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Distinct Differences in Self-Assembly of Aromatic Linear Dicarboxylic Acids Christoph Heininger,† Lorenz Kampschulte,†,‡ Wolfgang M. Heckl,†,‡ and Markus Lackinger*,† Department for Earth and EnVironmental Sciences, Ludwig-Maximilians-UniVersity and Center for NanoScience (CeNS), Theresienstrasse 41, DE-80333 Mu¨nchen, Germany, and Deutsches Museum, Museumsinsel 1, DE-80538 Mu¨nchen, Germany ReceiVed September 17, 2008. ReVised Manuscript ReceiVed October 31, 2008 Self-assembly into two-dimensionally ordered supramolecular structures of three aromatic dicarboxylic acids—2,6naphthalenedicarboxylic acid (NDA), 4,4′-biphenyldicarboxylic acid (BPDA), and 4,4′-stilbenedicarboxylic acid (SDA)—is studied at the liquid-solid interface by scanning tunneling microscopy. All compounds possess structural similarities, namely, two interconnected aromatic moieties and functionalization through two carboxylic groups in linear configuration. For all molecules, ordered monolayers were observed on a graphite substrate, and the resulting structures can be described as a dense packing of one-dimensionally hydrogen-bonded rows. However, concerning the stability of the adsorbate layers, the average domain size, and the degree of order, distinct differences were noticed. Supported by density functional theory (DFT) calculations, these differences are analyzed and explained as a consequence of molecular structure, adsorption geometry, and adsorption energy.
Introduction For applications in organic electronics, supramolecular selfassembly offers a promising approach for both rapid fabrication and for coping with the problems of ongoing miniaturization. Yet, device performance crucially depends on structural defects such as grain boundaries, which can deteriorate the electron mobility by orders of magnitude. In particular, as active regions in organic electronic components grow smaller and encompass fewer molecules down to the single molecule level as envisioned in molecular electronics, the influence of structural defects becomes more and more important. In order to achieve large, defect-free assemblies, it is essential to understand growth of the specific system on a molecular basis. In many cases, functionalization of molecular building blocks is the dominating aspect. For instance, functionalization can determine the dimensionality of supramolecular structures.1 Supramolecular nanowires, i.e., one-dimensional nanostructures, are frequently grown from either single ditopic molecules with complementary functional groups for hydrogen bonding or two different but likewise complementary molecules.2-4 Since they can interact with themselves because of their combined proton donor and acceptor character, carboxylic groups are widespread, very powerful synthons for supramolecular self-assembly.5,6 Furthermore, deprotonized carboxylic groups can form metal coordination bonds, widely exploited to create two- and three-dimensional nanostructures.7,8 Here, we compare interfacial self-assembly of three structurally similar aromatic dicarboxylic acids at the liquid-solid interface. Yet, the * Corresponding author. E-mail:
[email protected]. † Ludwig-Maximilians-University and Center for NanoScience. ‡ Deutsches Museum. (1) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619. (2) Weckesser, J.; De Vita, A.; Barth, J. V.; Cai, C.; Kern, K. Phys. ReV. Lett. 2001, 87, 096101. (3) Keeling, D. L.; Oxtoby, N. S.; Wilson, C.; Humphry, M. J.; Champness, N. R.; Beton, P. H. Nano Lett. 2003, 3, 9. (4) Canas-Ventura, M. E.; Xiao, W.; Wasserfallen, D.; Mullen, K.; Brune, H.; Barth, J. V.; Fasel, R. Angew. Chem., Int. Ed. 2007, 46, 1814. (5) Lackinger, M.; Griessl, S.; Markert, T.; Jamitzky, F.; Heckl, W. M. J. Phys. Chem. B 2004, 108, 13652. (6) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290.
