#Charting quantum signatures of electronic transport in graphene Over the last seven years, Javier Sanchez-Yamagishi has built several hundred nanoscale stacked graphene systems to study their electronic properties."
"What interests me a lot is that the properties of this combined system depend sensitively on the relative alignment between them,
"he says. Sanchez-Yamagishi, who received his Phd in January 2015, is now a postdoctoral associate in Associate professor Pablo Jarillo-Herrero's group.
He assembles sandwiches of graphene and boron nitride with various horizontal orientations.""The tricks we would use were making cleaner devices,
cooling them down to low temperatures and applying very large magnetic fields to them, "says Sanchez-Yamagishi,
who carried out measurements at the National High Magnetic field Laboratory in Tallahassee, Fla. The lab features the largest continuous magnet in the world, 45 Tesla,
which is about 10,000 times the strength of a refrigerator magnet. Sanchez-Yamagishi was a lead co-author of a 2014 paper in Nature("Tunable symmetry breaking and helical edge transport in a graphene quantum spin Hall state)
"which showed that having a component of the applied magnetic field in the graphene plane forced electrons at the edge of graphene to move in opposite directions based on their spins.
Lead co-authors were postdoctoral associate Benjamin M. Hunt and Pappalardo Fellow Andrea Young, both from MIT Physics Professor Raymond C. Ashoori's group.
The paper was the culmination of two years'work, Sanchez-Yamagishi says.""We were trying to realize some interesting quantum states in the graphene.
It's called a Quantum Spin Hall State, "Sanchez-Yamagishi explains. That would have applications in quantum computing,
an area of interest to the group because Jarillo-Herrero is a researcher in the NSF-funded Center for Integrated Quantum Materials.
Sanchez-Yamagishi also was a co-author of a 2013 Science paper in which Jarillo-Herrero, Ashoori,
and collaborators demonstrated that a certain alignment of layered graphene and hexagonal boron nitride created a unique bandgap in graphene,
which could be a precursor to developing the material for functional transistors. Sanchez-Yamagishi's co-authors again included Young
now assistant professor at the University of California at Santa barbara, and Hunt, who will join the faculty of the Carnegie mellon physics department this fall.
Hofstadter butterfly Graphene and boron nitride layers each have arranged atoms in a hexagonal, or six-sided, pattern.
When the lattice arrangement of graphene and hexagonal boron nitride layers are aligned closely, and the samples are exposed to a large out-of-plane magnetic field,
they exhibit electronic energy levels that are called"Hofstadter's butterfly, "because when they are plotted on a graph it resembles a butterfly.
What excites physicists is that this butterfly is one of the rare examples of a fractal pattern in quantum physics."
"These are physics that only come into play because the electrons are very small and we make them very cold.
So quantum physics takes a role and it is very different, shockingly different, "Sanchez-Yamagishi says.
In addition to the Hofstadter butterfly result, the same devices were also the first to show a bandgap in graphene.
Jarillo-Herrero says, "What was unexpected very was showed we that graphene, which usually conducts very well, under the conditions of that experiment with a very low angle of rotation between the graphene
and the HBN, became an insulator. It didn't conduct at all. That was a behavior
which was unexpected and it is still. Theorists are still trying to understand why. At a quantitative level
which is like a honeycomb or chicken-wire-shaped lattice of carbon atoms. When these honeycomb structures are stacked on top of each other,
if they are out of alignment, they create a so-called moire pattern, which varies with rotation of the layers with respect to each other."
the graphene has to be aligned very closely to hexagonal boron nitride. When it's closely aligned,
"While this honeycomb structure exists in graphite, a familiar bulk form of carbon, its special properties only show
when layers of graphene just one to few atoms thick are separated from the graphite.""Graphene conducts electricity better than graphite.
It conducts better than silver or gold, "Sanchez-Yamagishi says. Sanchez-Yamagishi built a machine in the lab that stacks extremely thin layers of graphene and similar materials.
"In graphite, normally all the layers are aligned with each other; electrons get slowed down, "he explains. It turns out that
and it can still conduct electricity basically as well as if it was still a single sheet of graphene,
Learning curve One of the first graduate students to join Jarillo-Herrero's group in 2008
since that is how they come off the natural graphite material. The graphite is rubbed on a sheet of silicon
and lifted off with special tape to create thin layers of graphene. Maximizing the amount of graphene that can be used for a device takes priority over making it look nice
"A big focus of our lab is just studying electricity in the form of how electrons move around
He is also mentoring current graduate students Yuan Cao and Jason Luo L
#One step closer to a single-molecule device Researchers have designed a new technique to create a single-molecule diode,
and, in doing so, they have developed molecular diodes that perform 50 times better than all prior designs.
