Bottom-Up Synthesis of Crystalline Covalent Organic Framework Nanosheets, Nanotubes, and Kippah Vesicles: An Odd-Even Effect Induction.

Few-layer organic nanosheets are becoming increasingly attractive as two-dimensional (2D) materials due to their precise atomic connectivity and tailor-made pores. However, most strategies for synthesizing nanosheets rely on surface-assisted methods or top-down exfoliation of stacked materials. A bottom-up approach with well-designed building blocks would be the convenient pathway to achieve the bulk-scale synthesis of 2D nanosheets with uniform size and crystallinity. Herein, we have synthesized crystalline covalent organic framework nanosheets (CONs) by reacting tetratopic thianthrene tetraaldehyde (THT) and aliphatic diamines. The bent geometry of thianthrene in THT retards the out-of-plane stacking, while the flexible diamines introduce dynamic characteristics into the framework, facilitating nanosheet formation. Successful isoreticulation with five diamines with two to six carbon chain lengths generalizes the design strategy. Microscopic imaging reveals that the odd and even diamine-based CONs transmute to different nanostructures, such as nanotubes and hollow spheres. The single-crystal X-ray diffraction structure of repeating units indicates that the odd-even linker units of diamines introduce irregular-regular curvature in the backbone, aiding such dimensionality conversion. Theoretical calculations shed more light on nanosheet stacking and rolling behavior with respect to the odd-even effects.

Covalent Organic Framework Thin-Film Photodetectors from Solution-Processable Porous Nanospheres.

The synthesis of homogeneous covalent organic framework (COF) thin films on a desired substrate with decent crystallinity, porosity, and uniform thickness has great potential for optoelectronic applications. We have used a solution-processable sphere transmutation process to synthesize 300 ± 20 nm uniform COF thin films on a 2 × 2 cm2 TiO2-coated fluorine-doped tin oxide (FTO) surface. This process controls the nucleation of COF crystallites and molecular morphology that helps the nanospheres to arrange periodically to form homogeneous COF thin films. We have synthesized four COF thin films (TpDPP, TpEtBt, TpTab, and TpTta) with different functional backbones. In a close agreement between the experiment and density functional theory, the TpEtBr COF film showed the lowest optical band gap (2.26 eV) and highest excited-state lifetime (8.52 ns) among all four COF films. Hence, the TpEtBr COF film can participate in efficient charge generation and separation. We constructed optoelectronic devices having a glass/FTO/TiO2/COF-film/Au architecture, which serves as a model system to study the optoelectronic charge transport properties of COF thin films under dark and illuminated conditions. Visible light with a calibrated intensity of 100 mW cm-2 was used for the excitation of COF thin films. All of the COF thin films exhibit significant photocurrent after illumination with visible light in comparison to the dark. Hence, all of the COF films behave as good photoactive substrates with minimal pinhole defects. The fabricated out-of-plane photodetector device based on the TpEtBr COF thin film exhibits high photocurrent density (2.65 ± 0.24 mA cm-2 at 0.5 V) and hole mobility (8.15 ± 0.64 ×10-3 cm2 V-1 S-1) compared to other as-synthesized films, indicating the best photoactive characteristics.

Electronic and Excitonic Properties of MSi2 Z4 Monolayers.

MA2 Z4 monolayers form a new class of hexagonal non-centrosymmetric materials hosting extraordinary spin-valley physics. While only two compounds (MoSi2 N4 and WSi2 N4 ) are recently synthesized, theory predicts interesting (opto)electronic properties of a whole new family of such two-dimensional (2D) materials. Here, the chemical trends of band gaps and spin-orbit splittings of bands in selected MSi2 Z4 (M = Mo, W; Z = N, P, As, Sb) compounds are studied from first-principles. Effective Bethe-Salpeter-equation-based calculations reveal high exciton binding energies. Evolution of excitonic energies under external magnetic field is predicted by providing their effective g-factors and diamagnetic coefficients, which can be directly compared to experimental values. In particular, large positive g-factors are predicted for excitons involving higher conduction bands. In view of these predictions, MSi2 Z4 monolayers yield a new platform to study excitons and are attractive for optoelectronic devices, also in the form of heterostructures. In addition, a spin-orbit induced bands inversion is observed in the heaviest studied compound, WSi2 Sb4 , a hallmark of its topological nature.

Structure-Imposed Electronic Topology in Cove-Edged Graphene Nanoribbons.

