khatana gurjar

7/29/2019 khatana gurjar

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Vivekananda Institute of Technology-East

1Department of Electronics & Communication

CHAPTER 1

INTRODUCTION

Molecular electronics, also called moletronics, is an interdisciplinary subject that spans

chemistry, physics and materials science. The unifying feature of molecular electronics is the useof molecular building blocks to fabricate electronic components, both active and passive. The

concept of molecular electronics has aroused great excitement, both in science fiction and among

scientists. This is because of the prospect of size reduction in electronics which is offered by

molecular-level control of properties. Molecular electronics provides means to extend Moores

Law beyond the foreseen limits of small-scale conventional silicon integrated circuits. Molecular

Electronics is recognized as a key candidate to succeed the silicon based technology once we

have arrived at the end of the semiconductor roadmap. The use of organic molecules in nano

scale nonlinear circuits offers many opportunities for new types of devices, which will differ in

fabrication, functionality, and architecture Molecular electronics involves the study and

application of molecular building blocks for the fabrication of electronic components .[1].

The concept of molecular electronics is not new in the field of technology and science. its core

concepts were originated with the concept of nano electronics. In 1997 the first ever theory for

the moelcular electronics was given by by Mark Reed and co-workers. Its one of the important

discpline that provides us way for dealing with all the electronic characteristics of conductors

,insulators and semi conductors .molecular electronics has two sub disciplines [1]

molecular materials for electronics molecular scale electronics

Both of them collectively give high tech and ultra intelligent devices to be used in various kind

of applications.This field is progressing rapidly and supporting nano electronics in order to

develop excellent applications.

Molecular electronics is a poorly defined term. Some authors refer to it as any molecular-based

system, such as a film or a liquid crystalline array. Other authors, including Tour J. M., prefer to

reserve the term molecular electronics for single-molecule tasks, such as single molecule-based

devices or even single molecular wires. Due to the broad use of this term, molecular electronics

are split into two related but separate subdi sciplines by Petty M. C.molecular materials for

electronicutilizes the properties of the molecules to affect the bulk properties of a material, while

molecular scale electronicsfocuses on single-molecule applications.[1]

Molecular electronics represent the ultimate challenge in device miniaturization. Molecular

devices can have any no of termini with current-voltage responses that would be expected to be

nonlinear due to intermediate barriers or hetero functionalities in the molecular framework while

molecular wires refer to especially tailored molecular nanostructures energetic properties.

Molecular-scale devices actually operating today include: FETs, junction transistors, diodes, and,

molecular and mechanical switches. Logic gates with voltage gain have been built, and many


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techniques have been demonstrated to assemble nanometer wide wires into large arrays.

Programmable and non-volatile devices which hold their state in a few molecules or in square

nanometers of material have been demonstrated[1].

This idea was tested by a 1974 paper entitled Molecular Rectifiers by Mark Ratner and Ari

Aviram.7 This paper illustrated a theoretical molecular rectifier and generalized molecular

conduction in molecular electronics. They discussed theoretically the possibility of constructing

a very simple electronic device, a rectifier, based on the use of a single organic molecule. It has

turned out in later years that observing true molecular rectification is very difficult. Their

proposal formed a brave attempt that would strengthen the foundations of the field with hopes of

electronic applications truly at the molecular scale. Later, in 1988, Aviram described in detail a

theoretical single-molecule field-effect transistor.8 Further concepts were proposed by Forrest

Carter5 of the Naval Research Laboratory, including single-molecule logic gates. These were all

theoretical constructs and not concrete devices. The direct measurement of the electronic

characteristics of individual molecules has to wait for the development of new techniques which

are capable of making reliable electrical contacts at the molecular scale. This was not an easytask. The first experiment measuring the conductance of a single molecule was only reported in

1997 by Mark Reed and co-workers. Since then, the development of nano-scale measuring

techniques has progressed rapidly and the theoretical predictions of the early workers have

mostly been confirmed. Rapid progress in molecular electronics has been made in the last three

decades owing to advent of new characterization techniques[1].

