Lowering the power consumption of CNT-based CMOS devices to subnanowatt

Researchers have demonstrated a new carbon nanotube (CNT)-based logic device that consumes just 0.1 nanowatts (nW) in its static ON and OFF states, representing the lowest reported value by 3 orders of magnitude for CNT-based CMOS logic devices. The device could serve as a building block for large-area, ultralow-power CNT logic circuits that can be used to realize a variety of nanoelectronics applications.

The researchers, Michael L. Geier, et al., at Northwestern University in Evanston, Illinois, and the University of Minnesota in Minneapolis, have published their paper on the subnanowatt CNT logic in a recent issue of Nano Letters.

'As the researchers explain, one of the biggest advantages of CMOS architecture is that it has intrinsically low power consumption. This benefit arises from the fact that, unlike other logic architectures, one of the two types of transistors (p-type or n-type) is turned off under steady state conditions in each logic gate in CMOS devices.

In order to fully take advantage of this potential for extremely low power consumption, the p-type and n-type transistors need to have precisely tuned and well-separated threshold voltages, which are the voltage levels that determine whether the device is ON or OFF. So far, this issue of the threshold voltages has not been addressed, and the researchers here identified it as the key challenge limiting the realization of highly integrated CNT-based CMOS electronics.

In their study, the researchers used a metal gate structure to achieve symmetric and clearly separated threshold voltages for p-type and n-type CNT transistors, resulting in the ultralow power consumption. In the static states, in which the device is either ON or OFF, power consumption is less than 0.1 nW. At the midpoint of the transfer state, when both p-type and n-type transistors are simultaneously in the ON state, the voltage reaches its peak at 10 nW.'

By connecting multiple CNT transistors in various configurations, the researchers demonstrated inverter, NAND and NOR logic gates. In the future, these gates can be integrated into complex circuits, where they can provide subnanowatt static power consumption along with the other advantages of CNTs, such as solution processability and flexibility.

More information: Michael L. Geier, et al. "Subnanowatt Carbon Nanotube Complementary Logic Enabled by Threshold Voltage Control." Nano Letters. DOI: 10.1021/nl402478p

Read more at: http://phys.org/news/2013-09-carbon-nanotube-logic-device-subnanowatt.html#jCp

Graphene could yield cheaper optical chips

Optoelectronic devices built from graphene could be much simpler in design than those made from other materials. If a method for efficiently depositing layers of graphene—a major area of research in materials science—can be found, it could ultimately lead to optoelectronic chips that are simpler and cheaper to manufacture.

"Another advantage, besides the possibility of making device fabrication simpler, is that the high mobility and ultrahigh carrier-saturation velocity of electrons in graphene makes for very fast detectors and modulators," says Dirk Englund, the Jamieson Career Development Assistant Professor of Electrical Engineering and Computer Science at MIT, who led the new research.

Graphene is also responsive to a wider range of light frequencies than the materials typically used in photodetectors, so graphene-based optoelectronic chips could conceivably use a broader-band optical signal, enabling them to move data more efficiently. "A two-micron photon just flies straight through a germanium photodetector," Englund says, "but it is absorbed and leads to measurable current—as we actually show in the paper—in graphene."

Read more at: http://phys.org/news/2013-09-graphene-yield-cheaper-optical-chips.html

More information: http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2013.253.html

Unbreakable and flexible phone is on the edge...

Breakthrough research at RMIT University is advancing transparent bendable electronics, bringing science fiction gadgets – such as unbreakable rubber-like phones, rollable tablets and even functional clothing – closer to real life.

Researchers from RMIT’s Functional Materials and Microsystems research group have developed a new method to transfer electronics with versatile functionality, which are usually made on rigid silicon, onto a flexible surface.

The result of their work is published today (13 September) in Nature Publishing Group’s Asia Materials, the leading materials science journal for the Asia-Pacific.

The ability of micro and nano-electronic devices to sense, insulate or generate energy is controlled by thin, transparent nanolayers of oxide materials, often much thinner than 1/100th of a human hair.

These oxide materials are brittle and their high processing temperatures – often in excess of 300 °C – have until now prevented their incorporation in flexible electronic devices.

Lead author, PhD researcher Philipp Gutruf, said the new process developed at RMIT could unleash the potential of fully functional flexible electronics, while providing a new way for the materials to mesh together.

"We have discovered a micro-tectonic effect, where microscale plates of oxide materials slide over each other, like geological plates, to relieve stress and retain electrical conductivity," he said.

"The novel method we have developed overcomes the challenges of incorporating oxide materials in bendable electronic devices, paving the way for bendable consumer electronics and other exciting applications."

Supervisor and co-leader of the research group, Dr Madhu Bhaskaran, said the new approach used two popular materials – transparent conductive indium tin oxide and rubber-like silicone which is also biocompatible.

“The ability to combine any functional oxide with this biocompatible material creates the potential for biomedical devices to monitor or stimulate nerve cells and organs. This is in addition to the immediate potential for consumer electronics applications in flexible displays, solar cells, and energy harvesters.”

Mr Gutruf is supported by an Australian Government Endeavour International Postgraduate Research Scholarship and the research was supported by Australian Post-Doctoral Fellowships from the Australian Research Council to Dr Bhaskaran and Dr Sharath Sriram, co-leader of the research group.

