Researchers map the physics of Tibetan singing bowls

Researchers have been investigating the connection between fifth century Himalayan instruments used in religious ceremonies and modern physics.

In a study published today, 1 July 2011, in IOP Publishing's journal Nonlinearity, researchers have captured high speed images of the dynamics of fluid-filled Tibetan bowls and quantified how droplets are propelled from the water's surface as the bowls are excited.

The first of five videos demonstrating the intriguing dynamics can be seen below:

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A Tibetan bowl, generally made from a bronze alloy containing copper, tin, zinc, iron, silver, gold and nickel, is a type of standing bell played by striking or rubbing its rim with a wooden or leather-wrapped mallet. This excitation causes the sides and rim of the bowl to vibrate, producing a rich sound.

The unique singing properties of Tibetan bowls were utilised as a way of investigating a liquid's interaction with solid materials ? a situation that arises in many engineering applications such as the wind-loading of bridges and buildings.

When a fluid-filled Tibetan bowl is rubbed, the slight changes in the bowl's shape disturb the surface at the water's edge, generating waves. Moreover, when these changes are sufficiently large, the waves break, leading to the ejection of droplets.

The new findings could benefit processes such as fuel injectors and perfume sprays where droplet generation plays an important role.

The high-speed videos allowed the researchers, from Université de Liège and the Massachusetts Institute of Technology, to quantify how the droplets were formed, ejected, accelerated, and bounced on the surface of the fluid.

A similar phenomenon exists when rubbing the edge of a wine glass, which inspired the design of the glass harmonica by Benjamin Franklin. However, the Tibetan singing bowl is easier to excite than the wine glass, since its resonant frequency is much smaller.

In order to generate the waves and resultant droplets, a loudspeaker was set up adjacent to the bowls, which emitted sound at specific frequencies. Once the sound hit the resonant frequency of the bowl?a sound wave vibrating in phase with the natural vibration of the bowl?the waves would be generated.

A high speed camera was used to capture images of the droplets, from which measurements could be taken.

Senior author Professor John Bush said, "Although our system represents an example of fluid-solid interactions, it was motivated more by curiosity than engineering applications.

"We are satisfied with the results of our investigation, which we feel has elucidated the basic physics of the system. Nevertheless, one might find further surprises by changing the bowl or fluid properties."

More information: The published version of the paper "Tibetan singing bowls" 2011 Nonlinearity 24 R51-66 will be freely available online from 1 July 2011. It will be available at iopscience.org/non/24/8/R01

Provided by Institute of Physics (news : web)

Researchers record two-state dynamics in glassy silicon

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University of Illinois chemistry professor Martin Gruebele led a group that directly observed two-state dynamics in glassy silicon for the first time. Credit: L. Brian Stauffer

Using high-resolution imaging technology, University of Illinois researchers have answered a question that had confounded semiconductor researchers: Is amorphous silicon a glass? The answer? Yes ? until hydrogen is added.

Led by chemistry professor Martin Gruebele, the group published its results in the journal Physical Review Letters.

Amorphous silicon (a-Si) is a semiconductor popular for many device applications because it is inexpensive and can be created in a flexible thin film, unlike the rigid, brittle crystalline form of silicon. But the material has its own unusual qualities: It seems to have some characteristics of glass, but cannot be made the way other glasses are.

Most glasses are made by rapidly cooling a melted material so that it hardens in a random structure. But cooling liquid silicon simply results in an orderly crystal structure. Several methods exist for producing a-Si from crystalline silicon, including bombarding a crystal surface so that atoms fly off and deposit on another surface in a random position.

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A time-lapse video of an amorphous silicon surface. The lumps are clusters of about five atoms of silicon. The "hopping" motion of the lumps shows that a-Si is a glass. Credit: Martin Gruebele

To settle the debate on the nature of a-Si, Gruebele's group, collaborating with electrical and computer engineering professor Joseph Lyding's group at the Beckman Institute for Advanced Science and Technology, used a scanning tunneling microscope to take sub nanometer-resolution images of a-Si surfaces, stringing them together to make a time-lapse video.

The video shows a lumpy, irregular surface; each lump is a cluster about five silicon atoms in diameter. Suddenly, between frames, one bump seems to jump to an adjoining space. Soon, another lump nearby shifts neatly to the right. Although few of the clusters move, the action is obvious.

