Electrical conductivity through π-π stacking in a two-dimensional porous gallium catecholate metal-organic framework

Metal-organic frameworks (MOFs) are hybrid materials known for their nanoscale pores, which give them high surface areas but generally lead to poor electrical conductivity. Recently, MOFs with high electrical conductivity were established as promising materials for a variety of applications in energy storage and catalysis. Many recent reports investigating the fundamentals of charge transport in these materials focus on the role of the organic ligands. Less consideration, however, is given to the metal ion forming the MOF, which is almost exclusively a late first-row transition metal. Here, we report a moderately conductive porous MOF based on trivalent gallium and 2,3,6,7,10,11-hexahydroxytriphenylene. Gallium, a metal that has not been featured in electrically conductive MOFs so far, has a closed-shell electronic configuration and is present in its trivalent state-in contrast to most conductive MOFs, which are formed by open-shell, divalent transition metals. Our material, made without using any harmful solvents, displays conductivities on the level of 3 mS/cm and a surface area of 196 m² /g, comparable to transition metal analogs.

Why conductivity is not always king - physical properties governing the capacitance of 2D metal-organic framework-based EDLC supercapacitor electrodes: a Ni3(HITP)2 case study

We report a systematic study on the variation of the physical properties of Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) in the context of their influence on the capacitive behavior of this material in supercapacitor electrodes prepared using the neat MOF. We find that, for this representative material, the sample morphology has a greater impact on the measured electrode performance than differences in bulk electrical conductivity.

Effect of different oxide and hybrid precursors on MOF-CVD of ZIF-8 films

Chemical vapor deposition of metal-organic frameworks (MOF-CVD) will facilitate the integration of porous and crystalline coatings in electronic devices. In the two-step MOF-CVD process, a precursor layer is first deposited and subsequently converted to a MOF through exposure to linker vapor. We herein report the impact of different metal oxide and metalcone layers as precursors for zeolitic imidazolate framework ZIF-8 films.

Porosimetry for Thin Films of Metal-Organic Frameworks: A Comparison of Positron Annihilation Lifetime Spectroscopy and Adsorption-Based Methods

Thin films of crystalline and porous metal-organic frameworks (MOFs) have great potential in membranes, sensors, and microelectronic chips. While the morphology and crystallinity of MOF films can be evaluated using widely available techniques, characterizing their pore size, pore volume, and specific surface area is challenging due to the low amount of material and substrate effects. Positron annihilation lifetime spectroscopy (PALS) is introduced as a powerful method to obtain pore size information and depth profiling in MOF films. The complementarity of this approach to established physisorption-based methods such as quartz crystal microbalance (QCM) gravimetry, ellipsometric porosimetry (EP), and Kr physisorption (KrP) is illustrated. This comprehensive discussion on MOF thin film porosimetry is supported by experimental data for thin films of ZIF-8.

Direct X-ray and electron-beam lithography of halogenated zeolitic imidazolate frameworks

Metal-organic frameworks (MOFs) offer disruptive potential in micro- and optoelectronics because of the unique properties of these microporous materials. Nanoscale patterning is a fundamental step in the implementation of MOFs in miniaturized solid-state devices. Conventional MOF patterning methods suffer from low resolution and poorly defined pattern edges. Here, we demonstrate the resist-free, direct X-ray and electron-beam lithography of MOFs. This process avoids etching damage and contamination and leaves the porosity and crystallinity of the patterned MOFs intact. The resulting high-quality patterns have excellent sub-50-nm resolution, and approach the mesopore regime. The compatibility of X-ray and electron-beam lithography with existing micro- and nanofabrication processes will facilitate the integration of MOFs in miniaturized devices.

Aqueous Flow Reactor and Vapour-Assisted Synthesis of Aluminium Dicarboxylate Metal-Organic Frameworks with Tuneable Water Sorption Properties

Energy-efficient indoors temperature and humidity control can be realised by using the reversible adsorption and desorption of water in porous materials. Stable microporous aluminium-based metal-organic frameworks (MOFs) present promising water sorption properties for this goal. The development of synthesis routes that make use of available and affordable building blocks and avoid the use of organic solvents is crucial to advance this field. In this work, two scalable synthesis routes under mild reaction conditions were developed for aluminium-based MOFs: (1) in aqueous solutions using a continuous-flow reactor and (2) through the vapour-assisted conversion of solid precursors. Fumaric acid, its methylated analogue mesaconic acid, as well as mixtures of the two were used as linkers to obtain polymorph materials with tuneable water sorption properties. The synthesis conditions determine the crystal structure and either the MIL-53 or MIL-68 type structure with square-grid or kagome-grid topology, respectively, is formed. Fine-tuning resulted in new MOF materials thus far inaccessible through conventional synthesis routes. Furthermore, by varying the linker ratio, the water sorption properties can be continuously adjusted while retaining the sigmoidal isotherm shape advantageous for heat transformation and room climatisation applications.

