22.  Molecular Nanoelectronics

Molecular electronics is a relatively young research area, which can be broadly defined as dealing with electronic devices in which molecular properties play a central role. The necessary criteria for considering a molecular system as a molecular device are decided in the context of the simplest conceivable molecular electronic device, the molecular switch. The main issues are placing the molecules in an immobile condition at pre-decided locations and connecting electrodes to them for current flow. The break junction method of nanogap electrode formation is described. The electrical properties of molecular contacts are treated in terms of HUMO and LUMO levels. Suitable materials serving as molecular wires and insulators in molecular devices are indicated. Appropriate molecules for forming N- and P-type regions are suggested. Two kinds of molecular switches are described, one triggered by electromagnetic radiation and the other via redox reaction. The pioneering theoretical and experimental work of Aviram and Ratner in the 1970s on molecular rectifying diode is elucidated with energy band diagrams. The fundamental demonstrations of the properties of electronic devices at the molecular scale make this field highly exciting and lucrative.