In cove-edged zigzag graphene nanoribbons (ZGNR-Cs), one terminal CH group per length unit is removed on each zigzag edge, forming a regular pattern of coves that controls their electronic structure. Based on three structural parameters that unambiguously characterize the atomistic structure of ZGNR-Cs, we present a scheme that classifies their electronic state (i.e., if they are metallic, topological insulators, or trivial semiconductors) for all possible widths N, unit lengths a, and cove position offsets at both edges b, thus showing the direct structure-electronic structure relation. We further present an empirical formula to estimate the band gap of the semiconducting ribbons from N, a, and b. Finally, we identify all geometrically possible ribbon terminations and provide rules to construct ZGNR-Cs with a well-defined electronic structure.

Shedding Light on the Enigmatic TcO2 ⋠xH2 O Structure with Density Functional Theory and EXAFS Spectroscopy.

The ß-emitting 99 Tc isotope is a high-yield fission product in 235 U and 239 Pu nuclear reactors, raising special concern in nuclear waste management due to its long half-life and the high mobility of pertechnetate (TcO4 - ). Under the conditions of deep nuclear waste repositories, Tc is retained through biotic and abiotic reduction of TcO4 - to compounds like amorphous TcO2 ⋅ xH2 O precipitates. It is generally accepted that these precipitates have linear (Tc(µ-O)2 (H2 O)2 )n chains, with trans H2 O. Although corresponding Tc-Tc and Tc-O distances have been obtained from extended X-ray absorption fine structure (EXAFS) spectroscopy, this structure is largely based on analogy with other compounds. Here, we combine density-functional theory with EXAFS measurements of fresh and aged samples to show that, instead, TcO2 ⋅ xH2 O forms zigzag chains that undergo a slow aging process whereby they combine to form longer chains and, later, a tridimensional structure that might lead to a new TcO2 polymorph.

Atomically Dispersed Pentacoordinated-Zirconium Catalyst with Axial Oxygen Ligand for Oxygen Reduction Reaction.

Single-atom catalysts (SACs), as promising alternatives to Pt-based catalysts, suffer from the limited choice of center metals and low single-atom loading. Here, we report a pentacoordinated Zr-based SAC with nontrivial axial O ligands (denoted O-Zr-N-C) for oxygen reduction reaction (ORR). The O ligand downshifts the d-band center of Zr and confers Zr sites with stable local structure and proper adsorption capability for intermediates. Consequently, the ORR performance of O-Zr-N-C prominently surpasses that of commercial Pt/C, achieving a half-wave potential of 0.91 V vs. reversible hydrogen electrode and outstanding durability (92 % current retention after 130-hour operation). Moreover, the Zr site shows good resistance towards aggregation, enabling the synthesis of Zr-based SAC with high loading (9.1 wt%). With the high-loading catalyst, the zinc-air battery (ZAB) delivers a record-high power density of 324 mW cm-2 among those of SAC-based ZABs.

Tuning Valleys and Wave Functions of van der Waals Heterostructures by Varying the Number of Layers: A First-Principles Study.

In van der Waals heterostructures of 2D transition-metal dichalcogenides (2D TMDCs) electron and hole states are spatially localized in different layers forming long-lived interlayer excitons. Here, the influence of additional electron or hole layers on the electronic properties of a MoS2 /WSe2 heterobilayer (HBL), which is a direct bandgap material, is investigated from first principles. Additional layers modify the interlayer hybridization, mostly affecting the quasiparticle energy and real-space extend of hole states at the Γ and electron states at the Q valleys. For a sufficient number of additional layers, the band edges move from K to Q or Γ, respectively. Adding electron layers to the HBL leads to more delocalized K and Q states, while Γ states do not extend much beyond the HBL, even when more hole layers are added. These results suggest a simple and yet powerful way to tune band edges and the real-space extent of the electron and hole wave functions in TMDC heterostructures, potentially affecting strongly the lifetime and dynamics of interlayer excitons.

Twist-angle-dependent interlayer exciton diffusion in WS2-WSe2 heterobilayers.

