Hierarchical Assembly of Ultranarrow Alkylamine-Coated ZnS Nanorods

Sep 30, 2008 - L. Saviot , G. Caputo , and N. Pinna ... Shape Dependent Synthesis and Field Emission Induced Rectification in Single ZnS Nanocrystals...
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NANO LETTERS

Hierarchical Assembly of Ultranarrow Alkylamine-Coated ZnS Nanorods: A Synchrotron Surface X-Ray Diffraction Study

2008 Vol. 8, No. 11 3858-3864

Nataly Belman,† Somobrata Acharya,†,‡ Oleg Konovalov,§ Alexei Vorobiev,§ Jacob Israelachvili,| Shlomo Efrima,⊥ and Yuval Golan*,† Department of Materials Engineering, Department of Chemistry, Ilse Katz Institute of Nanotechnology, Ben-Gurion UniVersity of the NegeV, Beer-SheVa 84105, Israel, World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan, European Synchrotron Radiation Facility, Beamline ID-10B, BP 220, 38043 Grenoble, France, and Department of Chemical Engineering, Materials Department and Materials Research Laboratory, UniVersity of California, Santa Barbara, California 93106 Received July 29, 2008; Revised Manuscript Received September 3, 2008

ABSTRACT The packing of anisotropic ultranarrow nanoparticles (r e 0.5 nm) in Langmuir films was investigated for two types of nanoparticles: short ZnS wires coated with tetradecylamine and ZnS rods coated with octadecylamine. In situ grazing incidence small-angle X-ray scattering (GISAXS) revealed the formation, even under zero pressure, of ordered superstructures on the water surface consisting of alternating nanoparticles. A hierarchical “packing model” is proposed, based on GISAXS, transmission electron microscopy, thermogravimetric analysis, and grazing incidence X-ray diffraction of pure surfactant Langmuir films.

Anisotropic semiconductor nanoparticles are attracting increasing interest for a variety of reasons. They are potential building blocks for nanobased structures, devices, and materials, offering tunability of properties via control of shape and other intrinsic properties, in addition to manifestation of size-dependent quantum confinement effects. A current major challenge is the production of nanorods and nanowires of uniform shape and size, and their assembly into well-defined and useful structures or assemblies.1-6 Zinc sulfide (ZnS) is a direct wide band gap (3.91 eV) compound semiconductor that has a high index of refraction and high transmittance in the visible range and is an important material for photonic applications. Nanocrystalline ZnS, and particularly anisotropic ZnS nanoparticles, are potentially useful in * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +972-8-6461474. Fax: +972-8-6472944. † Department of Materials Engineering and Ilse Katz Institute of Nanotechnology, Ben-Gurion University of the Negev. ‡ National Institute for Materials Science (NIMS). § European Synchrotron Radiation Facility. | University of California. ⊥ Department of Chemistry and Ilse Katz Institute of Nanotechnology, Ben-Gurion University of the Negev. 10.1021/nl802287h CCC: $40.75 Published on Web 09/30/2008

 2008 American Chemical Society

new nanomaterials-based devices such as solar cells and sensors.7-11 Surfactants are widely used as capping agents in order to confine nanomaterials to a restricted size and shape. Furthermore, assembly of nanomaterials into ordered structures can be controlled by using surfactants with appropriate chain length and headgroup.1,6 Thus, a comparative study of the structure of surfactant monolayers at the air-solution interface and their role in determining the spatial assembly of nanomaterials is of fundamental scientific interest and can potentially lead to technologically useful functional assemblies at the nanoscale. Tetradecylamine (TDA, C14H31N) and octadecylamine (ODA, C18H39N) are widely used as capping agents for nanoparticle synthesis,12-16 where the spacing between the resulting alkylamine-coated nanoparticles can be varied by the surfactant chain length.15 Several studies have focused on the properties of TDA and ODA. Pressure-area (Π-A) isotherms showed that the packing density of a monolayer increases with increasing subphase pH.17,18 At pH > 10, ODA monolayers are “condensed” with a high maximum collapse pressure of ∼60 mN/m, demonstrating the stability of the monolayer on compression.19-23

