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Tailored Dielectric Properties based on Microstructure Change in BaTiO3-Carbon Nanotube/Polyvinylidene Fluoride Three-Phase Nanocomposites Zhi-Min Dang,*,†,‡ Sheng-Hong Yao,‡,§ Jin-Kai Yuan,§ and Jinbo Bai§ Department of Polymer Science and Engineering, UniVersity of Science and Technology Beijing, Beijing 100083, People’s Republic of China, State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, People’s Republic of China, and Laboratoire de Me´canique des Sols, Structures et Mate´riaux, Ecole Centrale Paris, CNRS UMR8579, PRES UniVerSud, Grande Voie des Vignes, 92295 Chaˆtenay-Malabry Cedex, France ReceiVed: April 15, 2010; ReVised Manuscript ReceiVed: June 11, 2010
The carbon nanotube (CNT) has been chosen as an excellent candidate for acquiring high dielectric constant polymer matrix composites according to percolation theory. However, its nanometer-scale dimension makes it naturally form bundles, which makes it difficult to use. Compared with chemical modification of multiwalled carbon nanotube (MWNT), the incorporation of the third component (nanosized BaTiO3 (NBT)) particles into MWNT/polymer composites would realize the uniform dispersion of MWNT without sacrificing the inherent properties of MWNT. We reported a three-phase (NBT-MWNT)/polyvinylidene fluoride nanocomposite with a significantly enhanced dielectric constant (643 at 103 Hz) and a gradually decreased loss, which was extremely hard to be realized at the same time for composites only filled by conductive MWNT filler. Adjustable dielectric properties were discovered by employing the three-phase system due to the nanocomposites microstructure change. Furthermore, impedance analysis and simulated circuit confirmed the existence of microcapacitors comprised of MWNT- and NBT-rich composites. Introduction Polymeric materials with high dielectric constants are particularly appreciated for their unique combination of mechanical flexibility and tunable dielectric properties. These materials are being investigated for a wide range of potential applications such as static-charge dissipation,1,2 electromagnetic interference (EMI) shielding,3,4 actuators,5 capacitors,6 and photovoltaic devices,7,8 provided by the synergy between macromolecules and carbon-based fillers (e.g., carbon black or carbon nanotubes), which have at least one dimension in the nanometer regime. In the present work, carbon nanotubes (CNTs), which reveal an excellent mechanical strength and superior electrical and thermal conductivity values, are envisioned as revolutionary conductive filler in polymer-based composites. The CNTs reinforced composites possess much lower percolation threshold than those of composites containing conventional spherical fillers, such as metallic particles or carbon black, due to that the high-aspectratio conductive fillers are easy to produce a percolated, conducting network at much lower volume fraction. However, it has also been proven to be difficult to realize the attractive properties of the CNTs, as CNTs naturally exist as bundles because of the extremely tightly van der Waals attractions, which constitutes the current bottleneck to their application. Several efforts and methods have been devoted to improve the interfacial interactions and overall dispersion, as chemical modification of CNTs, to reduce the critical concentration of filler. For instance, Dang et al. have reported modified mutiwalled carbon nanotubes (MWNT) with 3,4,5-trifluorobromobenzene by a wet chemistry * To whom correspondence should be addressed. E-mail: dangzm@ mail.buct.edu.cn. Phone: +86-10-64452126. Fax: +86-10-64452126. † University of Science and Technology Beijing. ‡ Beijing University of Chemical Technology. § Laboratoire de Me´canique des Sols, Structures et Mate´riaux, Ecole Centrale Paris.
