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Self-Assembled Colloidal Superparticles from Nanorods

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Science  19 Oct 2012:
Vol. 338, Issue 6105, pp. 358-363
DOI: 10.1126/science.1224221

Abstract

Colloidal superparticles are nanoparticle assemblies in the form of colloidal particles. The assembly of nanoscopic objects into mesoscopic or macroscopic complex architectures allows bottom-up fabrication of functional materials. We report that the self-assembly of cadmium selenide–cadmium sulfide (CdSe-CdS) core-shell semiconductor nanorods, mediated by shape and structural anisotropy, produces mesoscopic colloidal superparticles having multiple well-defined supercrystalline domains. Moreover, functionality-based anisotropic interactions between these CdSe-CdS nanorods can be kinetically introduced during the self-assembly and, in turn, yield single-domain, needle-like superparticles with parallel alignment of constituent nanorods. Unidirectional patterning of these mesoscopic needle-like superparticles gives rise to the lateral alignment of CdSe-CdS nanorods into macroscopic, uniform, freestanding polymer films that exhibit strong photoluminescence with a striking anisotropy, enabling their use as downconversion phosphors to create polarized light-emitting diodes.

Directional bonding interactions dictate the structural complexity and functional specificity of naturally occurring materials at all length scales (14). On the nanometer scale, anisotropic nanoparticles are used to design directional bonding interactions through shape-specific (35) and surface-specific functionalization (6, 7). Because anisotropic nanoparticles exhibit shape-dependent physical and chemical properties (8, 9), the self-assembly of these nanoparticles can lead to metamaterials with important collective properties (3) such as spin-dependent electron transport (10), vibrational coherence (11), enhanced conductivity (12), and tandem catalysis (13). We report that anisotropy-driven self-assembly of CdSe-CdS core-shell semiconductor nanorods produces three-dimensional (3D) colloidal superparticles with multiple well-defined supercrystalline domains or needle-like superparticles with a single supercrystalline domain. The needle-like superparticles can be unidirectionally aligned and further assembled into centimeter-scale, uniform, freestanding polymer films, exhibiting a photoluminescence (PL) anisotropy ratio larger than that of single CdSe-CdS nanorods.

Colloidal CdSe-CdS superparticles were prepared using a previously described method with minor modifications (14), including two major steps: (i) synthesis of water-soluble nanorod micelles and (ii) growth of superparticles from nanorod micelles in an aqueous solution of ethylene glycol (Fig. 1A). We used highly fluorescent CdSe-CdS nanorods primarily capped with octylamine and octadecylphosphonic acid (ODPA), prepared via a literature method (15, 16), as precursors for making nanorod micelles (figs. S1 to S3). In a typical superparticle synthesis (17), a clear nanorod-micelle aqueous solution was prepared by mixing a chloroform solution of CdSe-CdS nanorods with length l = 28.0 ± 1.5 nm and diameter d = 6.7 ± 0.3 nm (10 mg/ml, 1 ml) with an aqueous solution (1 ml) containing varying amounts of dodecyl trimethylammonium bromide (DTAB), followed by bubbling Ar to evaporate chloroform. Under vigorous stirring, the nanorod micelle solution was injected into a three-neck flask with ethylene glycol (5.0 ml), causing the decomposition of nanorod micelles as a result of the loss of DTAB molecules into the growth solution. This led to the aggregation of nanorods and the eventual formation of superparticles (Fig. 1A). After stirring for 10 min, an aqueous solution of dithiol-functionalized Tween-20 (0.1 mM, 1.0 ml) was added into the growth solution to stabilize the superparticles (14). The resulting superparticles were isolated and purified by centrifugation and redispersed in polar solvents (such as water, ethanol, and ethylene glycol) at a variety of concentrations.

Fig. 1

(A) Scheme for the synthesis of superparticles from CdSe-CdS nanorods: (i) nanorod-micelle formation, (ii) superparticle formation. Numbered images: (1) Proposed model for a CdSe-CdS nanorod functionalized with ODPA and octylamine (17); (2) proposed model for a nanorod micelle prepared using DTAB; (3) proposed model for a double-domed cylinder; (4) proposed model for an irregular-multidomain particle. (B to P) TEM images of superparticles made from nanorods of l = 28.0 ± 1.5 nm and d = 6.7 ± 0.3 nm.

