Molecular ElectronicsPast, 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 Law2

• 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.

• 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

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.

•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?”

Molecules as Electronic Devices: Historical Perspective

Molecular Wire

Mechanically-Controlled Break Junction

Resistance is a few megohms. (Schottky Barrier)

Molecular Junction

Alkyl Tunnel Barriers

Conduction between the two ends of the moleculedepends on pi orbital overlap which in turn relies on a planar arrangement of the phenyl rings.

Resonant Tunneling Diode

mNDR = molecular Negative Differential ResistanceMeasured using a conducting AFM tip

Negative Differential Resistance

One electron reduction provides a charge carrier. A second reduction blocks conduction. Therefore, conduction occurs only between the two reduction potentials.

Voltage-DrivenConductivity Switch

Applied perpendicular field favorszwitterionic structure which is planarBetter pi overlap, better conductivity.

Dynamic Random Access Memory

Voltage pulse yieldshigh conductivityState - data bit stored

Bit is read as highin low voltage region

Device is fabricated by sandwiching a layerof catenane between an polycrystalline layer of n-dopedsilicon electrode and a metal electrode. The switch isopened at +2 V, closed at -2 V and read at 0.1 V.

Voltage-Driven Conductivity Switch

High/Low Conductivity Switching DevicesRespond to I/V Changes

Voltage-Driven Conductivity Switch

n-type

Voltage-Driven Conductivity Switch

Molecular Wire Crossbar Interconnect(MWCB)

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.

Carbon Nanotubes

Gentle contact needed

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 DevicesHydrogen-bonding interactions promote stacking of cyclic peptidesPi-systems stack face-to-face to allow conduction along the length of the tube

Cooper and McGimpsey - to be submittedCYCLIC BIOSYSTEMS

Spontaneous self-directed chemical growth allowing parallel fabrication of identical complex functional structures.