Materials in Focus
Ductile or brittle at the flip of a switch
(Technical University of Hamburg and Shenyang National Laboratory for Materials Science, China)
Photo credit: Technical University of Hamburg. Click image to enlarge.
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Generally, a material's basic properties are determined by its composition and microstructure during the manufacturing process. Changes in these properties may occur during extended use, but this is generally a slow process governed by creep, fatigue, or some other factor. Now, however, researchers Jörg Weissmüller at the Technical University of Hamburg and Hai-Jun Jin at the Shenyang National Laboratory for Materials Science in China have developed a hybrid nanostructure material that can change properties at the flip of a switch. As reported recently in Science, they have developed a material consisting of a nanoporous gold backbone filled with a liquid electrolyte that is capable of fast, reversible tuning of its yield strength, flow stress, and ductility through the application of an electric field. "The concept allows the user to select, for instance," the authors wrote, "a soft and ductile state for processing and a high-strength state for service as a structural material."
Starting with a gold/silver alloy, they removed the silver by corrosion, leaving a monolithic skeleton characterized by contiguous gold ligaments and an equally contiguous pore structure. The pores were then filled with a 1M HClO4 electrolyte. Compression of this hybrid nanostructure material under different electric potential conditions revealed the change in properties. Using a sample with gold ligaments having a diameter of 20 nm, compression under a constant applied voltage of 1.03 V showed ductility up to high strain conditions. The same process performed with an applied voltage of 1.48 V showed a 36% increase in yield strength and a loss of ductility. Also, the flow stress doubled when switching from 1.03 V to 1.48 V. These changes were completely reversible when the applied voltages were reversed.
The researchers noted that at 1.48 V the gold ligaments are covered with adsorbed oxygen, while at 1.03 V they are "clean." This led them to investigate the role of surface stress and surface tension on these property variations; they concluded that neither surface stress nor surface tension was responsible. The most likely explanation according to the researchers is that the adsorbed oxygen exerts a drag on dislocations that intersect the surface, resulting in "adsorption locking," which increases the yield strength and the flow stress at the higher voltage. "For the first time we have succeeded in in producing a material which, while in service, can switch back and forth between a state of strong and brittle behavior and one of soft and malleable," Weismuller said in a press release issued by the Technical University of Hamburg. "We are still at the fundamental research stage but our discovery may bring significant progress in the development of so-called smart materials." [Science]
Nano Focus
Si nanowire-based non-volatile memory devices reduce power consumption
(NIST and George Mason University)
Photo credit: Bonevich/NIST. Click image to enlarge.
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Using small, 20-nm diameter Si nanowires wrapped in HfO2 and Al2O3, researchers Curt Richter at NIST and Qiliang Li at George Mason University may have found a path to creating low-power, fast-writing, non-volatile memories that could eventually replace DRAM and SRAM. The DRAM devices require frequent refreshing to retain stored data, which consumes a large part of the power. The SRAM devices used for cache memory in computers' central processing units (CPUs) are volatile and need to be powered to retain data. The standby power for data remanence is a significant part of the total power dissipation. The lower power consumption of non-volatile memory could mean longer intervals between recharging batteries in computers and other electronic devices. This is very attractive for portable and stand-alone electronics.
As reported recently in Nanotechnology, Richter, Li, and their colleagues took advantage of electrical properties of the materials and the geometry of a small diameter nanowire to improve the electrostatics of gate control. The dielectric properties of HfO2 make it a good charge-trapping layer, and an Al2O3 layer acts as a blocking oxide. Richter says they can tune the stack through band engineering to produce the best possible charge trapping dielectric stacks. Then they take advantage of the small diameter of the Si nanowire to achieve 3D electrostatics, which Richter says, gives them better control than traditional 2D planar devices. "Better electrostatic control means faster, more effective turning on and off," Richter says. "And we've also tuned the gate stack so that most of the electric field is dropped just over the tunnel barrier so we have better control. That means we can hopefully operate at lower voltages and reduce power compared to more traditional dielectric stacks in planar structures."
Their goal is to achieve faster write/erase speeds for non-volatile memory with reduced power consumption. "Our plan is two-fold," Li says. "One is to reduce the channel length so we can achieve higher memory density. The second is to do more engineering on the dielectric stack so that we can get the non-volatile memory programming speed to below 1 ns, similar to SRAMs."[Nanotechnology]
Bio Focus
Attaching proteins to electrodes in ambient conditions
(University of Pennsylvania)
Image credit: Bonnell/University of Pennsylvania. Click image to enlarge.
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Most research involving the attachment of proteins to electrodes to measure their electrical properties has been done in liquid solutions to understand the biological principles of the operation of proteins inside cells. But for other potential applications, such as energy harvesting or toxic chemical sensing, the protein/electrode device must function in ambient, open air conditions. Now researchers at the University of Pennsylvania led by Dawn A. Bonnell have demonstrated successful operation of a single molecular layer of artificial proteins attached to electrodes as optoelectronic devices in an ambient environment. What's more, they've developed a new AFM-based technique to quantitatively measure the resistance, capacitance, and dielectric constant of such devices, as reported in ACS Nano.
