This document discusses molecular electronics and nanoelectronics. It describes two approaches to developing nanoelectronic devices - continuing to scale down existing solid-state devices, or developing new devices using molecules (molecular electronics). Molecular electronics involves using organic or organometallic molecules as the basic electronic components, such as in molecular wires, diodes, transistors, and LEDs. Challenges in molecular electronics include making reliable electrical contacts to organic molecules and improving carrier mobility and material stability.

1. Molecular Electronics

2. Conventional Electronics: Transistor
development
• In 1915 AT&T opened their transcontinental
telephone system; required signal amplification.
• 1945: AT&T and Bell Labs set up the Solid State
Physics group.
• First transistor invented in 1947 at Bell Labs.
• Junction transistors used to develop first integrated
circuit in 1958; Jack Kilby at Texas Instruments (2000
Nobel Prize in Physics).
• FET’s in 1961.
• 1965 Moore’s Law.

3. Moore’s LawMoore’s Law

4. Spring 2004 CS-603 Nanotechnology 4
What is Electronic Nanotechnology ?
 Electronic Nanotechnology  Nanoelectronics
 Nanoelectronics: Development of electronic devices
having smallest feature size between 1 to 10 nm
 Possible electronic devices in computers that can be
scaled down to nano levels
- CMOS
- Memory
- Switches

5. Spring 2004 CS-603 Nanotechnology 5
Approaches To Nanoelectronic Devices
 Two approaches:
- Develop “nano” descendants of present solid-state
microelectronics
- Fabricate nano devices from molecules  Molecular
electronics approach
Path I
Scaling down current S-State devices
Path II
Molecular Electronics

6. Spring 2004 CS-603 Nanotechnology 6
Promising Nanoelectronic Devices in Future
Computers
 Path I: Nanoelectronic Solid-State Devices
- Nano CMOS
- Resonant Tunneling Diode (RTD)
- Single Electron Transistor (SET)
 Path II: Molecular Electronic Devices
- Molecular Electronic RTD
- Spintronics
Quantum-Effect Devices

7. Boundaries of conventional techniquesBoundaries of conventional techniques
 Miniaturization achieved by “top down” approach usingMiniaturization achieved by “top down” approach using
improvements in lithography technique.improvements in lithography technique.
 Even with the development of ever-improving lithographicEven with the development of ever-improving lithographic
tools, silicon is approaching fundamental physical limitations oftools, silicon is approaching fundamental physical limitations of
operation. As gate widths decrease below 100 nm, bulkoperation. As gate widths decrease below 100 nm, bulk
properties yield to quantum phenomena and leakage currentsproperties yield to quantum phenomena and leakage currents
from electron tunneling prevent proper device operation.from electron tunneling prevent proper device operation.
 Chemistry operates at the nanometer scale by controlling theChemistry operates at the nanometer scale by controlling the
placement of individual atoms and functional groups onplacement of individual atoms and functional groups on
molecules through synthetic chemistry, allowing macroscopicmolecules through synthetic chemistry, allowing macroscopic
properties from rigidity to optical and electronic behavior to beproperties from rigidity to optical and electronic behavior to be
engineered.engineered.
 ““Bottom up” approach is promising instead of carvingBottom up” approach is promising instead of carving
lithographically bigger blocks into smaller and smaller chunks.lithographically bigger blocks into smaller and smaller chunks.

8. Molecular ElectronicsMolecular Electronics
 First coined by Mark Ratner, in 1974.First coined by Mark Ratner, in 1974.
 Molecular electronics involves the replacement of aMolecular electronics involves the replacement of a
wire, transistor or other basic solid-state (usuallywire, transistor or other basic solid-state (usually
silicon) electronic element with one or a fewsilicon) electronic element with one or a few
molecules.molecules.
 Molecular electronic device must exchangeMolecular electronic device must exchange
information, or transfer states or must be able toinformation, or transfer states or must be able to
interface with components at the macroscopic level.interface with components at the macroscopic level.
 Simple molecular electronic devices usually consist ofSimple molecular electronic devices usually consist of
organic molecules sandwiched between conductingorganic molecules sandwiched between conducting
electrodes.electrodes.

