【作者】 苏长荣；

【导师】 李家明；

【作者基本信息】 清华大学 ， 物理学， 2004， 博士

【摘要】 本论文应用第一原理计算方法研究了钠团簇的基态构型,一维铋纳米管的力学和电学性质和高指数Si（15,3,23）表面的再构现象,展示了第一原理计算对纳米科学和技术发展的促进作用。 纳米科技自上世纪90年代兴起以来取得了巨大的发展,论文第一章从纳米材料的制备、表征和组装三个方面介绍了纳米科技的实验进展,并介绍了现在理论研究纳米材料物性的主要第一原理方法。在第一章中还介绍了计算平台硬件和软件方面的进展。 论文第二章提出了金属键优选法系统地研究了钠团簇Na_n,n≤15的基态构形。在金属键优选法中,我们根据Na_n团簇稳定构形中金属键的特性来构建Na_n+1团簇的合适的初始几何构形,用第一原理结构优化方法得到相应的稳定构形,然后通过总能来确定团簇Na_n+1的基态构形。金属键优选法使得我们可以大大缩小团簇起始几何构形的尝试范围。对钠团簇的研究发现了一些有趣的性质,如在Na₁₃、Na₁₄和Na₁₅团簇中发现具有钠晶体（110）面特征的子结构。钠团簇基态构形的系统确定使得定量研究钠团簇质谱的结构细节成为可能,我们发现通过捕获和解离一个Na原子来达到平衡的过程决定了钠团簇质谱的主要特征,而为了理解质谱的细节还必须考虑通过捕获和解离一个Na₂团簇达到平衡的过程,这个过程对小尺寸钠团簇尤其重要。 第三章应用第一原理方法研究了铋纳米管Bi（n,n）,2≤n≤10和Bi（n,0）,4≤n≤18的力学和电学性质,发现铋纳米管是结构稳定的半导体性纳米材料。铋纳米管的应力能与碳纳米管相当,“扶手椅型”铋纳米管遵从经典的应变理论,但小尺寸的“之字型”铋纳米管却偏离了这一理论。计算发现铋纳米管的杨氏模量为0.06TPa（0.25TPa（?））,大概为碳纳米管的5%。计算表明铋纳米管具有有趣的电学性质,不同旋度和管径的铋纳米管都是半导体性的。在小尺寸铋纳米管中由于强杂化效应的存在,其能带结构和能隙有较大的变化。但是当铋纳米管的管径大于18（?）时,能隙稳定在0.63eV附近,对应于卷成铋纳米管的铋层在Γ点的能隙。我们预期铋纳米管将在未更多还原

【Abstract】 In this dissertation, we show the importance of the first-principles calculations to the nano-science and nano-technology with applying the first-principles methods to determine the groud-state atomic-structures of sodium clusters, investigate the mechanical and electronic properties of bismuth nanotubes and study the reconstruction of Si（15,3,23） surface.In Chapter One, we introduce the experimental developments of nano-science in generating, characterizing and assembling of nano-materials and the main first-principles methods to investigate the properties of nano-materials. The developments of computing conditions in hardwares and softwares are also introduced in this chapter.In Chapter Two, the optimum metallic-bond scheme is presented to investigate the ground-state atomic-structures of sodium clusters Na_n, n≤l5. In the optimum metallic-bond scheme, the characters of metallic bonds of Na_n cluster are combined to construct the initial geometrical coordinates of Na_n+1 cluster, and then the corresponding stable stucture of Na_n+1 is obtained by first-principles structure relaxation. The ground-state structure of Na_n+1 can be determined by comparing the total energies of its various stable stuctures. The optimum metallic-bond scheme reduces the initial guesses of geometrical coordinates of clusters dramatically. Some interesting features have been revealed from the ground-state structures of sodium clusters, for instance, there are plane-like subunits in Na₁₃, Na₁₄ and Na₁₅ that are similar to the frags of （110） surface of sodium crystal. The systematic research on the ground-state structures make it possible to elucidate the mass spectra of sodium clusters quantitatively. We find that the quasi-steady processes through capturing or dissociating a sodium atom dominate the main features of the mass spectra. The quasi-steady processes through capturing or dissociating a sodium dimer are also important to understand the detailed features of mass spectra, especially for small size clusters.In Chapter Three, the mechanical and electronic properties of bismuth nanotubes Bi（n,n）, 2≤n≤10 and Bi（n,0）, 4≤n≤18 are investigated with first-principles methods.We find that the bismuth nanotubes have comparative strain energies to carbon nanotubes, and the strain energies of armchair bismuth nanotubes follow the classical strain theory while the small size zigzag bismuth nanotubes deviate the theory with much larger strain energies. The bismuth nanotubes have a Young’s modulus in the order of 0.06TPa（0.25TPaA）, which is approximately 5% that of carbon nanotubes. All the bismuth nanotubes are found to be semi-conducting materials independent of their diameters and helicities. For bismuth nanotubes with small diameters, the band structures and bandgaps vary evidently with the strong hybridization effect. When the diameters are larger than 18A, the bandgaps of bismuth nanotubes approach 0.63 eV, corresponding to that of bismuth sheet at the T point. In addition, we expect the constant-bandgap bismuth nanotubes to be a potential semiconducting nano-material in future nano-electronics.In Chapter Four, a reconstruction model is presented to explain the experimental STM images of Si（15,3,23） surface. The calculated STM images of the model with first-principles methods agree well with experiment which indicates that our model is a possible candidate of the reconstruction of Si（15,3,23）. This chapter also introduces the stable high index silicon surfaces and their family territories, and the principles and theories of STM.更多还原