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Self-Assembly Patterns Formed upon Solvent Evaporation of Aqueous Cetyltrimethylammonium Bromide-Coated Gold Nanoparticles of Various Shapes Tapan K. Sau and Catherine J. Murphy* Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208 Received October 11, 2004. In Final Form: January 24, 2005 Gold nanocrystals of various shapes, which were produced in high yield in the presence of cetyltrimethylammonium bromide (CTAB), showed a range of two-dimensional self-assembly patterns upon drying from aqueous solution. The interparticle spacings were independent of the size and shape of the gold nanocrystals. Energy-dispersive X-ray analysis (EDAX) and Fourier-transform infrared (FTIR) spectroscopic studies revealed that the CTAB molecules adsorb onto surfaces of the gold nanocrystals in a bilayer or multilayer fashion, consistent with other groups’ results. Zeta potential measurements showed that CTABcoated nanocrystals were positively charged and the zeta potential remained almost the same upon two centrifugations and redispersion of the nanocrystals in deionized water, confirming the high stability of the surfactant-nanoparticle interaction. The nanocrystal shape strongly influenced the nature of the self-assembly patterns, in some cases in accord with theoretical predictions. CTAB is proposed as the medium for self-assembly, via interdigitation of its hydrophobic chains from adjacent nanocrystals for close contact, or via sharing a layer of counterions for larger inter-nanocrystal spacings.
Introduction Patterning of nanoscale materials on suitable surfaces is of great importance for many applications in science and technology.1 There is a great demand for the development of new building blocks as well as new fabrication techniques, especially nonlithographic techniques, to assemble, pattern, and integrate nanomaterials in functional and ordered networks, to develop practical and efficient electronic, photonic, or sensor devices. The colloidal chemical route of nanocrystal self-assembly can be an alternative means of nanofabrication due to its simplicity, versatility, and low cost. Several examples of one-, two- and three-dimensionally ordered arrays of metallic and semiconductor nanodots have been reported by the self-assembling method.2 Such self-assembly can be achieved via a variety of techniques (surface functionalization of nanocrystals with suitable molecules that bear specific groups, utilization of structures with suitable channels and cavities, application of electric and magnetic fields) in solution or on solid surfaces.3-5 The formation of ordered arrays of nanocrystals was first observed on a transmission electron microscope (TEM) grid, where an ordered 3D array of iron oxide nanocrystals resulted spontaneously on the grid after the evaporation of solvent from the sample drop.4 This simple approach * To whom correspondence should be addressed. Fax: +1 (803) 777-9521. Tel: +1 (803)777-3628. E-mail: murphy@ mail.chem.sc.edu. (1) (a) Pileni, M. P.; Lalatonne, Y.; Ingert, D.; Lisiecki, I.; Courty, A. Faraday Discuss. 2004, 125, 251. (b) Lin, X. M.; Parthasarathy, R.; Jaeger, H. M. Appl. Phys. Lett. 2001, 78, 1915. (c) Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 2003, 107, 7312. (d) Adams, M.; Dogic, Z.; Keller, S. L.; Fraden, S. Nature 1998, 393, 349. (e) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (f) Whitesides, G. M.; Boncheva, M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4769. (2) (a) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325. (b) Yin, J. S.; Wang, Z. L. J. Mater. Res. 1999, 14, 503. (c) Petit, C.; Taleb, A.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 1805. (d) Korgel, B. A.; Fitzmaurice, D. Adv. Mater. 1998, 10, 661. (e)Yin, J. S.; Wang, Z. L. Phys. Rev. Lett. 1997, 79, 2570. (f) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335.
