Fabrication of 3D Oriented MOF Micropatterns with Anisotropic Fluorescent Properties

Direct Photopatterning of Zeolitic Imidazolate Frameworks via Photoinduced Fluorination

Precise and effective patterning strategies are essential for integrating metal-organic frameworks (MOFs) into microelectronics, photonics, sensors, and other solid-state devices. Direct lithography of MOFs with light and other irradiation sources has emerged as a promising patterning strategy. However, existing direct lithography methods often rely on the irradiation-induced amorphization of the MOFs structures and the breaking of strong covalent bonds in their organic linkers. High-energy sources (such as X-rays or electron beams) and large irradiation doses - conditions unfavorable for scalable patterning - are thus required. Here, we report a photoinduced fluorination chemistry for patterning various zeolitic imidazolate frameworks (ZIFs) under mild UV irradiation. Using UV doses as low as 10 mJ cm-2, light-sensitive fluorine-containing molecules covalently bond to ZIFs and enhance their stability in water. This creates a water-stability contrast between ZIFs in exposed and unexposed regions, enabling scalable direct photolithography of ZIFs with high resolution (2 μm) on 4-inch wafers and flexible substrates. The patterned ZIFs preserve their original crystallinity and porous properties while gaining increased hydrophobicity. This allows for the demonstration of a water-responsive fluorescent MOFs array with implications in sensing and multicolor information encryption.

Nanoscale Resist-Free Patterning of Halogenated Zeolitic Imidazolate Frameworks by Extreme UV Lithography

Advancements in patterning techniques for metal-organic frameworks (MOFs) are crucial for their integration into microelectronics. However, achieving precise nanoscale control of MOF structures remains challenging. In this work, a resist-free method for patterning MOFs is demonstrated using extreme ultraviolet (EUV) lithography with a resolution of 40 nm. The role of halogen atoms in the linker and the effect of humidity are analyzed through in situ and near-ambient pressure synchrotron X-ray photoelectron spectroscopy. In addition to facilitating the integration of MOFs, the results offer valuable insights for developing the highly sought-after positive-tone EUV photoresists.

Regulated Charge Transfer in Donor-Acceptor Metal-Organic Frameworks for Highly-Sensitive Photodetectors

In photo-induced charge separation, organic thin films with donor and acceptor chromophores are vital for uses such as artificial photosynthesis and photodetection. The main challenges include optimizing charge separation efficiency and identifying the ideal acceptor/donor ratio. Achieving this is difficult due to the variability in molecular configurations within these typically amorphous organic aggregates. Metal-organic frameworks (MOFs) provide a structured solution by enabling systematic design of donor/acceptor blends with adjustable ratios within a crystalline lattice. We demonstrate this approach by incorporating donor and acceptor naphthalenediimide (NDI) chromophores as linkers in a highly oriented, monolithic MOF thin film. By adjusting the NDI acceptor linker concentration during the layer-by-layer assembly of surface-anchored MOF thin films (SURMOFs), we significantly enhanced charge separation efficiency. Surprisingly, the optimum acceptor concentration was only 3 %, achieving a forty-fold increase in photodetection efficiency compared to baseline NDI donor-based SURMOFs. This unexpected behaviour was clarified through theoretical analysis enabled by the well-defined crystalline structure of the SURMOFs. Using density functional theory and kinetic Monte Carlo simulations, we identified two opposing effects from acceptors: the positive effect of suppressing undesirable charge carrier recombination is offset at high concentrations by a reduction in charge-carrier mobility.

Interfacial Microenvironment-Regulated Coordinate Structures Dictate the Metal-Organic Framework Facet Orientation toward Efficient CO2 Cycloaddition

The preparation of high-quality highly oriented metal-organic framework (MOF) thin films is desirable for developing advanced functional devices. However, the pathways for controlling the oriented growth of MOFs are largely unknown, and determining their microcosmic evolution at the complex solid-liquid interface remains a challenge. Herein, we investigate the critical early growth stage of typical HKUST-1 on the COOH-functionalized Au substrate utilizing a combination of in situ surface-enhanced infrared spectroscopy, X-ray photoelectron spectroscopy, and photoinduced force microscopy. Detailed molecular level information indicates that it is not only the COOH-terminated SAM itself but also the distinct interfacial structures of the first metal coordinate layer and second ligand coordinate layer, which can be regulated in aprotic and protic solvents, that dominate the initial growth behavior of MOF and thus lead to the [111], [100], and polycrystal facet-oriented growth of HKUST-1. Moreover, the prepared HKUST-1 films exhibit a crystal facet-dependent catalytic rate in the chemical fixation of CO2 into cyclic carbonate. Our observations provide a reasonable guide for designing MOF-based anisotropic functional equipment.