The nanoscale periodic potentials introduced by moiré patterns in semiconducting van der Waals heterostructures have emerged as a platform for designing exciton superlattices. However, our understanding of the motion of excitons in moiré potentials is still limited. Here we investigated interlayer exciton dynamics and transport in WS2-WSe2 heterobilayers in time, space and momentum domains using transient absorption microscopy combined with first-principles calculations. We found that the exciton motion is modulated by twist-angle-dependent moiré potentials around 100 meV and deviates from normal diffusion due to the interplay between the moiré potentials and strong exciton-exciton interactions. Our experimental results verified the theoretical prediction of energetically favourable K-Q interlayer excitons and showed exciton-population dynamics that are controlled by the twist-angle-dependent energy difference between the K-Q and K-K excitons. These results form a basis to investigate exciton and spin transport in van der Waals heterostructures, with implications for the design of quantum communication devices.

Topological two-dimensional polymers.

There are more than 200 two-dimensional (2D) networks with different topologies. The structural topology of a 2D network defines its electronic structure. Including the electronic topological properties, it gives rise to Dirac cones, topological flat bands and topological insulators. In this Tutorial Review, we show how electronic properties of 2D networks can be calculated by means of a tight-binding approach, and how these properties change when 2nd-neighbour interactions and spin-orbit coupling are included. We explain how to determine whether or not the resulting electronic features have topological signatures by calculation of Chern numbers, Z2 invariants, and by the nanoribbon approach. This tutorial gives suggestions how such topological properties could be realized in explicit atomistic chemical 2D systems made of molecular frameworks, in particular in 2D polymers, where the edges and vertices of a given 2D net are substituted by properly selected molecular building blocks and stitched together in such a way that long-range π-conjugation is retained.

Identification of Prime Factors to Maximize the Photocatalytic Hydrogen Evolution of Covalent Organic Frameworks.

Visible-light-driven hydrogen (H2) production from water is a promising strategy to convert and store solar energy as chemical energy. Covalent organic frameworks (COFs) are front runners among different classes of organic photocatalysts. The photocatalytic activity of COFs depends on numerous factors such as the electronic band gap, crystallinity, surface area, exciton migration, stability of transient species, charge separation and transport, etc. However, it is challenging to fine tune all of these factors simultaneously to enhance the photocatalytic activity. Hence, in this report, an effort has been made to understand the interplay of these factors and identify the key factors for efficient photocatalytic H2 production through a structure-property-activity relationship. Careful molecular engineering allowed us to optimize all of the above plausible factors impacting the overall catalytic activities of a series of isoreticular COFs. The present study determines three prime factors: light absorption, charge carrier generation, and its transport, which influence the photocatalytic H2 production of COFs to a much greater extent than the other factors.

Dithiol-Dithione Tautomerism of 2,3-Pyrazinedithiol in the Synthesis of Copper and Silver Coordination Compounds.

A promising strategy for new electrically conductive coordination polymers is the combination of d10 metal ions, which tolerate short metal···metal distances, with dithiolene linkers, known for their "non-innocent" redox behavior. This study explores the coordination chemistry of 2,3-pyrazinedithiol (H2pdt) toward Cu+ and Ag+ ions, highlighting similarities and differences. The synthetic approach, starting with the fully protonated ligand, allowed the isolation of a homoleptic bis(dithiolene) complex with formal CuI atoms, [Cu(H2pdt)2]Cl (1). This complex was further transformed to a 1D coordination polymer with short metal···metal distances, 1D[Cu(Hpdt)] (2Cu). The larger Ag+ ion directly built up a very similar coordination polymer, 1D[Ag(Hpdt)] (2Ag), without any appearance of an intermediate metal complex. The coordination polymer 1D[Cu(H2pdt)I] (4), like complex 1, bears fully protonated H2pdt ligands in their dithione form. Upon heating, both compounds underwent auto-oxidation coupled with a dehydrogenation of the ligand to form the open-shell neutral copper(II) complex [Cu(Hpdt)2] (3) and the coordination polymer 1D[Cu2I2(H2pdt)(Hpdt)] (5), respectively. For all presented compounds, crystal structures are discussed in-depth. Furthermore, properties of 1, 3, and those of the three 1D coordination polymers, 2Ag, 2Cu, and 4, were investigated by UV-vis-NIR spectroscopy, cyclic voltammetry, and variable-temperature magnetic susceptibility, and direct current (dc)-conductivity measurements. The experimental results are compared and discussed with the aid of DFT simulations.

Analytical approach to phonon calculations in the SCC-DFTB framework.

Detailed derivation of the analytical, reciprocal-space approach of Hessian calculation within the self-consistent-charge density-functional based tight-binding framework (SCC-DFTB) is presented. This approach provides an accurate and efficient way for obtaining the SCC-DFTB Hessian of periodic systems. Its superiority with respect to the traditional numerical force differentiation method is demonstrated for doped graphene, graphene nanoribbons, boron-nitride nanotubes, bulk zinc-oxide, and other systems.

