The Next Generation Molecular Computers Using Nanotechnology

Nirupana.N(2nd year B.E IT)

Pandian Saraswathi Yadav Engineering College

nirupaanilu@gmail.com

ABSTRACT:In this revolutionary world anything at

anytime may happen, one of such thing

is the evolution of molecular computers

which is very much smaller than the

components of present technology. This

can be made with the help of

“Nanotechnology”, one of the emerging

technology which trends to open a new

world. In this paper we have used

aromatic and aliphatic molecules for the

formation of nano computing devices.

The poly phenylene molecular chain

takes the role of conductors and the

alkyl chain or aliphatic molecules as the

insulators which forms a barrier for the

conduction. The pi electron cloud over

each molecule interconnects each other

to form a massive electron cloud which

is used to transfer electron from one end

to other, this form the basis for

molecular electronic devices. These

molecules are used for the designing of

molecular diodes and resonant tunneling

devices. The molecular diodes are

having the same electrical response as

that of semiconductor diodes. The

resonant tunneling devices form an

electron channel to conduct in both

directions. These resonant tunneling

devices (RTD) have two barriers which

are formed by the aliphatic molecules.

These RTD is used in the formation of

XOR gates and the AND and OR gates

can be formed with the tour wires,

molecular diodes and aliphatic

molecular chain which acts like a

resistor. In RTD the electron transfer

occur due the change in LUMA energy

level between both sides of the

molecule. Usually the ends of the

molecules are plated using gold

molecules for better isolation. Using

these nano molecular devices like

Diodes, RTD, Conducting tour wires

and insulating aliphatic chain we can

form all the basic components of

computing. Here we have formed AND

gates, OR gates, XOR gates using only

molecular diodes and RTD.By the

formation of molecular gates we can

form ADDER, SUBTRACTORS

etc.Here we have designed molecular

Half adders and Full adders. From these

proposed models of basic computing

molecular devices, we can form flip

flops, registers, High density memory

and so on. Thus the basic computing

devices are designed for the formation

of the nanocomputers which will

revolutionize the future.

“Molecules are not only meant for

compounding but also for computing”

INTRODUCTION:

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There are several researches in progress

to increase the density of logic circuits

in order to increase the speed and to

hold the Moore law. The Moore’s law

states that the number of logic circuit in

the processor doubles for every one and

half year. The present technology has a

limit to the fabrication of the

components in a very dense manner.

There is a solution to all these problems

with Nanoelectronics using which we

can make any logic circuits with only

few molecules. When the formation of

basic logic circuits is possible at

molecular level we can also make a

“molecular computer” with these

gates. Let us discuss the formation of

logic circuits with few molecules and

their resemblance to the actual circuit

made using present technology.

Here we are using

aliphatic, aromatic compounds like

polypropylene chains for the formation

of molecular diodes from which we can

form any logic circuits by their

combination in certain manner. First we

will discuss about the formation of

molecular diodes and molecular

resonant tunneling device and then the

formation of basic logic gates and

adders with their combinations.

BASIC MOLECULES USED:

Fig 1 basic molecules used for conduction

Fig 2 conducting wires in nanoscale

MOLECULAR COMPUTERS: (Conjugated aromatic molecules)

The polyphenylene molecules are

formed by combining the benzene ring

with two hydrogen atom used for

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bonding. The polyphenylene based

molecules are formed by adding any

other molecular groups with very useful

properties and structure. These

polyphenylene based molecular wires

are called as “tour wires”. These tour

wires shows electrical conductivity in

the order of nano amperes which

corresponds to its size. The

polyphenylene based wires are formed

by arrays of such molecules in a

nanometer scale pore and adsorbed to

metal contacts on either side.

