Abstract: We report single-crystal X-ray diffuse scattering measurements on C-60.C8H8 fullerene-cubane showing that the C-60 molecules are orientationally disordered at 300 and 150K and get ordered at low temperatures. Monte Carlo simulations provide further insight in the orientational behavior of both C-60 and C8H8 molecules; low-temperature molecular orientations are predicted.

Abstract: The morphology and structure of hexagonal graphitic BN (h-BN) powders with graphitization indices GI <5, used as precursors for the synthesis of cubic BN (c-BN) crystals, has been investigated by transmission electron microscopy in diffraction contrast and high resolution. We show that besides the GI, which is a general parameter for controlling the structural quality of h-EN ponders, some other microstructural features strongly influence the synthesis of c-BN. In our opinion, the high reactivity of some h-BN powders results from the presence of some nucleation centers for c-BN, observed at the edges of the h-BN particles. They are formed by a rearrangement of the graphitic (0002) planes by bending back, joining in pairs and forming locally nanoarches (half nanotubes). In these particular places, the nature of bonding locally turns towards sp(3), as in the case of c-BN, (C) 2001 Elsevier Science B.V. All rights reserved.

Abstract: Despite great efforts in the research area of zeolite nanoparticles and their use in the synthesis of bimodal materials, still little is known about the impact of the synthesis conditions of the zeolite nanoparticles on its own characteristics, and on the properties and the formation mechanism of the final bimodal materials. A zeolite β nanoparticles solution is applied in a mesotemplate-free synthesis method, and the influence of the hydrothermal ageing temperature of the nanoparticles solution on both the zeolitic and porosity characteristics of the final bimodal material has been studied. Transmission electron microscopy in combination with 3-dimensional reconstructions obtained by electron tomography revealed that the zeolite β nanoparticles are connected by neck-like structures, thus creating a wormhole-like mesoporous material. Considering the zeolitic properties, a clear threshold is observed in the synthesis temperature series at 413 K. Below and at this threshold, the biporous materials show no apparent zeolitic characteristics, although these materials exhibit a more condensed and uniform SiOSi network in comparison to Al-MCF. Synthesis temperatures above the threshold lead to bimodal structures with defined zeolitic properties. Moreover, the dimensions of the nanoparticles are studied by TEM, revealing an increasing particle size with increasing temperature under the threshold of 413 K, which is in agreement with a sol-mechanism. This mechanism is disturbed after the threshold due to the start of the crystallisation process.

Abstract: Ion irradiation of carbon nanotubes very often leads to defect formation. However, we have recently shown that Ar ion irradiation in a limited energy window of 1025 eV may enhance the initial cap nucleation process, when the carbon network is in contact with the metal nanocatalyst. Here, we employ reactive molecular dynamics simulations to demonstrate that ion irradiation in a higher energy window of 1035 eV may also heal network defects after the nucleation stage through a non-metal-mediated mechanism, when the carbon network is no longer in contact with the metal nanocatalyst. The results demonstrate the possibility of beneficially utilizing ions in e.g. plasma-enhanced chemical vapour deposition of carbon nanotubes.

Abstract: Metalorganic frameworks (MOFs) have emerged as new porous materials for capture and separation of binary gas mixtures. Tuning the metal sites in MOF structures has an impact on properties, which enhance affinity of gas adsorption and selectivity (e.g., surface area, cavity, electric field, etc.). The synthesis and characterization of a M-DABCO series (M = Ni, Co, Cu, Zn) of MOFs are described in this study. The experiments were conducted using multicomponent gas mixtures and the Ideal Adsorbed Solution Theory (IAST) was applied to determine the CO2/CH4 selectivity. Experimental adsorption isotherms were fitted with a model equation to evaluate the characteristic adsorption energy (Isosteric, Qst) of this series. The Ni metal in the M-DABCO series reveals the best performance concerning CO2 adsorption and CH4/CO2 selectivity at ambient conditions based on IAST calculations. The combination of characterizations, calculations and adsorption experiments were used to discuss the metal impact on the adsorption sites in the M-DABCO series at ambient conditions.

