Graphene is a one atom thick sheet of carbon atoms, arranged in a periodic hexagonal lattice. It is famous not only for its remarkable electronic properties, but also for its exceptional mechanical strength and flexibility. To exploit these two last properties, researchers at Université Paris-Sud have draped a graphene sheet over square lattices of nanometer-sized pillars etched from silicon oxide (, "Strain Superlattices and Macroscale Suspension of Graphene Induced by Corrugated Substrates"). The pillars are either cones or cylinders, with a diameter of a the few tens of nanometer. They are produced by reactive ion etching of silicon oxide, through an electron-beam designed mask of aluminum nanocylinders. The large-area graphene sheet is produced by decomposition at high temperature of a methane/hydrogen mixture, on a copper substrate that acts as a catalyst. The graphene sheet is then transferred onto the pillar array using a polymer film. 
Transferred graphene on nanopillars. (a) Schematic view of graphene membrane deposited onto SiO2 nanopillar array. (b) Atomic force micrograph of graphene deposited on SiO2 nanopillars. (c) Schematics of graphene (in black) transferred onto nanopillars array (in blue). For dense array (a < a*), fully suspended graphene over large areas are observed. At low array density (a > a*), graphene conforms with the substrate and forms highly symmetric ripples. (d) and (e): Series of SEM micrographs showing the behavior of transferred graphene membrane for 270 nm height silicon pillars. (d) the pillar pitch a is respectively equal to 2.3 and 0.25 µm. Scale bars lengths are 2 µm. Scanning electron microscopy and atomic force microscopy are used to detect the different ways in which the graphene sheet rests on the pillars, whose separation in the arrays varies between 0.3 microns and 4 microns. The scientists found a characteristic pillar separation (characteristic lattice constant a*) of the order of one micrometer. For smaller pillar lattice constants (a < a*), the graphene lays flat, fakir-like, resting on the tips of the pillars. For less dense pillar arrays (a > a*), the graphene hugs the substrate and pillars tightly, without tearing. A high stress thus occurs at each pillar tip, leading to a deformation of the graphene hexagons that was detected by Raman spectroscopy. Stress is also released via the formation of folds that are oriented parallel to pillar lattice directions. The density of folds results from competing energies, the elastic deformation energy of graphene, and the attractive interaction between graphene and substrate (stemming from the energetics of the transfer process onto the irregular, sharp substrate: electrostatics, van der Waals, capillary, etc…). In this work a variety of nanostructures has been created that can modify the electronic properties of graphene, either by periodic modulation of the electrostatic potential, or by periodic mechanical stress that can generate an effective magnetic structure. Both electronic transport and magnetisation measurements of these structures are underway to reveal the consequences of such mesoscopically stressed graphene.
New projects support research in 'valleytronics'
Two new three-year research projects are supporting the role of the Stanford Institute for Materials and Energy Sciences (SIMES) as a leading center for studying exotic new materials that could enable future innovative electronic and photonic applications. SIMES is a joint institute of Stanford University and the Department of Energy's SLAC National Accelerator Laboratory. “These awards are very important for SIMES,” said Tom Devereaux, a professor at SLAC and director of SIMES. “We have been establishing leadership in scientific areas that make SLAC unique. The awards significantly strengthen our core efforts in ultrafast science and quantum materials.” The two complementary projects will explore several theoretical and experimental aspects of an emerging area called “valleytronics.” In valleytronics, electrons move through the lattice of a two-dimensional semiconductor as a wave with two energy valleys whose characteristics can be used to encode information. Prime valleytronic materials are chalcogenides (pronounced cal-CAW-gin-eyeds), materials composed of a heavy metal atom and one or more atoms of oxygen, sulfur, selenium or tellurium. Many chalcogenides naturally form atom-scale layers that, under the right circumstances, result in special properties of interest to the SIMES researchers. “For example,” said SIMES researcher Yi Cui, “shining certain types of light onto some chalcogenides can control their electrons’ movements in ways that produce properties favorable for their use in efficient photodetectors, low-energy computer logic and data storage chips or quantum computers.” The SIMES researchers will perform theoretical calculations, make new nanomaterials and perform experiments in SLAC’s laboratories and DOE Office of Science User Facilities, including the Stanford Synchrotron Radiation Lightsource and the Linac Coherent Light Source. Their ultimate goal is to learn how to tune the materials to optimize their electronic properties. “SIMES and SLAC provide a wonderful combination of expertise in material synthesis, advanced characterization capabilities and theory, bringing together the key ingredients to make progress in this exciting new field,” remarked Stanford/SLAC Professor and SLAC Chemical Sciences Division Director Tony Heinz. One project, titled “Induction and Dynamics of New States of Matter in Two-Dimensional Materials,” is led by Devereaux, with co-investigators Zhi-Xun Shen, Aaron Lindenberg and Tony Heinz. It has received funding under the DOE’s "Scientific Discovery through Ultrafast Materials and Chemical Sciences" program. SLAC was the only DOE national lab chosen as a sole principal investigator in this program. The second project, “Chalcogenide Nanomaterials,” is led by SIMES researcher Yi Cui with co-investigators Harold Hwang, Shoucheng Zhang, Jun-Sik Lee and Hongtao Yuan. After the project's success with last year’s seed funding, the DOE has established a core program at SLAC in this novel area.
