M.S. Thesis Defence by Yılmaz Can Yüksek
Thesis Advisor: Prof. Dr. Oğuz Gülseren
Thesis Topic: Electronic and Optical Properties of Example 2D Systems Under the Vertical Electric Field
From optics to nanoelectronics, two-dimensional (2D) materials have drawn attention due to their extraordinary properties such as high carrier mobility, good thermal and electrical conductivity, and mechanical strength. Electronic and optical properties of example 2D systems containing single-layer graphene, 2D molybdenum carbide (Mo2C), and 2D tungsten diselenide (WSe2) under the vertical static electric field (E-field) varying between 0.1 V/Å and 2.5 V/Å are investigated by the first principles calculations based on the density functional theory. Contributions of van der Waals interactions are included by selecting a suitable exchange-correlation functional. Electronic band structure and density of states information confirmed that monolayer graphene and single-layer Mo2C exhibit metallic properties whereas 2D WSe2 is a semiconductor with a direct band gap. For all systems up to some magnitude of the E-field, the bands in the valance band were found to be degenerate whereas shifts took place in the conduction band as the E-field was introduced to the system. By increasing E-field amplitudes, the Dirac point shifted upwards in graphene, and σ* band shifted below the Fermi level at 0.5 V/Å. In addition to four well-known interband transitions (π→ π*, σ → σ*, σ → π*, π → σ*), an indirect σ* → π* transition is observed. After an electric field amplitude (EZ) of 0.8 V/Å, bands due to the s-orbitals of Mo atoms in monolayer Mo2C shifted below the Fermi level. Additionally, π plasmon peaks redshifted up to 0.4 V/Å and blueshifted for 0.6 V/Å ≤ EZ ≤ 2.5 V/Å. For the monolayer WSe2 system, the band gap becomes zero when EZ ≥ 1.0 V/Å which indicates a semiconductor-to-metal transition under the E-field. Shifts below the Fermi level enabled us to n-dope those systems.
Keywords: Vertical static electric field, graphene, 2D Mo2C, 2D WSe2, electronic properties, optical properties, interband transitions, doping, density functional theory.
Date: July 31 Wednesday
Time: 11:00
Place: Department of Physics Meeting Room SA-211b