All-gas-phase synthesis of amino-functionalized UiO-66 thin films

Extrinsically conducting MOFs: guest-promoted enhancement of electrical conductivity, thin film fabrication and applications

Conductive metal-organic frameworks are of current interest in chemical science because of their applications in chemiresistive sensing, electrochemical energy storage, electrocatalysis, etc. Different strategies have been employed to design conductive frameworks. In this review, we discuss the influence of different types of guest species incorporated within the pores or channels of metal-organic frameworks (MOFs) and porous coordination polymers (PCPs) to generate charge transfer pathways and modulate their electrical conductivity. We have classified dopants or guest species into three different categories: (i) metal-based dopants, (ii) molecule and molecular entities and (iii) organic conducting polymers. Different types of metal ions, metal nano-clusters and metal oxides have been used to enhance electrical conductivity in MOFs. Metal ions and metal nano-clusters depend on the hopping process for efficient charge transfer whereas metal-oxides show charge transport through the metal-oxygen pathway. Several types of molecules or molecular entities ranging from neutral TCNQ, I2, and fullerene to ionic methyl viologen, organometallic like nickelcarborane, etc. have been used. In these cases, the charge transfer process varies with the guest species. When organic conducting polymers are the guest, the charge transport occurs through the polymer chains, mostly based on extended π-conjugation. Here we provide a comprehensive and critical review of these strategies to add electrical conductivity to the, in most cases, otherwise insulating MOFs and PCPs. We point out the guest encapsulation process, the geometry and structure of the resulting host-guest complex, the host-guest interactions and the charge transport mechanism for each case. We also present the methods for thin film fabrication of conducting MOFs (both, liquid-phase and gas-phase based methods) and their most relevant applications like electrocatalysis, sensing, charge storage, photoconductivity, photocatalysis,… We end this review with the main obstacles and challenges to be faced and the appealing perspectives of these 21st century materials.

Porous crystalline materials for memories and neuromorphic computing systems

Porous crystalline materials usually include metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs) and zeolites, which exhibit exceptional porosity and structural/composition designability, promoting the increasing attention in memory and neuromorphic computing systems in the last decade. From both the perspective of materials and devices, it is crucial to provide a comprehensive and timely summary of the applications of porous crystalline materials in memory and neuromorphic computing systems to guide future research endeavors. Moreover, the utilization of porous crystalline materials in electronics necessitates a shift from powder synthesis to high-quality film preparation to ensure high device performance. This review highlights the strategies for preparing porous crystalline materials films and discusses their advancements in memory and neuromorphic electronics. It also provides a detailed comparative analysis and presents the existing challenges and future research directions, which can attract the experts from various fields (e.g., materials scientists, chemists, and engineers) with the aim of promoting the applications of porous crystalline materials in memory and neuromorphic computing systems.

Recent Advances in Continuous MOF Membranes for Gas Separation and Pervaporation

Metal-organic frameworks (MOFs), a sub-group of porous crystalline materials, have been receiving increasing attention for gas separation and pervaporation because of their high thermal and chemical stability, narrow window sizes, as well as tuneable structural, physical, and chemical properties. In this review, we comprehensively discuss developments in the formation of continuous MOF membranes for gas separation and pervaporation. Additionally, the application performance of continuous MOF membranes in gas separation and pervaporation are analysed. Lastly, some perspectives for the future application of continuous MOF membranes for gas separation and pervaporation are given.

Aromatic sensitizers in luminescent hybrid films

Atomic layer deposition offers a unique set of design possibilities due to the vast range of metal and organic precursors that can be used and combined. In this work, we have combined lanthanides with aromatic aids as strongly absorbing sensitizers to form highly luminescent thin films. Terephthalic acid is used as a base sensitizer, absorbing shorter wavelengths than 300 nm. The absorption range is extended towards the near-UV and blue range by increasing the aromatic system and adding functional groups that have strong red-shifting effects. While terbium and europium provide green and red emission, yttrium allows emission from the sensitizer itself spanning the whole color range from purple, blue and green to red. Many organic dye molecules show very high luminescence quantum yields and several of the molecules and materials investigated in this work show bright luminescence.