Such cluster "hopping" between two positions is known as two-state dynamics, a signature property of glass. In a glass, the atoms or molecules are randomly positioned or oriented, much the way they are in a liquid or gas. But while atoms have much more freedom of motion to diffuse through a liquid or gas, in a glass the molecules or atom clusters are stuck most of the time in the solid. Instead, a cluster usually has only two adjoining places that it can ferry between.

"This is the first time that this type of two-state hopping has been imaged in a-Si," Gruebele said. "It's been predicted by theory and people have inferred it indirectly from other measurements, but this is the first time we're been able to visualize it."

The group's observations of two-state dynamics show that pure a-Si is indeed a glass, in spite of its unorthodox manufacturing method. However, a-Si is rarely used in its pure form; hydrogen is added to make it more stable and improve performance.

Researchers have long assumed that hydrogenation has little to no effect on the random structure of a-Si, but the group's observations show that this assumption is not quite correct. In fact, adding hydrogen robs a-Si of its two-state dynamics and its categorization as a glass. Furthermore, the surface is riddled with signs of crystallization: larger clusters, cracks and highly structured patches.

Such micro-crystalline structure has great implications for the properties of a-Si and how they are studied and applied. Since most research has been conducted on hydrogenated a-Si, Gruebele sees a great opportunity to delve into the largely unknown characteristics of the glassy state.

"In some ways, I think we actually know less about the properties of glassy silicon than we think we do, because a lot of what's been investigated of what people call amorphous or glassy silicon isn't really completely amorphous," Gruebele said. "We really need to revisit what the properties of a-Si are. There could yet be surprises in the way it functions and the kind of things that we might be able to do with it."

Next, the group hopes to conduct temperature-depended studies to further establish the activation barriers, or the energy "humps" that the clusters must overcome to move between positions.

More information: Direct Imaging of Two-State Dynamics on the Amorphous Silicon Surface, Phys. Rev. Lett. 106, 235501 (2011) DOI:10.1103/PhysRevLett.106.235501

Abstract

Amorphous silicon is an important material, amidst a debate whether or not it is a glass. We produce amorphous Si surfaces by ion bombardment and vapor growth, and image discrete Si clusters which hop by two-state dynamics at 295 K. Independent of surface preparation, these clusters have an average diameter of ~5 atoms. Given prior results for metallic glasses, we suggest that this cluster size is a universal feature. The hopping activation free energy of 0.93±0.15??eV is rather small, in agreement with a previously untested surface glass model. Hydrogenation quenches the two-state dynamics, apparently by increasing surface crystallinity.

Provided by University of Illinois at Urbana-Champaign (news : web)

Researchers clarify properties of 'confined' water within single-walled carbon nanotube pores

 

This global temperature-diameter (T-D) phase diagram of water inside SWCNTs shows that, depending on the water content, hollow or filled ice will form. On the right, hollow- and filled-ice nanotubes can be calculated at low temperature for SWCNTs with diameters indicated with (a) and (b) in the lower portion of the phase diagram. Credit: Yutaka Maniwa

Water and ice may not be among the first things that come to mind when you think about single-walled carbon nanotubes (SWCNTs), but a Japan-based research team hoping to get a clearer understanding of the phase behavior of confined water in the cylindrical pores of carbon nanotubes zeroed in on confined water's properties and made some surprising discoveries.

The team, from Tokyo Metropolitan University, Nagoya University, Japan Science and Technology Agency, and National Institute of Advanced Industrial Science and Technology, describes their findings in the American Institute of Physics' Journal of Chemical Physics.Although carbo nanotubes consist of hydrophobic (water repelling) graphene sheets, experimental studies on SWCNTs show that water can indeed be confined in open-ended carbon nanotubes.

This discovery gives us a deeper understanding of the properties of nanoconfined water within the pores of SWCNTs, which is a key to the future of nanoscience. It's anticipated that nanoconfined water within carbon nanotubes can open the door to the development of a variety of nifty new nanothings?nanofiltration systems, molecular nanovalves, molecular water pumps, nanoscale power cells, and even nanoscale ferroelectric devices.

"When materials are confined at the atomic scale they exhibit unusual properties not otherwise observed, due to the so-called 'nanoconfinement effect.' In geology, for example, nanoconfined water provides the driving force for frost heaves in soil, and also for the swelling of clay minerals," explains Yutaka Maniwa, a professor in the Department of Physics at Tokyo Metropolitan University. "We experimentally studied this type of effect for water using SWCNTs."

