First Fully-functional Radio From A Single Carbon Nanotube Created

Source:

http://www.sciencedaily.com/releases/2007/10/071031135307.htm

ScienceDaily (Oct. 31, 2007) — Make way for the real nanopod and make room in the Guinness World Records. A team of researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley have created the first fully functional radio from a single carbon nanotube, which makes it by several orders of magnitude the smallest radio ever made.
Wielding a single carbon nanotube 10,000 times smaller than a human hair, this is definitely the smallest radio yet. The nanotube vibrates at radio frequencies to receive the signal, then acts as both amplifier and demodulator. With only a battery and sensitive earphones, it can pick up AM or FM. With such a small receiver or transmitter, you could put a tracking collar on a bacterium.
“A single carbon nanotube molecule serves simultaneously as all essential components of a radio — antenna, tunable band-pass filter, amplifier, and demodulator,” said physicist Alex Zettl, who led the invention of the nanotube radio. “Using carrier waves in the commercially relevant 40-400 MHz range and both frequency and amplitude modulation (FM and AM), we were able to demonstrate successful music and voice reception.”
Given that the nanotube radio essentially assembles itself and can be easily tuned to a desired frequency band after fabrication, Zettl believes that nanoradios will be relatively easy to mass-produce. Potential applications, in addition to incredibly tiny radio receivers, include a new generation of wireless communication devices and monitors. Nanotube radio technology could prove especially valuable for biological and medical applications.
“The entire radio would easily fit inside a living cell, and this small size allows it to safely interact with biological systems,” Zettl said. “One can envision interfaces with brain or muscle functions, or radio-controlled devices moving through the bloodstream.”
It is also possible that the nanotube radio could be implanted in the inner ear as an entirely new and discrete way of transmitting information, or as a radically new method of correcting impaired hearing.
Zettl holds joint appointments with Berkeley Lab's Materials Sciences Division (MSD) and the UC Berkeley Physics Department where he is the director of the Center of Integrated Nanomechanical Systems. In recent years, he and his research group have created an astonishing array of devices out of carbon nanotubes - hollow tubular macromolecules only a few nanometers (billionths of a meter) in diameter and typically less than a micron in length – including sensors, diodes and even a motor. The nanotube radio, however, is the first that – literally – rocks!
“When I was a young kid, I got a transistor radio as a gift and it was the greatest thing I could imagine - music coming from a box I could hold in my hand!” Zettl said. “When we first played our nanoradio, I was just as excited as I was when I first turned on that transistor radio as a kid.”
The carbon nanotube radio consists of an individual carbon nanotube mounted to an electrode in close proximity to a counter-electrode, with a DC voltage source, such as from a battery or a solar cell array, connected to the electrodes for power. The applied DC bias creates a negative electrical charge on the tip of the nanotube, sensitizing it to oscillating electric fields. Both the electrodes and nanotube are contained in vacuum, in a geometrical configuration similar to that of a conventional vacuum tube.
Kenneth Jensen, a graduate student in Zettl’s research group, did the actual design and construction of the radio.
“We started out by making an exceptionally sensitive force sensor,” Jensen said.“Nanotubes are like tiny cat whiskers.Small forces, on the order of attonewtons, cause them to deflect a significant amount.By detecting this deflection, you can infer what force was acting on the nanotube. This incredible sensitivity becomes even greater at the nanotube’s flexural resonance frequency, which falls within the frequencies of radio broadcasts, cell phones and GPS broadcasting. Because of this high resonance frequency, Alex (Zettl) suggested that nanotubes could be used to make a radio.”
Although it has the same essential components, the nanotube radio does not work like a conventional radio. Rather than the entirely electrical operation of a conventional radio, the nanotube radio is in part a mechanical operation, with the nanotube itself serving as both antenna and tuner.
Incoming radio waves interact with the nanotube’s electrically charged tip, causing the nanotube to vibrate. These vibrations are only significant when the frequency of the incoming wave coincides with the nanotube’s flexural resonance frequency, which, like a conventional radio, can be tuned during operation to receive only a pre-selected segment, or channel, of the electromagnetic spectrum.
Amplification and demodulation properties arise from the needle-point geometry of carbon nanotubes, which gives them unique field emission properties. By concentrating the electric field of the DC bias voltage applied across the electrodes, the nanotube radio produces a field-emission current that is sensitive to the nanotube’s mechanical vibrations. Since the field-emission current is generated by the external power source, amplification of the radio signal is possible. Furthermore, since field emission is a non-linear process, it also acts to demodulate an AM or FM radio signal, just like the diode used in traditional radios.
“What we see then is that all four essential components of a radio receiver are compactly and efficiently implemented within the vibrating and field-emitting carbon nanotube,” said Zettl. “This is a totally different approach to making a radio - the exploitation of electro-mechanical movement for multiple functions. In other words, our nanotube radio is a true NEMS (nano-electro-mechanical system) device.”
Because carbon nanotubes are so much smaller than the wavelengths of visible light, they cannot be viewed with even the highest powered optical microscope. Therefore, to observe the critical mechanical motionof their nanotube radio, Zettl and his research team, which in addition to Jensen, also included post-doc Jeff Weldon and graduate student Henry Garcia, mounted their nanotube radio inside a high resolution transmission electron microscope (TEM). A sine-wave carrier radio signal was launched from a nearby transmitting antenna and when the frequencies of the transmitted carrier wave matched the nanotube resonance frequency, radio reception became possible.
“To correlate the mechanical motions of the nanotube to an actual radio receiver operation, we launched an FM radio transmission of the song Good Vibrations by the Beach Boys,” said Zettl. “After being received, filtered, amplified, and demodulated all by the nanotube radio, the emerging signal was further amplified by a current preamplifier, sent to an audio loudspeaker and recorded. The nanotube radio faithfully reproduced the audio signal, and the song was easily recognizable by ear.”
When the researchers deliberately detuned the nanotube radio from the carrier frequency, mechanical vibrations faded and radio reception was lost. A “lock” on a given radio transmission channel could be maintained for many minutes at a time, and it was not necessary to operate the nanotube radio inside a TEM. Using a slightly different configuration, the researchers successfully transmitted and received signals across a distance of several meters.
“The integration of all the electronic components of a radio happened naturally in the nanotube itself,” said Jensen. “Within a few hours of figuring out that our force sensor was in fact a radio, we were playing music!”
Added Zettl, “Our nanotube radio is sophisticated and elegant in the physics of its operation, but sheer simplicity in technical design. Everything about it works perfectly, without additional patches or tricks.”
Berkeley Lab’s Technology Transfer Department is now seeking industrial partners to further develop and commercialize this technology.
A paper on this work is now on-line at the Nano Letters Website. It will also be published in the November 2007 print edition of Nano Letters. The paper is entitled “Nanotube Radio” and the co-authors are Zettl, Jensen, Weldon and Garcia. In that same print edition, there appears a paper by Peter Burke and Chris Rutherglen of UC Irvine, reporting on the use of a carbon nanotube as a demodulator.
The nanotube radio research was supported by the U.S. Department of Energy and by the National Science Foundation within the Center of Integrated Nanomechanical Systems.
Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.
Adapted from materials provided by DOE/Lawrence Berkeley National Laboratory.

