Patterned Organosilane Monolayers as Lyophobic−Lyophilic Guiding

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Patterned Organosilane Monolayers as Lyophobic-Lyophilic Guiding Templates in Surface Self-Assembly: Monolayer Self-Assembly versus Wetting-Driven Self-Assembly† Assaf Zeira,‡ Devasish Chowdhury,‡, Stephanie Hoeppener,‡,^ Shantang Liu,‡,# Jonathan Berson,‡ Sidney R. Cohen,§ Rivka Maoz,*,‡ and Jacob Sagiv*,‡ Department of Materials and Interfaces and §Department of Chemical Research Support, The Weizmann Institute of Science, Rehovot 76100, Israel. Present address: Institute of Advanced Study in Science & Technology (IASST), Paschim Boragaon, Guwahati - 781035 (Assam), India. ^ Present address: Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. #Present address: Key Laboratory for Green Chemical Process of Ministry of Education, Hubei Key Laboratory of Novel Reactor & Green Chemical Technology, School of Chemical Engineering Wuhan Institute of Technology, Wuhan, Hubei 430073, P. R. China )

‡

Received June 11, 2009. Revised Manuscript Received August 22, 2009 Monolayer self-assembly (MSA) was discovered owing to the spectacular liquid repellency (lyophobicity) characteristic of typical self-assembling monolayers of long tail amphiphiles, which facilitates a straightforward visualization of the MSA process without the need of any sophisticated analytical equipment. It is this remarkable property that allows precise control of the self-assembly of discrete, well-defined monolayers, and it was the alternation of lyophobicity and lyophilicity (liquid affinity) in a system of monolayer-forming bifunctional organosilanes that allowed the extension of the principle of MSA to the layer-by-layer self-assembly of planed multilayers. On this basis, the possibility of generating at will patterned monolayer surfaces with lyophobic and lyophilic regions paves the way to the engineering of molecular templates for site-defined deposition of materials on a surface via either precise MSA or wetting-driven self-assembly (WDSA), namely, the selective retention of a liquid repelled by the lyophobic regions of the pattern on its lyophilic sites. Highly ordered organosilane monolayer and thicker layer-by-layer assembled structures are shown to be ideally suited for this purpose. Examples are given of novel WDSA and MSA processes, such as guided deposition by WDSA on lyophobic-lyophilic monolayer and bilayer template patterns at elevated temperatures, from melts and solutions that solidify upon cooling to the ambient temperature, and the possible extension of constructive nanolithography to thicker layer-by-layer assembled films, which paves the way to three-dimensional (3D) template patterns made of readily available monofunctional n-alkyl silanes only. It is further shown how WDSA may contribute to MSA on nanoscale template features as well as how combined MSA and WDSA modes of surface assembly may lead to composite surface architectures exhibiting rather surprising new properties. Finally, a critical evaluation is offered of the scope, advantages, and limitations of MSA and WDSA in the bottom-up fabrication of surface structures on variable length scales from nano to macro.

1. Introduction The concept of surface lyophobicity-lyophilicity (liquid repellency-liquid affinity) has played a central role throughout the history of monolayer self-assembly (MSA). Organosilanes have played a central role as well. Here we report a series of novel results that capitalize on the virtues of patterned organosilane monolayers as lyophobic-lyophilic guiding templates in surface self-assembly. It all started in 1946 with the realization, by Zisman and co-workers, that the oleophobicity (oil repellency) acquired by a polar (lyophilic) solid surface upon immersion in the solution of a long tail amphiphile in a hydrocarbon oil signifies adsorption onto the solid of a dense layer of oriented amphiphilic molecules.1 Such oleophobic monolayers have been studied for the past 63 years; however, the modern research area of MSA, as viewed from today’s perspective of its relevance to nanoscience and nanotechnology, was born with a series of nine papers published within a † Part of the “Langmuir 25th Year: Self-assembled monolayers: synthesis, characterization, and applications” special issue. *Corresponding authors. E-mail: [email protected] (J.S.); rivka. [email protected] (R.M.).

(1) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Sci. 1946, 1, 513– 538.

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period of ca. 5 years before the birth of this journal.2-10 While the work reported in these early papers centers on several types of both one-component and mixed oleophobic monolayers, much of its novelty undoubtedly derived from the introduction of long tail organosilanes as monolayer-forming components, which opened up unprecedented paths for the engineering of monolayer systems with new and rather unique properties. In fact, the term self-assembling monolayer itself was coined with reference to the then newly introduced paradigm of layer-by-layer self-assembly,11 proposed and demonstrated in 1982-1983 with a series of novel bifunctional organosilanes specially designed and synthesized for this purpose.6-8 The concept of surface lyophobicity-lyophilicity again played a central role in this development. (2) Polymeropoulos, E. E.; Sagiv, J. J. Chem. Phys. 1978, 69, 1836–1847. (3) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92–98. (4) Sagiv, J. Isr. J. Chem. 1979, 18, 339–345. (5) Sagiv, J. Isr. J. Chem. 1979, 18, 346–353. (6) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674–676. (7) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 99, 235–241. (8) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 100, 67–76. (9) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465–496. (10) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 101, 201–213. (11) New Sci. 1983, 98, 20.

Published on Web 10/16/2009

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The build-up principle underlying the chemically controlled selfassembly of stratified films made of discrete, well-defined molecular layers is based on the sequential repetition of a two-step process consisting of (i) the formation, via spontaneous self-assembly at a solid-fluid interface, of a compact monolayer of highly oriented and densely packed molecules, followed by (ii) in situ chemical activation of the exposed outer surface of the assembled monolayer, so as to generate suitable anchoring sites for the subsequent selfassembly of the next monolayer.6-8 In the typical case of long hydrocarbon tail silanes, effective self-control on the reproducible assembly of discrete monolayers with well-defined composition and structure is achieved as a result of the interplay between the multiple lateral (intralayer) and vertical (layer to underlying surface) covalent and/or hydrogen bonding of the silane head groups under the steric constraints imposed by the dense packing of the fully extended molecular tails,12 provided the outer exposed tail groups are inert toward additional binding of silane molecules from the fluid phase. To guarantee that no material is deposited in excess of a well-defined monolayer and so avoid possible surface fouling by accumulation of adventitious contamination, it is further required that the completion of a compact one-molecule-thick film of oriented molecules imparts liquid repellency to the coated surface. This implies the generation, in step 1 of the layer-by-layer assembly process, of a lyophobic monolayer whose nonpolar top surface is not wetted by liquids with surface tensions exceeding a certain critical value,13 including its own solution (organic or aqueous) or melt (autophobic), and water (hydrophobic).14 In step 2 of the process, the lyophobic monolayer is in situ converted, via a nondestructive chemical modification process, to a lyophilic one; a monolayer exposing active polar functions onto which an additional self-assembling monolayer can build up until the completion of a new autophobic outer surface terminates again the adsorption process. The alternating lyophobic/autophobic-lyophilic character of the outer film surface, upon the addition of each new monolayer and its subsequent functionalization, is thus a key feature ensuring the precise and reproducible layer-by-layer assembly of such stratified film structures. Highly ordered monolayers derived from bifunctional long tail trichlorosilane precursors terminated with vinyl (-CHdCH2) functions were found to be ideally suited as building blocks for this purpose.6-8,12,15-17 The top surfaces of such monolayers are lyophobic/autophobic, while their remarkable structural robustness allows nondestructive in situ chemical modification of the vinyl-rich surface via chemical routes conductive to lyophilic surfaces exposing diverse polar functions of interest.6,12,15-20 As the reactive trichlorosilane headgroup is compatible with MSA on a large variety of polar surfaces, high-quality silane monolayers may also form on various lyophilic monolayer surfaces generated by such in situ functionalization.21 (12) Wen, K.; Maoz, R.; Cohen, H.; Sagiv, J.; Gibaud, A.; Desert, A.; Ocko, B. M. ACS Nano 2008, 2, 579–599. (13) Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1960, 64, 519–524. (14) Gun, J.; Sagiv, J. J. Colloid Interface Sci. 1986, 112, 457–472. (15) Maoz, R.; Sagiv, J.; Degenhardt, D.; M€ohwald, H.; Quint, P. Supramol. Sci. 1995, 2, 9–24. (16) Maoz, R.; Yam, R.; Berkovic, G.; Sagiv, J. In Thin Films; Ulman, A., Ed.; Academic Press: San Diego, 1995; Vol. 20, pp 41-68. (17) Baptiste, A.; Gibaud, A.; Bardeau, J. F.; Wen, K.; Maoz, R.; Sagiv, J.; Ocko, B. M. Langmuir 2002, 18, 3916–3922. (18) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. Adv. Mater. 2000, 12, 725– 731. (19) Liu, S.; Maoz, R.; Schmid, G.; Sagiv, J. Nano Lett. 2002, 2, 1055–1060. (20) Liu, S.; Maoz, R.; Sagiv, J. Nano Lett. 2004, 4, 845–851. (21) This property of silane monolayers renders them suitable for fabrication of ordered multilayers with diverse interlayer functionality, such as, for example, alcohol,12,17 carboxylic acid,12,15,16 thiol (unpublished results), and amine (unpublished results).