apparently minor structural differences have substantial impact on the interaction strength with the substrate and consequently the assembling behavior. Since self-assembly crucially depends on the balance and interplay of interactions, this can have important consequences. In contrast to self-assembly processes at the solid-vacuum interface, adsorbed monolayers at the liquid-solid interface are commonly in thermodynamic equilibrium with the supernatant solution.9,10 A lowered desorption barrier facilitates exchange of molecules and can result in vertical mobility of the adsorbates. Most important, adsorption and subsequent monolayer formation at the liquid-solid interface only take place when the thermodynamic driving force is strong enough, i.e., the gain in enthalpy exceeds the loss of entropy.11 Contributions to the enthalpy arise from molecule-molecule and molecule-substrate interactions. Graphite, as a layered van der Waals material, exerts only weak forces on adsorbates, hence sufficiently strong intermolecular interactions are vital to stabilize the monolayer, especially when rather small molecules are to be adsorbed. Yet, despite the relative weakness of adsorbate-substrate interactions on graphite, for many molecules a distinct epitaxial relation between adsorbate and substrate lattice exists.12 For all compounds investigated here, intermolecular interactions are governed by two sets of 2-fold hydrogen bonds per molecule, and are thus comparable. However, the observed differences exemplify the importance of the molecule-substrate interaction, even on weakly physisorbing surfaces such as graphite. (7) Stepanow, S.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Delvigne, E.; Lin, N.; Deng, X. B.; Cai, C. Z.; Barth, J. V.; Kern, K. Nat. Mater. 2004, 3, 229. (8) Chae, H. K.; Siberio-Perez, D. Y.; Kim, J.; Go, Y.; Eddaoudi, M.; Matzger, A. J.; O’Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523. (9) Lei, S.; Tahara, K.; De Schryver, F. C.; Van der Auweraer, M.; Tobe, Y.; De Feyter, S. Angew. Chem., Int. Ed. 2008, 47, 2964–2964. (10) Kampschulte, L.; Werblowsky, T. L.; Kishore, R. S. K.; Schmittel, M.; Heckl, W. M.; Lackinger, M. J. Am. Chem. Soc. 2008, 130, 8502. (11) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (12) Lackinger, M.; Griessl, S.; Heckl, W. M.; Hietschold, M. Anal. Bioanal. Chem. 2002, 374, 685.
10.1021/la803055p CCC: $40.75 2009 American Chemical Society Published on Web 12/17/2008
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Figure 1. Structures of dicarboxylic acids studied. The upper row depicts the chemical structure, while the lower row represents DFT geometryoptimized structures of isolated molecules in the gas phase. Regarding the symmetry, all molecules exhibit at least an inversion center. (a) NDA, (b) BPDA, and (c) SDA. Between the aromatic moieties, dihedral angles of 0° (a), 32° (b), and 8° (c) were found.
Experimental Section Molecular monolayers were imaged in situ at the liquid-solid interface by scanning tunneling microscopy (STM). All experiments were conducted under ambient conditions, and constant current topographs were obtained with a home-built STM operated by a commercial RHK SPM 100 control unit. Typically, tunnelling voltages between +0.6 V and +0.8 V (with respect to the tip) and setpoint currents around 800 pA were applied. For all experiments, tips were used without further insulation and directly immersed into solution. As probes either mechanically cut Pt/Ir (90/10) wires or electrochemically etched tungsten tips (1 M, aqueous NaOH solution, ∼5 Vac) were used. Normally, tips were conditioned by short voltage pulses during the experiments. As solvents, two fatty acids (heptanoic and nonanoic acid) were used, both providing sufficient solubility for all dicarboxylic acids. No significant differences between both solvents were found, thereby excluding a decisive influence of the solvent. For all compounds, saturated solutions served as a starting point. Those were prepared by dissolving the solute until sedimentation, sonicating the solution for 5 min and subsequent centrifugation for 5 min at 5000 rpm. Concentrations were measured by means of UV-vis spectroscopy with a USB4000 spectrometer from Ocean Optics. By comparison with reference solutions of known concentration, the solubility of all three compounds was found to be in the order of 0.1 mmol/L. Prior to the STM experiments, small droplets (∼5 µL) of saturated solution were applied to the freshly cleaved basal plane of highly oriented pyrolytic graphite (HOPG). Density functional theory (DFT) calculations were carried out with the Dmol3 software package, based on a VWC local potential.13
