`
MOLETRONICS: FUTURE OF ELECTRONICS Oshin Sangha, Ruhee and Deep Kamal Kaur Randhawa Department of Electronics and communication Engineering, Guru Nanak Dev University, Regional Campus, Jalandhar, Punjab, India.
MOLECULAR ELECTRONICS ABSTRACT Zest to sustain Moore’s Law paved way for CMOS technology that has evolved from microelectronics to nanoelectronics. Further miniaturization is being limited in silicon based CMOS. Hence, alternative materials are being studied for use in electronic devices. Molecules are a very promising candidate. Molecular Electronics promise not only miniaturization but also property self assembly. Here we discuss the basics of molecular electronics, its components and some applications.
The next big thing is small...very small! Molecular electronics can be defined as technology utilizing single molecules or small groups of molecules to perform electronic functions. As silicon is approaching its fundamental limits, it can be said that silicon valley will soon have a sibling called “molecular valley”. Various types of molecules will act as different electronic devices and can be integrated into Molecular Electronic circuits.
Concept of Molecular Electronics is supported by : Size: The size scale of molecules is between 1 and 100 nm. Assembly and Recognition: One can exploit specific intermolecular interactions to form structures by nanoscale self assembly. Dynamical Stereochemistry: Many molecules have multiple distinct stable geometric structures or isomers. Such geometric isomers can have distinct optical and electronic properties. I)Molecular Diodes Molecular diode can be visualized as an assembly of a molecule inserted between a pair of metallic electrodes performing function of a rectifier. The suggested operation of rectifier is based on the principle that there is difference in energy of the frontier molecular orbitals of the ‘donor’ and ‘acceptor’- ‘π systems’. The spacer preserves the energy difference between the donor and the acceptor π systems, but allows electron transport to certain extent. This behaviour of D-ϭ-A assembly can be regarded as rectifier function. The acceptor part of the molecule considered as p-type semiconductor while donor part can be regarded as n-type, while the spacer can be compared to the junction barrier.
II) Three Terminal Devices Three terminal devices are integral element of an electronic circuit as they provide power amplification to the signal. The main problem in realization of three terminal devices is structuring three independent nanometer sized leads. Three terminal devices can be classified as 1) Molecular transistor 2) Molecular single electron transistor. Molecular Transistor : Molecular transistor has a molecule inserted between two metallic/ semiconducting terminals. The third electrode called gate is electrostatically coupled to the molecule. Molecular Single Electron Transistor : The transistor consist of metallic source and drain electrodes and a molecular island coupled with the two electrodes. Electrons can propagate from source to drain through the island.
Molecular Transistor
Molecular Single Electron Transistor
Molecular Diode
III) Molecular Wires Wire is one of the most essential parts of an electronic circuit that provides a path for transport of electric charge. It is expected that electron transport will be through the frontier molecular orbitals. So large delocalised π systems with reduced HOMO- LUMO GAPS (HLG) can be considered as promising candidate for molecular wires. The simplest chain for use as wire is polyene, consisting of alternating sequence of single and double bond, forming a π system.
IV) Molecular Resistor An electronic circuit designer is not always interested in property of conductance but also in the opposite property called resistor. The insulating properties of material are also of equal significance for design of analog and digital circuits. Resistance basically displays the characteristic of material which opposes flow of electrical charge. The spacer should be sufficiently insulating so as to preserve the energy difference between the π systems while allowing limited electron transport by tunnelling. Alkanes are known to display insulating properties. The molecules without delocalised π systems boast of excellent insulating properties.
V) Switches and Storage Elements The property of certain molecules to exist in two different states having different conductive properties leads us to the ideas for using these molecules as storage molecules. The two stable states of the molecule can be used to depict logic states 0 and 1. Rotaxanes and catenanes have been synthesize to switch as a function of applied potential between two states.
Molecular structures acting as Molecular Wires
Molecular Structures acting as Molecular Resistors.
Molecular structures acting as molecular switches and storage elements.
Conclusion: Molecular Electronics is predicted to change the trends of electronic circuits that will be smaller in size, will consume lesser energy and above all will be much cooler than the present electronic circuits.