takes advantage of the physical, chemical, and structural affinities (covalent as well as noncovalent van der Waals, hydrophobic or electrostatic interactions) of the building blocks and substrates.1f In principle, a dispersion of colloidal nanocrystals can lower its free energy through Brownian motion and by doing so, self-assemble.3l Nanocrystals can experience strong capillary forces through the surface of the drying liquid and are forced to organize side-by-side.1f The organization in the self-assembled structures is, in part, determined by the balance of van der Waals forces, capillary forces, surface tension, and others.1,3l,r Formation of the organized assembly with a stable structure is possible only when the collective interaction energy operating among the participating nanocrystals is sufficient to overcome the effect of the entropy loss due to ordering. (3) (a) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393. (b) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (c) Schmid, G.; Baumle, M.; Beyer, N. Angew. Chem., Int. Ed. 2000, 39, 181. (d) Lalatonne, Y.; Richardi, J.; Pileni, M. P. Nat. Mater. 2004, 3, 121. (e) Love, J. C.; Urbach, A. R.; Prentiss, M. G.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12696. (f) Lin, X. M.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 1999, 11, 198. (g) Sun, S.; Murray, C. B.; Weller, D.; Filks, L.; Moser, A. Science 2000, 287, 1989. (h) Murthy, V. S.; Cha, J. N.; Stucky, G. D.; Wong, M. S. J. Am. Chem. Soc. 2004, 126, 5292. (i) Li, X.; Li, Y.; Tan, Y.; Yang, C.; Li, Y. J. Phys. Chem. B. 2004, 108, 5192. (j) Pinna, N.; Maillard, M.; Courty, A.; Russier, V.; Pileni, M. P. Phys. Rev. B 2002, 66, 045415. (k) Courty, A.; Fermon, C.; Pileni, M. P. Adv. Mater. 2001, 13, 254. (l) van Blaaderen, A. MRS Bulletin 2004, 29, 85. (m) Brust, M.; Bethel, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795. (n) Helseth, L. E.; Wen, H. Z.; Hansen, R. W.; Johansen, T. H.; Heinig, P.; Fischer, T. M. Langmuir 2004, 20, 7323. (o) Smith, P. A.; Nordquist, C. D.; Jackson, T. N.; Mayer, T. S.; Martin, B. R.; Mbindyo, J.; Mallouk, T. E. Appl. Phys. Lett. 2000, 77, 1399. (p) Su, B.; Abid, J.-P.; Fermin, D. J.; Girault, H. H.; Hoffmannova, H.; Krtil, P.; Samec, Z. J. Am. Chem. Soc. 2004, 126, 915. (q) Salem, A. K.; Chen, M.; Hayden, J.; Leong, K. W.; Searson, P. C. Nano Lett. 2004, 4, 1163. (r) Rabani, E.; Reichman, D. R.; Geissier, P. L.; Brus, L. E. Nature 2003, 426, 271. (s) Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 13066. (4) Bentzen, M. D.; van Wonterghem, J.; Morup, S.; Thlen, A.; Koch, C. J. Philos. Mag. B 1989, 60, 169. (5) (a) Mirkin, C. A. MRS Bull. 2000, 25, 43. (b) Dujardin, E.; Hsin, L. B.; Wang, C. R. C. Mann, S. Chem. Commun. 2001, 1264. (c) Mbindyo, J. K. N.; Reiss, B. D.; Martin, B. R.; Keating, C. D.; Natan, M. J.; Mallouk, T. E. Adv. Mater. 2001, 13, 249.
