First-principles Study Of Elctronic Properties And Oxidation Behaviors Of Two-dimensional Chalcogenides

The boom of graphene has stimulated the interest in many other two-dimensional(2D)materials of atomic thickness,such as elemental group-IV materials,group III-V compound semiconductors,metal chalcogenides,and composite oxides.These findings not only break the long-standing claim that 2D materials could not be stable in nature,but also show many intrinsic physical and chemical properties,including semi-integer,fractional and fractal quantum Hall effects,high carrier mobility,and band structure evolution.2D chalcogenides have attracted much attention due to their high stability,less toxic,earth-abundant and excellent electronic properties.In this work,we investigated the electronic properties and oxidation behaviors of several 2D chalcogenides.The main results are summarized as following:1)We explored the oxidation behavior of monolayer group-IV monochalcogenides(GeS,GeSe,SnS,and SnSe)by first-principles calculations.We found superior oxidation resistance of the monolayer group-IV monochalcogenides with activation energies for the chemisorption of O2 on the 2D sheets in the range of 1.26-1.60 eV,about twice of the values of phosphorene and arsenene.The distinct oxidation behaviors of monolayer group-IV monochalcogenides and group-V phosphorene analogues originate from their different bonding natures.Moreover,the chemisorption of a moderate amount of oxygen atoms does not severely deteriorate the electronic band structures of these monolayers-the band gaps are intact in absence of any impurity states,and the carrier effective masses are only slightly changed.2)We systematically investigated the oxidization behaviors of perfect and defective group-Ⅲ monochalcogenide monolayers by first-principles calculations.The perfect monolayers show superior oxidation resistance with large barriers of 3.02～3.20 eV for the dissociation and chemisorption of O2 molecules.In contrast,the defective monolayers with single chalcogen vacancy are vulnerable to O2,showing small barriers of only 0.26-0.36 eV for the chemisorption of an O2 molecule.Interestingly,filling an O2 molecule to the chalcogen vacancy of group-Ⅲ monochalcogenide monolayers could preserve the electronic band structure of the perfect system-the bandgaps are almost intact and the carrier effective masses are only moderately disturbed.3)We showed theoretical evidence of a new phase of copper(Ⅰ)sulfide(Cu2S)monolayer,named as d’-Cu2S,with both novel electronic properties and superior oxidation resistance.We found that both monolayer and bilayer δ-Cu2S have much lower formation energy compared with the known β-Cu2S phase.Stability analysis indicates that δ-Cu2S is dynamically and thermally stable.Notably,δ-Cu2S exhibits superior oxidation resistance,due to the high activation energy of 1.98 eV for the chemisorption of O2 on δ-Cu2S.As for electronic properties,δ-Cu2S is a semiconductor with a modest direct band gap(1.26 eV)and ultrahigh electron mobility up to 6880 cmn2V-1s-1,about 27 times of that(246 cmn2V-1s-1)forβ-Cu2S bilayer.4)Starting from the group-Ⅲ monochalcogenide monolayers,we designed a series of Janus structures for piezoelectric materials,including Ga2SSe,Ga2STe,Ga2SeTe,In2SSe,In2STe,In2SeTe,GaInS2,GaInSe2]and GaInTe2.Our first-principles calculations show that these Janus structures are thermodynamically and dynamically stable.They have band gaps of 0.89～2.03 eV,lower than those of the perfect monolayers,and Ga2STe,Ga2SeTe,In2STe and In2SeTe monolayers are direct gap semiconductors.They possess piezoelectric coefficients up to 8.47 pm/V,over four times of the maximum value obtained for perfect group-III monochalcogenide monolayers.Moreover,the broken mirror symmetry of these Janus structures induces out-of-plane dipolar polarization,yielding additional out-of-plane piezoelectric coefficients of 0.07～0.46 pm/V.5)Using first-principles calculations,we predicted two types of 2D semiconductors,namely,ultrathin GeAsSe and SnSbTe nanosheets,with desirable electronic and optical properties.Both GeAsSe and SnSbTe sheets are energetically favorable with formation energy of-0.19 and-0.09 eV/atom,respectively;both of them have excellent dynamical and thermal stabilities,as demonstrated by phonon dispersion calculations and Born-Oppenheimer molecular dynamics simulations.Importantly,monolayer GeAsSe and SnSbTe possess direct band gaps(2.56 and 1.96 eV,respectively)and superior hole mobilities(～20000 cm2V-1s-1),both exhibiting notable absorption in the visible-light spectrum.By comparing the band edge positions with the redox potentials of water,layered GeAsSe and SnSbTe are potential photocatalysts for water splitting.