A comprehensive and up-to-date compilation of observed and calculated hydrogen (H)-related peaks that occur in the near- and middle-infrared spectra of diamond is presented. The experimental database ...contains >300 observed peaks attributed to H-related impurities in natural diamond. The database of calculated peaks includes data from first-principles simulations of the FTIR spectra of different VxNyHz defects in diamond and contains >300 peaks that correspond to different C-H, N-H, and B-H vibrational modes for each VxNyHz defect. Less than ~10 % of observed H-related peaks have been assigned to specific defects. Consequently, the computational database was constructed to better understand the properties of different VxNyHz defects that may correspond to different observed peaks. In general, hydrogen-rich diamonds with a dominant Type Ib character and thus poorly aggregated N, show a larger number of low-intensity, H-related peaks compared to diamonds with a dominant Type Ia character. For example, ~51 % of observed H-related peaks are only observed in Type Ib diamonds, ~9 % of peaks are only observed in diamonds with a dominant Type Ia character and ~40 % of peaks are observed in both Type Ia and Type Ib diamonds. There is a major increase in the number of distinct H-related peaks in Type IaA + Ib diamonds compared to Type Ib diamonds suggesting N-aggregation processes responsible for the formation of A-centers may also produce many distinct VxNyHz defects capable of trapping H. There is a major decrease in the number of H-related peaks in Type IaA > Ib diamonds compared to Type IaA + Ib diamonds suggesting that many of the VxNyHz defects associated with the initial production of A-centers become unstable with increasing mantle residence time and the progressive loss of C-centers. These defects likely combine (or disaggregate and then re-combine) to form fewer, relatively more intense, H-related peaks observed in Type IaA > Ib diamonds. Many of these peaks persist through continued annealing in the mantle and are observed in Type IaA, IaAB, and IaB diamonds. Our data suggests that N and H incorporation are not correlated during diamond growth and that in the early stages of diamond residence, H is incorporated into many distinct VxNyHz defects that involve C- and A-centers. With continued residence and annealing, and the progressive formation A-centers and loss of C-centers, these defects aggregate to form relatively fewer, presumably more stable, VxNyHz defects such as VN3H.
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•Only 10% of observed H-related IR peaks are assigned to specific defects.•VH3 and VH4 defects may not occur in natural diamond as H requires N to aggregate.•The VN3H defect may be produced by numerous distinct N/H-aggregation sequences.•H-rich Type Ib and IaA + Ib diamonds show a large number of H-related IR peaks.•H-related defects in Type Ia diamonds are relatively more stable during annealing.
•O, N, and P have great influence on the bond length and strength of Ti-C, Ta-C and Zr-C in W.•The bond lengths of Ti-C, Ta-C and Zr-C are generally increased when O meets TiC, TaC and ZrC.•The ...binding energy of O to TiC, TaC and ZrC are much larger than that of C to TiO, TaO and ZrO.•It is difficult for C to escape from TiO and ZrO, and for O to migrate away from TiC, TaC and ZrC.
Titanium carbide (TiC), tantalum carbide (TaC) and zirconium carbide (ZrC) in form of particles are widely added in W to improve its mechanical and anti-irradiation properties. However, TiC, TaC and ZrC particles are decomposed to Ti-C-O, Ta-C-O and Zr-C-O in W, respectively. To understand the micro-mechanisms, we carry out systematical simulations and find that the presence of impurities such as O, N, and P has great influence on the bond length and bond strength of Ti-C, Ta-C and Zr-C in W. The bond lengths of Ti-C, Ta-C and Zr-C are generally increased when O meets Ti-C, Ta-C and Zr-C. The bond strength of O to Ti-C, Ta-C and Zr-C is much larger than that of C to Ti-O, Ta-O and Zr-O. In contrast, N has little effect on the Ti-C bond, but has great influence on the Ta-C bond. P significantly increases Ta-C bond length, while its influence on Zr-C bond is negligible. The kinetics calculations elucidate that the diffusion barriers of C away from TiO are significantly increased by about 1 eV. It is very difficult for C to escape from TiO and ZrO, and it is also extremely difficult for O to escape from TiC, TaC and ZrC. This may be the reason why Ti-C-O, Ta-C-O, and Zr-C-O particles are found in W when TiC TaC and ZrC compounds are dispersed in W.
