 2 shows the TEM images collected from the bulk nanocomposites with different BTO loading concentrations. BTO llers are nanoscopically distributed and accommodated well in the PS-b-P4VP(PDP) supramolecules. Such dispersion of nanocubes in the host polymer is maintained when the BTO volume fraction (fBTO) is increased from 0.2 to 0.4. At fBTO ¼ 0.2, individual BTO nanocubes and arrays are spatially distributed throughout samples. As the ller loading increases, more particles begin to form nanocube arrays in the P4VP(PDP) microdomains and separation between them becomes narrower. BTO clusters in the size range of a few hundred nanometers are also observed in the nanocomposites probably due to the inherent instability of the oxide nanocubes. It is worthwhile to emphasize that the clusters are still nanoscopic in size and are much smaller than those agglomerated in homopolymer host matrices (typically larger than 1 mm). No evidence of macrophase separation of the nanollers from the polymer is observed for these bulk samples. These observations demonstrate successful incorporation of the high permittivity ceramic llers into the PS-b-P4VP(PDP) supramolecular matrix. Thin lms of the BTO nanocomposites were deposited onto SiO2 and ITO substrates and the morphology of the lms was characterized by atomic force microscopy (AFM) and crosssectional TEM and SEM. Fig. 3 shows AFM phase images obtained from the surfaces of the supramolecular thin lms with various ller concentrations (0.05, 0.1, 0.2, 0.3, and 0.5). When fBTO is low, AFM images indicate that BTO particles are incorporated well into the polymer matrix. Individual BTO particles observed throughout the nanocomposite lms suggest that BTO particles can be effectively dispersed into the P4VP(PDP) microdomains. BTO nanollers also form ordered 1D lines along with the lamellar domains of the polymer matrix, which is clear evidence of the BTO incorporation into the
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P4VP(PDP) microdomains by the favorable interaction between oleic acid on the surface of each BTO nanocrytal and the alkyl terminal groups of the PDP.27 To characterize the spatial organization of the BTO nanollers in the interior of the supramolecular nanocomposite thin lms, cross-sectional TEM/SEM was conducted. All crosssectional images in the bottom of Fig. 3 show that single BTO particles and aligned BTO arrays are embedded in the supramolecular nanocomposites. No dominant surface aggregation of the BTO nanocubes at the air/lm interface is observed. This suggests that BTO particles are effectively surrounded and insulated by the polymer matrix. Such distribution of BTO particles in the inside of the polymer keeps the composite energy loss reasonably low (the electrical aspect of the composite lms will be discussed shortly). It is noteworthy that dispersed or aligned BTO nanoparticles are relatively small with an average size close to 1020 nm. When the size of the BTO llers is larger than 30 nm, the BTO particles tend to form clusters. The BTO agglomerates can be identied on both surfaces and interior of the nanocomposites. As expected, more agglomerates are formed for lms with higher BTO loading. At low fBTO, the size of the agglomerates remains relatively small at 100 nm and they are efficiently surrounded by the polymer matrix. With increasing the nanocube loading to fBTO ¼ 0.5, the size of the BTO clusters becomes larger. In this case, polymeric insulation around the large agglomerates becomes less effective because incorporation of these agglomerates into a polymer microdomain is energetically costly.22 These large agglomerates are undesirable in energy storage because they can form pathways for current leakage, resulting in increased energy loss. In most agglomerates, big BTO particles >30 nm are found and surrounded by smaller particles implying that large BTO particles strongly attract particles nearby because of the electrical charge on their surfaces and the van der Waals interaction. This observation suggests that the quality (e.g. size) of the nanoparticle should be well controlled to further reduce the agglomeration of nanollers and to enhance the dispersion of the nanoller particles uniformly in the matrix of supramolecular nanocomposites.
Fig. 2 TEM images collected from bulk nanocomposite films based on BTO nanoparticles and PS-b-P4VP(PDP) with BTO concentrations of 0.2, 0.3 and 0.4. Scale bars are 200 nm.
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Fig. 3 AFM phase images (top) and corresponding cross-sectional TEM (ad)/SEM (e) images (bottom) of nanocomposite thin films based on BTO nanocubes and PS-b-P4VP(PDP) with different BTO concentrations of (a) 0.05, (b) 0.1, (c) 0.2, (d) 0.3, and (e) 0.5. Scale bars for the AFM and cross-sectional EM images are 200 and 100 nm, respectively.
To systematically investigate the dielectric response of the composite lms, impedance measurements were conducted on the supramolecular nanocomposite capacitors with concentrations of the BTO nanocubes varying from 0 to 0.5 and the results are displayed in Fig. 4. The Nyquist plots, or Z0 vs. Z00 plots, show straight vertical lines for all samples indicating that the nanocomposite capacitors behave like an ideal capacitor. This observation corroborates the calculation of the composite capacitance (C) and dielectric permittivity (3r) from the measured Z00 data according to the following equations: 1 C¼ 2pfZ 00 3r ¼
Cl 30 A
relatively slowly at low fBTO, but rises rapidly when the concentration of the nanoller becomes higher than 0.2. At fBTO ¼ 0.5 the permittivity value increases to 15, which is 5 times larger than that of the pure supramolecule. These results strongly suggest that the volume fraction of the high permittivity ller needs to be larger than a certain threshold (ca. 0.2) to effectively increase the dielectric permittivity of the nanocomposite.31–33 The permittivity increases can be explained by the percolation theory which follows the power law: 3r f ( fc fBTO)s for fBTO < fc
(3)
(1) where fc is the percolation threshold and s is the critical exponent of order unity.34,35 The experimental results follow the (2)
where f represents the frequency, 30 is the vacuum permittivity, l is the thickness of the composite and A is the area of the capacitor. Fig. 5a and b display the effect of BTO volume fraction on the dielectric permittivity and loss tangent of the nanocomposites measured at 1 kHz. The dielectric permittivity in Fig. 5a changes
Fig. 4 Nyquist plots, or Z0 vs. Z00 , for nanocomposite films with different BTO fractions. Impedance measurements were performed over the frequency range from 1 to 106 Hz with an AC amplitude of 100 mV.
