Recently in Nanoscale science and technology Category

The point defect in diamond known as the NV center—a nitrogen atom substituted for a carbon atom and adjacent lattice vacancy—has become a promising ingredient in recent efforts to develop atomic-scale sensors. When optically excited, the defect exhibits stable fluorescence, even in a crystal as small as 5 nm. And its ground state is magnetically sensitive—the spin 0 level is separated from degenerate spin ±1 levels by a microwave transition of 2.9 GHz. That sensitivity allows one to detect weak magnetic fields by observing the quantum spin state, which can be manipulated by microwave pulses and then read out optically by monitoring the fluorescence intensity. (The intensity depends on which of the three spin states is populated.) Researchers led by the University of Melbourne’s Lloyd Hollenberg have now performed such magnetic resonance experiments on individual nanodiamonds placed inside human cells. The quantum spin levels of the defects acted both as local magnetometers and as fingerprints to spectrally distinguish each nanodiamond in the complicated cell environment. Using a confocal microscope, the researchers were able to identify and track at nanometer precision individual NV centers in the cells from the optical emission (red). They were also able to measure the coherence time of the spin states; that work sets the stage for sensing the cells’ local magnetic fluctuations in response to, for example, the transport of charge through cell membrane ion channels, which are important drug targets. (L. P. McGuinness et al., Nat. Nanotechnol., in press, doi:10.1038/nnano.2011.64.)—R. Mark Wilson

An electromechanical system capable of sensing the mass of a single molecule or a few atoms typically consists of a nanometer-thick beam whose resonant frequency measurably shifts in response to the loaded mass. Naturally, such devices are sensitive to the adsorption and desorption of individual analytes or their diffusion along the beam's surface; both processes, shown in the schematic, cause the resonator's frequency to fluctuate. Now, Michael Roukes and colleagues at Caltech's Kavli Nanoscience Institute have determined the contribution of those processes to frequency noise for a nanoscale resonator vibrating at 190.5 MHz. The experiment, which was conducted in a temperature-controlled vacuum cryostat, relied on a nozzle that delivered a steady stream of xenon atoms to a silicon carbide resonator, shown in the inset. As the resonator was cooled, the average number of adsorbed xenon atoms increased; subsequently, so did the magnitude of the frequency shift. The researchers measured the fluctuation of surface adsorbates and found that diffusion is the favored route for atoms leaving the resonator; diffusion thus contributes the most to frequency noise in excess of the resonator's inherent thermal fluctuations. The combination of experimental data and analytical models also revealed previously unknown power-law dependence in the system's noise spectrum, a finding that could be used to probe the sensitivity limits of wide classes of nanoscale frequency-shift sensors. (Y. T. Yang et al., Nano Lett., in press, doi:10.1021/nl2003158.)—Jermey N. A. Matthews

In yet another application for micro- and nanostructures, recent experiments have demonstrated the potential for microresonators to serve as ultrasensitive temperature sensors. Last year, Ashok Pandey, Oded Gottlieb, and Eyal Buks of the Technion–Israel Institute of Technology showed that the resonant frequency of a suspended, microfabricated gold–palladium beam, hundreds of microns long but just a micron wide and a fraction of a micron high and supported at each end, was a strong function of temperature. The dominant contribution came from the temperature dependence of the tension in the beam, which is due to the difference between the thermal expansion coefficients of the beam and of the silicon substrate below it. The researchers could measure temperature changes by monitoring the relative frequency shift; as a temperature sensor, the beam's sensitivity was about a third that of commonly used, macroscopic platinum sensors. More recently, a team led by Anja Boisen of the Technical University of Denmark has reported aluminum microresonators, such as the ones seen here, whose resonant frequencies are even more sensitive to temperature. The improvement, of more than an order of magnitude, arises primarily from the larger difference in thermal expansion coefficients and from the Al beam's smaller initial tension compared with that of the Au–Pd beam. With their high quality factors, microresonator-based sensors would potentially have exceptional temperature resolution as well. (A. K. Pandey et al., Appl. Phys. Lett. 96, 203105, 2010; T. Larsen et al., Appl. Phys. Lett. 98, 121901, 2011.)—Richard J. Fitzgerald