9. Inorganic Vs. Organic
Material Properties

10. Limitations At Early Stage
 Organic materials have often proved to be
unstable.
 Making reliable electrical contacts to organic
thin films is difficult.
 When exposed to air, water, or ultraviolet light,
their electronic properties can degrade rapidly.
 The low carrier mobilities characteristic of
organic materials obviates their use in high-
frequency (greater than 10 MHz) applications.
These shortcomings are compounded by the difficulty of
both purifying and doping the materials.

11. 11
Molecular ElectronicsMolecular Electronics
Aviram-Ratner Diode: an acceptor-bridge-donor moleculeAviram-Ratner Diode: an acceptor-bridge-donor molecule
Chem.Phys.Lett. (1974) 29, 277.Chem.Phys.Lett. (1974) 29, 277.
1. Electrode charge-injection to donor
2. Donor-Acceptor ET
3. Acceptor-electrode charge-injection
A molecular rectifier

12. Molecular Rectifiers

13. Molecular RectifiersMolecular Rectifiers
 Metzger and co-workers haveMetzger and co-workers have
studied Langmuir Blodgettstudied Langmuir Blodgett
(LB) films of(LB) films of
(nhexadecyl)quinolinium(nhexadecyl)quinolinium
tricyanoquinodimethanidetricyanoquinodimethanide
between metal electrodes andbetween metal electrodes and
observed strong rectificationobserved strong rectification
behavior.behavior.
The donor is theThe donor is the quinoliniumquinolinium
moiety, connected to themoiety, connected to the
acceptor,acceptor,
tricyanoquinodimethanidetricyanoquinodimethanide
by a bridge.by a bridge.
D-π-A Rectifier
Metzger, R. M. Chem. Rev. 2003, 103, 3803-3834.

14. I/V curves from two different LB-film configurations. a) 1 LB monolayer b) 4 LB
monolayers.
Metzger, R. M. Chem. Rev. 2003, 103, 3803-3834.

15. Inorganic Vs. Organic
LEDs

16. Molecular Orbitals

17. Organic LED Energy Diagram

18. Organic Thin Film Transistors
(OTFTs)
Organic material Organic material

19. An Example of an I-V of OTFTs
Lg = 20 µm
W = 220 µm
400 nm SiO2
50 nm organic

21. (a) Structures of the long and short linked cobalt coordinated terpyridine thiols used
as gate molecules. (b) A topographic AFM image of the gold electrodes with a gap.
(c) A schematic representation of the assembled single atom transistor.
A Molecular Transistor

22. A schematic representation of Reed and Tour’s molecular junction containing
a benzene-1,4-dithiolate SAM that bridges two proximal gold electrodes.
Break Junctions
At the beginning of single molecule
electronics, break junctions were very
popular: Just crack a thin Au wire open
in a vice and adjust the width of the
crack with piezos (as in STM). Then
pour a solution of molecules over it.
Alternatively, one can burn out the
thinnest spot of a thin Au wire by
running a high current density through
it (using the effect of electromigration).
These days, many try to achieve a
well-defined geometry using a STM
or AFM, with a well-defined atom at the
end of the tip and another well-defined
atom at the surface as con-tacts to a
single molecule.

23. LangmuirLangmuir--Blodgett Monolayer PhotodiodeBlodgett Monolayer Photodiode
 ElectrochemicalElectrochemical
photodiode: D-S-Aphotodiode: D-S-A
 Under positive bias, eUnder positive bias, e--
moves from to D to Smoves from to D to S
(G.S).(G.S).
 PhotochemicalPhotochemical
excitation promotes eexcitation promotes e--
to first E.S. of S, to Ato first E.S. of S, to A
and finally to Au.and finally to Au.
 No current when light isNo current when light is
off.off.
Sakomura, M.; Lin, S.; Moore, T. A.; Moore, A. L.; Gust, D.; Fujihira, M.
J. Phys. Chem. A 2002, 106, 2218.