The source of conductivity in

polyphenylene molecules is a set of Pi-

type orbitals that lie below and above

the molecule which is shown in figure

3. In a planar conformation, the pi

orbitals associated with each individual

atom overlap or conjugate in various

combinations to create a set of extended

pi orbitals that span the length of the

molecule. This occurs due a significant

energetic advantage that rises from

delocalizing valence electrons. Long pi

orbitals are both located out of the plane

of the nuclei in the molecule and they

are relatively diffuse compared to the in

plane sigma type orbitals.Thus one or

more unoccupied or partially occupied

orbitals can provide channels that

permit the transport of additional

electrons from one end to the other end

when voltage bias is given. The sparse

unoccupied pi orbitals can be named as

conduction band. The unoccupied pi

orbitals with low energy are likely to

more effective for the flow of electron.

If the chain is formed only with

aromatic compounds static interference

will occur. This static interference will

leads to non-planar conformation of the

molecule which will reduce the

overlapping of pi orbitals and

conductivity of the chain. To avoid this

we are connecting aliphatic molecules

in-between to increase the conduction

due to it’s out of plane electron density.

Fig 3 conduction due to pi orbital

electrons in aromatic molecule

MOLECULAR INSULATORS:

Here the aliphatic molecules serve as

insulators. They contain only sigma

3

bonds which will not form an

uninterrupted channel outside the plane

of the nuclei. The positively charged

nuclei are obstacles to negatively

charged electrons traveling along the

axis of the plane. For this reason

aliphatic molecules are used as

insulators.

Fig 4 aliphatic molecules used as molecular insulators

FORMATION OF DIODES AND RTD:

The diodes can be formed from these

tour wires and aliphatic insulators. The

diode is a simple device which is used

to turn a current on or off as it attempts

to pass through the diode from the “in”

to “out” of the terminal. There are two

diodes which can be designed with

these molecules as the basic

components. They are

I. Rectifying diodes and

II. Resonant tunneling diodes.

MOLECULAR RESONANT

TUNNELING DIODE (RTD):

The resonant tunneling diode takes

advantage of energy quantization to

permit the amount of voltage bias across

source and drain. Unlike rectifying

diode, current passes equally well in

both directions. The RTD is formed by

inserting an aliphatic methylene group

on both sides of the aromatic ring. This

methylene group acts as the potential

energy barrier to the electron flow. They

establish aromatic ring in between them

and forms an “island” (0.5 nm) of lower

potential energy through which the

electrons must pass in order to traverse

the length of the molecule. These tour

wires are clipped to gold electrodes

through thiol (-SH) which adsorbs to

gold lattice. These molecules are called

as “molecular alligator clips”. This

method of clipping with gold electrode

will promote the conduction between

the metal and the molecules. Alternative

metal contact will leads to surface

oxidation and side reaction with organic

molecules.

OPERATION OF RTD:

Whenever electrons are confined

between two such closely spaced

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barriers, quantum mechanics restricts

their energies to one of a finite number

of discrete quantized levels. This energy

quantization is the basis of operation of

RTD.

The smaller the region in which the

electron are confined, the farther apart

in energy are the allowed quantized

energy levels.The unoccupied pi energy

levels are more densely spaced in

energy in the less confining low

potential energy regions of the molecule

to the left and right of the barrier

surrounding the island.

Fig 5 aromatic ring forming “island”

When the electron injected on the left

hand side of the barrier is not in

resonance with the an energy level

inside the well ,then that electron is not

transferred to the right side of the

barrier and this state is called as “OFF”

state. The only way for the electron to

transfer from left side to right side is

that the must be in resonance with any

of the quantized levels in the well.

Fig 6 operation of Resonant Tunneling Diode

. When this resonance occurs means the

electron is transferred to the well and in

turn the inside the well is in resonance

with the energy level to the right side of

the barrier and so the electron is

transferred to the right side of the

barrier and this implies the conduction

of current through the molecule. Thus

the electron is transferred through

tunneling process.

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Fig 7 peak point at resonance

Fig 8 VI characteristics of RTD

The fig 8 describes that it forms a peak

point at the resonance and valley at all

other points. Thus it shows that it will

conduct only at the resonant point and

switched of at all other points. This will

be basis for the operation of the XOR

gate formed using RTD.

