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An illustration of a device made from black phosphorus.

New Technique Reveals Black Phosphorus's Properties and How to Control Them

Ever since early 2014, when researchers were able to exfoliate black phosphorus into 10- to 20-atom-thick layers, it has been offering a new hope in the universe of silicon replacements. Not only does it have an inherent bandgap unlike graphene, but that bandgap is also highly tunable, depending on the number of layers used.

However, the property that really sets black phosphorus apart from graphene and nearly all two-dimensional materials is its intrinsically strong, in-plane anisotropy. That means its properties are directionally dependent. This in-plane anisotropy is sometimes considered both a blessing and a curse, and being able to control it could go a long way to ensuring it remains a benefit.

Now Philip Feng, whose research into black phosphorus we’ve previously written about in IEEE Spectrum, and his colleagues at Case Western University and the University of Science and Technology in Hefei, China, have shown that the anisotropy can be turned “on” and “off” so that black-phosphorus devices can enjoy the effect only when needed. To do this, Feng and his team had to come up with a new approach to investigating in-plane anisotropy.

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Growth of graphene on copper foils assisted by a continuous oxygen supply

Single-Crystal Graphene Films Grown More Than 100 Times as Fast as Previously Possible

The adaptation of chemical vapor deposition (CVD) production of graphene so that it’s compatible with roll-to-roll processing is transforming graphene manufacturing. That effort is being led by companies like Graphene Frontiers, based in Philadelphia.

However, the production of single-crystal graphene on copper foils in a CVD process remains a fairly time consuming procedure. Fabrication of centimeter-size single crystals of graphene still takes as much as a day.

Now researchers at Hong Kong Polytechnic University and Peking University have developed a technique that accelerates the process so that the growth happens at 60 micrometers per second—far faster than the typical 0.4 µm per second. The key to this 150-fold speed increase was adding a little oxygen directly to the copper foils.

In the research, which is described in the journal Nature Nanotechnology, the China-based researchers placed an oxide substrate 15 micrometers below the copper foil. The result: a continuous supply of oxygen that lowers the energy barrier to the decomposition of the carbon feedstock, thereby increasing the graphene growth rate.

The expectations were that the oxide substrate would release the oxygen at the high temperatures inside the CVD surface (over 800 degrees Celsius). The researchers confirmed this through the use of electron spectroscopy. While the measurements indicated that oxygen was indeed being released, the amount was still fairly minimal. Nevertheless, this minuscule amount of oxygen proved sufficient for their purposes because the very small space between the oxide substrate and the copper foil created a trapping effect that multiplied the effect of the oxygen.

In their experiments, the researchers were able to successfully produce single-crystal graphene materials as large as 0.3 millimeter in just five seconds. That, according to the researchers, is more than two orders of magnitude faster than other methods in which graphene is grown on copper foils.

The researchers believe that this ultrafast synthesis of graphene makes possible a new era of scalable production of high-quality, single-crystal graphene films by combining this process with roll-to-roll methods.

Counterintuitively, speeding up the process of producing single-crystal graphene films may not automatically lead to wider adoption of graphene in various devices. Just a few years ago, graphene production was stuck at around a 25-percent utilization rate, and there is no reason to believe that demand has increased enough to have dramatically changed those figures. (Graphene producers will tell you that if demand for CVD-produced graphene suddenly spiked, volume could be doubled nearly overnight.)

Nonetheless, speed in manufacturing is always an attractive option for any product. It just might not offer a change to the graphene landscape as much as a few “killer apps” might.

Diagram showing the one-dimensional atom chains: the oxygen molecules (red) separate the metal atoms – here cobalt (yellow) and iron (blue) – from the iridium substrate (grey). The arrows show the different magnetisation of the different metals.

One-Dimensional Magnetic Atom Chain Forged

Transition metals are 38 elements in the periodic table that are all conductive. What makes transition metals different from the other elements is that they have valence electrons (electrons that combine with other elements) in both their inner shell and their outer shell. This means that when they are combined with oxygen, an oxidation state arises that can give these metals unique properties.

