pubs.acs.org/Langmuir © 2009 American Chemical Society
Formation of Highly Ordered Rectangular Nanoparticle Superlattices by the Cooperative Self-Assembly of Nanoparticles and Fatty Molecules Takuya Harada and T. Alan Hatton* Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 Received January 2, 2009. Revised Manuscript Received March 27, 2009 We demonstrate the formation of highly ordered twofold symmetric rectangular nanoparticle superlattices by the slow evaporation of solvent from colloidal dispersions of oleic acid/oleylamine-coated Fe3O4 nanoparticles on a water surface. These superlattices covered regions of micrometers in size without any noticeable disorders or defects, with size controlled by the amount of oleic acid added to the colloidal dispersions. The superlattices were transformed into arrays of nanowires by subsequent calcination. The peculiar nanoparticle assemblies are discussed in terms of the cooperative self-assembly of nanoparticles and fatty molecules during the slow evaporation of solvent.
Introduction The development of monodisperse nanoparticle synthesis methods has expanded the range of possibilities for the fabrication of controlled nanoparticle assemblies.1-5 These assemblies are expected to lead to various advanced functional materials such as ultrahigh density information storage (magnetic recording media),6,7 single electron tunneling devices,8,9 ultrafine electrical circuits,10 novel electrochromic devices,11 and high-sensitivity sensors.12 It is well-known that the properties of these nanoparticle-based materials depend strongly on the manner in which the nanoparticles align in these assemblies. The existence of disorders or defects in the assembled structures will diminish their utility in these various applications. Thus, the exploration of methods to control nanoparticle assembly in a well-defined manner and the fabrication of highly ordered long-range periodic structures with nanoparticles, i.e., “nanoparticle superlattices”, is an important challenge for the further development of a range of advanced applications of nanoparticles. On the basis of these considerations, various approaches for the construction of ordered nanoparticle superlattices have been *To whom correspondence should be addressed. Phone: 617-253-4588, Fax: 617-253-8723, E-mail:
[email protected]. (1) Sun, S.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. J. Am. Chem. Soc. 2004, 126, 273. (2) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (3) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (4) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (5) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545. (6) Sun, S. Adv. Mater. 2006, 18, 393. (7) Matsushita, T.; Masuda, J.; Iwamoto, T.; Toshima, N. Chem. Lett. 2007, 36, 1264. (8) Black, C. T.; Murray, C. B.; Sandstorm, R. L.; Sun, S. Science 2000, 290, 1131. (9) Schmid, G. Adv. Eng. Mater. 2001, 3, 737. (10) Hoeppener, S.; Maoz, R.; Cohen, S. R.; Chi, L. F.; Fuchs, H.; Sagiv, J. Adv. Mater. 2002, 14, 1036. (11) Deb, S. K. Sol. Energy Mater. Sol. Cells 2008, 92, 245. (12) Tan, S.-Z.; Hu, Y.-J.; Chen, J.-W.; Shen, G.-L; Yu, R.-Q. Sens. Actuators, B 2007, 124, 68. (13) Motte, L.; Billoudet, F.; Lacaze, E.; Douin, J.; Pileni, M. P. J. Phys. Chem. B 1997, 101, 138. (14) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3358. (15) Kanehara, M.; Oumi, Y.; Sano, T.; Teranishi, T. J. Am. Chem. Soc. 2003, 125, 8708.
