Posted: Mar 14, 2018

Nanospears deliver genetic material to cells with pinpoint accuracy (Nanowerk News) In a step toward accelerating the production of new gene therapies, scientists report in ACS Nano („Precision-Guided Nanospears for Targeted and High-Throughput Intracellular Gene Delivery”) that they have developed remote-controlled, needle-like nanospears capable of piercing membrane walls and delivering DNA into selected cells. They say the new technique, which can ferry biological materials to cells with pinpoint accuracy, overcomes many of the existing barriers to effective gene modification.

Medical interventions based on the use of genetically modified cells are an emerging area of stem cell and cancer immunology research. Existing approaches to delivering DNA into cells for producing these gene therapies include viruses, external electrical fields or harsh chemical reagents. But these methods are often costly, inefficient and harmful to the cells.

Researchers have experimented with sharp-tipped nanoparticles stuck on surfaces to deliver biomolecules to cells, but it is difficult to remove the modified cells from the nanoparticle-coated surface for further study. Self-propelled nanoparticles also can deliver molecules to cells in the body. However, these devices are difficult to precisely control and can generate toxic byproducts.

To overcome these issues, Steven J. Jonas, Paul S. Weiss, Xiaobin Xu and colleagues sought to create biocompatible nanospears that can be configured to transport DNA into cells precisely using an external magnetic field without either damaging the cells or having to use chemical propellants.

The researchers fabricated nanospears using polystyrene beads as a template. They placed the beads onto silicon and etched them down into a tiny, sharp spear shape. The beads were removed, and the resulting silicon spears were coated with thin layers of nickel and gold. The gold was functionalized so that biomolecules, such as DNA, could attach.

Then, the researchers removed the nanospears from the silicon by mechanical scraping. Because the nickel layer is magnetic, the particles’ movement and orientation could be precisely controlled with a magnet. This capability allowed the researchers to maneuver the nanospears in a lab dish to modify brain cancer cells so that they expressed a green fluorescent protein. After making contact and penetrating the cells, the nanospears released their DNA cargo.

After the experiment, more than 90 percent of the cells remained viable and more than 80 percent exhibited green fluorescence, showing that the method is less harmful and more effective than other non-viral approaches. The researchers conclude this technique could eventually lead to new ways to prepare vast numbers of cells for the coordinated manufacture of gene therapies.

Source: American Chemical Society

Posted: Mar 14, 2018

Graphene flakes for future transistors (Nanowerk News) Tiny and very promising for possible applications in the field of nanoelectronics: they are the graphene nanoflakes studied by a SISSA’s team and protagonists of a study recently published in the Nano Letters journal („Quantum Interference Assisted Spin Filtering in Graphene Nanoflakes”).

These hexagonal shaped nanostructures would allow to exploit quantum effects to modulate the current flow. Thanks to their intrinsic magnetic properties, they could also represent a significant step forward in the field of spintronics, which is based on the electron spin. The study, conducted by a theoretical analysis and simulations at the computer, was led by Massimo Capone.

On the left: This is a spin filter made out of a magnetic nanoflake: a current of electrons with spin ‘up’ and ‘down’ in equal proportions flows through the device. Due to destructive interference in a spin channel (for example: down) the outgoing current is prevalently made of spin-up electrons. On the right: Schematic illustration of the device and plot of the spin-filtering efficiency. (Image: Angelo Valli)

„We have been able to observe two key phenomena by analysing the properties of graphene nanoflakes. Both are of great interest for possible future applications” explain Angelo Valli and Massimo Capone, authors of the study together with Adriano Amaricci and Valentina Brosco.

The first phenomenon deals with the so-called interference between electrons and is a quantum phenomenon: In nanoflakes, the electrons interfere with each other in a „destructive” manner if we measure the current in a certain configuration. This means that there is no transmission of current. This is a typically quantum phenomenon, which only occurs at very reduced sizes. By studying the graphene flakes we have understood that it is possible to bring this phenomenon to larger systems, therefore into the nano world and on a scale in which it is observable and can be exploited for possible uses in nanoelectronics.

The two researchers explain that in what are called „Quantum interference transistors” destructive interference would be the „OFF” status. For the „ON” status, they say it is sufficient to remove the conditions for interference, thereby enabling the current to flow.

Magnetism and spintronics

But there’s more. In the study, the researchers demonstrated that the nanoflakes present new magnetic properties which are absent, for example, in an entire sheet of graphene: The magnetism emerges spontaneously at their edges, without any external intervention. This enables the creation of a spin current.

