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2008, 112, 7557–7561 Published on Web 04/26/2008
Kinetics of PbCrO4 Nanorod Growth by Oriented Attachment at the Air-Water Interface Ranjit R. Hawaldar,† Shivaram D. Sathaye,§ Arti Harle,‡ R. S. Gholap,‡ and Kashinath R. Patil*,‡ Department of Physics, UniVersity of Pune, Pune 411007, India, Center for Materials Characterization, National Chemical Laboratory, Pune 411008, India, and 759/83 Deccan Gymkhana, Pune 411004, India ReceiVed: February 22, 2008; ReVised Manuscript ReceiVed: April 7, 2008
Oriented attachment is a new way of crystal growth to transform preformed nanoparticles into hierarchical assemblies. Here, we demonstrate the use of liquid-liquid interfaces toward the formation of PbCrO4 nanoparticles and their subsequent time-dependent self-assembly at the air-water interface into nanorods by oriented attachment. EDAX and XPS analysis indicate the formation of stoichiometric PbCrO4. TEM studies at different stages of aging reveal that the transformation from nanoparticles to nanorods is kinetically governed. HRTEM analysis indicates nanorod growth along the [110] plane. UV-visible spectra reveal the presence of peaks at 425 and 515 nm for nanorods, while for the nanoparticle sample, a single peak at 425 nm is evident. As formation of ultrathin films over a large area (typically equal to or greater than 1µm × 1µm) concomitantly accompanies this approach, it can be extended to other materials as well for nanostructured device applications. Introduction Bottom-up synthesis of nanoparticles with controlled size, shape, and crystallinity has been one of the main goals in nanochemistry, though this is only the first step toward their potential use in nanotechnology. In a subsequent step, these individual nanoparticles (building blocks) need to be arranged into well-defined hierarchical assemblies (e.g., nanoflowers, etc.) and superstructures (e.g., nanorods, nanowires, etc.) to obtain materials with unique improved properties.1 Among all of the reported procedures, one of the most promising methodologies is to exploit self-assembly-based processes.2 As all of these processes and subsequent organizations are solely governed by the foreplay of interactions between the primary building blocks, precise control of the surface properties of particles turns out to be a key issue. In the context of special superstructures, anisotropic nanostructures have aroused curiosity owing to their unique properties3 and anticipated imperative role in the fabrication of functional devices.4,5 In view of the importance of anisotropic nanostructures, it is not surprising, therefore, that a wide variety of synthetic approaches have been reported,5,6 and this currently remains a field of intense activity. However, the self-assembly of nanoparticles would offer some outstanding advantages; for example, the same nanobuilding blocks could be used for the fabrication of different nanostructures tailored for specific applications only by the control of the surface7 and, consequently, the assembly properties of the nanoparticles. Such a new way of crystal growth, different from the classical Ostwald ripening, termed “oriented attachment” was first demonstrated through the experiments of Penn and Banfield.8 Subsequent to * To whom correspondence should be addressed. E-mail: kr.patil@ ncl.res.in. † University of Pune. § 759/83 Deccan Gymkhana. ‡ National Chemical Laboratory.
