As new methods have become available for understanding and manipulating matter at its most fundamental levels, researchers working in the interdisciplinary field of materials science have been increasingly successful in synthesizing new kinds of materials. Often the goal of researchers in the field is to design materials that incorporate properties that can be useful for performing specific functions. Such materials can, for example, be more chemically stable or resistant to physical breakage, have advantageous electromagnetic characteristics, or react in predictable ways to specific environmental conditions.

Artist’s rendering of organic molecules adsorbing on a silicon surface. Credit: Image: Aaron Beller

Dr. Ralf Tonner and his research group at the University of Marburg are addressing the challenge of designing functional materials in an unusual way — by applying approaches based on computational chemistry. Using computing resources at the High-Performance Computing Center Stuttgart (HLRS), one of three German national supercomputing centers that make up the Gauss Centre for Supercomputing, Tonner models phenomena that happen at the atomic and subatomic scale to understand how factors such as molecular structure, electronic properties, chemical bonding, and interactions among atoms affect a material’s behavior.

“When you study how, for example, a molecule adsorbs on a surface,” Tonner explains, “other scientists will often describe that phenomenon with methods from physics, solid state theory, or band structures. We think it can also be very helpful to ask, how would a chemist look at what’s happening here?” From this perspective, Tonner is interested in exploring whether understanding chemical reactions — how atoms bond together into molecules and react when brought into contact with one another — can offer new and useful insights.

In a new publication in WIREs Computational Molecular Science, Tonner and his collaborator Lisa Pecher highlight the ability of computational chemistry approaches using high-performance computing to reveal interesting phenomena that occur between organic molecules and surfaces. They also demonstrate more generally how these interactions can be understood with respect to the molecular and solid state world. The knowledge they gained could be useful in designing patterned surfaces, a goal of scientists working on the next generation of more powerful, more efficient semiconductors.

Bringing computation to chemistry

Atoms bond together to form molecules and compounds when they approach one another and then trade or share electrons orbiting around their nuclei. The specific atoms involved, the physical shapes that the molecules take, their energetic properties, and how they interact with other nearby molecules are all properties that give a compound its unique properties. Such characteristics can determine whether compounds are likely to remain stable, or whether stresses such as changes in temperature or pressure could affect their reactivity.

Tonner uses a computational approach called density functional theory (DFT) to explore such characteristics at the quantum scale; that is, at the scale where Newtonian mechanics becomes replaced by the much stranger world of quantum mechanics (at distances of less than 100 nanometers). DFT uses information about variations in the density of electrons within a molecule — a quantity that can also be experimentally measured using a widely used technology called x-ray diffraction — to derive the energy of the system. This, in turn, enables the researchers to infer interactions among nuclei as well as interactions between electrons and nuclei, factors that are critical to understanding chemical bonds and reactions.

DFT can provide useful, though static, information about the energy profiles of the compounds they study. To gain a better understanding of how systems of molecules actually behave when interacting with a surface, Tonner’s group also uses high-performance computing at HLRS to perform molecular dynamics simulations. Here, the scientists look at how the system of molecules develops over time, at the level of atoms and electrons and at time scales of picoseconds (one picosecond is one trillionth of a second).

Such calculations typically use 2,000-3,000 computing cores, running on a problem for a week, and Tonner has been budgeted approximately 30 million CPU hours at HLRS for the current two-year funding cycle.

“Increasing computing power has made it possible for computational chemistry and quantum chemistry to describe real molecular systems. Just 15-20 years ago, people could only look at small molecules and had to make rather strong approximations,” Tonner explains. “In the last few years, the computational chemistry and solid state theory communities have solved the problem of parallelizing their codes to operate efficiently on high-performance computing systems. As supercomputers get bigger, we anticipate being able to develop increasingly realistic models for experimental systems in materials science.”

Toward light-based semiconductors

One area in which Tonner is currently using computational chemistry is to study ways to improve silicon for use in new kinds of semiconductors. This problem has gained urgency in recent years, as it has become clear that the microelectronics industry is reaching the limits of its ability to improve semiconductors using silicon alone.

