Am reușit să combinăm doi oxizi de metal de tranziție foarte diferiți: în timp ce în electronii titanați, legăturile chimice sunt puternic localizate în jurul ionilor, în partea de nickelat acești electroni sunt împărțiți între ionii de nichel și oxigen și, prin urmare, foarte covalent „, Explică Manuel Bibes.
Când se pun împreună ambele materiale, o anumită încărcare se transferă de la stratul de titanat la cel de aluminiu. Ei au investigat acest proces de transfer de sarcină pentru eșantioane care conțin diferite elemente de pământuri rare în stratul de nichelat, cum ar fi Lanthanum, Neodymium și Samarium la BESSY II.
Posted: Feb 04, 2016
Metal oxide sandwiches: New option to manipulate properties of interfaces
(Nanowerk News) Sandwich systems of thin film transition metal oxides display surprising properties at their interfaces. In case of the paradigmatic example of Lanthan-Aluminate ( LaAlO3) and Strontium-Titanate (SrTiO3) both materials are insulators and non-magnetic, while their interface has been observed to display ferromagnetism, high electrical conductivity and even superconductivity.
Now the team of Manuel Bibes, CNRS Thales at Palaiseau, France, in collaboration with scientists at HZB around Sergio Valencia and several European groups, devised a new approach to tailor interface properties. Together they designed a series of experiments at the synchrotron source BESSY II to shed more light on the emergence of such property changes, identifying a new „knob” for their control (Nature Physics, „Hybridization-controlled charge transfer and induced magnetism at correlated oxide interfaces”).
This is a sketch of the structure of both metal oxide layers. Interesting new properties can arise at the interface. (Image: M. Bibes)
Rare-Earth Elements influence charge transfer
The samples, which the team of Manuel Bibes did produce, consisted of a sandwich of 2 nm Gadolinium-Titanate (GdTiO3) and „R”-Nickelate (RNiO3) films, where R is a rare-earth element. „We have been able to combine two very different transition metal oxides: whereas in the titanate electrons in the chemical bonds are strongly localized around the ions, in the nickelate side these electrons are shared between Nickel- and Oxygen-ions, and thus highly covalent”, Manuel Bibes explains.
When putting both materials together some charge is transferred from the titanate layer to the nickelate one. They investigated this charge transfer process for samples containing different rare-earth elements in the nickelate layer such as Lanthanum, Neodymium and Samarium at BESSY II.
Their results show that the charge transfer at the interface between the materials strongly depends on the rare earth element in the nickelate layer. Different rare-earth elements have different atomic radii (size).This modifies the interaction between the Ni and O atoms and the degree of „covalency” between Ni and O changes. This was already known, but now the scientists have observed that this also affects the charge transferred from the GdTiO3 to the Nickelate film. „This is the key result”, Sergio Valencia from HZB explains. „We have found a new „knob”. Covalency (which is controlled by changing R) controls the charge transfer between the titanate and the nickelate.”
Ferromagnetism observed, superconductivity still searched
Tuning the charge transfer in this way might allow to control the formation of new interfacial phases too. For example, the scientists observed a new ferromagnetic phase at the interface. „Our work may help in the ongoing quest for cuprate-like superconductivity in nickelate heterostructures”, Valencia says. „We hope that this study will help to design better interfaces for exploring new exciting new phases of matter at interfaces between covalent materials”, Bibes adds.
Source: Helmholtz-Zentrum Berlin für Materialien und Energie
Hidrogenul din aceste reacții ar putea alimenta vehicule cu pile de combustie cu hidrogen curate sau ar putea fi combinat cu dioxid de carbon pentru a produce metanol, care ar putea fi distribuit prin sistemul nostru existent de combustibil lichid. Această din urmă abordare are avantajul suplimentar de a scoate dioxidul de carbon din atmosferă.
Posted: Feb 04, 2016
Researchers seek efficient means of splitting water with nanorods
(Nanowerk News) Photovoltaics promise to help meet our energy needs by turning sunlight into electricity. We can’t run everything that way, but with a little tweaking, photovoltaic materials can use solar energy to split water into hydrogen and oxygen.
