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Patterned Thin Films of Polyamidoamine Dendrimers Formed Using Microcontact Printing D. Arrington, M. Curry, and S. C. Street* Department of Chemistry, The Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, Alabama 35487-0209 Received April 29, 2002. In Final Form: August 21, 2002 Microcontact printing (µCP) is a soft lithographic technique used to transfer patterned thin organic films to surfaces with submicrometer resolution. Here, various concentrations of fourth-generation NH2terminated polyamidoamine dendrimers are used as the “ink” in µCP. A patterned monolayer is formed from dilute solution (1 µmol); however, this structure is not stable under ambient conditions. Increasing the dendrimer concentration (up to 1 mmol) results in stable multilayer structures up to roughly 60 nm in height, as characterized by atomic force microscopy. The relationship between dendrimer concentration and layer thickness is explored.
Introduction Dendrimers are monodisperse macromolecules with a central core and branching repeat units. Their polymeric architecture allows for control of chemical functionality and physical size (generation number). Thus, they are particularly suited for use as nanoscale building blocks in supramolecular assembly. This is potentially quite interesting since microscale (perhaps nanoscale) macromolecular patterning could be useful in many applications including, for example, polymeric components for microelectromechanical systems, microfluidic channels for labon-a-chip, and patterned array sensing elements or catalytic microreactors. There have been a number of reports on the molecular assembly of polyamidoamine (PAMAM) dendrimers onto silicon, mica, and gold surfaces.1-7 Stable, self-limiting monolayers of the dendrimer can be formed on these surfaces using simple adsorption techniques.1,6-8 The monolayers can be used, for instance, as adhesion promoters for continuous metal films and noble metal colloid monolayers.1,2,4 There is also evidence for multilayer formation through electrostatic interactions between positively charged dendrimers and negatively charged polyanhydrides,9,10 polyoxometalates,11,12 poly(styrenesulfonate),13 enzymes,14 and PAMAM dendrimers of (1) Baker, L. A.; Zamborini, F. P.; Sun, L.; Crooks, R. M. Anal. Chem. 1999, 71, 4403-4406. (2) Bar, G.; Rubin, S.; Cutts, R. W.; Taylor, T. N.; Zawodzinski, J.; Thomas, A. Langmuir 1996, 12, 1172-1179. (3) Evenson, S. A.; Badyal, J. P. S. Adv. Mater. 1997, 9, 1097-1099. (4) Rubin, S.; Bar, G.; Taylor, T. N.; Cutts, R. W.; Zawodzinski, J.; Thomas, A. J. Vac. Sci. Technol., A 1996, 14, 1870-1877. (5) Li, J.; Qin, D.; Baker, J. J. R.; Tomalia, D. A. Polym. Prepr. 2000, 41, 1446-1447. (6) Rahman, K. M. A.; Durning, C. J.; Turro, N. J.; Tomalia, D. A. Langmuir 2000, 16, 10154-10160. (7) Tokuhisa, H.; Zhao, M.; Baker, L. A.; Phan, V. T.; Dermody, D. L.; Garcia, M. E.; Peez, R. F.; Crooks, R. M.; Mayer, T. T. J. Am. Chem. Soc. 1998, 120, 4492-4501. (8) Street, S. C.; Rar, A.; Zhou, J. N.; Liu, W. J.; Barnard, J. A. Chem. Mater. 2001, 13, 3669-3677. (9) Zhao, M.; Liu, Y.; Crooks, R. M.; Bergbreiter, D. E. J. Am. Chem. Soc. 1999, 121, 923-930. (10) Liu, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2114-2116. (11) Cheng, L.; Cox, J. A. Electrochem. Commun. 2001, 3, 285-289. (12) Cheng, L.; Pacey, G. E.; Cox, J. A. Electrochim. Acta 2001, 46, 4223-4228. (13) Khopade, A. J.; Caruso, F. Nano Lett. 2002, 2, 415-418. (14) Yoon, H. C.; Kim, H.-S. Anal. Chem. 2000, 72, 922-926.