Figure 1. (a) Π-A isotherm of TDA and ODA on a NaOH aqueous solution subphase at pH ) 10.5 and 22 °C. (b,c) Diffraction intensity maps of qz vs qxy obtained in GIXD for TDA and ODA monolayers at the air-solution interface (solution same as in panel a) at 22 °C, compressed to 40 and 50 mN/m, respectively. Deconvoluted projection of the measured intensity onto the qxy axis, showing Bragg peaks for (d) TDA and (e) ODA LFs. (d, inset) Bragg rod intensity profiles along the vertical scattering vector qz for the single peak of the TDA LF at qxy ) 15.1 nm-1. (e, inset) Bragg rod intensity profiles along the vertical scattering vector qz for the three ODA LF peaks at qxy ) 15.0, 15.3, and 16.1 nm-1. (f) Schematic top view representation of the undistorted hexagonal unit cell of TDA. (g) Schematic top view representation of the oblique unit cell of ODA, as deduced from the GIXD data.

Previous grazing incidence X-ray diffraction (GIXD) reports on the in-plane structure of ODA Langmuir films (LFs) at the air-pure water interface showed a phase transition as a function of surface pressure. At a surface pressure of 6 mN/m there was coexistence of two liquid condensed phases: a rectangular phase with lattice parameters a ) 0.504 nm and b ) 0.414 nm and a hexagonal phase with lattice parameter a ) 0.488 nm. Both phases were tilted to about 5°. At 14 mN/m, an untilted hexagonal structure dominated with a ) 0.483 nm, while at 30 mN/m the hexagonal arrangement coexisted with an untilted rectangular phase (a ) 0.484 nm, b ) 0.409 nm), which became dominant at 40 mN/m.24 Hence, ODA forms increasingly stable monolayers at elevated surface pressure and pH.19,23,25 Small-angle X-ray diffraction was used to study the outof-plane stacking of ODA in Langmuir-Blodgett (LB) multilayer films prepared from a subphase of pH ) 10 and showed a lamellar spacing of d ) 52.0 Å with a tilt angle of 4.5° of the hydrocarbon chains from the surface normal.20,26 On the basis of X-ray diffraction (XRD) of ODA bulk powder samples, again a lamellar spacing of 52.0 Å was determined.19 The influence of chain length and ripening time on the self-assembly of TDA and ODA on mica has been studied by atomic force microscopy, showing that the kinetics of self-assembly of linear alkylamines is a strong function of Nano Lett., Vol. 8, No. 11, 2008

the alkyl chain length.27,28 Nevertheless, the structure of TDA LFs has not been reported to-date. In the present work, TDA and ODA LFs were studied by synchrotron GIXD at the interface of air - NaOH aqueous solution fixed at pH ) 10.5. This technique allowed us to determine the structure of the surfactant LFs in situ without the need to transfer the sample onto a solid support. Similarly, the structure of uniform, monodispersed ZnS nanoparticle films capped with alkylamines was studied directly at the air-water interface using grazing incidence small-angle X-ray scattering (GISAXS). Hierarchical packing models for the TDA-coated ZnS wires, ODA-coated ZnS rods, and the two-dimensional (2D) structure of the pure surfactant monolayers on the air-water interface were proposed, consistent with nanorod superlattice dimensions that are continuously tuned by the hydrocarbon chain lengths of the amine surfactant coating layer. The compression (Π-A) isotherms of the pure TDA and ODA surfactants on a NaOH solution subphase at pH ) 10.5 and 22 °C are presented in Figure 1a. This pH was chosen since it is close to the pKb of ODA, hence the molecules are not protonated and monolayer solubility is minimal.19-22,25 Extrapolation gives a limiting area per molecule of ∼0.22 nm2 and ∼0.21 nm2 for TDA and ODA, respectively. These values are only slightly larger than the cross sectional area of a hydrocarbon chain (∼0.208 nm2),29 indicating that the 3859