The union between the phenomena of quantum interference and of magnetism would allow to obtain almost complete spin polarization, with a huge potential in the field of spintronics, explain the researchers. These properties could be used, for example, in the memorising and processing information technologies, interpreting the spin as binary code. The electron spin, being quantised and having only two possible configurations (which we could call „up” and „down”), is very well suited for this kind of implementation.

Next step: the experimental test

To improve the efficiency of the possible device and the percentage of current polarization the researchers have also developed a protocol that envisages the interaction of the graphene flakes with a surface made of nitrogen and boron.

„The results obtained are really interesting. This evidence now awaits the experimental test, to confirm what we have theoretically predicted,” concludes Massimo Capone, head of the research and recently awarded the title of Outstanding Referee by the American Physical Society journal; in this way, each year, the journal indicates the male and female scientists who have distinguished themselves for their expertise in collaborating with the journal.

Source: Scuola Internazionale Superiore di Studi Avanzati (SISSA)

Posted: Mar 14, 2018

Chemists use abundant, low-cost and non-toxic elements to synthesize semiconductors (Nanowerk News) One of the problems for Javier Vela and the chemists in his Iowa State University research group was that a toxic material worked so well in solar cells.

And so any substitute for the lead-containing perovskites used in some solar cells would have to really perform. But what could they find to replace the perovskite semiconductors that have been so promising and so efficient at converting sunlight into electricity?

What materials could produce semiconductors that worked just as well, but were safe and abundant and inexpensive to manufacture?

“Semiconductors are everywhere, right?” Vela said. “They’re in our computers and our cell phones. They’re usually in high-end, high-value products. While semiconductors may not contain rare materials, many are toxic or very expensive.”

Vela, an Iowa State associate professor of chemistry and an associate of the U.S. Department of Energy’s Ames Laboratory, directs a lab that specializes in developing new, nanostructured materials. While thinking about the problem of lead in solar cells, he found a conference presentation by Massachusetts Institute of Technology researchers that suggested possible substitutes for perovskites in semiconductors.

Javier Vela and the chemists in his research group have been working to produce semiconductors from materials that are safe, abundant and inexpensive to manufacture. (Image: Iowa State University)

Vela and Iowa State graduate students Bryan Rosales and Miles White decided to focus on sodium-based alternatives and started an 18-month search for a new kind of semiconductor. The work was supported by Vela’s five-year, $786,017 CAREER grant from the National Science Foundation. CAREER grants are the foundation’s most prestigious awards for early career faculty.

They came up with a compound that features sodium, which is cheap and abundant; bismuth, which is relatively scarce but is overproduced during the mining of other metals and is cheap; and sulfur, the fifth most common element on Earth. The researchers report their discovery in a paper recently published online by the Journal of the American Chemical Society („Solution-Grown Sodium Bismuth Dichalcogenides: Toward Earth-Abundant, Biocompatible Semiconductors”).

“Our synthesis unlocks a new class of low-cost and environmentally friendly ternary (three-part) semiconductors that show properties of interest for applications in energy conversion,” the chemists wrote in their paper.

In fact, Rosales is working to create solar cells that use the new semiconducting material.

Vela said variations in synthesis – changing reaction temperature and time, choice of metal ion precursors, adding certain ligands – allows the chemists to control the material’s structure and the size of its nanocrystals. And that allows researchers to change and fine tune the material’s properties.

Several of the material’s properties are already ideal for solar cells: The material’s band gap – the amount of energy required for a light particle to knock an electron loose – is ideal for solar cells. The material, unlike other materials used in solar cells, is also stable when exposed to air and water.

So, the chemists think they have a material that will work well in solar cells, but without the toxicity, scarcity or costs.

“We believe the experimental and computational results reported here,” they wrote in their paper, “will help advance the fundamental study and exploration of these and similar materials for energy conversion devices.”

Source: Iowa State University

Posted: Mar 14, 2018

Turbocharging fuel cells with a multifunctional nanotechnology catalyst (Nanowerk News) Powering clean, efficient cars is just one way fuel cell technology could accelerate humanity into a sustainable energy future, but unfortunately, the technology has been a bit sluggish. Now, engineers may be able to essentially turbocharge fuel cells with a new catalyst.

The sluggishness comes from a chemical bottleneck, the rate of processing oxygen, a key ingredient that helps fuel cells, which are related to batteries, produce electricity. The new catalyst, a nanotechnology material developed by engineers at the Georgia Institute of Technology, markedly speeds up oxygen processing and is the subject of a new study (Joule, „A Highly Efficient Multi-phase Catalyst Dramatically Enhances the Rate of Oxygen Reduction”).

