Thermoelectric Properties of Metal-Organic-Framework Thin Films

Abstract

Bulk Metal-Organic-Framework (MOF) films are designed scaffold-like compounds which are composed by inorganic metal clusters connected with organic linker molecules, forming highly ordered, crystalline and porous structures. These bulk MOF frameworks were initially designed for gas storage due to their high storage capacity inside the pores of MOF bulk materials. However, the application for electrical devices were very limited resulting from their insulating character. Recently, it has been reported that the electrical properties of bulk host MOFs can be modulated by infiltrating guest molecules (metal clusters) inside the porous MOF framework. This renders MOF materials a novel and promising material for microelectronic devices, sensors, catalytically active materials and thermoelectrics. In our work, oriented SURMOF films and random polycrystalline MOF films Cu3(BTC)2 (BTC: benzene tricarboxylate, known as HKUST-1 MOF, were synthesized by a liquid phase epitaxy (LPE) spray method grown on surface functionalized gold coated silicon substrates and regular non-functionalized silicon substrates covered with a thick 484 nm thermal SiO2 layer respectively. The tetracyano-quinodimethane (TCNQ) guest molecules were infiltrated into the MOFs to modulate the electrical properties of the film. The horizontal Seebeck coefficient parallel to the sample surface of both oriented SURMOF films and random polycrystalline MOF films were measured. The random oriented MOF films exhibit a high positive Seebeck coefficient of 422.32 µV/K at 350 K. However, the horizontal Seebeck coefficient of oriented SURMOF films fluctuates around 0 µV/K. This can be interpreted that these highly oriented SURMOF films exhibit a large anisotropy with no charge carrier transport in horizontal direction parallel to the sample surface, but only carrier transport in vertical direction. The anisotropic nature of SURMOF films were further proven by IV characterization. The electrical conductivity of polycrystalline MOF film in horizontal direction was observed in the temperature range of 260 K to 350 K to calculate power factor (S2δ) of MOF films. The maximum value of power factor was found at 350 K about 1.527 µW/(m2K), which is about one order of magnitude higher than reported one. The thermal conductivity measurement on polycrystalline MOF films indicated the TCNQ infiltration not only contributes to enhance electrical conductivity, but also results in a reduction of thermal conductivity of MOF films. The reduced thermal conductivity can be attributed to enhanced phonon scattering due to TCNQ infiltration. The results demonstrate that random polycrystalline MOF film has a promising application potential in future thermoelectric and electronic devices.