Table 1. Summary of GIXD Data Obtained for TDA and ODA LFs: Miller Index for Each Reflection, Peak Positions in Reciprocal Space q, d-spacing, Average Peak Broadening FWHM(qxy), Coherence Length Lxy, average FWHM(qz) of the Bragg Rods and the Corresponding Monolayer Thickness Lz

molecules are close-packed and nearly perpendicular to the water surface. The molecules form condensed monolayers with a clear liquid expanded to close-packed transition.30 GIXD measurements of the TDA and ODA surfactant monolayers were also performed at the same air-NaOH solution interface. TDA LFs were compressed to Π ) 40 mN/m and ODA to Π ) 50 mN/m. It was observed that the resulting monolayers were close-packed and showed longrange order. The reflections were indexed by two Miller indices, (hk), that describe the repeat distances dhk ) 2π/qhk in the 2D lattice.31 A single peak at qxy ) 15.1 nm-1 in the diffraction intensity map qz versus qxy obtained for the TDA LF (Figure 1b) indicates an undistorted hexagonal packing. The ODA LF diffraction map (Figure 1c) with three distinct peaks at qxy ) 15.0, 15.3, and 16.1 nm-1, respectively indexed as (11j), (01), and (10), indicates an oblique unit cell.32-36 Projection of the diffracted intensities onto the qxy axis is useful for accurately determining the peak positions. The deconvoluted integrated intensities of the peaks verified the presence of three distinct peaks for the ODA LF (Figure 1e) and only one peak for the TDA LF (Figure 1d) with spacings consistent with the in-plane spacings we obtained from powder XRD.37 The coherence lengths Lxy of the TDA and ODA monolayers were deduced from X-ray line broadening (full width at half-maximum, FWHM) using the Scherrer equation,38 L ) 0.9 × 2π/fwhm(q), and are summarized in Table 1. The intensity profile of the observed Bragg rod near qz ) 0 nm-1 in Figure 1d inset indicates that the TDA hydrocarbon chains are aligned normal to the aqueous surface.31,32 The qz components of the three peaks obtained for ODA are shown in the inset in Figure 1e. While the largest qz component is observed for the (11j) reflection, the qz component along the (01) reflection is smaller and the (10) reflection practically peaks at zero. This indicates that the tilt of the ODA molecules lies within (or very close to) the (10) planes and that the molecules are tilted toward the [01] direction at φ ≈ 5° to the surface normal.31,32 This tilt angle is inline with the structure of ODA LB films previously reported by Takahashi20,26 and of ODA LFs at the air-pure water interface as measured by Ionov using GIXD.24 The fully extended lengths of the TDA and ODA molecules are 3860

approximately 0.20 + 0.1256 × 13 + 0.31 ) 2.14 nm and 0.20 + 0.1256 × 17 + 0.31 ) 2.64 nm, respectively.29,39 The approximate thickness, Lz, of the 2D crystalline film was calculated using the average fwhm(qz) of the Bragg rods, as summarized in Table 1. The deviation between these values and the chain lengths can be explained by the position of the amine headgroup (∼0.31 nm in diameter)29,39 under the water surface. The calculated length of fully extended tails is then ∼1.83 and ∼2.33 nm, which is close to the measured thicknesses of the TDA and ODA LFs based on the Bragg rod fwhm(qz). On the basis of the GIXD analysis, a schematic representation of the undistorted hexagonal unit cell of the TDA LF (a ) 0.482 nm, γ ) 120°) is shown in Figure 1f. Similarly, Figure 1g depicts the oblique unit cell: a ) 0.490 nm, b ) 0.465 nm, γ ) 123° of the ODA.32-35 The area per TDA molecule as calculated from the unit cell dimensions was 0.201 nm2, while the limiting area per molecule, measured from the compression isotherm in Figure 1a was somewhat larger (∼0.22 nm2). Similarly, the area per ODA molecule calculated from the unit cell dimensions was 0.192 nm2, and the ODA molecular area extrapolated from the Π-A isotherm was again slightly larger (∼0.21 nm2). The larger area per molecule obtained from the Π-A compression isotherms compared to the area per molecule deduced from GIXD is expected: GIXD gives the closepacking of the primary amine molecules within crystalline 2D domains, but these domains do not necessarily occupy the entire area of the trough with defects and grain boundaries, as well as hydration effects which can contribute to an increase in the overall area per molecule. These results are consistent with the area per molecule of ODA LFs at the air-pure water interface as a function of the surface pressure obtained from GIXD by Ionov et al. 24 It has been well established that surfactants, and specifically octadecylamines, play a crucial role in determining the size (arresting growth), the shape (by specific adsorption onto different crystal faces), and packing of nanomaterials.1,6 In this work, we describe the hierarchical structure of these anisotropic nanoparticles step-by-step: from the inorganic mineral core, through the surfactant molecules surrounding the individual particle, to their supercrystalline packing. Hence, the above comparison with 2D LF of the pure surfactant at Nano Lett., Vol. 8, No. 11, 2008

