Consequences of Varying Adsorption Strength and Adding Steric

Sep 10, 2012 - HUT molecules experience much stronger hydrogen bonding in addition. ..... W.; Wee, A. T. S. ACS Nano 2010, 4, 849−854. (24) Mao, J...
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Consequences of Varying Adsorption Strength and Adding Steric Hindrance on Self-Assembly of Supramolecular Polymers on Carbon Substrates Roozbeh Shokri,†,‡ Francois Vonau,‡ Marion Cranney,‡ Dominique Aubel,‡ Ashok Narladkar,‡ Benjamin Isare,§ Laurent Bouteiller,§ Laurent Simon,‡ and Günter Reiter*,†,∥ †

Physikalisches Institut, Universität Freiburg, Freiburg, Germany Institut de Science des Matériaux de Mulhouse (IS2M), Mulhouse, France § Laboratoire de Chimie des Polymères, Université Pierre et Marie Curie, Paris, France ∥ Freiburg Institute for Advanced Studies (FRIAS), Freiburg, Germany ‡

ABSTRACT: We investigated the consequences of changes in adsorption strength and the influence of steric hindrance with respect to ordering of supramolecular polymers on surfaces. The focus is on the kinetics of domain formation and the guidance of this selfassembly process by the substrate. To demonstrate general features, we compared two molecules, both forming supramolecular polymers, bis(hexylureido)toluene (HUT) and bis(2-ethylhexylureido)toluene (EHUT), differing only in the architecture of the alkyl side groups, on two substrates. Although highly oriented pyrolytic graphite (HOPG) and epitaxial graphene (EG) grown on silicon carbide have identical chemical composition and nearly the same crystal lattice parameters at the surface, they differ significantly in adsorption strength. Due to its higher polarizability, HOPG adsorbs molecules much more strongly than EG. Nonetheless, even on EG, the formation of supramolecular polymers was guided by the symmetry of the substrate lattice, but at a much slower rate. Accelerating ordering on EG through appropriate solvent vapor annealing, we eventually observed similar triangular patterns of HUT molecules on both substrates. This indicates that the orientation of supramolecular polymers is not controlled by the strength of substrate−molecule interactions but rather by the possibility to establish registry with the substrate. However, such guiding influence of the substrate was lost, even on strongly adsorbing HOPG, when steric hindrance, generated by adding ethyl branches to the side chains of the HUT molecule, caused weak adsorption. As a consequence, EHUT molecules did not adsorb in a parallel but in more or less perpendicular orientation to the substrate, and the resulting patterns were not guided anymore by the symmetry of the substrate. This demonstrates that small modifications of a molecule like the addition of short side branches can lead to drastic changes in the self-assembly process.

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INTRODUCTION Self-assembly of molecules on surfaces is often driven by a subtle balance of intermolecular interactions. During the past decades, enabled by the development of atomic force microscopy (AFM) and scanning tunneling microscopy (STM), details of the influence of various molecule−molecule and molecule−substrate interactions on the process of selfassembly of either small or long molecules have been studied.1−7 Self-assemblies based on weak and reversible supramolecular interactions, e.g., hydrogen bonds or π−π stacking, have attracted significant attention as suitable building blocks for a variety of applications, such as nanoelectronics, drug delivery, and solar cells.8−13 To improve such devices and to obtain the desired properties, a profound understanding and precise control of self-assembly up to macroscopic length scales is required. While three-dimensional molecular organization in solutions is quite well understood, organizing the same molecules in two dimensions on a surface is not yet fully predictable, particularly concerning aspects of long-range order of a supramolecular layer up to micrometer length scales. © 2012 American Chemical Society

