Semiconductor Metal-Organic Frameworks: Future Low-Bandgap Materials

Anchoring CoII Ions into a Thiol-Laced Metal-Organic Framework for Efficient Visible-Light-Driven Conversion of CO2 into CO

Using solar energy to convert CO₂ into valuable fuels or chemicals offers a powerful solution to urgent energy and environmental problems. However, the development of efficient and selective catalysts remains a considerable scientific challenge. To address this, catalytically active Co^II centers can be anchored into the porous matrix of metal-organic frameworks (MOFs) by utilizing a robust Zr-based MOF (Zr-DMBD) functionalized with freestanding thiol groups to enable efficient post-synthetic metal insertion. The thus-prepared Zr-DMBD-Co MOF solids are modified by well-defined Co-thiolate units and have the capability of photocatalytically converting CO₂ into CO with high efficiency and selectivity under visible-light irradiation in a water-containing system. The turnover number and CO selectivity reach as high as 97 941 and 98 %, respectively.

Integration of a (-Cu-S-)n plane in a metal-organic framework affords high electrical conductivity

Designing highly conducting metal–organic frameworks (MOFs) is currently a subject of great interest for their potential applications in diverse areas encompassing energy storage and generation. Herein, a strategic design in which a metal–sulfur plane is integrated within a MOF to achieve high electrical conductivity, is successfully demonstrated. The MOF {[Cu₂(6-Hmna)(6-mn)]·NH₄}_n (1, 6-Hmna = 6-mercaptonicotinic acid, 6-mn = 6-mercaptonicotinate), consisting of a two dimensional (–Cu–S–)_n plane, is synthesized from the reaction of Cu(NO₃)₂, and 6,6′-dithiodinicotinic acid via the in situ cleavage of an S–S bond under hydrothermal conditions. A single crystal of the MOF is found to have a low activation energy (6 meV), small bandgap (1.34 eV) and a highest electrical conductivity (10.96 S cm⁻¹) among MOFs for single crystal measurements. This approach provides an ideal roadmap for producing highly conductive MOFs with great potential for applications in batteries, thermoelectric, supercapacitors and related areas.

Metal–organic frameworks that contain metal–sulfur chains have been demonstrated to exhibit good electrical conductivity. Here, the authors integrate a 2D metal–sulfur plane into a metal–organic framework, reporting a single crystal with a high conductivity of 10.96 S/cm.

Sodium-coupled electron transfer reactivity of metal-organic frameworks containing titanium clusters: the importance of cations in redox chemistry

Stoichiometric reduction reactions of two metal-organic frameworks (MOFs) by the solution reagents (M = Cr, Co) are described. The two MOFs contain clusters with Ti8O8 rings: Ti8O8(OH)4(bdc)6; bdc = terephthalate (MIL-125) and Ti8O8(OH)4(bdc-NH2)6; bdc-NH2 = 2-aminoterephthalate (NH2-MIL-125). The stoichiometry of the redox reactions was probed using solution NMR methods. The extent of reduction is greatly enhanced by the presence of Na+, which is incorporated into the bulk of the material. The roughly 1 : 1 stoichiometry of electrons and cations indicates that the storage of e- in the MOF is tightly coupled to a cation within the architecture, for charge balance.

Supramolecular aggregation of a redox-active copper-naphthalenediimide network with intrinsic electron conduction

A sextuple ordered interpenetrated copper-naphthalenediimide network has been constructed by combining the features of porous metal-organic frameworks and π-conjugated supramolecular aggregation. The material exhibits intrinsic semiconductive features with narrow bandgap energy (1.2 eV) and outstanding electron transport. Theoretical calculations combined with experiments indicate that the high electron conduction may originate from π-d coupling and J-aggregation.

Single-Molecule-Based Electroluminescent Device as Future White Light Source

During the last two decades, spectacular development of light-emitting diodes (LEDs) has been achieved owing to their widespread application possibilities. However, traditional LEDs suffer from unavoidable energy loss because of the down conversion of photons, toxicity due to the involvement of rare-earth materials in their production, higher manufacturing cost, and reduced thermal stability that prevent them from all-inclusive applications. To address the existing challenges associated with current commercially available white LEDs, herein, we report a broad-band emission originating from an intrinsic lanthanide-free single-molecule-based LED. Self-assembly of a butterfly-shaped strontium-based compound {[Sr(H₂btc)₂(MeOH)(H₂O)₂]·2H₂O} (1) was achieved through the reaction of Sr(NO₃)₂ with 1,2,3-benzenetricarboxylic acid hydrate (1,2,3-H₃btc) under hydrothermal conditions. A white LED based on this single molecule exhibited a remarkable broad-band luminescence spectrum with Commission Internationale de l'Eclairage (CIE) coordinates at (0.33, 0.32) under 30 mA current injection. Such a broad luminescence spectrum can be attributed to the simultaneous existence of several emission lines originating from the intramolecular interactions within the structure. To further examine the nature of the observed transitions, density functional theory (DFT) calculations were carried out to explore the geometric and electronic properties of the complex. Our study thus paves the way toward a key step for developing a basic understanding and the development of high performance broad-band light-emitting devices with environment-friendly characteristics based on organic-inorganic supramolecular materials.