procedure and created a novel MWNT/polyvinylidene fluoride (PVDF) electroactive polymer composite, which displayed a giant dielectric constant (ε ≈ 8000) over a critical content of modified MWNT.9 Besides, polymers or surfactants are also usually introduced as a dispersion aid to make uniform conductive nanotube-filled polymer composites. Grunlan et al. adopted Gum Arabic as an effective stabilizing agent for CNTs,10 and a percolation threshold well below 0.1 wt % was demonstrated. To reach desired elastic modulus values and maintain the flexibility, it is essential for the polymer composites to exhibit a low percolation threshold. However, these are not ideal ways to improve the dielectric constant and electrical conductivity of composites. The atomic structural perfection of the nanotube might be impaired through chemical modification;11,12 meanwhile, the additional impurities may be introduced into the matrix, and the contacts between nanotubes can be degraded as the presence of a dispersing agent.13 More recently, a few studies have been dedicated to focus on the secondary particles, which have been used to improve electrical properties of the polymer composites containing conductive fillers.14-17 Liu et al. introduced clay into singlewalled carbon nanotube (SWNT)/epoxy composites, and with just 0.05 wt % SWNT and 0.2 wt % clay, electrical conductivity is increased by more than 4 orders of magnitude. The addition of clay could improve the dispersion of nanotubes without harming electrical or mechanical performance.16 Meanwhile, Shen et al. reported a new approach that used core/shell hybrid particles with metal Ag cores coated by organic dielectric shells as fillers to form a continuous interparticle-barrier-layer network and retain a stable high dielectric constant and low loss. The basic idea behind these works is to control the arrangement of the fillers in order to tailor the conductive network and consequently endow the composites with desired properties.18
10.1021/jp103411c 2010 American Chemical Society Published on Web 07/20/2010
BaTiO3-CNT/PVDF Three-Phase Nanocomposites In this work, we investigate the influence of nanosized barium titanate (NBT) with high dielectric constant on microstructure and dielectric properties of the MWNT/PVDF electroactive polymer materials without any functionalization of MWNT. According to the classical percolation power law principle, an abrupt variation of dielectric constant can only be achieved when the filler concentration is nearly close to the percolation threshold, which induces much challenge and risk in controlling the percolative composite, because of the fact that the dielectric property changes are very sensitive to the variation of composition, and by then the leakage current becomes remarkable accompanying with the increase of dielectric loss, both of which are not desirable for the practical applications. In addition, the effective dielectric constant (εeff) increases very slowly with the addition of ceramic fillers for ceramic/polymer composites. High loading of the ceramic fillers, usually over 50 vol %, can increase the εeff by about 5-10 times relative to the polymer matrix. Unfortunately, the high loading of fillers always brings out the high porosity and involves a mechanical deterioration. For example, Dang et al. obtained BaTiO3/PVDF composites with a dielectric constant of 120 when the volume fraction of BT is about 68 vol % with apparent microcracks between BT particles and PVDF polymer.19 Therefore, it would be of great interest to combine the advantages of carbon nanotubes and ceramic particles. Experimental Section MWNT was synthesized by a chemical vapor deposition method, having an aspect ratio 25-100, provided by Shenzhen Nanotech Port Company, China. Its diameter and length were about 20-40 nm and 1-2 µm, respectively. NBT fillers with an average particle size of 100 nm were provided by Hebei XiongWei Chemical Company. PVDF, which was chosen as polymer host due to its superior ferroelectric nature, was provided by Shanghai 3F New Materials Company. The (NBT-MWNT)/PVDF three-phase composites were prepared by suspending the appropriate amount of MWNT and NBT powders in organic solvent [N,N-dimethylformamide (DMF)] with ultrasonic treatment for 30 min. At the same time, the PVDF powder was also dissolved in DMF. Then, the suspension of fillers in DMF was added into PVDF solution, and the mixture solution was stirred by ultrasonic treatment for 40 min and further magnetic stirring for 6 h. The three-phase composites were obtained by precipitation of the mixed DMF solution with appropriate compositions into deionized water, and samples for characterization were made by molding the dried precipitate (by centrifugal procedure at 4000 rpm for 10 min and drying in oven for 4 h) at 200 °C and 15 MPa. The final samples with a disk-shape were 12 mm in diameter and around 1 mm in thickness. The microstructure of the (NBT-MWNT)/PVDF three-phase nanocomposites was characterized by transmission electron microscopy (TEM, Hitachi H-800). For dielectric measurements, electrodes were painted on both sides using silver paste, and then the alternating current (AC) electrical properties of the samples were measured using an impedance analyzer (Aglient 4294A) in the frequency ranges from 102-107 Hz at room temperature. The impedance magnitude and the impedance phase were measured by using an impedance analyzer (Solarton 1260) in a broader frequency range from 10 to 107 Hz at room temperature. Results and Discussion In our multiphases nanocomposites, we chose a MWNT/ PVDF composite as our matrix. As predicted by the percolation
