Role of metal contacts in the mechanical properties of molecular nanojunctions: Comparativeab initiostudy of Au/1,8-octanedithiol and Au/4,4-bipyridine

Abstract

A comparative study of the mechanical properties of $\text{Au}/4,{4}^{\ensuremath{'}}$-bipyridine ($4,{4}^{\ensuremath{'}}$ BPD) and Au/1,8-octanedithiol (1,8 ODT) molecular nanojunctions is developed using different metal wires and small clusters to represent the metal contact. Rupture of the junction at different bonds is analyzed. While in the case of 1,8 ODT, rupture at Au-Au bonds is always found; in the case of $4,{4}^{\ensuremath{'}}$ BPD, rupture of a N-Au bond also appears as possible. Comparison of rupture forces, maximum elongations and force constants with the experimental values lead to the conclusion that the most common geometrical arrangement in scanning tunneling microscopy break junctions should be that where the number of Au atoms is of the order of 4. Activation energies for the rupture of these structures are calculated at sample elongations.