Download molecular electronics (moletronics)


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 smallscale 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 binding
sites
•
Fig (a)
Phenylene group
•
Fig (b)
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 fairly conductive
•
Conduction in these molecules proceeds by electrons moving through
extended molecular orbitals that span the entire molecule
•
Extended molecular orbitals are called “¶-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 σ-bonds
•
σ-bonds lie along axes of molecule , cannot be extended b/w atoms
•
σ-bonds cannot be easily extended because at end of each σ-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 “electron donating
groups(donors)”
•
Groups that remove
groups(acceptors)”
electrons are called “electron withdrawing
•
A p-n junction is formed 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 “semi-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)
Semi-insulating group
I
Au
contact
SI
I
D
A
Au
contact
chain with withdrawing
group,d
chain with donating
group,d
•
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
 Consider
the energy level diagrams
 Under equilibrium

Fermi levels are aligned

fig
donating
Ef
I
LUMO STATES
withdrawing
SI
HOMO STATES
I
Ef
•
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 difference 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 flow is asymmetric and highly non-linear due to the
following facts:

The energy difference 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
Electron flow
donating
+V
I
LUMO STATES
SI
Ef
HOMO STATES
withdrawing
I
-V
Ef

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 “nonresonant current”

A very low bias is rqd in forward bias condn.

Reverse bias:
Fig
-V
Ef
donating
LUMO STATES
withdrawing
SI
I
I
+V
Ef
HOMO STATES

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
•
Fig
+V
Diode 1
VA
VA & VB are inputs
VC is the output
VC
Diode 2
VB
R
•
Operation : +ve
 If
voltage is applied across resistance R
either VA or VB are low or both are low then
 Diodes
 There
 VC
 If
1 & 2 are forward biased
is small or no voltage drop across diode in ideal case
is also low
both inputs VA & VB are high then

Both diodes 1 & 2 are reverse biased

The 2 diodes are open circuited in ideal case

VC is high