Dielectric Properties of Selected Metal–Organic Frameworks

The electronic structure of a class of [Zn₄O­(CO₂)₆] based metal–organic frameworks (MOFs) is theoretically analyzed by means of density functional perturbation theory. The calculated static dielectric constants vary in a range between 1.33 and 1.54, characterizing the structures as ultralow-<i>k</i> dielectric materials and confirming earlier estimates qualitatively. We also present the results of first-principle calculations of the real and imaginary parts of the dielectric function and give the frequency-dependent dielectric constant up to the near-ultraviolet, which is important for high frequency semiconductor and optical applications of MOFs. The dielectric and electronic properties are governed by the linker molecules, so that the band gap and the dielectric constant can be engineered

Proton Conduction in a MIL-53(Al) Metal–Organic Framework: Confinement versus Host/Guest Interaction

In this contribution, we present and discuss results from a computational study of proton transfers between imidazole molecules confined in a MIL-53­(Al) metal–organic framework. We combined molecular-dynamics simulations and a density-functional tight-binding method. The extensive analysis of trajectories resulted in two main competing effects: on the one hand, the one-dimensional channel structure of MIL-53­(Al) arranges the imidazole molecules to allow proton exchange by hopping transport; on the other hand, the interactions between the MIL-53­(Al) host system and the imidazole molecules influence the free movement retaining the molecules. We find that the retaining leads to an increase in proton transfers, when both vehicle mechanisms and hopping events are considered. Thus, a well-balanced relationship between these two effects is necessary for efficient proton transport in metal–organic frameworks. Furthermore, the lifetime of the transition state could be estimated to be on the order of 100 fs

Influence of Electric Fields on the Electron Transport in Donor–Acceptor Polymers

The influence of an electric field on different properties of the donor–acceptor polymer diketo-pyrrolo-pyrrole bithiophene thienothiophene (DPPT-TT) that are essential for the charge transport process is studied. The main focus is on whether the transport in DPPT-TT-based organic transistors can be tuned by electric fields in the gate direction. The considered electric fields are in the range 10⁸–10¹⁰ V m^–1. We show that strong electric fields (∼10⁹ V m^–1) which are parallel to the polymer backbone can influence the reorganization energy in a Markus-type approach. Weaker electric fields parallel to the polymer backbone result in minimal changes to the reorganization energy. The coupling element of DPPT-TT shows a pronounced affinity to be influenced by electric fields in the charge transport direction independent of the field strength

Water Multilayers on TiO₂ (101) Anatase Surface: Assessment of a DFTB-Based Method

A water/(101) anatase TiO₂ interface has been investigated with the DFT-based self-consistent-charge density functional tight-binding theory (SCC-DFTB). By comparison of the computed structural, energetic, and dynamical properties with standard DFT-GGA and experimental data, we assess the accuracy of SCC-DFTB for this prototypical solid–liquid interface. We tested different available SCC-DFTB parameters for Ti-containing compounds and, accordingly, combined them to improve the reliability of the method. To better describe water energetics, we have also introduced a modified hydrogen-bond-damping function (HBD). With this correction, equilibrium structures and adsorption energies of water on (101) anatase both for low (0.25 ML) and full (1 ML) coverages are in excellent agreement with those obtained with a higher level of theory (DFT-GGA). Furthermore, Born–Oppenheimer molecular dynamics (MD) simulations for mono-, bi-, and trilayers of water on the surface, as computed with SCC-DFTB, evidence similar ordering and energetics as DFT-GGA Car–Parrinello MD results. Finally, we have evaluated the energy barrier for the dissociation of a water molecule on the anatase (101) surface. Overall, the combined set of parameters with the HBD correction (SCC-DFTB+HBD) is shown to provide a description of the water/water/titania interface, which is very close to that obtained by standard DFT-GGA, with a remarkably reduced computational cost. Hence, this study opens the way to the future investigations on much more extended and realistic TiO₂/liquid water systems, which are extremely relevant for many modern technological applications

Line Defects in Molybdenum Disulfide Layers

Layered molecular materials and especially MoS₂ are already accepted as promising candidates for nanoelectronics. In contrast to the bulk material, the observed electron mobility in single-layer MoS₂ is unexpectedly low. Here we reveal the occurrence of intrinsic defects in MoS₂ layers, known as inversion domains, where the layer changes its direction through a line defect. The line defects are observed experimentally by atomic resolution TEM. The structures were modeled and the stability and electronic properties of the defects were calculated using quantum-mechanical calculations based on the Density-Functional Tight-Binding method. The results of these calculations indicate the occurrence of new states within the band gap of the semiconducting MoS₂. The most stable nonstoichiometric defect structures are observed experimentally, one of which contains metallic Mo–Mo bonds and another one bridging S atoms

Line Defects in Molybdenum Disulfide Layers

Layered molecular materials and especially MoS₂ are already accepted as promising candidates for nanoelectronics. In contrast to the bulk material, the observed electron mobility in single-layer MoS₂ is unexpectedly low. Here we reveal the occurrence of intrinsic defects in MoS₂ layers, known as inversion domains, where the layer changes its direction through a line defect. The line defects are observed experimentally by atomic resolution TEM. The structures were modeled and the stability and electronic properties of the defects were calculated using quantum-mechanical calculations based on the Density-Functional Tight-Binding method. The results of these calculations indicate the occurrence of new states within the band gap of the semiconducting MoS₂. The most stable nonstoichiometric defect structures are observed experimentally, one of which contains metallic Mo–Mo bonds and another one bridging S atoms

