As Francis Crick, one of Britain’s great scientists, once said: “If you want to understand function, study structure.” Within the realm of chemical physics, a clear example of this is the two forms of carbon — diamond and graphite. While they differ only in the atomic arrangement of atoms of a single element, their properties are quite different.

Differences between the properties of seemingly similar elements of a “family” can be intriguing. Carbon, silicon, germanium, tin and lead are all part of a family that share the same structure of their outermost electrons, yet range from acting as insulators (carbon) to semiconductors (silicon and germanium) to metals (tin and lead).

Is it possible to understand these and other trends within element families? In an article this week in The Journal of Chemical Physics, from AIP Publishing, a group of researchers from Peter Grünberg Institute (PGI) in Germany, and Tampere University of Technology and Aalto University in Finland, describe their work probing the relationship between the structure (arrangement of atoms) and function (physical properties) of a liquid metal form of the element bismuth.

“There are relatively few — less than 100 — stable elements, which means that their trends are often easier to discern than for those of alloys and compounds of several elements,” said Robert O. Jones, a scientist at PGI.

The group’s present work was motivated largely by the availability of high-quality experimental data — inelastic x-ray scattering (IXS) and neutron diffraction — and the opportunity to compare it with results for other liquids of the Group 15 nitrogen family (phosphorus, arsenic, antimony and bismuth). Phosphorus seems to have two liquid phases, and the amorphous form of antimony obtained by cooling the liquid crystallizes spontaneously and explosively.

Their structural studies use extensive numerical simulations run on one of the world’s most powerful supercomputers, JUQUEEN, in Jülich, Germany.

“We’re studying the motion of more than 500 atoms at specified temperatures to determine the forces on each atom and the total energy using density functional calculations,” Jones explained. “This scheme, for which Walter Kohn was awarded the 1998 Nobel Prize in chemistry, doesn’t involve adjustable parameters and has given valuable predictions in many contexts.” While density functional theory is in principle exact, it is necessary to utilize an approximate functional.

The positions and velocities of each atom, for example, are “stored at each step of a ‘molecular dynamics’ simulation, and we use this information to determine quantities that can be compared with experiment,” he continued. “It’s important to note that some quantities that are given directly by the simulation, such as the positions of the atoms, can only be inferred indirectly from the experiment, so that the two aspects are truly complementary.”

One of their most surprising and pleasing results was “the excellent agreement with recent IXS results,” Jones said. “One of the experimentalists involved noted that the agreement of our results with the IXS ‘is really quite beautiful,’ so that even small differences could provide additional information. In our experience, it’s unusual to find such detailed agreement.”

In terms of applications, the group’s work “provides further confirmation that simulations and experiments complement each other and that the level of agreement can be remarkably good — even for ‘real’ materials,” Jones pointed out. “However, it also shows that extensive, expensive, and time-consuming simulations are essential if detailed agreement is to be achieved.”

Jones and his colleagues have extended their approach to even longer simulations in liquid antimony at eight different temperatures, with the goal of understanding the “explosive” nature of crystallization in amorphous antimony (Sb).

“We’ve also run simulations of the crystallization of amorphous phase change materials over the timescale — up to 8 nanoseconds — that is physically relevant for DWD-RW and other optical storage materials,” he added, emphasizing that these types of simulations on computers today typically require many months. “They show, however, just how valuable they can be, and the prospects with coming generations of computers — with even better optimized algorithms — are very bright.”

The prospects of applications within other areas of materials science are extremely good, but the group is now turning its attention to memory materials of a different type — for which the formation and disappearance of a conducting bridge (a metallic filament) in a solid electrolyte between two electrodes could be the basis of future storage materials.

“Details of the mechanism of bridge formation are the subject of speculation, and we hope to provide insight into what really happens,” Jones said.

Researchers from the Moscow Institute of Physics and Technology (MIPT), Technological Institute for Superhard and Novel Carbon Materials (TISNCM), Lomonosov Moscow State University (MSU), and the National University of Science and Technology MISiS have shown that an ultrastrong material can be produced by “fusing” multiwall carbon nanotubes together. The research findings have been published in Applied Physics Letters.

According to the scientists, a material of that kind is strong enough to endure very harsh conditions, making it useful for applications in the aerospace industry, among others.