10.1021/jp801577y CCC: $40.75
this, Tang et al.9 reported a fine example of spontaneous anisotropic organization of nanoparticles into luminescent nanowires and free-floating nanosheets. Similar cases of oriented attachment of preformed nanoparticles into anisotropic nanostructures were reported for materials like ZnS, CdS, MnO2, TiO2, Y2O3:Tb, PbSe, CuO, R-Fe2O3, Au, Bi2Te3, Sb2S3, PbS, hydroxyapatite, and NiSe2.10 In most cases, the lattice planes of the individual particles are almost perfectly alinged with each other in the final nanostructure. All of these experiments documenting oriented attachment, however, result from the ligand-directed assembly of particles that is, selective partial removal of the organic ligand causes individual nanoparticles to assemble in a hierarchal manner rather than by the direct interaction between the crystal planes of the particles. The only report demonstrating oriented attachment through interaction between particle cores was reported for ZnO nanoparticles.11 Moreover, the introduction of surfactants, templates, or other additives into the synthetic route undoubtedly leads to more synthetic steps and might bring impurities into the final products. Thus, developing simple template-free solution-phase methods for synthesis of 1-D nanostructures and their thin films remains a challenge. According to Pacholski et al., oxide nanoparticles are very favorable for oriented attachment as (a) organic ligands that restrict intimate contact of crystal planes are usually not needed for stabilization and (b) crystalline fusion of correctly attached particles does lead to not only a gain in lattice free energy but also a gain in the free energy of polycondensation.11 Moreover, it is also necessary to avoid random collisions between nanoparticles by Brownian motion that may lead to growth by the Ostwald ripening process rather than by oriented attachment in forming superstructures. However, at the same time, it is a prerequisite that the rearrangement of individual nanoparticles needs mobility if such an oriented attachment is to take place. One probabilistic approach to realize this is confinement of 2008 American Chemical Society
7558 J. Phys. Chem. C, Vol. 112, No. 20, 2008 nanomaterial synthesis to interface(s) having one component in a liquid form and a study of their subsequent assembly at the interfaces, without the use of organic ligands/stabilizers/ templates. For example, one particular way of achieving this is synthesis at the liquid-liquid interface and concomitant assembly at the liquid-air interface. The liquid-liquid interface reaction technique (LLIRT)12 seems to be one simple approach that fits well into these parameters. We report, in this letter, the synthesis of ligand-free PbCrO4 nanoparticles at the liquid-liquid interface by LLIRT and their subsequent oriented assembly into nanorods at the air-water interface. PbCrO4 crystallizes in a monoclinic structure and has many important applications such as a photosensitizer, photoconductive dielectric material, humidity sensor, yellow pigment, and so forth.13 Also, PbCrO4 nanoparticles/ nanorods have been previously synthesized by the hydrothermal method, the microemulsion technique, using structure-guiding agents and surfactant additives, and also by adjusting the chemical environment by changing the pH, concentration of reacting species, and so forth.14 Incidentally, to the best of our knowledge, the suitability of the interface favoring the assembly of nanoparticles into superstructures is being reported for the first time. A remarkable feature of this approach is direct formation of nanorod thin films from nanoparticulate thin films in a ligand/ template-free environment, which is a pivotal step toward realizing nanotechnology-based applications. Experimental Section All chemicals used were analytical grade and used without any further processing. For the synthesis of PbCrO4 nanoparticulate thin films, we exploited the liquid-liquid interface reaction technique (LLIRT). In a typical procedure, a solution of K2CrO4 in deionized water (10-4 M) was placed in a polypropylene tray (15 cm × 15 cm × 2 cm) so that it formed a meniscus at the edges (subphase). The surface of the subphase was divided into two compartments by a Teflon thread barrier, which was conveniently fixed on the opposite edges of the tray. Since Pb(NO3)2 is insoluble in carbon tetrachloride and slightly soluble in ethyl alcohol, it was initially dissolved in alcohol. The alcoholic solution was subsequently added to carbon tetrachloride. Adjusting the weight of Pb(NO3)2 and volumes of ethyl alcohol and carbon tetrachloride, a ∼10-5 M solution of Pb(NO3)2 in an alcohol/carbon tetrachloride mixture was prepared. At room temperature and ambient atmosphere, 0.2 mL of precursor Pb(NO3)2 in an alcohol/carbon tetrachloride (mixture) solution (∼10-5 M) was spread onto the subphase surface in one compartment of the polypropylene tray with the help of a syringe. A precipitation of PbCrO4 in a thin film form was generated at the alcohol/carbon tetrachloride subphase interface. After all of the ethyl alcohol/carbon tetrachloride (CCl4) had evaporated, the film was then compressed laterally via a Teflon thread barrier by either spreading a drop of oleic acid on the surface of the subphase in the other compartment or by moving the Teflon rod along the surface of the tray as in the LB technique. The spreading oleic acid (pressure 30 dynes/ cm) acts as a piston. The (as-formed and subsequently) compressed film on the (water) subphase surface was transferred onto a glass substrate (1 cm × 1 cm × 0.25 cm) by immersing the substrate vertically in the solution at a constant rate of 0.5 cm/min and lifting it vertically at the same rate so that the film covered the dipped area (Blodgett technique). A similar procedure was adopted to deposit the film on the TEM grid. This operation was repeated many times to get the desired film thickness. The substrates used were silicon (100) wafer,
Letters microscopic glass slides, and quartz slides. The glass and silica slides were heated at 60 °C in a solution containing H2SO4 and H2O2 (30%) in the v/v ratio of 70:30 for two hours and rinsed with water followed by drying in a flow of nitrogen before use. Silicon substrates were cleaned by mild HF etching followed by rinsing by pure water and drying in dry N2. The structural characterization of the freshly formed film by powder XRD was not possible as the small particle size of the film does not produce an observable intensity buildup. The morphology and size of as-prepared products were observed by transmission electron microscopy (TEM), carried out on a JEOL 1200-EX transmission electron microscope with an accelerating voltage of 100 kV, and high-resolution transmission electron microscopy (HRTEM) (JEOL 2010F) at an acceleration voltage of 300 kV. For TEM and HRTEM analyses, samples were prepared on a carbon-coated copper grid that was temporarily fixed on the glass substrate. The grid fixed substrate was then coated by the film by the above-described procedure by dipping it twice in the solution containing film on the surface. The film-coated grid was processed for TEM analysis. The electron diffraction facility was employed during the morphological characterization of the film for assessment of the structure and the phases present. The absorption in the UV-vis region was studied on the films deposited on the quartz substrate using a JASCO dual-beam spectrophotometer (Model V-570) operated at a resolution of 1 nm. The surface characterization of the film was also done by XPS analysis using an ESCA-3000 (VG Scientific Ltd., England) with a base pressure of better than 1.0 × 10-9 Pa. Mg KR radiation (1253.6 eV) was used as an X-ray source and operated at 150 W. Results and Discussions The nucleation of PbCrO4 takes place through the basic reaction
K2CrO4 + Pb(NO3)2 f PbCrO4V + 2 KNO3
(1)
While PbCrO4 is precipitated during the growth process, KNO3 dissolves in the aqueous subphase, yielding phase-pure nanoparticulate PbCrO4 films at the air-water interface. To establish the chemical identity of the samples, the EDAX facility in HRTEM and X-ray photoelectron spectroscopy analysis of the samples were performed. EDAX analysis during HRTEM studies suggested the formation of PbCrO4. In the EDAX spectrum of the marked circled zone (Figure 1), Pb and Cr signal are observed on the particles as well as on the nanorod, while carbon and copper signals appear from carbon film on the copper grid. Additionally, the XPS analysis of the PbCrO4 sample (Figure 2) showed typical peaks of Pb2+ (BE at 138.6 and 143.5 eV) and Cr3+ (BE at 576.6 and 586.1 eV). The binding energy of Pb4f7/2, Cr2p3/2, and O1s levels were recorded. The BE of Pb4f7/2, Cr2p3/2, O1s, and spin-orbit splitting are in agreement with the reported binding energy values of PbCrO4.15 The atomic ratio of the Pb and Cr was estimated by considering the intensities of the Pb4f7/2 and Cr2p3/2 levels and their corresponding photoionization cross sections. The ratio was found to be close to 1, supporting the formation of stiochiometric PbCrO4. Furthermore, after confirming the chemical identity of the samples to envasige the crystallinity, morphology, and the detailed mechanism of nanorod formation, the samples were examined by transmission electron microscopy analysis with an aging period (Figure 3). A low-magnification TEM image of the product synthesized at room temperature and aged for
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Figure 1. EDAX spectra of PbCrO4 after 120 h (EDAX was recorded on the circled regions of the inset picture).
Figure 2. XPS of PbCrO4 thin films aged for 5 days.
