The field of molecular electronics seeks to use individual molecules to perform functions in electronic circuitry now performed by semiconductor devices. Individual molecules are hundreds of times smaller than the smallest features conceivably attainable by semiconductor technology. Because it is the area taken up by each electronic element that matters, electronic devices constructed from molecules will be hundreds of times smaller than their semiconductor based counterparts.
Moreover individual molecules are easily made exactly the same by billions & trillions. The dramatic reductions in size, and the sheer enormity of numbers in manufacture, are the principle benefits promised by the field of molecular electronics.
Will silicon technology become obsolete in future like the value technology done about 50 years ago? Scientists and technologists working in anew field of electronics, known as molecular electronics is a relatively new field, which emerged as an important area of research only in the 1980â„¢s. It was through the efforts of late professor Carter of the U.S.A that the field was born.
Conventional electronics technology is much indebted to the integrated circuit (IC) technology. IC technology is one of the important aspects that brought about a revolution in electronics. With the gradual increased scale of integration, electronics age has passed through SSI (small scale integration), MSI (medium scale integration), LSI (large scale integration), and ULSI (ultra large scale integration). These may be respectively classified as integration technology with 1-12 gates, 12-30 gates, 30-300 gates, 300-10000 gates, and beyond 10000 gates on a single chip.
The density of IC technology is increasing in pace with Famour Mooreâ„¢s law of 1965. till date Mooreâ„¢s law about the doubling of the number of components in an I.C every year holds good. He wrote in his original paper entitled ËœCramming More Components Onto Integrated Circuit â„¢, that, the complexity for minimum component costs has increased at the rate of roughly a factor of 2 per year .certainly, over the short term, this rate can be expected to continue, if not to increase. Over the longer term, the rate of increase is a bit more uncertain, although there is no reason to believe that it will not remain constant for at least ten more years.
It is now over 30 years since Moore talked of this so called technology-mantra. it is found that I.Câ„¢s are following his law and there is a prediction that Mooreâ„¢s law shall remain valid till 2010.the prediction was based on a survey of industries and is believed to be correct with research of properties of semiconductors and production processes. But beyond ULSI, a new technology may become competitive to semiconductor technology.
This new technology is known as Molecular electronics. Semiconductor integration beyond ULSI, through conventional electronic technology is facing problems with fundamental physical limitations like quantum effects, etc.
For a scaling technology beyond ULSI, prof.Forest Carter put forward a novel idea. In digital electronics, ËœYESËœ and ËœNOâ„¢ states are usually and respectively implemented and/or defined by ËœONâ„¢ and ËœOFFâ„¢ conditions of a switching transistor. Prof. Carter postulated that instead using a transistor, a molecule (a single molecule or a small aggregate of molecule) might be used to represent the two states, namely YES & NO of digital electronics.
For e.g. one can use positive spin & negative spin of a molecule to represent respectively ËœYESâ„¢ & ËœNOâ„¢ states of binary logic. As in the new concept a molecule rather than a transistor is proposed to be used, the scaling technology may go to molecular scale. It is therefore defined as MSE (molecular scale electronics). MSE is far beyond the ULSI technology in terms of scaling.
In order to augment his postulation Prof. Carter conducted a number of international conferences on the subject. The outcome of these conferences has been to establish the field of molecular electronics.
However, as of today, molecular electronics is a broad field. The field is a result of a search for alternative materials, devices and applications of electronics. The field deals with organic materials.
The field is a challenge but not a replacement for inorganic electronics on immediate terms. Molecular electronics is a technological challenge to explore the possible application of organic materials, non-linear optics and biologically important materials in the field of electronics. Therefore hopes run high for realization of plastic electronic systems, all optical computers, and chemical or bio-computers with inbuilt thinking functions and bio-chips etc..
In the field of communication the role of optical soliton, which is a by product of non-linear optics, will be used in the implementation of a very haul (say 50,000 kilometers) with T bits/sec data rate networks. Economic solar cells are another existing promise of molecular electronics.
Molecular electronics, which is a high investment and high-risk field, is at the same time a highly promising one. High investment and risks are involved in the initial phases. Under commercial phases the cost molecular systems shall be cheaper. The prospects of molecular electronics depend on the successful interaction and coordination of scientists of diverse fields like computer, electronics, physics, chemistry, biology, material science, etc.
