The Next Generation Molecular Computers Using Nanotechnology
Nirupana.N(2nd year B.E IT)
Pandian Saraswathi Yadav Engineering College
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:
1
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
2
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
4
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.
5
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
6
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
7
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.
8
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:
9
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:
10
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:
11
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.
12
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
13