Controlled Nanoparticle Assembly by Dewetting of Charged Polymer

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Controlled Nanoparticle Assembly by Dewetting of Charged Polymer Solutions Lay-Theng Lee,* Carlos A. P. Leite, and Fernando Galembeck Instituto de Quı´mica, Universidade Estadual de Campinas, P. O. Box 6154, 13084-971 Campinas, SP, Brazil Received January 21, 2004. In Final Form: March 2, 2004 In this paper, we present an alternative approach for controlled nanoparticle organization on a solid substrate by applying dewetting patterns of charged polymer solutions as a templating system. Thin films of charged polymer solutions dewet a solid substrate to form complex dewetting patterns that depend on the polymer charge density. These patterns, ranging from polygonal networks to elongated structures that are stabilized by viscous forces during dewetting, serve as potential templates for two-dimensional nanoparticle organization on a solid substrate. Thus, while nanoparticles dried in pure water undergo self-assembly to form close-packed arrays, addition of charged polymer in the dispersion leads to the formation of open structures that are directed by the dewetting patterns of the polymer solution. In this study, we focus on the application of elongated structures resulting from dewetting of high-charge-density polymer solutions to align nanoparticles of silica and gold into long chains that are several micrometers in length. The particle ordering process is a two-step mechanism: an initial confinement of the nanoparticles in the dewetting structures and self-assembly of the particles within these structures upon further drying by lateral capillary attractions.

Introduction In recent years, interest in nanoparticle self-assembly has centered on the generation of open and complex structures in two dimensions and of nanowires in one dimension. Such structures are of importance in view of their potential applications in micro- and nano-electronic devices.1,2 Self-assembly alone produces close-packed arrays; therefore, some degree of control over the assembly process is required to form open and complex structures. Different approaches for directed self-assembly have included application of external electric force fields,3-6 colloidal deposition on substrates that are prepatterned chemically7 or ionically,8 and making use of diblock copolymer microphases.9 Some interesting and promising methods for particle assembly in one dimension have also been reported. Here, alignment of nanoparticles into long chains to form nanowires has been achieved by using biomolecular templates such as phospholipid tubules,10 DNA molecules,11-17 protein fibers,18 and tobacco mosaic * Corresponding author. E-mail: [email protected]. Permanent address: Laboratoire Le´on Brillouin (CEA-CNRS), CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France. (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Heath, J. R.; Kuekes, P. J.; Snyder, G.; Williams, R. S. Science 1998, 280, 1717. (3) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706. (4) Yeh, S. R.; Seul, M.; Shraiman, B. I. Nature 1997, 386, 57. (5) Hermanson, K. D.; Lumsdon, S. O.; Williams, J. P.; Kaler, E. W.; Velev, O. D. Science 2001, 294, 1082. (6) Lumsdon, S. O.; Kaler, E. W.; Williams, J. P.; Velev, O. D. Appl. Phys. Lett. 2003, 82, 949. (7) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (8) Aizenberg, J.; Braun, P. V.; Wiltzius, P. Phys Rev. Lett. 2000, 84, 2997. (9) Zehner, R. W.; Lopes, W. A.; Morkved, T. L.; Jaeger, H.; Sita, L. R. Langmuir 1998, 14, 241. (10) Schnur, J. M.; Proce, R.; Schoen, P.; Yager, P.; Clavert, J. M.; Georger, J.; Singh, A. Thin Solid Films 1987, 152, 181. (11) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (12) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P., Jr.; Schultz, P. G. Nature 1996, 382, 609. (13) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775.