J. Phys. Chem. C, Vol. 114, No. 31, 2010 13205 theory, the εeff of the MWNT/PVDF composites consisting of the conductive MWNT filler and the insulating PVDF matrix is given by
εeff ∝ εb(fc - fMWNT)-s
(1)
Where εb is the dielectric constant of the PVDF matrix, fMWNT is the volume fraction of MWNT component, fc is the percolation threshold of compsoites (fc ) 0.50 in two dimensions and 0.16 in three dimensions), and s denotes a critical exponent in the insulating region. For the MWNT/PVDF composite, as reported in our previous study,20 its percolation threshold is usually around 0.015. Since the electric properties change significantly in the neighborhood of the percolation threshold, the volume concentrations of MWNT are kept as constant at fMWNT ) 0.01(lower than fc) and 0.02 (higher than fc), and the corresponding composites are defined as matrix I and matrix II, respectively. The conductive MWNT raises the εeff of the PVDF matrix via the percolation effect, while the NBT particles with the high dielectric constant enhances εeff of the MWNT/ PVDF composite by altering the conductive network already formed by MWNTs. In addition, we have reported research result about (BaTiO3-MWNT)/PVDF three-phase composite with 0.01 MWNT loading and the data is partly quoted here for a comparison.21 Figure 1 presents the εeff, AC conductivity, and dielectric loss of the (NBT-MWNT)/PVDF three-phase composites with two matrices as a function of frequency measured at room temperature. Compared with the reported results20 (see Figure 2a in ref 20), the εeff of matrix I, 17 at 103 Hz as shown in Figure 1a, is still lower than the lowest value (∼20) of dielectric constant obtained via a simple solution method at the same fMWNT.20 In contrast, the εeff of matrix II, 338 at 103 Hz as observed, is much higher than the highest one (∼280) obtained by solution method.20 Both phenomena can be mainly attributed to the distinct conductive network formed by MWNTs. Namely, the MWNT dispersion from the precipitation method is better than that from the solution method, which can be concluded by comparing the TEM images in our paper with those in ref 20. Because that dispersion state influences the dielectric performance of composites significantly, we can expect the similar variation of dielectric constant could appear after addition of NBT through altering the MWNTs dispersion state. It can be observed from Figure 1a that at every MWNT concentration the nanocomposites with NBT show higher dielectric constant than that without NBT. The maximum value of the dielectric constant for the three-phase nanocomposite with matrix II is about 924, at 100 Hz, which is about 90 times more than that for the pure PVDF polymer (ca. 10). The abundance of interfaces induced by emergence of NBT particles should be responsible for this high εeff. For matrix I, the dielectric constant of three-phase nanocomposite does not change remarkably with increasing frequency; however, matrix II present a strong dependence on frequency and becomes most significant when the fNBT is around 0.20. This phenomenon could be ascribed to the conductive characteristic of the composites, that is to say, weak dielectric constant frequency dependence of composites with matrix I is due to their insulator nature, while the strong one in composites with matrix II should arise from their conductor nature. That becomes reasonable after a comparison with the results of other conductive fillers loaded composites.6 Figure 1b presents the frequency dependence of AC conductivity; it increases significantly with increasing frequency for the three-phase nanocomposites with matrix I from
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Figure 1. (a) Dielectric constant, (b) AC conductivity, and (c) dielectric loss as a function of frequency from 102 to 107 Hz, measured at room temperature for the (NBT-MWNT)/PVDF three-phase composites with different volume concentration of NBT.
102-105 Hz; while for the nanocomposites with matrix II, it is almost independent of frequency in the same frequency range, which highlights the consistent inherent character of composites with that obtained from dielectric property. Of interest to note, the dielectric loss always exhibits rather low values relative to the high dielectric constant. For the composites with matrix I, the maximum value of loss is less than 0.60 in the measured frequency range,21 and for the nanocomposites with matrix II, the loss is lower than 3 with the dielectric constant as high as 507.9 at 104 Hz, with fNBT ) 0.15 (Figure 1c). The combination of high dielectric constant and low loss in this work is among the best ever reported.16,18 Figure 2a reveals that the dependence of dielectric constant of the nanocomposites on the volume fraction of NBT. The εeff of the three-phase nanocomposites, with two kinds of MWNT concentration, increases with the fNBT, and the variation becomes rapid at around fNBT ) 0.15 for the matrix I and fNBT) 0.10 for the matrix II, respectively. The variation of εeff with fNBT looks similar to the percolative composites, where a remarkable increase in εeff always occurs (usually several orders of
Dang et al.
