Electrostatic Self‐Assembly of Nanosized Carbon Nitride Nanosheet onto a Zirconium Metal–Organic Framework for Enhanced Photocatalytic CO2 Reduction

UiO‐66, a zirconium based metal–organic framework, is incorporated with nanosized carbon nitride nanosheets via a facile electrostatic self‐assembly method. This hybrid structure exhibits a large surface area and strong CO₂ capture ability due to the introduction of UiO‐66. We demonstrate that electrons from the photoexcited carbon nitride nanosheet can transfer to UiO‐66, which can substantially suppress electron–hole pair recombination in the carbon nitride nanosheet, as well as supply long‐lived electrons for the reduction of CO₂ molecules that are adsorbed in UiO‐66. As a result, the UiO‐66/carbon nitride nanosheet heterogeneous photocatalyst exhibits a much higher photocatalytic activity for the CO₂ conversion than that of bare carbon nitride nanosheets. We believe this self‐assembly method can be extended to other carbon nitride nanosheet loaded materials.     

Rapid Room‐Temperature Synthesis of Metal–Organic Framework HKUST‐1 Crystals in Bulk and as Oriented and Patterned Thin Films

Whereas the preparation of defined metal–organic framework (MOF) materials via hydrothermal or diffusion methods typically requires hours to days, our simple precipitation route opens the access to the well‐known HKUST‐1 frameworks within minutes. Crucial for the formation of a well‐defined, crystalline material is the choice of suitable precipitating solvents, with methanol and ethanol being the most favorable ones. This approach could be extended to the formation of dense, surface‐mounted MOF films (so‐called SURMOFs), in particular if the surfaces are decorated with suitable binding groups by formation of self‐assembled monolayers (SAMs). By combination with micro‐contact printing (μCP), patterned SURMOFs became accessible, in which the precipitating solvent is decisive on the formation of either spatially restricted films or single particles.     

Massive Anisotropic Thermal Expansion and Thermo‐Responsive Breathing in Metal–Organic Frameworks Modulated by Linker Functionalization

Functionalized metal–organic frameworks (fu‐MOFs) of general formula [Zn₂(fu‐L)₂dabco]_n show unprecedentedly large uniaxial positive and negative thermal expansion (fu‐L = alkoxy functionalized 1,4‐benzenedicarboxylate, dabco = 1,4‐diazabicyclo[2.2.2]octane). The magnitude of the volumetric thermal expansion is more comparable to property of liquid water rather than any crystalline solid‐state material. The alkoxy side chains of fu‐L are connected to the framework skeleton but nevertheless exhibit large conformational flexibility. Thermally induced motion of these side chains induces extremely large anisotropic framework expansion and eventually triggers reversible solid state phase transitions to drastically expanded structures. The thermo‐responsive properties of these hybrid solid–liquid materials are precisely controlled by the choice and combination of fu‐Ls and depend on functional moieties and chain lengths. In principle, this combinatorial approach allows for a targeted design of extreme thermo‐mechanical properties of MOFs addressing the regime between crystalline solid matter and the liquid state.     

Template‐Assisted Synthesis of π‐Conjugated Molecular Organic Nanowires in the Sub‐100 nm Regime and Device Implications

π‐conjugated molecular organics such as rubrene, Alq₃, fullerene, and PCBM have been used extensively over the last few decades in numerous organic electronic devices, including solar cells, thin‐film transistors, and large‐area, low‐cost flexible displays. Rubrene and Alq₃, have emerged as promising platforms for spin‐based classical and quantum information processing, which has triggered significant research activity in the relatively new area of organic spintronics. Synthesis of these materials in a nanowire geometry, with feature sizes in the sub‐100 nm regime, is desirable as it often enhances device performance and is essential for development of high‐density molecular electronic devices. However, fabrication techniques that meet this stringent size constraint are still largely underdeveloped. Here, a novel, versatile, and reagentless method that enables growth of nanowire arrays of the above‐mentioned organics in the cylindrical nanopores of anodic aluminum oxide (AAO) templates is demonstrated. This method 1) allows synthesis of high‐density organic nanowire arrays on arbitrary substrates, 2) provides electrical access to the nanowire arrays, 3) offers tunability of the array geometry in a range overlapping with the relevant physical length scales of many organic devices, and 4) can potentially be extended to synthesize axially and radially heterostructured organic nanowires. Thus prepared nanowires are characterized extensively with an aim to identify their potential applications in diverse areas such as organic optoelectronics, photovoltaics, molecular nanoelectronics, and spintronics.     