Molecular electronics is a branch of applied physics which aims at using molecules as passiveor active electronic component. These molecules will perform the functions currentlyperformed by semiconductors

In the natural word moleculse are used for many purpose.Using molecule based materials for

electronics sensing and optoelectronics is a new endeavor called molecular electronics and thesubject both of riveting new research and substantial popular press interest

[1].


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HISTORY OF MOLETRONICS

1950s- INVENTION OF TRANSISTOR 1956- ARTHUR VON GAVE IDEA ABOUT MOLECULAR ENGINEERING 1960s & 1970s- EXPERIMENTS ALL AROUND THE WORLD 1981-STM INVENTED (FIRST TOOL TO PROVIDE ABILITY TO SEE AT A[1]

The microelectronics and computer revolution arguably is one of the most importanttechnological advancement of our times, one that has drastically changed the way we work and

live. The success of the microelectronics industry is attributed in large parts to our ability to

shrink the transistor size in complementary metal-oxide semiconductor (CMOS) integratedcircuits down to an ever smaller dimension (currently at 60 nm). This increasing miniaturization

has allowed an exponential growth in computing power as the number of transistors in integrated

circuits doubles every 18 months. This is the famous Moores law However, as the size of the

transistor[2]

Figure 1.2: Schematic diagram of a Field Effect Transistor (N-type shown here)


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CHAPTER 2

ADVANTAGES OF MOLECULAR ELECTRONICS

Molecular structures are very important in determining the properties of bulk materials,

especially for application as electronic devices. The intrinsic properties of existing inorganic

electronic materials may not be capable of forming a new generation of electronic devices

envisioned, in terms of feature sizes, operation speeds and architectures. However, electronics

based on organic molecules could offer the following advantages: Compared to CMOS, the use

of molecules in Molecular electronics offers a number of advantages. Organic molecules are

extremely diverse and their behaviors can be tailored by chemical synthesis. There is also a great

potential for a bottom-up approach to manufacture integrated circuits using self-assembly. While

it is becoming prohibitively expensive to fabricate Si of increasingly smaller dimensions due to

stringent environmental conditions, self-assembly and chemical synthesis are relatively cheaper

to perform. More importantly, because of their small size, very dense circuits could be built. As

all molecules of one type are identical, they should also have identical characteristics, thus

avoiding the present problem of variability in components.[3]

Size-Molecules are in the nanometer scale between 1 and 100 nm. This scale permits small

devices with more efficient heat dissipation and less overall production cost to be made.Molecular Electronics is a way to extend Moores Law past the limits ofstandard semiconductor

Circuits.[3]

Power-One of the reasons that transistors are not stacked into 3D volumes today is that thesilicon would melt. The inefficiency of the modern transistor is staggering. It is much less

efficient at its task than the internal combustion engine. The brain provides an existence proof of

what is possible; it is 100 million times more efficient in power/calculation than our best

processors. Sure it is slow (under a kHz) but it is massively interconnected (with 100 trillion

synapses between 60 billion neurons), and it is folded into a 3D volume. Power per calculation

will dominate clock speed as the metric of merit for the future of computation .[3]

Assembly One can exploit different intermolecular interactions to form a variety of

structures by the array of self-assembly techniques which are reported in the literature. The scope

of application of the self-assembly technique is only limited by the researchers ability to

explore.[3]

Manufacturing Cost - Many of the molecular electronics designs use simple spin coating ormolecular self-assembly of organic compounds. The process complexity is embodied in the

synthesized molecular structures, and so they can literally be splashed on to a prepared siliconwafer. The complexity is not in the deposition or the manufacturing process or the systems


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engineering. Much of the conceptual difference of nanotech products derives from a biological

metaphor: complexity builds from the bottom up and pivots about conformational changes, weakbonds, and surfaces. It is not engineered from the top with precise manipulation and static

placement[3].

Low Temperature Manufacturing: Biology does not tend to assemble complexity at1000 degrees in a high vacuum. It tends to be room temperature or body temperature. In amanufacturing domain, this opens the possibility of cheap plastic substrates instead of expensive

silicon ingots[3].

