Growth mechanisms and anisotropic softness-dependent conductivity of orientation-controllable metal-organic framework nanofilms.

Conductive metal-organic frameworks (cMOFs) manifest great potential in modern electrical devices due to their porous nature and the ability to conduct charges in a regular network. cMOFs applied in electrical devices normally hybridize with other materials, especially a substrate. Therefore, the precise control of the interface between cMOF and a substrate is particularly crucial. However, the unexplored interface chemistry of cMOFs makes the controlled synthesis and advanced characterization of high-quality thin films, particularly challenging. Herein, we report the development of a simplified synthesis method to grow "face-on" and "edge-on" cMOF nanofilms on substrates, and the establishment of operando characterization methodology using atomic force microscopy and X-ray, thereby demonstrating the relationship between the soft structure of surface-mounted oriented networks and their characteristic conductive functions. As a result, crystallinity of cMOF nanofilms with a thickness down to a few nanometers is obtained, the possible growth mechanisms are proposed, and the interesting anisotropic softness-dependent conducting properties (over 2 orders of magnitude change) of the cMOF are also illustrated.

Light-Induced Transient Lattice Dynamics and Metastable Phase Transition in CH3NH3PbI3 Nanocrystals.

Methylammonium lead iodide (MAPbI3) perovskite nanocrystals (NCs) offer desirable optoelectronic properties with prospective utility in photovoltaics, lasers, and light-emitting diodes (LEDs). Structural rearrangements of MAPbI3 in response to photoexcitation, such as lattice distortions and phase transitions, are of particular interest, as these engender long carrier lifetime and bolster carrier diffusion. Here, we use variable temperature X-ray diffraction (XRD) and synchrotron-based transient X-ray diffraction (TRXRD) to investigate lattice response following ultrafast optical excitation. MAPbI3 NCs are found to slowly undergo a phase transition from the tetragonal to a pseudocubic phase over the course of 1 ns under 0.02-4.18 mJ/cm2 fluence photoexcitation, with apparent nonthermal lattice distortions attributed to polaron formation. Lattice recovery exceeds time scales expected for both carrier recombination and thermal dissipation, indicating meta-stability likely due to the proximal phase transition, with symmetry-breaking along equatorial and axial directions. These findings are relevant for fundamental understanding and applications of structure-function properties.

Two-Dimensional Covalent Organic Framework Solid Solutions.

Covalent organic frameworks (COFs) generally leverage one or two monomers with specific sizes and shapes to access highly symmetric and periodic polymer networks. Almost all reported COFs employ the minimum sets of monomers needed for the polymerization (usually two, sometimes one) and crystallize in high-symmetry topologies. COFs synthesized from more than two monomers usually employ mixtures with different pendant functionalities to distribute these groups statistically throughout the structure, or monomers with different sizes in ratios targeting lower symmetry topologies. Here, we demonstrate that mixtures of monomers with different lengths generate single-phase, hexagonal two-dimensional covalent organic framework (2D COF) solid solutions at continuously variable feed ratios. X-ray diffraction measurements, Fourier-transform infrared spectroscopy, and Pawley refinement indicate that both monomers distribute randomly within the same lattice, and the lattice parameters continuously increase as more of the larger linker is incorporated. Furthermore, COF solid solutions are accessed directly by polymerizing a mixture of monomers but not via linker exchange from a preformed COF. As strain develops from the lattice accommodating monomers with different sizes, the nonlinear relationship between the monomer incorporation and the COF's lattice parameters suggests that bond-bending of the monomers plays a role in incorporating monomers of different lengths into the solid solutions. Solid solution formation represents a new strategy to design 2D COFs and increase their complexity. Specifically, varying the monomer composition of a given network enables many properties, such as the average pore size, to be continuously tuned between those of corresponding pure COFs.

Anisotropic Transient Disordering of Colloidal, Two-Dimensional CdSe Nanoplatelets upon Optical Excitation.