A quantum dot is a nanostructure that confines a single conduction-band electron in three spatial dimensions. Researchers have long sought to use the spin on that electron as a quantum bit—or qubit—to store binary information for a quantum computer. First, however, they must show that they have precise and rapid control over the quantum-dot qubit. Unfortunately, flipping the spins with oscillating magnetic fields requires high-frequency fields and very low temperatures, posing a challenge to experimenters. An alternative is to use purely electric fields and exploit the spin–orbit coupling of electrons: The orbital motion relative to the background of the semiconductor’s nuclear charges causes the electron to see a magnetic field, which couples to its spin. In traditional gallium arsenide quantum dots, the spin manipulation times obtained with spin–orbit coupling are too slow. Recently, Leo Kouwenhoven and his colleagues at Delft University of Technology and at Eindhoven University of Technology, both in the Netherlands, have turned to indium arsenide, which is known to have much larger spin–orbit coupling. Furthermore, the team formed the qubits in an InAs nanowire, which offers interesting possibilities for combining with other semiconductors. For example, one might make an optoelectronic device that converts the spin state to a photon for long-distance transportation. A more exotic prediction is that InAs nanowires might be useful for topological quantum computing. (S. Nadj-Perge, S. M. Frolov, E. P. A. M. Bakkers, L. P. Kouwenhoven, Nature 468, 1084, 2010.)—Barbara Goss Levi

Since Andre Geim and Konstantin Novoselov first touched off the graphene “gold rush” in 2004—their pioneering work earned them this year’s Nobel Prize in Physics—researchers have been pursuing ways to scale up its production. Among graphene’s remarkable properties is its roughly 100-GPa tensile strength, which is 40 times greater than the value for steel. That, however, is for defect-free graphene sheets; when formed by chemical vapor deposition, a proven industrial technique, graphene sheets contain crystallites separated by grain boundaries (see the news story in Physics Today, August 2010, page 15). Now, a computational study by Rassin Grantab and Vivek Shenoy at Brown University and Rodney Ruoff at the University of Texas at Austin reveals that graphene sheets with highly misaligned boundaries are actually stronger than slightly misaligned ones. As the image shows, misaligned grain boundaries consist of repeating pairs of 5- and 7-member rings separated by hexagonal rings. In simulations of the stress–strain curves as a function of the misalignment, the researchers found that, surprisingly, tensile strength increases with increasing misalignment angle. According to their model, stress failure begins at critical bonds within the 7-member rings; and critical bond length, which decreases with increasing misalignment angle, is proportional to initial material strain. In one simulation, a graphene sheet with a boundary angle of 28.7° and strained by 15% resisted stress up to 95 GPa; conceivably, it might be more efficient for researchers to engineer controlled defects into a graphene sheet rather than trying to make a perfect one. (R. Grantab, V. B. Shenoy, R. S. Ruoff, Science 330, 946, 2010.)—Jermey N. A. Matthews

The public-health and biosecurity communities need biosensors that are sensitive, operate in real time, and can be easily deployed. Many approaches are being pursued, including one by a group from Lawrence Livermore National Laboratory that uses a photonic crystal (PC) slab connected to two waveguides. The idea is for viruses or other tiny pathogens to randomly infiltrate the pores of a silicon PC, whose optical properties—specifically, the in-plane transmission spectrum's band edge—change accordingly. First, the researchers used simulations to determine a PC geometry suitable for a specific virus, vaccinia, and for their laser's wavelength. They then fabricated an appropriate 17 × 17 array of 280-nm pores and exposed it to a flux of polystyrene beads with two different sizes; those with 260-nm diameter entered the pores (see the figure, with empty and filled pores enlarged) while 320-nm ones did not. The measured band-edge redshifts were then used to calibrate the simulations and predict detection limits. Theoretically, as few as 10 vaccinia viruses could be detected with the PC, comparable to other biodetection methods. Advantages accrue, however, from the small sensor size, the ability to tune the geometry for different particles, and the ease of integration into lab-on-a-chip setups. The authors say that their random-binding scheme is more practical than methods that rely on binding organisms to single PC defects. (S. E. Baker et al., Appl. Phys. Lett. 97, 113701, 2010.)—Stephen G. Benka