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Other types of self-assembling monolayers have been studied in the course of the past 25 years,2,3,9,22-26 most notably those based on the disulfide-thiol anchoring to gold,24-34 introduced in 1983 by Nuzzo and Allara.24 Organosilane monolayers, however, remain unmatched in what concerns their robustness and stability (chemical,12,16,18,35,36 electrochemical,18,37 and thermal19,36,38,39), compatibility with many different types of polar substrates (e.g., silicon,9,18,35,36,41 germanium,9,35 ZnSe,9,35 glass,3,9,35,36 metal oxides,2,10,38 gold,40,41 mica,42,43 functionalized monolayers,6,15-20 and flexible polymers such as PVA4), and performance as building blocks for planned assembly of organized multilayers.12,15-17,25,36,44,45 These exceptional qualities render silane-based systems rather unique with respect to the possible engineering of functional molecular components (monolayers and thicker layer-by-layer assembled architectures) for advanced future technologies (e.g., nanodevices). The main difficulty in the utilization of bifunctional monolayer forming silanes such as the extremely versatile vinyl-terminated long-tail derivatives has to do with their rather tedious synthesis and the lack of a commercial source for their regular supply. This led to the widespread utilization of short-tail bifunctional silanes, for example, aminopropyl and mercaptopropyl derivatives, which are abundant, inexpensive, bind to various polar surfaces like the long-tail derivatives, and are already equipped with terminal active functions that can be directly used without the need of further in situ derivatization. However, while such bifunctional silanes may be useful for the simple task of anchoring to a surface various nanoobjects of interest, they are inherently incompatible with the formation of inert, lyophobic outer surfaces upon the completion of a single well-defined monolayer and so are unsuitable for applications demanding precise surface functionalization or precise monolayer or layer-by-layer assembly with subnanoscale control of the resulting surface architecture. This laboratory has invested continuing efforts in the search of ways that would circumvent these difficulties associated with the use of organosilanes without losing the unique advantages offered by (22) Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597–2601. (23) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldman, K.; H€ahner, G.; Spencer, N. D. Langmuir 2000, 16, 3257–3271. (24) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481–4483. (25) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (26) Schwartz, D. K. Annu. Rev. Phys. Chem. 2001, 52, 107–137. (27) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365–385. (28) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426–429. (29) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550–575. (30) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1–68. (31) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A. Science 1999, 283, 661–663. (32) Liu, G.-Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457–466. (33) Friggeri, A.; van Manen, H.-J.; Auletta, T.; Li, X.-M.; Zapotoczny, S.; Sch€onherr, H.; Vancso, G. J.; Huskens, J.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 2001, 123, 6388–6395. (34) Kr€amer, S.; Fuierer, R. R.; Gorman, C. B. Chem. Rev. 2003, 103, 4367– 4418. (35) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621–1627. (36) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282–6304. (37) Zeira, A.; Chowdhury, D.; Maoz, R.; Sagiv, J. ACS Nano 2008, 2, 2554– 2568. (38) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054–3056. (39) Kluth, G. J.; Sung, M. M.; Maboudian, R. Langmuir 1997, 13, 3775–3780. (40) Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J. Electroanal. Chem. 1987, 219, 365–371. (41) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357–2360. (42) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354–3357. (43) Britt, D. W.; Hlady, V. J. Colloid Interface Sci. 1996, 178, 775–784. (44) Yitzchaik, S.; Marks, T. Acc. Chem. Res. 1996, 29, 197–202. (45) Maoz, R.; Matlis, S.; DiMasi, E.; Ocko, B. M.; Sagiv, J. Nature 1996, 384, 150–153.

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silane-based monolayer systems in the planned chemical fabrication of three-dimensional (3D) surface nanoarchitectures. Within this context, a major issue to which special attention has been paid is the advancement of a lateral patterning methodology that takes advantage of the superior performance of such monolayer systems and so allows their effective utilization as patterned templates for guided self-assembly on variable length scales from nano to macro. These efforts led to a series of significant advances centered on the straightforward functionalization of n-octadecyltrichlorosilane (OTS, SiCl3-(CH2)17-CH3) monolayers and the exploitation of the lyophilicity of functionalized OTS, as contrasted with the lyophobicity of the unfunctionalized monolayer, in the control of surface self-assembly. OTS is the best known low-priced monofunctional n-alkylsilane producing high-quality self-assembled monolayers with lyophobic/autophobic outer surfaces.3,9 The rather surprising discovery that an electrically biased scanning force microscope (SFM) tip or a conductive stamp may be used to nondestructively convert the lyophobic surface of an OTS monolayer to a lyophilic one via local electrochemical oxidation of the nonpolar (inert) -CH3 to polar (active) -COOH functions18,46 offers an ideal solution to both the problem of surface functionalization and that of effective lateral patterning at the level of a single monolayer, all this without any ex situ synthesized source of active surface functions. Lyophilic regions generated in this manner within the otherwise lyophobic OTS background may be used as template sites on which different postpatterning processes of site-defined self-assembly (of both organic and inorganic species) can take place, thus providing a basis for the generic bottom-up fabrication methodology named constructive lithography (CL).18,46,47 In a typical implementation mode of this approach, long-tail monolayer-forming compounds have been used to locally self-assemble on the lyophilic sites of such patterned OTS monolayers in a manner akin to MSA on unpatterned lyophilic monolayer surfaces of the kind generated in the layer-by-layer assembly process mentioned before. By using a long-tail bifunctional silane such as nonadecenyltrichlorosilane (NTS, SiCl3-(CH2)17-CHdCH2), access is again provided, now at the level of a site-confined bilayer, to a terminal vinyl function that may be further converted to various other desired functions and so allow planned fabrication of different well-defined templates for subsequent guided self-assembly steps.18-20 However, the use of NTS or similar bifunctional silanes brings us back to the problem of having to rely on a material that is expensive and not readily available. Recently, a different mode of implementation of site-defined assembly on a nanopatterned OTS monolayer was demonstrated (named wetting-driven self-assembly (WDSA)),48 based on a more general and less demanding deposition principle: selective retention, on the functionalized -COOH sites, of a liquid (neat, solution, melt) that wets such lyophilic surfaces without wetting the unmodified lyophobic surface (-CH3) of the OTS monolayer. For example, it was found that nanopatterns of eicosene (CH3-(CH2)17-CHdCH2, mp 28-29 C) assembled by retraction from the melt (at ca. 65 C) on nanopatterned OTS monolayers resemble in a number of significant aspects both the structure and function of NTS nanopatterns produced by MSA on similar nanopatterned OTS templates.48 Eicosene is a readily available long-tail hydrocarbon that differs from NTS only in that the silane headgroup is replaced in eicosene by a nonbinding methyl group. Despite the fact that eicosene is not an amphiphilic (46) Hoeppener, S.; Maoz, R.; Sagiv, J. Nano Lett. 2003, 3, 761–767. (47) Wouters, D.; Schubert, U. S. Langmuir 2003, 19, 9033–9038. (48) Chowdhury, D.; Maoz, R.; Sagiv, J. Nano Lett. 2007, 7, 1770–1778.

