Browsing by Author "Jahangirov, Seymur"

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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
Characterization and stability of Janus TiXY (X/Y = S, Se, and Te) monolayers
(American Chemical Society, 2019) Moğulkoç, A.; Moğulkoç, Y.; Jahangirov, Seymur; Durgun, Engin
The realization of Janus MoSSe monolayers has brought two-dimensional (2D), ternary transition metal dichalcogenides (TMDs) into focus. The addition of a third element can lead to superior properties, hence extensive analyses on the characterization of these sophisticated systems are required to reveal their full potential. In this study, we examine the structural, mechanical, electronic, thermal, and optical properties of TiXY (X/Y = S, Se, and Te) monolayers by using first-principles techniques. In addition to the common 1T form, the 2H phase is considered, and the stability of both phases is revealed by phonon spectrum analysis and molecular dynamics simulations. Following the investigation of the mechanical response, electronic structures are examined together with partial density of states analysis. While monolayers of 1T-TiXY are found to be semimetals, monolayers of 2H-TiXY are semiconductors with indirect band gap. The optical spectrum is obtained by calculating the frequency-dependent imaginary dielectric function and is correlated with the electronic structure. The variation of heat capacity with temperature is investigated, and low-/high-temperature response is shown. Finally, possible structural distortions/transformations are also taken into account, and charge density wave transition in 1T-TiSeS due to Peierls instability is demonstrated. Our results not only reveal the stable Janus monolayers of 2H- and 1T-TiXY but also point out these systems as promising candidates for nanoscale applications.
Open Access
Deterministic phase transitions and self-organization in logistic cellular automata
(American Physical Society, 2019) İbrahimi, Muhamet; Gülseren, Oğuz; Jahangirov, Seymur
We present a simple extension in which a single parameter tunes the dynamics of cellular automata (CA) by consequently expanding their discrete state space into a Cantor set. Such an implementation serves as a potent platform for further investigation of several emergent phenomena, including deterministic phase transitions, pattern formation, autocatalysis, and self-organization. We first apply this approach to Conway's Game of Life and observe sudden changes in the asymptotic dynamics of the system accompanied by the emergence of complex propagators. Incorporation of the new state space with system features is used to explain the transitions and formulate the tuning parameter range where the propagators adaptively survive by investigating their autocatalytic local interactions. Similar behavior is present when the same recipe is applied to Rule 90, an outer totalistic elementary one-dimensional cellular automaton. In addition, the latter case shows that deterministic transitions between classes of CA can be achieved by tuning a single parameter continuously.
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
Operator representation and class transitions in elementary cellular automata
(Complex Systems Publications,, 2022) İbrahimi, M.; Güçlü, A.; Jahangirov, Naide; Yaman, M.; Gülseren, Oğuz; Jahangirov, Seymur
We exploit the mirror and complementary symmetries of elementary cellular automata (ECAs) to rewrite their rules in terms of logical operators. The operator representation based on these fundamental symmetries enables us to construct a periodic table of ECAs that maps all unique rules in clusters of similar asymptotic behavior. We also expand the elementary cellular automaton (ECA) dynamics by introducing a parameter that scales the pace with which operators iterate the system. While tuning this parameter continuously, further emergent behavior in ECAs is unveiled as several rules undergo multiple phase transitions between periodic, chaotic and complex (class 4) behavior. This extension provides an environment for studying class transitions and complex behavior in ECAs. Moreover, the emergence of class 4 structures can potentially enlarge the capacity of many ECA rules for universal computation.
Embargo
Spectroscopic ellipsometry and raman spectroscopy of Bi1-xSbxTeI solid solutions with x≤0.1
(Elsevier BV, 2023-02-03) Aliev, Z. S.; Alizade, E. H.; Mammadov, D. A.; Jalilli, J. N.; Aliyeva, Y. N.; Abdullayev, N. A.; Ragimov, S. S.; Bagirova, S. M.; Jahangirov, Seymur; Mamedov, N. T.; Chulkov, E. V.
Spectroscopic ellipsometry supported by reflectivity measurements and Raman spectroscopy are applied at room temperature to n-type Rashba semiconductors BiTeI and Bi$_{1-x}$Sb$_{x}$TeI with nominal compositions x = 0.05 and 0.1. Complementary four probe Hall measurements are made using standard ac technique and the concentration of free carriers is determined. The Raman spectra, including TO-LO resonances are found to occupy a frequency range below 250 cm$^{-1}$ in the studied materials. The pseudodielectric function, retrieved at different angles of incidence is analyzed focusing on the free carrier absorption (intraband transitions) and the optical transitions between the Rashba-split branches of the conduction band (intersubband vertical transitions). The former transitions with screened plasma frequency anchored to the zero-crossing point of the real part of the pseudodielectric function are described within a simple Drude model and the important parameters such as electron effective mass, electron mobility and high frequency dielectric constant are obtained. Anchored to the Rashba energy, the intersubband transitions obtained for each material by refining the pseudodielectric function from Drude contribution, appear in the imaginary part as a broad peak in the photon energy range between 0.15 and 0.4 eV. A noticeable red shift of this peak for Bi$_{0.9}$Sb$_{0.1}$TeI as compared to BiTeI is proposed to be a manifestation of the reduction of the spin splitting after part of Bi atoms is replaced by lighter Sb atoms.
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
Temperature, strain and charge mediated multiple and dynamical phase changes of selenium and tellurium
(Royal Society of Chemistry, 2020) Demirci, Salih; Gürel, H. H.; Jahangirov, Seymur; Çıracı, Salim
Semiconducting 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.
Open Access
Two dimensional ruthenium carbide: structural and electronic features
(Royal Society of Chemistry, 2020) Görkan, T.; Demirci, Salih; Jahangirov, Seymur; Gökoğlu, G.; Aktürk, E.
The design and realization of novel 2D materials and their functionalities have been a focus of research inspired by the successful synthesis of graphene and many other 2D materials. In this study, in view of first principles calculations, we predict a novel 2D material ruthenium carbide (RuC) in graphene-like honeycomb hexagonal lattice with planar geometry. Phonon dispersion spectra display a dynamically stable structure. Comprehensive molecular dynamics calculations confirm the stability of the structure up to high temperatures as ≈1000 K. The system is a narrow gap semiconductor with a band gap of 53 meV (345 meV) due to GGA-PBE (HSE) calculations. Band gap exhibits significant changes by applied strain. Elastic and optical properties of the system are examined in monolayer form. RuC/RuC bilayer, RuC/graphene and RuC/h-BN heterostructures are also investigated. By calculating the phonon dispersion it is verified that RuC bilayer is the most stable in AA type-stacking configuration where Ru and C atoms of both layers have identical lateral coordinates. The effects of atomic substitutions on electronic band structures, acting as p-type and n-type doping, are revealed. A novel 3D RuCLi structure is also predicted to be stable and the isolation of its monolayer forms are discussed. Ruthenium carbide, as a 2D material which is dynamically and thermally stable, holds promise for applications in nanoelectronics.
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.