Fausto Intilla
www.oloscience.com

You Thought Your Nano Was Small: Nano-sized Detector Turns Radio Waves Into Music

Source:

http://www.sciencedaily.com/releases/2007/10/071017131851.htm

ScienceDaily (Oct. 19, 2007) — Researchers in California report development of the world's first working radio system that receives radio waves wirelessly and converts them to sound signals through a nano-sized detector made of carbon nanotubes.
The "carbon nanotube radio" device is thousands of times smaller than the diameter of a human hair. The development marks an important step in the evolution of nano-electronics and could lead to the production of the world's smallest radio, the scientists say.
Peter Burke and Chris Rutherglen developed a carbon nanotube "demodulator" that is capable of translating AM radio waves into sound. In a laboratory demonstration, the researchers incorporated the detector into a complete radio system and used it to successfully transmit classical music wirelessly from an iPod to a speaker several feet away from the music player.
Although other researchers have developed nano-sized radio wave detectors in the past, the current study marks the first time that a nano-sized detector has been demonstrated in an actual working radio system, the scientists say. The study demonstrates the feasibility of making other radio components at the nanoscale in the future and may eventually lead to a "truly integrated nanoscale wireless communications system," they say. Such a device could have numerous industrial, commercial, medical and other applications.
Their findings appeared online October 17 and are scheduled for publication in the Nov. 14 print edition of ACS' Nano Letters.
Adapted from materials provided by American Chemical Society.

