20-10-2010, 09:01 AM
Abstract
Researchers Assemble Building Blocks of Nanocomputers
In a flurry of new research, scientists have begun to assemble the tiniest electronic elements into simple logic circuits—the building blocks of the electronic mazes that power computers. Three independent groups of scientists have worked on this and come up with fascinating results.
Chemist Charles Lieber and co-workers at Harvard University created simple logic circuits incorporating up to six transistors by crisscrossing nanometer-wide wires of silicon and gallium-nitride, each junction of which forms a transistor. This technique works by catalyzing the growth of the crystal wires from solutions of each material with the assistance of a laser.
Physicist Adrian Bachtold and colleagues at Delft University of Technology in the Netherlands carved aluminum strips from a layer of the metal and deposited carbon nanotubes on top. They then attached strips of gold to both ends of each nanotube, creating a transistor, and linked up to three such devices in various ways to make circuits that would execute simple logical functions: flipping a signal from off to on or vice versa, turning two off signals into an on, storing a unit of information or creating an oscillating signal.
Physicist Jan Hendrik Schön, with help from other researchers at Bell Laboratories, has refined a technique he recently described for making transistors out of a layer of small carbon molecules. Diluting these transistor molecules with insulating carbon chains, Schön found that just one was enough to turn a signal on or off, making a rudimentary circuit element.
However scientists still have to reduce the complete circuit to molecular size. But the fact that several groups have assembled basic circuits from molecule-scale parts is an indicator of how far molecular electronics and nanotechnology have come and is very encouraging for the future.
Quantum Computers
A quantum computer - a new kind of computer far more powerful than any that currently exist - could be made today say Thaddeus Ladd of Stanford University, Kohei Itoh of Keio University in Japan, and their co-workers. They have sketched a blueprint for a silicon quantum computer that could be built using current fabrication and measurement techniques.
Quantum and conventional computers encode, store and manipulate information as sequences of binary digits, or bits, denoted as 1s and 0s. In a normal computer, each bit is a switch, which can be either 'on' or 'off'.
In a quantum computer, switches can be on, off or in a superposition of states - on and off at the same time. These extra configurations mean that quantum bits, or qubits, can encode more information than classical switches.
Single Electron Memory
The single-electron memory is the latest development in the field of microelectronics called ‘single electronics’, in which electrons are shunted around circuits one by one like strollers through a maze. The electrons pass through turnstiles, hop between resting places, and as they go on their way they flip switches and perform ‘logic operations’ just like the electrical currents in ordinary computers. The difference is that the currents are minute so very little power is consumed. And because the electrons pass through the system one at a time, the electrical current is ‘granular’ -- like a trickle of sand from an hourglass, rather than a stream of liquid gushing through a lock gate.
This low-power, low-heat ‘granular’ electronics would manipulate information, not in the form of electrical pulses representing the binary digits ‘1’ and ‘0’ that encode information in today’s computers, but instead by using single electrons to represent a ‘bit’ of information. That is to say, the presence of an electron in a channel would signify a ‘1’, the absence a ‘0’.
IBM’s Molecular Computer
IBM researchers have built and operated the world's smallest working computer circuits in which individual molecules move across an atomic surface like toppling dominoes.
The new "molecule cascade" technique makes the digital-logic elements some 260,000 times smaller than those used in today's most advanced semiconductor chips.
The circuits were made by creating a precise pattern of carbon monoxide molecules on a copper surface. Moving a single molecule initiates a cascade of molecule motions, just as toppling a single domino can cause a large pattern to fall in sequence. The scientists then designed and created tiny structures that demonstrated the fundamental digital-logic OR and AND functions, data storage and retrieval, and the "wiring" necessary to connect them into functioning computing circuitry.
The most complex circuit they built -- a 12 x 17-nanometer three-input sorter -- is so small that 190 billion could fit atop a standard pencil-top eraser.