Water within SWCNTs in the range of 1.68 to 2.40 nanometers undergoes a wet-dry type of transition when temperature is decreased. And the team discovered that when SWCNTs are extremely narrow, the water inside forms tubule ices that are quite different from any bulk ices known so far. Strikingly, their melting point rises as the SWCNT diameter decreases?contrary to that of bulk water inside a large-diameter capillary. In fact, tubule ice occurred even at room temperature inside SWCNTs.

"We extended our studies to the larger diameter SWCNTs up to 2.40 nanometers and successfully proposed a global phase behavior of water," says Maniwa. "This phase diagram (See Figure) covers a crossover from microscopic to macroscopic regions. In the macroscopic region, a novel wet-dry transition was newly explored at low temperature."

Results such as these contribute to a greater understanding of fundamental science because nanoconfined water exists and plays a vital role everywhere on Earth?including our bodies. "Understanding the nanoconfined effect on the properties of materials is also crucial to develop new devices, such as proton-conducting membranes and nanofiltration," Maniwa notes.

Next up, the team plans to investigate the physical properties of confined water discovered so far inside SWCNTs (such as dielectricity and proton conduction). They will pursue this to obtain a better understanding of the molecular structure and transport properties in biological systems.

Provided by American Institute of Physics

Researchers develop integrated nanomechanical sensor for atomic force microscopy

Scanning electron micrograph of the cantilever-microdisksystem. The image has a calculated z-component of the magnetic field overlaid on the structure.

The atomic force microscope (AFM) is an important tool for nanoscale surface metrology. Typical AFMs map local tip-surface interactions by scanning a flexible cantilever probe over a surface. They rely on bulky optical sensing instrumentation to measure the motion of the probe, which limits the sensitivity, stability, and accuracy of the microscope, and precludes the use of probes much smaller than the wavelength of light.

As reported in , CNST researchers have fabricated a novel integrated sensor combining a nanomechanical cantilever probe with a  nanophotonic on a single . Replacing the bulky laser detection system allowed them to build cantilevers orders of magnitude smaller than those used in conventional AFMs.

Because each of these smaller structures has an effective mass less than a picogram, the detection bandwidth is dramatically increased, reducing the system response time to a few hundred nanoseconds.

While probe  was kept comparable to conventional microcantilevers in order to maintain high mechanical gain (how much the tip moves when it senses a force change), the probe size was reduced to a mere 25 µm in length, 260 nm in thickness, and only 65 nm in width.

Readout is based on ?cavity optomechanics?, with the probe fabricated adjacent to a microdisk optical cavity at a gap of less than 100 nm. Due to this close separation, light circulating within the cavity is strongly influenced by the motion of the probe tip.

The cavity has a high optical quality factor (Q), meaning that the light makes tens of thousands of round-trips inside the cavity before leaking out of it, all the time accumulating information about the probe?s position.

The combination of small probe-cavity separation and high Q gives the device sensitivity to probe motion at less than 1 fm/vHz, while the cavity is able to sense changes in probe position with high bandwidth.

The entire device is nanofabricated as a single, monolithic unit on a silicon wafer. It is therefore compact (chip-scale), self-aligned, and stable.

Fiber optic waveguides couple light into and out of the sensor, so that it can be easily interfaced with standard optical sources and detectors.

Finally, through simple changes to the probe geometry, the mechanics of the probe tip can be greatly varied, allowing for the different combinations of mechanical gain and bandwidth needed for a variety of AFM applications.

Researchers develop biological circuit components, new microscope technique for measuring them

 

Rendering of protein assemblies under an atomic force microscope. Credit: Reprinted with permission from "Direct Probe of Molecular Polarization in De Novo Protein-Electrode Interfaces," Kendra Kathan-Galipeau, Sanjini Nanayakkara, Paul A. OBrian, Maxim Nikiforov, Bohdana M. Discher, Dawn A. Bonnell, ACS Nano, Copyright 2011 American Chemical Society

Electrical engineers have long been toying with the idea of designing biological molecules that can be directly integrated into electronic circuits. University of Pennsylvania researchers have developed a way to form these structures so they can operate in open-air environments, and, more important, have developed a new microscope technique that can measure the electrical properties of these and similar devices.