Fausto Intilla
www.oloscience.com

Novel Gate Dielectric Materials: Perfection Is Not Enough

Source:

http://www.sciencedaily.com/releases/2007/10/071016131407.htm

Science Daily — For the first time theoretical modeling has provided a glimpse into how promising dielectric materials are able to trap charges, something which may affect the performance of advanced electronic devices. This is revealed in a paper published in Physical Review Letters by researchers at the London Centre for Nanotechnology and SEMATECH, a company in Austin, Texas.

Through the constant quest for miniaturization, transistors and all their components continue to decrease in size. A similar reduction has resulted in the thickness of a component material known as the gate dielectric -- typically a thin layer of silicon dioxide, which has now been in use for decades. Unfortunately, as the thickness of the gate dielectric decreases, silicon dioxide begins to leak current, leading to unwieldy power consumption and reduced reliability. Scientists hope that this material can be replaced with others, known as high-dielectric constant (or high-k) dielectrics, which mitigate the leakage effects at these tiny scales.
Metal oxides with high-k have attracted tremendous interest due to their application as novel materials in the latest generation of devices. The impetus for their practical introduction would be further helped if their ability to capture and trap charges and subsequent impact on instability of device performance was better understood. It has long been believed that these charge-trapping properties originate from structural imperfections in materials themselves.
However, as is theoretically demonstrated in this publication, even if the structure of the high k dielectric material is perfect, the charges (either electrons or the absence of electrons -- known as holes) may experience 'self trapping'. They do so by forming polarons -- a polarizing interaction of an electron or hole with the perfect surrounding lattice. Professor Alexander Shluger of the London Centre for Nanotechnology and the Department of Physics & Astronomy at UCL says: "This creates an energy well which traps the charge, just like a deformation of a thin rubber film traps a billiard ball."
The resulting prediction is that at low temperatures electrons and holes in these materials can move by hopping between trapping sites rather than propagating more conventionally as a wave. This can have important practical implications for the materials' electrical properties. In summary, this new understanding of the polaron formation properties of the transition metal oxides may open the way to suppressing undesirable characteristics in these materials.
The article "Theoretical Prediction of Intrinsic Self-Trapping of Electrons and Holes in Monoclinic HfO2", authored by D. Muñoz Ramo, A. L. Shluger, J. L. Gavartin, and G. Bersuker was published in Physical Review Letters volume 99 issue 15, page 155504, on the 12 October 2007
The work at the London Centre for Nanotechnology and UCL Department of Physics & Astronomy was funded by the EPSRC. Access to computer time on the HPCx facility was awarded to the Materials Chemistry Consortium with funding from the EPSRC.
Note: This story has been adapted from material provided by University College London.

Fausto Intilla

www.oloscience.com

New Quantum Dot Transistor Counts Individual Photons

Source:

http://www.sciencedaily.com/releases/2007/10/071011180823.htm

Science Daily — A transistor containing quantum dots that can count individual photons (the smallest particles of light) has been designed and demonstrated at the National Institute of Standards and Technology (NIST). The semiconductor device could be integrated easily into electronics and may be able to operate at higher temperatures than other single-photon detectors--practical advantages for applications such as quantum key distribution (QKD) for "unbreakable" encryption using single photons.

