Langmuir 2007, 23, 2297-2299
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Covalent Formation of Nanoscale Fullerene and Dendrimer Patterns Scott A. Backer, Itai Suez, Zachary M. Fresco, Marco Rolandi, and Jean M. J. Fre´chet* Departments of Chemistry and Chemical Engineering, UniVersity of California, Berkeley, and the Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720-1460 ReceiVed NoVember 1, 2006. In Final Form: January 10, 2007 Localized patterns of amine-terminated monolayers obtained via the surface modification of a monolayer with the biased probe of an atomic force microscope were used to covalently attach buckminsterfullerene or dendrimers to the surface, affording lines as narrow as 20 nm.
The ability to fabricate and investigate at the nanoscale requires the development of chemical tools for the directed self-assembly of molecular components.1,2 Such tools may then be used in a host of applications such as molecular electronics, nanophotonics, and photovoltaics.3 Fullerenes have excellent electronic and optical properties and are used to fabricate high-performance devices.4,5 C60 and other fullerenoids are capable of self-organization6 and can arrange into ordered single layers and multilayers on surfaces.7 While the assembly of one-dimensional nanorods of fullerenes has been explored via noncovalent interactions including the crystallization of the long-chain alkyl groups of compounds8 containing C60, or by exploiting differential affinities to modified surfaces,9 the directed patterning of C60 remains a challenging target today. Nanoscale patterning and localized surface modification have been extensively investigated using scanning probes. For example, nanostructures may be grown locally via field-induced oxidation of a silicon surface and subsequently be transferred onto the underlying substrate with fluorine-based etchants.10 Similarly, the direct formation of carbonaceous etch-resistant nanoscale features has been demonstrated by degradation of common organic solvents.11 Finally, the direct deposition of material can also be achieved by “inking the tip” with specific molecules and writing on the target surface in a process known as dip-pen nanolithography.12 Although this serial form of patterning is slow, remarkable progress has been made towards increasing the throughput of scanning probe methods by using probe arrays with thousands of tips in parallel13,14 and through the development of ultra* Corresponding author. E-mail:
[email protected]. (1) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (2) Hawker, C. J.; Russel, T. P. MRS Bull. 2005, 30, 952. (3) Nakanishi, T; Miyashi, N.; Michinobu, T.; Wakayama, Y.; Tsuruoka, T.; Ariga, K.; Kurth, D. G. J. Am. Chem. Soc. 2006, 128, 6328. (4) Pierson, H. O. Handbook of Carbon, Graphite, Diamond and Fullerenes - Properties, Processing and Applications; Noyes Publishing: Park Ridge, NJ, 1993; pp 356-373. (5) Kurth, D. G.; Severin, N.; Rabe, J. P. Angew. Chem., Int. Ed. 2002, 41, 3681. (6) Brough, P.; Bonifazi, D.; Prato, M. Tetrahedron 2006, 62, 2110. (7) Langa, F.; de la Cruz, P.; Espildora, E.; de la Hoz, A.; Bourdelande, J. L.; Sanchez, L.; Martin, N. J. Org. Chem. 2001, 66, 5033. (8) Bonifazi, D.; Spillmann, H.; Kiebele, A.; de Wild, M.; Seiler, P.; Cheng, F.; Guntherodt, H.; Jung, T.; Diederich, F. Angew. Chem., Int. Ed. 2004, 43, 4759. (9) Nakanishi, T.; Miyashita, N.; Michinobu, T.; Wakayama, Y.; Tsuruoka, T.; Ariga, K.; Kurth, D. G. J. Am. Chem. Soc. 2006, 128, 6328. (10) Dagata, J.; Schneir, H. H.; Harary, C. J.; Evans, M. T.; Postek Bennett, J. Appl. Phys. Lett. 1990, 56, 2001. (11) Suez, I.; Backer, S. A.; Fre´chet, J. M. J. Nano Lett. 2005, 5, 321. (12) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (13) Salaita, K.; Wang, Y. H.; Fragala, J.; Vega, R. A.; Liu, C.; Mirkin, C. A. Angew. Chem., Int. Ed. 2002, 45, 7220.