Single Crystals of Electrically Conductive Two-Dimensional Metal-Organic Frameworks: Structural and Electrical Transport Properties

Crystalline, electrically conductive, and intrinsically porous materials are rare. Layered two-dimensional (2D) metal-organic frameworks (MOFs) break this trend. They are porous crystals that exhibit high electrical conductivity and are novel platforms for studying fundamentals of electricity and magnetism in two dimensions. Despite demonstrated applications, electrical transport in these remains poorly understood because of a lack of single crystal studies. Here, studies of single crystals of two 2D MOFs, Ni₃(HITP)₂ and Cu₃(HHTP)₂, uncover critical insights into their structure and transport. Conductivity measurements down to 0.3 K suggest metallicity for mesoscopic single crystals of Ni₃(HITP)₂, which contrasts with apparent activated conductivity for polycrystalline films. Microscopy studies further reveal that these MOFs are not isostructural as previously reported. Notably, single rods exhibit conductivities up to 150 S/cm, which persist even after prolonged exposure to ambient conditions. These single crystal studies confirm that 2D MOFs hold promise as molecularly tunable platforms for fundamental science and applications where porosity and conductivity are critical.

Chemiresistive Sensing of Ambient CO2 by an Autogenously Hydrated Cu3(hexaiminobenzene)2 Framework

A growing demand for indoor atmosphere monitoring relies critically on the ability to reliably and quantitatively detect carbon dioxide. Widespread adoption of CO₂ sensors requires vastly improved materials and approaches because selective sensing of CO₂ under ambient conditions, where relative humidity (RH) and other atmosphere contaminants provide a complex scenario, is particularly challenging. This report describes an ambient CO₂ chemiresistor platform based on nanoporous, electrically conducting two-dimensional metal-organic frameworks (2D MOFs). The CO₂ chemiresistive sensitivity of 2D MOFs is attained through the incorporation of imino-semiquinonate moieties, i.e., well-defined N-heteroatom functionalization. The best performance is obtained with Cu₃(hexaiminobenzene)₂, Cu₃HIB₂, which shows selective and robust ambient CO₂ sensing properties at practically relevant levels (400-2500 ppm). The observed ambient CO₂ sensitivity is nearly RH-independent in the range 10-80% RH. Cu₃HIB₂ shows higher sensitivity over a broader RH range than any other known chemiresistor. Characterization of the CO₂-MOF interaction through a combination of in situ optical spectroscopy and density functional theory calculations evidence autogenously generated hydrated adsorption sites and a charge trapping mechanism as responsible for the intriguing CO₂ sensing properties of Cu₃HIB₂.

Vapor-deposited zeolitic imidazolate frameworks as gap-filling ultra-low-k dielectrics

The performance of modern chips is strongly related to the multi-layer interconnect structure that interfaces the semiconductor layer with the outside world. The resulting demand to continuously reduce the k-value of the dielectric in these interconnects creates multiple integration challenges and encourages the search for novel materials. Here we report a strategy for the integration of metal-organic frameworks (MOFs) as gap-filling low-k dielectrics in advanced on-chip interconnects. The method relies on the selective conversion of purpose-grown or native metal-oxide films on the metal interconnect lines into MOFs by exposure to organic linker vapor. The proposed strategy is validated for thin films of the zeolitic imidazolate frameworks ZIF-8 and ZIF-67, formed in 2-methylimidazole vapor from ALD ZnO and native CoOx, respectively. Both materials show a Young's modulus and dielectric constant comparable to state-of-the-art porous organosilica dielectrics. Moreover, the fast nucleation and volume expansion accompanying the oxide-to-MOF conversion enable uniform growth and gap-filling of narrow trenches, as demonstrated for 45 nm half-pitch fork-fork capacitors.

