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Transcript of Molecular Electronics Semina Report
8/2/2019 Molecular Electronics Semina Report
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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 1980’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
Moore’s law of 1965. till date Moore’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 ‘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.
It is now over 30 years since Moore talked of this so called
technology-mantra. it is found that I.C’s are following his law and there is a
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prediction that Moore’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, ‘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.
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.
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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 attractiveoptical properties. Research the started in Soviet Union, Japan, U.K, France,
Germany and U.S. But Forest Carter who conducted in 1980’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
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ORGANIC DEVICES
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 1970’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.
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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 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 FET’s reported so far show a poor
current and a power handling capability in comparison with inorganic FETs, inaddition 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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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 hydrogen’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 µ A, 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.
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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.
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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
analogs.
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.
One group was led by R.M. Metzger at the University of Alabama 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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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 gate’s inputs are “1” (i.e., a high voltage)
at both inputs. This shuts off current flow through the RTD and makes the
XOR gate’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
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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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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 themolecule. 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 withpower 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-noiseratio 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.
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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.
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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.
Upcoming trends
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
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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 today’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. Butthe 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. It’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.
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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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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 communications—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 Moore’s prediction.
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ABSTRACT
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.
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CONTENTS
1. INTRODUCTION
2. ORGANIC DEVICES
3. POLYPHENYLENE–BASED CHAINS
4. CARBON-NANOTUBES
5. REALIZATION OF BASIC CIRCUITS
6. CHARACTERISTICS OF MOLECULAR DEVICES
7. UPCOMING TRENDS
8. CONCLUSION
9. REFERENCE
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