Metal-Organic Frameworks as Surface Enhanced Raman Scattering Substrates with High Tailorability

Surface enhanced Raman scattering (SERS) is a widely used analytical technique for detecting trace-level molecules based on an indispensable SERS substrate. SERS substrates with high tailorability are assumed to be attractive and desirable for SERS detection, because the substrates match the need for the selective detection of different species. Nevertheless, the rational design of such SERS substrates is rather challenging for both noble-metal and semiconductor substrates. Herein, expanding beyond conventional SERS substrates, we demonstrate that metal-organic framework (MOF) materials can serve as a type of SERS substrate with molecular selectivity, which are rarely realized for SERS detection without any special pretreatment. A salient structural characteristic of MOF-based SERS substrates benefiting the SERS selectivity is their high tailorability. By controlling the metal centers, organic ligands, and framework topologies of our MOF-based SERS substrates, we show that the electronic band structures of MOF-based SERS substrate can be purposively manipulated to match those of the target analytes, thus resulting in different detectable species. Going further, the SERS enhancement factors (EFs) of the MOF-based SERS substrates can be greatly enhanced to as high as 10⁶ with a low detection limit of 10^-8 M by pore-structure optimization and surface modification, which is comparable to the EFs of noble metals without "hot spots" and recently reported semiconductors. This selective enhancement is interpreted as being due to the controllable combination of several resonances, such as the charge-transfer, interband and molecule resonances, together with the ground-state charge-transfer interactions. Our study opens a new venue for the development of SERS substrates with high-design flexibility, which is especially important for selective SERS detection toward specific analytes.

Metal carbonyl cluster-based coordination polymers: diverse syntheses, versatile network structures, and special properties

This highlight surveys recent progress in groups 6–10 metal carbonyl cluster-based coordination polymers, focusing on diverse synthetic strategies, versatile structures, structural transformations, and semiconducting as well as magnetic properties.

Long-range ferromagnetism in nickel-based hybrid structure with semiconductor behavior

A new three-dimensional nickel-based hybrid structure exhibits semiconductor and long-range ferromagnetic behaviour.

Spectroelectrochemical studies of the redox active tris[4-(triazol-1-yl)phenyl]amine linker and redox state manipulation of Mn(ii)/Cu(ii) coordination frameworks

In situ spectroelectrochemical methods were conducted to investigate the redox active tris(4-(1H-1,2,4-triazol-1-yl)phenyl)amine (TTPA) ligand and its incorporation into the solid state of Mn(ii) and Cu(ii) frameworks.

Phenyl-grafted carbon nitride semiconductor for photocatalytic CO2-reduction and rapid degradation of organic dyes

Molecular engineering of graphitic carbon nitride (g-C₃N₄) is achieved by the copolymerization of π-conjugated phenyl urea, melamine, and urea.

Nanostructured and oriented metal-organic framework films enabling extreme surface wetting properties

We report on the synthesis of highly oriented and nanostructured metal-organic framework (MOF) films featuring extreme surface wetting properties. The Ni- and Co- derivatives of the metal-catecholate series (M-CAT-1) were synthesized as highly crystalline bulk materials and thin films. Oriented pillar-like nanostructured M-CAT-1 films exhibiting pronounced needle-like morphology on gold substrates were established by incorporating a crystallization promoter into the film synthesis. These nanostructured M-CAT-1 MOF films feature extreme wetting phenomena, specifically superhydrophilic and underwater superoleophobic properties with water and underwater oil-contact angles of 0° and up to 174°, respectively. The self-cleaning capability of the nanostructured, needle-like M-CAT-1 films was illustrated by measuring time-dependent oil droplet rolling-off a tilted surface. The deposition of the nanostructured Ni-CAT-1 film on a large glass substrate allowed for the realization of an efficient, transparent, antifog coating, enabling a clear view even at extreme temperature gaps up to ≈120 °C. This work illustrates the strong link between MOF film morphology and surface properties based on these framework materials.