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Fig. 5 Dielectric permittivity (a) and loss tangent (tan d) (b) of supramolecular nanocomposties as a function of BTO loading. Frequency dependent dielectric permittivity (c) and tan d (d) for the nanocomposites with different BTO concentrations. High frequency upturn in (d) is due to the lead resistance of the assembly.
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power law dependence with the t parameters of fc ¼ 0.62 and s ¼ 1.1. A physical origin for the increase of the dielectric permittivity in the composite lms can be explained by a myriad of nanocapacitors formed by neighboring BTO llers and the thin insulating polymer layer separating adjacent dielectric nanocrystals.36 As a result, the local electric eld across these nanocapacitors will be enhanced, which will then promote the migration and accumulation of the charge carriers in the polymer layer, thereby leading to an increased charge density or the dielectric permittivity. This phenomenon is known as space charge polarization of dielectrics and it becomes prominent when fBTO is high.37 Further insight into the dielectric response of the hybrid composites can be obtained from the dielectric loss. The dielectric loss is quantied by the loss tangent (tan d ¼ Z0 /Z00 ), also known as the dissipation factor, which represents the ratio between electrical energy dissipated and the energy stored in the system. Fig. 5b displays the dielectric loss tangent measured at 1 kHz with different BTO concentrations. Overall, tan d increases by adding BTO nanollers into the polymer probably due to ohmic current leakage through the llers or clusters. Measured tan d values are still low at 0.1 for all nanocomposites up to fBTO ¼ 0.5 implying that BTO nanocubes are effectively insulated by the polymer matrix. When the BTO fraction is further increased to above 0.5, however, composite lms behave like leaky resistors. This is probably because of the even larger BTO clusters which are not properly surrounded by the polymer host. Frequency dependent dielectric permittivity measurements were performed on the nanocomposites over the frequency range of 1 to 106 Hz and the results are displayed in Fig. 5c. When fBTO < 0.2 the permittivity values obtained from the composite lms are insensitive to the frequency change. However, the permittivity shows a weak dependence on the frequency at fBTO ¼ 0.3. The frequency effect is found to become signicant for samples corresponding to fBTO ¼ 0.4 and 0.5, which exhibits a substantial increase of the dielectric permittivity with decreasing the frequency. Such frequency dispersion is presumably attributed to the space charge polarization between BTO particles and the host polymer matrix. However, the observed frequency dependence is relatively weaker than or comparable to other polymer composites blended with high permittivity llers.1,6,38–40 For lms with fBTO < 0.2, the loss tangent in Fig. 5d shows frequency independent characteristics at low frequencies. As expected, when the fBTO is higher than 0.3 the loss tangent values increase with decreasing frequency due to the accumulated space charge in the nanocomposites. Such increase in the energy loss at low frequencies might be unsuitable for applications because it causes leakage currents which require an additional operating power and ultimately reduce the effectiveness and the life time of a power supply. A sharp increase in the loss tangent is observed in the high frequency regime (>105 Hz). These high nominal dielectric losses are due to the lead resistance (Rlead) of the assembly (in our case, Rlead is 20 U).
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Conclusions We have fabricated high permittivity hybrid composites using BCP-based supramolecules and BTO nanoparticles as building blocks. A small molecule PDP that mediates the interaction between the polymer and the BTO colloidal nanocrystals eliminates the need for either modication of the polymer or engineering of the nanoparticle surface chemistry, and provides a versatile approach to disperse dielectric nanollers. The AFM and EM images from both bulk and thin nanocomposite lms demonstrate the nanoscopic dispersion of the BTO particles with the size range of few hundred nanometers in the supramolecules. The dielectric permittivity of the nanocomposites increases with BTO fraction, reaching a value as high as 15 for the lms with fBTO ¼ 0.5. The loss tangent values also increase with the BTO loading, but still remain reasonably small at 0.1. To further improve the dielectric properties of the nanocomposites, monodisperse nanollers which are smaller than the size of the P4VP(PDP) domain need to be incorporated. Overall, this approach demonstrates the simplicity and versatility of a novel fabrication route enabling the design of functional polymer–ceramic nanocomposites for high permittivity dielectrics.
Acknowledgements The authors thank Prof. Jean M. J. Fr´ echet for access to his metal evaporator and impedance spectroscopy equipment. This work was supported by the Office of Naval Research Young Investigator program. GC thanks the Central Michigan University for supporting this work through start-up funds and the Macromolecular, Supramolecular and Nanochemistry Division of the National Science Foundation (CAREER award no. 1157300).