There, the competition between conformational changes potentially induced when molecules interact with the substrate and the packing order resulting from interactions with the nearest-neighbor molecules will determine the ordering process. As a result of such competition, often hierarchical patterns may appear which exhibit different features of order on increasing length scales. Chemical synthesis allows modifying the molecular structure progressively and in a highly controlled fashion. It is thus possible to systematically study the influence of molecular details (e.g., steric hindrance induced by additional side groups) on the self-assembly process. It is, however, often quite difficult or even impossible to modify the molecule−substrate interactions in a controlled and predictable manner. In most cases, a change of the substrate is required which introduces a change of polarizability. Thus, when adsorbing molecules on Received: June 13, 2012 Revised: September 4, 2012 Published: September 10, 2012 21594

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different substrates, one may induce significant changes in the conformations of molecules and also affect their orientation on the substrate. In the present work, we aim to demonstrate in a general fashion (i) the influence of the strength of interaction between substrate and molecules on substrate-guided selfassembly, by using chemically identical substrates differing in polarizability, i.e., graphite and graphene, and (ii) the influence of steric problems caused by adding a small ethyl branch to the side groups. We use two different substrates, graphene and highly oriented pyrolytic graphite (HOPG), to distinguish the contribution of the substrate from the influence of interactions between molecules. These substrates have identical chemical composition and nearly the same crystal lattice parameters at the surface but differ in adsorption strength. Graphene, here epitaxial graphene (EG) grown on SiC(0001), is an almost inert (“neutral”) substrate which only allows for weak adsorption of molecules. Graphene is a one-carbon-atom thick crystallographically perfect layer with amazing electronic properties like very low polarizability.14,15 As a consequence, a graphene substrate (or analogously epitaxial graphene) is only weakly interacting with molecules deposited onto its surface. Due to such almost complete electronic decoupling between substrate and molecules, STM measurements are able to directly access almost unperturbed molecular orbitals, e.g., of πconjugated molecules [Shokri et al., to be published].16 Recently, the interaction of molecules with EG has attracted much attention, motivated by the possibility to fabricate, e.g., graphene-based molecular electronic devices or other applications.16−24 Using STM, molecular self-assembly on EG has been studied and compared at the molecular level up to several tens of nanometers but not up to micrometer length scales.17−19 In contrast, strong interactions between substrate and molecules are present on HOPG which has a higher polarizability and thus stronger van der Waals forces between the substrate and adsorbate. For example, it has been demonstrated that alkanes and also other organic molecules can order in registry on HOPG, driven by substrate−molecule interactions with a suitable lattice match between the adsorbate and the substrate.1−5 However, it is not clear if much weaker substrate−molecule interactions like on EG would also lead to epitaxially ordered molecular layers. Thus, to determine if strong molecule−substrate interaction forces are necessary for obtaining highly ordered self-assembled structures, or if weak interactions in combination with lattice match can also induce order in random deposits of molecules, we compared the resulting self-assembled patterns on an epitaxial graphene and a graphite substrate. In the following, we present results on the self-assembly process on HOPG and EG using bis(hexylureido)toluene (HUT) and bis(2-ethylhexylureido)toluene (EHUT).25 The choice of the particular systems was guided by the fact that selfassembly of these molecules on gold has been studied in some detail previously.9,26,27 However, the chosen system can be considered as one possible choice for demonstrating general features. Our conclusions are expected to be valid for all systems having similar characteristics. Figure 1A depicts these molecules consisting of a toluene core connected to two urea groups which provide the dominating intermolecular interactions through four hydrogen bonds. As a result of hydrogen bonding, these molecules have the potential of forming long and straight supramolecular polymers which can also order

Figure 1. (A) Schematic representation of bis(hexylureido)toluene (HUT) and bis(2-ethylhexylureido)toluene (EHUT) molecules (carbon atoms are displayed in turquoise, hydrogen in gray, oxygen in red, and nitrogen in blue). The maximum length of fully extended HUT and EHUT molecules is 2.6 nm. (B) and (C) AFM images in air of bare HOPG and EG, respectively. The size of both images is 1.15 × 1.15 μm2. The color code reflects height variations between 0 and 6 nm. (D) and (E) Low-temperature STM topography images of a monolayer and a bilayer of epitaxial graphene on SiC(0001), respectively, imaged at UT = 100 mV and IT = 300 pA. The size of the images is 5 × 5 nm2.