The NIST device, described in a new paper,* can accurately count 1, 2 or 3 photons at least 83 percent of the time. It is the first transistor-based detector to count numbers of photons; most other types of single-photon detectors simply "click" in response to any small number of photons. (See table for a comparison of various types of single-photon detectors used at NIST.)
Counting requires a linear, stepwise response and low-noise operation. This capability is essential for advanced forms of precision optical metrology--a focus at NIST--and could be used both to detect photons and to evaluate single-photon sources for QKD. The new device also has the potential to be cooled electronically, at much higher temperatures than typical cryogenic photon detectors.
Dubbed QDOGFET, the new detector contains about 1,000 quantum dots, nanoscale clusters of semiconductors with unusual electronic properties. The NIST dots are custom-made to have the lowest energy of any component in the detector, like the bottom of a drain. A voltage applied to the transistor produces an internal current, or channel. Photons enter the device and their energy is transferred to electrons in a semiconductor "absorbing layer," separating the electrons from the "holes" they formerly occupied.
As each photon is absorbed, a positively charged hole is trapped by the quantum dot drain, while the corresponding electron is swept into the channel. The amount of current flowing in the channel depends on the number of holes trapped by quantum dots. By measuring the channel response, scientists can count the detected photons. NIST measurements show that, on average, each trapped hole boosts the channel current by about one-fifth of a nanoampere. The detector has an internal quantum efficiency (percentage of absorbed photons that result in trapped holes) of 68 ± 18 percent, a record high for this type of photon detector.
The QDOGFET currently detects single photons at wavelengths of about 800 nanometers. By using different semiconductor materials, NIST researchers hope to make detectors that respond to the longer near-infrared wavelengths used in telecommunications. In addition, researchers hope to boost the external quantum efficiency (percentage of photons hitting the detector that are actually detected), now below 10 percent, and operate the device at faster speeds.
The research is supported in part by the Disruptive Technology Office. The authors include one from Los Alamos National Laboratory and one from Heriot-Watt University, Edinburgh, UK.
* E.J. Gansen, M.A. Rowe, M.B. Greene, D. Rosenberg, T.E. Harvey, M.Y. Su, R.H. Hadfield, S.W. Nam and R.P. Mirin. Photon-number-discriminating detection using a quantum dot, optically gated, field-effect transistor. Nature Photonics. 1, 585 - 588 (2007). Published on-line Oct. 1, 2007.
Note: This story has been adapted from material provided by National Institute of Standards and Technology.

Fausto Intilla

www.oloscience.com

Robotic System On Space Station Improved

Source:

http://www.sciencedaily.com/releases/2007/10/071012084124.htm

Science Daily — Software for a robotic extension of existing NASA technology for remote operations on the International Space Station has been shown to improve astronauts' performance on high-precision tasks. Using graphical overlay information, researchers were able to achieve significant results in efficiency and accuracy. The new technology can be added to existing flight hardware.

Researchers from NASA and Lockheed Martin have successfully tested software the robotic extension device.
James C. Maida, Charles K. Bowen, and John Pace developed the method for use with the Special Purpose Dexterous Manipulator, which works in conjunction with the current Space Station Remote Manipulator System (SSRMS).
Robotic devices on the ISS make it possible for astronauts to perform tasks without leaving the vehicle. Manipulating these devices is challenging, particularly in bright sunlight and deep darkness. Maida and colleagues employed augmented reality techniques to create a graphical informational overlay that can be used in simulations of robotic installation tasks to improve operator performance.
The installation task requires intense concentration by the astronaut to align an external orbital replacement unit (ORU) within ¼ inch and ½ degree at its installation point. The task is accomplished by viewing the scene of the installation through a camera and manipulating robotic arms. The researchers used enhanced live video with dynamic overlay information superimposed on features in the operators' field of view to guide them regarding the direction of motion of the robotic arm, the type of motion, and the correct position for installation.
Twelve highly skilled robotics operators were tested on four installation tasks under conditions of dynamic sunlight and very dark nights with and without the overlay. In all cases, accuracy and efficiency improved significantly when using the new overlay system, and all 12 operators found the overlay information extremely helpful in performing the ORU alignment operation. Time to complete the task was also reduced.
The researchers conclude that because the graphics are relatively simple and the computational requirements are low, the overlay system could be implemented on existing flight hardware used on the space shuttle and the ISS.
They presented their research paper, “Improving Robotic Operator Performance Using Augmented Reality,” at the Human Factors and Ergonomics Society 51st Annual Meeting on October 3 in Baltimore, Maryland.
Note: This story has been adapted from material provided by Human Factors and Ergonomics Society.

Fausto Intilla

www.oloscience.com

New Prosthetic Devices Will Convert Brain Signals Into Action

Source:

www.sciencedaily.com/releases/2007/10/071003130747.htm

Science Daily — MIT researchers have developed a new algorithm to help create prosthetic devices that convert brain signals into action in patients who have been paralyzed or had limbs amputated.

