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A B S T R A C T S WEDNESDAY, JUNE 30 N A N O S E A 2 0 1 0 surface scale of molecules showing particular electronic properties (insulator, molecular wire, memory, diode, …) [3]. Using silicon as a SAM substrate appears to be a promising challenge in molecular electronics. Indeed, this hybrid approach benefits from both the well-developed silicon technology and the specific properties of molecules. Therefore, preparation of self-assembled monolayers of aromatic conjugated molecules on silicon is a key point in molecular electronics [4]. Moreover, regarding potential applications, it is important to be able to prepare nano-islands of such active molecules on silicon. Nevertheless few works addressed this subject [5]. To achieve these two points, strong interactions between aromatic molecules are mandatory. Varying these interactions in order to identify the right interaction strength suitable for preparing dense aromatic SAMs either at a large surface scale or within nano-islands is the scope of this work. For this purpose we used two aromatic molecules bearing a phenyl ring on a small alkyl chain, with (Fig. 1b) and without (Fig. 1a) fluorine substitution. F F Cl Si Cl Cl F F F Cl Cl Si Cl Figure 1. Aromatic trichlorosilane molecules used in this study: (a) phenylbutyltrichlorosilane (PBTCl), (b) pentafluoro-phenylpropyltrichlorosilane (FPPTCl). 2 – Abstract In a first part of our work, in order to control the formation of conjugated molecular nano-domains on native oxide covered silicon, we studied how various tri-functionalized silane molecules bearing a phenyl cycle, modified or not, interact during their self-assembly [6]. Concerning phenyl rings without alkyl chain, SAM growth is shown to occur in a single step: chemisorption on the surface. This step is thermally activated and does not depend on ring to ring interactions. We show that adding a short alkyl chain (3-4 carbon atoms) to the phenyl ring gives the molecules enough flexibility to generate an additional second growth step. The latter is independent from the deposition temperature and corresponds to the arrangement between molecules. We found that this packing step is accelerated by replacing phenyl by pentafluoro-phenyl rings, possibly due to quadrupolar interactions between fluorinated cycles. Furthermore we demonstrate that mixing phenyl and pentafluoro-phenyl molecules leads to an even faster packing step which is accounted for by hydrogen bonding CH FC in a face to face phenyl/pentafluoro-phenyl arrangement [7,8,9]. We believe these results allow improving charge delocalization over conjugated molecular domains. In a second part, we studied the phase separation between phenyl-alkylsilane and octadecyltrichlorosilane (OTS) molecules. Improving the phase separation was studied using two parameters: ring to ring interactions afore-analyzed and reactive heads with different grafting kinetics. Using the same trichlorosilane grafting moiety for phenyl molecules as for OTS, we show that phase separation is improved and OTS islands are smaller with phenyl species that involve stronger ring to ring interactions. The best case is obtained with mixing phenyl and pentafluoro-phenyl rings using hydrogen bonds for packing together the aromatic species of the SAM. Small phenyl species islands (40-100 nm in diameter) could be obtained inside the OTS SAM using a less reactive grafting head for the aromatic molecules. These two cases demonstrate an improved control of SAM composition and morphology essential to further use the obtained islands for building molecular devices. 67
A B S T R A C T S WEDNESDAY, JUNE 30 N A N O S E A 2 0 1 0 3 – Conclusion We have shown that the growth on silicon dioxide of SAMs of short phenyl-alkyltrichlorosilane species exhibits two steps: chemisorption depending on the grafting head, and a longer step of densification depending on the interactions involved between phenyl rings This second step is about height times quicker by introducing hydrogen bonding between phenyl rings while mixing phenylbutyltrichlorosilane and pentafluoro-phenylpropyltrichlorosilane molecules. Moreover, the interactions between aromatic rings in the monolayer modify the composition of the final SAM prepared with OTS. These interactions between phenyl rings impact on the size and quantity of alkyl nano-domains and moderate interactions can improve phase separation with the alkyl chains. Further work is addressed to improve this control. In particular, mixing molecules with reactive moieties having different grafting kinetics may offer another possibility to control the SAM structure and composition. Acknowledgments Equipment used for this study was mainly funded by the “Objectif 2” EEC program (FEDER), the “Conseil Général du Var” Council, the PACA Regional Council and Toulon Provence Méditerranée which are acknowledged. References 1. G.M. Whitesides, B. Grzybowski, Science 295, 2418 (2002). 2. A. Ulman, An introduction to ultrathin organic films (Academic Press: Boston, 1991) 3. H.B. Akkerman et al., Nature 69, 441, (2006). 4. M. Halik, H. Klauk, U. Zschieschang, G. Schmid, C. Dehm, M. Schütz, S. Maisch, F. Effenberger, M. Brunnbauer, F. Stellacci, Nature 431, 963 (2004) 5. F. Fan, C. Maldarelli, A. Couzis, Langmuir 19, 3254 (2003) 6. J. Moineau et al., Langmuir 20, 3202, (2004). 7. V.R. Thalladi et al., J. Am. Chem. Soc. 120, 8702, (1998). 8. J.D. Dunitz, ChemBioChem. 5, 614 (2004). 9. S. Zhu et al., Tetrahedr. Lett. 46, 2713, (2005). 11H40-12H00 Large-scale patterning of zwitterionic molecules on a Si(111)-7x7 surface. M. El Garah, Y. Makoudi, E. Duverger, F. Chérioux, F. Palmino, A. Rochefort (FEMTO-ST 32 avenue de l‟Observatoire, F-25044 Besançon France) mohamed.elgarah@pu-pm.univ-fcomte.fr 1 – Introduction Achievement of a large scale organic nano-structured pattern on semiconductors at room temperature is a major goal to realize molecular electronic nano-devices. To overcome the problem of the high reactivity of the Si(111)-7×7 reconstruction versus electron-rich molecules, original zwitterionic molecules were used.[1] The formation of a large scale pattern is successfully obtained thanks to the match of the molecular geometry with the surface topology and to electrostatic interactions between molecules and surface. 2 – Abstract High resolution STM images of MSP/Si(111)-7×7 surface carried out at room temperature and obtained, for a coverage 0.35 ML (Figure). 49% of single protrusions, 14% of couple protrusions and 37% of triangular nano-structures constituted by three protrusions are observed. One protrusion is attributed to one molecule adsorbed over the rest-atoms and out of the plane of the substrate. According to their negative charge, the 68
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