within larger assemblies.9,26,27 Both molecules have lateral alkyl chains of the same length which are symmetrically attached. To elucidate the relative importance of steric effects, we compare HUT molecules with the behavior of the quite similar EHUT molecule, a molecule derived from HUT by attaching an extra ethyl branch at the second carbon atom of each hexyl side group. By comparing the self-assembly behavior of these two molecules on two substrates of distinctly different adsorption strength, we are able to distinguish the relative importance of molecule−molecule and molecule−substrate interactions. The latter interactions are weakened in the case of EHUT due to the presence of a steric constraint resulting from the ethyl side branches.

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EXPERIMENTAL SECTION After dissolving HUT and EHUT molecules in a good solvent, they were deposited onto the substrate. We tested several methods like spin-casting, dip-coating, or drop casting. In the here presented experiments, toluene was used as the solvent, and solutions with a concentration of 0.04 mg/mL were used for preparing thin films by drop casting 4−15 μL of the solution. For AFM measurements (JPK, NANOWIZARD II) in air and at room temperature, we used sharp tips (SNL Bruker) in the tapping mode. The tips had a resonance frequency of 65 kHz and a tip radius of ca. 2 nm. STM experiments were performed with an LT-STM from Omicron working at 77 K at a base pressure in the 10−11 mbar range and employing W tips. 21595

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orientation changed at domain boundaries of the polycrystalline surface of HOPG. We note in passing that a similarly symmetric structure was observed for six other molecules having a bis-urea-toluene core when deposited on HOPG [Shokri et al., to be published]. Moreover, the distance between these parallel ribbons was rather constant, yielding a periodicity of 2.3 ± 0.3 nm. This period is consistent with the length of the molecule (ca. 2.6 nm), taking into account the convolution of the real size of the measured object with the size of the AFM tip. Thus, we conclude that the observed ribbons represent supramolecular polymers,9,26,27,33 consisting of a chain of extended molecules closely stuck together via hydrogen bonds. Analyzing the height variations in Figure 2A, we were able to identify at least three distinct levels differing by ca. 0.5 nm, suggesting that HUT molecules are lying flat on the substrate. Subsequent deposition of several additional drops from the 0.04 mg/mL solution, having a volume of 4 μL each, on the sample shown in Figure 2A resulted in the formation of further layers. Up to a total thickness of ca. 5 nm was checked, and the symmetry of the substrate was still detectable through the orientation of the supramolecular polymers. This suggests that stacks of supramolecular polymers were formed via a layer by layer growth mode in epitaxy with HOPG. Such ordering without any further sample treatment was occurring already during the process of depositing HUT molecules from a dilute solution. Figures 2C and 2D represent the results obtained when performing the same deposition process for HUT molecules, but now on EG. We also observed extended ribbons, indicating that supramolecular polymers formed also on EG. The width of the ribbons was again comparable to the length of HUT molecules. As on HOPG, the height of the ribbons was about 0.5 nm, suggesting that also on EG HUT molecules were lying flat on the substrate. However, the resulting ribbons were not at all oriented in preferred directions, although the crystalline surface of EG has the same symmetry as that of HOPG. The randomness in orientation was most probably indicating that HUT molecules were not in registry with the substrate which could be attributed to the much weaker interactions between HUT and graphene than those between HOPG and HUT. Intriguingly, the ribbons were frequently curved and almost always formed closed looplike structures. The topographic AFM images suggest that supramolecular polymers formed some kind of “fences” which enclosed additional (“free”) HUT molecules. To explore if the obviously weak coupling between HUT and EG is nonetheless sufficient for guiding self-assembly via the symmetry of the crystalline substrate, we have annealed the asdeposited molecules at 25 °C in a solvent-saturated atmosphere. The sample shown in Figures 2C and 2D was exposed to chloroform vapor in a tightly closed vessel for various times, progressively increased in steps of several hours. The resulting patterns are shown in Figure 3. Already after 4 h of solvent annealing a few triangular structures became visible, as highlighted in Figure 3A. However, the largest part of the sample still showed less ordered areas consisting of meandering ribbons. Subsequent solvent annealing for an additional 12 h led to the formation of long and straight ribbons as in Figure 3B. However, many distorted ribbons were still visible. Nonetheless, as indicated by the Fourier transform shown in the inset of Figure 3B, the 6-fold symmetry of the underlying graphene substrate could already be identified. Thus, HUT supramolecular polymers started to

