Knowing how crystals nucleate at the atomic scale is crucial for understanding, and in turn controlling, the structure and properties of a wide variety of materials. However, because of the scale and highly dynamic nature of nuclei, the formation and early growth of nuclei are very difficult to observe. Here, we have employed single-walled carbon nanotubes as test tubes, and an ‘atomic injector’ coupled with aberration-corrected transmission electron microscopy, to enable in situ imaging of the initial steps of nucleation at the atomic scale. With three different metals we observed three main processes prior to heterogeneous nucleation: formation of crystal nuclei directly from an atomic seed (Fe), from a pre-existing amorphous nanocluster (Au) or by coalescence of two separate amorphous sub-nanometre clusters (Re). We demonstrate the roles of the amorphous precursors and the existence of an energy barrier before nuclei formation. In all three cases, crystal nucleus formation occurred through a two-step nucleation mechanism.

了解晶体如何在原子尺度上成核对于理解并进而控制多种材料的结构和特性至关重要。但是，由于核的大小和高度动态性，很难观察到核的形成和早期生长。在这里，我们采用了单壁碳纳米管作为试管，并使用了“原子注入器”和像差校正后的透射电子显微镜，以实现原子级成核初始步骤的原位成像。对于三种不同的金属，我们观察到了异种成核之前的三个主要过程：直接由原子种子（Fe），先前存在的非晶纳米团簇（Au）或通过两个独立的非晶亚纳米簇（Re ）。我们证明了无定形前体的作用以及核形成之前能量垒的存在。在所有这三种情况下，晶核形成都是通过两步成核机制发生的。