Van der Waals Confinement Drives Anisotropic Crystallization in Two-Dimensional Bismuth Layers

The surprising ability to grow high-quality, single-crystal materials from substances not normally suited to layered structures has recently captivated materials scientists, and new research sheds light on the underlying mechanisms. Yuxiang Gao and Zhicheng Zhong, both from the Suzhou Institute for Advanced Research at the University of Science and Technology of China, and their colleagues, demonstrate how squeezing materials between surfaces, a technique known as van der Waals confinement, drives the formation of these crystals. Their work reveals that this confinement induces a unique anisotropic crystallization process, where layering occurs far more rapidly than in-plane ordering, leading to predictable phase transitions at specific pressures. These findings not only explain how large-area, high-quality crystals can form under confinement, but also provide crucial guidance for creating novel, metastable two-dimensional materials with potential applications in future nanoelectronic devices.

Van der Waals Epitaxy of Bismuth Crystals

This research details the synthesis and characterization of two-dimensional bismuth crystals grown using van der Waals (vdW) epitaxy, focusing on the mechanisms driving their formation and the resulting material properties. Researchers leverage vdW epitaxy, confining bismuth on a substrate with weak interlayer interactions, to induce 2D growth. The bismuth crystals exhibit a thickness-dependent structure, undergoing two critical phase transitions: a transformation at 1. 64 GPa and a collapse into a single atomic layer at 2. 19 GPa.

The crystallization process is anisotropic, proceeding in two steps: initial out-of-plane layering driven by confinement, and subsequent in-plane ordering governed by directional atomic interactions. Specific substrate angles (15° and 45°) energetically favor crystal growth, leading to orientational selection and the formation of single crystals. The vdW environment enhances thermal stability, allowing for higher-temperature processing and defect healing. Researchers employed molecular dynamics simulations, utilizing a machine-learning force field, to model the growth process, phase transitions, and material properties. This research demonstrates the importance of confinement, substrate engineering, and thermal control in achieving high-quality 2D crystals with tailored properties, with implications for novel electronic devices, quantum materials, and advanced technologies.

Anisotropic Crystallization Drives Bismuth Nanocrystal Growth

Researchers have uncovered key mechanisms driving the creation of high-quality two-dimensional bismuth crystals through van der Waals squeezing, a technique where materials are confined between layers. Simulations reveal that anisotropic crystallization, where layering occurs much faster than in-plane ordering, is key. This process involves the two critical phase transitions: a transformation at 1. 64 GPa and a subsequent collapse into a single-atomic layer at 2. 19 GPa.

Simulations demonstrate a step-wise progression as pressure increases, transitioning from multi-layered bismuth to single-atomic layers. These single-atomic layers initially exhibit disorder, but cool to form a square lattice structure. Further investigation revealed that out-of-plane layering arises intrinsically from the confinement itself, as restricting atomic movement creates energy wells that stabilize layered configurations. Substrate interactions contribute to orientational selection and accelerate grain boundary migration, ultimately enabling the formation of large-area, high-quality single crystals. These findings establish guiding principles for controlling the synthesis of metastable two-dimensional materials, with potential implications for the development of next-generation nanoelectronic devices.

Anisotropic Crystallization Drives Van der Waals Squeezing

Molecular dynamics simulations investigated the mechanisms behind the success of van der Waals squeezing in creating high-quality, two-dimensional bismuth crystals. Simulations revealed that anisotropic crystallization, where layering occurs much faster than in-plane ordering, is key. The simulations demonstrate that specific substrate angles promote single-crystal formation, while elevated temperatures enhance grain boundary migration and defect healing. Van der Waals pressure elevates the melting point, further facilitating this process. These findings provide fundamental insight into how confinement, substrate interactions, and thermal activation work together to enable the synthesis of high-quality, non-van der Waals 2D crystals.