24 h reveals the presence of qausi-rectangular particles with widths ranging from 10 to 30 nm (Figure 3a). Upon aging the film at room temperature for 48 h, the nanoparticles tend to self-assemble in a linear chain, indicating kinetic governance of the phenomena (Figure 3b). Under similar conditions, aging up to 120 h revealed the transformation of the linear pearl chain of particle aggregates into smooth nanorods (Figure 3c). The SAED pattern (inset of Figure 3c) recorded on the selected nanorods shows a spot pattern, indicating that the structure of the rod is single crystal. The HRTEM image of PbCrO4 nanorods exhibits clear lattice fringes with interfringe spacing of 5.1 Å, corresponding to the interplanar distance of (110) lattice planes of monoclinic PbCrO4 (Figure 3d), indicating that the rod grows preferentially in the [110] direction. Different
Figure 3. TEM images of the as-synthesized sample obtained at different aging times at room temperature. (a) Quasi-rectangular nanoparticles after 24 h. (b) Oriented attachment of nanoparticles after 48 h. Inset: SAED pattern of the same nanoparticles. (c) Nanorods formed after 120 h. (d) HRTEM image of the nanorods for 120 h.
growth planes have also been reported with the use of surfactants/templates, but therein, the interactions between the ligand and primary nuclei decide the plane of growth. Nanorod growth along different planes, namely, [0,1,0] and [1,-1,-1], under surfactant-free conditions in the bulk phase has also been reported.13e,f In the present study, since no surfactant/ligand is used, the phenomenon of preferential blocking/growth of planes does not take place. Thus, it is not surprising that the growth direction in the present case is different than that reported
7560 J. Phys. Chem. C, Vol. 112, No. 20, 2008 previously, especially owing to the fact that growth in the present experiment takes place on the surface of the interface (twodimensional), against the growth in solution (three-dimensional). Also, it can be inferred that many faces of monoclinic PbCrO4 are prone to form rods along different crystallographic axes depending upon the reaction conditions, unlike ZnO, which displays rod formation almost along the (200) plane. The formation of uniform nanorods over their entire length suggests that dipole-induced oriented attachment (OA) of individual nanoparticles has led to the formation of nanorods. van der Waals attractive forces can be a cause of self-assembly; however, the Brownian moment of particles in solution at room temperature contradicts its stability. Nonetheless, in the present experiments, the nanoparticles being nonspherical favors polar faces to give a dipole moment. The resulting electrostatic forces are proposed to facilitate the alignment of particles into chains, as observed after 2 days, which further tend to align and coalescence to form smooth rods after 5 days. In the present case, though the nucleation is homogeneous, the growth is nonhomogenous. The alignment would obviously be diffusioncontrolled, which explains the kinetics observed in the present experiments. It has been proposed that chemical potential and ionic motion could be possible reasons for nanostructure formation during growth in solution.16 In the present case, both factors seem to be of no consequence because the growth of nanostructures is taking place in two dimensions under static conditions and the distribution of chemical species/ions is presumed to be uniform. It is proposed that such charges on PbCrO4 crystal planes are the dominant driving force for the observed oriented attachment in the experiments. Thus, the dipole moment and van der Waals attractive forces overtake the destructive Brownian motion to make stable particle chains that are formed by oriented attachment. This is further supported by the fact that reaction takes place on the surface of the subphase so that disturbance by Brownian motion can occur only in the bulk of the subphase below. However, surface tension and the size of particles in the nanometer range make sure of the stability of the chains. On the other hand, Brownian motion would give translational/rotational movement to particles, helping favorable location/orientation to form chains. Obviously, obtaining a favorable location/orientation of particles would be a function of time, which explains the kinetics of oriented attachment. Also, it is understandable that some particles show their unattached presence in TEM as those particles do not obtain a favorable location/orientation within the time of study. Conversion of OA chains to rods occurs via an atomic diffusion process; Brownian motion playing a constructive role in it. The coalescence of chains formed by OA and small particles attached thereafter occurs as the surface energy is lowered in the process. Moreover, it is necessary to understand that the growth of anisotropic nanostructures is only possible since the substrate in the present case is a liquid surface, that is, water. The presence of a liquid substrate essentially assists the coming together of initially formed individual nanoparticles by dipolar interactions to form superstructures by oriented attachment. The lateral compression of the film/ nonspherical particles also favors alignment of particles. In fact, the similarity of the “logs-on-river” and LB technique has been reported for the alignment of rods.17 We propose that lateral compression and the dipole moment lead to a modified Brownian collision and coalescence, which facilitates alignment and oriented attachment of nanoparticles.18 Because of the nanosize of the particles, the contribution of surface energy to the total energy of the system is very high, which might lead to
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Figure 4. Illustration showing different stages of the nanoparticle to nanorod transformation: (a) formation of nanoparticles at the liquid-liquid interface by LLIRT (b); compression-induced alignment when a pressure of 30 dynes/cm2 is applied by spreading oleic acid on the opposite side of the Teflon barrier; (c) oriented attachment leading to formation of linear pearl chains, owing to the dipole moment and modified Brownian motion at the air-water interface (from 24 to 48 h); (d) coalescence/fusion of linear pearl chains to form aligned smooth nanorods (after 120 h).