Freshly cleaved HOPG (grade ZYH) and epitaxial graphene grown on 6H-SiC(0001) were used as substrates. Typical STM and AFM images of graphene and graphite are shown in Figure 1. Graphene layer preparation was carried out in ultra high vacuum (UHV) by annealing SiC at 650 °C for several hours for degassing followed by subsequent cycles of annealing for 10 s at 1250 °C to sublime Si atoms.28−31 The graphene layer was then analyzed by STM at 77 K under UHV conditions. The resulting epitaxial graphene (EG) substrate consisted mostly of large areas of monolayer graphene (ML) and only small areas of bilayer graphene (BL). As shown in Figure 1D + E, it is possible to distinguish ML from BL. Indeed, all atoms of the graphene unit cell are equivalent for ML and were detected by STM, whereas only one out of two atoms was observed for BL at low bias voltage, as in BL the atoms are not fully equivalent.29 Interestingly, we did not detect any differences in self-assembly of HUT on ML or BL graphene within the measured AFM images. The large-scale roughness of the EG substrate (Figure 1C) is due to atomic steps in the underlying SiC substrate. However, even such a rough surface was covered by at least a continuous monolayer of graphene over the step edges.32 Consequently, the supramolecular layer always adsorbed on graphene covering the SiC step edges.17,19

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RESULTS AND DISCUSSION We first focus on the differences in patterns obtained directly after depositing HUT molecules on HOPG and on EG (Figure 2). On HOPG, we found triangular arrays of parallel supramolecular ribbons (Figure 2B) with a length of up to about 1 μm. As can be clearly seen from Figure 2A, these ribbons were oriented exclusively in three distinct directions, which reflect the hexagonal symmetry of HOPG. Of course, the

Figure 2. AFM images of HUT molecules on HOPG [(A) and (B)] and on EG [(C) and (D)], right after deposition from a dilute toluene solution. The color code reflects height variations between 0 and 1.5 nm for (A), between 0 and 0.7 nm for (B), and between 0 and 6 nm for (C) and (D). The size of the images is 1.6 × 1.6 μm2 for both (A) and (C), 50 × 50 nm2 for (B), and 250 × 250 nm2 for (D), respectively. 21596

