#superlattices

A strikingly natural coincidence: Researchers find heating gallium nitride and magnesium forms a superlattice

A study led by Nagoya University in Japan revealed that a simple thermal reaction of gallium nitride (GaN) with metallic magnesium (Mg) results in the formation of a distinctive superlattice structure. This represents the first time researchers have identified the insertion of 2D metal layers into a bulk semiconductor. By carefully observing the materials through various cutting-edge characterization techniques, the researchers uncovered new insights into the process of semiconductor doping and elastic strain engineering. They published their findings in the journal Nature. GaN is an important wide bandgap semiconductor material that is poised to replace traditional silicon semiconductors in applications demanding higher power density and faster operating frequencies. These distinctive characteristics of GaN make it valuable in devices such as LEDs, laser diodes, and power electronics—including critical components in electric vehicles and fast chargers. The improved performance of GaN-based devices contributes to the realization of an energy-saving society and a carbon-neutral future.

Forging a dream material with semiconductor quantum dots

Researchers from the RIKEN Center for Emergent Matter Science and collaborators have succeeded in creating a "superlattice" of semiconductor quantum dots that can behave like a metal, potentially imparting exciting new properties to this popular class of materials.

Semiconducting colloidal quantum dots have garnered tremendous research interest due to their special optical properties, which arise from the quantum confinement effect. They are used in solar cells, where they can improve the efficiency of energy conversion, biological imaging, where they can be used as fluorescent probes, electronic displays, and even quantum computing, where their ability to trap and manipulate individual electrons can be exploited.

However, getting semiconductor quantum dots to efficiently conduct electricity has been a major challenge, impeding their full use. This is primarily due to their lack of orientational order in assemblies. According to Satria Zulkarnaen Bisri, lead researcher on the project, "making them metallic would enable, for example, quantum dot displays that are brighter yet use less energy than current devices."

Superlattice films developed to achieve macroscale superlubricity

A research group led by Prof. Wang Liping at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS), in collaboration with Prof. Li Qunyang at Tsinghua University, developed a universal superlattice film that can be applied to mechanical parts to achieve long-term macroscale superlubricity with self-rejuvenation. The study was published in Cell Reports Physical Science.

It is estimated that about one-third of global energy is consumed to get rid of friction, which is quite common in nature. Reducing friction to an ultra-low level or even a superlubricious state with a friction coefficient on the order of 10-3 can exert significant impacts on human life. However, achieving robust macroscale superlubricity between dry solid surfaces remains a challenge, which has been realized mainly at nanometer scales with lamellar materials in previous studies.

To tackle this challenge, the researchers designed a universal molybdenum disulfide (MoS2) /ceramic superlattice architecture, which enables long-term (close to 300,000 sliding cycles) macroscale superlubricity in mild vacuum after a short running-in period in air.

To really appreciate what a team of researchers led by Maksym Kovalenko and Maryna Bodnarchuk has achieved, it is best to start with something mundane: Crystals of table salt (also known as rock salt) are familiar to anyone who has ever had to spice up an overtly bland lunch. Sodium chloride -- NaCl in chemical terms -- is the name of the helpful chemical; it consists of positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). You can imagine the ions as beads that strongly attract each other forming densely packed and rigid crystals like the ones we can see in a saltshaker. Many naturally occurring minerals consist of ions -- positive metal ions and negative ions, which arrange themselves into different crystal structures depending on their relative sizes. In addition, there are structures such as diamond and silicon: These crystals consist of only one kind of atoms -- carbon in the case of diamond -, but, similar to minerals, the atoms are also held together by strong bonding forces.