IBM's molecule cascade works because carbon monoxide molecules can be arranged on a copper surface in an energetically meta-stable configuration that can be triggered to cascade into a lower energy configuration, just as with toppling dominoes. The meta-stability is due to the weak repulsion between carbon monoxide molecules placed only one lattice spacing apart. What enables computation is that each cascade carries a single bit of information. By analogy, a toppled domino can be thought of as a logical "1," and an untoppled domino can be thought of as a logical "0." Similarly, a cascaded or non-cascaded molecular array can represent a logical "1" or "0," respectively.
Since there is no reset mechanism, these molecule cascades can only perform a calculation once.
Carbon Nanotubes
One of the biggest discoveries to have aided the progress of electronics at the nano scale is that of carbon nanotubes. These are tiny tubular structures composed of a single layer of carbon atoms. Discovered in 1991 by Sumio Iijima of NEC Corporation, carbon nanotubes are an exotic variation of common graphite. The tubular structure imparts a number of mechanical and electronic properties that include super strength, combined with low weight, stability, flexibility, good heat conductance, large surface area and a host of intriguing electronic properties.
Carbon nanotubes are descendants of buckminsterfullerene, or "buckyball," the soccer-ball-shape molecule of 60 carbon atoms. It has been discovered that if a row of hexagons going down the tube's long axis were straight, the tube would behave as a metal and conduct electricity. If a line of hexagons formed a helix, however, the tube would act as a semiconductor. This gives rise to a wide variety of electronic applications in which the tubes can be used.
(To understand this completely refer our previous article on nano's like Nanotechnology )
Carbon Nanotubes Could Lengthen Battery Life
Carbon nanotubes could lengthen the life of batteries, according to new research. Recent findings suggest that the diminutive tubes can hold twice as much energy as graphite, the form of carbon currently used as an electrode in many rechargeable lithium batteries.
Conventional graphite electrodes can reversibly store one lithium ion for every six carbon atoms. By experiment it has been found that the tiny straws of carbon nanotubes manage to reversibly store one charged ion for every three carbon atoms. In explanation, the scientists note that the tubes' open ends facilitate the diffusion of lithium atoms into their interiors.
Carbon Nanotubes Could Serve as Ultrafast Oscillators
Carbon nanotubes are extremely small, measuring a few billionths of a meter in width. According to scientists these straws nestled inside one another with the inner set of tubes sliding in and out a billion times a second could constitute a gigahertz oscillator.
Scientists from the University of California have come up with the conclusion that if the inner core were pulled out of such a tube, it would not only retract back into the center of the tube, but it would also continue right out the other end. Nearly negligible friction between the tubes would enable a breakneck gigahertz oscillation frequency. And shorter tubes could move at even greater speeds.
However researchers have not yet figured out how exactly to excite the oscillator and how to couple it with the rest of the nanoscopic device. The actual implementation of this coupling represents another challenge in development.
Researchers Fashion the First Single Molecule Circuit
nano ckt.
In a remarkable feat of engineering, researchers at IBM have wired up a working computer circuit within a single carbon nanotube. Building on earlier work, Phaedon Avouris and colleagues turned the nanotube—essentially a sheet of carbon atoms rolled into a supertiny straw—into a voltage inverter, or NOT gate, one of the three fundamental types of logic gates on which all computers rely. FIG below shows a view of the circuit that was fabricated
Of importance, the current in the carbon nanotube NOT gate comes out stronger than it goes in—a necessary criterion for any circuit design. And because this gain is by as much as a factor of 1.6, researchers believe that more complex single-nanotube circuits will be possible.
Nanotube 'Peapods' Exhibit Surprising Electronic Properties
peapods
In yet another small step toward building nanoscale devices, scientists have determined that nanotube peapods—minute straws of carbon filled with spherical carbon molecules known as buckyballs—have tunable electronic properties. Recent findings
suggest that stuffing the straws provide greater control over the electronic states of single-walled carbon nanotubes (SWNT).
