Tag Archives: graphene

Double Layers Of Graphene Conduct Current Without Resistance

Scientists at the Helmholtz Zentrum Berlin (HZB) have found evidence that double layers of graphene have a property that may let them conduct current completely without resistance. They probed the bandstructure at BESSY II with extremely high resolution ARPES and could identify a flat area at a surprising location.

Carbon atoms have diverse possibilities to form bonds. Pure carbon can therefore occur in many forms, as diamond, graphite, as nanotubes, football molecules or as a honeycomb-net with hexagonal meshes, graphene. This exotic, strictly two-dimensional material conducts electricity excellently, but is not a superconductor. But perhaps this can be changed.

In April 2018, a group at MIT, USA, showed that it is possible to generate a form of superconductivity in a system of two layers of graphene under very specific conditions: To do this, the two hexagonal nets must be twisted against each other by exactly the magic angle of 1.1°. Under this condition a flat band forms in the electronic structure. The preparation of samples from two layers of graphene with such an exactly adjusted twist is complex, and not suitable for mass production. Nevertheless, the study has attracted a lot of attention among experts.

But there is one more, much simpler way of flat band formation. This was shown by a group at the HZB around Prof. Oliver Rader and Dr. Andrei Varykhalov with investigations at BESSY II. The samples were provided by Prof. Thomas Seyller, TU Chemnitz. There they are produced using a process that is also suitable for the production of larger areas and in large quantities: A silicon carbide crystal is heated until silicon atoms evaporate from the surface, leaving first a single-layer of graphene on the surface, and then a second layer of graphene. The two graphene layers are not twisted against each other, but lie exactly on top of each other.

This flat area is a prerequisite for superconductivity but only if it is situated exactly at the so-called Fermi energy. In the case of the two-layer graphene, its energy level is only 200 milli-electron volts below the Fermi energy, but it is possible to raise the energy level of the flat area to the Fermi energy either by doping with foreign atoms or by applying an external voltage, the so-called gate voltage.

The findings have been Published in Science Advances.

Source: https://www.helmholtz-berlin.de/

How to mass produce cell-sized robots

NanoRobots no bigger than a cell could be mass-produced using a new method developed by researchers at MIT. The microscopic devices, which the team calls “syncells” (short for synthetic cells), might eventually be used to monitor conditions inside an oil or gas pipeline, or to search out disease while floating through the bloodstream.

The key to making such tiny devices in large quantities lies in a method the team developed for controlling the natural fracturing process of atomically-thin, brittle materials, directing the fracture lines so that they produce miniscule pockets of a predictable size and shape. Embedded inside these pockets are electronic circuits and materials that can collect, record, and output data.  The system uses a two-dimensional form of carbon called graphene, which forms the outer structure of the tiny syncells. One layer of the material is laid down on a surface, then tiny dots of a polymer material, containing the electronics for the devices, are deposited by a sophisticated laboratory version of an inkjet printer. Then, a second layer of graphene is laid on top.

This photo shows circles on a graphene sheet where the sheet is draped over an array of round posts, creating stresses that will cause these discs to separate from the sheet. The gray bar across the sheet is liquid being used to lift the discs from the surface

People think of graphene, an ultrathin but extremely strong material, as being “floppy,” but it is actually brittle, Strano explains. But rather than considering that brittleness a problem, the team figured out that it could be used to their advantage. “We discovered that you can use the brittleness,” says Strano, who is the Carbon P. Dubbs Professor of Chemical Engineering at MIT. “It’s counterintuitive. Before this work, if you told me you could fracture a material to control its shape at the nanoscale, I would have been incredulous.”

The novel process, called “autoperforation,” is described in a paper published today in the journal Nature Materials, by MIT Professor Michael Strano, postdoc Pingwei Liu, graduate student Albert Liu, and eight others at MIT.

Source: http://news.mit.edu/

Let Your Skin Play Music

A variety of nanomaterials have been used over the years in loudspeakers and microphones. Nanoparticles have replaced permanent magnets in loudspeakers and a thin film of carbon nanotubes has done pretty much the same. And, of course, someone tried to use graphene to reproduce sound for microphones.