Figure 2. Bright Field (BF) TEM micrographs of (a) short nanowires of TDA-coated ZnS and (b) ODA-coated ZnS nanorods compressed to Π ) 50 mN/m. Samples were transferred from the air-water interface onto carbon-coated TEM grids. (c) Π-A compression isotherm of TDA-coated ZnS short nanowires and ODA-coated ZnS nanorods at the air-water interface. (d) Weight loss as a function of temperature in the TGA analyses of short nanowires of TDA-coated ZnS and nanorods of ODA-coated ZnS carried out in a pure, dry N2 environment. (d, insets) Schematic representation of the geometry and dimensions of (left) a short nanowire of TDA-coated ZnS and (right) a nanorod of ODA-coated ZnS. The black cylinders represent the inorganic ZnS cores, while the colored outer regions represent the surfactant coatings.

the air-water interface serves as a “baseline” for the longrange structure of the molecules in their pure form. The next step in the hierarchical assembly was the formation of two types of anisotropic ZnS nanoparticles: short ZnS wires coated with TDA and ZnS rods coated with ODA. The nanoparticles were dispersed in chloroform and spread on the air-water interface at 22 °C. Following compression of the nanoparticle films40 and transfer onto transmission electron microscope (TEM) grids, TEM analysis carried out using a Tecnai G2 TEM operating at 120 kV revealed a unique picture for both the short nanowires and the nanorods. Bright Field (BF) TEM micrographs of the TDA-coated ZnS nanowires are shown in Figure 2a. The nanoparticle dimensions were measured from TEM: nanowire width was 0.9 ( 0.1 nm and lengths were from 3 to 40 nm. The normal spacing between the nanowires was 3.4 ( 0.1 nm. ODA-coated ZnS nanorods had a narrower size and shape distribution, as shown in Figure 2b. The nanorod width was 1.0 ( 0.2 nm and length was 5.0 ( 1.0 nm. The rods assembled into highly ordered two-dimensional supercrystalline arrays organized in ribbonlike columns. Within each ribbon (width: 7.2 ( 0.2 nm) the ZnS nanorods were separated by a well-defined normal distance of 3.8 ( 0.1 nm. The corresponding Π-A compression isotherms are presented in Figure 2c and again show condensed behavior for both films. Figure 2d shows plots of the weight loss as a function of temperature during thermal gravimetric analysis (TGA) of Nano Lett., Vol. 8, No. 11, 2008