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triangular structures, like Figure 2A or 3C for graphite and graphene, respectively, was not observed. Within the plane of the substrate, alkanes interact only via van der Waals forces, but HUT molecules experience much stronger hydrogen bonding in addition. As a consequence of four hydrogen bonds per HUT molecule, rather stiff supramolecular polymers form. They grow along the lattice axes of the substrate until they are hitting another supramolecular polymer. As none of the three main axes is expected to be preferred, all three directions are equally probable, leading to the formation of triangular patterns. In addition, the growth of supramolecular polymers in the direction of hydrogen bonding is much faster than growth in the orthogonal direction via lateral arrangement of polymers. Thus, at low coverage, it is more likely that an individual polymer forms rather than wide bundles of such polymers. As a result of growth, basically in three directions only, empty triangular structures were obtained with the polymers forming the boundaries. Due to commensurable lattice parameters, ordered assemblies of HUT molecules were able to reflect the hexagonal symmetry of the graphene substrate, even for weak substrate−molecule interactions. Indeed, weaker interaction of molecules with EG during the stage of drop casting reduced the chance of molecules to get adsorbed on the substrate. Weak adsorption did not allow for largely enhanced HUT concentrations at the substrate during the initial stages of the drop casting process. A low concentration of HUT molecules close to the EG surface, in turn, was responsible for a low nucleation probability; i.e., more time was needed for initiating the growth of supramolecular polymers on EG. One possibility to provide enough time for nucleation and the subsequent formation and ordering of supramolecular polymers was to anneal the sample in solvent vapor. Solvent molecules from the vapor phase adsorbed on the sample and (partially) dissolved the HUT molecules. Thus, their mobility and diffusion coefficient increased which, in turn, provided the possibility to form patterns of improved order. Ordered assemblies of HUT molecules were able to replicate the hexagonal symmetry of the EG substrate, even for weak substrate−molecule interactions. Thus, concerning registry with the substrates, we are dealing with two aspects. “Lattice matching”, i.e., monomer/crystal matching, is possible due to the close commensurability of the lattice spacings of either graphite and graphene with the characteristic length of zigzag C−C−C bonds of the side groups of HUT. For long supramolecular polymers, however, even a small misfit in lattice parameters may cause difficulties in arranging these polymers in accordance with the substrate lattice. Nonetheless, even if there is no 100% perfect lattice match, the orientation of these rather rigid supramolecular polymers was definitely guided by the symmetry of the substrate. Particularly during solvent annealing, when the length of these polymers increased in time, this guidance became highly visible, expressed by the formation of large triangular patterns. As they became longer, they more sensitively detected the shallow variations in the molecule− substrate potential as these were multiplied by a factor proportional to the number of molecules linked together in the polymer. We would like to mention that this kinetic aspect is essential. We expect that such guidance will be observable for most if not all long rigid supramolecular polymers on crystalline substrates.

Figure 3. AFM images of HUT molecules on epitaxial graphene after (A) 4 h, (B) 16 h, and (C) and (D) 43 h annealing at 25 °C in chloroform vapor in a tightly closed vessel. The color code reflects for all images height variations from 0 to 3 nm. The size of all images in (A)−(C) is 1 × 0.5 μm2 on the left column and 250 × 250 nm2 on the right column. The size of image (D) is 2 × 1 μm2. The insets in (B) and (D) are the corresponding Fourier transforms reflecting the 6-fold symmetry of the molecular pattern. The different colors of image (D) indicate a layered molecular structure.

align along the crystallographic directions of graphene after 16 h of solvent annealing. Annealing in chloroform vapor for an additional 27 h led to a transformation of almost all meandering lines into straight ribbons. Due to guidance of the underlying substrate, a triangular pattern resulted (see Figures 3C and D). Accordingly, the Fourier transform of Figure 3D is sharper than the one of Figure 3B, reflecting the increasing order of supramolecular structures with solvent annealing time. All images recorded at different areas of EG exhibited triangles that were oriented in the same direction. This observation indicates that EG consisted of a single uniquely oriented domain as it was grown from a single crystal of SiC(0001). We note that the interior of most of these triangles in Figures 3C and 3D was at least partially empty. For the purpose of comparison, we note that epitaxial alignment of simple alkanes was reported on monocrystalline domains of HOPG.4 There, alkanes form large domains which are oriented at an angle of 60 or 120° with respect to each other. However, the formation of 21597

