No abstract

The structure of terephthalic acid (TPA) molecules adsorbed on rutile TiO 2 (110)-(1 × 1) has been investigated by scanning tunneling microscopy (STM). Molecularly resolved STM images show formation of monolayer with the tendency of TPA molecules to form dimer rows along the [001] substrate direction. The ability to image functional groups by the STM tip, and single molecule diffusion are demonstrated. The quality and stability of the monolayer are tested, including the resistance against air exposure. On the basis of results obtained, the model of TPA adsorption on the rutile TiO 2 (110)-(1 × 1) is proposed.

High resolution scanning tunneling microscopy has been applied to investigate adsorption and self-assembly of large organic molecules on the TiO(2)(011) surface. The (011) face of the rutile titania has been rarely examined in this context. With respect to possible industrial applications of rutile, quite often in a powder form, knowledge on behavior of organic molecules on that face is required. In the presented study we fill in the gap and report on experiments focused on the self-assembly of organic nanostructures on the TiO(2)(011) surface. We use three different kinds of organic molecules of potential interest in various applications, namely, PTCDA and CuPc representing flat, planar stacking species, and Violet Landers specially designed for new applications in molecular electronics. In order to reach a complete picture of molecular behavior, extended studies with different surface coverage ranging from single molecule up to 2 monolayer (ML) thick films are performed. Our results show that the adsorption behavior is significantly different from previously observed for widely used metallic templates. Creation of highly ordered molecular lines, quasi-ordered wetting layers, controlled geometrical reorientation upon thermal treatment, existence of specific adsorption geometries, and prospects for tip-induced molecule ordering and manipulation provide better understanding and add new phenomena to the knowledge on the (011) face of rutile titania.

Periodic DFT calculations were used to explore structural properties of terephthalic acid (TPA) deposited on the rutile TiO 2 ( 110)-(1 Â 1) surface at low and high coverage. Theoretical results were compared with scanning tunneling microscopy imaging data. At low loading the TPA molecules adsorb dissociatively as discrete entities adopting a flat-lying plane-on geometry. The resultant terephthalic anion is attached to the surface by two covalent bonds between the carboxylic oxygens and the 5-fold coordinated Ti atoms with an additional stabilization due to the hydrogen bond formation with the adjacent surface hydroxyls. When the saturation coverage is achieved, a well-ordered monolayer of the vertically oriented molecules is formed. In both cases the TPA admolecules bind to the surface via carboxylic groups as terephthalic anions. Formation of dimers results from the formation of hydrogen bonds between the adjacent TPA molecules. To elucidate the reactivity of the functionalized surface, we deposited zinc formate ions on top of the compact TPA monolayer. Calculations showed that the anchoring properties of the TiO 2 /TPA system are not perturbed by the dimer formation, auguring well for its prospective application as a promising chemically functionalized surface for on-top growth of metal organic frameworks.

We report on high-resolution friction force microscopy on a stepped NaCl(001) surface in ultrahigh vacuum. The measurements were performed on single cleavage step edges. When blunt tips are used, friction is found to increase while scanning both up and down a step edge. With atomically sharp tips, friction still increases upwards, but it decreases and even changes sign downwards. Our observations extend previous results obtained without resolving atomic features and are associated with the competition between the Schwöbel barrier and the asymmetric potential well accompanying the step edges.