Carbon nanotubes have shown promise as a possible medium for post-CMOS electronics, and some research projects have used them as conductors, resistors and semiconductors in novel approaches to transistor and circuit design. Although the tubes are versatile electronic components, integrating them can be a problem. Now researchers at Columbia University's Nanoscience and Engineering Center have come up with a promising approach by blending carbon nanotubes with organic molecules that naturally bind to carbon. The team reported successfully inserting several types of organic molecules and gave detailed test results for an application-specific organic molecule that changed its conductivity in response to pH, enabling that molecular transistor to act as a pH sensor. The researchers first positioned a carbon nanotube on a silicon substrate with metal contacts at each end. The tube was then covered with PMMA, a mask material used in lithography. Using high-resolution electron-beam equipment, they cut a 10-nanometer-long window in the PMMA. Oxygen plasma was then applied to vaporize the section of nanotube exposed in the window. The resulting gap is so small that it cannot be seen with an electron microscope. Even an atomic-force microscope has problems resolving the gap. Chemically, the oxygen plasma creates a particular molecular end structure on both sides of the gap consisting of attached carboxylic-acid molecules that narrow the gap to about 2 nm. These molecules bind readily to a wide variety of organic molecules, which makes the technique flexible in terms of different molecular operations.
Monday, January 30, 2006
Monday, January 23, 2006
Researchers have developed magnetic nanorings that promise to enable magnetic RAM densities to rival or surpass those of flash memories. MRAMs can store bits when the power is off, but their comparatively low densities have precluded their development for anything more than niche markets. Now researchers from Johns Hopkins and Carnegie Mellon Universities have found a way to form magnetic domains into asymmetrical 100-nanometer-diameter cobalt rings. The results are magnetic vortexes that become completely self-contained, permitting tightly packed densities of up to 30 Gbits of storage per square inch--ten times higher than flash's 3 Gbits/square inch. Other experimental MRAMs store bits in a magnetic tunnel junction--a sandwich structure that includes a pinned magnetic layer, an oxide tunnel barrier and a free magnetic layer that can switch between two linear magnetic polarization states. A bit is detected when the cell's electrical resistance changes. But MRAMs have only been successfully fabricated in samples at 1-Mbit and 4-Mbit densities, a far cry from the 1-Gbit flash chips fabricated today. Further, the magnetic vortexes can be harnessed to store bits in two states--either the vortex state or a neutral state of two opposing, onion-skin magnetic fields. Ironically, in order for the nanorings to encode the two states for memory cells, they must be so small that no magnetic field can exist inside each core. To ensure that no vortex could exist within the cores of the nanorings, the researchers kept the nanorings' diameter below 50 nm. To produce the rings, the researchers first coated the single-crystal silicon substrate with a monolayer of 100-nm-diameter polystyrene spheres. The spheres did not contact each other and had controllable average separation distances. Next, a 40-nm-thick film of cobalt was deposited by magnetron sputtering to cover all the spheres and open substrate areas. Finally, an argon ion beam was used to etch away the cobalt, including what was on top of the polystyrene spheres. The cobalt was protected by the bottom side of the spheres, however, resulting in 100-nm-diameter cobalt nanorings on the silicon substrate. The devices were found to switch reliably between two stable states. The asymmetry of the nanorings was also found to improve their switching behaviors, and the parameters that achieve optimal performance were explored, he said. Next, the researchers plan to prepattern a substrate with polystyrene spheres positioned over circuitry that can read and write the nanorings, thereby turning them into 100-nm- bit cells for MRAMs. The research was funded by the National Science Foundation.
Friday, January 20, 2006
Columbia University researchers have successfully married a carbon nanotube with an organic molecule, creating what they said is the world's first hybrid carbon-nanotube/molecular transistor. The experimental device enabled the chemical reaction in an application-specific organic molecule to be harnessed by the high carrier-mobility of carbon nanotubes, according to Shalom Wind, senior research scientist at Columbia University (New York). Wind joined Columbia in 2003 after a stint at IBM Research's T.J. Watson Research Center where he helped characterize nanotube transistors. Like IBM's original design, Columbia's molecular transistors used a carbon nanotube as the transistor channel. The researchers cut the nanotube and inserted an application-specific organic molecule to functionalize the resulting molecular transistor, leaving the nanotubes as the source — and drain — electrodes. The researchers successfully inserting several different types of organic molecules, and reported detailed test results for an application-specific organic molecule that changed its conductivity in response to Ph. This enabled the molecular transistor to act as a Ph sensor. The insertion technique used nanoscale lithography and a oxidation process that prepared the severed ends of the nanotube for chemical bonding to the inserted molecule. By ensuring the gap cut in the nanotube was similar in size to the molecule to be inserted, the researchers were able to marry the ends of the nanotube to a single organic molecule. The researcher was performed at Columbia University's Nanoscience Center.