24. Molecular WiresMolecular Wires
A molecule that connects two (or more)A molecule that connects two (or more)
continuous electron reservoirs, or metalliccontinuous electron reservoirs, or metallic
leadsleads
Organic compoundsOrganic compounds

Usually highly conjugatedUsually highly conjugated
Organometallic compoundsOrganometallic compounds

25. Organic ExamplesOrganic Examples

26. Synthesis of “Wires”Synthesis of “Wires”
PolymersPolymers

LinearLinear
PAH’sPAH’s

Conjugation (delocalization) importantConjugation (delocalization) important
Organometallic compoundsOrganometallic compounds
Electrical conduction is desiredElectrical conduction is desired

27. Insulating the WireInsulating the Wire
More efficientMore efficient
Use of cyclodextrin (a carbohydrate)Use of cyclodextrin (a carbohydrate)

28. Cyclodextrin used to insulate the molecular wire.
Used to produce green and blue LEDs.
Helps prevent “red shift” from molecular interactions.
CyclodextrinCyclodextrin

29. Tour WiresTour Wires
James M.Tour’s group over 15 years have been synthesizing moleculesJames M.Tour’s group over 15 years have been synthesizing molecules
with aromatic, alkene, and alkyne bridges, terminating in thiols at one orwith aromatic, alkene, and alkyne bridges, terminating in thiols at one or
both ends. These are known as Tour Wires.both ends. These are known as Tour Wires.
A wire is defined as a two-terminal entity that possesses a reasonably linearA wire is defined as a two-terminal entity that possesses a reasonably linear
I(V) curve prior to the breakdown limit.I(V) curve prior to the breakdown limit.
Precise molecular wires bearing protected alligator clips (SAc) at one and two ends.

30. Tour Wire: Molecular DevicesTour Wire: Molecular Devices
Molecular devices could be systems having two or more termini with currentMolecular devices could be systems having two or more termini with current--
voltage responses that would be expected to be nonlinear due to intermediatevoltage responses that would be expected to be nonlinear due to intermediate
barriers or heterofunctionalities in the molecular framework.barriers or heterofunctionalities in the molecular framework.
 Two terminal wire with tunnel barrier; wire with a quantum well: RTD; threeTwo terminal wire with tunnel barrier; wire with a quantum well: RTD; three
terminal system: switch; four terminal system: logic gateterminal system: switch; four terminal system: logic gate
Tour, J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998, 120, 8486-8493.
P(m,n) refers to the molecular electrostatic potential impedance of a system with m
1,4-phenylene moieties and n ethynylene moieties.

31. Spring 2004 CS-603 Nanotechnology 32
Resonant Tunneling Diode (RTD)
 Made by placing insulating barriers on a
semiconductor => creates island or
potential well between them
 Only finite number of discrete energy
levels are permitted in the island
 Electrons can pass through the island by
quantum tunneling
- If incoming electron energy matches (or
resonates) with an energy state inside the
island, then current flows through: “ON”
state
- If energy states inside and outside do not
match: “OFF” state
 Multiple logic states are possible
- As voltage bias is increased and
resonant states are established, switches
“ON. Then switches “OFF” and then
switches “ON” as soon as next level
energy states match

32. Spring 2004 CS-603 Nanotechnology 33
Molecular Electronic Devices
(…continued)
 Molecular Electronic Resonant Tunneling Diode
- Concept is similar to solid-state RTD
 Chains of Benzene ring act like conductive wires
- “CH2” (Methylene group) act as electron barriers
- Island or potential well formed between them
 Potential well in molecular RTDs is 10 to 100 times less than
solid-state RTDs

33. Resonant Tunnelling DiodeResonant Tunnelling Diode
 RTD allows voltage bias toRTD allows voltage bias to
switch “on” and “off” theswitch “on” and “off” the
current.current.
 Current passes equally wellCurrent passes equally well
in both directions.in both directions.
 Aliphatic groups with highAliphatic groups with high
P.E. establish aromatic ringP.E. establish aromatic ring
between them as narrowbetween them as narrow
“island” of lower P.E.“island” of lower P.E.
through which electronsthrough which electrons
must pass to traverse themust pass to traverse the
entire length of the wire.entire length of the wire.