Historically the concept of molecule electronics dates back to the last century. The familiar e.g. is the use of organic materials in displays of watches and calculators. During the 1950, material scientists started working on organic solids as alternative semiconductors because of their attractive optical properties. Research the started in Soviet Union, Japan, U.K, France, Germany and U.S. But Forest Carter who conducted in 1980â„¢s a number of international conferences on the subject mainly initiated the interest in molecular electronics as a separate and special subject. Since then although the progress of molecular electronics has always been smooth, the prospects of the future have vastly improved.
Molecular Electronics, as on date, can be divided into broad areas: Molecular materials of electronics (MME), and Molecular scale electronics (MSE). MME deals with the use of macroscopic or bulk properties of molecules or macro molecules or organic materials in electronic devices. MSE deals with microscopic properties, say spin or dipole moment, etc of a single molecule or a small aggregate of molecules for application in electronics. The main categories of MME are organic semiconductors or molecular semiconductors and metals. Liquid crystalline materials, piezo- and pyro- electric materials, photo and electro-chromic materials, non-linear optical materials and biologically important materials for electronics.
The use of molecular organic materials as active elements in electronic devices was actually augmented with the discovery of conducting polymers in mid 1970â„¢s. Traditionally polymers are flexible, versatile and easy to process. These properties, along with the electrical property of conducting polymers that behave like a conventional inorganic semiconductor (silicon or gallium arsenide ,etc.) , make the polymer a material of hot current research.
But the basic question is whether molecular organic materials will behave like real semi conductors. If any molecular material is to be considered as a semi conductor, it has to posses a reasonable charge carrier mobility and demonstrate the existence of controllable band gap of the order of 0.75 to 2 eV. Till date, no molecular material has come up to this expectation. We can see a comparison in Fig.1.
Here it can be pertinent to mention the functioning of p-n junction. The solid state error of electronics owes much to the discovery of p-n junction, which is based on the flow of electricity through silicon. The flow of electricity can be controlled by adding impurities to silicon.
Mobilities are seen to be low in molecular organic materials. Polymers took a leading high mobility charge carriers. But while some of these are insulators and cannot be doped, others are too impure and too inhomogeneous to access experimental high mobilities. Despite this, the conjugated or conducting polymers exhibited high carrier mobilities when doped. Several experiments confirm that synthesized conducting polymers could be employed as either metallic or semi conducting component of a metal-semiconductor junction device such as Schottky and p-n junction diode, with rectification ratios in excess of thousands
There are reports of polymer based MISFET (metal insulator semiconductor field effect transistor) devices with mobilities as high as 0.1 cm sq / volt sec, total organic (polymer) transistor and LED with quantum efficiencies in the region of 1% photons per electrons. Organics, which are intrinsically p-type in semi conducting behavior have been widely experimented with conjugated polymers.
There are recent reports of n-type organic semiconductors. This behavior is found when T N C Q (tetracyanoquinodimethane) is used as the active semi conducting materials in MISFETs. The maximum field mobility has been observed as 3x10-5 cm sq / volt sec.
An active polymer transistor was first reported by Burroughes et al in 1988. the device had some important features such as no chemical doping or side reactions and insensitivity to disorder. But the operating frequency was low due to low carrier mobility.
However a dramatic lead was achieved by Prof. Francis Garnier and co-workers in 1990. they reported a total organic transistor known as organic FET. The transistor is a metal insulator semiconductor structure comprising an oxidized silicon substrate and a semiconductor polymer layer. It has great flexibility and can even function when it is bent. The operating speed is still poor. There are also reports of organic FET from Dr.Friend and co-workers Cavendish Laboratory of Cambridge. All FETâ„¢s reported so far show a poor current and a power handling capability in comparison with inorganic FETs, in addition to low operating frequency. These problem need to be address before organic FETs can be used in place of inorganic FETs.
Recently, pure semi conducting polymers have channeled into display devices. These conjugated with improved impurity have shown very strong photoluminescence. The most exciting news is the possibility that conjugated polymers would be used to manufacture LEDs out of plastic. This has immense application computer and TV screens. To provide pixelled large area flat screen displays, two stumbling blocks which are yet to be overcome are efficiency and life time. LEDs should have at least 10% efficiency before they can be used in commercial areas. On the other hand, where as a minimum of 10000 hrs lifetime is required for flat screen or panel displays till date, the maximum life of polymer LEDs is reported to be only 1000 hrs.
