Substrate-enhanced supercooling in AuSi eutectic droplets

摘要

液体中局部原子排列与晶体中的序列有实质性rn差别。但当与一个晶体接触时，液体中相邻原rn子能模仿其序列，经常触发液体的固化。这种rn“种子”效应是人们所熟悉的，如当水接近冰rn点时所出现的那样以及在晶体成核中所出现的rn那样。比较意外的是本期Nature上所介绍的关rn于个相反效应的发现:如果个表面的原子rn结构与相接触液相的结构相似，那么该液体不rn是结晶，而是在远低于其融点时仍保持为液rn体，这样便实现了超冷状态。这个现象过去在rn金一硅共晶液滴中观察到过，这些液滴与金原rn子按五角形分层排列在一个硅基质上。这一发rn现对于关于凝固问题的基础研究及对于相变的rn实用控制都有意义。例如，经常用来获得超冷rn状态的“无容器”方法，也许就可以通过用rn种按二十面体排列的表面来涂覆容器而避免。%The phenomenon of supercooling in metals-that is, the preservation of a disordered, fluid phase in a metastable state well below the melting point-has led to speculation that local atomic structure configurations of dense, symmetric, but non-periodic packing act as the main barrier for crystal nucleation. For liquids in contact with solids, crystalline surfaces induce layering of the adjacent atoms in the liquid and may prevent or lower supercooling. This seed effect is supposed to depend on the local lateral order adopted in the last atomic layers of the liquid in contact with the crystal. Although it has been suggested that there might be a direct coupling between surface-induced lateral order and supercooling, no experimental observation of such lateral ordering at interfaces is available. Here we report supercooling in gold-silicon (AuSi) eutectic droplets, enhanced by a Au-induced (6 × 6) reconstruction of the Si(111) substrate. In situ X-ray scattering and ab initio molecular dynamics reveal that pentagonal atomic arrangements of Au atoms at this interface favour a lateral-ordering stabilization process of the liquid phase. This interface-enhanced stabilization of the liquid state shows the importance of the solid-liquid interaction for the structure of the adjacent liquid layers. Such processes are important for present and future technologies, as fluidity and crystallization play a key part in soldering and casting, as well as in processing and controlling chemical reactions for microfluidic devices or during the vapour-liquid-solid growth of semiconductor nanowires.