First-principles Study Of The Structural Design And Physical Properties Of Early Transition Metal-based MXenes-like Materials

Two-dimensional（2D）transition metal carbides,nitrides and carbonitrides（MXenes）are highly tunable and have broad applications in energy storage and capture,environment,optoelectronics,sensor,biomedicine,dielectric electromagnetic shielding and 5G antennas.At present,the regulations of MXenes are mainly focused on the“M”position,stoichiometric ratio,surface functional groups and solid solution formation.However,the regulation of the“X”position is very rare.Therefore,using first-principles calculations,We firstly extended the traditional MXenes to 2D early transition metal-based oxides,MOenes.Taking titanium-based MOenes,which are abundant on earth and environmentally friendly,as the research object,we systematically studied Ti_n+1O_n（n=1-3）MOenes and their functionalization,including their crystal structures,stability,electronic structures,superconductivity,light absorption,carrier lifetime,quantum phase transition,and quantum Hall effect,etc.In addition,motivated by the experimental progress,two layered structures,TaSi₂N₄ and C₂Si N/CSiN,were constructed and studied by saturating the surface of MXenes-like 2D TaN₂ and graphene with Si-N layers.The main research contents of this dissertation are as follows:1.Firstly,using layered Ti₂O bulk phase exfoliation and high-throughput structure search,we have designed single-layer 1T-and 2H-Ti₂O MOenes,which have good stability and synthesis feasibility.In addition,2H-and 1T-Ti₂O are 2D superconductors,with superconducting transition temperatures（T_c）of 4.7 and 9.8 K,respectively.At the same time,1T-Ti₂O is also an excellent anode material of lithium-ion batteries and sodium-ion batteries due to its surface-confined anions.2.When surface symmetrically halogenated,non-Janus 2H-and 1T-Ti₂OX₂（X=F,Cl）are direct bandgap semiconductors;2H-and 1T-Ti₂OBr₂ are indirect bandgap semiconductors.The band gap values of Ti₂OX₂ monolayers are 0.58-1.18 eV（HSE06）,which can be used in infrared detection and solar energy.In addition,2H-Ti₂OX₂（X=F,Cl）can occur quantum phase transitions under strains.By non-adiabatic molecular dynamics（NAMD）simulation,the carrier recombination lifetimes of 2H-and 1T-Ti₂OF₂ can reach to nanosecond level.Furthermore,Janus 2H-and 1T-Ti₂OFCl MOenes,constructed by surface asymmetric halogenation,are direct band gap semiconductors with band gaps of 0.75 and 1.12 eV（HSE06）,respectively.At the same time,Janus 2H-Ti₂OFCl can undergo a quantum phase transition under smaller strains,producing novel2D pseudospin-1 and Weyl fermions.Furthermore,1T-Ti₂OCl F/Ti₂OCl₂constitute a type-Ⅱ heterojunction,and its photoelectric conversion efficiency（PCE）can reach15.77%.3.By stoichiometric engineering,we have constructed 2H-and 1T-Ti_n+1O_n（n=2,3）MOenes and studied their thickness-dependent properties.However,only 2H Ti_n+1O_nand 2H Ti_n+1O_nT₂（T=F,OH,H）MOenes are kinetically stable.Among them,the T_c of2H-Ti₃O₂ and Ti₄O₃ are 4.3 and 3.7 K respectively,indicating that the T_c of 2H-Ti_n+1O_n（n=1-3）decreases slightly with increasing thickness.In particular,when spin-orbit coupling（SOC）effect is considered,2H-Ti_n+1O_nF₂（n=2,3）are topological insulators,originating from the d-d band inversions.4.By using Si-N layers to functionalize MXenes-like 1T-and 2H-TaN₂ with metal atoms as the central layer,we construct 2H-and 1T-TaSi₂N₄ crystal structures.The Si-N layers not only improve the stability of TaN₂,but also significantly improve the superconductivity of 2H-TaSi₂N₄,with T_c reaching 24.62 K.Further substitution of Nb element increases the T_c of 2H-Nb Si₂N₄ to 30.41 K.These results provide strategies for improving the superconductivity of 2D MN₂,including surface function and light element substitution.Furthermore,1T-TaSi₂N₄ is a typical charge density wave（CDW）material,and its CDW originates from momentum-dependent EPC,and the CDW transition temperature(T_CDW)of 1T-TaSi₂N₄ is about 500 K.Moreover,the CDW of 1T-TaSi₂N₄ can be suppressed by strain and carrier doping,leading to the emergence of superconducting states.5.Due to the unsaturation of the C-p_z orbitals,we use Si-N layers to saturate the surface of graphene.When the graphene surface is saturated on one side,C₂Si N is an intrinsic 2D superconductor,with a T_c of 1.01 K,providing a novel platform for regulating graphene-based superconductivity.When the graphene surface is saturated on both sides,CSiN has high stability and transforms into semiconductor properties with a band gap value of about 3.73 eV（HSE06）and strong ultraviolet light absorption ability.In particular,the valence band of CSiN has a ternary valley structure,and these valleys are connected by lattice symmetry.Therefore,the valley polarization in CSiN can be controlled by uniaxial strain.In this dissertation,in the framework of MXenes,we have extended traditional MXenes to 2D transition metal oxides MOenes.Titanium-based MOenes exhibit high regulations and have shown promising applications in infrared detection,solar energy and optoelectronic devices.For TaN₂ and graphene,surface function via Si-N layers greatly improves their stability and enriches their physical properties,such as superconductivity,CDW,semiconducting features and valley features.Our works will provide new clues for expanding and modulating the MXenes-like materials.