The technique, described in the October edition of the Journal of Neurophysiology, unifies seemingly disparate approaches taken by experimental groups that prototype these neural prosthetic devices in animals or humans.
"The work represents an important advance in our understanding of how to construct algorithms in neural prosthetic devices for people who cannot move to act or speak," said Lakshminarayan "Ram" Srinivasan, lead author of the paper.
Srinivasan, currently a postdoctoral researcher at the Center for Nervous System Repair at Massachusetts General Hospital and a medical student in the Harvard-MIT Division of Health Sciences and Technology, began working on the algorithm while a graduate student in MIT's Department of Electrical Engineering and Computer Science.
Trauma and disease can lead to paralysis or amputation, reducing the ability to move or talk despite the capacity to think and form intentions. In spinal cord injuries, strokes, and diseases such as amyotrophic lateral sclerosis (Lou Gehrig's disease), the neurons that carry commands from the brain to muscle can be injured. In amputation, both nerves and muscle are lost.
Neural prosthetic devices represent an engineer's approach to treating paralysis and amputation. Here, electronics are used to monitor the neural signals that reflect an individual's intentions for the prosthesis or computer they are trying to use. Algorithms form the link between neural signals that are recorded and the user's intentions that are decoded to drive the prosthetic device.
Over the past decade, efforts at prototyping these devices have divided along various boundaries related to brain regions, recording modalities, and applications. The MIT technique provides a common framework that underlies all these various efforts.
The research uses a method called graphical models that has been widely applied to problems in computer science like speech-to-text or automated video analysis. The graphical models used by the MIT team are diagrams composed of circles and arrows that represent how neural activity results from a person's intentions for the prosthetic device they are using.
The diagrams represent the mathematical relationship between the person's intentions and the neural manifestation of that intention, whether the intention is measured by an electroencephalography (EEG), intracranial electrode arrays or optical imaging. These signals could come from a number of brain regions, including cortical or subcortical structures.
Until now, researchers working on brain prosthetics have used different algorithms depending on what method they were using to measure brain activity. The new model is applicable no matter what measurement technique is used, according to Srinivasan. "We don't need to reinvent a new paradigm for each modality or brain region," he said.
Srinivasan is quick to underscore that many challenges remain in designing neural prosthetic algorithms before they are available for people to use. While the algorithm is unifying, it is not universal: the algorithm consolidates multiple avenues of development in prostheses, but it isn't the final and only approach these researchers expect to see in the years to come. Moreover, energy efficiency and robustness are key considerations for any portable, implantible bio-electronic device.
Through a better quantitative understanding of how the brain normally controls movement and the mechanisms of disease, he hopes these devices could one day allow for a level of dexterity depicted in movies, such as actor Will Smith's mechanical arm in the movie, "I, Robot."
The gap between existing prototypes and that final goal is wide. Translating an algorithm into a fully functioning clinical device will require a great deal of work, but also represents an intriguing road of scientific and engineering development for the years to come.
Other authors on the paper are Uri Eden Ph.D. , assistant professor in mathematics and statistics at Boston University; Sanjoy Mitter, professor in EECS and MIT's Engineering Systems Division; and Emery Brown, professor in brain and cognitive sciences, HST, and anesthesia and critical care at Massachusetts General Hospital.
This work was sponsored by the National Institutes of Health and the National Science Foundation.
Note: This story has been adapted from material provided by Massachusetts Institute of Technology.

Fausto Intilla

www.oloscience.com

Warped Fingerprints Identified At Warp Speed

Source:

http://www.sciencedaily.com/releases/2007/10/071001102348.htm

Science Daily — Researchers at the University of Warwick have devised a means of identifying partial, distorted, scratched, smudged, or otherwise warped fingerprints in just a few seconds.