Abstract: Highly ordered SBA-15-type ethane-bridged PMOs have been obtained by employing H3PO4 as acid to tune the pH in the presence of copolymer surfactant P123. The effects of the acidity and the addition of inorganic salt on the formation of the mesostructure are investigated. It is found that, compared with HCl, the polyprotic weak acid H3PO4 is preferable for the synthesis of highly ordered SBA-15-type ethane-bridged PMOs with larger pore size and surface areas under mild acidic conditions. Moreover, taking the advantages of the mild acidic condition, vanadium-containing SBA-15-type ethane-bridged PMOs were successfully prepared through a direct synthesis approach. The XRD, N2-sorption, UVVis and CW-EPR studies of the V-PMO show that part of the vanadium species are present in polymeric (VOV)n clusters, while part of the vanadium centers are well-dispersed and immobilized on the inner surface of the mesopores.

Abstract: The atomic structure of natural HEU-type zeolite and two ion-exchanged variants of the zeolite, Ag+ (Ag-HEU) and Zn2+ (Zn-HEU) ion exchanged HEU-type zeolites, are investigated using advanced transmission electron microscopy techniques in combination with X-ray powder diffraction and X-ray absorption fine structure measurements. In both ion-exchanged materials, loading of the natural HEU zeolite is confirmed. Using low-voltage, aberration-corrected transmission electron microscopy at low-dose conditions, the local crystal structure of natural HEU-type zeolite is determined and the interaction of the ion-exchanged natural zeolites with the Ag+ and Zn2+ ions is studied. In the case of Ag-HEU, the presence of Ag+ ions and clusters at extra-framework sites as well as Ag nanoparticles has been confirmed. The Ag nanoparticles are preferentially positioned at the zeolite surface. For Zn-HEU, no large Zn(O) nanopartides are present, instead, the HEU channels are evidenced to be decorated by small Zn(O) clusters. (c) 2015 Elsevier Inc. All rights reserved.

Abstract: Periodic mesoporous organosilicas provide the best of two worlds: the strength and porosity of an inorganic framework combined with the infinite possibilities created by the organic bridging unit. In this work we focus on post-synthetical modification of benzene bridged PMO, in order to create bromobenzene PMO. In the past, this proved to be very challenging due to unwanted structural deterioration. However, now we have found a way to brominate this material whilst keeping the structure intact. In-depth structural analysis by solid state NMR and XPS shows both vast progress over previous attempts as well as potential for improvement.

Abstract: Selective etching allows for obtaining carbon nanotubes with a specific chirality. While plasma-assisted etching has already been used to separate metallic tubes from their semiconducting counterparts, little is known about the nanoscale mechanisms of the etching process. We combine (reactive) molecular dynamics (MD) and force-bias Monte Carlo (tfMC) simulations to study H-etching of CNTs. In particular, during the hydrogenation and subsequent etching of both the carbon cap and the tube, they sequentially transform to different carbon nanostructures, including carbon nanosheet, nanowall, and polyyne chains, before they are completely removed from the surface of a substrate-bound Ni-nanocluster.We also found that onset of the etching process is different in the cases of the cap and the tube, although the overall etching scenario is similar in both cases. The entire hydrogenation/etching process for both cases is analysed in detail, comparing with available theoretical and experimental evidences.

Abstract: In order to optimize the plasma-synthesis and modification process of carbon nanomaterials for applications such as nanoelectronics and energy storage, a deeper understanding of fundamental hydrogengraphite/graphene interactions is required. Atomistic simulations by Molecular Dynamics have proven to be indispensable to illuminate these phenomena. However, severe time-scale limitations restrict them to very fast processes such as reflection, while slow thermal processes such as surface diffusion and molecular desorption are commonly inaccessible. In this work, we could however reach these thermal processes for the first time at time-scales and surface temperatures (1000 K) similar to high-flux plasma exposure experiments during the simulation of multilayer graphene etching by 5 eV H ions. This was achieved by applying the Collective Variable-Driven Hyperdynamics biasing technique, which extended the inter-impact time over a range of six orders of magnitude, down to a more realistic ion-flux of 1023m
2s
1. The results show that this not only causes a strong shift from predominant ion-to thermally induced interactions, but also significantly affects the hydrogen uptake and surface evolution. This study thus elucidates H ion-graphite/graphene interaction mechanisms and stresses the importance of including long time-scales in atomistic simulations at high surface temperatures to understand the dynamics of the ion-surface system.

