Many-body theory explored optical properties of selected 2D group II-VI monochalcogenides

Date
2022-09
Advisor
Gülseren, Oğuz
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Bilkent University
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English
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Abstract

Two-dimensional (2D) metal oxides (MOs) and metal chalcogenides (MChs) are emerging classes of 2D materials. Depending on the constituent elements, these materials can display various electronic and optical properties making them promising candidates in many device applications, such as solar cells and transparent circuits. Binary graphene-like structures of II-VI are the most straightforward structures of 2D MOs and MChs. We systematically examined the electronic and optical properties of selected 2D structures from this category: BeO, BaTe, CdO, CaO, CaS, MgO, SrS, SrSe and ZnO. The dynamical stability of these materials has been reported in previous studies. In 2D semiconductors, excitonic effects dominate the optical properties. Theoretical investigation of such phenomena requires employing many-body approaches beyond standard density functional theory. We utilized a single shot of GW approximation to predict the electronic band structure and solved the Bethe- Salpeter equation in the Tamm-Dancoff approximation to consider excitonic effects. Our results show that all structures possess indirect band gaps except ZnO and CdO. Furthermore, the considered structures have large exciton binding energies ranging from 0.72 eV in CdO to 2.84 eV in BeO. CdO has the smallest calculated optical band gap with a value of 1.43 eV. Analyzing the optical absorption spectra reveals that the CdO can absorb 7.9 % of the incident light in its optical band gap. The maximum amount of absorption appears in BeO, which can absorb 28% of incident light in the ultraviolet region. Among the structure mentioned above, there is a close matching between the lattice constants of ZnO and MgO, promising for creating lateral and vertical heterostructures. Due to the enhanced performance resulting from mixing distinct properties of individual monolayers, van der Waals heterostructures (vdWHs) are regarded as a revolutionary class among a plethora of presently fabricated or predicted 2D materials. Alongside vdWHs, recent studies have also reported 2D heterostructures with interlayer bonding. Motivated by the flourishing properties of vertical heterostructures, we comprehensively examined the mechanical, electronic and optical properties of ZnO/MgO structures in four different stackings. Structural relaxation has indicated two vdWHs and two structures with interlayer binding. All considered structures are mechanically stable. In addition, phonon dispersion curves show that the AB stacking formed by placing the Mg atom on top of the O atom of the ZnO layer is also dynamically stable at zero temperature. The s orbital of Zn atom dominates the minimum of the first conduction band of these structures. The optical absorbance spectra show that strong excitonic effects reduce the optical band gap to the visible light spectrum range, and all structures can absorb around 8% of incident light.

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2D materials, 2D metal chalcogenides and metal oxides, Optical properties, Density functional theory, Excitonic effects, Many-body theory, GW and Bethe-Salpeter equation
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Published Version (Please cite this version)