1H and 1T polymorphs, structural transitions and anomalous properties of (Mo,W)(S,Se) ₂ monolayers: first-principles analysis

Among the 1H and 1T structures exhibited by monolayers of transition metal dichalcogenides, the group VI compounds MX 2 (M = Mo, W and X = S, Se) largely occur in the 1H form. Recently, transformation of the 1H form to the 1T form with metallic electronic structure at high temperatures was demonstrated in MoS 2 with Re substitution and electron irradiation by Lin et al (2014 Nat. Nanotechnology 9 391 ). Here, we use first-principles calculations to determine the energy landscape associated with the 1H to 1T phase transition, predict novel 1T structures and relate the observed by Lin et al (2014 Nat. Nanotechnology 9 391 ) intermediate structures to structural instabilities of the 1T structure of MX 2 compounds. We show that the metallic centrosymmetric 1T (c 1T ) structure of these compounds is unstable with respect to dimerization or trimerization of metal atoms, leading to a competing metallic 1T form and ferroelectric semiconducting 1T form respectively. While the former is a more stable 1T form of MoSe 2 , WS 2 and WSe 2 , the latter is a more stable 1T form of MoS 2 exhibiting rich ferroelectric dipolar domain structure. In the vicinity of metal-semiconductor transitions, their semiconducting forms are shown to exhibit an anomalous response to electric fields. To facilitate the experimental verification of these subtle features of the 1T forms of MX 2 monolayers, we present comparative analysis of their vibrational properties, and identify their Raman and infra-red spectroscopic signatures.