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Nanometer-Scale Fabrication by Simultaneous Nanoshaving and Molecular Self-Assembly Song Xu and Gang-yu Liu* Department of Chemistry, Wayne State University, Detroit, Michigan 48202 Received November 22, 1996X Nanostructures down to a few nanometers in size and composed of close-packed and well-ordered molecules have been fabricated by simultaneous nanoshaving using an atomic force microscopy (AFM) tip and alkanethiols’ self-assembly on gold. Compared with other microfabrication methods, this procedure allows more precise control in terms of the size and geometry of the fabricated features. An edge resolution better than 2 nm can be routinely obtained. In addition, the fabricated nanostructures can be quickly changed, modified, and characterized in situ. These advantages should make this method very useful in the development of prototypical nanoelectronic devices and, perhaps more importantly, in the study of spatially confined chemical reactions.
Microfabrication of self-assembled monolayers (SAMs) has recently attracted tremendous attention because of its scientific importance and potential applications.1,2 Microscopic patterns, e.g. pits or trenches, can be formed by decomposing or removing SAMs using electron beams,3,4 metastable Ar beams,5 photolithography,6 or scanning probe microscopy (SPM).7 Recently, several methods such as microcontact printing,8 micromachining,9 and microwriting1,9,10 have been developed to prepare desired patterns (0.1-100 µm in dimensions) of two or more SAMs on gold. The challenging task of forming higher resolution, e.g. nanometer or molecular-scale, patterns within the plane of the adsorbates still remains. In this report, we introduce a new procedure that uses an atomic force microscopy (AFM)11 tip as a nanoshaver. As illustrated in Figure 1, this nanoshaver is operated on a matrix monolayer immersed in a solution containing the desired molecules different from the matrix. As the AFM tip plows through the matrix monolayer, the matrix molecules are removed and replaced by these reactive * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, January 1, 1997. (1) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H. A.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219-226. (2) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705-1707. (3) Sondag-Huethorst, J. A. M.; Van Helleputte, H. R. J.; Fokkink, L. G. J. Appl. Phys. Lett. 1994, 64, 285-287. (4) Tiberio, R. C.; Craighead, H. G.; Lercel, M.; Lau, T.; Sheen, C. W.; Allara, D. L. Appl. Phys. Lett. 1993, 62, 476-481. (5) Berggren, K. K.; et al. Science 1995, 269, 1255-1257. (6) (a) Wollman, E. W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395-4404. (b) Huang, J. Y.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626-628. (c) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305-5306. (7) (a) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632636. (b) Schoer, J. K.; Ross, C. B.; Crooks, R. M.; Corbitt, T. S.; HampdenSmith, M. J. Langmuir 1994, 10, 615-618. (c) Muller, W. T.; Klein, D. L.; Lee, T.; Clarke, J.; Mceven, P. L.; Schultz, P. G. Science, 1995, 268, 272-273. (8) (a) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60-62. (b) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 3274-3279. (c) Jeon, N. L.; Nuzzo, R. G.; Xia, Y.; Mrksich, M.; Whitesides, G. M. Langmuir 1995, 11, 3024-3026. (d) Jackman, R. J.; Wilbur, J. L.; Whitesides, G. M. Science 1995, 269, 664-666. (9) Abbott, N. L.; Kumar, A.; Whitesides, G. M. Chem. Mater. 1994, 6, 596-602. (10) Kumar, A.; Biebuyck, M. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188-9189. (11) Our instrument is a home-constructed state-of-the-art atomic force microscopy. Sharpened, microfabricated cantilevers were purchased from Park Scientific Instrument. The cantilevers were made of Si3N4 with a force constant of 0.1 N/m. C10SH (96%) and C18SH (98%) were purchased from Aldrich and used without further purification.
Figure 1. Procedure for nanografting. The schematic diagram provides an example of the fabrication of a C18S nanostructure inlaid in a C10S monolayer. The drawings are not to scale. (a) Well-ordered C10S on gold imaged via AFM with a low imaging force of 0.3 nN in a 2-butanol solution containing C18SH. (b) At the image force of 5.2 nN (higher than the displacement force threshold of 5.1 nN), C10S molecules can be displaced during the scan, and C18SH molecules (0.1 mM) self-assemble on the exposed gold surface. (c) The resulting nanofeature of C18S can be imaged by AFM at a low imaging force, e.g. 0.3 nN. Under these conditions, the gold substrate is not deformed. The typical time to complete procedure (a-c) is ∼5 min.
molecules in solution. Hereafter we will refer to this method as “nanografting”. As an example of the fabricated patterns, two CH3(CH2)17S (C18S) islands inlaid in the matrix CH3(CH2)9S/ Au(111) (C10S/Au(111)) monolayer, are shown in Figure 2a. The dimensions of the two islands are 3 × 5 nm2 and
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Figure 2. (a) Topographic image of two fabricated C18S nanoislands (brighter areas) inlaid in the matrix C10S monolayer. The steps, shown more clearly in part b, are due to the single atomic step of Au(111). As shown in the cursor profile in part c, the C18S islands are 8.8 Å higher than the surrounding C10S monolayer, consistent with the theoretical value for crystalline-phase SAMs. Molecular-resolution images (50 × 50 Å2) acquired from C10S (white square in part a) and C18S (black square in part a) covered areas are shown in parts d and e.
