几种低维体系自组装和功能化的理论模拟

低维体系的本质特征是某一个或几个方向上电子运动受到限制，被限制方向上的尺寸与诸多物理量的特征长度相当，如物质波长、电子关联长度等。空间受限导致体系费米能附近的电子状态发生离散化；同时大体表比和高密度的活性位点导致低维体系表面具有良好活性；另外，低维材料的界面外场折射率远强于体相材料，从而形成局域场的增强和吸收峰的等离子共振。这些独特的物理化学铁性导致低维体系表现出新颖的磁电、光电和催化性能。鉴于低维材料在包括自旋电子学、表面催化和多铁性等领域具有潜在的优势，本论文基于第一性原理和动力学蒙特卡洛等理论模拟方法，系统地研究了低维体系在自旋界面上的自组装和调控、异质结光解水以及磁电耦合等功能化应用，得到了以下结论： 零维富勒烯C60子在氧钝化的Fe(001)-O 表面的自组装过程，呈现随机团簇和有序岛相共存的混合生长现象。通过考察 C60/Fe(001)-O 吸附体系的哈密顿量，密度泛函理论的模拟结果表明，当毗邻分子数目一定时，分子间和分子-衬底间作用大小相当，且不同吸附位置（Otop、Fetop 和 Obri）的稳定构型间的动力学参数存在明显差异；基于动力学蒙特卡洛长时间演化的模拟，得到与实验一致的 C60 分子自组装结构；我们进一步将该理论推广到其它分子生长过程，证明了在较大的温度和覆盖度范围内，依然存在混合生长模式的可能性。本工作是对传统逐层生长模式理论的有效补充，为合成和设计新型自旋功能器件提供了理论的基础。与钝化表面降低自旋输运性能不同的是，C60吸附可以有效地调控二维磁性金属衬底Fe2C的物理性质，包括自旋极化率、磁各向异性和磁耦合强度等物理性质。理论模拟结果表明，C60分子的𝜋轨道和衬底 Fe原子的𝑑轨道之间形成较强杂化耦合，能够有效地重组体系的能带排列，从而造成较高的自旋极化；磁性金属衬底的易磁化方向由面外翻转为面内，其主要贡献来自于体系费米面附近自旋平行电子态的跃迁；同时由于吸附增加了衬底磁性原子间的超交换路径，致使磁耦合强度下降。本工作是对有机自旋界面中三维磁性金属衬底研究的拓展，证明了分子吸附能够有效地调控二维磁性金属材料的物理性质。 基于二维层状 MoSSe/WSSe 过渡金属硫化物 (TMDCs) 构建的异质结，我们通过平移和翻转来构建不同的堆积方式，可以组成 TypeI、TypeII 和 TypeZ 三类异质结光催化模型。低维体系的本征极化造成异质结两侧真空能级发生显著的相对 偏移，改变了异质结两侧的氧化还原电位，从而有效地调节异质结的光生载流子的分离和复合。通过引入点缺陷、pH 值和应变等方法可以提高三类异质结的光解水催化性能。计算结果表明，Se空位较S空位能更好地降低还原半反应的过电势， 但氧化半反应对点缺陷并不敏感,并从𝑑带模型和线性标度关系等角度给出了相应的解释。另外，异质结光催化转换效率优于单层材料，三类异质结的能量转换效率分别提高至 16.9%, 26.3% 和 14.7%。本工作证明了异质结堆积方式能调控体 系光催化性能，为实验构建高效光解水模型提供理论依据。 基于二维过渡金属磷化物 (TMPCs) 单层 CuVP2S6 材料，我们研究了其铁电和铁磁共存以及磁电强耦合机制。当Cu原子偏离中心平面，体系形成面外极化的铁电相；同时铁电比反铁电和顺电相在能量上更稳定，不同构型间的活化势垒较大， 可以防止体系铁电序的自发转变。铁磁序来源于过渡金属原子V与周围配位的 S原子之间 𝑝-𝑑局域杂化所介导的交换耦合作用，并且较大的磁晶各向异性能也有利于单层CuVP2S6 形成稳定的长距离磁序。更为重要的是，理论模拟的结果显示 在铁电相变前后，体系的磁晶各向异性能发生了跃变，为提高单层材料的铁磁相变温度提供了一种可行方案。另外，磁电耦合导致了体系铁谷状态在外场下的可调性，为构建基于反常铁谷霍尔效应的谷电子器件创造了可能性。

The essential characteristic of low-dimensional system is the confined movement space of electronics in a (several) certain direction(s). It leads to the system size is similar as many characteristic lengths of matter, such as material wavelength, electron correlation length and so on. It destroys the translation periodicity of material, and causes the evo_x0002_lution of electronic states near Fermi energy. The atom distribution of low-dimensional system is dispersed, resulting in the large surface ratio and high density of active sites. In addition, the field refractive index at the interface of low-dimensional system is stronger than that of bulk materials, causing the enhancement of local field and the plasmon resonance of absorption peak. The unique physicochemical effects of low-dimensional system lead to their abundant and novel properties, such as magnetoelectric, photoelectric and catalytic performances. In view of the potential values of low-dimensional system in spintronics, surface catalysis and magnetoelectric devices, this dissertation systematically studies the applications of low-dimensional system in self-assembly and regulation of spinterface, photocatalytic water splitting and magnetoelectric coupling based on theoretical simulation, including density functional theory and kinetics Monte Carlo. The specific results are as follows: The self-assembly of fullerene C60 on the oxygen passivated Fe(001)-O surface presents a mixed growth phenomenon as the coexistence of random clusters and ordered islands. The total Hamiltonian of the low-dimension adsorption system is investigated, showing the value of the interaction between molecules are similar to that between molecule and substrate when existing adjacent molecules. Meanwhile, there are remarkable difference among the dynamical parameters of the stable adsorption configuration including Otop, Fetop and Obri. It results the mixed mode of molecular self-assembly, which is consistent with the experimental observation based on the kinetics Monte Carlo of the long-term evolution. In addition, we extend the mixed mode to the general molecule, by proving the existence of a stable dynamical region of mixed mode in a large range of temperature and coverage. Our study remedies the traditional