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Article pubs.acs.org/JPCC

STM Studies of Self-Assembled Tetrathiafulvalene (TTF) Derivatives on Graphene: Influence of the Mode of Deposition Maya N. Nair,† Cristina Mattioli,‡ Marion Cranney,† Jean-Pierre Malval,† François Vonau,† Dominique Aubel,† Jean-Luc Bubendorff,† André Gourdon,‡ and Laurent Simon*,† †

Institut de Sciences des Matériaux de Mulhouse, IS2M CNRS-UMR 7361, UHA, 3bis, rue Alfred Werner, 68093 Mulhouse, France Nanosciences group, CEMES CNRS-UPR 8011, Bât. PicoLab, BP 94347, 31055 Toulouse, France

‡

S Supporting Information *

ABSTRACT: The conformations and the self-assembly process of tetrathiafulvalene (TTF) derivatives functionalized by lateral alkylthio chains deposited on graphene/ SiC(0001) in ultrahigh vacuum (UHV) and at the solid−liquid interface are studied by scanning tunneling microscopy (STM). The study in UHV evidences a “molecular fastener” effect induced by the increase of van der Waals interactions between the alkylthio side chains which forces the major part of the molecules to self-organize in π−π stacked edge-on conformation. The study at the solid−liquid interface reveals a drastically different behavior with molecules lying flat on the surface as the solvent is involved in the stabilization of the molecular layer. This work raises a burning issue concerning the choice of the deposition method for graphene functionalization with such molecules.

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the reduction of the dimensionality? Is this “molecular fastener” effect strong enough to remain in the 2D molecular layer whatever its environment? This will be answered by changing the conditions of deposition on a surface, i.e., by evaporation in UHV or by deposition of a drop of solution in ambient conditions. Although one could think that the dependence of the conformation with the mode of deposition (i.e., by evaporation or by solvation) is not surprising, only few articles study the influence of the mode of deposition on the molecular self-assembly. No easy generalization can in fact be drawn as some articles report the same self-assembly process and the same conformation of molecules deposited on the same substrate using both methods. For example, trimethyl acid (TMA) was deposited on HOPG both by evaporation in UHV15 and by deposition in solution.16 The same self-assembly and conformations were observed, with a dependence on the solvent used. Another striking example is the study by Stecher et al.,17 who investigated donor−acceptor-substituted bi- and terthiophene monolayers obtained by sublimation in UHV and by liquid deposition on HOPG. The same self-assembly and conformations were reported with both methods in the case of bithiophenes but not in the case of terthiophenes. Then the outcome of the self-assembly seems to depend more strongly on the molecule, the solvent, or the substrate than on the deposition method itself. That is why a complementary study at the solid−liquid interface is of high interest to check if there are any specific conditions for the “molecular fastener” effect to take place in 2D self-assemblies.

INTRODUCTION Since the early synthesis in the 1970s,1 tetrathiafulvalene (TTF) molecules are extensively studied due to their strong electron-donor abilities with a high-lying HOMO arising from the core sulfur atoms.2 Depending on the side groups grafted to the TTF core, TTF derivatives develop interesting properties for applications in many fields, such as electronics (organic conductors and superconductors), optoelectronics (photochromism, NLO materials), chemistry (catalysts), magnetism (organic ferromagnets), or molecular devices (switches, shuttles).2−8 In this work, we studied 4,4′,5,5′-tetrakis(dodecylthio)-2,2′-bi(1,3-dithiolylidene)tetrathiafulvalene, labeled TTC12-TTF, a TTF derivative functionalized with four alkylthio substitutional groups, as schematized in Figure 1a. Single crystals of these TTF modified compounds (see Figures 1b,c) have shown an increase of the electrical conduction with the length of the lateral alkyl chains and bulk carrier mobility up to 6−20 cm2 V−1 s−1 was measured.6,9−11 This value is considerably high when compared with several other organic compounds in different forms, like single crystals or films, obtained by different methods, such as drop-casting or Langmuir−Blodgett deposition and by using numerous solvents.12 Authors claimed that this high electronic conductivity is due to a so-called “molecular fastener” effect of the long alkylthio chains, which forces, due to van der Waals interactions between lateral alkylthio chains, the molecules to adopt a conformation and a stacking where the intermolecular overlap of the π orbitals in the TTF core is enhanced.9,11,13,14 One question that may arise in our study concerns the role of the long alkylthio chains in a self-assembled molecular layer on a surface: will they keep their role of “molecular fastener” in 2D, or will they perturb the self-assembly by steric hindrance due to © 2015 American Chemical Society

