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Two-dimensional van der Waals materials have been the focus of work by numerous research groups for some time. Standing just a few atomic layers thick, these structures are produced in the laboratory by combining atom-thick layers of different materials (in a process referred to as “atomic Lego”).

Interactions between the stacked layers allow the heterostructures to exhibit properties that the individual constituents lack.

Continue reading “Coupling of electron-hole pairs” »

The adoption rate of fuel cells has increased owing to the rising need for clean energy.

In a research that could jump-start the work on a range of technologies, including fuel cells, which are key to storing solar and wind energy, MIT researchers have found a simple way to significantly increase the lifetimes of fuel cells and other devices – changing the pH of the system.

Fuel/electrolysis cells made of materials known as solid metal oxides are in interest for several reasons. In electrolysis mode, they are very efficient at converting electricity from a renewable source into a storable fuel like hydrogen or methane. This storable fuel can be used in the fuel cell mode to generate electricity when the sun is not shining, or the wind isn’t blowing.

Two-dimensional van der Waals materials have been the focus of work by numerous research groups for some time. Standing just a few atomic layers thick, these structures are produced in the laboratory by combining atom-thick layers of different materials (in a process referred to as “atomic Lego”). Interactions between the stacked layers allow the heterostructures to exhibit properties that the individual constituents lack.

Two-layered molybdenum disulfide is one such van der Waals material, in which electrons can be excited using a suitable experimental setup. These negatively charged particles then leave their position in the valence band, leaving behind a positively charged hole, and enter the conduction band. Given the different charges of electrons and holes, the two are attracted to one another and form what is known as a quasiparticle. The latter is also referred to as an electron-hole pair, or exciton, and can move freely within the material.

In two-layered molybdenum disulfide, excitation with light produces two different types of electron-hole pairs: intralayer pairs, in which the electron and hole are localized in the same layer of the material, and interlayer pairs, whose hole and electron are located in different layers and are therefore spatially separate from one another.

A new theoretical model explains why chiral molecules favour transporting electrons in one spin state over the other, providing a quantitative fit to experiments.¹

Many current-carrying materials show no bias towards the spin state of the electrons they transport, allowing spin-up and spin-down species to flow in equal numbers. But chiral molecules can be more discriminating, offering easier passage to one spin orientation than to its counterpart.

A multidisciplinary team of Indiana University researchers have discovered that the motion of chromatin, the material that DNA is made of, can help facilitate effective repair of DNA damage in the human nucleus — a finding that could lead to improved cancer diagnosis and treatment. Their findings were recently published in the Proceedings of the National Academy of Sciences.

DNA damage happens naturally in human body and most of the damage can be repaired by the cell itself. However, unsuccessful repair could lead to cancer.

“DNA in the nucleus is always moving, not static. The motion of its high-order complex, chromatin, has a direct role in influencing DNA repair,” said Jing Liu, an assistant professor of physics in the School of Science at IUPUI. “In yeast, past research shows that DNA damage promotes chromatin motion, and the high mobility of it also facilitates the DNA repair. However, in human cells this relationship is more complicated.”

Astronomers may soon have the answer to what is perhaps the greatest mystery of modern science –is dark energy a uniform force across space and time, or has its strength evolved over eons?

The universe is not only expanding – it is accelerating outward, driven by what is commonly referred to as “dark energy.” The term is a poetic analogy to the label for dark matter, the mysterious material that dominates the matter in the Universe and that really is dark because it does not radiate light (it reveals itself via its gravitational influence on galaxies).

A new technique makes it possible to capture videos of single atoms “swimming” at the interface between a solid and a liquid for the first time. The approach uses stacks of two-dimensional materials to trap the liquid, making it compatible with characterization techniques that usually require vacuum conditions. It could enable researchers to better understand how atoms behave at these interfaces, which play a crucial role in devices such as batteries, catalytic systems and separation membranes.

Several techniques exist to image single atoms, including scanning tunnelling microscopy (STM) and transmission electron microscopy (TEM). However, they involve exposing atoms on the surface of the sample to a high-vacuum environment, which can change the material’s structure. Techniques that do not require a vacuum, meanwhile, are either lower-resolution or only work for short time periods, meaning that the atoms’ motion cannot be captured on video.

Researchers led by materials scientists Sarah Haigh of the University of Manchester’s National Graphene Institute (NGI) have now developed a new approach that enables them to track the motion of single atoms on a surface when that surface is surrounded by liquid. They showed that the atoms behave very differently under these circumstances than they do in vacuum. “This is crucial,” explains Haigh, “since we want to understand atomic behaviour for realistic reaction/environmental conditions that the material will experience in use – for example, in a battery, supercapacitor and membrane reaction vessels.”

Breakthroughs in modern microelectronics depend on understanding and manipulating the movement of electrons in metal. Reducing the thickness of metal sheets to the order of nanometers can enable exquisite control over how the metal’s electrons move. By doing so, one can impart properties that aren’t seen in bulk metals, such as ultrafast conduction of electricity. Now, researchers from Osaka University and collaborating partners have synthesized a novel class of nanostructured superlattices. This study enables an unusually high degree of control over the movement of electrons within metal semiconductors, which promises to enhance the functionality of everyday technologies.

Precisely tuning the architecture of metal nanosheets, and thus facilitating advanced microelectronic functionalities, remains an ongoing line of work worldwide. In fact, several Nobel prizes have been awarded on this topic. Researchers conventionally synthesize nanostructured superlattices—regularly alternating layers of metals, sandwiched together—from materials of the same dimension; for example, sandwiched 2D sheets. A key aspect of the present researchers’ work is its facile fabrication of hetero-dimensional superlattices; for example, 1D nanoparticle chains sandwiched within 2D nanosheets.

“Nanoscale hetero-dimensional superlattices are typically challenging to prepare, but can exhibit valuable physical properties, such as anisotropic electrical conductivity,” explains Yung-Chang Lin, senior author. “We developed a versatile means of preparing such structures, and in so doing we will inspire synthesis of a wide range of custom superstructures.”

Thermoelectric devices convert thermal energy into electricity by generating a voltage from the difference in temperature between the hot and cold parts of a device.

To better understand how the conversion process occurs at the atomic scale, researchers used neutrons to study single crystals of tin sulfide and tin selenide. They measured changes that were dependent on temperature.

The measurements revealed a strong correlation between changes in the structure at certain temperatures and the frequency of atomic vibrations (phonons). This relationship affects how the materials conduct heat.