Nanotechnology Research Center (NANOTAM)

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Browsing Nanotechnology Research Center (NANOTAM) by Subject "2-dimensional systems"

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    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, Engin
    In 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.
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    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, Engin
    Structural 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.
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    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, Engin
    The 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.