© Copyright 1998 American Chemical Society
JANUARY 20, 1998 VOLUME 14, NUMBER 2
Letters Selective Decoration of a Phase-Separated Diblock Copolymer with Thiol-Passivated Gold Nanocrystals Robert W. Zehner,† Ward A. Lopes,‡ Terry L. Morkved,‡ Heinrich Jaeger,‡ and Lawrence R. Sita*,† The Searle Chemistry Laboratory, Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, and The James Franck Institute, The University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637 Received October 6, 1997 Nanocrystalline gold particles passivated with alkane- and arenethiols were deposited on microphaseseparated ultrathin films of poly(styrene-block-methyl methacrylate). The nanocrystals preferentially adsorbed to the polystyrene phases over areas of the substrate as large as 10 µm2, as seen by transmission electron microscopy. The basis for the separation is postulated to be the difference in interaction energies between the thiol passivants and the two polymers. The resulting patterns of nanocrystals are more than an order of magnitude smaller than features which can be produced by optical lithography.
Introduction The emerging field of nanotechnology will require a toolbox of methods capable of producing and manipulating patterns on the nanometer scale. Optical lithographic techniques, the de facto methods of microscale patterning, are limited by the diffraction limit of light to roughly 100 nanometer resolution. Electron-beam and scanning probe lithography have already shown promise in producing smaller-scale features. However, both of these methods pattern the substrate sequentially, and therefore the patterning time scales linearly with the area to be patterned. Herein, we describe a technique that employs patterns in block copolymers as a template for the directed self-assembly of thiol-passivated metal nanocrystals. The limits of resolution for such patterning are dictated by the size of the metal particles and the width of the copolymer interface, both of which can approach 1 nm. Accordingly, by using this new technique, we are able to show that patterned assemblies can be produced that are an order of magnitude smaller than those that can be † ‡
The Searle Chemistry Laboratory. The James Franck Institute.
achieved by conventional optical lithography. In addition, the patterning takes place simultaneously over the entire surface, eliminating the effect of size on processing time. The source of the template is microphase separation in a diblock copolymer, a well-studied phenomenon.1,2 The morphologies produced by phase separation are determined by the relative lengths of the polymer blocks and can range from micelles to lamellae. The size of the polymer domains is determined by the overall chain length. Thus, block copolymers provide a wide range of selforganized nanoscale templates, which have already been utilized as masks for lithography3 or as reaction vessels for the production or binding of metal and semiconductor nanoparticles.4-6 However, these methods require extensive processing or the synthesis of specially functionalized copolymers, respectively. We take a new approach that involves adjusting the surface chemistry of the nanocrystals to tailor their interaction with the substrate. (1) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525-557. (2) Lohse, D. J.; Hadjichristidis, N. Curr. Opin. Polym. Interface Sci. 1997, 2, 171-176. (3) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401-1404.
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Figure 1. (a) TEM micrograph of a phase-separated, 600 Å thick, PS-b-PMMA copolymer film. The light areas correspond to the PMMA phase. Image dimensions were 1.7 µm × 1.75 µm. (b) TEM micrograph of alkanethiol-coated gold nanocrystals on a microphaseseparated PS-b-PMMA substrate, deposited from a dilute solution in toluene/ethanol. Image dimensions were 360 nm × 350 nm.
Our technique utilizes commercially available polymers and can be completed on the benchtop in a matter of minutes. An additional benefit is that, with this proofof-concept now demonstrated, it should be possible in the future to vary the chemical properties of both the passivating thiol (e.g., with ω-functionalized thiols) and the diblock copolymers in order to enhance the interaction between the polymer domains and passivated metal and semiconductor nanocrystals in a variety of ways. Experimental Section Dodecanethiol and gold tetrachloride trihydrate were obtained from Aldrich. Absolute ethanol (Aaper) and toluene (HPLC grade, Fisher) were used as received. The capping agent 4-(phenylethynyl)benzenethiol was synthesized by a previously published procedure.7 The poly(styrene-block-methyl methacrylate) (PSb-PMMA) diblock copolymer used as a substrate was from two separate batches, one purchased from Polymer Labs, Inc., and one purchased from PolySciences. The Polymer Labs material was purified by Soxhlet extraction with cyclohexane to remove polystyrene homopolymer and had a PS volume fraction of 0.66, an average molecular weight of 1.01 × 105, and a polydispersity of 1.09. The PolySciences polymer had a PS volume fraction of 0.74, an average molecular weight of 8.43 × 104, and a polydispersity of 1.08 and was used as received. Alkanethiol-coated gold nanocrystals were prepared via a twophase reduction in the presence of dodecanethiol.8 Nanocrystals were also synthesized with a conjugated, rigid-rod capping agent by substituting an equivalent amount of 4-(phenylethynyl)benzenethiol for the alkanethiol in the synthesis. This arenethiol has previously been demonstrated to form ordered monolayers on Au(111).9 These arenethiol-stabilized particles are qualitatively similar to the corresponding alkanethiol-coated particles but are less soluble in organic solvents. Their other (4) Moller, M.; Spatz, J. P. Curr. Opin. Polym. Interface Sci. 1997, 2, 177-187. (5) Cummins, C. C.; Beachy, M. D.; Schrock, R. R.; Vale, M. G.; Sankaran, V.; Cohen, R. E. Chem. Mater. 1991, 3, 1153-1163. (6) Fogg, D. E.; Radzilowski, L. H.; Blanski, R.; Schrock, R. R.; Thomas, E. L. Macromolecules 1997, 30, 417-426. (7) Hsung, R. P.; Babcock, J. R.; Chidsey, C. E. D.; Sita, L. R. Tetrahedron Lett. 1995, 36, 4525-4528. (8) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036-7041. (9) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319-3320.
