Mechanism of Anisotropic Crystallization and Phase Transitions under Van der Waals Squeezing

TL;DR

This study uncovers how van der Waals squeezing induces anisotropic crystallization and phase transitions in bismuth, enabling the controlled synthesis of high-quality 2D single crystals through molecular dynamics simulations.

Contribution

It reveals the quantum confinement-driven anisotropic crystallization mechanism and critical phase transitions under vdW confinement, advancing understanding of 2D material synthesis.

Findings

Pressure induces alpha-to-beta phase transition at 1.64 GPa.

Single-atomic layer collapse occurs at 2.19 GPa.

Large-area single crystals form via substrate-induced orientation and grain boundary migration.

Abstract

Mechanical confinement strategies, such as van der Waals (vdW) squeezing, have emerged as promising routes for synthesizing non-vdW two-dimensional (2D) layers, surprisingly yielding high-quality single crystals with lateral sizes approaching 100 micrometer. However, the underlying mechanisms by which such a straightforward approach overcomes the long-standing synthesis challenges of non-vdW 2D materials remains a puzzle. Here, we investigate the crystallization dynamics and phase evolution of Bi under vdW confinement through molecular dynamics (MD) simulations powered by a machine-learning force filed fine-tuned and distilled from a pre-trained model with DFT-level accuracy. We reveal that pressure-dependent layer modulation arises from a quantum confinement-driven anisotropic crystallization mechanism, in which out-of-plane layering occurs nearly two orders of magnitude faster than in-plane ordering. Two critical transitions are identified: an alpha-to-beta phase transformation at 1.64 GPa, and a subsequent collapse into a single-atomic layer at 2.19 GPa. The formation of large-area single crystals is enabled by substrate-induced orientational selection and accelerated grain boundary migration, driven by atomic diffusion at elevated temperatures. These findings resolve the mechanistic origin of high-quality 2D crystal growth under confinement and establish guiding principles for the controlled synthesis of metastable 2D single crystals, with implications for next-generation quantum and nanoelectronic devices.