Topics Molecular Electronics Photonic Crystals Spintronics* *Sorry, there is no time for this!
Molecular Electronics Artist’s Depiction of a Long Molecule Between Metal Contacts
Molecular Electronics From Wikipedia: “Molecular Electronics (sometimes calledmoletronics)involves the study & application of molecular building blocks for the fabrication of electronic components. This includes both passive & 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 molecular-scale 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, Even if Made by Very Intelligent, Educated people!! For example consider some “memorable quotes” from perhaps The Greatest Scientist of the 19th Century: Lord Kelvin (William Thompson) Typical “Fortune Teller” or “Psychic" • “Heavier-than-air flying machines are impossible.”(1895) • “I have not the smallest molecule of faith in aerial navigationother than ballooning...I do not care to be a member of the Aeronautical Society.” (1896) • “There is nothing new to be discovered in physics. • All that remains is more precise measurement.”(1900)
Another 19th Century Example “Everything that can be invented has been invented.” Charles H. Duell, Commissioner U.S. Office of Patents, 1899
More examples of the risk in predicting the future: Some quotes from the 20th Century: More Prophecies of the Future of Technology! “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! “I think there is a world market for maybe five computers.” T.J. Watson, President & CEO, IBM Corporation, 1941-1956 “There is no reason anyone would want a computer in their home.” Ken Olson, co-Founder, Digital Equipment Corporation (DEC) “640K of computer memory ought to be enough for everybody!” Bill Gates, co-Founder, Microsoft Corporation. One of the wealthiest men in the world.
More 20th Century Examples “Computers in the future may weigh no more than 1.5 tons.” The Magazine Popular Mechanics, 1949 “I have traveled the length and breadth of this country and talked with the best people, and I can assure you that data processing is a fad that won't last out the year.” Business book editor, Prentice Hall, 1957
More 20th Century Examples “I believe OS/2 is destined to be the most important operating system, and possibly program, of all time”. Bill Gates, 1987 “.. and it probably never will support anything other than AT-hard disks, ..” Linus Torvalds in his Linux release note from August 26, 1991
More 20th Century Examples “Windows NT addresses 2 Gigabytes of RAM, which is more than any application will ever need. .” Microsoft on the development of Windows NT, 1992 “.. and it probably never will support anything other than AT-hard disks, ..” Linus Torvalds in his Linux release note from August 26, 1991
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 4 decades, solid-state microelectronics has advanced to the point at which more than 100 million transistors, with feature size around 120 nm can be put onto a few square centimeters of silicon.
Smaller, Denser, Cheaper Electronics Moore’s “Law” (1965) Every 1.5 years the number of transistors on a chip is doubled. Does this mean that there could be a transistor the size of a single-atom by 2020?
Silicon & Moore’s Law: Practical Problems • Heat dissipation. • At present, a state-of-the-art a 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 & smaller it’s not possible to dope silicon uniformly. Conclusion: This is the end of the line for bulk behavior!!
Related to Moore’s “Law” isMoore’s “Second Law.”Moore’s 2nd “Law” is a financial “law”! X 1000$ Plant Cost Mask Cost Billions of Dollars!!
Silicon & Moore’s Law Moore’s “Second Law” • Continued exponential decrease in silicon device size is achieved by a continuing exponential increase in financial investment. An estimated cost for a fabrication facility by 2015 is $200 billion!!!!! • In addition, transistor densities achievable under the present & foreseeable silicon format are not sufficient to allow microprocessors to do the things imagined for them.
Nearing the End of Moore's Law • So far, history has proved Gordon Moore more or less right. But transistor growth may soon slow for a number of reasons: • Difficulties to contend with the heat produced and power consumed by transistor-crammed chips • Photolithography as we know it is expected to reach its ultimate limits before 2020 • Chip voltages cannot be reduced forever
The Limits of Silicon Technology Still in 2002 Intel's chief technology officer Pat Gelsinger said, "We're on track, by 2010, for 30-gigahertz devices, 10 nanometers or less, delivering a tera-instruction of performance." But Gelsinger was wrong. By 2010 Intel and its competitors were making processors that topped out at less than four gigahertz, and 22 nm had only reached the design lab.
Hope for Moore's Law in New Technologies? Miniaturization of integrated circuits based on photolithography may soon come to an end. However, new technologies have emerged that may push miniaturization to the nanoscale. On the forefront are memristors and graphene. In 2011 the first quantum computer was announced and shortly after researchers at IBM presented the first graphene integrated circuit. The graphene IC has only one transistor (framed in the bottom picture) and two coils, but it operates at 10 GHz. Image source: IBM
Single-Atom Transistor Miniaturization reached its ultimate limit in February 2012, when scientists at University of New South Wales, Australia, reported having created a single-atom transistor – at least under the eye of a scanning tunneling microscope. It is a phosphorous atom that has replaced a silicon atom in a group of six, and which acts as a switch when a voltage is applied. Ok, it might take a while before this device finds its way to computer stores. Image source: UNSW/Sydney Morning Herald
Death to Moore's Law! Despite impressive progress in hardware technology, there are alternatives that should be considered. Steve Wozniak, the inventor of the Apple II, once said: "The repeal of Moore's Law would create a renaissance for software development. Only then will we finally be able to create software that will run on a stable and enduring platform."
Finally: An Accurate Statement ”Bill Gates is a very rich man today ... and do you want to know why? The answer is one word: versions.” Dave Barry Welcome Windows 1, 2, 3, NT, 95, 98, 2000, ME, Xp, Vista, 7, 8. We’re so happy to pay for all of you!
Electronics Development Strategies • Top-Down • Continued reduction in size of bulk semiconductor devices. • Bottom-Up • Molecular Scale Electronics • Design of molecules with specific electronic functions. • Design of molecules for self assembly into supramolecular structures with specific electronic functions. • 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 DevicesHistorical Perspective 1950’s: Inorganic Semiconductors • To make p-doped material, one dopes Group IV (14) elements (Si, Ge) with electron-poor Group III elements (Al, Ga, In) • To make n-doped material, one uses electron-rich dopants such as the Group V elements N, P, As. 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: Organic semiconductors weren’t as good as the inorganic standards (more expensive & less efficient)
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
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 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
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.
Nanopore Scanning Probe STM Break Junction Cross-wire Cui et al. Science 294 (2001) 571 Wang et al. PRB 68 (2003) 035416 Kushmerick et al. PRL 89 (2002) 086802 B. Xu & N. J. Tao Science (2003) 301, 1221 Electromigration H. S. J. van der Zant et al. Faraday Discuss. (2006) 131, 347 Molecular Electronics:Measuring single molecule conduction Nanocluster Mechanical Break Junction Dadosh et al. Nature 436 (2005) 677 Reichert et al. PRL 88 176804
Single-Molecule Conductivity L ELECTRODE R ELECTRODE MOLECULE
L ELECTRODE R ELECTRODE MOLECULE Molecular Orbitals Fermi energy
L ELECTRODE R ELECTRODE MOLECULE Molecular Orbitals eV V I
Finding a true molecular signature:Inelastic Electron Tunnelling Spectroscopy (IETS) I h/e V Elastic h/e V dI/dV Inelastic h/e V d2I/dV2 h/e V