Cornell researchers are studying how this works, hoping to make the process more efficient and less expensive. “There are several existing systems, but none actually work well,” said Peng Chen, the Peter J. W. Debye Professor of Chemistry.
Hydrogen from these reactions could power vehicles with clean-burning hydrogen fuel cells, or could be combined with carbon dioxide to make methanol, which could be distributed through our existing liquid-fuel system. The latter approach has the added benefit of taking carbon dioxide out of the atmosphere.
Chen and colleagues studied water-splitting activity on the surface of nanorods of the semiconductor titanium dioxide, finding that variations in the structure of the surface determine where and how the reaction takes place. They report their results in the Feb. 4 issue of the journal Nature („Sub-particle reaction and photocurrent mapping to optimize catalyst-modified photoanodes”).
Flashes of fluorescence seen under a microscope show where chemical reactions to (b) split water molecules or (c) electric currents take place on a nanorod of photovoltaic material. The outline of the rod laid over flash images is taken from the scanning electron microscope image at right.
When a photon of light hits a crystal of titanium dioxide, the added energy kicks an electron into a higher energy level, leaving behind a “hole” – a space where an electron ought to be but isn’t, that looks like a positive charge. If electrodes are attached to the crystal, the free electrons will flow through a circuit as an electric current.
But in the presence of a catalyst – in this case cobalt borate – the array of positive charges can withdraw electrons from a water molecule (whose formula can be written as H-O-H), pulling it apart into a positively charged proton – the nucleus of a hydrogen atom – and a neutral hydroxyl radical, or OH. The OH radicals combine into hydrogen peroxide (H-O-O-H), which the catalytic reaction again pulls apart into protons and oxygen.
To study the reaction, Chen’s research team focused a powerful microscope on titanium dioxide rods about 127×1,735 nanometers in a microfluidic chamber through which water was flowed. A light microscope can’t see chemical reactions, so Chen employed a technique he calls “fluorescence microscopy,” adding chemicals to the water that change into a fluorescent material when the reactions take place. One changes in the presence of free electrons; the other changes in the presence of OH radicals, which are produced when water is split. When the sample is zapped with a laser to induce fluorescence, the microscope sees flashes of light that reveal where the reactions are taking place.
Alternating from one probe to the other, they observed that the reactions were not distributed evenly across the surface. Water splitting occurred only at certain “hotspots” that seem to be characterized by some irregularities in the surface or the presence of a few “doping” atoms.
“This gives us a way to improve the process,” Chen said. “Now we know where to deposit the catalyst according to the map of the hole and electron reactions on each nanorod.”
The researchers propose a method for selectively depositing catalyst on the areas that will be most productive.
poate deschide o cale către noi materiale care să profite de proprietățile optice și mecanice ale acestei Structura cristalină pentru aplicații precum tranzistoare optice, materiale de schimbare a culorii și materiale ușoare, dar dure.
Posted: Feb 04, 2016
Scientists guide gold nanoparticles to form ‘diamond’ superlattices
(Nanowerk News) Using bundled strands of DNA to build Tinkertoy-like tetrahedral cages, scientists at the U.S. Department of Energy’s Brookhaven National Laboratory have devised a way to trap and arrange nanoparticles in a way that mimics the crystalline structure of diamond. The achievement of this complex yet elegant arrangement, as described in a paper published February 5, 2016, in Science („Diamond family of nanoparticle superlattices”), may open a path to new materials that take advantage of the optical and mechanical properties of this crystalline structure for applications such as optical transistors, color-changing materials, and lightweight yet tough materials.
„We solved a 25-year challenge in building diamond lattices in a rational way via self-assembly,” said Oleg Gang, a physicist who led this research at the Center for Functional Nanomaterials (CFN, www.bnl.gov/cfn/) at Brookhaven Lab in collaboration with scientists from Stony Brook University, Wesleyan University, and Nagoya University in Japan.
