This is an artist’s concept of two kinds of monolayer atomic crystal molecular superlattices. On the left, molybdenum disulfide with layers of ammonium molecules, on the right, black phosphorus with layers of ammonium molecules.
A team of scientists from UCLA has developed a new faster and more efficient way to make artificial superlattices, comprised of alternating layers of ultra-thin 2D sheets that are only a few atoms thick.
The new superlattices—called monolayer atomic crystal molecular superlattices—feature a molecular layer that becomes the second “sheet” that is held in place by van der Waals forces—weak electrostatic forces that keep otherwise neutral molecules attached to each other.
“Traditional semiconductor superlattices can usually only be made from materials with highly similar lattice symmetry, normally with rather similar electronic structures,” Yu Huang, UCLA professor of materials science and engineering at the UCLA Samueli School of Engineering, said in a statement. “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,” she added.
The researchers used an electrochemical intercalation process where a negative voltage is applied, injecting the negatively charged electrons into the 2D material. This attracts positively charged ammonium molecules, which automatically assemble into new layers in the ordered crystal structure in the spaces between the atomic layers.
“Think of a two-dimensional material as a stack of playing cards,” Xiangfeng Duan, UCLA professor of chemistry and biochemistry, said in a statement. “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 new method could yield improved and new classes of electronic and optoelectronic devices, including applications for superfast and ultra-efficient semiconductors for transistors in computers and smart devices, as well as advanced LEDs and lasers.
The researchers demonstrated the new technique with black phosphorus as a base 2D atomic crystal material. Using the negative voltage, positively charged ammonium ions were attracted to the base material and inserted themselves between the layered atomic phosphorous sheets.
They then inserted different types of ammonium molecules with various sizes and symmetries into a series of 2D materials to create a broad class of superlattices.
“The resulting materials could be useful for making faster transistors that consume less power, or for creating efficient light-emitting devices,” Duan said.
The new method could also yield superlattices with thousands of alternating layers, which is not possible using traditional approaches.
Currently, superlattices feature alternating layers that have similar atomic structures and similar electronic properties. However, the new superlattices can have radically different structures, properties and functions.
For example, one layer could allow a fast flow of electrons through it, while the other type of layer could act as an insulator. The new design confines the electronic and optical properties to single active layers and prevents them from interfering with other insulating layers.
Superlattices are currently built by manually stacking the ultrathin layers on top of each other. However, this method is labor-intensive and the flake-like sheets are fragile.
The other method is to grow one new layer on top of the other, using a process called chemical vapor deposition, but different conditions, including heat, pressure or chemical environments are required for each layer to grow. The process could result in altering or breaking the layer underneath. The process is also labor-intensive with low yield rates.
