According to the team, the lack of a bandgap in graphene limits its application to electronics while the presence of a bandgap in TMDs allows them to be stacked into heterostructures. It is, however, difficult to experimentally determine band alignment between these layers because the results depend on the quality of the TMDs.
The team has now proven that the concept known as the Anderson model – a simple way of determining band alignment – is applicable to this system.
The Anderson model assumes that when two semiconductors are placed together, they share a common zero in their energy-band structure known as the vacuum level. Bandgap alignment can then be determined directly from calculated values of bandgaps and offsets. Until now, it was unclear whether this assumption would hold true in atomic-layer TMDs.
The researchers addressed this by measuring the energy of the bandgap in three TMDs – molybdenum disulphide, tungsten disulphide and tungsten diselenide – using a method called ultraviolet photoelectron spectroscopy. They then applied the Anderson model to predict the band alignment. They compared these calculated values with direct experimental measurements.
Agreement between the values obtained by the two methods seems to indicate that the Anderson model holds true. The team suggests that this is because of van der Waals surfaces, which ensure an absence of dangling atomic bonds that would otherwise prevent the vacuum levels in the two materials from aligning.
"Our next step is to build heterojunctions based on the knowledge gained from the theory," says Professor Lance Li. "We will research several heterostructures for various applications, such as solar cells and light-emitting diodes."