Ordered Transfer from 3D-Oriented MOF Superstructures to Polymeric Films: Microfabrication, Enhanced Chemical Stability, and Anisotropic Fluorescent Patterns

Films and patterns of 3D-oriented metal-organic frameworks (MOFs) afford well-ordered pore structures extending across centimeter-scale areas. These macroscopic domains of aligned pores are pivotal to enhance diffusion along specific pathways and orient functional guests. The anisotropic properties emerging from this alignment are beneficial for applications in ion conductivity and photonics. However, the structure of 3D-oriented MOF films and patterns can rapidly degrade under humid and acidic conditions. Thus, more durable 3D-ordered porous systems are desired for practical applications. Here, oriented porous polymer films and patterns are prepared by using heteroepitaxially oriented N3-functionalized MOF films as precursor materials. The film fabrication protocol utilizes an azide-alkyne cycloaddition on the Cu2(AzBPDC)2DABCO MOF. The micropatterning protocol exploits the X-ray sensitivity of azide groups in Cu2(AzBPDC)2DABCO, enabling selective degradation in the irradiated areas. The masked regions of the MOF film retain their N3-functionality, allowing for subsequent cross-linking through azide-alkyne coupling. Subsequent acidic treatment removes the Cu ions from the MOF, yielding porous polymer micro-patterns. The polymer has high chemical stability and shows an anisotropic fluorescent response. The use of 3D-oriented MOF systems as precursors for the fabrication of oriented porous polymers will facilitate the progress of optical components for photonic applications.

Multimaterial Digital-Light Processing of Metal-Organic Framework (MOF) Composites: A Versatile Tool for the Rapid Microfabrication of MOF-Based Devices

Patterning Metal-Organic Frameworks (MOFs) is essential for their use in sensing, electronics, photonics, and encryption technologies. However, current lithography methods are limited in their ability to pattern more than two MOFs, hindering the potential for creating advanced multifunctional surfaces. Additionally, balancing design flexibility, simplicity, and cost often results in compromises. This study addresses these challenges by combining Digital-Light Processing (DLP) with a capillary-assisted stop-flow system to enable multimaterial MOF patterning. It demonstrates the desktop fabrication of multiplexed arbitrary micropatterns across cm-scale areas while preserving the MOF's pore accessibility. The ink, consisting of a MOF crystal suspension in a low volatile solvent, a mixture of high molecular weight oligomers, and a photoinitiator, is confined by capillarity in the DLP projection area and quickly exchanged using syringe pumps. The versatility of this method is demonstrated by the direct printing of a ZIF-8-based luminescent oxygen sensor, a 5-component dynamic information concealment method, and a PCN-224-based colorimetric sensor for amines, covering disparate pore and analyte sizes. The multi-MOF capabilities, simplicity, and accessibility of this strategy pave the way for the facile exploration of MOF materials across a wide range of applications, with the potential to significantly accelerate the design-to-application cycle of MOF-based devices.

Rise of Metal-Organic Frameworks: From Synthesis to E-Skin and Artificial Intelligence

Metal-organic frameworks (MOFs) have attained broad research attention in the areas of sensors, resistive memories, and optoelectronic synapses on the merits of their intriguing physical and chemical properties. In this review, recent progress on the synthesis of MOFs and their electronic applications is introduced and discussed. Initially, the crystal structures and properties of MOFs encompassing optical, electrical, and chemical properties are discussed in brief. Subsequently, advanced synthesis methods for MOFs are introduced, categorized into hydrothermal approach, microwave synthesis, mechanochemical synthesis, and electrochemical deposition. After that, the various roles of MOFs in widespread applications, including sensing, information storage, optoelectronic synapses, machine learning, and artificial intelligence, are discussed, highlighting their versatility and the innovative solutions they provide to long-standing challenges. Finally, an outlook on remaining challenges and a future perspective for MOFs are proposed.