notable property is a strong optical absorption at 300 nm, corresponding to the π-π* transition for the arenethiols,10 in addition to the gold plasmon absorbance at 525 nm.11 Polymers to be decorated were prepared as thin films on transparent silicon nitride substrates to allow for transmission electron microscopy (TEM) imaging.12,13 When the films were annealed under an argon atmosphere at 245 °C for 12 h, halfcylinders of PMMA surrounded by PS form in the film. The polymer domains exhibit a repeat spacing of between 53 and 60 nm, depending on the molecular weight employed and whether or not homopolymer has been extracted. To decorate the polymer substrates, 0.3-1 mg of nanocrystals was dissolved in 0.3 mL of toluene. The deep red solution was diluted to 3 mL with absolute ethanol and dropped with a pipet onto a 3 mm × 4 mm polymer substrate. After approximately 30 s, the solution was wicked off from the substrate by placing one edge in contact with a paper towel, and the sample was briefly allowed to air dry. Micrographs of the decorated polymer were taken in a Philips CM 120 transmission electron microscope operated at 120 keV.
Results and Discussion Figure 1a shows a representative area of a phaseseparated polymer film before decoration. The PMMA phase appears lighter than the PS in the micrograph due to electron beam thinning of the PMMA.14 After addition of a dilute solution of 1-2 nm diameter alkanethiol-coated gold nanocrystals (0.1 mg/mL), portions of the PS phase are densely covered with particles (Figure 1b). On some substrates, this low-concentration solution resulted in more even coverage of the surface, without signs of aggregation (Figure 2a). For both types of coverage, the selectivity of the nanocrystals for the PS phase is greater than 99%. (10) Dhirani, A.-A.; Lin, P.-H.; Guyot-Sionnest, P.; Zehner, R. W.; Sita, L. R. J. Chem. Phys. 1996, 106, 5249-5253. (11) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189-197. (12) Morkved, T. L.; Lopes, W. A.; Hahm, J.; Sibener, S. J.; Jaeger, H. M. Polymer, in press. (13) Morkved, T. L.; Lu, M.; Urbas, A. M.; Ehrics, E. E.; Jaeger, H. M.; Mansky, P.; Russell, T. P. Science 1996, 273, 931-933. (14) Thomas, E. L.; Talmon, Y. Polymer 1978, 19, 225-227.
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Figure 2. TEM micrographs of alkanethiol-coated gold nanocrystals deposited on a phase-separated PS-b-PMMA block copolymer film. (a) Disperse coverage observed for deposition from dilute solution. Image dimensions 615 nm × 690 nm. (b) Dense coverage resulting from deposition from a more concentrated solution. Note the presence of nanocrystals on the PMMA phases. Image dimensions 205 nm x 230 nm.
At higher concentrations (0.3 mg/mL), the particles are close-packed on the PS over more than 1 µm2 but are no longer entirely excluded from the PMMA (Figure 2b). At still higher concentrations, selective coverage decreases rapidly and the number of large aggregates of particles increases. Nonselective aggregation was also observed for solutions of larger (5 nm diameter) particles. Application of arenethiol-stabilized nanocrystals resulted in a disperse, low-density selective coverage of the surface, identical to that observed in Figure 2a. Attempts to produce more dense coverage of these particles were unsuccessful. The toluene-ethanol solvent system used in this procedure was chosen to avoid dissolving the thin polymer substrate while maintaining the solubility of the gold nanocrystals. Initial dissolution of the particles in toluene, followed by dilution in ethanol, produces a suspension of nanocrystals which slowly flocculate and precipitate from solution. While very few individual particles appear on the PMMA phase except in the most dense coverage case, some large aggregates of particles are evenly distributed across the surface. As the concentration of nanoparticles increases, the rate of aggregation increases, and correspondingly the number of individual particles available for decoration decreases. The degree of flocculation has also been shown to depend upon the size of the nanocrystals.15 These two effects explain the observed decrease in selective decoration of the surface with increasing concentration and particle size. Regardless of particle size or concentration, however, any nonaggregated particles were deposited preferentially on the PS stripes. A reasonable hypothesis for the driving force for this surface ordering is that the nonpolar alkane and arene coatings on the nanocrystals have much more energetically favorable interactions with the nonpolar PS than the fairly polar PMMA. If we make the estimation that the alkane coating of the nanocrystals is similar to linear polyethylene (15) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. Rev. Lett. 1995, 75, 3466-3469.
