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.