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attempted by many groups.2,13-30 Solvent evaporation from colloidal dispersions of nanoparticles on smooth solid substrates has been used most commonly as a reasonable way to obtain relatively well ordered 2D and 3D nanoparticle superlattices.2,13-19 In this approach, it is suggested that the utilization of lowvolatility solvents,16 an increase in the surfactant concentration,17 the utilization of a number of different kinds of surfactant molecules,18 and heat annealing19 can have beneficial effects on the construction of well-ordered nanoparticle superlattices. Likewise, the evaporation of solvent from a colloidal dispersion at an airwater interface has also been proposed as a potential method for the fabrication of homogeneous nanoparticle superlattices.20-26 In this case, the nanoparticle assemblies are obtained by the deposition and drying of colloidal dispersions of nanoparticles coated with amphiphilic surfactant molecules on the water surface. Methods for the construction of large-scale ordered nanoparticle assemblies include a three-solvent-layer technique for controlled oversaturation with nonsolvent diffusion,27-29 as well as nanoparticle assembly in a freely suspended soap film.30 In these previous attempts at self-assembly, however, the nanoparticle superlattices obtained were usually highly symmetric structures, such as close-packed sixfold hexagonal lattices, (16) Puntes, V. F.; Krishnan, K. M.; Alivisatos, P. Appl. Phys. Lett. 2001, 78, 2187. (17) Lin, X. M.; Jaeger, H. M.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 2001, 105, 3353. (18) Kim, Y. H.; Lee, D. K.; Cha, H. G.; Kim, C. W.; Kang, Y. S. Chem. Mater. 2007, 19, 5049. (19) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325. (20) Santhanam, V.; Liu, J.; Agarwal, R.; Andres, R. P. Langmuir 2003, 19, 7881. (21) Yang, Y.; Kimura, K. J. Phys. Chem. B 2006, 110, 24442. (22) Fried, T.; Shemer, G.; Markovich, G. Adv. Mater. 2001, 13, 1158. (23) Lee, D. K.; Kim, Y. H.; Kim, C. W.; Cha, H. G.; Kang, Y. S. J. Phys. Chem. B 2007, 111, 9288. (24) Chen, S. Langmuir 2001, 17, 2878. (25) Kanehara, M.; Kodzuka, E.; Teranishi, T. J. Am. Chem. Soc. 2006, 128, 13084. :: (26) Aleksandrovic, V.; Greshnykh, D.; Randjelovic, I.; Fromsdorf, A.; Kornowski, A.; Roth, S. V.; Klinke, C.; Weller, H. ACS Nano 2008, 2, 1097. (27) Talapin, D. V.; Shevchenko, E. V.; Kornowski, A.; Gaponik, N.; Haase, M.; Rogach, A. L.; Weller, H. Adv. Mater. 2001, 13, 1868. (28) Shevchenko, E. V.; Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2002, 124, 11480. (29) Talapin, D. V.; Shevchenko, E. V.; Murray, C. B.; Titov, A. V.; Kral, P. Nano Lett. 2007, 7, 1213. (30) Wei, Q. -H.; Cupid, D. M.; Wu, X. L. Appl. Phys. Lett. 2000, 77, 1641.
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fourfold cubic square lattices, and threefold quasi-honeycomb lattices. To enhance the versatility of nanoparticle superlattices, more complex and low symmetry ordered structures are also strongly desired,31-33 especially in the case of magnetic materials, where it has been reported that the magnetic reversal and magneto-transport properties are strongly affected by the symmetry of the lattice arrangements,34-39 and the presence of anisotropic periodic grooves or surface scratches can enhance the coercivity and remanent fields.40 For structural control of nanoparticle assembly, various procedures have been proposed, such as utilization of anisotropic nanoscale templates,30,31,41 the application of external magnetic fields,42-44 the tuning of interligand interactions,45,46 