Notes and references 1 Q. M. Zhang, H. F. Li, M. Poh, F. Xia, Z. Y. Cheng, H. S. Xu and C. Huang, Nature, 2002, 419, 284. 2 C. Huang and Q. M. Zhang, Adv. Mater., 2005, 17, 1153. 3 F. Carpi, G. Gallone, F. Galantini and D. De Rossi, Adv. Funct. Mater., 2008, 18, 235. 4 P. Kim, X.-H. Zhang, B. Domercq, S. C. Jones, P. J. Hotchkiss, S. R. Marder, B. Kippelen and J. W. Perry, Appl. Phys. Lett., 2008, 93, 013302. 5 B. Chu, X. Zhou, K. Ren, B. Neese, M. Lin, Q. Wang, F. Bauer and Q. M. Zhang, Science, 2006, 313, 334. 6 Z.-M. Dang, H.-Y. Wang and H.-P. Xu, Appl. Phys. Lett., 2006, 89, 112902. 7 M. Dietze and M. Es-Souni, Sens. Actuators, A, 2008, 143, 329. 8 Z.-M. Dang, T. Zhou, S.-H. Yao, J.-K. Yuan, J.-W. Zha, H.-T. Song, J.-Y. Li, Q. Chen, W.-T. Yang and J. Bai, Adv. Mater., 2009, 21, 2077. 9 P. Kim, S. C. Jones, P. J. Hotchkiss, J. N. Haddock, B. Kippelen, S. R. Marder and J. W. Perry, Adv. Mater., 2007, 19, 1001.
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10 P. Kim, N. M. Doss, J. P. Tillotson, P. J. Hotchkiss, M.-J. Pan, S. R. Marder, J. Li, J. P. Calame and J. W. Perry, ACS Nano, 2009, 3, 2581. 11 S. Siddabattuni, T. P. Schuman and F. Dogan, ACS Appl. Mater. Interfaces, 2013, 5, 1917. 12 J. Y. Li, L. Zhang and S. Ducharme, Appl. Phys. Lett., 2007, 90, 132901. 13 M. E. Mackay, A. Tuteja, P. M. Duxbury, C. J. Hawker, B. Van Horn, Z. Guan, G. Chen and R. S. Krishnan, Science, 2006, 311, 1740. 14 R. Krishnamoorti, MRS Bull., 2007, 32, 341. 15 R. B. Thompson, V. V. Ginzburg, M. W. Matsen and A. C. Balazs, Science, 2001, 292, 2469. 16 A. Bansal, H. Yang, C. Li, K. Cho, B. C. Benicewicz, S. K. Kumar and L. S. Schadler, Nat. Mater., 2005, 4, 693. 17 L. Xie, X. Huang, C. Wu and P. Jiang, J. Mater. Chem., 2011, 21, 5897. 18 N. Guo, S. A. DiBenedetto, D.-K. Kwon, L. Wang, M. T. Russell, M. T. Lanagan, A. Facchetti and T. J. Marks, J. Am. Chem. Soc., 2007, 129, 766. 19 S. Ramesh, B. A. Shutzberg, C. Huang, G. Jie and E. P. Giannelis, IEEE Trans. Adv. Packag., 2003, 26, 17. 20 P. Akcora, H. Liu, S. K. Kumar, J. Moll, Y. Li, B. C. Benicewicz, L. S. Schadler, D. Acehan, A. Z. Panagiotopoulos, V. Pryamitsyn, V. Ganesan, J. Ilavsky, P. Thiyagarajan, R. H. Colby and J. F. Douglas, Nat. Mater., 2009, 8, 354. 21 A. C. Balazs, T. Emrick and T. P. Russell, Science, 2006, 314, 1107. 22 M. R. Bockstaller, R. A. Mickiewicz and E. L. Thomas, Adv. Mater., 2005, 17, 1331. 23 J. Kao, K. Thorkelsson, P. Bai, B. J. Rancatore and T. Xu, Chem. Soc. Rev., 2013, 42, 2654.
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24 F. S. Bates and G. H. Fredrickson, Annu. Rev. Phys. Chem., 1990, 41, 525. 25 F. S. Bates, Science, 1991, 251, 898. 26 K. Thorkelsson, B. J. Rancatore, C. E. Mauldin, J. M. Luther, D. Poulsen, J. M. J. Frechet, A. P. Alivisatos, and X. Ting, 10th IEEE Conference on Nanotechnology (IEEE-NANO), 2010, vol. 51. 27 Y. Zhao, K. Thorkelsson, A. J. Mastroianni, T. Schilling, J. M. Luther, B. J. Rancatore, K. Matsunaga, H. Jinnai, Y. Wu, D. Poulsen, J. M. J. Frechet, A. Paul Alivisatos and T. Xu, Nat. Mater., 2009, 8, 979. 28 J. Kao, P. Bai, V. P. Chuang, Z. Jiang, P. Ercius and T. Xu, Nano Lett., 2012, 12, 2610. 29 J. Ruokolainen, M. Saariaho, O. Ikkala, G. ten Brinke, E. L. Thomas, M. Torkkeli and R. Serimaa, Macromolecules, 1999, 32, 1152. 30 S. Adireddy, C. Lin, B. Cao, W. Zhou and G. Caruntu, Chem. Mater., 2010, 22, 1946. 31 R. Yang, J. Qu, T. Marinis and C. P. Wong, IEEE Trans. Adv. Packag. Technol., 2000, 23, 680. 32 N. Jayasundere and B. V. Smith, J. Appl. Phys., 1993, 73, 2462. 33 J. P. Calame, J. Appl. Phys., 2006, 99, 084101. 34 S. Kirkpatrick, Rev. Mod. Phys., 1973, 45, 574. 35 C.-W. Nan, Prog. Mater. Sci., 1993, 37, 1. 36 C. Pecharrom´ an and J. S. Moya, Adv. Mater., 2000, 12, 294. 37 Y. S. C.-W. Nan and J. Ma, Annu. Rev. Mater. Res., 2010, 40, 131. 38 H. Stoyanov, D. M. Carthy, M. Kollosche and G. Kofod, Appl. Phys. Lett., 2009, 94, 232905. 39 Z. M. Dang, Y. H. Lin and C. W. Nan, Adv. Mater., 2003, 15, 1625. 40 L. Wang and Z.-M. Dang, Appl. Phys. Lett., 2005, 87, 042903.