StereochemistryA large number of molecules can be made with indistinguishable chemicalstructures and properties. On the other hand, many molecules can exist as distinct stable

geometric structures or isomers. Such geometric isomers exhibit unique electronic properties.Moreover, electronic properties of conformers can be affected by pressure and temperature. We

can therefore make use of stereochemistry to tune properties.[3]


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CHAPTER3

MOLECULAR ELECTRONIC SYSTEMS

In order to perform as an electronic material, molecules need a set of overlapping electronic

states. These states should connect two or more distant functional points or groups in the

molecule. A conjugated orbital system is required for a typical candidate of molecular

electronics. This conjugated system needs to extend on an -framework with terminal functional

groups. Molecules for electronic applications generally have 1-, 2-, or 3-dimensional shapes as

depicted which provides stable connection of the material to the metallic electrodes or inorganic

substrates, is the caudal functional group of the organic electronic material. It is important to

note that each part of an organic molecule used as the active component in nano scale electronic

device has their own contribution. In general, by measuring the conductivity of a series of

systematically modified molecules, the contribution of each component can be determined. For

example, by varying the molecular alligator clip.[4]

3.1Electronics Structures

The simplest molecules studied in molecular electronics are the alkylthiols, which only have -

bonds. The others are organic molecules represented by alternating double and single bonds or

alternating triple and single bonds. These are indicative of an -bonded C-C backbone with -

electron delocalization. The conjugation length is defined as the extent over which the -

electrons are delocalized. The double or triple bonds between carbon atoms in the molecules

have an electron excess to that normally required for just -bonds. These extra electrons are in

the pz orbitals which are mainly perpendicular to the bonding orbitals between adjacent carbon

atoms. These electrons overlap with adjacent pz orbitals to form a delocalized -electron cloud.

This cloud spreads over several units along the backbone. When this happens, delocalized

valence (bonding) and * conduction (anti-bonding) bands with defined bandgap are formed

which meets the requirements for (semi)conducting behavior. Normally the electrons reside in

the lower energy valence band. If given sufficient energy, they can be excited into the normally

empty upper conduction band, giving rise to a * transition. Intermediate states are forbidden

by quantum mechanics. The delocalized -electron system confers the (semi)conducting

properties on the molecule and gives it the ability to support charge transport[4].

3.2 Different Alligator Clips in SAMs

Scheme 2 shows some common alligator clips used in molecular electronics for forming SAMs.

The acetyl-protected thiols and dithiols can be deprotected in situ under acid or base conditions

to form SAMs on gold substrate. The diazonium salt generates an aryl radical by loss of N2 and

ultimately produces an irreversible gold-aryl bond. Isocyanide and diisocyanide also perform

gold-carbon bond. Among all the alligator clips, sulfur compounds have a strong affinity to

transition metal surfaces. This is probably because of the possibility to form multiple bonds with


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surface metal clusters. The number of reported surface active organ sulfur compounds and their

derivatives that form monolayers on gold include, di-n-alkyl sulfide, di-n-alkyl disulfides,

thiophenols, thiophenes, mercaptopyridines, mercaptoanilines, xanthates,

cysteines,thiocarbamates, thiocarbaminates, thioureas, mercaptoimidazoles, ditellurides and

alkaneselenols. SAMs of alkanethiolates on Au surfaces are the most studied and well

understood[4]

.

Figure 3.1Representative alligator clips for forming SAMs. 1,2-dioctyldisulfane (a); bis(4,4-biphenyl)ditelluride (b); benzenethiol (c); benzene-1,4-dithiol (d); S-phenylethanethioate (e);

S,S-1,4-phenylene diethanethioate (f); 4,4-biphenyl selenoacetate (g); phenyl isocyanide

(h);1,4-phenylene diisocyanide (i); 2-nitro-1,4-bis(phenylethynyl)benzene diazonium

tetrafluoroboride (j)

3.3 Electrode Effects

There has been great interest in molecular electronics since the observation of electrical

conductivity of the molecules from early experiments with the junction formed by sandwichingthe molecule between two metal electrodes. However, it has been shown that in some systems, it

was not the molecules themselves but the metal contacts that mainly contribute to the junction

conductivity. The misleading observations from early experiments are due to the so called metal

nanofilaments effect. The metal nanofilaments effect is caused by the movements of metal

atoms from the contacts to the tiny gap (several nanometers) between the two contacts with a

bundle of molecules in between when an electric field is applied. The metal atoms in the gap act


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as a low resistance bridge between the two contacts. Instead of flowing through the molecule,

electrical current tends to pass through the low-resistance bridge. More recently,proposed a

metal-free system in which the two sides of a molecular monolayer attached to single-crystal

silicon and a mat of single-walled carbon nanotubes, respectively Figure .Such a design

eliminated the metal nanofilaments effect and switching property was observed under an applied

field.[5]

(a) (b)

Figure 3.2 (a) Metal-molecule-metal junction with metal nanofilaments effect. (b) Carbon

nanotube-molecule-silicon junction.

