First-principles Study Of Quasi-two-dimensional Chalcogenides

Due to their unique quantum properties and high flexibility,two-dimensional?2D?materials provide an ideal platform for next generation electronic devices.Searching for new 2D materials and manipulating their properties have been a hotspot of research in communities of condensed matter physics and material science.Based on first-principles calculations,we systematically investigated the structural phase transitions,stability,electronic structure,and optical properties of several typical 2D chalcogenides.In ad-dition,we predicted a class of stable 2D hexagonal structures of group-IVA monochalco-genides?MXs?.Among the known transition metal dichalcogenides,monolayer VS₂has attracted considerable attention because of its intrinsic ferromagnetism.we studied the structural and electronic phase transitions in monolayer VS₂induced by charge doping.We show that without electron or hole doping,VS₂stabilizes in the 2H phase and is a bipolar magnetic semiconductor?BMS?whose valence and conduction states near the Fermi level carry opposite spin polarization.Within the experimentally accessible doping regime,we found that hole doping can trigger a structural phase transition in monolayer VS₂from the 2H phase to the 1T phase,which concomitantly results in an electronic BMS-to-half metal-to-normal ferromagnetic metal phase transition,while electron doping can only induce an electronic phase transition from semiconductor to half metal.The different effects of hole and electron dopings on the structural phase transitions result from the fact that the Fermi level of the 1T phase is higher than the conduction band minimum of the2H phase.Saddle-point in the joint density of states of a 2D semiconductor will result in di-vergent optical absorptions.Moreover,excitonic effect is significant in 2D materials due to decreased dielectric screening,which induces a unique excitonic saddle-point state.We found saddle-point excitons in 2D?-MXs,which is another class of materials that possesses significant saddle-point excitons after graphene.The optical absorbance at the saddle-points in the visible range that reaches values as large as 61%in the monolayer form.In addition,we found thickness is an effective method to tune the properties of saddle-point excitons.All these results not only suggest 2D?-MXs could act as promis-ing candidates in future optoelectronic devices,but also open avenues for the future ex-plorations of many-body physics associated with 2D saddle-point excitions.IVA-VIA compounds exhibit a variety of crystal structures with distinct physical properties.Therefore,the search for new stable structures in these materials is of great significance.We found a class of hexagonal structured 2D MXs??-MXs;M=Ge,Sn;X=S,Se,Te?.The?phase are energetically more favorable compared to their?and?phase counterparts in all of the studied MXs except SnS.We reveal that low tempera-ture is a key factor to synthesize?-MXs experimentally by calculating their Helmholtz free energy.2D?-MXs are indirect gap semiconductors with relatively small electron effective mass??0.14 m_e?,which results in a relatively high electron mobility in these materials.