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Molecular Electronics
From Wikipedia:
“Molecular Electronics (sometimes called moletronics)
involves the study and application of molecular
building blocks for the fabrication of electronic
components. This includes both passive and
active electronic components. Molecular
electronics is a branch of nanotechology.”
Wikipedia Continued
“An interdisciplinary pursuit, molecular electronics spans physics, chemistry,
and materials science. The unifying feature is the use of molecular building
blocks for the fabrication of electronic components. This includes both passive
(e.g. resistive wires) and active components such as transistors and molecularscale switches. Due to the prospect of size reduction in electronics
offered by molecular-level control of properties, molecular electronics has
aroused much excitement both in science fiction and among scientists.
Molecular electronics provides means to extend “Moore's Law” beyond
the foreseen limits of small-scale conventional silicon integrated circuits.
Molecular electronics is split into two related but separate subdisciplines:
1. Molecular materials for electronics utilizes the properties
of the molecules to affect the bulk properties of a material.
2. Molecular scale electronics focuses on single-molecule
applications.
Prophecies of the Future of Technology are Risky!! For
example:
“There is no reason anyone would want a computer in their home”.
--- Ken Olson, co-Founder, Digital Equipment Corporation (DEC)
“I think there is a world market for maybe
five computers.” - T.J. Watson, President &
CEO, IBM Corporation, 1941-1956
“640K ought to be enough for everybody.”
--- Bill Gates, co-Founder, Microsoft Corporation.
One of the wealthiest men in the world.
“There is not the slightest indication that nuclear energy will
ever be obtainable.”--- Albert Einstein, Nobel Laureate
& one of the greatest scientists who ever lived!
Source: Quantum Computing: “A short Course from Theory to
Experiment, Joachim Stolze and Dieter Suter.
Source: Quantum Computing. 2004. A Short Course from Theory to
Experiment. Joachim Stoltze and Dieter Stuter.
Moore’s “Law”
• The number of transistors that can be fabricated on a silicon integrated
circuit--and therefore the computing speed of such a circuit--is
doubling every 18 to 24 months.
• After four decades, solid-state microelectronics has advanced to the
point at which 100 million transistors, with feature size measuring 180
nm can be put onto a few square centimeters of silicon
Source: European Commission. Community Research.
2004. Nanotechnology. Innovation for tomorrow’s world.
Silicon and Moore’s Law
• Heat dissipation.
– At present, a state-of-the-art 500 MHz microprocessor with 10
million transistors emits almost 100 watts--more heat than a stovetop cooking surface.
• Leakage from one device to another.
– The band structure in silicon provides a wide range of allowable
electron energies. Some electrons can gain sufficient energy to hop
from one device to another, especially when they are closely
packed.
• Capacitive coupling between components.
• Fabrication methods (Photolithography).
– Device size is limited by diffraction to about one half the
wavelength of the light used in the lithographic process.
• ‘Silicon Wall.’
– At 50 nm and smaller it is not possible to dope silicon uniformly.
(This is the end of the line for bulk behavior.)
X 1000$
Moore’s “Second Law"
generation
Plant cost
Mask cost
Silicon and Moore’s Law
• Moore’s second law.
– Continued exponential decrease in silicon device size is
achieved by exponential increase in financial
investment. $200 billion for a fabrication facility by
2015.
• Transistor densities achievable under the present and
foreseeable silicon format are not sufficient to allow
microprocessors to do the things imagined for them.
Electronics Development Strategies
• Top-Down.
– Continued reduction in size of bulk semiconductor
devices.
• Bottom-up (Molecular Scale Electronics).
– Design of molecules with specific electronic function.
– Design of molecules for self assembly into
supramolecular structures with specific electronic
function.
– Connecting molecules to the macroscopic world.
Bottom-Up (Why Molecules?)
• Molecules are small.
– With transistor size at 180 nm on a side, molecules are some 30,000 times
smaller.
• Electrons are confined in molecules.
– Whereas electrons moving in silicon have many possible energies that will
facilitate jumping from device to device, electron energies in molecules
and atoms are quantized - there is a discrete number of allowable energies.
• Molecules have extended pi systems.
– Provides thermodynamically favorable electron conduit - molecules act as
wires.
• Molecules are flexible.
– pi conjugation and therefore conduction can be switched on and off by
changing molecular conformation providing potential control over
electron flow.
•
Molecules are identical.
– Can be fabricated defect-free in enormous numbers.
• Some molecules can self-assemble.
– Can create large arrays of identical devices.
Molecules as Electronic Devices:
Historical Perspective
• 1950’s: Inorganic Semiconductors
• To make p-doped material, one dopes Group IV
(14) elements (Silicon, Germanium) with electronpoor Group III elements (Aluminum, Gallium,
Indium)
• To make n-doped material, one uses electron-rich
dopants such as the Group V elements nitrogen,
phosphorus, arsenic.