Anisotropic Thermoelectric Response in Two-Dimensional Puckered Structures

Two-dimensional semiconductor materials with puckered structure offer a novel playground to implement nanoscale thermoelectric, electronic, and optoelectronic devices with improved functionality. Using a combination of approaches to compute the electronic and phonon band structures with Green’s function based transport techniques, we address the thermoelectric performance of phosphorene, arsenene, and SnS monolayers. In particular, we study the influence of anisotropy in the electronic and phononic transport properties and its impact on the thermoelectric figure of merit <i>ZT</i>. Our results show no strong electronic anisotropy, but a strong thermal one, the effect being most pronounced in the case of SnS monolayers. This material also displays the largest figure of merit at room temperature for both transport directions, zigzag (<i>ZT</i> ∼ 0.95) and armchair (<i>ZT</i> ∼ 1.6), thus hinting at the high potential of these new materials in thermoelectric applications

Porous Graphene Oxide/Diboronic Acid Materials: Structure and Hydrogen Sorption

Solvothermal reaction of graphite oxide (GO) with benzene-1,4-diboronic acid (DBA) was reported previously to result in formation of graphene oxide framework (GOF) materials. The theoretical structure of GOFs consists of graphene layers separated by benzene-diboronic “pillars” with ∼1 nm slit pores thus providing the opportunity to use it as a model material to verify the effect of a small pore size on hydrogen adsorption. A set of samples with specific surface area (SSA) in the range of ∼50–1000 m²/g were prepared using variations of synthesis conditions and GO/DBA proportions. Hydrogen storage properties of GOF samples evaluated at 293 and 77 K were found to be similar to other nanocarbon trends in relation to SSA values. Structural characterization of GO/DBA samples showed all typical features reported as evidence for formation of a framework structure such as expanded interlayer distance, increased temperature of thermal exfoliation, typical features in FTIR spectra, etc. However, the samples also exhibited reversible swelling in polar solvents which is not compatible with the idealized GOF structure linked by benzene-diboronic molecular pillars. Therefore, possible alternative nonframework models of structures with pillars parallel and perpendicular to GO planes are considered

Molybdenum Carbide-Embedded Nitrogen-Doped Porous Carbon Nanosheets as Electrocatalysts for Water Splitting in Alkaline Media

Molybdenum carbide (Mo₂C) based catalysts were found to be one of the most promising electrocatalysts for hydrogen evolution reaction (HER) in acid media in comparison with Pt-based catalysts but were seldom investigated in alkaline media, probably due to the limited active sites, poor conductivity, and high energy barrier for water dissociation. In this work, Mo₂C-embedded nitrogen-doped porous carbon nanosheets (Mo₂C@2D-NPCs) were successfully achieved with the help of a convenient interfacial strategy. As a HER electrocatalyst in alkaline solution, Mo₂C@2D-NPC exhibited an extremely low onset potential of ∼0 mV and a current density of 10 mA cm^–2 at an overpotential of ∼45 mV, which is much lower than the values of most reported HER electrocatalysts and comparable to the noble metal catalyst Pt. In addition, the Tafel slope and the exchange current density of Mo₂C@2D-NPC were 46 mV decade^–1 and 1.14 × 10^–3 A cm^–2, respectively, outperforming the state-of-the-art metal-carbide-based electrocatalysts in alkaline media. Such excellent HER activity was attributed to the rich Mo₂C/NPC heterostructures and synergistic contribution of nitrogen doping, outstanding conductivity of graphene, and abundant active sites at the heterostructures

Molecular Doping of a High Mobility Diketopyrrolopyrrole–Dithienylthieno[3,2‑<i>b</i>]thiophene Donor–Acceptor Copolymer with F6TCNNQ

Herein we present a molecular doping of a high mobility diketopyrrolopyrrole–dithienylthieno­[3,2-<i>b</i>]­thiophene donor–acceptor copolymer poly­[3,6-(dithiophene-2-yl)-2,5-di­(6-dodecyl­octadecyl)­pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-dione-<i>alt</i>-thieno­[3,2-<i>b</i>]­thiophene], PDPP­(6-DO)₂TT, with the electron-deficient compound hexafluoro­tetracyano­naphthoquino­dimethane (F6TCNNQ). Despite a slightly negative HOMO_donor–LUMO_acceptor offset of −0.12 eV which may suggest a reduced driving force for the charge transfer (CT), a partial charge CT was experimentally observed in PDPP­(6-DO)₂TT:F6TCNNQ by absorption, vibrational, and electron paramagnetic resonance spectroscopies and predicted by density functional theory calculations. Despite the modest CT, PDPP­(6-DO)₂TT:F6TCNNQ films possess unexpectedly high conductivities up to 2 S/cm (comparable with the conductivities of the benchmark doped polymer system P3HT:F4TCNQ having a large positive offset). The observation of the high conductivity in doped PDPP­(6-DO)₂TT films can be explained by a high hole mobility in PDPP­(6-DO)₂TT blends which compensates a lowered (relatively to P3HT:F4TCNQ) concentration of free charge carriers. We also show that F6TCNNQ-doped P3HT, the system which has not been reported so far to the best of our knowledge, exhibits a conductivity up to 7 S/cm, which exceeds the conductivity of the benchmark P3HT:F4TCNQ system