Therefore it is critical to understand the correlation of the interface energy levels which demandsboth theoretical and experimental study. A relevant consideration involves how the chemical

nature of the molecule-electrode interface affects the rest of the molecule. The zero-bias coherent

conductance of a molecular junction may be described as a product of functions that describe the

molecules electronic structure and the molecule-electrode interfaces. However, it is likely that

the chemical interaction between the molecule and the electrode will modify the molecules

electron density in the vicinity of the contacting atoms and, in turn, modify the molecular energy

levels or the barriers within the junction. There is little doubt that the molecular and interface

functions must be considered in tandem in theoretical studies.[5]


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CHAPTER 4

MOLECULES AS ELECTRONICS DEVICES

It is important to recognize that molecules can conduct current. Mechanisms of electron transport

through single organic molecules include direct tunneling, Fowler-Nordheim tunneling,

thermionic emission and hopping conduction. For a detailed treatment of electron transport

theory, please refer to reference . Here we are interested in the fact that the conductance of a

molecule can be changed with an applied electric field, exhibiting an FET-like behavior.

Molecular electronics devices have been implemented using carbon nanotubes , nanowires, or

organic molecules. Among these, methods using thin-film organic molecule are currently the

most developed[6]

.

4.1 Carbon nanotubes

Carbon nanotube (CNT) belongs to fullerene, a class of allotrope of Carbon. Its structure

assembles a sheet of graphene rolled up into a tube. There are two main types: single-walled and

multi-walled nanotubes. Single-walled carbon nanotubes hold great promise for future

electronics as they exhibit a range of conductivities from metallic to semi-conducting depending

their atomic arrangement . CNTs have been synthesized using electric arc discharge or chemical

vapour deposition methods. The metallic tubes are then burnt off leaving behind the

semiconducting ones. By using semiconducting CNT as the channel connecting the source and

the drain electrodes, it has been possible to make FET-like transistors. CNTFETs have been

implemented with bottom gate top gate and vertical gate electrodes .CNTFETs can be made to

be P-type if exposed to air or or N-type if annealed in vacuum. NOT, NOR, NAND, and ANDgates as well as memory cells such as flip-flops have also been fabricated from these

transistors.[7]

The advantage of using CNT lies in its mechanical strength. The length of CNTs makes it easy to

connect them to electrodes. In addition, they are simple to fabricate and allow good control of

electronic properties. On the other hand, there is no good reproducibility. Right now the coupling

between the gate and the nanotube is still not strong enough for amplification. Thus it is difficult

to use the output of one device to control another, the requirement for higher level of integration.

Currently, there is no method to mass-produce CNTs and there is also a lack of method for

accurate placing during fabrication. Once made, carbon nanotubes are stable but they are madeonly 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


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backbone, the polyphenylene backbone, is not the most conductive and the most conductive, the

carbon nanotube, is not the most flexible chemically[7].

Figure 4.1: Carbon nanotube field effect transistor (a) bottom gate (b) top gate Source

4.2 Nanowires

Nanowires are wires of thickness in the range of nanometers. Semiconductor nanowires (GaP,

GaN, Si, InP) have been produced for use in electronics. The common technique for the

synthesis of nanowires is Vapour-Liquid-Solid growth Just like solid-state Si, semiconductor

nanowires can be doped chemically to give p- and n-nanowires. Employing the same principlesas the Unlike carbon nanotubes which can be semiconducting or metallic, inorganic nanowires

are always semiconducting. Another advantage is the relative ease at which Si-nanowires can be

integrated into the current silicon industry process and fabrication. Moreover, nanowire FETs

have been found to respond faster than conventional MOSFET One disadvantage is the high cost

of large-scale production of nanowires FETs.[8]