Partly to accommodate oxygen’s limitations, fuel cells usually require pure hydrogen fuel, which reacts with the oxygen taken in from the air, but the costs of producing the hydrogen have been prohibitive. The new catalyst is a potential game-changer.

The new Georgia Tech fuel cell catalyst, a coating only about two dozen nanometers thick, works in two phases. First, the nanoparticles on top grab molecular oxygen from the air and make it very easy and tear apart into single oxygen ions. Then oxygen vacancies in the nanoparticle rapidly pass the oxygen ions to the second phase, a layer full of oxygen vacancies which shuttle the ions to their meeting with ionic hydrogen to complete the chemical process that powers fuel cells. (Image: Georgia Tech / Liu / Chen)

“It can easily convert chemical fuel into electricity with high efficiency,” said Meilin Liu, who led the study and is a Regents’ Professor in Georgia Tech’s School of Material Science and Engineering. “It can let you use readily available fuels like methane or natural gas or just use hydrogen fuel much more efficiently,” Liu said.

Catalyst 8 times as fast

The catalyst achieves the efficiency by rushing oxygen through a fuel cell’s system. “It’s more than eight times as fast as state-of-the-art materials doing the same thing now,” said Yu Chen, a postdoctoral research associate in Liu’s lab and the study’s first author.

There are a few types of fuel cells, but the researchers worked to improve solid oxide fuel cells, which are found in some prototypical fuel cell cars. The research insights could also aid in honing supercapacitors and technology paired with solar panels, thus advancing sustainable energy beyond the new catalyst’s immediate potential to improve upon fuel cells.

Naturally sluggish oxygen

Though they work differently from fuel cells and are much less efficient and clean, combustion engines make a useful metaphor to aid in understanding how fuel cells and the new catalyst work.

In a combustion engine, fuel from a tank and oxygen from the air come together to react in an explosion, producing energy that turns a crankshaft. Adding a turbocharger speeds the process up by mixing fuel and oxygen together more quickly and rushing them to combustion.

Currently, in fuel cells, hydrogen fuel from a tank and oxygen from the air also drive a process that produces energy, in this case, electricity. The two ingredients do come together in a reaction, but one very different from combustion, and much cleaner.

One end of the fuel cell, the anode, removes electrons from the hydrogen atoms in what’s called oxidation and sends the electrons through an external circuit as electric current to the cathode on the other side. There, oxygen, which is notoriously electron hungry, sucks the electrons up in what’s called reduction, and that keeps the electricity flowing.

The hydrogen, now positively charged, and the oxygen, now negatively charged, meet up to form water, which is the fuel cell’s exhaust.

In that reaction chain, oxygen is the slow link in two ways: Oxygen’s reduction takes longer than hydrogen’s oxidation, and the reduced oxygen moves more slowly through the system to meet with hydrogen. Analogous to the turbocharger, the new catalyst pushes the oxygen forward.

A simple diagram depicts the basic functioning of a solid oxide fuel cell. (Image: The National Museum of American History)

Oxygen rush nanotechnology

The catalyst is applied as a sheer coating only about two dozen nanometers thick and is comprised of two connected nanotechnology solutions that break both oxygen bottlenecks.

First, nanoparticles highly attractive to oxygen grab the O2 molecule and let inflowing electrons quickly jump onto it, easily reducing it and tearing it into two separate oxygen ions (each one an O2-). Then a series of chemical gaps called oxygen vacancies that are built into the nanoparticles’ structures suck up the oxygen ions like chains of vacuum cleaners passing the ions hand to hand to the second phase of the catalyst.

The second phase is a coating that is full of oxygen vacancies that can pass the O2- even more rapidly toward its final destination.

“The oxygen goes down quickly through the channels and enters the fuel cell, where it meets with the ionized hydrogen or another electron donor like methane or natural gas.”

The ions meet to make water, which exits the fuel cell. In the case of methane fuel, pure CO2 is also emitted, which can be captured and recycled back into fuel.

Interesting rare metals

In the first stage, there are two different flavors of nanoparticle at work. Both have cobalt, but one contains barium and the other praseodymium, a rare-earth metal that can be pricey in high quantities.

“Praseodymium is in such very small amounts that it doesn’t impact costs,” Liu said. “And the catalyst saves lots of money on fuel and on other things.”