viruses.19 Carbon nanotubes have also been used as templates to assemble bundles of gold particles.20-22 Other methods of controlled particle alignment apply directional flows23 and artificial24 and molecular templates.25,26 In this paper, we present another potential method for nanoparticle alignment by applying complex dewetting patterns of charged polymer solutions as a templating system. This method is based on a competition of confinement of particles within the dewetting morphologies and particle self-assembly. Dewetting of charged polymer solutions on a hydrophilic solid substrate produces a rich variety of patterns. It has been shown in our recent study27 that thin films of aqueous charged polymer solutions dewet a hydrophilic solid substrate to form complex morphologies that include classic features such as holes, polygonal networks, bicontinuous structures, and droplets. Further, interesting novel patterns consisting of elongated structures are observed when the polymer charge density is (14) Ford, W. E.; Harnack, O.; Yasuda, A.; Wessels, J. M. Adv. Mater. 2001, 13, 1793. (15) Monson, C. F.; Woolley, A. T. Nano Lett. 2003, 3, 359. (16) Nakao, H.; Shiigi, H.; Yamamoto, Y.; Tokonami, S.; Nagaoka, T.; Sugiyama, S.; Ohtani, T. Nano Lett. 2003, 3, 1391. (17) Karanas, A. G.; Wang, Z. X.; Bates, A. D.; Brust, M. Angew. Chem., Int. Ed. 2003, 42, 191. (18) Scheibel, T.; Parthasarathy, R.; Sawicki, G.; Lin, X.-M.; Jaeger, H.; Lindquist, S. L. Biophys. 2003, 100, 4527. (19) Dujardin, E.; Peet, C.; Stubbs, G.; Culver, J. N.; Mann, S. Nano Lett. 2003, 3, 413. (20) Fullam, S.; Cottell, D.; Rensmo, H.; Fitzmaurice, D. Adv. Mater. 2000, 12, 1430. (21) Jiang, K.; Eitan, A.; Schadler, L. S.; Ajayan, P. M.; Siegel, R. W. Nano Lett. 2003, 3, 275. (22) Ellis, A. V.; Vijayamohanan, K.; Goswami, R.; Chakrapani, N.; Ramanathan, L. S.; Ajayan, P. M.; Ramanath, G. Nano Lett. 2003, 3, 279. (23) Burghard, M.; Philipp, G.; Roth, S.; von Klitzing, K. Appl. Phys. A 1998, 67, 591. (24) Hutchinson, T. O.; Liu, Y.-P.; Kiely, C.; Kiely, C. J.; Brust, M. Adv. Mater. 2001, 13, 1800. (25) Reuter, T.; Vidoni, O.; Torma, V.; Schmid, G.; Nan, L.; Gleiche, M.; Chi, L.; Fuchs, H. Nano Lett. 2002, 2, 709. (26) Osterloh, F. E.; Martino, J. S.; Hiramatsu, H.; Hewitt, D. P. Nano Lett. 2003, 3, 125. (27) Lee, L. T.; da Silva, M. D. C. V.; Galembeck, F. Langmuir 2003, 19, 6717.

10.1021/la049806t CCC: $27.50 © 2004 American Chemical Society Published on Web 04/27/2004

