Probing voltage induced bond rupture in a molecular junction*

We use scanning tunneling microscope break junction to study electric field breakdown at the single molecule level. We investigate breakdown phenomena in atomic chains composed of Si---Si, Si---O, Si---C, Ge---Ge and C---C bonds that are commonly found in the low-$\kappa$ dielectric material. We see different bond rupture behaviors in a range of molecular backbones, and use the results from a statistically large number of measurements to determine which bond breaks. We find that Si---Si and Ge---Ge bonds rupture above a 1V bias. We also find that the Si---C bond is more robust than Si---O or Si---Si bond at above 1V. Finally, we illustrate how an additional conductance pathway in parallel to the Si---Si bond changes bond rupture behavior under an electric field. We carry out ab initio calculations on these systems and demonstrate that the mechanism for bond rupture under electric field involves {\lq\lq}heating{\rq\rq} of the molecule through electron-vibrational mode coupling.