NANO LETTERS
Spontaneous Fabrication of Microscopic Arrays of Molecular Structures with Submicron Length Scales
2002 Vol. 2, No. 6 635-639
Massimiliano Cavallini,*,† Fabio Biscarini,† Massimiliano Massi,† Angeles Farran-Morales,‡ David A. Leigh,‡ and Francesco Zerbetto§ Consiglio Nazionale delle Ricerche-Istituto per lo Studio dei Materiali Nanostrutturati, Sezione di Bologna, Italy, Department of Chemistry, UniVersity of Edimburgh, UK, and Dipartimento di Chimica “G. Ciamician”, UniVersita´ degli Studi di Bologna, Italy Received March 12, 2002; Revised Manuscript Received April 30, 2002
ABSTRACT We show the spontaneous fabrication of organized structures with multiple lengthscales from a solution of rotaxanes cast onto a holey mesh in contact with a substrate. The mesh controls the spatial distribution of deposited material on a micrometer scale. The borders around a hole in the mesh pin the solution, and dewetting within the hole generates droplets of rotaxane with nanometer length scale. The droplet size is controlled by the radial concentration profile.
Patterning molecular thin films by simple fabrication methods is crucial for the development of organic electronics and photonics. Amongst possible strategies, bottom-up approaches appear particularly promising. These approaches are mainly based onto two different paradigms: (i) selfassembly and the length scale of the phenomenon controlled by templates or stamps with well-defined features,1-3 else (ii) exploitation of the interplay between self-organization and cooperativity, competing interactions, spinodal dewetting, where the emergence of a pattern is intrinsic to a characteristic length scale of the relevant phenomena.4-6 In this letter we discuss the possibility to merge the two paradigms in order to spontaneously fabricate well-defined arrays of molecular domains with a microscopic pitch and a spatially organized structure at nanometer length scales. Such a proposition is very attractive in view of large area fabrication of arrays of devices or memory cells based on organics. Moreover, it is relevant for devising functional surfaces which respond to external stimuli at small length scales, and can be addressed at specific locations and at a larger length scale, for instance, by optical methods. Among possible candidates for bottom-up patterning, benzilic amide macrocycle-containing rotaxanes are particu* Corresponding author. Address: Consiglio Nazionale delle RicercheIstituto per lo Studio dei Materiali Nanostrutturati, Sezione di Bologna, Via P. Gobetti 101, I-40129 Bologna, Italy. Tel.++39-051-6398523 Fax. ++39-051-6398539. E-mail:
[email protected]. † Consiglio Nazionale delle Ricerche-Istituto per lo Studio dei Materiali Nanostrutturati. ‡ University of Edimburgh. § Universita ´ degli Studi di Bologna. 10.1021/nl0255496 CCC: $22.00 Published on Web 05/21/2002
© 2002 American Chemical Society
larly interesting for their rich phenomenology of organization at a surface controlled by their intrinsic multistability. Rotaxanes form a family of hydrogen-bonded architectures based on a macrocycle interlocked with a thread.7-9 The different accessible co-conformations make a rotaxane to be an intrinsically bistable or multistable system.10 Such architectures undergo wetting/dewetting transitions at a surface with dramatic morphological changes, when co-conformational changes are triggered either chemically11,12 or mechanically. The ease in inducing such co-conformational changes arises from their interactions in the solid state and the mobility of the macrocycle.13 Recently it was demonstrated that such architectures, such as the [2]catenanes-based monolayer sandwiched between two electrodes, can be electronically switched.14 Moreover, rotaxanes are suitable for electronically configurable molecular-based logic gates, which is the first step for the realization of molecular computers.15 Here, we propose a very simple approach based on the dewetting of a thin film confined inside a microscopic cavity. We demonstrate how a spontaneous pattern formation occurs and exhibits both micro- and submicrometer length scales, in a single step and a few seconds, starting from a template mesh. Dewetting is initiated by solvent evaporation, and the interplay between the shrinking droplet of rotaxane solutions and the finite boundaries of the cavity select the pattern and the submicrometer length scale which dominate at the end of the process. For these experiments, we used a series of rotaxanes,7 and their respective threads (Chart 1). Both rotaxanes and threads
Chart 1.
Schematic Diagram of the Rotaxane Structure
Figure 1. Optical micrograph (200 times magnification) of an array obtained by deposition of rotaxane 1 (a) 20 µL and (b) 60 µL droplet of a solution 0.15 mg/cm3, and the thread of rotaxane 1 (c) 20 µL and (d) 60 µL droplet of a solution 0.17 mg/cm3 onto a Cu grid. The grid has been removed.
