KOOBE Nanotechnology Digest

Stretchy, High-Quality Conductors

Friday, August 15th, 2008 | Electronic Devices, Nanotechnology | No Comments

By adding carbon nanotubes to a stretchy polymer, researchers at the University of Tokyo made a conductive material that they used to connect organic transistors in a stretchable electronic circuit. The new material could be used to make displays, actuators, and simple computers that wrap around furniture, says Takao Someya, a professor of engineering at the University of Tokyo. The material could also lead to electronic skin for robots, he says, which could use pressure sensors to detect touch while accommodating the strain at the robots’ joints. Importantly, the process that the researchers developed for making long carbon nanotubes could work on the industrial scale.

A researcher stretches a mesh of transistors connected by elastic conductors that were made at the University of Tokyo. Credit: Science/AAAS

“The measured conductivity records the world’s highest value among soft materials,” says Someya. In a paper published last week in Science, Someya and his colleagues claim a conductivity of 57 siemens per centimeter, which is lower than that of copper, the metal normally used to connect transistors, but two orders of magnitude higher than that of previously reported polymer-carbon-nanotube composites. Someya says that the material is able to stretch up to about 134 percent of its original shape without significant damage.

Electronics that can bend and flex are already used in some applications, but they can’t be wrapped around irregular shapes, such as the human body or complex surfaces, says John Rogers, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign. Rogers, who recently demonstrated a spherical camera sensor using his own version of an elastic circuit, says that Someya’s approach is a creative addition to the science of stretchable electronic materials. “It’s a valuable contribution to an important, emerging field of technology,” he says.

To make the stretchable polymer conductive, Someya’s group combined a batch of millimeter-long, single-walled carbon nanotubes with an ionic liquid–a liquid containing charged molecules. The resulting black, paste-like substance was then slowly added to a liquid polymer mixture. This produced a gel-like substance that was poured into a cast and air-dried for 24 hours.

The benefit of adding the nanotubes to a polymer before it is cast, says Someya, is that the nanotubes, which make up about 20 percent of the weight of the total mixture, are more evenly distributed. And because each nanotube is about a millimeter in length, there’s a high likelihood that in aggregate they will form an extensive network that allows electrical charge to propagate reliably throughout the polymer.

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Drawing Circuits with Nano Pens

Friday, August 15th, 2008 | Nanotechnology | No Comments

The demand for ever faster, cheaper electronics is pushing the lithography-based manufacturing techniques standard in the semiconductor industry to their limits. Now researchers report a cheap, fast lithography technique that uses arrays of flexible polymer nano pens to precisely pattern millions of complex structures in parallel. The technique, which the researchers have used to create an integrated circuit (and lilliputian versions of the Olympics logo), can be employed to make lines whose sizes range from a few nanometers to millimeters thick.

These 15,000 gold replicas of the Olympics logo (top) were created using a new lithography technique that relies on large arrays of polymer nano pens writing in parallel. The bottom image shows the logos in close-up, and the inset further zooms in on the “e” in “Beijing,” showing that its bottom stroke is only 90 nanometers wide. The logo of the running man is 36 micrometers high. These images, taken with a scanning electron microscope, demonstrate the range in feature sizes that can be made with the technique. Credit: Science/AAAS

The technique, developed by Chad Mirkin, a chemist at Northwestern University and director of the International Institute for Nanotechnology, uses arrays of pyramid-shaped polymer pens whose tips are dipped in solutions of chemicals that may feature almost any molecule, including proteins and acids; the pens are then traced over a surface by a mechanical arm to create millions of structures in parallel. The width of the lines drawn by each pen can be carefully controlled by varying the force exerted on the flexible pen tips. Because Mirkin’s pens trace out designs programmed by computer software, they can quickly switch between complicated designs, making possible the creation of complex patterns whose features are very close together.

Mirkin has used the pens to pattern acid on a silicon wafer coated with gold; he then etched, based on the pattern, a gold integrated circuit. Polymer-pen lithography also shows promise for patterning biological molecules. Indeed, says Mirkin, the technique could work with almost any molecular “ink,” including proteins for capturing and studying cells. The arrays of polymer pens cost less than a dollar each to make.

Polymer-pen lithography is an improvement over dip-pen lithography, a technique that Mirkin has been developing since 1999. Dip-pen lithography uses arrays of sharp, stiff cantilevered probes–the same ones used for atomic force microscopy. Mirkin created a company, NanoInk, to commercialize the technology. But, he acknowledges, “its ultimate utility has been limited by problems with throughput, cost, and complexity.” The size of its molecular strokes has been restricted to a relatively narrow range, the cantilevers are prone to breaking, and the number of structures that can be made in parallel is limited.

