Materials in Focus
Thermoelectric properties of half-Heusler alloys enhanced
(Physics World)
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To be of practical use, a thermoelectric material must be good at conducting electricity but poor at conducting heat. "Half-Heusler" alloys have promising thermoelectric properties but they suffer from having relatively high thermal conductivities. One way of reducing their conductivity is to squish together a fine powder of the material to form a nanocomposite containing many small grains. Heat has a hard time travelling across grain boundaries, thereby reducing the overall thermal conduction of the nanocomposite. Researchers have now used this technique on an extremely fine powder of a half-Heusler alloy, producing a nanocomposite with the best ZT (thermoelectric figure of merit) yet for a half-Heusler. [Nano Letters]
Embedded microvoids make LEDs more efficient
(North Carolina State University)
LED lighting relies on GaN thin films to create the diode structure that produces light. A new technique now reduces the number of defects in GaN films by two to three orders of magnitude by embedding microvoids. This improves the quality of the material that emits light, and for a given input of electrical power, the output of light can be increased by a factor of two – which is very big. This is particularly true for low electrical power input and for LEDs emitting in the ultraviolet range. The researchers started with a GaN film that was two microns thick and embedded half of that thickness with large voids – empty spaces that were one to two microns long and 0.25 microns in diameter. The researchers found that defects in the film were drawn to the voids and became trapped – leaving the portions of the film above the voids with far fewer defects. [Applied Physics Letters]
Growth, characterization of LiMnAs: A useful pyramid scheme
(Physics)
All electronics technologies have, at their heart, critical materials that make their function possible. These can be "old" materials such as silicon, whose major materials development was achieved by previous generations, or "new" materials such as gallium-nitride, which has been developed by our contemporaries.If the discovery and development of new materials comes to a stop, then the introduction and growth of new technologies will almost certainly come to a halt as well. Spintronics is an example of such a critical current technology, driving the creation of increased density, faster electronic memories through the electronic manipulation of magnetic moments. Researchers now report the successful growth and characterization of LiMnAs, a new candidate material for spintronic applications. They show convincing evidence of epitaxy and good film quality, and show that LiMnAs is a semiconductor, by performing optical spectroscopy. They also show that it is antiferromagnetic in thin film form by measuring its temperature-dependent magnetization. [Physical Review B]
Electrical phenomena in silicon oxide in electronics explored
(Eurekalert/ACS)
Researchers have found that silicon dioxide in computer chips, long regarded as an electrical insulator, can actually be made to act like a switch and take part in electronic processes. They have documented various electrical phenomena such as resistive switching and related nonlinear conduction, current hysteresis, and negative differential resistance, that are intrinsic to a thin layer of SiOx. This is more crucial in the area of nanoelectronics, wherein researchers thought that switching observed was due to the nano-additive but it turns out that the source of the switching might be from the underlying silicon oxide itself. The work clarifies the possible nature behind switching events in molecular and nano-scale systems investigated so far, that were not well understood. [J. American Chemical Society]
Nano Focus
Silver nanoparticles-coated paper for food packaging
(American Chemical Society)
It is known that silver nanoparticles show excellent microbicidal properties, much better than those of larger particles. Researchers have now demonstrated an effective, long-lasting method for depositing silver nanoparticles on the surface of paper that involves ultrasound waves. The coated paper showed potent antibacterial activity against E. coli and S. aureus, two causes of bacterial food poisoning, killing all of the bacteria in just three hours. This suggests its potential application as a food packaging material for promoting longer shelf life. [Langmuir]
Bio Focus
Nanoparticle divides to penetrate into tumors
(Chemistry World)
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Researchers have created a nanoparticle that breaks up into smaller units once it reaches its target, allowing it to penetrate deeper into tumor tissue and deliver treatment more effectively. The new nanoparticles are 100 nm balls of gelatin that contain small particles that are only 10 nm in diameter. The gelatin nanoparticles get to the tumors, and then tumor enzymes digest the gelatin and release the smaller constituents, that then move through the tumor. In vitro studies showed that the particles penetrated tumor tissue much better traditional larger nanoparticles that remain one size. [Proceedings of the National Academy of Sciences]
New method for tethering and stretching DNA
(Nanotechweb.org)
Researchers have developed a reproducible surface chemistry technique for tethering DNA molecules onto surfaces and a new way to stretch the molecules to various lengths. DNA can be used as a molecular scaffold to make metal contacts to organic semiconductors. A key step in this process involves being able to tether the DNA to various surfaces and stretch the molecule to varying lengths. The new strategy involves synthesizing hybrid DNA-organic molecule-DNA (DOD) structures, then stretching and tethering the DOD assemblies between two microscopic metal electrodes. The researchers then make metal electrode-organic molecule-metal electrode (MOM) structures by further metallizing the DNA segments within the DOD structures. The team then exploited so-called biotin-Streptavidin linkage chemistry to tether the DNA assemblies to device surfaces. The method could eventually be used to make large-scale nanoelectronic devices based on single organic molecules. [ACS Nano]
Nanoscale transistors used to study single-molecule interactions
(Columbia University/Eurekalert)
Researchers have figured out a way to study single-molecule interactions on very short time scales using nanoscale transistors. 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. Previously, scientists have used fluorescence techniques to look at interactions at the level of single molecules. 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 transistors employed in this study were fashioned from carbon nanotubes which are exquisitely sensitive because the biomolecule can be directly tethered to the carbon nanotube wall creating enough sensitivity to detect a single DNA molecule. [Nature Nanotechnology]
Energy Focus
Packings of carbon nanotubes for hydrogen storage
(Chemistry World)
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Researchers have designed a 3D carbon nanotube matrix that can store and release hydrogen extremely efficiently. They used a computer-based approach to design a 3D carbon nanotube structure that can store more hydrogen at room temperature than any other carbon-based material. This is a top down approach from advanced mathematics, to geometry, to computer modeling, to chemical properties. The US Department of Energy's target for hydrogen storage materials by 2015 is 6wt% while the new nanotube material has a total hydrogen uptake of 5.5wt% at room temperature. Inspired by natural sponges, the team designed a computer model that placed carbon nanotubes in the hole positions of a theoretical sponge network. [Advanced Materials]
Relativity powers lead-acid battery
(Physical Review Focus)
The lead-acid battery that starts most car engines gets about 80 percent of its voltage from relativity, according to theoretical work using computer simulations. The relativistic effect comes from fast-moving electrons in the lead atom. The computer simulations also explain why tin-acid batteries do not work, despite apparent similarities between tin and lead. The researchers are the first to derive theoretical models of the lead-acid battery from fundamental physics principles. By switching relativistic parts of their models "on" and "off", the team found that relativity accounts for 1.7 volts of a single cell, which means that about 10 of the 12 volts in a car battery come from relativistic effects. [Applied Physics Letters]
Image in Focus
ZnO Nanoflowers
Stem of nanoflowers made by coloring and combining different SEM images of a variety of ZnO nanostructures grown by thermal Chemical Vapor Deposition.
Credit: Abhishek Prasad, Michigan Technological University
(One of three Science as Art competition first place winners at the 2010 MRS Fall Meeting)
[We invite you to submit your images to the Editor for possible inclusion in this feature]