Received: January 27, 2015 Revised: April 10, 2015 Published: April 11, 2015 9334

DOI: 10.1021/acs.jpcc.5b00857 J. Phys. Chem. C 2015, 119, 9334−9341

Article

The Journal of Physical Chemistry C

Figure 1. (a) Structure of the TTC12-TTF molecule. The length of the molecule is 38.8 Å, and the length of a dodecylthio side chain is 15.3 Å. (b, c) Stacking and the conformations of the TTCn-TTF molecules in single crystals, depending on n.9,13,29,30 When n is small (the sketch in part b is done for n = 1), the molecules are in boatlike conformation and the distance between TTF cores is high. When n increases, the molecules change to chairlike conformation and the distance between TTF cores decreases due to the “molecular fastener” effect. The red arrows in (c) symbolize this effect originating from van der Waals interactions between the long lateral alkylthio chains, which force the π−π stacking of the TTF cores.

Figure 2. STM topographic images of TTC12-TTF molecules on epitaxial graphene at low coverage. (a) (21.2 nm × 15.2 nm; −2.4 V) shows one island of the self-assembled molecules on graphene and (b) (5.5 nm × 3.3 nm; 1.4 V) is a zoom of the molecular monolayer, showing that the molecules are tightly packed forming parallel rows. (c) presents a possible model of the assembly of the TTC12-TTF molecules at low coverage on graphene in linear edge-on conformation. The sulfur atoms are displayed in yellow and the carbon in gray. The graphene network is in blue.

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EXPERIMENTAL SECTION The molecule used in this study is a derivative of TTF molecules with four dodecylthio side chains, labeled TTC12TTF,13 as shown in Figure 1a. Prior to molecules deposition, graphene samples were prepared in ultrahigh vacuum (UHV) by annealing of n-doped 6H-SiC(0001) at 900 K for several hours and subsequent annealing at 1500 K for 30 s.31−35 We deposited the TTC12-TTF molecules in UHV on pristine epitaxial graphene by evaporation from 420 to 490 K with the samples held at room temperature. Then the samples were in situ analyzed using a scanning tunneling microscope working at 77 K at a base pressure in the 10−11 mbar range. Images were acquired employing Pt−Ir tips with bias voltage applied to the sample. For the study at the solid−liquid interface, the molecules were first dissolved in 1-phenyloctane (Aldrich, 98% purity, used as received) at concentrations of 0.2−6 mol L−1, which is slightly below the concentration at saturation. We were not able to image at 0.2 mol L−1, and stable self-assembled monolayers were observed starting at 0.6 mol L−1 (see Supporting Information). A drop of these solutions was applied either on pristine epitaxial graphene or on a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG). We used 1-phenyloctane as solvent as it has a high boiling point and does not form an immobilized monolayer on HOPG at room temperature and, therefore, does not compete with the TTC12-

Because of its high inertness, graphene is one of the most eligible substrate to grow a self-assembled layer of these molecules, allowing anticipating what would be the selforganization in 3D crystals. Indeed, graphene promotes the molecule−molecule interactions,18,19 and in comparison with metallic substrates, conjugated molecules were observed to be electronically decoupled from the substrate, allowing a direct visualization of the molecular orbitals.20 Moreover, it has been found that TTF derivatives are usually in strong interaction with metallic substrates, hindering the interactions between molecules.21,22 An important outcome of this work deals with the doping of epitaxial graphene by depositing molecules on top of it, in particular with TTF derivatives. Indeed, several experiments23−25 have shown that a charge-transfer doping occurs from TTF molecules to a graphene surface, and theoretical papers26−28 claim that this transfer of charge occurs only if the π orbitals of the substrate and of the molecules are parallel and aligned. These results indicate that the knowledge of the molecules conformation with respect to the substrate is crucial to determine if the charge transfer to the graphene layer occurs. Here we report an STM study of the self-organization process of tetrakis(alkylthio)TTF molecules on epitaxial graphene on SiC(0001) by evaporation in UHV or at the solid−liquid interface with an emphasis on a probable 2D “molecular fastener” effect of the alkylthio side chains. 9335


