Interlayer vacancy defects in AA-stacked bilayer graphene: Density functional theory predictions

© 2017 IOP Publishing Ltd.AA-stacked graphite and closely related structures, where carbon atoms are located in registry in adjacent graphene layers, are a feature of graphitic systems including twisted and folded bilayer graphene, and turbostratic graphite. We present the results of ab initio density functional theory calculations performed to investigate the complexes that are formed from the binding of vacancy defects across neighbouring layers in AA-stacked bilayers. As with AB stacking, the carbon atoms surrounding lattice vacancies can form interlayer structures with sp 2 bonding that are lower in energy than in-plane reconstructions. The sp 2 interlayer bonding of adjacent multivacancy defects in registry creates a type of stable sp 2 bonded 'wormhole' or tunnel defect between the layers. We also identify a new class of 'mezzanine' structure characterised by sp 3 interlayer bonding, resembling a prismatic vacancy loop. The V 6 hexavacancy variant, where six sp 3 carbon atoms sit midway between two carbon layers and bond to both, is substantially more stable than any other vacancy aggregate in AA-stacked layers. Our focus is on vacancy generation and aggregation in the absence of extreme temperatures or intense beams

Roadmap on carbon molecular nanostructures in space

In this roadmap article, we consider the main challenges and recent breakthroughs in understanding the role of carbon molecular nanostructures in space and propose future avenues of research. The focus lies on small carbon-containing molecules up to fullerenes, extending to even larger, more complex organic species. The roadmap contains forty contributions from scientists with leading expertize in observational astronomy, laboratory astrophysics/chemistry, astrobiology, theoretical chemistry, synthetic chemistry, molecular reaction dynamics, material science, spectroscopy, graph theory, and data science. The concerted interdisciplinary combination of the state-of-the-art of these astronomical, laboratory, and theoretical studies opens up new ways to advance the fundamental understanding of the physics and chemistry of cosmic carbon molecular nanostructures and touches on their wider relevance and impact in nanotechnology and catalysis

Investigation of the Fe³⁺ centers in perovskite KMgF₃ through a combination of <i>ab initio</i> (density functional theory) and semi-empirical (superposition model) calculations

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Analysis of paramagnetic 3d ions (Cr3+ and Fe3+) centers in fluoroelpasolite Cs2NaGaF6 crystal by both DFT and SPM calculations

The structural properties of 3d ions (Cr3+ and Fe3+) centers in Cs2NaGaF6 have been investigated by performing density functional theory (DFT) and semi-empirical superposition model (SPM) calculations. The local geometry (i.e. equilibrium ligand distance and angles between the host cation ion (Cs+, Na+, or Ga3+) and F ligands) for pure, Cr3+ doped and Fe3+ doped structures have been determined after a fully relaxed geometry optimization at each lattice site. The previous experimental zero-field splitting (ZFS) parameters (ZFSPs) have been analyzed by SPM calculations using the obtained structural properties from DFT. The combination of two methods provide us to confirm the main result of the previous electron magnetic resonance and optical investigations stating that transition metal (TM) ions substitute for octahedral sites instead of being distributed randomly in the lattice of Cs2NaGaF6. (C) 2017 Elsevier B.V. All rights reserved

The role of charge distribution on the friction coefficients of epitaxial graphene grown on the Si-terminated and C-terminated faces of SiC

The friction coefficients of single-layer epitaxial graphene grown on the Si-terminated and C-terminated faces of Silicon Carbide (SiC) substrate were measured under ambient conditions using Friction Force Microscope (FFM). The lateral friction force measurements acquired in the applied normal force range between 4.0 and 16.0 nN showed that the friction coefficient of graphene on the C-terminated face of SiC is about two times smaller than the one grown on its Si-terminated face. The lateral friction was found to be decreased as the average of root mean square roughness increases suggesting the observed difference in the friction coefficients cannot be related to the roughness of the graphene layers. DFT calculations demonstrated that the altered periodicity of charge distribution on graphene due to the specific interactions with two distinct polar faces of SiC substrate might explain the observed difference in the friction coefficients

Anion sensing with cobalt corrinoid grafted quartz crystal microbalances

Corrin macrocycles are well known for their interesting and multifaceted coordination chemistry at the axial positions of the cobalt center. In this work, the use of such cobalt corrinoids as sensitive elements for sorption based chemical sensors like the quartz crystal microbalance (QCM) is described for the first time. This approach of fast and reversible ion detection extends the versatility and applicability of this class of metal-containing ionophores and colorimetric indicators. QCMs were grafted with aquacyano and dicyano cobalt corrinoids and assessed in their sensing characteristics during exposure to cyanide, thiocyanate, and other ionic species as well as selected organic compounds. The liquid sensing characteristics of the corronoids were found to be excellent. Sensors reach equilibrium in <3 s from the stable baseline after analyte exposure and the responses are fully reversible. Sensors of high sensitivity and selectivity for cyanide have been obtained with detection limits at 1 μM. At the same time, the sensitivities to common organic compounds are negligible. The results demonstrate that corrin grafted QCMs exhibit enormous potential as chemical sensors for the detection of anions, especially cyanide, in aqueous samples

A novel selective fluorescent chemosensor for Fe3+ions based on phthalonitrile dimer: Synthesis, analysis, and theoretical studies

Phenyl-4,4-di(3,6-dibutoxyphthalonitrile) (3) was synthesized by the reaction of 1,4-phenylenebisboronic acid (1) and 4-bromo-3,6-dibutoxyphthalonitrile (2), using Suzuki cross-coupling reaction. The newly synthesized compound (3) was characterized by FT-IR, MALDI-MS, ESI-MS, 1H-NMR, 13C-NMR, and 13C-DEPT-135-NMR. The fluorescence property of phenyl-4,4-di(3,6- dibutoxyphthalonitrile) (3) towards various metal ions was investigated by fluorescence spectroscopy, and it was observed thatthe compound (3) displayed a significantly 'turn-off' response to Fe3+, which was referred to 1:2 complex formation between ligand (3) and Fe3+. The compound was also studied via density functional theory calculations revealing the interaction mechanism of the molecule with Fe3+ions. © 2020 TUBITAK. All rights reserved