(Top) Rotaxanes consist of a macrocycle (depicted as black ring) interlocked onto a linear chain (the thread) with two bulky stoppers. The thread is depicted as a “dumbell”. Bottom: the three rotaxanes used in this work.
are soluble in acetone (Aldrich for chromatography quality), and solutions with a concentration from 0.1 to 0.4 mg/mL were used. The substrate is an SiOx/Si wafer (Wacker Chemitronics, GmbH Germany) cleaned by sonication in acetone for two minutes, then in 2-propanol for two more minutes. The template mesh is a Cu grid for transmission electron microscopy (TAAB G1000HS, UK), which is placed on to the substrate without any mechanical or chemical bonding. The grid is ≈2 mm diameter, 1000 mesh (which implies about 5000 holes), each hole being a square with a 20 µm side, 20 ( 2 µm thick. The total effective volume of the holes is 0.04 µL. We put 20 µL (and integer multiple of it) of solution on the grid, then the grid was removed after the solvent had evaporated completely. Optical micrographs were taken in the reflection mode with an optical microscope and recorded with a CCD camera. An atomic force microscope (AFM) operated in contact mode was used to image the samples on a smaller scale. All images shown are unfiltered. The pattern formation occurs from a droplet of acetone solution cast onto a square grid with micrometer rule: after the solvent evaporation, an ordered pattern with holes surrounded by rings of molecular material appears (Figure 1a and 1c). The deposition is extremely effective and reproducible. Very rarely, we observed a flow of material under the grid, which was ascribed to grids deformed during the manipulation. In the arrays in Figure 1 all the holes are filled with rings with a narrow dispersion of diameter and thickness. Figure 636
1b and 1d show how, by increasing the cast volume, the holes can be completely filled with material. The AFM images in Figure 2 show details with submicron resolution of the typical structures generated from solutions of the rotaxane 1 (Figure 2 a-c) and its thread (Figure 2 d-e), respectively. The morphologies of rotaxanes 2 and 3 are similar to that of rotaxane 1. The morphology of the thread of rotaxane 1 is similar to that of the thread of 3, whereas the thread of 2 forms rings with a nanometer-sized lamellae oriented radially. The different morphologies, viz. fragmented vs continuous rings, appear to be a feature of the distinct nature of the rotaxane with respect to its thread. In Figure 2 we show a view at higher magnification of the cells forming the patterns in Figure 1. In the case of rotaxanes (Figure 2a-c), the hole of the mesh is filled with material organized in droplet patterns. At low cast volume (Figure 2a), a “dry” (e.g., material depleated) patch bound by a continuous ring of material forms at the interior of the hole and its radius decreases with increasing cast volume (as shown in Figure 2a-c), viz. the amount of deposited material. The dispersion of the droplet size depends on the radial distance from the center, and droplet size increases with the radial distance from the center. Casting larger volumes (Figure 2c-e), the ring disappears and a homogeneous dispersion of spherical-capped droplets is deposited over the squared patch. The droplet pattern exhibits a characteristic interdroplet distance at submicrometer length scale. On the basis of this evidence, the mechanism of droplet formation is discussed below. Conversely, the thread forms rings (Figure 2 d,e) whose diameter is almost invariant with the cast volume of the solution. The ring diameter is determined by the template mesh, their outer diameter being constant (15 ( 1 µm) and matching the size of the holes in the grid. The maximum height of the rim is approximately constant 210 ( 30 nm. At the center of the ring there is the dry patch. A thin layer spreading between the ring and the outer boundary of the dry patch is formed as the volume of cast solution increases. Nano Lett., Vol. 2, No. 6, 2002
Figure 3. Schematic drawing of the pattern formation within the holes, in the case of thread (left) and rotaxane (right) solutions: (a, e) formation of the meniscus; (b, f) the meniscus is pinned at the grid boundary, and the contact angle is constant. Capillary flow compensates solvent losses at the thinner points; (c, g) the solid phase is formed with the dry patch at the center surrounded by the ring formed by capillary flow; (d, h) larger cast volume shrinks or prevents the formation of the “dry” patch.
Figure 2. AFM images of the structures formed within the holes of a Cu grid (the dashed lines show where the grid was placed): rings from rotaxane 1 solution (vertical scale z ) 85 nm): (a) 20 µL, (b) 40 µL, (c) 60 µL; droplet patterns from thread of rotaxane 1 (vertical scale z ) 310 nm) (d) 20 µL, (rim height ) 180 nm), (e) 40 µL (rim height ) 225 nm), and (f) 60 µL (rim height ) 216 nm). Dashed white lines draw the edge of the grid, which has been removed.
