Long n-Alkane Adlayers as Templates for Tailoring Supramolecular Self-Assemblies on Surfaces
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0937-M12-05
Long n-Alkane Adlayers as Templates for Tailoring Supramolecular Self-Assemblies on Surfaces Luc Piot1, Alexandr Marchenko1, Jishan Wu2, Klaus Müllen2, and Denis Fichou1 1 DSM/DRECAM/SPCSI, CEA-SACLAY, LRC Nanostructures et Semi-Conducteurs Organiques(CNRS-CEA-UPMC), CEA-SACLAY, Gif-sur-Yvette, 91191, France 2 Max Planck Institut for Polymer Research, Ackermannweg 10, Mainz, 55128, Germany ABSTRACT In this paper we report the heteroepitaxial growth and the structural evolution of hexakisdodecyl-peri-hexabenzocoronene (HBC-C12) adsorbed on a template monolayer (n-C50H102). STM measurements at the liquid/solid interface made it possible to follow in real time the early stage of deposition of the HBC-C12 molecules. Additionally, molecule manipulations were realized using the tip of the STM. Our measurements give direct evidence of two successive phase transitions occurring in the upper HBC monolayer weakly bonded to the underlying template layer. INTRODUCTION Organic thin films are of interest for various applications like field effect transistors [1] or photovoltaic [2] devices because of their long-term stability, inertness and easy preparation. It is well known that the electronic properties of such devices strongly depend on the degree of order in the organic materials used. Unfortunately the building of welldefined highly ordered organic materials remains problematic. Recently, composite-organic layers have been presented as a possible solution [3-6]. We propose here an approach for controlling the self-assembly process of disc-like graphite molecule, hexakis-dodecyl-perihexabenzocoronene (HBC-C12) [7], using an organic template formed by long n-alkanes (nC50H102) on highly ordered pyrolitic graphite (HOPG). Unlike the UHV systems the liquid/solid interface STM technique allows us to follow at the nanoscale and in real time the deposition process of HBC-C12 [8, 9]. It is shown that molecules are on top of the template monolayer and that HBC-C12 nucleation occurs on specific regions on the surface. Finally careful measurements on a time scale of 10 hours reveal two phase transitions occurring one after the other and leading to a densification of the packing. The resulting HBC packings contrast strongly with the ones seen on bare graphite [10]. EXPERIMENTAL DETAILS STM measurements were performed using a Pico-SPM (Molecular Imaging) equipped with a home-built liquid-cell. Mechanically etched Pt/Ir (80:20) wire was used as STM tip and all experiments were performed at the n-tetradecane/solid interface (n-C14H30, 99+%, Acros) at room temperature. The n-tetradecane solvent was chosen because it does not adsorb on the surface under the present experimental conditions [11-13]. This paper presents only STM measurements acquired in the constant current mode. Highly oriented pyrolitic graphite substrates used in this experiment are obtained commercially (Good Fellow). The preparation of the substrate begins with the deposition of a self-assembled monolayer of n-pentacontane (n-C50H102, >97%, Fluka) on a freshly
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