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Nanocrystals made in solution as colloids require capping agents to limit nanoparticle growth and to passivate dangling bonds. The morphology of a nanocrystal is believed to influence the packing of stabilizing molecules bound to its surface, which in turn affects nanocrystalnanocrystal interactions.6 Stoeva et al. reported that dodecanethiol-stabilized gold nanocrystals with similar average size organized into different superlattice structures, which resulted from differences in nanoparticle core morphologies, which in turn depended upon the method of preparation of the nanocrystals.7 Various liquid crystalline ordered structures with nematic, smectic, and columnar phases have been observed for rod- and plateshaped nanocrystals depending on their volume fractions.8 Surface functionalization of rod-shaped nanocrystals can promote end-to-end assembly instead of side-to-side liquid crystalline assembly.3q,9 Recently, we have been able to prepare surfactant-stabilized Au nanocrystals with several shapes, such as small aspect ratio rods, rectangular blocks, cubes, tetrapods, and platelets in high yield.10 In this paper we examine the self-assembly behavior of this family of gold nanocrystals, all capped with the same surfactant, as a function of core nanocrystal shape. Experimental Section HAuCl4‚3H2O (99.9%), NaBH4 (99%), L-ascorbic acid (AA, 99+%), cetyltrimethylammonium bromide (CTAB, 99%), and AgNO3 (99+%) were used as received (Aldrich). Ultrapure deionized and distilled water (Continental Water Systems) was used for all solution preparations and experiments. Glassware was cleaned by soaking in aqua regia and finally washing with distilled, deionized water. The shape-controlled syntheses of the gold nanocrystals were done via the seed- mediated method. Essentially, in the presence of CTAB and gold seeds that are 3-4 nm spheres, gold ions are reduced on the surface of the seeds by ascorbic acid. Gold seeds were prepared by the reduction of appropriate quantities of gold ions by ice-cold NaBH4 in the presence of CTAB. The detailed synthesis procedures are described elsewhere.10 The presence of a large excess of CTAB in the as-prepared particle samples makes their observation under transmission electron microscopy a very difficult task. CTAB-coated nanocrystals were, therefore, separated from excess CTAB present in solution by centrifugation. The solid residue was redispersed with distilled, deionized water and centrifuged again at a lower speed. Then the separated solid mass was dispersed in a suitable volume (0.5 or 1.0 mL) of distilled, deionized water again. This dispersion was used for the preparation of samples for TEM, EDAX, and FTIR. TEM samples were prepared by placing 7 µL of this solution on a carbon-coated copper grid (200 or 300 mesh, Ted Pella). The solution on the grid was allowed to evaporate in open laboratory atmosphere, which takes ∼45 min. TEM images were obtained either with a Hitachi H-8000 or a JEOL JEM100CX II transmission electron microscope. FTIR and EDAX studies were carried out in order to check the presence, and the nature of interaction, of CTAB molecules with the gold nano(6) (a) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1996, 100, 13904. (b) Harfenist, S. A.; Wang, Z. L.; Whetten, R. L.; Vezmar, I.; Alvarez, M. M. Adv. Mater. 1997, 9, 817. (c) Luedtke, W. D.; Landman, U. J. Phys. Chem. B 1996, 100, 13323. (7) Stoeva, S. I.; Prasad, B. L. V.; Uma, S.; Stoimenov, P. K.; Zaikovski, V.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 2003, 107, 7441. (8) (a) Jana, N. R.; Gearheart, L.; Obare, S. O.; Johnson, C. J.; Edler, K. J.; Mann, S.; Murphy, C. J. J. Mater. Chem. 2002, 12, 2909. (b) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 8635. (c) Li, L. S.; Walda, J.; Manna, L.; Alivisatos, A. P. Nano Lett. 2001, 1, 349. (d) Kim, F.; Kwan, S.; Akana, J.; Yang, P. D. J. Am. Chem. Soc. 2001, 123, 4360. (e) Jana, N. R. Angew. Chem., Int. Ed. 2004, 43, 1536. (f) Pradhan, N.; Efrima, S. J. Phys. Chem. B 2004, 108, 11964. (9) Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914. (10) (a) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414. (b) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648.
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Figure 1. FTIR spectra of drop-dried samples of CTAB (A) and CTAB-coated gold nanocrystals (B, rectangle-shaped; C, cube-shaped). Gold nanocrystals were originally prepared in 9.5 × 10-2 M (B) and 1.6 × 10-2 M (C) of CTAB solution. crystals. In situ FTIR spectral studies of aqueous sample solutions were made with a Shimadzu 8400 FTIR instrument. FTIR spectral characteristics of drop-dried samples were collected in reflectance mode with a Nexus Thermo-Nicolet 470 series FTIR instrument coupled with a Thermo-Nicolet continuum FTIR microscope. The samples were prepared by placing a few drops of the above dispersion on Si(111) wafers and drying in a covered Petri dish. FTIR spectra were recorded over 128 scans of each sample with a resolution of 4 cm-1 and the background spectra were automatically subtracted. Elemental analysis was carried out on a Hitachi 2500 Delta scanning electron microscope by X-ray energy dispersive analysis (EDAX). Zeta potentials (effective surface charge) of the CTAB-coated nanocrystals were measured with a ZetaPALS Zeta Potential Analyzer, Brookhaven Instruments Corporation.