Now researchers at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) in Germany have taken this oxidation process for four transition metals (manganese, iron, cobalt and nickel) to a new level. For the first time, they have successfully created a one-dimensional magnetic atom chain of these metals by using oxygen. The researchers claim that this feat will prove to have a fundamental impact on magnetic data storage as well as chemistry in general.

In experiments described in the journal Physical Review Letters, the researchers grew these transition metals on an iridium surface. In a departure from the usual results of separation from a substrate, the researchers used the oxygen to separate the metals from the substrate while maintaining their magnetic properties. The entire process occurs through a process of self-assembly in which the atoms arrange themselves into these one-dimensional chains.

“Evaporating metals onto a metallic surface in a vacuum is a common procedure,” explained Alexander Schneider, a professor at FAU who led the research, in a press release. “However, this often produces a two-dimensional layer of metal. For the first time, with the help of oxygen, we have managed to produce atom chains that cover the entire iridium surface, are arranged with a regular distance of 0.8 nanometres between each atom and can be up to 500 atoms long, without a single structural fault.”

The researchers observed that the oxygen serves as a kind of lifting mechanism to separate the transition metals from the substrate. This lifting mechanism involves the bonding geometry that dictates the position of the atoms in that system.

“Essentially the oxygen clings to the magnetic atom (binds strongly and sucks away electrons) which in turn looses interest (electrons) in binding to the iridium,” Schneider told IEEE Spectrum in an e-mail interview.  “As a consequence the position of the magnetic atom is determined by the bond angles to the oxygen which are least bent if the magnetic atom is lifted from the iridium.”

In their one-dimensional form, these transitional metals take on slightly different properties: nickel takes on a non-magnetic state, cobalt maintains its ferromagnetic properties, while iron and manganese become antiferromagnetic. It is the alternating state of magnetism that each link possesses that is a unusual feature of the chain.

“This means that we can create mixed systems in which ferromagnetic sections of chains can be separated from antiferromagnetic or non-magnetic sections, for example,” said Schneider.

The restriction of the dimensionality of a material produces much of its unique properties. For instance, a two-dimensional material like graphene,  has very different properties from its carbon cousin, the one-dimensional carbyne. Now that it’s possible to remove a one-dimensional atom chain from a thin film substrate that is in two dimensions, it’s possible to foresee more research into the potential of one-dimensional systems.

For instance, the researchers expect that this research will lead to further experiments in which a number of different pieces of chains with different lengths and magnetic properties can be tested to see how they can lead to further miniaturization in data storage.

Schneider added: “We will look for other similar exciting configurations in other nanostructured oxides and also investigate molecular systems for which we may use the found system as a substrate.”

Oxygen frees silicene from its metal substrate

Breakthrough in Silicene Production

In the expanding universe of two-dimensional materials, perhaps none have been more tempting than silicon and its 2D version, known as silicene. The attraction is obvious: Silicon is the boat on which the computer age has floated for more than 50 years. And its proximity on the periodic table to carbon, whose 2D version is the wonder material graphene, has led researchers to investigate whether it might have the same attractive properties.

If researchers can overcome more of the obstacles standing in the way of silicene production, it has some pretty attractive qualities. First, unlike graphene, it has an intrinsic bandgap, which makes it attractive for digital electronics to stop and start the flow of electrons. Its semiconductor nature also comes with many of the same properties that make graphene so attractive—including, potentially, superconductivity.

While silicene is indeed tempting, it has proven frustratingly difficult to produce. Now researchers at the University of Wollongong, in Australia, have overcome one of the main obstacles: separating it from its substrate.

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A hexagon-shaped silicon-based virus sorter

IBM Making Silicon to Sort Viruses and Other Nanoscale Biological Targets

It’s long been understood that early disease detection is the key to successful treatments. But annual checkups with a doctor might not be frequent enough to help. So imagine if you could forego a trip to the doctor’s office and detect any disease with a simple urine or saliva test at home.

