Crystal with a twist: scientists grow spiral…

Crystal with a twist: scientists grow spiraling new material

With a simple twist of the fingers, one can create a beautiful spiral from a deck of cards. In the same way, scientists at the University of California, Berkeley, and Lawrence Berkeley National Laboratory (Berkeley Lab) have created new inorganic crystals made of stacks of atomically thin sheets that unexpectedly spiral like a nanoscale card deck.

Their surprising structures, reported in a new study appearing online Wednesday, June 20, in the journal Nature, may yield unique optical, electronic and thermal properties, including superconductivity, the researchers say.

These helical crystals are made of stacked layers of germanium sulfide, a semiconductor material that, like graphene, readily forms sheets that are only a few atoms or even a single atom thick. Such “nanosheets” are usually referred to as “2-D materials.”

“No one expected 2-D materials to grow in such a way. It’s like a surprise gift,” said Jie Yao, an assistant professor of materials science and engineering at UC Berkeley. “We believe that it may bring great opportunities for materials research.”

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Exfoliating 2D materials.


I finally got to try my hand at sample preparation these past 2 weeks. Materials can exhibit striking electronic ​and mechanical properties when stripped to few layers / monolayer. Graphene (single layer graphite) is one popular example, first to be discovered among the family of 2D materials in 2004. But the search for conducting 2D materials has now extended beyond graphene.

I tried to exfoliate a single layer of molybdenum disulfide (MoS2), one type of transition metal dichalcogenides (TMD), under the guidance of my lab colleague. The properties of these TMD in the form of 2D sheets have application in more efficient electronics, semiconductors, solar cells, and touch screen display panels, just to name a few.

Exfoliation in this context is actually applying the material of interest onto a scotch tape, then folding and opening the tape several times until the flakes on it are as thin as possible. Judging with my naked eyes, this usually just means for the flakes to not appear as shiny (for metal compounds) anymore. Then, I will have to transfer the flakes on the Scotch tape to a tiny piece of transparent gel (PDMS gel). The procedure is pretty simple, but the flakes produced are not guaranteed to be thin enough. Through my many trials, I find random thickness of flakes distributed all over the gel under optical microscope. And among them, we need to locate the thinnest of all that could potentially be a monolayer.

Below is the flake that I suspect to be a single layer of MoS2 at different magnification level. ​ ​Can you find it in the last picture?

Here is the relative size of the PDMS gel on which all these flakes lie.

This also happened when I managed to transfer plenty of flakes onto the PDMS gel, while most of the time I do not get enough to choose from when observed under the microscope. After identifying this flake, I had to transfer it onto another substrate (such as a silicon nitride grid), before I can put it into an electron microscope that offer higher resolution through electrons scattering method. The transfer patience takes some patience, as pressing or lifting the gel too quickly may break the hard-found flake. So it is just a lot of going back and forth between the sample preparation table and microscope to get one nice specimen. But it is important to get through this process for us to get to the characterization part, which is where all the fun stuff about science comes in!


Special nanotubes could improve solar power …

Special nanotubes could improve solar power and imaging technology

Physicists have discovered a novel kind of nanotube that generates current in the presence of light. Devices such as optical sensors and infrared imaging chips are likely applications, which could be useful in fields such as automated transport and astronomy. In future, if the effect can be magnified and the technology scaled up, it could lead to high-efficiency solar power devices.

Working with an international team of physicists, University of Tokyo Professor Yoshihiro Iwasa was exploring possible functions of a special semiconductor nanotube when he had a lightbulb moment. He took this proverbial lightbulb (which was in reality a laser) and shone it on the nanotube to discover something enlightening. Certain wavelengths and intensities of light induced a current in the sample—this is called the photovoltaic effect. There are several photovoltaic materials, but the nature and behavior of this nanotube is cause for excitement.

“Essentially our research material generates electricity like solar panels, but in a different way,” said Iwasa. “Together with Dr. Yijin Zhang from the Max Planck Institute for Solid State Research in Germany, we demonstrated for the first time nanomaterials could overcome an obstacle that will soon limit current solar technology. For now solar panels are as good as they can be, but our technology could improve upon that.”

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Opposite piezoresistant effects of rhenium d…

Opposite piezoresistant effects of rhenium disulfide in two principle directions

Using optical and electrical measurements, a two-dimensional anisotropic crystal of rhenium disulfide was found to show opposite piezoresistant effects along two principle axes, i.e. positive along one axis and negative along another. Piezoresistance was also reversible; it appeared upon application of a strain, but the relative resistance returned to its original value on strain removal. This novel finding is expected to lead to wide application of rhenium disulfide.

Upon application of mechanical stress such as pressure on crystals and some kinds of ceramics, a surface charge proportional to the applied strain is induced; this phenomenon is called the piezoelectric effect. The piezoelectric effect has been known since the mid-18th century and has found use, for example, in the ignition device of cigarette lighters. Today it is widely applied in sensors, actuators, etc. On the other hand, when mechanical strain is applied to semiconducting materials, some of them show a change in electrical resistance, called the piezoresistive effect. Materials showing the piezoresistive effect are used in pressure sensors, strain sensors etc.

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Adding a carbon atom transforms 2-D semicond…

Adding a carbon atom transforms 2-D semiconducting material

A technique that introduces carbon-hydrogen molecules into a single atomic layer of the semiconducting material tungsten disulfide dramatically changes the electronic properties of the material, according to Penn State researchers at Penn State who say they can create new types of components for energy-efficient photoelectric devices and electronic circuits with this material.

