NANO LETTERS
Self-Assembled Stripe Patterns of CdS Nanorods
2006 Vol. 6, No. 8 1832-1836
Ali Ghezelbash, Bonil Koo, and Brian A. Korgel* Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The UniVersity of Texas at Austin, Austin, Texas 78712-1062 Received May 8, 2006; Revised Manuscript Received June 17, 2006
ABSTRACT Sterically stabilized CdS nanorods, 3−5 nm in diameter with aspect ratios ranging from 4 to 15, were observed to self-align into networks of stripes several micrometers long when drop-cast from dispersions at submonolayer coverage. The stripes are two to three nanorods thick with nanorods assembled side-by-side with their long axes parallel to the stripe direction. Energetic forces between nanorods, such as van der Waals and dipole−dipole attractions, favor parallel nanorod alignment; however, we propose that pattern formation is kinetically limited, or mediated, by solvent evaporation.
a distinct anisotropic phase with positional disorder and longrange orientational order (the nematic phase). The disorder f order phase transition of these noninteracting hard rods, that is, those with zero interaction potential until touching, at which point they infinitely repel each other, is driven by entropy. The configurational entropy of each individual rod favors misalignment, but the excluded volume entropy in a dense collection favors orientational order. In a real colloidal system, attractive forces between rods, such as van der Waals and dipole-dipole interactions, also influence the phase behavior of the system.
Colloidal nanocrystals with characteristic dimensions less than ∼10 nm provide interesting model systems for studying phase behavior.1-4 Repulsive ligands adsorbed to the nanocrystal surfaces prevent interparticle sticking, thus enabling very stable dispersions without aggregation. Particles can be observed experimentally using various microscopy techniques, such as transmission and scanning electron microscopy (TEM and SEM),5,6 scanning probe microscopies like atomic force microscopy (AFM),7 or scattering techniques like small-angle X-ray scattering (SAXS),8-10 and their small size eliminates gravitational settling and makes their diffusion rates fast. Equilibration, for example, can often occur during the course of a simple drop-casting experiment on a substrate, at least on micrometer length-scales, to give significant thermodynamic insight into self-assembly and crystallization processes.6,9,11 Because the solvent-mediated two-body interactions between particles can be approximated with relative ease using model equations for steric stabilization and dispersion forces,2,12 and then incorporated into statistical physical models, experimental measurements can often be compared directly to theoretical predictions.13-16 The influence of kinetics on nanocrystal self-assembly is more complicated, but has also been considered recently.7,11,17 Synthetic advances for colloidal nanocrystals with anisotropic shape, such as nanorods, can now provide stable dispersions with relatively narrow size and shape distributions, opening up the possibility of systematically studying the influence of shape on their phase behavior.18-20 Shape is an important parameter: Onsager21 first showed theoretically that hard rods at very low volume fractions can form
Orientational “self-alignment” has been observed for nanorods of Ag,22 BaCrO4,18,23 Au,24-26 Co,27 CdSe19,20,28,29 in evaporated films,20,22-27 concentrated dispersions,19,28,29 and at the air-water interface of a Langmuir-Blodgett trough.18 The attractive forces between nanorods in many of these systems is undoubtedly strong, often leading to a “bunching” of nanorods.20,27,29 For spherical nanoparticles, spatially complex structures such as “stripes”30,31 and kinetically limited bicontinuous networks7,17,32 have been observed, directed in large part by attractive interactions between particles. These kinds of structures have not yet been observed for rods. Here, we show self-assembled networks of “stripes” of sterically stabilized narrow diameter (3-5 nm) CdS nanorods with relatively high aspect ratios (4-15) that form when evaporated onto a substrate at submonolayer coverage (Figure 1). We propose that inter-rod attractions influence this structural organization but that the patterns are in fact kinetically limited by solvent evaporation.
* Corresponding author. E-mail:
[email protected]. Tel: (512) 4715633. Fax: (512) 471-7060.