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Cite this: Nanoscale, 2014, 6, 3526
Received 17th October 2013 Accepted 16th January 2014
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Dielectric properties of barium titanate supramolecular nanocomposites Keun Hyung Lee,†a Joseph Kao,a Saman Salemizadeh Parizi,b Gabriel Caruntub and Ting Xu*acd
DOI: 10.1039/c3nr05535c www.rsc.org/nanoscale
Nanostructured dielectric composites can be obtained by dispersing high permittivity fillers, barium titanate (BTO) nanocubes, within a supramolecular framework. Thin films of BTO supramolecular nanocomposites exhibit a dielectric permittivity (3r) as high as 15 and a relatively low dielectric loss of 0.1 at 1 kHz. These results demonstrate a new route to control the dispersion of high permittivity fillers toward high permittivity dielectric nanocomposites with low loss. Furthermore, the present study shows that the size distribution of nanofillers plays a key role in their spatial distribution and local ordering and alignment within supramolecular nanostructures.
Dielectric materials with high permittivity, low loss, light weight and good processability are highly desirable for a broad range of applications including electromechanical actuators, gate dielectrics, and energy storage devices.1–4 Polymers oen have excellent insulating nature (i.e. low loss), solution processability, mechanical exibility, and lightweight.5 However, they usually exhibit low permittivity, typically below 10. To overcome the limitation, ferroelectric metal oxide nano- or micro-particles such as barium titanate (BTO), lead zirconium titanate (PZT), and calcium copper titanate (CCTO) have been incorporated into a host polymer matrix as llers to enhance the dielectric permittivity.6–11 To effectively increase the permittivity of the composites, the volume fraction of the high dielectric constant ller should be large enough to be close to a percolation limit. Using an effective medium approximation, theoretical studies suggested that the well-dispersed nanollers shi the percolation threshold
a
Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA. E-mail: [email protected]; Fax: +1 510-643-5792; Tel: +1 510-6421632
b
Department of Chemistry and the Science of Advanced Materials Program, Central Michigan University, Mount Pleasant, MI, 48858, USA
c
Department of Chemistry, University of California, Berkeley, CA 94720, USA
d
Materials Sciences Division, Lawrence Berkeley National Laboratory, CA 94720, USA
† Current address: Department of Chemical Engineering, Inha University, Incheon 402-751, Korea.
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toward lower volume fraction of the nanollers than that based on macroscale particles. This can be attributed to the high surface area to volume ratio of the nanoscopic llers.12 Thus, it is requisite to achieve good dispersion of high dielectric nanollers in the polymer matrix to obtain nanocomposites with desirable dielectric properties. A range of homopolymers or random copolymers have been used as host matrices to fabricate dielectric materials. Simply mixing nanollers with the host polymers usually yields nonuniform distribution of the nanollers and poor lm quality due to the chemical incompatibility between the nanollers and the polymers and the entropic penalty for the polymer chains to deform upon nanoller incorporation.13–15 Unfunctionalized silica nanoparticles at a weight concentration of 10 wt% were shown to aggregate into large clusters in the size range of 1100 mm upon blending with a polystyrene homopolymer.16 These aggregated nanollers increase the leakage current and dielectric loss of the nanocomposites. Homogeneous dispersion of nanollers in a polymer matrix can be achieved by attaching ligand molecules on the surface of the particles to engineer favorable ller/polymer interactions. The surface chemistry of the nanollers has been modied either by directly attaching molecular ligands onto or growing polymers from the surface of the nanoller.9,17–19 Kumar et al. showed both theoretically and experimentally that the molecular weight of graed polystyrene on each nanoparticle should be large enough to achieve spatially uniform particle dispersion in a polystyrene homopolymer matrix.20 Phosphoric acid ligands were used to passivate BTO nanollers to prevent the particle agglomeration in nanocomposites.9,11 However, surface modication of nanollers, especially for metal oxide nanoparticles, is not trivial and the surface modication process can lead to the aggregation of oxide nanoparticles. Nanoscopic distribution of nanollers with macroscopic alignment of the ller assembly can be advantageous to further optimize the macroscopic dielectric properties of nanocomposites. Upon blending with homopolymers, nanollers at best are randomly distributed. Functional nanocomposites were
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also prepared by blending nanollers with block copolymers (BCPs) or BCP-based supramolecules.21–23 The BCP macrophase separates to form well-ordered arrays of nanostructures, providing nanostructural frameworks to disperse nanollers.24,25 BCP-based supramolecules were used to achieve controlled dispersion of a wide range of nanoparticles without modifying the chemistry of neither constituent by employing small molecule linkers that favorably interact with both nanollers and one of the polymer blocks.26–28 Pre-synthesized nanollers with common alkane-based ligands, like alkyl acid or amine, can be readily dispersed to obtain supramolecular nanocomposites. We have successfully achieved ordered assemblies of spherical semiconducting and metallic nanoparticles in BCP-based supramolecules.26–28 However, there are challenges to uniformly disperse metal oxide nanollers in polymer hosts due to the surface charge and the faceted sides of the oxide nanollers that make them unstable and aggregate even in colloidal solutions. Furthermore, most of the studies are based on small nanoparticles with very low polydispersity in size. There has been limited investigation on how the large nanoparticles with high polydispersity in size can be assembled within the supramolecular framework. The particle size distribution may also contribute to the local organization of nanoparticles. In addition, the cube-shaped oxide nanollers may have stronger interparticle interactions than those of small spherical particles due to the entropic contribution originated from the particle shape. Here, functional dielectric composites using oleic acid-capped BTO nanocubes as high permittivity llers were fabricated using the supramolecular approach. A schematic illustration of nanocomposite formation and chemical structures of the supramolecule are shown in Fig. 1. The supramolecule is composed of BCP polystyrene-b-poly(4-vinylpyridine) PS-b-P4VP and 3-pentadecylphenol (PDP). The alkyl tail of the PDP interacts favorably with the oleic acid ligand on BTO nanocubes.27–29 In bulk and thin lms of supramolecular nanocomposites, BTO nanocubes were dispersed and incorporated reasonably well and remain preferentially within the P4VP(PDP) microdomain.
