Mechanistic Insights into Layered Growth of Imine-Linked Bilayer 2D Covalent Organic Frameworks.

The crystallinity and stacking order of two-dimensional covalent organic frameworks (2D COFs) critically govern their physical properties and performance. However, direct molecular-resolution insight into the growth of layered 2D COFs remains elusive, particularly for imine-linked systems. Herein, an imine-linked bilayer 2D COF is constructed at the solid-liquid interface, enabling time-dependent scanning tunneling microscopy (STM) visualization of interlayer growth. STM imaging reveals a layered growth mechanism involving defect-mediated nucleation, lattice-guided directional propagation, cross-defect growth, and oriented domain coalescence. This growth pathway gives rise to local ordered-on-disordered stacking embedded within predominantly AA stacking domains. These findings provide direct molecular-level insights into layered growth under surface-confined conditions and offer a conceptual perspective for considering stacking disorder in bulk imine-linked COFs.