CdS nanorods were prepared by high-temperature “multiple injection” arrested precipitation in the presence of
10.1021/nl061035l CCC: $33.50 Published on Web 07/01/2006
© 2006 American Chemical Society
Figure 1. TEM images of CdS nanorods drop-cast from anhydrous chloroform onto a TEM grid. The black box identifies a region on the substrate imaged at three different magnifications (A-C). The nanorods are 31.8 ( 8.0 nm long with a diameter of 3.2 ( 0.6 nm. These stripe network structures were reproduced many times by drop-casting nanorods from different synthetic batches under varying deposition conditions.
tetradecylphosphonic acid (TDPA), trioctylphosphine (TOP), and trioctylphosphine oxide (TOPO) (see the Supporting Information).33 In the synthesis, a substoichiometric amount of chalcogen reactant is injected into a reaction flask containing a Cd source, which provides an initial burst of particle nucleation. After nucleation, more chalcogen reactant is injected by syringe pump. This chalcogen precipitates more CdS, which deposits selectively on the hexagonal (001) surfaces of the nanoparticles/nanorods.33 The nanorod length can be tuned to some extent by the amount of additional chalcogen added. The nanorods are dispersible in various organic solvents and have diameters typically between 3 and 5 nm with aspect ratios up to ∼15. When drop-cast from anhydrous chloroform at relatively low surface coverage, the CdS nanorods formed networks of “stripes.” As in the TEM images in Figures 1 and 2, the stripes were typically several micrometers in length and were networked. The networks covered areas as large as ∼36 µm2. The stripes consist of a few nanorods packed next to each other, aligned parallel to the stripe direction. If we think of the stripe direction as the direction of a “lattice plane” (rigorously, because the structure is in 2D, it’s not a “plane” but rather a “line”) one sees that the rods are oriented parallel to the “lattice direction.” In a 3D smectic phase, the rods orient perpendicular to the lattice planes; there is periodic order only in one dimension, between close-packed planes of rods.1 When the CdS nanorods were drop-cast at much higher concentrations, the nanorods changed orientational alignment with respect to their packing direction, with perpendicular orientation as would occur in a smectic phase (see Figure 2c). The nanorods in Figures 1 and 2 had aspect ratios of 10 and 4.5. Nanorods with both lower and higher aspect ratios formed stripes: Figure 3 shows TEM images of stripes with shorter (20.8 ( 6.7 nm by 4.6 ( 0.8 nm; aspect ratio of ∼4.7) and longer (48.8 ( 12.9 nm by 3.4 ( 0.6 nm; aspect ratio of 14.4) nanorods. Nanorods with “hard” interactions, that is, zero attraction and infinite repulsion upon touching, are not expected to exhibit orientational alignment and “clustering” at these low surface coverages. Bates and Frenkel34 computed that hard Nano Lett., Vol. 6, No. 8, 2006
Figure 2. TEM images of CdS nanorods (20.8 ( 4.6 nm by 4.6 ( 0.8 nm) drop-cast from anhydrous chloroform dispersions at increasing concentration: (A) 0.68 mg/mL, (B) 1.78 mg/mL, and (C) 8.92 mg/mL.
spherocylinders in two dimensions with an aspect ratio of L/D ) 10 (L is length and D is diameter) undergo an isotropic f nematic phase transition above a normalized concentration of FL2/D2 ≈ 4.5 (the concentration F, has units of 1/D2). For the CdS nanorods, 1/D2 ≈ 1/25 nm-2, and nematic ordering (assumes a “hard” interaction potential) should require a surface coverage of at least one nanorod per 555 nm2. The surface coverage of the nanorods that formed the stripes was only 1 nanorod per 630 nm2. Nanorods drop-cast at higher surface coverage exhibited qualitatively different orientational alignment. As in Figure 2c, the densely packed CdS nanorods 1833
Figure 3. CdS nanorods of two different aspect ratios drop-cast from anhydrous chloroform onto TEM grids: (A and C) 20.8 ( 6.7 nm by 4.6 ( 0.8 nm wide (average aspect ratio ) 4.7); (B and D) 48.8 ( 12.9 nm by 3.4 ( 0.6 nm (average aspect ratio ) 14.9).
