Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
Nanotechnology. 2012 May 4;23(17):175302. doi: 10.1088/0957-4484/23/17/175302. Epub 2012 Apr 5.
It is a well-known fact that a sphere offers less surface area, and thus less surface energy, than any other arrangement of the same volume. From this perspective, all other shapes are metastable objects. In this paper, we present and discuss a manifestation of this metastability: the spontaneous alignment of free-standing amorphous nanowires towards, and ultimately parallel to, a flux of directional ion irradiation. The behavior expected from surface energy reduction is the opposite of that predicted by both theory and experiment regarding defect generation in crystalline nanowires, but is consistent with other observations on non-crystalline materials. We verify our expectations by bending and aligning finely stranded amorphous silica nanowires, noting that such nanostructures are particularly susceptible to bending through ion-induced surface energy reduction. We offer support for this mechanism through bending rate studies, thermal annealing experiments and mathematical modeling. Experimentally, we also demonstrate selective reorientation of nanowires in patterned areas, as well as conformal coating of reoriented arrays with functional materials. These capabilities offer the prospect of exploiting engineered surface anisotropies in optical, fluidic and micromechanical applications.
这是一个众所周知的事实，即与相同体积的任何其他排列方式相比，球体的表面积更小，因此表面能也更小。从这个角度来看，所有其他形状都是亚稳物体。在本文中，我们介绍并讨论了这种亚稳性的一种表现形式：自由-standing 无定形纳米线自发地朝向并最终平行于定向离子辐照流的对齐。从表面能降低的角度来看，这种行为与理论和实验对晶状纳米线中缺陷生成的预测相反，但与非晶材料的其他观察结果一致。我们通过弯曲和对齐细绞合的无定形二氧化硅纳米线来验证我们的预期，注意到这种纳米结构特别容易通过离子诱导的表面能降低而弯曲。我们通过弯曲速率研究、热退火实验和数学建模为该机制提供了支持。在实验中，我们还证明了在图案化区域中纳米线的选择性重新取向，以及用功能材料对重新取向的阵列进行共形涂层。这些功能为在光学、流体和微机械应用中利用工程表面各向异性提供了前景。
