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The details in contact geometry play a crucial role in the electron transport through single molecules attached to metal electrodes. Eventually, control on the contact formation will not only have to include the atomic-scale geometry itself, but also coherent (strong coupling) versus incoherent (weak coupling) electron transport. In
addition, the behaviour of molecular orbitals in the coupling, and also the phase of the orbital wavefunction at the contact point may influence the electron flow.
Such a controlled contacting can be achieved when connecting metal atoms (gold) to pi-conjugated molecules (pentacene) on insulating NaCl films by means of single-molecule chemistry in a scanning tunneling microscope (STM) junction. The gold atom can be brought into various positions within a few Ångstroms from the molecule facilitating an electron tunneling current between the atom and the molecule (weak coupling). Alternatively, the gold atom can be covalently bound to the pentacene molecule to form a metal-organic complex, which is accompanied by a strong and coherent electronic coupling between the constituents. Moreover, the possibility to create different structural isomers by bringing together the reactants in different orientations enables control of the phase of the molecular orbital at the contact point. The influence of the contact formation on the electronic
structure of the complex is evident from the different frontier orbitals of the different isomers as can be directly seen in the corresponding STM images.
Here we report of experimental work in which a covalent bond between an individual pentacene molecule and a gold atom, and of theoretical work in which we reveal how the electronic structure of the system is influenced upon the bond formation. Density functional calculations provide a reliable geometrical and electronic structure of the contact, while a tight-binding model can be used for a detailed information of behaviour of molecular orbitals upon the bond formation. The correspondence between experimental setup and calculations is verified by comparison of the experimental and simulated STM images. |