Bringing Order to the Framework: Nanoscale Charge Transport Characterization of MOFs and COFs
by Jonas Fredrik Pöhls
Date of Examination:2024-09-10
Date of issue:2024-10-22
Advisor:Prof. Dr. Thomas Weitz
Referee:Prof. Dr. Thomas Weitz
Referee:Prof. Dr. Christian Jooß
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Abstract
English
Reticular chemistry, the synthesis of highly ordered, porous, crystalline networks with a previously identified target structure, has given rise to various classes of open framework materials, including the best-known metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). While MOFs consist of functional metal nodes linked by organic connector molecules, COFs are composed purely of organic compounds. Both material classes are characterized by a high porosity and large surface area, making MOFs and COFs particularly suitable for applications in gas storage, detection and catalysis. For a long time, they played no significant role in electrical applications; in particular MOFs were initially regarded as classic insulators. In the meantime, however, there are various electrically conductive representatives in both material classes, making these materials interesting for new areas of application. As a result, there is a growing need for a fundamental understanding of charge transport in MOFs and COFs and the limiting factors. In this thesis, materials are electrically characterized in the various forms in which MOFs and COFs can currently be synthesized. The tetrathiafulvalene-based COF TUS-48 is synthesized in powder form and pressed into a pellet for electrical characterization. It can be shown that the electrical conductivity can be improved by doping with I2 and by increasing the temperature. It can also be seen that the charge transport is strongly influenced by the disorder in the pellet due to the measurement as a pressed powder. The two COFs BTT TTA and BTT TTTBA have a 2D structure and are synthesized as ordered thin films. They both exhibit directional charge transport, in which the electrical conductivity within the 2D plane clearly exceeds the one in the perpendicular direction. Thus, the experimental findings deviate from the simulations for these materials, which had expected the charge transport in the perpendicular direction to be more efficient. Grain boundaries can be identified as the reason for the discrepancy. Grain boundaries also play a major role in the electrical characterization of the MOF Cu5BHT, which can be grown as a single crystal. The electrical properties of a single crystal are compared with those of a crystal with various grain boundaries. This demonstrates that grain boundaries are capable of reversing the temperature dependence of the electrical conductivity and masking the true nature of the charge transport. A MOF with a very similar structure, Cu3BHT, is synthesized as a monolayer. A major influence of grain boundaries on the electrical charge transport can also be demonstrated for this material. However, if this influence is minimized, it is possible to reveal quantum effects like oscillations of the electrical resistance due to magnetic fields in the MOF. This effect is characterized in more detail at low temperatures and high magnetic fields. Overall, this thesis highlights the influence of grain boundaries in MOFs and COFs on the charge transport. It thus raises the question of how suitable the common method of using MOF powders for electrical characterization really is.
Keywords: Charge Transport; MOF; COF; Grain Boundaries