Covalent Organic Frameworks (COFs) are versatile two-dimensional (2D) materials for flexible electronics, catalysis, and sensing, owing to their tunable architectures and large surface areas. However, like most materials, COFs inevitably contain synthesis-induced defects, which-similar to graphene-can strongly influence intrinsic properties, such as thermal transport and mechanical strength. To address this challenge, we have assessed the performance of a set of machine learning interatomic potentials (MLIP) capable of efficient large-scale simulations of COFs with quantum accuracy. In doing so, QCOF models (Quantum COF) were developed by tuning the state-of-the-art MACE architecture on an extensive dataset of non-equilibrium COF conformations generated from high-fidelity density functional theory calculations. The accuracy, computational efficiency, memory footprint, and transferability to unseen chemical environments of these models were benchmarked against general-purpose MACE models and their fine-tuned variants. Our results show that an invariant QCOF model with a small descriptor dimensionality and cutoff outperforms all other models in most validation tasks, including scalability to large systems, force prediction in defective COFs, and phonon dispersion calculations. The best-performing QCOF model was then used to run large-scale simulations of thermal conductivity for defective CTF-1 and COF-LZU1 systems via non-equilibrium MD, revealing a more pronounced sensitivity of CTF-1 to structural defects. Stress-strain curves were also investigated, showing that the mechanical response remains nearly invariant at low defect densities, while asymmetric behaviour emerges at large strains. This work thus provides a foundation for the design of robust quantum-informed MLIP for large-scale property simulations of defective of extended network materials.

Two-dimensional polymers (2DPs), comprising mono- or multilayer covalent polymeric networks with long-range order in two orthogonal directions, are of considerable interest due to their unique physicochemical properties. However, achieving precise thickness control from monolayer to bilayer, crucial for exploring proximity effect-driven phenomena beyond the monolayer limit, remains synthetically challenging. Here we report the on-water surface synthesis of crystalline mechanically interlocked monolayer and bilayer 2DP (MI-M2DP and MI-B2DP) films by embedding macrocyclic molecules with one and two cavities into 2DP backbones. The incorporation of bulky macrocyclic molecules introduces periodic mechanical bonds that precisely control interlayer interlocking, enabling selective monolayer or bilayer 2DP formation. Both MI-M2DP and MI-B2DP exhibit homogeneous, large-area films with ordered hexagonal pores and high modulus. MI-B2DP demonstrates an exceptionally high effective Young’s modulus of 151 ± 16 GPa (indentation method), surpassing MI-M2DP (90 ± 14 GPa), van der Waals-stacked MI-M2DPs (46 ± 11 GPa) and other reported multilayer 2DPs (<50 GPa). Modelling confirms that the mechanical interlocking minimizes interlayer sliding and reinforces the structure. Mechanically interlocked monolayer and bilayer two-dimensional polymers (2DPs) are synthesized on the water surface by embedding macrocyclic molecules with one and two cavities into the backbones. The resulting bilayer 2DP displays a high effective Young’s modulus, exceeding other reported multilayer 2DPs.

Two-dimensional polymers (2DPs) are an interesting class of polymers due to their reticular synthetic assembly, which make them an ideal platform for designing materials with specific target properties. Predicting and understanding their elastic behavior is crucial for their application. However, a realistic calculation of their properties remains computationally challenging due to the ubiquitous presence of defects in synthesized 2DPs. Here, we introduce a coarse-graining (CG) approach based on elastic beams called MikadoRR with parameters extracted from a simple regression-based fitting. This approach allows us to accurately calculate the elastic properties of defective 2DPs up to the microscale. Furthermore, we show that design principles of 2DPs for tailored elastic properties can be derived from this CG model.