Results and Discussion Bilayers of CTAB Exist on Gold Nanocrystals of Various Shapes. EDAX studies showed that carbon and bromine from CTAB were present on the Au nanocrystals even after two centrifugations and washings in distilled, deionized water. A comparison of the FTIR spectra of dropdried samples of washed, CTAB-coated Au nanocrystals with that of pure CTAB not only supports the presence but also reveals the nature of interaction of CTAB molecules with the Au nanocrystals (Figure 1). The FTIR spectral features in the region of CH2 symmetric and antisymmetric vibrations (∼3000-2800 cm-1) appeared similar for pure as well as surface-bound CTAB molecules; however the peak positions slightly shifted to lower frequencies (∼4 to 6 cm-1) in the case of surface-bound molecules.11 For drop-dried CTAB, the FTIR spectral characteristics of nanorod-bound CTAB molecules were nearly identical to that observed by Nikoobakht and ElSayed.12a However, some differences in the peak positions and in the nature of packing of the surfactant tails were observed in our cases for different nanoparticle shapes. The peaks at 1487 and 1430 cm-1 are assigned respectively to the anti-symmetric and symmetric modes of vibrations (11) Kung, K. H. S.; Hayes, K. F. Langmuir 1993, 9, 263. (12) (a) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368. (b) Venkataraman, N. V.; Vasudevan, S. Proc. Indian Acad. Sci. (Chem. Sci.) 2001, 113, 539. (c) Cheng, W.; Dong, S.; Wang, E. Langmuir 2003, 19, 9434.
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Figure 2. Self-assembly patterns formed by gold nanorods, as visualized by transmission electron microscopy. (a) Ribbon structure formed from nanorods of different aspect ratios; (b) globally isotropic but locally ordered arrangement of nanorods; and (c) smectic arrays formed from short rods. Scale bars are all 100 nm.
of the headgroup methylene moiety (N+-CH3); and the peaks at 1473 and 1464 cm-1 arise from the CH2 scissoring modes.12 The doublet peaks at 730 and 719 cm-1 correspond to the rocking mode of the methylene (-CH2-)n, chain.11,12 The presence of the doublet peaks in all our samples, unlike those observed in ref 12a, indicates that the methylene chains of CTAB have packed, restricted (crystalline) structures. The positions of the CH2 scissoring vibrational peaks are also used as sensitive indicators of the ordering of the alkyl chains. The CH2 scissoring mode peaks of CTAB with gold nanorods/blocks (which are prepared in the presence of a large quantity of CTAB) were observed to shift slightly to lower frequencies with respect to pure CTAB. These CH2 scissoring peaks hardly shifted in the presence of Au nanocubes (which are prepared in the presence of a small quantity of CTAB). The shift of vibrations to lower frequency suggests that alkyl chains have more ordered structures and experience a more hydrophobic environment for gold nanorods and blocks.11 The peaks of pure CTAB at 1068, 1045, 1038, 964, 937, 912, and 881 cm-1 can be assigned to the C-N+ stretching modes.12 Many of these peak positions were red-shifted by 4-6 cm-1 in the gold nanocrystal samples, supporting the notion that the CTAB headgroups face the metal nanoparticle surface. Since such an orientation of the CTAB molecules gives rise to unfavorable interactions by putting the hydrophobic tails of the surfactants toward the water environment, a double layer arrangement with a second layer of CTAB molecules pointing their headgroups toward the water environment can be invoked, as proposed by others in similar systems.12b,c In fact, the doublet peaks at 730 and 719 cm-1 in our samples indicates the existence of an almost crystalline structure for the organic core made of surfactant tails.12a,b A bilayer of CTAB should make the CTAB-coated gold nanocrystals positively charged. We have measured zeta potential values for CTAB-coated gold nanocrystals in the range +49 mV to +71 mV, depending on nanoparticle shape and size.These zeta potential values hardly changed even after centrifugation and redispersion of the nanocrystals in distilled, deionized water, suggesting that the bilayer structure is quite stable, as we have stated before.13 Self-Assembly of Nanorods. Rod-shaped nanocrystals assemble in a number of ways, such as end-to-end or sideto-side, depending on their concentrations in the dispersions.8b,e,f Side-to-side assemblies can form ribbonlike structures or smectic arrays. Bates and Frenkel reported (13) Gao, J.; Bender, C. M.; Murphy, C. J. Langmuir 2003, 19, 9065.