High‐Resolution Patterning of Various Large‐Area,Highly Ordered Structural Motifs by Directional Photofluidization Lithography: Sub‐30‐nm Line,Ellipsoid, Rectangle,and Circle Arrays

A major challenge in nanolithography is to overcome the resolution limit of conventional patterning methods. Herein, we demonstrate a simple and convenient approach to generate sub‐30‐nm various structural motifs with precisely controlled sizes, shapes, and orientations. The proposed method, the “directional photofluidization” of an azopolymer, follows the same philosophy as a path‐changing approach, for example, thermal‐reflow of polymer arrays, in that post‐treatment simultaneously leads to a reduction of the feature sizes and line‐edge roughness (LER) of nanostructures. However, in contrast to thermal‐induced isotropic reflow, directional photofluidization provides unprecedented flexibility to control the structural features, because the direction of photofluidization can be arbitrary controlled according to the light polarization. Furthermore, this approach offers good control of the final features due to a gradual reduction in the rate of photofluidization during light irradiation. More importantly, the photofluidic behavior of the azopolymer significantly reduces the LER, and thus it can improve the quality of nanostructures. Finally, the far‐field process of directional photofluidization enables hierarchical nanofabrication, in contrast to mechanical contact fabrication, because the patterned light can reconfigure the polymer arrays selectively. Our approach is potentially advantageous for the fabrication of various structural motifs with well‐controlled dimensions on the nanoscale and with minimized LER.     

Structure–Property Relationship of Pyridine‐Containing Triphenyl Benzene Electron‐Transport Materials for Highly Efficient Blue Phosphorescent OLEDs

Three triphenyl benzene derivatives of 1,3,5‐tri(m‐pyrid‐2‐yl‐phenyl)benzene (Tm2PyPB), 1,3,5‐tri(m‐pyrid‐3‐yl‐phenyl)benzene (Tm3PyPB) and 1,3,5‐tri(m‐pyrid‐4‐yl‐phenyl)benzene (Tm4PyPB), containing pyridine rings at the periphery, are developed as electron‐transport and hole/exciton‐blocking materials for iridium(III) bis(4,6‐(di‐fluorophenyl)pyridinato‐N,C^2′)picolinate (FIrpic)‐based blue phosphorescent organic light‐emitting devices. Their highest occupied molecular orbital and lowest unoccupied molecular orbital (LUMO) energy levels decrease as the nitrogen atom of the pyridine ring moves from position 2 to 3 and 4; this is supported by both experimental results and density functional theory calculations, and gives improved electron‐injection and hole‐blocking properties. They exhibit a high electron mobility of 10^?4–10^?3 cm² V^?1 s^?1 and a high triplet energy level of 2.75 eV. Confinement of FIrpic triplet excitons is strongly dependent on the nitrogen atom position of the pyridine ring. The second exponential decay component in the transient photoluminescence decays of Firpic‐doped films also decreases when the position of the nitrogen atom in the pyridine ring changes. Reduced driving voltages are obtained when the nitrogen atom position changes because of improved electron injection as a result of the reduced LUMO level, but a better carrier balance is achieved for the Tm3PyPB‐based device. An external quantum efficiency (EQE) over 93% of maximum EQE was achieved for the Tm4PyPB‐based device at an illumination‐relevant luminance of 1000 cd m^?2, indicating reduced efficiency roll‐off due to better confinement of FIrpic triplet excitons by Tm4PyPB in contrast to Tm2PyPB and Tm3PyPB.     

Praseodymium Silicate as a High‐k Dielectric Candidate: An Insight into the Pr2O3‐Film/Si‐Substrate Interface Fabricated Through a Metal–Organic Chemical Vapor Deposition Process

Praseodymium‐containing thin films have been deposited on Si(001) substrates by metal–organic chemical vapor deposition (MOCVD) from the Pr(tmhd)₃ (H‐tmhd = 2,2,6,6‐tetramethyl‐3,5‐heptanedione) precursor. The structural, compositional, and morphological film characterization has been investigated using X‐ray diffraction (XRD), angle‐resolved X‐ray photoelectron spectroscopy (AR‐XPS), and transmission electron microscopy (TEM). Detailed studies of the deposition parameters indicate that the MOCVD process is governed by a kinetic regime and that some reactive phenomena occur at the film/substrate interface, forming a praseodymium silicate layer. A possible explanation for interfacial interaction has been proposed, taking into account the diffusion of Si from the substrate towards the bulk and that of oxygen from the film surface toward the substrate/film interface. Finally, the electrical characterization of the praseodymium silicate layer has been carried out in order to evaluate its potential implementation as an alternative dielectric. Its dielectric constant has been evaluated to be ～ 8.     

Dielectric Relaxation Processes,Electronic Structure,and Band Gap Engineering of MFU‐4‐type Metal‐Organic Frameworks: Towards a Rational Design of Semiconducting Microporous Materials

The electronic structures and band gaps of MFU‐4‐type metal‐organic frameworks can be systematically engineered leading to a family of isostructural microporous solids. Electrical properties of the microcrystalline samples are investigated by temperature‐dependent broad‐band dielectric and optical spectroscopy, which are corroborated by full band structure calculations performed for framework and cluster model compounds at multiple levels of density functional theory. The combined results glean a detailed picture of relative shifts and dispersion of molecular orbitals when going from zero‐dimensional clusters to three‐dimensional periodic solids, thus allowing to develop guidelines for tailoring the electronic properties of this class of semiconducting microporous solids via a versatile building block approach. Thus, engineering of the band gap in MFU‐4 type compounds can be achieved by adjusting the degree of conjugation of the organic ligand or by choosing an appropriate metal whose partially occupied d‐orbitals generate bands below the LUMO energy of the ligand which, for example, is accomplished by octahedral Co(II) ions in Co‐MFU‐4.     