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Study Details How Gold Nanocages ‘Cook’ Cancer Cells

Friday, August 15th, 2008 | Nanotechnology | No Comments

Platinum-based anticancer agents have a long history as proven therapeutic agents, but their toxicity and short lifetime in the body and the ability of tumors to develop resistance to these drugs limit the ultimate utility of these agents.

In an attempt to overcome these limitations, a multi-institutional research team comprising members from Stanford University, the Massachusetts Institute of Technology (MIT), and the University of Duisburg-Essen in Germany is using targeted carbon nanotubes as delivery agents for an inactive form of platinum that cancer cells themselves convert into a toxic anticancer agent.

Reporting its work in the Journal of the American Chemical Society, the research team headed by Stanford’s Hongjie Dai, Ph.D., a member of the Center for Cancer Nanotechnology Excellence Focused on Therapy Response, and Stephen Lippard, Ph.D., MIT, describes its development of methods to attach platinum-containing compounds firmly to the surface of carbon nanotubes to create what they call a “longboat delivery system” for the platinum warhead.

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Study reveals principles behind stability and electronic properties of gold nanoclusters

Tuesday, July 15th, 2008 | Nanotechnology | No Comments

A report published in the July 8 issue of the journal Proceedings of the National Academy of Sciences (PNAS) is the first to describe the principles behind the stability and electronic properties of tiny nanoclusters of metallic gold. The study, which confirms the “divide and protect” bonding structure, resulted from the work of researchers at four universities on two continents.

(a) Spacefilling and (b) ball-and-stick representations of 102-atom gold nanoparticle; (c, d) 79-atom gold core surface with 23-atom protective layer; (e) Close-up of protective layer units; and (f, g) 79-atom core. Image courtesy of Hannu Häkkinen

“While gold nanoparticles are being used by so many researchers – chemists, materials scientists and biomedical engineers – no one understood their molecular and electronic structures until now,” said Robert Whetten, a professor in the Georgia Institute of Technology’s School of Physics and School of Chemistry and Biochemistry. “This research opens a new window for nanoparticle chemistry.”

Gold and sulfur atoms tend to aggregate in specific numbers and highly symmetrical geometries. Sometimes these clusters are called “superatoms” because they can mimic the chemistry of single atoms of a completely different element.

Researchers commonly use gold nanoparticles because they are stable and exhibit distinct optical, electronic, electrochemical and bio-labeling properties. However, understanding the physicochemical properties of such clusters is a challenge, according to Whetten, because that requires knowledge of their atomic structures.

A significant advance came in late 2007 though, when Stanford University researchers reported the first-ever total structure determination of a 102-atom gold cluster. The X-ray structure study revealed that pairs of organic sulfur (”thiolate”) groups extracted gold atoms from the gold layer to form a linear thiolate-gold-thiolate bridge while interacting weakly with the metal surface below. These gold–thiolate complexes formed a sort of protective crust around the nanoparticles.

“This discovery contradicted what most chemists believed was going on – which was that the sulfur atom merely sat atop the uppermost gold layer, bound to three adjacent metal atoms,” said Whetten.

With the experimentally determined structural coordinates, an international team of researchers from Georgia Tech, Stanford University, the University of Jyväskylä in Finland and Chalmers University of Technology in Sweden set out to determine the electronic principles underlying the 102-atom gold compound and others like it. The team conducted large-scale electronic structure calculations in supercomputing centers in Espoo, Finland; Stockholm, Sweden; and Juelich, Germany.

The researchers found that the 102-atom gold cluster was a “superatom” with a core of 79 gold atoms arranged into a truncated decahedron: two pyramids with pentagonal bases joined together into a faceted shape, but with the pyramids’ tips chopped off. Around the core, 23 gold atoms formed an unusual pattern, joining the thiolates in shapes that resemble handles.

The results confirmed the “divide and protect” structure first predicted by team member Hannu Häkkinen, a professor at the University of Jyväskylä and former senior research scientist at Georgia Tech in the laboratory of Uzi Landman. Häkkinen and Henrik Grönbeck of the Chalmers University of Technology previously proposed that a cluster of 38-atom gold contained a central metallic core of 14 gold atoms and a protective layer of 24 gold atoms bound to sulfur.

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Nanowires may boost solar cell efficiency, engineers say

Thursday, May 15th, 2008 | Nanotechnology | No Comments

University of California, San Diego electrical engineers have created experimental solar cells spiked with nanowires that could lead to highly efficient thin-film solar cells of the future.