Now researchers at Ulsan National Institute of Science and Technology (UNIST) in South Korea have made a nanomembrane out of silver nanowires to serve as flexible loudspeakers or microphones. The researchers even went so far as to demonstrate their nanomembrane by making it into a loudspeaker that could be attached to skin and used it to play the final movement of a violin concerto—namely, La Campanella by Niccolo Paganini.

In research described in the journal Science Advances, the Korean researchers embedded a silver nanowire network within a polymer-based nanomembrane. The decision to use silver nanowires rather than the other types of nanomaterials that have been used in the past was based on the comparative ease of hybridizing the nanowires into the polymer. In addition, the researchers opted for nanowires because the other materials like graphene and carbon nanotubes are not as mechanically strong at nanometer-scale thickness when in freestanding form, according to Hyunhyub Ko, an associate professor at UNIST and coauthor of the research. It is this thickness that is the critical element of the material.

The biggest breakthrough of our research is the development of ultrathin, transparent, and conductive hybrid nanomembranes with nanoscale thickness, less than 100 nanometers,” said Ko. “These outstanding optical, electrical, and mechanical properties of nanomembranes enable the demonstration of skin-attachable and imperceptible loudspeaker and microphone.”

The nanomembrane loudspeaker operates by emitting thermoacoustic sound through the oscillation of the surrounding air brought on by temperature differences. The periodic Joule heating that occurs when an electric current passes through a conductor and produces heat leads to these temperature oscillations.

Source: https://spectrum.ieee.org/

Colorful 3D Printing

People are exploring the use of 3D printing for wide-ranging applications, including manufacturing, medical devices, fashion and even food. But one of the most efficient forms of 3D printing suffers from a major drawback: It can only print objects that are gray or black in color. Now, researchers have tweaked the method so it can print in all of the colors of the rainbow.

THIS BRIGHTLY COLORED DRAGON WAS PRODUCED BY 3D PRINTING, USING GOLD NANORODS AS PHOTOSENSITIZERS.

Selective laser sintering (SLS) printers use a laser to heat specific regions of a powdered material, typically nylon or polyamide, so that the powder melts or sinters to form a solid mass. The printer adds then selectively sinters new powdered material layer by layer until the desired 3D structure is obtained. To reduce the energy requirements of the process, researchers have added compounds called photosensitizers to the polymer powders. These materials, such as carbon nanotubes, carbon black and graphene, absorb light much more strongly than the polymers and transfer heat to them, enabling the use of cheaper, lower-power lasers. However, the carbon-based photosensitizers can only produce printed objects that are gray or black. Gerasimos Konstantatos, Romain Quidant and their coworkers at The Institute of Photonic Sciences (IFCO) wanted to find a photosensitizer that would enable color printing by the SLS method.

The researchers designed gold nanorods to strongly absorb in the near-infrared region of the spectrum while being almost transparent to visible light. They coated them with silica and then mixed them with polyamide powders to print 3D objects. They found that the gold nanorods were much better at converting light from the laser to heat than carbon black, the industry standard. Also, the new photosensitizers could produce much whiter and — when mixed with dyes — brightly colored 3D objects. Importantly, the materials are cost-effective for large-scale production. The researchers have filed several patent applications related to the new technology.

The findings are reported in the ACS journal Nano Letters.

Source: https://www.acs.org/

2D Material Revolutionizes Solar Fuel Generation

Following the isolation of graphene in 2004, a race began to synthesize new two-dimensional materials. 2D materials are single-layer substances with a thickness of between one atom and a few nanometers (billionths of a meter). They have unique properties linked to their reduced dimensionality and play a key role in the development of nanotechnology and nanoengineering.

An international group of researchers including Brazilian scientists affiliated with the University of Campinas (UNICAMP) have succeeded in producing a new material with these characteristics.

The researchers extracted a 2D material they call hematene from ordinary iron ore like that mined in many parts of the world, including Brazil. The material is only three atoms thick and is thought to have enhanced photocatalytic properties.

International group of researchers including Brazilian scientists obtain new material from iron ore with application as a photocatalyst

The research was conducted at the Center for Computational Engineering and Sciences (CCES), one of the Research, Innovation and Dissemination Centers (RIDCs) funded by FAPESP, and during a research internship abroad that was also supported by FAPESP via a specific scholarship.

