Here we discuss, based on first-principles calculations, two-dimensional (2D) kagome lattices composed of polymerized heterotriangulene units, planar molecules with D3h point group containing a B, C, ...or N center atom and CH2, O, or CO bridges. We explore the design principles for a functional lattice made of 2D polymers, which involves control of π-conjugation and electronic structure of the knots. The former is achieved by the chemical potential of the bridge groups, while the latter is controlled by the heteroatom. The resulting 2D kagome polymers have a characteristic electronic structure with a Dirac band sandwiched by two flat bands and are either Dirac semimetals (C center), or single-band semiconductorsmaterials with either exclusively electrons (B center) or holes (N center) as charge carriers of very high mobility, reaching values of up to ∼8 × 103 cm2 V–1 s–1, which is comparable to crystalline silicon.
Highly Emissive Covalent Organic Frameworks Dalapati, Sasanka; Jin, Enquan; Addicoat, Matthew ...
Journal of the American Chemical Society,
05/2016, Letnik:
138, Številka:
18
Journal Article
Recenzirano
Highly luminescent covalent organic frameworks (COFs) are rarely achieved because of the aggregation-caused quenching (ACQ) of π–π stacked layers. Here, we report a general strategy to design highly ...emissive COFs by introducing an aggregation-induced emission (AIE) mechanism. The integration of AIE-active units into the polygon vertices yields crystalline porous COFs with periodic π-stacked columnar AIE arrays. These columnar AIE π-arrays dominate the luminescence of the COFs, achieve exceptional quantum yield via a synergistic structural locking effect of intralayer covalent bonding and interlayer noncovalent π–π interactions and serve as a highly sensitive sensor to report ammonia down to sub ppm level. Our strategy breaks through the ACQ-based mechanistic limitations of COFs and opens a way to explore highly emissive COF materials.
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.
The noble art: Noble‐metal dichalcogenides and phosphochalcogenides are a novel class of two‐dimensional materials. This Minireview summarizes the progress in their synthesis, characterization, and application, highlighting their unique structural features and relationships.
The stacking orders in layered hexagonal boron nitride bulk and bilayers are studied using high-level ab initio theory local second-order Møller-Plesset perturbation theory (LMP2). Our results show ...that both electrostatic and London dispersion interactions are responsible for interlayer distance and stacking order, with AA' being the most stable one. The minimum energy sliding path includes only the AA' high-symmetry stacking, and the energy barrier is 3.4 meV per atom for the bilayer. State-of-the-art density functionals with and without London dispersion correction fail to correctly describe the interlayer energies with the exception of a Perdew-Burke-Ernzerhof functional intended for solid state and surface systems that agrees very well with our LMP2 results and experiment.
One of the major obstacles to a wide application range of the quantum spin Hall (QSH) effect is the lack of suitable QSH insulators with a large bulk gap. By means of first-principles calculations ...including relativistic effects, we predict that methyl-functionalized bismuth, antimony, and lead bilayers (Me-Bi, Me-Sb, and Me-Pb) are 2D topological insulators (TIs) with protected Dirac type topological helical edge states, and thus suitable QSH systems. In addition to the explicitly obtained topological edge states, the nontrivial topological characteristic of these systems is confirmed by the calculated nontrivial Z2 topological invariant. The TI characteristics are intrinsic to the studied materials and are not subject to lateral quantum confinement at edges, as confirmed by explicit simulation of the corresponding nanoribbons. It is worthwhile to point out that the large nontrivial bulk gaps of 0.934 eV (Me-Bi), 0.386 eV (Me-Sb), and 0.964 eV (Me-Pb) are derived from the strong spin–orbit coupling within the p x and p y orbitals and would be large enough for room-temperature application. Moreover, we show that the topological properties in these three systems are robust against mechanical deformation. These novel 2D TIs with such giant topological energy gaps are promising platforms for topological phenomena and possible applications at high temperature.
Two-dimensional topological insulators (2D TIs) are a remarkable class of atomically thin layered materials that exhibit unique symmetry-protected helical metallic edge states with an insulating ...interior. Recent years have seen a tremendous surge in research of this intriguing new state of quantum matter. In this Perspective, we summarize major milestones and the most significant progress in the latest developments of material discovery and property characterization in 2D TI research. We categorize the large number and rich variety of theoretically proposed 2D TIs based on the distinct mechanisms of topological phase transitions, and we systematically analyze and compare their structural, chemical, and physical characteristics. We assess the current status and challenges of experimental synthesis and potential device applications of 2D TIs and discuss prospects of exciting new opportunities for future research and development of this fascinating class of materials.
