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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.

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.

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, and 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.

We investigate the structural, mechanical, and electronic properties of the two-dimensional hexagonal structure of group III-VI binary monolayers, MX (M = B, Al, Ga, In and X = O, S, Se, Te) using first-principles calculations based on the density functional theory. The structural optimization calculations and phonon spectrum analysis indicate that all of the 16 possible binary compounds are thermally stable. In-plane stiffness values cover a range depending on the element types and can be as high as that of graphene, while the calculated bending rigidity is found to be an order of magnitude higher than that of graphene. The obtained electronic band structures show that MX monolayers are indirect band-gap semiconductors. The calculated band gaps span a wide optical spectrum from deep ultraviolet to near infrared. The electronic structure of oxides (MO) is different from the rest because of the high electronegativity of oxygen atoms. The dispersions of the electronic band edges and the nature of bonding between atoms can also be correlated with electronegativities of constituent elements. The unique characteristics of group III-VI binary monolayers can be suitable for high-performance device applications in nanoelectronics and optics.

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