alkylamine-coated ZnS nanoparticles in dry (powder) form, performed using a Merrler TGA/SDTA 851E instrument. For both samples, the first weight loss is presumably due to water evaporation at ∼100 °C (2.6% for the TDA-coated and 0.5% for the ODA-coated nanoparticles). Since TDA has a lower melting point compared to ODA, pyrolysis of TDA-coated nanoparticles indeed started at lower temperature. The weight loss as a result of surfactant pyrolysis was 89.9% for the TDA-coated and 89.0% for the ODA-coated nanoparticles. The remaining weight, 7.5 and 10.5% respectively, corresponded to the ZnS mineral cores. TGA analysis revealed that the mass fraction of the ZnS cores is significantly smaller than that of the surfactants. This matches well with a calculated geometric model in which the dimensions of the ZnS nanoparticles (Figure 2d insets) were measured from TEM micrographs (Figures 2a,b) assuming a cylindrical shape for the nanoparticles. The ZnS cores of the TDA-coated nanowires have a radius of rZnS_wire ) 0.45 nm and the average length used for calculations was hZnS_wire ) 15 nm. For the ODA-coated nanorods, rZnS_rod ) 0.5 nm and length hZnS_rod ) 5.0 nm were measured from TEM micrographs. Considering the fully extended lengths of the TDA and ODA molecules (see above), the maximum thicknesses of the TDA and ODA coatings should then be approximately 2.14 and 2.64 nm, respectively.29,39 The overall radii of the TDA- and ODA-coated ZnS nanoparticles with their surfactant layers were: RZnS_wire+TDA ) 0.45 + 2.14 ) 2.59 nm and RZnS_rod+ODA ) 0.50 + 2.64 ) 3.14 nm. The 3861

Figure 3. Diffraction intensity maps of qz vs qxy obtained in a GISAXS experiment at the air-water interface from short nanowires of TDA-coated ZnS at (a) zero pressure and (b) compressed to Π ) 50 mN/m, and nanorods of ODA-coated ZnS at (c) zero pressure and (d) compressed to Π ) 50 mN/m. Schematic illustration depicting the superstructure formed by (e) TDA-coated ZnS wires and (f) ODAcoated ZnS rods. The schemes on top represent the side views and the bottom schemes represent the calculated top views (see TEM micrographs in Figure 2a,b).

length of the ODA-coated nanorods with the ODA surfactant was taken as the spacing between the ribbons, as measured from the TEM micrographs: HZnS_rod+ODA ) 7.2 nm, which means that the ODA surfactant contribution to this length was 2.2 nm. Since the TDA-coated nanowires were not arranged in ordered ribbons, this value could not be measured for the TDA-coated nanowires, and a similar length contribution was assumed for TDA: HZnS_wire+TDA ) (15 + 2.2) ) 17.2 nm. The insets in Figure 2d show schematics of the shapes and dimensions of a TDA-coated ZnS wire and an ODA-coated ZnS rod. Using bulk densities of FZnS ) 4.09 g/cm3, FODA ) 0.86 g/cm3, and FTDA ) 0.81 g/cm3,41 the mass fractions calculated for the surfactant coating and ZnS mineral core were ∼12% for the TDA-coated nanowires and ∼8% for the ODA-coated nanorods, in reasonable agreement with our TGA results. The calculation of the effective area per nanoparticle requires input of the exact amount of spreading solution, solution concentration, and molecular weight of the surfactant-coated nanoparticles. However, determining these parameters for nanoparticles is complicated due to possible presence of excess surfactant. Below we estimate these parameters and, consequently, the number of nanoparticles that were spread on the water surface. The trough area divided by the number of nanoparticles provides the area per nanoparticle for the x-axis in Figure 2c. The number of nanoparticles spread was obtained using the weight contribution of the ZnS mineral core relative to the overall weight, as measured by TGA. The mass of all the ZnS cores was obtained from (amount of surfactantcoated nanoparticles solution spread) × (solution concentra3862