Using a low-temperature scanning tunneling microscope, Ali Yazdani of the University of Illinois at Urbana-Champaign and colleagues imaged the physical structure of individual peapods (FIG ). They mapped the motion of electrons within the pipes and, as Yazdani explains, showed "that an ordered array of encapsulated molecules can be used to engineer electron motion inside nanotubes in a predictable way." Though the harbored buckyballs modify the electronic properties of the nanotube, the atomic structure of the straw remains unchanged
The researchers also utilized the microscope to move the buckyballs, which allowed them to compare the same section of a SWNT when it was filled and unfilled. "The encapsulated balls have a much stronger effect on the electronic structure of the tube than we had expected," says study co-author Eugene Mele of the University of Pennsylvania. Indeed, the authors conclude that their calculation not only shows how a peapod's electronic properties differ from those of its constituent parts, "it also provides possible design rules for proposing hybrid structures having a specific type of electronic functionality."
In addition to those listed some possible uses of carbon nanotubes in the future are:
• Field emitter for flat panel displays.
• Cellular phone signal amplifier.
• Ion storage for batteries.
• Materials strengthener.
In the future, these tubes could well replace silicon. Thus carbon nanotubes are of great importance in the field of nanoelectronics.
One nano step toward efficient LED lighting
Engineers at Kopin Corp. (Taunton, MA) are using nanotechnology (patent pending as NanoPockets) to produce "CyberLites" — blue light-emitting diodes (LEDs) smaller than a grain of sand . The new LEDs are as bright as 3.3V commercially available devices, yet can be driven by <2.9V (using 20mA of current) and still have 100 mC brightness. In addition, CyberLites have achieved ESD resistance >4000V compared to ~2000V resistance with commercially available LEDs; high ESD resistance is critical for industrial applications.
This work was done in cooperation with Jagdish Narayan of North Carolina State University and director of the NSF Center of Advanced Materials and Smart Structures. FIG 5 shows a ‘CyberLite’ on a US dime.NanoPockets is based on Kopin's patented wafer engineering process, already being used by the company for displays and HBT transistors; this process significantly reduces the number of natural atomic level defects when different semiconductor materials are combined. CyberLites are fabricated on gallium nitride grown — via organometallic chemical vapor deposition — on low-cost aluminum oxide. The process provides confinements ("NanoPockets") for production of light away from defects. The nanostructures, which are naturally formed as a result of internal strains, are spaced less than the separation of material defects, such as dislocations.
A blue CyberLite can be combined with yellow phosphor to create a white LED. Blue and white CyberLites are ideal for compact battery-powered portable light-using devices, such as wireless phones, games, camcorders, cameras and laptops.
Nano Solar Cells
nano solar cell :fraunhofer.
Paul Alivisatos, a chemist at the University of California, Berkeley, has an idea in which he aims to use nanotechnology to produce a photovoltaic material that can be spread like plastic wrap or paint.
His approach begins with electrically conductive polymers. To improve the efficiency, Alivisatos is adding a new ingredient to the polymer: nanorods, bar-shaped semiconducting inorganic crystals measuring just seven nanometers by 60 nanometers. The result is a cheap and flexible material that could provide the same kind of efficiency achieved with silicon solar cells. The prototype solar cells he has made so far consist of sheets of a nanorod-polymer composite just 200 nanometers thick. Thin layers of an electrode sandwich the composite sheets. When sunlight hits the sheets, they absorb photons, exciting electrons in the polymer and the nanorods, which make up 90 percent of the composite. The result is a useful current that is carried away by the electrodes. FIG 6 shows the hybrid nanocrystal-polymer solar cell which is made by blending CdSe nanocrystals with P3HT, a conducting polymer, to form a 200nm thick film sandwiched between an aluminum top contact and a transparent bottom contact.
By adjusting the diameter of the nanorods, Alivisatos' lab has tuned their cells' absorption spectrum to have as large an overlap with the solar energy spectrum as possible, enabling them to collect more light than typical plastic solar cells. This tuning will also enable the fabrication of nanocrystal / polymer/ nanoparticle combinations that absorb different wavelengths of light more efficiently. Multiple layers of varied composition can then be stacked on top of one another to form a more efficient cell. To further boost performance Alivisatos and his collaborators have switched to a new nanorod material, cadmium telluride, which absorbs more sunlight than cadmium selenide, the material they used initially. The scientists are also aligning the nanorods in branching assemblages that conduct electrons more efficiently than do randomly mixed nanorods.The nanorod solar cells could be rolled out, ink-jet printed, or even painted onto surfaces, so a billboard on a bus could be a solar collector.