two types of phase behavior for spherocylinders confined to a plane using Monte Carlo simulations.15 According to them, long rods with aspect ratio >7 behave similar to infinitely thin needles and exhibit a 2D nematic phase at high density. Shorter rods (aspect ratio 5 have a tendency to align parallel to each other with their long axes perpendicular to the array axis; and this side-byside array can extend over a few microns.8a Lower-aspectratio gold nanorods (especially those prepared in the presence of a small quantity of silver nitrate10) show a tendency to fill the available space by all sorts of assemblies, such as end-to-end, side-to-side, and so forth, exhibiting short-range order locally but being isotropic globally (Figure 2b), as was predicted by Bates and Frenkel.15 However, smectic arrays were also found for small aspect ratio rods, possibly arising from the variation in the local particle concentrations (Figure 2c).3r Self-Assembly of Nanocubes/Hexagons. Cubic nanocrystals were observed to form either hexagonal or square networks, whereas nanocrystals with hexagonal profiles showed only 2D hexagonal close packing (Figure 3). Figure 3a shows that cubic nanocrystals formed hexagonal networks in two different ways. In one kind, cubic nanocrystals form chains by sharing only two diagonally opposite corners; and in the other kind, all the four corners of a cubic nanoparticle are shared with four other nanocubes. The square arrangement was frequently observed for cube-shaped nanocrystals, where they share maximal surface area with each other (Figure 3b). Preliminary HRTEM studies show that the cubic nanocrystals are bounded by {100} planes and the corners by (14) Weidemaier, K.; Tavernier, H. L.; Fayer, M. D. J. Phys. Chem. B 1997, 101, 9352. (15) Bates, M. A.; Frenkel, D. J. Chem. Phys. 2000, 112, 10034. (16) (a) Veerman, J. A. C.; Frenkel, D. Phys. Rev. A 1992, 45, 5632. (b) van der Beek, D.; Lekkerkerker, H. N. W. Langmuir 2004, 20, 8582.
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Table 1. Conditions for Self-Assembled Patterns of CTAB-Coated Gold Nanocrystals
nanocrystal shape
a
self-assembly pattern
[Au], M
% frequencya of pattern observation
figure no.
rod, 15 nm × 70-480 nm rod, 14 nm × 45 nm rod, 9 nm × 42 nm rod, aspect ratio up to 5 rod, 29 nm × 60 nm rod, 29 nm × 60 nm
ribbon isotropic smectic end-to-end swirling cluster end-to-end cementing
1.5 × 10-3 1.5 × 10-3 1.5 × 10-3 7.5 × 10-4 2.0 × 10-3 2.0 × 10-3
90 100 80 50 80 80
2a 2b 2c 6c 9b 9c
cube
hexagonal square
1.5 × 10-3 1.5 × 10-3
30 70
3a 3b
hexagon
hexagonal
1.5 × 10-3
100
3c
10-3
rectangle
side-to-side
1.5 ×
90
4
platelet
stacking
1.5 × 10-3
90
5
sphere/nearly sphere
hexagonal nanoring chain
1.5 × 10-3 7.5 × 10-4 7.5 × 10-4
80 40 30
7a 7b 6b
sphere + phi-shape + star (∼20 + 45 + 35)% sphere + spiked (∼50%) sphere + rod (∼50%)
end-to-end
7.5 × 10-4
40
6a
nanoring shape-separated
7.5 × 10-4 1.5 × 10-3
40 70
7c 8
Calculation based on the observations from ten TEM grids of each sample.