Enhancement in Organic Photovoltaic Efficiency through the Synergistic Interplay of Molecular Donor Hydrogen Bonding and π‐Stacking

For organic photovoltaic (OPV) cells based on the bulk heterojunction (BHJ) structure, it remains challenging to rationally control the degree of phase separation and percolation within blends of donors and acceptors to secure optimal charge separation and transport. Reported is a bottom‐up, supramolecular approach to BHJ OPVs wherein tailored hydrogen bonding (H‐bonding) interactions between π‐conjugated electron donor molecules encourage formation of vertically aligned donor π‐stacks while simultaneously suppressing lateral aggregation; the programmed arrangement facilitates fine mixing with fullerene acceptors and efficient charge transport. The approach is illustrated using conventional linear or branched quaterthiophene donor chromophores outfitted with terminal functional groups that are either capable or incapable of self‐complementary H‐bonding. When applied to OPVs, the H‐bond capable donors yield a twofold enhancement in power conversion efficiency relative to the comparator systems, with a maximum external quantum efficiency of 64%. H‐bond promoted assembly results in redshifted absorption (in neat films and donor:C₆₀ blends) and enhanced charge collection efficiency despite disparate donor chromophore structure. Both features positively impact photocurrent and fill factor in OPV devices. Film structural characterization by atomic force microscopy, transmission electron microscopy, and grazing incidence wide angle X‐ray scattering reveals a synergistic interplay of lateral H‐bonding interactions and vertical π‐stacking for directing the favorable morphology of the BHJ.     

Synthesis of Semiconducting Functional Materials in Solution: From II‐VI Semiconductor to Inorganic–Organic Hybrid Semiconductor Nanomaterials

This Feature Article provides a brief overview of the latest development and emerging new synthesis solution strategies for II–VI semiconducting nanomaterials and inorganic‐organic semiconductor hybrid materials. Research on the synthesis of II–VI semiconductor nanomaterials and inorganic–organic hybrid semiconducting materials via solution strategies has made great progress in the past few years. A variety of II–VI semiconductor and a new family of [MQ(L)_0.5] (M = Mn, Zn, Cd; Q = S, Se, Te; L = diamine, deta) hybrid nanostructures can be generated using solution synthetic routes. Recent advances have demonstrated that the solution strategies in pure solvent and a mixed solvent can not only determine the crystal size, shape, composition, structure and assembly properties, but also the crystallization pathway, and act as a matrix for the formation of a variety of different II–VI semiconductor and hybrid nanocomposites with diverse morphologies. These II–VI semiconductor nanostructures and their hybrid nanocomposites display obvious quantum size effects, unique and tunable optical properties.     

Exciton–Polaron‐Induced Aggregation of Wide‐Bandgap Materials and its Implication on the Electroluminescence Stability of Phosphorescent Organic Light‐Emitting Devices

The degradation mechanisms of phosphorescent organic light‐emitting devices (PhOLEDs) are studied. The results show that PhOLED degradation is closely linked to interactions between excitons and positive polarons in the host material of the emitter layer (EML), which lead to its aggregation near the EML/electron transport layer (ETL) interface. This exciton–polaron‐induced aggregation (EPIA) is associated with the emergence of new emission bands at longer wavelengths in the electroluminescence spectra of these materials, which can be detected after prolonged device operation. Such EPIA processes are found to occur in a variety of wide‐bandgap materials commonly used as hosts in PhOLEDs and are correlated with device degradation. Quite notably, the extent of EPIA appears to correlate with the material's bandgap rather than with the glass‐transition temperature. The findings uncover a new degradation mechanism, caused by polaron‐exciton interactions, that appears to be behind the lower stability of OLEDs utilizing wide‐bandgap materials in general. The same degradation mechanism can be expected to be present in other organic optoelectronic devices.     

Manipulation of the Open‐Circuit Voltage of Organic Solar Cells by Desymmetrization of the Structure of Acceptor–Donor–Acceptor Molecules

The synthesis of acceptor–donor–acceptor (A–D–A) molecules based on a septithiophene chain with terminal electron acceptor groups is reported. Using a dicyanovinyl‐ (DCV) substituted molecule as reference, another symmetrical A–D–A donor containing thiobarbituric (TB) groups is synthesized and these two acceptor groups are combined to produce the unsymmetrical A–D–A′ compound. The electronic properties of the donors are analyzed by cyclic voltammetry and UV‐Vis absorption spectroscopy and their photovoltaic properties are characterized on bilayer planar heterojunction cells that include spun‐cast donor films and vacuum‐deposited C₆₀ as acceptor. Optical and electrochemical data show that replacement of DCV by TB leads to a small increase of the HOMO level and to a larger decrease of the LUMO, which result in a reduced band‐gap. The desymmetrized compound presents the lowest oxidation potential in solution but the highest oxidation onset in the solid state, which leads to a significant increase of the open‐circuit voltage of the resulting solar cells.