tion) × (ZnS wt % measured from TGA) ) 85 µL × 0.5 g/L × 0.075 ) 3.2 × 106 g of ZnS mineral cores of the short nanowires, and 85 µL × 0.5 g/L × 0.105 ) 4.5 × 106 g of ZnS cores of the nanorods. The volume of all ZnS cores is equal to the mass of all the ZnS cores divided by the bulk density of ZnS, giving Vwire_cores ) (3.2 × 106) g/4.09 g/cm3 ) 0.8 × 10-6 cm3 and Vrod_cores ) (4.5 × 106) g/4.0 g/cm3 ) 1.1 × 10-6 cm3. Assuming a cylindrical shape for the nanoparticles, and using the dimensions obtained from TEM (Figure 2a,b,d), the volume of individual ZnS cores were calculated to be Vsingle_wire_core ) 9.54 nm3 and Vsingle_rod_core ) 3.93 nm3. Dividing the volume of all the ZnS nanoparticle cores spread by the volume of a single nanoparticle, we calculate that 8.2 × 1013 short nanowires and 2.8 × 1014 nanorods were spread. These values were used for calibrating the area-per-nanoparticle axis of the Π-A isotherms. By extrapolating the slopes of the Π-A isotherms in Figure 2c we obtain effective or limiting areas of ∼79 nm2 per TDAcoated ZnS short nanowire and ∼32 nm2 per ODA-coated ZnS nanorod. For the GISAXS measurements, LFs of short nanowires of TDA-coated ZnS and nanorods of ODA-coated ZnS were spread on the air-water interface at 5 °C. In the uncompressed state, the TDA-coated nanowire film did not show any GISAXS diffraction peaks (Figure 3a). After compression to a surface pressure of Π ) 50 mN/m, the GISAXS diffraction intensity map showed a peak at qxy ) 1.3 nm-1 and qz ) 0 and two high-order peaks (Figure 3b), corresponding to a fundamental interparticle spacing of d ) 4.8 nm. Nano Lett., Vol. 8, No. 11, 2008

The film of ODA-coated ZnS nanorods showed a high degree of order even in the uncompressed state, manifested by two orders of the fundamental diffraction Bragg peak at qxy ) 1.2 nm-1 and qz ) 0, which correspond to a spacing of d ) 5.2 nm (Figure 3c). This important result indicates that the nanorods form ordered islands at the air-water interface even before any compression takes place. When the nanorod film was compressed to a pressure of 50 mN/ m, the diffraction intensity map showed a peak at qxy ) 1.2 nm-1 and qz ) 0 with four additional high-order peaks corresponding to the same fundamental spacing of d ) 5.2 nm (Figure 3d). The increase in diffraction intensity and in the number of high-order diffraction peaks confirmed the high degree of order and monodispersity of the ODA-coated nanoparticles and, as expected, indicates that a larger number of ordered islands are present under the footprint of the beam at Π ) 50mN/m. We also found that by placing a droplet of chloroform suspension of nanoparticles on a carbon-coated TEM grid and allowing the solvent to slowly evaporate, the nanoparticles formed ordered arrays (not shown) similar to those formed at the air-water interface and transferred onto TEM grids (Figure 2a,b). A schematic representation of the nanowire superstructure formed on the water surface, based on the spacings derived from GISAXS and the nanowire dimensions from TEM (Figure 2d inset), is shown in Figure 3e. Since the centerto-center distance of the nanowires, (2 × 2.59 nm) ) 5.18 nm, is larger than the distance measured by GISAXS (4.8 nm), neighboring nanowires are not at the same distance from the water surface. Their center-to-center radius vector makes an angle of 22° with the water surface. Similarly, a schematic representation of the nanorod superstructure on the water surface is presented in Figure 3f with GISAXS projection distance and center-to-center distance of (2 × 3.14 nm) ) 6.28 nm, which corresponds to an angle of 34° with respect to the water surface. This packing was suggested previously by Pradhan and Efrima15 for ODA-coated ZnS nanorods and is in good agreement with our TEM and powder XRD measurements.37 Pradhan15 estimated that the angle between neighboring rods should be in the range of 31-39°, in good agreement with the model shown in Figure 3f. Synchrotron X-ray reflectivity measurements of this system (not shown) clearly indicated film roughness, supportive of this packing model. Nevertheless, we note that alternative packing schemes which involve strong interdigitation of the surfactant chains42 cannot be ruled out, although these models are unlikely based on the results of this work. The area per nanoparticle calculated from the model presented in Figure 3e was ∼83 nm2 in the case of TDAcoated ZnS wires and ∼37 nm2 (Figure 3f) for the ODAcoated nanorods. Those values are in reasonable agreement with the values measured from the Π-A isotherms of the ZnS nanoparticles (Figure 2c) of ∼79 and ∼32 nm2, respectively. The GISAXS maps indicate that the interparticle spacings scale with the length of the surfactant molecule. Thus, a larger spacing was obtained for ODA-coated nanorods Nano Lett., Vol. 8, No. 11, 2008