the hybridization of multiple-sized nanoparticles,47-50 and the utilization of nonspherical (cubic or rodlike) nanoparticles.51-57 Nevertheless, to our knowledge, the spontaneous self-assembly of single-sized spherical nanoparticles into low-symmetry structures, such as twofold symmetric rectangular nanoparticle superlattices, with long-range periodic order has never been established. We have developed a simple procedure to prepare large, highly ordered magnetic nanoparticle superlattices with low symmetry order. We discovered that highly ordered planar anisotropic rectangular nanoparticle superlattices can be constructed when colloidal dispersions of Fe3O4 nanoparticles coated with oleic acid and oleylamine in a low-volatility hydrocarbon (n-decane) are dried slowly on a water surface. We further clarified that the ordered superlattice can be enlarged by dissolving additional oleic acid in the initial colloidal dispersions and can be transformed into arrays of nanowires by subsequent calcination. In this article, we report the details of the formation of these highly ordered rectangular nanoparticle superlattices and arrays of nanowires, and discuss a possible mechanism for such peculiar nanoparticle assemblies. (31) Ye, Y. -H.; Badilescu, S.; Truong, Vo-Van; Rochon, P.; Natansohn, A. Appl. Phys. Lett. 2001, 79, 872. (32) Teranishi, T.; Sugawara, A.; Shimizu, T.; Miyake, M. J. Am. Chem. Soc. 2002, 124, 4210. (33) Zhao, L. L.; Kelly, K. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 7343. (34) Jun, Y. -w.; Choi, J.-s.; Cheon, J. Chem. Commun. 2007, 1203. (35) Yang, T.; Shen, C.; Li, Z.; Zhang, H.; Xiao, C.; Chen, S.; Xu, Z.; Shi, D.; Li, J.; Gao, H. J. Phys. Chem. B 2005, 109, 23233. (36) Wang, C. C.; Adeyeye, A. O.; Singh, N. Nanotechnology 2006, 17, 1629. (37) Poddar, P.; Telem-Shafir, T.; Fried, T.; Markovich, G. Phys. Rev. B 2002, 66, 060403. (38) Russier, V.; Petit, C.; Pileni, M. P. J. Appl. Phys. 2003, 93, 10001. (39) Luo, Y.; Misra, V. Nanotechnology 2006, 17, 4909. (40) Twisselmann, D. J.; Farhoud, M.; Smith, H. I.; Ross, C. A. J. Appl. Phys. 1999, 85, 4292. (41) Rezende, C. A.; Lee, L. -T.; Galembeck, F. Langmuir 2007, 23, 2824. (42) Lalatonne, Y.; Richardi, J.; Pileni, M. P. Nat. Mater. 2004, 3, 121. (43) Petit, C.; Legrand, J.; Russier, V.; Pileni, M. P. J. Appl. Phys. 2002, 91, 1502. (44) Keng, P. Y.; Shim, I.; Korth, B. D.; Douglas, J. F.; Pyun, J. ACS Nano 2007, 1, 279. (45) Yamada, M.; Shen, Z.; Miyake, M. Chem. Commun. 2006, 2569. (46) Shen, Z.; Yamada, M.; Miyake, M. J. Am. Chem. Soc. 2007, 129, 14271. (47) Shevchenko, E. V.; Talapin, D. V.; Murray, C. B.; O’Brien, S. J. Am. Chem. Soc. 2006, 128, 3620. (48) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature (London) 2006, 439, 55. (49) Shevchenko, E. V.; Kortright, J. B.; Talapin, D. V.; Aloni, S.; Alivisatos, A. P. Adv. Mater. 2007, 19, 4183. (50) Chen, Z.; Moore, J.; Radtke, G.; Sirringhaus, H.; O’Brien, S. J. Am. Chem. Soc. 2007, 129, 15702. (51) Li, M.; Schnablegger, H.; Mann, S. Nature (London) 1999, 402, 393. (52) Wang, Z. L.; Dai, Z.; Sun, S. Adv. Mater. 2000, 12, 1944. (53) Zeng, H.; Rice, P. M.; Wang, S. X.; Sun, S. J. Am. Chem. Soc. 2004, 126, 11458. (54) Chen, M.; Kim, J.; Liu, J. P.; Fan, H.; Sun, S. J. Am. Chem. Soc. 2006, 128, 7132. (55) Halder, A.; Ravishankar, N. J. Phys. Chem. B 2006, 110, 6595. (56) Song, Q.; Ding, Y.; Wang, Z. L.; Zhang, Z. J. J. Phys. Chem. B 2006, 110, 25547. (57) An, K.; Lee, N.; Park, J.; Kim, S. C.; Hwang, Y.; Park, J.-G.; Kim, J.-Y.; Park, J.-H.; Han, M. J.; Yu, J.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 9753.