“We have successfully introduced the carbon species into the monolayer of the semiconducting material,” said Fu Zhang, doctoral student in materials science and engineering lead author of a paper published online today in Science Advances.

Prior to doping—adding carbon—the semiconductor, a transition metal dichalcogenide (TMD), was n-type—electron conducting. After substituting carbon atoms for sulfur atoms, the one-atom-thick material developed a bipolar effect, a p-type—hole—branch, and an n-type branch. This resulted in an ambipolar semiconductor.

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New Flatland material: Physicists obtain qua…

New Flatland material: Physicists obtain quasi-2D gold

Researchers from the MIPT Center for Photonics and 2-D Materials have synthesized a quasi-2-D gold film, revealing how materials not usually classified as two-dimensional can form atomically thin layers. Published in Advanced Materials Interfaces, the study shows that by using monolayer molybdenum disulfide as an adhesion layer, quasi-2-D gold can be deposited on an arbitrary surface. The team says the resulting ultrathin gold films, which are only several nanometers thick, conduct electricity very well and are useful for flexible and transparent electronics. The finding might contribute to a new class of optical metamaterials with the unique potential to control light.

The first 2-D material discovered, graphene is a one-atom-thick sheet of carbon atoms in a honeycomb formation. Its synthesis and the study of its exciting properties have given rise to an entirely new field of science and technology. The groundbreaking experiments regarding graphene earned MIPT graduates Andre Geim and Kostya Novoselov the 2010 Nobel Prize in physics.

Since then, more than 100 graphene cousins have been discovered. Their intriguing properties had applications in biomedicine, electronics and the aerospace industry. These materials belong to the class of layered crystals whose layers are weakly bound to one another but have strong internal integrity. For example, the graphite in a pencil is essentially many stacked-up layers of graphene bound so weakly that Geim and Novoselov famously used sticky tape to peel them off.

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2D Materials: Molybdenum disulfide

2D Materials: Molybdenum disulfide

Though molybdenum disulfide (MoS2) is also used in bulk form, it is perhaps the most well known two-dimensional transition metal dichalcogenide. As a bulk material its most common application is likely that of a solid state lubricant, but when MoS2 is produced in monolayer form it transitions from an indirect bandgap semiconductor to a direct bandbap semiconductor.

This transition in its properties – specifically its usage as a semiconductor – is what makes MoS2 stand apart from other common 2D materials such as graphene. Because of it’s structure (see the image above), monolayers of MoS2 are still three atoms thick, not quite as thin as flatter structures such as graphene. MoS2 is commonly produced through exfoliation.

Current applications of mono- or multi-layer MoS2 include field effect transistors, memresistors, electrolysis, and photovoltaics and photodetectors. However, it is still a relatively new material and applications are limited by the difficultly in dealing with such small materials.

Sources/Further Reading: (1 – image 1) (2 – image 2) (3 – images 3 and 4) ( 4 )

New discovery makes fast-charging, better pe…

New discovery makes fast-charging, better performing lithium-ion batteries possible

Materials could boost battery performance in consumer electronics, electric vehicles, and more

Creating a lithium-ion battery that can charge in a matter of minutes but still operate at a high capacity is possible, according to research from Rensselaer Polytechnic Institute just published in Nature Communications. This development has the potential to improve battery performance for consumer electronics, solar grid storage, and electric vehicles.

A lithium-ion battery charges and discharges as lithium ions move between two electrodes, called an anode and a cathode. In a traditional lithium-ion battery, the anode is made of graphite, while the cathode is composed of lithium cobalt oxide.

These materials perform well together, which is why lithium-ion batteries have become increasingly popular, but researchers at Rensselaer believe the function can be enhanced further.

“The way to make batteries better is to improve the materials used for the electrodes,” said Nikhil Koratkar, professor of mechanical, aerospace, and nuclear engineering at Rensselaer, and corresponding author of the paper. “What we are trying to do is make lithium-ion technology even better in performance.”

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Catalyst research for solar fuels: Amorphous…

Catalyst research for solar fuels: Amorphous molybdenum sulfide works best

Sunlight not only can be used to generate electricity, but also hydrogen. Hydrogen is a climate-neutral fuel that stores energy chemically and releases it again when needed, either directly via combustion (where only water is produced) or as electrical energy in a fuel cell. But to produce hydrogen from sunlight, catalysts are needed that accelerate the electrolytic splitting of water into oxygen and hydrogen.

One particularly interesting class of catalysis materials for hydrogen generation are the molybdenum sulphides (MoSx). They are considerably cheaper than catalysts made of platinum or ruthenium. In a comprehensive study, a team led by Prof. Dr. Sebastian Fiechter at the HZB Institute for Solar Fuels has now produced and investigated a series of molybdenum sulphide layers. The samples were deposited at different temperatures on a substrate, from room temperature to 500 °C. The morphology and structure of the layers change with increasing deposition temperature (see transmission electron microscopy (TEM) images). While crystalline regions are formed at higher temperatures, molybdenum sulphide deposited at room temperature is amorphous. It is precisely this amorphous molybdenum sulphide deposited at room temperature that has the highest catalytic activity.

A catalyst made of amorphous molybdenum sulphide not only releases hydrogen during electrolysis of water, but also hydrogen sulphide gas in the initial phase. The sulphur for this had to come from the catalyst material itself, and astonishingly – this process improves the catalytic activity of the molybdenum sulphide considerably. Fiechter and his team have now taken a close look at this and are proposing an explanation for their findings.

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