New C 2h phase of group III monochalcogenide monolayers AlX (X = S, Se, and Te) with anisotropic crystal structure: first-principles study.

In this paper, we introduce a new phase of two-dimensional aluminum monochalcogenide, namely C 2h-AlX (X = S, Se, and Te). With the C 2h space group, C 2h-AlX possesses a large unit cell containing 8 atoms. The C 2h phase of AlX monolayers is found to be dynamically and elastically stable based on the evaluation of its phonon dispersions and elastic constants. The anisotropic atomic structure of C 2h-AlX leads to a strong anisotropy in its mechanical properties with Young's modulus and Poisson's ratio strongly dependent on the directions examined in the two-dimensional plane. All three monolayers of C 2h-AlX are found to be direct band gap semiconductors, which are compared with the indirect band gap semiconductors of available D 3h-AlX. Particularly, the transition from direct to indirect band gap is observed in C 2h-AlX when a compressive biaxial strain is applied. Our calculated results indicate that C 2h-AlX exhibits anisotropic optical characteristics and its absorption coefficient is high. Our findings suggest that C 2h-AlX monolayers are suitable for applications in next-generation electro-mechanical and anisotropic opto-electronic nanodevices.

Structural, electronic, and transport properties of Janus GaInX2(X=S, Se, Te) monolayers: first-principles study.

Two-dimensional Janus monolayers have outstanding electronic and transport properties due to their asymmetric atomic structures. In the present work, we systematically study the structural, electronic, and transport properties of the Janus GaInX2(X= S, Se, Te) monolayers by using the first-principles calculations. The stability of the investigated monolayers is confirmed via the analysis of vibrational spectrum and molecular dynamics simulations. Our calculations demonstrate that while GaInS2and GaInSe2monolayers are direct semiconductors, GaInTe2monolayer exhibits the characteristics of an indirect semiconductor. The band gap of GaInX2decreases when the chalcogen elementXvaries from S to Te. Obtained results reveal that small spin-orbit splitting energy in the valence band is found around the Γ point of the Brillouin zone when the spin-orbit coupling is included. Interestingly, GaInS2and GaInSe2have high and directional isotropic electron mobility meanwhile the directional anisotropy of the electron mobility is found in the Janus GaInTe2monolayer. Our findings not only present superior physical properties of GaInX2monolayers but also show promising potential applications of these materials in nanoelectronic devices.

First-principles insights onto structural, electronic and optical properties of Janus monolayers CrXO (X = S, Se, Te).

The lacking of the vertical mirror symmetry in Janus structures compared to their conventional metal monochalcogenides/dichalcogenides leads to their characteristic properties, which are predicted to play significant roles for various promising applications. In this framework, we systematically examine the structural, mechanical, electronic, and optical properties of the two-dimensional 2H Janus CrXO (X = S, Se, Te) monolayers by using first-principles calculation method based on density functional theory. The obtained results from optimization, phonon spectra, and elastic constants demonstrate that all three Janus monolayers present good structural and mechanical stabilities. The calculated elastic constants also indicate that the Janus CrTeO monolayer is much mechanically flexible than the other two monolayers due to its low Young's modulus value. The metallic behavior is observed at the ground state for the Janus CrSeO and CrTeO monolayers in both PBE and HSE06 levels. Meanwhile, the Janus CrSO monolayer exhibits a low indirect semiconducting characteristic. The bandgap of CrSO after the correction of HSE06 hybrid functional is the average value of its binary transition metal dichalcogenides. The broad absorption spectrum of CrSO reveals the wide activated range from the visible to near-ultraviolet region. Our findings not only present insight into the brand-new Janus CrXO monolayers but can also motivate experimental research for several applications in optoelectric and nanoelectromechanical devices.

A first-principles study on defect association and oxygen ion migration of Sm3+ and Gd3+ co-doped ceria.

First-principles calculations based on density functional theory were performed to investigate the co-doping effects of Sm and Gd in ceria on its oxygen ion conduction. The focus of this study is on the interactions between the cation dopants and an oxygen vacancy within the two adjacent tetrahedral sites of fluorite structure surrounding the oxygen migration path. Vacancy formation energies, dopant-vacancy association energies, and migration energies were calculated to elucidate the doping effects on oxygen ion conduction. The migration energies show remarkable dependences on the ionic radii of the cations located at the edges of the migration path. A simple relation between migration energy and vacancy formation energy is proposed. This work provides an informative insight into vacancy diffusion that could be useful in optimizing doping materials for improving oxygen ion conductivity in doped ceria.