Nanoplatelets (NPLs)-colloidally synthesized, spatially anisotropic, two-dimensional semiconductor quantum wells-are of intense interest owing to exceptionally narrow transition line widths, coupled with solution processability and bandgap tunability. However, given large surface areas and undercoordinated bonding at facet corners and edges, excitation under sufficient intensities may induce anisotropic structural instabilities that impact desired properties. We employ time-resolved X-ray diffraction to study the crystal structure of CdSe NPLs in response to optical excitation. Photoexcitation induces greater out-of-plane than in-plane disordering in 4 and 5 monolayer (ML) NPLs, while 3 ML NPLs display the opposite behavior. Recovery dynamics suggest that out-of-plane cooling slightly outpaces in-plane cooling in 5 ML NPLs with recrystallization occurring on indistinguishable time scales. In comparison, for zero-dimensional CdSe nanocrystals, disordering is isotropic and recovery is faster. These results favor the use of NPLs in optoelectronic applications, where they are likely to exhibit superior performance over traditional, zero-dimensional nanocrystals.

Solvothermal depolymerization and recrystallization of imine-linked two-dimensional covalent organic frameworks.

Mechanistic understanding into the formation and growth of imine-linked two-dimensional (2D) covalent organic frameworks (COFs) is needed to improve their materials quality and access larger crystallite sizes, both of which limit the promise of 2D COFs and 2D polymerization techniques. Here we report a previously unknown temperature-dependent depolymerization of colloidal 2D imine-linked COFs, which offers a new means to improve their crystallinity. 2D COF colloids form at room temperature but then depolymerize when their reaction mixtures are heated to 90 °C. As the solutions are cooled back to room temperature, the 2D COFs repolymerize and crystallize with improved crystallinity and porosity, as characterized by X-ray diffraction, infrared spectroscopy and N2 porosimetry. The evolution of COF crystallinity during the solvothermal depolymerization and repolymerization processes was characterized by in situ wide angle X-ray scattering, and the concentrations of free COF monomers as a function of temperature were quantified by variable temperature 1H NMR spectroscopy. The ability of a 2D COF to depolymerize under these conditions depends on both the identity of the COF and its initial materials quality. For one network formed at room temperature (TAPB-PDA COF), a first depolymerization process is nearly complete, and the repolymerization yields materials with dramatically enhanced crystallinity and surface area. Already recrystallized materials partially depolymerize upon heating their reaction mixtures a second time. A related 2D COF (TAPB-DMTA COF) forms initially with improved crystallinity compared to TAPB-PDA COF and then partially depolymerizes upon heating. These results suggest that both high materials quality and network-dependent properties, such as interlayer interaction strength, influence the extent to which 2D COFs resist depolymerization. These findings offer a new means to recrystallize or solvent anneal 2D COFs and may ultimately inform crystallization conditions for obtaining large-area imine-linked two-dimensional polymers from solution.

Trends in the thermal stability of two-dimensional covalent organic frameworks.

Two-dimensional covalent organic frameworks (2D COFs) are synthetically diverse, layered macromolecules. Their covalent lattices are thought to confer high thermal stability, which is typically evaluated with thermogravimetric analysis (TGA). However, TGA measures the temperature at which volatile degradation products are formed and is insensitive to changes of the periodic structure of the COF. Here, we study the thermal stability of ten 2D COFs using a combination of variable-temperature X-ray diffraction, TGA, diffuse reflectance infrared spectroscopy, and density functional theory calculations. We find that 2D COFs undergo a general two-step thermal degradation process. At the first degradation temperature, 2D COFs lose their crystallinity without chemical degradation. Then, at higher temperatures, they chemically degrade into volatile byproducts. Several trends emerge from this exploration of 2D COF stability. Boronate ester-linked COFs are generally more thermally stable than comparable imine-linked COFs. Smaller crystalline lattices are more robust to thermal degradation than chemically similar larger lattices. Finally, pore-functionalized COFs degrade at significantly lower temperatures than their unfunctionalized analogues. These trends offer design criteria for thermally resilient 2D COF materials. These findings will inform and encourage a broader exploration of mechanical deformation in 2D networks, providing a necessary step towards their practical use.

