Theoretical Study On Structural Design And Physical Properties Of New Two-Dimensional Materials

Advanced materials are particularly important for the sustainable and environment-friendly economic growth and,eventually,the human well-being,such as the applications in solar energy,high performance catalyst and medicine.However,it can take tens of years to move a material to the market after its initial discovery/synthesis.“Accelerating the pace of discovery and deployment of advanced material systems will therefore be crucial to achieving global competitiveness in the 21^st century”.The Materials Genome Initiative（MGI）is thus proposed for supporting institutions all over the world in the effort to discover,manufacture,and deploy advanced materials faster and at lower cost.The isolation and characterization of graphene have stimulated the huge fundamental and technological interest on the two-dimensional（2D）materials with atomic-thick layered structures and inspired the explosive explorations of the huge family of new 2D materials including boron nitride,transition metal dichalcogenides,group IV,II-VI and III-V compounds.Each of these materials has its own virtues and shortcomings,and it is essential to look for more new 2D materials to satisfy different requirements for applications.In this dissertation,from first-principles calculations,we have designed the 2D metallic and semiconducting Au₆S₂ polymorphs,2D AuMX₂（M=Al,Ga,In;X=S,Se）monolayers featuring intracrystalline aurophilic interactions,versatile titanium silicide monolayers,and 2D transition metal dihydride monolayers with intrinsic room-temperature half-hetallicity.Besides,we also collaborate actively with the experimental groups.The main results are summarized as below:（1）Au₆S₂ monolayer sheets:metallic and semiconducting polymorphs.Gold-sulfur interfaces,including self-assembled monolayers of thiol molecules on gold surface,thiolate-protected gold nanoclusters,and gold sulfide complexes have attracted intensive interest for their promising applications in electrochemistry,bioengineering and nanocatalysis.We predict two hitherto unreported 2D Au₆S₂ monolayer polymorphs,named as G-Au₆S₂ and T-Au₆S₂.The global-minimum G-Au₆S₂ monolayer can be viewed as a series of[-S-Au-]_n and[-Au₄-]_n chains parallel packed together.The metastable T-Au₆S₂ monolayer resembles the widely studied T-MoS₂ monolayer structure by substituting each Mo atom with an octahedral Au₆ cluster while S atom is bonded with three Au atoms in aμ₃ bridging mode.The G-Au₆S₂ monolayer is predicted to be metallic.The T-Au₆S₂ monolayer is predicted to be a semiconductor with a direct bandgap of 1.48 eV and high carrier mobility of 2721 cm²V^-1S^-1,¹0 times higher than that of semiconducting H-MoS₂.Moreover,the T-Au₆S₂ monolayer can absorb sunlight efficiently over almost entire solar spectrum.These properties render the G-and T-Au₆S₂monolayers promising materials for advanced applications in microelectronics and optoelectronics.（2）Two-dimensional AuMX₂（M=Al,Ga,In;X=S,Se）monolayers featuring intracrystalline aurophilic interactions with novel electronic and optical properties.Aurophilicity,known as aurophilic interaction,is a strong attractive van der Waals interaction between cationic gold（I）centers,whose strength is comparable to the hydrogen bond.We show that aurophilicity can serve as an engineering approach to expand structural dimensionality for nanomaterials design.Specifically,based on a global-structure search method and density functional theory calculations,we predict a series of stable 2D AuMX₂（M=Al,Ga,In;X=S,Se）structures featuring intracrystalline aurophilic interactions.All AuMX₂ monolayers designed are semiconductors with moderate bandgaps,excellent carrier mobilities,and good optical properties.The intriguing chemistry of aurophilicity coupled with novel electronic properties render AuMX₂ monolayers a potentially new series of 2D materials that are of fundamental importance in gold chemistry and technology importance for nanoelectronics.（3）Versatile titanium silicide monolayers with prominent ferromagnetic,catalytic,and superconducting properties.On the basis of global structure search and density functional theory calculations,we predict a new class of 2D materials:titanium silicide（Ti₂Si,TiSi₂ and TiSi₄）monolayers.They are proved to be energetically,dynamically and thermally stable and own excellent mechanical properties.Among them,Ti₂Si is ferromagnetic metal with a magnetic moment of 1.37μ_B/cell,while TiSi₂ is an ideal catalyst for hydrogen evolution reaction with a nearly zero free energy of hydrogen adsorption.More importantly,electron-phonon coupling calculations suggest that TiSi₄ is a robust 2D phonon-mediated superconductor with a transition temperature of 5.8 K and the transition temperature can be enhanced up to 11.7 K under a suitable external strain.（4）Intrinsic room-temperature half-metallicity and wide spin gap in two-dimensional transition metal dihydride monolayer.2D ferromagnetic materials with intrinsic half-metallicity are highly desirable for nanoscale spintronic applications.By using density functional theory computation,we predict a series of 2D transition metal dihydride（MH₂;M=Sc,Ti,V,Cr,Fe,Co,Ni）monolayers with novel magnetic properties.CoH₂ and ScH₂monolayers are ferromagnetic metals,while the others are antiferromagnetic semiconductors.Interestingly,CoH₂ monolayer is predicted to be half-metallic with 100%spin filtering and a wide spin gap of 3.48 eV,while ScH₂ monolayer can entail half-metallicity as well through low concentration hole doping.Moreover,our Monte Carlo simulations based on 2D Heisenberg Hamiltonian model show that CoH₂ monolayer possesses an above-room-temperature Curie point（339 K）,while that of ScH₂ monolayer can also reach as high as 160 K.Dynamic and thermal stabilities of MH₂ monolayers are confirmed by phonon dispersion calculations and ab initio molecular dynamics simulations.We further propose practical approaches for synthesizing CoH₂ and ScH₂ monolayers.We show that their half-metallicity can be well maintained on substrates.Overall,many of these MH₂ monolayers are promising candidates for spintronic applications due to their novel magnetic properties.2D atomic crystals are extensively studied in recent years due to their exciting physics and device applications.However,a molecular counterpart,with scalable processability and competitive device performance,is still very challenging.The experimental measurements demonstrate that high-quality few-layer C₈-BTBT molecular crystals can be grown on the top of graphene or boron nitride substrate via van der Waals epitaxy,with precisely controlled thickness down to monolayer,large-area single crystal,low process temperature and patterning capability.By performing further theoretical simulations,we reveal that the crystalline layers are atomically smooth and effectively decoupled from the substrate due to weak van der Waals interactions,affording a pristine interface for high-performance organic transistors.Our work unveils an exciting new class of 2D molecular materials for electronic and photonic applications.