The research was conducted by Dawn Bonnell, Trustee Chair Professor and director of the Nano/Bio Center, graduate students Kendra Kathan-Galipeau and Maxim Nikiforov and postdoctoral fellow Sanjini Nanayakkara, all of the Department of Materials Science and Engineering in Penn?s School of Engineering and Applied Science. They collaborated with assistant professor Bohdana Discher of the Department of Biophysics and Biochemistry at Penn?s Perelman School of Medicine and Paul A. O?Brien, a graduate student in Penn's Biotechnology Masters Program.

Their work was published in the journal. The development involves artificial proteins, bundles of peptide helices with a photoactive molecule inside. These proteins are arranged on electrodes, which are common feature of circuits that transmit electrical charges between metallic and non-metallic elements. When light is shined on the proteins, they convert photons into electrons and pass them to the electrode.

?It's a similar mechanism to what happens when plants absorb light, except in that case the electron is used for some chemistry that creates energy for the plant,? Bonnell said. ?In this case, we want to use the electron in electrical circuits.? 

Similar peptide assemblies had been studied in solution before by several groups and had been tested to show that they indeed react to light. But there was no way to quantify their ambient electrical properties, particularly capacitance, the amount of electrical charge the assembly holds.

?It?s necessary to understand these kinds of properties in the molecules in order to make devices out of them. We've been studying silicon for 40 years, so we know what happens to electrons there,? Bonnell said.  ?We didn?t know what happens to electrons on dry electrodes with these proteins; we didn't even know if they would remain photoactive when attached to an electrode.? 

Designing circuits and devices with silicon is inherently easier than with proteins. The electrical properties of a large chunk of a single element can be measured and then scaled down, but complex molecules like these proteins cannot be scaled up. Diagnostic systems that could measure their properties with nanometer sensitivity simply did not exist.

The researchers therefore needed to invent both a new way of a measuring these properties and a controlled way of making the photovoltaic proteins that would resemble how they might eventually be incorporated into devices in open-air, everyday environments, rather than swimming in a chemical solution.  

To solve the first problem, the team developed a new kind of atomic force microscope technique, known as torsional resonance nanoimpedance microscopy. Atomic force microscopes operate by bringing an extremely narrow silicon tip very close to a surface and measuring how the tip reacts, providing a spatial sensitivity of a few nanometers down to individual atoms.  

?What we've done in our version is to use a metallic tip and put an oscillating electric field on it. By seeing how electrons react to the field, we?re able to measure more complex interactions and more complex properties, such as capacitance,? Bonnell said.

Bohdana Discher?s group designed the self-assembling proteins much as they had done before but took the additional step of stamping them onto sheets of graphite electrodes.  This manufacturing principle and the ability to measure the resulting devices could have a variety of applications.  

?Photovoltaics ? solar cells ? are perhaps the easiest to imagine, but where this work is going in the shorter term is biochemical sensors,? Bonnell said. 

Instead of reacting to photons, proteins could be designed to produce a charge when in the presence of a certain toxins, either changing color or acting as a circuit element in a human-scale gadget.  

Sharpening the nanofocus: Researchers use nanoantenna to enhance plasmonic sensing

 

Top figure shows hydrogen molecules (red) absorbed on a palladium nanoparticle, resulting in weak light scattering and barely detectable spectral changes. Bottom figure shows gold antenna enhancing light scattering and producing an easy to detect spectral shift. Credit: Image courtesy of Alivisatos group

Such highly coveted technical capabilities as the observation of single catalytic processes in nanoreactors, or the optical detection of low concentrations of biochemical agents and gases are an important step closer to fruition. Researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab), in collaboration with researchers at the University of Stuttgart in Germany, report the first experimental demonstration of antenna-enhanced gas sensing at the single particle level. By placing a palladium nanoparticle on the focusing tip of a gold nanoantenna, they were able to clearly detect changes in the palladium's optical properties upon exposure to hydrogen.

"We have demonstrated resonant antenna-enhanced single-particle hydrogen sensing in the visible region and presented a fabrication approach to the positioning of a single palladium nanoparticle in the nanofocus of a gold nanoantenna," says Paul Alivisatos, Berkeley Lab's director and the leader of this research. "Our concept provides a general blueprint for amplifying plasmonic sensing signals at the single-particle level and should pave the road for the optical observation of chemical reactions and catalytic activities in nanoreactors, and for local biosensing."