Results and Discussion The three solute molecules investigated have their functionalization with two carboxylic groups in linear configuration and two aromatic moieties in common; their chemical structures are depicted in Figure 1. However, the compounds differ in the way the two benzene rings are interconnected. For the smallest molecule, 2,6-naphthalenedicarboxylic acid (NDA), the two benzene rings are directly linked into a naphthalene unit. As anticipated, a DFT-based geometry optimization of an isolated molecule yields a planar geometry of the aromatic core. In 4,4′(13) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
biphenyldicarboxylic acid (BPDA), the two phenyl groups are interconnected by a single carbon-carbon σ-bond. Since in BPDA the two phenyl rings are in close proximity, a planar geometry is very unfavorable due to steric hindrance of the hydrogen atoms at the 2,6 and 2′,6′ positions, respectively. A DFT-based geometry optimization of an isolated molecule yields a dihedral angle of 32° between the otherwise planar phenyl moieties. In other words, the energetic gain through an extended aromatic system in a hypothetical fully planar geometry is not strong enough to overcome the steric hindrance. Restricted Hartree-Fock calculations find a somewhat higher dihedral angle of 44.8° between the two phenyl rings.14 The largest compound investigated is 4,4′-stilbenedicarboxylic acid (SDA), where two phenyl rings are interconnected by an ethenyl unit. This rather flexible linker group with two freely rotatable σ-bonds allows both cis and trans configuration. All three compounds self-assemble into ordered monolayers at the nonanoic and heptanoic acid/graphite interface, respectively, and high-resolution topographs of each compound are presented in Figure 2a-c. As illustrated by the molecular overlay, all monolayers can be described as a dense packing of onedimensional molecular rows, and monomolecular unit cells suffice for a crystallographic description. Proposed structural models are derived by molecular overlays that have been scaled to the actual size of the STM topographs. Then an arrangement is postulated that is consistent with the submolecular contrast and also allows for 2-fold hydrogen bonds between carboxylic groups of adjacent molecules, the driving force for chain formation. The linear configuration of the two COOH groups in all dicarboxylic acids facilitates continuation of the binding principle into onedimensional supramolecular polymers. Since for the 2-fold hydrogen bond between carboxylic groups a straight geometry is energetically preferred, and, apparently, other constraints are not dominating, the rows grow linear. At the liquid-solid interface, dense packing of these molecular wires into closed two-dimensional monolayers provides the necessary additional stabilization through van der Waals interaction among the rows. Cross-linking between the rows via weak C-H · · · O hydrogen (14) Matos, M. A. R.; Miranda, M. S.; Martins, D. V. S. S.; Pinto, N. A. B.; Morais, V. M. F.; Liebman, J. F. Org. Biomol. Chem. 2004, 2, 1353.
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Figure 2. STM topographs of dicarboxylic acid monolayers obtained at the liquid-solid interface. (a,d) NDA (unit cell parameters: a ) 1.1 ( 0.2 nm, b ) 0.8 ( 0.2 nm, γ ) 82° ( 2°); (b,e) BPDA (unit cell parameters: a ) 1.0 ( 0.3 nm, b ) 0.8 ( 0.1 nm, γ ) 78° ( 4°); and (c,f) SDA (unit cell parameters: a ) 1.40 ( 0.05 nm, b ) 0.75 ( 0.1 nm, γ ) 75° ( 4°). The upper row depicts high-resolution topographs, whereas, in the lower row, typical large-scale images of the respective compound are depicted in order to demonstrate the long-range order, domain size distribution, defect density, and coverage for all dicarboxylic acids. Because the quality of STM data on BPDA monolayers is generally inferior as compared to that on NDA or SDA, the image depicted in panel b has been averaged over equivalent parts of a larger scan.