POLYPHENYLENE BASED

RECTIFYING DIODES:

The rectifying diode is the device

which rectifies the flow of current in

one direction (Reverse direction) and

allow the passage of current in other

direction(forward direction).Let us now

see the structure of molecules

equivalent to diode and the reason

behind their operation as a diode.

Fig 7 VI characteristics of rectifying diode

The molecular equivalent of the diode

consists of an electron donating group

and electron withdrawing group. These

two are called as intramolecular

dopants,the electron donating

substituent X and electron withdrawing

substituent Y.The X substituents are n-

type doped and Y substituents are p-

type doped.

The donor sub-complex

consists of an electrically conductive

molecular backbone with one or more

electron donating intra-molecular

dopants covalently bound to it. The

acceptor sub-complex consists of

similar backbone with one or more

electron withdrawing intramolecular

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dopant substituents covalently bound to

it. The Donor and acceptor sub-

complexes are separated within the

diode structure by a semi insulating

bridging group, to which they both are

chemically bonded. Usually this three

part “donor acceptor complex” is

envisioned as in contact with metal

terminals at both ends.

Fig 8 diode formation using molecules

The insulating layer in the middle is

associated with an energy barrier. The

barrier is intended to preserve the

voltage drop induced by donor and

acceptor. The barrier group prevents the

differing electron densities in the

substituted complexes on either side

from coming to equilibrium. While it

still permits added electrons under a

voltage bias to tunnel through. The

aliphatic dimethylene group(R) is

selected as the central bridging group

because it is the smallest non

conducting group. The gold molecules

at the end on two sides provide good

isolation between molecules and

prevents the equilibrium of electron

densities.Coplanarity of the donor

substituent and acceptor substituent

aromatic ring is desirable in order to

enhance the extend of pi orbital there by

increasing the conductivity of the

molecule.Dimethylene group permits

internal rotation of the chain with out

changing the electron density there by

increasing the flexibility of the circuit.

OPERATION:

The chain is divided in to two halves by

the aliphatic group. One lie on the left

hand side and the other on right hand

side. The aliphatic molecule forms a

barrier in between these two halves. The

valence energy levels of the electron

donating dopants are elevated in energy.

This affects the molecular orbitals on

the left side. This includes highest

occupied orbitals (HOMO), the lowest

unoccupied orbitals (LUMO) and the

associated low energy unoccupied pi

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orbitals on the left hand, donor side of

the molecule.

An electron donating group bonded to

an aromatic ring tends to place more

electrons upon the ring. This increases

the mutual repulsion among the

molecules, these additional repulsive

interactions raise the total energy, as

well as the components orbital energies.

FORWARD BIASED OPERATION:

When no supply is provided there will

be a dopant induced difference in

relative energetic positions of the pi

orbitals in the donor and acceptor halves

of the molecule.

Where ELUMO is the energy of

the lowest unoccupied orbital.

This energy difference forms the basis

of operation. When high voltage is

applied to the left hand side with low

voltage on right hand side, the process

of setting field will increase the energy

of electrons on right hand side than left

hand side. Since the increase in energy

is in opposite direction, this induces an

electron on the right hand side which

will pass through the barrier to the left

side.

The highest occupied level of

energy is called Fermi energy level.

When voltage bias is applied, it will

increase the Fermi level on the low

voltage region and decreases on the

other side. The electrons that have

migrated from the right to the acceptor

may further tunnel through the barrier to

donor side. Since the density of

unoccupied energy levels of the donor is

too high, the transmitted electron will be

in resonance with any one of the energy

levels. By this the electron can be

transferred from right to left which

implies that the current flows from left

to right.

REVERSE BIASED OPERATION:

When the reverse voltage is applied

to the molecule, the energy on the donor

side is raised and the energy on the

acceptor side is depressed. When the

same voltage as applied before is

applied here no electrons will be

induced on the donor side. This is due to

that the unoccupied energy levels are

greatly reduced or the energy induced is

more than that induced on the right hand

side during forward biased condition.

