IBM Scientists Find New Way to Shrink Transistors

A set of ultratiny nanotube transistors made by IBM. Credit IBM Research

A set of ultratiny nanotube transistors made by IBM. Credit IBM Research

In the semiconductor business, it is called the “red brick wall” — the limit of the industry’s ability to shrink transistors beyond a certain size.

On Thursday, however, IBM scientists reported that they now believe they see a path around the wall. Writing in the journal Science, a team at the company’s Thomas J. Watson Research Center said it has found a new way to make transistors from parallel rows of carbon nanotubes.

The advance is based on a new way to connect ultrathin metal wires to the nanotubes that will make it possible to continue shrinking the width of the wires without increasing electrical resistance.

One of the principal challenges facing chip makers is that resistance and heat increase as wires become smaller, and that limits the speed of chips, which contain transistors.

The advance would make it possible, probably sometime after the beginning of the next decade, to shrink the contact point between the two materials to just 40 atoms in width, the researchers said. Three years later, the number will shrink to just 28 atoms, they predicted.

The ability to reduce electrical resistance will not only make it possible to extend the process of shrinking transistors beyond long-held beliefs about physical limits. It may also be the key to once again increasing the speed of computer processors, which has been stalled for the last decade.

The report represents a big advance for an exotic semiconductor material that has long held great promise but has also proved maddeningly difficult for scientists to work with. Single-wall carbon nanotubes are strawlike structures that are a composed of a one-atom thick matrix of carbon atoms rolled into an infinitesimally small tube.

The challenge of carbon nanotubes in their typical state is that they form what scientists call a giant “hairball” of interwoven molecules.

However, researchers have found ways to align them closely and in regularly spaced rows and deposit them on silicon wafers with great precision. They then serve the crucial role of a semiconductor, allowing electrical current to be switched on and off in a computer circuit.

Until now, however, they have been just one of a range of new materials that have been seen as candidates to replace silicon, which has for more than half a century been the material of choice for chip makers.

“Of all the possible materials, this one is at the top of the list by a long shot,” said Dario Gil, vice president for science and technology at IBM Research.

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First Optical Rectenna – Combined Rectifier and Antenna – Converts Light to DC Current

VIDEO -Using nanometer-scale components, researchers have demonstrated the first optical rectenna, a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current.

Using nanometer-scale components, researchers have demonstrated the first optical rectenna, a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current.

Using nanometer-scale components, researchers have demonstrated the first optical rectenna, a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current.

Potential to achieve 40 percent broad spectrum efficiency . . .

Based on multiwall carbon nanotubes and tiny rectifiers fabricated onto them, the optical rectennas could provide a new technology for photodetectors that would operate without the need for cooling, energy harvesters that would convert waste heat to electricity – and ultimately for a new way to efficiently capture solar energy.

In the new devices, developed by engineers at the Georgia Institute of Technology, the carbon nanotubes act as antennas to capture light from the sun or other sources. As the waves of light hit the nanotube antennas, they create an oscillating charge that moves through rectifier devices attached to them. The rectifiers switch on and off at record high petahertz speeds, creating a small direct current.

Billions of rectennas in an array can produce significant current, though the efficiency of the devices demonstrated so far remains below one percent. The researchers hope to boost that output through optimization techniques, and believe that a rectenna with commercial potential may be available within a year.

“We could ultimately make solar cells that are twice as efficient at a cost that is ten times lower, and that is to me an opportunity to change the world in a very big way” said Baratunde Cola, an associate professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech. “As a robust, high-temperature detector, these rectennas could be a completely disruptive technology if we can get to one percent efficiency. If we can get to higher efficiencies, we could apply it to energy conversion technologies and solar energy capture.”

The research, supported by the Defense Advanced Research Projects Agency (DARPA), the Space and Naval Warfare (SPAWAR) Systems Center and the Army Research Office (ARO), was reported September 28 in the journal Nature Nanotechnology.