High operating temperatures in existing fuel cells require expensive protective casings and cooling materials. The researchers believe the catalyst could help lower the temperatures by reducing electrical resistance inherent in current fuel cell chemistry. That could, in turn, reduce overall material costs.

Protective cathode coating

The second stage of the catalyst is a lattice that contains praseodymium and barium, as well as calcium and cobalt (PBCC). In addition to its catalytic function, the PBCC coating protects the cathode from degradation that can limit the lifetime of fuel cells and similar devices.

The underlying original cathode material, which contains the metals lanthanum, strontium, cobalt, and iron (LSCF), has become an industry standard but comes with a caveat.

“It’s very conductive, very good, but the problem is that strontium undergoes a diminishment called segregation in the material,” Liu said. “One component of our catalyst, PBCC, acts as a coating and keeps the LSCF a lot more stable.”

LSCF manufacturing is already well-established, and adding the catalyst coating to production could be likely reasonably achieved. Liu also is considering replacing the LSCF cathode completely with the new catalyst material, and his lab is developing a yet another catalyst to boost fuel oxidation reactions at the fuel cell’s anode.

Source: Georgia Tech

Posted: Mar 13, 2018

In search of a photocatalyst to get hydrogen from water with visible light (Nanowerk News) Hydrogen is the cleanest potential fuel we have; the challenge is to get a clean process to stop using fossil fuels. The COMPHOCAT project, led by Professor Francesc Illas, director of the Institute of Theoretical and Computational Chemistry of the University of Barcelona (IQTCUB), has worked on the computational model of titanium dioxide nanoparticles (TiO2) to search for a catalyst which, with visible light, favours the division of water to generate hydrogen.

TiO2 is a material with unique photocatalysis properties and is now being applied to many areas of technology. One of the main properties it has is the ability to photocatalyse the breaking of the water molecule to obtain a hydrogen molecule and molecular oxygen. The main objective in COMPHOCAT was to study TiO2 properties at the nanoscale and build computational models to simulate the behaviour of the nanoparticles to be used.

Titanium dioxide nanoparticles.

“So far, it has not been possible to identify photocatalysts with activity in the area of visible light, which restricts our practical applications”, says Francesc Illas. “To get a photocatalyst to be practical –he continues- we need to get light in the ultraviolet range, where the TiO2 works better, or we should change the compound so that it has the same properties as visible light”. Illas believes the first option is ruled out “because we cannot get enough ultraviolet light from solar light, since the atmosphere absorbs this range’s light”. The second option –changing TiO2’s nanoparticles- is being studied at the IQTCUB.

The research team led by Illas is formed by the IQTCUB researchers Stefan Bromley (ICREA), Francesc Viñes, Kyoung Chul Co, Ángel Morales, Rosendo Valero, and the researchers in training Antoni Macià and Oriol Lamiel. These researchers created realists models of TiO2 nanoparticles in which they could study the electronic properties using models based on the density functional theory, and conducted precise calculations considering all electrons and adding relativist effects.

“Each particle’s geometry has been determined by the minimization of energy, a technique which is used to get the most stable atom distribution for a molecule”, says Illas. Electronic properties were studied as the function of the shape and size of the nanoparticle. “These TiO2 structure models allowed researchers build a database with nanoparticles which, if suitably modified, can lead to candidates with an absorption spectrum that works well with the window of visible light”, says the UB researcher.

The COMPHOCAT project was carried out thanks to the 52 million hours of calculation with the MareNostrum supercomputer from the Barcelona Supercomputingn Center (BSCN-CNS) given by the Partnership for Advanced Computing in Europe (PRACE). In order to continue with this study, PRACE platform gave the team led by Illas 40 million additional calculation hours for the new project EXCIPHOCAT, which will enable researcher characterize the compounds that were designed in this first stage.

Full bibliographic information

A. Morales-García, O. Lamiel-Garcia, R. Valero and F. Illas. “Properties of single oxygen vacancies on a realistic (TiO2)84 nanoparticle: a challenge for density functionals”. The Journal of Physical Chemistry C, January 2018. DOI: 10.1021/acs.jpcc.7b11269

F. Viñes, O. Lamiel-Garcia, F. Illas and S. T. Bromley. “Size dependent structural and polymorphic transitions in ZnO: from nanocluster to bulk”. Nanoscale, 2017. Doi: 10.1039/C7NR02818K

O. Lamiel-Garcia, K. C. Ko, J. Y. Lee, S. T. Bromley and F. Illas. “When anatase nanoparticles become bulk-like: properties of realistic TiO2 nanoparticles in the 1-6 nm size range from all electron relativistic density functional theory based calculations”. Journal of Chemical Theory and Computation, 2017. Doi: 10.1021/acs.jctc.7b00085

Source: Universidad de Barcelona

Posted: Mar 09, 2018

A bright idea for on-demand nanopatterns (Nanowerk News) An A*STAR team has assembled zinc sulfide (ZnS) quantum dots into nanoscale grids and arrays of lens-like disks by performing electron-beam lithography on a multipurpose thin film (ACS Nano, „Direct patterning of zinc sulfide on a sub-10 nanometer scale via electron beam lithography”). The photoluminescent properties of these patterns could make them useful components in applications such as biosensors and solar cells.