growth modes and provides a theoretical guidance for the synthesis of electronic functional devices. Unlike passivated surface reducing spin transport properties, C60 adsorption can effectively control the physical properties of the two-dimensional (2D) magnetic metal substrate Fe2C, including spin polarizability, magnetic anisotropy (MAE) and magnetic coupling strength. The results of density functional theory simulation show that there is the strong hybrid coupling between the orbital of C60 molecule and the orbital of Fe atom on the substrate. It’s able to rearrange the band arrangement, resulting in the high spin polarization. The reversal of the magnetic direction from out-of-plane to in-plane is mainly attributed from the transition between occupied state to unoccupied with parallel spin. In addition, the magnetic coupling strength decreases due to the super-exchange paths between the Fe atoms on the substrate by C60 adsorption. It is proved that the physical properties of 2D magnetic metal materials can be effectively regulated by molecular adsorption. Therefore, our study is an extension of three-dimensional (3D) magnetic metal substrate applied in spinterface.Based on the transition metal dichalcogenides (TMDCs) heterostructure composed by 2D layered MoSSe/WSSe, we can construct three types of heterostructure for photocatalysis including TypeI, TypeII and TypeZ by sliping and flipping the stacking modes. The intrinsic polarization of low-dimensional system significantly shifts the vacuum energy levels on both sides of the heterostructures. Thus, it results the effective separation and recombination of photogenerated carriers in heterostructures. By introducing point defects, pH value and strain, the photocatalytic performance of the three heterojunctions can be tuned. The calculated results show that Se vacancy can reduce the overpotential of the reduction half reaction better than S vacancy, but the oxidation half reaction is not sensitive to point defects. These results are explained by the 𝑑-band model and linear scaling relationship. In addition, the photocatalytic conversion efficiency of the heterostructure is better than that of the monolayer of MoSSe and WSSe. The energy conversion efficiency of the three heterostructures are increased to 16.9%, 26.3% and 14.7% respectively. Our study proves that the stacking modes can regulate the photocatalytic performance of low-dimension system, which should guide the construction of efficient photolysis model. Based on the 2D single-layer transition metal phosphorous chalcogenides (TMPCs) CuVP2S6 material, we study the coexistence of ferroelectric and ferromagnetic and the strong magnetoelectric coupling. When the Cu atom deviates from the central plane, the system forms out-of-plane ferroelectric polarization ferroelectric. The energy of ferroelectric phase is more stable than that of anti-ferroelectric and paramagnetic phase. In addition, the activation barrier between different configurations is so large that it can prevent the spontaneous transition of ferroelectric order. The ferromagnetic order originated from the exchange coupling effect, which mediated by the p-d local hybridization between the transition atom V and the surrounding S atoms. Meanwhile, the large magnetocrystalline anisotropy is also conducive to the formation of a stable long-distance magnetic order in the monolayer CuVP2S6. More importantly, the theoretical simulation results show that the MAE take place of the remarkable change before and after the ferroelectric phase transition, which provides a feasible scheme for improving the ferromagnetic phase transition temperature of single-layer materials. Moreover, agnetoelectric coupling also leads to the spontaneous response of the ferrovalley state in the external field. It is possible to construct vallytronic devices based on anomalous valley hall effect.