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CHAPTER 5

TECHNIQUES FOR ELECTRICAL CHARACTERIZATION OF

MOLECULES

A major part of research in molecular electronics is the development of experimental techniques

for measuring electrical conductance of organic molecules. As contact with a single molecule is

difficult, a self-assembled monolayer (SAM) is often used. These are single layers of molecules

on a substrate arranged in regular ways. The physical process that forms SAMs is thought to beVan der Waal interaction between the chains of molecules. The most well-studied SAM system

is alkanedithiol on gold Proposed methods for studying molecular electron conductance include

scanning tunneling microscope, break junction, nanopore and mercury drop methods[9]

Figure 5.1: Various techniques to measure electronic properties of molecules.(A) Hg drop junction. (B) Mechanically controlled break junctions. (C) Nanopore.

(D) Nanowire. (E) Nanoparticle bridge. (F) Crossed wires.(G) STM. (H) Contact CP-AFM.

(I) Nanoparticle coupled CP-AFM.


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5.1 Scanning tunneling microscopeThis method uses an STM tip to scan over the SAM-covered surface, keeping the tunneling

current constant or monitoring the current. The surface acts as the first electrode while the STM

tip acts as the second one. The junction is thus asymmetric as the surface and the STM tip hasdifferent chemical potentials due to their difference in shape. This is a disadvantage of scanning

tunneling microscope method. The advantage is that both the image and the information aboutconductance can be provided at the same time

.[10]

Atomic force microscope (AFM) can also be used in two ways(i) conductive probe ATM(ii) nanoparticle coupled conductive probe AFM.

5.2 Mercury-drop junction methodA mercury drop is used as an electrode on which a SAM develops Another SAM is developed on

a metal substrate. Mechanical contact is then form between the two SAMs to form a Mercury-SAM-SAM-Metal structure. This method is fast and the junction is easily constructed, allowing

multiple SAMs to be investigated concurrently. However, it can only be used for ensemble

measurements and cannot be performed at cryogenic temperatures[10].

5.3 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[10]

5.4 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.Molecular Scale

Electronics[10]

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 junctiongate 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 .[10]


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CHAPTER 6

APPLICATIONS OF MOLECULAR ELECTRONICS

Molecular electronics seeks to be the next technology in the electronics industry where

molecules assemble themselves into devices using environmentally friendly and low cost

fabrication techniques. It goes beyond the limitations of rigid silicon-based solutions. It

implements one or a few molecules to function as connections, switches, and other logic devices

in future computational devices. Molecular electronics can be used in emerging technologies

ranging from novel optical discs based on bistable biomolecules to conceptual design of the

computers based on molecular switches and wires.[11]

Molecular electronics seeks to be the next technology in the electronics industry where

molecules assemble themselves into devices using environmentally friendly and low cost

fabrication techniques. It goes beyond the limitations of rigid silicon-based solutions. It

implements one or a few molecules to function as connections, switches, and other logic devices

in future computational devices. Molecular electronics can be used in emerging technologies

ranging from novel optical discs based on bistable biomolecules to conceptual design of the

computers based on molecular switches and wires. The processing speed of existing computers is

limited by the time it takes for an electron to travel between devices. Molecular electronics-based

computation addresses the ultimate requirements in a 16 dimensionally scaled system: ultra

dense, ultra fast and molecular-scale[11]

.

By the use of molecular scale electronic interconnects, the transmittance times could beminimized. This could result in novel computational systems operating at far greater speeds than

conventional inorganic electronics. The design of a molecular CPU can bring great technical

renovation in computer science. Table 1 shows the main differences between the present bulkelectronic devices and the proposed molecular electronic devices. Novel molecular electronics

would approach the density of ~1013 logic gates/cm2. It offers a 105 decrease in the size

dimensions compared to the present feature of a silicon-based microchip. In addition, the presentfastest devices can only operate in nanosecond while the response times of molecular-sized

systems can reach the range of femtoseconds. Thus, the speed may be attained to a 106 increase.

On the basis of these estimates, a 1011 fold increase in the performance can be expected withmolecular electronics, which offers an exciting impetus for intense research and development

though numerous obstacles remain[11].

