Modeling the electrically tunable optical properties of monolayer MoS2

Christiansen, Mads-Peter Verner ; Jørgensen, Jannick Kjær

4. term

Nanotechnology, Master

2018

2018-06-01

86

Nylige studier viser, at den optiske respons i overgangsmetal-dichalcogenider (TMD'er) kan indstilles over et bredt område. Denne afhandling søger at forklare årsagen med en kvantemekanisk beskrivelse med fokus på excitoner – bundne elektron-hul-par, hvis egenskaber bestemmes af elektron-hul-interaktionen. Jeg udvikler en beregningsramme, der kombinerer ab initio-DFT-bølgefunktioner (tæthedsfunktionelteori) med en analytisk model for dielektrisk screening, dvs. materialets evne til at skærme elektriske felter. For intrinsiske, udopede materialer forudsiger metoden exciton-egenskaber i rimelig overensstemmelse med eksperimenter. For at beskrive elektrostatisk gating (elektrisk doping) indarbejdes dopingen direkte i DFT-beregningerne, og to dopingsmodeller foreslås og sammenlignes. Mange-legeme-beregninger af typen G0W0 bruges derefter til at undersøge, hvordan doping ændrer båndstrukturen (båndgab-renormalisering). I tilfælde af n-doping findes den effektive elektronsmasse at falde markant. Da doping også ændrer den dielektriske screening, udvides den analytiske screeningsmodel til at inkludere denne effekt. Samlet set reproducerer modellen kvalitativt den eksperimentelt observerede tunérbarhed af de optiske spektre. Eksperimentelt viser reflektionsspektroskopi på intrinsisk MoS2 tydelige excitonovergange. Der blev deponeret gate-elektroder på MoS2-prøven for at muliggøre elektrisk doping under målingerne, men der kræves yderligere arbejde for at opnå endelige resultater.

Recent studies show that the optical response of transition metal dichalcogenides (TMDs) can be tuned over a wide range. This thesis seeks the underlying cause using a quantum-mechanical description centered on excitons—bound electron–hole pairs whose properties are set by the electron–hole interaction. I develop a computational framework that combines ab initio density-functional-theory (DFT) wave functions with an analytical model of dielectric screening, i.e., how a material reduces electric fields. For intrinsic, undoped materials, the approach predicts excitonic properties in fair agreement with experiments. To describe electrostatic gating (electrical doping), the doping is included directly in the DFT calculations, and two doping models are proposed and compared. Many-body G0W0 calculations are then used to study how doping changes the band structure (band-gap renormalization). In the case of n-doping, the effective electron mass is found to decrease significantly. Because doping also alters the dielectric screening, the analytical screening model is extended to include this effect. With all ingredients combined, the model qualitatively reproduces the experimentally observed tunability of the optical spectra. Experimentally, reflection (reflectance) spectroscopy on intrinsic MoS2 reveals clear excitonic transitions. Gate electrodes were added to the MoS2 sample to enable in situ electrical doping during reflectance measurements, but further work is needed to obtain definitive gating results.

[This abstract was generated with the help of AI]

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

Download
View record in AAU Student Projects