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So we need more reverse biased voltage

to induce an electron in the left hand

side which similar to that of our

semiconductor diode for which the

voltage needed to conduct on reverse

direction is high and is called reverse

breakdown voltage. The anticipated

potential drops across the diode is

DESIGN OF LOGIC CIRCUITS:

The logic circuits such as gates,

adder, and subtractor can be designed

using these molecular diodes and

molecular RTD. These devices are

combined accordingly to form the

operation of the basic gates. When these

gates are successfully build, then we can

design flip flops, registers, processors

and at last we will be able to build a

nanocomputer.Let us see the formation

for the basis of nanocomputers.

FORMATION OF “AND” GATES

AND “OR” GATES:

The “AND gates

and “OR” gates are formed with the

help of molecular diodes and molecular

RTD.The molecular gates formed by

this method is one million times smaller

than that of the gates produced by

fabrication of semi conducting

materials. The “AND” and “OR” are

formed by the diode-diode logic. The

aliphatic groups are used as the resistive

element in the circuit. The more

complex circuits can be formed with the

components like molecular diodes,

molecular RTD, aromatic rings as the

conductors and aliphatic molecules as

the insulators.

“AND” GATE:

Fig 9 Molecule Diagram for AND gateTruth table AND gate:

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Circuit Diagram for AND gate:

“OR” GATE:

Circuit Diagram of OR gate:

Molecule Diagram of OR gate:

Truth table OR gate:

FORMATION OF XOR GATE:

The “AND” and “OR” gates are not

only sufficient to form more complex

circuits, so we are using RTD to form

“NOT” gates. From these components

more complex circuits can be formed.

The operation of the XOR gate is

similar to that “OR” gates and only

difference is at the input 1,1 where the

inputs put the operating point in the

valley region and thus the RTD shuts

the flow of current there by it makes the

logic value at c low.

Molecular Diagram for XOR gate:

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Circuit Diagram for XOR gate:

Truth table for XOR gate

FORMATION OF ADDERS:

Here the adders are formed with the

molecular diodes, molecular RTD’s,

aromatic rings as a conductor and

aliphatic molecules as insulators. The

adders and subtractors are formed by

linking the gates with tour wires. This

sequential circuit responds exactly as

the semiconductor devices but with low

voltage and current. The dimension of

these molecular adder and subtractor

will be in the order of 10nmX10nm

area.

Molecule diagram for XOR gate:

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IMPLEMENTATION OF FULL ADDERS:

The full adders are formed by

combining two half adders as shown in

the figure.

Molecule Diagram for Full Adder:

ENERGY DISSIPATION:

The power

dissipation in these molecular devices

are due to the lattice vibrations. When

an electron travels through the chain its

some part of energy is used to vibrate

the other electrons and nucleus. Thus

the power dissipation will leads to a

weaker signal. The amount of power

dissipation depends conductance

mechanism. In case of super exchange

mechanism the electrons are not directly

coupled to the wire, thus the losses can

be reduced.

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CONCLUSION:

Thus molecules which

are of prime importance to us are not

only used for compounding but also

these can be used for computing. Using

these aromatic phenylene molecules and

aliphatic molecules a model for

molecular diode is proposed from which

we have designed the gates. Using the

gates we have implemented half adders

and full adders. Since the basic

components of computing is found this

will form the first step towards

molecular computers. These molecular

computers are formed by forming flip

flops, from which the registers can be

designed which can pave way for the

high density memory. Thus the basic for

the construction of nano computers have

been proposed.

REFERENCE:

[1] www.mitre.org/nanotechnology

[2] Architecture for molecular

computers, J.christopher love, James

C.Ellenbogan.

[3] Electrical properties of molecular

devices, H.A.Reed.

[4] Unimolecular electrical rectification,

R.M.Metzger

[5]”A brief overview of nano electronic

devices”, J.C.Ellenbogen.

[6] www.foresight.org

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