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Figure 3. (a) Self-assembled molecules on graphene form domains of parallel rows with different orientations, as shown by the lines. (b) and (c) are a zoom of the molecular layer, showing that the molecules are tightly packed and that the molecular monolayer covers the surface and the steps like a carpet. We place on (b) dotted lines to show the distance of 1.78 nm between molecular rows. (c) is the ∂z/∂x derivative representation of (b) to highlight the undisturbed continuity of the molecular layer over the step. (d) is a zoom of the molecular layer shown by the dashed rectangle in (b). We place on (d) schemes of TTC12-TTF molecules drawn to scale in edge-on conformation. The sulfur atoms are displayed in yellow and the carbon in gray. It seems that the molecules prefer to adsorb and to form a molecular layer in chairlike conformation with the dodecylthio chains slightly interdigitated. We observe two different chairlike conformations indicated by the blue arrows: the two TTF cores are symmetric with respect to the green dotted line. Defects in the molecular row due to the intercalation of molecules between the rows are highlighted by dotted ellipses. (e) presents a possible model of the assembly of the TTC12-TTF molecules at high deposited quantity on graphene in chairlike conformation. The graphene network is in blue. Image (a): 110 nm × 110 nm at −2.0 V; (b, c): 22 nm × 22 nm at 1.6 V; (d): 7.0 nm × 5.1 nm at 1.6 V.

cores within a molecular row is 0.45 ± 0.07 nm, which means that the TTC12-TTF molecules are in edge-on conformation, as illustrated in Figures 2b,c. From STM images such as in Figure 2b, it seems that the TTF cores and the superimposed dodecylthio chains are slightly tilted from the direction perpendicular to the surface. On the basis of measurements using a self-correlation filter on STM images of the molecular layer and of the graphene surface prior to deposition (see Supporting Information), we present in Figure 2c a possible model of the assembly of the TTC12-TTF molecules on graphene at low deposited quantity. All molecules have their dodecylthio chains in registry with the graphene substrate in a geometry known to be the epitaxial ordering of alkanes on graphite, i.e., in commensuration with the “zigzag” shape of a graphene lattice direction. This means that the assembly of the molecules is guided and stabilized by the underlying substrate (there are van der Waals interactions between TTC12-TTF molecules and the substrate)41,42 and that the molecular monolayer is also stabilized by van der Waals interactions between the dodecylthio chains.43−45 Because of the too large distance between TTF cores inside a row, there is no π−π interaction between adjacent molecules in the row. When increasing the deposited quantity of TTC12-TTF molecules on graphene, the self-organized molecules are adsorbed on the whole pristine graphene surface, covering like a carpet terraces and steps as high as 3.3 Å (see Figures 3a,b). They form domains of parallel rows with different orientations (see Figure 3a, where lines are drawn to point out the different orientations). As seen in Figure 3a, there are very few defects in the molecular layer, mostly at the junctions of different domains. Using self-correlation measurements, we

TTF molecules for the adsorption on the surface.36−39 STM experiments were performed using a Nanoscope V Veeco working in ambient conditions with a Picoamp amplifier module and using a Pt−Ir tip immersed in the droplet. Images were acquired in constant current mode with the bias voltage applied to the sample. The experiments were repeated several days using different tips to check for reproducibility and to avoid artifacts. All STM images are processed with WSxM software.40

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RESULTS AND DISCUSSION

Self-Assembly of TTC12-TTF Molecules on Graphene under UHV. After evaporating TTC12-TTF molecules on pristine epitaxial graphene with a low deposited quantity, molecules self-assemble forming molecular islands that may spread over several dozens of nanometers, these organized islands covering around 13 ± 3% of the surface (see Supporting Information). These molecular islands are quite unstable under the STM tip and break easily, forcing us to work at quite low tunneling current (usually at 0.1 nA). These islands are composed of rows formed by a continuous stacking of molecules (see Figure 2a). Using self-correlation measurements, we measure an average distance between the molecular rows of 3.36 ± 0.14 nm, which is slightly less than the length of a TTC12-TTF molecule. This indicates that the brightest feature inside a row, as in Figure 2b, corresponds to the TTF core of the molecule. From STM images, it seems that the molecules are in linear conformation with their long axis parallel to the graphene surface, their dodecylthio chains being slightly interdigitated (they are well distinguishable between the rows in Figure 2b). The average distance between the TTF 9336