Figure 2. (a) Dielectric constant, (b) dielectric loss (left) and AC conductivity (right) as a function of the volume fraction of NBT measured at room temperature and 103 Hz, and (c) the variation of log εeff vs log ω for the (NBT-MWNT)/PVDF three-phase composites with fNBT ) 0.15 for matrix I and fNBT ) 0.05 for matrix II, respectively. The straight lines have slopes of -0.163 and -0.357, respectively.
magnitude higher than the insulator matrix) near fc as demonstrated by eq 1 induced by the inclusion of conductive fillers. Here, the εeff increases from 17 to 123 for the three-phase nanocomposites with matrix I and from 338 to 643 for the threephase nanocomposites with matrix II, respectively. However, this quasipercolation threshold (fc(NBT) ) 0.15, 0.10, respectively) of the (NBT-MWNT)/PVDF three-phase nanocomposite arises from the addition of high dielectric ceramic filler and approaches the universal value (fc ≈ 0.16) for spherical particles as fillers.10,20 Another exceptional feature of our results is that the dielectric loss of the three-phase nanocomposites with matrix II does not remain stable at a high value but abruptly drops as the fNBT increases especially beyond fc(NBT) (Figure 2b), which is completely different from common percolative composites where the large leakage current caused by the direct connection between the conductive fillers leads to a significant rise of loss at fNBT > fc(NBT). This variation of εeff and dielectric loss in such nanocomposites with matrix II could be interpreted by three main factors. First, a large number of effective three-dimensional conductive networks formed by MWNTs (fNBT > 0.05) can be destroyed because of the addition of NBT particles, and this change is confirmed by consistent decrease of conductivity and
BaTiO3-CNT/PVDF Three-Phase Nanocomposites
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Figure 3. TEM images of the freeze-fractured (NBT-MWNT)/PVDF composites with matrix II at (a) fNBT ) 0, (b) fNBT ) 0.05, (c) fNBT ) 0.20, and the schematic illustration of the microstructure of the (NBT-MWNT)/PVDF composites with matrix II at (a′) fNBT ) 0, (b′) fNBT ) 0.05, (c′) fNBT ) 0.20, in which the gray lines stand for the MWNTs and the green balls are NBTs.
loss, as presented in Figure 2b. In addition, NBT particles seems to not only destroy the networks but also somewhat improve the dispersion of MWNT when NBT loading is low, which is in favor of a better connection among the clusters of MWNT, as verified by the increase of conductivity when fNBT < 0.05. Second, the model of the interfacial polarization also should take charge of the abrupt increase of dielectric constant, which arises from the significant blockage of charge carriers at internal interfaces (namely, the MWS effect), because of the existence of a large number of interfaces among MWNT, NBT, and PVDF. To estimate the effect of interfacial polarization, we fit the dielectric constant and frequency to eq 2 according to percolation theory
εeff ∝ ωu-1
(2)
where ω is the angular frequency (equal to 2πf) and u is a critical exponent, always between 0 and 1. Fitting the dielectric constant data of three-phase composites with fNBT ) 0.15 for matrix I and fNBT ) 0.05 for matrix II in Figure 1a, respectively, to eq 2 yields u ) 0.84 for matrix I and 0.64 for matrix II (obtained from the slopes of fitting lines in Figure 2c), which are in the neighborhood of the theoretical value u ) 0.70 predicted by the percolation theory,22,23 indicating the effective influence of space charge polarization on the dielectric response in both three-phase composites systems. Finally, the microcapacitor model is proposed to be responsible for high dielectric constant and low dielectric loss. In these nanocomposites, both of the ceramic NBT particles and PVDF material are electrically insulated, while the MWNT are conductive. For the percolative composites, the MWNT particles are isolated by polymer matrix as a thin dielectric layer, which is identical to the case where a large number of nanocapacitors are connected with each other, and can be described by the relationship between capacitance C and the dielectric constant shown as
C ) ε0εrA/t
(3)
