This is an artist's concept of two different monolayer atomic crystal molecular superlattices: (left) molybdenum disulfide with layers of ammonium molecules; (right) black phosphorus with layers of ammonium molecules. Image: UCLA Samueli Engineering.
This is an artist's concept of two different monolayer atomic crystal molecular superlattices: (left) molybdenum disulfide with layers of ammonium molecules; (right) black phosphorus with layers of ammonium molecules. Image: UCLA Samueli Engineering.

A research team led by scientists and engineers at the University of California, Los Angeles (UCLA) has developed a method to make new kinds of artificial ‘superlattices’ – materials comprised of alternating layers of ultra-thin two-dimensional (2D) sheets that 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 achievable.

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 novel design confines the electronic and optical properties to single active layers, and prevents them from interfering with each other.

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

Compared with the conventional layer-by-layer assembly or growth approaches currently used to create 2D superlattices, the new UCLA-led process for manufacturing superlattices from 2D materials is much faster and more efficient. Most importantly, the new method could easily yield superlattices with tens, hundreds or even thousands of alternating layers, which is not yet possible with other approaches.

This new class of superlattice is made up of 2D atomic crystal sheets interspaced with molecules of varying shapes and sizes. In effect, this molecular layer becomes the second ‘sheet’ because it is held in place by ‘van der Waals’ forces, weak electrostatic forces that keep otherwise neutral molecules ‘attached’ to each other. These new superlattices are called ‘monolayer atomic crystal molecular superlattices’.

The study, reported in a paper in Nature, was led by Xiangfeng Duan, UCLA professor of chemistry and biochemistry, and Yu Huang, UCLA professor of materials science and engineering at the UCLA Samueli School of Engineering.

"Traditional semiconductor superlattices can usually only be made from materials with highly similar lattice symmetry, normally with rather similar electronic structures," Huang said. "For the first time, we have created stable superlattice structures with radically different layers, yet nearly perfect atomic-molecular arrangements within each layer. This new class of superlattice structures has tailorable electronic properties for potential technological applications and further scientific studies."

One existing method for building a superlattice involves manually stacking the ultrathin layers one on top of the other, but this is labor-intensive. In addition, since the flake-like sheets are fragile, the superlattice takes a long time to build because many sheets will break during the placement process. Another method involves growing one new layer on top of the other, using a process known as ‘chemical vapor deposition’. But since that means different conditions, such as heat, pressure or chemical environments, are needed to grow each layer, the process can alter or break the layer underneath. This method is also labor-intensive with low yield rates.

The new method for creating monolayer atomic crystal molecular superlattices uses a process called ‘electrochemical intercalation’, in which a negative voltage injects negatively charged electrons into the 2D material. The electrons then attract positively charged ammonium molecules into the spaces between the atomic layers, and these ammonium molecules automatically assemble into new layers in the ordered crystal structure, creating a superlattice.

"Think of a two-dimensional material as a stack of playing cards," Duan said. "Then imagine that we can cause a large pile of nearby plastic beads to insert themselves, in perfect order, sandwiching between each card. That's the analogous idea, but with a crystal of 2D material and ammonium molecules."

The researchers first demonstrated the new technique using black phosphorus as a base 2D atomic crystal material. By applying a negative voltage, positively charged ammonium ions were attracted into the base material, inserting themselves between the layered atomic phosphorous sheets.

Following that success, the team inserted different types of ammonium molecules with various sizes and symmetries into a series of 2D materials to create a broad class of superlattices. They found they could tailor the structures of the resulting monolayer atomic crystal molecular superlattices, giving them a diverse range of desirable electronic and optical properties. "The resulting materials could be useful for making faster transistors that consume less power, or for creating efficient light-emitting devices," Duan said.

This story is adapted from material from UCLA, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.