Transient Lattice Response upon Photoexcitation in CuInSe2 Nanocrystals with Organic or Inorganic Surface Passivation.

CuInSe2 nanocrystals offer promise for optoelectronics including thin-film photovoltaics and printed electronics. Additive manufacturing methods such as photonic curing controllably sinter particles into quasi-continuous films and offer improved device performance. To gain understanding of nanocrystal response under such processing conditions, we investigate impacts of photoexcitation on colloidal nanocrystal lattices via time-resolved X-ray diffraction. We probe three sizes of particles and two capping ligands (oleylamine and inorganic S2-) to evaluate resultant crystal lattice temperature, phase stability, and thermal dissipation. Elevated fluences produce heating and loss of crystallinity, the onset of which exhibits particle size dependence. We find size-dependent recrystallization and cooling lifetimes ranging from 90 to 200 ps with additional slower cooling on the nanosecond time scale. Sulfide-capped nanocrystals show faster recrystallization and cooling compared to oleylamine-capped nanocrystals. Using these lifetimes, we find interfacial thermal conductivities from 3 to 28 MW/(m2 K), demonstrating that ligand identity strongly influences thermal dissipation.

Large Exciton Diffusion Coefficients in Two-Dimensional Covalent Organic Frameworks with Different Domain Sizes Revealed by Ultrafast Exciton Dynamics.

Large singlet exciton diffusion lengths are a hallmark of high performance in organic-based devices such as photovoltaics, chemical sensors, and photodetectors. In this study, exciton dynamics of a two-dimensional covalent organic framework, 2D COF-5, is investigated using ultrafast spectroscopic techniques. After photoexcitation, the COF-5 exciton decays via three pathways: (1) excimer formation (4 ± 2 ps), (2) excimer relaxation (160 ± 40 ps), and (3) excimer decay (>3 ns). Excitation fluence-dependent transient absorption studies suggest that COF-5 has a relatively large diffusion coefficient (0.08 cm2/s). Furthermore, exciton-exciton annihilation processes are characterized as a function of COF-5 crystallite domain size in four different samples, which reveal domain-size-dependent exciton diffusion kinetics. These results reveal that exciton diffusion in COF-5 is constrained by its crystalline domain size. These insights indicate the outstanding promise of delocalized excitonic processes available in 2D COFs, which motivate their continued design and implementation into optoelectronic devices.

In Situ Grazing-Incidence Wide-Angle Scattering Reveals Mechanisms for Phase Distribution and Disorientation in 2D Halide Perovskite Films.

2D hybrid halide perovskites with the formula (A')2 (A)n -1 Pbn I3 n +1 have remarkable stability and promising efficiency in photovoltaic and optoelectronic devices, yet fundamental understanding of film formation, key to optimizing these devices, is lacking. Here, in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) is used to monitor film formation during spin-coating. This elucidates the general film formation mechanism of 2D halide perovskites during one-step spin-coating. There are three stages of film formation: sol-gel, oriented 3D, and 2D. Three precursor phases form during the sol-gel stage and transform to perovskite, first giving a highly oriented 3D-like phase at the air/liquid interface followed by subsequent nucleations forming slightly less oriented 2D perovskite. Furthermore, heating before crystallization leads to fewer nucleations and faster removal of the precursors, improving orientation. This outlines the primary causes of phase distribution and perpendicular orientation in 2D perovskite films and paves the way for rationally designed film fabrication techniques.

Acid Exfoliation of Imine-linked Covalent Organic Frameworks Enables Solution Processing into Crystalline Thin Films.