Of course this has been the aim of lab-on-a-chip technologies for years now, but now scientists at IBM Research may have tipped the scales in the technology that could make such at-home tests real.

In cross-disciplinary research described in the journal Nature Nanotechnology, a team at IBM led by research scientist Joshua Smith and Gustavo Stolovitzky, program director of IBM Translational Systems Biology and Nanobiotechnology, has been able to retool silicon-based technologies to create a diagnostic device that can separate viruses, DNA, and other nanoscale-size biological targets from saliva or urine. This could enable the device to detect the presence of diseases before any physical symptoms are visible.

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Turn printer paper into a flexible electronic display by slapping on a coat of graphene

Graphene-Enabled Paper Makes for Flexible Display

Graphene has been building quite a reputation for itself in flexible displays. Among the ways graphene has been used in this field is as an alternative to the relatively scarce indium tin oxide (ITO), a transparent conductor that controls display pixels. Graphene has also been used in a display’s pixel electronics, or backplane, where a solution-processed graphene is used as an electrode.

Now researchers at Bilkent University in Ankara, Turkey, have demonstrated that an ordinary sheet of paper that is sandwiched between two films of multilayer graphene can act as a rudimentary flexible electronic display.

In an interview with Nature Photonics, the corresponding author, Coskun Kocabas, says that this system could serve as a framework for turning ordinary printing paper into an optoelectronic display.

Kocabas explained:

We would like to fabricate a display device that can reconfigure the displayed information electronically on a sheet of printing paper. Several technologies based on electrophoretic motion of particles, thermochromic dyes and electrowetting of liquids have been developed to realize electronic paper, or e-paper, which has great potential for consumer electronics. Contrasting with the primary aim of e-paper, these technologies, however, are not compatible with conventional cellulose-based printing papers.

The researchers described their device in the journal ACS Photonics. It operates by applying a bias voltage to the graphene to trigger an intercalation of ions so that the optical absorption of the graphene layers is altered. That turns them from transparent to dark and back again. (Intercalation is the reversible inclusion of a molecule or ions between two other molecules in multilayered structures or compounds.)

In the experiments, the display’s transition to transparent takes a bit of time— about 4 seconds; reverting to its darker form takes under half a second. While this may be suitable for signs that don’t need to change their images that often, the lapse is still too long for display applications that require quick refresh times.

The multilayer graphene was produced through chemical vapor deposition in which the graphene is grown on a metal surface inside a furnace. After it’s removed from the furnace, the metal is etched away, leaving a thin film of graphene on the surface of the water in which the etching occurs. Then the paper is simply submersed into the liquid, which transfers the thin film of graphene onto the paper.

While the initial experiments showed that there were some issues with oxidation of the doped graphene layers, the researchers believe that this hiccup can be overcome with the addition of a simple polymer coating.

In future research, Kocabas and his colleagues are planning to make a fully functional sheet of e-paper with pixels and an integrated driving circuit. They would like to see the process they have developed adapted into a roll-to-roll-compatible manufacturing process.

Examples of handheld devices that could benefit from a new oxide compound for power electronics.

New Material Offers a Revolutionary Approach to Power Electronics

Researchers at the University of Utah and the University of Minnesota have discovered that when two oxide compounds—strontium titanate (STO) and neodymium titanate (NTO)—are joined together, they make an extraordinary conductive material that could vastly improve power transistors.  The researchers have shown that these two materials—which on their own operate as insulators—are up to five times more conductive than silicon.

In research described in the journal APL Materials, scientists found that the bonds between the atoms from the oxide compounds arrange themselves in a way that generates 100 times more free electrons than conventional semiconductors, which means the new material can transport more electrical current.

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Organic nanowires bound for supramolecular electronics

Promise of Nanowires in Optoelectronics Realized By Getting Them Connected

Supramolecular electronics has been solidifying as the bridge between molecular electronics—in which molecules become the basic building blocks of electronics—and the use polymers for the fabrication of nanoscale circuitry. A supramolecule is actually a number of different molecules that are fused together to act as a single molecule and carry out a particular programmed function. These supramolecules are used, for instance, in block copolymer-based supramolecular solutions that direct the self-assembly of nanoparticles.