Fig. 1 Schematic illustration of nanocomposite formation (top) and a thin film capacitor (bottom). The supramolecular nanocomposite is a blend of cube-like barium titanate (BTO) particles capped with oleic acid and a common supramolecule, PS(242 kDa)-b-P4VP(43 kDa)(PDP)1.2, constructed by hydrogen bonding between 3-pentadecylphenol (PDP) and the 4VP units of a diblock copolymer, polystyrene-b-poly(4-vinylpyridine) PS-b-P4VP at a PDP : 4VP stoichiometry of 1.2.
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The dielectric permittivity and loss tangent of the nanocomposites were systematically studied by impedance spectroscopy as functions of the excitation frequency and the BTO volume fraction. Hybrid composites show a dielectric permittivity value as high as 15 at 1 kHz that is 5 times higher than that of the pure supramolecule, while maintaining a relatively low dielectric loss tangent of 0.1. These results reveal an attractive strategy to generate functional hybrid dielectrics based on metal oxide nanocubes and polymer hosts. Oleic acid-capped BTO nanoparticles were synthesized by a previously reported low temperature hydrothermal technique and then suspended in chloroform.30 The size distribution of cubic-like BTO particles was quite broad (10100 nm). A supramolecular solution was prepared by co-dissolving PS(252 kDa)-b-P4VP(43 kDa) and PDP in chloroform. The molar ratio between the 4VP unit and the PDP small molecule was kept at 1 : 1.2. To rst evaluate the morphology of supramolecular nanocomposites, PS-b-P4VP(PDP) and BTO solutions were mixed and cast in a Teon beaker. The solvent was then slowly evaporated overnight at room temperature. Bulk nanocomposites were characterized by transmission electron microscopy (TEM) to investigate the co-assembly of BTO nanollers and PS-bP4VP(PDP). Thin lms of nanocomposites were directly spin coated on SiO2 or glass substrates from chloroform solutions at spinning speeds ranging from 1000 to 3000 rpm and subsequently placed in a vacuum oven at 60 C for 2 h to remove the residual solvent. Thicknesses of the spin coated lms were measured by a Veeco Dektak 150 surface proler. The typical thickness of the composite thin lms was 100200 nm. For TEM analysis, bulk nanocomposite lms were embedded in an epoxy resin (Araldite 502, Electron Microscopy Sciences) and cured at 60 C overnight. A thin section of the nanocomposites was microtomed using an RMC MT-X Ultramicrotome (Boeckler Instruments) and then picked up on a copper TEM grid. For the cross-sectional samples, composite lms were rst oated off from the SiO2 substrate by using a 5% HF solution, and then transferred onto an epoxy block. The epoxy/ sample assemblies were subsequently cured at 60 C for 4 h to ensure a good contact between the epoxy and the nanocomposites and microtomed to obtain thin slices of the nanocomposites. The TEM images were collected using a FEI Tecnai 12 transmission electron microscope. For a nanocomposite lm with a BTO concentration of 0.5, a freeze-fractured cross-section was imaged by using a JEOL JSM-6340F scanning electron microscope (SEM). To investigate the dielectric response of the nanocomposite lms, parallel-plate capacitors were fabricated on indium tin oxide (ITO) coated glass slides as bottom electrodes. A schematic diagram of the capacitor is shown in Fig. 1 (bottom). ITO substrates were sequentially washed with copious amounts of de-ionized water, acetone, and isopropyl alcohol prior to the active layer coating. Composite lms were then directly spin coated on the bottom electrode from chloroform solutions. Two-terminal capacitors were designed by thermally depositing a top Al electrode (thickness: 100 nm, area: 1 cm2) on the nanocomposite lms by using a stainless steel shadow mask. Dielectric measurements were conducted on these capacitors by
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using a Solartron 1260 impedance gain-phase analyzer controlled by SMaRT soware over the frequency range of 1–106 Hz with an AC amplitude of 100 mV at room temperature. The impedance (Z*) was collected as Z* ¼ Z0 + iZ00 , where i is the imaginary unit, and Z0 and Z00 are the real and imaginary components of the impedance, respectively. Fig. 2 shows the TEM images collected from the bulk nanocomposites with different BTO loading concentrations. BTO llers are nanoscopically distributed and accommodated well in the PS-b-P4VP(PDP) supramolecules. Such dispersion of nanocubes in the host polymer is maintained when the BTO volume fraction (fBTO) is increased from 0.2 to 0.4. At fBTO ¼ 0.2, individual BTO nanocubes and arrays are spatially distributed throughout samples. As the ller loading increases, more particles begin to form nanocube arrays in the P4VP(PDP) microdomains and separation between them becomes narrower. BTO clusters in the size range of a few hundred nanometers are also observed in the nanocomposites probably due to the inherent instability of the oxide nanocubes. It is worthwhile to emphasize that the clusters are still nanoscopic in size and are much smaller than those agglomerated in homopolymer host matrices (typically larger than 1 mm). No evidence of macrophase separation of the nanollers from the polymer is observed for these bulk samples. These observations demonstrate successful incorporation of the high permittivity ceramic llers into the PS-b-P4VP(PDP) supramolecular matrix. Thin lms of the BTO nanocomposites were deposited onto SiO2 and ITO substrates and the morphology of the lms was characterized by atomic force microscopy (AFM) and crosssectional TEM and SEM. Fig. 3 shows AFM phase images obtained from the surfaces of the supramolecular thin lms with various ller concentrations (0.05, 0.1, 0.2, 0.3, and 0.5). When fBTO is low, AFM images indicate that BTO particles are incorporated well into the polymer matrix. Individual BTO particles observed throughout the nanocomposite lms suggest that BTO particles can be effectively dispersed into the P4VP(PDP) microdomains. BTO nanollers also form ordered 1D lines along with the lamellar domains of the polymer matrix, which is clear evidence of the BTO incorporation into the