Two-Dimensional Noble-Metal Chalcogenides and Phosphochalcogenides.

Noble-metal chalcogenides, dichalcogenides, and phosphochalcogenides are an emerging class of two-dimensional materials. Quantum confinement (number of layers) and defect engineering enables their properties to be tuned over a broad range, including metal-to-semiconductor transitions, magnetic ordering, and topological surface states. They possess various polytypes, often of similar formation energy, which can be accessed by selective synthesis approaches. They excel in mechanical, optical, and chemical sensing applications, and feature long-term air and moisture stability. In this Minireview, we summarize the recent progress in the field of noble-metal chalcogenides and phosphochalcogenides and highlight the structural complexity and its impact on applications.

On the Chemistry and Diffusion of Hydrogen in the Interstitial Space of Layered Crystals h-BN, MoS2 , and Graphite.

Recent experiments have demonstrated transport and separation of hydrogen isotopes through the van der Waals gap in hexagonal boron nitride and molybdenum disulfide bulk layered materials. However, the experiments cannot distinguish if the transported particles are protons (H+ ) or protium (H) atoms. Here, reported are the theoretical studies, which indicate that protium atoms, rather than protons, are transported through the gap. First-principles calculations combined with well-tempered metadynamics simulations at finite temperature reveal that for h-BN and MoS2 , the diffusion mechanism of both protons and protium (H) atoms involves a hopping process between adjacent layers. This process is assisted by low-energy phonon shear modes. The extracted diffusion coefficient of protium matches the experiment, while for protons it is several orders of magnitude smaller. This indicates that protium atoms are responsible for the experimental observations. These results allow for a comprehensive interpretation of experimental results on the transport of hydrogen isotopes through van der Waals gaps and can help identify other materials for hydrogen isotope separation applications.

Bridging the Green Gap: Metal-Organic Framework Heteromultilayers Assembled from Porphyrinic Linkers Identified by Using Computational Screening.

In organic photovoltaics, porphyrins (PPs) are among the most promising compounds owing to their large absorption cross-section, wide spectral range, and stability. Nevertheless, a precise adjustment of absorption band positions to reach a full coverage of the so-called green gap has not been achieved yet. We demonstrate that a tuning of the PP Q- and Soret bands can be carried out by using a computational approach for which substitution patterns are optimized in silico. The most promising candidate structures were then synthesized. The experimental UV/Vis data for the solvated compounds were in excellent agreement with the theoretical predictions. By attaching further functionalities, which allow the use of PP chromophores as linkers for the assembly of metal-organic frameworks (MOFs), we were able to exploit packing effects resulting in pronounced redshifts, which allowed further optimization of the photophysical properties of PP assemblies. Finally, we use a layer-by-layer method to assemble the PP linkers into surface-mounted MOFs (SURMOFs), thus obtaining high optical quality, homogeneous and crystalline multilayer films. Experimental results are in full accord with the calculations, demonstrating the huge potential of computational screening methods in tailoring MOF and SURMOF photophysical properties.

Conformational isomerism controls collective flexibility in metal-organic framework DUT-8(Ni).

Metal-organic frameworks (MOFs) are coordination networks with organic ligands containing potential voids. Some MOFs show pronounced structural flexibility that may result in closing and re-opening these pores. Here, we show that collective flexibility in a MOF-DUT-8(Ni) - is controlled by conformational isomerism. DUT-8(Ni), a pillared-layer MOF with Ni2 paddle-wheels, dabco pillars and naphthalene dicarboxylate (ndc) linkers, can crystallize in many conformational isomers that depend on the orientation of the non-linear ndc linkers with respect to each other. While the open form is compatible with several of these conformations, only one of them, with alternating linker orientations, is stable as the closed form. We show, by means of first principles calculations, that in the stable closed form, the appreciable lattice strain is compensated by London-dispersion forces between the ndc linkers that arrange with maximum overlap in a stacking order similar to the stacking in graphite. We substantiate these results by well-tempered metadynamics calculations on the DFT-based Born-Oppenheimer potential energy surface, by refined X-ray diffraction data and by nitrogen adsorption data obtained by experiment and grand-canonical Monte-Carlo simulations based on the DFT-optimized and PXRD-derived geometries. While the reported origin of flexibility cannot be generalized to all flexible MOFs, it offers a rational design concept of folding mechanisms in switchable MOFs by exploitation of the stabilization effect of linker stacking in the closed form.

