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Self-Assembly of Uniform Monolayer Arrays of Nanoparticles Venugopal Santhanam, Jia Liu, Rajan Agarwal, and Ronald P. Andres* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907 Received February 1, 2003. In Final Form: July 8, 2003 Five nanometer diameter gold particles encapsulated by alkanethiol molecules have been self-assembled into well-ordered monolayers on a water surface, and these nanoparticle films have been transferred intact onto various solid substrates. The method involves spreading a thin layer of an organic solvent containing the gold nanoparticles on a water subphase that has a controlled surface curvature. As the solvent evaporates, a nanoparticle monolayer free of microscopic cracks and voids nucleates at the center of the apparatus and grows radially outward until it covers practically the entire water surface. This monolayer is transferred from the water surface to a polydimethylsiloxane (PDMS) stamp pad by the Langmuir-Schaefer (LS) technique and is applied to the solid substrate by microcontact printing (µCP). Uniform centimeter-scale films have been produced with this method. The quality of the transferred films has been verified by transmission electron microscopy (TEM). The nanoparticle monolayer is a hexagonal close-packed array with a center-to-center spacing that is approximately equal to the sum of the diameter of the gold particles and twice the height of a self-assembled monolayer (SAM) of the alkanethiol molecules on Au(111). The transferred films are free of multilayer regions and of microscopic voids and grain boundaries over their entire area and exhibit crystalline order across the openings in the TEM grid (∼4000 µm2).
Introduction The goal of nanotechnology is the creation of useful materials, devices, and systems through the control of matter on the nanometer length scale. One way to accomplish this goal is to synthesize a large number of identical nanoparticles and to fabricate macroscopic assemblies from these nanoscale components. Significant progress has been made in the past few years in developing synthesis and purification schemes, particularly solutionphase methods, for producing uniform populations of nanoparticles with controllable size, composition, shape, structure, and surface chemistry.1 An important technological challenge that remains is to develop effective ways to self-assemble these nanoscale components into larger structures and systems. Of particular interest are ordered 2-D and 3-D arrays or crystalline superlattices in which the nanoparticles take the place of the atoms in traditional solids.2 Here we discuss a method for the self-assembly of large area, hexagonal close-packed, monolayers of monodisperse nanoparticles on an arbitrary solid surface. There are several published techniques for self-assembling ordered monolayer arrays of nanoparticles on solid surfaces. These fall into three general categories: (1) spreading a colloidal suspension on the solid substrate either by drop-casting1c,d,3 or spin-coating4 and allowing * Corresponding author. E-mail:
[email protected]. (1) (a) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (b) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (c) Murray, C. B.; Sun, S.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R. IBM J. Res. Dev. 2001, 45, 47. (d) Stoeva, S.; Klabunde, K. J.; Sorensen, C. M.; Dragieva, I. J. Am. Chem. Soc. 2002, 124, 2305. (2) (a) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. Rev. Phys. Chem. 1998, 49, 371. (b) Roldughin, V. I. Russ. Chem. Rev. 2000, 69. 821. (c) Pileni, M.-P. Nano-Surface Chemistry; Rosoff, M., Ed.; Marcel Dekker Inc.: New York, 2000; p 315. (3) (a) Huang, S.; Sakaue, H.; Shingubara, S.; Takahagi, T. Jpn. J. Appl. Phys. 1998, 37, 7198. (b) Wang, Z. L. Adv. Mater. 1998, 10, 13. (c) Whetten, R. L.; Shafogullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilikinson, A. Acc. Chem. Res. 1999, 32, 397. (d) Lin, X. M.; Jaeger, H. M.; Sorensen, C. M.; Kablunde, K. J. J. Phys. Chem. B 2001, 105, 3353.
