Graphene-based technique creates the smallest gaps in nanostructures

A new procedure will enable researchers to fabricate smaller, faster, and more powerful nanoscale devices - and do so with molecular control and precision. Using a single layer of carbon atoms, or graphene, nanoengineers at the University of California, San Diego have invented a new way of fabricating nanostructures that contain well-defined, atomic-sized gaps. The results from the UC San Diego Jacobs School of Engineering were published in the January issue of the journal ("Using the Thickness of Graphene to Template Lateral Subnanometer Gaps between Gold Nanostructures"). A single layer of graphene shown on a slide A single layer of graphene shown on a slide. Structures with these well-defined, atomic-sized gaps could be used to detect single molecules associated with certain diseases and might one day lead to microprocessors that are 100 times smaller than the ones in today’s computers. The ability to generate extremely small gaps - known as nanogaps - is highly desirable in fabricating nanoscale structures, which are typically used as components in optic and electronic devices. By decreasing the spacing between electronic circuits on a microchip, for example, one can fit more circuits on the same chip to produce a device with greater computing power. A team of Ph.D. students and undergraduate researchers led by UC San Diego nanoengineering professor Darren Lipomi demonstrated that the key to generating a smaller nanogap between two nanostructures involves using a graphene spacer, which can be etched away to create the gap. Graphene is the thinnest material known: it is simply a single layer of carbon atoms and measures approximately 0.3 nanometers (nm), which is about 100,000 times thinner than a human hair. The technique developed by Lipomi’s team overcomes some of the limitations of standard fabrication methods, such as photolithography and electron-beam lithography. By comparison, the smallest nanogaps that can be generated using the standard methods are 10–20 nm wide. “Making a nanogap is interesting from a philosophical standpoint,” said Lipomi. “While most efforts in nanotechnology focus on making materials, we’ve essentially made nothing - but with controlled dimensions.” Making “nothing” The method for making nanogaps begins with the production of thin films in which a single layer of graphene is sandwiched between two gold metal sheets. First, graphene is grown on a copper substrate, and then layered on top with a sheet of gold metal. Because graphene sticks better to gold than to copper, the entire graphene single-layer can be easily removed and remains intact over large areas. Compared to other techniques that are used to produce similar layered structures, this method allows graphene to be transferred to gold film with minimal defects or contamination. “This new method, which we developed in our lab, is called metal-assisted exfoliation. This is the only way so far in which we can place single-layer graphene between two metals and ensure that it contains no rips, cracks, folds, or unwanted chemical species,” said Alex Zaretski, a graduate student in Lipomi’s research group who pioneered the technique and is the first author of the study. “Metal-assisted exfoliation can potentially be useful for industries that use large areas of graphene.” Once the gold/graphene composite is separated from the copper substrate, the newly exposed side of the graphene layer is sandwiched with another gold sheet to produce the gold:single-layer graphene:gold thin film. The films are then sliced into 150 nm-wide nanostructures. Finally, the structures are treated with oxygen plasma to remove graphene. Scanning electron micrographs of the structures reveal extremely small nanogaps between the gold layers. Nanogap applications One potential application for this technology is in ultra-sensitive detection of single molecules, particularly those that are characteristic of certain diseases. When light is shined upon structures with extremely small gaps, the electromagnetic field that is confined within the gap becomes enormously enhanced. This enhanced electromagnetic field, in turn, increases the signal produced by any molecule within the gap. “If some disease marker comes in and bridges the gap between the nanostructures, you would observe a change in the light scattering from the nanogap that would correspond to whether the disease was present or not,” said Lipomi. While the technique reported in this study can produce nanostructures suitable for optical applications, it exhibits a major drawback for electronic applications. Raman spectroscopic measurements of the gold nanostructures reveal that small amounts of graphene still remain between the gold layers after being treated with oxygen plasma. This means that only the graphene exposed near the surfaces of the gold nanostructures can be removed so far. Having graphene still in the structures is not desirable for electronic devices, which require an entire gap between the structures. The team is working to figure out how to solve this problem. In the future, the team would also like to explore ways to vary the thickness of the well-defined gap between the structures by increasing the number of graphene layers. “For optical applications, it would be desirable to have gaps that are a little bit bigger than what we’ve generated. We just wanted to show, in principle, the smallest gap size that is possible to achieve,” said Lipomi.
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Researchers model new atomic structures of gold nanoparticle