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P4VP(PDP) microdomains by the favorable interaction between oleic acid on the surface of each BTO nanocrytal and the alkyl terminal groups of the PDP.27 To characterize the spatial organization of the BTO nanollers in the interior of the supramolecular nanocomposite thin lms, cross-sectional TEM/SEM was conducted. All crosssectional images in the bottom of Fig. 3 show that single BTO particles and aligned BTO arrays are embedded in the supramolecular nanocomposites. No dominant surface aggregation of the BTO nanocubes at the air/lm interface is observed. This suggests that BTO particles are effectively surrounded and insulated by the polymer matrix. Such distribution of BTO particles in the inside of the polymer keeps the composite energy loss reasonably low (the electrical aspect of the composite lms will be discussed shortly). It is noteworthy that dispersed or aligned BTO nanoparticles are relatively small with an average size close to 1020 nm. When the size of the BTO llers is larger than 30 nm, the BTO particles tend to form clusters. The BTO agglomerates can be identied on both surfaces and interior of the nanocomposites. As expected, more agglomerates are formed for lms with higher BTO loading. At low fBTO, the size of the agglomerates remains relatively small at 100 nm and they are efficiently surrounded by the polymer matrix. With increasing the nanocube loading to fBTO ¼ 0.5, the size of the BTO clusters becomes larger. In this case, polymeric insulation around the large agglomerates becomes less effective because incorporation of these agglomerates into a polymer microdomain is energetically costly.22 These large agglomerates are undesirable in energy storage because they can form pathways for current leakage, resulting in increased energy loss. In most agglomerates, big BTO particles >30 nm are found and surrounded by smaller particles implying that large BTO particles strongly attract particles nearby because of the electrical charge on their surfaces and the van der Waals interaction. This observation suggests that the quality (e.g. size) of the nanoparticle should be well controlled to further reduce the agglomeration of nanollers and to enhance the dispersion of the nanoller particles uniformly in the matrix of supramolecular nanocomposites.
Fig. 2 TEM images collected from bulk nanocomposite films based on BTO nanoparticles and PS-b-P4VP(PDP) with BTO concentrations of 0.2, 0.3 and 0.4. Scale bars are 200 nm.
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Fig. 3 AFM phase images (top) and corresponding cross-sectional TEM (ad)/SEM (e) images (bottom) of nanocomposite thin films based on BTO nanocubes and PS-b-P4VP(PDP) with different BTO concentrations of (a) 0.05, (b) 0.1, (c) 0.2, (d) 0.3, and (e) 0.5. Scale bars for the AFM and cross-sectional EM images are 200 and 100 nm, respectively.
To systematically investigate the dielectric response of the composite lms, impedance measurements were conducted on the supramolecular nanocomposite capacitors with concentrations of the BTO nanocubes varying from 0 to 0.5 and the results are displayed in Fig. 4. The Nyquist plots, or Z0 vs. Z00 plots, show straight vertical lines for all samples indicating that the nanocomposite capacitors behave like an ideal capacitor. This observation corroborates the calculation of the composite capacitance (C) and dielectric permittivity (3r) from the measured Z00 data according to the following equations: 1 C¼ 2pfZ 00 3r ¼
Cl 30 A
relatively slowly at low fBTO, but rises rapidly when the concentration of the nanoller becomes higher than 0.2. At fBTO ¼ 0.5 the permittivity value increases to 15, which is 5 times larger than that of the pure supramolecule. These results strongly suggest that the volume fraction of the high permittivity ller needs to be larger than a certain threshold (ca. 0.2) to effectively increase the dielectric permittivity of the nanocomposite.31–33 The permittivity increases can be explained by the percolation theory which follows the power law: 3r f ( fc fBTO)s for fBTO < fc
(3)
(1) where fc is the percolation threshold and s is the critical exponent of order unity.34,35 The experimental results follow the (2)
where f represents the frequency, 30 is the vacuum permittivity, l is the thickness of the composite and A is the area of the capacitor. Fig. 5a and b display the effect of BTO volume fraction on the dielectric permittivity and loss tangent of the nanocomposites measured at 1 kHz. The dielectric permittivity in Fig. 5a changes
Fig. 4 Nyquist plots, or Z0 vs. Z00 , for nanocomposite films with different BTO fractions. Impedance measurements were performed over the frequency range from 1 to 106 Hz with an AC amplitude of 100 mV.