Previous studies have shown that the amount of DTAB affects the kinetics of superparticle formation from isotropic spherical nanocrystals and thus can be used to achieve a size-controlled synthesis of superparticles (18), and we apply this to the synthesis of superparticles from nanorods. Accordingly, we have prepared CdSe-CdS superparticles with an average size ranging from 180 ± 23 nm to 1100 ± 150 nm by varying the amount of DTAB in nanorod micelle solutions (18). Low-magnification transmission electron microscope (TEM) images show that the resulting superparticles are nearly spherical (fig. S4), whereas higher-magnification images from tilting experiments reveal that these superparticles exhibit multiple supercrystalline domains with configurations and sizes that are dependent on the total number (N) of constituent nanorods inside each superparticle (Fig. 1, B to P, and fig. S5).

When N is less than ~80,000, superparticles typically exhibit a morphology of double-domed cylinders, wherein the cylindrical domain occupies a volume larger than that of the corresponding domes (Fig. 1, B to K). The cylindrical domain adopts a lamellar structure that consists of stacked multilayer disks formed from nanorods via lateral association, where the thickness of each disk is consistent with the length of a single nanorod. With an increase of N, the radius of the cylindrical domain increases, and so does the number of stacked-disk monolayers but with some fluctuations (Fig. 1, B to K). For example, the superparticle shown in Fig. 1G possesses more constituent rods than that shown in Fig. 1F, but the cylindrical domain of the superparticle in Fig. 1F has one more stacked-disk monolayer. When N is larger than ~80,000, CdSe-CdS superparticles appear as either irregular-multidomain particles (Fig. 1, L, M, and O) or double-domed cylinders (Fig. 1, N and P).

TEM tilting experiments further reveal that the domes in a double-domed cylinder typically consist of three supercrystalline domains together forming a curved, arc-shaped architecture (Fig. 2, A to D, and figs. S5 and S6). The middle domain possesses a lamellar structure that comprises stacked multilayer arches formed from close-packed nanorods lying perpendicular to the rods in the cylindrical domain, and the thickness of each arch is nearly identical to the length of a single nanorod (Fig. 2D). The two small side domains appear as closely packed dots, indicating that the constituent nanorods are aligned parallel to the direction of the electron beam (Fig. 2D).

Fig. 2

(A) Proposed double-domed cylinder model in TEM tilting experiments. (B to D) TEM images of a double-domed cylinder superparticle at different tilting angles, as indicated. (E to H) SAED patterns taken from the double-domed cylinder superparticle at the selected areas labeled with colored circles in (C). A [0001] zone ring pattern of a wurtzite CdS crystal shown in (E) indicates that the nanorods in the small side domain of the double-domed cylinder’s dome are arranged randomly at the atomic level, whereas their long axes are aligned parallel to the electron beam. Two sets of diffraction dots from the Embedded Image and Embedded Image zones, shown in (F) and (G), also suggest that the nanorods are arranged randomly at the atomic level in the selected areas, whereas their long axes are aligned in parallel along the direction of the two (0002) diffraction dots. (I) TEM image of an irregular-multidomain particle. (J to L) SAED patterns taken from this irregular-multidomain particle at the selected areas indicated in (I). (M) The integrated data from the SAXS pattern (inset), wherein the color scale indicates the intensity. This diffraction pattern characterizes a 1D lamellar structure with peak positions corresponding to Bragg reflections on planes specified by Miller indices 01, 02, 03, and 05 (indicated in red), and a 2D hexagonally packed structure with Bragg reflection peaks on planes specified by Miller indices 10, 11, 20, and 21 (indicated in blue). See table S1 for the values of interplanar spacing associated with each peak. (N) Schemes of the 1D lamellar and 2D hexagonally packed structures created from nanorods.

These TEM observations are consistent with selected-area electron diffraction (SAED) measurements, which also provide structural information of nanorod packing inside a superparticle (19). The SAED data show that although the alignment of nanorods in this superparticle is random at the atomic level (largely because of the rotational freedom around their long axes), the long axes of the nanorod constituents within a given domain are parallel to each other, whereas the orientation of long axes of nanorods from neighboring domains is perpendicular (Fig. 2, E to H). Taken together, these results further suggest that a typical double-domed cylinder consists of seven supercrystalline domains where the nanorods of two neighboring domains are arranged in an orthogonal configuration (Fig. 2, A to H). In contrast, although irregular-multidomain particles exhibit multiple lamellar supercrystalline domains, the long axes of constituent nanorods in two neighboring domains are not at right angles (Fig. 2, I to L).