clustered as lines of side-by-side rods, similar to previous observations for CdSe nanorods.20,29 Talapin et al.20 and Bunge et al.29 proposed that parallel alignment of CdSe nanorods, similar to that in Figure 2c, resulted at least in part from antiparallel coupling between permanent dipole moments along the long axis of the rods. Electric dipole coupling between Cu2S nanodisks has also been proposed as one driving force for orientationally aligned ordering,35 and magnetic dipole coupling has been proposed as a major driving force for side-by-side alignment of Co nanorods27 and nanodisks.36,37 A large permanent dipole moment in CdSe nanorods has been measured by transient electric birefringence.38 Like CdSe nanorods, the CdS nanorods have a wurtzite crystal structure with the hexagonal [001] crystal direction oriented down the length of the rod33 and are also expected to exhibit a relatively strong permanent dipole moment.39 However, the rods are primarily oriented end-to-end along the stripes and not side-by-side. We wanted to estimate the difference in dipole-dipole attractive force between rods oriented end-to-end and side-by-side. Figure 4 shows a schematic illustration of a nanorod network with head-to-tail coupling of the rods along the length of the stripes and the side-by-side alignment of rods within the stripes. According to Nann and Schneider’s model,39 CdS nanorods 3.2 nm in diameter and 31.8 nm long will have a dipole moment, µ, of 711 D. Using a dipole approximation, the interaction energy for two nanorods oriented end-to-end is40 Edipole )
-2µ2 4π0r3
(1)
where 0 is the vacuum permittivity, is the dielectric constant of the intervening solvent, and r is the center-tocenter separation. At the close separations of the nanorods in the dried films, the dipole approximation has significant 1834
Figure 4. Dipole-dipole interactions between nanorods (represented by the black arrows). The end-to-end dipole coupling is in fact relatively weak and is probably not responsible for the stripe assembly. Within the stripes, a few nanorods are packed side-byside, an orientation favored by both antiparallel dipole interactions and van der Waals attraction.
error but still provides a qualitative estimate of the energetic attractions between nanorods with the different orientations.40 By TEM, the edge-to-edge separation (δ) was 2 nm. Plugging in the appropriate quantities into eq 1, the nanorods have an attraction of ∼3.5 meV, which is only about a tenth of a kT at room temperature. When the nanorods are aligned sideby-side, the dipole-dipole attraction is40 Edipole )
-µ2 4π0r3
(2)
which gives an attraction of ∼470 meV (∼18 kT at room temperature). The dipole-dipole coupling between nanorods aligned parallel is nearly 130 times stronger than that when oriented end-to-end. The difference in interaction energy is due to the fact that the center-to-center distance between the two dipoles can be much shorter when the rods are aligned parallel as opposed to end-to-end, making the attractions much stronger. One should note that we have not explicitly accounted for the repulsive potential between rods due to the ligands. The actual interaction potential will have a value less than what we have calculated due to the steric repulsion of the ligand layer;2,12 however, the repulsive force is expected to be nearly independent of how the rods are oriented with respect to each other and the attractions between rods aligned side-by-side should be 2 orders of magnitude stronger than rods aligned end-to-end. It is therefore surprising that the nanorods orient primarily endto-end along the stripes as side-by-side bunching, as observed experimentally20,29 in more concentrated dried films is more energetically favored. van der Waals attractions between nanorods also favor parallel alignment. Two parallel nanorods have an interaction Nano Lett., Vol. 6, No. 8, 2006
Figure 5. TEM images of self-assembled CdS nanorods. The insets show fast Fourier transforms (FFTs) of the regions enclosed within the outlined squares. The FFTs reveal “scattering peaks” that indicate domains with a characteristic length scale. The distances indicated by the spot positions are noted. The 1D alignment of the stripe domains gives rise to spots in the FFTs as opposed to a diffuse ring.
