Single Molecule Electronics
Measurement of the electrical properties of single molecules sandwiched between metal contacts has recently become an experimental reality. In recent years, we have developed and exploited novel scanning-tunnelling-microscopy-based methods for achieving this feat. In our methods, a gold substrate is coated with a low-coverage monolayer of molecules bearing two terminal thiol moieties, which interact strongly with gold. A gold STM tip is employed, and in the course of the experiments, one (or a few) molecule(s) forms a junction between tip and substrate, as illustrated schematically in Figure 1 (below). In one implementation, which we named theI(s) method, the tip is brought close to the surface (without making contact), then withdrawn while the junction conductance (Figure 1) is measured.
Figure 1. Our method for forming single molecule junctions which we call the I(s) technique (I=current and s=distance). (A) shows the approach of the STM tip towards the molecular target on the metal surface. In (B) a molecule is trapped and then pulled up in the junction (C and D) as the STM tip is retracted. The graph shows junction conductance versus extension, with the upper curve being for a molecular junction and the lower curve in the absence of molecules.
Using this and a related method (the I(t) technique) we have made several landmark findings in this high-profile area. The following key issues affecting single molecule conductance have been addressed:
- The effect of environment, notably water, on molecular conductance.
- Photo-switching and molecular conductance.
- Thermal effects on single molecular junctions.
- Investigation of highly-transmissive molecular backbones.
- Electronic effects of substituents.
- The role of the molecule-electrode contact and molecular tilting in the junction.
- Varying contact chemistry (e.g. direct metal-carbon contacts and use of 4–pyridyl or COO– as contacts in place of RS–).
- Effect of redox state on the conductance of redox-active molecules.
- Reasons for observed multiple conductance values for metal|molecule|metal junctions.