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Experimental Section Materials. Iron tri(acetylacetonate) (Fe(acac)3)(97%), 1,2tetradecanediol (90%), oleic acid (OA) (90%), oleylamine (OAm) (70%), and benzyl ether (99%) were purchased from Sigma Aldrich. n-Decane (99%) was purchased from Alfa Aesar. Methanol (99.8%) was purchased from Mellinkrod. All chemicals were used as received. All water utilized in the experiments was Milli-Q (Millipore) deionized water. Magnetic Nanoparticle Preparation. For the formation of highly ordered nanoparticle superlattices, we first prepared colloidal dispersions of Fe3O4 nanoparticles coated with oleic acid and oleylamine as described elsewhere.58 In brief, iron tri(acetylacetonate) (2 mmol), 1,2-tetradecanediol (10 mmol), oleic acid (6 mmol), oleylamine (6 mmol), and benzyl ether (20 mL) were mixed and stirred magnetically under flowing nitrogen. The mixture was heated gradually to 200 °C and kept at this temperature for 2 h. Then, the temperature was increased up to the reflux condition (∼300 °C) under a blanket of nitrogen, and kept for 1 h at reflux. The black reacted liquid was cooled to room temperature by air-cooling and transferred from the reaction flask to a centrifugation bottle. On addition of methanol (∼40 mL) to the reaction mixture, the black nanoparticles precipitated and were separated via centrifugation (9000 rpm, 10 min). To remove the residual reacting materials, the precipitated nanoparticles were rinsed with methanol several times. After the precipitated nanoparticles were well-dried, 10 mL of n-decane was added to the precipitate and the mixtures were ultrasonicated. Finally, the colloidal dispersions of Fe3O4 nanoparticles used for the formation of nanoparticle superlattices were obtained by mixing 0.09 mL of above dispersions and 0.01 mL of various concentrations of oleic acid solutions in n-decane. Magnetic Nanoparticle Superlattice Formation. The formation of nanoparticle superlattices from the colloidal dispersions was carried out on the water surface as follows. First, polystyrene Petri dishes (100 mm 20 mm) were half-filled with deionized water. Then, 0.02 mL of the colloidal dispersion of Fe3O4 nanoparticles was deposited gently on the water surface without further dilution. The system dish and contents were kept for 12 h at room temperature with dish covers lightly attached to reduce the evaporation rate of the solvent. After the solvent (n-decane) was totally evaporated, assembled nanoparticle films with nanoparticle superlattices floating on the water surface were obtained. The nanoparticle films were transferred to carboncoated copper grids and glass substrates by the horizontal lifting method for the transmission electron microscopy (TEM) and atomic force microscopy (AFM) measurements. TEM measurements were performed on a JEOL JEM2011 (200 kV) microscope, and AFM measurements were performed on a Veeco Dimension DI3100 atomic force microscope using the tapping mode.
Results and Discussion Figure 1 shows TEM images under different magnifications for a nanoparticle film obtained using a colloidal dispersion prepared by mixing with 0.053 mol/L oleic acid solutions. Figure 1a shows a low-magnification image of the nanoparticle film. We observe a multidomain structure that is distinguished by the different contrasts (dark, gray, and bright). The nanoparticle film includes multiple numbers of discrete dark and gray islands within a continuous sea of the bright domain. The dark area is composed of a single domain of a highly ordered nanoparticle superlattice as shown in the medium magnification image in Figure 1b. The spherical nanoparticles of 6 nm in average diameter are aligned quite regularly to form a highly ordered nanoparticle superlattice with no discernible defects or disarrangements, homogeneous over the entire dark area. On the other hand, the peripheral gray (58) Lattuada, M.; Hatton, T. A. Langmuir 2007, 23, 2158.
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Figure 1. TEM images of assembled nanoparticle films obtained by the slow evaporation of solvent from drops of a colloidal dispersion mixed with a 0.053 mol/L oleic acid solution placed on the water surface. (a) Low-magnification image showing three distinct assembled nanoparticle domains. (b) Medium-magnification image showing highly ordered array of assembled nanoparticles (superlattice). (c) Highmagnification image of the highly ordered nanoparticle superlattice.