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increased. These elongated structures are formed during expansion of holes in the ruptured film, a phenomenon referred to as fingering that is attributed to Marangoni effects. However, instead of fragmenting into droplets as observed for low-charge-density polymer, these fingers are unusually stable and resist fragmentation at an elevated polymer charge density. Very long structures are, thus, formed as the dewetted hole expands. The unusual stability of these elongated structures is attributed to viscous effects arising from the stiffness of the charged polymer molecules in the drying film. These viscous forces counter capillary forces that are responsible for the fragmentation of elongated cylindrical structures. We show in this study the potential of these elongated dewetting structures as templates to align nanoparticles that are dispersed in the polymer solution. Because the template morphology depends on easily controlled parameters such as the polymer charge density, concentration, and dewetting rate,27 this method of controlled particle alignment holds an attractive feature in its relative simplicity, compared to other methods that require chemical modifications and ligand grafting onto the particles and on the templating substrate. Experimental Section The charged polymer is poly(N-isopropylacrylamide) (PNIPAM) (Mr ) 90K) decorated with negatively charged aggregates of dodecyl sulfate with sodium counterions (SDS). Native PNIPAM is a thermosensitive polymer with a lower critical solution temperature at around 32 °C. It interacts with SDS above the critical aggregation concentration to form a charged PNIPAMSDS complex that exhibits polyelectrolyte behavior in water.28 The polymer charge density is controlled by the mass ratio of surfactant to polymer, S/P. In this study, the initial polymer concentration, Cp, is varied from 10-5 to 10-4 g/mL, and the surfactant/polymer mass ratio is varied from S/P ) 0.5-2. This corresponds to 4-16 charged aggregates per polymer chain, respectively, estimated by taking the bound surfactant aggregation number to be 35.29 Most of the results presented in this paper are obtained at S/P ) 2 and at an enhanced drying rate at 40 °C because these are optimal conditions for forming stable elongated structures.27 The nanoparticles used in this study include Sto¨ber silica (1580 nm),30 commercial colloidal Au particles (5 and 15 nm) from Ted Pella, and Au particles grafted with β-cyclodextrin molecules (∼5 nm).31 The particles are dispersed in the respective polymer solutions at a particle concentration around 0.01%. A total of 10 µL of the dispersion is deposited on freshly cleaved mica and allowed to dry at 40 °C. The dry sample is imaged with an atomic force microscope (AFM; Topometrix Discoverer) in noncontact mode. Morphology images over larger areas of the sample are also acquired using a scanning electron microscope (SEM; JEOL JSM 6360 LV) operating at 15 keV. For SEM imaging, the same samples used for AFM measurements are mounted on a brass stub and coated with Au and Pd for 10 s. For samples containing Au nanoparticles, backscattered electron images are also acquired as a result of better contrast because backscattering depends on the atomic number of the element.

Results and Discussions In the present study, the objective is to control nanoparticle organization to produce open structures, such as unidimensional chains, using dewetting patterns of polymer solutions as templates. Two basic ideas are involved: particle self-assembly and dewetting behavior (28) Lee, L. T.; Cabane, B. Macromolecules 1997, 30, 6559. (29) Lissi, E. A.; Abuin, E. J. Colloid Interface Sci. 1985, 89, 41. (30) Costa, C. A. R. Private communication: the Sto¨ber silica was synthesized under basic conditions. (31) Gimenez, I.; Alves, O. L. Private communication: the β-cyclodextrin was grafted onto the Au particles by a dithiol functional C6 chain.

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of thin liquid films on a solid substrate. When an aqueous dispersion of particles in the absence of polymer is deposited on freshly cleaved mica and allowed to dry by evaporation, regions of two-dimensional aggregates are formed. These close-packed domains result from the selfassembly of particles, a process that has been described by a two-stage mechanism:32 (i) formation of a nucleus of an ordered phase and (ii) convective transport of particles toward the ordered domain. The formation of a nucleus takes place when the liquid level falls below the particle height, dp. If neighboring particles are close together, deformations of their menisci result in capillary attraction of the particles, forming a nucleus cluster. Once the nucleus is formed, further evaporation within the nucleus increases the local capillary suction, resulting in convective transport of particles to the ordered cluster. Thus, to form open structures from a drying dispersion of particles, a control over the nucleation stage of the ordering process is required. The approach adopted in the present study involves templating the nucleation process, by confinement of the particles within the dewetting morphologies of a charged polymer solution. Lateral capillary attractions of the particles confined in the dewetting structures further complete the organization process. The dewetting of thin films of charged polymer solutions offers a potential templating system for nanoparticle organization because of the variety of patterns that can be formed. In a recent paper,27 we reported on the dewetting behavior of thin films of aqueous PNIPAMSDS solutions on a hydrophilic solid substrate. It is shown that the charged polymer film undergoes spontaneous dewetting as a result of spinodal instability at film thicknesses less than 10 nm, a value that compares well with the theoretically calculated value for the spinodal regime. The dewetted film forms a wide range of patterns consisting of holes, polygonal networks, bicontinuous structures, and droplets, depending on the film thickness. The latter refers to the average thickness of the dewetted film structures, obtained by a line profile measurement or by critical dimension analysis of the AFM images.27 Note that although the relevant thickness should be the film thickness immediately prior to rupture, such a parameter is experimentally difficult to assess for a volatile system where, upon drying, the evaporation of solvent and thinning of the film occur simultaneously. Therefore, the value determined after film rupure is used, bearing in mind that this value is larger than the relevant dewetting film thickness because of the accumulation of polymeric materials from the dewetted areas. Beyond the spinodal regime, thicker films can exhibit metastability and dewet by heterogeneous or thermal hole nucleation, the former initiated by surface defects or contamination and the latter by thermal activation sufficient to overcome the potential barrier for nucleation of a dry spot.33 For the present system, film rupture by hole nucleation takes place at film thicknesses of 10-20 nm. These values are determined by line profile analysis performed on a partially ruptured film, and the thickness of the unruptured area in the vicinity of the hole is taken as the dewetting film thickness (see Figures 1a,e). Irrespective of the mode of hole nucleation, the final dewetting pattern depends on polymer charge density; these patterns vary from less organized droplets to droplets arranged in polygons to elongated structures with in(32) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. (33) Seemann, R.; Herminghaus, S.; Jacobs, K. Phys. Rev. Lett. 2001, 86, 5534.