the solvent to evaporate; (2) field-enhanced5 or molecularinteraction-induced6 deposition from a colloidal suspension onto the solid substrate; and (3) spreading a colloidal suspension of hydrophobic particles on a water surface, allowing the solvent to evaporate, and subsequently transferring the nanoparticle array that forms on the water surface to the solid substrate using the LangmuirSchaefer technique.7,8 Each of these techniques fails to a greater or lesser degree as the size of the particles is decreased and the areal size of the desired monolayer is increased. Although equiaxed nanoparticles often assemble into hexagonal close-packed layers on a flat surface, it has proven difficult to form well-ordered, dense monolayers that cover a macroscopic area. None of the methods proposed to date can assemble on an arbitrarily chosen solid substrate a close-packed nanoparticle monolayer that is free of microscopic defects and that spans macroscopic dimensions. The inherent problem in any method that involves selfassembly on a solid substrate is the fact that few solid surfaces are uniform on the nanoscale. As a result, the particles are preferentially attracted to certain sites on (4) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (5) (a) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408. (b) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1196, 272, 706. (6) (a) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353. (b) Rubin, S.; Bar, G.; Taylor, T. N.; Cutts, R. W.; Zawodzinski, T. A., Jr. J. Vac. Sci. Technol., A 1996, 14, 1870. (c) Sato, T.; Hasko, D. G.; Ahmed, H. J. Vac. Sci. Technol., B 1997, 15, 45. (d) Sato, T.; Brown, D.; Johnson, B. F. G. Chem. Commun. 1997, 1007. (e) He, H. X.; Zhang, H.; Li, Q. G.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (f) Schmid, G.; Baumle, M.; Beyer, N. Angew. Chem., Int. Ed. 2000, 39, 181. (7) (a) Lee, W. Y.; Hostetler, M. J.; Murray, R. W.; Majda, M. Isr. J. Chem. 1997, 37, 213. (b) Bourgoin, J. P.; Kergueris, C.; Lefevre, E.; Palacin, S. Thin Solid Films 1998, 327-329, 515. (c) Markovich, G.; Collier, C. P.; Hendrichs, S. E.; Remacle, F.; Levine, D. R.; Heath, J. R. Acc. Chem. Res. 1999, 32, 415. (d) Huang, S.; Tsutsui, G.; Sakaue, H.; Shingubara, S.; Takahagi, T. J. Vac. Sci. Technol., B 2001, 19, 115. (e) Chen, S. Langmuir 2001, 17, 2878. (f) Brown, J. J.; Porter, J. A.; Daghlian, C. P.; Gibson, U. J. Langmuir 2001, 17, 7966. (8) Schmid, G.; Beyer, N. Eur. J. Inorg. Chem. 2000, 835.
10.1021/la0341761 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/16/2003
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the surface. In the process of forming a monolayer array by evaporation of the solvent from a colloidal suspension or by field-enhanced or molecular-interaction-induced deposition, the lateral mobility of the nanoparticles becomes frozen at some point. If this does not happen uniformly over the entire surface, it results in the formation of multilayer domains and/or microscopic voids. This problem gets worse as the size of the particles becomes smaller. This has led researchers to attempt to assemble the particle monolayer on a water surface and then transfer it to the desired solid substrate.7,8 To prevent formation of multilayer domains, the number of nanoparticles initially spread on the water surface is usually taken to be substantially less than the number needed to form a dense monolayer. As the organic solvent in which the nanoparticles are suspended evaporates, small monolayer islands or rafts of nanoparticles form on the water surface. These monolayer domains can be well-ordered but cover only a fraction of the surface area. A dense monolayer is obtained by decreasing the area available to the particles using a Langmuir trough.7 As the area of the trough is decreased, the monolayer domains collide with each other and coalesce. Unfortunately, without an organic solvent present, these nanoparticle domains typically exhibit solidlike behavior and resist deformation.7d This results in microscopic voids in the final film and/or regions in which the monolayer has buckled to produce multilayer domains. A potential solution to this problem is to nucleate only a single monolayer domain and allow this domain to grow as a single 2-D crystal until it either incorporates all the nanoparticles or covers the entire water surface. Duskin et al.9 and Yamaki et al.10 have shown that it is possible to self-assemble a single 2-D crystal of micronscale particles on a solid surface by controlling the surface contour of the colloidal suspension covering the substrate during the evaporation process. The key to the success of their scheme is construction of a circular cell in which the depth of the colloidal suspension covering the substrate is always thinnest in the center of the cell, gradually becoming thicker with radial distance from this central point out to the periphery of the cell. The dynamics of the self-assembly process in such a cell was studied by Duskin et al.11 They found that as the solvent evaporates, a monolayer array of particles nucleates on the solid substrate at the center of the cell. A circular contact line forms around this monolayer domain, and this contact line expands radially as evaporation proceeds. Convective flow of solvent toward the contact line carries particles to the growing monolayer where they self-assemble onto its outer edge. As the solvent evaporates, the diameter of the monolayer expands and the contact line moves outward. To adapt this scheme for use with particles a few nanometers in diameter, it is necessary to replace the solid surface with a water surface that is nearly flat but that has a slight upward convex curvature. This can be accomplished by placing a Teflon disk with a circular hole machined in it on an open stand in a reservoir and slowly filling the reservoir with water. Increasing the level of water in the reservoir until the contact line between the water and the disk becomes pinned at the bottom edge of (9) Dushkin, C. D.; Yoshimura, H.; Nagayama, K. Chem. Phys. Lett. 1993, 204, 455. (10) Yamaki, M.; Matsubara, K.; Nagayama, K. Langmuir 1993, 9, 3154. (11) (a) Dushkin, C. D.; Lazarov, G. S.; Kotsev, S. N.; Yoshimura, H.; Nagayama, K. Colloid Polym. Sci. 1999, 277, 914. (b) Maenosono, S.; Dushkin, C. D.; Yamaguchi, Y.; Nagayama, K.; Tsuji, Y. Colloid Polym. Sci. 1999, 277, 1152.