While individual ZnS nanoparticles have intriguing optical properties, due to the effects of quantum coupling, their light-emitting capabilities become more potent when positioned into ordered assemblies. Instead of conventional bottom-up approaches that use wet chemicals to generate nanoparticle ensembles on silicon chips, many researchers now approach this problem from the top down, using nanoscale lithography to strip away unwanted material and write quantum dots directly on to surfaces.

Light-emitting ZnS nanostructures can be patterned into nearly any shape, including grids as above, using a multipurpose photoresist. (© ACS)

Carving shapes into semiconductor surfaces smaller than 10 nanometers is a particular expertise of M. S. M. Saifullah from the A*STAR Institute of Materials Research and Engineering, and colleagues. They direct high-powered electron beams on to special thin films called ‘resists’. Areas of the resist exposed to the focused beams undergo chemical changes that enable tiny features to stay in place while the surrounding film is washed away by solvents.

In most electron-beam lithography techniques, the patterned resist is transferred to another substrate and a chemical etching step generates the final nanoscale shapes. Saifullah and the team, however, had a different strategy. “We developed a resist that can decompose and form a metal sulfide right under the electron beam,” he notes. “This was a challenge because most resists don’t have such functionalities.”

The team found a compound called zinc butylxanthate that could meet their needs. In this molecule, zinc and sulfur atoms are connected to long-chain organic groups that potentially can be detached using the energy from an electron beam. Experiments with the new resist proved the efficiency of this conversion process: by gradually increasing the electron beam exposure, the starting film was transformed into ZnS nanocrystals with a conversion rate of almost 100 per cent

The A*STAR-led researchers exploited the properties of the zinc butylxanthate resist to produce lines of ZnS nanocrystals with diameters as thin as 6 nanometers. Then, after characterizing the structures with electron microscopy, they made another serendipitous discovery — the nanopatterns emitted bright photoluminescent light when exposed to ultraviolet radiation. Defect states on the nanocrystal surfaces were pinpointed as the cause of the new optical behavior.

“The nice thing about photoluminescent ZnS nanocrystals is they can be arranged into practically any shape,” says Saifullah. “In the future, we would like to combine these nanostructures with plasmonics.”

Source: A*STAR

Posted: Mar 09, 2018

Flat gallium joins roster of new 2-D nanomaterials (Nanowerk News) Scientists at Rice University and the Indian Institute of Science, Bangalore, have discovered a method to make atomically flat gallium that shows promise for nanoscale electronics.

The Rice lab of materials scientist Pulickel Ajayan and colleagues in India created two-dimensional gallenene, a thin film of conductive material that is to gallium what graphene is to carbon.

Extracted into a two-dimensional form, the novel material appears to have an affinity for binding with semiconductors like silicon and could make an efficient metal contact in two-dimensional electronic devices, the researchers said.

A model of the structure of gallenene is shown after exfoliation from bulk gallium. Scientists at Rice University and the Indian Institute of Science, Bangalore, discovered a method to make atomically flat gallium that shows promise for nanoscale electronics. (Image: Ajayan Research Group)

The new material was introduced in Science Advances („Atomically thin gallium layers from solid-melt exfoliation”).

Gallium is a metal with a low melting point; unlike graphene and many other 2-D structures, it cannot yet be grown with vapor phase deposition methods. Moreover, gallium also has a tendency to oxidize quickly. And while early samples of graphene were removed from graphite with adhesive tape, the bonds between gallium layers are too strong for such a simple approach.

So the Rice team led by co-authors Vidya Kochat, a former postdoctoral researcher at Rice, and Atanu Samanta, a student at the Indian Institute of Science, used heat instead of force.

Rather than a bottom-up approach, the researchers worked their way down from bulk gallium by heating it to 29.7 degrees Celsius (about 85 degrees Fahrenheit), just below the element’s melting point. That was enough to drip gallium onto a glass slide. As a drop cooled just a bit, the researchers pressed a flat piece of silicon dioxide on top to lift just a few flat layers of gallenene.