The field of nanotechnology is in part rooted in the 1985 Nobel Prize–winning laboratory synthesis of buckyballs—the soccer-ball-shaped carbon molecule, C₆₀—by Rice University chemists Richard Smalley and Robert Curl and their collaborator, University of Sussex chemist Harold Kroto. The synthesis was guided by Kroto’s hypothesis that complex carbon chains could naturally form in the interstellar medium of aging carbon-rich, hydrogen-poor giant branch stars. Now, 25 years later, Jan Cami at the University of Western Ontario and his colleagues have reported the clearest evidence yet of such complex carbon structures in space. The research team analyzed IR spectroscopic data—collected by the Spitzer Space Telescope—of the circumstellar region of a planetary nebula known as Tc 1. As the image shows, the spectrum contains several prominent peaks of C₆₀ (red arrows) and peaks of the rugby-ball-shaped C₇₀ (blue arrows); both molecules were uncharged and in the solid phase. Previous spectra of other carbon-rich planetary nebulae indicated strong emission peaks of volatile polycyclic hydrocarbons, which were completely absent in the monitored region of Tc 1. Cami and his colleagues suggest that the planetary nebula may have ejected its hydrogen envelope a few thousand years ago and that a recent thermal pulse prompted the ejection of the pure carbon dust they’re now observing. (J. Cami et al., Science, in press, doi:10.1126/science.1192035.)—Jermey N. A. Matthews

In the presence of a suitable nucleating agent, a liquid in a metastable state below its thermodynamically defined melting point freezes. That’s what happens when atmospheric aerosol particles cause supercooled water droplets in clouds to form snowflakes. Researchers have suspected that the atomic surface structure of such seeding particles acts as a template, inducing local order in the disordered liquid and catalyzing its crystallization. Conversely, a solid with a different structure can inhibit crystallization, as has now been observed at the European Synchrotron Radiation Facility in Grenoble, France, by Tobias Schülli and his colleagues. The researchers coupled x-ray scattering data with molecular-dynamics simulations to study supercooled gold-silicon droplets on a silicon substrate, a system that is used to grow Si nanowires. Surprising results emerged when they heated the AuSi alloy above 676 K: As it cooled, the Si atoms leached onto the substrate and, as the figure shows, rearranged its surface atoms into pentagonal clusters. The alloy’s atoms near the interface mimicked the substrate’s surface structure (see inset), but the resulting local order did not promote crystallization in the droplets, which froze at 513 K, about 120 K below the freezing temperature for the AuSi alloy. Apparently, the pentagonal geometry inhibits freezing because it is not conducive to crystal packing. That finding suggests that substrates with such atomic structures offer a simpler method of maintaining and observing the supercooling process than such techniques as magnetically levitating or otherwise suspending the liquid droplet. (T. Schülli et al., Nature 464, 1174, 2010.)—Jermey N. A. Matthews

In materials, as the axiom goes, structure follows function: A metal’s tightly bonded atomic crystal lattice gives it strength, and a polymer’s mesh of macromolecular chains makes it elastic. Medical implants, electronic components, and other similar devices call for multifunctional materials that are both strong and stretchy. One such material is the shape-memory alloy (SMA), a polycrystalline arrangement of assorted metals that, when stressed, undergoes a structural phase transition from high to low symmetry. The transition is reversible, and above a critical temperature SMAs are superelastic—they fully recover after being stretched well beyond the reversible-deformation strain values of pure metals. Now, materials scientists at Tohoku University in Sendai, Japan, have presented evidence for an iron-based SMA that is 35 times as elastic as pure metals. The new alloy, which also features nickel, cobalt, aluminum, tantalum, and boron, has an elastic strain of 13%, as shown in the figure, almost double the value of the more expensive commercial-standard nickel–titanium alloy. Furthermore, the material’s yield strength, 800 MPa, is 1.5 times that of the nickel–titanium SMA. The researchers say that microstructured precipitates similar in composition to the bulk matrix and interspersed through it are a key to the improved mechanical strength. The greater elastic strain and strength could be exploited for mechanical damping in building materials. Also, the ferrous SMA’s magnetism is phase dependent, which makes it potentially useful for electromechanical sensing applications. (Y. Tanaka et al., Science 327, 1488, 2010.)—Jermey N. A. Matthews