The group, under the direction of Latha Venkataraman, associate professor of applied physics at Columbia Engineering, is the first to develop a single-molecule diode that may have real-world technological applications for nanoscale devices.
Their paper,"Single-Molecule Diodes with High On-Off Ratios through Environmental Control,"is published May 25 in Nature Nanotechnology."
"Our new approach created a single-molecule diode that has a high(>250) rectification and a high"on"current (0. 1 micro Amps),"says Venkataraman."
"Constructing a device where the active elements are only a single molecule has long been a tantalizing dream in nanoscience.
This goal, which has been the'holy grail'of molecular electronics ever since its inception with Aviram and Ratner's 1974 seminal paper, represents the ultimate in functional miniaturization that can be achieved for an electronic device."
"With electronic devices becoming smaller every day, the field of molecular electronics has become ever more critical in solving the problem of further miniaturization,
and single molecules represent the limit of miniaturization. The idea of creating a single-molecule diode was suggested by Arieh Aviram
and Mark Ratner who theorized in 1974 that a molecule could act as a rectifier, a one-way conductor of electric current.
Researchers have since been exploring the charge-transport properties of molecules. They have shown that single-molecules attached to metal electrodes (single-molecule junctions) can be made to act as a variety of circuit elements
including resistors, switches, transistors, and, indeed, diodes. They have learned that it is possible to see quantum mechanical effects, such as interference, manifest in the conductance properties of molecular junctions.
Since a diode acts as an electricity valve, its structure needs to be asymmetric so that electricity flowing in one direction experiences a different environment than electricity flowing in the other direction.
In order to develop a single-molecule diode, researchers have designed simply molecules that have asymmetric structures.""While such asymmetric molecules do indeed display some diode-like properties,
they are not effective, "explains Brian Capozzi, a Phd student working with Venkataraman and lead author of the paper."
"A well-designed diode should only allow current to flow in one direction-the'on'direction
-and it should allow a lot of current to flow in that direction. Asymmetric molecular designs have suffered typically from very low current flow in both'on and off'directions,
and the ratio of current flow in the two has typically been low. Ideally, the ratio of'on'current to'off'current, the rectification ratio, should be very high."
"In order to overcome the issues associated with asymmetric molecular design, Venkataraman and her colleagues-Chemistry Assistant professor Luis Campos'group at Columbia and Jeffrey Neaton's group at the Molecular Foundry at UC Berkeley-focused on developing an asymmetry in the environment around the molecular junction.
They created an environmental asymmetry through a rather simple method-they surrounded the active molecule with an ionic solution
and used gold metal electrodes of different sizes to contact the molecule. Their results achieved rectification ratios as high as 250: 50 times higher than earlier designs.
The"on"current flow in their devices can be more than 0. 1 microamps which, Venkataraman notes, is a lot of current to be passing through a single-molecule.
And, because this new technique is implemented so easily, it can be applied to all nanoscale devices of all types,
including those that are made with graphene electrodes.""It's amazing to be able to design a molecular circuit,
using concepts from chemistry and physics, and have it do something functional, "Venkataraman says.""The length scale is so small that quantum mechanical effects are absolutely a crucial aspect of the device.
So it is truly a triumph to be able to create something that you will never be able to physically see
An illustration of the molecule used by Columbia Engineering professor Latha Venkataraman to create the first single-molecule diode with a non-trivial rectification ratio overlaid on the raw current versus voltage data.
Diodes are fundamental building blocks of integrated circuits; they allow current to flow in only one direction.
#Researchers develop a semiconductor chip made almost entirely of wood Portable electronics-typically made of nonrenewable,
non-biodegradable and potentially toxic materials-are discarded at an alarming rate in consumers'pursuit of the next best electronic gadget.
In an effort to alleviate the environmental burden of electronic devices, a team of University of Wisconsin-Madison researchers has collaborated with researchers in the Madison-based U s. Department of agriculture Forest Products Laboratory (FPL) to develop a surprising solution:
a semiconductor chip made almost entirely of wood. The research team, led by UW-Madison electrical
and computer engineering professor Zhenqiang"Jack"Ma, described the new device in a paper published May 26, 2015 by the journal Nature Communications("High-performance green flexible electronics based on biodegradable
cellulose nanofibril paper")."The paper demonstrates the feasibility of replacing the substrate, or support layer, of a computer chip, with cellulose nanofibril (CNF), a flexible, biodegradable material made from wood."