Now researchers at the University of Strasbourg and the Le Centre National de la Recherche Scientifique (CRNS) in France, along with collaborators from the University of Nova Gorica in Slovenia, have buoyed the prospects of supramolecular electronics by addressing one of the chief problems of supramolecular organic nanowires for optoelectronics: getting them connected.

In research described in the journal Nature Nanotechnology, the international team of researchers fabricated a photovoltaic device in which they managed to connect and integrate hundreds of organic nanowires.

This is a significant achievement, because supramolecular organic nanowires have long tantalized researchers in the field of optoelectronics. Their highly efficient generation of excitons—essentially energized electrons that are formed when light hits a semiconductor—make devices like solar cells very sensitive to light and boost their light absorption coefficient.

While this is indeed tempting, the rub has been that you couldn’t harvest that photocurrent from the supramolecular nanowires unless they were connected to nanoelectrodes (anodes and cathodes) that carry out different functions. And the best anyone had previously been able to do was connect just a few. Thus, these nanowires were limited to use in very rudimentary devices.

To the rescue is this new approach: a nanomesh scaffold that supports and connects the supramolecular nanowires between two nanoelectrodes with different work functions.

The researchers were able to fabricate this nanomesh scaffold using a technique known as nanosphere lithography, in which nanoscale spheres are used as a mask to fabricate nanoparticle arrays. The result is a nanomesh comprising millions of hole-shaped nanoelectrodes patterned into a hexagonal array with channel lengths less than 100 nanometers.

By using a commercially available n-type organic semiconductor that self-assembles into supramolecular nanowires in combination with the nanomesh scaffold so that the nanowires become connected to the nanoelectrodes, the researchers have been able to fabricate a photovoltaic device with very promising characteristics.

One of the attractive properties of the device is that the polymer/nanowire p-n junction provides a fast photoresponse because the anode and cathode are quite close together. Another feature of this device is that it is possible to chemically modify the anode and cathode separately. This enables tailoring of interfaces that, in turn, makes it possible to replace calcium and aluminum cathodes. It also makes the use of transparent electrodes such as indium tin oxide unnecessary.

In future research, the team intends to optimize the device in a number of different areas, such as using polymer thin films to impart flexibility . The researchers are also considering the use of thinner dielectric layers to increase the photocurrent.

Porous silicon nanoparticles offer harmless theraphy and diagnostic solution for many types of cancer

Silicon Nanoparticles Provide Biocompatible Solution to Cancer Detection and Treatment

When it comes to cancer treatment and nanoparticles, the preference has been to use gold nanoparticlesSilicon nanoparticles, on the other hand, have been limited mainly to the realm of electronics applications.

Now researchers at Lomonosov Moscow State University and the Leibniz Institute of Photonic Technology in Germany have demonstrated that silicon nanoparticles can be applied to the diagnosis and treatment of cancer. The researchers claim that this work represents the first time nanoparticles have penetrated into diseased cells and completely dissolved after delivering their cancer treatment drug payload.

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Professor Shubhra Gangopadhyay of the University of Missouri holding plasmonic gratings.

Plasmonics Enable Optical Microscopes to Perform Like Electron Microscopes

Optical microscopes are  a key tool in biological studies. But because they are limited by approximately half the wavelength of light used (200 to 400 nanometers), they can’t resolve molecules that are typically much smaller than these dimensions. While electron microscopes can reach resolutions far below an optical microscope, they are large, expensive pieces of equipment that require a vacuum to operate, limiting the ability to examine live samples.

Now researchers at the University of Missouri have developed a way to make an optical microscope resolve images down to 65 nanometers. In the process, they may have extended access to high resolution imaging to a much larger group of scientists who may not have access to electron microscopes.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
Editor
Dexter Johnson
Madrid, Spain
 
Contributor
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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