Developed in the 1960s and 1970s, rectennas have operated at wavelengths as short as ten microns, but for more than 40 years researchers have been attempting to make devices at optical wavelengths. There were many challenges: making the antennas small enough to couple optical wavelengths, and fabricating a matching rectifier diode small enough and able to operate fast enough to capture the electromagnetic wave oscillations. But the potential of high efficiency and low cost kept scientists working on the technology.

“The physics and the scientific concepts have been out there,” said Cola. “Now was the perfect time to try some new things and make a device work, thanks to advances in fabrication technology.”

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Realizing Carbon Nanotube Integrated Circuits



Encapsulation layers keep carbon nanotube transistors stable in open air

Individual transistors made from carbon nanotubes are faster and more energy efficient than those made from other materials. Going from a single transistor to an integrated circuit full of transistors, however, is a giant leap.

“A single microprocessor has a billion transistors in it,” said Northwestern Engineering’s Mark Hersam. “All billion of them work. And not only do they work, but they work reliably for years or even decades.”

When trying to make the leap from an individual, nanotube-based transistor to wafer-scale integrated circuits, many research teams, including Hersam’s, have met challenges. For one, the process is incredibly expensive, often requiring billion-dollar cleanrooms to keep the delicate nano-sized components safe from the potentially damaging effects of air, water, and dust. Researchers have also struggled to create a carbon nanotube-based integrated circuit in which the transistors are spatially uniform across the material, which is needed for the overall system to work.

Now Hersam and his team have found a key to solving all these issues. The secret lies in newly developed encapsulation layers that protect carbon nanotubes from environmental degradation.

Supported by the Office of Naval Research and the National Science Foundation, the research appears online in Nature Nanotechology on September 7. Tobin J. Marks, the Vladimir N. Ipatieff Research Professor of Chemistry in the Weinberg College of Arts and Sciences and professor of materials science and engineering in the McCormick School of Engineering, coauthored the paper. Michael Geier, a graduate student in Hersam’s lab, was first author.

“One of the realities of a nanomaterial, such as a carbon nanotube, is that essentially all of its atoms are on the surface,” said Hersam, the Walter P. Murphy Professor of Materials Science and Engineering. “So anything that touches the surface of these materials can influence their properties. If we made a series of transistors and left them out in the air, water and oxygen would stick to the surface of the nanotubes, degrading them over time. We thought that adding a protective encapsulation layer could arrest this degradation process to achieve substantially longer lifetimes.”

Hersam compares his solution to one currently used for organic light-emitting diodes (LEDs), which experienced similar problems after they were first realized. Many people assumed that organic LEDs would have no future because they degraded in air. After researchers developed an encapsulation layer for the material, organic LEDs are now used in many commercial applications, including displays for smartphones, car radios, televisions, and digital cameras. Made from polymers and inorganic oxides, Hersam’s encapsulation layer is based on the same idea but tailored for carbon nanotubes.

To demonstrate proof of concept, Hersam developed nanotube-based static random-access memory (SRAM) circuits. SRAM is a key component of all microprocessors, often making up as much as 85 percent of the transistors in the central-processing unit in a common computer. To create the encapsulated carbon nanotubes, the team first deposited the carbon nanotubes from a solution previously developed in Hersam’s lab. Then they coated the tubes with their encapsulation layers.

Using the encapsulated carbon nanotubes, Hersam’s team successfully designed and fabricated arrays of working SRAM circuits. Not only did the encapsulation layers protect the sensitive device from the environment, but they improved spatial uniformity among individual transistors across the wafer. While Hersam’s integrated circuits demonstrated a long lifetime, transistors that were deposited from the same solution but not coated degraded within hours.

“After we’ve made the devices, we can leave them out in air with no further precautions,” Hersam said. “We don’t need to put them in a vacuum chamber or controlled environment. Other researchers have made similar devices but immediately had to put them in a vacuum chamber or inert environment to keep them stable. That’s obviously not going to work in a real-world situation.”