Using 60 µL, the thin layer fills all space within the ring (Figure 2f). Its thickness increases with the radial distance from the center. The morphology as well as the thickness profile of the deposited material can be explained in the two cases by two concurrent mechanisms: (i) the ring is the result of mass transport via capillary flow;16 (ii) The material is deposited as a film, the coverage and the radial thickness profile depending on the concentration profile of the saturated solution within the hole. In the case of rotaxanes, droplets form as a consequence of the film rupture by dewetting. The pattern formation is schematically depicted in Figure 3. After the drop casting, the solution covers the grid completely because the cast volume (g20 µL) is much larger than the total volume of the holes (≈ 0.04 µL). When the volume of the solution is reduced close to the volume of the holes, due to solvent evaporation, a more concentrated Nano Lett., Vol. 2, No. 6, 2002
Figure 4. Optical micrograph showing the meniscus shapes upon deposition of 20 µL solution onto a Cu grid: (a) convex meniscus for the thread and (b) concave (inverted) meniscus for rotaxane 1. The images were recorded when the volumes of cast solutions were comparable with the volume of the holes. A movie showing the evolution as the solvent evaporates is enclosed as supporting information.
solution fills each hole. At this stage the effective concentration of solution is roughly proportional to the cast volume of solution. In the case of the thread, the solution forms a convex meniscus within each hole (Figures 3a and 4a). The outer rim arises from the pinning of the contact line at the boundaries of the grid phenomenon which is well described in ref 16 and has been exploited in order to generate a variety of ring motifs.17,18 Under this constraint, the strongest evaporation losses occur in the proximity of the contact line. The need to compensate such radially inhomogeneous losses, together with the constraint of a constant contact angle, forces an outward radial capillary flow of the solution at the bottom of the droplet (Figure 3b). This flow of solution accumulates 637
material at the border (Figure 3c). As long as the solution is diluted, this mechanism of mass transport is dominant, but as super-saturation is reached, the solute is deposited directly onto the substrate (Figure 3d). The homogeneous distribution of ring sizes, as in Figure 1, is explained on the basis that the contact angle and evaporation rate is the same for each hole in the grid. This evidence indicates that the micrometer scale pattern and the ring formation arise both from the evaporation of individual droplets of solution confined within each hole. In the case of rotaxanes, the ring formation has the same origin, with the solution pinned at the grid boundary. However, in this case the flow of material is inward, due to the inverted meniscus (Figures 3e and 4b) and the constraint of constant contact angle at the boundary. The fastest loss of solvent occurs at the center of the holes (Figure 3f). The origin of inverted meniscus is due to different surface and interfacial energies between the solutions and the grid, which determine the contact angle. This is confirmed by changing the nature of grid material: using a polymeric grid, the behavior of rotaxane solutions is similar to the thread solutions, with the ring forming in contact with the grid boundary. The dry patch arises because of recession of the contact line, and the ring diameter is set by the balance between the velocity of the receding front and the capillary flow inward (Figure 3f). Once the ring is formed, the evaporation of the solvent makes the solution to precipitate as a film, which then undergoes dewetting. Dewetting of a thin film4-6 gives rise to patterns with spatial correlation. In the case of droplet patterns, it results into the appearance of two length scales: the nearest neighbor interdroplet distance (viz. the correlation length) and the droplet diameter. In the case of rotaxanes on the grid, both the droplet size and interdroplet distance exhibit a radial dependence from the center outward. The interdroplet distance and droplet size in Figure 2a both increase monotonically with the radial distance. The radial distribution of droplet size in Figure 5 shows that there is indeed a correlation between the droplet diameter D and the radial distance R, which can be fitted by a power law. In this case, R is the variable of the film thickness profile, and the exact exponent in the power law will depend on the radial concentration profile. We find that for all the series of rotaxanes, D ≈ R1/3. Table 1 summarizes the best fit coefficients for our experimental data. It appears that these parameters are relatively insensitive to their detailed structure. The concentration controls the radial profile of the deposited layer, and hence, via dewetting, the characteristic length scales. Since both the characteristic size and distance are within the few hundred nanometer range, dewetting of the deposited film introduces a sub-microscopic lengthscale in direct superposition with the microscopic scale imposed by the grid. Noticeably, at large cast volume of rotaxane solutions (Figure 2c) there is a homogeneous (in space) dispersion of sizes and a single interdroplet characteristic distance. In this case, supersaturation is reached before the “dry” patch can form, and hence a homogeneous film is deposited within the hole. 638