A sulfonic-acid-based covalent organic framework (TpPa-SO3H) has been synthesized that exhibits intrinsic proton conductivity under anhydrous conditions. The sulfonic acid groups are aligned on the ...two-dimensional (2D) layers at periodic intervals and promote the proton hopping inside the hexagonal one-dimensional channel. The intrinsic proton conductivity of TpPa-SO3H was measured as 1.7 × 10–5 S cm–1 at 120 °C under anhydrous conditions. To enhance the proton conductivity, we have synthesized a hybrid COF TpPa-(SO3H-Py) by a ligand-based solid-solution approach that contains sulfonic acid as the acidic site, as well as pyridine as the basic site, in order to immobilize acidic proton carrier molecules. Impregnation of phytic acid molecules inside the framework increases the anhydrous proton conductivity up to 5 × 10–4 S cm–1 at 120 °C. Such an approach highlights the advantage and first-time use of hybrid COF for interplaying intrinsic to extrinsic proton conductivity.
Covalent organic frameworks (COFs) have emerged as a tailor‐made platform for designing layered two‐dimensional polymers. However, most of them are obtained as neutral porous materials. Here, we ...report the construction of ionic crystalline porous COFs with positively charged walls that enable the creation of well aligned yet spatially confined ionic interface. The unconventional reversed AA‐stacking mode alternately orientates the cationic centers to both sides of the walls; the ionic interface endows COFs with unusual electrostatic functions. Because all of the walls are decorated with electric dipoles, the uptake of CO2 is enhanced by three fold compared to the neutral analog. By virtue of sufficient open space between cations, the ionic interface exhibits exceptional accessibility, efficiency, and selectivity in ion exchange to trap anionic pollutants. These findings suggest that construction of the ionic interface of COFs offers a new way to structural and functional designs.
A scaffold for ionic interfaces: Covalent organic frameworks were synthesized. They bear ionic interfaces that are well aligned and spatially confined on the one‐dimensional channel walls. The ionic interfaces exert profound effects on the frameworks and trigger unusual electrostatic functions, such as the adsorption of CO2 and the selective removal of anionic pollutants.
We report that an external electric field applied normal to bilayers of transitionmetal dichalcogenides TX sub(2)(T = Mo, W, X = S, Se) creates significant spin-orbit splittings and reduces the ...electronic band gap linearly with the field strength. Contrary to the TX sub(2) monolayers, spin-orbit splittings and valley polarization are absent in bilayers due to the presence of inversion symmetry. This symmetry can be broken by an electric field, and the spin-orbit splittings in the valence band quickly reach values similar to those in the monolayers (145 meV for MoS sub(2), ..., 418 meV for WSe sub(2)) at saturation fields less than 500 mV A super(-1). The band gap closure results in a semiconductor-metal transition at field strength between 1.25 (WX sub(2)) and 1.50 (MoX sub(2))V A super(-1). Thus, by using a gate voltage, the spin polarization can be switched on and off in TX sub(2) bilayers, thus activating them for spintronic and valleytronic applications.
Two-dimensional redox-active covalent organic frameworks (COFs) are ideal materials for energy storage applications due to their high surface area, extended π conjugated structure, tunable pore size, ...and adjustable functionalities. Herein, we report the synthesis and supercapacitor application of two redox active COFs TpPa-(OH) 2 and TpBD-(OH) 2 along with the role of their redox active functional groups for the enrichment of specific capacitance. Of these COFs, TpPa-(OH) 2 exhibited the highest specific capacitance of 416 F g–1 at 0.5 A g–1 current density in three electrode configuration while the highest specific capacitance was 214 F g–1 at 0.2 A g–1 current density in two electrode configuration. Superior specific capacitance was due to emergence of excellent pseudocapacitance by virtue of precise molecular level control over redox functionalities present in the COF backbone. This COF also demonstrated 66% capacitance retention after 10 000 cycles along with 43% accessibility of the redox-active hydroquinone (H2Q) moieties in three electrode configuration while the capacitance retention was 88% after 10 000 cycles in two electrode configuration. Exceptionally high specific capacitance of TpPa-(OH) 2 was due to the reversible proton-coupled electron transfer (2H+/2e–) of hydroquinone/benzoquinone (H2Q/Q) moieties wherein H2Q and Q had comparable chemical stabilities during redox cycling that originated from H-bonding, which was supported by calculated structures.
PDF