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2D layer is low, the molecules get organized in linear conformation with a large distance between the TTF cores, as for TTCn-TTF molecules in single crystals with small n, where the “molecular fastener” effect of the alkylthio chains is prevented. For higher density, a non-negligible number of molecules can be intercalated between the initial molecular rows, the interactions between dodecylthio chains are thereby increased, and the stacked molecules change into chairlike conformation. In this case the distance between the TTF cores is reduced enough for a strong π−π interaction, as for the case of single crystals of TTCn-TTF molecules with large n. Thus, we may conclude that the dodecylthio chains keep their “molecular fastener” role in 2D. Self-Assembly of TTC12-TTF Molecules on Graphene and Graphite at the Solid−Liquid Interface. In order to check the possible role of the solvent in the 2D selforganization process and particularly its influence on the occurrence of the “molecular fastener” effect, we study the selfassembly of TTC12-TTF in a drop of 1-phenyloctane deposited on epitaxial graphene and on HOPG in ambient conditions. As it is much faster, easier, and cheaper to work on HOPG than on epitaxial graphene on SiC(0001), we have repeated these experiments much more often with HOPG than with graphene, and thus the quality of the STM images and the reproducibility of the measurements were improved on HOPG. Then the STM images shown here were done on HOPG, but the results are the same on both substrates, as can be seen in Figures 4a,b. This indicates that the interactions governing the self-assembly

measure an average distance between the molecular rows of 1.78 ± 0.06 nm, which is approximately the length of a dodecylthio chain. By looking more accurately on STM images such as Figure 3d, one observes that the rows are discontinuous, the longest rows separated in fact by 3.56 nm with few molecules intercalated in the middle of the two rows. These intercalated TTC12-TTF molecules are perturbing the initial self-assembled rows, creating defects (as can be seen in Figure 3d, where they are encircled by dotted ellipses) or even making them end by beginning an intercalated new row. Moreover, we measure an average distance between the dodecylthio chains within a molecular row of 0.44 ± 0.03 nm. This indicates that the TTC12-TTF molecules are once again in edge-on conformation with their long axis parallel to the graphene surface. Indeed, in Figure 3d, we show a submolecular resolution of the self-organized molecules where both the TTF cores (bright features) and all four lateral alkylthio chains with half of the carbon atoms are resolved. The superimposed structural model strongly supports that the molecules are once again stacked in edge-on conformation with their straight lateral dodecylthio chains this time not superimposed. The quite large spatial shifting between the lower and upper dodecylthio chains increases the number of van der Waals interactions between alkylthio chains of neighboring molecules inside a row. This increase is furthermore enhanced by the intercalation of molecules between the rows, as the dodecylthio chains are fully interdigitated. Then, these intercalated molecules are participating in the stability of the 2D molecular layer and initiate the change of molecular conformation from linear to chairlike, which is the “molecular fastener” process. Along the bright lines indicated by blue arrows, the central eight sulfur atoms of the TTF core are tilted 35 ± 2° with respect to the lateral dodecylthio chains with the molecules being stacked in chairlike conformation as schematized in Figure 1c. Using the same procedure of measurements as in the previous study at low deposited quantity (see Supporting Information), we present in Figure 3e a possible model of the assembly of the TTC12-TTF molecules on graphene at high deposited quantity, where the position of dodecylthio chains with respect to carbon hexagonal rings of graphene does not favor van der Waals interactions between TTC12-TTF molecules and the substrate. This means that the assembly of the molecules is mainly guided by intermolecular interactions such as van der Waals interactions between the dodecylthio chains and above all π−π interactions between adjacent molecules in the row. (When a molecule is intercalated between the rows, there are probably in addition hydrogen bonds between S atoms of the TTF cores nearby and H atoms of the dodecylthio side chains). Indeed, the distance between TTF cores in the molecular row is 0.36 ± 0.04 nm. For such close stacking of TTF cores, experiments and several modelizations of interactions between two stacked “free” TTF molecules suggest that π−π interactions between the TTF cores of adjacent molecules in the row take place with a strong overlap of the molecular orbitals.36,46−48 The fact that the molecules are more tightly packed when increasing the quantity of deposited molecules was already observed both in UHV49−53 and in solution.49,54−60 Our observation about the self-assembly of a 2D monolayer of TTC12-TTF on epitaxial graphene in UHV is the first direct observation of the so-called “molecular fastener” effect described by Saito et al. for 3D single crystals as schematized in Figures 1b,c.6,9,14 Indeed, when the density of molecules in

