Molecular Nanoelectronics

Abstract

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.

Review Exercises

• 22.1 Give a broad definition of molecular electronics . Explain with reference to a molecular switch, the essential properties that a molecular device must possess in order to qualify as a molecular electronic device.

• 22.2 Highlight the difficulties faced in the placement and contacting of molecules in molecular electronics. How is the top junction with a molecule formed using scanning probe techniques? Which type of SPM offers the opportunity to see the molecule before contacting?

• 22.3 What is a break junction? How is a mechanical break junction formed? How is the gap between the metallic wires established? What is the accuracy with which the distance between the electrodes can be varied? Suggest a means of forming a break junction without mechanical force.

• 22.4 What is the name of the molecular orbital in an organic semiconductor, which corresponds to the valence band of an inorganic semiconductor? Which molecular orbital of an organic semiconductor represents the conduction band of an inorganic semiconductor?

• 22.5 What is the condition under which the current flows from the contact to the molecule through tunneling mechanism? What charge carriers participate in the tunneling current flow between the contact material and the molecular orbitals through LUMO? What are the charge carriers taking part in the tunneling current flow through HUMO?

• 22.6 What is a molecular wire ? Name an organic material, which can serve as a molecular wire.

• 22.7 What organic molecules are used as insulating materials in molecular electronics? Explain the origin of their insulating behavior.

• 22.8 (i) Give a few examples of electron-donating and electron-withdrawing moieties. (ii) “An electron-donating moiety should have a low ionization potential.” Justify the statement. (iii) Explain why an electron-withdrawing moiety should have a high electron affinity.

• 22.9 What is photochromism ? Explain the operation of a photochromic switch .

• 22.10 What is a redox reaction ? Explain the operation of an anthraquinone-based redox switch.

• 22.11 What is the basic structure of a molecular device using a single organic molecule that was proposed by Aviram and Ratner from computational analysis? Elaborate the functions of the π-conjugating segments and the tunneling bridge made through σ-bond.

• 22.12 Explain with the help of energy band diagrams the rectifying behavior of a molecular device made from a non-centrosymmetric molecule with the donor and acceptor moieties fastened on the opposite sides of a σ-bridge.