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http://www.scienceda...40107072303.htm
Source: University Of California - Berkeley
Date: 2004-01-07

Researchers Develop First Integrated Silicon Circuit With Nanotube Transistors

Berkeley - The discovery of carbon nanotubes has heralded a new era of scientific discovery that includes the promise of ultra-sensitive bomb detectors and super-fast computer memory chips. But finding a way to incorporate nanomaterials into a working nanoelectronic system has been a frustratingly elusive achievement - until now. In an important milestone in the fields of nanosciences and nanoengineering, researchers at the University of California, Berkeley and Stanford University have created the first working, integrated silicon circuit that successfully incorporates carbon nanotubes in its design.

"Until our work, no group has publicly reported success in directly integrating nanotubes onto silicon circuits," said Jeffrey Bokor, UC Berkeley professor of electrical engineering and computer sciences and principal investigator of the project. "It is a critical first step in building the most advanced nanoelectronic products, in which we would want to put carbon nanotubes on top of a powerful silicon integrated circuit so that they can interface with an underlying information processing system."

Researchers say the development brings them a significant step closer to using carbon nanotubes for memory chips that can hold orders of magnitude more data than current silicon chips - 10,000 times greater, according to some estimates - or for sensors sensitive enough to detect traces of explosives or biochemical agents at the molecular level.
UC Berkeley engineers teamed up with chemists at Stanford to develop an integrated circuit that can dramatically speed the analysis of thousands of synthesized carbon nanotubes. The description of this work appears in the January 2004 issue of Nano Letters, a publication of the American Chemical Society.

"These results represent a dream come true," said Hongjie Dai, associate professor of chemistry at Stanford and co-principal investigator of the project. "This achievement opens up a vast number of possible applications in nanotechnology."

A carbon nanotube, which looks like rolled chicken wire when examined at the atomic level, is tens of thousands of times thinner than a human hair, yet remarkably strong. It has attractive electrical properties, which several research groups - including the one led by Dai at Stanford - have harnessed to create high performance transistors.

The road to creating the first nano-silicon hybrid circuit began as a solution to a practical research problem: How to refine the process of growing nanotubes so that they are created with predictable qualities.

Depending on the molecular structure specific to each carbon nanotube, it can either be metallic and capable of conducting electricity, or act like semiconductors, with conductivity that can be turned on and off. But the current synthesis process results in an unpredictable proportion of metallic and semiconducting nanotubes, leaving researchers uncertain as to how much of each type they'll get in any one batch.

Analyzing whether a batch yielded metallic or semiconducting nanotubes involved a labor-intensive processing of manually checking the electrical conductivity of each carbon nanotube.

To resolve this problem, the researchers set out to build a device that would automate the process of decoding thousands of carbon nanotubes on a silicon chip. Working with UC Berkeley's Microfabrication Laboratory, they created a chip with silicon metal oxide semiconductor (MOS) circuitry. The chip, dubbed the random access nanotube test chip, or RANT, contains a network of silicon wires and switches that form a circuit.

Researchers then proceeded to grow carbon nanotubes directly onto "islands" on the circuit platform that contained the necessary catalyst for nanotube synthesis. The extreme heat required to grow nanotubes would typically melt the circuitry of traditional semiconductors, but the researchers got around that problem by interconnecting the silicon transistors with molybdenum, a refractory metal that can withstand very high temperatures.

"We first envisioned a patterned growth of carbon nanotubes on silicon wafers five years ago, but it wasn't clear at that time whether that approach would work as an integrated nanotube-silicon hybrid circuit," said Dai. "It was the combined expertise in chemistry, materials science and electrical engineering that made this a reality."

The resulting chip contained thousands of carbon nanotubes connected to the circuit on a 1-square-centimeter silicon chip. By turning certain switches on and off, researchers were able to isolate the path that leads to an individual nanotube. Not only could researchers pinpoint which nanotube was responding to electrical current passing through the system, they could tell whether the conductivity could be turned on or off. If they were able to change the conductivity of the nanotube, they knew that it was a semiconductor and not metallic.

"The circuit is interconnected in such a way that only 22 control signals are needed in testing more than 2,000 nanotubes," said Yu-Chih Tseng, a UC Berkeley graduate student in electrical engineering and computer sciences and lead author of the paper. "The key is that this can all be done by a machine and computer. We succeeded in making a tool for nanotechnology researchers, and in the process, we demonstrated the broader proof of principle that nanotubes can be successfully integrated in a complex circuit."

Research such as this is an important component of the UC Berkeley-based Center for Information Technology Research in the Interest of Society, or CITRIS. The center includes a major emphasis on nanosciences and nanoengineering, and is funding the construction of a new nanofabrication laboratory on the UC Berkeley campus that would significantly enhance researchers' ability to conduct such fundamental and innovative work, said Ruzena Bajcsy, director of CITRIS.

Bokor cautions that the integrated circuit they have built is not a likely candidate for commercialization just yet. For one, the molybdenum they used to protect the circuit from heat damage is not a typical material used in the semiconductor industry because it is a high-resistance metal.