Covalent organic frameworks (COFs) are highly modular porous crystalline polymers that are of interest for applications such as charge-storage devices, nanofiltration membranes, and optoelectronic devices. COFs are typically synthesized as microcrystalline powders, which limits their performance in these applications, and their limited solubility precludes large-scale processing into more useful morphologies and devices. We report a general, scalable method to exfoliate two-dimensional imine-linked COF powders by temporarily protonating their linkages. The resulting suspensions were cast into continuous crystalline COF films up to 10 cm in diameter, with thicknesses ranging from 50 nm to 20 µm depending on the suspension composition, concentration, and casting protocol. Furthermore, we demonstrate that the film fabrication process proceeds through a partial depolymerization/repolymerization mechanism, providing mechanically robust films that can be easily separated from their substrates.

Emissive Single-Crystalline Boroxine-Linked Colloidal Covalent Organic Frameworks.

The synthesis of periodic two-dimensional (2D) polymers and characterization of their optoelectronic behaviors are challenges at the forefront of polymer chemistry and materials science. Recently, we showed that layered 2D polymers known as 2D covalent organic frameworks (COFs) can be synthesized as single crystals by preparing COF particles as colloidal suspensions. Here we expand this approach from the condensation of boronic acids and catechols to the dehydrative trimerization of polyboronic acids. The resulting boroxine-linked colloids are the next class of 2D COFs to be obtained as single-crystalline particles, as demonstrated here for four 2D COFs and one 3D COF. Colloidal stabilization enables detailed structural analysis by synchrotron X-ray diffraction and high-resolution transmission electron microscopy. Solution fluorescence spectroscopy revealed that the COF crystallites are highly emissive compared to their respective monomer solutions. Excitation-emission matrix fluorescence spectroscopy indicated that the origin of this enhanced emission can be attributed to through-space communication of chromophores between COF sheets. These observations will motivate the development of colloidal COF systems as a platform to organize functional aromatic systems into precise and predictable assemblies with emergent properties.

A Dinuclear Mechanism Implicated in Controlled Carbene Polymerization.

Carbene polymerization provides polyolefins that cannot be readily prepared from olefin monomers; however, controlled and living carbene polymerization has been a long-standing challenge. Here we report a new class of initiators, (π-allyl)palladium carboxylate dimers, which polymerize ethyl diazoacetate, a carbene precursor in a controlled and quasi-living manner, with nearly quantitative yields, degrees of polymerization >100, molecular weight dispersities 1.2-1.4, and well-defined, diversifiable chain ends. This method also provides block copolycarbenes that undergo microphase segregation. Experimental and theoretical mechanistic analysis supports a new dinuclear mechanism for this process.

Controlled growth of imine-linked two-dimensional covalent organic framework nanoparticles.

Covalent organic frameworks (COFs) consist of monomers arranged in predictable structures with emergent properties. However, improved crystallinity, porosity, and solution processability remain major challenges. To this end, colloidal COF nanoparticles are useful for mechanistic studies of nucleation and growth and enable advanced spectroscopy and solution processing of thin films. Here we present a general approach to synthesize imine-linked 2D COF nanoparticles and control their size by favoring imine polymerization while preventing the nucleation of new particles. The method yields uniform, crystalline, and high-surface-area particles and is applicable to several imine-linked COFs. In situ X-ray scattering experiments reveal the nucleation of amorphous polymers, which crystallize via imine exchange processes during and after particle growth, consistent with previous mechanistic studies of imine-linked COF powders. The separation of particle formation and growth processes offers control of particle size and may enable further improvements in crystallinity in the future.

Photoinduced, reversible phase transitions in all-inorganic perovskite nanocrystals.

