The vast majority of single crystalline metal nanoparticles adopt shapes in the O_h point group as a consequence of the symmetry of the underlying face-centered cubic (FCC) crystal lattice. Tetrahedra are a notable exception to this rule, and although they have been observed in several syntheses, their growth mechanism, and the symmetry-reduction process that necessarily characterizes it, is poorly understood. Here, a symmetry breaking mechanism is revealed by in situ liquid flow cell transmission electron microscopy (TEM) observation of seeded growth in which tetrahedra nanoparticles are formed from higher symmetry seeds. Real-time observation of the growth demonstrates a kinetically driven pathway during which rhombic dodecahedra nanoparticles transition to tetrahedra through tristetrahedra intermediates, with an accompanying surface facet evolution from {110} to {111} via {hhl} (where h > l), respectively. On the basis of these data, we propose a mechanism that relies on a rapid loss of inversion symmetry in the initial stages of the reaction, followed by differential reactivity of tips vs faces under conditions of relatively high supersaturation and moderate ligand concentration. The application of these insights to ex situ synthesis conditions allowed for an improved yield of tetrahedra nanoparticles. This work sheds an important mechanistic light on the crystallographic underpinnings of nanoparticle shape and symmetry transformations and highlights the importance of single-particle characterization tools for monitoring nanoscale phenomena.

中文翻译：

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通过从更高对称性前体生长四面体形纳米晶体了解单粒子水平的对称性破坏

由于底层面心立方 (FCC) 晶格的对称性，绝大多数单晶金属纳米颗粒都采用O _h点群中的形状。四面体是这一规则的一个显着例外，尽管它们已在几种合成中观察到，但它们的生长机制以及必然表征它的对称性减少过程却知之甚少。在这里，原位揭示了对称破坏机制液体流动池透射电子显微镜 (TEM) 观察种子生长，其中四面体纳米粒子由更高对称性的种子形成。对生长的实时观察表明了一个动力学驱动的途径，在此过程中菱形十二面体纳米粒子通过三四面体中间体转变为四面体，伴随着分别通过{hhl}（其中 h > l）从 {110} 到 {111} 的表面小面演变. 基于这些数据，我们提出了一种机制，该机制依赖于反应初始阶段反转对称性的快速丧失，然后在相对高的过饱和度和中等配体浓度的条件下，尖端与面的不同反应性。这些见解在异地的应用合成条件允许提高四面体纳米粒子的产率。这项工作揭示了纳米粒子形状和对称变换的晶体学基础的重要机制，并强调了单粒子表征工具对监测纳米级现象的重要性。