Deformation behavior of an amorphous zeolitic imidazolate framework - from a supersoft material to a complex organometallic alloy

Zeolitic imidazolate frameworks (ZIFs)-a subset of metal-organic frameworks (MOFs)-have recently attracted immense attention. Many crystalline ZIFs (c-ZIFs) have highly porous zeolite structures that are ideal for molecular encapsulation. Recently emerging non-crystalline or amorphous ZIFs (a-ZIFs) with a similar short-range order are of interest because they can be converted from c-ZIFs for large-scale production. Here, we present a computational study of the deformation behavior of a unique a-ZIF model by simulating step-wise compression and expansion with strains between -0.389 and +0.376. An insulator-to-metal transition is observed at 51 GPa leading to a multicomponent light amorphous alloy of only 3.68 g (cm)-3. A high-density amorphous-to-amorphous phase transition is observed due to the sudden formation of N-N bond pairs. The systematic expansion of the a-ZIF retains the framework softness until it fractures at high strain. Based on the expansion data, we propose an empirical formula for super-soft materials, which is in line with available experimental data.

Accelerated discovery of stable lead-free hybrid organic-inorganic perovskites via machine learning

Rapidly discovering functional materials remains an open challenge because the traditional trial-and-error methods are usually inefficient especially when thousands of candidates are treated. Here, we develop a target-driven method to predict undiscovered hybrid organic-inorganic perovskites (HOIPs) for photovoltaics. This strategy, combining machine learning techniques and density functional theory calculations, aims to quickly screen the HOIPs based on bandgap and solve the problems of toxicity and poor environmental stability in HOIPs. Successfully, six orthorhombic lead-free HOIPs with proper bandgap for solar cells and room temperature thermal stability are screened out from 5158 unexplored HOIPs and two of them stand out with direct bandgaps in the visible region and excellent environmental stability. Essentially, a close structure-property relationship mapping the HOIPs bandgap is established. Our method can achieve high accuracy in a flash and be applicable to a broad class of functional material design.

High- k Gate Dielectrics for Emerging Flexible and Stretchable Electronics

Recent advances in flexible and stretchable electronics (FSE), a technology diverging from the conventional rigid silicon technology, have stimulated fundamental scientific and technological research efforts. FSE aims at enabling disruptive applications such as flexible displays, wearable sensors, printed RFID tags on packaging, electronics on skin/organs, and Internet-of-things as well as possibly reducing the cost of electronic device fabrication. Thus, the key materials components of electronics, the semiconductor, the dielectric, and the conductor as well as the passive (substrate, planarization, passivation, and encapsulation layers) must exhibit electrical performance and mechanical properties compatible with FSE components and products. In this review, we summarize and analyze recent advances in materials concepts as well as in thin-film fabrication techniques for high- k (or high-capacitance) gate dielectrics when integrated with FSE-compatible semiconductors such as organics, metal oxides, quantum dot arrays, carbon nanotubes, graphene, and other 2D semiconductors. Since thin-film transistors (TFTs) are the key enablers of FSE devices, we discuss TFT structures and operation mechanisms after a discussion on the needs and general requirements of gate dielectrics. Also, the advantages of high- k dielectrics over low- k ones in TFT applications were elaborated. Next, after presenting the design and properties of high- k polymers and inorganic, electrolyte, and hybrid dielectric families, we focus on the most important fabrication methodologies for their deposition as TFT gate dielectric thin films. Furthermore, we provide a detailed summary of recent progress in performance of FSE TFTs based on these high- k dielectrics, focusing primarily on emerging semiconductor types. Finally, we conclude with an outlook and challenges section.

Design of Single-Site Photocatalysts by Using Metal-Organic Frameworks as a Matrix

Single-site photocatalysts generally display excellent photocatalytic activity and considerably high stability compared with homogeneous catalytic systems. A rational structural design of single-site photocatalysts with isolated, uniform, and spatially separated active sites in a given solid is of prime importance to achieve high photocatalytic activity. Intense attention has been focused on the design and fabrication of single-site photocatalysts by using porous materials as a platform. Metal-organic frameworks (MOFs) have great potential in the design and fabrication of single-site photocatalysts due to their remarkable porosity, ultrahigh surface area, extraordinary tailorability, and significant diversity. MOFs can provide an abundant number of binding sites to anchor active sites, which results in a significant enhancement in photocatalytic performance. In this focus review, the development of single-site MOF photocatalysts that perform important and challenging chemical redox reactions, such as photocatalytic H2 production, photocatalytic CO2 conversion, and organic transformations, is summarized thoroughly. Successful strategies for the construction of single-site MOF photocatalysts are summarized and major challenges in their practical applications are noted.