Synthesis and Characterization of Ru-Loaded Anodized Aluminum Oxide for Hydrogenation Catalysis

Anodized aluminum oxides (AAOs) are synthesized and used as catalyst support in combination with Ru as metal in hydrogenation catalysis. SEM and TEM analysis of the as-synthesized AAOs reveal uniform, ordered nanotubes with pore diameters of 18 nm, which are further characterized with Kr physisorption, XRD and FTIR spectroscopy. After impregnation of the AAOs with Ru, the presence of Ru nanoparticles inside the tubular pores is evidenced clearly for the first time via HAADF-STEM-EDX. The Ru-AAOs have been tested for catalytic activity, which showed high conversion and selectivity for the hydrogenation of toluene and butanal.

Vapour-phase deposition of oriented copper dicarboxylate metal–organic framework thin films

Vapour-phase deposition of copper dicarboxylate metal–organic framework thin films with an out-of-plane orientation and accessible porosity.

Photopatterning of fluorescent host-guest carriers through pore activation of metal-organic framework single crystals

Encoded fluorescent particles are fabricated through the selective uptake of dyes in photopatterned metal-organic framework single crystals. The concept is based on spatially controlled photochemical cleavage of pore-blocking pendant groups. Because of the crystalline and porous nature of the host, this approach enables guest uptake that is tunable and can be triggered though controlled irradiation.

Gel-based morphological design of zirconium metal-organic frameworks

The ability of metal-organic frameworks (MOFs) to gelate under specific synthetic conditions opens up new opportunities in the preparation and shaping of hierarchically porous MOF monoliths, which could be directly implemented for catalytic and adsorptive applications. In this work, we present the first examples of xero- or aerogel monoliths consisting solely of nanoparticles of several prototypical Zr4+-based MOFs: UiO-66-X (X = H, NH2, NO2, (OH)2), UiO-67, MOF-801, MOF-808 and NU-1000. High reactant and water concentrations during synthesis were observed to induce the formation of gels, which were converted to monolithic materials by drying in air or supercritical CO2. Electron microscopy, combined with N2 physisorption experiments, was used to show that irregular nanoparticle packing leads to pure MOF monoliths with hierarchical pore systems, featuring both intraparticle micropores and interparticle mesopores. Finally, UiO-66 gels were shaped into monolithic spheres of 600 μm diameter using an oil-drop method, creating promising candidates for packed-bed catalytic or adsorptive applications, where hierarchical pore systems can greatly mitigate mass transfer limitations.

Highlights from the Faraday Discussion on New Directions in Porous Crystalline Materials, Edinburgh, UK, June 2017

A lively discussion on new directions in porous crystalline materials took place in June 2017, with the beautiful city of Edinburgh as a backdrop, in the context of the unique Faraday Discussions format.

An updated roadmap for the integration of metal–organic frameworks with electronic devices and chemical sensors

This review highlights the steps needed to bring the properties of MOFs from the chemical lab to the microelectronics fab.

Correction: An updated roadmap for the integration of metal–organic frameworks with electronic devices and chemical sensors

Correction for ‘An updated roadmap for the integration of metal–organic frameworks with electronic devices and chemical sensors’ by Ivo Stassen et al., Chem. Soc. Rev., 2017, DOI: 10.1039/c7cs00122c.

Towards metal-organic framework based field effect chemical sensors: UiO-66-NH2 for nerve agent detection

Kelvin probe monitoring of metal-organic framework coated electrodes is demonstrated as a route for ppb-level detection of alkyl phosphonates.

We present a highly sensitive gas detection approach for the infamous ‘nerve agent’ group of alkyl phosphonate compounds. Signal transduction is achieved by monitoring the work function shift of metal–organic framework UiO-66-NH₂ coated electrodes upon exposure to ppb-level concentrations of a target simulant. Using the Kelvin probe technique, we demonstrate the potential of electrically insulating MOFs for integration in field effect devices such as ChemFETs: a three orders of magnitude improvement over previous work function-based detection of nerve agent simulants. Moreover, the signal is fully reversible both in dry and humid conditions, down to low ppb concentrations. Comprehensive investigation of the interactions that lead towards this high sensitivity points towards a series of confined interactions between the analyte and the pore interior of UiO-66-NH₂.