Organic materials have not being able to compete with silicon or inorganic materials to form active electronic devices. Moreover, the materials to be studied, if at all, are yet to be finalized. But there is a world wide trend towards organics, at least in research areas. Two of the molecules that have been used to demonstrate current carrying molecular scale structures are poly phenylene-based chains and carbon nanotubes.
Polyphenylene based molecular wires and switches use chains of organic aromatic benzene rings. Recently, it has been shown by several research groups that molecules of this type conduct electrical currents. In addition, polyphenylenes as well as similar organic molecules have been shown to be capable of switching small currents.
An individual benzene ring less one of its hydrogens, giving the phenyl group C6H5, can be bonded as a group to other molecular components. By removing two hydrogenâ„¢s, giving the group C6H4, you have two binding sites in the ring.
Polyphenylenes are obtained by binding phenylenes to each other on both sides and ending the chain-like structures with phenyl groups. These can be made in different shapes and lengths. Other types of molecular groups (e.g., singly-bonded aliphatic groups, doubly-bonded ethanol groups, and triple bonded ethanol or acetylene groups) may be inserted into a Polyphenylene chain to make Polyphenylene-based aromatic molecules with useful structures and properties. Recently, sensitive experiments by various investigators have shown that Polyphenylene based molecules conduct electricity. In one experiment, an electrical current was passed through a monolayer of approximately 1,000 Polyphenylene-based molecular wires that were arranged in a nanometer-scale pore and adsorbed to metal contacts on either end. The system was prepared so that all the molecules of the nanopore were identical three benzene-ring polyphenylene-based chain molecules. The measured current that passed through the molecular-wires was 30 mA, or about 30 nA per molecule. This works out to about 200 billion electrons per second being transmitted across the short polyphenylene-based molecular wire.
For comparison, a larger molecule, the carbon nanotube (bucky tube) has been measured transmitting currents in the range 20 to 500 nA, or 120 billion to 3 trillion electrons per second. The polyphenylene-based molecular-wires do not carry as much current as the bucky tubes however, because of their very small cross-sectional areas, their current densities are the same as those of the carbon nanotubes. These current densities are quite high - about a half a million times greater than that of a copper wire.
Polyphenylene-based molecules also have the advantage of a well-defined chemistry, synthetic flexibility, and more than a century of experience studying and manipulating them. The synthetic techniques for conductive polyphenylene-based chains have been refined by J.M. Tour who has made mole quantities of these molecules. These Polyphenylene-based chains have come to be known as Tour wires".
The way energy is transferred or channeled from one end of a molecule to the other is via p-type orbitals lying above and below the plane of the molecule. These p-type orbitals can extend over the length of the molecule thus connecting with the neighboring molecule creating a polyphenylene-based chain. Polyphenylenes will conduct current as long as conjunction among p-bonded components is maintained.
Polyphenylene-based molecules bonded with multiply bonded groups (such as ethenyl, -HC=CH-, or ethynyl, -C=C-) are also conductive. Because of this, triply bonded ethynyl or acetylenic linkages can be inserted as spacers between phenyl rings in a Tour wire. Spacers are needed to eliminate steric interference between hydrogen atoms bonded to adjacent rings. Steric interference can affect the extent of p-orbital overlap between adjacent rings thereby reducing conductiveness.
A second type of molecule that can be used for a molecular electronic backbone is the carbon nanotube or bucky tube. When used on micropattened semiconductor surfaces, these carbon nanotube structures make a very conductive wire. They differ in diameters and chiralities and come in a range of conductive properties ranging from excellent conduction to pretty good insulation. Bucky tubes are fairly new to the world of chemistry having only been discovered and characterized in the last two decades. It is not yet known how to selectively make a particular structure while excluding others.
Once made, carbon nanotubes are stable but they are made only under extreme conditions. Their synthesis is neither selective nor precise. During synthesis many molecules form in a range of structures. To get the precision required to function in electronic circuits, the use of physical inspection and manipulation of the molecules, one at a time, is needed. So far, there is no bulk chemical method for this purpose.
