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Öğe Monolayer diboron dinitride: Direct band-gap semiconductor with high absorption in the visible range(AMER PHYSICAL SOC, 2020) Demirci, Salih; Rad, Soheil Ershad; Kazak, Sahmurat; Nezir, Saffet; Jahangirov, SeymurWe 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.Öğe Nanostructures of Molybdenum, Chromium, and Tungsten Constructed by a Basic Structural Unit(Amer Chemical Soc, 2024) Demirci, Salih; Jahangirov, Seymur; Ciraci, SalimWe predicted a stable, zigzag chain structure of Molybdenum Mo, which, being a basic structural unit, can construct stable 0D or quantum dot (segments of atomic chains and rings), 1D (various infinite chains), quasi-1D (nanoribbons and nanotubes), and 2D (bilayers) structures entailing critical magnetic, elastic, and electronic properties. This zigzag chain, constituted by strong Mo-Mo covalent bonds, is a nonmagnetic semiconductor but undergoes an insulator-metal transition under compression acquiring a ferromagnetic state. Although Mo cannot form stable, suspended, 2D monolayers of a single atomic plane, parallel zigzag chains bound by metallic interchain interaction can construct stable bilayers of different symmetry, and also their stacks forming slabs. These bilayers, being elemental Dirac materials and displaying directional properties, clarify the ambiguous situation in recent studies synthesizing and/or predicting different 2D molybdenene structures. Notably, other Group VIB elements, chromium, and tungsten, also construct similar atomic chains and 2D bilayers with diverse physical properties. Intriguingly, an atomic chain structure serves as a building block for many low-dimensional materials with diverse physical properties, making it unique in nanoscience.Öğe Strain engineering of electronic and optical properties of monolayer diboron dinitride(Amer Physical Soc, 2021) Demirci, Salih; Rad, Soheil Ershad; Jahangirov, SeymurWe 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.Öğe Structural and electronic properties of monolayer group III monochalcogenides (conferenceObject)(Amer Chemical Soc, 2019) Demirci, Salih; Avazli, Nurlan; Durgun, Engin; Jahangirov, Seymur…Öğe Temperature, strain and charge mediated multiple and dynamical phase changes of selenium and tellurium(ROYAL SOC CHEMISTRY, 2020) Demirci, Salih; Gürel, Hikmet Hakan; Jahangirov, Seymur; Çıracı, SalimSemiconducting selenium and tellurium in their 3D bulk trigonal structures consist of parallel and weakly interacting helical chains of atoms and display a number of peculiarities. We predict that thermal excitations, 2D compressive strain and excess charge of positive and negative polarity mediate metal-insulator transitions by transforming these semiconductors into different metallic crystal structures. When heated to high temperature, or compressed, or charged positively, they change into a simple cubic structure with metallic bands, which is very rare among elemental crystals. When charged negatively, they transform first into body-centered tetragonal and subsequently into the body-centered orthorhombic structures with increasing negative charging. These two new structures stabilized by excess electrons also have overlapping metallic bands and quasi 2D and 1D substructures of lower dimensionality. Since the external charging of crystals can be achieved through their surfaces, the effects of charging on 2D structures of selenium and tellurium are also investigated. Similar structural transformations have been mediated also in 2D nanosheets and free-standing monolayers of these elements. These phase changes assisted by phonons are dynamical, reversible and tunable; the resulting metal-insulator transitions can occur within very short time intervals and may offer important device applications.