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molecule, the SFM images of eicosene nanopatterns produced by the WDSA process are compatible with the retention of a single monolayer of perpendicularly oriented eicosene molecules on top of the OTS background, possibly resembling the organization of an ordered NTS monolayer.48 Furthermore, the vinyl functions of eicosene immobilized on the surface in this manner could be in situ reacted to generate polar functions such as -COOH and -SH, which were further used as templates for in situ generation of silver nanoparticles,48 in a manner also resembling the use of NTS for this purpose.18 These intriguing findings prompted us to undertake a comparative study of the WDSA and ordered MSA on patterned organosilane monolayer templates exposing lyophilic regions of variable lateral size between ca. 10 nm to more than 100 μm. Here we briefly report a series of representative main results of this study, focusing on the scope, advantages and limitations of each of these different approaches to site-defined immobilization of materials on a surface. An important new finding emerging from the present work is that the nondestructive electrochemical patterning of OTS monolayers can be extended to bilayer and possibly even thicker layerby-layer assembled films, which paves the way to 3D CL with stratified template patterns made of readily available monofunctional n-alkyl silanes only. In this more complex mode of 3D sitedefined deposition, the assembly process also proceeds in a precisely self-regulated manner as a consequence of the fact that material deposition on lyophilic surface sites created by patterning at different height levels of a stratified structure comes to a halt with the formation of a lyophobic/autophobic layer of oriented molecules on each such functionalized site. Thus, in both WDSA and MSA on lyophobic-lyophilic templates, the interplay between lyophobicity and lyophilicity constitutes the main regulatory force driving the deposition of material to preselected surface regions, as defined by the patterning process. The basic conceptual difference between the self-assembly of lyophobic/autophobic monolayers and WDSA is that, in the latter case, the material deposited is not lyophobic, meaning that its amount and detailed surface distribution within the lyophilic regions of a lyophobiclyophilic template pattern are not precisely controlled by a single prevailing parameter (i.e., the autophobicity of the deposited material), but may vary depending on many experimental variables. Some of the key variables controlling the size and shape of simple line and loop liquid features produced by the WDSA process were discussed theoretically and tested in a series of experiments with glycerol assembled on micropatterned surfaces with wettable regions in the range of tens to hundreds of micrometers.49,50 WDSA experiments on the micrometric scale were also performed with hydrophilic-hydrophobic monolayer patterns withdrawn from volatile aqueous solutions51 or immiscible organic-aqueous two-phase systems.52-54 We developed a WDSA methodology that allows direct formation of stable solid deposits of diverse materials, down to the nanoscale.48 The high thermal stability of organosilane template patterns19 permits the assembly to be carried out at relatively high temperatures (in this (49) Darhuber, A. A.; Troian, S. M.; Miller, S. M.; Wagner, S. J. Appl. Phys. 2000, 87, 7768–7775. (50) Darhuber, A. A.; Troian, S. M.; Davis, J. M.; Miller, S. M.; Wagner, S. J. Appl. Phys. 2000, 88, 5119–5126. (51) Qin, D.; Xia, Y.; Xu, B.; Yang, H.; Zhu, C.; Whitesides, G. M. Adv. Mater. 1999, 11, 1433–1437. (52) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 2790–2793. (53) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Chem. Mater. 1995, 7, 252–254. (54) Braun, H.-G.; Meyer, E. Thin Solid Films 1999, 345, 222–228.

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work up to 112 C) from melts48 or solutions37 that solidify upon cooling to the ambient temperature. This considerably expands the range of applicability of the WDSA process while also facilitating a detailed nondestructive characterization of the assembled patterns by SFM and quantitative micro-Fourier transform infrared (FTIR) techniques. It was thus possible to devise new modes of implementation and reveal interesting new aspects of the WDSA process, associated with specific properties of the assembled material and the length scale of the template pattern, which point to both benefits and inherent limitations arising from the multifaceted character of this methodology. We further show how subtle WDSA edge effects can also come into play in laterally confined MSA on patterned templates, the relative contribution of such effects becoming evident as the lyophilic regions of the template shrink to nanometric dimensions. The present comparative study unequivocally identifies such effects, along with experimental conditions that are likely to enhance or diminish their contribution to MSA on patterned templates. Finally, we show how WDSA combined with MSA may be used both as an interesting new tool of research and in the engineering of composite surface architectures exhibiting rather surprising new properties. In line with the exploratory nature of this research, the results obtained are illustrated with specific examples that emphasize the inherent differences between WDSA and precise MSA.

2. Methods and Materials 2.1. Materials. The experimental results reported in this paper were obtained with three monolayer-forming compounds known to produce high-quality self-assembling monolayers on suitable unpatterned surfaces: OTS, NTS,12,14-17 and C20NH2 (eicosylamine, NH2-(CH2)19-CH3),55 and three very different materials that lend themselves to WDSA on monolayer template patterns: eicosene, agarose, and gallium. Here, OTS and NTS fulfill the double role of both monolayer template materials and test materials for MSA on the template patterns, whereas C20NH2 was selected as an additional test material that differs from the long tail organosilanes both in its mode of anchoring to -COOH-terminated templates55 and in that the amine headgroup is devoid of lateral bonding capability. Eicosene (a hydrocarbon structurally related to NTS (vide supra), assembled by retraction from the melt at 65 C),48 agarose (a hydrogel forming high molecular weight polysaccharide polymer, assembled by retraction from dilute aqueous solutions at 90 C),37 and Ga (a metal with melting point like that of eicosene (29.8 C), assembled from the melt at either ambient or high temperature using two experimental protocols that differ from those applied to eicosene and agarose as well as from one another (Section 3.3.3.)) were selected as test materials for WDSA in order to identify expected differences in the mode of assembly of such very different materials, along with eventual common features characteristic of WDSA in general. The static contact angles of eicosene, agarose solution (0.1 wt %), and Ga on the unpatterned OTS/Si monolayer, each measured at the respective deposition temperature, are 46 (eicosene), 78 (agarose),56 and 160 (Ga). Under the deposition conditions, droplets of each of these materials move freely on unmodified OTS/Si monolayer surfaces while wetting (0 contact angle) -COOH-terminated regions or bare silicon oxide regions of the monolayer template patterns employed in this study (Schemes 1 and 2). 2.2. Experimental Approach. A comprehensive quantitative structural characterization of self-assembling monolayers and (55) Maoz, R.; Cohen, H.; Sagiv, J. Langmuir 1998, 14, 5988–5993. (56) Higher contact angles were measured with more diluted solutions of agarose, up to a maximal contact angle of 114 with pure water at 90 C. This trend points to a monotonic lowering of the solution surface tension and, most probably, also of the solution-OTS monolayer interfacial tension with the addition of agarose.

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Article multilayers of the kind employed in this study has been achieved using a methodology mainly based on the combined application of quantitative FTIR spectroscopy and synchrotron X-ray scattering techniques to continuous unpatterned films covering large silicon surfaces.12,15,17 These techniques are not applicable on the nanoscale, while the SFM imaging techniques employed in the study of nanopatterned monolayer architectures offer a direct but often less accurate and less reliable tool of structural characterization, mainly because of artifactual SFM effects20,37,57 that may lead to erroneous conclusions. To establish a valid comparative basis for the structural characterization of the assembled surface structures on different lateral length scales, we adopted a combined nano-micro-macro methodology whereby the SFM imaging is complemented by and correlated with quantitative FTIR spectral measurements performed on analogously fabricated surface features. This can be done on the microscale, as monolayer template features with lateral dimensions of the order of tens of micrometers are amenable to both SFM imaging (like their nanoscale counterparts) and quantitative micro-FTIR spectral measurements using an FTIR microscope (like corresponding unpatterned macroscale films).37 By bridging the information gap between the nanoscale and the macroscale, such microscale patterns provide a reliable calibration tool that can help elucidate ambiguities arising from the exclusive use of SFM imaging only (vide infra). Electrochemical patterning of OTS monolayers on silicon (OTS/Si) with an electrically biased SFM tip18,20,48,57 was employed in the preparation of monolayer template patterns with -COOH-terminated lyophilic domains (electrooxidized OTS (OTSeo)) spanning lateral dimensions in the range between ca. 10 nm to more than 1000 nm (Scheme 1). Micropatterned monolayer templates with lyophilic domains on the order of tens of micrometers were fabricated as shown in Scheme 2, by mask-defined local photocleavage of the alkyl tails of a self-assembled OTS/Si monolayer, followed by backfilling of the bare silicon oxide-hydroxide spaces thus created with a selfassembled NTS/Si monolayer and final conversion of NTS to oxidized NTS (NTSox) via chemical oxidation of the terminal vinyl function of NTS to -COOH.37 This conservative route, based on proven macroscale protocols that use NTS to generate lyophilic monolayer features with well-defined structure and -COOH top functionality (NTSox),15,37,45,58 was chosen in order to provide an unambiguous calibration standard for the correlation of SFM with FTIR data. 2.3. Experimental Procedures. All patterns were fabricated on double-side-polished silicon wafer substrates coated on both sides with a highly ordered (lyophobic) OTS monolayer assembled as described in ref 37. The OTS monolayer protects the unpatterned back side of the substrate from contamination that would interfere with the micro-FTIR spectral measurements performed in transmission through the silicon substrate.37 The tip-assisted electrochemical patterning of the OTS monolayers (Scheme 1) was performed with a SOLVER P47 instrument (NT-MDT) as described in ref 20. SOLVER P47 was also used in the SFM imaging of all nanopatterns (Figures 3, 6, 7, 8, 9). Two different experimental protocols were employed for the assembly of top OTS or NTS monolayers on OTSeo@OTS template nanopatterns (Scheme 1, routes 1 and 2), depending on environmental conditions:59 (I) The following sequence of operations is repeated three times: immersion for 10 min in pure (57) Wouters, D.; Willems, R.; Hoeppener, S.; Flipse, C. F. J.; Schubert, U. S. Adv. Funct. Mater. 2005, 15, 938–944. (58) Maoz, R.; Cohen, S. R.; Sagiv, J. Adv. Mater. 1999, 11, 55–61. (59) These assembly protocols are the result of trial-and-error experimentation under different environmental conditions. Protocol I was developed while working under conditions of low relative humidity (RH), which necessitated supply of additional water to the silane assembly sites, while protocol II was found to optimize the hydration layer needed for the formation of high-quality silane monolayers12 when the monolayer assembly operations were carried out at 50-60% RH and 22 ( 1 C in a humidity/temperature-controlled clean atmosphere (in a vertical laminar flow hood specially designed for this task12,37).