TEM observations further show that the orientations of the nanorods in domains from different domes of a double-domed cylinder do not correlate with each other (Fig. 1, B to I), thereby suggesting that the domes have degenerate free energy levels for lying at different rotational angles along the double-domed cylinder’s axis. The supercrystalline structure of double-domed cylinders was further characterized using synchrotron-based small-angle x-ray scattering (SAXS). The SAXS data show that these double-domed cylinders do not adopt perfect 3D superlattices, but instead exhibit both 1D and 2D supercrystalline orders along directions parallel and perpendicular to the long axis of constituent nanorods, respectively (Fig. 2, M and N). The 1D superlattice possesses a lamellar period of 31.1 ± 0.2 nm, and the 2D ordered structure adopts a hexagonal lattice with a unit cell constant of 8.7 ± 0.1 nm (17) (table S1). These results further suggest that the end-to-end distance between neighboring constituent nanorods is 3.1 ± 1.5 nm, confirming that the long axes of nanorods are parallel to the lamellar normal as observed from TEM. In addition, the side-by-side distance of neighboring rods is 2.0 ± 0.5 nm, which is about twice the length of an octylamine ligand, thus indicating that the long chains of ODPA ligands are intercalated between neighboring nanorods.

Superparticles with multiple supercrystalline domains are mesoscopic analogs of multiply twinned particles with atomic lattices (20). The formation of multidomain superparticles may occur through two different mechanisms: (i) a nucleation-growth process in which larger supercrystalline particles are created from the growth of smaller particles or (ii) an embryo-crystallization process wherein nonsupercrystalline embryos are formed by decomposition of nanorod micelles, after which colloidal crystallization of nanorods occurs inside the embryos to form multiple supercrystalline domains (fig. S7A). To distinguish between these formation mechanisms, we performed a superparticle synthesis with two separate injections of nanorod micelles (17). Our results show that the second injection did not increase superparticle size, but instead increased the amount of particles (fig. S8), thus favoring the embryo-crystallization mechanism (17). This formation mechanism is also consistent with our previous results from the synthesis of superparticles with spherical nanoparticles, showing that supercrystalline particles form via a crystallization process from particles without a long-range supercrystalline order (18).

We propose that the colloidal crystallization of nanorods inside an embryo is under thermodynamic control. Hence, the surface morphology and constituent nanorod packing configuration of a superparticle adopt an equilibrium structure proposed by a modified Wulff construction (21), wherein this superparticle has a minimized Gibbs free energy (G): G = Gb + S · γ (where Gb represents the bulk Gibbs free energy, and S and γ are the surface area and surface tension of the superparticle, respectively). The surface free energy is determined by the repulsive solvophobic interactions between the superparticle and the surrounding water and ethylene glycol molecules (14), whereas Gb is determined by anisotropic interactions between CdSe-CdS nanorods (including van der Waals and dipole interactions) that favor formation of a single-layer disk with hexagonal close-packed nanorods (14, 22). Because these anisotropic interactions exhibit an energy level comparable to the repulsive solvophobic interactions, the interplay of the bulk and surface free energy minimizations governs the formation of multiple supercrystalline domains that consist of stacked multilayer structures of hexagonal-close-packed nanorods (Figs. 1, 2, and 3A).