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Figure 1. AFM images (5 × 5 µm) of the dewetting of the polymer solution alone at Cp ) 10-4 g/mL at various polymer charge densities: (a) S/P ) 0.5 (early stage); (b) S/P ) 0.5 (late stage); (c) S/P ) 1.0; and (d) S/P ) 2.0. Late-stage dewetting shows droplets arranged in a polygonal pattern for S/P ) 0.5 and 1.0 and elongated structures for S/P ) 2.0. Part e shows a line-scan profile of the partially ruptured film shown in part a (solid line from top to bottom). The unruptured area in the vicinity of the holes is about 10-20 nm; this range of values is taken as the thickness of the dewetting film.

creasing polymer charge density. Similar structures have been reported in our previous studies27 but are shown here in Figures 1a-d for easier reference. In this study, we focus on the elongated structures produced at an elevated polymer charge density as a template for the alignment of nanoparticles into unidimensional chains. The particles are dispersed in the polymer solution, and the dispersion is allowed to dry under similar conditions on freshly cleaved mica. Recall that this process entails confinement of the particles in the dewetting morphologies prior to interparticle attractions: liquid dewetting of the solid substrate precedes particle self-assembly. This sequence requires the particle size to be smaller than the thickness of the dewetting film; for the present system, this thickness is considered to be around 10-20 nm, as shown in Figure 1. On the other hand, if the particle size is larger than the thickness of the dewetting film, the liquid dewets the particle before it dewets the solid substrate; as mentioned above, this results in nucleation of the particle clusters. An illustration of this concept is shown in Figure 2. Figure 3a shows an AFM image of silica particles of size dp ) 80 nm dried in a high-charge-density polymer solution at Cp ) 10-4 g/mL, S/P ) 2. Here, the particle size is much larger than the thickness of the dewetting film, h ≈ 10-20 nm. The liquid level, thus, falls below the particle height before the liquid film reaches its dewetting thickness (illustrated in Figure 2a). Deformations of liquid menisci around the particles result in interparticle attractions, and close-packed domains are formed, as seen in the lower right-hand corner of Figure 3a. Subsequent dewetting of the polymer solution on the solid substrate, producing elongated morphologies, does not influence particle organization. These features can be seen in the upper left-hand corner of the same figure.

Figure 2. Schematic drawing of liquid dewetting versus particle ordering; dp is the particle height and h is the thickness of the dewetting liquid. (a) dp . h, particle self-assembly precedes dewetting, close-packed domains are formed; (b) dp ∼ h, simultaneous dewetting and self-assembly, some controlled particle ordering; (c) dp < h, dewetting precedes self-assembly, controlled particle ordering.