small {111} planes. This suggests that the different crystal faces, with their attendant surface-bound molecules, are available for assembly. Self-Assembly of Rectangle-Shaped Nanocrystals. Rectangle-shaped nanocrystals can be thought as intermediate between rod- and cube-shaped nanocrystals. Figure 4 shows two kinds of rectangular nanocrystals of different dimensions. These rectangular nanocrystals mainly showed side-to-side organization, with minor components of end-to-end assembly. Thus the rectangular nanocrystals behaved more like rods than cubes. As it was previously noted, a more ordered and hydrophobic environment of the capping CTAB molecules existed in this case, perhaps leading to principally side-to-side assembly through interdigitation of CTAB chains from adjacent nanocrystals (see below).
Figure 3. Self-assembled patterns formed by gold nanocrystals with cubic profiles (a, hexagonal assembly; b, square assembly) and hexagonal profiles (c) hexagonal assembly).
Self-Assembly of Nanoplatelets. Isolated nanoplatelets were rarely present in our samples; rather, they formed slanted stacks consisting of ∼3 to 20 platelets over the grids (Figure 5). Examples of such directional assembly have been previously observed in the case of prolate silver and oblate ruthenium nanocrystals and nanodisks of cobalt.3j,17 It was argued in those cases that magnetic platelets can minimize their interaction energy by aligning parallel to each other.17c In the cases of Ru and Ag platelets, a self-assembled monolayer of capping agents such as dodecanethiol or CTAB on the basal plane was suggested to be responsible for the formation of platelet stacking.17b,d Recently, triangular platelets with “patchy” surfaces, that promote face-face interactions, have been predicted to stack in a similar manner.18 Thus such stacking assemblies arise from the combined effects of the nanocrystal shape and the nature of the capping agents, irrespective of the material contents of the nanocrystals. End-to-End Self-Assembly. End-to-end self-assembly of nanocrystals can be obtained with bifunctional capping agents.3s We have previously observed end-to-end assembly of long gold nanorods linked with biotin-streptavidin connectors,9 which we believe was due to preferential biotinylation of the nanorods at their ends; the CTAB bilayer, we believe, has a surprisingly high stability on the nanorod side surfaces.13 Thus, these nanorods may be an experimental example of “patchy” nanocrystals that are being examined theoretically.18 In this study, end-to-end self-assembly was observed for nanocrystals capped with CTAB alone. Figure 6 shows typical examples of these end-to-end chainlike assemblies formed by gold nanocrystals of several different shapes. At relatively low concentrations, especially nanocrystals with anisotropic shapes and sharp tips, showed a tendency to form one-dimensional chains via end-to-end linkages. Chainlike assemblies of nanocrystals, even for spheres, can occur if they have local inhomogeneities in their capping groups.18 (17) (a) Korgel, B. A.; Fitzmaurice, D. Adv. Mater. 1998, 10, 661. (b) Viau, G.; Brayner, R.; Poul, L.; Chakroune, N.; Lacaze, E.; FievetVincent, F.; Fievet, F. Chem. Mater. 2003, 15, 486. (c) Gao, Y.; Bao, Y.; Beerman, M.; Yasuhara, A.; Shindo, D.; Krishnan, K. M. Appl. Phys. Lett. 2004, 84, 3361. (d) Chen, S.; Fan, Z.; Carroll, D. L. J. Phys. Chem. B 2002, 106, 10777. (18) Zhang, Z. L.; Glotzer, S. C. Nano Lett. 2004, 4, 1407.