generation [X - 1/2].15 Multilayers have also been fabricated upon complexing dendrimers with Pt ions.16 Even upon immobilization as either a monolayer or a multilayer, dendrimer adlayers retain their chemical functionality and are open to further surface grafting chemistry.9-12,17,18 Microcontact printing (µCP) is a soft lithographic technique used to transfer patterned thin organic films to surfaces with submicrometer resolution.19-22 This technique uses an elastomeric stamp, formed over a patterned master, to deliver a molecular “ink” to a substrate upon contact. µCP has been shown to form highquality self-assembled monolayers with well-defined lateral dimensions. The technique has generally been used to direct deposition of thiols and silanes onto coinage metals and silicon substrates, respectively.19,23-26 µCP has also been used to pattern proteins,27-29 biological cells,29 polymer thin films30,31 and multilayer structures,32 2D (15) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171-2176. (16) Watanabe, S.; Regen, S. L. J. Am. Chem. Soc. 1994, 116, 88558856. (17) Fail, C. A.; Evenson, S. A.; Ward, L. J.; Schofield, W. C. E.; Badyal, J. P. S. Langmuir 2002, 18, 264-268. (18) Liu, Y.; Zhao, M.; Bergbreiter, D. E.; Crooks, R. M. J. Am. Chem. Soc. 1997, 119, 8720-8721. (19) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 20022004. (20) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550-575. (21) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. Adv. Mater. 1994, 6, 600-604. (22) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (23) St. John, P. M.; Craighead, H. G. Appl. Phys. Lett. 1996, 68, 1022-1024. (24) Xia, Y.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater. 1996, 8, 601-603. (25) Yang, X. M.; Tryk, D. A.; Hasimoto, K.; Fujishima, A. Appl. Phys. Lett. 1996, 69, 4020-4022. (26) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 9576-9577. (27) Bernard, A.; Renault, J. P.; Michel, B.; Bosshard, H. R.; Delamarche, E. Adv. Mater. 2000, 12, 1067-1070. (28) James, C. D.; Davis, R. C.; Kam, L.; Craighead, H. G.; Isaacson, M.; Turner, J. N.; Shain, W. Langmuir 1998, 14, 741-744. (29) Kane, R. S.; Takayama, S.; Ostuni, E.; Ingber, D. E.; Whitesides, G. M. Biomaterials 1999, 20, 2363-2376. (30) Ghosh, P.; Lackowski, W. M.; Crooks, R. M. Macromolecules 2001, 34, 1230-1236. (31) Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. L. Angew. Chem., Int. Ed. Engl. 1999, 38, 647649. (32) Huck, W. T. S.; Yan, L.; Stroock, A.; Haag, R.; Whitesides, G. M. Langmuir 1999, 15, 6862-6867.
10.1021/la0258749 CCC: $22.00 © 2002 American Chemical Society Published on Web 09/17/2002
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arrays of metal nanoparticles,33,34 3D metallic microstructures,35 and ordered condensation figures.36 It has also been employed in directing deposition of metal37 and ceramic films38 by chemical vapor deposition. Such monolayers have been extensively used as ultrathin resists.32,39-41 The µCP technique is inexpensive, has high throughput, and has adaptability to planar and nonplanar substrates. The µCP technique does not suffer from the optical diffraction constraints that limit standard photolithographic techniques. The chemical nature of the dendrimer pattern itself may be an advantage, as in the electroless deposition of metals.42,43 µCP is restricted by limitations in master template fabrication, defects in the patterned features caused by stress on the elastomeric stamp, solvent-induced swelling of the stamp, and molecular diffusion contributing to ink