Single Molecule “Fishing”: The I(s) and I(t) Methods
The picture below schematically shows a single molecule which has been fished by the STM tip to form a metal│molecule│metal junction. This can be achieved with the I(s) or I(t) methods, developed in Liverpool.
We have developed these two methods for determining single molecule conductance. Both are based on using the STM to form metal│molecule│metal junctions. In the I(s) technique current (I) is recorded and the tip is rapidly retracted (s = distance) from a given set-point current (I0) following temporarily disablement of the feedback loop. The set-point parameters can be used to control the initial distance between tip and substrate, and for a given bias voltage, higher set-point current (I0) results in closer initial approach of the tip to substrate. If molecules become attached between the gold STM tip and gold substrate at the start of an I(s) retraction sweep they are then pulled up in the junction until the molecular bridge is broken. This results in characteristic current plateaus, when molecular wires bridge the gap between the tip and substrate, an example of which is shown below. On the other hand in the absence of molecular bridge formation the current simply decreases exponentially with separation. The key difference between the I(s) technique and the in-situ break junction (BJ) method of Xu and Tao (Science, 2003), which also uses an STM as the method of junction formation, is that direct metal-to-metal contact between tip and substrate is avoided, with molecular junctions being formed when the STM tip is brought sufficiently close to the surface covered by the target molecules and then withdrawn while the junction current is measured.
The figure below shows a current-versus-distance curves for obtained for a single molecule bridge, with the break-off distance (s) and the plateau current (IM) marked. As the STM tip is retracted in the I(s) experiment, a plateau is observed, with the target molecule bridging between STM tip and substrate surface. The tip is retracted until this molecular junction breaks, as indicated by a sharp drop in the current at the break-off distance. Many of these I(s) curves are analysed statistically to produce conductance histograms, which employ in their construction all current values from the collected I(s) curves.
The I(t) Method
The I(t) method also uses an STM but rather than sweeping the tip back a fixed gap separation is employed. The stochastic formation and breaking of molecule bridges between an STM tip and substrate is observed as steps in the tunnelling current. These are analysed using histograms to obtain single molecule conductance.
