Phase transformation in two-dimensional covalent organic frameworks under compressive loading

TL;DR

This study investigates how 2D covalent organic frameworks, specifically DTPA sheets, undergo phase transformations under compression, significantly altering their mechanical and electronic properties, with implications for other similar 2D materials.

Contribution

It reveals a novel compression-induced phase transformation in 2D COFs and analyzes its effects on their properties using molecular dynamics and density functional theory.

Findings

Phase transformation occurs under large in-plane compression.

Transformed phases show reduced Young's modulus, band gap, and thermal conductivity.

A large in-plane negative Poisson's ratio is observed in transformed phases.

Abstract

As a new class of two-dimensional (2D) materials, 2D covalent organic frameworks (COFs) are proven to possess remarkable electronic and magnetic properties. However, their mechanical behaviours remain almost unexplored. In this work, taking the recently synthesised dimethylmethylene-bridged triphenylamine (DTPA) sheet as an example, we investigate the mechanical behaviours of 2D COFs based on molecular dynamics simulations together with density functional theory calculations. A novel phase transformation is observed in DTPA sheets when a relatively large in-plane compressive strain is applied to them. Specifically, the crystal structures of the transformed phases are topographically different when the compressive loading is applied in different directions. The compression-induced phase transformation in DTPA sheets is attributed to the buckling of their kagome lattice structures and is found to have significant impacts on their material properties. After the phase transformation, Young's modulus, band gap and thermal conductivity of DTPA sheets are greatly reduced and become strongly anisotropic. Moreover, a large in-plane negative Poisson's ratio is found in the transformed phases of DTPA sheets. It is expected that the results of the compression-induced phase transformation and its influence on the material properties observed in the present DTPA sheets can be further extended to other 2D COFs, since most 2D COFs possess a similar kagome lattice structure.