bonds is geometrically feasible and might additionally stabilize the structure. According to the proposed monolayer structures, the H · · · O distance measures ∼0.25 nm both for NDA and SDA, which is well within the range for weak C-H · · · O hydrogen bonds.15 However, because of their weak nature, it is hard to discriminate whether these additional hydrogen bonds are structure determinant or a mere consequence of crystal packing. Isolated supramolecular wires are often observed under ultrahigh vacuum conditions; for instance, NDA was found to form truly one-dimensional supramolecular structures on Ag(111), which even overgrow step edges.16 Interestingly, for NDA, SDA, and BPDA on graphite, significant differences with respect to stability, defect density, and microstructure (i.e., domain size distribution) are apparent, as exemplified by the large-scale scans in Figure 2e,f. With NDA solutions, the surface is fully covered by adsorbate structures; however, domains remain comparatively small, with sizes down to 10 nm. At some domain boundaries, assemblies of NDA molecules with deviating azimuthal orientation can be found. On the other hand, antiphase domain boundaries between translationally shifted domains can ideally be closed by fitting in rotated NDA molecules; an example is depicted in Figure 3. A similar arrangement is also found at nondensely packed domain boundaries, indicating a general preference for a domain
termination by azimuthally rotated NDA molecules. The submolecular STM contrast of NDA exhibits two distinct lobes, as also observed for unfunctionalized naphthalene on Cu(111) in solution.17 Within the framework of nucleation theory, such a small average domain size can be explained by a high nucleation density. When kinetically allowed, Ostwald ripening (the growth of larger domains or grains at the expense of smaller ones) subsequently reduces the number of small domains and eliminates boundaries. This was experimentally observed for the monoaromatic analog to NDA, terephthalic acid (TPA; 1,4-dicarboxylic acid) on graphite,18 whereas, for NDA, ripening processes are significantly slowed down if not inhibited at room temperature. Small domains were still observable after several hours during the course of extended experiments. In principle, this high nucleation density is caused by a relatively high adsorption rate in combination with a slow growth rate. For growth of stable nuclei into extended islands, surface diffusion is a vitally important process. However, if surface diffusion is comparatively slow, the nucleation rate exceeds the growth rate, resulting in a fine-grained microstructure. Experiments with diluted NDA solutions (1 part saturated solution + 1 part pure solvent) yield a comparable domain size distribution, thereby proving that the high nucleation density is not primarily caused by rapid adsorption. Also, besides minor changes, the
(15) Steiner, T.; Saenger, W. J. Am. Chem. Soc. 1992, 114, 10146. (16) Schnadt, J.; Rauls, E.; Xu, W.; Vang, R. T.; Knudsen, J.; Laegsgaard, E.; Li, Z.; Hammer, B.; Besenbacher, F. Phys. ReV. Lett. 2008, 100, 046103.
(17) Wan, L. J.; Itaya, K. Langmuir 1997, 13, 7173. (18) Lackinger, M.; Griessl, S.; Kampschulte, L.; Jamitzky, F.; Heckl, W. M. Small 2005, 1, 532.
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Figure 3. STM topograph of a NDA antiphase domain boundary. The white dashed line illustrates the lateral offset between the two domains. Interestingly, the gap between translationally shifted domains is ideally closed by inclusion of a row of azimuthally rotated NDA molecules (two example molecules are encircled). It is conceivable that the rotated NDA molecules are additionally stabilized by weak C-H · · · O hydrogen bonds with NDA molecules at the domain boundary in a head-to-edge arrangement.