34. Resonant Tunnelling Diode; OperationResonant Tunnelling Diode; Operation
 Smaller the region in which theSmaller the region in which the
electrons are confined, farther apartelectrons are confined, farther apart
are the allowed quantized energyare the allowed quantized energy
levels, eg. “island” and regions tolevels, eg. “island” and regions to
left and right of barrier.left and right of barrier.
 Electrons injected under bias intoElectrons injected under bias into
LUMO on LHS.LUMO on LHS.
 If the K.E. is’nt enough, noIf the K.E. is’nt enough, no
tunneling occurs; switched “off”.tunneling occurs; switched “off”.
 If bias is high enough, incomingIf bias is high enough, incoming
electron’s energy resonate withelectron’s energy resonate with
energy levels inside well, tunnelingenergy levels inside well, tunneling
ocuurs, etc.; switched “on”.ocuurs, etc.; switched “on”.
“Peak” to “valley” ratio ~1.3:1

35.  A negative differential resistance (NDR) is characterized by a
discontinuity in the monotonic increase of current as the
voltage is increased.
 Several of these devices can be combined to give I/V curves
with multiple peaks–this behavior has been proposed to lead
to multi-state memory and logic devices.
 Reed and Tour et al. reported the clearest example of
molecule-based NDR to date.
Negative Differential Resistance

36. At 60 K, assembly was found to display a very
strong NDR with a peak-to-valley
ratio (PVR) of 1030:1.
Control molecules (having no
nitro or amine moieties) showed no NDR.
In the singly reduced state, the LUMO
becomes fully delocalized, allowing
enhanced conduction, thus creating the
onset of the NDR peak. As the bias voltage
is increased the molecule becomes doubly
reduced, the LUMO becomes localized
across the molecule and decreases the
conductivity of the molecule, reducing the
current passed through the molecule.

37. Rotaxane: Molecular SwitchRotaxane: Molecular Switch
 Docking stations:Docking stations:
Benzidine andBenzidine and
Benzophenol.Benzophenol.
 Bulky stopper groups.Bulky stopper groups.
 Bead: tetracationicBead: tetracationic
cyclophane.cyclophane.
 Protonation/Oxidation:Protonation/Oxidation:
bead shifts tobead shifts to
benzophenolbenzophenol
 Molecular shuttleMolecular shuttle
switched electrostaticallyswitched electrostatically
Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed. 2002, 41, 4378-4440.

38. Rotaxane: Logic DeviceRotaxane: Logic Device
 ““Ring” and “Thread”Ring” and “Thread”
fluoresce separately.fluoresce separately.
 Upon threading (CTUpon threading (CT
complex), fluorescencecomplex), fluorescence
extinguished.extinguished.
 Addition of protons or baseAddition of protons or base
recovers the fluorescence.recovers the fluorescence.
 Neutralization removesNeutralization removes
fluorescence again.fluorescence again.
 If the fluorescence is takenIf the fluorescence is taken
as an indicator of truth, andas an indicator of truth, and
B and HB and H++
are taken as inputs,are taken as inputs,
then the system has thethen the system has the
same behavior as an XORsame behavior as an XOR
gate.gate.
Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed. 2002, 41, 4378-4440.

39. Rotaxane: Logic DeviceRotaxane: Logic Device
 Tetracationic cyclophaneTetracationic cyclophane
with two bipyridiniumwith two bipyridinium
units interlocked with aunits interlocked with a
crown ether containing acrown ether containing a
TTF and a NP unit onTTF and a NP unit on
opposite sides.opposite sides.
 TTF inside : ATTF inside : A00
 On oxidation, TTFOn oxidation, TTF
outside: Boutside: B++
 At 0 V , goes to BAt 0 V , goes to B00
 Bistability is the basis ofBistability is the basis of
the device.the device.
Stoddart et al. Science 2000, 289, 1172-1175.