Much of the light emitted from stars and other astrophysical objects is absorbed by dust and reemitted at far-IR or submillimeter wavelengths—radiation that is notoriously difficult to detect. Last year researchers from the Jet Propulsion Laboratory proposed a new type of detector for that regime, with an eye toward future, more sensitive space missions. The team has now built a prototype microdevice (see figure), called a quantum capacitance detector (QCD), which would be one pixel in an eventual array. The detection chain goes like this: Photons are received at an antenna and fed into a superconducting absorber where they break Cooper pairs and generate quasiparticles. A superconducting island, called a single Cooper-pair box (SCB), is connected to the absorber in such a way that, at most, one quasiparticle at a time can tunnel onto it; that changes the island’s capacitance, which is so small that the charging energy of a single electron has a large effect. With a resonant circuit, the physicists monitor the frequency of capacitance changes from which they can determine the density of quasiparticles in the absorber and thus the photon flux at the antenna. The device’s performance is already comparable to that of other superconducting detectors. The advantage of the QCD, say the researchers, is the ease with which it can be read out from an array of detectors. For example, each pixel detector could be fabricated with a different resonance and simultaneous readout could be done with a frequency comb. (J. Bueno et al., Appl. Phys. Lett. 96, 103503, 2010.) —Stephen G. Benka

Biological and medical researchers have long sought to study or control cellular function by inserting biomolecular probes inside the cell. But those probes, which include peptides and nucleic acids, must first cross the cell’s highly selective membrane. Traditional approaches to breaching that barrier are to chemically modify the probe or membrane and to pack the probe into a virus, which fuses to a cell’s membrane before depositing its load; both methods induce unwanted side effects and are limited to delivering specific molecular cargo. Now a team of US and South Korean scientists, led by Harvard University’s Hongkun Park, has developed a minimally invasive delivery method that exploits the ability of silicon nanowires to physically penetrate the cell’s membrane. The researchers prepared vertically aligned nanowire arrays with a density of roughly 25 million nanowires/cm² and altered their surface chemistries to enable noncovalent binding of a broad spectrum of molecules. With the nanowire platform, they were able to simultaneously assay the intracellular effects of distinct molecular probes. In one experiment, the researchers layered human fibroblasts, shown green in the scanning electron microscope image, across the nanowires, shown in blue. Nearly all of the cells were impaled within one hour and received the bound probes within 24 hours. Impaled cells continued to grow for several weeks, albeit at a slightly slower rate. (A. K. Shalek et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0909350107.) — Jermey N. A. Matthews

Several methods exist for growing nanowires, whether attached to a substrate or dispersed in a liquid. Using those wires to make designated electrical connections in a circuit, however, has been difficult. Yves Galerne and his colleagues at the University of Strasbourg, France, now demonstrate a procedure that produces conducting wires across a gap between two electrodes. The chemical physicists first paint the electrodes with a polymer so as to create "anchors" in predetermined locations; when the gap is filled with nematic liquid crystals, an isolated defect line—a disclination—connects the anchors and therefore the electrodes. Next, silica particles coated with a conducting polymer are introduced and gather along the disclination like beads on a necklace. In the third step, a voltage across the electrodes welds the necklace beads together into a robust wire. Although ragged with extra polymer aggregates, the central region of a 150-micron-long wire, shown in the photo, demonstrates the team’s initial result. The researchers note that the wire’s size, smoothness, and conductivity can be improved—for example, by decreasing the silica particles’ size and concentration and by electroplating them. (J.-B. Fleury, D. Pires, Y. Galerne, Phys. Rev. Lett., in press.) —Stephen G. Benka

In recent years, notions of the ultrafast, the ultraintense, and the ultrasmall have been recurring themes in physics as those envelopes have been relentlessly pushed to reveal new phenomena. Caltech’s Brett Barwick, David Flannigan, and Ahmed Zewail have combined all three notions into a new technique they dub photon-induced near-field electron microscopy. PINEM exploits the fact that free–free interactions of electrons and photons are greatly enhanced when a third body, like a nanostructure, is present and when the electrons are more energetic than the photons. The physicists illuminated a carbon nanotube with an intense femtosecond laser pulse that generated an evanescent plasmonic field at the CNT’s surface. Simultaneously, a similar-duration pulse of 200-keV electrons from an electron microscope traversed the sample. During the few-hundred-attosecond interaction time, some of those electrons absorbed energy quanta from the 2.4-eV photon field. By selecting only those electrons that gained energy, the researchers could image the evanescent surface field with the spatial resolution of electron microscopy. That field extends about 50 nm into the vacuum from the dark surface of the roughly 150-nm-diameter CNT. As shown in the images, Zewail and colleagues also monitored the temporal decay of the surface field by varying the delay times between the exciting laser pulse and the probing electron pulse, from zero (top) to 400 fs (bottom) and beyond. With tunable and temporally controlled light pulses, PINEM enables visualization of dynamical optical responses of various nanostructures. (B. Barwick, D. J. Flannigan, A. H. Zewail, Nature 462, 902, 2009.) —Stephen Benka