Significant interest exists in lead trihalides that present the perovskite structure owing to their demonstrated potential in photovoltaic, lasing, and display applications. These materials are also notable for their unusual phase behavior often displaying easily accessible phase transitions. In this work, time-resolved X-ray diffraction, performed on perovskite cesium lead bromide nanocrystals, maps the lattice response to controlled excitation fluence. These nanocrystals undergo a reversible, photoinduced orthorhombic-to-cubic phase transition which is discernible at fluences greater than 0.34 mJ cm-2 through the loss of orthorhombic features and shifting of high-symmetry peaks. This transition recovers on the timescale of 510 ± 100 ps. A reversible crystalline-to-amorphous transition, observable through loss of Bragg diffraction intensity, occurs at higher fluences (greater than 2.5 mJ cm-2). These results demonstrate that light-driven phase transitions occur in perovskite materials, which will impact optoelectronic applications and enable the manipulation of non-equilibrium phase characteristics of the broad perovskite material class.

High aspect ratio nanotubes assembled from macrocyclic iminium salts.

One-dimensional nanostructures such as carbon nanotubes and actin filaments rely on strong and directional interactions to stabilize their high aspect ratio shapes. This requirement has precluded making isolated, long, thin organic nanotubes by stacking molecular macrocycles, as their noncovalent stacking interactions are generally too weak. Here we report high aspect ratio (>103), lyotropic nanotubes of stacked, macrocyclic, iminium salts, which are formed by protonation of the corresponding imine-linked macrocycles. Iminium ion formation establishes cohesive interactions that, in organic solvent (tetrahydrofuran), are two orders of magnitude stronger than the neutral macrocycles, as explained by physical arguments and demonstrated by molecular dynamics simulations. Nanotube formation stabilizes the iminium ions, which otherwise rapidly hydrolyze, and is reversed and restored upon addition of bases and acids. Acids generated by irradiating a photoacid generator or sonicating chlorinated solvents also induced nanotube assembly, allowing these nanostructures to be coupled to diverse stimuli, and, once assembled, they can be fixed permanently by cross-linking their pendant alkenes. As large macrocyclic chromonic liquid crystals, these iminium salts are easily accessible through a modular design and provide a means to rationally synthesize structures that mimic the morphology and rheology of carbon nanotubes and biological tubules.

Seeded growth of single-crystal two-dimensional covalent organic frameworks.

Polymerization of monomers into periodic two-dimensional networks provides structurally precise, layered macromolecular sheets that exhibit desirable mechanical, optoelectronic, and molecular transport properties. Two-dimensional covalent organic frameworks (2D COFs) offer broad monomer scope but are generally isolated as powders comprising aggregated nanometer-scale crystallites. We found that 2D COF formation could be controlled using a two-step procedure in which monomers are added slowly to preformed nanoparticle seeds. The resulting 2D COFs are isolated as single-crystalline, micrometer-sized particles. Transient absorption spectroscopy of the dispersed COF nanoparticles revealed improvement in signal quality by two to three orders of magnitude relative to polycrystalline powder samples, and suggests exciton diffusion over longer length scales than those obtained through previous approaches. These findings should enable a broad exploration of synthetic 2D polymer structures and properties.

Equilibration of Imine-Linked Polymers to Hexagonal Macrocycles Driven by Self-Assembly.

Macrocycles based on directional bonding and dynamic covalent bond exchange can be designed with specific pore shapes, sizes, and functionality. These systems retain many of the design criteria and desirable aspects of two-dimensional (2D) covalent organic frameworks (COFs) but are more easily processed. Here we access discrete hexagonal imine-linked macrocycles by condensing a truncated analogue of 1,3,5-tris(4-aminophenyl)benzene (TAPB) with terephthaldehyde (PDA). The monomers first condense into polymers but eventually convert into hexagonal macrocycles in high yield. The high selectivity for hexagonal macrocycles is enforced by their aggregation and crystallization into layered structures with more sluggish imine exchange. Their formation and exchange processes provide new insight into how imine-linked 2D COF simultaneously polymerize and crystallize. Solutions of these assembled macrocycles were cast into oriented, crystalline films, expanding the potential routes to 2D materials.