Figure 2. Effect of oleic acid concentration on (a) the diameter and (b) the area of superlattice domains (dark areas) in the assembled nanoparticle films. Points and histograms in (a) indicate the average value and size distributions for the assembled nanoparticle films, respectively.
and bright areas are both composed of relatively disordered assemblies of nanoparticles. Figure 1c shows a high-magnification image of a small section of the highly ordered nanoparticle superlattice in the dark areas. It is interesting to note that the interparticle spacing in the square lattice of nanoparticles is different in the two orthogonal directions defining the superlattice structure. Nanoparticles are closely adjacent in one direction, whereas they are separated by about 2 nm in the other direction. The anisotropic interparticle spacing results in a twofold symmetric rectangular lattice array of nanoparticles. The domain size of the ordered nanoparticle superlattices can be controlled by the amount of oleic acid added to the colloidal dispersions. Figure 2 shows the (a) diameters and (b) areas of the circular superlattice domains (dark areas) in the nanoparticle assembled films made from colloidal dispersions with different concentrations of oleic acid. The size distributions of the superlattice domains in the nanoparticle assembled films are also shown in Figure 2a. The results clearly indicate that the ordered superlattices span several micrometers in size and are larger the Langmuir 2009, 25(11), 6407–6412
higher the oleic acid concentration in the colloidal dispersions, as shown in Figure 2b, where the area of an ordered superlattice is seen to be linearly proportional to the amount of added oleic acid. It is important to note that the features of the nanoparticle assemblies are not affected by the amount of oleic acid added under the conditions examined, i.e., the dark areas in the nanoparticle assembled films are composed of highly ordered rectangular nanoparticle superlattice with the same interparticle distances and the lattice aspect ratio for all the nanoparticle films. To further understand the details of the assembled nanoparticle structures, we examined the features of the three-dimensional nanoparticle stacks by AFM. Figure 3 shows (a) a typical threedimensional AFM image measured for a nanoparticle film and (b) the height profile of this region along the dashed line in Figure 3a. We can identify a specific mesa-like island structure with an inner circular capping area similar to the island domains observed in the low magnification TEM image (gray and dark areas in Figure 1a). The island structure has essentially a two-dimensional flat top surface, with a height above the surrounding sea area of particles DOI: 10.1021/la900013r
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Figure 3. (a) Typical three-dimensional AFM image of nanoparticle assemblies obtained using a colloidal dispersion prepared by mixing with 0.053 mol/L oleic acid solutions and (b) the height profile for the cross-sectional area denoted by the dashed line in (a). Trigonal marks in Figure 3b indicate the edge positions of superlattice domain.
of about 150 nm, which indicates a stacking of about 25 layers of nanoparticles. The height of the island structures increases gradually with increasing addition of oleic acid to the colloidal dispersions. However, the features of the mesa-like island structures are identical in all of the nanoparticle films. As noted in Figure 3b, the height difference between the inner circular capping area and the peripheral island surface is about 10 nm. This height difference varied only slightly with the concentration of oleic acid added to the nanoparticle colloidal dispersions and is within 6-14 nm in all of the nanoparticle films. These results indicate that the ordered nanoparticle superlattices are present as single or double layers of nanoparticles on the mesa-like flat tops of the three-dimensional stacked nanoparticle assemblies. The highly ordered nanoparticle superlattices can be transformed into arrays of nanowires by calcination at 450 °C for 1 h under flowing nitrogen, as shown in the TEM image in Figure 4. The nanowires are several micrometers in length and 6 nm in diameter and are aligned regularly in a parallel array. It is thought that the nanoparticles in the superlattice were accreted preferentially in their proximal directions through the calcination and transformed into these nanowires. As noted earlier, there are many reports on methods for the fabrication of ordered nanoparticle assemblies by solvent evaporation from nanoparticle colloidal dispersions. In particular, solvent evaporation at the air-water interface similar to our method has often been used as a useful technique to obtain extensive homogeneous nanoparticle assemblies. Such assemblies of single-sized nanoparticles obtained by other groups, however, were limited to relatively high symmetry structures (such as sixfold hexagonal or fourfold tetragonal), and the highly ordered twofold rectangular nanoparticle superlattices with long-range periodic order as observed in our work have never been reported. The specific three-dimensional nanoparticle stacks composed of single or double layers of nanoparticle superlattices on the mesalike flat-top nanoparticle assemblies is also a unique feature of our 6410
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Figure 4. TEM image of array of nanowires obtained by calcination of a nanoparticle superlattice at 450 °C for 1 h under flowing nitrogen. (The calcined superlattice was obtained using a colloidal dispersion prepared by mixing with 0.053 mol/L oleic acid solutions.)
