Charge Transport, Conductivity, and Carrier Density in Tcnq Loaded MOF Films

Abstract

Bulk Metal-organic-framework (MOF) films are designed scaffold-like compounds that consist of metal ions connected by organic ligands in the form of crystalline powders, while Surface Anchored Metal-organic-framework (SURMOF) films consist of quasi-liquid phase epitaxially grown highly ordered crystalline films. The ability to conduct electrical charge is one of the most important but least understood properties of MOF films and coordination network materials. Very little work exists on electrical characterization of MOF films, because the early synthesized pristine MOF films exhibited mostly insulating characteristics. Recently, it has been reported that the electrical properties of bulk host MOF films can be modulated by infiltrating guest molecules (metal clusters) inside the porous MOF framework. This renders MOF materials a novel and potentially promising material for microelectronic devices, sensors, and thermoelectrics. For this case study we used MOF Cu3(BTC)2 (BTC: benzene tricarboxylate) films, also known as HKUST-1 (Hong Kong University of Science and Technology-1), which were infiltrated with tetracyano-quinodimethane (TCNQ) guest molecules to modulate the charge transport characteristics. HKUST-1 MOF thin films have been grown using a liquid phase epitaxy (LPE) spray method in which the metal-containing solution [1 mmol Cu(OAc)2] and the linker solution [0.1 mmol BTC] are sprayed alternately on modified silicon substrates. Subsequently, the samples were immersed into an ethanoic TCNQ solution. For benchmarking we have investigated the electrical characteristics of pristine HKUST-1 and TCNQ loaded compact HKUST-1 SURMOF films by Hall-Effect measurements and Seebeck measurements. Both Hall Effect and Seebeck coefficient measurements indicate that electrical conduction in dense compact MOF films involves positive charge carriers. The Hall coefficient, mobility as well as the charge carrier density, the electrical conductivity and the sheet resistance for HKUST-1 MOF films will be discussed.