(PE), we can compare the interfacial tension of the two components with this reference. The interfacial tension is 3.5 erg/cm2 larger for PE/PMMA than for PE/PS16 (cf. 8.3 vs 11.8 erg/cm2). A contact area of 3 nm2 per nanocrystal gives an energy on the order of kT. In the micrographs, however, it is clear that, in the lowconcentration limit, the fraction of individual particles on the PMMA domains is less than 1:1000, indicating that the relevant energy must be at least an order of magnitude larger than kT. In this calculation, we are assuming that the particles are in the dilute limit, i.e., not interacting. As the surface concentration increases, interparticle repulsive contacts will increase, making adsorption on the PS less favorable. Again, the micrographs of samples with a high surface concentration exhibit some adsorption on the PMMA, but still at a much lower density than on the PS. It is also important to note that using the bulk interfacial tension can only be an approximation since we are considering a microscopic system where the interactions between mere tens or hundreds of molecules are determining the behavior of the nanocrystals. The resolution of these patterns is defined by both the minimum feature width and by the edge definition. The features shown in the figures are roughly 25 nm wide, which is the same width as the underlying polymer phases. The edge definition is limited by two factors: the size of the particles deposited on the surface and the width of the interface between neighboring polymer blocks. The gold nanocrystals used in this study were 1-2 nm in diameter and can be routinely produced at this size and other studies of PS/PMMA copolymers have determined the interfacial width to be 5 nm,17 which is anomalously large when compared with many other common copolymers that exhibit interface widths of 1-1.5 nm.18 Altogether, these properties of the present system allow us to create metallic (16) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; Wiley: New York, 1989. (17) Russell, T. P.; Hjelm, R. P., Jr.; Seeger, P. A. Macromolecules 1990, 23, 890-893. (18) Helfand, E.; Sapse, A. M. J. Chem. Phys. 1975, 62, 1327-1331.
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features with an edge definition approaching 1 nm thatare an order of magnitude smaller than those that can be realized with optical patterning techniques. Regarding where the passiviated nanocrystals are residing, in a previous study of gold islands on diblock copolymers, Morkved and co-workers found that they could generate 5-10 nm diameter gold particles within the PS phases of a PS-b-PMMA diblock copolymer by evaporating a 5 Å gold layer onto phase-separated polymer films and then annealing the structure again to allow the gold to cluster and migrate.19 On a poly(styrene-block-vinylpyridine) film, these particles were determined by x-ray photoelectron spectroscopy (XPS) to lie inside the PVP phase, rather than at the surface.20 However, another study found that colloidal particles deposited onto PS homopolymer do not penetrate the polymer surface.21 It seems likely, therefore, that the nanocrystals in the present example have remained at the surface considering that both PS and PMMA have glass transition temperatures over 100 °C and that the polymer is only in contact with the nanocrystal solution for a minute or less. In conclusion, the proof-of-concept presented here clearly demonstrates that phase-separated copolymers can direct the self-assembly of passivated metal nanoparticles. If these particles are indeed present on the surface of the
polymer, then they should be amenable to solution-phase chemistry. Further, it has already been demonstrated that the polymer domains can be aligned to a high degree by applying an electric field while annealing.13 Accordingly, the combination of the ability to apply an arbitrary alignment to the substrate and a suitable methodology for linking the deposited nanocrystals and releasing the products from the surface should provide a way to design and produce large, ordered arrays of nanoparticles.22 In addition, efforts are also underway to vary the chemical properties of both the passivating thiol (e.g., with ω-functionalized thiols) and the diblock copolymers in order to enhance the interaction between the polymer domains and passivated metal and semiconductor nanocrystals in a variety of other differet ways. Studies along these lines are now in progress, the results of which will be reported in due course.
(19) Morkved, T. L.; Wiltzius, P.; Jaeger, H. M.; Grier, D. G.; Witten, T. A. Appl. Phys. Lett. 1994, 64, 422-424. (20) Morkved, T. L. Unpublished. (21) Kunz, M. S.; Shull, K. R.; Kellock, A. J. J. Colloid Interface Sci. 1993, 156, 240-249.
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Acknowledgment. We thank Professor Robert Josephs, University of Chicago, for assistance with TEM imaging. This work was supported by the MRSEC program under the National Science Foundation (DMR-9400379) for which we are grateful. L.R.S. is a Beckman Young Investigator (1995-1997) and a Camille Dreyfus TeacherScholar (1995-2000).
(22) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P., Jr.; Schultz, P. G. Nature 1996, 382, 609-611.