They may deal in gold, atomic staples and electron volts rather than cement, support beams and kilowatt-hours, but chemists have drafted new nanoscale blueprints for low-energy structures capable of housing pharmaceuticals and oxygen atoms. Led by UNL's Xiao Cheng Zeng and former visiting professor Yi Gao, new research has revealed four atomic arrangements of a gold nanoparticle cluster. The arrangements exhibit much lower potential energy and greater stability than a standard-setting configuration reported last year by a Nobel Prize-winning team from Stanford University. The modeling of these arrangements could inform the cluster's use as a transporter of pharmaceutical drugs and as a catalyst for removing pollutants from vehicular emissions or other industrial byproducts, Zeng said. atomic arrangements of a gold nanocluster This rendering shows the atomic arrangements of a gold nanocluster as reported in a new study led by UNL chemist Xiao Cheng Zeng. The cluster measures about 1.7 nanometers long - roughly the same length that a human fingernail grows in two seconds. Zeng and his colleagues unveiled the arrangements for a molecule featuring 68 gold atoms and 32 pairs of bonded sulfur-hydrogen atoms. Sixteen of the gold atoms form the molecule's core; the remainder bond with the sulfur and hydrogen to form a protective coating that stems from the core. Differences in atomic arrangements can alter molecular energy and stability, with less potential energy making for a more stable molecule. The team calculates that one of the arrangements may represent the most stable possible structure in a molecule with its composition. "Our group has helped lead the front on nano-gold research over the past 10 years," said Zeng, an Ameritas University Professor of chemistry. "We've now found new coating structures of much lower energy, meaning they are closer to the reality than (previous) analyses. So the deciphering of this coating structure is major progress." The researchers reported their findings in the April 24 edition of ("Unraveling structures of protection ligands on gold nanoparticle Au68(SH)32"), an online journal from the American Association for the Advancement of Science. The structure of the molecule's gold core was previously detailed by the Stanford team. Building on this, Zeng and his colleagues used a computational framework dubbed "divide-and-protect" to configure potential arrangements of the remaining gold atoms and sulfur-hydrogen pairs surrounding the core. The researchers already knew that the atomic coating features staple-shaped linkages of various lengths. They also knew the potential atomic composition of each short, medium and long staple -- such as the fact that a short staple consists of two sulfur atoms bonded with one gold. By combining this information with their knowledge of how many atoms reside outside the core, the team reduced the number of potential arrangements from millions to mere hundreds. "We divided 32 into the short, middle and long (permutations)," said Zeng, who helped develop the divide-and-protect approach in 2008. "We lined up all those possible arrangements, and then we computed their energies to find the most stable ones. "Without those rules, it's like finding a needle in the Platte River. With them, it's like finding a needle in the fountain outside the Nebraska Union. It's still hard, but it's much more manageable. You have a much narrower range." The researchers resorted to the computational approach because of the difficulty of capturing the structure via X-ray crystallography or single-particle transmission electron microscopy, two of the most common imaging methods at the atomic scale. Knowing the nanoparticle's most stable configurations, Zeng said, could allow biomedical engineers to identify appropriate binding sites for drugs used to treat cancer and other diseases. The findings could also optimize the use of gold nanoparticles in catalyzing the oxidation process that transforms dangerous carbon monoxide emissions into the less noxious carbon dioxide, he said.
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Study explores the interaction of carbon nanotubes and the blood-brain barrier

A research published in ("The interaction of carbon nanotubes with an in vitro blood-brain barrier model and mouse brain in vivo") studies the interaction of carbon nanotubes and the blood-brain barrier through two different proceedings. The study was carried by the Institute of Pharmaceutical Science at the King's College London and Elzbieta Pach and Belén Ballesteros, members of the ICN2 Electron Microscopy Division, participated on the electron microscopy characterization studies. translocation of carbon nanotubes cross porcine brain endothelial cells membrane Translocation of “individual” MWNTs-NH3+ across porcine brain endothelial cells membrane. Images acquired using the STEM detection system on the Magellan HRSEM at 20 kV. The study investigates the ability of amino-functionalized multi-walled carbon nanotubes (MWNTs-NH3+) to cross the Blood-Brain Barrier (BBB) by two ways: in vitro using a co-culture BBB model comprising primary porcine brain endothelial cells (PBEC) and primary rat astrocytes and, in vivo, following a systemic administration of radiolabelled f-MWNTs. The study carried out at ICN2 has allowed the corroboration of the results and the better understanding of the processes. Images by Transmission Electron microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) showed that the cells or the tight junction assemblies were not damaged, that the interaction of MWNTs-NH3+ and the plasma membrane of the endothelial cells took place after 4 h of incubation and confirmed that MWNTs-NH3+ crossed the PBEC monolayer via energy-dependent transcytosis. Also, high resolution TEM (HRTEM) and Electron Energy Loss Spectroscopy (EELS) showed that the graphitic structure of the MWNTs-NH3+ was preserved following uptake into PBEC. To sum up, researchers were able to demonstrate, for the first time, the ability of MWNTs-NH3+ to cross the BBB in vitro with low voltage STEM imaging, thus providing solid evidence using electron microscopy for each step of the transcytosis process. This research also stands out because its results could lead to the use of CNTs in new applications. For instance, they could work as nanocarriers for delivery of drugs and biologics to the brain, after systemic administration.
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