The synthesis of thin crystalline two-dimensional polymers (2DPs) typically relies on reversible dynamic covalent reactions. While substantial progress has been made in solution-based and interfacial syntheses, achieving 2DPs through irreversible carbon-carbon coupling reactions remains a formidable challenge. Herein, we present an on-liquid surface (a mixture of N,N-dimethylacetamide and water, DMAc-H2O) synthesis method for constructing diyne-linked 2DP (DY2DP) crystals via Glaser coupling, assisted by a perfluoro-surfactant (PFS) monolayer. In-situ spectroscopic and diffraction techniques reveal that the well-ordered PFS monolayer facilitates the accumulation of Cu+ ions and subsequent vertical coupling of acetylenic monomers on the DMAc-H2O surface. Building on these findings, we successfully synthesized micro-scale rod-shaped DY2DP-Por or graphdiyne (GDY) crystals through the polymerization of porphyrin- or benzene-based monomers, respectively. Our study represents a significant advancement in the field of on-liquid surface chemistry and opens up enormous opportunities for constructing C-C bond linked 2DP crystals with unique functionalities.

2D polymers have emerged as a highly promising category of nanomaterials, owing to their exceptional properties. However, the understanding of their fracture behavior and failure mechanisms remains still limited, posing challenges to their durability in practical applications. This work presents an in-depth study of the fracture kinetics of a 2D polyimine film, utilizing in situ tensile testing within a transmission electron microscope (TEM). Employing meticulously optimized transferring and patterning techniques, an elastic strain of ≈6.5% is achieved, corresponding to an elastic modulus of (8.6 ± 2.5) GPa of polycrystalline 2D polyimine thin films. In step-by-step fractures, multiple cracking events uncover the initiation and development of side crack near the main crack tip which toughens the 2D film. Simultaneously captured strain evolution through digital image correlation (DIC) analysis and observation on the crack edge confirm the occurrence of transgranular fracture patterns apart from intergranular fracture. A preferred cleavage orientation in transgranular fracture is attributed to the difference in directional flexibility along distinct orientations, which is substantiated by density functional-based tight binding (DFTB) calculations. These findings construct a comprehensive understanding of intrinsic mechanical properties and fracture behavior of an imine-linked polymer and provide insights and implications for the rational design of 2D polymers.

Preserving the superior mechanical properties of monolayer two-dimensional (2D) materials when transitioning to bilayer and layer-stacked structures poses a great challenge, primarily arising from the weak van der Waals (vdW) forces that facilitate interlayer sliding and decoupling. Here, we discover that mechanically interlocked 2D polymers (2DPs) offer a means for structural reinforcement from monolayer to bilayer. Incorporating macrocyclic molecules with one and two cavities into 2DPs backbones enables the precision synthesis of mechanically interlocked monolayer (MI-M2DP) and bilayer (MI-B2DP). Intriguingly, we have observed an exceptionally high effective Young's modulus of 222.4 GPa for MI-B2DP, surpassing those of MI-M2DP (130.1 GPa), vdW-stacked MI-M2DPs (2 MI-M2DP, 8.1 GPa) and other reported multilayer 2DPs. Modeling studies demonstrate the extraordinary effectiveness of mechanically interlocked structures in minimizing interlayer sliding ( 0.1 Å) and energy penalty (320 kcal/mol) in MI-B2DP compared to 2 MI-M2DP ( 1.2 Å, 550 kcal/mol), thereby suppressing mechanical relaxation and resulting in prominent structural reinforcement.

Conductive metal-organic frameworks (MOFs) are emerging electroactive materials for (opto)electronics; however, it is challenging to achieve MOF-based devices using existing synthesis methods. Here we develop an on-liquid-gallium surface synthesis (OLGSS) strategy under chemical vapour deposition conditions for the controlled growth of two-dimensional conjugated MOF (2D c-MOF) thin films, which gives a tenfold improvement in surface flatness compared with traditionally synthesized c-MOFs. The basis for constructing these flatter surfaces is a layer-by-layer chemical vapour deposition growth mode, which is triggered by the high adhesion energy between gallium and aromatic ligands. We demonstrate the generality of the OLGSS strategy by reproducing flat surfaces for nine different 2D c-MOF films with variable thicknesses (similar to 2-208 nm). Compared to traditionally synthesized MOF films, the resultant ultrasmooth films enable the formation of high-quality electrical contacts with contact resistance reduced by over 13-fold. Furthermore, due to the efficient interfacial interaction, the prepared van der Waals heterostructure of OLGSS c-MOF and MoS2 shows intriguing photoluminescence enhancement, photoluminescence peak shift and work function modulation. This robust OLGSS method provides the opportunity to develop MOF electronics and shows promise for the construction of multicomponent MOF-based heterostructure materials. The controlled growth of thin films of conjugated metal-organic frameworks is reported using an on-liquid-gallium surface synthesis strategy under chemical vapour deposition conditions. The surface flatness of the thin films is a tenfold improvement compared with samples synthesized by traditional routes.