Currently, the molecular electronic community is in a situation where the most chemically flexible molecular backbone, the polyphenylene backbone, is not the most conductive and the most conductive, the carbon nanotube, is not the most flexible chemically. Development has been undertaken by several researchers on a variety of molecular electronic components for use in molecular circuits. Here, two particular components, aliphatic molecular insulators and diode switches, that in concept can be used with Tour wires to build the computational devices are focused on.
Aliphatic Molecular Insulators
Aliphatic organic molecules have nodes in their electron densities above the atomic nuclei. For this reason, they cannot transport unimpeded electrical current when placed under a voltage bias. This enables aliphatic molecules or groups to act like resistors.
A diode is a two terminal device in which current may pass in one direction through the device, but not the in the other direction, and in which the conduction of current may be switched on or off. Two important types of molecular-scale diode switches have been demonstrated: rectifying diodes and resonant tunneling diodes. Both are modeled after familiar solid-state analogs.
Rectifying diodes, also called molecular rectifiers, use structures that make it more difficult for an electric current to go through them in one direction, usually termed reverse direction from terminal B to A, than it is to go the opposite forward direction from A to B. Rectifying diodes have been elements of analog and digital circuits since the beginning of the electronic revolution. They have also had a role in the forming and testing of strategies for molecular scale electronics. In fact, the first theoretical paper on molecular electronics was a paper entitled Molecular Rectifiers by A. Aviram and M.A. Ratner that appeared in the journal Chemical Physics Letters in November 1974. But it was only in 1997 that, building on earlier experiments; two separate groups demonstrated practical molecular rectifiers. One group was led by R.M. Metzger at the University of Alabama and the other led by M.A. Reed at Yale University.
Resonant Tunneling Diodes (RTDs)
Unlike the rectifying diode, current can pass just as easily in both directions through an RTD. The RTD uses electron energy quantization to permit the amount of voltage bias across the source and drain to control the diode so as to switch current on and off, and so as to keep electrical current going from the source to the drain. An experimental RTD of a working electronic device has been recently synthesized by Tour and demonstrated by Reed. The device is a molecular analog of a larger solid-state RTD that has commonly been fabricated in III-V semiconductors and used in solid-state, quantum-effect circuitry.
Advantages of Polyphenylene-Based Structures
With Polyphenylene-based molecules, it is relatively easy to propose complex molecular structures that are needed for digital logic and to know ahead of time that the needed structures can be synthesized. For their size, polyphenylene-based molecular devices conduct an impressive current of electrons.
Tour-wire-based molecular digital logic has another advantage. Since polyphenylene-based molecules are so much smaller than carbon nanotubes, when electronic logic structures are finally synthesized and operated, they will represent the ultimate in digital electronic logic miniaturization. Any other structure will likely be as large or larger. It is unlikely that any working structure will be smaller.
REALIZATION OF BASIC CIRCUITS
Molecular AND and OR Gates Using Diode-Diode Logic
The circuits for the AND and OR digital logic gates which use diode-diode logic structures have been known for decades. Molecular logic gates constructed from the selected diode molecule would measure about 3 nm x 4 nm. That area is about one million times smaller than would be the area of a corresponding semiconductor logic element.
Molecular XOR Gates Using Molecular RTDs and Molecular Rectifying Diodes
To complete the diode-based family of logic gates, you need a NOT gate. To make a NOT gate with diodes, you need to use resonant tunneling diodes. Using a Reed-Tour molecular RTD and two polyphenylene-based rectifying diodes, an XOR gate measuring about 5 nm x 5 nm can be built. The three switching devices used are built with polyphenylene-based Tour wire backbones. Except for the insertion of the molecular RTD, the molecular circuit for the XOR gate is similar to the OR gate. The XOR and OR gates operate alike except when the XOR gateâ„¢s inputs are 1 (i.e., a high voltage) at both inputs. This shuts off current flow through the RTD and makes the XOR gateâ„¢s output 0, or low voltage. With the XOR gate added to the AND and OR gates, you have a complete set which can be made the same as the complete set AND, OR, and NOT.