Schematic illustration of the experimental strategy: Double stranded DNA bundles (gray) form tetrahedral cages. Single stranded DNA strands on the edges (green) and vertices (red) match up with complementary strands on gold nanoparticles. This results in a single gold particle being trapped inside each tetrahedral cage, and the cages binding together by tethered gold nanoparticles at each vertex. The result is a crystalline nanoparticle lattice that mimics the long-range order of crystalline diamond. The images below the schematic are (left to right): a reconstructed cryo-EM density map of the tetrahedron, a caged particle shown in a negative-staining TEM image, and a diamond superlattice shown at high magnification with cryo-STEM. (click on image to enlarge)
The scientists employed a technique developed by Gang that uses fabricated DNA as a building material to organize nanoparticles into 3D spatial arrangements [ www.bnl.gov/newsroom/news.php?a=11728]. They used ropelike bundles of double-helix DNA to create rigid, three-dimensional frames, and added dangling bits of single-stranded DNA to bind particles coated with complementary DNA strands.
„We’re using precisely shaped DNA constructs made as a scaffold and single-stranded DNA tethers as a programmable glue that matches up particles according to the pairing mechanism of the genetic code-A binds with T, G binds with C,” said Wenyan Liu of the CFN, the lead author on the paper. „These molecular constructs are building blocks for creating crystalline lattices made of nanoparticles.”
The difficulty of diamond
As Liu explained, „Building diamond superlattices from nano- and micro-scale particles by means of self-assembly has proven remarkably difficult. It challenges our ability to manipulate matter on small scales.”
The reasons for this difficulty include structural features such as a low packing fraction-meaning that in a diamond lattice, in contrast to many other crystalline structures, particles occupy only a small part of the lattice volume-and strong sensitivity to the way bonds between particles are oriented. „Everything must fit together in just such a way without any shift or rotation of the particles’ positions,” Gang said. „Since the diamond structure is very open, many things can go wrong, leading to disorder.”
„Even to build such structures one-by-one would be challenging,” Liu added, „and we needed to do so by self-assembly because there is no way to manipulate billions of nanoparticles one-by-one.”
Gang’s previous success using DNA to construct a wide range of nanoparticle arrays suggested that a DNA-based approach might work in this instance.
DNA guides assembly
The team first used the ropelike DNA bundles to build tetrahedral „cages”-a 3D object with four triangular faces. They added single-stranded DNA tethers pointing toward the interior of the cages using T,G,C,A sequences that matched up with complementary tethers attached to gold nanoparticles. When mixed in solution, the complementary tethers paired up to „trap” one gold nanoparticle inside each tetrahedron cage.
The arrangement of gold nanoparticles outside the cages was guided by a different set of DNA tethers attached at the vertices of the tetrahedrons. Each set of vertices bound with complementary DNA tethers attached to a second set of gold nanoparticles.
When mixed and annealed, the tetrahedral arrays formed superlattices with long-range order where the positions of the gold nanoparticles mimics the arrangement of carbon atoms in a lattice of diamond, but at a scale about 100 times larger.
Brookhaven Lab’s Oleg Gang describes the idea of using DNA to drive the self-assembly of materials from nanoscale particles.
„Although this assembly scenario might seem hopelessly unconstrained, we demonstrate experimentally that our approach leads to the desired diamond lattice, drastically streamlining the assembly of such a complex structure,” Gang said.
The proof is in the images. The scientists used cryogenic transmission electron microscopy (cryo-TEM) to verify the formation of tetrahedral frames by reconstructing their 3D shape from multiple images. Then they used in-situ small-angle x-ray scattering (SAXS) at the National Synchrotron Light Source (NSLS, www.bnl.gov/ps/), and cryo scanning transmission electron microscopy (cryo-STEM) at the CFN, to image the arrays of nanoparticles in the fully constructed lattice.
„Our approach relies on the self-organization of the triangularly shaped blunt vertices of the tetrahedra (so called ‘footprints’) on isotropic spherical particles. Those triangular footprints bind to spherical particles coated with complementary DNA, which allows the particles to coordinate their arrangement in space relative to one another. However, the footprints can arrange themselves in a variety of patterns on a sphere. It turns that one particular placement is more favorable, and it corresponds to the unique 3D placement of particles that locks the diamond lattice,” Gang said.
The team supported their interpretation of the experimental results using theoretical modeling that provided insight about the main factors driving the successful formation of diamond lattices.
