Molecular Electronics
Past, Present, Future?
W. Grant McGimpsey
Professor, Department of Chemistry and Biochemistry
Worcester Polytechnic Institute
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
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 stove-top 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.)
Silicon and Moore’s
•
2
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.
Top-Down
From the Wall Street Journal, February, 2002
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 electron-poor 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 electron-rich 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.
The Corporate World at Present
•
Strong Academic Alliances and Backgrounds
•
Molecular Electronics (Tour and Reed)
– Switches, mDRAM
•
California Molecular Electronics CALMEC
– Ratner, Metzger, Mirkin, Michl, etc
– Chiroptycene switches
•
ZettaCore
– NC State/UC Riverside alliance
– Porphyrin wires
The Big Corporate Player
•
Hewlett-Packard
– Stan Williams, Phil Kuekes (HP)
– Jim Heath, Fraser Stoddard (UCLA)
– Circuits!
The Big Issues
•
Commercialization and the ‘DOS/Windows’ Paradigm
– Marketing and Science must proceed in lock-step
•
‘The Single Molecule Holy Grail’
– Individual molecules versus ensembles
– Stochastic behavior of individual molecules
– Connectivity
– Defect tolerance
•
Molecular Self-Assembly of Devices
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.