high-speed scanners.15 Furthermore, lithographic schemes initially developed using scanning probes have been easily employed for high-resolution patterning of larger areas by using metallic stamps.16,17 Recent explorations have involved self-assembled monolayers (SAMs) that can be locally modified to template further assembly from the patterned functionalities via ionic and covalent attachments.18-21 In particular, we have demonstrated the fabrication of amine patterns from a SAM of 3,5-dimethoxyR,R-dimethyl-benzyloxycarbonyl (DDZ) molecules.22 The high electric field generated by applying a voltage bias between the surface and the conducting atomic force microscope (AFM) probe causes localized heterolytic bond cleavage of the DDZ-protecting group, exposing the desired amine functionality. This has previously enabled the patternwise assembly of nanoscale objects containing carboxylic acids through ionic interactions.22 In this work, we explore the covalent attachment of functional macromolecules and fullerenes to the latent amine patterns created by the scanning probe on the DDZ-protected monolayer. In a representative experiment, 3,5-dimethoxy-R,R-dimethylbenzyl aminopropyltriethoxysilyl carbamate was self-assembled onto a freshly cleaned and oxidized p-type silicon (100) wafer having a ca. 2 nm oxide layer. Patterning of the SAM was performed using a Digital Instruments multimode AFM operated in tapping mode. The AFM was placed in a low humidity environment to prevent capillary formation of a water meniscus between the tip and the sample during patterning, as such would lead to fieldenhanced oxidation of the sample10 rather than liberation of the desired surface amine groups22 via the removal of the labile DDZ protecting group. Deprotection of the monolayer (Figure 1) was achieved by applying a +12 V bias to the surface using a Nanoscope IIIa controller while the grounded tip (Tap 300, VeecoProbes) was translated across the desired location at 2-5 µm/s at an amplitude of vibration of ca. 2% of the imaging setpoint. (14) Vettiger, P.; Despont, M.; Drechsler, U.; Durig, U.; Haberle, W.; Lutwyche, M. I.; Rothuizen, H. E.; Stutz, R.; Widmer, R.; Binnig, G. K. IBM J. Res. DeV. 2000, 44, 323. (15) Fanter, G. E.; Schitter, G.; Kindt, J. H.; Ivanov, T.; Ivanov, K.; Patel, R.; Holten-Andersen, N.; Adams, J.; Thurner, P. J.; Rangelow, I. W.; Hansma, P. K. Ultramicroscopy 2006, 106, 881. (16) Cavallini, M.; Mei, P.; Biscarini, F.; Garcia, R. Appl. Phys. Lett. 2003, 83, 5286. (17) Hoeppener, S.; Maoz, R.; Sagiv, J. Nano Lett. 2003, 3, 761. (18) McCoy, K.; Gumieny, C.; Hess, D. W.; Tolbert, L. M.; Henderson, C. L. Proc. SPIE-Int. Soc. Opt. Eng. 2002, 4690, 1025. (19) Osruni, E.; Yan, L.; Whitesides, G. M. Colloids Surf., B 1999, 15, 3. (20) Fresco, Z. M.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2005, 125, 8302. (21) Liu, S.; Maoz, R.; Sagiv, J. Nano Lett. 2004, 4, 845. (22) Fresco, Z. M.; Suez, I.; Backer, S. A.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2004, 126, 8374.
10.1021/la0631973 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/02/2007
2298 Langmuir, Vol. 23, No. 5, 2007
Letters Scheme 1. Synthesis of Third-Generation Formyl Dendrimer 3
Figure 1. Schematic of the patterning procedure: the protected SAM undergoes heterolytic cleavage of the carbamate bond, forming a highly stabilized carbocation and a carbamic acid oxyanion, which decompose to give off CO2 and 3,5-dimethoxy-R-methylstyrene, leaving a latent amine image suitable for directed self-assembly.
