51639309 molecular electronics abstract
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1. INTRODUCTION
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 1980s. 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 Famous Moores law of
1965. Till date Moores law about the doubling of the number of components in an I.C
every year holds good. He wrote in his original paper entitled Cramming More
Components Onto Integrated Circuit , 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.
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It is now over 30 years since Moore talked of this so called technology-mantra.
It is found that I.Cs are following his law and there is a prediction that Moores 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.
Molecular based electronics can overcome the fundamental physical and economic
issues limiting Silicon Technology.
For a scaling technology beyond ULSI, prof. Forest Carter put forward a novel
idea. In digital electronics, YES and NO states are usually and respectively
implemented and/or defined by ON and OFF 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 YES & NO 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.
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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 biochips 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
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Soviet Union, Japan, U.K, France, Germany and U.S.But Forest Carter who conducted
in 1980s 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.
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Typical resistivity
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
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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 Prof. Francis Garnier and co-workers achieved a dramatic lead 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 FETs 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.
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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 lifetime. 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 lifetimes 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 worldwide 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.
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3. POLYPHENYLENE-BASED CHAINS
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
hydrogens, 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 obtain Polyphenylenes. 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 A, or about 30 n A per molecule. This
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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 n A, 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. J.M. Tour who has made mole quantities of these molecules has
refined the synthetic techniques for conductive polyphenylene-based chains. 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 orbital
lying above and below the plane of the molecule. These p-type orbital 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 conduciveness.
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4. CARBON NANOTUBES
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.
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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.
Diode Switches
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 analoges.
Rectifying Diodes
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. R.M. Metzger at the
University of Alabama led one group and the other led by M.A. Reed at Yale
University.
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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.
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5. 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 gates inputs are 1 (i.e., a high
voltage) at both inputs. This shuts off current flow through the RTD and makes the
XOR gates 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.
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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.
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6. 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.
Energy Dissipation
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 molecules 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.
Speeds
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
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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
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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 forever 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.
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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.
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7. FUTURE DEVELOPMENTS
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.
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The micro electronic devices on todays 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. Its 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.
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8. CONCLUSION
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
communicationsthe two wings of information technology. For e.g., polymer optical
fiber has a number of advantages over glass fibers like better ductivity, light weight,
higher flexibility is in splicing and insensitivity to stresses 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 Moores prediction.
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9. REFERENCES
"Large-scale synthesis of carbon nanotubes", T W Ebbesen and P M Ajayan
Nature, vol.358, p220 (1992
Scientific forum http://www.calmec.com/scientif.htm
Search http://www.calmec.com/search.htm
www.ieee.org
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CONTENTS
1. INTRODUCTION 1
2. ORGANIC DEVICES 5
3. POLYPHENYLENE-BASED CHAINS 9
4. CARBON-NANOTUBES 11
5. REALIZATION OF BASIC CIRCUITS 14
6. CHARACTERISTICS OF MOLECULAR DEVICES 16
7. FUTURE DEVELOPMENTS 20
8. CONCLUSION 22
9. REFERENCES 23
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ACKNOWLEDGEMENT
I express my sincere gratitude to Dr.Nambissan, Prof. & Head,
Department of Electrical and Electronics Engineering, MES College of
Engineering, Kuttippuram, for his cooperation and encouragement.
I would also like to thank my seminar guide Ms. Sunitha M.M.
(Lecturer, Department of EEE), Asst. Prof. Gylson Thomas. (Staff in-charge,
Department of EEE) for their invaluable advice and wholehearted cooperation
without which this seminar would not have seen the light of day.
Gracious gratitude to all the faculty of the department of EEE &
friends for their valuable advice and encouragement.
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ABSTRACT
Semiconductor integration beyond Ultra Large Scale Integration (ULSI),
through conventional electronic technology facing some problems with fundamental
physical limitations. Beyond ULSI, a new technology may become competitive to
semiconductor technology. This new technology is known is as Molecular Electronics.
Molecular based electronics can overcome the fundamental physical and
economic issues limiting Si technology. Here, molecules will be used in place of
semiconductor, creating electronic circuit small that their size will be measured in
atoms. By using molecular scale technology, we can realize molecular AND gates, OR
gates, XOR gates etc.
The dramatic reduction in size, and the sheer enormity of numbers in
manufacture, are the principle benefits promised by the field of molecular electronics.