50 × 60 nm2, respectively, with an edge resolution of 1 nm. By zooming into the C18S and surrounding C10S areas, molecular-resolution AFM images have been acquired. The results, shown in Figure 2d and e, reveal a twodimensional close-packed structure with a lattice constant of 5.0 Å. Such periodicity is consistent with the wellknown (x3 × x3)R30°-based structure, in which hydrocarbon chains are close-packed and tilted ∼27° with respect to the surface normal.12-16 No C18SH-C10S exchange reaction was observed in the nonfabricated area during the entire course of the experiment (e.g. 2 h), which assures the high spatial selectivity and precision of nanografting. By changing the thiol solution before each fabrication step, we have successfully produced multiple nanostructures with different shapes and/or thiolate components. In addition, nanografting enables us to quickly change and/or modify the fabricated patterns in situ. An example illustrating this concept is shown in Figure 3. First, two parallel C18S nanolines were fabricated, and then the distance between them was increased. In contrast to lithography or previously mentioned fabrication methods,1,3-6,8-10 such changes do not require changes of mask or repetition of the entire fabrication procedure. Thus, it is convenient and useful to use nanografting in the development of prototypical nanoelectronic devices and to systematically change or modify the nanostructures to achieve optimum performance. We noted that the chemical reaction involved in the key fabrication step (Figure 1b) is not a conventional chemi(12) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (13) Camillone, N.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503-3511. (14) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (15) Poirier, G. E.; Taylor, M. J. Langmuir 1994, 10, 2853-2856. (16) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216-1218. (17) (a) Liu, G. Y.; Salmeron, M. B. Langmuir 1994, 10, 367-370. (b) Liu, G. Y.; Fenter, P.; Eisenberger, P.; Chidsey, C. E. D.; Ogletree, D. F.; Salmeron, M. B. J. Chem. Phys. 1994, 101, 4301-4306.
Figure 3. Changing the nanostructures in situ. (A) AFM image of C10S/Au(111) before fabrication. The bright area is 2.5 Å higher than the rest of the surface, which corresponds to the height of a single atomic Au(111) step. We used this area as our reference to perform nanografting. (B) Following the procedure in Figure 1, two parallel C18S nanolines are fabricated with the dimensions 10 × 50 nm2 and a separation of 20 nm. (C) Erasure of one line by scanning the nanoline area under high imaging force in a C10SH solution. The erase is complete, and the frame of the scanned areas can be very vaguely identified if one searches carefully. (D) Refabrication of the second line by a scanning under high imaging force in C18SH solution. The interline spacing now is increased to 60 nm.
sorption because the self-assembly occurs in a nanometerconfined environment. As illustrated in Figure 1b, a clean Au(111) surface is uncovered as the AFM tip plows through the C10S SAM. The fresh gold surface is spatially confined by the surrounding C10S monolayer and the AFM tip (transient confinement). The C18SH molecules in solution self-assemble on the exposed Au(111) substrate following the shaving track of the tip. In comparison with the
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conventional method of self-assembly,18-20 the gold surface is less subject to contamination because a clean Au(111) surface is produced and instantly exposed to the thiol solution during the scan under high force. In addition, the thiol adsorption pathway may be changed due to the spatial confinement, as well as the orientation of C10S molecules at the boundary. According to recent helium diffraction21 and scanning tunneling microscopy (STM)22 studies, SAM formation follows a two-step process on an unconstrained gold surface.21,22 Thiol molecules first lie down on the Au(111) surface (striped phase) and then stand up with the molecular axis tilted ∼27° from the surface normal (crystalline phase) as the coverage increases.21,22 In our fabrication process, the striped phase was not observed. We infer that C18SH may not go through the surface-aligned striped phase21,22 due to the spatial confinement and the orientation of C10S molecules at the boundary. Thiol molecules may instead form the crystalline phase directly following the orientation of the neighboring C10S molecules. These studies demonstrate that nanografting, i.e. simultaneous molecular displacement using an AFM tip and self-assembly, can be used to fabricate nanostructures of SAMs. Compared with other methods used to fabricate microstructures of SAMs, nanografting has several advantages over lithography and other microfabrication techniques.3-6,8-10 First, edge resolutions of 2 nm are routinely obtained and molecular precision is likely to be achieved with a sharp tip. Second, nanostructures can be characterized in situ and with molecular resolution using the same AFM tip. Third, once set up, one can quickly change and/or modify the fabricated patterns in situ (18) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (19) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (20) Ulman, A. Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic: San Diego, CA, 1991. (21) Camillone, N.; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737-2746. (22) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148.
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without changing the mask or repeating the entire fabrication procedure. We believe that this method may be very useful in forming novel nanoelectronic devices for research purposes. For instance, by combining nanografting with follow-up treatments, such as etching of shorter chain SAM-covered gold in an aqueous solution of CN- and O2,9 this protocol could be used to prepare gold nanostructures. Besides technical advantages, this new nanometer-scale fabrication method has special scientific value because the chemical reaction involved in the key fabrication step is not a conventional chemisorption but rather a spatially confined molecular self-assembly. The latter may show interesting kinetic and/or mechanistic deviations from selfassembly occurring on an “infinite” surface. One of the difficulties in the study of surface kinetics is the “spatial confinement” provided by lattice steps and other defects. Therefore it is logical to conclude that the possibility of varying the size and nature of the confinement in situ should prove beneficial to our understanding of this class of surface chemistry. Furthermore, the ability to chemically functionalize surfaces with different terminal groups on the nanometer length scale opens the possibility to use these nanostructures as templates for the construction of three-dimensional molecular nanoassemblies, which should prove useful in areas as diverse as electron transfer and polymer and biological sciences. Acknowledgment. We thank Dr. Nan Li for his help in preparation of Au(111)/mica. We appreciate many helpful discussions with Professors P. E. Laibinis, G. Scoles, C. Chow, and R. Levis. G.Y.L. gratefully acknowledges the Camille and Henry Dreyfus Foundation for a New Faculty Award and the Arnold and Mabel Beckman Foundation for a Young Investigator Award. This work is also supported by Wayne State University, the Institute for Manufacturing Research, and National Science Foundation Grant CHE-9510402. LA962029F