While we had previously used the patterned amine surface to attach dendritic macromolecules via the formation of ionic bonds, we have now explored the formation of more robust covalent bonds. In a first demonstration, a dendrimer bearing an aldehyde functionality at its focal point was designed to test its reactivity with the surface-bound amino groups obtained by exposure of the surface to the voltage bias. The dendritic benzaldehyde derivative 3 was prepared by esterification of Fre´chet-type23 dendron 1 with 4-formylbenzoic acid 2 serving as both a spacer unit and an anchoring point. (Scheme 1) Dendrimer 3 was chosen for its significant bulk, as such large molecules deposited on a surface can easily be observed using the AFM. In addition, its highly aromatic character confers etch resistance, thus allowing image transfer into the silicon as previously described for ionically held dendritic patterns.22 Anchoring of dendrimer 3 to the amine surface was achieved as shown in Figure 2a through imine formation24 by immersing the amine-patterned wafer into a 3 mM solution of 3 in tetrahydrofuran for 12 h at ca. 20 °C. In initial experiments, the reaction of third-generation benzyl ether dendron 3 with the patterns produced lines that are approximately 1.4 nm tall and 110 nm wide (Figure 2b), as measured by AFM using the same broad tip used for the initial patterning of the DDZ surface. The measured height of the deposited layer is consistent with the ellipsometric thickness of a monolayer prepared in a control experiment with the self-assembly of dendrimer 3 on a blanket amine-modified surface. The covalent nature of the attachment of dendron 3 to the surface was tested by vigorous rinsing of the sample with polar and nonpolar solvents. This treatment was shown in the past to fully remove physisorbed species and ionically bound patterns.22 In this instance, however, no change or damage to the imine-anchored dendrimers was observed. Further confirmation of imine bond formation was obtained by exposing the sample to water-ethanol mixtures that can react with the imine bond, reversing its formation. As expected, the patterns lifted off as the covalent imine bonds were cleaved by hydrolysis, thus breaking the anchoring point of the molecules to the surface. These experiments with bulky model compound 3, which confirmed the high reactivity of the surfacebound amino groups, enabled us to attempt surface functionalization using the known reaction of C60 with primary amines; the ability of primary amines to chemically bond to C60 has been shown to be an effective means of binding fullerenes to small
molecules, polymers, particles, and modified metallic surfaces.25,26 The mechanism of bond formation is thought to involve a single electron transfer between the amine and the fullerene, followed by the formation of an aminofullerene from the amine radical cation and the reduced C60 species via radical recombination and proton transfer.27 The formation of fullerene lines was therefore achieved as shown schematically in Figure 3a by exposing the patterned wafer to a solution of C60 in toluene (1 mg/mL) and heating at 50 °C for 24 h in a dry nitrogen environment. The first inspection of the samples with an AFM revealed a complete coverage of the surface by physisorbed C60. The wafer was then washed overnight with refluxing hot toluene, followed by rinsing with acetone and isopropanol prior to re-examination of the surface using the same AFM. This treatment was shown to remove the nonspecifically adsorbed C60 from the surface, and AFM imaging revealed raised features matching the length and pitch of the patterns originally produced by the AFM tip on the DDZfunctionalized surface (Figure 3b). The stability of the fullerene lines to conditions that remove the C60 physisorbed onto the
(23) Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638. (24) Jung, Y. J.; La, Y.; Kim, H. J. T.; Kyuwook Ihm, K.; Kim, K. J.; Kim, B.; Park, J. W. Langmuir 2003, 19, 4512.
(25) Geckeler, K. E.; Hirsch, A. J. Am. Chem. Soc. 1993, 115, 3850. (26) Chen, K.; Caldwell, W. B.; Mirkin, C. A. J. Am. Chem. Soc. 1993, 115, 1193. (27) Hirsch, A.; Li, Q.; Wudl, F. Angew. Chem., Int. Ed. Engl. 1992, 30, 1309.
Letters
Figure 2. (a) Covalent self-assembly of formyl-modified G-3 benzyl ether dendrimer 3 via imine formation. (b) AFM image and crosssection analysis of covalently self-assembled dendritic patterns. Lines are 1.4 nm tall and 110 nm wide.
DDZ-protected monolayer is suggestive of the formation of the covalently bound aminofullerene species previously observed for fully aminated surfaces.26 Using this process and standard AFM tips, we were able to create lines as narrow as 20 nm. The lines are taller than expected (1.6 nm) for the assembly of a single monomolecular layer. This might indicate a stacking pattern based on the crystallization of free C60 molecules on the layer of fullerenes bonded to the patterned region.3,28 In summary, the covalent assembly of functional building blocks on latent amine images fabricated with scanning probe lithography has been demonstrated. C60 nanowires as small 20 nm have been obtained through this simple process requiring no functionalization of C60 prior to assembly. To our knowledge, this is the first report of covalently bound nanoscale fullerene patterns. The approach demonstrated herein with the covalent (28) Forro, L.; Mihaly, L. Rep. Prog. Phys. 2001, 64, 649.
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Figure 3. (a) Covalently bound fullerene nanoarrays formed via hydroamination of C60 by surface-bound primary amines. (b) AFM image and cross-section analysis of covalently self-assembled dendritic patterns. Lines are 1.6 nm tall and 20 nm wide.
attachment of dendritic aldehydes or buckminsterfullerene is particularly attractive because it only requires the use of a dilute solution of readily accessible compounds to modify the surface. This enables assembly with minimal material waste since no coupling agents or side products need to be removed via a purification step during each patterning experiment. Acknowledgment. We thank the NSF Center for Scalable Integrated Nanomanufacturing, the US Department of Energy (DE-AC03-76SF00098), and SRC/DARPA for their support of this research. M.R. thanks Intel for postdoctoral funding through the Molecular Foundry, Lawrence Berkeley National Laboratory. Supporting Information Available: Details of the experimental procedures. This material is available free of charge via the Internet at http://pubs.acs.org. LA0631973