Previous techniques have tried to identify a few key features on a finger print and laboriously match them against a database of templates. The University of Warwick researchers consider the entire detailed pattern of each print and transform the topological pattern into a standard co-ordinate system.
This allows the researchers to "unwarp" any finger print that has been distorted by smudging, uneven pressure, or other distortion and create a clear digital representation of the fingerprint that can then be mapped on to an "image space" of all other finger prints held on a database.
This unwarping is so effective that it also for the first time allows comparison of the position of individual sweat pores on finger print. This has not previously been possible as the hundreds of pores on an individual finger are so densely packed that the slightest distortion prevented analysts from using them to differentiate finger prints.
The "unwarping" of distorted, damaged or partial prints is not the only benefit of the new technology. The system created by the Warwick researchers is also able to give almost instantaneous results. Instead of laboriously comparing a print against each entry in a database any new print scanned by the system is unwarped and over laid onto a virtual "image space" that includes all the fingerprints available to the database. It does not matter whether it’s a thousand or a million fingerprints in the database the result comes back in seconds.
The University of Warwick researchers have set up a spin out company "Warwick Warp" to take the technology to market. This summer they took part in a 3 day exhibition at the London Science Museum to test their technology. Dr Li Wang, Chief Technology Officer at Warwick Warp said:
"We tested our system on nearly 500 visitors from all over the world and achieved 100% accuracy. Many of the visitors were children and children's fingerprints are particularly challenging as they generally contain finer features on a smaller area than adult fingers. Children often tend to twist their finger when placing the finger on the scanner, creating an elastic deformation which provides a great testing ground for our technology. "
Dr Li Wang also said: "Our technology also provides high speed and more importantly, our system’s accuracy and speed doesn't degrade when the size of database increases."
The researchers are exploring a number of commercial opportunities for their new technology including commercial access control systems, financial transaction authorization systems and possibly even ID cards passports or border control systems and are now seeking venture capital to assist such commercial developments.
Note: This story has been adapted from material provided by University of Warwick.

Fausto Intilla

www.oloscience.com

Magnetic Properties Of Extemely Thin Films Explored

Source:

http://www.sciencedaily.com/releases/2007/09/070928104301.htm

Science Daily — Materials researchers at the National Institute of Standards and Technology, together with colleagues from IBM and the Massachusetts Institute of Technology, have pushed the measurement of thin films to the edge--literally--to produce the first data on how the edges of metallic thin films contribute to their magnetic properties. Their results may impact the design of future nanoscale electronics.

Ferromagnetic thin films of metallic materials--ranging in thickness from fractions of a nanometer to several micrometers--are layered in patterns on a substrate (such as silicon) during the manufacture of many microelectronic devices that use magnetic properties, such as computer hard drives.
While methods for measuring the magnetic properties of ferromagnetic thin films have existed for some time, there currently is no way to define those properties for the edges of the film. On a relatively large-scale device, this doesn't matter much.
However, as microelectronic components get smaller and smaller, the edge becomes a bigger and bigger fraction of the surface, eventually becoming the thin film's dominant surface and the driver of its magnetic character. (Shrink a disk by half and the top surface area is reduced by a factor of four while the length of the edge is only halved.)
A research team from NIST, IBM and MIT recently demonstrated a spectroscopic technique for measuring the magnetic properties of the edges of nickel-iron alloy thin films patterned in an array of parallel nanowires (called "stripes") atop a silicon disk.
The researchers beamed microwaves of different frequencies over the stripes and measured the magnetic resonances that resulted. Because a thin film's edge resonates differently from its center, the researchers were able to determine which data--and subsequently, which magnetic behaviors--were attributable to the edge.
In its first trials, the new technique has been used to measure how the magnetic properties of the thin film edge are affected by the thickness of the film and the processing conditions during the stripe patterning. Data gained from the study of stripes with widths of 250 to 1,000 nanometers will be used to predict the behavior of similar structures at the nanoscale level (100 nanometers or less).
Note: This story has been adapted from material provided by National Institute of Standards and Technology.

Fausto Intilla

www.oloscience.com

Nanotube Forests Grown On Silicon Chips For Future Computers, Electronics

Source:

http://www.sciencedaily.com/releases/2007/10/071001152247.htm

Science Daily — Engineers have shown how to grow forests of tiny cylinders called carbon nanotubes onto the surfaces of computer chips to enhance the flow of heat at a critical point where the chips connect to cooling devices called heat sinks.

