It is a brief overview on molecular electronics with special focus to molecular rectifiers.

1. THE GENESIS OF
MOLETRONICS
Saurav Chandra Sarma
1

2. Introduction
Introduction
Molecular Rectifiers
Fabrication of molecular junction
Some common terms
Factors affecting Conductance
Research Highlights
Other molecular Devices
Conclusion
2
Sketch of my talk
Molecular
Rectifiers
Fabrication
Research
Highlights
Logic Gates
Molecular
Switches
Molecular
Wires
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

3. 3
Moore’s Law
3The Wall Street Journal
The number of transistor that can be
placed on a single integrated circuit
double about every two year.
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

4. 4
Moore’s Law
4Google images
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

5. • Term coined by Mark Ratner, in 1974.
• Can be defined as technology utilizing
 Single molecules,
 Small groups of molecules,
 Carbon nanotubes, or
 Nanoscale metallic or
 Semiconductor wires to perform electronic functions.
Molecular Electronics
• This device must exchange information, or transfer states or must be able to
interface with components at the macroscopic level.
• Usually consist of organic molecules sandwiched between conducting
electrodes.
5
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

6. 1959 1971 1974
Late
1970s
1980s 1997 2000
R. Feynmann
lecture
Kuhn and
Mann
experiment
Aviram and Ratner
first paper on
moletronics
Conferences
arranged by
F. Carter
Shirakawa, Heeger
and MacDiarmid
awarded Nobel Prize
in Chemistry
First attempt to
measure single
molecule transport by
Reed and Tour
Development of
STM and AFM by
IBM
Pioneers of
Moletronics

7. Bottom-Up ( Why molecules..??)
7
 Speed:
Good molecular wires can reduce the transit time of typical transistors.
 New functionalities:
New property can easily be assigned to a molecule that are not possible to
implement in conventional solid state physics.
 Flexibility:
Pi conjugation and therefore conduction can be switched on and off by changing
molecular conformation providing potential control over electron flow.
 Self-assembly:
Can create large arrays of identical devices.
 Size:
Molecules are small that leads to higher packing density
of devices.
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

8. Molecular
Rectifiers
Molecular
Wires
Molecular
Switches
Molecular
Devices
Molecular
Transistor
Molecular
Sensors
Molecular Devices
8
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

9. What is a Rectifier…???
9
HALF-WAVE RECTIFIER
RECTIFIERS: It converts alternating waveform to direct waveform.
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

10. First pioneering paper in Moletronics
10Aviram, A. et. al., Chem. Phys. Lett. 29, 277–283 (1974).
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

11. Proposal of Molecular Rectifier
An organic molecule to have rectifier properties should have roughly the
properties of p-n junction.
11Aviram, A. et. al., Chem. Phys. Lett. 29, 277–283 (1974).
 By the use of EDG/EWG substituents in the aromatic ring, we can
increase/decrease electron density in the aromatic ring and thus create p-
type and n-type molecular junctions.
Hemiquinone
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

12. Mechanism of rectification by
substitution group
12

13. 13Aviram, A. et. al., Chem. Phys. Lett. 29, 277–283 (1974)
Alligator group
Molecular bridge
Left electrode
Right electrode
Potential Barrier
Energy Levels
D
A

14. Y
X
Zero Biased
14Aviram, A. et. al., Chem. Phys. Lett. 29, 277–283 (1974)
D
AX=EDG
Y= EWG
D= Donor half
A= Acceptor half

15. Y
X
V
I
Forward Biased
Current Flows
15
X=EDG
Y= EWG
D= Donor half
A= Acceptor half

16. Y
X
X=EDG
Y= EWG
D= Donor half
A= Acceptor half
V
I
Reverse Biased
No Current Flows
16
D
A

17. Fabrication of molecular junction
DEPOSITION TECHNIQUE:
 Self-assembled monolayer (SAM)
 Langmuir-Blodgett (LB) Technique
SOPHISTICATED TECHNIQUES:
 The size of the molecule is less than the resolution of the lithographic
method
 Following sophisticated techniques are used:
• Electro-migration technique
• Scanning Probe technique
• Mechanically Controllable break-junctions (MCBJs)
17
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

18. Langmuir- Blodgett film technique
• LANGMUIR-BLODGETT (LB) TECHNIQUE : A LB film consists of one or more
monolayers of an organic material, deposited from the surface of a liquid onto a metal
surface by immersing the the solid substrate into the liquid.
18
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

19. Self-assembly monolayer technique
• SELF-ASSEMBLED MONOLAYERS (SAM): The substrate covered with the metal layer is
dipped into the molecular solution. Ampiphilic molecules equipped with one
anchoring group adsorbs chemically onto the metal surface.
19
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

20. Mechanically Controllable Break Junction technique
20Reed et. al., Science, 278, 252-253 (1997)
Schematic of MCB junction
a  Bending beam
b  Counter support
c  Notched gold-wire
d  Glue contacts
e  Piezo-element
f  Glass tube containing solution
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