Posted by R. Colin Johnson at 9:01 AM
Monday, January 16, 2006
After an exhaustive study of all varieties of locomotion in both living and mechanical systems, two researchers have concluded this month that the same set of engineering principles applies to all systems capable of independent motion. The study, which was carried out by engineering professor Adrian Bejan of Duke University and professor James Marden, a biologist at Penn State University, builds on a 1996 theory proposed by Bejan. Bejan's "constructal theory" states that "For a finite-size system to persist in time [to live], it must evolve in such a way that it provides easier access to the imposed currents that flow through it." For example, the current that flows through an IC package is heat. If the package does not provide an efficient structure for dissipating that heat, the system will overheat and cease to function. In another example, in the circulatory system of the body, blood flow must be optimized by the geometric configuration of blood vessels and capillaries and arteries. The theory aims to be as fundamental as thermodynamics, providing basic design principles rooted in physical laws. Constructal principles apply across virtually any functioning system-biological or man-made. In 2004, Bejan began cooperating with Marden to extend the theory from its origins in comparing airplanes and the flight of birds, moving to comparisons of the gaits of animals and robots. This month Bejan and Marden reported that all forms of locomotion obey the same ratios between the energy destroyed at each step or flap and the energy lost to friction against the ground or air. Even swimmers obey the same body-mass scaling principles as runners or fliers, whether they're living or robotic.
Monday, January 09, 2006
Researchers at Argonne National Laboratory have succeeded in tuning the luminescence from gold nanorods by adjusting their lengths. By varying length from 300 nanometers down to 50 nm while fixing the diameter at 30 nm, the team demonstrated photoluminescence at 400- to 700-nm wavelengths. Further enhancements could make it possible to use the optical emissions from gold nanorods as a light source inside a silicon chip, transducing light for optical network switches, routers and all-optical computers. Other researchers had shown that surface plasmons could influence luminescence emission spectra, but the Argonne group wanted to verify that the spectra of photoluminescent gold nanorods could be controlled by surface plasmon resonances. When excited by two-photon-induced photoluminescence, the nanorods exhibited two spectral resonances, corresponding to their longitudinal and transverse dimensions, demonstrating sensitivity to both excitation energy and polarization. Surface plasmons are collective oscillations of electrons at the boundary between a conductor and an insulator. Plasmons themselves are collections of electrons that meld with photons to form a new order of object, called a surface plasmon polariton, that can enhance transmission in certain optical bands, acting as optical "bandpass filters." The Argonne team examined the photoluminescence in gold by exciting nanorods of various aspect ratios with 120-picosecond pulses from a 785-nm-wavelength laser. The rods responded with two-photon-induced photoluminescence that the team showed was dependent on their size and shape, thus confirming that surface plasmons — likewise dependent on size and shape — had allowed them to control spectral emission.
The Department of Defense is attempting to leverage silicon-germanium ICs to create a low-cost, high-performance technology to handle radar and communications on earth and radiation-hardened electronics in space. The program represents a new research strategy for the DOD, which traditionally has funded expensive new technologies that had a low probability of seeing the light of day because of their exorbitant development costs. In a reversal of that trend, the Pentagon now is looking to develop less-expensive solutions with the potential for commercial as well as military applications. In particular, the program has the potential to lead to the development of cheap silicon-germanium chips for less-expensive weather radar for aircraft or as collision-avoidance radar for automobiles. The program, called the Silicon-Germanium Transmit-Receive Module Project by the DOD, should dramatically lower the cost of modern phased-array radar systems. And since a specially equipped vehicle would no longer be needed just to transport the antenna, it should also enable the systems to be portable. If the silicon-germanium chips work as well as the researchers hope, then the naturally radiation-hardened chips could also downsize and lower the cost of critical space applications. Silicon-germanium devices are not likely to replace gallium-arsenide devices for most optical applications, however, because their power-handling capabilities are about 10-fold lower than for high-power gallium-arsenide discrete devices. But for low-power applications like radar, the ability to integrate the silicon-germanium optics on the same chip with pure-silicon electronics could enable developers to dramatically lower the cost.
