Conductance of single 1,4-benzenedithiol ͑BDT͒ molecules is investigated in a wide range ͑0-0.3͒G 0 , exploiting mechanically controllable break junction technique. The authors observed a series of clear conductance steps both in low-͑ϳ0.01G 0 ͒ and high-conductance ͑ϳ0.1G 0 ͒ regimes and corresponding two sets of peak structures in the conductance histograms. The two distinct conductance states are attributable to different Au-S bonding configurations of Au/BDT/Au junctions. The high-bias measurements reveal that the high-conductance state of single BDT molecules is stable up to 1.6 V and prospective for molecular device applications.

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We have studied the high-bias breakdown of Au/1,4-benzenedithiol ͑BDT͒/Au junctions at room temperature. Exploiting the break junction technique, we held a Au/BDT/Au junction and ruptured it by applying a voltage ramp. The conductance first changes gradually with the bias and then abruptly increases at breakdown. We found that the breakdown voltage shows a broad distribution and takes a maximum at ϳ͑1.2-1.5͒V. The breakdown voltage is unaffected by the ambient atmosphere but tends to slightly decrease with increasing the junction conductance. We consider that the Au electrode becomes unstable at the breakdown voltage and collapses to crush the junction.

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