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Figure 4. Arraying behavior of rectangular blocks of various dimensions.
Figure 5. Stacking assembly of gold nanoplatelets as observed by TEM.
Figure 7. Self-assemblies of spherical and anisotropic gold nanocrystals: hexagonal close packing (a); and nanorings (b and c).
Figure 6. Linear chain assemblies formed by gold nanocrystals of different shapes.
Self-assembly of Spheres and Mixtures of Spheres with Other Shapes. We have observed that spheres, or nearly spherical nanocrystals, often assemble into hexagonal close-packed and ring-like structures (Figure 7). Nanorings were observed with nanocrystals of other shapes too. Figure 7c shows such a typical self-assembled nanoring pattern formed from completely anisotropic, spike-shaped nanocrystals; no nanocrystals were observed inside the nanorings. The formation of rings depends on several factors including hydrodynamics and surface tension.19 Self-assembly of oleic acid/oleylamine coated FePt nanocrystals into monolayer, submonolayer, and multilayer nanorings have been reported by Zhou et al.20 (19) Maillard, M.; Motte, L.; Ngo, A. T.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 11871.
According to the authors, holes nucleate in the liquid thin film over the grid in order to restore the equilibrium film thickness and grow bigger due to evaporation-driven instability. According to Deegan et al., when the contact line pinning of the evaporating droplet occurs, an outward flow of the solvent develops, since the solvent is predominantly removed via evaporation from the edge of the droplet.21 Govor et al. reported that ring formation was due to a retraction of the droplet contact line, not due to the outward solvent flow.22 Simulation and experiment show that a variety of self-assembly patterns from small spherical nanocrystals can exist, depending on surface energies and the degree of local homogeneity in the drying process.3r A sample containing a mixture of sphere-like and rodshaped nanocrystals assembled into exclusively sphereand rod-rich regions (Figure 9). Lekkerkerker et al. have presented experimental evidence for the existence of such rod-sphere phase separation in dilute suspensions of silica spheres and silica-coated boehmite rods.23 This phenomenon is termed “depletion attraction”. Adams et al. stated that such entropically driven orderings occur via demixing of colloidal species, which vary sufficiently in at least one (20) Zhou, W. L.; He, J.; Fang, J.; Huynh, T.-A.; Kennedy, T. J.; Stokes, K. L.; O’Connor, C. J. J. Appl. Phys. 2003, 93, 7340. (21) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. Rev. E 2000, 62, 756. (22) Govor, L. V.; Reiter, G.; Bauer, G. H.; Parisi, J. Appl. Phys. Lett. 2004, 84, 4774.
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Figure 8. Phase separation by a mixture of gold nanospheres and short nanorods.
physical property.1d Rodlike nanocrystals are thought to be very efficient depletion agents.23 The physical origin of such phase separation lies in the increase in free volume and thereby increased translational entropy of the colloidal species, which compensates for the loss of mixing entropy.1d Effects of Local Concentrations on Self-Assembly. Different regions of the grid sometimes showed different self-assembly designs even for a given sample with a unique concentration of nanocrystals. This happens especially for relatively high concentrations of nanocrystals. This could possibly be due to the difference in local concentrations created by inhomogeneities of the thin solvent film, leading to its breakage into unsymmetrical regions over the grid, before complete evaporation of the
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solvent takes place.3r For example, Figure 9 shows four different kinds of self-assembly designs formed at four different regions of the same grid. It was not clear why the interparticle distance (side-wise or end-to-end) in Figure 9a,b varied over such a wide range, ∼8 to 30 nm. It is interesting to note that under such higher rod concentration condition, the rods showed a clustering arrangement or an end-to-end cementing process, so-called “oriented attachment” by the fusion of the rod ends, forming parallel wavy structures (Figure 9c,d); although rods have been observed to prefer side-to-side ordering. The direct reduction of total surface area thereby decreasing the surface energy could be the driving force for such attachment.24 Role of CTAB in Self-Assembly. Surface-bound CTAB molecules provide steric as well as electrostatic repulsion between nanocrystals when the nanocrystals approach each other as the solution becomes more concentrated. Upon solvent evaporation, these CTAB molecules can paradoxically also assist in drawing the nanocrystals closer, to share a common layer of counterions, or through the inter-digitation of CTAB tails from neighboring nanocrystals. In our samples, upon drying, typical nearest interparticle distances varied between 3.4 and 9.0 nm. The length of the fully stretched CTA+ ion is ∼2.2 nm.12b,14 The interparticle distance of ∼3.4 nm is shorter than twice the length of the fully stretched CTA+ ion. It suggests that in some cases there were only two layers of the CTAB molecules between nanocrystals, and the alkyl chains of CTAB molecules from adjacent nanocrystals were inter-
Figure 9. Self-assembly patterns formed over different regions of a single TEM grid: (a) “swirling” and smectic, (b) “swirling clustering”, and (c) “end-to-end cementing”.
