Browsing by Subject "2-dimensional systems"
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Item Open Access Columnar antiferromagnetic order of a MBene monolayer(American Physical Society, 2021-04-16) Ozdemir, I.; Kadioglu, Y.; Yüksel, Y.; Akıncı, Ü.; Üzengi Aktürk, O.; Aktürk, E.; Çıracı, SalimFirst-principles density functional theory, combined with the Monte Carlo method, predicts that the Fe2B2 monolayer of the MBene family has a stable columnar antiferromagnetic (AFM) ground state. Below the critical temperature, Tc=115 K in equilibrium, the spins rotate by the same amount in every other column of Fe atoms, but they retain the same direction in the same column. Under applied tensile strains, Tc and the order parameter can increase nonmonotonically. The onset of the columnar order can result in a transition from two dimension (2D) to 1D in magnetic, electronic, and conduction properties. The ordered magnetic state itself can be tuned by external magnetic field, whereby the columnar magnetic order changes to ferromagnetic order with a double hysteresis behavior. When terminated by Fluorine atoms, the columnar order changes to the AFM order with Tc rising above room temperature. This situation is rather unusual and insofar is fundamental for a realistic, strictly 2D monolayer and can have critical consequences in spin conduction.Item Open Access Design of nanoscale capacitors based on metallic borophene and insulating boron nitride layers(American Physical Society, 2021-12-13) Mogulkoc, Y.; Mogulkoc, A.; Guler, H. E.; Durgun, EnginIn alignment with the efforts on miniaturizing the components of electronic devices with enhanced performance, we investigate a dielectric nanocapacitor (DNC) based on metallic borophene electrodes separated with insulating hexagonal boron nitride (h-BN) monolayers (n=1–5). The capacitive performance of the proposed DNC as a function of applied electric field (→E) and thickness of the dielectric material is examined by using ab initio methods. The borophene plates and h-BN monolayers are commensurate and coupled only with van der Waals interaction, which constitutes an ideal configuration as a DNC. It is found that a single h-BN layer is not thick enough as a spacer to hinder quantum tunneling effects, and similar to the case with no insulating layer, borophene electrodes are shorted. Being effective from two h-BN layers, the charge separation on borophene plates is attained via →E in the vertical direction. The capacitance of the DNC rapidly saturates at →E≥0.1V/Å and reaches its maximum value of 0.77μF/cm2 for n=2. The capacitance decreases with an increasing number of insulating layers as the distance between electrodes enlarges and shows a similar trend that is expected from the classical Helmholtz model. Our results suggest metallic and lightweight borophene and insulating h-BN monolayers as ideal constituents for the DNC design.Item Open Access Hydrogen-induced sp2-sp3 rehybridization in epitaxial silicene(American Physical Society, 2017) Solonenko, D.; Dzhagan, V.; Cahangirov, S.; Bacaksiz, C.; Sahin, H.; Zahn, D. R. T.; Vogt, P.We report on the hydrogenation of (3×3)/(4×4) silicene epitaxially grown on Ag(111) studied by in situ Raman spectroscopy and state-of-the-art ab initio calculations. Our results demonstrate that hydrogenation of (3×3)/(4×4) silicene leads to the formation of two different atomic structures which exhibit distinct spectral vibrational modes. Raman selection rules clearly show that the Si atoms undergo a rehybridization in both cases from a mixed sp2-sp3 to a dominating sp3 state increasing the distance between the two silicene sublattices. This results in a softening of the in-plane and a stiffening of the out-of-plane phonon modes. Nevertheless, hydrogenated epitaxial silicene retains a two-dimensional nature and hence can be considered as epitaxial silicane. The level of hydrogenation can be determined by the intensity ratio of the Raman modes with different symmetries. © 2017 American Physical Society.Item Open Access Janus two-dimensional transition metal dichalcogenide oxides: first-principles investigation of WXO monolayers with X=S, Se, and Te(American Physical Society, 2021-05-26) Varjovi Jahangirzadeh, Mirali; Yagmurcukardes, M.; Peeters, F. M. F. M.; Durgun, EnginStructural symmetry breaking in two-dimensional materials can lead to superior physical properties and introduce an additional degree of piezoelectricity. In the present paper, we propose three structural phases (1H,1T, and 1T′) of Janus WXO (X=S, Se, and Te) monolayers and investigate their vibrational, thermal, elastic, piezoelectric, and electronic properties by using first-principles mods. Phonon spectra analysis reveals that while the 1H phase is dynamically stable, the 1T phase exhibits imaginary frequencies and transforms to the distorted 1T′ phase. Ab initio molecular dynamics simulations confirm that 1H- and 1T′−WXO monolayers are thermally stable even at high temperatures without any significant structural deformations. Different from binary systems, additional Raman active modes appear upon the formation of Janus monolayers. Although the mechanical properties of 1H−WXO are found to be isotropic, they are orientation dependent for 1T′−WXO. It is also shown that 1H−WXO monolayers are indirect band-gap semiconductors and the band gap narrows down the chalcogen group. Except 1T′-WSO, 1T′−WXO monolayers have a narrow band gap correlated with the Peierls distortion. The effect of spin-orbit coupling on the band structure is also examined for both phases and the alteration in the band gap is estimated. The versatile mechanical and electronic properties of Janus WXO monolayers together with their large piezoelectric response imply that these systems are interesting for several anoelectronic applications.Item 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, 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.Item Open Access Tuning structural and electronic properties of two-dimensional aluminum monochalcogenides: prediction of Janus Al2 X X′ (X / X′ : O, S, Se, Te) monolayers(American Physical Society, 2020) Demirtaş, Mehmet; Varjovi, M. Jahangirzadeh; Çiçek, Mert Miraç; Durgun, EnginThe realization of ternary, single-layer transition metal dichalcogenides has suggested a promising strategy to develop two-dimensional (2D) materials with alternative features. In this study, we design and investigate Janus aluminum monochalcogenide monolayers, Al2XX′ (X/X′=O,S,Se,Te) by using first-principles methods. Starting from binary constituents, the ternary structures are optimized without any constraint and ground-state configurations are obtained. The stability of these systems is tested by performing phonon spectra analysis and ab initio molecular dynamics simulations and all Al2XX′ monolayers other than AlTeO are confirmed to be dynamically stable. Mechanical properties are examined by calculating Young's modulus and Poisson's ratio and subsequently compared with binary counterparts. Monolayers of Al2XX′ have a brittle character but oxygenation makes them less stiff. The electronic structure is also analyzed and variation of the band gap with the type of chalcogen atoms is revealed. It is found that different from their binary counterparts, Al2XO monolayers are direct band-gap semiconductors. Additionally, modification of the electronic structure in the presence of biaxial compressive or tensile strain is investigated by taking into account possible indirect-direct band-gap transitions. Our results not only predict stable 2D ternary Al2XX′ structures but also point out them as promising materials for optoelectronic applications.