Nevertheless, the achievement opens the door for other promising research on nanotechnology devices, including those made of silicon nanowires and organic polymers, researchers said.

"Carbon nanotubes have fascinated many scientists and those interested in science ever since they were discovered," said Ali Javey, a graduate student in chemistry at Stanford and co-author of the paper. "This work takes us an important step forward by proving the compatibility of the nanotube synthesis process with modified silicon technology and leading the way to future nanotube-based commercial applications."

[Omnido]

This area of technology might be the hub of the wheel which brings us the necessary computing power needed to help design and faciliate the development of MNT.

[chubtoad]

http://www.scienceda...40330090701.htm
Source: Duke University
Date: 2004-03-30

Duke Chemists Describe New Kind Of 'Nanotube' Transistor

Duke University researchers exploring ways to build ultrasmall electronic devices out of atom-thick carbon cylinders have incorporated one of these "carbon nanotubes" into a new kind of field effect transistor. The Duke investigators also reported new insights into their previously published technique for growing nanotubes in straight structures as long as half an inch.

Field effect transistors, among the workhorse devices of microelectronics technology, are tiny switches in which the passage of electric current between a "source" and a "drain" is controlled by an electric field in a middle component called a "gate."

Carbon nanotubes -- so named because of their billionths-of-a-meter dimensions ("nano" means billionths) -- combine exceptional strength, minuscule size and flexible electronic properties. They can behave either like conducting metals or like semiconductors, depending on how carbon atoms are arranged on their walls. As a result, they offer great promise as components in electronic devices even smaller than those available today.

The Duke research group headed by Liu is among a number that have incorporated a semiconducting nanotube as a component in an experimental field effect transistor. The nanotube is grown on a surface of silicon dioxide with metal electrodes evaporated on the nanotube's surface serving as the device's electron source and drain. Meanwhile, a layer of silicon fabricated under the silicon dioxide serves as the transistor's gate, also called a "back gate."

However, other groups have found that this back gate of silicon, which is "doped" with other chemicals to fine-tune its electronic properties, is poorly coupled with the rest of the device. The result is excess power demand. "To turn the device from off to on, you need five to ten volts," Liu said in an interview.

To address this shortcoming, teams at two other universities have found they can reduce the power demand to between 0.3 and 0.5 volts by adding an additional gate made of a tiny droplet of salty water.

"That's an order of magnitude of difference," Liu said of what he termed a "water gate." But "the disadvantage is that water is a liquid. So we looked for a way of replacing this water droplet with something that has similar properties but is a solid."

In a new paper in the research journal Nanoletters, Liu, graduate students Chenguang Lu and Qiang Fu, and research associate Shaoming Huang describe substituting an electrically conducting polymer that has been developed for dry lithium battery technology.

This substitute compound, called lithium perchlorate/polyethylene oxide (PEO), "can achieve similarly good device performance and avoid the problem of using liquid in the device," the Duke authors wrote in their paper. This PEO "polymer gate" is placed directly over the carbon nanotube.

Liu's team found the polymer gate's electronic properties can also be more easily fine-tuned to control the direction of the electric current by doping the underlying nanotube with other small carbon-containing molecules.

Doping silicon-based semiconductors in that way requires fabricators to precisely incorporate chemicals into those materials' internal crystal structures. "For a nanotube, you just coat it on the surface, which is a lot easier," Liu said.

Also at the Anaheim meeting, Liu presented an update on research his group reported in the Journal of the American Chemical Society in April 2003 on growing straight and exceptionally long nanotubes that can be potentially cut into smaller lengths for splicing into electronic nanoarrays.

That 2003 journal report described how quick heating the emerging nanotubes in a continuously flowing feeding gas of carbon monoxide and hydrogen to a temperature hot enough to melt glass made the tubes grow in unusually long and true alignment. "We now have a much better understanding of why this fast heating technology performs differently," Liu said in an interview before his 2004 presentation.

In previous methods of using this chemical vapor deposition (CVD) process to grow nanotubes, the tubes extend along a surface of silicon dioxide. In the process, they encounter "physical resistance caused by the friction of bumping into other surface features," he explained. "That stops the growth of the nanotubes."

But quick-heating in the flowing gas makes the incipient nanotube lift up slightly above the surface as it begins to grow, he said. The growing nanotube follows the direction of the gas and stays slightly suspended, thus avoiding interacting with surface that is rough at molecular dimensions. "It's like flying a kite," he added.

[chubtoad]

http://www.scienceda...40421234637.htm

Stacked, Packed Nanowires Hold Triplexed Megadata

novel transistor architecture using molecular-scale nanowire memory cells holds the promise of unprecedently compact data storage.

Researchers at the University of Southern California and the NASA Ames Research Center have successfully tested a self-assembled molecular memory device they say has the potential of holding 40 Gigabits per square centimeter -- a far greater density than any achieved on silicon.

Furthermore, says Chongwu Zhou, an assistant professor in the USC Viterbi School department of electrical engineering, because of the self-assembly feature, such ultra dense memory devices can likely be cheaper than the silicon flash memories now widely used in digital cameras, "memory sticks" and other applications.