Photoactive Zr-aromatic hybrid thin films made by molecular layer deposition

The principle of antimicrobial photodynamic therapy (PDT) is appealing because it can be controlled by an external light source and possibly the use of durable materials. However, to utilise such surfaces requires a process for their production that allows for coating on even complex geometries. We have therefore explored the ability of the emerging molecular layer deposition (MLD) technique to produce and tune PDT active materials. This study demonstrates how the type of aromatic ligand influences the optical and antimicrobial properties of photoactive Zr-organic hybrid thin films made by MLD. The three aromatic dicarboxylic acids: 2,5-dihydroxy-1,4-benzenedicarboxylic acid, 2-amino-1,4-benzenedicarboxylic acid and 2,6-naphthalenedicarboxylic acid have been combined with ZrCl₄ to produce hybrid coatings. The first system has not been previously described by MLD and is therefore more thoroughly investigated using in situ quartz crystal microbalance (QCM), Fourier transform infrared (FTIR) and UV-Vis spectroscopy. The antibacterial phototoxic effects of Zr-organic hybrids have been explored in the Staphylococcus aureus bacteria model using a UVA/blue light source. Films based on the 2,6-naphthalenedicarboxylic acid linker significantly reduced the number of viable bacteria by 99.9%, while no apparent activity was observed for the two other photoactive systems. Our work thus provides evidence that the MLD technique is a suitable tool to produce high-quality novel materials for possible applications in antimicrobial PDT, however it requires a careful selection of aromatic ligands used to construct photoactive materials.

Photoactive organic–inorganic hybrid thin films for potential use in antimicrobial photodynamic therapy (PDT) were fabricated based on Zr clusters and three different aromatic dicarboxylic acid linkers using the molecular layer deposition (MLD) technique.

Utilizing Zirconium MOF-functionalized Fiber Substrates Prepared by Molecular Layer Deposition for Toxic Gas Capture and Chemical Warfare Agent Degradation

Metal-organic frameworks (MOFs) are a class of porous organic-inorganic solids extensively explored for numerous applications owing to their catalytic activity and high surface area. In this work MOF thin films deposited in a one-step, molecular layer deposition (MLD), an all-gas-phase process, on glass wool fibers are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and their capabilities towards toxic industrial chemical (TIC) capture and chemical warfare agents (CWA) degradation are investigated. It is shown that despite low volume of the active material used, MOFs thin films are capable of removal of harmful gaseous chemicals from air stream and CWA from neutral aqueous environment. The results confirm that the MLD-deposited MOF thin films, amorphous and crystalline, are suitable materials for use in air filtration, decontamination, and physical protection against CWA and TIC.

Molecular layer deposition of photoactive metal-naphthalene hybrid thin films

We here report on photoactive organic-inorganic hybrid thin films prepared by the molecular layer deposition (MLD) method. The new series of hybrid films deposited using 2,6-naphthalenedicarboxylic acid (2,6-NDC) and either hafnium chloride (HfCl₄), yttrium tetramethylheptanedionate (Y(thd)₃) or titanium chloride (TiCl₄) were compared with the known zirconium chloride (ZrCl₄) based system. All metal-naphthalene films are amorphous as-deposited and show self-saturating growth as expected for an ideal MLD process with varied growth rates depending on the choice of metal precursor. The growth was studied in situ using quartz crystal microbalance (QCM) and the films were further characterised using spectroscopic ellipsometry (SE), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and UV-Vis and photoluminescence (PL) spectroscopy to obtain information on their physicochemical properties. The hybrid thin films display intense blue photoluminescence, except for the Ti-organic complex in which titanium clusters were found to be an effective PL quencher for the organic linker. We demonstrate how the optical properties of the films depend on the choice of metal component to make a foundation for further studies on these types of organic-inorganic hybrid materials for applications as photoactive agents.

Stretchable and Multi-Metal-Organic Framework Fabrics Via High-Yield Rapid Sorption-Vapor Synthesis and Their Application in Chemical Warfare Agent Hydrolysis

Protocols to create metal-organic framework (MOF)/polymer composites for separation, chemical capture, and catalytic applications currently rely on relatively slow solution-based processing to form single MOF composites. Here, we report a rapid, high-yield sorption-vapor method for direct simultaneous growth of single and multiple MOF materials onto untreated flexible and stretchable polymer fibers and films. The synthesis utilizes favorable reactant absorption into polymers coupled with rapid vapor-driven MOF crystallization to form high surface area (>250 m²/g_composite) composites, including UiO-66-NH₂, HKUST-1, and MOF-525 on spandex, nylon, and other fabrics. The resulting composites are robust and maintain their functionality even after stretching. Stretchable MOF fabrics enable rapid solid-state hydrolysis of the highly toxic chemical warfare agent soman and paraoxon-methyl simulant. We show that this approach can readily be scaled by solution spray-coating of MOF precursors and to large area substrates.

Benzenedisulfonic Acid as an ALD/MLD Building Block for Crystalline Metal-Organic Thin Films*

Two new atomic/molecular layer deposition processes for depositing crystalline metal-organic thin films, built from 1,4-benzenedisulfonate (BDS) as the organic linker and Cu or Li as the metal node, are reported. The processes yield in-situ crystalline but hydrated Cu-BDS and Li-BDS films; in the former case, the crystal structure is of a previously known metal-organic-framework-like structure, while in the latter case not known from previous studies. Both hydrated materials can be readily dried to obtain the crystalline unhydrated phases. The stability and the ionic conductivity of the unhydrated Li-BDS films were characterized to assess their applicability as a thin film solid polymer Li-ion conductor.

