Ab Initio Study Of Novel Low Dimensional Dialkali Metal Monchalcogenides

Two-dimensional(2D)materials have triggered tremendous research activities among inter-disciplinary communities in exploring their divergent novel properties for scientific breakthroughs and for prototypical device applications.The flourishing of novel 2D materials have led to the discoveries of linear energy dispersion with high mobility,tunable magnetism beyond room temperature,non-volatile ferroelectricity and valleytronics for manipulating spins and excitations,etc With the development of high-performance computing and the refinement of density-functional theory calculations,many 2D materials are theoretically predicted and studied before experimentally synthesized and characterized,representing a new paradigm in material science studiesAmong archetypal 2D materials,metallic MXenes represent a rather large family of metal-shrouded main-group elements to form the structure of anti-1T TMDC structure,which have great potentials for catalysis and energy storage.Using fist-principles calculations,we discover a new family of dialkali-metal monochalcogenides(DMMCs,with the chemical formula of M2X)in 1T-TMDC structure.DMMC compounds show excellent dynamical stability and energy bandgaps within the visible-(near)infrared light region,which is very attractive for optoelectronic applications.DMMCs also exhibit stark different electron and hole mobility,which exceeds 1 x 103 cm2V-1s-1 for electron and is about 1 cm2V-1s-1 for hole.Due to the large electronegativ-ity difference between anions and cations in the 1T structure,DMMCs monolayers with unique'Mexican-hat' shaped valence band top are expected to be easily exfoliated from the bulk.Using electrostatic gating to introduce moderate hole doping ·(?1.0 ×1012 cm-2)in DMMCs mono-layers,it is possible to trigger spontaneous and reversible ferromagnetic phase transitions,as a result of doping enhanced electron-electron interactions to satisfy the Stoner criterion.Such a 1T-DMMCs family may provide us an unparalleled platform for exploring low-dimensional electronic,optoelectronic devices and gate-tunable spintronicsUsing DFT,we have also found that the group of dialkali-metal monoxides,which crystalize into the well-known 2H-MoS2 lattice(2H-DMMOs),host multiple symmetry-protected topological phases,which can be readily tuned by strain engineering.Typically,2H-Na2O and 2H-K2O in the equilibrium state exhibit massive Dirac(double Weyl)and pseudospin-one fermions,respectively.These exotic fermions are promising for the study of universal optical absorbance,super Klein tunneling effect,and super Andreev reflection.By introducing biaxial strain,we found that quantum phase transitions can be reversibly triggered between Dirac metal,massive Dirac semimetal,pseudospin-1 semimetal,and semiconductor phases.Our results suggest 2D materials may provide a fascinating platform for studying topological phases.