Browsing by Author "Varjovi, Mirali Jahangirzadeh"

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Open Access
First-principles investigation on the structural, vibrational, mechanical, electronic, and optical properties of MSi2Z4 (M: Pd and Pt, Z: N and P) monolayers
(American Physical Society, 2023-03-20) Varjovi, Mirali Jahangirzadeh; Kilic, M.E.; Durgun, Engin
The recent synthesis of two-dimensional (2D) layered crystals of $MoSi_{2}N_{4}$ and $WSi_{2}N_{4}$ has received significant attention due to their novel properties and potential applications. The inclusion of silicon during chemical vapor deposition growth enhances the ambient stability of transition metal nitrides and enables large-scale growth in monolayer limit. The transition metal and nitrogen elements can potentially be replaced with other metal and pnictogen atoms, which can constitute a new and large 2D family. In this respect, based on first-principles calculations, we design $MSi_{2}Z_{4}$ (M: Pd and Pt, Z: N and P) monolayers and investigate their structural, vibrational, mechanical, electronic, and optical properties. The dynamical, thermal, and mechanical stabilities of the proposed systems are confirmed through phonon band dispersion calculations, ab initio molecular dynamics simulations, and elastic tensor analyses, respectively. The computed Raman and infrared spectra of the nanosheets reveal that the frequency and the intensity of the most prominent peaks are specified by the type of pnictogen atom in the structures. The mechanical response of the considered systems in the elastic regime is investigated in terms of in-plane stiffness ($Y_{2D}$) and the Poisson's ratio ($_{ν}$). The obtained values of $Y_{2D}$ indicate that $MSi_{2}N_{4}$ monolayers are stiffer than MSi2P4 nanosheets and the ν values are found to be within the same range and close to the ductile-brittle transition limit. The electronic structure investigation shows that while $MSi_{2}N_{4}$ monolayers are quasidirect wide-band-gap semiconductors, the $MSi_{2}P_{4}$ monolayers are semiconductors with an indirect narrow band gap. Additionally, these monolayers have isotropic optical spectra and, depending on the type of pnictogen atom, significant optical absorption peaks within the infrared, visible, and ultraviolet regions can be attained. Additionally, it is found that $MSi_{2}N_{4}$ monolayers possess multiple bound excitons with strong exciton binding when electron-hole interactions are taken into account. Our results not only expand the class of 2D $MSi_{2}Z_{4}$ crystals but also offer valuable insights on the physical properties of the designed systems and suggest them as promising materials in diverse optoelectronic applications.
Open Access
First-principles study on structural, vibrational, elastic, piezoelectric, and electronic properties of the Janus BiXY (X= S, Se, Te and Y = F, Cl, Br, I) monolayers
(American Physical Society, 2021-03-09) Varjovi, Mirali Jahangirzadeh; Durgun, Engin
Broken inversion symmetry in atomic structure can lead to the emergence of specific functionalities at the nanoscale. Therefore, realizing 2D materials in Janus form is a growing field, which offers unique features and opportunities. In this paper, we investigate the structural, vibrational, elastic, piezoelectric, and electronic properties of Janus BiXY (X=S, Se, Te and Y=F, Cl, Br, I) monolayers based on first-principle methods. The structural optimization and vibrational frequency analysis reveal that all of the proposed structures are dynamically stable. Additionally, ab initio molecular dynamics simulations verify the thermal stability of these structures even at elevated temperatures. The mechanical response of the Janus BiXY crystals in the elastic regime is investigated in terms of in-plane stiffness and the Poisson ratio, and the obtained results ascertain their mechanical flexibility. The piezoelectric stress and strain coefficient analysis demonstrates the appearance of strong out-of-plane piezoelectricity, which is comparable with the Janus transition metal dichalcogenide monolayers. The calculated electronic band structures reveal that except for BiTeF, all Janus BiXY monolayers are indirect band gap semiconductors, and their energy band gaps span from the infrared to the visible part of the optical spectrum. Subsequently, large Rashba spin splitting is observed in electronic band structures when the spin-orbit coupling is included. The obtained results point out Janus 2D BiXY structures as promising materials for a wide range of applications in nanoscale piezoelectric and spintronics fields.