Bottom-Up Fabrication of Semiconductive Metal-Organic Framework Ultrathin Films

Though generally considered insulating, recent progress on the discovery of conductive porous metal-organic frameworks (MOFs) offers new opportunities for their integration as electroactive components in electronic devices. Compared to classical semiconductors, these metal-organic hybrids combine the crystallinity of inorganic materials with easier chemical functionalization and processability. Still, future development depends on the ability to produce high-quality films with fine control over their orientation, crystallinity, homogeneity, and thickness. Here self-assembled monolayer substrate modification and bottom-up techniques are used to produce preferentially oriented, ultrathin, conductive films of Cu-CAT-1. The approach permits to fabricate and study the electrical response of MOF-based devices incorporating the thinnest MOF film reported thus far (10 nm thick).

A general microwave synthesis of metal (Ni, Cu, Zn) selenide nanoparticles and their competitive interaction with human serum albumin

Simple microwave irradiation technique was used to synthesize a series of selenide nanoparticles (platelet-like NiSe nanoparticles, uniform CuSe nanorods, and distorted ZnSe nano-hexagons) and their competitive interaction with human serum albumin was studied.

Fast and efficient direct formation of size-controlled nanostructures of coordination polymers based on copper(i)–iodine bearing functional pyridine terminal ligands

We report on the direct formation of 1D nanostructures of two coordination polymers based on copper(i)–iodine double chains decorated with ethyl isonicotinate or 2-amino-5-nitropyridine as terminal ligands.

Surface-tension-confined assembly of a metal–organic framework in femtoliter droplet arrays

The key factors determining HKUST-1 single-crystal growth are the internal flows in an evaporating droplet and consequently aggregation induced by the combination of metallic Cu(ii) and BTC ions.

Efficient heterogeneous catalysis by dual ligand Zn(ii)/Cd(ii) MOFs for the Knoevenagel condensation reaction: adaptable synthetic routes, characterization, crystal structures and luminescence studies

Chemically stable Zn(ii)/Cd(ii) MOFs as a heterogeneous catalysts for Knoevenagel condensation.

The role of redox hopping in metal–organic framework electrocatalysis

A perspective on redox hopping charge transport through metal organic frameworks and its role in driving efficient electrocatalysis.

A Co(ii) framework derived from a tris(4-(triazol-1-yl)phenyl)amine redox-active linker: an electrochemical and magnetic study

This is a rare case of a solid-state electrochemical and spectroelectrochemical study of coordination polymers. Furthermore, the framework exhibits an interesting field-induced magnetic spin-flop transition.

Thiophene insertion for continuous modulation of the photoelectronic properties of triphenylamine-based metal–organic frameworks for photocatalytic sulfonylation–cyclisation of activated alkenes

Ligand-engineering through the insertion of thiophenes precisely modulates the photoelectronic properties of metal–organic frameworks for efficient and diastereoselective photocatalytic sulfonylation–cyclisation of activated alkenes.

Direct Formation of Sub-Micron and Nanoparticles of a Bioinspired Coordination Polymer Based on Copper with Adenine

We report on the use of different reaction conditions, e.g., temperature, time, and/or concentration of reactants, to gain control over the particle formation of a bioinspired coordination polymer based on copper(II) and adenine, allowing homogeneous particle production from micro- to submicro-, and up to nano-size. Additionally, studies on this reaction carried out in the presence of different surfactants gives rise to the control of the particle size due to the modulation of the electrostatic interactions. Stability of the water suspensions obtained within the time and pH has been evaluated. We have also studied that there is no significant effect of the size reduction in the magnetic properties of the Cu(II)-adenine coordination polymer.