As chairman of the physics department at Mercer University, Randall Peters is more accustomed to helping scientists than designing commercial products. But when he developed an instrument for NASA that detects the Earth's acceleration, he found he'd invented a marketable earthquake detector (see Jan. 17, 2005, page 36). Peters' patented design is now slated to become a commercial product this year. Priced at under $500 for a bare-bones version, it will be the world's least expensive earthquake detector: The price of equally sensitive full-function seismic instruments is closer to $10,000. Peters had set up his instrument to register the Earth's acceleration — but instead it registered an incredibly large earthquake and its aftershocks. By checking the exact timing of the quakes, Peters confirmed that his instrument — which he had built for less than $200 in parts — had detected the Dec. 26, 2004, earthquake in the Indian Ocean that caused the devastating tsunami in Southern Asia. Since then Peters has used his instrument from the comfort of his Macon, Ga., lab to monitor earthquakes worldwide. Peters' design uses a novel means of varying the surface area of a capacitor. Rather than varying its gap as in standard capacitive sensors, Peters' design varies the capacitor's surface area. Because the capacitor's gap is constant, detection is not accompanied by a drop-off in sensitivity, as is the case with other capacitive sensors. Most of those become less sensitive when their gap widens. Since sensitivity and dynamic range don't have to be treated as a design trade-off — as in traditional capacitive sensors — Peters' design sets sensitivity by the constant size of the gap. By changing the surface area, it separately determines dynamic range.
Monday, January 02, 2006
Just as a poem is composed from a toolbox of 26 characters or a fugue from a 12-tone scale, molecular-scale chips will one day be built from the four-letter alphabet of DNA, a Duke University EE professor believes. To prove the point, Chris Dwyer and his collaborator, Duke professor Thom LaBean, recently demonstrated how to pattern 100 trillion 16-cell (4 x 4) programmable arrays with 2- to 10-nanometer features — compared with 65-nm features today — using just $40 worth of commercially available dioxyribonucleic acid. The 4 x 4 grids in the demonstration were programmed to spell out "DNA," but the researchers claim that eventually an entire memory bit cell could be programmed into larger versions of the grids. Once the grids are populated with the necessary components to form programmable bit cells, they could be deposited on silicon substrates as thin films, or they could potentially be left in the solution in which they were programmed and then addressed with a scanning laser. The circuitry realized on the grids could have optical qualities too. For instance, the circuits might yield liquid-crystal displays with floating pixels that could be toggled to the correct color, depending on where the pixels happened to be floating at any particular moment. Each of the 16 tiles comprised nine coded strands of DNA, obtained from a DNA sample that the researchers had purchased commercially. By performing various wet-chemistry steps in the lab — because DNA can be damaged by direct exposure to air — the researchers were able to coax the floating DNA strands to self-assemble into the grids of 16 tiles, each coded for attaching an electronic nanoparticle, an optical molecule or a nanowire connection.
Electrical engineers at the University of Texas (Austin) have demonstrated what they claim is the world's smallest silicon modulator. The device features a photonic-crystal waveguide with an electrode configuration that they hope will make it easily manufacturable. Such a compact modulator might be the key to building practical all-silicon lasers, they said. Photonic crystals are periodic structures in silicon. They are usually defined by a regular set of holes that interact with photons in the same way that the much smaller periodic structure of silicon atoms interacts with electrons. The structures can produce superior optical cavities when optical bandgaps are introduced into silicon. The architecture realizes a Mach-Zehnder interferometer (MZI), which modulates by comparing the phase at the end of two arms of a signal path. One is unobstructed and the second is electrically controlled by the photonic-crystal waveguide, thereby enabling an electrical control signal to modulate the optical signal. To make the design the world's smallest, and thus manufacturable, the two electrical connections (one on each side of the 80-micron-long photonic crystal in one arm of the MZI) had to go through an extra step that created an external connection to the central electrode. The ultracompact silicon electro-optical modulator works by slowing light down in one of the two arms of a Mach-Zehnder interferometer, thereby shifting its phase to control the frequency of modulation of the optical signal passing through the photonic crystal. The electrical signal injected just .15 milliampere into one arm of the dual photonic crystals to achieve a 92 percent modulation depth in optical communication wavelengths of 1.3 to 1.55 microns.