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Figure 1. Schematic illustration of (a) the shape of the airwater interface before addition of the organic suspension of nanoparticles and (b) the shape of the lens formed after addition of the nanoparticle suspension.
Figure 2. Schematic illustration of the two step process for transferring a nanoparticle monolayer from the water surface to a solid substrate.
the hole produces an air-water interface within the hole that is nearly flat but has a slight upward convex curvature, as illustrated in Figure 1a. A thin layer of a suspension of hydrophobic nanoparticles in an organic solvent can be spread on the water surface to produce a liquid lens shaped as illustrated in Figure 1b. With this lens structure it is possible to nucleate a single monolayer array at the center of the hole and grow the array to macroscopic size. Once a uniform nanoparticle monolayer has formed on the water surface, it still must be transferred intact to the desired solid substrate. The classical Langmuir-Schaefer technique of touching a hydrophobic surface to the nanoparticle film and then lifting the film from the water surface is used. This method works well if a small hydrophobic substrate such as a carbon-coated TEM grid is used; however, it is not satisfactory for large area or hydrophilic substrates. In such cases the nanoparticle monolayer tends to buckle and tear during the transfer process. This problem is avoided if a hydrophobic, deformable substrate is used to transfer the nanoparticle film. We have found that a PDMS pad is ideal for this purpose. The monolayer is subsequently transferred to the desired substrate by pressing the substrate onto the PDMS pad, as in conventional microcontact printing (Figure 2).12 Experimental Section Gold Colloid Preparation. A citrate-stabilized gold sol with a nominal size of 5 nm was purchased from Ted Pella Inc. All chemicals, of the highest available quality, were purchased from Sigma-Aldrich and used as is. Dodecanethiol-coated gold particles were prepared by a slight modification of a reported procedure.5a 5 mL of the gold sol (5 × 1013 particles/mL) was gently mixed with 100 µL of an ∼20 mM solution of n-dodecanethiol in ethanol for 20 min. Ethanol (30 mL) was added, and the mixture was allowed to sit for 2 h and was then centrifuged at 3400 rpm for 1 h. The supernatant solution was decanted, and the precipitate was dried in air overnight. The dried precipitate was dissolved in n-hexane and size selected using hexane/ethanol as the solvent/ nonsolvent pair.13 The size-selected nanoparticles were stored (12) (a) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823. (b) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker, D.; Kind, H.; Renault, J.-P.; Rothuizen, H.; Schmid, H.; Schmidt-Winkel, P.; Stutz, R.; Wolf, H. IBM. J. Res. Dev. 2001, 45, 697. (13) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706.