Researcher Bodhana Discher fabricated the artificial proteins used in these experiments. Device manufacture involved the self-assembly of amphiphilic protein helices in groups of four on a highly oriented pyrolitic graphite surface using microcontact printing. A single molecular layer of these helices measured 6.6 ± 0.5 nm—the height of a protein helix standing vertically on the graphite surface. The optically active molecule zinc (II) protroporphyrin (ZnPP) was inserted into the interior of the scaffold formed by these four helices; later measurements showed that approximately five ZnPP molecules occupied a single scaffold.
Using their new AFM-based technique called torsional resonance nanoimpedance microscopy, the researchers oscillated a metal AFM tip sideways rather than up and down, so as not to damage the delicate protein structures. A blue LED with a wavelength of 425 nm emitted light near the sample-tip junction to excite the ZnPP molecules. "We use a technique we call 'force stabilization' to get very near the surface," Bonnell says," without disrupting or damaging it. We call it 'soft contact.'" When combined with special circuitry that maximized the signal-to-noise ratio at a higher frequency, they were able to measure the dielectric constant quantitatively by "measuring the polarization volume change between the ground state when there is no light on the ZnPP and the excited state when the light is on it and it is absorbing photons," Bonnell says.
"You'll see lots of characterization papers on lots of different properties in these systems," Bonnell concludes, "but what was different here and I think is going to be generalized in a broader context is that we developed a technique that can measure the dielectric constant of a single-molecule-thick layer." [ACS Nano]
To hear Dawn Bonnell explain her views of the possible applications of this research (mp3, 59 sec.), click the soundwave icon:
"Sounds of Science"
Energy Focus
Dark plasmons trap more light
(Northwestern University)
Photo credit: Northwwestern University. Click image to enlarge.
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Researchers Teri Odom and Wei Zhou of Northwestern University recently reported in Nature Nanotechnology a new type of subradiant (dark) plasmon that is easily tunable by modification of the height of gold nanoparticles arranged in a large-scale, two-dimensional array. Previous attempts to make dark plasmons have involved structuring single nanoparticles or nanoparticle arrays in complex ways, in an attempt to take advantage of broken symmetries in the structure. "In our case we just change the height of the nanoparticles," Odom says. "That's easier than trying to manipulate sub-wavelength features in individual particles."
Abandoning the traditional electron beam lithographic methods, which limit the height of nanoparticles that can be made, the researchers used a template-stripping nanofabrication technique to obtain two-dimensional arrays of gold particles with heights ranging from 65 to 175 nm on transparent substrates. Experimenting with an array of 100-nm high, 160-nm diameter gold particles spaced at 400-nm intervals and covering a total area greater than 18 cm2, Odom and Zhou found an out-of-plane (E0z) electric component of tranverse-magnetic polarized light that excited out-of-plane plasmon modes. These plasmon modes are narrow (FWHM~5 nm) at resonance, and strong coupling between their dipolar moments suppresses the radiative decay of the radiant (bright) plasmons, trapping light in the x-y plane of the nanoparticle array. "We're finally accessing the third dimension," Odom says. "Because we could make the gold nanoparticles so tall, we were able to discover this out-of-plane lattice mode which happens to have this dark plasmon character. We're uncovering some of the unique outcomes of being able to manipulate structure in the z-dimension."
Odom thinks these arrays, with their concentrated, in-plane local energy fields, might be valuable platforms on which to study the mechanisms of chemical reactions. Also, the scalability of the fabrication technique could lead to a coupling of plasmonic and photovoltaic applications. "Because these arrays can trap the light in a much more efficient way and because we can scale them," Odom says, speculating about the distant future, "they could provide a practical first step for plasmonics-based photovoltaics." [Nature Nanotechnology]
Graphene oxide "glue" makes stacking tandem solar cells easier
(Northwestern University)
Photo credit: Huang/Northwestern University. Click image to enlarge.
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Mixing graphene oxide (GO) and the common polymer PEDOT:PSS in water produces a sticky thin film upon casting that may make it simpler to fabricate tandem solar cells, according to research published recently in the Journal of the American Chemical Society. Jiaxing Huang and his colleagues at Northwestern University describe a proof-of-concept using direct adhesive lamination of the layers of tandem devices with GO/PEDOT gel as the glue, a process which they say is much easier than creating tandem architectures via solution processes, as is now commonly done.
Tandem solar cells are multijunction devices in which two sub-cells are stacked for increased solar energy absorption. This stacking requires that the "glue" interlayers be orthogonally processable, which is not easy to achieve in solution with organic solar cells. Also, careful choice of solvents is needed at each step to avoid damaging components in other layers. No such problems arise when aqueous solutions of GO (0.1 -2 wt%) and PEDOT:PSS (1.3-1.7wt.%) are mixed to form a viscous gel that can be easily applied to many substrates. Heat treatment at 60°C turns the gel into a sticky adhesive to bond stacks together. Furthermore, despite the electrically insulating nature of GO, the conductivity of PEDOT:PSS films increases by an order of magnitude when GO is added. The authors suggest that this may be due to a conformational change in PEDOT upon contact with GO. More generally, the GO:PEDOT gel could serve as a non-metallic solder for electrical and mechanical connections in any organic electronic device. [Journal of the American Chemical Society]