NDA microstructure remains mostly unaffected over a time span of approximately one hour and coalescence of islands is rarely observed, as shown by a series of subsequent STM images (cf. Supporting Information). This provides more experimental evidence for the hypothesis that surface diffusion of NDA on graphite is comparatively slow and molecules are not very mobile. On the contrary, STM experiments on SDA reveal large domains with superior stability during imaging. Also, in contrast to NDA, Ostwald ripening is observed for SDA by means of subsequent STM topographs of the same area. In overview images, rather large domains with a diameter of at least 30 nm were identified. In some cases, no SDA domain boundaries were found within the maximum scan area of the instrument (0.5 × 0.5 µm2). In high-resolution images, the π-system of each phenyl ring and the double bond are clearly discernible. However, compared to NDA, SDA monolayers require longer time to grow, and full coverage is only observed after some time. Owing to the thermodynamic equilibrium between solution and surface, SDA monolayers also exhibit self-healing properties of defects such as vacancies through adsorption of excess molecules from the supernatant solution, possibly assisted by the scanning process (cf. Figure S1 of Supporting Information). It is reasonable to assume SDA molecules adsorb in trans configuration for several reasons. Experimentally determined unit cell parameters and submolecular contrast are in excellent agreement with SDA molecules in trans configuration. Moreover, without optical excitation, the trans configuration is thermodynamically stable. Third, because cis-SDA is not planar anymore, the trans configuration maximizes adsorbate-substrate and adsorbate-adsorbate interactions. Its planar geometry exhibits increased area of contact with the substrate and also facilitates the formation of four hydrogen bonds per SDA molecule with adjacent molecules. Compared to NDA and SDA, STM topographs of BPDA monolayers were more difficult to obtain, and the structures were
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not very stable during imaging. Disintegration of BPDA monolayers during image acquisition was frequently observed (cf. Figure S2 of Supporting Information). Normally, it took between 30 and 60 min until the first images of molecular monolayers could be obtained. It is conceivable that, during this time span, solvent was already evaporating, and the resulting supersaturation enforced adsorption. In order to promote monolayer self-assembly through adsorption from gently undercooled solutions, preheated (∼50 °C) saturated BPDA solutions were applied to the substrate held at room temperature, thereby shortening the time span until monolayers were observable. Because their functionalizations are similar and the overall sizes are comparable, the diffusion constants of all compounds in solution are expected to be very comparable. Also, the solute concentrations of saturated solutions are of the same order of magnitude for all compounds. Since the adsorption rate at the liquid-solid interface depends both on concentration and solute mobility in solution, an equal adsorption rate on the substrate of all compounds can be concluded. Yet, the question arises how can the differences in self-assembly behavior of the three dicarboxylic acids—large domains for SDA, small domains for NDA, and weak adsorption for BPDA—be explained? For BPDA, the DFT calculations of isolated molecules already reveal an unfavorable geometry for adsorption. Because of steric hindrance, the dihedral angle between the two phenyl rings is rather large, and the molecule is nonplanar, in agreement with other studies.14 Moreover, rotation around the interconnecting σ-bond is energetically too costly to allow for planarization upon adsorption. Since the physisorption of aromatic molecules on graphite is mediated by van der Waals and π-π interaction, a nonplanar geometry of the adsorbate results in reduced adsorption energy. Because these monolayers also require stabilization through molecule-substrate interaction, a low adsorption energy seriously inhibits monolayer formation even when the moleculemolecule interaction is relatively strong as in the case of BPDA. Furthermore, slow nucleation, possibly caused by a relatively large size of the critical nuclei, promotes the formation of large domains, as experimentally observed for BPDA monolayers (cf. Figure 2d). On the other hand, both NDA and SDA are planar molecules, and the observed differences in self-assembly are not as obvious as for BPDA. One possible explanation for the observed comparatively small NDA domains is offered by adsorption energy differences between SDA and NDA. Temperature programmed desorption (TPD) experiments of various polyaromatic molecules on HOPG indicate a particularly strong adsorption of naphthalene.19 The normalized (either to molecular weight or van der Waals surface) binding energy of naphthalene exceeds benzene, coronene, and ovalene. Since the energy barrier for surface diffusion can be approximated as a fraction of the adsorption energy,20 these results already suggest a low surface diffusion constant for NDA. Also, the aromatic system of NDA is largest and extends over two benzene rings, which favors π-π interaction with the likewise aromatic substrate. Furthermore, the center-to-center distance between the two benzene rings in NDA (0.266 nm) is very close to the graphite lattice parameter (0.246 nm), resulting in a favorable registry with the substrate. Both effects contribute to a high NDA adsorption energy and, consequently, a high energy barrier for surface diffusion. Competitive solvent adsorption provides indirect experimental evidence that the adsorption energy of SDA is lower than that for NDA. Occasionally, mixed monolayers of SDA and nonanoic acid were observed, and an example along (19) Ulbricht, H.; Zacharia, R.; Cindir, N.; Hertel, T. Carbon 2006, 44, 2931. (20) Miyabe, K.; Takeuchi, S. J. Phys. Chem. B 1997, 101, 7773.