They successfully exfoliated gallenene onto other substrates, including gallium nitride, gallium arsenide, silicone and nickel. That allowed them to confirm that particular gallenene-substrate combinations have different electronic properties and to suggest that these properties can be tuned for applications.

“The current work utilizes the weak interfaces of solids and liquids to separate thin 2-D sheets of gallium,” said Chandra Sekhar Tiwary, principal investigator on the project he completed at Rice before becoming an assistant professor at the Indian Institute of Technology in Gandhinagar, India. “The same method can be explored for other metals and compounds with low melting points.”

Gallenene’s plasmonic and other properties are being investigated, according to Ajayan. “Near 2-D metals are difficult to extract, since these are mostly high-strength, nonlayered structures, so gallenene is an exception that could bridge the need for metals in the 2-D world,” he said.

 

Source: By Mike Williams, Rice University

Posted: Mar 08, 2018

Researchers develop a new class of two-dimensional materials

(Nanowerk News) A research team led by UCLA scientists and engineers has developed a method to make new kinds of artificial „superlattices” – materials comprised of alternating layers of ultra-thin „two-dimensional” sheets, which are only one or a few atoms thick. Unlike current state-of-the art superlattices, in which alternating layers have similar atomic structures, and thus similar electronic properties, these alternating layers can have radically different structures, properties and functions, something not previously available.

For example, while one layer of this new kind of superlattice can allow a fast flow of electrons through it, the other type of layer can act as an insulator. This design confines the electronic and optical properties to single active layers, and prevents them from interfering with other insulating layers.

Such superlattices can form the basis for improved and new classes of electronic and optoelectronic devices. Applications include superfast and ultra-efficient semiconductors for transistors in computers and smart devices, and advanced LEDs and lasers.

Compared with the conventional layer-by-layer assembly or growth approach currently used to create 2D superlattices, the new UCLA-led process to manufacture superlattices from 2D materials is much faster and more efficient. Most importantly, the new method easily yields superlattices with tens, hundreds or even thousands of alternating layers, which is not yet possible with other approaches.

two kinds of monolayer atomic crystal molecular superlattices

This is an artist’s concept of two kinds of monolayer atomic crystal molecular superlattices. On the left, molybdenum disulfide with layers of ammonium molecules, on the right, black phosphorus with layers of ammonium molecules. (Image: UCLA Samueli Engineering)

This new class of superlattices alternates 2D atomic crystal sheets that are interspaced with molecules of varying shapes and sizes. In effect, this molecular layer becomes the second „sheet” because it is held in place by „van der Waals” forces, weak electrostatic forces to keep otherwise neutral molecules „attached” to each other. These new superlattices are called „monolayer atomic crystal molecular superlattices.”

The study, published in Nature („Monolayer atomic crystal molecular superlattices”), was led by Xiangfeng Duan, UCLA professor of chemistry and biochemistry, and Yu Huang, UCLA professor of materials science and engineering at the UCLA Samueli School of Engineering.

„Traditional semiconductor superlattices can usually only be made from materials with highly similar lattice symmetry, normally with rather similar electronic structures,” Huang said. „For the first time, we have created stable superlattice structures with radically different layers, yet nearly perfect atomic-molecular arrangements within each layer. This new class of superlattice structures has tailorable electronic properties for potential technological applications and further scientific studies.”

One current method to build a superlattice is to manually stack the ultrathin layers one on top of the other. But this is labor-intensive. In addition, since the flake-like sheets are fragile, it takes a long time to build because many sheets will break during the placement process. The other method is to grow one new layer on top of the other, using a process called „chemical vapor deposition.” But since that means different conditions, such as heat, pressure or chemical environments, are needed to grow each layer, the process could result in altering or breaking the layer underneath. This method is also labor-intensive with low yield rates.

The new method to create monolayer atomic crystal molecular superlattices uses a process called „electrochemical intercalation,” in which a negative voltage is applied. This injects negatively charged electrons into the 2D material. Then, this attracts positively charged ammonium molecules into the spaces between the atomic layers. Those ammonium molecules automatically assemble into new layers in the ordered crystal structure, creating a superlattice.

„Think of a two-dimensional material as a stack of playing cards,” Duan said. „Then imagine that we can cause a large pile of nearby plastic beads to insert themselves, in perfect order, sandwiching between each card. That’s the analogous idea, but with a crystal of 2D material and ammonium molecules.”