The tunable elasticity and porosity of colloidal gels lead to some interesting applications, among them tissue scaffolding and drug delivery. Conventionally, colloidal particles interact and assemble under entropic and electrostatic forces to form predictable structures. But greater control can be achieved from an approach developed by Paul Clegg, Michael Cates, and their collaborators at the University of Edinburgh in the UK. The researchers disperse silica particles in the single-phase region of two partially miscible solvents—water and the organic base 2,6-lutidine. When the solution is heated above a critical temperature, the solvents separate and the particles become trapped at the liquid–liquid interfaces. The bulky particle domains then jam together and arrest the solvent separation, forming a two-phase network the researchers call a bijel. But cool the solution and remix the solvents too soon and the distinct structure disappears, as shown in movie 1 and the two left images in which the colloids appear green, the water black, and the lutidine red. Now the researchers have discovered an approach to stabilize the bijel structure. When the phase-separated solution is allowed to sit for at least 24 hours before it is cooled, the bijel surprisingly keeps its shape, as shown in the two right images and movie 2. From Monte Carlo simulations, the researchers deduce how the resulting network of colloidal monolayers, or monogel, stays intact: the particles become compressed by capillary forces, remain attracted by van der Waals forces, and are kept from collapsing into each other by repulsive electrostatic forces. (E. Sanz et al., Phys. Rev. Lett., in press.) —Jermey N. A. Matthews

Movie 1

Movie 2

A common step in industrial cooling processes is the liquefaction of a vapor on a condenser. If, however, a liquid film forms on the condenser, the cooling may be compromised. The problem can be addressed by coating the condenser with a hydrophobic material conducive to drop formation and then letting the drops slide off due to gravity. Now Chuan-Hua Chen of Duke University and his student Jonathan Boreyko report a different approach. By depositing carbon nanotubes on silicon micropillars and coating both with hexadecanethiol (C₁₆H₃₄S), they engineered a rough “superhydrophobic” surface. The water drops that condensed on it were about a hundred times smaller than those on a conventional hydrophobic surface that the Duke team considered as a standard; the surface roughening offers the promise for more efficient cooling. Furthermore, as the figure and video show, when two sufficiently large drops coalesce into a single drop, that drop literally springs off the condenser—no external prompting needed.

The post-combination drop has less surface energy than do the two drops from which it forms. Most of the released surface energy is dissipated, but Chen and Boreyko observed that the vertical component of the drop’s velocity can be as much as one-sixth of the theoretical maximum. Nature has her own version of the jumping trick. Coalescence of a wet portion of a spore with a dew drop provides the energy for spore ejection in certain mushrooms. (J. B. Boreyko, C.-H. Chen, Phys. Rev. Lett., in press.) —Steven K. Blau

In principle, setting a droplet in motion inside a microfluidic channel is straightforward: Apply pressure and the liquid flows. In practice, however, precise control of droplet flow simultaneously along multiple channels is technically challenging; conventional pressure pumps are not feasible for microfluidic systems. Inspired by the potential of finely tunable acoustic-pressure generators, a group of engineers at the University of Michigan set out to control droplet motion with music. First, they composed a computer-synthesized sequence of single notes and chords. That signal was then sent to four resonance cavities that were tuned according to their lengths to extract and amplify narrow, non-overlapping frequency bands from the input tones. As shown in the figure and the movie, unidirectional droplet flow was generated from the difference between positive air pressure in the oscillating cavity and relative negative pressure at vent ports near the cavity's outlet. Although the relatively high frequencies of the selected tones produced steady flow, the researchers adjusted the relative amplitudes of the input tones as needed to compensate for variations in average flow velocity. Maybe someday, conducting complex lab-on-chip microfluidic operations will be as simple as stringing together a few musical notes. (S. M. Langelier et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0900043106.) — Jermey N. A. Matthews

