Jul 2, 2014
MoS2's dielectric constant is not constant
Researchers at Harvard University in the US have put forward the first thorough theory of how electric fields affect the dielectric response of molybdenum disulphide – a new, technologically important 2D layered material. Their calculations have revealed that the dielectric constant of MoS2 is not fixed but depends very much on the gate voltage applied to the system. The findings will be important for when it comes to designing MoS2-based devices in the future since many of the material’s electronic and optical properties depend on its dielectric constant.
MoS2 belongs to the family of dichalcogenides – layered semiconducting films that might be used to make circuits for low-power electronics, low-cost or flexible displays, sensors and even flexible electronics that can be coated onto a wide variety of surfaces. These so-called van der Waals materials have the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is a chalcogen (such as S, Se and Te). They go from being indirect bandgap semiconductors in the bulk to direct bandgap semiconductors when scaled down to monolayers. These monolayers efficiently absorb and emit light and so could also be ideal for making a variety of optoelectronics devices such as light-emitting diodes and solar cells.
Elton Santos and Efthimios Kaxiras have now used state-of-the-art first principles simulations to reveal the full electronic spectrum of MoS2 multilayers for the first time. “These are complex and time-consuming calculations because applied electric fields modify interlayer distances in such 2D materials,” explains Santos. “The modifications generate forces that tend to keep the layers separated at distances that vary as a function of how intense the applied field actually is.”
Electrically tuneable optoelectronics
The Harvard researchers overcame this problem by introducing layer “relaxation” factors as they increased the magnitude of the applied field in their simulation. They also took into account van der Waals dispersion forces and the electrical polarization of the MoS2 layers.
“The mechanisms we unearthed in our work suggest that layered materials like MoS2 might come in handy for applications such as electrically tuneable optoelectronics,” Santos told nanotechweb.org. “Other potential technologies that could benefit include flexible interactive displays and wearable smart fabrics, sensors and electro-optic modulators.”
Santos says that he is now busy trying to measure the effects predicted by the calculations in collaboration with colleagues from Stanford University.
Further details about the present research are available in ACS Nano DOI: 10.1021/nn403738b.
About the author
Belle Dumé is contributing editor at nanotechweb.org