compared to TDA-coated nanowires. Importantly, this demonstrates that inter-rod spacings in nanorod superlattices can be tuned simply by using different hydrocarbon chain lengths of the amine surfactant. Preliminary powder X-ray diffraction studies suggest that similar criteria apply to three-dimensional supercrystalline assemblies of these materials.37 In summary, GIXD and GISAXS studies allowed us to determine the 2D structures of pure alkylamine LFs and the superstructures formed by alkylamine-coated ZnS nanoparticles at the air-water interface. Two-dimensional structures of ordered close-packed TDA and ODA LFs are indicated with an undistorted hexagonal unit cell for TDA and an oblique unit cell for ODA. In situ monitoring of the nanoparticles compression isotherms coupled with GISAXS measurements in real-time showed that ODAcoated nanorods form ordered islands at the air-water interface even before any compression takes place, that is, at zero surface pressure. Schematic “packing schemes” were proposed for the TDA-coated ZnS wires and ODAcoated ZnS rods at the water surface, consistent with nanorod superlattice dimensions that can be continuously tuned by the hydrocarbon chain lengths of the amine surfactant coating layer. Acknowledgment. The help of A.B. Panda in nanoparticle synthesis is highly appreciated. We thank A. Upcher and Y. Lifshitz for assistance in GIXD and for helpful discussions and J. Doyle for help with TGA measurements. This work was supported by the US-Israel Binational Science Foundation, Grant 2006032. We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities at beamline ID-10B. This work made use of MRL Central Facilities supported by the MRSEC Program of the National Science Foundation under award No. DMR05-20415. Note Added after ASAP Publication: Changes were made in Table 1 in the version published ASAP September 30, 2008; the corrected version was published ASAP October 22, 2008. Supporting Information Available: Detailed description of materials and methods. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Acharya, S.; Panda, A. B.; Efrima, S.; Golan, Y. AdV. Mater. 2007, 19, 1105–1108. (2) Wu, Y.; Messer, B.; Yang, P. AdV. Mater. 2001, 13 (19), 1487–1489. (3) Huang, M.; Wu, Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. AdV. Mater. 2001, 13 (2), 113–116. (4) Morales, A. M.; Lieber, C. M. Science 1998, 279 (5348), 208–211. (5) Peng, H. Y.; Zhou, X. T.; Wang, N.; Zheng, Y. F.; Liao, L. S.; Shi, W. S.; Lee, C. S.; Lee, S. T. Chem. Phys. Lett. 2000, 327, 263–270. (6) Patla, I.; Acharya, S.; Zeiri, L.; Israelachvili, J.; Efrima, S.; Golan, Y. Nano Lett. 2007, 7, 1459–1462. (7) Zhou, T. Y.; Yuan, X.; Hong, J. M.; Xin, X. Q. Mater. Lett. 2006, 60, 168–172. (8) Wang, Z.; Daemen, L. L.; Zhao, Y.; Zha, C. S.; Douns, R. T.; Wang, X.; Wang, Z. L.; Hemley, R. J. Nat. Mater. 2005, 4, 922–927. (9) Ma, C.; Moore, D.; Li, J.; Wang, Z. L. AdV. Mater. 2003, 15 (3), 228–231. (10) Ding, Y.; Wang, X. D.; Wang, Z. L. Chem. Phys. Lett. 2004, 398, 32–36. 3863

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