As Tonner and experimental colleagues report in a recent paper in the Beilstein Journal of Organic Chemisty, functionalizing silicon with compounds such as gallium phosphide (GaP) or gallium arsenide (GaAs) could enable the design of new kinds of semiconductors. This research, based in a field called silicon photonics, posits that such new materials would make it possible to use light instead of electrons for signal transport, supporting the development of improved electronic devices.

“To do this,” Tonner explains, “we really need to understand how the interfaces between silicon and these organic compounds look and behave. The reaction between these two material classes needs to proceed in a very controlled manner so that the interface is as perfect as possible. With computational chemistry we can look at the elemental details of these interactions and processes.”

For example, to cover a slab of silicon, liquid precursor molecules for the constituent atoms of gallium arsenide are placed in a bubbler, where they are then brought into the gas phase. These precursor molecules are composed of the atoms required for the new material (gallium, arsenic) and ions or molecules called ligands to stabilize them in the liquid and gas phase. These ligands are subsequently lost in the deposition process and when silicon is placed in the system, the precursor molecules are adsorbed onto the solid silicon surface. After adsorption and loss of the ligands, gallium and arsenide atoms attach to the silicon, forming a GaAs film.

How atoms are arranged when they adsorb to a surface is determined by chemical bonding. The strength of these bonds and the density with which the GaAs precursor molecules are adsorbed is affected not only by the distance between them and the silicon surface but also by interactions among the precursor molecules themselves. In one type of interaction, called Pauli repulsion, clouds of electrons overlap and repel each other, causing the available energy for bonding to decline. In another, called attractive dispersion interaction, changes in the electronic positions in one atom cause electrons to be redistributed in other atoms, bringing the electron movements into harmony and lowering the energy of the total system.

Previously, it had been suggested that repulsive relationships among atoms is the most important factor in “steering” atoms into place when they adsorb on a surface. By using density functional theory and looking at intriguing features of how electrons are distributed, the researchers determined that the ability of atoms to steer other atoms into place on the surface can also result from attractive dispersive interactions.

Gaining a better understanding of these fundamental interactions should help designers of optically active semiconductors to improve adsorption of the precursor molecules onto silicon. This, in turn, would make it possible to combine light signal conduction with silicon based microelectonics, bringing together the best of both worlds in optical and electronic conduction.

For Tonner, using first principles methods in chemistry for materials science applications holds great promise. “Theory today is very often taken as a complement to experimental investigation,” he says. “Although experimentation is extremely important, our ultimate goal is for theory to be predictive in ways that enable us to make the first steps in first principles-inspired materials design. I see this as a long term goal.”

A team of researchers from Siberian Federal University (SFU) obtained thin copper/gold and iron/palladium films and studied the reactions that take place in them upon heating. Knowing these processes, scientists will be able to improve the properties of materials currently used in microelectronics. The article of the scientists was published in the Journal of Solid State Chemistry.

Materials based on thin metal films are widely used in microelectronics (e.g. copper and gold – in the manufacture of electrical contacts). Nanomaterials based on iron and palladium have unique magnetic properties and potentially can be used for high-density magnetic recording of information. One of the main factors that affects the properties of thin film materials is alteration of the phase composition as a result of chemical reactions and atomic structure realignment. The work of the researchers covers solid phase reactions in two-layer thin metal films – copper/gold (Cu/Au) and iro/palladium (Fe/Pl).

The scientists obtained the Cu/Au and Fe/Pd films in SFU common use center. To do so, they used the method of electron-beam deposition in high vacuum, i.e. evaporated the alloy using a beam of electrons and then deposited it on a carrying base as a thin layer. The thickness of the layer could be regulated. After obtaining the films the scientists made an experiment to study the course of chemical reactions in the interface region of the initial elements. For the reactions to take place, materials had to be heated to high temperatures which was done directly in the column of a transmission electron microscope. The team used a special sample holder that allowed for controlled heating of each sample from room temperature to 1,000 °. Along with the heating, the team registered electron diffraction images and measured the temperature. Thus, the scientists managed to combine the initiation of the reaction and the registration of changes in a solid-phase reaction within one experiment and to secure high data precision.

