Unraveling Intrinsic Thermal Conductivity in Layered Conductive MOF Single Crystals

TL;DR

This study measures the intrinsic thermal conductivity of layered conductive MOF single crystals, revealing ultralow values and complex phonon scattering mechanisms, advancing understanding of their thermoelectric properties.

Contribution

First direct measurement of thermal conductivity in single-crystalline layered conductive MOFs, highlighting the role of structural disorder in phonon scattering.

Findings

Ultralow thermal conductivities (0.075-0.194 W/m·K) along the stacking direction.

Nd3HHTP2 exhibits high electrical conductivity but similar thermal conductivity to less conductive counterparts.

Structural disorder and incommensurate modulation are key to strong phonon scattering.

Abstract

Layered conductive metal-organic frameworks (LCMOFs) show great promise in energy and electronics due to their high electrical conductivity and tunable pore structures. They are considered ideal "phonon-glass, electron-crystal" materials. However, their intrinsic thermal transport properties, particularly the thermal conductivity in the single-crystalline state, have never been explored before. The applicability of the Wiedemann-Franz law to such complex porous materials is a key scientific question to describe their thermoelectric relationship. We investigated single crystals of three LCMOFs (Cu3HHTP2, Co9HHTP4, Nd3HHTP2) using the microfabricated suspended device. Results showed ultralow thermal conductivities (0.075-0.194 W m-1 K-1) along the {\pi}-{\pi} stacking direction. Crucially, Nd3HHTP2 exhibited a high electrical conductivity of 398 S cm-1, yet its thermal conductivity (0.148 W m-1 K-1) was comparable to the other two LCMOFs with significantly lower electrical conductivities. Structural characterization revealed that the incommensurate modulation, and in-plane correlated disorder within the Nd3HHTP2 structure are the potential causes of strong phonon scattering and the observed ultralow thermal conductivity.