"The majority of material in a chip is support. We only use less than a couple of micrometers for everything else,
"Ma says.""Now the chips are so safe you can put them in the forest
and fungus will degrade it. They become as safe as fertilizer.""Zhiyong Cai, project leader for an engineering composite science research group at FPL,
has been developing sustainable nanomaterials since 2009.""If you take a big tree and cut it down to the individual fiber,
the most common product is paper. The dimension of the fiber is in the micron stage,
"Cai says.""But what if we could break it down further to the nano scale?
At that scale you can make this material, very strong and transparent CNF paper.""Working with Shaoqin"Sarah"Gong, a UW-Madison professor of biomedical engineering, Cai's group addressed two key barriers to using wood-derived materials in an electronics setting:
surface smoothness and thermal expansion.""You don't want it to expand or shrink too much. Wood is a natural hydroscopic material
and could attract moisture from the air and expand,"Cai says.""With an epoxy coating on the surface of the CNF,
"Gong and her students also have been based studying bio polymers for more than a decade. CNF offers many benefits over current chip substrates, she says."
"The advantage of CNF over other polymers is that it's a bio-based material and most other polymers are based petroleum polymers.
Bio-based materials are sustainable, biocompatible and biodegradable, "Gong says.""And, compared to other polymers,
CNF actually has a relatively low thermal expansion coefficient.""The group's work also demonstrates a more environmentally friendly process that showed performance similar to existing chips.
The majority of today's wireless devices use gallium arsenide-based microwave chips due to their superior high-frequency operation and power handling capabilities.
However, gallium arsenide can be environmentally toxic, particularly in the massive quantities of discarded wireless electronics.
Yei Hwan Jung, a graduate student in electrical and computer engineering and a co-author of the paper,
says the new process greatly reduces the use of such expensive and potentially toxic material."
"I've made 1, 500 gallium arsenide transistors in a 5-by-6 millimeter chip. Typically for a microwave chip that size,
there are only eight to 40 transistors. The rest of the area is wasted just, "he says."
"We take our design and put it on CNF using deterministic assembly technique, then we can put it wherever we want
and make a completely functional circuit with performance comparable to existing chips.""While the biodegradability of these materials will have a positive impact on the environment,
Ma says the flexibility of the technology can lead to widespread adoption of these electronic chips."
"Mass-producing current semiconductor chips is so cheap, and it may take time for the industry to adapt to our design,
"he says.""But flexible electronics are the future, and we think we're going to be well ahead of the curve
#Nanotechnology helps protect patients from bone infection Leading scientists at the University of Sheffield have discovered nanotechnology could hold the key to preventing deep bone infections,
after developing a treatment which prevents bacteria and other harmful microorganisms growing. The pioneering research,
led by the University of Sheffield School of Clinical Dentistry, showed applying small quantities of antibiotic to the surface of medical devices,
from small dental implants to hip replacements, could protect patients from serious infection. Scientists used revolutionary nanotechnology to work on small polymer layers inside implants
which measure between 1 and 100 nanometers (nm) a human hair is approximately 100, 000 nm wide.
Lead researcher Paul Hatton, Professor of Biomaterials Sciences at the University of Sheffield, said: icroorganisms can attach themselves to implants
or replacements during surgery and once they grab onto a nonliving surface they are notoriously difficult to treat
which causes a lot of problems and discomfort for the patient. y making the actual surface of the hip replacement or dental implant inhospitable to these harmful microorganisms,
the risk of deep bone infection is reduced substantially. ur research shows that applying small quantities of antibiotic to a surface between the polymer layers
which make up each device could prevent not only the initial infection but secondary infection it is like getting between the layers of an onion skin. one infection affects thousands of patients every year and results in a substantial cost to the NHS.
Treating the surface of medical devices would have a greater impact on patients considered at high risk of infection such as trauma victims from road traffic collisions or combat operations,
and those who have had previous bone infections. Professor Hatton added: eep bone infections associated with medical devices are increasing in number,
especially among the elderly. s well as improving the quality of life, this new application for nanotechnology could save health providers such as the NHS millions of pounds every year. ource:
http://www. sheffield. ac. uk/news/..e
#World smallest spirals could guard against identity theft Take gold spirals about the size of a dime...
and shrink them down about six million times. The result is the world's smallest continuous spirals:'
'nano-spirals'with unique optical properties that would be almost impossible to counterfeit if they were added to identity cards, currency and other important objects.