results. In the previous reports presenting the formation of threedimensional nanoparticle stacks following the evaporation of solvent from nanoparticle colloidal dispersions,27-29,59,60 the nanoparticle assemblies obtained were dome-like in shape59 or hexagonal platelets,27-29 and did not have the mesa-like structure observed in our samples. The specific nanoparticle assemblies in our samples were constructed through the gradual self-assembly of nanoparticles with fatty molecules during the slow evaporation of the solvent. The nanoparticle colloidal dispersions placed on the water surface included not only the suspended nanoparticles but also freely dispersed oleic acid and oleylamine. The dispersed fatty molecules arise partly from those used in the nanoparticle synthesis and partly from the additionally added oleic acid solutions. The nanoparticle colloidal dispersions spread quickly to form a homogeneous thin liquid film over the water surface due to a reduction in surface energy as the fatty molecules in the colloidal dispersions adsorbed at the interface between the water and the colloidal dispersion. The increase in particle concentration within this film as the solvent evaporated resulted in an increase in the interparticle attractive interactions that caused fluctuations in the thickness of the floating film, which eventually broke up into a large number of smaller, particle-enriched droplets. We postulate that the specific three-dimensional nanoparticle assemblies (gray and dark areas in Figure 1a) reflect directly the formation of these droplets and that the disordered nanoparticle assemblies in the sea area (bright regions in Figure 1a) were the nanoparticles stranded on the water surface during the droplet formation process. It is important to note that the highly ordered nanoparticle superlattices were present as single or double layers on the dozen or more layers of relatively disordered nanoparticle stacks. We suggest here that the formation of the specific three-dimensional nanoparticle assemblies occurs by two different sequential nanoparticle self-assembly processes within the clusters of droplets. In the first instance, as the solvent evaporates from the clustered (59) Motte, L.; Billoudet, F.; Lacaze, E.; Pileni, M. P. Adv. Mater. 1996, 8, 1018. (60) Yamamuro, S.; Farrell, D. F.; Majetich, S. A. Phys. Rev. B 2002, 65, 224431.
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Figure 5. Schematic illustrations of (a) molecular assembly of oleic acid and oleylamine and (b) nanoparticle superlattice constructed by the cooperative self-assembly of nanoparticles and fatty molecules.
