Electron transmission through individual 1,4-benzenedithiol molecules bridging between two gold electrodes (Au/BDT/Au junctions) has been studied by measuring the current-voltage (I-V) characteristics. Measurements were made at room temperature on three junction states of conductance 0.005G(0), 0.01G(0), and 0.1G(0), respectively, where G(0) is the quantum unit of conductance. All I-V curves are linear around zero bias and nonlinearly increase upward for biases above approximately 0.2 V. Absence of plateaus in the observed I-V characteristics up to +/- 1 V indicates that the electron transmission spectrum of Au/BDT/Au has no peaks within +/- 0.5 eV from the Fermi level.

We have studied electron transport through Ni/1,4-benzenedithiol (BDT)/Ni single molecule junctions at cryogenic temperatures under magnetic field up to 250 mT. Instead of examining magnetoresistance (MR) of individual junctions, we measured the conductance of many junctions under a constant magnetic field and investigated how a single-molecule peak in a conductance histogram shifts with the field strength. We found that the single-molecule resistance at 77 K, deduced from the conductance peak position, shows a hysteresis against the field strength and takes a maximum around 50 mT when the magnetic field increases from 0 T to 150 mT. The observed resistance change yields a MR of $ð80À90Þ% for Ni/BDT/Ni single molecule junctions. This MR is higher than experimental MR reported for non-conjugating molecules but consistent with high theoretical MR predicted for p-conjugated molecules such as BDT. We have also investigated the nonlinearity of the current-voltage (IÀV) characteristics of Ni/BDT/Ni junctions under 0 T and 150 mT and found that the nonlinearity changes its sign from negative at 0 T to positive at 150 mT. This result suggests that the junction transmission spectrum at 0 T should have a low-lying peak within 60:1 eV from the Fermi level, but the peak moves out of the bias window when the magnetic field increases to 150 mT. The observed field-induced change in the IÀV nonlinearity is qualitatively consistent with theoretical IÀV curves of Ni/BDT/Ni calculated for magnetized and non-magnetized Ni electrodes. V

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.

We have investigated the low bias conductance and I-V characteristics of single 4 , 4Ј-bipyridine ͑BPY͒ molecule connected to gold electrodes at room temperature. Exploiting the mechanical controllable break junction technique we statistically determined the most preferred conductance value of Au/BPY/Au single molecule junctions. The measured conductance, 0.01G 0 ͑G 0 is the conductance quantum unit͒, is in good agreement with two of the previous experiments using scanning tunnel microscopy break junctions. The I-V characteristics of the Au/BPY/Au junction have been directly measured by varying the bias up to Ϯ0.5 V while holding the junction at its single molecule state. By comparing the obtained I-V curves with those of the Au/1,4-benzenedithiol ͑BDT͒/Au junction, we observed differences in the energy level alignment of BPY and BDT molecules with respect to the Au electrode.