Students and faculty at Vanderbilt University fabricated these tiny Archimedesspirals and then used ultrafast lasers at Vanderbilt and the Pacific Northwest National Laboratory in Richland, Washington,
to characterize their optical properties. The results are reported in a paper published online by the Journal of Nanophotonics on May 21. hey are certainly smaller than any of the spirals wee found reported in the scientific literature,
said Roderick Davidson II, the Vanderbilt doctoral student who figured out how to study their optical behavior.
The spirals were designed and made at Vanderbilt by another doctoral student, Jed Ziegler, now at the Naval Research Laboratory.
Most other investigators who have studied the remarkable properties of microscopic spirals have done so by arranging discrete nanoparticles in a spiral pattern:
similar to spirals drawn with a series of dots of ink on a piece of paper. By contrast, the new nano-spirals have solid arms
and are much smaller: A square array with 100 nano-spirals on a side is less than a hundredth of a millimeter wide.
When these spirals are shrunk to sizes smaller than the wavelength of visible light, they develop unusual optical properties.
For example, when they are illuminated with infrared laser light, they emit visible blue light. A number of crystals produce this effect, called frequency doubling or harmonic generation, to various degrees.
The strongest frequency doubler previously known is the synthetic crystal beta barium borate, but the nano-spirals produce four times more blue light per unit volume.
When infrared laser light strikes the tiny spirals it is absorbed by electrons in the gold arms.
The arms are so thin that the electrons are forced to move along the spiral. Electrons that are driven toward the center absorb enough energy
so that some of them emit blue light at double the frequency of the incoming infrared light. his is similar to
what happens with a violin string when it is bowed vigorously, said Stevenson Professor of Physics Richard Haglund,
who directed the research. f you bow a violin string very lightly it produces a single tone.
The electrons at the center of the spirals are driven pretty vigorously by the laser electric field.
Because of the tiny quantities of metal actually used, they can be made inexpensively out of precious metals,
paper and a number of other substrates. f nano-spirals were embedded in a credit card or identification card,
Scanning electron microscope image of an individual nano-spiral. Haglund Lab/Vanderbilt) Source: http://news. vanderbilt. edu/..t
#Engineers show how'perfect'materials begin to fail at the nanoscale Crystalline materials have atoms that are lined neatly up in a repeating pattern.
Now that nanotechnological advances have made such materials a reality, however, researchers at the University of Pennsylvania and Germany Max Planck Institute for Intelligent Systems have shown how these defects first form on the road to failure.
In a new study, published in Nature Materials("Measuring surface dislocation nucleation in defect-scarce nanostructures),
"they stretched defect-free palladium nanowires, each a thousand times thinner than a human hair, under tightly controlled conditions.
Contrary to conventional wisdom, they found that the stretching force at which these wires failed was unpredictable
This thermal uncertainty in the failure limit suggests that the point where a failure-inducing defect first appears is on the nanowire surface,
which cuts across the nanowire, causing it to break. The study was led by graduate student Lisa Chen and associate professor Daniel Gianola of the Department of Materials science and engineering in Penn School of engineering and Applied science.
Other members of Gianola lab postdoctoral researcher Mo-Rigen He and graduate student Jungho Shin, contributed to the study.
They collaborated with Gunther Richter of the Max Planck Institute for Intelligent Systems. anotechnology is not just about making things smaller,
t also about different properties that arise in materials at the nanoscale. hen you make these really small structures,
and can control the properties of the nanoscale material. The researchers grew palladium nanowires through a vapor deposition method at high temperature,
which provided each atom with the time and energy to move around until it found its preferred spot in the metal crystalline structure.
Sprouting from a substrate like blades of grass, the team used a microscopic robotic manipulator to painstakingly pluck the wires
and attach them to their testing platform inside an electron microscope. This platform, developed in conjunction with Sandia National Laboratory
functions like an industrial mechanical testing machine at the nanoscale. Welding a nanowire to a grip attached to a series of slanted bars that expand
when heated by an electric current, the researchers could then stretch the nanowire in a controlled way.
By repeatedly ramping up the voltage to a different maximum and bringing it down at the same rate,
the researchers could pinpoint when the first irreversible deformation in the wire occurred. ust pulling it until it fails doesn tell you exactly where
Gianola said. ur goal was to deduce the point where the first of the nanowire atoms begin to shift out of their original positions
Absent defect-free nanowires to run physical experiments upon, earlier theories and analyses suggested that the relationship between temperature
knowing the temperature would allow one to estimate a nanowire failure limit. By conducting their stretching experiments at various temperatures,
but you have to take a different approach to specify the strength of the nanowire.
the amount of energy necessary to jumpstart the nucleation of the first defect, was relatively low.
