Structural, electronic, and magnetic properties of vanadium-based Janus dichalcogenide monolayers: A first-principles study

The structural, electronic, and magnetic properties of VSSe, VSeTe, and VSTe monolayers in both 2H and 1T phases are investigated via first-principles calculations. The 2H phase is energetically favorable in VSSe and VSeTe, whereas the 1T phase is lower in energy in VSTe. For V-based Janus monolayers in the 2H phase, calculations of the magnetic anisotropy show an easy-plane for the magnetic moment. As such, they should not exhibit a ferromagnetic phase transition, but instead, a Berezinskii-Kosterlitz-Thouless (BKT) transition. A classical XY model with nearest-neighbor coupling estimates critical temperatures (${\mathrm{T}}_{\text{BKT}}$) ranging from 106 K for VSSe to 46 K for VSTe.

Figure 1

Crystal structure of the (a) 1T and (b) 2H polymorphs of V-based Janus ($\mathrm{V}\mathit{XY}$) monolayers. V atoms are depicted in golden yellow, $X$ atoms in dark green, and $Y$ atoms in red. The $X$-V-$X, Y$-V-$Y$, and $X$-V-$Y$ bond angles are indicated as ${\theta }_{XX}, {\theta }_{YY}$, and ${\theta }_{XY}$, respectively. The distances between V and $X$ ($Y$) atoms are shown as ${\mathrm{d}}_X$ (${\mathrm{d}}_Y$). The crystal field environment for V atoms in the two phases is displayed on the bottom right side of (a) and (b) panels—octahedral for the 1T phase, trigonal prismatic for the 2H phase.

Figure 2

Phonon dispersions from GGA calculations in a FM state for VSSe (a), (b); VSeTe (c), (d); and VSTe (e), (f) monolayers in 1T (left panel) and 2H (right panel) phases along the high-symmetry direction $\mathrm{\Gamma }-\mathrm{M}-\mathrm{K}-\mathrm{\Gamma }$.

Figure 3

Top view of the effective triangular lattice formed by V ions in $\mathrm{V}\mathit{XY}$ monolayers showing (a) a ferromagnetic state, (b) a stripe antiferromagnetic state (${\mathrm{AFM}}_1$), and (c) a noncollinear ${120}^{\text{o}}$ antiferromagnetic state (${\mathrm{AFM}}_2$). V ions are shown as golden spheres.

Figure 4

Electronic band structures in the FM state for 2H-VSSe (a), (b); 2H-VSeTe (c), (d); and 2H-VSTe (e), (f); monolayers within GGA (left panels) and $\mathrm{GGA}+U$ (right panels). Red solid lines represent the majority spin channel and blue dotted lines the minority spin channel. (g) Brillouin zone of 2H-$\mathrm{V}\mathit{XY}$ monolayers showing the high-symmetry points used in the band-structure plots: $\mathrm{\Gamma }=(0,0,0), M=$ (1/2, 0, 0), $K=$(1/3, 1/3, 0), $A=$(0, 0, 1/2), $L=$(1/2, 0, 1/2), and $H=$(1/3, 1/3, 1/2).

Figure 5

Majority-spin band structures (right panels) and orbital-projected DOS for V $d$ states (middle panels) and anion (S, Se, Te) $p$ states (right panels) from FM $\mathrm{GGA}+U$ calculations in 2H-VXY monolayers. (a)–(c) correspond to plots for 2H-VSSe, (d)–(f) for 2H-VSeTe, and (g)–(i) for 2H-VSTe.