Direct observation of vapor-liquid-solid nanowire growth
Direct observation of vapor-liquid-solid nanowire growth
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DOI：
10.1021/ja0059084
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发表时间：
2001-04-04
影响因子：
15
通讯作者：
Yang, PD
中科院分区：
文献类型：
--
作者：
Wu, YY;Yang, PD
纳米管和半导体纳米线对于研究尺寸和维度相关的化学和物理现象具有重要意义。1，2如何合理地合成这些一维纳米结构一直是一个重大挑战，尽管最近已经采取了几种策略。3-16例如，碳纳米管是通过热的碳等离子体在某些金属的存在下凝聚而成的，尽管真正的生长机制一直难以捉摸。3-5最近，不同组成的半导体纳米线已经成功地用汽相或基于溶液的方法合成了。13-16这些合成的一个关键特征是使用金属纳米颗粒作为催化剂来促进各向异性晶体的生长。虽然也有人提出了氧化物辅助生长机制，但从20世纪60-70年代提出的气-液-固(VLS)机制推断出了生长机制。2，20然而，除了这些纳米线的尖端通常有合金液滴这一事实之外，仍然缺乏关于纳米线生长机制的直接证据。因此，更好地了解纳米线在气相中的生长过程，对于确定纳米线的生长机理并能够合理地控制其成分、尺寸、晶体结构和生长方向是必要的。在此，我们首次在原位高温电子显微镜上实时观察到了半导体纳米线的生长，这明确地证明了纳米尺度上VLS生长机制的有效性。21在这一过程中明确确定了三个明确的阶段：金属合金化、晶体形核和轴向生长。在此基础上，以单分散的金纳米团簇为催化剂，实现了不同直径硅纳米线的选择性生长。在高温透射电子显微镜(JEOL CX200)上进行了纳米级丝形核/生长的原位观察。少量微米级的Ge粒子与溶液制备的单分散Au纳米团簇一起分散在电子显微镜网格上。22金团簇的平均尺寸为20.2(3.1 nm)。虽然纯Ge的蒸汽压在900℃以下可以忽略不计，但我们发现在显微镜下，一层薄薄的碳涂层可以促进Ge的蒸发，这可能是由于Ge/C界面的相互作用。23，24事实上，在真空中对这些碳涂层Ge颗粒进行长时间的加热
Nanotubes and semiconductor nanowires are of fundamental importance to the study of size-and dimensionality-dependent chemical and physical phenomena. 1, 2 How to rationally synthesize these 1-dimensional nanostructures has been a major challenge, although several strategies have been pursued recently. 3-16 For example, carbon nanotubes have been prepared via condensation of hot carbon plasmas in the presence of certain metals, although the real growth mechanism has been elusive. 3-5 Recently, semiconductor nanowires with different compositions have been successfully synthesized using either vapor6-12 or solution-based methodologies. 13-16 One key feature of these syntheses is the promotion of anisotropic crystal growth using metal nanoparticles as catalysts. The growth mechanism has been extrapolated from the vapor-liquid-solid (VLS) mechanism which was proposed in the 1960s-1970s for large whisker growth, 17-19 although an oxide-assisted growth mechanism has also been proposed. 2, 20 Direct evidence for the nanowire growth mechanism, however, is still lacking except for the fact that these nanowires generally have alloy droplets on their tips. Hence, a better understanding of the nanowire growth process in the vapor phase is necessary to pin down the growth mechanism and to be able to rationally control their compositions, sizes, crystal structures, and growth directions. Herein we report the first real-time observation of semiconductor nanowire growth in an in situ high-temperature transmission electron microscope (TEM), which unambiguously demonstrates the validity of the VLS growth mechanism at nanometer scale. 21 Three well-defined stages have been clearly identified during the process: metal alloying, crystal nucleation, and axial growth. On the basis of this mechanism study, selective growth of Si nanowires with different diameters has been demonstrated using monodispersed gold nanoclusters as catalysts. In situ observation of wire nucleation/growth at nanometer scale was conducted within a high-temperature transmission electron microscope (JEOL CX200). A small amount of micrometer-sized Ge particles were dispersed on TEM grids together with solutionmade monodispersed Au nanoclusters. 22 The gold clusters have average sizes of 20.2 (3.1 nm. Although pure Ge has negligible vapor pressure up to 900 C, we found that a thin layer of carbon coating could promote Ge evaporation within the microscope, presumably due to Ge/C interfacial interaction. 23, 24 In fact, prolonged heating of these carbon-coated Ge particles in a vacuum