Thin Films of Metal-Organic Framework Interfaces Obtained by Laser Evaporation

Properties such as large surface area, high pore volume, high chemical and thermal stability, and structural flexibility render zeolitic imidazolate frameworks (ZIFs) well-suited materials for gas separation, chemical sensors, and optical and electrical devices. For such applications, film processing is a prerequisite. Herein, matrix-assisted pulsed laser evaporation (MAPLE) was successfully used as a single-step deposition process to fabricate ZIF-8 films. By correlating laser fluency and controlling the specific transfer of lab-synthesized ZIF-8, films with user-controlled physical and chemical properties were obtained. Films' characteristics were evaluated by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The analysis showed that frameworks of ZIF-8 can be deposited successfully and controllably to yield polycrystalline films. The deposited films maintained the integrity of the individual ZIF-8 framework, while undergoing minor crystalline and surface chemistry changes. No significant changes in particle size were observed. Our study demonstrated control over both the MAPLE deposition conditions and the outcome, as well as the suitability of the listed deposition method to create composite architectures that could potentially be used in applications ranging from selective membranes to gas sensors.

Current Progress and Future Directions in Gas-Phase Metal-Organic Framework Thin-Film Growth

Deposition of materials as a thin film is important for various applications, such as sensors, microelectronic devices, and membranes. There have been breakthroughs in gas-phase metal-organic framework (MOF) thin-film growth, which is more applicable to micro- and nanofabrication processes and also less harmful to the environment than solvent-based methods. Three different types of gas-phase MOF thin film deposition methods have been developed using chemical vapor deposition (CVD), atomic layer deposition (ALD), and physical vapor deposition (PVD)-CVD combined techniques. The CVD-based method basically converts metal oxide layers into MOF thin films by exposing the surface to ligand vapor. The ALD-based method allows growing MOF thin films following layer-by-layer (LBL) growth by sequentially exposing gas-phase metal and ligand precursors. The PVD-CVD method uses PVD for metal deposition and CVD for ligand deposition, which is similar to LBL growth. These gas-phase growth methods can broaden the use of MOFs in diverse areas. Herein, the current progress of gas-phase MOF thin film growth is discussed and future directions suggested.

Vapor phase processing: a novel approach for fabricating functional hybrid materials

Materials science is nowadays facing challenges in optimizing properties of materials which are needed for numerous technological applications and include, but are not limited to, mechanics, electronics, optics, etc. The key issue is that for emerging applications materials are needed which incorporate certain properties from polymers or biopolymers and metals or ceramics at the same time, thus fabrication of functional hybrid materials becomes inevitable. Routes for the synthesis of functional hybrid materials can be manifold. Among the explored routes vapor phase processing is a rather novel approach which opts for compatibility with many existing industrial processes. This topical review summarizes the most important approaches and achievements in the synthesis of functional hybrid materials through vapor phase routes with the goal to fabricate suitable hybrid materials for future mechanical, electronic, optical or biomedical applications. Most of the approaches rely on atomic layer deposition (ALD) and techniques related to this process, including molecular layer deposition (MLD) and vapor phase infiltration (VPI), or variations of chemical vapor deposition (CVD). The thus fabricated hybrid materials or nanocomposites often show exceptional physical or chemical properties, which result from synergies of the hybridized materials families. Even though the research in this field is still in its infancy, the initial results encourage further development and promise great application potential in a large variety of applications fields such as flexible electronics, energy conversion or storage, functional textile, and many more.

Composition-tuned metal-organic thin-film structures based on photoswitchable azobenzene by ALD/MLD

We demonstrate the fabrication of in-situ crystalline thin films of various azobenzene (AZO) based metal-organic structures using the atomic/molecular layer deposition (ALD/MLD) technique; these are highly intriguing materials as azobenzene is one of the prototype organic molecules experiencing photoisomerization. Our Li-, Ca and Fe-AZO thin films deposited from Li(thd) (thd = 2,2,6,6-tetramethyl-3,5-heptanedione), Ca(thd)2, FeCl3 and azobenzene-4,4'-dicarboxylic acid precursors in the temperature range of 250-360 °C exhibit metal-AZO structures not known from bulk samples. In all these structures, the AZO linker molecules are free to undergo the characteristic trans-cis photoisomerization reaction upon UV (360 nm) irradiation. However, this lowers the degree of crystallinity. To address the issue, we investigate hetero-organic structures where TPA (terephthalic acid) is used as another linker component together with AZO. This allows the trans-cis reaction of the AZO moieties to occur without compromising the film crystallinity. Finally, we demonstrate the growth of MOF-on-MOF type Ca-ADA@Ca-TPA thin films also showing the efficient photoisomerization reaction.