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The above results convincingly demonstrate that even weak molecule−substrate interactions are able to lead to ordered assemblies which reflect the symmetry of the substrate. Of course, the formation of long supramolecular polymers was advantageous for sensitively exploring the weak modulations in the substrate potential over larger areas. Thus, one might conclude that supramolecular polymers will always order according to the substrate potential, even if it is a weak one. However, in the following we will show that such weak molecule−substrate interactions represent a necessary but not a sufficient condition for generating order which reflects the substrate symmetry. To this end, we have chosen a second possibility for weakening the interaction between supramolecular polymers and the substrate. Such weakening of interaction was achieved by a slight modification of the molecule: we grafted ethyl branches on HUT (leading to EHUT, see Figure 1A). Indeed, these side branches introduced steric constraints which did not allow intimate contact between molecules and substrate for all parts of the molecule and thus led to a weaker adsorption of EHUT onto the substrate. Due to the close commensurability of graphite or graphene with the characteristic length of C−C−C zigzag for both HUT and EHUT, EHUT molecules have also the potential to orient along the hexagonal symmetry directions of HOPG and to form patterns of long-range order similar to HUT. Surprisingly, right after drop casting of EHUT molecules, we never observed any patterns of long-range order which would have indicated guidance by the substrate, even when deposited on strongly adsorbing HOPG as in Figure 4A. The importance of steric hindrance introduced by the grafted ethyl branches on the self-assembly process was already recognized in previous STM studies on self-assembly of EHUT and HUT molecules on a Au(111) substrate.9,26,27 There, different types of organization were observed at hierarchical levels; i.e., large areas of ordered, zipper-like supramolecular assemblies were found. Due to the methyl group on the phenyl ring, the two attached urea groups cannot be in the plane of this toluene core. To form a supramolecular polymer, the urea groups need to slightly rotate to allow for efficient hydrogen bonding between adjacent molecules adsorbed on a planar substrate. This rotation of the urea groups induces a rotation of the alkyl side groups. Then, in the case of the ethyl-hexyl branches of EHUT, these branches cannot easily adsorb flatly on the substrate. The ethyl and the hexyl part of the lateral chain cannot adsorb both simultaneously on the substrate, as confirmed by simulation results.26 We assume that, similar to a gold substrate, steric problems were responsible for the weak adsorption observed for EHUT molecules on HOPG. When interacting with HOPG, the existence of ethyl groups on the side branches limits the rotational degrees of freedom of these side branches. For HUT molecules such hindrance does not exist, and thus both hexyl branches of a HUT molecule can adsorb strongly onto the substrate. Upon annealing in saturated chloroform vapor for 36 h, the disordered arrangement of EHUT molecules seen in Figure 4A changed into rather smooth domains of closely packed EHUT molecules, labeled “UO” in Figure 4B, surrounded by areas containing only a few disordered molecules. As can be seen from the histogram of the height distribution shown in Figure 4C, the mean height difference between these two regions is about 1.9 nm. This value is rather close to the length of the fully extended EHUT molecule (2.6 nm), indicating that EHUT

Figure 4. AFM topographic images of patterns formed by EHUT molecules deposited on HOPG (A) right after deposition and (B) after annealing at 25 °C with exposure to saturated chloroform vapor for 36 h. The bright area labeled UO is related to molecules being in an upward orientation. (C) Histogram showing the height distribution of (B), indicating a step height of ca. 1.9 nm. N (in arbitrary units) is related to the number of pixels having a certain height h. (D) and (E) show the results for higher EHUT coverage after exposure to saturated chloroform vapor at 25 °C for 36 h. A layered structure was formed during solvent annealing with a step height of ca. 1.8 nm, as given by the corresponding histogram of the height distribution (F). The width of the supramolecular ribbons in (D) and (E) was measured to be 30 ± 3 nm. The size of the images in (A) is 690 × 690 nm2, (B) 915 × 915 nm2, (D) 3 × 3 μm2, and (E) 1 × 1 μm2, respectively.