Figure 5. Double log plot of the droplet diameter D vs the radial distance R from the center shows a power law dependence, with best fit exponents reported in Table 1. Table 1. Best Fit Coefficients from log D ) A + B log R Plot of the Average Droplet Diameter D vs the Radial Distance R from the Center rotaxane
A
B
1 2 3
1.42 ( 0.07 1.52 ( 0.10 1.20 ( 0.24
0.35 ( 0.02 0.31 ( 0.03 0.41 ( 0.07
In conclusion, we showed a fast and reproducible procedure capable of generating an ordered pattern of rings with both microscopic and submicroscopic length scales. In principle, the process shown here should be general and could be extended to other materials deposited on a surface in a partial wetting regime. We have demonstrated that a submicroscopic length scale, related to dewetting emerges in the case of the rotaxanes, but not with their thread, owing to a wetting/dewetting transition of the depositing layer. This contrasting behavior is also observed in films cast on the unstructured substrate: rotaxanes generate droplet patterns, whereas the thread forms smooth and continuous films. The molecular origin of dewetting in the case of rotaxanes can be ascribed to their relative ease to change their conformation in the solid state,13 which possibly changes their interaction with the underlying substrate, and hence surface tension. This origin is the subject of an ongoing investigation. Acknowledgment. We thank Carlo Taliani and Roberto Zamboni for their encouragement and discussions. This work was partly supported by the TMR initiative of the European Union through contracts FMRX-CT96-0059 and FMRXCT97-0097, and CNR Progetto Coordinato Nanotecnologie Project SCRIBA. F.Z. also acknowledges partial support from MURST project “Dispositivi Supramolecolari” and the University of Bologna “Funds for selected research topics” initiative. D.A.L. is an EPSRC Advanced Research Fellow (AF/982324). Supporting Information Available: A movie showing the evolution as the solvent evaporates is enclosed as supporting information. This material is available free of charge via the Internet at http://pubs.as.org. Nano Lett., Vol. 2, No. 6, 2002
References (1) Xia, Y.; Whitesides, G. M. Angew. Chem. Int. Ed. Engl. 1998, 37, 550. (2) Cavallini, M.; Murgia, M.; Biscarini, F. Nano Lett. 2001, 1, 193. (3) Schmidt, G.; Tormen, M.; Mueller, G.; Molenkamp, L. W.; Chen, Y.; Lebib, A.; Launois, H. Electron. Lett. IEE 1999, 17, 1731. (4) Herminghaus, S.; Jacobs, K.; Mecke, K.; Bischof, J.; Fery, A.; IbnElhaj, M.; Schlagowski, S. Science 1998, 282, 916. (5) Xie, R.; Karim, A.; Douglas, J. F.; Han, C. C.; Weiss, R. A. Phys. ReV. Lett. 1998, 81, 1251. (6) Higgins, A. M.; Jones, R. A. Nature 2000, 404, 476. (7) Clegg, W.; Gimenez-Saiz, C.; Leigh, D. A.; Murphy, A.; Slawin, A. M. Z.; Teat, S. J. J. Am. Chem. Soc. 1999, 121, 4124. (8) Blanco, M. J.; Jimenez, M. C.; Chambron, J. C.; Heitz, V.; Linke, M.; Sauvage, J. P. Chem. Soc. ReV. 1999, 28, 293. (9) Gatti, F. G.; Leigh, D. A.; Nepogodiev, S. A.; Slawin, A. M. Z.; Teat, S. J.; Wong, J. K. Y. J. Am. Chem. Soc. 2001, 123, 5983. (10) Caciuffo, R.; Degli Esposti A.; Deleuze, M. S.; Leigh, D. A.; Murphy, A.; Paci, B.; Parker, S.; Zerbetto, F. J. Chem. Phys. 1998, 109, 11094. (11) Balzani, B.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem.
Nano Lett., Vol. 2, No. 6, 2002
Int. Ed. Engl. 2000, 112(19), 3484. (12) Cavallini, M.; Lazzaroni, R.; Zamboni, R.; Biscarini, F.; Timpel, D.; Zerbetto, F.; Clarkson, G. J.; Leigh, D. A. J. Phys. Chem. B 2001, 105, 10826. (13) Biscarini, F.; Cavallini, M.; Leigh, D. A.; Le´on, S.; Teat, S. J.; Wong, J. K. Y.; Zerbetto, F., J. Am. Chem. Soc. 2002, 124, 225. (14) Collier, C. P.; Mattersteig, G.; Wong, E. W.; Luo, Y.; Beverly, K.; Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000, 289, 1172. (15) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (16) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, Th. A. Nature 1997, 389, 827. (17) Maillard, M.; Motte, L.; Pilleni, M. P. AdV. Mater. 2001, 13, 200. (18) Hofkens, J.; Latterini, L.; Vanoppen, P.; Faes, H.; Jeuris, K.; De Feyter, S.; Kerimo, J.; Barbara, P.; De Schryver, F. C.; Rowan, A. E.; Nolte, R. J. M. J. Phys. Chem. B 1997, 101, 10588.
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