- The paper presents the first systematic measurement of intrinsic thermal conductivity in layered conductive MOFs, reporting ultralow values along the stacking axis.
- The paper employs advanced microfabricated techniques and characterization methods, such as TEM and PXRD, to quantify phonon scattering and decoupling of electron transport.
- The paper identifies structural factors like incommensurate modulation and correlated disorder as key drivers in suppressing phonon transport while maintaining high electrical conductivity.
Intrinsic Thermal Conductivity in Layered Conductive MOF Single Crystals
Introduction
Layered conductive metal-organic frameworks (LCMOFs) have emerged as tunable, porous materials with high charge carrier mobility and substantial electrical conductivity, presenting opportunities in energy storage, sensing, electrocatalysis, and thermoelectric conversion. Despite their widespread application, fundamental knowledge regarding their intrinsic thermal transport, particularly in the single-crystalline state, has remained elusive. This work addresses the gap by systematically examining the thermal conductivity of three archetypal LCMOFs—Cu₃HHTP₂, Co₉HHTP₄, and Nd₃HHTP₂—probing the underlying mechanisms of phonon transport suppression and electron/phonon decoupling.
Experimental Approach
High-quality single crystals were synthesized via hydrothermal methods and characterized by OM, SEM, TEM, PXRD, BET, and XPS. Room-temperature electrical conductivity was measured using four-probe techniques with microfabricated devices; smaller crystals were contacted via EBL and magnetron sputtering, whereas larger Nd₃HHTP₂ crystals utilized silver paste on prefabricated gold electrodes. Thermal conductivity along the T-T stacking direction was determined via microfabricated suspended platforms employing lock-in amplification under high vacuum, utilizing Fourier's law for computation.
Numerical Results and Analysis
All LCMOFs studied exhibit extremely low thermal conductivity along the T-T stacking axis: 0.075 W·m⁻¹·K⁻¹ (Cu₃HHTP₂), 0.194 W·m⁻¹·K⁻¹ (Co₉HHTP₄), and 0.148 W·m⁻¹·K⁻¹ (Nd₃HHTP₂). Nd₃HHTP₂ is particularly notable for its electrical conductivity of 398 S·cm⁻¹, three orders of magnitude higher than the other two LCMOFs, yet its thermal conductivity is similar, demonstrating strong decoupling of charge and heat transport.
Crucially, the measured total thermal conductivity of Nd₃HHTP₂ is below the predicted electronic contribution based on the Wiedemann-Franz law (Ke=σLT): the theoretical Ke at measured conductivities (311–398 S·cm⁻¹) ranges from 0.226 to 0.289 W·m⁻¹·K⁻¹, yet the total measured value is 0.148 W·m⁻¹·K⁻¹. This constitutes clear empirical evidence for deviation from the Wiedemann-Franz law, indicative of phonon-dominated transport and substantial lattice suppression.
Structural Factors in Thermal Transport Suppression
PXRD, SCXRD, Cryo-HRTEM, and SAED reveal key features in Nd₃HHTP₂'s crystal architecture:
- Incommensurate modulation: Satellite spots in X-ray diffraction indicate periodic atomic deviations with irrational modulation vectors (q=0.393c∗), signifying structural modulation not locked to the lattice periodicity. This origin of strong electron-phonon coupling, possibly associated with Kohn anomaly and phonon softening, incites lattice instability and increased phonon scattering.
- Correlated disorder: SAED reveals diffuse scattering and elongated spots, ascribed to correlated disorder in Nd³⁺ occupation within the ab-plane. This type of disorder acts as additional scattering centers beyond the intrinsic anharmonicity and mass/bond mismatch.
- Low density/high porosity: Large pores and mass mismatch (metal and light elements), complex bonding (covalent, coordination, van der Waals), and low volumetric heat capacity further underpin the suppression of phonon mean free path and group velocity.
Decoupling Electron and Phonon Transport
The results confirm the "phonon-glass, electron-crystal" paradigm at the single-crystal level. Nd₃HHTP₂, with a high electrical conductivity typically indicating substantial electronic contribution to thermal transport, exhibits suppressed total thermal conductivity due to structural mechanisms that scatter phonons more efficiently than electrons. Electron transport is maintained via strong band dispersion and narrow interlayer spacing, but phonons are hindered by incommensurate modulation and correlated disorder.
Implications and Future Outlook
These findings provide a rigorous foundation for designing LCMOF-based thermoelectric materials capable of maximizing the figure of merit (ZT) through intrinsic decoupling of carrier and phonon transport. The single-crystalline measurements, unimpeded by grain boundary effects, reveal fundamental structure-property relationships as targets for theoretical modeling and synthetic control. Practically, materials such as Nd₃HHTP₂ may be exploited in advanced thermal management, low-diffusion energy storage, and other applications necessitating ultralow thermal conductivity combined with high electrical performance.
Potential research directions include: (1) engineering of structural modulation and disorder to optimize phonon scattering; (2) exploration of other lanthanide-based LCMOFs for higher ZT values in thermoelectric devices; (3) systematic studies correlating modulation vector and charge density waves with transport properties; (4) integration into device architectures with precise orientation control.
Conclusion
This work presents the first systematic measurement and analysis of intrinsic thermal conductivity in LCMOF single crystals, establishing ultralow phonon transport as a consequence of structural modulation and correlated disorder. The results highlight the fundamental electron/phonon decoupling in porous conductive MOFs, clarify deviations from classical transport relations, and extend the theoretical and practical landscape for materials optimization in advanced thermoelectric and energy applications.