The carpetlike growth of nanotubes has been shown to outperform conventional "thermal interface materials." Like those materials, the nanotube layer does not require elaborate clean-room environments, representing a possible low-cost manufacturing approach to keep future chips from overheating and reduce the size of cooling systems, said Placidus B. Amama, a postdoctoral research associate at the Birck Nanotechnology Center in Purdue's Discovery Park.
Researchers are trying to develop new types of thermal interface materials that conduct heat more efficiently than conventional materials, improving overall performance and helping to meet cooling needs of future chips that will produce more heat than current microprocessors. The materials, which are sandwiched between silicon chips and the metal heat sinks, fill gaps and irregularities between the chip and metal surfaces to enhance heat flow between the two.
The method developed by the Purdue researchers enables them to create a nanotube interface that conforms to a heat sink's uneven surface, conducting heat with less resistance than comparable interface materials currently in use by industry, said doctoral student Baratunde A. Cola.
Findings were detailed in a research paper that appeared in September's issue of the journal Nanotechnology. The paper was written by Amama; Cola; Timothy D. Sands, director of the Birck Nanotechnology Center and the Basil S. Turner Professor of Materials Engineering and Electrical and Computer Engineering; and Xianfan Xu and Timothy S. Fisher, both professors of mechanical engineering.
Better thermal interface materials are needed either to test computer chips in manufacturing or to keep chips cooler during operation in commercial products.
"In a personal computer, laptop and portable electronics, the better your thermal interface material, the smaller the heat sink and overall chip-cooling systems have to be," Cola said.
Heat sinks are structures that usually contain an array of fins to increase surface contact with the air and improve heat dissipation, and a fan often also is used to blow air over the devices to cool chips.
Conventional thermal interface materials include greases, waxes and a foil made of a metal called indium. All of these materials, however, have drawbacks. The greases don't last many cycles of repeatedly testing chips on the assembly line. The indium foil doesn't make good enough contact for optimum heat transfer, Fisher said.
The Purdue researchers created templates from branching molecules called dendrimers, forming these templates on a silicon surface. Then, metal catalyst particles that are needed to grow the nanotubes were deposited inside cavities between the dendrimer branches. Heat was then applied to the silicon chip, burning away the polymer and leaving behind only the metal catalyst particles.
The engineers then placed the catalyst particle-laden silicon inside a chamber and exposed it to methane gas. Microwave energy was applied to break down the methane, which contains carbon. The catalyst particles prompted the nanotubes to assemble from carbon originating in the methane, and the tubes then grew vertically from the surface of the silicon chip.
"The dendrimer is a vehicle to deliver the cargo of catalyst particles, making it possible for us to seed the carbon nanotube growth right on the substrate," Amama said. "We are able to control the particle size - what ultimately determines the diameters of the tubes - and we also have control over the density, or the thickness of this forest of nanotubes. The density, quality and diameter are key parameters in controlling the heat-transfer properties."
The catalyst particles are made of "transition metals," such as iron, cobalt, nickel or palladium. Because the catalyst particles are about 10 nanometers in diameter, they allow the formation of tubes of similar diameter.
The branching dendrites are tipped with molecules called amines, which act as handles to stick to the silicon surface.
"This is important because for heat-transfer applications, you want the nanotubes to be well-anchored," Amama said.
Researchers usually produce carbon nanotubes separately and then attach them to the silicon chips or mix them with a polymer and then apply them as a paste.
"Our direct growth approach, however, addresses the critical heat-flow path, which is between the chip surface and the nanotubes themselves," Fisher said. "Without this direct connection, the thermal performance suffers greatly."
Because the dendrimers have a uniform composition and structure, the researchers were able to control the distribution and density of catalyst particles.
The research team also has been able to control the number of "defect sites" in the lattice of carbon atoms making up the tubes, creating tubes that are more flexible. This increased flexibility causes the nanotube forests to conform to the surface of the heat sink, making for better contact and improved heat conduction.
"The tubes bend like toothbrush bristles, and they stick into the gaps and make a lot of real contact," Cola said.
The carbon nanotubes were grown using a technique called microwave plasma chemical vapor deposition, a relatively inexpensive method for manufacturing a thermal-interface material made of carbon nanotubes, Fisher said.
"The plasma deposition approach allows us great flexibility in controlling the growth environment and has enabled us to grow carbon nanotube arrays over a broad range of substrate temperatures," Fisher said.
The research has been funded by NASA through the Institute for Nanoelectronics and Computing, based at Purdue's Discovery Park. Cola also received support through a fellowship from Intel Corp. and Purdue.
Note: This story has been adapted from material provided by Purdue University.

Fausto Intilla

www.oloscience.com