Tuning the structural, electronic and dynamical properties of Janus M₄X₃Y₃ (M = Pd, Ni and Co; X,Y = S, Se and Te) monolayers: a DFT study

Based on density functional theory, the structural, electronic and vibrational properties of two-dimensional transition metal chalcogenides M₂X₃ and their Janus type M₄X₃Y₃, where M = Pd, Co and Ni and X = Se, S and Te, are investigated. Motivated by the successful synthesis of a 2D Pd₂Se₃ monolayer and the proof of the dynamical stability of Ni₂Se₃ and Co₂Se₃ monolayers, in terms of the phonon band dispersions, we have systemically studied the fundamental physical properties of Janus transition metal chalcogenides, such as their structural, phonon and thermodynamic stability and their electronic and mechanical properties. Our results show that Janus structures of M₄X₃Y₃ are energetically favorable and dynamically stable. The ab initio molecular dynamic simulations (AIMD) results clearly prove that they kept their thermal stability at room temperature. We have demonstrated their structural, electronic and vibrational properties and Raman spectra. The electronic band dispersions show that monolayer Co₂Se₃ shows half-metal properties with a moderate band gap (1.01 eV), Pd₂Se₃ has a 1.42 eV direct band gap, while Ni₂Se₃ has a 1.38 eV indirect band gap. Pd₄Se₃S₃, Pd₄Se₃Te₃ and Pd₄S₃Te₃ are indirect band gap semiconductors with band gaps of 1.22 eV, 1.05 eV and 0.61 eV, respectively. Ni₄Se₃S₃, Ni₄Se₃Te₃ and Ni₄S₃Te₃ are indirect band gap semiconductors with band gaps of 1.61 eV, 0.77 eV and 0.49 eV, respectively. While pristine Co₂Se₃ is shown to have half-metallicity (HM), the HM behaviour of the Janus Co₄Se₃Te₃ and Co₄S₃Te₃ monolayers disappear and Co₄Se₃S₃ remains a HM with a moderate band gap of 0.85 eV. In addition, the Raman spectra of these Janus materials are shown to exhibit totally distinctive features as compared to those of the pristine materials. This work reveals the important material properties of Janus type M₄X₃Y₃ monolayers, where M = Pd, Co and Ni and X = Se, S and Te, which could have wide applications in new functional devices.