Hersam imagines that his solution-processed, air-stable SRAM could be used in emerging technologies. Flexible carbon nanotube-based transistors could replace rigid silicon to enable wearable electronics. The cheaper manufacturing method also opens doors for smart cards — credit cards embedded with personal information to reduce the likelihood of fraud.

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New nanomaterial maintains conductivity in three dimensions



An international team of scientists has developed what may be the first one-step process for making seamless carbon-based nanomaterials that possess superior thermal, electrical and mechanical properties in three dimensions.

The research holds potential for increased energy storage in high efficiency batteries and supercapacitors, increasing the efficiency of energy conversion in solar cells, for lightweight thermal coatings and more. The study is published today (Sept. 4) in the online journal Science Advances.

In early testing, a three-dimensional (3D) fiber-like supercapacitor made with the uninterrupted fibers of carbon nanotubes and graphene matched or bettered–by a factor of four–the reported record-high capacities for this type of device.

Used as a counter electrode in a dye-sensitized solar cell, the material enabled the cell to convert power with up to 6.8 percent efficiency and more than doubled the performance of an identical cell that instead used an expensive platinum wire counter electrode.

Carbon nanotubes could be highly conductive along the 1D nanotube length and two-dimensional graphene sheets in the 2Dplane. But the materials fall short in a three-dimensional world due to the poor interlayer conductivity, as do two-step processes melding nanotubes and graphene into three dimensions.

“Two-step processes our lab and others developed earlier lack a seamless interface and, therefore, lack the conductance sought,” said Liming Dai, the Kent Hale Smith Professor of Macromolecular Science and Engineering at Case Western Reserve University and a leader of the research.

“In our one-step process, the interface is made with carbon-to-carbon bonding so it looks as if it’s one single graphene sheet,” Dai said. “That makes it an excellent thermal and electrical conductor in all planes.”

Dai has worked for nearly four years with Zhong Lin Wang, the Hightower Chair in Materials Science and Engineering, and Yong Ding, a senior research scientist, at Georgia Institute of Technology; and Zhenhai Xia, professor of materials science and engineering, at the University of North Texas; Ajit Roy, principal materials research engineer in the Materials and Manufacturing Directorate, Air Force Research Laboratory, Dayton; and others on a U.S. Department of Defense-Multidisciplinary University Research Initiative (MURI) program (Joycelyn Harrison, Program Manager). Close collaboration was also made with Yuhua Xue, the Research Associate at CWRU and visiting scholar from the Institute of Advanced Materials for Nano-Bio Applications, School of Ophthalmology & Optometry, Wenzhou Medical University, along with Jia Qu and Hao Chen, professors in the Wenzhou Medical University.

To make the 3-D material, the researchers etched radially aligned nanoholes along the length and circumference of a tiny aluminum wire, then used chemical vapor deposition to cover the surface with graphene using no metal catalyst that could remain in the structure.

“Radially-aligned nanotubes grow in the holes. The graphene that sheathes the wire and nanotube arrays are covalently bonded, forming pure carbon-to-carbon nodal junctions that minimize thermal and electrical resistance,” Wang said.

The architecture yields a huge surface area, adding to the transport properties, the researchers say. Using the Brunauer, Emmett and Teller theory, they calculate the surface area of this architecture to be nearly 527 square meters per gram of material.

Testing showed the material makes an ideal electrode for highly efficient energy storage. Capacitance by area reached as high as 89.4 millifarads per square centimeter and by length, up to 23.9 millifarads per centimeter in the fiber-like supercapacitor.

The properties can be customized. With the one-step process, the material can be made very long, or into a tube with a wider or narrower diameter, and the density of nanotubes can be varied to produce materials with differing properties for different needs.

The material can be used for charge storage in capacitors and batteries or the large surface could enable storage of hydrogen. “The properties could be used for an even wider variety of applications, including sensitive sensors, wearable electronics, thermal management and multifunctional aerospace systems”, Roy said.