Scanning electron microscope (SEM) image of n-type InP nanowire growth on indium tin oxide (ITO) taken at a 45 degree tilt with scale bar of 500 nanometers. Credit: UC San Diego

Indium phosphide (InP) nanowires can serve as electron superhighways that carry electrons kicked loose by photons of light directly to the device’s electron-attracting electrode – and this scenario could boost thin-film solar cell efficiency, according to research recently published in NanoLetters.

The new design increases the number of electrons that make it from the light-absorbing polymer to an electrode. By reducing electron-hole recombination, the UC San Diego engineers have demonstrated a way to increases the efficiency with which sunlight can be converted to electricity in thin-film photovoltaics.

Including nanowires in the experimental solar cell increased the “forward bias current” – which is a measure of electrical current – by six to seven orders of magnitude as compared to their polymer-only control device, the engineers found.

The online journal NanoLetters published this new work on polymer/nanowire hybrid photovoltaics in February 2008.

“If you provide electrons with a defined pathway to the electrode, you can reduce some of the inefficiencies that currently plague thin-film solar cells made from polymer mixtures. More efficient transport of electrons and holes – collectively known as carriers – is critical for creating more efficient solar cells,” said Clint Novotny the first author of the NanoLetters paper, and a recent electrical engineering Ph.D. from UC San Diego’s Jacobs School of Engineering. Novotny is now working on solar technologies at BAE Systems.

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Held together by metal-metal bonds: a large ring containing 36 gold atoms

Thursday, May 15th, 2008 | Nanotechnology | No Comments

Chinese researchers have recently made a “golden crown” with a diameter of only a few nanometers. It is a large ring-shaped molecule containing 36 gold atoms. The lords of the ring, a team of researchers from the Universities of Beijing, Hong Kong, and Nanjing report their unusual compound in the journal Angewandte Chemie: the molecular ring structure is held together exclusively by gold–gold bonds and is thus the largest ring system made of gold atoms produced to date.

A giant, crown-like Au36 ring aggregate with continuous metal-metal contacts (see picture; gold(I) centers are in ball and stick representation) is formed by an Au(I)-Au(I) bonding interaction directed self-assembly. (c) Wiley-VCH 2008

Large molecular rings have fascinated chemists for over 40 years—ever since the discovery of crown ethers in 1967. The pioneers in this area, C. J. Pederson, J.-M. Lehn, and D. J. Cram received the Nobel Prize in Chemistry for their discovery in 1987.

In the meantime, large molecular ring systems have played an important role in the search for new functional materials and in nanotechnology. The synthesis of ring systems held together exclusively by metal–metal bonds has remained a challenge.

Small rings made of positively charged gold atoms have been know for some time, but only recently could the Chinese team make a ring containing 16 gold atoms. Now, the researchers, led by Shu-Yan Yu, Yi-Zhi Li, and Vivian Wing-Wah Yam, have introduced a new representative of this class of compounds, the biggest gold ring to date that is held together by means of gold–gold bonds: a ring system containing 36 univalent gold atoms.

The researchers started their synthesis with a ring system containing six gold atoms. Three of the gold atoms are linked into a triangle. Each of these gold atoms is attached to another gold atom that sticks out from the corner of the triangle. Three organic ligands are then bound to this flat double triangle to form a molecule that resembles a three-blade propeller. 

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Physicists show electrons can travel over 100 times faster in graphene than in silicon

Monday, March 24th, 2008 | Nanotechnology | No Comments

University of Maryland physicists have shown that in graphene the intrinsic limit to the mobility, a measure of how well a material conducts electricity, is higher than any other known material at room temperature. Graphene, a single-atom-thick sheet of graphite, is a new material which combines aspects of semiconductors and metals.

Their results, published online in the journal Nature Nanotechnology, indicate that graphene holds great promise for replacing conventional semiconductor materials such as silicon in applications ranging from high-speed computer chips to biochemical sensors.

A team of researchers led by physics professor Michael S. Fuhrer of the university’s Center for Nanophysics and Advanced Materials, and the Maryland NanoCenter said the findings are the first measurement of the effect of thermal vibrations on the conduction of electrons in graphene, and show that thermal vibrations have an extraordinarily small effect on the electrons in graphene.

In any material, the energy associated with the temperature of the material causes the atoms of the material to vibrate in place. As electrons travel through the material, they can bounce off these vibrating atoms, giving rise to electrical resistance. This electrical resistance is “intrinsic” to the material: it cannot be eliminated unless the material is cooled to absolute zero temperature, and hence sets the upper limit to how well a material can conduct electricity.