Douglas Soares Galvão, a researcher at CCES and one of the authors of the study, told Agência FAPESP about the discovery. “The material we synthesized can act as a photocatalyst to split water into hydrogen and oxygen, so that electricity can be generated from hydrogen, for example, as well as having several other potential applications,” he said.

The new material was exfoliated from hematite, one of the most common minerals on earth and the main source of iron, which is the cheapest metal, used in many products and above all to make steel.

Unlike carbon and its 2D form graphene, hematite is a non-van der Waals material, meaning it is held together by 3D bonding networks rather than by nonchemical and comparatively weaker atomic van der Waals interactions, which are noncovalent (they do not involve the sharing of one or more pairs of electrons by the atoms that participate in the bond).

Because it is a naturally occurring mineral, has highly oriented, large crystals and is a non-van der Waals material, the researchers believe that hematite is an excellent precursor for the exfoliation of novel 2D materials.

Most of the 2D materials synthesized to date were derived from samples of van der Waals solids. Non-van der Waals 2D materials with highly ordered atomic layers and large grains are still rare,” Galvão said.

Hematene was synthesized by the liquid-phase exfoliation of hematite ore in an organic solvent, N,N-dimethylformamide (DMF). Transmission electron microscopy confirmed the exfoliation and formation of hematene in single sheets with a thickness of only three iron and oxygen atoms (monolayer) and in randomly stacked sheets (bilayer).

The innovation is described in an article published in Nature Nanotechnology.

Source: http://agencia.fapesp.br/

 

Super Conductive Graphene Will Boost Solar Technology

In 2010, the Nobel Prize in Physics went to the discoverers of graphene. A single layer of carbon atoms, graphene possesses properties that are ideal for a host of applications. Among researchers, graphene has been the hottest material for a decade. In 2017 alone, more than 30,000 research papers on graphene were published worldwide.

Now, two researchers from the University of Kansas (KU), Professor Hui Zhao and graduate student Samuel Lane, both of the Department of Physics & Astronomy, have connected a graphene layer with two other atomic layers (molybdenum diselenide and tungsten disulfide) thereby extending the lifetime of excited electrons in graphene by several hundred times. The finding will be published on Nano Futures, a newly launched and highly selective journal.

The work at KU may speed development of ultrathin and flexible solar cells with high efficiency.

For electronic and optoelectronic applications, graphene has excellent charge transport property. According to the researchers, electrons move in graphene at a speed of 1/30 of the speed of light — much faster than other materials. This might suggest that graphene can be used for solar cells, which convert energy from sunlight to electricity. But graphene has a major drawback that hinders such applications – its ultrashort lifetime of excited electrons (that is, the time an electron stays mobile) of only about one picosecond (one-millionth of one-millionth of a second, or 10-12 second).

These excited electrons are like students who stand up from their seats — after an energy drink, for example, which activates students like sunlight activates electrons,” Zhao said. “The energized students move freely in the classroom — like human electric current.

The KU researcher said one of the biggest challenges to achieving high efficiency in solar cells with graphene as the working material is that liberated electrons — or, the standing students — have a strong tendency to losing their energy and become immobile, like students sitting back down.

The number of electrons, or students from our example, who can contribute to the current is determined by the average time they can stay mobile after they are liberated by light,” explains Zhao. “In graphene, an electron stays free for only one picosecond. This is too short for accumulating a large number of mobile electrons. This is an intrinsic property of graphene and has been a big limiting factor for applying this material in photovoltaic or photo-sensing devices. In other words, although electrons in graphene can become mobile by light excitation and can move quickly, they only stay mobile too short a time to contribute to electricity.”

In their new paper, Zhao and Lane report this issue could be solved by using the so-called van der Waals materials. The principle of their approach is rather simple to understand. “We basically took the chairs away from the standing students so that they have nowhere to sit,” Zhao said. “This forces the electrons to stay mobile for a time that is several hundred times longer than before.”

To achieve this goal, working in KU’s Ultrafast Laser Lab, they designed a tri-layer material by putting single layers of MoSe2, WS2 and graphene on top of each other.

Source: https://news.ku.edu/