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Zeira et al. Scheme 1. Assembly Routes of Planned Surface Nanopatterns via Constructive Nanolithographya

a (center) Nanoelectrochemical patterning of a self-assembled OTS monolayer on silicon with an electrically biased SFM tip creates a monolayer template nanopattern consisting of lyophilic OTSeo sites surrounded by the unmodified lyophobic OTS monolayer (OTSeo@OTS/Si). (1,2,3) Selective self-assembly of a top organized monolayer on the OTSeo sites results in the formation of a bilayer nanopattern (e.g., eicosylamine on OTSeo bilayer, surrounded by the OTS monolayer; C20NH2/OTSeo@OTS/Si). (4) WDSA, effecting selective deposition of a liquid (depicted as a droplet) on the lyophilic OTSeo sites (e.g. agarose on OTSeo, surrounded by the OTS monolayer: Agarose/OTSeo@OTS/Si).

water and then in analytical grade toluene (surplus liquid being blown off with clean nitrogen after each immersion), immersion in the respective silane solution (0.005 M in purified bicyclohexyl12) for 15 min, followed by twice sonication in fresh analytical-grade toluene for ca. 2 min, drying in a stream of clean nitrogen, and final removal of eventual impurities adhering to the surface with Scotch tape. (II) The following sequence of operations is repeated twice: immersion in the respective silane solution (0.005 M in purified bicyclohexyl12) for 15-30 s, followed by twice sonication in fresh analytical-grade toluene for ca. 2 min, drying in a stream of clean nitrogen, and final removal of eventual impurities adhering to the surface with Scotch tape. The OTS employed throughout this work was Merck “For Synthesis” grade, NTS was obtained (as 10% solution in chloroform) from Prof. K. Ogawa, Kagawa Univ., Takamatsu, Japan. NTS was in situ converted to NTSox via oxidation with the KMnO4/crown ether (dicyclohexano18-crown-6) complex in benzene, as described in ref 37. Eicosylamine (synthesized in this laboratory) was assembled on OTSeo@OTS template nanopatterns (Scheme 1, route 3) by immersion for 5-10 s in a 0.01 M solution of the amine in purified bicyclohexyl,12 followed by twice immersion in bicyclohexyl for ca. 30 s. The deposition of eicosene and agarose (WDSA process) was done by retraction from melt at 65 C (eicosene) or from dilute aqueous solutions (0.1-0.2 wt %) at 90 C (agarose), as described 13988 DOI: 10.1021/la902107u

in refs 48 and 37, respectively. The deposition of gallium is described in Section 3.3.3. Micropatterns (Scheme 2) were fabricated as described in ref 37. All monolayer, bilayer and agarose micropattern images (Figures 1, 4, 5, 10, 11) were acquired on an NTEGRA instrument (NT-MDT), as described in ref 37. A DI Multi Mode IIIa instrument (Veeco) operated in the torsion resonance (TR) mode was used for the noncontact imaging of tall eicosene features (Figure 4). This mode was chosen since, as opposed to semicontact mode, the tip does not contact the surface during each oscillation cycle, but rather hovers a few nanometers above contact. The micro-FTIR spectra were acquired (in transmission through the silicon wafer substrate) on a Bruker Equinox 55 spectrometer equipped with a IRscope II infrared microscope, as described in ref 37.

3. Results and Discussion 3.1. MSA on Monolayer Template Micropatterns: Correlation of SFM with FTIR Data. Figure 1 shows typical SFM images of monolayer and bilayer micropatterns fabricated according to Scheme 2, assembly route a f b f c f e f f. The high contrast between bars (low friction) and squares (high friction) in the lateral force image acquired from the Si@OTS monolayer pattern (a) (corresponding to Scheme 2b) is consistent with the Langmuir 2009, 25(24), 13984–14001

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a Photocleavage-based patterning of OTS/Si monolayer using a transmission electron microscope (TEM) grid as a contact mask (a f b). The initial monolayer micropattern consisting of lyophilic silicon oxide-hydroxide regions surrounded by the unmodified lyophobic OTS monolayer (Si@OTS/Si) is subsequently used as template in a series of surface self-assembly and in situ chemical modification steps (notations are as in Scheme 1). Materials deposited by WDSA are depicted as liquid droplets.

presence of an ordered lyophobic monolayer on the bars (OTS) and a monolayer-free lyophilic surface (Si) within the squares. According to the corresponding contact mode topographic image (a), the depth of the squares (h = 110-120 nm) exceeds, however, by a factor of more than 40, the expected thickness of an OTS monolayer (!?), while switching to the semicontact mode (b) gives for exactly the same squares a depth consistent with a normal monolayer height (h ≈ 2.5-2.8 nm). Following the self-assembly, within the squares, of an NTS monolayer (Scheme 2c), the height (semicontact mode) of the squares is seen to equal that of the bars, with an apparently imperfect boundary between the NTS (squares) and OTS (bars) regions (c, topographic image). Although displaying now virtually equal heights, the contrast between squares and bars in the corresponding phase image (c) confirms the presence of different materials in each of these surface regions. Finally, the formation of a bilayer within the squares, upon the self-assembly of a top NTS monolayer (Scheme 2f), is seen to reverse the topographic contrast (d), the squares now being 2.2-2.6 nm higher than the bars (compare with topographic image b). Langmuir 2009, 25(24), 13984–14001

To check the SFM topographic results, micro-FTIR spectra (Figure 2) were acquired from a square region of the micropattern at the same stages as the SFM images: bare square (Si + back-side OTS/Si), monolayer filled square (NTS/Si + back-side OTS/Si), and bilayer filled square (NTS/NTSox/Si + back-side OTS/Si). The infrared data confirm the formation, within the bare squares, of a highly ordered NTS monolayer and then a highly ordered NTS/NTSox bilayer, with a molecular organization resembling that of the removed OTS monolayer.12 This is evident from the identical band widths and peak positions of the methylene H-C-H stretch modes, at 2916 cm-1 and 2848 cm-1, in all three curves in Figure 2, and their proportional growth.12,15,37,45 Practically identical spectral curves were recorded in control micro-FTIR measurements performed on unpatterned films on silicon consisting of the same monolayer and bilayer configurations.60 (60) Highly ordered OTS, NTS, and NTSox (prepared by the present employed oxidation route with the organic KMnO4-crown ether complex) monolayers display practically identical methylene stretch bands,15,37,45 as their molecular organization is virtually the same12,15 and each of these monolayer-forming compounds has 17 methylene units in its molecular tail.

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Figure 1. SFM images of monolayer and bilayer micropatterns corresponding to steps b, c, and f in Scheme 2, and height-width profiles along the marked lines.

According to X-ray diffraction data, highly ordered OTS, NTS, and NTSox monolayers have average thicknesses in the range 2.5-2.7 nm, with a characteristic roughness parameter (at the top monolayer surface) of ca. 0.2-0.4 nm.12,15,17 The cumulative effects of surface roughness and measurement noise in the present SFM images (Figure 1, see height-width profiles taken from images b, c, and d) would not allow the monolayer thickness to be locally determined with an accuracy better than ca. 0.5 nm (see also Figures 3, 7 and 8). It follows that SFM-measured height values of ca. 2.5 ( 0.3 nm compare well with those derived from the X-ray data and are indeed representative of the heights of compact, highly ordered OTS, NTS, and NTSox monolayer domains with dense packing and perpendicular orientation of their molecular tails. The infrared data (Figure 2) thus fully confirm the conclusions reached on the basis of the SFM data (Figure 1), with the exception of the contact mode topographic image (a), which is obviously artifactual. It is now well documented that the apparent negative topographic contrast usually displayed by the hydrophilic features in hydrophilic-hydrophobic monolayer patterns imaged in the contact mode represents an imaging artifact.20,37,57 However, such a huge artifactual effect has never been observed before, which emphasizes the need of an independent verification 13990 DOI: 10.1021/la902107u

of the validity of the SFM observations. A calibration of the SFM measured monolayer thickness against the corresponding X-ray derived thickness is thus achieved here via the correlation with the quantitative infrared spectral data. 3.2. 3D CL: Layer-by-Layer Electrochemical Patterning and Self-Assembly. A trilayer pyramid (Figure 3) was constructed according to assembly route 1 in Scheme 1, by repeating the tip-assisted electrochemical patterning at the level of the second assembled OTS monolayer (L2) and then concluding the process with the selective self-assembly of a third OTS monolayer (L3) on the bilayer template created by the patterning of the second layer. The successful inscription of a smaller OTSeo square on top of the bilayer square (L2) is indicated by the considerably higher friction displayed by this OTSeo square compared with that of both the L1 and L2 OTS (b, lateral force image, and lateral force-width profile) along with its weak negative height contrast (b, topography), which are characteristic features of the CL patterning of OTS monolayers.20,57 That this small apparent depression represents an imaging artifact (like the similar but much stronger effect discussed above in connection with the contact mode images in Figure 1) is confirmed by its disappearance upon the assembly of the third OTS monolayer (c). The successful layer-by-layer assembly of the entire pyramid Langmuir 2009, 25(24), 13984–14001