Alivisatos, who is also the Larry and Diane Bock Professor of Nanotechnology at the University of California, Berkeley, is the corresponding author of a paper in the journal Nature Materials describing this research. The paper is titled "Nanoantenna-enhanced gas sensing in a single tailored nanofocus." Co-authoring the paper with Alivisatos were Laura Na Liu, Ming Tang, Mario Hentschel and Harald Giessen.

One of the hottest new fields in technology today is plasmonics ? the confinement of electromagnetic waves in dimensions smaller than half-the-wavelength of the incident photons in free space. Typically this is done at the interface between metallic nanostructures, usually gold, and a dielectric, usually air. The confinement of the electromagnetic waves in these metallic nanostructures generates electronic surface waves called "plasmons." A matching of the oscillation frequency between plasmons and the incident electromagnetic waves gives rise to a phenomenon known as localized surface plasmon resonance (LSPR), which can concentrate the electromagnetic field into a volume less than a few hundred cubic nanometers. Any object brought into this locally confined field ? referred to as the nanofocus - will influence the LSPR in a manner that can be detected via dark-field microscopy.

"Nanofocusing has immediate implications for plasmonic sensing," says Laura Na Liu, lead author of the paper who was at the time the work was done a member of Alivisatos' research group but is now with Rice University. "Metallic nanostructures with sharp corners and edges that form a pointed tip are especially favorable for plasmonic sensing because the field strengths of theare so strongly enhanced over such an extremely small sensing volume."

Plasmonic sensing is especially promising for the detection of flammable gases such as hydrogen, where the use of sensors that require electrical measurements pose safety issues because of the potential threat from sparking. Hydrogen, for example, can ignite or explode in concentrations of only four-percent. Palladium was seen as a prime candidate for the plasmonic sensing of hydrogen because it readily and rapidly absorbs hydrogen that alters its electrical and dielectric properties. However, the LSPRs of palladium  yield broad spectral profiles that make detecting changes extremely difficult.

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This is a scanning electron microscopy image showing a palladium nanoparticle with a gold antenna to enhance plasmonic sensing. Image courtesy of Alivisatos group

"In our resonant antenna-enhanced scheme, we use double electron-beam lithography in combination with a double lift-off procedure to precisely position a single palladium nanoparticle in the nanofocus of a gold nanoantenna," Liu says. "The strongly enhanced gold-particle plasmon near-fields can sense the change in the dielectric function of the proximal palladium nanoparticle as it absorbs or releases hydrogen. Light scattered by the system is collected by a dark-field microscope with attached spectrometer and the LSPR change is read out in real time."

Alivisatos, Liu and their co-authors found that the antenna enhancement effect could be controlled by changing the distance between the palladium nanoparticle and the gold antenna, and by changing the shape of the antenna.

"By amplifying sensing signals at the single-particle level, we eliminate the statistical and average characteristics inherent to ensemble measurements," Liu says. "Moreover, our antenna-enhanced plasmonic sensing technique comprises a noninvasive scheme that is biocompatible and can be used in aqueous environments, making it applicable to a variety of physical and biochemical materials."

For example, by replacing the palladium nanoparticle with other nanocatalysts, such as ruthenium, platinum, or magnesium, Liu says their antenna-enhanced plasmonic sensing scheme can be used to monitor the presence of numerous other important gases in addition to hydrogen, including carbon dioxide and the nitrous oxides. This technique also offers a promising plasmonic sensing alternative to the fluorescent detection of catalysis, which depends upon the challenging task of finding appropriate fluorophores. Antenna-enhanced plasmonic sensing also holds potential for the observation of single chemical or biological events.

"We believe our antenna-enhanced sensing technique can serve as a bridge between plasmonics and biochemistry," Liu says. "Plasmonic sensing offers a unique tool for optically probing biochemical processes that are optically inactive in nature. In addition, since plasmonic nanostructures made from gold or silver do not bleach or blink, they allow for continuous observation, an essential capability for in-situ monitoring of biochemical behavior."

Researchers produce world's first programmable nanoprocessor

Researchers produce world's first programmable nanoprocessor

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This is a false-color scanning electron microscopy image of a programmable nanowire nanoprocessor super-imposed on a schematic nanoprocessor circuit architecture. Credit: Photo courtesy of Charles M. Lieber, Harvard University

Engineers and scientists collaborating at Harvard University and the MITRE Corporation have developed and demonstrated the world's first programmable nanoprocessor.

