Interactions between molecular wires and a gold surface
Introduction
The molecular wire based on conjugated molecules is a fast developing research area in modern electronics 1, 2. The electronic conduction of many different metal/molecule/metal systems has been studied experimentally [3]. Theoretically, the emphasis has been on the molecular wire itself [4] and very little is known about the detailed interaction between the metal contacts and the molecular wire. This interaction can be discussed in terms of chemisorption, in which the molecule forms a chemical bond to the surface, and physisorption in which the molecule interacts much more weak with the metal contact. The standard approach to chemisorb molecules to a metal contact is to use gold as a contact material and to attach a thiol group to the molecule. The thiol group forms a strong chemical bond to the gold surface. In the case of physisorption, the molecule is bound to the surface by weak Van der Waals forces. This situation applies for instance to the studies of conductance through C60[4] but also to all cases in which an STM tip is used as one of the leads to contact the molecular wire [4]. Molecular/metal interaction in the case of physisorption is usually described as a weak hopping between neighboring atoms on the metal and the molecule. However, in the case of chemisorption of thiols on gold, the situation becomes much more complex. The symmetry of the gold orbitals forming the bond to sulfur is important as well as the coupling of the sulfur to the rest of the molecular wire.
The purpose of this Letter is to achieve a better understanding of the gold–thiol–molecular wire interaction, in particular to what extent the π electronic wavefunctions of the molecule interact with the sulfur and gold orbitals. The molecule investigated is a phenyl ring with a thiol(-SH) group attached to one of the carbon atoms of the ring. By studying the ground state geometry, the charge transfer and the molecular orbitals of the gold–thiol–phenyl system we show possible ways for electronic transport from the gold surface into the molecule.
Section snippets
Methodology
The calculations have been performed at the ab initio Hartree–Fock level (HF) [5], using a double-zeta basis set of contracted Gaussian functions including a relativistic effective core potential for gold (LanL2DZ 6, 7, 8).
The (111) gold surface was modeled by a 10-atom cluster. The geometry of the cluster, i.e. the bond lengths and bond angles was fixed to that of a gold lattice (Au–Au bond length is 2.88 Å). It has been shown [9] that an underlying layer of atoms is of great importance for
Results and discussion
Geometry optimization of the free molecule (C6H5SH) results in an S–C distance of 1.83 Å. All C–C bond lengths are almost identical in the phenyl ring, around 1.39 Å. As an intermediate step we have re-optimized the molecular geometry replacing the hydrogen with a single gold atom. This resulted in a very small change of the S–C bond length to 1.84 Å. The ring structure also remains unchanged with an aromatic structure in which the C–C bond lengths are 1.40 Å. The Au–S bond length is 2.41 Å,
Acknowledgements
Computational resources were provided by the Swedish Council for High Performance Computing (NSC). Financial support from the Swedish Research Council for Engineering Science (TFR) and the Swedish Natural Science Research Council (NFR) is gratefully acknowledged.
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