Understanding the interaction mechanisms between volatile organic compounds (VOCs) with graphene-based materials is the primary and crucial step for human health and the advancement of digital olfaction. In this study, we investigated the adsorption behavior of four common odor molecules (toluene, ethanol, 2-Furfurylthiol, and guaiacol) on various graphene-based substrates, including pristine graphene, graphene doped with single graphitic-N atom (GR-N), and multiple pyrodinic-N atoms (1pd-N, 2pd-N, 3pd-N, and 4pd-N) using density functional theory. The adsorption energies and Bader charge analysis for all adsorption cases demonstrated that the molecules were weak physisorbed on all substrates. Through the work function change comparison, 2pd-N and N-gra presents likely promising sensing performance towards the odor molecules, while the selectivity declines by further introducing 3 and 4 pyrodinic-N atoms into graphene. The surface dipole moment analysis shed light on the underlying mechanism of work function change and explained the reduced sensitivity and selectivity observed for 3pd-N and 4pd-N, which can be attributed to a decrease in the molecule-induced dipole moment and increase in spatial charge redistribution. These findings could contribute to the fundamental understanding of odor molecule-graphene interactions and provide insights for the design and optimization of graphene-based electronic olfaction devices.

Changes in the work function provide a fingerprint to characterize analyte binding in charge transfer-based sensor devices. Hence, a rational sensor design requires a fundamental understanding of the microscopic factors controlling the modification of the work function. In the current investigation, we address the mechanisms behind the work function change (WFC) for the adsorption of four common volatile organic compounds (toluene, ethanol, 2-Furfurylthiol, and guaiacol) on different nitrogen-doped graphene-based 2D materials using density functional theory. We show that competition between the surface dipole moment change induced by spatial charge redistribution, the one induced by the pure adsorbate, and the one caused by the surface deformation can quantitatively predict the work function change. Furthermore, we also show this competition can explain the non-growing work function change behavior in the increasing concentrations of nitrogen-doped graphenes. Finally, we propose possible design principles for WFC of VOCs interacting with N-doped graphene materials.

The Hofstadter butterfly is one of the first and most fascinating examples of the fractal and self-similar quantum nature of free electrons in a lattice pierced by a perpendicular magnetic field. However, the direct experimental verification of this effect on single-layer materials is still missing as very strong and inaccessible magnetic fields are necessary. For this reason, its indirect experimental verification has only been realized in artificial periodic 2D systems, like moiré lattices. The only recently synthesized 2D covalent organic frameworks might circumvent this limitation: Due to their large pore structures, magnetic fields needed to detect most features of the Hofstadter butterfly are indeed accessible with today technology. This work opens the door to make this exotic and theoretical issue from the 70s measurable and might solve the quest for the experimental verification of the Hofstadter butterfly in single-layer materials. Moreover, the intrinsic hierarchy of different pore sizes in 2D covalent organic framework adds additional complexity and beauty to the original butterflies and leads to a direct accessible playground for new physical observations.

A series of hybrid organic–inorganic perovskites, based on naphthalene ammonium cations have been investigated. All materials exhibit edge- or face-sharing 1D chain structures and broadband light emission arising from the self-trapped excitons.

