【作者】 胡娟梅；

【导师】 吴锋民；

【作者基本信息】 浙江师范大学 ， 理论物理， 2007， 硕士

【摘要】 基底表面有序的纳米结构薄膜在现代科技领域中有着广泛的应用,具研究成果转化为生产力的速度越来越快。在原子尺度上揭示纳米薄膜形成初期粒子的扩散、成核等微观过程对改进和优化薄膜生长工艺、提高薄膜的性能具有重要意义。实验上可以利用扫描隧道显微镜(STM)和原子力显微镜(AFM)来观察薄膜的微观形成过程,而理论上,由于不同的理论方法适合不同的时间和空间尺度,需要用与实际生长过程相匹配的理论来研究这一运动规律。本文采用了密度泛函理论(DFT)和动力学蒙特卡罗(kMC)模拟相结合的方法。先用DFT计算了沉积原子在Cu(111)表面的扩散势垒,然后结合计算结果,用kMC方法模拟了长程相互作用下磁性原子在Cu(111)表面自组装形成有序纳米薄膜的微观过程及其成核规律。首先,用密度泛函理论结合超元胞方法,对Cu原子在Cu(111)表面fcc洞位和hcp洞位的吸附模型进行了几何优化,并用从fcc洞位到hcp洞位过渡态计算的方法,研究了Cu(111)表面扩散势垒的计算模型。通过对不同基底层数、表面弛豫层数、表面覆盖率、真空层厚度的比较,认为基底层数为6层,表面弛豫2层,表面覆盖率为1/4ML(monolayer),真空层为1.0nm的计算模型满足计算Cu(111)表面的扩散势垒的要求。并且,根据以上模型,计算了Fe、Co、Ni等磁性原子在Cu(111)表面的扩散势垒和磁矩。其次,结合计算得到的扩散势垒和动力学蒙特卡罗方法,模拟了长程相互作用下Mn原子在Cu(111)表面的自组装过程。模拟结果发现,表面覆盖率和基底温度对Mn原子的自组装过程有着重要的影响。在基底温度为12K-18K时,Mn原子先形成链状有序结构,但是这种结构不稳定,容易演化成团簇状的纳米超结构。并且,当沉积的原子数目大于饱和覆盖率后,纳米超结构的周期随着覆盖率的增大而减小。此外,我们系统的研究了长程相互作用下超晶格的形成条件,认为排斥环的大小,原子到最稳定点的距离以及扩散势垒的大小是影响大面积超晶格形成的主要因素。最后,结合计算得到的扩散势垒,用动力学蒙特卡罗方法模拟了长程相互作用下Ni原子在Cu(111)表面的成核规律。模拟结果发现,由于长程相互作用中排斥势垒的影响,Ni原子的成核推迟,岛密度增大,岛的尺寸变得非常均匀,集中分布在2-6个原子。并且岛密度对基底温度的依赖关系变的不明显,不再满足经典成核理论。更多还原

【Abstract】 Ordered nanostructure thin films on substrate are widely used in scientific and technological fields, and the study fruits of thin film science are increasingly translated into productivity. The understanding of the initial stage of thin film process such as diffusion and nucleation from atomic-level have much significance in optimizing the growth technics and capability. In experiments, scanning tunnel microscope(STM) and atom force microscope (AFM) are used to observe the atom actions in the thin film process. However, in theory, different methods are fit for different time and space scale, and the methods which match the actual growth process must be selected. In this paper, the method that combined density-functional theory(DFT) and kinetic Monte Carlo(kMC) simulation is proposed. Firstly, the diffusion barrier of adatoms on Cu(111) surface is calculated using DFT, and then the self-organization and nucleation rule of magnetic adatoms on Cu(111) surface are simulated using kMC method in which long-range interactions are considered.The diffusion barrier of Cu on Cu(111) surface is calculated by means of geometrically optimizing the absorption models for fcc and hcp absorbed sites, and then calculating the transition state from fcc site to hcp site, which is based on the DFT and supercell approach. The results show that the model of diffusion barrier is accurate when the supercell is of six total layers with two relaxed layers, the coverage is 1/4ML (monolayer) and the vacuum space is 1.0nm. In addition, the diffusion barriers and magnetic moments of magnetic adatoms, such as Fe、Co、Ni on Cu(111) surface are obtained with the calculation model above.Then the self-organization of Mn adatoms on Cu(111) surface is simulated by kinetic Monte Carlo method with the activation barrier obtained above and the long-range interactions between Mn adatoms. The simulation results show that the self-organization of Mn adatoms is sensitively dependent on the coverage and temperature. At low coverage the Mn adatoms are likely to form linear chains at first and to locally ordered nano-superstructures when it becomes steady at about 12K and 18K. If the number of Mn adatoms is larger than the saturated coverage, the periods of the superstructures will decrease with the increase of coverage. Analyzing the formation mechanism of superlattice, we find that the size of the repulsive ring, the distance to the first minimal potential, and the activation barrier are crucial parameters to form superlattice. At last, with the diffusion barrier obtain above the nucleation rule of Ni adatoms with the long range interactions on Cu(111) surface are simulated with kinetic Monte Carlo method. The results show that the nucleation is delayed, the island density is increased and the island size becomes much uniform, which distributes in the range of 2-6 atoms if the repulsive potential of the long-range interactions is considered. In addition, the relationship between island density and substrate temperature becomes much nonsensitive and no longer obeys the classical nucleation law.更多还原