Novel building blocks for a new kind of matter

What if all these strong bonding forces between atoms could be eliminated? In the realm of atoms, with all the quantum mechanics at play, this would not yield a molecule or a solid-state matter, at least at ambient conditions. "But modern chemistry can produce alternative building blocks that can indeed have vastly different interactions than those between atoms," says Maksym Kovalenko, Empa researcher and professor of chemistry at ETH Zurich. "They can be as hard as billiard balls in a sense that they sense each other only when colliding. Or they can be softer on the surfaces, like tennis balls. Moreover, they can be built in many different shapes: not just spheres, but also cubes or other polyhedra, or more anisotropic entities." Such building blocks are made of hundreds or thousands of atoms and are known as inorganic nanocrystals. Kovalenko's team of chemists at Empa and ETH is able to synthesize them in large quantities with a high degree of uniformity. Kovalenko and Bodnarchuk, and some of their colleagues the world over, have been working for about 20 years now with these kinds of building blocks. The scientists call them "Lego materials" because they form long-range ordered dense lattices known as superlattices. It had long been speculated that mixing different kinds of nanocrystals would allow the engineering of completely new supramolecular structures.

Three-dimensional superlattice engineering with block copolymer epitaxy

Three-dimensional (3-D) structures at the nanoscale are important in modern devices, although their fabrication with traditional top-down approaches is complex and expensive. Block copolymers (BCPs) that are analogous to atomic lattices can spontaneously form a rich variety of 3-D nanostructures to substantially simplify 3-D nanofabrication. In a new report on Science Advances, Jiaxing Ren and a research team in molecular engineering, chemical engineering and materials science at the University of Chicago, Technion-Israel Institute of Technology and the Argonne National Laboratory in the U.S. and Israel formed a 3-D superlattice using BCP micelles. They controlled the process using lithographically defined 2-D templates that matched a crystallographic plane in the 3-D superlattice. Using scanning transmission electron microscopy tomography, the team demonstrated precise control across the lattice symmetry and orientation. They achieved excellent ordering and substrate registration through 284-nanometer-thick films. To mediate lattice stability, the scientists tapped into molecular packing frustration of the superlattice and observed surface-induced lattice reconstruction, which led to form a unique honeycomb lattice.

Electron microscopy of nanoparticle superlattice formation at a solid-liquid interface in non-polar liquids

Nanoparticle superlattice films that form at the solid-liquid interface are important for mesoscale materials but are challenging to analyze on the onset of formation at a solid-liquid interface. In a new report on Science Advances, E. Cepeda-Perez and a research team in materials, physics and chemistry in Germany studied the early stages of nanoparticle assembly at solid-liquid interfaces using liquid-phase electron microscopy. They observed oleylamine-stabilized gold nanoparticles to spontaneously form thin layers on a silicon nitride membrane window of the liquid enclosure. In the first monolayer, the assembly maintained dense packings of hexagonal symmetry independent of the nonpolar solvent type. The second layer displayed geometries ranging from dense packing in a hexagonal honeycomb structure to quasi-crystalline particle arrangements—based on the dielectric constant of the liquid. The complex structures made of weaker interactions remained preserved, while the surface remained immersed in liquid. By fine-tuning the properties of materials involved in nanoparticle superlattice formation, Cepeda-Perez et al. controlled the three-dimensional (3-D) geometry of a superlattice, including quasi-crystals (a new state of matter).

UCLA researchers develop a new class of two-dimensional materials: New kinds of 'superlattices' could lead to improvements in electronics, from transistors to LEDs

A research team led by UCLA scientists and engineers has developed a method to make new kinds of artificial "superlattices" -- materials comprised of alternating layers of ultra-thin "two-dimensional" sheets, which are only one or a few atoms thick. Unlike current state-of-the art superlattices, in which alternating layers have similar atomic structures, and thus similar electronic properties, these alternating layers can have radically different structures, properties and functions, something not previously available.

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For example, while one layer of this new kind of superlattice can allow a fast flow of electrons through it, the other type of layer can act as an insulator. This design confines the electronic and optical properties to single active layers, and prevents them from interfering with other insulating layers.

Such superlattices can form the basis for improved and new classes of electronic and optoelectronic devices. Applications include superfast and ultra-efficient semiconductors for transistors in computers and smart devices, and advanced LEDs and lasers.