Janus two-dimensional transition metal dichalcogenide oxides: First-principles investigation of WX O 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 1T′) of Janus WXO (X=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 1T′ phase. Ab initio molecular dynamics simulations confirm that 1H- and 1T′-WXO 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-WXO are found to be isotropic, they are orientation dependent for 1T′-WXO. It is also shown that 1H-WXO monolayers are indirect band-gap semiconductors and the band gap narrows down the chalcogen group. Except 1T′-WSO, 1T′-WXO 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 WXO monolayers together with their large piezoelectric response imply that these systems are interesting for several nanoelectronic applications.