Abstract
A detailed scanning tunneling microscopy (STM) study of two variants of oligo(phenylene ethynylene) (OPE) molecules is presented. These molecules might serve as molecular wires up to ≈ 5 nm in length. Self-assembled arrangements as well as single molecules on a Au(111) surface were analyzed. The molecular orbitals were directly imaged and are compared to density functional theory calculations. Sub-molecular resolution images of both molecules directly display the chemical structure. One of the OPE variants was lifted off the surface by the STM tip to measure the single-molecule conductance in order to explain previously reported low conduction values. Furthermore, we present a detailed analysis of a tip-induced conformational switching of the hexyl side groups from all-trans to a nonlinear conformation, which was observed for both variants.
Highlights
Knowledge of electronic properties and conformation of a molecule attached to metallic electrodes is essential for its integration into a functional molecular electronic device
Similar molecules have been extensively studied in break junctions [10, 11], scanning-probe studies of Oligo(phenylene ethynylene)s (OPEs) molecules have so far been limited to the self-assembled monolayers (SAMs) of OPEs or OPEs inserted into SAMs of other molecules [12,13,14,15,16,17,18] while the adsorption of single, isolated OPE molecules has not been studied at all
scanning tunneling microscopy (STM) measurements of the electronic structure of single, isolated OPE molecules on a Au(111) surface were presented. Both OPE variants we studied tend to arrange in regular patterns on the fcc regions of the reconstructed Au(111) surface
Summary
Knowledge of electronic properties and conformation of a molecule attached to metallic electrodes is essential for its integration into a functional molecular electronic device. With regard to possible applications in those devices, it is of crucial importance to understand how a metallic substrate affects the properties of a molecular building block. This holds all the more for larger and more complex molecules with their much larger degree of freedom in conformation and bonding configurations compared to smaller compact molecules. They might on the other hand at the same time occur as an undesired sideeffect In both cases, knowledge of these processes and their effect on the electronic properties of the molecule is crucial for the design of molecular electronic units. Theoretical calculations have not been tested against experimental data
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