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45256-G5
Single-Molecule Studies of Surface Restructuring and Molecular Self-Assembly
The ACS-PRF grant has helped support three separate projects over the last year. They all have a common theme of using low-temperature scanning tunneling microscopy (LT-STM) to study molecular adsorption and interaction with surfaces at the single-molecule level, but are distinct in their goals.
Extraordinary Restructuring of Gold Surfaces by Styrene:
We discovered how the presence of styrene, a weakly adsorbed molecule, dramatically restructures the Au{111} surface at temperatures as low as 80 K. The restructuring manifests itself both in mobility of step edge atoms, as well as changes in the position of the herringbone reconstruction over time. These effects are understood in terms of the preferential adsorption sites of styrene allowing it to assist in atom detachment from step edges, as well as lowering of the energetic barrier for movement of the herringbone reconstruction. This work has important consequences for studies in which Au is used as a support for, or as an electrical contact to molecules.
Single-Molecule Ferroelectric Self-Assembly:
By studying hydrocarbons with weak dipole moments we have investigated ferroelectric ordering and ferroelectric transitions at the single-molecule level. Low-temperature scanning tunneling microscopy imaging of individual styrene molecules reveals their internal structure, and hence the orientation of their dipole moment parallel to the Au{111} surface. At near monolayer coverages, both local ferroelectric ordering of the molecules and long-range antiferroelectric ordering of the domains are observed, and the dipole-dipole interaction energies quantified. A piezoelectric transition from ferroelectric to paraelectric ordering is observed upon further increase of the coverage. The effect of the STM tip in randomizing ordered domains was also investigated. This work demonstrates that important ferroelectric properties such as spontaneous polarization, long-range ordering and piezoelectricity can be achieved in nanoscale domains of a weakly polar molecule on a metal surface.
Adsorption, Interaction, and Manipulation of Dibutyl Sulfide on Cu{111}:
The literature is full of data about thiol-based monolayers; however, relatively little is known about thioether self-assembly. Thioethers are more resilient to oxidation than thiols and offer the potential for control over nanoscale assembly in two dimensions parallel to the surface. Therefore, robust assembly schemes derived from thioethers may offer a new class of self-assembled systems with novel and useful properties. This study was a low-temperature scanning tunneling microscopy investigation of a simple thioether, dibutyl sulfide, on a Cu{111} surface. At a medium surface coverage and a temperature of 78 K, dibutyl sulfide grows in small, highly ordered islands in which the ordering is driven by both the molecule–surface dative bonds and intermolecular van der Waals bonding. Annealing to around 120 K allows diffusion and reordering of the molecules and the formation of large, very well ordered domains with little or no defects. We recorded high-resolution images of the molecular arrays and proposed a model for their packing structure. These data suggest the potential use of thioethers for a variety of self-assembly applications that require control over molecular spacing parallel to the surface. We also demonstrated how the STM tip can be used to manipulate individual molecules within the ordered structures and that the arrays can act as a nanoscale abacus. The range of motion of the manipulated molecules inside a regular array reflects the potential imposed upon them by their neighbors.
