One of the areas of interest is transition metal dichalcogenides (TMDs) which can be made into sheets a few molecules thick. “It was thought that graphene could be used in transistors, but in transistors, you need to be able to switch the electric current on and off," said Dr Fan Zhang. “With graphene, however, the current cannot be easily switched off.”
“TMDs have an energy gap that allows the flow of electrons to be controlled,” said researcher Armin Khamoshi said. “This gap makes TMDs ideal for use in transistors. TMDs are also very good absorbers of circularly polarised light, so they could be used in detectors.”
In their latest project, Dr Zhang and Khamoshi provided the theoretical work to guide a group at the Hong Kong University of Science and Technology on the layer-by-layer construction of a TMD device and on the use of magnetic fields to study how electrons travel through the device. Each TMD layer is three molecules thick, with the layers sandwiched between two sheets of boron nitride.
The team discovered that how electrons behave in the TMDs depends on whether an even or odd number of TMD layers is used. “This is a very surprising finding,” said Dr Zhang. “It doesn’t matter how many layers you have; rather, it’s whether there are an odd or even number of layers.”
At the quantum scale, the electrical transverse conductance of the 2D material in the presence of a magnetic field changes in discrete steps, Dr Zhang said; a phenomenon called quantum Hall conductance.
“Quantum Hall conductance might change one step by one step, or two steps by two steps, and so on,” he said. “We found that if we used an even number of TMD layers, there was a 12 step quantum conductance. If we applied a strong enough magnetic field, it would change by six steps at a time.”
Using an odd number of layers combined with a low magnetic field resulted in a six step quantum Hall conductance, but under stronger magnetic fields, it became a three by three step phenomenon.“The type of quantum Hall conductance we predicted and observed in our TMD devices has never been found in any other material,” Dr Zhang said. “These results not only decipher the intrinsic properties of TMD materials, but also demonstrate that we achieved high electron mobility. This gives us hope that we can, one day, use TMDs for transistors.”