Understanding the structural and dynamic factors that govern the formation of covalent and non-covalent organic 2D crystalline materials is key to controlling their quality, including defect density and lateral dimensions. Gaining such insight enables the rational tuning of their physicochemical properties.
In this contribution, we showcase how scanning probe microscopy—particularly scanning tunneling microscopy (STM) and atomic force microscopy (AFM)—can be leveraged to investigate the structure and evolution of substrate-supported metal–organic frameworks (sMOFs) and covalent organic frameworks (sCOFs) at the liquid–solid interface.
We outline approaches for tracking and steering the growth of sMOFs and sCOFs across diverse scenarios: from achiral to chiral, from single layers to multilayer architectures, and from monocomponent to multicomponent systems. Beyond offering high-resolution visualization, scanning probe microscopy can also play an active role in directing on-surface growth processes, with very high spatial resolution.
Precisely tailoring the macroscopic morphology of covalent organic frameworks (COFs) fundamentally drives their physicochemical properties. However, the robust and highly directional nature of covalent bonds makes such control at the single-crystal level a formidable challenge. Resolving this bottleneck, we establish a synergistic Brønsted and Lewis dual-acid catalytic strategy to dictate the controlled axial growth and morphological evolution of large (≥50 μm) three-dimensional(3D) COFs single crystals (the XNU-375-X; X = 1-6, a, p, EtOH) featuring a dia topology. Modulating the concentration of dysprosium trifluoromethanesulfonate (Dy(OTf)3), acting as the Lewis acid, drastically suppresses the twinning rate. Consequently, this targeted regulation drives a continuous morphological transition from octahedral to tetragonal bipyramidal geometries. Single-crystal X-ray diffraction (SCXRD) explicitly confirms the microscopic structural consistency throughout this macroscopic evolution. Crucially, during guest solvent removal and exchange, these crystallographic analyses directly capture a rare structural flexibility and dynamic "breathing" effect, evidenced by a massive 46% volume variation. Density functional theory (DFT) calculations elucidate the underlying growth kinetics. Conditional on the specific exposed facets ({100} versus {001}), Dy3+ exhibits differential adsorption behaviors that effectively passivate lateral free amine sites. To the extent that these sites govern horizontal proliferation, this selective binding simultaneously promotes ordered c-axis stacking and intrinsic self-correction. Ultimately, this work bridges the gap in the precision morphological tailoring of 3D COFs single crystals, providing a robust platform for the anisotropic growth and targeted synthesis of complex porous architectures.
Shijiao Fan, Chenyang Yan, Hanzhen Liu et al.· Journal of the American Chem...· 1 citation
The on-surface synthesis of atomically precise carbon nanomaterials has emerged as a powerful strategy to overcome the limitations of solution-based chemistry, enabling the fabrication of low-dimensional polymers and networks with unprecedented structural control. In particular, the development of single-layer two-dimensional (2D) covalent organic frameworks (COFs) remains a central challenge due to their promising applications in a variety of fields, as sensing, catalysis, and (opto)electronics. [1,2]
Here, we report a stepwise on-surface synthetic approach that exploits steric hindrance and sequential thermal activation to engineer and transform two COFs with atomic precision by scanning probe techniques. An ex-professo designed molecular precursor, functionalized with gem-dibromoalkene groups and a phenanthroline moiety, is employed to guide controlled coupling reactions under ultra-high vacuum conditions. Initial debromination leads to the formation of sterically controlled one-dimensional covalent chains, which self-assemble into a two-dimensional supramolecular network. Subsequent annealing yields a 2D organometallic network. Further thermal activation drives carbon–carbon coupling reactions between wires to form a 2D-COF composed of linear chains connected through ethynylene bridges. Remarkably, continued annealing triggers an unprecedented COF-to-COF transformation, in which the ethynylene linkages convert into antiaromatic pentalene moieties, demonstrating in situ chemical transformations of 2D-COFs in a stepwise manner.
This work establishes a general strategy for the bottom-up synthesis and thermal transformation of 2D-COFs at interfaces, validating the combination of steric control and sequential reactions as a viable alternative to symmetric precursor design, and opens new avenues for tailoring the structure and electronic properties of atomically precise 2D-COFs by thermal stimuli.
