Materials - New Trends

Materials in Focus

Artificial skin made of semiconductor nanowires
(University of California, Berkeley)
Researchers have developed a pressure-sensitive electronic material from semiconductor nanowires. The artificial skin, dubbed "e-skin", is the first such material to be made out of inorganic single crystalline semiconductors. They utilized an innovative fabrication technique that works somewhat like a lint roller in reverse. Instead of picking up fibers, nanowire "hairs" are deposited. The researchers started by growing germanium/silicon nanowires on a cylindrical drum, which was then rolled onto a sticky substrate. As the drum rolled, the nanowires were deposited, or "printed," onto the substrate in an orderly fashion, forming the basis from which thin, flexible sheets of electronic materials could be built. The researchers demonstrated the ability of the e-skin to detect pressure from 0 to 15 kilopascals, a range comparable to the force used for such daily activities as typing on a keyboard or holding an object. [Nature Materials]

Ultrasensitive, highly flexible electronic skin developed
(Stanford University)
By sandwiching a precisely molded, highly elastic rubber layer between two parallel electrodes, researchers were able to create an electronic sensor that can detect the slightest touch. It was able to detect pressures well below the pressure exerted by a 20 milligram bluebottle fly carcass that they experimented with, and with unprecedented speed. The key innovation in the new sensor is the use of a thin film of rubber molded into a grid of tiny pyramids. The thin rubber film between the two electrodes stores electrical charges, much like a battery. When pressure is exerted on the sensor, the rubber film compresses, which changes the amount of electrical charges the film can store. That change is detected by the electrodes and is what enables the sensor to transmit what it is "feeling." [Nature Materials]

Cracking the case on fracture
(Physics)
Many material engineering studies are carried out within a model of continuum plasticity, yet such models often lack sufficient microscopic detail to account for crack propagation and fracture resistance. A study now reports computer simulations showing more clearly what processes influence fracture in plastic deformation, and on what length scales. The authors modeled plastic deformation as the movement of discrete dislocations along slip planes. Specifically, a set of "obstacles" arrayed with some selected spacing restricts the movement of dislocations and modifies the plasticity. They then examined fracture by including an initial crack in the material and observing it propagate as a function of material cohesive strength, fracture energy, and obstacle spacing. They found that it is the obstacle spacing length scale that most strongly affects fracture toughness. Moreover, they propose that their model could serve as a more general simulation environment for fracture studies in various materials. [Physical Review Letters]

Energy Focus

Self-repairing photovoltaics rival conventional solar cells
(NanotechWeb)
During photosynthesis, plants harness solar radiation and convert it into energy. However, the Sun's rays damage and gradually destroy solar-cell components over time. Naturally occurring plants have developed a highly elaborate self-repair mechanism to overcome this problem that involves constantly breaking down and reassembling photodamaged light-harvesting proteins. Researchers have now succeeded in mimicking this process for the first time by creating novel self-assembling complexes that convert light into electricity. The complexes can be repeatedly broken down and reassembled by simply adding a surfactant (a solution of soap molecules). The researchers found that they can indefinitely cycle between assembled and disassembled states by adding and removing the surfactant, but the complexes are only photoactive in the assembled state. [Nature Chemistry]

Laser welding boosts efficiency of TiO₂ solar cells
(NanotechWeb)
Dye-sensitized solar cells (DSSCs) have excellent charge collection capabilities, high open-circuit voltages and good fill-factors. However, they do not completely absorb all of the photons from visible and near-infrared ranges and consequently have lower short-circuit photocurrent densities than inorganic photovoltaics. Increasing the short-circuit current density of DSSCs is a key factor in improving the efficiency of these devices. Researchers have recently demonstrated that the inter-electrode contact resistance arising from poor interfacial adhesion is responsible for a considerable portion of the total resistance in the DSSC. The group has shown that the current flow can be greatly improved by welding the interface with a laser. TiO₂ films formed on transparent conducting oxide (TCO)-coated glass substrates were irradiated with a pulsed UV laser beam at 355 nm, which transmits through TCO and glass, but is strongly absorbed by TiO₂. It was found that a thin continuous TiO₂ layer is formed at the interface as a result of the local melting of TiO₂ nanoparticles. This layer completely bridges the gap between the two electrodes and improves current flow by reducing the contact resistance. [Nanotechnology]

Carbon dioxide-free production of iron
(Highlights in Chemical Technology)
Iron metal has been conventionally produced by melting iron ore at temperatures over 2000°C in a blast furnace. This however produces large amounts of CO₂, which is released into the atmosphere and contributes to climate change. A research team has demonstrated that iron ores (Fe₂O₃ and Fe₃O₄) can be dissolved in molten lithium carbonate at temperatures of around 800°C - a process that was previously thought impossible. Adding an electrical current to the molten mix separates the iron ore into its component parts, iron and oxygen, which can be collected by two electrodes in the solution. Less energy is required to generate the lower temperatures and power the electrolysis, but the researchers also demonstrate that these can be achieved using renewable energy. The team employed their recently developed solar technique, called solar thermal electrochemical photo (STEP) - which uses the Sun's thermal energy to melt the lithium carbonate solution while the visible light energy powers the electrolysis. Using the STEP process no CO₂ is produced. [Chemical Communications]

