Soon after the synthesis of two-dimensional (2D) ultrathin black phosphorus and fabrication of field effect transistors thereof, theoretical studies have predicted that other group-VA elements (or pnictogens), N, As, Sb, and Bi can also form stable, single-layer (SL) structures. These were nitrogene in a buckled honeycomb structure, arsenene, antimonene, and bismuthene in a buckled honeycomb, as well as washboard and square-octagon structures with unusual mechanical, electronic, and optical properties. Subsequently, theoretical studies are followed by experimental efforts that aim at synthesizing these novel 2D materials. Currently, research on 2D pnictogens has been a rapidly growing field revealing exciting properties, which offers diverse applications in flexible electronics, spintronics, thermoelectrics, and sensors. This review presents an evaluation of the previous experimental and theoretical studies until 2019, in order to provide input for further research attempts in this field. To this end, we first reviewed 2D, SL structures of group-VA elements predicted by theoretical studies with an emphasis placed on their dynamical and thermal stabilities, which are crucial for their use in a device. The mechanical, electronic, magnetic, and optical properties of the stable structures and their nanoribbons are analyzed by examining the effect of external factors, such as strain, electric field, and substrates. The effect of vacancy defects and functionalization by chemical doping through adatom adsorption on the fundamental properties of pnictogens has been a critical subject. Interlayer interactions in bilayer and multilayer structures, their stability, and tuning their physical properties by vertical stacking geometries are also discussed. Finally, our review is concluded by highlighting new research directions and future perspectives on the challenges in this emerging field.

We investigate Janus and alloy structures of $\mathrm{Pt}$$X_n{Y}_{2-n}$ ($X$, $Y$ = $\mathrm{S}$, $\mathrm{Se}$, $\mathrm{Te}$; $0\le n\le 2$) materials on the basis of first-principles plane-wave simulations. Using cluster-expansion theory to study alloys of $\mathrm{Pt}$$X_n{Y}_{2-n}$ monolayers at various concentrations, for half coverage ($n=1$), our results indicate that Janus-type structures are not energetically the most favorable for $\mathrm{Pt}$$XY$ monolayers; however, they are dynamically and thermally stable. To distinguish Janus $\mathrm{Pt}$$XY$ structures, we report the Raman-active modes and compared them with those of bare $\mathrm{Pt}$$X_2$ monolayers. The electronic band gaps calculated with use of hybrid functionals are on par with available experimental data. Spin-orbit coupling significantly modifies the electronic band structure of $\mathrm{Pt}$$XY$ monolayers. Because of the electronegativity differences of different chalcogen atoms on each surface of Janus $\mathrm{Pt}$$XY$ structures, the arising dipole moment significantly modifies the band alignments on both surfaces. We find that hydrogen-evolution and oxygen-evolution reactions occur on different surfaces and that applied strain enhances the catalytic activity. We also investigate the monovacancy and stacking effects on the electronic properties of $\mathrm{Pt}$$X_2$ and $\mathrm{Pt}$$XY$ structures. Our results indicate that due to their intrinsic dipole moments and band gaps, Janus $\mathrm{Pt}$$XY$ monolayers are perfect candidates for water-splitting reactions.

Two-dimensional (2D) transition metal carbides and nitrides, called as MXenes, displaying astonishing properties are emerged as a new class of 2D layered materials. In this study, we have studied the structural, electronic, and magnetic properties of 2D bare $M_2\mathrm{C}$ and Janus Cr-based ${MM}^{\prime }\mathrm{C}$ metal carbides $(M\ne M^{\prime }=\mathrm{Cr}, \mathrm{Ti}, \mathrm{Sc}, \mathrm{V})$ based on density functional theory. We found that 2D Janus ${MM}^{\prime }\mathrm{C}$ MXenes are dynamically and thermally stable and show unique electronic and magnetic properties which are not seen in their individual bare cases. Our calculated electronic band structures indicate that the Janus ${MM}^{\prime }\mathrm{C}$ MXenes have Dirac-type band dispersion as seen in stanene with tiny band gaps. In addition, all considered bare $M_2\mathrm{C}$ MXenes show metallic property. However, Janus CrScC has dilute magnetic semiconducting property, CrVC shows half-metallicity, and CrTiC shows metallic property which can easily turn to half-metallicity by external effects. Our extensive density functional theory and Monte Carlo calculations indicate that Janus ${MM}^{\prime }\mathrm{C}$ MXenes also show different magnetic properties than their bare cases. While ${\mathrm{Cr}}_2\mathrm{C}$ and ${\mathrm{Sc}}_2\mathrm{C}$ have ferromagnetic spin orientations with $T_C=2000$ and 226 K Curie temperatures, respectively, CrScC shows ferrimagnetic character with antiferromagnetic Néel spin orientation and has $T_N=1120$ K magnetic phase transition temperature between ferrimagnetic and paramagnetic phases. Finally, external magnetic field effects on each crystal have also been elucidated in detail. The numerical outcomes reveal that Janus Cr-based ${MM}^{\prime }\mathrm{C}$ metal carbides have some unusual and interesting hysteresis characters. Our investigations presented here are not only interesting from a theoretical perspective, but also they show that Janus ${MM}^{\prime }\mathrm{C}$ MXenes are promising candidates for future spintronic applications, which should encourage their synthesis.

The electronic and magnetic properties of a material can be altered by strain engineering. We elucidate the strain dependence of electronic and magnetic properties in $\alpha \text{-}{\mathrm{RuCl}}_3$ monolayer by varying the biaxial in-plane tensile strain from 1% to 8%. The magnetic ground state of the $\alpha \text{-}{\mathrm{RuCl}}_3$ monolayer evolves from antiferromagnetic zigzag (AFM-ZZ) configuration to ferromagnetic (FM) under a biaxial in-plane tensile strain higher than 2%. In a strain-free state, the FM configuration has a direct bandgap of 0.54 eV, and the AFM-ZZ configuration has an indirect bandgap of 0.73 eV. The energy bandgap of the $\alpha \text{-}{\mathrm{RuCl}}_3$ monolayer undergoes a change by the variation of the tensile strain. Furthermore, a detailed Monte Carlo simulation has been implemented to investigate the magnetic properties of the considered system for varying values of tensile strain. Temperature dependencies of the thermodynamic quantities of interest as functions of strains display strong evidence supporting the first-principles calculations within density functional theory. Our Monte Carlo findings also suggest that the Curie temperature of the $\alpha \text{-}{\mathrm{RuCl}}_3$ monolayer tends to get higher up to 20.11 K with a tensile strain 8%, which means that applying a strain leads to getting a more stable FM ground state. In addition, we find that magnetocrystalline anisotropy in the $\alpha \text{-}{\mathrm{RuCl}}_3$ monolayer can be controlled by the applied strain.