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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molecular electronics semina report

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