Figure 3b,c shows AFM images of silica particles of 30 nm dried in the same polymer solution. In this case, the particles are arranged in open structures and elongated bundles of several tens of micrometers in length. Thus, some templating effect is obtained when the particle size approaches the thickness of the dewetting film (illustrated in Figure 2b). With a further decrease in the particle size to 15 nm, long chains of silica particles are formed. These

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Figure 3. Sto¨ber silica particles dried in the polymer solution at Cp ) 10-4 g/mL, S/P ) 2. Effect of the particle size (dp) with respect to the thickness of polymer dewetting film (h ≈ 10-20 nm). AFM images (5 × 5 µm) of (a) dp ) 80 nm (dp . h), dewetting of polymer solution (upper left) does not affect particle organization, close-packed aggregates are formed (lower right); (b and c) dp ) 30 nm (dp f h), some controlled particle organization, bundles of silica particles are formed (z scale ≈ 80 nm); (d) dp ) 15 nm (dp ≈ h), particle ordering controlled by polymer dewetting, silica particles are aligned in chains several tens of micrometers in length (z scale ≈ 18-40 nm); (e) SEM (secondary electrons) of sample d (scale bar ) 5 µm).

Figure 4. Colloidal Au particles (≈15 nm) dried in the polymer solution at S/P ) 2 at different total polymer concentrations. AFM images (5 × 5 µm): (a) Cp )10-4 g/mL (z scale ≈ 40 nm); (b) Cp ) 5 × 10-5 g/mL (z scale ≈ 15 nm); and (c) SEM (backscattered electrons) of sample a (scale bar ) 5 µm).

chains are several tens of micrometers in length, and neighboring chains are arranged in a two-dimensional array. These results show the efficiency of the dewetting polymer solution as a templating system when the particle size is of the order of, or smaller than the thickness of, the dewetting film, as evidenced by a transition of particle oraganization from the close-packed domain to unidimensional chains. In all the above, although the images are obtained from 5 × 5 µm scans, several scans of different areas of the same sample show similar features. Thus, the features shown in the figures are representative over much larger areas. This is further supported by scanning electron micrographs obtained from the same samples used for AFM imaging. Figure 3e shows a SEM (secondary electrons) micrograph of sample 3d, demonstrating the alignment of long chains of silica particles over very large areas. Templating the nanoparticle organization by the dewetting of a polymer solution can also be achieved with other colloidal particles. Figure 4a shows similar organization of colloidal Au nanoparticles (15 nm) into long chains that are several tens of micrometers in length, although in this case they appear to be thicker than those of silica particles. The size of the chain can be decreased

Figure 5. AFM image (1.2 × 1.2 µm) of colloidal Au particles (≈5 nm) dried in the polymer solution at Cp ) 5 × 10-5 g/mL, S/P ) 2. The chains are several micrometers in length, z scale ≈ 4-7 nm.

by a small reduction in polymer concentration but keeping the charge density constant at a S/P ) 2. Figure 4b shows thin chains, obtained at a reduced polymer concentration (Cp ) 5 × 10-5 g/mL), containing single Au particles in the cross section, as indicated by the z scale (≈18 nm). A larger field of the Au chains is shown in the SEM (backscattered electrons) image (Figure 4c). In this case, as a result of

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Figure 6. AFM image (5 × 5 µm) of Au particles grafted with β-cyclodextrin molecules (≈5 nm) dried in the polymer solution at Cp ) 5 × 10-5 g/mL, S/P ) 2. The linear chains are several tens of micrometers in length and arranged in a two-dimensional array (z scale ≈ 6 nm). The figure on the right shows a line scan profile perpendicular to the chains.