Highly Sensitive Electromechanical Piezoresistive Pressure Sensors Based on Large-Area Layered PtSe2 Films.

Two-dimensional (2D) layered materials are ideal for micro- and nanoelectromechanical systems (MEMS/NEMS) due to their ultimate thinness. Platinum diselenide (PtSe2), an exciting and unexplored 2D transition metal dichalcogenide material, is particularly interesting because its low temperature growth process is scalable and compatible with silicon technology. Here, we report the potential of thin PtSe2 films as electromechanical piezoresistive sensors. All experiments have been conducted with semimetallic PtSe2 films grown by thermally assisted conversion of platinum at a complementary metal-oxide-semiconductor (CMOS)-compatible temperature of 400 °C. We report high negative gauge factors of up to -85 obtained experimentally from PtSe2 strain gauges in a bending cantilever beam setup. Integrated NEMS piezoresistive pressure sensors with freestanding PMMA/PtSe2 membranes confirm the negative gauge factor and exhibit very high sensitivity, outperforming previously reported values by orders of magnitude. We employ density functional theory calculations to understand the origin of the measured negative gauge factor. Our results suggest PtSe2 as a very promising candidate for future NEMS applications, including integration into CMOS production lines.

2D Crystals in Three Dimensions: Electronic Decoupling of Single-Layered Platelets in Colloidal Nanoparticles.

2D crystals, single sheets of layered materials, often show distinct properties desired for optoelectronic applications, such as larger and direct band gaps, valley- and spin-orbit effects. Being atomically thin, the low amount of material is a bottleneck in photophysical and photochemical applications. Here, the formation of stacks of 2D crystals intercalated with small surfactant molecules is proposed. It is shown, using first principles calculations, that the very short surfactant methyl amine electronically decouples the layers. The indirect-direct band gap transition characteristic for Group 6 transition metal dichalcogenides is demonstrated experimentally by observing the emergence of a strong photoluminescence signal for ethoxide-intercalated WSe2 and MoSe2 multilayered nanoparticles with lateral size of about 10 nm and beyond. The proposed hybrid materials offer the highest possible density of the 2D crystals with electronic properties typical of monolayers. Variation of the surfactant's chemical potential allows fine-tuning of electronic properties and potentially elimination of trap states caused by defects.

Probing charge transfer characteristics in a donor-acceptor metal-organic framework by Raman spectroelectrochemistry and pressure-dependence studies.

The stimuli responsive behaviour of charge transfer donor-acceptor metal-organic frameworks (MOFs) remains an understudied phenomenon which may have applications in tuneable electronic materials. We now report the modification of donor-acceptor charge transfer characteristics in a semiconducting tetrathiafulvalene-naphthalene diimide-based MOF under applied electrochemical bias and pressure. We employ a facile solid state in situ Raman spectroelectrochemical technique, applied for the first time in the characterisation of electroactive MOFs, to monitor the formation of a new complex TTFTC˙+-DPNI from a largely neutral system, upon electrochemical oxidation of the framework. In situ pressure-dependent Raman spectroscopy and powder X-ray diffraction experiments performed in a diamond anvil cell revealed blue shifts in the donor and acceptor vibrational modes in addition to contractions in the unit cell which are indicative of bond shortening. This study demonstrates the utility of in situ Raman spectroscopic techniques in the characterisation of redox-active MOFs and the elucidation of their electronic behaviours.

Defect Healing and Charge Transfer-Mediated Valley Polarization in MoS2/MoSe2/MoS2 Trilayer van der Waals Heterostructures.

Monolayer transition metal dichalcogenides (TMDCs) grown by chemical vapor deposition (CVD) are plagued by a significantly lower optical quality compared to exfoliated TMDCs. In this work, we show that the optical quality of CVD-grown MoSe2 is completely recovered if the material is sandwiched in MoS2/MoSe2/MoS2 trilayer van der Waals heterostructures. We show by means of density functional theory that this remarkable and unexpected result is due to defect healing: S atoms of the more reactive MoS2 layers are donated to heal Se vacancy defects in the middle MoSe2 layer. In addition, the trilayer structure exhibits a considerable charge-transfer mediated valley polarization of MoSe2 without the need for resonant excitation. Our fabrication approach, relying solely on simple flake transfer technique, paves the way for the scalable production of large-area TMDC materials with excellent optical quality.