Investigations Of Novel Structures And Physical Properties In Two-dimensional Multinary Semiconductors

(GaN)_1-x（ZnO）_x alloys are more promising semiconductors than their counterparts for optoelectronic applications.Unfortunately,the strong structural internal electrostatic field（IEF）seriously hinders to further improve the optoelectronic performance,two-dimensional（2D）stable configurations as well as complete stoichiometries with better properties have also not been inverstigated,and the anomalous linear thermal conductivity of 2D hexagonal GaN and ZnO monolayer makes a suitable substrate material also worth further research.In this study,based on the structural search method and density functional theory,we focus on the stable configurations as well as complete stoichiometries and the novel Haeckelite（8|4）configuration in 2D（GaN）_1-x（ZnO）_x alloys,to reveal the convex hull of alloys and find the novel functional materials with the excellent optical,mechanical,and thermal performance.We provide a basic understanding of the stability of the thickness-driven structural transition from the multilayer wurtzite to 8|4 configurations in few-layer limit,unveils the relationship of the structural,optical,and thermal properties versus ZnO composition,and proposes 3-X configuration to achieve the anomalous thermal transport,which appears in 2D hexagonal GaN and ZnO monolayers,in 2D boron carbides.The innovative results are as follows:（1）In 2D nonisovalent（GaN）₁（ZnO）₁ alloys,we point out a structural model to extremely improve the visible-light absorption by overcoming the bottleneck of the IEF.We explore the thickness-driven structural transitions from the planar hexagonal to the8|4 and to the wurtzite configurations.The visible-light absorption efficiency quickly rises up from the bulk wurtzite to the bulk 8|4 to the 2D 8|4 to the Mo S₂-based heterostructures with the different-layer 8|4 configurations.The further improved visible-light absorption in heterostructures activates by enlarging the population of band edge states with the heterointerfacial coupling.（2）To explore the stable configurations as well as complete stoichiometries in（GaN）_1-x（ZnO）_x alloys and its optical,electronic,thermal,and permeation properties,we identify the stable wurtzite-like Pm-（GaN）₃（ZnO）₁,Pmc2₁-（GaN）₁（ZnO）₁,multilayer planar P3m1-（GaN）₁（ZnO）₂,and 8|4 C2/m-（GaN）₁（ZnO）₃.We show that P3m1-（GaN）₁（ZnO）₂ shares the excellent thermoelectrics with the figure of merit as high as 3.08 at 800 K for the p-type doping due to the ultralow lattice thermal conductivity,which mainly arises from the strong anharmonicity by the interlayer asymmetrical charge distribution.We reveal the anomalous relationship of the band gap versus ZnO composition originates from the forbidden N:2p-Zn:3d coupling according to the group theory and p-d coupling theory.We deeply explore the adsorpotion and permeation properties of stable configurations,and point out that the difference of potential wells and barriers in structures arises from the distinct ionic size and Coulomb interaction with the cases of H⁺,Na⁺,and OH^-ions.（3）Inspired by synthesized T-carbon and H-boron,the 3-X structural models are proposed to unify the 2D multitriangle materials.Employing structural searches,we identify the stability of the 3-X configuration in 2D boron carbides as 3-9 BC₃monolayer,which exhibits a linear thermal conductivity versus temperature,not the traditional 1/T trend,while the similar thermal transport behavior also appears in 2D hexagonal GaN and ZnO monolayers.By analyzing the absolute and percentage contributions from the optical phonon branches,we reveal that the linear thermal conductivity originates from the huge contribution from the out-of-plane vibrational mode FO to thermal transport.We point out the FO contribution activates by the structural anharmonicity by the introducing 3-X model and the atomic resonance in the carbon triangles.The results differ with the anomalous behavior in 2D GaN and ZnO monolayers,which have the large difference in atomic size and electronegativity.Our results further explain why the linear behavior is absent in 3-9 Al C₃ and graphene,and further unveil that the so-called anomalous thermal conductivity versus temperature is usually a linear relationship because of the second-order trend in heat capacity.