Surface diffusion is atoms hopping around, site to site, somewhat chaotically, almost like a fluid.
and has to break most of those bonds to move around. But one on the surface might have only three or four to break.
Understanding the origin of the distribution of strengths in nanostructures will allow for more rational design of devices. ntil recently,
t been very difficult to make defect-free nanowires. But now that we can, there a reason to care about how they fail.
the highest strengths ever measured in that crystal structure of metal so theye going to be attractive to use in all sorts of devices
#Unlocking nanofiberspotential Prototype boosts production of versatile fibers fourfold, while cutting energy consumption by 92 percent.
Nanofibers-polymer filaments only a couple of hundred nanometers in diameter have a huge range of potential applications, from solar cells to water filtration to fuel cells.
But so far, their high cost of manufacture has relegated them to just a few niche industries.
In the latest issue of the journal Nanotechnology, MIT researchers describe a new technique for producing nanofibers that increases the rate of production fourfold
while reducing energy consumption by more than 90 percent, holding out the prospect of cheap, efficient nanofiber production. e have demonstrated a systematic way to produce nanofibers through electrospinning that surpasses the state of the art,
says Luis Fernando Velásquez-García, a principal research scientist in MIT Microsystems Technology Laboratories, who led the new work. ut the way that it done opens a very interesting possibility.
Our group and many other groups are working to push 3-D printing further, to make it possible to print components that transduce,
that actuate, that exchange energy between different domains, like solar to electrical or mechanical. We have something that naturally fits into that picture.
We have an array of emitters that can be thought of as a dot matrix-printer printer where you would be able to individually control each emitter to print deposits of nanofibers. angled talenanofibers are useful for any application that benefits from a high ratio of surface area to volume solar cells, for instance,
which try to maximize exposure to sunlight, or fuel cell electrodes, which catalyze reactions at their surfaces.
Nanofibers can also yield materials that are permeable only at very small scales, like water filters,
or that are remarkably tough for their weight, like body armor. The standard technique for manufacturing nanofibers is called electrospinning,
and it comes in two varieties. In the first a polymer solution is pumped through a small nozzle,
and then a strong electric field stretches it out. The process is slow, however, and the number of nozzles per unit area is limited by the size of the pump hydraulics. The other approach is to apply a voltage between a rotating drum covered by metal cones and a collector electrode.
The cones are dipped in a polymer solution, and the electric field causes the solution to travel to the top of the cones,
where it emitted toward the electrode as a fiber. That approach is erratic however, and produces fibers of uneven lengths;
it also requires voltages as high as 100,000 volts. Thinking smallvelásquez-García and his co-authors Philip Ponce de Leon, a former master student in mechanical engineering;
Frances Hill, a former postdoc in Velásquez-García group who now at KLA-Tencor; and Eric Heubel, a current postdoc adapt the second approach,
but on a much smaller scale, using techniques common in the manufacture of microelectromechanical systems to produce dense arrays of tiny emitters.
The emitterssmall size reduces the voltage necessary to drive them and allows more of them to be packed together, increasing production rate.
At the same time, a nubbly texture etched into the emitterssides regulates the rate at which fluid flows toward their tips,
yielding uniform fibers even at high manufacturing rates. e did all kinds of experiments, and all of them show that the emission is uniform,
Velásquez-García says. To build their emitters, Velásquez-García and his colleagues use a technique called deep reactive-ion etching.
and a dissolved polymer. When an electrode is mounted opposite the sawteeth and a voltage applied between them,
the water-ethanol mixture streams upward, dragging chains of polymer with it. The water and ethanol quickly dissolve, leaving a tangle of polymer filaments opposite each emitter, on the electrode.
The researchers were able to pack 225 emitters, several millimeters long, on a square chip about 35 millimeters on a side.
At the relatively low voltage of 8, 000 volts, that device yielded four times as much fiber per unit area as the best commercial electrospinning devices.
The work is n elegant and creative way of demonstrating the strong capability of traditional MEMS microelectromechanical systems fabrication processes toward parallel nanomanufacturing
says Reza Ghodssi, a professor of electrical engineering at the University of Maryland. Relative to other approaches, he adds,
there is n increased potential to scale it up while maintaining the integrity and accuracy by
which the processing method is applied. mage: A scanning electron micrograph of the new microfiber emitters, showing the arrays of rectangular columns etched into their sides.
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