Molecules as Electronic Devices:
Historical Perspective
• 1960’s: Organic Equivalents.
– Inorganic semiconductors have their organic molecular
counterparts. Molecules can be designed so as to be electronrich donors (D) or electron-poor acceptors (A).
– Joining micron-thick films of D and A yields an organic
rectifier (unidirectional current) that is equivalent to an
inorganic pn rectifier.
– Organic charge-transfer crystals and conducting polymers
yielded organic equivalents of a variety of inorganic
electronic systems: semiconductors, metals,
superconductors, batteries, etc.
• BUT: they weren’t as good as the inorganic standards.
– more expensive
– less efficient
Molecules as Electronic Devices:
Historical Perspective
•1970’s: Single Molecule Devices?
• In the 1970’s organic synthetic
techniques start to grow up
prompting the idea that device
function can be combined into a
single molecule.
• Aviram and Ratner suggest a
molecular scale rectifier. (Chem.
Phys. Lett. 1974)
• But, no consideration as to how
this molecule would be
incorporated into a circuit or
device.
Molecules as Electronic Devices:
Historical Perspective
•1980’s
Single Molecule Detection.
How to image at the molecular level.
How to manipulate at the molecular
level.
• Scanning Probe Microsopy.
STM (IBM Switzerland, 1984)
AFM
Molecules as Electronic Devices:
Historical Perspective
1990’s: Single Molecule Devices.
• New imaging and manipulation techniques
• Advanced synthetic and characterization techniques
• Advances in Self-Assembly »» Macroscopic/Supramolecular Chemistry
These developments have finally allowed scientists to address the
question:
“How can molecules be synthesized and assembled into
structures that function in the same way as solid state silicon
electronic devices and how can these structures be integrated
with the macroscopic regime?”
Molecular Wire
Molecular Junction
Mechanically-Controlled Break Junction
Resistance is a few megohms.
(Schottky Barrier)
Resonant Tunneling Diode
Alkyl Tunnel Barriers
Conduction between the two ends of the molecule
depends on pi orbital overlap which in turn relies
on a planar arrangement of the phenyl rings.
Negative Differential Resistance
mNDR = molecular Negative Differential Resistance
Measured using a conducting AFM tip
One electron reduction provides a charge carrier.
A second reduction blocks conduction.
Therefore, conduction occurs only between the
two reduction potentials.
Voltage-Driven
Conductivity Switch
Applied perpendicular field favors
zwitterionic structure which is planar
Better pi overlap, better conductivity.
Dynamic Random Access
Memory
Voltage pulse yields
high conductivity
State - data bit stored
Bit is read as high
in low voltage region
Voltage-Driven
Conductivity Switch
Device is fabricated by sandwiching a layer
of catenane between an polycrystalline layer of n-doped
silicon electrode and a metal electrode. The switch is
opened at +2 V, closed at -2 V and read at 0.1 V.
Voltage-Driven
Conductivity Switch
High/Low Conductivity Switching Devices
Respond to I/V Changes
Voltage-Driven
Conductivity Switch
n-type
Molecular Wire Crossbar Interconnect
(MWCB)
Carbon Nanotubes
Gentle contact needed
Nanotube conductivity is quantized.
Nanotubes found to conduct current ballistically and do not dissipate heat.
Nanotubes are typically 15 nanometers wide and 4 micrometers long.
Molecular Self-Assembly
•
Self-Assembly on Metals
– (e.g., organo-sulfur compounds on
gold)
•
Assembly Langmuir-Blodgett Films
– Requires amphiphilic groups for
assembly
•
Carbon Nanotubes
– Controlling structure
Cyclic Peptide Nanotubes as Scaffolds for Conducting Devices
Hydrogen-bonding interactions promote stacking of cyclic peptides
Pi-systems stack face-to-face to allow conduction along the length of the tube
Cooper and McGimpsey - to be submitted
CYCLIC BIOSYSTEMS
Spontaneous self-directed chemical growth allowing
parallel fabrication of identical complex functional structures.
Molecular Electronics:
Measuring single molecule conduction
Nanopore
Cross-wire
STM Break Junction
Scanning Probe
Cui et al.
Science 294
(2001) 571
Wang et al.