Atomic/molecular layer deposition and electrochemical performance of dilithium 2-aminoterephthalate

Crystalline Li-terephthalate and amino-functionalized Li-terephthalate thin film electrodes are fabricated from gaseous precursors with ALD/MLD to show that the electron-donating amino group lowers the redox potential.

Electrical conductivity and magnetic bistability in metal–organic frameworks and coordination polymers: charge transport and spin crossover at the nanoscale

This review aims to reassess the progress, issues and opportunities in the path towards integrating conductive and magnetically bistable coordination polymers and metal–organic frameworks as active components in electronic devices.

Metal-organic framework UiO-66 membranes

Metal-organic frameworks (MOFs) have emerged as a class of promising membrane materials. UiO-66 is a prototypical and stable MOF material with a number of analogues. In this article, we review five approaches for fabricating UiO-66 polycrystalline membranes including in situ synthesis, secondary synthesis, biphase synthesis, gas-phase deposition and electrochemical deposition, as well as their applications in gas separation, pervaporation, nanofiltration and ion separation. On this basis, we propose possible methods for scalable synthesis of UiO-66 membranes and their potential separation applications in the future.

Facile synthesis of acid-modified UiO-66 to enhance the removal of Cr(VI) from aqueous solutions

The adsorption behavior and mechanism of Cr(VI) on different acid-modified UiO-66s (Form-UiO-66 and Ac-UiO-66) were systematically investigated for the first time through a series of characterizations, and theoretical calculations of batch experiments. The characterization results demonstrate that acid-modified UiO-66 exhibited a larger specific surface area than did unmodified UiO-66. In addition, since the regulator (formic acid) of Form-UiO-66 was the stronger competition, the specific surface area of Form-UiO-66 (1138 m² g^-1) was larger than that of Ac-UiO-66 (915 m² g^-1). Under optimal experimental conditions, the maximum adsorption capacity of Cr(VI) was 243.9 mg g^-1 on Form-UiO-66, and 151.52 mg g^-1 on Ac-UiO-66, which was far higher than on the reported unmodified UiO-66 (36.4 mg g^-1). The results of pH testing, zeta potential, and X-ray photoelectron spectroscopy analysis indicate that Cr(VI) ions were fixed to adsorbent surfaces via electrostatic adsorption. Acid-modified UiO-66 increased the surface active site via the increase in its specific surface area to enhance adsorption capacity of Cr(VI). These results indicated that both the surface charge and specific surface area of the adsorbent primarily determined the Cr(VI) adsorption capacity. Acid modified UiO-66 exhibited enhanced adsorption capacity, stability, and regeneration, compared to traditional adsorbents, and these results provide new insights into adsorption by MOFs.

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.

Cluster nuclearity control and modulated hydrothermal synthesis of functionalized Zr12 metal–organic frameworks

Cluster nuclearity control over the SBUs of zirconium MOFs to target Zr₆-based and Zr₂-based phases is demonstrated for the Zr terephthalate system (Zr-BDC) using a modulated hydrothermal synthesis method.

Indium sulfide nanotubes with sulfur vacancies as an efficient photocatalyst for nitrogen fixation

We have designed and manufactured In₂S₃ nanotubes containing sulfur vacancies as effective and stable photocatalysts for nitrogen fixation and ammonia production. In the preparation process of In₂S₃, a self-templated strategy was used to obtain the nanotubes. The sulfur vacancies were then manufactured by calcination under a nitrogen atmosphere. The existence of sulfur vacancies enhances the light absorption and promotes the separation and migration of the photoinduced charge carriers. In addition, sulfur vacancies can serve as the active sites to achieve strong N₂ adsorption and activation. Thus the obtained samples show enhanced photocatalytic performance with a high NH₃ generation rate (52.49 μmol h⁻¹ g⁻¹) and excellent stability under UV-vis light.

We have designed and manufactured In₂S₃ nanotubes containing sulfur vacancies as effective and stable photocatalysts for nitrogen fixation and ammonia production.

Iron-Terephthalate Coordination Network Thin Films Through In-Situ Atomic/Molecular Layer Deposition

Iron terephthalate coordination network thin films can be fabricated using the state-of-the-art gas-phase atomic/molecular layer deposition (ALD/MLD) technique in a highly controlled manner. Iron is an Earth-abundant and nonhazardous transition metal, and with its rich variety of potential applications an interesting metal constituent for the inorganic-organic coordination network films. Our work underlines the role of the metal precursor used when aiming at in-situ ALD/MLD growth of crystalline inorganic-organic thin films. We obtain crystalline iron terephthalate films when FeCl₃ is employed as the iron source whereas depositions based on the bulkier Fe(acac)₃ precursor yield amorphous films. The chemical composition and structure of the films are investigated with GIXRD, XRR, FTIR and XPS.