Janus two-dimensional transition metal dichalcogenide oxides: First-principles investigation of WXO monolayers with X=S , Se, and Te

Structural symmetry breaking in two-dimensional materials can lead to superior physical properties and introduce an additional degree of piezoelectricity. In the present paper, we propose three structural phases ($1H, 1T$, and $1{T}^{\ensuremath{'}}$) of Janus $\mathrm{W}X\mathrm{O}$ ($X=\mathrm{S}$, Se, and Te) monolayers and investigate their vibrational, thermal, elastic, piezoelectric, and electronic properties by using first-principles methods. Phonon spectra analysis reveals that while the $1H$ phase is dynamically stable, the $1T$ phase exhibits imaginary frequencies and transforms to the distorted $1{T}^{\ensuremath{'}}$ phase. Ab initio molecular dynamics simulations confirm that $1H$- and $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ monolayers are thermally stable even at high temperatures without any significant structural deformations. Different from binary systems, additional Raman active modes appear upon the formation of Janus monolayers. Although the mechanical properties of $1H\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ are found to be isotropic, they are orientation dependent for $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$. It is also shown that $1H\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ monolayers are indirect band-gap semiconductors and the band gap narrows down the chalcogen group. Except $1{T}^{\ensuremath{'}}$-WSO, $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ monolayers have a narrow band gap correlated with the Peierls distortion. The effect of spin-orbit coupling on the band structure is also examined for both phases and the alteration in the band gap is estimated. The versatile mechanical and electronic properties of Janus $\mathrm{W}X\mathrm{O}$ monolayers together with their large piezoelectric response imply that these systems are interesting for several nanoelectronic applications.