The formation and growth of water-ice layers on surfaces and of low-dimensional ice under confinement are frequent occurrences^1,2,3,4. This is exemplified by the extensive reporting of two-dimensional (2D) ice on metals^{5,6,7,8,9,10,11}, insulating surfaces^{12,13,14,15,16}, graphite and graphene^17,18 and under strong confinement^{14,19,20,21,22}. Although structured water adlayers and 2D ice have been imaged, capturing the metastable or intermediate edge structures involved in the 2D ice growth, which could reveal the underlying growth mechanisms, is extremely challenging, owing to the fragility and short lifetime of those edge structures. Here we show that noncontact atomic-force microscopy with a CO-terminated tip (used previously to image interfacial water with minimal perturbation)¹², enables real-space imaging of the edge structures of 2D bilayer hexagonal ice grown on a Au(111) surface. We find that armchair-type edges coexist with the zigzag edges usually observed in 2D hexagonal crystals, and freeze these samples during growth to identify the intermediate edge structures. Combined with simulations, these experiments enable us to reconstruct the growth processes that, in the case of the zigzag edge, involve the addition of water molecules to the existing edge and a collective bridging mechanism. Armchair edge growth, by contrast, involves local seeding and edge reconstruction and thus contrasts with conventional views regarding the growth of bilayer hexagonal ices and 2D hexagonal matter in general.

表面上的水冰层和限制下的低维冰的形成和生长是经常发生的^1,2,3,4。^{这可以通过广泛报道金属5、6、7、8、9、10、11}、绝缘表面^{12、13、14、15、16}、石墨和石墨烯^17、18上的二维 (2D) 冰和强约束下^{14,19,20,21,22}. 尽管已经对结构化水层和二维冰进行了成像，但由于这些边缘结构的脆弱性和短寿命，捕获二维冰生长中涉及的亚稳态或中间边缘结构（这可以揭示潜在的生长机制）极具挑战性。在这里，我们展示了具有 CO 端接尖端的非接触式原子力显微镜（以前用于以最小的扰动对界面水进行成像）¹²，能够对在 Au(111) 表面上生长的二维双层六边形冰的边缘结构进行实空间成像。我们发现扶手椅型边缘与通常在 2D 六角晶体中观察到的锯齿形边缘共存，并在生长过程中冻结这些样品以识别中间边缘结构。结合模拟，这些实验使我们能够重建生长过程，在锯齿形边缘的情况下，涉及将水分子添加到现有边缘和集体桥接机制。相比之下，扶手椅边缘生长涉及局部播种和边缘重建，因此与关于双层六角冰和一般二维六角物质生长的传统观点形成对比。