According to a recent paper by Zhou and his group in Applied Physics Letters describing the technology, the density is achieved by the nanoscale (one millionth of a millimeter) size of the building blocks used,

( Ten nanometers is 0.0000004 inch; an average bacterium is about 1000 nanometers long; the smallest known virus about 20 nanometers long).

The USC/Ames system is still more compact because each memory cell can hold not just one bit of data but three, by virtue of having 8 separate, stable identifiable electronic states.

The USC/Ames system is already quite stable, holding information up to 600 hours. "We believe further work can increase the stability still further," the scientist said.

The USC/Ames researchers synthesized nanowires of indium oxide (In2O3) 10 nanometers in diameter and about 2000 nanometers long, by a "laser ablation" process that first vaporizes an indium containing compound, and then precipitates the indium out in a catalyzed process in which the wires form spontaneously as the indium reacts with ambient oxygen.

The researchers then placed the nanowires on a thin layer of quartz, and activated them by simply submerging them in a solutions of redox materials — various were tested — which self-assembled a layer of coating onto the wires, creating transistors.

The resulting transistors could be placed not in one activated state, but three distinct ones, by using different voltages to stimulate them. "We repeated tens of cycles for the endurance test for each memory operation and found that all the levels were distinguishable in the tested cycles," the authors wrote in their APL paper.

In the same paper, they also noted that the assembly process — a cold one — "represents a significant departure from the channel hot electron injection commonly used for silicon flash memory," The paper claims that the USC/Ames process requires lower power and is inherently less likely to introduce defects that can cause errors in the device.

[chubtoad]

http://center.acs.or...43&categoryid=2

High-speed nanotube transistors could lead to better cell phones, faster computers

Scientists have demonstrated, for the first time, that transistors made from single-walled carbon nanotubes can operate at extremely fast microwave frequencies, opening up the potential for better cell phones and much faster computers, perhaps as much as 1,000 times faster.

The findings, reported in the April issue of Nano Letters, a peer-reviewed journal of the American Chemical Society, the world's largest scientific society, add to mounting enthusiasm about nanotechnology's revolutionary potential.

"Since the invention of nanotube transistors, there have been theoretical predictions that they can operate very fast," says Peter Burke, Ph.D., a professor of electrical engineering and computer science at the University of California, Irvine, and lead author of the paper. "Our work is the first to show that single-walled nanotube transistor devices can indeed function at very high speeds."

Burke and his colleagues built an electrical circuit with a carbon nanotube between two gold electrodes. When they varied the voltage, the circuit operated at a frequency of 2.6 gigahertz (GHz), which means electrical current could be switched on and off in about one billionth of a second. This is the first demonstration of a nanotube operating in the frequency range of microwaves — electromagnetic waves with faster frequencies than radio waves.

Although Burke's group demonstrated that nanotube transistors could work in the GHz range, he believes that much faster speeds are possible. "I estimate that the theoretical speed limit for these nanotube transistors should be terahertz [1 THz=1,000 GHz], which is about 1,000 times faster than modern computer speeds." His team is currently doing related research on the theoretical prediction of the cutoff frequency, or so-called speed limit, for these transistors.

Every transistor has a cutoff frequency, which is the maximum speed at which it can operate. For silicon, the cutoff is about 100 GHz, but current circuits typically operate at much slower speeds, according to Burke. For example, some of today's newest processor chips still operate below 5 GHz.

Nanotechnology is the science of the very small: a nanometer is one billionth of a meter, or about 1,000 times smaller than the width of a human hair. A nanotube is another form of carbon, like graphite or diamond, where the atoms are arranged like a rolled-up tube of chicken wire.

Electrons move without losing energy inside nanotubes, which makes them perfect candidates for connections in electrical devices. A semiconducting carbon nanotube can act as a transistor — the key component in all modern electronics — because it can be switched on and off.

High-speed nanotube transistors could be useful in a number of applications. "Theoretically, this can translate into very low noise microwave amplifiers that could increase the range in which cell phones operate," Burke says. A cell phone receives its radio signal at a very low strength, so a microwave amplifier is needed to boost the signal for further processing.

Nanotube transistors could also lead to very high quality microwave filters that can separate out many different phone conversations more efficiently than current filters, and at lower cost, according to Burke. "Right now, this one function requires a separate chip inside a cell phone," he says. If the filter could be integrated with the other processing parts, the entire radio system would be on one chip, saving power, space and cost.

This type of "integrated nanosystem" is a goal of Burke's research. "Ultimately, we would like more sophisticated circuits on a single chip," he says. "Our nanotube transistor is on a silicon substrate, but there are no active silicon devices." If all the transistors and electrical connections on a chip were made of nanotubes or nanowires, there would be no silicon parts to slow things down.

Burke expects to have a prototype transistor available within two years. "We still need to demonstrate operation at room temperature, which we are working on in my lab now. Also, we need to show that we can achieve amplification," he says. "But these are both achievable goals given one or two years of work."