molecules did not adsorb flatly on the substrate but rather were oriented with their long axis almost perpendicular to the substrate. Assuming that the EHUT molecule would be a rigid rod of 2.6 nm, tilting all rods within a domain by ca. 43° with respect to the substrate normal would give a 1.9 nm thick layer. When applying the same solvent-annealing procedure as in Figure 4B to a sample with a larger amount of deposited molecules, we obtained a layered morphology of elongated domains (Figures 4D and 4E). The histogram shown in Figure 4F indicates an average thickness of each layer of ca. 1.8 nm. Each layer is composed of parallel ribbons which are stacked inline with the ribbons of the underlying EHUT layer. Obviously, due to steric hindrance caused by the branched side chains, difficulties in packing arise when trying to form large ordered domains consisting of flatly adsorbed EHUT molecules. When the size of the domain became sufficiently large, lateral stresses within such a domain must have developed. Consequently, to avoid such stresses at least partially, EHUT molecules preferred to orient their long axis in the direction away from the substrate. In addition, this 21598

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the formation of large and rather smooth layers having a bimodal height distribution (see Figure 5B), with peaks at ca. 1 nm and ca. 1.5 nm above the substrate level. In addition, in the surrounding areas some flatly adsorbed molecules were still detectable. Annealing the sample at 90 °C yielded domains of rather uniform height of ca. 1.9 nm formed by the assembly of many parallel nanoribbons as can be seen in Figure 5C and similar to the ones visible in Figure 4D and 4E. Directional hydrogen bonding was responsible for the formation of supramolecular polymers, and thus the long axis of the nanoribbons most likely reflects the length of the supramolecular polymers. The width of the nanoribbons was measured to be around 30 ± 3 nm and probably reflects the interplay between much weaker van der Waals interactions between the supramolecular polymers and the packing constraints induced by the ethyl side groups which might be responsible for tilting and might result in periodic changes in tilt angle.34

upward orientation probably allowed a more effective intermolecular hydrogen bonding. To avoid the complex interplay between substrate, solvent, and EHUT molecules occurring during the stage of solvent annealing, we alternatively increased the mobility of molecules by systematically annealing samples at different temperatures in vacuum (10−3 mbar) for a fixed time of 15 h (Figure 5). We

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CONCLUSIONS In conclusion, by comparing characteristic features (morphologies, thicknesses, etc.) of the resulting self-assembled patterns we were able to identify the consequences of changes in adsorption strength and steric hindrance with respect to kinetics of the self-assembly process and the orientation of molecules in ordered domains. We have shown that even weak molecule−substrate interactions can induce self-assembly guided by the symmetry of the substrate. However, due to weakness of the interactions, the process proceeds at a slower rate though. At early stages of assembly, only a low degree of order is achieved on EG which may erroneously lead to the conclusion that weak substrate−molecule interactions are not sufficient for guiding the self-assembly process (see, e.g., studies on perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on EG17,19). Guidance by the symmetry of the substrate lattice allowed the formation of similar triangular patterns of supramolecular polymers of HUT on weakly adsorbing EG and on strongly adsorbing HOPG. Of course, stronger adsorption from solution causes higher coverage of molecules on the substrate, and thus, for a given concentration of the solution, the density of molecules deposited on HOPG is higher than on EG. The lower coverage on EG clearly allowed us to identify more clearly individual supramolecular polymers of lengths up to several hundreds of nanometers which contained almost all molecules from their surroundings. As these polymers were growing along the axes of the underlying substrate at an angle of 60° or 120° relative to each other, they eventually touched and formed triangular patterns. Most of the triangular structures were “empty” because most HUT molecules were attracted by the stronger hydrogen bonds to form polymers and were thus only available at a much lower extent for lateral assembly driven by weaker van der Waals interactions. Accordingly, at higher substrate coverage, the triangular domains were filled with additional polymers oriented parallel to the edges of the domains, similar to the patterns found on HOPG shown in Figure 2A. Interactions with the substrate, even if these are weak, represent a necessary condition for generating order guided by the substrate symmetry. However, when adding extra ethyl branches on the side groups of HUT molecules, we induced steric hindrance leading to weak adsorption and difficulties in ordering. When EHUT molecules adsorbed flatly onto the