Self-Assembly of Uniform Nanocrystal Arrays as dry particles until just before casting a monolayer film. They were then dissolved in either hexane or a 50:50%v mixture of hexane and dichloromethane to produce an ∼1 µM colloid solution of encapsulated gold nanoparticles with a mean diameter of ∼5.5 nm (σ < 10%). Monolayer Formation. A 2 mm thick disk of Teflon, 5 cm in diameter with a 2 cm diameter circular hole machined in its center, is supported on an open stand in a Petri dish. Care is taken to ensure that the Teflon disk is level. Deionized water is added to the Petri dish until the water contacts the underside of the Teflon disk. At this point, the water surface inside the hole assumes a concave upward curvature. As the water level is increased further, the contact line between the air-water interface and the bottom of the disk moves to the edge of the hole and becomes pinned. Further increase in the water level results in the curvature of the air-water interface within the hole changing from concave upward to convex upward. Control of the water level in the reservoir provides excellent control of the shape of the air-water interface within the hole. For experiments reported in this paper, the water level in the reservoir was adjusted to produce a water surface with only a very slight convex upward curvature. The nanoparticle array is formed by dropping ∼0.4 mL of the colloid solution on the water surface and allowing the organic solvent to evaporate inside a fume hood. Care is taken to minimize air currents that might disturb the water surface. PDMS Stamp Pad Preparation. Sylgard 184 (Dow Corning) is used to prepare the PDMS stamp pads. A silicon chip covered with a thermally grown silicon dioxide layer is placed in a plastic weighing cup and covered with a 7:1 by weight mixture of PDMS oligomer and catalyst. The polymerization process is allowed to proceed at room temperature for ∼48 h. The pads are peeled off the silicon chip and cut to the desired shape. The PDMS pads are cleaned and any unreacted components removed by sequential immersion in hexane and ethanol. The pads are blown dry in a stream of nitrogen before use. Preparation of Samples for TEM Analysis. Samples for TEM characterization are prepared by lightly touching the monolayer film on the water surface either with a carbon-filmcoated 200 mesh TEM grid (Ted Pella Inc.) or with a PDMS stamp pad. In the latter case the film was subsequently transferred to a 100 nm thick silicon nitride membrane window TEM grid (SPI Supplies). Sample Characterization. TEM characterization of the nanoparticle arrays is performed on a JEOL 2000 FX operating at 200 kV. The size distribution and mean separation of clusters are obtained from TEM micrographs using a commercially available image analysis program, OPTIMAS. Optical images of the nanoparticle film on the water subphase are obtained using a digital camera, and optical micrographs are obtained using a bright field Normanski microscope with an attached CCD camera.
Results and Discussion Optimum Conditions for Formation of WellOrdered Monolayers. The experimental parameters that were studied to determine their effect on the quality of the monolayer array included colloid concentration, solvent composition, and solvent evaporation rate. Figure 3 shows an optical image of a nanoparticle film on the water surface taken after all the organic solvent has evaporated. The picture indicates two concentric regions. TEM micrographs of samples taken from these two regions reveal that the central region is a uniform monolayer while the region surrounding it consists of alternating multilayer and monolayer bands. The nanoparticle film in Figures 3 and 4 was formed under optimum conditions. These conditions are as follows: (1) a colloid volume of ∼0.4 mL and nanoparticle concentration of ∼1 µM, (2) a solvent composition of 50: 50%v of hexane and dichloromethane, and (3) a solvent evaporation time of 5 min. The low magnification image (Figure 4a) of the sample taken from the center of the cell illustrates the uniformity of the array over the entire
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Figure 3. Optical image of a nanoparticle film floating on a water subphase. Two concentric regions with distinct hues are visible. The inner region is a uniform nanoparticle monolayer about 1 cm in diameter. The outer region contains alternating multilayer and monolayer rings.
surface of the TEM grid. The hexagonal close-packed structure of the array is illustrated by the high magnification image (Figure 4c). The mean diameter of the particles is 5.6 nm with a standard deviation of 0.4 nm. The average separation between adjacent particles is 2.9 nm. This corresponds to minimal interpenetration of the dodecanethiol monolayers on adjacent clusters, in contrast to the substantial interpenetration present in arrays formed by solvent evaporation from a solid substrate.4 The low magnification image (Figure 4b) from the edge of the cell shows multilayer bands. The high magnification image (Figure 4d) reveals that the multilayer bands consist of steps ranging from a bilayer to a four-layer region and then back to a bilayer. These bands are attributed to instabilities that arise as the contact line approaches the edge of the hole. Figure 5 shows a hole in the monolayer left after transfer of a small portion of the array to a TEM grid. Such holes in the monolayer film do not shrink with time and are an indication of the solidlike character of the close-packed monolayer film left after all the solvent has evaporated. Effect of Changing the Colloid Concentration. Figure 6 contains TEM micrographs of nanoparticle arrays formed at 50%, 200%, and 400% of the optimum nanoparticle concentration while all other parameters are kept at their optimum values. At 50% of the optimum concentration the clusters form small close-packed regions separated by sparsely populated domains, while at 200% of the optimum concentration small patches of bilayers are randomly dispersed on a compact monolayer. At 400% of the optimum concentration a uniform bilayer array is formed on the water surface. Effect of Changing Solvents. A good solvent for forming monolayer arrays of 5 nm diameter gold particles was found to be a mixture of dichloromethane and hexane. Hexane is an excellent solvent for suspending the particles, is relatively volatile, and is lighter than water. On observing the evaporation of a pure hexane film with an optical microscope, however, micron sized objects were seen being generated at the evaporating front. This is tentatively attributed to formation of tiny hexane-water micelles. This phenomenon does not occur with a 50:50%v mixture of dichloromethane and hexane. Effect of Changing Evaporation Rate. The volatility of the solvent and the maximum air velocity in our fume hood (98 fpm) determine the minimum evaporation time possible with our present setup. Longer evaporation times
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Figure 4. (a and c) Low and high magnification TEM images, respectively, of a sample taken from the center region of the film in Figure 3. These images indicate that the central region of the film is a well-ordered monolayer of nanoparticles. (b and d) Low and high magnification TEM images, respectively, of a sample taken from the edge of the film. These images show concentric rings of nanoparticle multilayers and monolayers.