“We’ve established the value of the long-range order parameter and the temperature of the order-disorder transition in atomically ordered phases formed in the course of the reaction. The atoms of such phases form ordered structures of certain shapes. We also suggested a mechanism for the formation of such ordered structures. For instance, in the case of the Cu/Au system we demonstrated how mutual diffusion of copper and gold on the initial stages of the reaction leads to the refinement of grains of the initial materials and the formation of Cu-Au solid solution nanocrystallites within the material. Later on, a new ordered structure occurs and starts to grow on the basis of these components,” explains Evgeny Moiseenko, a co-author of the work, candidate of physics and mathematics, and a research assistant at SFU.

The work of the scientists will help identify the features of the studied thin film systems that may be used in the design of microelectronic devices.

In optics, the era of glass lenses may be waning.

In recent years, physicists and engineers have been designing, constructing and testing different types of ultrathin materials that could replace the thick glass lenses used today in cameras and imaging systems. Critically, these engineered lenses — known as metalenses — are not made of glass. Instead, they consist of materials constructed at the nanoscale into arrays of columns or fin-like structures. These formations can interact with incoming light, directing it toward a single focal point for imaging purposes.

But even though metalenses are much thinner than glass lenses, they still rely on “high aspect ratio” structures, in which the column or fin-like structures are much taller than they are wide, making them prone to collapsing and falling over. Furthermore, these structures have always been near the wavelength of light they’re interacting with in thickness — until now.

Four ultrathin metalenses developed by University of Washington researchers and visualized under a microscope. Credit: Liu et al., Nano Letters, 2018

In a paper published Oct. 8 in the journal Nano Letters, a team from the University of Washington and the National Tsing Hua University in Taiwan announced that it has constructed functional metalenses that are one-tenth to one-half the thickness of the wavelengths of light that they focus. Their metalenses, which were constructed out of layered 2D materials, were as thin as 190 nanometers — less than 1/100,000ths of an inch thick.

“This is the first time that someone has shown that it is possible to create a metalens out of 2D materials,” said senior and co-corresponding author Arka Majumdar, a UW assistant professor of physics and of electrical and computer engineering.

Their design principles can be used for the creation of metalenses with more complex, tunable features, added Majumdar, who is also a faculty researcher with the UW’s Molecular Engineering & Sciences Institute.

Majumdar’s team has been studying the design principles of metalenses for years, and previously constructed metalenses for full-color imaging. But the challenge in this project was to overcome an inherent design limitation in metalenses: in order for a metalens material to interact with light and achieve optimal imaging quality, the material had to be roughly the same thickness as the light’s wavelength in that material. In mathematical terms, this restriction ensures that a full zero to two-pi phase shift range is achievable, which guarantees that any optical element can be designed. For example, a metalens for a 500-nanometer lightwave — which in the visual spectrum is green light — would need to be about 500 nanometers in thickness, though this thickness can decrease as the refractive index of the material increases.

Majumdar and his team were able to synthesize functional metalenses that were much thinner than this theoretical limit — one-tenth to one-half the wavelength. First, they constructed the metalens out of sheets of layered 2D materials. The team used widely studied 2D materials such as hexagonal boron nitride and molybdenum disulfide. A single atomic layer of these materials provides a very small phase shift, unsuitable for efficient lensing. So the team used multiple layers to increase the thickness, although the thickness remained too small to reach a full two-pi phase shift.

“We had to start by figuring out what type of design would yield the best performance given the incomplete phase,” said co-author Jiajiu Zheng, a doctoral student in electrical and computer engineering.

To make up for the shortfall, the team employed mathematical models that were originally formulated for liquid-crystal optics. These, in conjunction with the metalens structural elements, allowed the researchers to achieve high efficiency even if the whole phase shift is not covered. They tested the metalens’ efficacy by using it to capture different test images, including of the Mona Lisa and a block letter W. The team also demonstrated how stretching the metalens could tune the focal length of the lens.

