Browsing by Author "Rad, Soheil Ershad"

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Open Access
Boron-pnictogens: Highly anisotropic two-dimensional semiconductors for nanoelectronics and optoelectronics
(American Physical Society, 2022-06-16) Kılıç, M. E.; Rad, Soheil Ershad; İpek, S.; Jahangirov, Seymur
Two-dimensional materials open up tremendous opportunities for nanoelectronics and optoelectronics. Using first-principles density functional methods, we predict a family of two-dimensional boron-pnictogen materials. Our results show that these materials have excellent energetic, dynamical, thermal, mechanical, and chemical stabilities. The intrinsic structural anisotropy found in these materials leads to highly direction-dependent mechanical, electronic, and optical properties. They possess highly anisotropic Young's modulus and Poisson's ratio. The tensile strength under uniaxial and biaxial deformations is found to be very high for these materials. Electronically, they are all semiconductors with narrow band gaps. The band gap energies can be tuned by alloying, strain engineering, and chemical functionalization. They exhibit anisotropic and high carrier mobility. All these electronic properties make them promising candidates for nanoelectronic device applications. Using state-of-the-art GW- Bethe-Salpeter equation approach, taking the electron-hole effect into account, the prominent optical absorption structure with strong anisotropy in the visible light region endow the boron-pnictogen materials with great potential in optoelectronics.
Open Access
Monolayer diboron dinitride: direct band-gap semiconductor with high absorption in the visible range
(American Physical Society, 2020) Demirci, Salih; Rad, Soheil Ershad; Kazak, Sahmurat; Nezir, S.; Jahangirov, Seymur
We predict a two-dimensional monolayer polymorph of boron nitride in an orthorhombic structure (o-B2N2) using first-principles calculations. Structural optimization, phonon dispersion, and molecular dynamics calculations show that o-B2N2 is thermally and dynamically stable. o-B2N2 is a semiconductor with a direct band gap of 1.70 eV according to calculations based on hybrid functionals. The structure has high optical absorption in the visible range in the armchair direction while low absorption in the zigzag direction. This anisotropy is also present in electronic and mechanical properties. The in-plane stiffness of o-B2N2 is very close to that of hexagonal boron nitride. The diatomic building blocks of this structure hint at its possible synthesis from precursors having B-B and N-N bonds.
Open Access
Strain engineering of electronic and optical properties of monolayer diboron dinitride
(American Physical Society, 2021-11-29) Demirci, Salih; Rad, Soheil Ershad; Jahangirov, Seymur
We studied the effect of strain engineering on the electronic, structural, mechanical, and optical properties of orthorhombic diboron dinitride (o-B2N2) through first-principles calculations. The 1.7-eV direct band gap observed in the unstrained o-B2N2 can be tuned up to 3 eV or down to 1 eV by applying 12% tensile strain in armchair and zigzag directions, respectively. Ultimate strain values of o-B2N2 were found to be comparable with that of graphene. Our calculations revealed that the partial alignment of the band edges with the redox potentials of water in pristine o-B2N2 can be tuned into a full alignment under the armchair and biaxial tensile strains. The anisotropic charge carrier mobility found in o-B2N2 prolongs the average lifetime of the carrier drift, creating a suitable condition for photoinduced catalytic reactions on its surface. Finally, we found that even in extreme straining regimes, the highly anisotropic optical absorption of o-B2N2 with strong absorption in the visible range is preserved. Having strong visible light absorption and prolonged carrier migration time, we propose that strain engineering is an effective route to tune the band gap energy and band alignment of o-B2N2 and turn this two-dimensional material into a promising photocatalyst for efficient hydrogen production from water splitting.
Open Access
Two-dimensional tetrahexagonal CX2 (X= P, As, Sb) semiconductors for photocatalytic water splitting under visible light
(American Physical Society, 2022-03-07) Kılıç, M. E.; Rad, Soheil Ershad; Jahangirov, Seymur
In this paper, we introduce a family of two-dimensional group-V carbides with an ordered sequence of tetragons and hexagons (th-CX2, where =P, As, and Sb). We demonstrate that th-X2 monolayers exhibit robust energetic, dynamical, thermal, and mechanical stability. Our calculations show that the intrinsic structural anisotropy of the th-CX2 family induces strongly anisotropic mechanical, electronic, and optical behavior. These monolayers offer ultrahigh ultimate tensile strength, comparable with that of graphene, making them suitable for strain engineering of electronic and optical properties. They are semiconductors in nature, where th-CP2, th-CAs, and th-CSb2 possess quasidirect, direct, and indirect band gaps, respectively. The band gaps of thCP2 and th-CAs2 are wide enough to provide the photogenerated energy required for the splitting of water. Besides, the positions of band edges are in alignment with the water oxidation and reduction potentials. For th-CSb2, however, the suitable width of the band gap and the appropriate band edge positions for photocatalytic water splitting are achieved by strain engineering. Both indirect-to-direct and direct-to-indirect band gap transitions can be induced in th-CX2 compounds through strain engineering. The th-CX2 monolayers offer anisotropic high charge carrier mobility, which prolongs the average lifetime of charge carrier drift. They have good optical absorption (∼105 cm−1) in the visible and ultraviolet regions of the light spectrum. W0+BSE (Bethe-Salpeter equation) calculations reveal that they exhibit strong excitonic effects where the first bright excitonic binding energy is calculated as 0.27, 0.52, and 0.22 eV for th-C2, th-CAs2, and th-CSb2, respectively. Having all these features in one package, the th-CX2 monolayers are among the best candidates for high-performance photocatalytic water splitting.