PRB 68 (2003) 035416
Kushmerick et al. PRL 89
(2002) 086802
Electromigration
H. S. J. van der Zant et al.
Faraday Discuss. (2006)
131, 347
B. Xu & N. J. Tao Science (2003) 301, 1221
Nanocluster
Mechanical Break
Junction
Dadosh et al. Nature
436 (2005) 677
Reichert et al. PRL 88
176804
Single-Molecule Conductivity
L
ELECTRODE
MOLECULE
R
ELECTRODE
L
ELECTRODE
MOLECULE
R
ELECTRODE
Molecular
Orbitals
Fermi
energy
L
ELECTRODE
MOLECULE
R
ELECTRODE
Molecular
Orbitals
eV
I
V
I
Finding a true molecular signature:
Inelastic Electron Tunnelling Spectroscopy (IETS)
Inelastic
h/e V
2
h/e
2
V
d I/dV
Elastic
dI/dV
h/e V
h/e V
Molecular level structure between electrodes
energy
LUMO
HOMO
Cui et al (Lindsay),
Science 294, 571 (2001)
“The resistance of a single
octanedithiol molecule was 900  50
megaohms, based on measurements
on more than 1000 single molecules.
In contrast, nonbonded contacts to
octanethiol monolayers were at least
four orders of magnitude more
resistive, less reproducible, and had
a different voltage dependence,
demonstrating that the measurement
of intrinsic molecular properties
requires chemically bonded
contacts”.
6
I / arb. units
I
Ratner and
Troisi, 2004
5
4
3
2
1
0
-1
-1
- 0.5
-0.5
0.0
0
0.5
0.5
1
V (V)
Dynamics of current voltage
switching response of single
bipyridyl-dinitro oligophenylene
ethynylene dithiol (BPDN-DT)
molecules between gold contacts.
In A and B the voltage is changed
relatively slowly and bistability
give rise to telegraphic switching
noise. When voltage changes
more rapidly (C) bistability is
manifested by hysteretic behavior
Lortscher et al (Riel), Small, 2, 973 (2006)
Switching with light
Chem. Commun., 2006, 3597 - 3599, DOI: 10.1039/b609119a
Uni- and bi-directional light-induced switching of
diarylethenes on gold nanoparticles
Tibor Kudernac, Sense Jan van der Molen, Bart J. van Wees and Ben L.
Feringa
“In conclusion, photochromic
behavior of diarylethenes
directly linked to gold nanoparticles
via an aromatic spacer has
been investigated. Depending on the
spacer, uni- (3) or bidirectionality
(1,2) has been observed.”
Nanotechnology 16 (2005) 695–702
Switching of a photochromic molecule on gold electrodes: single-molecule
measurements
J. He, F. Chen, P. Liddell, J. Andr´easson, S D Straight, D. Gust, T. A. Moore,
A. L. Moore, J. Li, O. F Sankey and S. M. Lindsay
Current–voltage data (open circles) for (a) open
molecules 1o and (b) closed molecules 1c
Temperature and chain length dependence
MichelBeyerle
et al
Selzer
et al
2004
Giese
et al,
2002
Xue
and
Ratner
2003
Electron transfer in DNA
DNA-news-1
DNA-news-4
DNA-news-2
Conjugated vs. Saturated Molecules: Importance of Contact
Bonding S
S
S
S
Au//
S/Au
Au/S
S/Au
Kushmerick
et al., PRL
(2002)
negative bias
Positive bias
2- vs. 1-side Au-S
bonded conjugated
system gives at most 1
order of magnitude
current increase
compared to 3 orders for
C alkanes!
Au/S(CH2)8SAu
Au//CH3(CH2)7S/Au
Lindsay & Ratner 2007
Where does the potential bias falls, and how?
•Image effect
•Electron-electron interaction (on the Hartree level)
Vacuum
Excess electron
density

L
Xue, Ratner
(2003)
Potential
profile
Galperin et al
JCP 2003
Galperin et al
2003
Experiment
Theoretical Model
Experimental i/V behavior
Experimental (Sek&Majda)
junction
aCurrent
Ratio of current:
i(-1.0 V)/i(+1.0 V)a
Hg-SC12/C12S-Au
0.98  0.13
Hg-SC12/C10S-Au
1.03  0.07
Hg-SC16/C12S-Au
1.22  0.16
Hg-SC12/C9S-Au
1.44  0.20
Hg-SC16/C10S-Au
1.34  0.19
Hg-SC16/C9S-Au
2.03  0.27
at the negative bias refers to the measurement with the
Hg side of the junction biased negative relative to the Au side.
Cui et al (Science 2001):
The sulfur atoms (red dots) of
octanethiols bind to a sheet of
gold atoms (yellow dots), and the
octyl chains (black dots) form a
monolayer. The second sulfur
atom of a 1,8-octanedithiol
molecule inserted into the
monolayer binds to a gold
nanoparticle, which in turn is
contacted by the gold tip of the
conducting AFM.
J. G. Kushmerick et al., Nano Lett. 3, 897
(2003). A. S. Blum, J. G. Kushmerick, et al.,
The J. Phys. Chem. B 108, 18124 (2004).