In addition to achieving a wholly new approach to metalens design at record-thin levels, the team believes that its experiments show the promise of making new devices for imaging and optics entirely out of 2D materials.

“These results open up an entirely new platform for studying the properties of 2D materials, as well as constructing fully functional nanophotonic devices made entirely from these materials,” said Majumdar.

Additionally, these materials can be easily transferred on any substrate, including flexible materials, paving a way towards flexible photonics.

Researchers at Linköping University, Sweden, are working to develop a method to convert water and carbon dioxide to the renewable energy of the future, using the energy from the sun and graphene applied to the surface of cubic silicon carbide. They have now taken an important step towards this goal, and developed a method that makes it possible to produce graphene with several layers in a tightly controlled process.

Jianwu Sun at Linköping University inspecting the growth reactor for growth of cubic silicon carbide. Credit: Thor Balkhed/LiU

The research group has also shown that graphene acts as a superconductor in certain conditions. Their results have been published in the scientific journals Carbon and Nano Letters.

Carbon, oxygen and hydrogen. These are the three elements you would get if you took apart molecules of carbon dioxide and water. The same elements are the building blocks of chemical substances that we use for fuel, such as ethanol and methane. The conversion of carbon dioxide and water to renewable fuel, if possible, would provide an alternative to fossil fuels, and contribute to reducing our emission of carbon dioxide to the atmosphere. Jianwu Sun, senior lecturer at Linköping University, is trying to find a way to do just that.

The first step is to develop the material they plan to use. Researchers at Linköping University have previously developed a world-leading method to produce cubic silicon carbide, which consists of silicon and carbon. The cubic form has the ability to capture energy from the sun and create charge carriers. This is, however, not sufficient. Graphene, one of the thinnest materials ever produced, plays a key role in the project. The material comprises a single layer of carbon atoms bound to each other in a hexagonal lattice. Graphene has a high ability to conduct an electric current, a property that would be useful for solar energy conversion. It also has several unique properties, and possible uses of graphene are being extensively studied all over the world.

In recent years, the researchers have attempted to improve the process by which graphene grows on a surface in order to control the properties of the graphene. Their recent progress is described in an article in the scientific journal Carbon.

“It is relatively easy to grow one layer of graphene on silicon carbide. But it’s a greater challenge to grow large-area uniform graphene that consists of several layers on top of each other. We have now shown that it is possible to grow uniform graphene that consists of up to four layers in a controlled manner”, says Jianwu Sun, of the Department of Physics, Chemistry and Biology at Linköping University.

One of the difficulties posed by multilayer graphene is that the surface becomes uneven when different numbers of layers grow at different locations. The edge when one layer ends has the form of a tiny, nanoscale, staircase. For the researchers, who want large flat areas, these steps are a problem. It is particularly problematic when steps collect in one location, like a wrongly built staircase in which several steps have been united to form one large step. The researchers have now found a way to remove these united large steps by growing the graphene at a carefully controlled temperature. Furthermore, the researchers have shown that their method makes it possible to control how many layers the graphene will contain. This is the first key step in an ongoing research project whose goal is to make fuel from water and carbon dioxide.

In a closely related article in the journal Nano Letters, the researchers describe investigations into the electronic properties of multilayer graphene grown on cubic silicon carbide.

“We discovered that multilayer graphene has extremely promising electrical properties that enable the material to be used as a superconductor, a material that conducts electrical current with zero electrical resistance. This special property arises solely when the graphene layers are arranged in a special way relative to each other”, says Jianwu Sun.

Theoretical calculations had predicted that multilayer graphene would have superconductive properties, provided that the layers are arranged in a particular way. In the new study, the researchers demonstrate experimentally for the first time that this is the case. Superconducting materials are used in, among other things, superconducting magnets – extremely powerful magnets found in magnet resonance cameras for medical investigations, and in particle accelerators for research. There are many potential areas of application for superconductors, such as electrical supply lines with zero energy loss, and high-speed trains that float on a magnetic field. Their use is currently limited by the inability to produce superconductors that function at room temperature: currently available superconductors function only at extremely low temperatures.