mobility and resultant loss of patterned feature resolution. During the preparation of this paper, a study of contact printing of PAMAM dendrimers has appeared.44 Our findings are similar; however, our study addresses the instability of monolayer structures and the formation of stable multilayer structures upon varying the dendrimer ink concentration. We present results concerning the deposition of PAMAM dendrimer mono- and multilayers onto the native oxide of silicon using µCP. Experimental Procedure Si(100) wafers were cleaned by degreasing them in an ultrasonic acetone bath, rinsed with acetone, and dried in a stream of nitrogen gas. They were then treated with UV-generated ozone (UVO-Cleaner model 42, Jelight Company, Irvine, CA) to remove hydrocarbon contaminants and create a continuous silicon oxide layer approximately 2 nm thick. Substrates were cut into 1 cm × 1 cm squares, placed in Fluoroware containers, and stored under ambient conditions. Patterned poly(dimethylsiloxane) (PDMS) stamps were fabricated by pouring a 10:1 mixture of Sylgard 184 elastomer/ curing agent (Dow Corning, Midland, MI) over a patterned master. The mixture cured for approximately 24 h at room temperature and was carefully peeled away from the master. The masters were created using conventional photolithographic methods and consisted of features (parallel lines) of varying sizes from 1 µm to tens of µm. Fourth-generation (G4) amine-terminated PAMAM dendrimer (Aldrich Chemical, Milwaukee, WI) was obtained as a 10% w/w methanolic solution. Ethanolic solutions of G4 PAMAM were prepared by diluting the original solution to various concentrations (from 1 × 10-3 to 1 × 10-6 M). The dendrimer ink was applied to the stamp by covering the stamp with the solution using a pipet. The solution remained on the stamp for 30 s and was blown dry under a stream of nitrogen. The ink was then transferred to the silicon substrate by bringing (33) Qin, D.; Xia, Y.; Xu, B.; Yang, H.; Zhu, C.; Whitesides, G. M. Adv. Mater. 1999, 11, 1433-1437. (34) Zhong, Z.; Gates, B.; Xia, Y.; Qin, D. Langmuir 2000, 16, 1036910375. (35) Rogers, J. A.; Jackman, R. J.; Whitesides, G. M. Adv. Mater. 1997, 9, 475-477. (36) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60-62. (37) Jeon, N. L.; Nuzzo, R. G.; Xia, Y.; Mrksich, M.; Whitesides, G. M. Langmuir 1995, 11, 3024-3026. (38) Jeon, N. L.; Clem, P. G.; Nuzzo, R. G.; Payne, D. A. J. Mater. Res. 1995, 10, 2996-2999. (39) Xia, Y.; Zhao, X.-M.; Kim, E.; Whitesides, G. M. Chem. Mater. 1995, 7, 2332-2337. (40) Xia, Y.; Zhao, X.-M.; Whitesides, G. M. Microelectron. Eng. 1996, 32, 255-268. (41) Sugimura, H.; Hanji, T.; Takai, O.; Masuda, T.; Misawa, H. Electrochim. Acta 2001, 47, 103-107. (42) Wu, X. C.; Bittner, A. M.; Kern, K. Langmuir 2002, 18, 49844988. (43) Bittner, A. M.; Wu, X. C.; Kern, K. Adv. Funct. Mater. 2002, 12, 432-436. (44) Li, H.; Kang, D.-J.; Blamire, M. G.; Huck, W. T. S. Nano Lett. 2002, 2, 347-349.
Figure 1. Tapping-mode AFM image (20 µm × 20 µm) of a G4-patterned monolayer. This structure was formed by microcontact printing using a PDMS stamp and a 1 µmol solution of the dendrimer ink. the stamp into conformal contact with the silicon for 30 s. After removal of the stamp, the surface was subsequently rinsed with ethanol and dried under nitrogen. Atomic force microscopy (AFM) images were obtained with a Dimension 3000 microscope and a Nanoscope III controller (Digital Instruments, Santa Barbara, CA). The microscope was equipped with a standard cantilever tip and was operated in tapping mode.
