Study of the formation process and mechanical properties of nanoscale materials by means of atomistic simulation

(1)采用分子动力学模拟方法研究了硅原子在硅衬底上的沉积过程。结果表明，在低温条件下，沉积的初始三层或四层附着原子呈外延生长，而随后的层形成无定形结构，这与实验中确定的有限厚度外延结构一致。考虑到二聚体行在瞬态生长表面上的断裂和更新的重复，可以理解纳米尺度外延的机制，并且随着沉积的进行，二聚体键逐渐稳定。如果二聚体行断裂和更新被中断，外延就会发生破坏。(2)采用分子动力学方法分析了金簇在金衬底上软沉积的形态演变过程。结果表明：晶体和液体状态下的团簇都成功地将自身外延安置在衬底上，在低温条件下，晶体团簇与衬底之间存在能量势垒，团簇最终成为一个面形外延岛；在低温条件下，液体团簇与衬底之间不存在能量势垒，团簇顺利地成为一个外延岛。通过改变自由团簇的状态和衬底温度，可以控制沉积的纳米结构的形貌。(3)采用分子动力学方法模拟了金纳米线的伸长率。结果表明，具有螺旋多壳结构的Au纳米线的杨氏模量小于中心轴<110>和表面为{111}的fcc结构的Au纳米线的杨氏模量，且随着温度的升高，前者比后者下降得更快，具有螺旋多壳结构的Au纳米线可以被拉伸形成单原子Au原子链。

(1) Deposition of Si atoms on a Si substrate was investigated by means of molecular-dynamics simulation. It was concluded that at low temperatures, the initial three or four monolayers of deposited adatoms grow epitaxially while the subsequent layers form an amorphouslike structure, which is consistent with the limited-thickness epitaxy identified in experiments, that the mechanism of epitaxy at nanoscale can be understood by considering repetition of the breaking and renewal of dimer rows on transient growing surfaces, and that the dimer bonds become gradually stabilized as deposition proceeds, and breakdown of epitaxy occurs if the breaking and renewal of dimer rows are interrupted.(2) The molecular-dynamics analysis of morphological evolution of a Au cluster softly-deposited onto a Au substrate was performed. It was concluded that both the clusters in a crystalline state and in a liquid state succeed in epitaxially accommodating themselves to the substrate, that in the accommodation of a crystalline cluster to the substrate at low temperatures, an energy barrier exists, and the cluster becomes finally a facetted epitaxial island, that in the accommodation of a liquid cluster to the substrate at low temperatures, no energy barriers exist, and the cluster becomes an epitaxial island smoothly, and that the morphology of such deposited nanostructures can be controlled by changing the state of free clusters and the substrate temperature.(3) Elongation of Au nanowires was simulated by the molecular-dynamics method. It was concluded that the Young's modulus of a Au nanowire with a helical multi-shell structure is smaller than the Young's modulus of a Au nanowire of the fcc structure with a <110> center axis and with {111} surfaces, that with the increase of temperature, the former decreases more rapidly than the latter, and that a Au nanowire with a helical multi-shell structure can be elongated to produce a single-atom chain of Au atoms.