Self-Assembly of Uniform Nanocrystal Arrays
Figure 5. Optical image of a nanoparticle film floating on a water subphase showing a hole left by the transfer of a portion of the film to a carbon film coated TEM grid.
were achieved by evaporating into still air and by enclosing the cell in a bell jar. The effect of evaporation time on the quality of the array is illustrated in Figure 7. For a nanoparticle concentration of ∼1 µM and an evaporation time of ∼5 min, a closepacked monolayer forms while, at evaporation times of ∼1 h, a loosely ordered monolayer forms. A more compact monolayer is produced at the long evaporation time if the nanoparticle concentration is doubled, but the array that forms is not as rigid as an array formed at the shorter evaporation time.
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Examples of Nanoparticle Films Printed Using a PDMS Pad. Two samples obtained by microcontact printing of nanoparticle arrays using a smooth PDMS pad are shown in Figure 8. The silicon nitride membrane window grid was gently placed on top of the PDMS pad and was left in contact for 10 s. The SiO2 chip, which had been coated with a monolayer of APTMS,6d was lightly pressed onto the PDMS pad and left in contact for 10 s. Figure 8a is a high magnification TEM image of a nanoparticle monolayer printed onto a silicon nitride membrane window grid. It shows that the hexagonal closepacked structure of the monolayer is maintained when the monolayer is transferred by means of a PDMS pad. Figure 8b is a high magnification TEM image of a nanoparticle bilayer printed in the same way. This bilayer was formed by sequentially transferring two monolayers from the water surface to the stamp pad and then printing the resulting bilayer onto a silicon nitride membrane window grid. Figure 8b shows that not only monolayers but also multilayers can be transferred intact to a solid surface. Figure 8c is an optical micrograph of a bilayer film printed onto a SiO2 chip on which gold electrodes have been photolithographically patterned. This bilayer film is quite robust. When Scotch tape is applied to the surface of the film and removed, no damage was apparent. The bilayer also resists most organic solvents except alkanes. Conclusions In this paper we describe a simple technique for producing uniform nanoparticle monolayer films on a
Figure 6. Effect of nanoparticle concentration on the structure of the central region of a nanoparticle film formed on a water subphase with a controlled surface curvature: (a) 50% of optimum concentration; (b) 200% of optimum concentration; (c) 400% of optimum concentration.
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Figure 7. Effect of solvent evaporation time on array quality: (a) evaporation time of 2 h; (b) evaporation time of 5 min.
Figure 8. (a) TEM image of a nanoparticle monolayer printed onto a silicon nitride membrane window. (b) TEM image of a nanoparticle bilayer printed onto a silicon nitride membrane window. (c) Optical micrograph of a nanoparticle bilayer printed onto a silicon dioxide substrate containing lithographically patterned gold electrodes.
water subphase. These films are free of the microscopic defects that characterize nanoparticle monolayers formed by previous methods and can span macroscopic areas. We also demonstrate how these nanoparticle films can be transferred onto a solid substrate by means of microcontact printing using a PDMS stamp pad. By patterning the PDMS stamp pad, it should be possible to print lines and patterns of close-packed monolayers of gold nanoparticles on solid substrates and construct circuits and structures of arbitrary complexity. We have shown that multilayer
films and structures may also be fabricated by transferring multiple monolayers from the water surface onto a PDMS stamp pad before “printing” the nanoparticles onto a solid surface. This technique can be extended to combine monolayers of different nanoclusters and provides a method for producing multilayer nanoparticle films with control over the properties of the individual layers. The techniques described above allow one to integrate nanoscale components with traditional semiconductor fabrication procedures to produce hybrid devices. In particular,
Self-Assembly of Uniform Nanocrystal Arrays
the mechanical and chemical robustness of printed arrays of alkanethiol-encapsulated gold nanoparticles can provide a technology, using molecular exchange reactions, for fabricating nanoscale chemical sensors and nanocontacts for molecular electronics.
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Acknowledgment. This research was supported in part by the National Science Foundation, the Department of Energy, and the Indiana 21st Century Fund. LA0341761