[1] R.-R. Liang et al. "Two-Dimensional Covalent Organic Frameworks With Hierarchical Porosity" Chem. Soc. Rev. 2020, 49, 3920
[2] C. Wang et al. " 2D Covalent Organic Frameworks: From Synthetic Strategies to Advanced Optical-Electrical-Magnetic Functionalities" Adv. Mater. 2022, 34, 2102290
Ana Barragán, Elena Pérez Elvira, Diego J. Vicent et al.· ECS Meeting Abstracts· 0 citations
Metal-organic frameworks (MOFs) provide powerful templates for constructing porous inorganic materials, yet the nanoscale thermal conversion pathways remain unclear. We reveal the mechanistic origins of structure-property emergence during thermal transformation of Cu-BTC (HKUST-1 MOF) into nanoporous copper oxides, uncovering transient nanocomposite states that dictate the mechanical and optoelectronic properties. By integrating various characterization techniques, we establish a unified multiscale framework linking phase evolution, pore architecture, nanomechanics, and electronic structure. Controlled calcination (300°C-500°C) transforms MOF to nanoporous oxide with non-monotonic mechanical evolution. At 300°C, partial decomposition produces a heterogeneous Cu2O/CuO-carbon nanocomposite that retains mesostructural connectivity and exhibits high local Young's modulus (∼41 GPa). This reinforcement arises from residual carbon networks, heterophase interfaces, and mesoporosity. At 500°C, phase-pure porous CuO forms with improved optical absorption but reduced stiffness due to pore coarsening and sintering. These results reveal that functional performance in MOF-derived oxides is not simply governed by bulk phase composition but by transient nanoscale connectivity and heterogeneity formed during partial conversion. This establishes a structure-property-function paradigm for MOF-templated oxides, demonstrating that controlled intermediate states provide a powerful route for engineering mechanically robust and optoelectronically active nanoporous semiconductors for catalysis, sensing, and energy conversion applications.
The multilevel assembly of conjugated polymers critically determines their electronic performance. However, establishing a direct correlation from molecular-level interactions to macroscopic morphology and charge transport properties remains challenging. In this work, advanced characterization techniques were employed to identify and visualize two distinct assembly processes of a representative n-type conjugated polymer, F4BDOPV-2T, in both solution and thin film states, allowing for a structural correlation between solvent affinities and assembly structures. In 1-chloronaphthalene, strong solvation of the conjugated backbones promoted sidechain-dominated growth of nanoscale assemblies and ordered lamellar packing, resulting in compact 3D cluster aggregates in solution and well-oriented, large-scale ordered crystalline grains in the solid state. Conversely, toluene favored sidechain solvation, leading to backbone-dominated assembly growth that formed fiber networks and interconnected crystalline grains. The superior grain connectivity during backbone-dominated growth outweighed the effect of smaller grain size, yielding a three-fold enhancement in electron mobility (μe = 2.05 cm2 V–1 s–1). Through real-space electron microscopy visualization, this work bridges intermolecular interactions with multilevel assembly structures of conjugated polymers, providing fundamental insights into rational solvent regulation and morphological control for high-performance organic electronics.
Nai-Fu Liu, Li Ding, Xiao-Yan Zhang et al.· Macromolecules· 0 citations
Silver nanocubes have emerged as a model system for understanding how surface chemistry governs shape-controlled synthesis and shape preservation, in addition to their widespread use in a range of applications. This review discusses our recent progress in directly probing the surface of Ag nanocubes, with emphasis on surface-enhanced Raman scattering studies carried out under actual experimental conditions. We first discuss how the native surface is established during polyol synthesis, where chemisorbed chloride and poly(vinylpyrrolidone) act cooperatively to define and stabilize the cubic shape. We then show that sample collection, washing, and redispersion also reorganize the surface and determine whether the Ag nanocubes can retain their shape during storage. We also examine aqueous seed-mediated growth, in which rapid ligand exchange, transient AgCl formation, interfacial electron transfer, and surface reduction collectively govern the enlargement of Ag nanocubes. Taken together, these studies provide a mechanistic framework for rational synthesis of colloidal metal nanocrystals with controlled sizes and shapes.
Qijia Huang, Younan Xia· Journal of Materials Researc...· 0 citations
The advancement of stretchable organic photovoltaics is hindered by a fundamental trade-off between high optoelectronic performance and mechanical durability, in particular, a challenge rooted in the complex microstructure of conjugated polymers and their blends. This review addresses a critical literature gap on the microstructure of various conjugated polymer systems by systematically examining how multiscale microstructural evolution, from solution-state aggregation to solid-state morphology and dynamic response under strain, can be characterized, understood, and rationally controlled. Analysis begins with how advanced X-ray and neutron scattering techniques elucidate the formative solution-aggregation structures that dictate the final solid-state morphology. Subsequently, multiscale characterization methods for thin films are reviewed, alongside material design strategies, such as elastomeric modifiers, engineered to optimize phase-separated morphologies for simultaneous conductivity and mechanical flexibility. A key focus is the transition beyond static X-ray scattering analysis to explore the dynamic evolution of microstructure under mechanical stretching, thereby underscoring the pivotal role of in situ and operando characterization in revealing deformation and failure mechanisms. By integrating insights across these scales, this review aims to distill rational design principles, offer a coherent processing-structure-performance framework, and provide a complete picture for the predictive design of next-generation intrinsically robust stretchable photovoltaics and related electronics.
M. Gao, Junjiang Wu, Yanhou Geng et al.· Advances in Materials· 0 citations