Unique Thermoelectric Properties Induced by Intrinsic Nanostructuring in a Polycrystalline Thin‐Film Two‐Dimensional Metal–Organic Framework, Copper Benzenehexathiol

Electrically conductive two-dimensional (2D) metal-organic frameworks (MOFs) have emerged as good candidates for thermoelectric applications due to their high electrical and low thermal conductivities. This work studies the microscopic origin of the thermoelectric properties of copper benzenehexathiol (Cu-BHT), a highly electrically conductive 2D MOF. 2D MOFs usually have polycrystalline domains because of the bottom-up synthesis, and the polycrystallinity makes it challenging to understand the intrinsic properties of 2D MOFs. Mesoscopic-scale devices are fabricated to measure the thermal conductivity, electrical conductivity, and Seebeck coefficient of Cu-BHT by exfoliating the synthesized Cu-BHT samples into thin films of thickness ranging from 30 to 400 nm. It is verified that the low thermal conductivity of Cu-BHT originates from the unique intrinsic structure of 2D MOFs, while our data indicates that the electrical conductivity is largely controlled by the polycrystallinity. The Seebeck effect measurement reveals the presence of robust electronic bands arising from definite crystallinity, which differentiates 2D MOFs from conductive polymers. This work points to opportunities in optimizing the thermoelectric figure of merit of 2D MOFs through the appropriate combination of metal ions and organic ligands.