Nanotechnology quick test for Ebola
When diagnosing a case of Ebola, time is of the essence. However, existing diagnostic tests take at least a day or two to yield results, preventing health care workers from quickly determining whether a patient needs immediate treatment and isolation. A new test from MIT researchers could change that: The device, a simple paper strip similar to a pregnancy test, can rapidly diagnose Ebola, as well as other viral hemorrhagic fevers such as yellow fever and dengue fever. “As we saw with the recent Ebola outbreak, sometimes people present with symptoms and it’s not clear what they have,” says Kimberly Hamad-Schifferli, a visiting scientist in MIT’s Department of Mechanical Engineering and a member of the technical staff at MIT’s Lincoln Laboratory. “We wanted to come up with a rapid diagnostic that could differentiate between different diseases.” 
A new paper diagnostic device can detect Ebola as well as other viral hemorrhagic fevers in about 10 minutes. The device (pictured here) has silver nanoparticles of different colors that indicate different diseases. On the left is the unused device, opened to reveal the contents inside. On the right, the device has been used for diagnosis; the colored bands show positive tests. (Photo courtesy of Jose Gomez-Marquez, Helena de Puig, and Chun-Wan Yen) Hamad-Schifferli and Lee Gehrke, the Hermann L.F. von Helmholtz Professor in MIT’s Institute for Medical Engineering and Science (IMES), are the senior authors of a paper describing the new device in the journal ("Multicolored silver nanoparticles for multiplexed disease diagnostics: distinguishing dengue, yellow fever, and Ebola viruses"). The paper’s lead author is IMES postdoc Chun-Wan Yen, and other authors are graduate student Helena de Puig, IMES postdoc Justina Tam, IMES instructor Jose Gomez-Marquez, and visiting scientist Irene Bosch. Color-coded test Currently, the only way to diagnose Ebola is to send patient blood samples to a lab that can perform advanced techniques such as polymerase chain reaction (PCR), which can detect genetic material from the Ebola virus. This is very accurate but time-consuming, and some areas of Africa where Ebola and other fevers are endemic have limited access to this kind of technology. The new device relies on lateral flow technology, which is used in pregnancy tests and has recently been exploited for diagnosing strep throat and other bacterial infections. Until now, however, no one has applied a multiplexing approach, using multicolored nanoparticles, to simultaneously screen for multiple pathogens. “For many hemorrhagic fever viruses, like West Nile and dengue and Ebola, and a lot of other ones in developing countries, like Argentine hemorrhagic fever and the Hantavirus diseases, there are just no rapid diagnostics at all,” says Gehrke, who began working with Hamad-Schifferli four years ago to develop the new device. Unlike most existing paper diagnostics, which test for only one disease, the new MIT strips are color-coded so they can be used to distinguish among several diseases. To achieve that, the researchers used triangular nanoparticles, made of silver, that can take on different colors depending on their size. The researchers created red, orange, and green nanoparticles and linked them to antibodies that recognize Ebola, dengue fever, and yellow fever. As a patient’s blood serum flows along the strip, any viral proteins that match the antibodies painted on the stripes will get caught, and those nanoparticles will become visible. This can be seen by the naked eye; for those who are colorblind, a cellphone camera could be used to distinguish the colors. “When we run a patient sample through the strip, if you see an orange band you know they have yellow fever, if it shows up as a red band you know they have Ebola, and if it shows up green then we know that they have dengue,” Hamad-Schifferli says. This process takes about 10 minutes, allowing health care workers to rapidly perform triage and determine if patients should be isolated, helping to prevent the disease from spreading further. Warren Chan, an associate professor at the University of Toronto Institute of Biomaterials and Biomedical Engineering, says he is impressed with the device because it not only offers faster diagnosis, but also requires smaller patient blood samples, as just one test strip can detect multiple diseases. “It’s a step up from what everyone else is doing,” says Chan, who was not involved in the research. “They’re targeting diseases that are really relevant to what’s going on in the world at this point, and have shown that they can detect them simultaneously.” Faster triage The researchers envision their new device as a complement to existing diagnostic technologies, such as PCR. “If you’re in a situation in the field with no power and no special technologies, if you want to know if a patient has Ebola, this test can tell you very quickly that you might not want to put that patient in a waiting room with other people who might not be infected,” says Gehrke, who is also a professor of microbiology and immunology at Harvard Medical School. “That initial triage can be very important from a public health standpoint, and there could be a follow-up test later with PCR or something to confirm.” The researchers hope to obtain Food and Drug Administration approval to begin using the device in areas where the Ebola outbreak is still ongoing. In order to do that, they are now testing the device in the lab with engineered viral proteins, as well as serum samples from infected animals. This type of device could also be customized to detect other viral hemorrhagic fevers or other infectious diseases, by linking the silver nanoparticles to different antibodies. “Thankfully the Ebola outbreak is dying off, which is a good thing,” Gehrke says. “But what we’re thinking about is what’s coming next. There will undoubtedly be other viral outbreaks. It might be Sudan virus, it might be another hemorrhagic fever. What we’re trying to do is develop the antibodies needed to be ready for the next outbreak that’s going to happen.”
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