One current method to build a semiconductor superlattice — materials comprised of alternating layers of ultra-thin, two-dimensional sheets only one or a few atoms thick — is to manually stack ultrathin layers one on top of the other, but this is labor-intensive. In addition, since the flake-like sheets are fragile, it takes a long time to build because many sheets will break during the placement process. The other method is to grow one new layer on top of the other, using a process called chemical vapor deposition. But since that means different conditions such as heat, pressure, or chemical environments are needed to grow each layer, the process could result in altering or breaking the layer underneath. This method is also labor-intensive with low yield rates.
A method was developed to make new kinds of artificial superlattices. Unlike current state-of-the art super-lattices 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.
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 they do not interfere with other insulating layers.
Compared with the conventional layer-by-layer assembly or growth approach currently used to create 2D superlattices, the new process is much faster and more efficient. Most importantly, the new method easily yields superlattices with tens, hundreds, or even thousands of alternating layers, which is not yet possible with other approaches.
This new class of superlattices alternates 2D atomic crystal sheets that are 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 — to keep otherwise neutral molecules attached to each other. These new superlattices are called monolayer atomic crystal molecular superlattices.
The new method to create these superlattices uses a process called electrochemical intercalation in which a negative voltage is applied. This injects negatively charged electrons into the 2D material. This attracts positively charged ammonium molecules into the spaces between the atomic layers. Those ammonium molecules automatically assemble into new layers in the ordered crystal structure, creating a superlattice.
The technique was demonstrated using black phosphorus as a base 2D atomic crystal material. Using the negative voltage, positively charged ammonium ions were attracted into the base material, and inserted themselves between the layered atomic phosphorous sheets. Different types of ammonium molecules with various sizes and symmetries were inserted into a series of 2D materials. The structures of the resulting monolayer atomic crystal molecular superlattices — which had a diverse range of desirable electronic and optical properties — could be tailored.