Electronic, vibrational, elastic, and piezoelectric properties of monolayer Janus MoSTe phases: A first-principles study

By performing density functional theory based first-principles calculations, the electronic, vibrational, elastic, and piezoelectric properties of two dynamically stable crystal phases of monolayer Janus MoSTe, namely $1H$-MoSTe and $1T^{\prime }$-MoSTe, are investigated. Vibrational frequency analysis reveals that the other possible crystal structure, $1T$-MoSTe, of this Janus monolayer does not exhibit dynamical stability. The $1H$-MoSTe phase is found to be an indirect band-gap semiconductor while $1T^{\prime }$-MoSTe is predicted as small-gap semiconductor. Notably, in contrast to the direct band-gap nature of monolayers $1H-{\mathrm{MoS}}_2$ and $1H-{\mathrm{MoTe}}_2, 1H$-MoSTe is found to be an indirect gap semiconductor driven by the induced surface strains on each side of the structure. The calculated Raman spectrum of each structure shows unique character enabling us to clearly distinguish the stable crystal phases via Raman measurements. The systematic piezoelectric stress and strain coefficient analysis reveals that out-of-plane piezoelectricity appears in $1H$-MoSTe and the noncentral symmetric $1T^{\prime }$-MoSTe has large piezoelectric coefficients. Static total-energy calculations show clearly that the formation of $1T^{\prime }$-MoSTe is feasible by using $1T^{\prime }-{\mathrm{MoTe}}_2$ as a basis monolayer. Therefore, we propose that the Janus MoSTe structure can be fabricated in two dynamically stable phases which possess unique electronic, dynamical, and piezoelectric properties.

Figure 1

For the three structural phases of monolayer Janus MoSTe crystal, (a) top and side views of the structures and (b)–(d) corresponding phonon band dispersions. The Brillouin zones of the hexagonal and rectangular lattices are given on the right-hand panel of the phonon bands.

Figure 2

For the monolayers of ${\mathrm{MoS}}_2, {\mathrm{MoTe}}_2$, and Janus MoSTe crystals, (a) Raman spectra for the $1H$ phases and the vibrational characteristics of the Raman-active phonon modes. (b) Those for their $1T^{\prime }$ phases.

Figure 3

The electronic band dispersions and the corresponding partial density of states (PDOS) of monolayer Janus; (a) $1H$-MoSTe and (b) $1T^{\prime }$-MoSTe. The Fermi level is set to zero.

Figure 4

For the monolayers of Janus $1H$- and $1T^{\prime }$-MoSTe crystals the orientation angle-dependent (a) in-plane stiffness $C(\theta$) and (b) Poisson ratio $\nu (\theta$). $AC$ and $ZZ$ in $1H$ phase stand for the armchair and zigzag directions, respectively, while $a$ and $b$ in $1T^{\prime }$ phase represent the directions perpendicular and parallel to the Mo chains, respectively.