energy of40 EvdW )
-A121LR1/2 24δ3/2
(3)
Taking the solvent (or ligand)-screened Hamaker constant, A121, to be ∼0.05 eV (see the Supporting Information for details), the vdW attraction at the closest interparticle separation observed in the dried film is ∼140 meV (∼5.4 kT). Two rods positioned end-to-end have a van der Waals attraction of only ∼5 meV (∼0.2 kT). Again, we have not accounted explicitly for the repulsive potential of the ligands,2,12 but it should not depend on the relative orientation of the rods. The attractive energy between nanorods clearly favors their parallel alignment, by about an order of magnitude. Both vdW and dipole-dipole attractions favor side-byside clustering as opposed to end-to-end alignment, yet the nanorods predominantly align end-to-end in the stripes. Therefore, we believe that the stripe patterns are not equilibrium structures like the self-assembled stripe patterns of sterically stabilized silver nanocrystals at the air-water interface of a Langmuir-Blodgett trough30,31 but are rather kinetically limited structures. For example, evaporated thin films of spherical nanocrystals can form kinetically limited stripe-like patterns and networks of particles reminiscent of spinodal decomposition patterns of two phase-separating materials.7,17,32,41 During the drying process, the nanocrystal concentration becomes high enough to enter a phase boundNano Lett., Vol. 6, No. 8, 2006
ary between regions of high and low nanocrystal concentration, which then promotes a “phase separation” or clustering.7 As in a molecular solution of two immiscible phases, the phase separation can occur by either nucleation or spinodal decomposition.7,17,42 The phase separation kinetics depend on the energy barrier to forming the new phase. If there is an activation energy barrier, then the new phase evolves by nucleation and growth. If there is no energy barrier, then the phase separation occurs by spinodal decomposition. In spinodal decomposition, the new phases evolve from small concentration fluctuations that grow with a characteristic “wavelength” of concentration variation that ripples through the system to produce a bicontinuous network of high and low concentration regions.43 The bicontinuous structure gives rise to a “scattering peak” with mesoscopic (i.e., greater than the molecular or nanocrystal size) dimensions that distinguishes it from nucleation and growth. The stripes of CdS nanorods exhibit a mesoscopic correlation length that is characteristic of spinodal decomposition. Figure 5 shows TEM images of six stripe structures and their corresponding fast Fourier transforms (FFTs). The FFTs correspond to small angle scattering patterns of the system, and all six images reveal “diffraction” peaks. The correlation length corresponds to the separation between stripes. The structures are consistent with a system undergoing spinodal decomposition. It is noteworthy that the FFTs give rise to spots as opposed to diffuse rings, which indicates preferential orientation of the domains on a local length scale. When the FFT is taken 1835
on an extended region of the film, a ring-like pattern does indeed appear as multiple domains of oriented stripes are taken into account (e.g., in the top left image in Figure 5). The anisotropic rod shape may be responsible for breaking the symmetry in the patterns and giving rise to the extended linear structure of the stripes. It is reasonably certain that the CdS nanorod stripes are kinetically trapped structures. The stripes form at relatively low concentrations, well below the critical concentration required for an isotropic f nematic phase transition for hard rods, and interparticle attractions are most likely very important in the their formation. However, the structures are surprising because the strong orientationally dependent interrod attractions due to van der Waals and dipole-dipole attractions both favor side-by-side alignment, whereas the nanorods are primarily aligned end-to-end in the stripes. At much higher deposition concentrations that give thick films, the nanorods do indeed tend to cluster and align into bunches. The stripes have a surprisingly consistent separation distance, exhibiting a correlation length that is larger than the nanorod dimensions. The appearance of such a correlation length is consistent with the idea that the stripe patterns formed by the nanorods are spinodal decomposition patterns. We have considered alternative explanations for the structures, such as solvent dewetting,44 convective instabilities in the drying film,45,46 and lateral capillary forces;47,48 however, none of these mechanisms are expected to give rise to the correlation lengths observed in these structures. Certainly, additional experimental and theoretical studies will help reveal the role of the inter-rod attractions and the dynamic interactions that determine the nanorod assembly. One conclusion from the observation of the stripe patterns of nanorods is that clustering and inter-rod attractions appear to be relatively strong and will be important considerations for future applications of these materials, such as polymer composites or mineral liquid crystals. Acknowledgment. This work was supported in part by funds provided by the National Science Foundation through their STC program (CHE-9876674), the Welch Foundation, the Advanced Processing and Prototyping Center (AP2C; DARPA: HR0011-06-1-0005), and the Office of Naval Research (N000N-05-1-0857). Supporting Information Available: Experimental details of CdS nanorod synthesis, deposition and imaging, and Hamaker constant calculations. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Gelbart, W. M.; Ben-Shaul, A. J. Phys. Chem. 1996, 100, 1316913189. (2) Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D. J. Phys. Chem. B 1998, 102, 8379-8388. (3) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397-406. (4) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55-59. (5) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335-1338. 1836
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NL061035L Nano Lett., Vol. 6, No. 8, 2006