Sparkling implications
„This work brings to the nanoscale the crystallographic complexity seen in atomic systems,” said Gang, who noted that the method can readily be expanded to organize particles of different material compositions. The group has demonstrated previously that DNA-assembly methods can be applied to optical, magnetic, and catalytic nanoparticles as well, and will likely yield the long-sought novel optical and mechanical materials scientists have envisioned.
„We’ve demonstrated a new paradigm for creating complex 3D-ordered structures via self-assembly. If you can build this challenging lattice, the thinking is you can build potentially a variety of desired lattices,” he said.
Deși grafina monocristalină pură poate avea mai puține defecte, autorii au studiat grafina policristalină, deoarece este mai ieftină și sintetizată în mod obișnuit cu depunerea chimică de vapori. Ritchie este conștient de o singură măsură experimentală a durității materialului.
Posted: Feb 04, 2016
Graphene is strong, but is it tough?
(Nanowerk News) Graphene, a material consisting of a single layer of carbon atoms, has been touted as the strongest material known to exist, 200 times stronger than steel, lighter than paper, and with extraordinary mechanical and electrical properties. But can it live up to its promise?
Scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed the first known statistical theory for the toughness of polycrystalline graphene, which is made with chemical vapor deposition, and found that it is indeed strong (albeit not quite as strong as pristine monocrystalline graphene), but more importantly, its toughness—or resistance to fracture—is quite low. Their study was published recently in Nature Communications („Toughness and strength of nanocrystalline graphene”).
Polycrystalline graphene contains inherent nanoscale line and point defects that lead to significant statistical fluctuations in toughness and strength. (Image: Berkeley Lab)
“This material certainly has very high strength, but it has particularly low toughness—lower than diamond and a little higher than pure graphite,” said Berkeley Lab scientist Robert Ritchie. “Its extremely high strength is very impressive, but we can’t necessarily utilize that strength unless it has resistance to fracture.”
Ritchie, a senior scientist in the Materials Sciences Division of Berkeley Lab and a leading expert on why materials fail, was co-author of the study along with Ashivni Shekhawat, a Miller Research Fellow in his group. Together they developed a statistical model for the toughness of polycrystalline graphene to better understand and predict failure in the material.
“It’s a mathematical model that takes into account the nanostructure of the material,” Ritchie said. “We find that the strength varies with the grain size up to a certain extent, but most importantly this is a model that defines graphene’s fracture resistance.”
Toughness, a material’s resistance to fracture, and strength, a material’s resistance to deformation, are often mutually incompatible properties. “A structural material has to have toughness,” Ritchie explained. “We simply don’t use strong materials in critical structures—we try to use tough materials. When you look at such a structure, like a nuclear reactor pressure vessel, it’s made of a relatively low-strength steel, not an ultrahigh-strength steel. The hardest steels are used to make tools like a hammer head, but you’d never use them to manufacture a critical structure because of the fear of catastrophic fracture.”
As the authors note in their paper, many of the leading-edge applications for which graphene has been suggested—such as flexible electronic displays, corrosion-resistant coatings, and biological devices—implicitly depend on its mechanical properties for structural reliability.
Although pure monocrystalline graphene may have fewer defects, the authors studied polycrystalline graphene as it is more inexpensively and commonly synthesized with chemical vapor deposition. Ritchie is aware of only one experimental measurement of the material’s toughness.
“Our numbers were consistent with that one experimental number,” he said. “In practical terms these results mean that a soccer ball can be placed on a single sheet of monocrystalline graphene without breaking it. What object can be supported by a corresponding sheet of polycrystalline graphene? It turns out that a soccer ball is much too heavy, and polycrystalline graphene can support only a ping pong ball. Still remarkable for a one-atom thick material, but not quite as breathtaking anymore.”
Next, Shekhawat and Ritchie are studying the effects of adding hydrogen to the material. “We don’t know a lot about the fracture of graphene, so we’re trying to see if it’s sensitive to other atoms,” he said. “We’re finding the cracks grow more readily in the presence of hydrogen.”