40. Spring 2004 CS-603 Nanotechnology 41
Molecular Electronic Devices for Future Computers
 Molecular Electronics – Uses covalently bonded molecules to
act as wires and switching devices
- Molecules are natural nanometer-scale structures
E.g., A molecular switching device is only 1.5 nm wide!
 Molecular electronics will bring the ultimate revolution in
computing power
- 1 trillion switching devices on a single CPU chip!
- Terabyte level memory capacities!
 Primary advantage – can be synthesized in large numbers; in
the order of Avagadro’s number (1023
)
 Present day challenge is to develop methods to incorporate
these devices in circuits

41. Spring 2004 CS-603 Nanotechnology 42
Molecular Electronic Devices
(…continued)
 Spintronics
- Spintronics  Spin electronics  Magneto-electronics
- Discovered in 1988 by German and French physicists; IBM
commercialized the concept in 1997
- Exploits the “spin” of electrons, rather than “charge” in information
circuits
- Information is stored into spins as a particular spin orientation (up
or down)
- Spins, being attached to mobile electrons, carry the information
along a wire
 Spin orientation of electrons survive for a relatively longer time,
which makes Spintronic devices attractive for memory storage
devices in computers

42. 40 nm line width, 40 Gbit/inch2
HP Molecular Memory

43. Output:
Stored
Data
Input:
Address
Molecular Memory
MRAM
(Magnetic Random Access Memory)
Crossbar Memory
Architecture
DRAM
1
0

44. HP Molecular Memory
The blue ring can shuttle back
and forth along the axis of the
rotaxane molecule, between
the green and red groups.
Rotaxane molecules switch
between high and low resis-
tance by receiving a voltage
pulse.

45. Collier et al., Science 289, 1172 (2000).
(Many Molecules)
HP Molecular Memory
Change the resistance between
low and high by voltage pulses.
Is the resistance change really due
to the rotaxane ring shuttling back
and forth? Other molecules exhibit
the same kind of switching.
One possible model is the creation
and dissolution of metal filaments
which create a short between the
top and bottom electrodes. (Some-
thing like that happens in batteries).

46. Quantum
Dot
Molecular Switch
Self-Organizing Memory + Data Processor
Heath et al., Science
280, 1716 (1998)
People have been thinking about
how to combine memory with logic
(= a microprocessor) in a molecular
device.
Self-assembly is the preferred
method. It generates errors, though.
They need to be absorbed by a
fault-tolerant architecture (e.g. in the
HP Teramac)

47. “Conductivity”
of DNA
Berlin et al., Chem. Phys. 275, 61 (2002)
Tunneling at short distances (independent of temperature)
Hopping at large distances (thermally activated)

48. Molecular ConductivityMolecular Conductivity
 Electron Transfer:Electron Transfer:
 Coherent nonresonant tunneling :Coherent nonresonant tunneling :
Electronic states of the molecule are far from the energy of theElectronic states of the molecule are far from the energy of the
tunneling electrons; rate of electron transport exponentiallytunneling electrons; rate of electron transport exponentially
dependent on the length of the molecule.dependent on the length of the molecule.
 Coherent resonant tunnelingCoherent resonant tunneling
Energy of tunneling electrons resonant with the energy of theEnergy of tunneling electrons resonant with the energy of the
molecular orbitals’ rate of electron transport is essentiallymolecular orbitals’ rate of electron transport is essentially
independent of length .independent of length .

49. Spring 2004 CS-603 Nanotechnology 50
Snapshot of Active Research in Nano Devices
Nano
CMOS
RTDs SETs Molecular
Devices
MRAM Hard Drive

    
  
 
 

 
  

50. ConclusionConclusion
 Molecular electronics will mature into aMolecular electronics will mature into a
powerful technology only if its development ispowerful technology only if its development is
based on sound scientific conclusions thatbased on sound scientific conclusions that
have been tried and tested at every step.have been tried and tested at every step.
 Detailed understanding of theDetailed understanding of the
molecule/electrode interface, as well asmolecule/electrode interface, as well as
developing methods for manufacturing reliabledeveloping methods for manufacturing reliable
devices needed.devices needed.

51. Possibilities
ENDLESS!!!