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Fig. 5 Dielectric permittivity (a) and loss tangent (tan d) (b) of supramolecular nanocomposties as a function of BTO loading. Frequency dependent dielectric permittivity (c) and tan d (d) for the nanocomposites with different BTO concentrations. High frequency upturn in (d) is due to the lead resistance of the assembly.
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power law dependence with the t parameters of fc ¼ 0.62 and s ¼ 1.1. A physical origin for the increase of the dielectric permittivity in the composite lms can be explained by a myriad of nanocapacitors formed by neighboring BTO llers and the thin insulating polymer layer separating adjacent dielectric nanocrystals.36 As a result, the local electric eld across these nanocapacitors will be enhanced, which will then promote the migration and accumulation of the charge carriers in the polymer layer, thereby leading to an increased charge density or the dielectric permittivity. This phenomenon is known as space charge polarization of dielectrics and it becomes prominent when fBTO is high.37 Further insight into the dielectric response of the hybrid composites can be obtained from the dielectric loss. The dielectric loss is quantied by the loss tangent (tan d ¼ Z0 /Z00 ), also known as the dissipation factor, which represents the ratio between electrical energy dissipated and the energy stored in the system. Fig. 5b displays the dielectric loss tangent measured at 1 kHz with different BTO concentrations. Overall, tan d increases by adding BTO nanollers into the polymer probably due to ohmic current leakage through the llers or clusters. Measured tan d values are still low at 0.1 for all nanocomposites up to fBTO ¼ 0.5 implying that BTO nanocubes are effectively insulated by the polymer matrix. When the BTO fraction is further increased to above 0.5, however, composite lms behave like leaky resistors. This is probably because of the even larger BTO clusters which are not properly surrounded by the polymer host. Frequency dependent dielectric permittivity measurements were performed on the nanocomposites over the frequency range of 1 to 106 Hz and the results are displayed in Fig. 5c. When fBTO < 0.2 the permittivity values obtained from the composite lms are insensitive to the frequency change. However, the permittivity shows a weak dependence on the frequency at fBTO ¼ 0.3. The frequency effect is found to become signicant for samples corresponding to fBTO ¼ 0.4 and 0.5, which exhibits a substantial increase of the dielectric permittivity with decreasing the frequency. Such frequency dispersion is presumably attributed to the space charge polarization between BTO particles and the host polymer matrix. However, the observed frequency dependence is relatively weaker than or comparable to other polymer composites blended with high permittivity llers.1,6,38–40 For lms with fBTO < 0.2, the loss tangent in Fig. 5d shows frequency independent characteristics at low frequencies. As expected, when the fBTO is higher than 0.3 the loss tangent values increase with decreasing frequency due to the accumulated space charge in the nanocomposites. Such increase in the energy loss at low frequencies might be unsuitable for applications because it causes leakage currents which require an additional operating power and ultimately reduce the effectiveness and the life time of a power supply. A sharp increase in the loss tangent is observed in the high frequency regime (>105 Hz). These high nominal dielectric losses are due to the lead resistance (Rlead) of the assembly (in our case, Rlead is 20 U).
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Conclusions We have fabricated high permittivity hybrid composites using BCP-based supramolecules and BTO nanoparticles as building blocks. A small molecule PDP that mediates the interaction between the polymer and the BTO colloidal nanocrystals eliminates the need for either modication of the polymer or engineering of the nanoparticle surface chemistry, and provides a versatile approach to disperse dielectric nanollers. The AFM and EM images from both bulk and thin nanocomposite lms demonstrate the nanoscopic dispersion of the BTO particles with the size range of few hundred nanometers in the supramolecules. The dielectric permittivity of the nanocomposites increases with BTO fraction, reaching a value as high as 15 for the lms with fBTO ¼ 0.5. The loss tangent values also increase with the BTO loading, but still remain reasonably small at 0.1. To further improve the dielectric properties of the nanocomposites, monodisperse nanollers which are smaller than the size of the P4VP(PDP) domain need to be incorporated. Overall, this approach demonstrates the simplicity and versatility of a novel fabrication route enabling the design of functional polymer–ceramic nanocomposites for high permittivity dielectrics.
Acknowledgements The authors thank Prof. Jean M. J. Fr´ echet for access to his metal evaporator and impedance spectroscopy equipment. This work was supported by the Office of Naval Research Young Investigator program. GC thanks the Central Michigan University for supporting this work through start-up funds and the Macromolecular, Supramolecular and Nanochemistry Division of the National Science Foundation (CAREER award no. 1157300).
Notes and references 1 Q. M. Zhang, H. F. Li, M. Poh, F. Xia, Z. Y. Cheng, H. S. Xu and C. Huang, Nature, 2002, 419, 284. 2 C. Huang and Q. M. Zhang, Adv. Mater., 2005, 17, 1153. 3 F. Carpi, G. Gallone, F. Galantini and D. De Rossi, Adv. Funct. Mater., 2008, 18, 235. 4 P. Kim, X.-H. Zhang, B. Domercq, S. C. Jones, P. J. Hotchkiss, S. R. Marder, B. Kippelen and J. W. Perry, Appl. Phys. Lett., 2008, 93, 013302. 5 B. Chu, X. Zhou, K. Ren, B. Neese, M. Lin, Q. Wang, F. Bauer and Q. M. Zhang, Science, 2006, 313, 334. 6 Z.-M. Dang, H.-Y. Wang and H.-P. Xu, Appl. Phys. Lett., 2006, 89, 112902. 7 M. Dietze and M. Es-Souni, Sens. Actuators, A, 2008, 143, 329. 8 Z.-M. Dang, T. Zhou, S.-H. Yao, J.-K. Yuan, J.-W. Zha, H.-T. Song, J.-Y. Li, Q. Chen, W.-T. Yang and J. Bai, Adv. Mater., 2009, 21, 2077. 9 P. Kim, S. C. Jones, P. J. Hotchkiss, J. N. Haddock, B. Kippelen, S. R. Marder and J. W. Perry, Adv. Mater., 2007, 19, 1001.
