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Recent studies have revealed that Janus structures and heterobilayers made from them might have properties superior to those of two-dimensional (2D) materials. We construct 2D monolayer boron phosphide (MBP)/MoSSe and MoSSe/MBP heterobilayers and describe comprehensively their optoelectronic properties in the presence of biaxial in-plane and uniaxial out-of-plane strain and the effects of electric fields using first-principles methods. The electronic bands of both MBP/MoSSe and MoSSe/MBP heterobilayers display a peculiarity in the direct gap, with different band-gap values in the pristine forms. Remarkably, it is shown that different varieties of band alignments are induced for different orders of the van der Waals (vdW) heterobilayers. Electric fields and in-plane and out-of-plane strain give rise to important changes in the electronic and optical properties. The band alignments can transform from type I to type II under the influence of an electric field and strain. The main absorption peaks of pristine forms of both heterobilayers have in the visible region (approximately 2.67 eV), while in the presence of biaxial strain are redshifted. Since the absorption peaks of the pristine forms of the heterobilayers are in the visible region, the heterobilayers recommended here can be used in photovoltaic applications, and the presence of effects of external electric fields and strain promises functional optoelectronic devices.

By using density functional theory (DFT) based first-principles calculations, electronic, vibrational, piezo-electric, and optical properties of monoclinic Janus single-layer Ga2TeX (X = S or Se) are investigated. The dynamical, mechanical, and thermal stability of the proposed Janus single layers are verified by means of phonon bands, stiffness tensor, and quantum molecular dynamics simulations. The calculated vibrational spectrum reveals the either pure or coupled optical phonon branches arising from Ga-Te and Ga-X atoms. In addition to the in-plane anisotropy, single-layer Janus Ga2TeX exhibits additional out-of-plane asymmetry, which leads to important consequences for its electronic and optical properties. Electronic band dispersions indicate the direct band-gap semiconducting nature of the constructed Janus structures with energy band gaps falling into visible spectrum. Moreover, while orientation-dependent linear-elastic properties of Janus single layers indicate their strong anisotropy, the calculated in-plane stiffness values reveal the ultrasoft nature of the structures. In addition, predicted piezoelectric coefficients show that while there is a strong in-plane anisotropy between piezoelectric constants along armchair (AC) and zigzag (ZZ) directions, there exists a tiny polarization along the out-of-plane direction as a result of the formation of Janus structure. The optical response to electromagnetic radiation has been also analyzed through density functional theory by considering the independent-particle approximation. Finally, the optical spectra of Janus Ga2TeX structures is investigated and it showed a shift from the ultraviolet region to the visible region. The fact that the spectrum is between these regions will allow it to be used in solar energy and many nanoelectronics applications. The predicted monoclinic single-layer Janus Ga2TeX are relevant for promising applications in optoelectronics, optical dichroism, and anisotropic nanoelasticity.

The possibility of breaking structural symmetry with realization of Janus monolayers offers new possibilities in the field of two-dimensional (2D) materials, and various ternary systems including the class of group-III monochalcogenides have been suggested. However, interaction of oxygen was shown to modify optoelectronic properties of gallium monochalcogenides, and design of ternary systems with oxygen as a third component has not been considered yet. In this paper, we design and investigate 2D Ga2XO (X = S, Se, Te) systems by using first-principles calculations. Phonon spectra analysis and molecular dynamics simulations indicate that while Ga2SO and Ga2SeO are stable even at high temperatures Ga2TeO is dynamically unstable. Inclusion of oxygen makes Ga2SO and Ga2SeO less brittle when compared to their binary constituents. While GaX monolayers have indirect band gaps, Ga2SO and Ga2SeO become direct band-gap semiconductors and the band gap can be further tuned by tensile/compressive strain. Additionally, depending on the type of the system, strong optical absorption within the infrared, visible, and/or ultraviolet region is also predicted. Finally, structural and electronic properties of bilayers of Ga2XO are examined and compared with monolayers. Our results not only predict stable 2D ternary Ga2XO structures but also suggest them as promising materials for optoelectronic applications.

We propose a spin valve based on a blue-phosphorus monolayer sandwiched between two half-metallic two-dimensional CrN layers. We use density-functional theory combined with Boltzmann transport theory to investigate both the structural and the magnetic stability of the CrN/P/CrN heterostructure and to study its spin-dependent transport properties. Among the different possible layer stackings considered, only one is shown to be thermodynamically stable, corresponding to the AA stacking. In this geometry, the critical temperature of magnetic ordering is estimated to be around 150 K. The electronic structure of CrN/P/CrN is strongly dependent on the mutual orientation of the magnetic moments in individual CrN layers. If the alignment is parallel, only one spin channel predominantly contributes to the electronic bands in the vicinity of the Fermi energy. In the case of an antiparallel alignment, both spin channels contribute to the electronic states. The alteration of magnetic moments affects electronic transport, causing magnetoresistance of up to 12% at moderate dopings. © 2019 American Physical Society. The authors acknowledge Ankara University for use of the high-performance computing facility through the AYP under Grant No. 17A0443001. A.N.R. acknowledges support from the FLAG-ERA JTC2017 Project “Gransport”.

WOS: 000435668100011 An ab-initio study of structural, elastic, electronic and optical properties of KCaP compound with space group F (4) over bar 3m have been reported using the plane-wave pseudo-potential technique based on density functional theory under pressure effect. The elastic anisotropy of compound is investigated in terms of Poisson's ratio, shear modulus and Young's modulus with pressure effect. The electronic band structure calculations and charge densities are obtained. Optical properties are also investigated and refraction index, extinction coefficient, reflectivity and loss function of KCaP compound are determined. Ankara UniversityAnkara University [17A0443001] Y. Mogulkoc acknowledges Ankara University for financial support under AYP with 17A0443001 project number.

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International audience; no abstract

WOS: 000349310200030, First-principles calculations are used to investigate the structural, elastic and vibration properties of the binary intermetallic compounds AgRE (RE = Ho, Er, Tm) with a B2 (CsCl) structure using density functional theory. The calculated structural parameters, the bulk modulus and its derivative with respect to pressure are in good agreement with experimental and numerical data. Additionally, the negative energy of formation of AgRE (RE = Ho, Er, Tm) with a B2 structure is examined. The independent second order elastic constants and their related properties, which are essential for mechanical stability, are investigated under pressure (0-60 GPa). The phonon spectra and phonon density of states are also discussed. All calculated phonon modes for AgRE (RE = Ho, Er) are positive in the phonon dispersion diagrams., Gazi University Research-Project Unit [05/2010-51]; Ankara University Research-Project Unit [12B4343020], This work was supported by Gazi University Research-Project Unit under Project No. 05/2010-51 and by Ankara University Research-Project Unit under Project No. 12B4343020. The numerical calculations reported in this paper were performed at TUBITAK ULAKBIM, High Performance and Grid Computing Center (TR-Grid e-Infrastructure).