Scanning Tunneling Microscopy of Chiral Pair Self-Assembled Monolayers
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Long-chain alkanes [CH3(CH2)nCH3, n > 8] and their derivatives such as alkenes, alkanols, ethers, and carboxylic acids physisorb on highly oriented pyrolitic graphite (HOPG) surfaces to form very stable self-assembled monolayers (SAMs). These monolayer structures, with nanometer dimensions, form an entire class of nanoscale objects whose properties are of considerable interest. These nanoscale objects provide important models that are relevant to the understanding of molecular electronic devices, biological recognition chips, tribology, and corrosion inhibition systems. These very interesting nanoscale structures have been extensively studied in recent years. Unlike the close packed self-assembled monolayer in the alkanethiol/Au(111) system, where the alkane chain stands up from the surface, in the alkane derivative/HOPG SAMs, the alkane chain adsorbs parallel to the graphite basal plane. The alkane molecule adsorbs in an all-gauche conformation, which has the lowest energy. In this conformation, the carbon–carbon backbone of the molecule has a zigzag shape. Theoretical and experimental results indicate that in the long-chain alkane self-assembled monolayer on HOPG, the zigzag is parallel to the HOPG basal plane. Under such packing conditions, the distance between the hydrogens of the neighboring methylene units in the alkane chain is 2.52 Å and the distance between sixfold hollows of HOPG is 2.46 Å. This good match between the substrate lattice and the organic molecule adsorbate dimension results in a relatively strong interaction between the substrate and the adsorbate. Such interaction is the basis for formation of a stable monolayer on the HOPG surface. The van der Waals interactions between the long-chain alkane molecules provide the basis for the self-assembly process.
Sometimes these stable self-assembled monolayers exhibit chirality, or handedness. This can result from the segregation of chiral molecules into chiral domains on the surface, from the distortion of achiral molecules to form an overall chiral structure, or from the pairing or self-assembly within the unit cell to make a chiral structure from an enantiomeric mixture. This article discusses several examples of self-assembled monolayers formed on HOPG that exhibit chirality. These nanoscaled materials may have interesting applications in chirally selective catalysis and separations, and certainly help us to understand the interactions between molecules and the substrate that govern the self-assembly process. In the following article, the structures of three separate racemic mixtures of iodine substituted octadecanol and octadecanoic acids adsorbed on the (0001) plane of HOPG are examined. Chiral pairs are formed and the pairs form ordered monolayers, in which different faces of the same molecule are exposed and imaged. These examples illustrate the formation of chiral nanostructured monolayers in this self-assembly process.