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10 P. Kim, N. M. Doss, J. P. Tillotson, P. J. Hotchkiss, M.-J. Pan, S. R. Marder, J. Li, J. P. Calame and J. W. Perry, ACS Nano, 2009, 3, 2581. 11 S. Siddabattuni, T. P. Schuman and F. Dogan, ACS Appl. Mater. Interfaces, 2013, 5, 1917. 12 J. Y. Li, L. Zhang and S. Ducharme, Appl. Phys. Lett., 2007, 90, 132901. 13 M. E. Mackay, A. Tuteja, P. M. Duxbury, C. J. Hawker, B. Van Horn, Z. Guan, G. Chen and R. S. Krishnan, Science, 2006, 311, 1740. 14 R. Krishnamoorti, MRS Bull., 2007, 32, 341. 15 R. B. Thompson, V. V. Ginzburg, M. W. Matsen and A. C. Balazs, Science, 2001, 292, 2469. 16 A. Bansal, H. Yang, C. Li, K. Cho, B. C. Benicewicz, S. K. Kumar and L. S. Schadler, Nat. Mater., 2005, 4, 693. 17 L. Xie, X. Huang, C. Wu and P. Jiang, J. Mater. Chem., 2011, 21, 5897. 18 N. Guo, S. A. DiBenedetto, D.-K. Kwon, L. Wang, M. T. Russell, M. T. Lanagan, A. Facchetti and T. J. Marks, J. Am. Chem. Soc., 2007, 129, 766. 19 S. Ramesh, B. A. Shutzberg, C. Huang, G. Jie and E. P. Giannelis, IEEE Trans. Adv. Packag., 2003, 26, 17. 20 P. Akcora, H. Liu, S. K. Kumar, J. Moll, Y. Li, B. C. Benicewicz, L. S. Schadler, D. Acehan, A. Z. Panagiotopoulos, V. Pryamitsyn, V. Ganesan, J. Ilavsky, P. Thiyagarajan, R. H. Colby and J. F. Douglas, Nat. Mater., 2009, 8, 354. 21 A. C. Balazs, T. Emrick and T. P. Russell, Science, 2006, 314, 1107. 22 M. R. Bockstaller, R. A. Mickiewicz and E. L. Thomas, Adv. Mater., 2005, 17, 1331. 23 J. Kao, K. Thorkelsson, P. Bai, B. J. Rancatore and T. Xu, Chem. Soc. Rev., 2013, 42, 2654.
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24 F. S. Bates and G. H. Fredrickson, Annu. Rev. Phys. Chem., 1990, 41, 525. 25 F. S. Bates, Science, 1991, 251, 898. 26 K. Thorkelsson, B. J. Rancatore, C. E. Mauldin, J. M. Luther, D. Poulsen, J. M. J. Frechet, A. P. Alivisatos, and X. Ting, 10th IEEE Conference on Nanotechnology (IEEE-NANO), 2010, vol. 51. 27 Y. Zhao, K. Thorkelsson, A. J. Mastroianni, T. Schilling, J. M. Luther, B. J. Rancatore, K. Matsunaga, H. Jinnai, Y. Wu, D. Poulsen, J. M. J. Frechet, A. Paul Alivisatos and T. Xu, Nat. Mater., 2009, 8, 979. 28 J. Kao, P. Bai, V. P. Chuang, Z. Jiang, P. Ercius and T. Xu, Nano Lett., 2012, 12, 2610. 29 J. Ruokolainen, M. Saariaho, O. Ikkala, G. ten Brinke, E. L. Thomas, M. Torkkeli and R. Serimaa, Macromolecules, 1999, 32, 1152. 30 S. Adireddy, C. Lin, B. Cao, W. Zhou and G. Caruntu, Chem. Mater., 2010, 22, 1946. 31 R. Yang, J. Qu, T. Marinis and C. P. Wong, IEEE Trans. Adv. Packag. Technol., 2000, 23, 680. 32 N. Jayasundere and B. V. Smith, J. Appl. Phys., 1993, 73, 2462. 33 J. P. Calame, J. Appl. Phys., 2006, 99, 084101. 34 S. Kirkpatrick, Rev. Mod. Phys., 1973, 45, 574. 35 C.-W. Nan, Prog. Mater. Sci., 1993, 37, 1. 36 C. Pecharrom´ an and J. S. Moya, Adv. Mater., 2000, 12, 294. 37 Y. S. C.-W. Nan and J. Ma, Annu. Rev. Mater. Res., 2010, 40, 131. 38 H. Stoyanov, D. M. Carthy, M. Kollosche and G. Kofod, Appl. Phys. Lett., 2009, 94, 232905. 39 Z. M. Dang, Y. H. Lin and C. W. Nan, Adv. Mater., 2003, 15, 1625. 40 L. Wang and Z.-M. Dang, Appl. Phys. Lett., 2005, 87, 042903.
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