Fig. 3

(A) Left: Proposed double-domed cylinder model with seven supercrystalline domains. Right: Top view of a dome and a schematic diagram of the 2D nanorod packing in the bottom layer of the dome; the angles ϕ and θ are defined. (B) Calculated Φ in a dome with R = 160 nm versus the number of layers in the middle domain (i) at a corresponding j value associated with maximum packing densities in the two side domains. (C) Experimentally determined i values (red dots) and numerically simulated i values (dashed lines) versus R. (D) Simulated surface area of a superparticle (N = 17,425) versus its radius R, determined by a numerical simulation using a linear regression (18). Insets are calculated structures of a right cylinder, a single-domain sphere, and a double-domed cylinder with R = 160 nm and n = 5, and TEM image of a superparticle with N = 17.2 (±0.5) × 103. (E) R versus N. (F) n versus N. (G) The volume ratio, 2Vd/Vc, of the domes (2Vd) and cylinder (Vc) of a double-domed cylinder versus N. In (E) to (G), experimental data are labeled in red: open triangles, double-domed cylinders; open circles, irregular-multidomain particles. Simulated data are labeled in black: solid triangles, double-domed cylinders with volume ratio smaller than 1; dots, double-domed cylinders with volume ratio larger than 1, wherein only irregular-multidomain particles were observed in the corresponding size regime experimentally. The smallest irregular-multidomain particle determined by numerical simulation, shown as a green star, has N = 74,677. The smallest observed irregular-multidomain particle (see Fig. 1L), shown as a yellow star, has N = 80.4 (±0.6) × 103, which is close to a simulated irregular-multidomain particle with N = 80,456 shown in (G). The regime of N < 74,677 is shown in blue. In the regime of N ≥ 74,677, the subregime consisting of the “stable islands” for double-domed cylinders is shown in pale blue, and the subregime for the existence of irregular-multidomain particles is shown in yellow.

To explore how colloidal crystallization of nanorods leads to the formation of double-domed cylinders, we conducted numerical simulations based on the assumption that surface area minimization is the dominant factor in minimizing the Gibbs free energy of a superparticle. The 3D architecture of a double-domed cylinder is characterized by nine parameters (Fig. 3A and figs. S9 to S11): the number of lamellar layers in the cylinder (n), the numbers of lamellar arch layers in the middle (i) and two small side (j) domains, the height (H) and radius (R) of its cylindrical domain, the length (l) and diameter (d) of constituent nanorods, the relative curvature of the domes (κd), and the relative curvature of the cylinder wall (κc). The minimization of the surface area of a double-domed cylinder requires maximization of the nanorod packing factor (Φ) on its cylinder base. In turn, this requires that the long axes of nanorods in the middle domain are aligned perpendicular to the long axes of nanorods in the side domains (Fig. 3A), which agrees with our experimental observations (Fig. 2, B to H). The nanorod packing factor (Φ) is given by

Φ(i,j,R,l)=12lπ(2cosϕcosθ)2π(cosϕsinθ)(cosθsinϕ) (1)

where θ and ϕ are the angles determined by i and j, respectively; sin θ = i · l/(2 · R); and sin ϕ = j · l/(2 · R) (17) (fig. S9). Our numerical simulations show that Φ exhibits a parabola-like shape as a function of i, whereas for each i value, a corresponding j value is chosen to maximize the packing factor in the two small side domains (Fig. 3B). If the i values for the top three simulated packing densities are chosen for a superparticle of a given R, the trend for simulated i as a function of R encompasses all the experimental data (Fig. 3C and fig. S6).

Using Lagrange multipliers (17), we determined that there are two necessary conditions for a double-domed cylinder of a given size to have minimal surface area: (i) Its two domes must have identical H values, and (ii) H must be smaller than R (fig. S10). These mathematically determined conditions are consistent with our TEM observations of more than 400 individual superparticles, for which the two domes in a double-domed cylinder have nearly identical sizes (Fig. 1). In addition, the rule of surface area minimization allows for the two domes to rotate at different angles around the axis of a double-domed cylinder while still maintaining a minimized Gibbs free energy. As observed experimentally, nanorod packing directions in the domains of different domes are not necessarily related to each other (Fig. 1).

To further understand the formation of double-domed cylinders, we built a superparticle with hexagonal close-packed nanorods of a given size (l and d). The superparticle structure is defined with seven variables (R, H, i, j, n, κc, and κd) that dictate the evolution of superparticle morphology and domain configuration during surface-area minimization (figs. S11 and S12). The global minimal surface area of the superparticle as a function of N was determined using a method of mixed-integer linear programming of these variables (17) (figs. S13 to S16). As an example, we show that the surface area of a superparticle (N = 17,425) is parabolically related to the value of its R. The superparticle having the minimal surface area adopts a double-domed cylinder structure with R and n values in good agreement with an experimentally observed particle, whereas the right cylinder and single-domain sphere display substantially larger surface areas (Fig. 3D). Although the simulated lamellar layer number (i = 7) does not match the experimental values of 6 and 8 in the observed particle, these experimental values are still within the range of simulated values that are associated with the top three nanorod packing densities on the base of cylinder (Fig. 3, C and D, inset). This minor deviation is likely caused by the small size difference in the cylinder diameters at the two bases adjacent to the domes (Fig. 2D and Fig. 3D, inset).