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Figure 2. Micro-FTIR spectra acquired from a square region of a micropattern such as that in Figure 1 at the stage of bare square (Si + OTS/Si), after the assembly of the NTS/Si monolayer (NTS/ Si + OTS/Si), and after the assembly of the NTS/NTSox/Si bilayer (NTS/NTSox/Si + OTS/Si). OTS/Si is the added spectral contribution of the back-side OTS monolayer coating of the silicon wafer, the spectra being recorded in transmission through the silicon substrate36 (see Section 2.3.). The spectral contribution of the uncoated silicon substrate is mathematically subtracted from all curves.

structure is finally confirmed by an analysis of the topographic images acquired at each stage of the build-up process: As indicated by both the height-width profiles and the height histograms, the average height of the L2 square relative to the background OTS monolayer (L1) is ca. 2.7 nm in image a and ca. 2.6 nm in image c; that of the L2+L3 bilayer relative to L1 is ca. 5.0 nm (c), and that of L3 relative to L2 is ca. 2.4 nm (c). It is further apparent in these SFM images that the surface roughness does not increase on going from L1 to L3 (peakto-peak roughness/noise features of ca. 0.5 nm), which indicates that the layers are similarly structured. Within the accuracy of the SFM imaging, these figures concur favorably with those derived on the basis of the monolayer and bilayer SFM data discussed before in connection with Figure 1. This analysis thus provides convincing evidence for the layer-by-layer self-assembly of discrete highly ordered OTS monolayers with dense packing and perpendicular orientation of their molecular tails, the assembly of each successive layer being strictly confined to the surface region of the underlying layer defined in the respective patterning step. Additional preliminary results point to the possible extension of this CL strategy based on the exclusive utilization of OTS monolayers to even thicker film structures. As expected from the decrease of the electronic current across a thicker film barrier, longer times appear to be needed (at a given applied bias voltage) for the patterning of thicker films compared to that of a single monolayer. 3.3. WDSA. 3.3.1. Eicosene and Agarose Micropatterns and Nanopatterns. Figure 4 shows examples of typical eicosene and agarose micropatterns assembled on patterned monolayer templates with variable size of lyophilic squares (in the range of tens of micrometers) according to Scheme 2, assembly routes a f b f d and a f b f c f e f g. As indicated by both the drop-like profiles of the eicosene features (Figure 4a-d) and their infrared spectra, upon cooling to the ambient temperature (22 ( 0.5 C), solidified eicosene appears to largely maintain (61) The peak positions and widths of the C-H stretch bands at 2917 cm-1 and 2849 cm-1 are indicative of a rotator phase of long chain n-alkanes, similar to the organization of the molecular tails in highly ordered NTS and OTS monolayers.12,15,17

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the overall bulk morphology of a supercooled liquid, with a local molecular organization reminiscent of that of a rotator rather than a crystalline phase.48,61 Upon cooling to the ambient temperature and equilibrating with the ambient humidity, the agarose solution retained on the hydrophilic squares forms a compressible, sponge-like hydrogel (Figure 4e,f).37 A characteristic WDSA feature, apparent in both the SFM images and micro-FTIR spectra collected from these patterns, is the monotonic growth of the thickness (h) of retained material with the size (w) of the lyophilic template region on which deposition occurs.50 The eicosene squares are, however, at least 10 times thicker than agarose squares of similar size (compare squares d and e). This is mainly a consequence of the fact that eicosene was assembled from the neat melt, while agarose was from a dilute aqueous solution that may lose much of its initial water content by evaporation. Indeed, at the ambient relative humidity (RH) of ca. 55%, the water content of the agarose features (determined from the intensity of the water bending mode at 1644 cm-1 in the respective infrared spectra37) accounts for no more than 22% of the height of the small square and 19% of that of the large one (Figure 4e,f, respectively). The thickness of eicosene assembled on the bare silicon oxide squares (Scheme 2d) was found to be higher than that on the -COOH-terminated monolayer squares (Scheme 2g) of identical size (Figure 4: compare squares a and b with c and d, respectively). This trend suggests that the effect of small topographic differences between the lyophilic and lyophobic regions of a template pattern may not be neglected49,50 even in the case of a monolayer height of no more that 3 nm, which is 4 orders of magnitude smaller than the lateral dimensions of the lyophilic features under consideration and almost 3 orders of magnitude smaller than the thickness of the retained liquid. All eicosene features were assembled at the lowest retraction speed provided by the motor-driven lift used in this study (ca. 66 μm/s), which was found to give optimal results in terms of the selectivity of deposition on the lyophilic regions and the thickness uniformity of deposited material.48 With agarose, different retraction speeds were needed to reach optimal deposition conditions on different length scales. A general observed trend, on both the microscale (Figure 5) and the nanoscale (Figure 6), is the decreasing thickness of deposited agarose the higher the retraction speed. This is just the opposite of what was predicted theoretically and confirmed experimentally for the simple case of glycerol deposited on an isolated 49 μm wide and 4 mm long lyophilic strip.50 On the microscale, dramatic differences were observed in both the lateral distribution profile and total amount of agarose deposited at different retraction speeds on patterns consisting of tens of micrometers wide lyophilic squares (NTSox/Si) separated by somewhat narrower lyophobic (OTS/Si) bars (Figure 5). Clear kinetic effects of accumulation of material at the lyophilic/lyophobic boundaries, with preference for the downlocated square edges, are apparent at high retraction speeds (4160 μm/s - 770 μm/s). This behavior most probably reflects the complex dynamics of the readjusting shape of the liquid-solid contact line and the resulting meniscus as the patterned surface emerges from the agarose solution and the liquid retracts under the constraints imposed by the lateral and vertical alternation of the contact angle between the lyophilic and lyophobic values, gravitation, and water evaporation. The situation is seen to gradually improve at lower retraction speeds, finally reaching a rather homogeneous distribution of material on the lyophilic squares at speeds around 430 μm/s. DOI: 10.1021/la902107u

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Figure 3. Fabrication of a trilayer pyramid by 3D CL: Contact-mode SFM images of the (a) bilayer stage, (b) patterned bilayer, and (c) final trilayer pyramid (see text).

An inspection of agarose nanopatterns with features of different size and shape (Figure 6) reveals interesting effects of the assembly on monolayer template nanopatterns (Scheme 1, assembly route 4), some of which, again, are rather different from theoretical predictions and experimental observations with glycerol deposited on pattern features spanning tens of micrometers.49,50 As with the agarose micropatterns (Figure 5), more material is seen to be retained on exactly the same monolayer template nanopattern when the retraction speed is reduced from 770 to 66 μm/s (Figure 6a,b, respectively). Likewise, the thickness of deposited agarose features scales up with their lateral size (Figure 6c; compare dots A and B), which is a general feature of the WDSA process, observed with all materials, on all length scales (Figure 4). However, unlike the results obtained with glycerol deposited on lyophilic closed loops made of lines measuring tens of micrometers in width and hundreds of micrometers in length,50 here the agarose is rather uniformly deposited on the differently shaped template nanopatterns, including grids (Figure 6a,b) and more complex loop geometries (Figure 6d), without coating the 13992 DOI: 10.1021/la902107u

interior of these closed loop features.62 No dependence of the amount of deposited material on the orientation of the template pattern with respect to the direction of retraction from the liquid is apparent in any of these agarose nanopatterns either.50 A final interesting observation pertains to the uniform thickness of agarose assembled on template nanopatterns with interconnected features of very different lateral dimensions (Figure 6d), or with disconnected but closely located such features (Figure 6e). This is remarkable in view of the predicted variations of the liquid height in patterns with wide and narrow interconnected sections,49 as well as the generally observed monotonic dependence of the thickness of deposited material on the size of the template. In contrast with the unequal heights of the dots in Figure 6c, the uniform heights of the different pattern features in Figure 6d,e would suggest that, in the latter case, there is effective “crosstalk” between the different lyophilic features of the pattern during the short time these are traversed by the receding meniscus (62) Similar results were obtained with eicosene assembled on nanometric closed loop template features.

