Molecular Electronics Moletronics - Copy

8/3/2019 Molecular Electronics Moletronics - Copy

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Moletronics involves the study and application of molecular building blocks for the fabrication of electronic components.

Includes

conductive polymers

single-molecule electronic components

2 most promising conducting molecular species are:

Polyphenylene

Carbon nanotubes

It is useful in the prospect of size reduction.

Extends Moore's Law beyond the foreseen limits of small-

scale conventional silicon integrated circuits.



It¶s a conductive polymer

It forms chain-like molecules

Formed by linking basic molecular unit,C6H4, i.e Phenylene

Phenylene is a derivative of Benzene ring and has 2 free bindingsites

Fig (a)

Fig (b)

Phenylene group

Polyphenylene



2 binding sites of Phenylene are represented by open circles on ends of

the ring

With 2 binding sites,each Phenylene can be bound to 2 others hence

forming a chain as shown in fig (b)

These chains are then used for molecular wires

Polyphenylene wires are f airly conductive

Conduction in these molecules proceeds by electrons moving through

extended molecular orbitals that span the entire molecule

Extended molecular orbitals are called ³¶-type´

When atoms come close enough spatially, the wave-function overlaps

leading to extended states

These extended states have large energy



Aliphatic molecules are singly bond molecule

They do not contain -bonds , but have -bonds

-bonds lie along axes of molecule , cannot be extended b/w atoms

-bonds cannot be easily extended because at end of each -bond there is a

positively charged nucleus and hence its spatial extent is interrupted by the

nucleus

Example, chaining together of Methylene,CH2,molecules can form an

aliphatic molecule

When these aliphatic molecules are inserted in C6H4 molecules they

interrupt the extended states formed by -bonds thus breaking the conductive

pathway in poly-phenylene chain.



Its a primary molecular device type

2 different doped regions are rqd:

P-type region

N-type region

Electron concentraton can be varied by introduction of foreign agents

In molecular s/m , molecular groups are added

These molecular groups attach themselves at specific places within chains

thus altering the electron concentration

Groups that add electron to s/m are called ³electron donatinggroups(donors)´

Groups that remove electrons are called ³electron withdrawing

groups(acceptors)´



A p-n junction is f ormed by placing a chain of

withdrawing group together with donating groups

A potential barrier is formed b/w donor and acceptor

group chains by using ³semi-insulating groups´

The potential barrier which is formed by semi-insulating

group maintains charge imbalance b/w the 2 sides of the

junction

If the potential barrier is removed, then 2 sides of the

junction,would equilibrate in terms of electron densities thus

removing any diode action



Fig (c)

Molecular arrangement for rectifying diode structure

Insulating groups ,I , provide potential barriers for tunneling into and

out of the diode

SI,semi-insulating group in center of device maintains electron density

imbalance in the jn.

Donating group side is at higher energy than the withdrawing group

side

I

SI

I

Aucontact

Au

contact

chain with donating

group,d

chain with withdrawing

group,d

A

D

Semi-insulating group



Consider the energy level diagrams

Under equilibrium

Fermi levels are aligned

fig

I SI I

LUMO STATES

HOMO STATES

Ef Ef

donating withdrawing



Lowest unoccupied molecular orbital(LUMO) levels in donating

chain are at higher energy than those in withdrawing chain

This is because, electron density is higher in donating chain which

results in increase electron-electron repulsion and hence a higher total

electron energy

Similarly, lower electron density in withdrawing chain results in lower

total electron energy

Hence there exists an energy diff erence b/w 2 sides of molecular

diode i.e. the levels in donating chain is at higher energy than

withdrawing chain energy levels



Molecular diode has 2 bias conditions:

Forward

Reverse

Current f low is asymmetric and highly non-linear due to the

following facts:

The energy diff erence b/w Fermi level & LUMO levels on acceptor group

side is less as compared to energy difference b/w Fermi level & LUMO

levels on donating group side

Bias rqd to align Fermi level with LUMO level on acceptor side is less

compared to bias rqd on donor side

Therefore for same magnitude of bias , current flow in device will be

highly non-linear & asymmetric.



BIAS CONDITIONS:

Insulator groups , I, and semiconductor-insulator-group,SI,act as

potential barriers in structure

Donating chains and acceptor chains are conducting with highest

occupied molecular orbital(HOMO) and (LUMO) states

Forward bias:-

Fig

SII I

LUMO STATES

HOMO STATES

Electron f low

Ef

Ef

-V+V




Occurs when high potential is applied to left hand contact w.r.t right hand

contact

+ve potential lowers electron energies

Hence, occupied energy levels in left hand contact are lower than those in

right hand contact

When Fermi level in right hand contact is raised so as to align with LUMO

levels in acceptor chain, electrons tunnel from contact into LUMO levels

e- then tunnel through SI potential barrier into donator chain LUMO levels

Finally, e- travel through last potential barrier into +vely biased left hand

contact

When levels are not aligned a very small current flows which is called ³non-

resonant current´

A very low bias is rqd in forward bias condn.



Reverse bias:-

Fig

High voltage is applied to right hand contact w.r.t left hand contact

Fermi level is lowered on right hand contact & raised on left hand contact

For same magnitude of bias

Fermi level is not aligned with LUMO levels

No resonant alignment occurs

Little current flows in device

I SI I

HOMO STATES

LUMO STATES

Ef

Ef

-V +V




Fig

Diode 2

Diode 1

VA

VB

VC

R

+V

VA & VB are inputs

VC is the output



Operation :-

+ve voltage is applied across resistance R

If either VAor VB are low or both are low then

Diodes 1 & 2 are forward biased

There is small or no voltage drop across diode in ideal case

VC is also low

If both inputsVA & VB are high then

Both diodes 1 & 2 are reverse biased

The 2 diodes are open circuited in ideal case

VC is high



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Molecular Electronics Moletronics - Copy