where ε0 is the dielectric constant of the free space (8.85 × 10-12 F/m), A is the area of the electrical conductor, t is the thickness of the insulator layer, and εr is the dielectric constant of the insulator layer. On one hand, as the ffiller increases, it is expected that the thickness t of insulation layer between the conductive particles decreases, which causes the augment of the effective capacitance of both the microcapacitors and whole composite, and then the enhancement of composite’s dielectric constant due to the proportional relationship of the capacitance and dielectric constant as shown in eq 3. On the other hand, by replacing the polymer with NBT-PVDF blend, the εr of the insulator layer increases, which is implying that the dielectric constant of the three-phase nanocomposite will be further increased. Meanwhile, there are isolated CNT-rich regions along with a vast majority composite that would be largely NBTrich, which forms an interparticle barrier layer nanocapacitor network, thus preventing the MWNTs from directly connecting to each other, the formation of conductive paths, and the increase of dielectric loss. These explanations can be confirmed by variations of microstructures in the three-phase nanocomposites as presented in Figure 3. TEM images of the fractured two-component and threecomponent composites with a fixed fMWNT ) 0.02 and different fBT are shown in Figure 3. In the absence of NBT, MWNTs trend to be poorly dispersed and a few of aggregated MWNT bundles are observed (Figure 3a). Then, a more effective threedimensional conductive network is formed because of the additional connection of the MWNT bundles (as indicated by arrows in Figure 3b) when fNBT increases up to 0.05. In contrast, the image of nanocomposites with fNBT ) 0.20 shows that a significantly improved MWNT dispersion is realized, and MWNT particles (as pointed by arrows in Figure 3c) are separated so well by NBT-rich areas that the aggregated MWNT bundles mentioned above could not be seen any more. Meanwhile, plenty of microcapacitors with polymer and NBT as a medium between the MWNT plates are formed simultaneously (see Figure 3c). To ensure this excellent dispersion of NBT and MWNT, as the density divergence exists between NBT and MWNT (5.85 and 1.89 g/cm3, respectively), a coagulation
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Dang et al. TABLE 1: Simulated Values of Each Element According to the Data and Equivalent Circuit in Figure 4 fBT 0 0.05 0.10 0.15 0.20
Figure 4. (a) Cole-Cole plot, (b) the imaginary part of complex impedance as a function of frequency, and (c) an equivalent circuit for the (NBT-MWNT)/PVDF three-phase composites with matrix II and different fNBT.
method for composite preparation is employed, in which the homogeneously mixed solution after ultrasonic procedure is rapidly precipitated into a large amount of water to avoid the reaggregation of NBT and MWNT. Parts a′-c′ of Figure 3 give schematic images of the microstructure of the (NBT-MWNT)/ PVDF nanocomposites corresponding to the TEM images in parts a-c of Figure 3, in which the threads and solid circles represents the MWNT and NBT particulate, respectively, and the region in dash circles shows interfaces and microcapacitor structure. From the microstructure evolution processes, a critical concentration of NBT particles (fNBT ) 0.05) is found. Before fNBT ) 0.05, additional conductive network of MWNT bundles is induced (see Figure 3b′), but after that the conductive network is interrupted rapidly with increasing the fNBT(see Figure 3c′). Recently, although several theoretical studies have focused on the relationship between dispersion states of CNT and electrical properties of nanocomposites, such as an excluded volume approach and the limiting concentration model,20,25 it is still hard to realize the quantitative analysis of the microstructure corresponding to various properties. In the present study, AC impedance spectroscopy (AC-IS) is employed to investigate the microstructure, which is always expected to be used widely in electrochemistry field. To our knowledge, this is the first report on the AC-IS response of the (NBT-MWNT)/ PVDF three-phase nanocomposites. Figure 4a presents the Cole-Cole plots with frequency increasing from right to left
R1 (Ω) 3294 52.79 2322 5306 10435
CPE1 (F) -9
3.758 × 10 1.010 × 10-9 4.090 × 10-9 2.319 × 10-9 5.015 × 10-9
R2 (Ω)
CPE2 (F)
2223 1214 1842 3645 9204
9.534 × 10-9 1.4787 × 10-8 1.2016 × 10-8 1.3978 × 10-8 1.2359 × 10-8
