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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.
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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
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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
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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.
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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)´
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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
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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
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Consider the energy level diagrams
Under equilibrium
Fermi levels are aligned
fig
I SI I
LUMO STATES
HOMO STATES
Ef Ef
donating withdrawing
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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
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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.
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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
donating withdrawing
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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.
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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
donating withdrawing
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Fig
Diode 2
Diode 1
VA
VB
VC
R
+V
VA & VB are inputs
VC is the output
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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