Figure 4. TTC12-TTF molecules are dissolved in 1-phenyloctane with a concentration of 2 mol L−1, and a drop is deposited on epitaxial graphene and on HOPG. (a) and (b) are STM images obtained at the solid−liquid interface of the self-assembled molecules on epitaxial graphene and on HOPG, respectively, where the lattice is shown by the same blue parallelogram. (c) is an STM image obtained on HOPG with the 2 mol L−1 solution and showing two domains, one being closely packed (right, light blue) and one being more loosely packed (left, navy blue). The measured loose-packed domain unit cell parameters are a = 1.54 ± 0.05 nm, b = 1.87 ± 0.18 nm, and α = 73 ± 4°. In the case of the close-packed domain, we measure c = 1.55 ± 0.03 nm, d = 1.90 ± 0.20 nm, and β = 65 ± 2°. Image (a): 26.5 nm × 13 nm at −1.7 V; (b): 26.5 nm × 13 nm at −1.0 V; (c): 70 nm × 70 and 10 nm × 10 nm, both at −1.1 V. 9337


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Figure 5. TTC12-TTF molecules are dissolved in 1-phenyloctane and a drop is deposited on HOPG. (a) is an STM image (10.5 nm × 6.4 nm) of the loose-packed domain, which lattice is shown by the blue arrows (the dotted one for a⃗ and the dashed one for b⃗). (d) is an STM image of the same size of the close-packed domain, which lattice is shown by the blue arrows (the dotted one for c⃗ and the dashed one for d⃗). Defects in the molecular layers are encircled. We place on (a) and (d) schemes of TTC12-TTF molecules drawn to scale. The sulfur atoms are displayed in yellow and the carbon in blue. It seems that the molecules preferentially lie flat on the HOPG and on the epitaxial graphene surfaces with their dodecylthio chains fully interdigitated. Both models (b) and (e) of the self-assembly corresponding to the STM images (a) and (d), respectively, are drawn with the same length scale. The probable protruding dodecylthio chains are drawn more transparent. (c) and (f) are a zoom of (b) and (e), respectively, to see better the hydrogen bonds (symbolized by dotted lines) between S atoms of the TTF core and dodecylthio chains. For more clarity, we remove from the model the protruding dodecylthio chains and we slightly shift the neighboring molecules. Image (a): at −1.5 V with a concentration of 2 mol L−1; (d): at −1.4 V with a concentration of 6 mol L−1.

close-packed domain is observed. Using self-correlation measurements, we measure the mean unit-cell parameters of the loose-packed domain a = 1.54 ± 0.05 nm, b = 1.87 ± 0.18 nm, and α = 73 ± 4°, and they are c = 1.55 ± 0.03 nm, d = 1.90 ± 0.20 nm, and β = 65 ± 2° for the close-packed domain (see Figures 4c and 5a,d). These measurements in the loose-packed and in the close-packed domains tend to show that the TTC12TTF molecules are this time lying flat on the surface, both on HOPG and on epitaxial graphene, with their dodecylthio chains fully interdigitated. We present possible models of the assembly of the TTC12-TTF molecules on HOPG both in the loosepacked (Figures 5a−c) and in the close-packed (Figures 5d−f) domains. On the basis of these models, we may assume that the intermolecular interactions governing the self-assembly of the molecules result from the probable conjunction of van der Waals interactions between the dodecylthio chains and hydrogen bonds between S atoms of the TTF core and H atoms of the dodecylthio chains (see Figures 5c,f).

of TTC12-TTF are the same on epitaxial graphene and on HOPG. This has been already reported for other molecules.61,62 However, a slight difference in the kinetic of ordering for long-range order has been reported for another family of molecules.20 When depositing a drop of 1-phenyloctane with a concentration of 2 mol L−1 of TTC12-TTF, the surface of both substrates is already completely covered by a monolayer of the self-organized molecules at the liquid−solid interface. Contrary to the observed self-organization process in UHV on graphene, the molecules follow a close-packed structure with an oblique lattice. This result is in full accordance with previous studies.36,39,63,64 Moreover, we have observed that the sizes of the primitive unit cell of this lattice (axial distances and angles) are varying with the concentration of molecules in solution. Indeed, Figure 4c is an STM image obtained with the 2 mol L−1 solution which shows a domain wall boundary between a closely packed (right) and a more loosely packed (left) monolayers. When depositing the 6 mol L−1 solution, only the 9338


