Electron transport through single phenylene–ethynylene molecular junctions at low temperature

We present low-temperature electron transport measurements of individual phenylene–ethynylene molecular wires, connected to nanometer-spaced gold electrodes. Low-bias current–voltage $(I–V)$ characteristics measured at $4.2K$ are stable and show irregular steps. After application of a large voltage, the low-bias $I–V$ curves switch between different stable configurations, some of which show negative differential resistance (NDR). Similar behavior, including the NDR, has been observed in molecules irrespective of whether they contain a $NO2$ side group or not. We suggest that different $I–V$ curves measured, including the NDR, could be due either to conformational changes in the molecules or a change in coupling of the molecular junction.