The spontaneous migration and interconversion of oxygen functional groups over graphene sheet have promising high-tech applications. There is a remarkable activation energy barrier against epoxy ...movement over the graphene. To overcome this energy barrier, different methods have been proposed. In this paper, using first principles calculations, we study the effects of external electric fields and solvents on the activation energy barrier. Our findings show that: (i) epoxy migration on one side of the surface of a graphene sheet is facilitated when a hydroxyl group is adsorbed on the other side of the surface, (ii) applying a perpendicular electric field along the opposite direction of the adsorbed epoxy, weakens the corresponding C–O bonds strength and consequently decreases the activation energy barrier, (iii) the solvents with a larger dielectric constant cause decreasing the activation energy barrier, (iv) under applying an electric field for any given solution, the oxidized graphene is polarized and its solubility increases with decreasing activation energy barrier, and (v) the presence of a single vacancy in the graphene sheet, decreases the barrier. Our work reveals different physical aspects of the solubility of the oxidized graphene in the presence of electric field and vacancy defect on the surface of the graphene sheet.
•Adsorption of a hydroxyl group facilitates the epoxy migration over graphene sheet.•Presence of a vacancy defect in the graphene lowers the epoxy migration barrier.•Applying a perpendicular electric field changes the epoxy migration energy barrier.•Solvents with a larger dielectric constant cause a decrease in migration barrier.•Applying a negative electric field increases the solubility of oxidized graphene.
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•Ab initio simulations of the surface of reduced graphene oxide were carried out.•The goal was to study formation of good leaving groups (GLG): CO, CO2, H2O etc.•The dynamical ...simulations were initiated at 300 K as well as at 1373 K.•It was established that the reactions of H-atom transfer enhanced formation of GLG.
In the present work, the surface chemistry of reduced graphene oxide, modified with hydrocarbon, hydroxyl, aldehyde and carboxyl groups is studied by means of computational chemistry. The simulations by ab-initio molecular dynamics show that the reactions depend on the proximity of chemical groups and possibilities for H-atom transfer and gas evolution. Defects in close proximity can also promote certain reactions, especially in case of good leaving groups as products (CO2, H2O, CH3OH, CO). Parts of the surface, rich in sp2 carbon atoms (i.e. regular graphene surface) can significantly decrease the scission energy of CC bonds from the leaving groups, compared to gas phase ethane molecule or to the corresponding CC- bonds located at the edge (or defect) of the two-dimensional carbon material. This effect is caused by the unpaired electron, formed after bond dissociation, joining the global π system and restoring of the regular graphene structure. Decarboxylation reactions are found to be energetically favorable at both edge and surface. Generally, reactions at the edge are found to be disfavored energetically if they involve the participation of a C atom from the surface of the two-dimensional carbon material.
Ge2Sb2Te5 (GST) is an important phase‐change material used in optical and electronic memory devices. In this work, crystal growth of GST at 600 K is investigated by ab initio molecular dynamics. ...Simulations of two different crystallization processes are performed. In the first set of simulations, the growth of crystalline nuclei generated using the metadynamics method is studied. In the second set, models containing a planar amorphous–crystalline interface are considered and the crystallization at the interface is investigated. The extracted crystal growth velocities are in the range of 1 m s−1 in both cases and compare well with recent experimental measurements. It is also found that GST crystallizes into a disordered cubic phase in all the simulations.
The crystallization properties of the phase‐change material Ge2Sb2Te5 (GST) at high temperatures are investigated by advanced ab initio molecular dynamics simulations. The crystal growth processes from both a quasi‐spherical nucleus and a planar amorphous–crystalline interface are considered. The obtained growth velocities are compatible with recent experiments. The simulations elucidate the fast crystal growth of the phase change GST compound at the atomistic level.