6.1 Switch using MoltronicsBenzene ring of six carbon at is held together in part by a pi bond a sort of smeared bond in

which some of the electrons are loosely shared by all the atoms in a kind of cloud that circles

above and below the carbon ring. Its not a broken bond. Instead, its a sort of bond within whichelectrons are somewhat more able to move[11].

By changing the structure of the molecule, the researchers found that they were able to alter its

behaviour. They hung molecular fragments an NH2 group and an NO2 group from the middlebenzene ring. This distorted the electron cloud, making the molecule more susceptible to


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twisting. By applying a voltage to the molecule, for example, they could cause a change, a bend

or a twist in the molecule .This disrupted the flow of electrons. And, this twist was reversible.When the voltage was removed, the molecule returned to its original shape, allowing current to

pass through once again. In other words, this molecule can act as a switch. It turns electricity on

and offa basic characteristic that a computer needs to process information in bits of 1 and 0.[11]

Figure 6.1 A perposed molecular switch based on molecular stereochemistry rather than aexternal gate

6.2 Memory ChipData storage is done by multiporphyrin nanostructures into electronic memory. The application

of a voltage causes the molecules to oxidize, or give up electrons. The molecules then retain theirpositive charge after the electric field is removed, producing a memory effect[11].

Figure 6.2 Architecture for a proposed molecular memory


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6.2.1 Memory Hold Time

Silicon memory devices retain charged bits for only a millisecond before the charge leaks away.

That means that each piece of information must be restored ten to a hundred times a second,

which requires substantial amounts of power. Moletronic device retains its electrons for about

nearly fifteen minutes. It has the ability to get the information in and out of the systems and

using significantly less[11].

Figure 6.3 Circuits contacting 64 bit molecular observed at increasing magnification

power. Compared to, say, current equipment, which only runs for a few hours before the

batteries wear out, Reed says, machines using molecular memory could run for a week. Theresan energy structure that explains how long a device either silicon, or molecular will hold

electrons. They leak out at a certain rate and when you go to a molecular structure, the energies

become much bigger. So the leak-out rate is slower.[11]


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CHAPTER7

FUTURE OF MOLECULAR ELECTRONICS

The drive toward yet further miniaturization of silicon-based electronics has led to a revival of

efforts to build devices with molecular-scale organic components. However, the fundamentalchallenges of realizing a true molecular electronics technology are daunting. Controlled

fabrication within specified tolerances and its experimental verification are major issues. Self-

assembly schemes based on molecular recognition will be crucial for that task. Ability tomeasure electrical properties of organic molecules more accurately and reliably is paramount in

future developments. Fully reproducible measurements of junction conductance are just

beginning to be realized in labs at Purdue, Harvard, Yale, Cornell, Delft, and Karlsruhe

Universities and at the Naval Research Laboratory and other centers.[12]

Working molecular electronic devices exist today. Research progress is steady and strong, giving

us cause to believe that molecular electronic systems may be practical in five to ten years. If

lithography reaches fundamental physical or economic limits, molecular electronics may allow

us to continue observing Moores Law. Regardless, molecular bottom-up fabrication could giveus a much better alternative, whose price would depend mainly on design and test cost, instead

of billion-dollar factories.[12]

Robust modeling methods are also necessary in order to bridge the gap between the synthesis

and understanding of molecules in solution and the performance of solid-state molecular devices.

In addition, the searching of fabrication approaches which can couple the densities achievable

through lithography with those achievable through molecular assembly is also a great challenge.Controlling the properties of molecule-electrode interfaces and constructing molecular-electronic

devices that can exhibit signal gain are also crucial to the development in the field[12].

An even more important impact will be a qualitative transformation of the nature of computing.

This could be an even greater transformation than the one in which microcomputers replaced

mainframes during the 1980s. In the forthcoming nano computer revolution led by DARPA,computers will become small enough to be placed on top of a human cell. Computation will

literally become a property of matter[12].