Figure 5. AFM topography images of patterns formed by EHUT molecules on HOPG annealed in vacuum (10−3 mbar) for 15 h at (A) 50 °C, (B) 70 °C, and (C) 90 °C. The inset in (C) represents the corresponding phase image (900 × 400 nm2) indicating that this layer is composed of nanoribbons. The color code reflects for all topographic images height variations between 0 and 3.6 nm. Histograms to the right of (A), (B), and (C) show the corresponding height distribution within the AFM images. N (in arbitrary units) is related to the number of pixels having a certain height h. The evolution of average layer thickness with temperature of annealing indicates that reorientation of the molecules from flat adsorbed to almost vertically oriented is gradual. The size of the images is 650 × 650 nm2 for (A) and 1 × 1 μm2 for (B) and (C), respectively.

used samples having an initial state similar to Figure 4A. Annealing at 35 °C did not lead to any changes in morphology, and no organization of molecules was detectable. Starting around 50 °C, we observed indications of assembly of molecules into long supramolecular polymers which were adsorbed flatly on the substrate. These supramolecular polymers followed the crystallographic directions of HOPG as highlighted by the white triangle in Figure 5A. Elongated structures (polymers) coexisted with small regions of accumulated molecules with an average thickness larger than the cross-sectional diameter of EHUT molecules (ca. 0.5 nm) but still significantly less than the thickness of layers of molecules oriented in an almost upward direction, i.e., around 1.8 nm as seen in Figure 4. Further annealing at 70 °C led to 21599

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The Journal of Physical Chemistry C

Article

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HOPG substrate, the attached ethyl branches caused distortions in the orientation of the urea parts of the molecule and thus made hydrogen bonding for molecules lying flat on the substrate more difficult. By orienting the molecules more or less perpendicular to the substrate, such distortions could be largely avoided. However, the energetic costs for orienting a single EHUT molecule perpendicular to the substrate are enormous. Thus, we anticipate that a minimum number of molecules have to orient collectively along the substrate normal to nucleate growth of domains consisting of molecules in the approximately upright orientation. In contrast to supramolecular polymers of EHUT molecules lying flat on the HOPG substrate, the formation and orientation of such domains is not guided by the symmetry of the substrate lattice. Consequently, the resulting patterns have a distinctly different and variable shape. The formation of the observed rounded domains was mainly driven by minimizing the number of molecules at domain boundaries, i.e., by minimizing line and surface tension. We would like to underline that these results demonstrate that even slight modifications of a molecule like the addition of side branches can lead to drastic changes in the self-assembly process up to micrometer length scales. Thus, one should not disregard that, e.g., improving the solubility of molecules by grafting side groups may also have a significant impact on self-assembly.35 Finally, in accordance with a large number of other systems, the orientation of the first layer adsorbed on the substrate was guiding the ordering of additional layers above this substrate layer, as can be seen for both cases: HUT formed stacks of several layers having a step height of ca. 0.5 nm which all were in registry with the underlying layers, both on HOPG and on EG. Some guidance of the direction of upper layers was also observed for domains consisting of EHUT molecules in an almost upright orientation. Guiding the orientation of molecules via a layer by layer growth of ordered molecular domains may be considered as a route for templating the deposition of additional molecules, also of a different kind, and may open up new strategies for fabricating, e.g., molecular nanoelectronic structure on substrates like graphene.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Région Alsace and the CNRS. The Agence Nationale de la Recherche supports this work under the ANR Blanc program, reference ANR-2010-BLAN1017-ChimiGraphN. We are grateful for fruitful discussions we had with Dr. Maryam Roghani (Hamburg, Germany), Dr. Hossein Fazli (IASBS, Zanjan, Iran), Dr. Samad Bazargan (Waterloo, Canada), and Kaiwan Jahanshahi (Freiburg, Germany). We would like to thank to Emmanuel Denys and Alban Florentin (Mulhouse, France) for the technical support.

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REFERENCES

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dx.doi.org/10.1021/jp305799x | J. Phys. Chem. C 2012, 116, 21594−21600