droplets, the concentration of dispersed nanoparticles increases significantly until it exceeds the dispersion limit, and the excess nanoparticles begin to precipitate and accumulate at the bottom of the droplets. We conclude that the relatively disordered lower layers of the mesa-like island structures grew by this accumulation. The second process is a coprecipitation, or “cooperative selfassembly”, of nanoparticles and fatty molecules during the final stages of the solvent evaporation. As noted earlier, the colloidal dispersions of nanoparticles include the freely dispersed oleic acid and oleylamine, as well as suspended nanoparticles. With the solvent evaporation from the colloidal dispersions, the fatty molecules are also gradually concentrated, and eventually reach their solubility limit. On the basis of several reports that fatty molecules precipitate and self-assemble into highly ordered molecular arrays from saturated solutions,61,62 it is reasonable to infer that similar ordered molecular assemblies emerge in our highly condensed droplets. The rectangular conformation with the specific anisotropic interparticle spacing observed in our nanoparticle superlattices is not the thermodynamically and kinetically favored packing symmetry for spherical nanoparticles.29,60,63 On the basis of these considerations, we conclude that the nanoparticle superlattices are constructed by the hybridization of these assembled fatty molecules and nanoparticles. Figure 5 shows a schematic illustration of the expected structures of our heterogeneous fatty molecule-nanoparticle assemblies. The oleic acid and oleylamine can be anticipated to align in parallel and alternating directions, and result in the ladder-like molecular arrays shown in Figure 5a.61,62 These molecules interact strongly with each other through the van der Waals forces between the adjacent alkyl chains. The fatty molecules surrounding the nanoparticles in the colloidal dispersions should also take part in the molecular assemblies, in which hydrophilic carboxyl and amine groups locate at the outer edges of the arrays. Since these functional groups have an affinity for the Fe3O4 nanoparticle surface through hydrogen bonding and chelation, the nanoparticles are expected to align with the functional groups, wedged between the ladder-like molecular arrays. This assembly process results in the formation of linear, parallel, and interdigitated assemblies of molecular arrays and nanoparticles, as shown in (61) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (62) Hibino, M.; Sumi, A.; Hatta, I. Jpn. J. Appl. Phys. 1995, 34, 610. (63) Norris, D. J.; Arlinghaus, E. G.; Meng, L.; Heiny, R.; Scriven, L. E. Adv. Mater. 2004, 16, 1393.
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Figure 5b. The molecular length of oleic acid and oleylamine is about 2 nm, and thus, the schematic illustration of nanoparticle assembly is consistent with the actual nanoparticle assembly structure observed by TEM. The formation of nanowires by calcination of the superlattices also provides supportive evidence for our conclusion that the nanoparticle assembled chains are separated by the arrays of fatty molecules. It is important to note that the nanoparticles in the highly ordered nanoparticle superlattices aligned not only linearly in a direction parallel to the fatty molecules arrays (y-axis), but also maintain alignments in the perpendicular direction (x-axis). This arrangement of nanoparticles can be attributed to the interparticle van der Waals attractive force. It is well-known that the van der Waals interaction between nanoparticles acts over relatively large length scales and encourages a close-packed arrangement of the nanoparticles.29,64,65 In this case, however, the interactions are modulated by the fatty molecule arrays, and the energetically most favorable arrangement for the nanoparticles is in the direction normal to the fatty molecule arrays. The highly ordered nanoparticle superlattices observed here are present as single or double layers of nanoparticles, regardless of the size of the ordered area. The results may indicate that the cooperative self-assembly process occurs at specific boundary regions, such as at the droplet air-solvent interface.
Conclusion We have studied the self-assembly of Fe3O4 nanoparticles during the slow evaporation of solvent from droplets of a colloidal dispersion of oleic acid/oleylamine-coated Fe3O4 nanoparticles on a water surface. Highly ordered nanoparticle superlattices with twofold rectangular symmetry are formed during the evaporation process. The ordered superlattices exist as single or double layers of nanoparticles on a mesa-like three-dimensional disordered stack of nanoparticles. The superlattices span micrometers in size with no discernible disorders or defects; the size is controlled by the amount of oleic acid in the colloidal dispersion. These superlattices can be transformed into well-aligned arrays of nanowires by subsequent calcination. The formation of the specific rectangular nanoparticle superlattices is consistent with a cooperative self-assembly of the nanoparticles and fatty (64) Hamaker, H. C. Physica 1937, 4, 1058. (65) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. Rev. Lett. 1995, 75, 3466.
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molecules during the slow evaporation of solvent from the droplets such that the nanoparticles are wedged between ladderlike arrays of the fatty molecules, and the line-by-line assembly leads to the formation of twofold symmetry rectangular lattices.
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Acknowledgment. We thank Dr. Yong Zhang at the Center for Materials Science and Engineering in MIT for his technical support on the TEM measurements. We also thank The Furukawa Electric Co., Ltd., for their financial support to this work.
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