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previously estimated for CTAB-coated long gold nanorods from thermogravimetric analysis.8a Since interparticle spacings were found to be similar through out the range of particle shapes and patterns, we suggest that CTAB is the “glue” that holds nanocrystals together upon drying (Figure 10). Nanocrystals with sharp tips presumably have a smaller two-dimensional area to support the bilayer of CTAB on the tip faces, which would affect tip-tip interactions between adjacent nanocrystals. Others have observed that mixed self-assembled monolayers organize themselves into domains as small as 0.5 nm on metal nanocrystals, as a function of surface curvature.25 Thus, it is possible that the CTAB bilayer is more fluid, or less stable, on a small crystal face, which would in turn affect its ability to “glue” nanocrystals together. Conclusion
Figure 10. Cartoon of CTAB-mediated self-assembly of gold nanocrystals (not to scale). As adjacent nanocrystals (large ovals) approach each other, CTAB bilayers on the nanocrystals (open circles denoting the cationic headgroups, lines denoting hydrophobic tails) can either share a layer of counterions (filled circles), to give 9 nm nanocrystal-nanocrystal spacings; or the outer CTAB monolayers can be expelled, and the remaining monolayers interdigitate their hydrophobic chains, to give 3.4 nm nanocrystal-nanocrystal spacings. For clarity, only one side of the bilayer on each nanocrystal is shown.
digitated. An interparticle spacing of ∼9 nm is approximately four times the length of the CTAB molecule. This supports the concept of double layer formation by CTAB molecules surrounding the gold nanocrystals. Similar multilayer arrangements of CTAB were also (23) Koenderink, G. H.; Vliegenthart, G. A.; Kluijtmans, S. G. J. M.; van Blaaderen, A.; Philipse, A. P.; Lekkerkerker, H. N. W. Langmuir 1999, 15, 4693. (24) (a) Chang, Y.; Zeng, H. C. Cryst. Growth Des. 2004, 4, 397. (b) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188. (c) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969.
This study provides information regarding the shape effect on the self-assembly behavior of nanocrystals. EDAX and FTIR studies and the zeta potential measurements revealed that surfactant CTAB molecules, which were used to regulate particle growth as well as stabilize particle size, adsorb onto surfaces of the gold nanocrystals and play major roles in the direction-specific self-assembly of the coated nanocrystals via inter-digitation of the tails or possibly by sharing a common layer of counterions. The interparticle spacings were similar irrespective of the size and shape of the gold nanocrystals. Though self-assembly was achieved by simple solvent evaporation, the shape of the nanocrystals assist in governing the final twodimensional pattern. These experiments demonstrate that the self-assembly patterns formed by these surfactantcoated gold nanoparticle systems are reproducible and ultimately (in combination with theory) can be predictable. Acknowledgment. We thank the University of South Carolina, the USC NanoCenter, and the National Science Foundation for funding. LA047488S (25) Jackson, A. M.; Myerson, J. W.; Stellacci, F. Nat. Mater. 2004, 3, 330.