Figure 7. AFM image (5 × 5 µm) of colloidal Au particles (≈5 nm) dried in the solution of polymer at a reduced charge density, Cp ) 5 × 10-5 g/mL, S/P ) 1. The particles are ordered in a polygonal network (z scale ≈ 6 nm).

the high atomic number of Au, backscattered electrons can be acquired to produce better constrast in the image. From the illustration in Figure 2c, good control of particle alignment can also be expected when the particle size is smaller than the thickness of the dewetting film, where dewetting of the polymer film clearly precedes nucleation of close-packed clusters. This is seen from the chain of 5-nm Au particles formed by drying in the polymer solution at Cp ) 5 × 10-5 g/mL and S/P ) 2 (Figure 5). Figure 6 shows chains of cyclodextrin-grafted Au particles (≈5 nm). Here, not only is there linear alignment of particles but also the chains form a long-range twodimensional array. As in previous cases, the chains are several tens of micrometers in length. These linear chains resemble those obtained for silica particles but are spaced

closer together. The interchain spacing is about 1.2 µm, as shown by the line profile taken perpendicular to the chains. The above results show formation of long chains of nanoparticles of various natures and particle sizes. This is achieved using as templates the elongated dewetting structures of high-charge-density polymer solutions. However, the templating system is not limited to elongated structures; other dewetting morphologies can also be applied as a template. For instance, at a low polymer charge density, elongated structures are unstable and the development of fingers during hole expansion is reduced. The dewetted film forms holes that meet to form a polygonal pattern, and the subsequent breakup of the ridges results in droplets arranged in polygons. Such a polygonal morphology provides another type of twodimensional open structure for nanoparticle ordering. The possibility to employ this dewetting morphology to template particle organization is shown in Figure 7, where 5-nm Au particles are dried in a solution of polymer at a reduced charge density, S/P ) 1. Here, the nanoparticles are arranged in a two-dimensional polygonal network. The above results show conclusively that nanoparticle organization, upon drying, can be controlled by the dewetting behavior of the liquid phase on the solid substrate. A transition from a close-packed structure to an open polygonal network to unidimensional long chains is obtained by variation of the polymer charge density (see Figure 8). Note that the application of dewetting as a template for particle organization requires zero or very weak interaction of the particle and the solid substrate.

Figure 8. AFM images of colloidal Au particles (≈5 nm) dried in (a) pure water and in polymer solutions at Cp ) 5 × 10-5 g/mL at different charge densities: (b) S/P ) 0.5; (c) S/P ) 1.0; (d) S/P ) 2.0. In the absence of nanoparticles, the dewetted polymer solution forms polygonal patterns for S/P ) 0.5 and S/P ) 1.0 and elongated structures for S/P ) 2. (a-c) 5 × 5 µm; (d) 1.2 × 1.2 µm.

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For the particles used in this study, the silica and the Au colloids are negatively charged and the cyclodextringrafted Au particles are neutral. It seems reasonable, therefore, to assume the absence of strong interactions of these particles with the mica surface. However, further studies in this aspect are warranted. Properties of the particles at the solid-liquid and air-liquid interfaces may play a role in the organization process and may help explain some differences in the alignment of the gold particles and that of the silica and cyclodextrin-grafted Au particles where long-range two-dimensional networks of linear chains are obtained. Conclusions The dewetting of charged polymer solutions is shown to be a potential method for controlled nanoparticle assembly on a solid substrate. The complex dewetting morphologies can be used as templates to direct nanoparticle ordering in one-dimensional linear chains as well as in two-dimensional open patterns such as a polygonal network. A two-step mechanism, an initial confinement of the nanoparticles in the dewetting structure and self-

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assembly of the confined particles upon further drying by lateral capillary attractions, is involved. For particles larger than the thickness of the dewetting film, selfassembly precedes dewetting and close-packed structures are formed. For particles of the order of, or smaller than, the thickness of the dewetting film, dewetting and pattern formation precede interparticle attraction. Thus, while dewetting morphologies are controlled by polymer solution properties, the templating of the nanoparticle assembly by these polymer films is controlled by the particle size with respect to the thickness of the dewetting film. The easy control of these parameters renders this approach highly attractive as a potential method for directed nanoparticle organization. Acknowledgment. We thank C. Costa for the Sto¨ber silica particles and I. Gimenez and O. L. Alves for the cyclodextrin-grafted Au particles. Financial support of FAPESP, pronex/Finep/MCT, and CNPq is acknowledged. This is a contribution from the Instituto Mileˆno de Materiais Complexo (PADCT). LA049806T