Carbon Nanotubes  

Researchers have developed scalable and rapid deposition process to coat substrate surfaces with aligned carbon nanotubes. The results could lead to advances like in longer battery life, faster wireless communication and faster processing speeds for devices like smartphones and laptops.

For the first time, materials engineers have created carbon nanotube transistors that outperform state-of-the-art silicon transistors.

University of Wisconsin–Madison material scientists Michael Arnold and Padma Gopalan, created carbon nanotube transistors achieved current that’s 1.9 times higher than silicon transistors. The researchers reported their breaktrough in a paper published in the journal Science Advances.

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“This achievement has been a dream of nanotechnology for the last 20 years,” says Arnold. “Making carbon nanotube transistors that are better than silicon transistors is a big milestone. This breakthrough in carbon nanotube transistor performance is a critical advance toward exploiting carbon nanotubes in logic, high-speed communications, and other semiconductor electronics technologies.”

This development could open the door for carbon nanotube transistors to replace silicon transistors and continue delivering the performance gains the computer industry relies on and that consumers demand. The new transistors are particularly promising for wireless communications technologies that require a lot of current flowing across a relatively small area.

Carbon nanotubes have long been recognized as a promising material for next-generation transistors and are some of the best electrical conductors ever discovered,.

Carbon nanotube transistors should be able to perform five times faster or use five times less energy than silicon transistors, according to extrapolations from single nanotube measurements. The nanotube’s ultra-small dimension makes it possible to rapidly change a current signal traveling across it, which could lead to substantial gains in the bandwidth of wireless communications devices.

Material scientists have struggled to isolate purely carbon nanotubes, which are crucial, because metallic nanotube impurities act like copper wires and disrupt their semiconducting properties — like a short in an electronic device.

The UW–Madison team used polymers to selectively sort out the semiconducting nanotubes, achieving a solution of ultra-high-purity semiconducting carbon nanotubes.

"There has been a lot of hype about carbon nanotubes that hasn’t been realized, and that has kind of soured many people’s outlook. But we think the hype is deserved."

“We’ve identified specific conditions in which you can get rid of nearly all metallic nanotubes, where we have less than 0.01 percent metallic nanotubes,” says Arnold. Placing and aligning the nanotubes is also difficult to control.

To make a good transistor, the nanotubes need to be aligned in just the right order, with just the right spacing, when assembled on a wafer. In 2014, the UW–Madison researchers overcame that challenge when they announced a technique, called “floating evaporative self-assembly,” that gives them this control.

The nanotubes must make good electrical contacts with the metal electrodes of the transistor. Because the polymer the UW–Madison researchers use to isolate the semiconducting nanotubes also acts like an insulating layer between the nanotubes and the electrodes, the team “baked” the nanotube arrays in a vacuum oven to remove the insulating layer. The result: excellent electrical contacts to the nanotubes.

The researchers also developed a treatment that removes residues from the nanotubes after they’re processed in solution.

“In our research, we’ve shown that we can simultaneously overcome all of these challenges of working with nanotubes, and that has allowed us to create these groundbreaking carbon nanotube transistors that surpass silicon and gallium arsenide transistors,” says Arnold.

Arnold says it’s exciting to finally reach the point where researchers can exploit the nanotubes to attain performance gains in actual technologies.

“There has been a lot of hype about carbon nanotubes that hasn’t been realized, and that has kind of soured many people’s outlook,” says Arnold. “But we think the hype is deserved. It has just taken decades of work for the materials science to catch up and allow us to effectively harness these materials.”

SOURCE  University of Wisconsin–Madison

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 Nanotechnology  

Researchers have created nanowire-bridging transistors grown out of semiconducting materials. These nanopillars will allow for smaller, faster and more robust complex circuits that will pave the way for next-generation technology.

A new approach to integrated circuits, combining atoms of semiconductor materials into nanowires and structures on top of silicon surfaces, shows promise for a new generation of fast, robust electronic and photonic devices.

Engineers at the University of California, Davis, have recently demonstrated three-dimensional nanowire transistors using this approach that open exciting opportunities for integrating other semiconductors, such as gallium nitride, on silicon substrates.

"Silicon can't do everything."

The work has been published in Advanced Materials and Applied Physics Letters.

"Silicon can't do everything," said Saif Islam, professor of electrical and computer engineering at UC Davis. Circuits built on conventionally etched silicon have reached their lower size limit, which restricts operation speed and integration density. Additionally, conventional silicon circuits cannot function at temperatures above 250 degrees Celsius (about 480 degrees Fahrenheit), or handle high power or voltages, or optical applications.