Red – single molecule; black –
molecular layer. Dashed black
is molecular layer per molecule
1-nitro-2,5-di(phenylethynyl4’-mercapto)benzene
Red – single molecule; black –
molecular layer per molecule
Y. Selzer et al., Nano Letters 5, 61
(2005).
Resonant tunneling?
V(x)
|1>
L
R
....
|0>
x
r
l 
1
{l }
V1l
V1r
Carbon Nano Tubes (CNT)
Issues:
•Production of Single Walled CNTs yield a mixture of
types (dimensions to less than 1nm)
•
•
Metallic
Semiconductive
•Separation of types is time consuming
Benefits:
•Novel electronic devices
•High temperature applications
•Improved microscopy
Potential Solutions
•Continue development efforts
Solar Cells (Organic)
Issues:
•Efficiencies
•Material development
•Manufacturing processes
Potential Solutions
•Development of organic plastics with improved
efficiency
•Development of adsorptive dyes
•Flexible conductors
•Enhanced property covering material
Benefits:
•Low cost energy
•Inexpensive to manufacture yielding to wide
spread applications
Credit: Nicole Cappello and the Georgia Institute of Technology
New Material Properties
Issues:
•Unanticipated properties are being found in nano
materials – Example:
•
•
Potential Solutions:
•Quantify and classify the material properties in the
range between bulk material properties and quantum
phenomena
•Establish a program to employ theoretical projections
to verify experimental data
Thirteen atoms of Silver have been
shown theoretically to be magnetic
Thirteen atoms of Platinum have
been experimentally shown to be
magnetic
Benefits:
•Improve the time to develop nano based devices, due
to eliminating the duplication of research efforts
•Creation of new products based on applying novel
nano properties
Example: The creation of new memory devices that
are 100x more dense than current technology
Silver properties reported May 30, 2006 in NanoTechWeb
Platinum experiments reported by University of Stuttgart
Metrology
Au dot structure
&
Nanowire Twinning
Potential Solutions:
•New solutions for metrology
•Enhancements to equipment
•New technologies
Aberration Corrected HR-TEM
Korgel Group Si Nanowire
Issues:
•Imaging realm is at limits of resolution, in the
1nm range
•Time per image is long >one hour
•Effective imaging applications require multiple
images in minutes or less
Benefits:
•Improved resolution of material properties
•Capability to employ in manufacturing processes
•If one can not measure something, it can not be
manufactured
Metrology
Aberration Corrected TEM Imaging
Corrected
Not corrected
K & I in nanotube
Potential Solutions
•Development and execution of validation plan
•Improved algorithms
•Improved equipment for rapid imaging
Sloan, et al., MRS Bulletin, April 2004
Issues:
•Imaging is slow and computations are time
consuming
•Unique structures can not be verified
•No validation results
•Dimensions extend to below 1nm
Benefits:
•Improved understanding of materials
•Ability to identify unique nano structures
•Ability to create and verify novel materials
Proposal for Molecular Computers
Nanotechnology
+ cheap
+ high-density
+ low-power
– unreliable
Reconfigurable
Computing
+ defect tolerant
+ high performance
– low density
_
_
_
++++
+ +
_
Computer architecture
+ vast body of knowledge
– expensive
– high-power
Reconfigurable Computing
• Back to ENIAC-style computing
• Synthesize one machine to solve one
problem
Defect Tolerance
Despite having >70% of the chips
defective, Teramac works
flawlessly.
Compilation has two phases:
• defect detection through
self-testing
• placement for
defect-avoidance
Single-walled Carbon Nanotube
d
d = 0.4nm - 10nm
L=?
L
Lattice of covalently bonded
carbon atoms
Nano-wires
• carbon nanotubues, Si, metal
• >2nm diameter, up to mm length
• excellent electrical properties
A carbon nanotube: one molecule
Independent Claims
1. A transistor that uses a carbon nanotube ring as a
semiconductor material, the carbon nanotube ring having
semiconductor characteristics.
12. A transistor that uses a carbon nanotube ring as an
electrode material, the carbon nanotube ring having
conductivity or semiconductor characteristics.
18. A carbon nanotube ring having p-type semiconductor
characteristics.
19. A semiconductor device in which a carbon nanotube
ring having p-type semiconductor characteristics is placed
on an n-type semiconductor substrate thereof.
Nanotechnology in Electronics
Alternatives for transistors
 Carbon nanotube transistors
 Single electron transistors (SET)
Memory devices
 MRAM (various different approaches
 Phase change RAM
Photonics
Nano-electromechanical system (NEMS)
Fuel cells
Thermo-photovoltaics
Quantum computers
Software
Nano-switch
Nano-switch Between
Nano-wires
Self-assembly