Nano Focus

High-strength Al-alloy includes core/double-shell nanoparticles
(Northwestern University/Small)

Researchers have created a new high-strength aluminum alloy by engineering it at the nano level to give it high-strength and corrosion resistance to high temperatures. They combined aluminum with lithium, scandium, and ytterbium and they were able to create nano-particles with a core surrounded by two shells. The core is ytterbium-rich, while the first shell is rich in scandium and the second shell contains mostly lithium. This core/shell-shell structure has been achieved previously in liquid solutions but this is the first time it has been achieved by processing solely in the solid-state. They also found that some nano-particles had an unexpected structure — a single particle with two cores and two outer shells, like a double-yolked egg. This novel structure consists of two Yb-rich Al3(Li,Yb,Sc) cores with 4--5 nm diameter, two Sc-rich Al3(Li,Sc,Yb) inner shells surrounding their respective cores and one Li-rich Al3Li outer shell enfolding the previous regions and contained within an Al matrix. This is the first time this type of structure has been observed. [Small]

Nanoscale ion diffusion behavior in Li-ion battery revealed
(Oak Ridge National Laboratory)
A research team has developed the new electrochemical strain microscopy (ESM) to examine the movement of lithium ions through a battery's cathode material. The method can provide a detailed picture of ionic motion in nanometer volumes, which exceeds state-of-the-art electrochemical techniques by six to seven orders of magnitude. They achieved the results by applying voltage with an ESM probe to the surface of the battery's layered cathode. By measuring the corresponding electrochemical strain, or volume change, the team was able to visualize how lithium ions flowed through the material. Conventional electrochemical techniques, which analyze electric current instead of strain, do not work on a nanoscale level because the electrochemical currents are too small to measure. These are the first measurements of lithium ion flow at this spatial resolution, according to the authors. [Nature Nanotechnology]

Electric shock resets nanotube sensor
(Chemistry World)

Single-walled carbon nanotube (SWNTs) can be used in very small, highly sensitive chemical sensors for a variety of gases and other chemicals. The SWNTs, attached to a silicon substrate, absorb chemicals onto their surface, however many chemicals are irreversibly absorbed resulting in lengthy processes before the sensor can be reused. A study now shows that the SWNTs could be 'reset' at the simple flick of a switch. The team found that organic molecules bound to the nanotube surface are shaken off when an electric current is passed through the material, resetting the sensor ready for further use. Their technique - current-stimulated desorption (CSD) - passes a strong electric current through the SWNTs. As electrons jump across defects built into the nanotubes, they collide with molecules on the surface. When they hit an absorbed molecule, they transfer excess energy to it, and it flies off the surface. [Science]

High-speed filter uses electrified nanostructures to purify water
(Stanford University)
By dipping plain cotton cloth in a broth full of silver nanowires and carbon nanotubes, researchers have developed a new high-speed, low-cost filter that could easily be implemented to purify water. Instead of physically trapping bacteria as most existing filters do, the new filter lets them flow on through with the water. By the time the pathogens have passed through, the device kills them with an electrical field that runs through the highly conductive "nano-coated" cotton. In lab tests, over 98 percent of Escherichia coli bacteria that were exposed to 20 volts of electricity in the filter for several seconds were killed. Multiple layers of fabric were used to make the filter 2.5 inches thick. [Nano Letters]

Image in Focus

  
Dark Night in Desert
Colorized SEM image (5,000x) of a nano-skyline of cactus in a seemingly extraterrestrial landscape formed by Si pillars created by deep reactive ion etching decorated with Ge nanowires by vapor-liquid-solid growth.
Credit: Lucia Romano, University of Catania, Italy

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Industry Focus

LED technology used in illumination system for fluorescence microscopy
Fluorescence microscopy requires an intense light source at the specific wavelength that will excite fluorescent dyes and proteins. The traditional method employs a white light, typically from a Mercury or Xenon arc lamp. Although such broad spectrum lamps can generate ample light at desired wavelengths, only a small percentage of the projected light is useful in any particular application. The other wavelengths need to be suppressed to avoid background noise that reduces image contrast and obscures the fluorescent light emissions. This process of suppressing extraneous light is complex, expensive and only partially effective: even after decades of refinements, the best filters are not 100% percent successful at blocking the bleed through of non-specific photons. Some mitigation techniques end up not only suppressing peripheral light, but also significantly diminishing the intensity of the desired wavelengths. A radically different approach is now coming to light. Recent advances in high performance Light Emitting Diode (LED) technology have enabled the practical implementation of this theoretical model. High-intensity monochromatic LEDs are now available in a variety of colors that match the excitation bandwidth of many commonly-used fluorescent dyes and proteins.

Carl Zeiss MicroImaging has incorporated this new LED technology in their Colibri illumination system, a light source system for widefield fluorescence microscopy that uses specific wavelength windows with a considerably decreased need to suppress unwanted peripheral wavelengths from a white light arc lamp. The modular Colibri system employs up to four LEDs, without any of the mechanical switching devices like filterwheels or shutters required by traditional illumination systems. With LED technology, users can now take advantage of an excellent alternative for live cell imaging, high-speed or multi-channel fluorescence microscopy, and many other applications.

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