In addition, the simulated data—for R and n of superparticles as a function of their N—show strong consistency with the experimental values (Fig. 3, E and F). The simulations further reveal that the fluctuations of n as a function of N, as observed experimentally (Fig. 1), are attributable to the strong intercorrelations among R, H, and n during the minimization of superparticle surface area (figs. S17 to S19). For instance, with an increase in N, the corresponding decrease in n is associated with an increase of R and/or H, and thus a substantial increase in the volume ratio of the domes and cylindrical domain (fig. S18). The consistency between experimental data and simulation results confirms that the formation of double-domed cylinder superparticles is a thermodynamically controlled process that minimizes double-domed cylinder surface area.

However, there are cases when the surface energy term does not dominate the minimization of superparticle free energy. Because of the strong anisotropic interactions between neighboring rods aligned side-by-side, the stacked arches in the domes of a superparticle can have a specific bulk free energy substantially higher than that of stacked disks in the cylindrical domain (fig. S20A). Thus, the increase in the volume of the two domes in a double-domed cylinder with a given N can increase the overall bulk free energy of this superparticle. When the volume percentage of the domes reaches a threshold, the minimization of superparticle surface energy no longer dominates the bulk free energy term, and the Gibbs free energy of the double-domed cylinder structure can be higher than that of an irregular-multidomain particle, thus leading to irregular-multidomain particle formation (fig. S20B).

If we assume this threshold to be a 1:1 volume ratio between the domes and the cylinder, our simulation results match all experimental data on the formation of double-domed cylinders and irregular-multidomain particles (Fig. 1, B to P, and Fig. 3G). Moreover, our simulations show that when N < 74,677, all superparticles have a volume ratio smaller than 1 between the domes and the cylinder, and thus only adopt a double-domed cylinder structure (Fig. 1, B to K). In the regime of N ≥ 74,677, superparticles can have volume ratios larger than 1 and thus appear as irregular-multidomain particles (Fig. 1, L, M, and O). Our simulations also reveal that in this regime, an increase in N can periodically lead to superparticles having volume ratios smaller than 1, thereby creating “stable islands” for double-domed cylinders (Fig. 3G), which is in agreement with our experimental observations (Fig. 1, L to P).

Our numerical simulations can also predict the morphology and supercrystalline domain configurations of superparticles made from constituent nanorods of other sizes (fig. S21). The excellent agreement between the numerical simulations and experimental data further suggests that (i) multiple supercrystalline domains in double-domed cylinders and irregular-multidomain particles are formed through a spontaneous process in a cooperative manner under thermodynamic equilibrium (23), and (ii) the anisotropic interactions between nanorods and their anisotropic geometry play critical roles in this process.

Moreover, the surfaces of CdSe-CdS nanorods exhibit atomic packing factor anisotropy, enabling their bottom and top {0001} faces to have a higher ligand packing density than their side faces, such as {101¯0} and {112¯0} (fig. S22). Therefore, appropriately functionalized nanorods may allow the intercalation of DTAB hydrocarbon chains into the surface ligand layer on the side faces but not on the bottom and top faces of CdSe-CdS nanocrystals (fig. S7B). As a result, during nanorod micelle decomposition, the rates for DTAB leaving from the side faces are slower than those from the bottom and top faces, thus kinetically creating anisotropic and crystal face–specific functionality on the nanorods whose bottom and top faces exhibit greater solvophobicity than the side faces. This kinetic process can, in turn, lead to a rapid growth of supercrystalline domains along the long axes of constituent nanorods during colloidal crystallization (Fig. 4A and fig. S7B).

