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Register a new userHydrogen Bonding: A Mechanism for Tuning Electronic and Optical Properties of Hybrid Organic-Inorganic Frameworks
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Here we demonstrate that significant progress in this area may be achieved by introducing structural elements that form hydrogen bonds with environment. Considering several examples of hybrid framework materials with different structural ordering containing protonated sulfonium cation H3S+ that forms strong hydrogen bonds with electronegative halogen anions (Cl-, F-), we found that hydrogen bonding increases the structural stability of the material and may be used for tuning electronic states near the bandgap. We suggest that such a behavior has a universal character and should be observed in hybrid inorganic-organic framework materials containing protonated cations. This effect may serve as a viable route for optoelectronic and photovoltaic applications.
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Boron subphthalocyanine chloride is an electron donor material used in small molecule organic photovoltaics with an unusually large molecular dipole moment. Using first-principles calculations, we investigate enhancing the electronic and optical properties of boron subphthalocyanine chloride, by substituting the boron and chlorine atoms with other trivalent and halogen atoms in order to modify the molecular dipole moment. Gas phase molecular structures and properties are predicted with hybrid functionals. Using positions and orientations of the known compounds as the starting coordinates for these molecules, stable crystalline structures are derived following a procedure that involves perturbation and accurate total energy minimization. Electronic structure and photonic properties of the predicted crystals are computed using the GW method and the Bethe-Salpeter equation, respectively. Finally, a simple transport model is use to demonstrate the importance of molecular dipole moments on device performance.
Much of our understanding of complex structures is based on simplification: for example, metal-organic frameworks are often discussed in the context of nodes and linkers, allowing for a qualitative comparison with simpler inorganic structures. Here we show how such an understanding can be obtained in a systematic and quantitative framework, by combining atom-density based similarity (kernel) functions and unsupervised machine learning with the long-standing idea of coarse-graining atomic structure. We demonstrate how the latter enables a comparison of vastly different chemical systems, and use it to create a unified, two-dimensional structure map of experimentally known tetrahedral AB2 networks - including clathrate hydrates, zeolitic imidazolate frameworks (ZIFs), and diverse inorganic phases. The structural relationships that emerge can then be linked to microscopic properties of interest, which we exemplify for structural heterogeneity and tetrahedral density.
Behaving like atomically-precise two-dimensional quantum wells with non-negligible dielectric contrast, the layered HOIPs have strong electronic interactions leading to tightly bound excitons with binding energies on the order of 500 meV. These strong interactions suggest the possibility of larger excitonic complexes like trions and biexcitons, which are hard to study numerically due to the complexity of the layered HOIPs. Here, we propose and parameterize a model Hamiltonian for excitonic complexes in layered HOIPs and we study the correlated eigenfunctions of trions and biexcitons using a combination of diffusion Monte Carlo and very large variational calculations with explicitly correlated Gaussian basis functions. Binding energies and spatial structures of these complexes are presented as a function of the layer thickness. The trion and biexciton of the thinnest layered HOIP have binding energies of 35 meV and 44 meV, respectively, whereas a single exfoliated layer is predicted to have trions and biexcitons with equal binding enegies of 48 meV. We compare our findings to available experimental data and to that of other quasi-two-dimensional materials.
Porphyrinic materials show a range of interesting and useful optical and electrical properties. The less well-known sub-class of porphyrin diacids has been used in this work to construct an ionic hybrid organic-inorganic material in combination with a halogenidometalate anion. The resulting compound, $[H_6TPyP][BiCl_6]_2$ (1) (TPyP = tetra(4-pyridyl)porphyrin) has been obtained via a facile solution based synthesis in single crystalline form. The material exhibits a broad photoluminescence emission band between 650 and 850 nm at room temperature. Single crystals of $[H_6TPyP][BiCl_6]_2$ show a photocurrent in the fA and a much higher dark current in the nA range. They also display an unexpected reversible color change upon wetting with different liquids. This phenomenon has been investigated with optical spectroscopy, SEM, XPS and NEXAFS techniques, showing that a surface-based structural coloration effect is the source of the color change. This stands in contrast to other materials where structural coloration typically has to be introduced through elaborate, multi-step processes or the use of natural templates. Additionally, it underscores the potential of self-assembly of porphyrinic hybrid compounds in the fabrication of materials with unusual optical properties.
The two-dimensional (2D) C3N has emerged as a material with promising applications in high performance device owing to its intrinsic bandgap and tunable electronic properties. Although there are several reports about the bandgap tuning of C3N via stacking or forming nanoribbon, bandgap modulation of bilayer C3N nanoribbons (C3NNRs) with various edge structures is still far from well understood. Here, based on extensive first-principles calculations, we demonstrated the effective bandgap engineering of C3N by cutting it into hydrogen passivated C3NNRs and stacking them into bilayer heterostructures. It was found that armchair (AC) C3NNRs with three types of edge structures are all semiconductors, while only zigzag (ZZ) C3NNRs with edges composed of both C and N atoms (ZZ-CN/CN) are semiconductors. The bandgaps of all semiconducting C3NNRs are larger than that of C3N nanosheet. More interestingly, AC-C3NNRs with CN/CN edges (AC-CN/CN) possess direct bandgap while ZZ-CN/CN have indirect bandgap. Compared with the monolayer C3NNR, the bandgaps of bilayer C3NNRs can be greatly modulated via different stacking orders and edge structures, varying from 0.43 eV for ZZ-CN/CN with AB-stacking to 0.04 eV for AC-CN/CN with AA-stacking. Particularly, transition from direct to indirect bandgap was observed in the bilayer AC-CN/CN heterostructure with AA-stacking, and the indirect-to-direct transition was found in the bilayer ZZ-CN/CN with AB-stacking. This work provides insights into the effective bandgap engineering of C3N and offers a new opportunity for its applications in nano-electronics and optoelectronic devices.
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