Controlling site-selectivity and reactivity in chemical reactions continues to be a key challenge in modern synthetic chemistry. Here, we demonstrate the discovery of site-selective chemical reactions on the water surface via a sequential assembly approach. A negatively charged surfactant monolayer on the water surface guides the electrostatically driven, epitaxial, and aligned assembly of reagent amino-substituted porphyrin molecules, resulting in a well-defined J-aggregated structure. This constrained geometry of the porphyrin molecules prompts the subsequent directional alignment of the perylenetetracarboxylic dianhydride reagent, enabling the selective formation of a one-sided imide bond between porphyrin and reagent. Surface-specific in-situ spectroscopies reveal the underlying mechanism of the dynamic interface that promotes multilayer growth of the site-selective imide product. The site-selective reaction on the water surface is further demonstrated by three reversible and irreversible chemical reactions, such as imide-, imine-, and 1, 3-diazole (imidazole)- bonds involving porphyrin molecules. This unique sequential assembly approach enables site-selective chemical reactions that can bring on-water surface synthesis to the forefront of modern organic chemistry.

The interest in two-dimensional conjugated polymers (2D CPs) has increased significantly in recent years. In particular, vinylene-linked 2D CPs with fully in-plane sp(2)-carbon-conjugated structures, high thermal and chemical stability, have become the focus of attention. Although the Horner-Wadsworth-Emmons (HWE) reaction has been recently demonstrated in synthesizing vinylene-linked 2D CPs, it remains largely unexplored due to the challenge in synthesis. In this work, we reveal the control of crystallinity of 2D CPs during the solvothermal synthesis of 2D-poly(phenylenequinoxaline-vinylene)s (2D-PPQVs) and 2D-poly(phenylenevinylene)s through the HWE polycondensation. The employment of fluorinated phosphonates and rigid aldehyde building blocks is demonstrated as crucial factors in enhancing the crystallinity of the obtained 2D CPs. Density functional theory (DFT) calculations reveal the critical role of the fluorinated phosphonate in enhancing the reversibility of the (semi)reversible C-C single bond formation.

Two-dimensional covalent organic frameworks (2D COFs) are an interesting class of 2D materials since their reticular synthesis allows the tailored design of structures and functionalities. For many of their applications the mechanical stability and performance is an important aspect. Here, we use a computational approach involving a density-functional based tight-binding method to calculate the in-plane elastic properties of about 40 COFs with a honeycomb lattice. Based on those calculations, we develop two coarse-grained descriptions: one based on a spring network and the second using a network of elastic beams. The models allow us to connect the COF force constants to the molecular force constants of the linker molecules and thus enable an efficient description of elastic deformations. To illustrate this aspect, we calculate the deformation energy of different COFs containing the equivalent of a Stone-Wales defect and find very good agreement with the coarse-grained description.

Computerpraktika stellen einen wichtigen Bestandteil vieler Lehrveranstaltungen dar, welche die Grundlagen und Details von computergestützten Methoden vermitteln sollen. In den Materialwissenschaften spielen solche Methoden eine zunehmend wichtige Rolle. Typischerweise setzen die Praktika eine physische Präsenz in den PC Pools voraus, u.a. da eine Vielzahl von verschiedenen Programmen lokal installiert und bereitgestellt werden muss. Um Computerpraktika auch in der Online-Lehre vollumfänglich und weitestgehend unabhängig von den Gegebenheiten der Studierenden einsetzen zu können, wurde im Wintersemester 2020/21 ein virtueller PC Pool auf Basis von virtuellen Maschinen mit Web-basiertem Zugang eingerichtet. Dieser virtuelle PC Pool wurde in verschiedenen Lehrveranstaltungen erfolgreich eingesetzt und kann auch bei hybriden Lehrformaten in verschiedenen Disziplinen verwendet werden. Computer practicals are an important part of many courses that are designed to teach the basics and details of computer-based methods. In the materials sciences, such methods play an increasingly important role. Typically, the practical courses require a physical presence in the PC pools, among other things because a large number of different programmes have to be installed and made available locally. In order to be able to use computer practicals fully and as far as possible independently of the students' circumstances in online teaching, a virtual PC pool based on virtual machines with web-based access was set up in the winter semester 2020/21. This virtual PC pool has been successfully used in various courses and can also be used in hybrid teaching formats in various disciplines. *Corresponding author: gianaurelio.cuniberti@tu-dresden.de This article was originally submitted in German. S. Kampmann et al. / Virtual PC pools for computer practicals 1/2.17-2 Lessons Learned | Volume 1 (2021) | Issue 1&2