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Figure 4. SFM images, height (h)-width (w) profiles along the marked lines, and micro-FTIR spectra of eicosene and agarose squares in patterns fabricated by WDSA: (a,b) Eicosene/Si@OTS/ Si micropattern (Scheme 2d); (c,d) Eicosene/NTSox@OTS/Si micropattern (Scheme 2g); (e,f) Agarose/NTSox@OTS/Si micropattern (Scheme 2g). The eicosene images were acquired with a DI Multi Mode IIIa instrument in the TR mode, and the agarose images were acquired with an NTEGRA instrument in the semicontact mode. The micro-FTIR spectral curves (c,d,e,f) were acquired from the respective eicosene and agarose squares. A variable square aperture was used to define the IR acquisition area in each measurement, as shown in the optical micrograph taken from agarose square f. The spectral curves represent neat eicosene or agarose contributions, after mathematical subtraction of the spectral contributions of the NTSox/Si template monolayer and the back-side OTS/Si monolayer coating.

(as the template is pulled out from the agarose solution),63 whereas, in the former case, the lyophobic gap between the (63) One may note that, at a retraction speed of 66 μm/s, lyophilic template features measuring less than 1 μm in the vertical direction (as in Figure 6d,e) emerge from the liquid phase within less than 0.015 s.

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individual dots is sufficiently large (compared with the dot size) to permit separate equilibration of each dot with the liquid phase. 3.3.2. WDSA Edge Effects and Their Contribution to MSA on Nanopatterned Templates. As discussed before, the selfassembly of a highly ordered monolayer such as OTS, NTS, and C20NH2 is a self-regulating process in which the generation of a lyophobic top surface upon the completion of a well-defined compact layer of oriented molecules prevents further deposition of molecules from solution. Keeping in mind that the lyophobicity of such self-assembling monolayers is a property associated with the dense packing of the terminal -CH3 groups forming the top monolayer surface, this simple scenario should strictly apply to monolayer assembly on smooth, unpatterned (practically infinite) surfaces only. What will then happen at the edge of a highly ordered monolayer domain formed by laterally confined self-assembly on a nanopatterned lyophilic-lyophobic template (Scheme 1, assembly routes 1-3)? To answer this question, it may be instructive to first examine the WDSA of eicosene on a bilayer rather than monolayer template nanopattern (Figure 7a,b,c). A comparative analysis of the SFM images of eicosene assembled on an array of NTSox/OTSeo bilayer dot-lines (Figure 7c) and those of the respective bilayer template precursor NTS/OTSeo (Figure 7b) shows that each dot in image c is ca. 2-3 nm higher and ca. 4-10 nm broader than the corresponding dot in image b. On this basis, we may conclude that the assembly of eicosene on the NTSox/OTSeo bilayer dots resembles the previously studied eicosene assembly on arrays of OTSeo monolayer dots (Scheme 1, assembly route 4),48 the added height following the eicosene assembly being consistent with a monolayer of perpendicularly oriented eicosene molecules deposited on the lyophilic top surface of the NTSox monolayer, and the broadening of the dots pointing to the spillover of eicosene into the surrounding lyophobic area (as shown in Figure 7, scheme c). As expected, in a control experiment in which a similar array of OTS/OTSeo dot-lines (Figure 7d) was used as the bilayer template, no eicosene was retained on the lyophobic top surface of the OTS/ OTSeo bilayer. However, despite their unchanged heights, the dots broadened considerably (Figure 7e), almost as in the assembly of eicosene on the lyophilic NTSox/OTSeo dots (Figure 7; compare images e and d with c and b, respectively). This clear lateral growth implies that eicosene molecules stick to the edges of the OTS dots like they stick to those of NTSox (Figure 7, scheme c), even though eicosene does not stick to their top surfaces. This is understandable in view of the fact that the methylene-rich (-CH2-) inner core of the OTS monolayer exposed at the edge of a cylindrical OTS dot is not oleophobic like its methyl-rich (-CH3) top surface. Thus, notwithstanding their small molecular heights, the wettable perimeters of isolated monolayer domains in topographically modulated surface patterns such as the bilayer nanopattern in Figure 7b are active in WDSA processes and may retain significant amounts of material from liquid phases that do not wet their top surfaces.64 Coming back to the issue of laterally confined MSA on monolayer template nanopatterns with isolated lyophilic domains such as that depicted in Figure 7a (Scheme 1, assembly routes 1-3), we note that the NTS/OTSeo bilayer dots in Figure 7b are also considerably (64) A related mechanism for the retention of molecules of a nonwetting liquid on a surface coated with an imperfect lyophobic monolayer was proposed in ref 14. (65) It should be noted that the hydrolysis and lateral bonding of the silane headgroups may be completed upon contact with the ambient humidity after withdrawal of the assembled monolayer from the silane solution.12 Such postassembly fixation of surplus material adhering to the periphery of a monolayer domain may be minimized by thorough rinsing immediately after withdrawal from the silane solution.

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Figure 5. SFM images and height-width profiles along the marked lines of Agarose/NTSox@OTS/Si micropatterns (Scheme 2g) produced with variable retraction speeds (between 4160 and 430 μm/s) from the agarose solution.

wider than the corresponding OTSeo template dots (Figure 7a), and the same trend was noted in the assembly of the OTS/OTSeo bilayer dots (Figure 7d) as well. In a previous publication it was suggested that this broadening might be an artifactual imaging effect caused by the convolution with the SFM tip.20 However, no such broadening could be detected in the self-assembly of eicosyl amine (C20NH2) on an array of OTSeo template dots (compare panels g and f of Figure 7), although the added height of the eicosyl amine monolayer (2.7-2.8 nm) is comparable with that of NTS or OTS. Thus the convolution with the tip does not seem to significantly affect the measured widths of monolayer features with heights on the order of 2-3 nm, which further means that the observed broadening of the NTS and OTS dots must be real. The basic difference between the amine and the silane selfassembly is that the amine binds ionically to the top -COOH functions of the OTSeo template, via formation of -COO+ H3N- ion pairs (Scheme 1, assembly route 3),55 whereas silanes 13994 DOI: 10.1021/la902107u

such as OTS and NTS can bind to both the template and to adjacent silane molecules in the monolayer via multiple and exchangeable covalent and hydrogen bonds (involving -Si-O-C-, -Si-O-Si-, and -SiOH species).12 In other words, the anchoring of the amine headgroups to the template is exclusively vertical, while that of the trichlorosilane headgroups involves both vertical and lateral bonding.12 We may then envisage a kinetically controlled mechanism of broadening of the silane bilayer dots that involves the initial rapid selfassembly of a highly ordered silane monolayer strictly confined to the lyophilic area of each template dot, followed by a slower WDSA-assisted process whereby additional silane molecules from solution adhere to the lyophilic edges of the initially formed monolayer dots (with preferential tail alignment with the perpendicularly oriented tails of the dot molecules) and eventually form lateral covalent/hydrogen bonds with those of the dot (Figure 7, scheme b). As no similar mechanism of lateral Langmuir 2009, 25(24), 13984–14001

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Figure 6. SFM images of agarose nanopatterns assembled on OTSeo@OTS/Si monolayer templates with OTSeo features of variable size and shape (Scheme 1, assembly route 4), and height (h)-width (w) profiles along the marked lines. Patterns a and b were assembled with two different retraction speeds on the same monolayer template pattern. Patterns b, c, and d are located on same silicon wafer specimen; pattern e (on a different silicon specimen) was produced with same retraction speed (66 μm) as b, c, and d. The vertical direction in the figure corresponds to the direction of retraction from the agarose solution. All listed height and width figures are in nanometers.

fixation is operative in the case of the long-tail amine, C20NH2 molecules weakly sticking to the edge of an initially assembled C20NH2/OTSeo bilayer dot are easily removed by the solvent rinse applied as a routine final step in the monolayer assembly protocol (see Section 2.3.). Strong support to the basic validity of this kinetically controlled mechanism of broadening of the silane dots comes from the observation that, with much shorter immersion times in the solution of the trichlorosilane monolayer precursor, and by limiting the access of water to the surface of the OTSeo template (see monolayer assembly protocols I and II in Section 2.3.), it was possible to fabricate NTS/OTSeo and OTS/OTSeo bilayer dots with widths practically identical to those of the respective OTSeo template dots (Figure 8). Such experimental conditions are likely to minimize the probability of lateral bonding, by both Langmuir 2009, 25(24), 13984–14001

suppressing the hydrolytic conversion of the chlorosilane headgroups to the respective silanols12 and limiting the interaction time of silane molecules from solution with those in the already assembled monolayer dots.65 3.3.3. Gallium Nanopatterns. Gallium melts at ca. 30 C; however, if heated to ca. 112 C and then allowed to cool, it remains in a supercooled liquid state that facilitates its surface manipulation at ambient temperature (22 ( 0.5 C). Several experimental procedures were tried for the assembly of gallium on monolayer template nanopatterns (Scheme 1, assembly route 4). Here we show examples of nanodots obtained with two very different assembly protocols, which permit one to drastically vary the amount of metal retained on each OTSeo template dot: (a) gentle tapping of the silicon substrate with a drop of liquid gallium placed on the site of the template nanopattern DOI: 10.1021/la902107u