Syntheses of Exceptionally Stable Aluminum(III) Metal-Organic Frameworks: How to Grow High-Quality, Large, Single Crystals

The difficulty of obtaining large single crystals of aluminum carboxylate metal-organic frameworks (MOFs) for structure determinations has limited the development of these water and thermally stable MOFs. Herein, how large single crystals of known MIL-53(Al) and the first two tetrahedral ligand-based, visible-light-absorbing 3D Al-MOFs, [Al₃ (OH)₃ (HTCS)₂ ] (AlTCS-1) and [Al₅ O₂ (OH)₃ (TCS)₂ (H₂ O)₂ ] (AlTCS-2; TCS=tetrakis(4-oxycarbonylphenyl)silane), are obtained in the presence of hydrofluoric or formic acid for conventional single-crystal diffraction measurements is presented. The technique of obtaining those single crystals has potential to be a general method for obtaining large and good-quality single crystals of Al-MOFs. AlTCS-1 and -2 are stable over a wide pH range (1-11), and AlTCS-1 is even stable in aqua regia solution for at least 24 h. The BET specific surface areas of AlTCS-1 and -2 are 11 and 1506 m²  g^-1 , respectively. AlTCS-2 takes up 51 cm³  (STP) g^-1 CO₂ and 15 cm³  (STP) g^-1 CH₄ at 298 K and 1 bar, which is relatively high among MOF materials. AlTCS-1 takes up 30 cm³  g^-1 CO₂ and 4.2 cm³  g^-1 CH₄ at 298 K and 1 bar. The rapid and stable photocurrent responses of AlTCS-1 and -2 under UV and visible-light illumination are observed. Moreover, AlTCS-1 photocatalyzes the water-splitting reaction under visible light with an average hydrogen evolution efficiency of 50 μmol g^-1  h^-1 for the first 10 h in a mixture of water and triethanolamine.

Lowering Band Gap of an Electroactive Metal-Organic Framework via Complementary Guest Intercalation

A new honeycomb-shaped electroactive metal-organic framework (MOF) has been constructed from an electron deficient naphthalenediimide (NDI) ligand equipped with two terminal salicylic acid groups. π-Intercalation of electron-rich planar tetrathiafulvalene (TTF) guests between the NDI ligands stacked along the walls lowers the electronic band gap of the material by ca. 1 eV. An improved electron delocalization through the guest-mediated π-donor/acceptor stacks is attributed to the diminished band gap of the doped material, which forecasts an improved electrical conductivity.

Accelerating Palladium Nanowire H2 Sensors Using Engineered Nanofiltration

The oxygen, O₂, in air interferes with the detection of H₂ by palladium (Pd)-based H₂ sensors, including Pd nanowires (NWs), depressing the sensitivity and retarding the response/recovery speed in air-relative to N₂ or Ar. Here, we describe the preparation of H₂ sensors in which a nanofiltration layer consisting of a Zn metal-organic framework (MOF) is assembled onto Pd NWs. Polyhedron particles of Zn-based zeolite imidazole framework (ZIF-8) were synthesized on lithographically patterned Pd NWs, leading to the creation of ZIF-8/Pd NW bilayered H₂ sensors. The ZIF-8 filter has many micropores (0.34 nm for gas diffusion) which allows for the predominant penetration of hydrogen molecules with a kinetic diameter of 0.289 nm, whereas relatively larger gas molecules including oxygen (0.345 nm) and nitrogen (0.364 nm) in air are effectively screened, resulting in superior hydrogen sensing properties. Very importantly, the Pd NWs filtered by ZIF-8 membrane (Pd NWs@ZIF-8) reduced the H₂ response amplitude slightly (ΔR/R₀ = 3.5% to 1% of H₂ versus 5.9% for Pd NWs) and showed 20-fold faster recovery (7 s to 1% of H₂) and response (10 s to 1% of H₂) speed compared to that of pristine Pd NWs (164 s for response and 229 s for recovery to 1% of H₂). These outstanding results, which are mainly attributed to the molecular sieving and acceleration effect of ZIF-8 covered on Pd NWs, rank highest in H₂ sensing speed among room-temperature Pd-based H₂ sensors.

Missing Linkers: An Alternative Pathway to UiO-66 Electronic Structure Engineering

UiO-66 is a promising metal-organic framework for photocatalytic applications. However, the ligand-to-metal charge transfer of an excited electron is inefficient in the pristine material. Herein, we assess the influence of missing linker defects on the electronic structure of UiO-66 and discuss their ability to improve ligand-to-metal charge transfer. Using a new defect classification system, which is transparent and easily extendable, we identify the most promising photocatalysts by considering both relative stability and electronic structure. We find that the properties of UiO-66 defect structures largely depend on the coordination of the constituent nodes and that the nodes with the strongest local distortions alter the electronic structure most. Defects hence provide an alternative pathway to tune UiO-66 for photocatalytic purposes, besides linker modification and node metal substitution. In addition, the decomposition of MOF properties into node- and linker-based behavior is more generally valid, so we propose orthogonal electronic structure tuning as a paradigm in MOF design.

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.