Late Stone Age metal smiths added a little tin to copper to usher in the eponymous Bronze Age; over the ensuing five millennia, many new combinations and applications of the two metals have appeared. Today, for example, a thin tin coating on a copper substrate often serves to interconnect electronic components of various kinds, such as are found in medical devices and satellite equipment. Unfortunately, micron-sized tin whiskers (see figure) sometimes arise spontaneously and can short out the equipment, with great technological and economic repercussions. After decades of widespread effort, the actual mechanism underlying such whisker growth has only now been elucidated. Led by Eric Mittemeijer, a group from the Max Planck Institute for Metals Research in Stuttgart, Germany, working with the Robert Bosch company, examined growing whiskers and their crystallographic environment. Using Laue diffraction measurements made at the Advanced Photon Source at Argonne National Laboratory in Illinois, the researchers noted that at the Cu–Sn interface, Cu₆Sn₅ develops along the tin grain boundaries and is most pronounced directly beneath a whisker's root. That observation, coupled with residual strain measurements, led the team to propose the following mechanism: Deep penetration of Cu₆Sn₅ into the 3-μm-thick tin layer induces in-plane compressive strains near the Cu–Sn interface and in-plane tensile strains nearer the surface. Out-of-plane and in-plane strain gradients—not the strains themselves—then provide the driving force that leads to whisker growth by transporting Sn atoms to the whisker nucleation site as a strain-relief mechanism. (M. Sobiech et al., Appl. Phys. Lett., in press.) —Stephen G. Benka

Before they form snowflakes and other hexagonal crystals, water molecules nucleate in smaller configurations. Determining the structure of those precursors—even in the outwardly simple case of water on a clean metal surface—is an area of ongoing interest and controversy. For example, at submonolayer coverage on a copper (110) surface, water molecules form chains that can grow to many tens of nanometers in length but are just 1 nm wide. The chains’ structure has been a mystery, since no arrangement of water molecules into hexagonal units entirely explains the experimental data. Now, Andrew Hodgson and colleagues of the University of Liverpool in the UK have collaborated with Angelos Michaelides’ group at University College London to find the structure. Michaelides and postdoc Javier Carrasco ran calculations on some 50 possible chain structures; they found that the most energetically stable one also gave the best fit to the Liverpool group’s high-resolution scanning tunneling microscopy images (as shown in the top panel) and vibrational spectra. That structure (bottom panel) is an arrangement of pentagons, not hexagons. The water molecules shown in red and yellow are perpendicular to the Cu surface—the hydrogen atoms pointing up are responsible for the bright spots in the STM images, and the ones pointing down (not visible in the figure) interact with the Cu atoms. The researchers suggest that nonhexagon arrangements might be involved at other water–metal interfaces where the structure of water is unknown. (J. Carrasco et al., Nat. Mater., doi:10.1038/nmat2403.) — Johanna Miller

Lightness, strength, and moldability are among the most desired material properties for aircraft, sporting equipment, and many structural applications. Those sometimes opposing properties converge in bulk metallic glasses—supercooled amorphous metal alloys that can be cast into complex shapes and are resilient under large elastic strains. However, their toughness is suspect: Under repeated stress, BMGs fatigue and develop fatal cracks much more quickly than crystalline metal alloys do. To control crack propagation, Caltech's William Johnson, Lawrence Berkeley National Laboratory's Robert Ritchie, and their collaborators focused on controlling the microstructure of a particularly tough BMG composite made of zirconium, titanium, and other metals. Its fingerlike crystalline dendrites (67% by volume) are surrounded by an amorphous matrix, as seen in this optical micrograph. By heating the precursor alloys between their melting points then rapidly quenching the solution, the researchers were able to control dendrite size and the spacing between the glassy and crystalline phases. The width of the glassy region between the much tougher dendrite fingers was tailored to be short enough to serve as a "microstructural arrest barrier" for just-formed cracks. Compared with existing dendrite-containing BMGs, the new material holds up under three times more stress cycles and is comparable in toughness to high-strength steel or aluminum. (M. E. Launey et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0900740106.) — Jermey N. A. Matthews