Figure 5. UV-visible absorption spectra of the PbCrO4 (A) nanoparticles and (B) nanorods.
coalescence of the oriented attached nanodots to nanorod formation. On the basis of our experiments, it can be inferred that pressure-induced oriented attachment has taken place in a similar manner to the alignment of nanowires. A schematic illustration of the inference from the TEM analysis demonstrating nanoparticle to nanorod transformation is shown in Figure 4. The UV-vis absorption spectrum for the as-formed PbCrO4 nanoparticle and nanorod thin films is shown in Figure 5. The nanoparticle samples display a single peak at 425 nm (curve A), whereas two peaks at 425 and 515 nm are evident for the nanorods (curve B). However, the presence of the nanorods is reflected in the features of the curve B. We attribute two peaks in the absorption to size effects. As particles transform into rods, the size increases and the shape changes, resulting in a red shift to 515 nm. Also, it may be noted that all particles do not transform into rod-shape and therefore continue to absorb at 425 nm as in curve A. The red shift in the absorption at 425 nm is in agreement with the reported literature14 and is ascribed to the presence of nanorods in the film material. Conclusion In summary, we demonstrate the formation of PbCrO4 nanorod thin films from nanoparticulate thin films at the air-water interface. Detailed kinetic studies of nanoparticle to nanorod transformation with the help of TEM analysis revealed
Letters that oriented attachment (OA) of nanoparticles having a dipole, assisted by modified Brownian motion and a subsequent merger, has led to the formation of nanorods, with times ranging from 24 to 120 h. Further studies toward achieving complete conversion of nanoparticles into nanorods and their subsequent ordered superstructure assembly for nanostructured technology are underway. We hope that the present studies open a more general route toward surmounting the formidable challenge of hierarchial organization of nanoscale building blocks into functional assemblies and, ultimately, useful systems over a large area (typically equal to or greater than 1µm × 1µm). Acknowledgment. One of the authors, R.R.H. thanks the Centre for Quantum and Nano Systems, Department of Physics, University of Pune for providing a fellowship. Supporting Information Available: Scanning slectron micrograph (SEM) of 2 dip film after 120 h of aging. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Antonietti, M.; Ozin, G. A. Chem.sEur. J. 2004, 10, 28. (2) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (3) Hu, J. T.; Odom, T. W.; Liber, C. M. Acc. Chem. Res. 1999, 32, 435. (4) Kovtyukhova, N. I.; Mallouk, T. E. Chem.sEur. J. 2002, 8, 4354. (5) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (6) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (7) Niederberger, M.; Garnweitner, G.; Krumeich, F.; Nesper, R.; Colfen, H.; Antonietti, M. Chem. Mater. 2004, 16, 1202. (8) (a) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751. (b) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (c) Penn, R. L.; Banfield, J. F. Geochim. Cosmochim. Acta 1999, 63, 1549. (9) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237.
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