New atom-flat compounds show promise for optoelectronics, advanced computing
A laboratory at Rice University wants its products to have a clean appearance, even at the nanoscale. His latest creation is on time.
Pulickel Ajayan's rice research laboratory has created unique two-dimensional flakes with two distinct personalities: molybdenum diselenide on one side of a marked demarcation line and rhenium diselenide on the other.
From all appearances, the two-tone material likes it this way, developing naturally - albeit under tight conditions - in a chemical vapor deposition furnace.
The material is a two-dimensional transition metal dichalcogenide heterostructure, a crystal having more than one chemical component. This is not unusual in itself, but the zigzag boundary between the material reported in the American Chemical Society's Nano Letters journal is unique.
Dichalcogenides are semiconductors that incorporate transition metals and chalcogens. They are a promising component for optoelectronic applications such as solar cells, photodetectors and detection devices. Lead author Amey Apte, a graduate student of Rice, said they can also be appropriate materials for quantum computing or neuromorphic computing, which mimics the structure of the human brain.
Apte said that well known and atomically flat molybdenum tungsten dichalcogenide heterostructures may be more similar to alloys, with diffuse boundaries between their crystalline domains. However, the new material - technically, 2H MoSe2-1T' ReSe2 - has atomically pointed interfaces that give it a smaller electronic bandgap than other dichalcogenides.
"Instead of having a single bandgap based on the composition of an alloy, we can adjust the bandgap in this material in a very controllable way," says Apte. "The strong dissimilarity between two adjacent atomically thin domains opens new paths." He said that the voltage range probably extends from 1.5 to 2.5 electron volts.
To achieve reliable material growth, it was necessary to create a phase diagram that explained how each parameter - the balance of the chemical precursor of the gas, temperature and time - affects the process. Sandhya Susarla, a graduate student and co-author of Rice, said the diagram serves as a roadmap for manufacturers.
"The biggest problem with these two-dimensional documents is that they are not very reproducible," she says. "They are very sensitive to many parameters, because the process is controlled kinetically.
"But our process is scalable because it is thermodynamically controlled," explains Susarla. "Manufacturers don't have many parameters to consider. They just have to look at the phase diagram, check the composition and they will have the product every time."
Researchers believe that they can better control the shape of the material by adapting the substrate to epitaxial growth. The fact that the atoms fall into place in accordance with the atomic arrangement of the surface would allow for much better personalization.