Molecular Electronic Half Adder
With a complete set of molecular logic gates, larger structures can be made that implement higher binary digital functions. An electronic half adder can be built using Tour wires and molecular AND and XOR gates and measuring only 10 nm x 10 nm. When currents and voltages representing two addends are passed through the molecular half adder, they will be added electronically. The half adder has two inputs that split the current introduced so that the current passes through both of the logic gates regardless of which input receives the current. Results from the AND and XOR gates are delivered to separate outputs. By using an out-of-plane connector structure, an in-plane molecular wire can be passed over making it possible to connect the gates. Even though the input to each molecular lead is split, signal loss should not be a problem because the signal is recombined on the output side of the structure. In our half adder design, a three-methylene aliphatic chain resistor is embedded in the output lead that goes to the ground to help minimize signal loss.
Molecular Electronic Full Adder
By combining two half adders plus an OR gate, you can make a molecular electronic full adder measuring about 25 nm x 25 nm.
Combining Individual Devices
By bonding together existing functional devices, it is thought that devices of higher functions can be made. But when put together, these individual molecular devices will not behave as they do by themselves. The characteristic properties of each device will in general be altered by the quantum wave interference from the electrons in the devices. It is expected that Fermi levels will be affected as well. Software is being developed to deal with quantum mechanical issues so that complete molecular electronic circuits may be understood and built.
CHARACTERISTICS OF MOLECULAR DEVICES
Nonlinear I-V Behavior
Unlike solid-state electronics, the I-V behavior of a molecular wire is nonlinear. Some molecular devices will take advantage of this nonlinearity.
When electrons move through a molecule, some of their energy can be lost to other electrons motions and the motion of the nuclei of the molecule. The amount of energy lost depends on the electronic energy levels of the molecule and how they interact with the moleculesâ„¢ vibrational modes. Depending on the mechanism of conductance, the energy loss can range from very small to significantly large.
Gain in Molecular Electronic Circuits
In large molecular structures deploying molecular devices with power gain, such as molecular transistors, there will be a need to restore signal loss. Gain is needed in order to achieve signal isolation, maintain signal-to-noise ratio, and to achieve fan-out.
Energy dissipation relates closely to the speed at which a molecular electronic circuit can operate. If strong couplings cause the signal-to-noise ratio to dramatically decrease, a greater total charge flow would be needed to ensure the reading of a bit. This would require more time. Because of their scale and density, molecular electronic computers may not need to be faster than semiconductor computers to be highly important. The molecular half-added described earlier is one million times smaller than one in a Pentium processor.
Optical information technology
The ever growing demand of increased computing speed is mainly limited by memory accessing time and storage capacity. Optical storage and accessing can remove these problems as optical speed is the ultimate speed.
Photo chromic materials show a bistable property. They undergo reversible color changes under irradiation at an appropriate wavelength. The photon absorption technique of photo chromic material, in order to build a three-dimensional optical memory, appears appropriate to build a three-dimensional optical memory. Applications of electronic materials in displays and optical filters have also been conceptualized.
With the advent of optical fiber communication an interest in components for processing optical signals has arisen. On the other hand, in order to avoid the drawbacks of conventional electronics IC technology such as problems of parasitic capacitance, inductance and resistance, less reliability and power dissipation there has arisen the need to use optical integrated circuits (OICs) in proposed all optical computers where full advantage of the fundamental speed of light is proposed to be achieved. Nonlinear optics (NLO) is a new frontier of science and technology, multi-disciplinary in nature, which has potential applications in computer communication and information technology. Current research has made available organic NLO materials with properties superior to those of inorganic NLO materials. Discovery of laser in 1960s has given a thrust to the research of NLO materials and their applications.
Nonlinearity can be used basically in two ways for electronic devices: frequency conversion and refractive index modulation. Frequency conversion technique which is due to second order linearity, may be used for second harmonic generation, frequency mixing and parametric amplification, etc. the prime interest of second harmonic generation is for optical data storage.
Molecular Scale Electronics
The quest for ever decreasing size but more complex electronic component with high speed ability gave birth to MSE. The concept that molecules may be designed to operate as self constrained devices was put forward by Carter, who proposed some molecular analogues of conventional electronic switches, gates and connections. Accordingly a molecular p-n junction gate was proposed by Aviram and Rather. MSE is a simple interpolation of IC scaling. Scaling is an attractive technology. Scaling of FET and MOS transistors is more rigorous and well defined than that of bipolar transistors.