Improved optical quality of heteroepitaxially grown metal-organic framework thin films by modulating the crystal growth

Fabricating high-quality thin films of metal-organic frameworks (MOFs) is important for integrating MOFs in various applications. Specifically, optical/electrical devices require MOF thin films that are crystallographically oriented, with closely packed crystals and smooth surfaces. Although the heteroepitaxial growth approach of MOFs on metal hydroxides has been demonstrated to control the orientation of the three crystallographic axes, the fabrication of MOF thin films with both three-dimensional crystallographic orientation and smooth surfaces remains a challenge. In this study, we report the fabrication of high-quality thin films of MOFs with closely packed MOF crystals, smooth surfaces, optical transparency, and crystal alignment by modulating the crystal growth of MOFs using the heteroepitaxial growth approach. High-quality thin films of Cu-paddlewheel-based pillar-layered MOFs are fabricated on oriented Cu(OH)2 thin films via epitaxial growth using acetate ions as modulators to control the crystal morphology. Increasing the modulator concentration results in a uniform crystal shape with a relatively long one-dimensional pore direction and uniform heterogeneous nucleation over the entire film. The MOF thin films fabricated using the modulator exhibit high optical transparency. High-quality MOF thin films with dense and flat surfaces will pave the way for integrating MOFs into sophisticated optical and electrical devices.

Direct Optical Patterning of Metal-Organic Frameworks via Photoacid-Induced Etching

Metal-organic frameworks (MOFs) are a class of porous materials constructed from organic linkers and inorganic building blocks. Coordinative competition labilizes some MOFs under harsh chemical conditions because of their weak bonding. However, instability is not always a negative property of a material. In this study, we demonstrated the use of the acidic lability of MOFs for direct optical patterning. The controllable acid release from the photoacid generator at the exposed area causes bond cleavage between the linkers and metal ions/clusters, leading to solubility changes and pattern formation after development. This process avoids redundant steps and possible contamination in traditional photolithography, while maintaining the original properties of patterned MOFs. The preserved porosity and crystallinity promoted the development of MOFs for gas sensors and solid displays.

Coreactant-Free Zirconium Metal-Organic Framework with Dual Emission for Ratiometric Electrochemiluminescence Detection of HIV DNA

Owing to the limitations of dual-signal luminescent materials and coreactants, constructing a ratiometric electrochemiluminescence (ECL) biosensor based on a single luminophore is a huge challenge. This work developed an excellent zirconium metal-organic framework (MOF) Zr-TBAPY as a single ECL luminophore, which simultaneously exhibited cathodic and anodic ECL without any additional coreactants. First, Zr-TBAPY was successfully prepared by a solvothermal method with 1,3,6,8-tetra(4-carboxyphenyl)pyrene (TBAPY) as the organic ligand and Zr4+ cluster as the metal node. The exploration of ECL mechanisms confirmed that the cathodic ECL of Zr-TBAPY originated from the pathway of reactive oxygen species (ROS) as the cathodic coreactant, which is generated by dissolved oxygen (O2), while the anodic ECL stemmed from the pathway of generated Zr-TBAPY radical itself as the anodic coreactant. Besides, N,N-diethylethylenediamine (DEDA) was developed as a regulator to ECL signals, which quenched the cathodic ECL and enhanced the anodic ECL, and the specific mechanisms of its dual action were also investigated. DEDA can act as the anodic coreactant while consuming the cathodic coreactant ROS. Therefore, the coreactant-free ratiometric ECL biosensor was skillfully constructed by combining the regulatory role of DEDA with the signal amplification reaction of catalytic hairpin assembly (CHA). The ECL biosensor realized the ultrasensitive ratio detection of HIV DNA. The linear range was 1 fM to 100 pM, and the limit of detection (LOD) was as low as 550 aM. The outstanding characteristic of Zr-TBAPY provided new thoughts for the development of ECL materials and developed a new way of fabricating the coreactant-free and single-luminophore ratiometric ECL platform.

Crosslinking-induced patterning of MOFs by direct photo- and electron-beam lithography

Metal-organic frameworks (MOFs) with diverse chemistry, structures, and properties have emerged as appealing materials for miniaturized solid-state devices. The incorporation of MOF films in these devices, such as the integrated microelectronics and nanophotonics, requires robust patterning methods. However, existing MOF patterning methods suffer from some combinations of limited material adaptability, compromised patterning resolution and scalability, and degraded properties. Here we report a universal, crosslinking-induced patterning approach for various MOFs, termed as CLIP-MOF. Via resist-free, direct photo- and electron-beam (e-beam) lithography, the ligand crosslinking chemistry leads to drastically reduced solubility of colloidal MOFs, permitting selective removal of unexposed MOF films with developer solvents. This enables scalable, micro-/nanoscale (≈70 nm resolution), and multimaterial patterning of MOFs on large-area, rigid or flexible substrates. Patterned MOF films preserve their crystallinity, porosity, and other properties tailored for targeted applications, such as diffractive gas sensors and electrochromic pixels. The combined features of CLIP-MOF create more possibilities in the system-level integration of MOFs in various electronic, photonic, and biomedical devices.