The researchers first demonstrated the new technique using black phosphorus as a base 2D atomic crystal material. Using the negative voltage, positively charged ammonium ions were attracted into the base material, and inserted themselves between the layered atomic phosphorous sheets.”

Following that success, the team inserted different types of ammonium molecules with various sizes and symmetries into a series of 2D materials to create a broad class of superlattices. They found that they could tailor the structures of the resulting monolayer atomic crystal molecular superlattices, which had a diverse range of desirable electronic and optical properties.”The resulting materials could be useful for making faster transistors that consume less power, or for creating efficient light-emitting devices,” Duan said.

Source: UCLA Henry Samueli School of Engineering of Applied Science

 

Posted: Mar 08, 2018
Nanostructures made of previously impossible material
(Nanowerk News) When you bake a cake, you can combine the ingredients in almost any proportions, and they will still always be able to mix together. This is a little more complicated in materials chemistry.
Often, the aim is to change the physical properties of a material by adding a certain proportion of an additional element; however, it isn’t always possible to incorporate the desired quantity into the crystal structure of the material.
At TU Wien, a new method has been developed using which previously unattainable mixtures can be achieved between germanium and desired foreign atoms. This results in new materials with significantly altered properties (ACS Nano, „Direct Synthesis of Hyperdoped Germanium Nanowires”).
A low-temperature chemical vapor growth of Ge nanowires using Ga as seed material
A low-temperature chemical vapor growth of Ge nanowires using Ga as seed material. (© ACS)
More tin or gallium in the germanium crystal
„Incorporating foreign atoms into a crystal in a targeted manner to improve its properties is actually a standard method,” says Sven Barth from the Institute of Materials Chemistry at TU Wien. Our modern electronics are based on semiconductors with certain additives. Silicon crystals into which foreign atoms such as phosphorus or boron are incorporated are one example of this.
The semiconductor material germanium was also supposed to fundamentally change its properties and behave like a metal when a sufficient amount of tin was mixed in – that was already known; however, in practice, that was previously not attained.
One could of course attempt to simply melt the two elements, thoroughly mix them together in liquid form and then let them solidify, as has been done for thousands of years in order to produce simple metal alloys. „But in our case, this simple thermodynamic method fails, because the added atoms do not efficiently blend into the lattice system of the crystal,” explains Sven Barth. „The higher the temperature, the more the atoms move inside the material. This can result in these foreign atoms precipitating out of the crystal after they have been successfully incorporated, leaving behind a very low concentration of these atoms within the crystal.”
Sven Barth’s team have therefore developed a new approach that links particularly rapid crystal growth to very low process temperatures. In the process, the correct quantity of the foreign atoms is continuously incorporated as the crystal grows.
The crystals grow in the form of nano-scale threads or rods, and specifically at considerably lower temperatures than before, in the range of just 140-230°C. „As a result, the incorporated atoms are less mobile, the diffusion processes are slow, and most atoms stay where you want them to be,” explains Barth.
Using this method, it has been possible to incorporate up to 28% tin and 3.5% gallium into germanium. This is considerably more than was previously possible by means of the conventional thermodynamic combination of these materials – by a factor of 30 to 50.
Lasers, LEDs, electronic components
This opens up new possibilities for microelectronics: „Germanium can be effectively combined with existing silicon technology, and also the addition of tin and/or gallium in such high concentrations offers extremely interesting potential applications in terms of optoelectronics,” says Sven Barth. The materials would be used for infrared lasers, for photodetectors or for innovative LEDs in the infrared range, for example, since the physical properties of germanium are significantly changed by these additives.
Source: Vienna University of Technology

Posted: Mar 07, 2018

Dog’s nose inspires new graphene gas sensor materials

(Nanowerk News) It is well known that dogs have a better sense of smell than humans. For years, researchers have been trying to develop an artificial detector that is just as good as a canine’s nose.

Now, one group reports in ACS Nano („Mimicking a Dog’s Nose: Scrolling Graphene Nanosheets”) that they were able to mimic a dog’s sniffer with graphene-based nanoscrolls.

The inside of a dog’s nose is lined with millions of tiny capillaries, which creates a super-sensitive sense of smell. Since the capillaries cover such a large surface area, they can detect odors at extremely low concentrations.

Drawing inspiration from the capillary structure in the dog nose, scientists have been trying to mimic it to create a sensitive gas detector.