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Figure 4. (a) STM topograph of a mixed nonanoic acid and SDA monolayer. (b) Corresponding model. Nonanoic acid molecules adsorb parallel aligned along rows in head-to-tail geometry, thereby maximizing intermolecular van der Waals interaction. These rows are interconnected by 2-fold hydrogen bonds, visible as dark troughs in the STM contrast. In these intermingled monolayers, some of the nonanoic acid rows are interconnected by ditopic SDA molecules, which likewise bridge the rows via hydrogen bonds. SDA molecules are tilted with respect to the row axis in order to adapt to the nonanoic acid row spacing.
with the corresponding model is presented in Figure 4. Fatty acids by themselves can form hydrogen-bonded monolayers on graphite as well.21,22 Only, when the gain in free energy of solvent and solute monolayers is comparable, competitive adsorption plays a role.23 For the three compounds investigated, nonanoic acid coadsorption was only observed for SDA but never for NDA, indicating a comparatively high free energy of NDA monolayers. Despite the lower stabilization energy of BPDA monolayers, mixed monolayers with nonanoic acid have never been observed. This can be explained by the nonplanar geometry of BPDA, which hampers effective hydrogen bonding on both carboxylic groups. As further indication for strong NDA-graphite bonding, adsorption of single NDA molecules inbetween domains, which thereby lack stabilization through molecule-molecule interaction, is frequently observed. Since similar observations have never been made at SDA domain boundaries, this provides further experimental evidence for enhanced interaction between NDA and the graphite substrate. There is one more aspect supporting a higher nucleation rate of NDA. As frequently observed at domain boundaries (cf. Figure 3), NDA molecules can also bind in an edge-to-head geometry to each other where the molecules are azimuthally rotated with respect to each other. This arrangement is reminiscent of the electrostatically driven 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) herringbone structure.24 Since, for NDA, various relative orientations yield stable supramolecular arrangements, the agglomeration rate on the surface is significantly larger compared to SDA, where apparently a head-to-head geometry is required. Consequently, the formation of critical nuclei is expected to be more rapid, which also yields a higher nucleation rate of NDA. (21) Wintgens, D.; Yablon, D. G.; Flynn, G. W. J. Phys. Chem. B 2003, 107, 173. (22) Volcke, C.; Simonis, P.; Thiry, P. A.; Lambin, P.; Culot, C.; Humbert, C. Nanotechnology 2005, 16, 2596. (23) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608. (24) Wagner, T.; Bannani, A.; Bobisch, C.; Karacuban, H.; Moller, R. J. Phys.: Condens. Matter 2007, 19, 056009.
Conclusion All three different aromatic dicarboxylic acids were found to form ordered monolayers at the solution-graphite interface. In all cases, 2-fold hydrogen bonds between carboxylic groups dominate the intermolecular interaction. The linear configuration of the two carboxylic groups in all compounds investigated facilitates the growth of linear hydrogen bonded arrangements, whereas the aromatic moieties contribute to the stabilization by means of π-π interaction with the graphite substrate. Despite their structural similarities, the three compounds exhibit pronounced differences in their self-assembly behavior. BPDA as the only nonplanar molecule shows diminished interaction with the substrate; consequently, the monolayers were not very stable. The other two compounds, NDA and SDA, are planar, but their monolayers exhibit differences with respect to the microstructure, i.e., domain size distribution. The comparatively small NDA domains arise from a high nucleation rate as a consequence of slow surface diffusion due to enhanced adsorbate-substrate interaction. In order to grow large, defect free assemblies of organic molecules on crystalline surfaces, a profound understanding of the processes involved in their growth is very important. Interactions between molecule and substrate are essential to stabilize the initial monolayer, but on the other hand, if too strong, they might hamper the formation of large domains and induce high defect densities. However, despite tremendous progress, fine-tuning of interactions by either functionalization and/or choice of substrate still mostly relies on intuition and experience rather than appropriate computational simulations. Acknowledgment. We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) within the SFB 486 and the Nanosystems Initiative Munich (NIM). Supporting Information Available: Additional STM topographs depicting disintegration of BPDA monolayers, self-healing in SDA monolayers, and frustrated Ostwald ripening in NDA monolayers. This material is free of charge via the Internet at http://pubs.acs.org. LA803055P
