Molecule@MOF: A New Class of Electronic Materials
Authors: Talin Albert Alec , Stavila Vitalie , Foster Michael E. , El Gabaly Farid , Ford Alexandra C. , Léonard François , Allendorf Mark D 
Published: February 2020

Conducting organic polymers are highly attractive electronic materials, having the advantages of low-cost fabrication routes, tunable electronic properties, and mechanical flexibility. Unfortunately, their charge mobility relative to traditional inorganic semiconductors is poor due in part to microstructural disorder. A new class of synthetically flexible, crystalline materials known as Metal-Organic Frameworks (MOFs) could solve the disorder problem, providing within a single material the highly ordered structure of inorganic conductors with the tunable properties and low cost of organics. MOFs are inorganic-organic hybrids consisting of metal cations linked by organic groups such as carboxylates and amines, forming a crystalline, nanoporous structure. With record-setting surface areas (>7000 m2/g), they are ideal for applications such as CO2sequestration and catalysis. While efforts to develop MOFs for these applications made major strides over the past decade, electrically conducting frameworks would open up a host of other promising avenues, such as novel electronic devices, photovoltaics, supercapacitors, and electrocatalysts. Recently, we discovered that infiltrating the pores of the copper-containing MOF HKUST-1 with redox-active guest molecules, such as TCNQ (7,7,8,8-tetracyanoquinododimethane), increases the electrical conductivity of thin film devices by as much as seven orders of magnitude [1]. This emergent property results from a novel donor-bridge-acceptor geometry, in which TCNQ binds to two Cu(II) dimer units within the MOF pore. In this presentation I will describe the synthesis and characterization of Molecule@MOF materials. Coupling these results with first-principles electronic structure calculations demonstrates that conductivity occurs via a hopping mechanism facilitated by the bridging TCNQ. From these results it will be clear that Molecule@MOF represents a novel class of electronic materials with the potential to bridge the properties gap between inorganic and organic conductors, providing a high degree of electronic tailorability combined with long-range order for high charge mobility.

Figure 1 Process for fabrication of conductive metal-organic framework thin film devices, and electrical characterization data. (A) A thin film of metal-organic framework is grown on an insulating wafer pre-patterned with electrodes. Molecules are then infiltrated by exposing the thin film device to a solution containing the molecules. (B) A TCNQ molecule shown above a Cu3(BTC)2
MOF with arrow pointing to the pore. Atom color code: white – hydrogen, blue – nitrogen, cyan – carbon, red – oxygen, ochre (light brown) – copper. (C) SEM image of MOF thin film device with optical images of devices before and after TCNQ infiltration (the rectangular features are Au electrodes overcoated with the MOF film (D) Current-voltage characteristics before and after TCNQ infiltration indicating an increase in conductivity of ×107 or larger. Trace colors represent different electrode gaps

[1] A. A. Talin, A. Centrone, A. Ford, M. E. Foster, V. Stavila, P. Haney, R. A. Kinney, V. Szalai, F. El Gabaly, H. P. Yoon, F. Léonard, M. D. Allendorf, Science
343, 66 (2014)