Tuning the structural, electronic and dynamical properties of Janus MXY (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 MX and their Janus type MXY, where M = Pd, Co and Ni and X = Se, S and Te, are investigated. Motivated by the successful synthesis of a 2D PdSe monolayer and the proof of the dynamical stability of NiSe and CoSe 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 MXY are energetically favorable and dynamically stable. The 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 CoSe shows half-metal properties with a moderate band gap (1.01 eV), PdSe has a 1.42 eV direct band gap, while NiSe has a 1.38 eV indirect band gap. PdSeS, PdSeTe and PdSTe are indirect band gap semiconductors with band gaps of 1.22 eV, 1.05 eV and 0.61 eV, respectively. NiSeS, NiSeTe and NiSTe are indirect band gap semiconductors with band gaps of 1.61 eV, 0.77 eV and 0.49 eV, respectively. While pristine CoSe is shown to have half-metallicity (HM), the HM behaviour of the Janus CoSeTe and CoSTe monolayers disappear and CoSeS 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 MXY monolayers, where M = Pd, Co and Ni and X = Se, S and Te, which could have wide applications in new functional devices.