The several approaches being pursued for fuel cells vary in their chemical reactions, materials, and optimal operating conditions, but they share a basic configuration (see Physics Today, November 1994, page 54, and October 2006, page 38). A fuel, often hydrogen, is oxidized at the anode, where it liberates electrons. The electrons travel through and power an external circuit and eventually reach the cathode, where oxygen is reduced. Meanwhile, to complete the redox reaction, ions travel through an electrolyte that separates the electrodes. In alkaline fuel cells, first developed for the Apollo space missions in the 1960s, oxygen combines with water and electrons at the cathode to form hydroxyl ions (OH^-) that travel through an aqueous alkaline electrolyte to produce water by combining with protons from the anode. Commercialization of those fuel cells, however, has been limited by the high cost of the platinum used for the cathode. New work by Liming Dai of the University of Dayton and colleagues at the Air Force Research Laboratory and the University of Akron has shown that vertically aligned carbon nanotubes doped with nitrogen provide an efficient, lower-cost alternative for the cathode. Nitrogen-doped nanotubes have better long-term stability and, unlike Pt, are not harmed by the presence of carbon monoxide or any fuel molecules that cross the electrolyte from the anode to the cathode. The researchers attribute the catalytic performance to the relatively high positive charge density on the carbon atoms adjacent to the nitrogen atoms. (K. Gong et al., Science 323, 760, 2009.) — Richard J. Fitzgerald

Inorganic crystal aggregates known as biomorphs earn their name by virtue of a remarkable resemblance to the fossils of primitive organisms. But although the structures can be varied and complex—leaflike sheets, wormy ropes, and helical filaments, among others—biomorphs are exceedingly simple to make. They self assemble when an alkaline earth halide such as barium chloride is mixed with a silica-rich solution under high pH conditions at ambient pressure and temperature. As carbon dioxide from the air dissolves into solution, barium carbonate and silica precipitate out and produce the complex structures. A long-standing question is how. Juan Manuel García-Ruiz, his postdoc Emilio Melero-García (both at the University of Granada), and Stephen Hyde (Australian National University) now propose a mechanism for the morphogenesis. As the carbonate crystallizes, it lowers the pH of the local environment and triggers the precipitation of silica. The silica precipitation, in turn, raises the local pH, which prompts another round of carbonate formation. The sensitivity of silica and carbonate species to opposite trends in pH fluctuation creates a chemical feedback that eventually produces rodlike, carbonate particles, each coated with silica. Freed from the hexagonal symmetry restrictions imposed by carbonate growth, the silica-coated nanoparticles form clusters that can adopt various shapes. For reasons unexplained by their mechanism, the clusters align themselves on the micron scale and grow as two-dimensional sheets. The edges of those sheets can then curl like a scroll to create the sort of curved and twisted filaments captured here by optical microscopy. (J. M. García-Ruiz, E. Melero-García, S. T. Hyde, Science 323, 362, 2009.) — R. Mark Wilson

To map molecules in cells and tissue, researchers prefer biomedical imaging techniques that rely solely on the intrinsic responses of chemical bonds to optical stimulation. Although fluorescence microscopy and other chemical tagging methods yield high-resolution images, they also introduce foreign species or synthetic derivatives that can alter the dynamics of intracellular processes. Spontaneous Raman scattering, which uses a single laser beam to excite the vibrational and rotational modes in chemical bonds, requires no chemical labels but generates a weak signal that gets muddled by Rayleigh scattering. A more sensitive technique known as coherent anti-Stokes Raman scattering uses multiple laser beams to generate coherent optical signals that enhance resonant frequencies in the sample; that method, however, also produces nonresonant background noise. Recently a team led by Harvard University chemist Sunney Xie demonstrated a new technique based on stimulated Raman scattering that tunes the difference between the frequencies of two laser beams to match a desired molecule's resonant frequency, thus amplifying the Raman signal. The measurable intensities of the transmitted beams change only when a match occurs; nonresonant signals are not picked up. The images show the top view (a) and the depth profile (b) of an acne medication (blue) that penetrated a mouse's skin, thus demonstrating the potential of the new technique to monitor drug delivery. (C. W. Freudiger et al., Science 322, 1857, 2008.)
— Jermey N. A. Matthews