The authors of the paper performed a series of experiments to study the effect of high pressure on multiwall carbon nanotubes (MWCNTs). In addition, they simulated nanotube behavior in high pressure cells, finding that the shear stress strain in the outer walls of the MWCNTs causes them to connect to each other as a result of the structural rearrangements on their outer surfaces. The inner concentric nanotubes, however, retain their structure completely: they simply shrink under pressure and restore their shape once the pressure is released.

The main feature of this study is that it demonstrates the possibility of covalent intertube bonding giving rise to interconnected (polymerized) multiwall nanotubes; these nanotubes being cheaper to produce than their single-wall counterparts.

“These connections between the nanotubes only affect the structure of the outer walls, whereas the inner layers remain intact. This allows us to retain the remarkable durability of the original nanotubes,” comments Prof. Mikhail Y. Popov of the Department of Molecular and Chemical Physics at MIPT, who heads the Laboratory of Functional Nanomaterials at TISNCM.

A shear diamond anvil cell (SDAC) was used for the pressure treatment of the nanotubes. The experiments were performed at pressures of up to 55 GPa, which is 500 times the water pressure at the bottom of the Mariana Trench. The cell consists of two diamonds, between which samples of a material can be compressed. The SDAC is different from other cell types in that it can apply a controlled shear deformation to the material by rotating one of the anvils. The sample in an SDAC is thus subjected to pressure that has both a hydrostatic and a shear component, i.e., the stress is applied both at a normal and parallel to its surface. Using computer simulations, the scientists found that these two types of stress affect the structure of the tubes in different ways. The hydrostatic pressure component alters the geometry of the nanotube walls in a complex manner, whereas the shear stress component induces the formation of sp³-hybridized amorphized regions on the outer walls, connecting them to the neighboring carbon tubes by means of covalent bonding. When the stress is removed, the shape of the inner layers of the connected multiwall tubes is restored.

Carbon nanotubes have a wide range of commercial applications by virtue of their unique mechanical, thermal and conduction properties. They are used in batteries and accumulators, tablet and smartphone touch screens, solar cells, antistatic coatings, and composite frames in electronics.

Adding hydrogen to graphene could improve its future applicability in the semiconductor industry, when silicon leaves off. Researchers at the Center for Multidimensional Carbon Materials (CMCM), within the Institute for Basic Science (IBS) have recently gained further insight into this chemical reaction. Published in Journal of the American Chemical Society, these findings extend the knowledge of the fundamental chemistry of graphene and bring scientists perhaps closer to realizing new graphene-based materials.

The images show a graphene flake before (a), two minutes (b), and eight minutes (c), after exposure to a solution of lithium and liquid ammonia (Birch-type reaction). Graphene gets gradually hydrogenated starting from the edges. (Reprinted with permission from Zhang X. et al., JACS, Copyright 2016 American Chemical Society) Credit: IBS

Understanding how graphene can chemically react with a variety of chemicals will increase its utility. Indeed, graphene has superior conductivity properties, but it cannot be directly used as an alternative to silicon in semiconductor electronics because it does not have a bandgap, that is, its electrons can move without climbing any energy barrier. Hydrogenation of graphene opens a bandgap in graphene, so that it might serve as a semiconductor component in new devices.

While other reports describe the hydrogenation of bulk materials, this study focuses on hydrogenation of single and few-layers thick graphene. IBS scientists used a reaction based on lithium dissolved in ammonia, called the “Birch-type reaction”, to introduce hydrogen onto graphene through the formation of C-H bonds.

The research team discovered that hydrogenation proceeds rapidly over the entire surface of single-layer graphene, while it proceeds slowly and from the edges in few-layer graphene. They also showed that defects or edges are actually necessary for the reaction to occur under the conditions used, because pristine graphene with the edges covered in gold does not undergo hydrogenation.

Using bilayer and trilayer graphene, IBS scientists also discovered that the reagents can pass between the layers, and hydrogenate each layer equally well. Finally, the scientists found that the hydrogenation significantly changed the optical and electric properties of the graphene.

“A primary goal of our Center is to undertake fundamental studies about reactions involving carbon materials. By building a deep understanding of the chemistry of single-layer graphene and a few layer graphene, I am confident that many new applications of chemically functionalized graphenes could be possible, in electronics, photonics, optoelectronics, sensors, composites, and other areas,” notes Rodney Ruoff, corresponding author of this paper, CMCM director, and UNIST Distinguished Professor at the Ulsan National Institute of Science and Technology (UNIST).

An electric current will not only heat a hybrid metamaterial, but will also trigger it to change state and fade into the background like a chameleon in what may be the proof-of-concept of the first controllable metamaterial device, or metadevice, according to a team of engineers.