Mimicking a Dog’s Nose with Scrolling Graphene Nanosheets

Inspired by the densely covered capillary structure inside a dog’s nose, the authors report an artificial nanostructure, i.e., poly(sodium p-styrenesulfonate)-functionalized reduced graphene oxide nanoscrolls (PGNS), with high structural perfection and efficient gas sensing applications. (© American Chemical Society)

Previous studies have had some success in using graphene-based nanoscrolls (GNS), which are nanosheets of graphene rolled up in continuous and uniform manner. These nanoscrolls have a large surface area, are stable at high temperatures, and are strong and durable. But they are also difficult to manufacture, consume a lot of energy and difficult to scale up.

And past studies have used raw graphene or modified graphene that either left behind some unrolled structures, or shriveled up and aggregated, respectively. So Yao Wang, Lei Jiang, Guofu Zhou and colleagues wanted to modify the graphene with a polymer to make high-quality nanoscrolls.

The group prepared graphene-based nanoscrolls with the addition of poly(sodium-p-stryrenesulfonate) using the freeze-drying method to create uniform, unaggregated structures. Upon examination, the nanoscrolls had a wide, tubular shape, and almost all of the graphene was rolled up. The researchers then incorporated the nanoscrolls into a gas sensor, which was highly selective and sensitive. Lastly, the team notes that this method has the potential for large-scale production.

Source: American Chemical Society

 

Posted: Mar 07, 2018

Graphene promise for more efficient fertilisers (Nanowerk News) Fertilisers with lower environmental impacts and reduced costs for farmers are being developed by University of Adelaide researchers in the world-first use of the new advanced material graphene as a fertiliser carrier.

In partnership with industry, the researchers have demonstrated effective slow release fertilisers can be produced from loading essential trace elements onto graphene oxide sheets.

Using graphene as a carrier means the fertilisers can be applied in a more targeted fashion, with overall increased fertiliser efficiency and great nutrient uptake by the plants. The graphene-based carriers have so far been demonstrated with the micronutrients zinc and copper. Work is continuing with macronutrients such as nitrogen and phosphate.

The slow release fertiliser granules with zinc loaded onto graphene oxide sheets. (Image: Shervin Kabiri)

“Fertilisers that show slower, more controlled release and greater efficiency will have reduced impact on the environment and lower costs for farmers over conventional fertilisers, bringing significant potential benefit for both agriculture and the environment,” says Professor Mike McLaughlin, Head of the University of Adelaide’s Fertiliser Technology Research Centre at the Waite campus.

“Our research found that loading copper and zinc micronutrients onto graphene oxide sheets was an effective way to supply micronutrients to plants. It also increased the strength of the fertiliser granules for better transport and spreading ability.”

Professor Dusan Losic, nanotechnology leader in the University’s School of Chemical Engineering and Director of the University’s Australian Research Council (ARC) Research Hub for Graphene Enabled Industry Transformation, says: “Graphene is a novel new material only discovered in 2004 and has incredible properties, including a very high surface area, strength and adaptability to bind to different nutrients. We started exciting research on a broad range of applications of graphene four years ago – this is the first time graphene has been developed as a carrier for fertiliser nutrients.”

The research, carried out by PhD student Shervin Kabiri, has been published in the journal Applied Materials and Interfaces („Graphene Oxide: A New Carrier for Slow Release of Plant Micronutrients”). It is a collaboration between the University of Adelaide’s Fertiliser Technology Research Centre and the University’s Australian Research Council Research Hub for Graphene Enabled Industry Transformation.

The Fertiliser Technology Research Centre was established in 2007 in partnership with The Mosaic Company, the world’s largest combined producer of phosphate and potash, to develop and evaluate more efficient fertiliser products. A new five-year, $8.5 million partnership agreement was reached with The Mosaic Company in 2015. Mosaic has an option to license the new technology and is further examining the use of graphene-based materials in fertilisers.

“This decade-long partnership is testament to the University’s strength in this area of research and our success in partnering with industry for research translation that benefits the wider community,” says Professor Mike Brooks, Deputy Vice-Chancellor Research.

“Combining plant research with our new Graphene Research Hub is a great example of how the University assembles interdisciplinary teams to deliver innovation solutions for industry.”

Professor McLaughlin says: “It’s still early days but there is no doubt that fertilisers with release rates more tailored to crop demand, and fertilisers with greater physical strength and robustness, will both improve grower efficiency of fertiliser application and efficiency of nutrient uptake.

“Successful commercialisation will depend on cost of graphene/graphene oxide and the ability to scale this process up, and integrate it into the commercial fertiliser production process.”

Source: University of Adeleide