Folosind o inginerie inteligentă, cercetătorii au depășit acest impas prin plasarea unui film subțire de polimeri electrochromi pe un nanograt de aluminiu. În configurația lor, lumina întâlnește întâi nanograma care, în funcție de spațierea fantelor, filtrează numai o singură culoare a luminii. Acoperirea electrochromică servește pentru a modula această lumină permițând tuturor sau unuia (sau niciunuia) să treacă. Un afișaj bazat pe această schemă ar avea o serie de nanograme care acționează ca pixeli cu o singură culoare, ale căror culori, adăugând sau scăzând, ar produce toate culorile curcubeului, pixelii individuali având niveluri reglabile de luminozitate sau contrast.
Posted: Feb 04, 2016
New twist on old tech could bring living color at low power
(Nanowerk News) By marrying state-of-the-art nanometer-scale gratings with a Space Age-era thin-film polymer, researchers working at the National Institute of Standards and Technology (NIST) have developed a new technology for building power-sipping full-color video displays, switches and routers for optical signals, as well as smart windows and coatings (Nature Communications, „High-contrast and fast electrochromic switching enabled by plasmonics”).
In this schematic, light enters from above and encounters a nanometer-scale grating made of aluminum and coated with a special ‘electrochromic’ material. Light passes through the nanograting as surface plasmons, oscillations that travel at the interface of a metal and a dielectric (insulating) material. The spacing of the slits serves to filter out all but one color of light, and an additional layer of silicon nitride further spectrally purifies the escaping light. (Image: T. Xu and A. Agrawal/NIST) (click on image to enlarge)
Electrochromic polymers make up a special class of materials that can switch from clear to colored and back again when their electrical charge changes. Their wide adoption, however, has been hindered because this transformation depends on how quickly electrons can get into and out of the material, which can take several seconds depending on the thickness of the film. Why not just make the films thinner so they’ll change color faster? Good idea, but unfortunately, reducing the film’s thickness robs the colors of their contrast, giving you a semitransparent black tint when what you wanted was an impenetrable, opaque black.
If the choice between being slow or weakly colored weren’t bad enough, an electrochromic display using an additive color scheme—in which red, green and blue are mixed to achieve full color—would need up to six transparent electrically conducting layers to work, adding substantially to the cost of manufacturing.
Using some clever engineering, the researchers overcame this impasse by layering a thin film of electrochromic polymer over an aluminum nanograting. In their setup, the light first encounters the nanograting which, depending on the spacing of the slits, filters out all but one color of light. The electrochromic coating serves to modulate that light by allowing all or some (or none) of it to pass. A display based on this scheme would have an array of nanogratings acting as single-color pixels whose colors would, by adding or subtracting them, produce all the colors of the rainbow, with the individual pixels having adjustable levels of brightness or contrast.
But how does combining a thin film of electrochromic polymer with a nanograting make up for the just-described deficiencies of using said thin film? According to co-lead author Amit Agrawal, it’s all about the architecture.
“Because the electrochromic film that coats the sidewalls of the nanograting is so thin, it’s very easy for electrons to make their way through and change its opacity,” Agrawal says. “The slits are much narrower (about 60 nanometers) than the wavelengths of light they are transmitting (from about 400 to 700 nanometers). This forces the incoming light to cling to the interface between the metal and the electrochromic layer on its way down the slits. And because the grating is so much deeper (about 250 nanometers) than it is wide, and the walls of the slits are coated with electrochromic polymer all the way down, it can reduce the light’s intensity to the same degree as if the light were passing through a much thicker layer of electrochromic film, enabling a great amount of control.”
NIST Fellow Henri Lezec, an author of the paper, says the setup they have now uses a liquid electrolyte, a chemical that facilitates the proper flow of electrons, which is not ideal for manufacturability and reliability, but that it could easily be replaced with a more physically stable and compact solid-state electrolyte.
Alec Talin, a co-author based at Sandia National Laboratories, says that the next step in extending this principle would be toward reflective, sunlight-readable displays that do not require an energy-consuming backlight.
