Single Molecule Junctions Formed via Au−Thiol Contact: Stability and Breakdown Mechanism

The stability and breakdown mechanism of a single molecule covalently bound to two Au
electrodes via Au-S bonds were studied at room temperature. The distance over which a molecular junction
can be stretched before breakdown was measured using a scanning tunneling microscopy break junction
approach as a function of stretching rate. At low stretching rates, the stretching distance is small and
independent of stretching rate. Above a certain stretching rate, it increases linearly with the logarithm of
stretching rate. At very high stretching rates, the stretching distance reaches another plateau and becomes
insensitive to the stretching rate again. The three regimes are well described by a thermodynamic bondbreaking model. A comparative study of Au-Au atomic point contacts indicates that the breakdown of the
molecular junctions takes place at Au-Au bonds near the molecule-electrode contact. By fitting the
experimental data with the model, the lifetime and binding energy were extracted. Both quantities are found
to have broad distributions, owing to large variations in the molecule-electrode contact geometry. Although
the molecular junctions are short-lived on average, certain contact geometries are considerably more stable.
Several types of stochastic fluctuations were observed in the conductance of the molecule junctions, which
are attributed to the atomic level rearrangement of the contact geometry, and bond breakdown and
reformation processes. The possibility of bond reformation increases the apparent lifetime of the molecular
junctions.