In typical loudspeakers, a coil surrounds the apex of a flexible cone; when a varying current flows through the coil, the cone moves toward and away from a fixed permanent magnet and produces pressure waves we hear as sound. But researchers from Tsinghua University and Beijing Normal University have demonstrated a radically simpler loudspeaker design based on nanotubes: They showed that a thin film of nanotubes can reproduce sounds over a wide frequency range--including the full human audible range--with high sound pressure level, low total harmonic distortion, and no magnets. The team created the film by drawing nanotubes from a so-called superaligned array grown on a wafer, a technique the group introduced six years ago (see also PHYSICS TODAY, October 2005, page 23). The resulting film, only tens of nanometers thick but up to 10 cm wide, is transparent and has a nearly purely resistive impedance. When electrodes are placed along its ends and an alternating current is applied, the film produces clear tones that can be as loud as a conventional speaker. Moreover, since the film is flexible, the nanotube speaker can be configured into arbitrary shapes or mounted onto curved substrates; the figure shows an omnidirectional cylindrical loudspeaker 9 cm in diameter and 8.5 cm high. The film can even be stretched with essentially no degradation of the sound reproduction. The researchers attribute the sound generation not to vibration but to a thermoacoustic effect first proposed nearly a century ago: Thanks to the nanotube film's extremely low heat capacity per unit area, changes in the current flowing through the film are reflected in the film's temperature, and those temperature changes excite pressure waves in the surrounding air. The mechanism is independent of the sign of the current, which leads to a frequency doubling of the input signal, but that drawback can be overcome by applying a constant current bias. The movie shows a nanotube loudspeaker being periodically stretched with almost no noticeable effect on the sound intensity. (L. Xiao et al., Nano Lett., in press, doi:10.1021/nl802750z.) -- Richard J. Fitzgerald

Overuse of antibiotics has spawned strains of bacteria whose cell walls are impervious to the crippling blows once delivered by penicillin and its derivatives. One such so-called superbug, methicillin-resistant staphylococcus aureus, although found primarily in prisons and hospitals, has now spread beyond those confines. Despite the controlled use of the drug vancomycin, a last line of defense against MRSA, the latest threat are vancomycin-resistant bacteria, which mutate by deleting a key hydrogen bond that allows the drug to bind and inhibit cell wall growth, thereby mechanically weakening the bacteria . Rachel McKendry at University College London and her collaborators recently demonstrated a nanoscale cantilever system that is sensitive enough to detect the difference between the native drug-sensitive bacteria and the mutated resistant form with the missing hydrogen bond. The researchers coated silicon cantilevers with vancomycin-resistant (DLac in the schematic) and vancomycin-sensitive (DAla ) bacterial cell-wall analogues, then immersed them in a solution containing free vancomycin molecules. As expected, the molecules preferentially bound to the cantilevers coated with the drug-sensitive analogue; those cantilevers experienced a marked deflection—as measured by an optical detector—that equated to an 800-fold difference in binding compared with the cantilevers coated with the drug-resistant analogue. The researchers believe their system will lead to sensitive, nondestructive, and rapid nanomechanical biosensors for high-throughput drug– target interaction studies and will aid in the design of more effective drugs. (J.W. Ndieyira et al., Nat. Nanotechnol., doi: 10.1038/nnano.2008.275 .) — Jermey N.A. Matthews

In scanning microscopy, images are put together by sweeping a single narrow beam back and forth over a sample. If you had a wide array of multiple beams, one scan would suffice. And if the sample moved over the array, you wouldn't need to scan at all. That idea is behind a new optofluidic imaging scheme developed for biological applications by Caltech's Changhuei Yang and his colleagues. At the heart of the scheme is an off-the-shelf sensor whose CMOS pixels are read out individually. Contrast is achieved when a sample, under constant illumination, momentarily shadows the pixels as it passes over them in a microfluidic channel. At 10 × 10 µm², the pixel size is too big to resolve the parts of amoebae and other tiny organisms. To boost the resolution, Yang masks the pixels with a commensurate array of 1-µm-diameter holes. Although 97% of the sensor is masked, 100% of a sample is imaged because the sample's path over the lines of holes is canted at a slight angle. Thanks to the angle, an organism or cell is scanned not only along but also across its whole body. The Caltech team built and demonstrated two types of imager; they differ in how they stabilize the samples' orientation during a scan. In one type, suitable for imaging tiny worms and other elongated samples, gravity pulls the samples. Confinement suffices to prevent the samples from tumbling. In the other type, suitable for squatter, more rounded samples, pressure pushes the samples. Tumbling is forestalled by a strong DC electric field, which polarizes and aligns the samples. Both imagers are barely bigger than a US quarter and, as the accompanying images show, provide resolution comparable to that of a conventional optical microscope. (X. Cui et al., Proc. Natl. Acad. Sci. USA, in press.) — Charles Day