Even beyond these radical advances in the quantitative and qualitative nature of computation,Moletronics is stimulating a revolution in materials. To compute with molecular-scale structures,

we have found that we must learn how to characterize and organize them on similar scales, oneby one, and in vast arrays. This is creating a whole new science and industry of "nanostructured

materials."[12]

But the most important impact of our efforts in the DARPA Moletronics Program possibly will

be seen in generations to come. The nanocomputers and the nanomaterials developed by our

moletronics investigators will stimulate a new wave of innovation as our children and their

children begin to assimilate these revolutionary changes and harness them with their ownimaginations.

In summary, my collaborators and I in the DARPA Moletronics Program are working onrealizing the dream of nanotechnology successfully executing a systematic plan to transform the

vision of molecular electronic nanocomputers to reality. We will deliver a prototype ultradense

memory in September 2004 using molecular-scale components and we hope (and expect) thisimportant development will seed a revolution in materials as well as in computation[12].


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While these currents are small in absolute magnitude, their densities are enormous. As many as

one trillion electrons per second pass through a single square nanometer each second in thesemonomolecular semiconductors. Using appropriate nanoscale units, the current density

through[12]

Figure 7.1 Programmable/erasable polyphenylene-based switch used by Yale-Rice-PSMoletronics team in molecular RAM cell(a)porphyrin multi-bit memory cell by UCRiverside-NCSU Moletronics team(b)ruthenium-based dimer being used in efforts


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CHAPTER 8

REALIZATION OF BASIC CIRCUITS

The circuits for the AND and OR digital logic gates which use diode-diode logic structure

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.[14]

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. [14]

8.1 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.[14]

8.2 Molecular Electronic Full AdderBy combining two half adders plus an OR gate, you can make a molecular electronic full addermeasuring about 25 nm x 25 nm.

8.3 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


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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.[14]


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CHAPTER 9

ORGANIC DEVICES

Molecular Electronics, as on date, can be divided into broad areas: Molecular materials ofelectronics (MME), and Molecular scale electronics (MSE). MME deals with the use ofmacroscopic or bulk properties of molecules or macromolecules 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 maincategories 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.[15]

The use of molecular organic materials as active elements in electronic devices was actually

augmented with the discovery of conducting polymers in mid 1970s. 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.[15]

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

reasonable charge carrier mobility and demonstrate the existence of controllable band gap of the

order of 0.75 to 2 e V. Till date, no molecular material has come up to this expectation [15].

Figure 9.1 it can be pertinent to mention the functioning of p-n junction


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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[15]

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

[15].

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 stillpoor. 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.[15]

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.


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CHAPTER 10

CONCLUSION

Molecular electronics is an exciting emergent field of study. The reward of research in this areais enormous as the birth of molecular computer implies unprecedented processing power that

may enable breakthroughs in artificial intelligence. This paper has given a glimpse at how such

an endeavor might be accomplished by introducing the basic ideas in molecular device

implementation and electrical characterization methods. The path towards a full working system

is still a long one, yet the prospects are bright and great strides have been taken. Even a lot of

approach has been proposed in moletronic computer. But there still exists critical problem: most

of the technologies are valid only in laboratory condition, and cannot be produced massively


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REFERENCES

1 http://www.ctcase.org/bulletin/16_4/moletronics.html

2 http://seminarprojects.net/t-molecular-electronics-full-report3 http://seminarprojects.com/s/advantages-of-moletronics

4 http://en.wikipedia.org/wiki/Molecular_scale_electronics5 http://www.alcatel-lucent.com/bstj/vol05-1926/articles/bstj5-4-555.pdf6 en.wikipedia.org/wiki/Molecular_electronics7 www.personal.reading.ac.uk/~scsharip/tubes.htm

8 www.personal.reading.ac.uk/~scsharip/tubes.htm

9 ieeexplore.ieee.org ... Applied Physics Letters

10 www.ruf.rice.edu/~natelson/theses/ward_thesis.pdf

11 http://www.wifinotes.com/nanotechnology/what-is-molecular-electronics.html

12 www.lorentzcenter.nl/lc/web/2012/.../info.php3?.13 seminarprojects.net/q/advantages-and-disadvantages-of-moletronics

14 mitre.org/work/best_papers/00/ellenbogen_arch/99W0207.pdf

15 www.sigmaaldrich.com/content/dam/.../material_matters_v4n3.pdf

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