“Previous metamaterials work focused mainly on cloaking objects so they were invisible in the radio frequency or other specific frequencies,” said Douglas H. Werner, John L. and Genevieve H. McCain Chair Professor of electrical engineering, Penn State. “Here we are not trying to make something disappear, but to make it blend in with the background like a chameleon and we are working in optical wavelengths, specifically in the infrared.”

Metamaterials are synthetic, composite materials that possess qualities not seen in natural materials. These composites derive their functionality by their internal structure rather than by their chemical composition. Existing metamaterials have unusual electromagnetic or acoustic properties. Metadevices take metamaterials and do something of interest or value as any device does.

“The key to this metamaterial and metadevice is vanadium dioxide, a phase change crystal with a phase transition that is triggered by temperatures created by an electric current,” said Lei Kang, research associate in electrical engineering, Penn State.

The metamaterial is composed of a base layer of gold thick enough so that light cannot pass through it. A thin layer of aluminum dioxide separates the gold from the active vanadium dioxide layer. Another layer of aluminum dioxide separates the vanadium from a gold-patterned layer that is attached to an external electric source. The geometry of the patterned mesh screen controls the functional wavelength range. The amount of current flowing through the device controls the Joule heating effect, the heating due to resistance.

“The proposed metadevice integrated with novel transition materials represents a major step forward by providing a universal approach to creating self-sufficient and highly versatile nanophotonic systems,” the researchers said in today’s (Oct. 27) issue of Nature Communications.

As a proof of concept, the researchers created a .035 inch by .02 inch device and cut the letters PSU into the gold mesh layer so the vanadium dioxide showed through. The researchers photographed the device using an infrared camera at 2.67 microns. Without any current flowing through the device, the PSU stands out as highly reflective. With a current of 2.03 amps, the PSU fades into the background and becomes invisible, while at 2.20 amps, the PSU is clearly visible but the background has become highly reflective.

The response of the vanadium dioxide is tunable by altering the current flowing through the device. According to the researchers, vanadium dioxide can change state very rapidly and it is the device configuration that limits the tuning.

Thermoelectric materials, which can directly convert thermal energy into electrical energy (Seebeck effect), can be effectively used for the development of a clean and environmentally compatible power-generation technology.

Picture of the synthesized bulk CaMgSi thermoelectric material through the procedure developed in this study. CREDIT: TOYOHASHI UNIVERSITY OF TECHNOLOGY.

However, these materials are not commonly used for practical applications as they mostly include toxic and/or expensive elements.

Recently, researchers at the Materials Function Control Laboratory at the Toyohashi University of Technology and the Nagoya Institute of Technology have successfully synthesized a new thermoelectric material, CaMgSi, which is an intermetallic compound. The key to this development was the synthesis procedure; bulk CaMgSi intermetallic compound was synthesized by combining mechanical ball-milling (MM) and pulse current sintering (PCS) processes.

“Appearance of thermoelectric property in the intermetallic compound, CaMgSi, has been predicted by both theoretical and experimental studies”, explain the researchers of this work, Nobufumi Miyazaki and Nozomu Adachi. ” However, the biggest issue in front of us was the synthesis of thermoelectric CaMgSi of optimal size “, they continued. In general, alloys are produced by mixing the constituent elements in their molten forms. However, when a temperature is raised up to the melting temperature of Si, Mg vapors; liquids of Ca, Mg, and Si cannot exists at same time.

Associate Professor Yoshikazu Todaka says “To overcome the aforementioned problem, we chose the mechanical ball milling process to mix the elements homogeneously, without melting, and then a chemical reaction between Ca, Mg, and Si was induced using the pulse current sintering process”.

Consequently, the intermetallic compound, CaMgSi, with sufficient size was synthesized. The thermoelectric property of the synthesized CaMgSi exhibited a performance comparable to that of the previously developed Mg-based thermoelectric materials. It is expected that an addition of a fourth element to CaMgSi renders it with superior thermoelectric properties. Interestingly, they found that the novel thermoelectric can exhibit both n- and p-type conductivity with a slight change in the composition of CaMgSi. Such a property for the material is very significant for its application in power-generation modules.

The new thermoelectric material synthesized in this study is composed of lightweight elements, and has a low density of 2.2 g/cm3. Therefore, one of the possible applications of the material is in automobiles to utilize waste heat emitted from engines. These findings could contribute to the development of green energy technology.