Silver-induced reconstruction of an adeninate-based metal-organic framework for encapsulation of luminescent adenine-stabilized silver clusters

Bright luminescent silver-adenine species were successfully stabilized in the pores of the MOF-69A (zinc biphenyldicarboxylate) metal-organic framework, starting from the intrinsically blue luminescent bio-MOF-1 (zinc adeninate 4,4'-biphenyldicarboxylate). Bio-MOF-1 is transformed to the MOF-69A framework by selectively leaching structural adenine linkers from the original framework using silver nitrate solutions in aqueous ethanol. Simultaneously, bright blue-green luminescent silver-adenine clusters are formed inside the pores of the recrystallized MOF-69A matrix in high local concentrations. The structural transition and concurrent changes in optical properties were characterized using a range of structural, physicochemical and spectroscopic techniques (steady-state and time-resolved luminescence, quantum yield determination, fluorescence microscopy). The presented results open new avenues for exploring the use of MOFs containing luminescent silver clusters for solid-state lighting and sensor applications.

Chemical vapour deposition of zeolitic imidazolate framework thin films

Integrating metal-organic frameworks (MOFs) in microelectronics has disruptive potential because of the unique properties of these microporous crystalline materials. Suitable film deposition methods are crucial to leverage MOFs in this field. Conventional solvent-based procedures, typically adapted from powder preparation routes, are incompatible with nanofabrication because of corrosion and contamination risks. We demonstrate a chemical vapour deposition process (MOF-CVD) that enables high-quality films of ZIF-8, a prototypical MOF material, with a uniform and controlled thickness, even on high-aspect-ratio features. Furthermore, we demonstrate how MOF-CVD enables previously inaccessible routes such as lift-off patterning and depositing MOF films on fragile features. The compatibility of MOF-CVD with existing infrastructure, both in research and production facilities, will greatly facilitate MOF integration in microelectronics. MOF-CVD is the first vapour-phase deposition method for any type of microporous crystalline network solid and marks a milestone in processing such materials.

Resolving Interparticle Heterogeneities in Composition and Hydrogenation Performance between Individual Supported Silver on Silica Catalysts

Supported metal nanoparticle catalysts are commonly obtained through deposition of metal precursors onto the support using incipient wetness impregnation. Typically, empirical relations between metal nanoparticle structure and catalytic performance are inferred from ensemble averaged data in combination with high-resolution electron microscopy. This approach clearly underestimates the importance of heterogeneities present in a supported metal catalyst batch. Here we show for the first time how incipient wetness impregnation leads to 10-fold variations in silver loading between individual submillimeter-sized silica support granules. This heterogeneity has a profound impact on the catalytic performance, with 100-fold variations in hydrogenation performance at the same level. In a straightforward fashion, optical microscopy interlinks single support particle level catalytic measurements to structural and compositional information. These detailed correlations reveal the optimal silver loading. A thorough consideration of catalyst heterogeneity and the impact thereof on the catalytic performance is indispensable in the development of catalysts.

A zirconium squarate metal-organic framework with modulator-dependent molecular sieving properties

We report the first zirconium metal-organic framework based on squaric acid, representing the member with the smallest unit cell in the isoreticular UiO-66 family. Its molecular sieving properties are strongly influenced by the monocarboxylic acid modulator incorporated during synthesis.

Bio-based nitriles from the heterogeneously catalyzed oxidative decarboxylation of amino acids

The oxidative decarboxylation of amino acids to nitriles was achieved in aqueous solution by in situ halide oxidation using catalytic amounts of tungstate exchanged on a [Ni,Al] layered double hydroxide (LDH), NH4 Br, and H2 O2 as the terminal oxidant. Both halide oxidation and oxidative decarboxylation were facilitated by proximity effects between the reactants and the LDH catalyst. A wide range of amino acids was converted with high yields, often >90 %. The nitrile selectivity was excellent, and the system is compatible with amide, alcohol, and in particular carboxylic acid, amine, and guanidine functional groups after appropriate neutralization. This heterogeneous catalytic system was applied successfully to convert a protein-rich byproduct from the starch industry into useful bio-based N-containing chemicals.

Ruthenium-catalyzed aerobic oxidative decarboxylation of amino acids: a green, zero-waste route to biobased nitriles

The oxidative decarboxylation of amino acids into biobased nitriles was performed using molecular oxygen and a supported ruthenium hydroxide-based catalyst. A range of amino acids was successfully oxidized in a salt-free aqueous system under mild conditions.