The new technology could be used, for example, to build sensors that can operate under high temperatures, for example inside aircraft engines.

"In the foreseeable future, society will be dependent on a variety of sensors and control systems that operate in extreme environments, such as motor vehicles, boats, airplanes, terrestrial oil and ore extraction, rockets, spacecraft, and bodily implants," Islam said.

Devices that include both silicon and nonsilicon materials offer higher speeds and more robust performance. Conventional microcircuits are formed from etched layers of silicon and insulators, but it's difficult to grow nonsilicon materials as layers over silicon because of incompatibilities in crystal structure (or "lattice mismatch") and differences in thermal properties.

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Instead, Islam's laboratory at UC Davis has created silicon wafers with nanopillars of materials such as gallium arsenide, gallium nitride or indium phosphide on them, and grown tiny nanowire "bridges" between nanopillars.

"We can't grow films of these other materials on silicon, but we can grow them as nanowires," Islam said.

The researchers have been able to make these nanowires operate as transistors, and combine them into more complex circuits as well as devices that are responsive to light. They have developed techniques to control the number of nanowires, their physical characteristics and consistency.

Islam said the suspended structures have other advantages: They are easier to cool and handle thermal expansion better than planar structures — a relevant issue when mismatched materials are combined in a transistor.

The technology also leverages the well-established technology for manufacturing silicon integrated circuits, instead of having to create an entirely new route for manufacturing and distribution, Islam said.




SOURCE  UC Davis

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 Nanocomptuers  

By using a new bottom-up nanotech approach, researchers have created the first nanocomputer, the nanoFSM. The ultra-small electronic computer systems that push beyond the imminent end of Moore’s Law.

Ateam of scientists and engineers from The MITRE Corporation and Harvard University has taken key steps toward ultra-small electronic computer systems that push beyond the imminent end of Moore's Law, which states that the device density and overall processing power for computers will double every two to three years.

The team designed and assembled, from the bottom up, a functioning, ultra-tiny control computer that is the densest nanoelectronic system ever built.

A technical paper has been published online in the Proceedings of the National Academy of Sciences on the research.

The ultra-small, ultra-low-power control processor—termed a nanoelectronic finite-state machine or "nanoFSM"—is smaller than a human nerve cell. It is composed of hundreds of nanowire transistors, each of which is a switch about ten-thousand times thinner than a human hair. The nanowire transistors use very little power because they are "nonvolatile." That is, the switches remember whether they are on or off, even when no power is supplied to them.

In the nanoFSM, these nanoswitches are assembled and organized into circuits on several "tiles." Together, the tiles route small electronic signals around the computer, enabling it to perform calculations and process signals that could be used to control tiny systems, such as nanorobot medical therapeutic devices, other tiny sensors and actuators, or even insect-sized robots.

In the image at the top, an SEM image of the final chip having 204 contact pads on the outer periphery of the chip is shown. The pads match the pins of a probe card that is connected to the researcher's test system. The metal pads and fan-in interconnect lines appear bright in the image. On the right, the SEM image of the inner layout of the fabricated chip as indicated in the dashed box on the left.

Nanocircuit fabrication - Image Source - Yao et al./PNAS

In 2011, the MITRE-Harvard team demonstrated a single such tiny tile capable of performing simple logic operations. In their recent collaboration they combined several tiles on a single chip to produce a first-of-its-kind complex, programmable nanocomputer.

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"It was a challenge to develop a system architecture and nanocircuit designs that would pack the control functions we wanted into such a very tiny system," according to Shamik Das, chief architect of the nanocomputer, who is also principal engineer and group leader of MITRE's Nanosystems Group. "Once we had those designs, though, our Harvard collaborators did a brilliant job innovating to be able to realize them."

Construction of this nanocomputer was made possible by significant advances in processes that assemble with extreme precision dense arrays of the many nanodevices required. These advances also made it possible to manufacture multiple copies of the nanoFSM, using a groundbreaking approach in which, for the first time, complex nanosystems can be economically assembled from the bottom up in close conformity to a preexisting design. Until now, this could be done using the industry's expensive, top-down lithographic manufacturing methods, but not with bottom-up assembly.

For this reason, the nanoFSM and the means by which it was made represent a step toward extending the very economically important five-decade-long trend in miniaturization according to Moore's Law, which has powered the electronics industry. Because of limitations on its conventional lithographic fabrication methods and on conventional transistors, many industry experts have suggested that the Moore's Law trend soon may come to an end. Some assert that this might occur in as little as five years and have negative economic consequences, unless there are innovations in both device and fabrication technologies, such as those demonstrated by the nanoFSM.


SOURCE  MITRE

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