chapter  3

Unimolecular Electronics: Results and Prospects

WithRobert M. Metzger

“Molecular electronics,” “molecular-scale electronics,” or “unimolecular electronics” promises electronic devices with dimensions of 1 to 3 nm, useful for the ultimate miniaturization of electrical circuits. Three-electrode measurements have been made—where two electrodes are prepared beforehand, the molecule is placed between them by physisorption or chemisorption; the third “gate” electrode is an scanning tunneling microscopy or conducting-tip- atomic force microscopy tip. In the 1960s, Henry Taube proved by kinetic studies that electron transfer rates between metal ions across alkane ligands occur more slowly than across unsaturated ligands. The current-voltage plot is asymmetric as a function of bias, and a succession of read-write cycles shows that the resistance changes stepwise, as the train(s) move from the lower-conductivity station(s) to the higher-conductivity station(s). A field-effect transistor requires a semiconducting channel connecting source and drain electrodes whose “thickness” can be modified by an applied bias on the gate electrode.