Atomic-resolution imaging reveals nucleus-free crystallization in two-dimensional amorphous ice on graphite.

Two-dimensional (2D) crystallization at interfaces or in thin films plays a critical role in many natural phenomena and technological applications, yet its microscopic mechanism remains elusive due to the challenges of directly observing atomic-scale transient states during crystallization. Here, we present atomic-resolution imaging of 2D ice crystallization on graphite surface using qPlus-based cryogenic atomic force microscopy (AFM) combined with molecular dynamics (MD) simulations. The crystallization of 2D amorphous bilayer ice undergoes a fractal-to-compact transition as temperature increases. Instead of forming a critical nucleus as predicted by classical theories, the crystallization firstly proceeds via the dendritic extension of fractal islands, followed by compact growth with defect healing at the percolated edges. We find that this process is significantly assisted by out-of-plane adsorbed (ad-) water molecules, which, like a spider weaving its web, facilitate the rearrangement of hydrogen-bonding network from disordered pentagons or heptagons to ordered hexagons. This fractal-to-compact crystallization pathway, mediated by ad-molecules, presents a non-classical ordering mechanism beyond classical nucleation theory, and may offer general insights into the crystallization at the 2D limit.