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Scientists Stretch Electrically Conducting Fibers to New Lengths

UT Dallas scientists have constructed novel fibers by wrapping sheets of tiny carbon nanotubes to form a sheath around a long rubber core. This illustration shows complex two-dimensional buckling, shown in yellow, of the carbon nanotube sheath/rubber-core fiber. The buckling results in a conductive fiber with super elasticity and novel electronic properties.

UT Dallas scientists have constructed novel fibers by wrapping sheets of tiny carbon nanotubes to form a sheath around a long rubber core. This illustration shows complex two-dimensional buckling, shown in yellow, of the carbon nanotube sheath/rubber-core fiber. The buckling results in a conductive fiber with super elasticity and novel electronic properties.

Researchers Wrap Nanotubes Around Rubber Core Sparking a Creation That May Lead to Artificial Muscles, Sensors

An international research team based at The University of Texas at Dallas has made electrically conducting fibers that can be reversibly stretched to over 14 times their initial length and whose electrical conductivity increases 200-fold when stretched.

The research team is using the new fibers to make artificial muscles, as well as capacitors whose energy storage capacity increases about tenfold when the fibers are stretched. Fibers and cables derived from the invention might one day be used as interconnects for super-elastic electronic circuits; robots and exoskeletons having great reach; morphing aircraft; giant-range strain sensors; failure-free pacemaker leads; and super-stretchy charger cords for electronic devices.

In a study published in the July 24 issue of the journalScience, the scientists describe how they constructed the fibers by wrapping lighter-than-air, electrically conductive sheets of tiny carbon nanotubes to form a jelly-roll-like sheath around a long rubber core.

The new fibers differ from conventional materials in several ways. For example, when conventional fibers are stretched, the resulting increase in length and decrease in cross-sectional area restricts the flow of electrons through the material. But even a “giant” stretch of the new conducting sheath-core fibers causes little change in their electrical resistance, said Dr. Ray Baughman, senior author of the paper and director of the Alan G. MacDiarmid NanoTech Institute at UT Dallas.

One key to the performance of the new conducting elastic fibers is the introduction of buckling into the carbon nanotube sheets. Because the rubber core is stretched along its length as the sheets are being wrapped around it, when the wrapped rubber relaxes, the carbon nanofibers form a complex buckled structure, which allows for repeated stretching of the fiber.

“Think of the buckling that occurs when an accordion is compressed, which makes the inelastic material of the accordion stretchable,” said Baughman, the Robert A. Welch Distinguished Chair in Chemistry at UT Dallas.

“We make the inelastic carbon nanotube sheaths of our sheath-core fibers super stretchable by modulating large buckles with small buckles, so that the elongation of both buckle types can contribute to elasticity. These amazing fibers maintain the same electrical resistance, even when stretched by giant amounts, because electrons can travel over such a hierarchically buckled sheath as easily as they can traverse a straight sheath.”

Dr. Zunfeng Liu, lead author of the study and a research associate in the NanoTech Institute, said the structure of the sheath-core fibers “has further interesting and important complexity.” Buckles form not only along the fiber’s length, but also around its circumference.

“Shrinking the fiber’s circumference during fiber stretch causes this second type of reversible hierarchical buckling around its circumference, even as the buckling in the fiber direction temporarily disappears,” Liu said. “This novel combination of buckling in two dimensions avoids misalignment of nanotube and rubber core directions, enabling the electrical resistance of the sheath-core fiber to be insensitive to stretch.”

By adding a thin overcoat of rubber to the sheath-core fibers and then another carbon nanotube sheath, the researchers made strain sensors and artificial muscles in which the buckled nanotube sheaths serve as electrodes and the thin rubber layer is a dielectric, resulting in a fiber capacitor. These fiber capacitors exhibited a capacitance change of 860 percent when the fiber was stretched 950 percent.

“No presently available material-based strain sensor can operate over nearly as large a strain range,” Liu said.

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