Modeling the electrically tunable optical properties of monolayer MoS2

Christiansen, Mads-Peter Verner ; Jørgensen, Jannick Kjær

4. term

Nanotechnology, Master

2018

2018-05-31

86 pages

The motivation of this thesis is the experimental realization of highly tunable optical properties of transition metal dichalcogenides as reported in recent literature. In order to understand the root cause of this phenomenon a quantum mechnical description is required. The tunability of the optical properties is controlled by excitons which require including the electron-hole interaction responsible for their existence. A framework based on ab-initio DFT wave functions and an analytical expression for the dielectric screening has been developed which predicts intrinsic properties in fair agreement with experiments. The inclusion of doping by electrostatic gating requires including the effect in the DFT calculation and two models of doping have been proposed and compared. G0W0 calculations have been performed to study the effect of doping on bandgap renormalization and it has been found that doping also significantly reduces the effective electron mass in the case of n-doping. Importantly the effect of doping must also be included in the dielectric screening which is done through an analytical model. The fully developed model qualitatively reproduces the tunability of the optical properties. Reflection spectroscopy measurements have been carried out on intrinsic MoS2 showing the excitonic transitions. Gates have been deposited on the MoS2 sample in order to electrically dope during reflectance measurements, however more work is required to obtain definitive results.

MoS2 ; DFT ; Density Functional ; Bethe-Salpeter ; Exciton ; Monolayer ; Numerical ; Optical

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