Source: http://www.rmit.edu.au/browse;ID=2p5rc967irk1;STATUS=A

Reference: http://www.nature.com/am/journal/v5/n9/full/am201341a.html

Autonomic Restoration of Electrical Conductivity

Self-healing of an electrical circuit is demonstrated with nearly full recovery of conductance less than one millisecond after damage. Crack damage breaks a conductive pathway in a multilayer device, interrupting electron transport and simultaneously rupturing adjacent microcapsules containing gallium–indium liquid metal (top). The released liquid metal flows to the area of damage, restoring the conductive pathway (bottom).

Autonomic Restoration of Electrical Conductivity Blaiszik 2011 Advanced Materials Wiley Online Library

Stanford scientists use DNA to assemble a transistor from graphene

Stanford scientists use DNA to assemble a transistor from graphene

DNA meets electronics

DNA is the blueprint for life. Could it also become the template for making a new generation of computer chips based not on silicon, but on an experimental material known as graphene?

That's the theory behind a process that Stanford chemical engineering professor Zhenan Bao reveals in Nature Communications.

Bao and her co-authors, former post-doctoral fellows Anatoliy Sokolov and Fung Ling Yap, hope to solve a problem clouding the future of electronics: consumers expect silicon chips to continue getting smaller, faster and cheaper, but engineers fear that this virtuous cycle could grind to a halt.

Why has to do with how silicon chips work.

Everything starts with the notion of the semiconductor, a type of material that can be induced to either conduct or stop the flow of electricity. Silicon has long been the most popular semiconductor material used to make chips.

The basic working unit on a chip is the transistor. Transistors are tiny gates that switch electricity on or off, creating the zeroes and ones that run software.

To build more powerful chips, designers have done two things at the same time: they've shrunk transistors in size and also swung those gates open and shut faster and faster.

The net result of these actions has been to concentrate more electricity in a diminishing space. So far that has produced small, faster, cheaper chips. But at a certain point, heat and other forms of interference could disrupt the inner workings of silicon chips.

"We need a material that will let us build smaller transistors that operate faster using less power," Bao said.

Graphene has the physical and electrical properties to become a next-generation semiconductor material – if researchers can figure out how to mass-produce it.

Graphene is a single layer of carbon atoms arranged in a honeycomb pattern. Visually it resembles chicken wire. Electrically this lattice of carbon atoms is an extremely efficient conductor.

Bao and other researchers believe that ribbons of graphene, laid side-by-side, could create semiconductor circuits. Given the material's tiny dimensions and favorable electrical properties, graphene nano ribbons could create very fast chips that run on very low power, she said.

"However, as one might imagine, making something that is only one atom thick and 20 to 50 atoms wide is a significant challenge," said co-author Sokolov.

To handle this challenge, the Stanford team came up with the idea of using DNA as an assembly mechanism.

Physically, DNA strands are long and thin, and exist in roughly the same dimensions as the graphene ribbons that researchers wanted to assemble.

Chemically, DNA molecules contain carbon atoms, the material that forms graphene.

The real trick is how Bao and her team put DNA's physical and chemical properties to work.

The researchers started with a tiny platter of silicon to provide a support (substrate) for their experimental transistor. They dipped the silicon platter into a solution of DNA derived from bacteria and used a known technique to comb the DNA strands into relatively straight lines.

Next, the DNA on the platter was exposed to a copper salt solution. The chemical properties of the solution allowed the copper ions to be absorbed into the DNA.

Next the platter was heated and bathed in methane gas, which contains carbon atoms. Once again chemical forces came into play to aid in the assembly process. The heat sparked a chemical reaction that freed some of the carbon atoms in the DNA and methane. These free carbon atoms quickly joined together to form stable honeycombs of graphene.

"The loose carbon atoms stayed close to where they broke free from the DNA strands, and so they formed ribbons that followed the structure of the DNA," Yap said.

So part one of the invention involved using DNA to assemble ribbons of carbon. But the researchers also wanted to show that these carbon ribbons could perform electronic tasks. So they made transistors on the ribbons.

"We demonstrated for the first time that you can use DNA to grow narrow ribbons and then make working transistors," Sokolov said.

The paper drew praise from UC Berkeley associate professor Ali Javey, an expert in the use of advanced materials and next-generation electronics.

"This technique is very unique and takes advantage of the use of DNA as an effective template for controlled growth of electronic materials," Javey said. "In this regard the project addresses an important research need for the field."

Bao said the assembly process needs a lot of refinement. For instance, not all of the carbon atoms formed honeycombed ribbons a single atom thick. In some places they bunched up in irregular patterns, leading the researchers to label the material graphitic instead of graphene.

Even so, the process, about two years in the making, points toward a strategy for turning this carbon-based material from a curiosity into a serious contender to succeed silicon.

"Our DNA-based fabrication method is highly scalable, offers high resolution and low manufacturing cost," said co-author Yap. "All these advantages make the method very attractive for industrial adoption."

Source : http://www.eurekalert.org/pub_releases/2013-09/ssoe-ssu090613.php