The groundbreaking prototype computer system, described in a paper appearing today in the journal Nature, represents a significant step forward in the complexity of computer circuits that can be assembled from synthesized nanometer-scale components.

It also represents an advance because these ultra-tiny nanocircuits can be programmed electronically to perform a number of basic arithmetic and logical functions.

"This work represents a quantum jump forward in the complexity and function of circuits built from the bottom up, and thus demonstrates that this bottom-up paradigm, which is distinct from the way commercial circuits are built today, can yield nanoprocessors and other integrated systems of the future," says principal investigator Charles M. Lieber, who holds a joint appointment at Harvard's Department of Chemistry and Chemical Biology and School of Engineering and Applied Sciences.

The work was enabled by advances in the design and synthesis of nanowire building blocks. These nanowire components now demonstrate the reproducibility needed to build functional electronic circuits, and also do so at a size and material complexity difficult to achieve by traditional top-down approaches.

Moreover, the tiled architecture is fully scalable, allowing the assembly of much larger and ever more functional nanoprocessors.

"For the past 10 to 15 years, researchers working with nanowires, carbon nanotubes, and other nanostructures have struggled to build all but the most basic circuits, in large part due to variations in properties of individual nanostructures," says Lieber, the Mark Hyman Professor of Chemistry. "We have shown that this limitation can now be overcome and are excited about prospects of exploiting the bottom-up paradigm of biology in building future electronics."

An additional feature of the advance is that the circuits in the nanoprocessor operate using very little power, even allowing for their miniscule size, because their component nanowires contain transistor switches that are "nonvolatile."

This means that unlike transistors in conventional microcomputer circuits, once the nanowire transistors are programmed, they do not require any additional expenditure of electrical power for maintaining memory.

"Because of their very small size and very low power requirements, these new nanoprocessor circuits are building blocks that can control and enable an entirely new class of much smaller, lighter weight electronic sensors and consumer electronics," says co-author Shamik Das, the lead engineer in MITRE's Nanosystems Group.

"This new nanoprocessor represents a major milestone toward realizing the vision of a nanocomputer that was first articulated more than 50 years ago by physicist Richard Feynman," says James Ellenbogen, a chief scientist at MITRE.

Researchers use nanoscale transistors to study single-molecule interactions

Researchers use nanoscale transistors to study single-molecule interactions

An interdisciplinary team from Columbia University that includes electrical engineers from Columbia's Engineering School, together with researchers from the University's departments of Physics and Chemistry, has figured out a way to study single-molecule interactions on very short time scales using nanoscale transistors. In a paper to be published online January 23 in Nature Nanotechnology, they show how, for the first time, transistors can be used to detect the binding of the two halves of the DNA double helix with the DNA tethered to the transistor sensor. The transistors directly detect and amplify the charge of these single biomolecules.

Prior to this work, scientists have largely used fluorescence techniques to look at interactions at the level of single molecules. These studies have yielded fundamental understanding of folding, assembly, dynamics, and function of proteins and other cellular machinery. But these techniques require that the target molecules being studied be labeled with fluorescent reporter molecules, and the bandwidths for detection are limited by the time required to collect the very small number of photons emitted by these reporters.

The Columbia researchers, including Professor of Electrical Engineering Ken Shepard, Professor of Chemistry Colin Nuckolls, and graduate students Sebastian Sorgenfrei and Chien-Yang Chiu, realized that transistors, like those used in modern integrated circuits, have reached the same nanoscale dimensions as single molecules. "So this raised the interesting question," said Sorgenfrei, the lead author on the study, "as to whether these very small transistors could be used to study individual molecules."

They have discovered that the answer is "yes." The transistors employed in this study are fashioned from carbon nanotubes, which are cylindrical tubes made entirely of carbon atoms. While these are still emerging devices for electronics applications, they are exquisitely sensitive because the biomolecule can be directly tethered to the carbon nanotube wall creating enough sensitivity to detect a single DNA molecule.

The Columbia team expects this new technique to be a powerful tool for looking at single molecule interactions and is looking at instrumentation applications that currently rely almost exclusively on fluorescence such as protein assays and DNA sequencing. They also plan to study interactions at time scales several orders of magnitude greater than current techniques based on fluorescence.