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controlled modification of the electronic properties of a surface by the deposition of molecules, such as the molecular functionalization of graphenea hot topic as evidenced by the number of recent review papers62,68−71or for an efficient realization of molecular devices.72,73

Why is there such a discrepancy between the self-assembly of TTC12-TTF molecules in UHV and at the solid−liquid interface? As can be seen on the STM images in Figures 5a,d, there is no coadsorption of the 1-phenyloctane molecules on the surface with the TTC 12-TTF molecules. This observation is in accordance with a previous study about the role of the solvent in the self-assembly of TTC18-TTF molecules on HOPG, where weakly adsorbed solvents such as 1-phenyloctane were used.39 This means that solvent molecules are not involved in the self-assembly of TTC12TTF molecules at the molecule/graphene interface. In fact, the solvent−molecules interactions do not govern the formation of the molecular layer but rather seem to stabilize the 2D assembly, keeping the molecules in flat conformation. Indeed, in the study about the self-assembly of TTC18-TTF solvated in 1-phenyloctane on HOPG showing the same kind of closepacked self-assembly,36,63 Abdel-Mottaleb et al. claim that at least two octadecylthio chains are fully adsorbed on the surface; the other chains are either partially or fully protruding above the adsorbate layer, a conformation that was already observed with different molecules.53,56,60,65−67 It should be the case here as well, as dodecylthio chains are intersecting and crossing over the molecular backbone on both models in Figures 5b,e. In order to stabilize further the 2D molecular layer, these desorbed dodecylthio chains should be in van der Waals interactions with the solvent molecules on top. This supplementary interaction out of the surface seems to be necessary to compensate for the increase of Gibbs energy due to the translational reduction of symmetry in the transition conformation from 3D to 2D. The solvent molecules keep the TTC12-TTF molecules in this flatlying conformation, preventing therefore the “molecular fastener” effect.

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ASSOCIATED CONTENT

S Supporting Information *

Chemical synthesis; absorption−emission spectroscopy measurements and comparison with scanning tunneling spectroscopy measurements in UHV; complementary data about the selfassembly of evaporated TTC12-TTF on epitaxial graphene under UHV; complementary data about the self-assembly of solvated TTC12-TTF on epitaxial graphene and on HOPG at the solid−liquid interface. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +0033 (0)389336603 (L.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Région Alsace, the CNRS, and the Agence Nationale de la Recherche (ANR) through Grant ANR ChimiGraphN (ANR2010-BLAN-1017-ChimiGraphN) support this work. We thank W. Hourani for his participation in the STM measurements at the solid−liquid interface and M. Salé (Moltech-Anjou, Fr) for help in the syntheses.

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CONCLUSIONS By studying the deposition of TTC12-TTF on epitaxial graphene in different conditions, we have observed the selforganization of 1D rows in 2D layers. At high coverage we have shown for the first time a direct visualization of the so-called “molecular fastener” effect observed for a long time in 3D crystals. The long lateral alkylthio chains and their van der Waals interactions force the molecules to adopt an edge-on conformation, which reduces the distance separation between TTF cores lower than 3.8 Å, leading to a full π−π overlapping. This effect is no longer observed when the self-organization is studied at the liquid−solid interface. In the latter case the molecules remain in a flat-lying conformation. The role of the solvent has been discussed. An important outcome of this work deals with the doping of epitaxial graphene by depositing molecules on top of it, in particular with TTF derivatives. Indeed, several experiments using different characterization techniques (X-ray photoelectron spectroscopy, Raman spectroscopy, electrical resistivity measurements, etc.) evidenced a charge-transfer doping in few-layer graphene covered with TTF molecules and TTF derivatives.23−25 Recently, theoretical papers26−28 about the physisorption of a TTF molecule on graphene have shown that charge is transferred from the molecule to the substrate only when their π orbitals are parallel and aligned, i.e., only when the TTF molecules are lying flat on the graphene layer. Then the “molecular fastener” effect of the alkylthio chains, by favoring edge-on conformation of the TTF core, is expected to impact on the doping of the graphene substrate by the molecular layer. This work illustrates the importance of the choice of the deposition method for a

REFERENCES

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