Results and Discussion Figure 1 shows an AFM image of a patterned surface stamped from a 1 × 10-6 M ethanolic G4 solution. The light areas are indicative of the pattern formation (dendrimer) while the dark areas represent the substrate beneath the recessed portion of the stamp where no contact is made. The height of the stamped region is approximately 2.3 nm, corresponding to a monolayer adsorbed on the surface.2 The lateral size of the feature is approximately 2.5 µm, the original feature size of the stamp. A recent report indicates that much better feature size resolution is possible.44 The monolayer structure dissipates into an incomplete monolayer film across the surface after 14 days. A similar result is found upon rinsing a stamped G4 monolayer with distilled water. We conclude that adsorbed water from ambient is enough to induce pattern failure by mobilizing the dendrimer on the surface. Figure 2a shows an AFM image of G4 stamped from 1 × 10-3 M solution. The width of this feature is approximately 15 µm (comparable to the master image in Figure 3), and the height of the features is 56.7 nm. This height corresponds to approximately 25 “monolayers” of G4. Figure 2b shows a cross-sectional height profile of the multilayer film. The linescan for this image shows the regularity of the pattern. The linescan does not reflect the true shape of the feature since there is tip convolution. Narrower area scans show that the tops and bottoms (heights and troughs) of the features are flat. Figure 4 shows an optical micrograph of the G4 multilayer structure, indicating that the pattern exists over a large scale, 1 cm × 1 cm (not shown in full). The more concentrated solution obviously allows more dendrimer molecules to dry on the PDMS stamp for transfer.
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Figure 4. Optical micrograph (250 µm × 250 µm) of pattern stamped G4 multilayer. The pattern extends over a macroscopic area.
Figure 2. (a) Tapping-mode AFM image (100 µm × 100 µm) of a G4-patterned multilayer. The structure was formed using a 1 mmol solution of dendrimer ink. The PDMS stamp was formed using the master shown in Figure 3. (b) AFM linescan of the patterned structure. While the effects of tip convolution are apparent, the features are very regular.
Figure 5. Patterned G4 film thickness as a function of dendrimer ink concentration. For all concentrations inking time was 30 s, printing time 30 s.
Figure 3. Optical micrograph (100 µm × 100 µm) of the pattern master used to form the PDMS stamp (see Figure 2). The master was fabricated using standard lithographic techniques.
sonication in ethanol. Self-assembly studies have shown that dendrimer multilayers do not form from ethanolic solutions. We postulate that the stability of the multilayers is due to cross-linking of the dendrimer at the exposed amine functionalities through reaction with CO2 present in ambient conditions. The reaction is well-known45-47 and has recently been used to explain findings concerning poly[(aminopropyl)siloxane] films.48 This could result in a final structure that exhibits increased density and stability. The structure heights obtained from AFM data are shown in Figure 5. By varying the concentration of dendrimer in solution, the height of the resulting structure can be controlled. The results show that for the G4 PAMAM in concentration ranges from micromoles to millimoles and using constant ink exposure and stamp contact times, the feature height appears to saturate. The nature of this correlation is likely due to diffusion-limited behavior. That is, either during the time the dendrimer ink solution is
Across a single multilayered surface, the height variance is roughly 6 nm (on the order of two monolayers). Interestingly, these multilayer patterns are quite stable. Unlike the monolayer structures, the multilayer films are stable in ambient conditions. Moreover, the pattern does not change with solvent (ethanol) rinsing or even
(45) Brousseau, L. C., III; Aurentz, D. J.; Benesi, A. J.; Mallouk, T. E. Anal. Chem. 1997, 69, 688-694. (46) Alper, E. Chem. Eng. J. 1990, 44, 107-111. (47) Versteeg, G. F.; Van Swaaij, W. P. M. Chem. Eng. Sci. 1988, 43, 573-585. (48) Cabibil, H. L.; Pham, V.; Lozano, J.; Celio, H.; Winter, R. M.; White, J. W. Langmuir 2000, 16, 10471-10481.
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wetting the stamp or during the time the stamp is in contact with the substrate for pattern transfer (or both), the mass transfer is limited by diffusion. Conclusions Patterned deposition of PAMAM dendrimer multilayers onto silicon oxide surfaces can be accomplished by microcontact printing. We have shown a correlation between dendrimer ink concentration and dendrimer film thickness. AFM images indicate that as the concentration increases, the film thickness increases. Multilayer structures are stable with respect to sonication and solvent
rinsing. Investigations of the long-term stability, further characterization of the stamped structures and their fidelity to the stamp masters, and in situ chemical reaction of these films are currently underway. Acknowledgment. This work was supported by the NSF (DMR-9809423) at the University of Alabama. The authors would like to thank Dr. Gary Mankey for supplying the masters and various materials for PDMS stamp fabrication. LA0258749