"The area of single molecule research is an important one and pushes the envelope on our sensing systems," commented Ken Shepard, Professor of Electrical Engineering at Columbia Engineering. "There is a huge potential for modern nanoelectronics to play an important role in this field. Our work, which has been a terrific collaboration between groups from Electrical Engineering, Chemistry, and Physics, is a great example of how nanoelectronics and biotechnology can be combined to produce new, exciting results."

Shepard hopes that this research, which was funded primarily by the National Science Foundation and the National Institutes of Health, will lead to exciting new applications for nanoscale electronic circuits.

What a ride! Researchers take molecules for a spin (w/ Video)

What a ride! Researchers take molecules for a spin (w/ Video)

Kolomeisky and Rice graduate student Alexey Akimov have taken a large step toward defining the behavior of these molecular whirligigs with a new paper in the American Chemical Society's Journal of Physical Chemistry C. Through molecular dynamics simulations, they defined the ground rules for the rotor motion of molecules attached to a gold surface.

"This is no cartoon. It's a real molecule, with all the interactions taking place correctly," said Anatoly Kolomeisky as he showed an animation of atoms twisting and turning about a central hub like a carnival ride gone mad.

Kolomeisky, a Rice University associate professor of chemistry, was offering a peek into a molecular midway where atoms dip, dive and soar according to a set of rules he is determined to decode.

Kolomeisky and Rice graduate student Alexey Akimov have taken a large step toward defining the behavior of these molecular whirligigs with a new paper in the American Chemical Society's Journal of Physical Chemistry C. Through molecular dynamics simulations, they defined the ground rules for the rotor motion of molecules attached to a gold surface.

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An animation of a rotor simulation

It's an extension of their work on Rice's famed nanocars, developed primarily in the lab of James Tour, Rice's T.T. and W.F. Chao Chair in Chemistry as well as a professor of mechanical engineering and materials science and of computer science, but for which Kolomeisky has also constructed molecular models.

Striking out in a different direction, the team has decoded several key characteristics of these tiny rotors, which could harbor clues to the ways in which molecular motors in human bodies work.

The motion they described is found everywhere in nature, Kolomeisky said. The most visible example is in the flagella of bacteria, which use a simple rotor motion to move. "When the flagella turn clockwise, the bacteria move forward. When they turn counterclockwise, they tumble." On an even smaller level, ATP-synthase, which is an enzyme important to the transfer of energy in the cells of all living things, exhibits similar rotor behavior -- a Nobel Prize-winning discovery.

Understanding how to build and control molecular rotors, especially in multiples, could lead to some interesting new materials in the continuing development of machines able to work at the nanoscale, he said. Kolomeisky foresees, for instance, radio filters that would let only a very finely tuned signal pass, depending on the nanorotors' frequency.

"It would be an extremely important, though expensive, material to make," he said. "But if I can create hundreds of rotors that move simultaneously under my control, I will be very happy."

The professor and his student cut the number of parameters in their computer simulation to a subset of those that most interested them, Kolomeisky said. The basic-model molecule had a sulfur atom in the middle, tightly bound to a pair of alkyl chains, like wings, that were able to spin freely when heated. The sulfur anchored the molecule to the gold surface.

While working on a previous paper with researchers at Tufts University, Kolomeisky and Akimov saw photographic evidence of rotor motion by scanning tunneling microscope images of sulfur/alkyl molecules heated on a gold surface. As the heat rose, the image went from linear to rectangular to hexagonal, indicating motion. What the pictures didn't indicate was why.

That's where computer modeling was invaluable, both on the Kolomeisky lab's own systems and through Rice's SUG@R platform, a shared supercomputer cluster. By testing various theoretical configurations -- some with two symmetrical chains, some asymmetrical, some with only one chain -- they were able to determine a set of interlocking characteristics that control the behavior of single-molecule rotors.

First, he said, the symmetry and structure of the gold surface material (of which several types were tested) has a lot of influence on a rotor's ability to overcome the energy barrier that keeps it from spinning all the time. When both arms are close to surface molecules (which repel), the barrier is large. But if one arm is over a space -- or hollow -- between gold atoms, the barrier is significantly smaller.