Solar cells based on inorganic nanorods combine the processing advantages of small molecules and organic polymers with the performance advantages of bulk inorganic materials. Because of their solubility in various common solvents, nanorods can be used to make semiconductors using low cost processing techniques such as spin coating, blade casting, and screen printing on substrates of various flexibility, including plastic. They do not require a clean room, a vacuum chamber, or high temperatures for fabrication and the electrode and nanorod/polymer layers of the solar cell can be applied in separate coats for ease of production.
Drawback of working at the nano scale
The main drawback of nanoelectronics is that it is prone to damage due to external electric discharge. On a dry winter day, walking on a new carpet can generate a whopping 35,000-volt discharge. This high voltage does not harm us because the amount of charge that flows is puny. Still, it is large enough to destroy sensitive micro-electronic components. Modern microelectronics is extremely sensitive and can be ruined by the pulse of electricity of an electrostatic discharge (ESD) that can occur from mere handling of a chip.
ESD is an issue not only for finished products but also during their manufacture, from wafer fabrication to packaging to the assembly of complete systems. Each step has its own electrostatic hazards. The main cause of failure of electronics when ESD occurs is the heat generated by the electric current of the discharge, which can be enough to melt the material. Damage occurs even without melting. The properties of diodes and transistors are determined by the doping of the semiconductor: carefully introduced impurity atoms, or dopants, produce regions having specific electronic properties. Excessive heating can allow dopants to migrate, ruining the precise pattern of regions that is essential for the device to function properly.
Processes known as electro-current constriction and thermal runaway make matters worse by concentrating the heating in a hot spot: when one location of a semiconductor heats up significantly, its resistance falls, so that more of the current flows through the hottest place, heating it even more.
The solution to protect these delicate transistors is to include ESD protection circuits on the chip, to divert currents from discharges away from the transistors toward the ground. Since 1995 "smart" circuits known as ESD power clamps have been used to discharge the ESD current through the final stage, from the power rail to the ground. For example, some power clamps use a simple frequency-dependent filter to discriminate an ESD pulse from normal signals. Others detect the excess voltage of the discharge. Once the device senses the pulse, a signal powered by the pulse turns on robust transistor circuits to discharge the current safely to the ground.
David V. Cronin of Polaroid invented a mechanical solution to protect individual diodes when they are being handled: When the diode is not in its socket, conductive metal springs short the electrodes to the diode's metal casing (). Any ESD on the electrodes will flow to the casing instead of to the diode's semiconductor. When the laser diode is inserted into its socket, the metal spring disengages.
In years to come, traditional methods of ESD protection for semiconductors may not be acceptable with smaller, faster devices. Alternatively, designers might use new materials to make intrinsically sturdier transistors and rely on off-chip devices to prevent ESD pulses from reaching the nanocircuitry.
:Conclusion:
It appears that the future of electronics is ‘nano’ i.e. electronics at the nano scale could revolutionise the way the world works. Microelectronics has been miniaturized to such an extent that it now works at the molecular level to make use of fundamental properties, phenomena, and processes. Nanoelectromechanical systems (NEMS)--the smaller cousins of microelectromechanical systems (MEMS)--are paving the way for a revolution in applications such as sensors, medical diagnostics, displays and data storage. Current integrated circuits have minimum dimensions of the order of 0.35 microns. Based on current rates of development, people have projected that around the year 2005 companies will be manufacturing in high volume integrated circuits that have dimensions around 0.1 micron.
Nanoelectronics will be a strategic branch of science and engineering for the current century, one that will fundamentally restructure the technologies currently used in computation , medicine, energy production, communication, and education.
As the twenty-first century unfolds, the impact of nanoelectronics on our society is expected to be as significant as that of the silicon chip, antibiotics, man made polymers or integrated circuits on the twentieth century.
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