Source: The National Institute of Standards and Technology
inginerii mecanici ar putea reduce dependența de baterii de pe dispozitivele noastre mobile, asigurându-ne că avem putere pentru dispozitivele noastre, indiferent unde suntem.Tehnologia ar putea permite o combinație de energie încorporată în încălțăminte care să surprindă energia produsă de oameni în timpul mersului pe jos și să o stocheze pentru o utilizare ulterioară
au dezvoltat prima teorie statistică cunoscută despre duritatea grafenului policristalin, care se face cu depunere chimică de vapori și a constatat că acesta este într-adevăr puternic
Înlătură termen: particulele cu cea mai mare energie sunt îndepărtate din gaz. Ceilalți interacționează particulele cu cea mai mare energie sunt îndepărtate din gaz. Ceilalți interacționeazăÎnlătură termen: răcirea gazelor reci: cu ajutorul câmpurilor electromagnetice răcirea gazelor reci: cu ajutorul câmpurilor electromagneticeÎnlătură termen: redistribuie energia și gazul se relaxează din nou la o distribuție tipică a energiei – dar la o temperatură ușor mai scăzută decât înainte. redistribuie energia și gazul se relaxează din nou la o distribuție tipică a energiei – dar la o temperatură ușor mai scăzută decât înainte.
Posted: Feb 01, 2016
The quantum fridge
(Nanowerk News) When cold milk is poured into a hot cup of tea, a temperature equilibrium is reached very quickly. The milk droplets and the tea particles interact, and after a few moments they all have the same average energy. This process is called thermalization. It plays a crucial role in cooling down gases to ultra-low temperatures. But surprisingly, even gases for which this effect is suppressed can be cooled. Scientists at TU Wien (Vienna) took a closer look at this phenomenon and found a special quantum-mechanical kind of cooling at work (Physical Review Letters, „Cooling of a One-Dimensional Bose Gas”).
Kicking Out Hot Particles
“The particles that make up a liquid or a gas have different energies”, says Professor Jörg Schmiedmayer (TU Wien). The distribution of these energies depends on the temperature. The hotter the gas, the higher the number of particles with high energies. Therefore, a simple trick can be used to cool down cold gases: with the help of electromagnetic fields, the particles with the highest energy are removed from the gas. The remaining ones interact, redistribute the energy, and the gas relaxes again to a typical energy distribution—but at a slightly lower temperature than before.
“It is a bit like blowing on your tea in order to cool it”, says Bernhard Rauer, who led the experiments in Schmiedmayer’s research group. “The particles with the highest energy manage to leave the liquid and are blown away. The remaining tea quickly reaches an equilibrium state at a slightly lower temperature.”
Bernhard Rauer in the lab at TU Wien.
However, there are cases, in which reaching such a thermal equilibrium is not possible. A Newton’s cradle for example is a device that has several spheres hanging from strings, arranged in a straight line. When one of the spheres is set in motion and hits the others, the last sphere in line is kicked away, while the others do not move. “In this case, the spheres can only exchange energies between each other. There will never be a thermal distribution of many different energies”, says Bernhard Rauer.
Rauer studied a similar system: a one dimensional gas of atoms, kept in a straight line by an electromagnetic trap. The atoms can just exchange their energies, just like the spheres in Newton’s cradle. Therefore, one would expect the cooling mechanism of removing the most energetic particles to fail in this case. When the fastest particles are gone, no other particles in the gas will ever have the same speed again. According to this simple model, an energy that is missing will be gone forever.
Astonishingly, this is not true for the one-dimensional gas. It can be cooled down by continuously removing particles—to much lower energies than one would expect, according, to the simplified picture of fast and slow particles.
It’s a Wave!
The reason for this effect is that the particles can only be understood quantum mechanically. “We should not think of individual particles colliding like the spheres in Newton’s cradle. Instead, we have to consider collective excitations, which are distributed across many particles – like a water wave, which is carried by many water molecules at the same time”, says Jörg Schmiedmayer. These quantum waves store the energy of the system, and the more particles that are removed from the gas, the less intense these waves become. This is a quantum mechanical cooling mechanism which should be impossible according to simple classical laws of nature.
“For us it is crucial that the gas behaves more and more quantum mechanically at low temperatures”, says Jörg Schmiedmayer. “That is exciting, because that is precisely what we are interested in: often quantum physics is studied in simple systems, consisting only of a few particles, such as an atom with a few electrons. We have a system which undeniably exhibits quantum behaviour, and it is made of thousands of atoms.”
