"The most critical significance of this work was in finding the band gap," said graduate student Kaiyuan Yao. "Optoelectronic device engineers need to know what the band gap is in orderly to properly connect the 2D material with other materials and components in a device.”
In this study, the researchers found the band gap for a monolayer of moly sulphide to be about 30% higher than expected based on previous experiments. They also quantified how the band gap changes with electron density - a phenomenon known as band gap renormalisation.
Using photoluminescence excitation spectroscopy, the research team measured both the exciton and band gap signals, and then detangled the separate signals.
The scientists observed how light was absorbed by electrons in the moly sulphide sample as they adjusted the density of electrons in the sample by changing the electrical voltage on a layer of charged silicon that sat below the moly sulphide monolayer.
"The large degree of tunability really opens people's eyes," said James Schuck, director of the Imaging and Manipulation of Nanostructures facility at the Molecular Foundry.
"And because we could see both the band gap's edge and the excitons simultaneously, we could understand each independently and also understand the relationship between them. It turns out all of these properties are dependent on one another."
Applications for the material include ultrasensitive biosensors and smaller transistors.