Abstract: Two-dimensional (2D) carbon materials play an important role in nanomaterials. We propose a new carbon monolayer, named hexagonal-4,4,4-graphyne (H-4,H-4,H-4-graphyne), which is a nanoporous structure composed of rectangular carbon rings and triple bonds of carbon. Using first-principles calculations, we systematically studied the structure, stability, and band structure of this new material. We found that its total energy is lower than that of experimentally synthesized beta-graphdiyne and it is stable at least up to 1500 K. In contrast to the single Dirac point band structure of other 2D carbon monolayers, the band structure of H-4,H-4,H-4-graphyne exhibits double Dirac points along the high-symmetry points and the corresponding Fermi velocities (1.04-1.27 x 10(6) m/s) are asymmetric and higher than that of graphene. The origin of these double Dirac points is traced back to the nodal line states, which can be well explained by a tight-binding model. The H-4,H-4,H-4-graphyne forms a moire superstructure when placed on top of a hexagonal boron nitride substrate. These properties make H-4,H-4,H-4-graphyne a promising semimetal material for applications in high-speed electronic devices. (C) 2018 Elsevier Ltd. All rights reserved.

Abstract: Using the first-principles calculations, we explore the structural and novel electronic/optical properties of the C2N nanosheet. To this goal, we systematically investigate the affect of layer thickness, electrical field and strain on the electronic properties of the C2N nanosheet. By increasing the thickness of C2N, we observed that the band gap decreases. Moreover, by applying an electrical field to bilayer C2N, the band gap decreases and a semiconductor-to-metal transition can occur. Our results also confirm that uniaxial and biaxial strain can effectively alter the band gap of C2N monolayer. Furthermore, we show that the electronic and magnetic properties of C2N can be modified by the adsorption and substitution of various atoms. Depending on the species of embedded atoms, they may induce semiconductor (O, C, Si and Be), metal (S, N, P, Na, K, Mg and Ca), dilute-magnetic semiconductor (H, F, B), or ferro-magnetic-metal (Cl, Li) character in C2N monolayer. It was also found that the inclusion of hydrogen or oxygen impurities and nitrogen vacancies, can induce magnetism in the C2N monolayer. These extensive calculations can be useful to guide future studies to modify the electronic/optical properties of two-dimensional materials. (C) 2019 Elsevier Ltd. All rights reserved.

Abstract: The free energy surface (FES) for carbon segregation from nickel nanoparticles is obtained from advanced molecular dynamics simulations. A suitable reaction coordinate is developed that can distinguish dissolved carbon atoms from segregated dimers, chains and junctions on the nanoparticle surface. Because of the typically long segregation time scale (up to ms), metadynamics simulations along the developed reaction coordinate are used to construct FES over a wide range of temperatures and carbon concentrations. The FES revealed the relative stability of different stages in the segregation process, and free energy barriers and rates of the individual steps could then be calculated and decomposed into enthalpic and entropic contributions. As the carbon concentration in the nickel nanoparticle increases, segregated carbon becomes more stable in terms of both enthalpy and entropy. The activation free energy of the reaction also decreases with the increase of carbon concentration, which can be mainly attributed to entropic effects. These insights and the methodology developed to obtain them improve our understanding of carbon segregation process across materials science in general, and the nucleation and growth of carbon nanotube in particular.

Abstract: Using evolutionary algorithm for crystal structure prediction, we present a new stable two-dimensional (2D) carbon allotrope composed of polymerized as-indacenes (PAI) in a zigzag pattern, namely PAI-graphene whose energy is lower than most of the reported 2D allotropes of graphene. Crucially, the crystal structure realizes a nonsymmorphic layer group that enforces a nontrivial global topology of the band structure with two Dirac cones lying perfectly at the Fermi level. The absence of electron/hole pockets makes PAI-graphene a pristine crystalline topological semimetal having anisotropic Fermi velocities with a high value of 7.0 x 10(5) m/s. We show that while the semimetallic property of the allotrope is robust against the application of strain, the positions of the Dirac cone and the Fermi velocities can be modified significantly with strain. Moreover, by combining strain along both the x- and y-directions, two band inversions take place at G leading to the annihilation of the Dirac nodes demonstrating the possibility of strain-controlled conversion of a topological semimetal into a semiconductor. Finally we formulate the bulk-boundary correspondence of the topological nodal phase in the form of a generalized Zak-phase argument finding a perfect agreement with the topological edge states computed for different edge-terminations. (C) 2020 The Author(s). Published by Elsevier Ltd.