In graphene, the vibrating atoms at room temperature produce a resistivity of about 1.0 microOhm-cm (resistivity is a specific measure of resistance; the resistance of a piece material is its resistivity times its length and divided by its cross-sectional area). This is about 35 percent less than the resistivity of copper, the lowest resistivity material known at room temperature.

“Other extrinsic sources in today’s fairly dirty graphene samples add some extra resistivity to graphene,” explained Fuhrer, “so the overall resistivity isn’t quite as low as copper’s at room temperature yet. However, graphene has far fewer electrons than copper, so in graphene the electrical current is carried by only a few electrons moving much faster than the electrons in copper.”

In semiconductors, a different measure, mobility, is used to quantify how fast electrons move. The limit to mobility of electrons in graphene is set by thermal vibration of the atoms and is about 200,000 cm2/Vs at room temperature, compared to about 1,400 cm2/Vs in silicon, and 77,000 cm2/Vs in indium antimonide, the highest mobility conventional semiconductor known.

“Interestingly, in semiconducting carbon nanotubes, which may be thought of as graphene rolled into a cylinder, we’ve shown that the mobility at room temperature is over 100,000 cm2/Vs” said Fuhrer (T. Dürkop, S. A. Getty, Enrique Cobas, and M. S. Fuhrer, Nano Letters 4, 35 (2004)).

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Scientists First To Measure Force Required To Move Individual Atoms

Saturday, February 23rd, 2008 | Nanotechnology | No Comments

IBM scientists, in collaboration with the University of Regensburg in Germany, are the first ever to measure the force it takes to move individual atoms on a surface. This fundamental measurement provides important information for designing future atomic-scale devices: computer chips, miniaturized storage devices, and more.

Illustration of an Atomic Force Microscope (AFM) tip measuring the force it takes to move a cobalt atom on a crystalline surface. The ability to measure the exact force it takes to move individual atoms is one of the keys to designing and constructing the small structures that will enable future nanotechnologies. Credit: IBM

Some twenty years ago at IBM’s Almaden Research Center in San Jose, in a small lab packed with high-tech equipment in the hills of Silicon Valley, IBM Fellow Don Eigler achieved a landmark in mankind’s ability to build small structures. On September 29, 1989 he demonstrated the ability to manipulate individual atoms with atomic-scale precision, and went on to write I-B-M with individual Xenon atoms, an event likened to the Wright brothers’ first flight at Kitty Hawk.

Now, a new crop of researchers in that same lab – with help from the University of Regensburg –have taken the extraordinary step of measuring the tiny forces needed to manipulate the atoms. These findings will be published in the February 22 issue of Science magazine.

Understanding the force necessary to move specific atoms on specific surfaces is one of the keys to designing and constructing the small structures that will enable future nanotechnologies. The problem is akin to what scientists and engineers needed to learn about construction at macroscopic sizes many decades ago. For example, building a modern bridge would be impossible without first measuring the strength of different materials, understanding the relevant forces, and comprehending how everything interacts. In the nanotechnology realm, to make structures that you want to remain rigidly in place you would use strongly bonded (“sticky”) atoms while for groups of atoms that need to move you would use atoms held in place only by weak chemical bonds.

“This result provides fundamental information about atomic scale fabrication and could pave the way for new data storage and memory devices,” said Andreas Heinrich, lead scientist in the scanning tunneling microscopy lab at the IBM Almaden Research Center. “Our mission is to create the foundation for what could someday be called the IBM nanoconstruction company.”

In the paper, “The Force Needed to Move an Atom on a Surface,” the scientists show that the force required to move a cobalt atom over a smooth platinum surface is 210 piconewtons, while moving a cobalt atom over a copper surface takes only 17 piconewtons. To put this in perspective, the force required to lift a copper penny that weighs just three grams is nearly 30 billion piconewtons – 2 billion times greater than the force to move a single cobalt atom over a copper surface.

This knowledge will enable a deeper understanding of the atomic-scale processes at the heart of future nanotechnology endeavors, furthering progress toward nanoscale computing and medical devices. The well-known trend in computer hardware – the exponentially increasing number of ever-shrinking transistors that can be placed on an integrated circuit – is commonly known as Moore’s Law. Shrinking the transistors allows them to use less power while having higher speed and lower cost. One of the IT industry’s most pressing challenges is to find designs and manufacturing methods that will allow the industry to continue making these devices smaller and smaller.

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