Computerpraktika stellen einen wichtigen Bestandteil vieler Lehrveranstaltungen dar, welche die Grundlagen und Details von computergestützten Methoden vermitteln sollen. In den Materialwissenschaften spielen solche Methoden eine zunehmend wichtige Rolle. Typischerweise setzen die Praktika eine physische Präsenz in den PC Pools voraus, u.a. da eine Vielzahl von verschiedenen Programmen lokal installiert und bereitgestellt werden muss. Um Computerpraktika auch in der Online-Lehre vollumfänglich und weitestgehend unabhängig von den Gegebenheiten der Studierenden einsetzen zu können, wurde im Wintersemester 2020/21 ein virtueller PC Pool auf Basis von virtuellen Maschinen mit Web-basiertem Zugang eingerichtet. Dieser virtuelle PC Pool wurde in verschiedenen Lehrveranstaltungen erfolgreich eingesetzt und kann auch bei hybriden Lehrformaten in verschiedenen Disziplinen verwendet werden.

Hydrogen bonds are of great scientific interest, determining the free energy landscape and hence chemical and physical properties of many materials systems, for example, the hybrid organic-inorganic perovskites. Although these interactions are critical, understanding them is difficult in complex, multi-component systems; hydrogen halides are ideal as simple binary model compounds for understanding the role of hydrogen bonding in physical properties like phase transitions. Here we investigate the orthorhombic low-temperature phase and the cubic high-temperature phase in HX (X = F, Cl, Br, or I) systems to understand how different hydrogen-halide bonds influence free energy profiles. We show that hydrogen fluoride has a qualitatively different behavior due to strong hydrogen bonding and hence a very different vibrational entropy. Heavier halides are in contrast rather similar in their physical properties; however, dispersion interactions play a more crucial role in these. These results have implications for the rational design of materials with hydrogen-halide bonds and tuning material properties in systems like mixed anion CH3NH3PbX3 perovskites.

The modular building principle of metal-organic frameworks (MOFs) presents an excellent platform to explore and establish structure-property relations that tie microscopic to macroscopic properties. Negative thermal expansion (NTE) is a common phenomenon in MOFs and is often ascribed to collective motions that can move through the structure at sufficiently low energies. Here, we show that the introduction of additional linkages in a parent framework, retrofitting, is an effective approach to access lattice dynamics experimentally, in turn providing researchers with a tool to alter the NTE behavior in MOFs. By introducing TCNQ (7,7,8,8-tetracyanoquinodimethane) into the prototypical MOF Cu3BTC2 (BTC = 1,3,5-benzenetricarboxylate; HKUST-1), NTE can be tuned between αV = -15.3 × 10-6 K-1 (Cu3BTC2) and αV = -8.4 × 10-6 K-1 (1.0TCNQ@Cu3BTC2). We ascribe this phenomenon to a general stiffening of the framework as a function of TCNQ loading due to additional network connectivity, which is confirmed by computational modeling and far-infrared spectroscopy. Our findings imply that retrofitting is generally applicable to MOFs with open metal sites, opening yet another way to fine-tune properties in this versatile class of materials.

The post-synthetic installation of linker molecules between open-metal sites (OMSs) and undercoordinated metal-nodes in a metal-organic framework (MOF) - retrofitting - has recently been discovered as a powerful tool to manipulate macroscopic properties such as the mechanical robustness and the thermal expansion behavior. So far, the choice of cross linkers (CLs) that are used in retrofitting experiments is based on qualitative considerations. Here, we present a low-cost computational framework that provides experimentalists with a tool for evaluating various CLs for retrofitting a given MOF system with OMSs. After applying our approach to the prototypical system CL@Cu3BTC2 (BTC = 1,3,5-benzentricarboxylate) the methodology was expanded to NOTT-100 and NOTT-101 MOFs, identifying several promising CLs for future CL@NOTT-100 and CL@NOTT-101 retrofitting experiments. The developed model is easily adaptable to other MOFs with OMSs and is set-up to be used by experimentalists, providing a guideline for the synthesis of new retrofitted MOFs with modified physicochemical properties.