21. • Changing the environment around the electrodes.
• Changing the alligator group attached to the metal electrode
• Inducing asymmetry in the molecular design.
Rectification Ratio
21
• Rectification Ratio can be defined as the ratio of the current at conducting
voltage to the current at insulating voltage.
RR=
𝐼(𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑛𝑔 𝑏𝑖𝑎𝑠)
𝐼(𝐼𝑛𝑠𝑢𝑙𝑎𝑡𝑖𝑛𝑔 𝑏𝑖𝑎𝑠)
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

22. Conductance Quantum
22
• Conductance given by the Landauer expression
G=
2𝑒2
ℎ
Σ𝑇𝑖
G= Conductance
Ti=Transmission probabilities in the conductance channels.
• Monovalent metal single atom ( such as Au, Cu and Ag ) possess a single channel
for electron transmission. Therefore,
Conductance Quantum= 1 G0 =
2𝑒2
ℎ
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

23. Conductance Trace
• It is the graph of conduction as a function of electrode separation.
23https://www.weizmann.ac.il/chemphys/orental/research.html
Conductance histogramConductance vs. Displacement graph
Repeated ‘n’
times
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

24. Conductance Quantization at molecular level
24Tao et. al., Science, 301, 1221-1223 (2003)
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion
Electrode Separation
X

25. Alkanedithiols:
SHHS
RC6=10.5 MW
HS SH
N=6:
N=8:
RC8=51 MW
SHHS
N=10:
RC10=630 MW
Factor affecting molecular conductance- Chain Length
25Zhou et. al., J Chem. Phys., 128, 044704-907 (2008)
G(L) = Ae-βL
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

26. Effect of Twist angle
26Venkataraman et. al., Nature, 442, 904-907 (2006)
G α cos2θ
Conductance for the series decreases with increasing twist angle
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

27. Factor affecting molecular conductance- Aromaticity
27Chen et al, JACS 442, 918-920 (2014)
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion
The highest conductance is achieved by structure that can
easily acquire quinoid like structure and hence attain a
better coupling between gold electrodes and other cyclic
units.

28. 28Ratner, A. et. al., Nano Lett., 15 (3), 1577–1584 (2015)
New design of molecular rectifier
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

29. Fermi-Level Pinning
29Ratner, A. et. al., Nano Lett., 15 (3), 1577–1584 (2015)
• Energy offset between the electrode fermi-level and the frontier orbital of the
molecule is unchanged even when bias is applied.
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion
Transmission
Window

30. 30Ratner, A. et. al., Nano Lett., 15 (3), 1577–1584 (2015)
New design of molecular rectifier
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion
Transmission Window

31. The environment does the trick
31Venkataraman, L. et al. Nat. Nanotechnol. 10, 522–527 (2015)
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

32. The environment does the trick
Single-molecule junctions with high
rectification ratios can be realized by
exposing different electrode surface
areas to an ionic liquid.
32Venkataraman, L. et al. Nat. Nanotechnol. 10, 522–527 (2015)
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion
Polar Solvent
Non-Polar Solvent

33. 33Whitesides, G. et. al., J. Am. Chem. Soc., 133 (39), 15397–15411 (2011)
Molecular half-wave rectifier
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion
S
Fc2

34. 34Whitesides, G. et. al., J. Am. Chem. Soc., 133 (39), 15397–15411 (2011)
Molecular half-wave rectifier
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion
S
Fc

35. 35Whitesides, G. et. al., J. Am. Chem. Soc., 133 (39), 15397–15411 (2011)
Molecular half-wave rectifier
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion
S
CH3

36. 36Whitesides, G. et. al., J. Am. Chem. Soc., 133 (39), 15397–15411 (2011)
Molecular half-wave rectifier
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

37. 37Nijhuis, C. et. al., Nano Lett., 10, 3611—3619 (2010)
Mechanism of Rectification
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

38. Other Molecular Devices
38Venkataraman, L. et. al., Nano Lett., 11 (4), 1575–1579 (2011)
A Single molecule
potentiometer
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion
Higher conductance state:
Electrode Olefin SulfideElectrode
Low conductance state:
ElectrodeSulfide Olefin Sulfide
Electrode

39. 39
Other molecular devices
Molecular Wires
Molecular Switches
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion
Cuerva et. al., Nanoscale, 3, 4003-4014 (2011)

40. Challenges of moletronics
40
Stability of molecules.
Reproducibility of results.
Controlled fabrication within specified tolerances.
Hard experimental verification.
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion

41. 41
“The Next Big Thing is very, very small
Trillions of
transistors
THz processors
Infinite storage
capacity
High computation
power
Concluding Remarks
It is the birth of a new technological revolution
And the death of silicon”
Introduction
Molecular
Rectifiers
Fabrication
Common
Terms
Factors
Research
Highlights
Molecular
Devices
Conclusion