Restricted
Prediction of new generation two-dimensional ternary structures and investigation of their fundamental properties
(2023-05) Varjovi, Mirali Jahangirzadeh
Consecutive to the isolation of graphene and uncovering its extraordinary properties, the dynasty of two-dimensional (2D) materials has expanded rapidly. A realization of every new member suggests novel features, holding the promise to be used in current and prospective nanodevices. In parallel with the attempts on exploring new 2D systems, the formation of ternary configurations has been suggested as an alternative approach to tailor the inherent properties of the already existing 2D structures. In accordance with recent advancements in ternary 2D systems, in this dissertation, we design and investigate the 2D systems which possess three types of elements in their crystal structure. In this regard, we design Janus Al2XX′ (X/X′: O, S, Se, Te) crystals, 1H, 1T, and 1T′ phases of Janus WXO (X = S, Se, and Te) monolayers and Janus BiXY (X= S, Se, Te, and Y = F, Cl, Br, I ) nanosheets and investigate their structural, vibrational, elastic, piezoelectric, and electronic properties by first-principle methods. In addition, inspired by the synthesis of penta-Si nanoribbons, advancements in 2D pentagonal systems, and recent developments on ternary structures, we propose and investigate a new ternary pentagon-based 2D monolayer, namely penta-BNSi. We study the mechanical, electronic, piezoelectric, photocatalytic, and optical properties of penta-BNSi crystal and reveal its suitability to be used in optoelectronics and photocatalytic applications. Then, we focused our attention on new family of MA2Z4 monolayers. Based on this motivation, we perform a comprehensive study on physical properties of MSi2Z4 (M: Pd and Pt, Z: N and P) monolayers and suggested novel single layer of InSiN2 (In2Si2N4). For this purpose, in both studies, the ground state configurations of the designed materials are determined, and then the dynamical and thermal stability of these nanosheets are investigated using phonon spectrum analysis and ab initio molecular dynamic (AIMD) simulations, respectively. Next, each structure’s Raman and infrared (IR) spectrum are analyzed, and the corresponding atomic displacements of the optical phonon modes are presented. Then, the mechanical properties are studied in terms of in-plane stiffness and Poisson’s ratio. The electronic band structures are computed in the electronic properties section, and the corresponding energy band gaps are reported. For MSi2Z4 (M: Pd and Pt, Z: N and P) monolayers, the optical response is examined via calculation of the complex dielectric function by taking many-body interactions into account. In the final study, the effect of an external biaxial strain on the electronic and vibrational properties of the InSiN2 nanosheet is investigated, and the variation of the obtained properties under strain is illustrated. As a result of a thorough theoretical study focusing on ternary 2D materials, it can be said that the examined crystals are stable systems with potential applications in a wide range of nanoelectronics and nanomechanical devices.
Open Access
Structural, vibrational, elastic, electronic, and piezoelectric properties of binary γ-GeX and ternary γ-Ge2XX′ monolayers (X, X′= S, Se, and Te)
(American Physical Society, 2023-05-11) Varjovi, Mirali Jahangirzadeh; Ershadrad, S.; Sanyal, B.
The recent synthesis of a new polymorph of two-dimensional (2D) germanium monochalcogenides, namely, γ-GeSe with a four-atomic-layer-thick hexagonal lattice, has received considerable attention due to its novel properties and potential applications. This exciting advancement paves the path for extensive experimental and theoretical investigations on the family of γ-MX crystals in which M and X are elements of group IV and VI, respectively. In this regard, herein we conduct first-principles-based calculations to explore the structural, vibrational, mechanical, electronic, and piezoelectric properties of γ-GeX and Janus γ-Ge2XX′ (X/X′: S, Se, and Te) monolayers. We performed a detailed analysis of the suggested systems' dynamical, thermal, and mechanical stability through phonon-band-dispersion calculations, ab initio molecular dynamics (AIMD) simulations, and elastic tensor analyses, respectively, and all six possible nanosheets are found to be stable. The computed Raman spectra of the monolayers reveal that, different from binary systems, the formation of Janus monolayers results in the appearance of additional Raman active modes. The mechanical response of the proposed crystals is examined by calculating in-plane stiffness (Y2D) and the Poisson's ratio (ν) within the elastic regime, and the obtained results ascertain their flexibility. It is found that similar to their binary counterparts, Janus monolayers are indirect-band-gap semiconductors, and their valence-band maxima show a Mexican hat dispersion along the high-symmetry points of the Brillouin zone. Additionally, it is demonstrated that the construction of Janus crystals enhances the piezoelectric coefficients of γ-GeX monolayers, both in the in-plane and out-of-plane directions. Our findings not only provide a comprehensive insight into physical and electronic properties of γ-GeX and γ-Ge2XX′ monolayers but also reveal their promising features for various nanoelectronic and nanoelectrochemical applications.
Open Access
Ternary pentagonal BNSi monolayer: Two-dimensional structure with potentially high carrier mobility and strong excitonic effects for photocatalytic applications
(American Physical Society, 2022-03-11) Varjovi, Mirali Jahangirzadeh; Kılıç, M. E.; Durgun, Engin
In recent years many attempts have been made to discover new types of two-dimensional (2D) nanostructures with novel properties beyond the hexagonal crystals. The prediction of pentagraphene has sparked a great deal of research interest to investigate 2D pentagonal systems. In line with these efforts, in this paper, we propose a 2D ternary pentagonal monolayer of BNSi (penta-BNSi) and systematically investigate its structural, vibrational, mechanical, piezoelectric, electronic, photocatalytic, and optical properties by performing first-principles methods. We verify the stability of the penta-BNSi monolayer from the dynamical, thermal, and mechanical aspects based on phonon dispersion, ab initio molecular dynamics, and elastic tensor analysis, respectively. The mechanical properties are examined by calculating in-plane stiffness (Y2D), Poisson's ratio (ν), and ultimate tensile strength and penta-BNSi is found to be soft and ductile. The electronic structure and electronic transport calculations indicate that the penta-BNSi monolayer possesses a quasidirect band gap and anisotropic, potentially high carrier mobility. Due to the noncentral symmetric character and semiconducting feature, an intrinsic piezoelectric response emerges in the structure. In addition, penta-BNSi has a suitable band gap as well as proper band edge positions for photocatalytic water splitting within practical pH levels. The analysis of optical properties, including many-body effects, points out strong exciton binding and high light absorption in the visible and near-UV parts of the spectrum. Our findings not only expand the family of 2D pentagonal materials but also uncover an ideal ultrathin material for photocatalytic applications.