to describe the electrical behavior of the (NBT-MWNT)/PVDF three-phase nanocomposites with matrix II. In the highfrequency region, they are shown as semicircles indicating the existence of interparticle current flow and the polarization resistance (double layer capacitance on the fiber surfaces),26 whose diameters correspond to the bulk resistivity of the components in the composite. With enhancement of the NBT loading, the variation of this bulk resistivity is identical to Figure 1b. Furthermore, this phenomenon of each nanocomposite has been confirmed by other conductive filler/polymer composite, such as carbon black reinforced EPDM rubber.27 Figure 4b exhibits the frequency dependence of the imaginary part of complex impendence, which normally carries a negative sign as the system is capacitive. It is noticeable that the maximum of each plot shows a significant increase when increasing the fNBT from 0.05 to 0.20, which confirms the breakage of the conducting paths and the formation of microcapacitors as the incorporation of NBT. Simultaneously, the shift of the peak to low frequencies in the figure indicates the densification effect of BT when blended with matrix II.28 In addition, the equivalent circuit, shown as in Figure 4c, gives a more direct and easy understanding of the conduction processes involved in this threephase nanocomposites. On the basis of the physical and chemical process proposed by several researches,26-29 the equivalent circuit is presented as an inductance element, which presents the intrinsic electrical properties of MWNT, and two resistorcapacitors in parallel for our composites. The variation of each element value is shown in Table 1 with the increase of fNBT. It should be noted that for the change of resistor value, R1 and R2, we obtain the lowest resistance at fNBT ) 0.05, which fits well with the variation of conductivity in Figure 2b. Meanwhile, the value of constant phase elements 2 (CPE2) is an order magnitude lager than that of CPE1, which illustrates the much more effect of the interfacial polarization on dielectric constant than that of conductive network. Here, the R2 and CPE2 in the circuit of Figure 4c represent the polarization of the interfacial layer between fillers and PVDF, and the R1 and CPE1 represent the resistance and capacitance produced by the polarization of fillers at high frequency, respectively. Conclusions The addition of NBT particles effectively improves the dispersion of MWNT in the (NBT-MWNT)/PVDF nanocomposites with fixed MWNT concentrations. A quasipercolative enhancement in the dielectric constant is obtained as the concentration of NBT fillers increases, and a critical NBT concentration of 5 vol % is found. When the volume concentration of NBT is 0.20, the uniform dispersion of MWNT in matrix can be realized, and the dielectric constant enhances significantly with the loss tangent decreasing abruptly. The above study can shed light onto realizing both the high dielectric constant and low dielectric loss by adding the secondary ceramic particulates into percolative composite. Furthermore, an equivalent circuit of an inductance element and two parallel RC in series is established to quantitatively describe the electrical nature in the
BaTiO3-CNT/PVDF Three-Phase Nanocomposites three-component nanocomposites, and the extracted data fit well with the microstructure analysis from the physical comprehension view. The present approach has potential to quantitatively describe microstructures of composites filled by conductive fillers, thus realizes the fabrication of properties-tailored materials. Acknowledgment. This work was financially supported by NSF of China (Grant No. 50977001), State Key Laboratory of Power System (SKLD09KZ03), the Ministry of Sciences and Technology of China through 863-project (Grant No. 2008AA03Z307), Program for New Century Excellent Talents in University (NCET), and the Project-sponsored by SRF for ROCS, SEM. References and Notes (1) Kwon, J.; Kim, H. J. Polym. Sci., Part A 2005, 43, 3973. (2) Yu, A.; Hui, H.; Bekyarova, E.; Itkis, M. E.; Gao, J.; Zhao, B.; Haddon, R. C. Compos. Sci. Technol. 2006, 66, 1190. (3) Li, N.; Huang, Y.; Du, F.; He, X.; Lin, X.; Gao, H.; Ma, Y.; Li, F.; Chen, Y.; Eklund, P. C. Nano Lett. 2006, 6, 1141. (4) Yang, Y.; Gupta, M. C.; Dudley, K. L.; Lawrence, R. W. Nano Lett. 2005, 5, 2131. (5) Landi, B. J.; Raffaelle, R. P.; Heben, M. J.; Alleman, J. L.; VanDerveer, W.; Gennett, T. Nano Lett. 2002, 2, 1329. (6) Dang, Z. M.; Lin, Y. H.; Nan, C. W. AdV. Mater. 2003, 15, 1625. (7) Kymakis, E.; Amaratunga, G. A. J. Appl. Phys. Lett. 2002, 80, 112. (8) Baibarac, M.; Gomez-Romero, P. J. Nanosci. Nanotechnol. 2006, 6, 289. (9) Dang, Z. M.; Wang, L.; Yin, Y.; Zhang, Q.; Lei, Q. Q. AdV. Mater. 2007, 19, 852.
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JP103411C
