【作者】 宋艳；

【导师】 米文博；

【作者基本信息】 天津大学 ， 材料物理与化学， 2018， 博士

【摘要】 新型二维半导体具有优异的电子性质,是新一代自旋电子、谷电子器件的候选材料。由于尺度降低到纳米量级,特征能量也随之减小到电子伏特量级,二维材料的内禀属性容易与外场发生耦合。由此激发出的可被外场调节的物理特性为设计与优化自旋电子、谷电子器件提供了平台。本文利用密度泛函理论,研究了单层过渡金属二硫化物和砷烯、锑烯等二维材料在铁磁/铁电基底、掺杂/吸附、降低维度、应变和电场等作用下的电子结构、能谷结构和磁各向异性等。所得研究结果为自旋电子、谷电子器件的设计与实现提供理论依据。本文的主要工作如下:（1）通过研究室温亚铁磁性Fe₃O₄基底对单层WTe₂能谷结构的影响,发现与Fe₃O₄的交换相互作用使单层WTe₂出现能谷劈裂。基底终端和界面接触方式对单层WTe₂的电子结构有影响。因此选择合适的基底终端并控制具体接触方式对单层WTe₂/Fe₃O₄谷电子器件十分必要。外加应变和电场将界面态和能谷态对单层WTe₂传导特性的贡献分离。能谷劈裂在电场下保持稳定;（2）通过研究多铁性YMn O₃基底的磁电耦合对单层WTe₂能谷结构的影响,发现Ti掺杂使单层WTe₂出现能谷劈裂,且谷劈裂能可由基底的电极化方向控制。能谷电学调控的首次实现为谷电子器件的设计提供了理论指导;（3）通过研究5d元素掺杂的单层WSe₂的磁各向异性,揭示了磁各向异性随杂质电子填充状态改变所表现出的奇偶规律,并利用二阶微扰理论进行了解释。通过对掺杂的单层WSe₂进行电子与空穴掺杂,证实了奇偶规律的可靠性。5d元素掺杂的单层WSe₂具有垂直各向异性,在新型磁存储器件中有应用前景;（4）3d元素吸附能影响Td相单层WTe₂的电子结构。Ni元素吸附使单层WTe₂恢复体相中导致不饱和巨磁电阻的电子-空穴补偿能带。此结果为二维材料巨磁电阻效应的研究提供了理论基础;（5）在砷烯/Fe Cl₂范德华异质结中,砷烯的导带底受单层Fe Cl₂的半金属性影响,出现自旋劈裂。自旋劈裂和二者的接触势垒均可被外加垂直电场调控。因此砷烯/Fe Cl₂异质结中有可能出现电场可控的反常霍尔效应;（6）自旋轨道耦合效应对锑烯纳米带的电子结构影响较大。空间反演对称性缺失的锑烯纳米带具有自旋轨道劈裂。自旋密度波是锯齿型纳米带的基态。特定结构的纳米带在面内电场作用下出现电子-空穴补偿能带,在巨磁电阻效应方面具有研究价值。更多还原

【Abstract】 Novel two-dimensional semiconductors possess prominent electronic properties.They are potential candidates for the next generation of spintronic and valleytronic devices.As two-dimensional materials scale down to nanometer,the characteristic energy changes accordingly to the level of electron volt.Therefore,the external field is prone to a strong coupling with the local field and thereby remarkably changes the properties of systems.Tunable properties are highly desirable in design and optimization of spintronic and valleytronic devices.The electronic properties,valley structure and magnetic anisotropy of monolayer transition-metal dichalcogenides,arsenene and antimonene influenced by the ferromagnetic/ferroelectric substrate,doping/adsorption,dimension reduction,strain and electric field are investigated by density functional theory.The results show the potential applications in spintronic and valleytronic devices.The main works of the dissertation are as follows:（1）The valley structure of monolayer WTe₂ influenced by room-temperature ferrimagnetic Fe₃O₄ substrate is investigated.It is found that the exchange interaction with Fe₃O₄ introduces valley splitting in monolayer WTe₂.Substrate termination and contact manner play a key role in the detailed electronic structure.Hence,it is essential to choose suitable substrate termination and control the stacking pattern for monolayer WTe₂/Fe₃O₄valleytronic devices.Mechanical strain and electric field effectively separate the contributions of the interfacial and valley states to the conductivity of monolayer WTe₂.Valley splitting is robust against the electric field.（2）The valley structure of monolayer WTe₂ influenced by the magnetoelectric coupling of YMn O₃ substrate is investigated.It is found that the valley splitting energy can be controlled by the switch of substrate polarization in Ti-doped monolayer WTe₂/YMn O₃ heterostructures.The first demonstration of the electrical control of valley degree provides the foundation of electrically controlled valleytronics.（3）The magnetic anisotropy of 5d transition-metal-doped monolayer WSe₂ is investigated.The magnetic anisotropy obeys an electron-filling-rule that the parity of the occupation number of the dopant in-plane d orbital determines the preference between in-plane and out-of-plane anisotropy.This rule is understood by the second-order perturbation theory and charge doping analysis,which shows the great potentials in the novel magnetic memory devices.（4）The 3d transition-metal adsorption is an effective method to modify the electronic properties of Td phase monolayer WTe₂.The reemergence of the electron-hole pockets appears in Ni-adsorbed system,which serves as a platform for testing the extraordinary magnetoresistance effect and the e-h theory.（5）In the arsenene/Fe Cl₂ van der Waals heterostructures,the spin splitting appears at the conduction band minimum of arsenene.The splitting energy and the contact barrier can be modulated by the electric field.The electrical control of anomalous Hall effect is predicted.（6）Spin-orbit coupling plays an important role in the electronic properties of antimonene nanoribbons.Spin-orbit splitting appears in the nanoribbons with no inversion symmetry.Spin density wave is the ground state of the zigzag nanoribbons.Electron-hole pockets could be triggered in the particular antimonene nanoribbon by an in-plane electric field,which is interesting for exploiting the magnetoresistance effect.更多还原