Silicon technology has offered us SSI, LSI, VLSI and finally we have ULSI. Such technologies make even the logic gate minimization technique redundant. Today integration barrier of 2.5 million transistors on a chip is over. But there are some problems now in further scaling in silicon technology. For instance, power dissipation and quantum effect are posing problems for increasing packing density.
MSE is a remedial measure. Molecules possess great variety in the structure and properties. Therefore finding molecules and their appropriate properties for electronics, opto-electronics and bio-electronics is possible the study of a single molecule is not a problem now as we have STM (scaling tunneling microscope),AFM(atomic force microscope),L-B technique etc.
At some of the top laboratories around the country, scientists are publicly expressing beliefs that before now they would only express in private: electronics technology is on the edge of a molecular revolution where molecules will be used in place of semiconductors, creating electronics circuit small that their size will be measured in atoms not microns. They are boldly predicting that the impact on computing speed and memory resulting from circuits so small would stagger virtually all fields of technology and business. Research teams from Rice and Yale Universities say that they have successfully created molecular size switches that can be opened and closed repeatedly. The HP/UCLA group had only reported being able to switch once, not repeatedly. Repeated switching is necessary to build functioning digital computers. These breakthroughs in the field of molecular electronics seem to be giving researches a new sense of confidence.
There are several research groups working in laboratories under top-secret conditions. They are making progress on several fronts. One of them is said to be working on molecular scale Random Access Memory (RAM). RAM, on a molecular scale, could offer incredibly huge storage capacities. Molecular methods could make it available at costs so low as to be pocket change. Because of the very small scale of such devices, it might be possible to store, for e.g., a DVD movie on something the size of a grain of rice.
The micro electronic devices on todayâ„¢s silicon chips have components that are 0.18 microns in size or about one thousandth the width of a human hair. They could go as small as 0.10 microns or hundred nanometers. In molecular electronics, the components could be as tiny as 1 nanometer. This would make for a new breed of super powerful chips and computers so small that could be incorporated into all manmade items.
The semiconductor world predicts it will continue to advance the silicon based chip, making ever smaller device, through the year 2014. But the costs involved with these advancements are enormous. Currently semiconductor chips are made in multibillion dollar fabrication plants by etching circuitry into layers of silicon with light waves. Itâ„¢s a very expensive process and each new generation requires huge amounts of money to upgrade to newer fab-plants. The world of computers is in for a change.
Several computer semiconductor companies, including Sun Microsystems and Motorola have been meeting to consider forming a consortium that would look for commercial uses for molecular electronics.
Researches say that this is still only the beginning in the making of molecular computers. There are still many obstacles to over come before molecular computers become reality.
Some researches believe that in order for molecular systems to work as computers, they will need to have fault tolerant architectures. Several groups are working on such devices.
The progress made recently has caused a lot of excitements among researches in molecular electronics. For a long time, they have had the vision but have had few results. Now they are looking towards the future and have results that are helping to map the way for them.
The subject of molecular electronics has moved from mere conjuncture to an experimental stage. Research in molecular electronics will naturally dominate the next century. Today is the age of information explosion. Polymer materials hold hopes of rapid development of improved systems and techniques of computing and communicationsâ€the two wings of information technology. for e.g., polymer optical fibre has a number of advantages over glass fibres like better ductivity,light weight, higher flexibility is in splicing and insensitivity to stress,etc. all these show that polymers will play a vital role in the coming years and MSE shall compete with IC technology which is growing in accordance with Mooreâ„¢s prediction.
. T. Rueckes, K. Kim, E. Joselevich, G.Y. Tseng, C.-L. Cheung, and C.M. Lieber "Carbon Nanotube-Based Nonvolatile Random Access Memory for Molecular Computing", Science 289, 94-97 (2000).
. "Large-scale synthesis of carbon nanotubes", T W Ebbesen and P M Ajayan Nature, vol.358, p220 (1992
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2. ORGANIC DEVICES
3. POLYPHENYLENEâ€œBASED CHAINS
5. REALIZATION OF BASIC CIRCUITS
6. CHARACTERISTICS OF MOLECULAR DEVICES
7. UPCOMING TRENDS