Second, symmetric rotors spin faster than asymmetric ones. The longer chain in an asymmetric pair takes more energy to get moving, and this causes an imbalance. In symmetric rotors, the chains, like rigid wings, compensate for each other as one wing dips into a hollow while the other rises over a surface molecule.

Third, Kolomeisky said, the nature of the chemical bond between the anchor and the chains determines the rotor's freedom to spin.

Finally, the chemical nature of rotating groups is also an important factor.

Kolomeisky said the research opens a path for simulating more complex rotor molecules. The chains in ATP-synthase are far too large for a simulation to wrangle, "but as computers get more powerful and our methods improve, we may someday be able to analyze such long molecules," he said.

Researchers take molecule's temperature

Researchers take molecule's temperature

You can touch a functioning light bulb and know right away that it's hot. Ouch! But you can't touch a single molecule and get the same feedback.

Rice University researchers say they have the next best thing -- a way to determine the temperature of a molecule or flowing electrons by using Raman spectroscopy combined with an optical antenna.

A new paper from the lab of Douglas Natelson, a Rice professor of physics and astronomy, details a technique that measures the temperature of molecules set between two gold nanowires and heated either by current applied to the wires or laser light. The paper was published this week in the online edition of Nature Nanotechnology.

Natelson, postdoctoral research associate Dan Ward and their colleagues found that while measuring heat at the nanoscale can be much more complicated than taking the temperature of macro objects, it can be done with a level of accuracy that will be of interest to the molecular electronics community or anyone who wants to know how heating and dissipation work at very small scales.

"When you get down to making small electronic devices or tiny junctions, you have to worry about how energy ends up in the form of heat," Natelson said. "In the case of macroscopic objects, like the filament in a light bulb, you can attach a thermocouple -- a thermometer -- and measure it." When light bulbs get hot, they also glow. "If you look at the spectrum of the light coming out, you can figure out how hot it is," he said.

That's an over-simplified version of what Natelson and Ward are doing. One can't see the glow of a molecule. However, the researchers can send in light as a probe and detect the wavelength of the light that molecule is returning when heated. "In Raman scattering, you send in light that interacts with your target. When it comes back, it will either have more energy than you put in, or the same, or less. And we can see that and figure out the effective temperature of whatever is scattering the light."

The new work follows a paper published in September about the lab's creation of nano antennas that concentrate and magnify light up to 1,000 times. That paper focused on the intensity of laser light shot into a gap between the tips of two gold nanowires.

This time, Natelson and Ward spread molecules -- either oligophenylene vinylene or 1-dodecanethiol -- on the surface of a gold nanowire and then broke the wire, leaving a nanoscale gap. When they were fortunate enough to find molecules in the gap -- "the sweet spot" being where the metal wires are closest, Natelson said -- they'd power up and read the resulting spectra.

The experiments were carried out in a vacuum with materials cooled to 80 kelvins (-315 degrees Fahrenheit). The researchers found they could easily detect temperature fluctuations of up to 20 degrees in the molecules.

On the macro level, Natelson said, "You're usually looking at something that's essentially cold. You send in light, it dumps some of the energy into the thing you're looking at and the light comes out with less energy than when you started. With Raman scattering, you can actually see particular molecular vibrational modes."

But the opposite can happen if the atoms are already vibrating with stored energy. "The light can grab some of that and come out with more energy than when it started," he explained.

The effect is most dramatic when current is supplied through the nanowires. "As we crank up the current through this junction, we can watch these different vibrations shaking more and more. We can watch this thing heat up."

Natelson, named by Discover magazine in 2008 as one of the nation's top 20 scientists under age 40, said the experiments show not only how molecules wedged into the nanogap heat up, but also their interaction with the metal wires. "The vibrations show up as sharp peaks in the spectra," he said. "They have very definite energies. Underneath all that, there's this sort of diffuse smear where the light instead is interacting with the electrons in the metal, the actual metal wires."

Natelson said it's extremely hard to get direct information about how heating and dissipation work on nano scales. "In general, you can't do it. There's a lot of modeling, but in terms of experimental things you can actually measure that tell you what's happening, everything is very indirect. This is an exception. This is special. You can see what's happening.

"In our fantasy experiment, we'd say, 'Boy, I wish I could go in with a thermometer,' or, 'I wish I could see each molecule and see how much it's shaking.' And this is effectively a way of doing that. We can really watch these things heat up."