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Figure 7. SFM images of dot-line nanopatterns acquired at different consecutive stages of processing: (a) OTSeo@OTS/Si monolayer template nanopattern; (b) NTS/OTSeo@OTS/Si bilayer nanopattern assembled on monolayer template a (Scheme 1, assembly route 2; NTS assembled via protocol I (Section 2.3)); (c) eicosene deposited (WDSA) on the bilayer template nanopattern generated by the KMnO4 oxidation of the top NTS monolayer of b to NTSox; (d) OTS/OTSeo@OTS/Si bilayer nanopattern (Scheme 1, assembly route 1; OTS assembled via protocol I (Section 2.3)); (e) eicosene deposited (WDSA) on bilayer nanopattern d; (f) OTSeo@OTS/Si monolayer template nanopattern; (e) C20NH2/OTSeo@OTS/Si bilayer nanopattern assembled on monolayer template f (Scheme 1, assembly route 3). Examples are given of height (h)-width (w) profiles (figures in nanometers) showing the same pair of dots at different consecutive stages of processing from the monolayer template to the bilayer and the final eicosene/bilayer nanopattern. The growing dot width (w) on going from a to c is indicated in schemes a, b, c.

(at ambient temperature), in order to promote effective contact of the gallium with the lyophilic features of the template, followed by removal of the gallium drop by pushing it away from the nanopattern site with the tip of a glass pipet; (b) a piece of 13996 DOI: 10.1021/la902107u

Scotch-tape on which a thin film of liquid gallium was applied is pressed against the silicon substrate bearing the template nanopattern, this entire sandwich being then placed for a few minutes in a hot oven at 112 C. The tape is finally peeled off while hot, Langmuir 2009, 25(24), 13984–14001

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Figure 8. SFM images and height (h)-width (w) profiles along the marked lines of OTSeo@OTS/Si monolayer template nanodots and the corresponding NTS/OTSeo@OTS/Si bilayer nanodots (as in Figure 7a,b, respectively) with NTS assembled via protocol II (Section 2.3.).

shortly after the silicon-tape sandwich is taken out from the oven. As a result of the extremely phobic character of unmodified OTS monolayer toward gallium, no traces of metal are left on the surface, except for the wetted OTSeo nanodots, regardless of the assembly protocol employed. Protocol “a” yields gallium nanodots with heights on the order of 20 nm on OTSeo template dots with widths on the order of 25 nm (Figure 9a), by far the highest aspect ratio (height/width) reached to date in surface assembly via the WDSA process. Protocol “b”, by contrast, yields dot heights of ca. 2 nm on ca. 40 nm wide OTSeo template dots (Figure 9b), which is less than the molecular height of eicosene assembled on smaller template dots48 (Figure 7). The drop-like profiles of both the tall and short gallium nanodots suggest that the deposited metal is still in a supercooled liquid state. This is supported by the observation that dot features with clear crystalline morphology are obtained upon the in situ conversion of such gallium nanodots to silver or gold via a process of galvanic displacement (paper in preparation). However, the facile SFM imaging of these gallium features and the fact that closely spaced gallium dots in very dense dotlines do not coalesce (Figure 9b, phase image) suggests that each such discrete gallium nanodot may have the structure of a mushroom-shaped solidified glass rather than that of a flowing liquid droplet. The possible assembly of very dense but uniform gallium dot-lines circumvents the problem of coalescence and bulging occurring in the assembly of liquids on continuous template wires, and is thus of interest as a route to uniform conducting nanowires.48 3.4. Composite Surface Architectures: Combining WDSA with MSA. A few representative examples were given of the use of patterned organosilane monolayers as guiding templates for sitedefined deposition of material from a liquid phase by either Langmuir 2009, 25(24), 13984–14001

precise MSA or WDSA. One may, however, design more complex modes of guided self-assembly, in which sequential WDSA and MSA, starting from an initial monolayer template layout, can lead to composite surface architectures with unusual new properties. In the example described here, agarose hydrogel assembled on the NTSox squares of an NTSox@OTS/Si monolayer micropattern (Figure 10, top row) plays the role of template in the self-assembly of a top NTS monolayer that renders the outer surface of the water-containing hydrogel hydrophobic (Figure 10, middle row). The hydrophilicity of the hydrogel surface is then restored by the in situ chemical conversion of NTS to NTSox (Figure 10, bottom row). As the water condensation images in Figure 10 clearly show, following the assembly of the NTS monolayer on the agarose squares, most water droplets (dark spots) reside on the OTS bars at the boundaries with the agarose squares (Figure 10, middle row). The NTS-coated agarose is thus more hydrophobic than the OTS/Si monolayer! Upon the oxidation of the terminal vinyl function of NTS to -COOH with the organic KMnO4 reagent, this situation is reversed to that before the assembly of the NTS monolayer, all agarose squares being now completely wetted by the condensed water (Figure 10, bottom row). Most interestingly, in response to variations of the ambient RH, agarose coated with NTS contracts and swells somewhat less (Figure 11) than the uncoated hydrogel;37 however, despite its pronounced hydrophobicity toward liquid water, the NTS monolayer is obviously leaky with respect to permeation of water vapor and does not effectively protect the hydrogel from equilibrating with the ambient humidity. The thickness of the agarose hydrogel is thus subject to variations associated with variations in its water content, which makes it practically impossible to rely on SFM DOI: 10.1021/la902107u

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Figure 9. SFM images of gallium nanopatterns and the corresponding monolayer template nanopatterns (Scheme 1, assembly route 4). Examples are given of height (h)-width (w) profiles (figures in nanometers) of the same dots in the OTSeo@OTS/Si template and the corresponding gallium/template nanopattern. Patterns a and b were produced with different gallium assembly protocols (see text).

measurements as an indicator of the presence and structure of a 2-3 nm-thick monolayer assembled on the hydrogel surface. In fact, a comparison of the height-width profiles recorded from the same agarose square before and after its coating with NTS, and after the conversion of NTS to NTSox (Figure 10, middle column), points to a 2.5% thickness decrease following the NTS assembly, instead of the expected 2.5% increase as a result of the added thickness of the NTS monolayer. The thickness remains constant following the oxidation of NTS to NTSox; however, the oxidation was performed after the exposure of the pattern to variable levels of ambient humidity, upon which the thickness decreased by an additional 3% (compare the height-width profiles in Figures 10 and 11). Quite obviously, the added monolayer thickness of 2-3 nm is much too small to be detectable by such SFM measurements when superimposed on a 4050 times thicker gel feature that may contract and swell independently of the presence of the monolayer. The NTS coating is evident from the hydrophobicity it imparts to the coated gel surface; however, upon its conversion to NTSox, the hydrophilicity of the gel surface is restored and the SFM measurements would not offer any evidence as to the fate of the monolayer under the conditions of the oxidation reaction. The presence of an undamaged NTSox monolayer coating is convincingly demonstrated by the quantitative micro-FTIR spectra taken from one agarose square (in parallel with the SFM measurements and the 13998 DOI: 10.1021/la902107u

wettability observations) before and after the NTS assembly, and after the oxidation of NTS to NTSox (Figure 12). Both the initially assembled (NTS) and the oxidized (NTSox) monolayer display sharp methylene bands centered at 2916 cm-1 and 2848 cm-1, like the corresponding bands of the template NTSox/Si monolayer and the back-side OTS/Si monolayer (see discussion in Section 3.1.). Thus the infrared spectra unequivocally demonstrate the quantitative formation of a highly ordered NTS monolayer on top of the agarose hydrogel, which remains intact upon its conversion to NTSox. These results emphasize once more the remarkable performance of organosilane monolayers in surface self-assembly as well as the important role of a combined analytical methodology that can offer independent verifications of the meaning of SFM observations.

4. Conclusions MSA and WDSA are two basically different modes of sitedefined deposition of material from a liquid phase on a patterned solid surface consisting of lyophilic and lyophobic regions. The rationale behind the present comparative study has to do with the fact that, in both cases, the selectivity of material deposition is governed by its affinity to lyophilic surface sites as contrasted with the repellency of the lyophobic portions of the patterned surface, which led to the realization that, in certain particular situations, WDSA may simulate some of the features of MSA. Langmuir 2009, 25(24), 13984–14001

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Figure 10. SFM images and optical micrographs: (top row) Agarose assembled on an NTSox@OTS/Si monolayer template micropattern (Scheme 2, assembly route a f b f c f e f g). The optical micrograph taken with the SFM video camera shows three cantilevers with the red laser beam focused on the middle one. (middle row) Same agarose pattern after the assembly of a top NTS monolayer. The water condensation micrograph was taken in a purpose designed humidity chamber.37 (bottom row) Same as the middle row, after the KMnO4 oxidation of NTS to NTSox. The height-width profiles (middle column) were taken from the same agarose square at each successive stage of its modification.

