Unveiling the electron-phonon coupling anisotropy in 2D covalent organic frameworks

Area of Science:

• Materials Science
• Condensed Matter Physics
• Organic Chemistry

Background:

• Covalent organic frameworks (COFs) are promising organic semiconductors for energy applications.
• Understanding electron-phonon interactions (EPI) in 2D COFs is crucial due to their anisotropic nature.
• Anisotropy arises from orthogonal covalent and van der Waals interactions in 2D COFs.

Purpose of the Study:

• To investigate the electron-phonon interaction (EPI) properties of six synthesized 2D COFs.
• To elucidate the relationship between EPI and thermal transport in 2D COFs.
• To explore the potential of 2D COFs as anisotropic thermal conductors.

Main Methods:

• Synthesis of six 2D COFs with varying structures and conjugation.
• Experimental investigation of electron-phonon coupling (EPC) properties.
• Measurement of thermal conductivity using the laser flash method.

Main Results:

• Electron-phonon coupling (EPC) in 2D COFs is direction-dependent, strong along the interlayer direction and weak in-plane.
• A positive correlation between thermal conductivity and EPC strength was observed, unlike conventional semiconductors.
• Interlayer EPC was found to harden phonon frequencies and enhance in-plane acoustic phonon bunching.
• A highly oriented COF film demonstrated extreme thermal transport anisotropy, with a two-order-of-magnitude difference in conductivity.

Conclusions:

• 2D COFs exhibit significant directional anisotropy in electron-phonon coupling and thermal transport.
• Interlayer EPC plays a key role in enhancing thermal conductivity in these materials.
• These findings provide fundamental insights into 2D COF transport mechanisms and their potential as efficient thermal conductors.