Fig. 4

(A) Scheme of needle-like superparticle synthesis: (ia) incubation with octylamine, (ib) nanorods-micelle formation, and (ii) superparticle formation. (B) Scanning electron microscope (SEM) image of needle-like superparticles. (C) TEM image of a needle-like superparticle. (D) Scheme of the lateral alignment of superparticles into the unidirectional line patterns on a solid substrate. (E) SEM image of laterally aligned needle-like superparticles inside a line pattern on a Si3N4 substrate made using photolithography (groove width = 2.0 μm, depth = 1.2 μm, gap = 2.0 μm). (F) PL intensity versus polarization angle as the polarization was manually rotated while measuring a typical superparticle-embedded PDMS thin film under excitation wavelength of 380 nm. (G to J) Optical images of a light panel consisting of 10 LEDs [λ = 380 nm, bandwidth (full width at half maximum) = 12 nm, radiant power = 20 mW; Super Bright LEDs Inc., St. Louis, MO] with superparticle-embedded PDMS thin films as energy downconversion phosphors, taken without a polarizer (H) and with a polarizer [(I) and (J)] at the angles indicated by the direction of the compass needles in each panel. The LEDs emit orange lights at 579 nm with a bandwidth of 40 nm and a polarization ratio of ~0.88.

Incubation of CdSe-CdS nanorods (l = 78.0 ± 2.1 nm, d = 5.4 ± 0.3 nm; 10 mg) with octylamine (1 μl) for 6 days under Ar modified the particles, leading to the formation of single-domain, elongated needle-like superparticles with l = 11 ± 4 μm and d = 1.1 ± 0.3 μm (Fig. 4, B and C). These single-domain superparticles have a different morphology from those multidomain superparticles made from identical nanorods without incubation treatment (fig. S21, C to E).

The needle-like superparticles from unoptimized syntheses exhibit a PL quantum yield of ~40% and are indefinitely stable in solvents with strong polarity, such as water or ethanol. However, these particles can undergo intraparticle ripening in lower-polarity solvents such as ethylene glycol, demonstrating that the needle-like morphology is not an equilibrium shape (fig. S23). The mesoscopic sizes of these needle-like superparticles allow them to be easily aligned into unidirectional line patterns on Si3N4 substrates through capillary forces (24) (fig. S24), which can be readily transferred into uniform and removable thin films of polydimethylsiloxane (PDMS) with sizes as large as 5.0 cm × 5.0 cm (Fig. 4, D and E, and fig. S25). The resulting thin films exhibit strong linearly polarized PL at 579 nm with a typical emission polarization ratio [ρ = (I||I)/(I|| + I), where I|| and I are the intensities parallel (I||) and perpendicular (I) to the nanorod long axis] of 0.88 (Fig. 4F and fig. S26), which is substantially higher than that of individual single CdSe-CdS nanorods [0.75 (25, 26)]. This PL anisotropy enhancement can be attributed to a combination of dielectric effect and collective electric dipole coupling effects among the CdSe-CdS nanorods inside the elongated needle-like superparticles embedded in PDMS films (2628). In addition, we show that the superparticle-embedded PDMS films can be used as energy downconversion phosphors to build polarized light-emitting diodes (Fig. 4, G to J, and fig. S27).

Our results show that anisotropic interactions of CdSe-CdS nanorods can be used to synthesize colloidal superparticles with multiple well-defined supercrystalline domains under thermodynamic equilibrium. Functionality-based anisotropic interactions between these nanorods can be kinetically introduced during the superparticle synthesis, leading to the formation of single-domain, needle-like particles. We anticipate that these findings can be extended for the self-assembly of nano-objects having other anisotropic shapes, as well as the self-assembly of two or more types of anisotropic nano-objects into well-defined mesoscopic and macroscopic complex architectures (13).

Supplementary Materials

www.sciencemag.org/cgi/content/full/338/6105/358/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S27

Table S1

References (2932)

References and Notes

  1. See supplementary materials on Science Online.
  2. Acknowledgments: Supported by Office of Naval Research grant N00014-09-1-0441 (Y.C.C.), NSF Career Award DMR-0645520, and the Cornell High Energy Synchrotron Source through NSF award DMR-0936384. We thank S. Zou for helpful discussions and X. Liu for providing line-patterned Si3N4 substrates. Transmission electron microscope work was conducted at the Major Analytical Instrumentation Center at the University of Florida. A PCT International Patent Application has been filed (docket no. 995XC1PCT, “Laterally Aligned Colloidal Nanorods Assemblies,” serial no. PCT/US2012/042958).
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