Using patterned organosilane monolayers produced by selfassembly as lyophobic-lyophilic templates for both further MSA and WDSA enabled us to devise a consistent experimental framework applicable on practically any length scale of interest from nanometer to centimeter, as well as an internal calibration system of the SFM measured thicknesses, based on the known thickness and structure of the monolayer template itself. In addition, quantitative micro-FTIR spectroscopy was used to determine the structure of monolayer entities assembled on micrometric length scales and so check the validity of the SFM measurements. This methodology allowed fine-tuning of the template architecture with unprecedented precision and reproducibility, down to molecular dimensions, and, consequently, attainment of similar molecular resolution in the characterization of the various materials assembled on the template. A number of different examples of self-assembling organosilane monolayer entities were analyzed using the present combined SFM-FTIR approach: monolayer and bilayer micropatterns on the length scale of tens of micrometers, comprising OTS, NTS, and NTSox species (Figures 1 and 2); monolayer, bilayer, and trilayer nanopatterns on the length scale of hundreds of Langmuir 2009, 25(24), 13984–14001

nanometers, comprising OTS and OTSeo species (Figure 3); monolayer and bilayer nanopatterns on the length scale of tens of nanometers, comprising OTS, OTSeo, NTS, and NTSox species (Figures 7 and 8); complex micropatterns on the length scale of tens of micrometers, including OTS and NTSox species on silicon and NTS and NTSox species assembled on agarose deposited on the NTSox on silicon (Figures 10-12). The entire body of evidence obtained supports the view that all these monolayer entities have essentially the same structure, regardless of the nature of the underlying surface on which the monolayer was assembled (oxidized silicon, another monolayer or bilayer, or agarose) and its lateral size. Both the SFM and the IR spectral data are consistent with a densely packed monolayer structure virtually identical to that of highly ordered monolayers and multilayers of the same long-tail silanes assembled on virtually infinite surfaces. Thus the site-defined deposition of material via the self-assembly of lyophobic monolayers of long-tail silanes such as OTS and NTS is characterized by the formation of similarly structured molecular films with well-defined thickness (equal to the extended molecular length of the respective monolayer forming compound), regardless of the nature and lateral DOI: 10.1021/la902107u

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Figure 11. Height-width profiles along the marked line across the same NTS-coated agarose square as in Figure 10, middle row, under exposure to variable levels of ambient RH.

Figure 12. Micro-FTIR spectra recorded from an agarose square of the pattern shown in Figure 10, before (A) and after (B) its coating with the NTS monolayer, and after the conversion of NTS to NTSox (C). OTS/Si is the added spectral contribution of the back-side OTS monolayer coating of the silicon wafer (as in Figure 2). The difference spectra shown in the magnified spectral region around 2900 cm-1 give the net monolayer contributions, after mathematical subtraction of the agarose contribution from each of the three curves, A, B, and C.

dimensions of the lyophilic template regions on which the material is deposited.66 These findings can be rationalized in terms of the dynamic bonding model of the trivalent silane headgroups, involving exchangeable covalent and hydrogen bonds, vertical as well as lateral, which is a characteristic feature of such organosilane monolayer systems.12 The lateral bonding of the silane head groups manifests itself also in the observed broadening of monolayer features assembled on lyophilic template domains of nanometric dimensions (Figure 7), this effect being minimized by conducting the monolayer assembly under conditions that limit the formation of lateral covalent and hydrogen bonds (Figure 8). Monolayers of other long-tail amphiphiles, e.g., C20NH2, anchored with a one-to-one molecular stoichiometry to the top functions of an organosilane monolayer template necessarily adopt the molecular organization of the template (Figure 7). The structural insensitivity of highly ordered organosilane monolayers to the nature and size of the underlying lyophilic substrate, along with their self-healing and high resilience, are further remarkable manifestations of the dynamic nature of the lateral bonding in such films.12 These properties render (66) The smallest lyophilic template domains thus far produced, with areas in the range of 100-200 nm2, can accommodate 500-1000 closely packed molecular tails. It is conceivable that, below a certain critical area, the total number of surface -bound molecules may not be sufficient to maintain the highly ordered structure of an infinite monolayer.

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self-assembling organosilane monolayers rather unique as both templates and building blocks for the bottom-up fabrication of precisely defined nanoarchitectures. To fully exploit the benefits offered by such monolayer systems, it is, however, necessary to invest considerable efforts in the synthesis of suitable monolayer components, which constitutes a main obstacle to their wide utilization. The successful electrochemical functionalization of simple n-alkylsilanes such as OTS in both monolayer and thicker film structures (Figure 3) may circumvent this difficulty and pave the way to useful new modes of application of organosilane monolayers. Unlike MSA, WDSA is applicable to a multitude of materials that may be deposited under a multitude of experimental conditions, leading to a multitude of different results. This is emphasized by the present results obtained with materials as different as eicosene, agarose, and gallium, assembled from the melt or aqueous solutions, at temperatures between 65 and 112 C, using different deposition procedures. The complexities characterizing this mode of surface assembly leave little hope for theoretical considerations that might have real predictive value in the selection of specific materials and experimental conditions. Consequently, while WDSA is extremely versatile and useful, it is also experimentally demanding, as only by trial and error experimentation one may find out whether and how it may be applied to a particular assembly task. Langmuir 2009, 25(24), 13984–14001

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In WDSA the thickness of deposited material scales up with the lateral dimensions of the lyophilic regions of the template; however, as the present study shows, relative to lateral dimensions, much more material is usually deposited on nanometric than on micrometric template features. For example, the ratio film thickness/template width is 0.080-0.090 in the eicosene nanodots (Figure 7c) and 0.028-0.034 in the micrometric eicosene squares (Figure 4). This trend reflects the fact that the thickness of the eicosene film retained on template features with lateral dimensions below ca. 30 nm reaches the limiting molecular dimension of ca. 2.5 nm, which is consistent with a dense monolayer of perpendicularly orientated eicosene molecules. Additional evidence in support of the ordered monolayer structure of the thinnest eicosene film retained on nanometric template regions comes from the observation that eicosene nanopatterns appear to be indefinitely stable under ambient conditions, while the micrometric patterns steadily loose material by slow sublimation of the eicosene, until the same stable monolayer thickness of ca. 2.5 nm is finally reached. Infrared spectra acquired from such residual films are consistent with this interpretation. It thus appears that the adhesion forces between a single eicosene monolayer and the underlying OTSeo or NTSox template surface are considerably stronger that the intermolecular cohesion forces operating in bulk eicosene. The results obtained with agarose and gallium resemble those obtained with eicosene in the sense that WDSA on nanometric template regions with lateral dimensions on the order of tens of nanometers and smaller can yield quite uniform film thicknesses on the order of several nanometers (Figures 6, 7, 9), which are comparable with typical film thicknesses obtained by MSA. Thus, on the nanoscale, both assembly modes may be used to deposit comparable amounts of materials, and care should therefore be

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exercised not to confuse WDSA with MSA. WDSA gives access to a wide variety of materials with different desired properties, without, however, allowing precise control of the structure and amount of deposited material. In contradistinction to WDSA, the self-assembly of highly ordered organosilane monolayer systems is restricted to a particular class of materials; however, it offers the structural uniformity, stability and reproducibility needed for the planned fabrication of 3D nanoarchitectures with subnanometric precision, including template structures that can guide the deposition by both assembly modes. These attributes of lyophobic MSA are conserved on all lateral length scales. The thickness invariability is thus a simple and generally applicable criterion by which genuine MSA can be distinguished from WDSA. Combined modes of surface self-assembly involving both WDSA and MSA are also possible (Figure 10) and may provide access to composite surface architectures exhibiting unusual properties and functions. Note Added after ASAP Publication. This article was published ASAP on October 16, 2009. The second paragraph in section 3.3.2 has been modified. The correct version was published on October 22, 2009. Acknowledgment. This research was supported by the Israel Science Foundation (Grant No. 1109/04), the G. M. J. Schmidt Minerva Center of Supramolecular Architectures, and the Minerva Foundation with funding from the Federal German Ministry of Education and Research. This research is made possible in part by the historic generosity of the Harold Perlman family. The NTS material was kindly supplied by Prof. Kazufumi Ogawa of Kagawa University, Takamatsu, Japan.

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