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

Area of Science:

• Surface Science
• Materials Science
• Physical Chemistry

Background:

• Two-dimensional (2D) crystallization is crucial for natural phenomena and technology.
• Understanding the atomic-scale mechanisms of 2D crystallization is challenging.
• Classical nucleation theory often fails to explain 2D crystallization processes.

Purpose of the Study:

• To investigate the microscopic mechanism of 2D ice crystallization on a graphite surface.
• To observe atomic-scale transient states during crystallization.
• To elucidate the role of adsorbed water molecules in the crystallization pathway.

Main Methods:

• Atomic-resolution imaging using qPlus-based cryogenic atomic force microscopy (AFM).
• Molecular dynamics (MD) simulations.
• Combined AFM and MD approach to study 2D amorphous bilayer ice.

Main Results:

• Observed a fractal-to-compact transition during 2D ice crystallization with increasing temperature.
• Identified a non-classical crystallization pathway involving dendritic extension of fractal islands and subsequent compact growth.
• Demonstrated the crucial role of out-of-plane adsorbed water molecules in facilitating hydrogen-bonding network rearrangement.

Conclusions:

• The study reveals a non-classical ordering mechanism for 2D crystallization, deviating from classical nucleation theory.
• Adsorbed water molecules act as mediators, guiding the transition from disordered to ordered hexagonal ice structures.
• The findings provide general insights into crystallization phenomena at the 2D limit, applicable to various interfaces and thin films.