A linker selective retention strategy to construct hierarchical porous metal-organic frameworks with high catalytic activity for oxidative desulfurization

In order to improve the oxidative desulfurization (ODS) performance of MOF materials, an effective way is to convert a microporous MOF into a hierarchical porous MOF (HP-MOF) by utilizing the linker selective retention strategy. Herein, UiO-66 with the introduction of an unstable linker ligand (dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate, dhtz) can selectively remove dhtz ligands to form HP-MOF (HP-UiO-66-dhtz) through heat treatment at high temperature. While maintaining the original structure of UiO-66, HP-UiO-66-dhtz features mesopores and abundant Lewis acid sites, showing excellent ODS performance for diphenylthiophene (DBT).

Polymorphism and orientation control of copper-dicarboxylate metal-organic framework thin films through vapour- and liquid-phase growth

Precise control over the crystalline phase and crystallographic orientation within thin films of metal-organic frameworks (MOFs) is highly desirable. Here, we report a comparison of the liquid- and vapour-phase film deposition of two copper-dicarboxylate MOFs starting from an oriented metal hydroxide precursor. X-ray diffraction revealed that the vapour- or liquid-phase reaction of the linker with this precursor results in different crystalline phases, morphologies, and orientations. Pole figure analysis showed that solution-based growth of the MOFs follows the axial texture of the metal hydroxide precursor, resulting in heteroepitaxy. In contrast, the vapour-phase method results in non-epitaxial growth with uniplanar texture only.

Fabrication of Oriented Polycrystalline MOF Superstructures

The field of metal-organic frameworks (MOFs) has progressed beyond the design and exploration of powdery and single-crystalline materials. A current challenge is the fabrication of organized superstructures that can harness the directional properties of the individual constituent MOF crystals. To date, the progress in the fabrication methods of polycrystalline MOF superstructures has led to close-packed structures with defined crystalline orientation. By controlling the crystalline orientation, the MOF pore channels of the constituent crystals can be aligned along specific directions: these systems possess anisotropic properties including enhanced diffusion along specific directions, preferential orientation of guest species, and protection of functional guests. In this perspective, we discuss the current status of MOF research in the fabrication of oriented polycrystalline superstructures focusing on the specific crystalline directions of orientation. Three methods are examined in detail: the assembly from colloidal MOF solutions, the use of external fields for the alignment of MOF particles, and the heteroepitaxial ceramic-to-MOF growth. This perspective aims at promoting the progress of this field of research and inspiring the development of new protocols for the preparation of MOF systems with oriented pore channels, to enable advanced MOF-based devices with anisotropic properties.

Water sensitivity of heteroepitaxial Cu-MOF films: dissolution and re-crystallization of 3D-oriented MOF superstructures

3D-oriented metal-organic framework (MOF) films and patterns have recently emerged as promising platforms for sensing and photonic applications. These oriented polycrystalline materials are typically prepared by heteroepitaxial growth from aligned inorganic nanostructures and display anisotropic functional properties, such as guest molecule alignment and polarized fluorescence. However, to identify suitable conditions for the integration of these 3D-oriented MOF superstructures into functional devices, the effect of water (gaseous and liquid) on different frameworks should be determined. We note that the hydrolytic stability of these heteroepitaxially grown MOF films is currently unexplored. In this work, we present an in-depth analysis of the structural evolution of aligned 2D and 3D Cu-based MOFs grown from Cu(OH)2 coatings. Specifically, 3D-oriented Cu2L2 and Cu2L2DABCO films (L = 1,4-benzenedicarboxylate, BDC; biphenyl-4,4-dicarboxylate, BPDC; DABCO = 1,4-diazabicyclo[2.2.2]octane) were exposed to 50% relative humidity (RH), 80% RH and liquid water. The combined use of X-ray diffraction, infrared spectroscopy, and scanning electron microscopy shows that the sensitivity towards humid environments critically depends on the presence of the DABCO pillar ligand. While oriented films of 2D MOF layers stay intact upon exposure to all levels of humidity, hydrolysis of Cu2L2DABCO is observed. In addition, we report that in environments with high water content, 3D-oriented Cu2(BDC)2DABCO recrystallizes as 3D-oriented Cu2(BDC)2. The heteroepitaxial MOF-to-MOF transformation mechanism was studied with in situ synchrotron experiments, time-resolved AFM measurements, and electron diffraction. These findings provide valuable information on the stability of oriented MOF films for their application in functional devices and highlight the potential for the fabrication of 3D-oriented superstructures via MOF-to-MOF transformations.