Hydrogen hydrates are among the basic constituents of our solar system's outer planets, some of their moons, as well Neptune-like exo-planets. The details of their high-pressure phases and their ...thermodynamic conditions of formation and stability are fundamental information for establishing the presence of hydrogen hydrates in the interior of those celestial bodies, for example, against the presence of the pure components (water ice and molecular hydrogen). Here, we report a synthesis path and experimental observation, by X-ray diffraction and Raman spectroscopy measurements, of the most H2-dense phase of hydrogen hydrate so far reported, namely the compound 3 (or C3). The detailed characterisation of this hydrogen-filled ice, based on the crystal structure of cubic ice I (ice Ic), is performed by comparing the experimental observations with first-principles calculations based on density functional theory and the stochastic self-consistent harmonic approximation. We observe that the extreme (up to 90 GPa and likely beyond) pressure stability of this hydrate phase is due to the close-packed geometry of the hydrogen molecules caged in the ice I
The atomic‐scale response of inhomogeneous fluids at interfaces and surrounding solute particles plays a critical role in governing chemical, electrochemical, and biological processes. Classical ...molecular dynamics simulations have been applied extensively to simulate the response of fluids to inhomogeneities directly, but are limited by the accuracy of the underlying interatomic potentials. Here, we use neural network potentials (NNPs) trained to ab initio simulations to accurately predict the inhomogeneous responses of two distinct fluids: liquid water and molten NaCl. Although NNPs can be readily trained to model complex bulk systems across a range of state points, we show that to appropriately model a fluid's response at an interface, relevant inhomogeneous configurations must be included in the training data. In order to sufficiently sample appropriate configurations of such inhomogeneous fluids, we develop protocols based on molecular dynamics simulations in the presence of external potentials. We demonstrate that NNPs trained on inhomogeneous fluid configurations can more accurately predict several key properties of fluids—including the density response, surface tension and size‐dependent cavitation free energies—for liquid water and molten NaCl, compared to both empirical interatomic potentials and NNPs that are not trained on such inhomogeneous configurations. This work therefore provides a first demonstration and framework to extract the response of inhomogeneous fluids from first principles for classical density‐functional treatment of fluids free from empirical potentials.
Simulation of inhomogeneous liquids under external potentials in solvation and classical density‐functional theory has been unable to capture ab‐initio level accuracy for many fluid types. Neural network potentials now allow for the possibility, but require care to design and characterization of their limitations. This article develops methods for training and testing neural network potentials under relevant conditions such as cavitation free energy and surface tension, while assessing their limitations through uncertainty analysis.
Superior field electron emission (FEE) characteristics are achieved in edge‐rich diamond‐enhanced carbon nanowalls (D‐ECNWs) grown in a single‐step chemical vapor deposition process co‐doped with ...boron and nitrogen. The structure consists of sharp, highly conductive graphene edges supplied by a solid, diamond‐rich bottom. The Raman and transmission electron microscopy studies reveal a hybrid nature of sp3‐diamond and sp2‐graphene in these nanowalls. The ab‐initio calculations were carried out to support the experimental observations of diamond‐graphene hybrid structure. Finally, this hybrid D‐ECNWs is employed as a cathode in an FEE device resulting in a low turn‐on field of 3.1 V µm−1, a large field enhancement factor, a high FEE Je of 2.6 mA cm−2, and long lifetime stability of 438 min. Such an enhancement in the FEE originates from the unique materials combination, resulting in good electron transport from the graphene phases and efficient FEE of electrons from the sharp edges on the nanowalls. The prospective application of these materials is displayed by employing these hybrids as cathodes in a microplasma device ensuing a low threshold voltage of 160 V and high plasma stability of 140 min, which confirms the role of these hybrid structured nanowalls in the enhancement of electron emission.
Investigation of the novel diamond‐enhanced carbon nanowall nanostructures grown in a microwave plasma is demonstrated. Nanowalls show a noticeable enhancement in the field electron emission characteristics viz. a low turn‐on field and a large field enhancement factor. Graphene phases and diamond sharp edges enhance the field electron emission properties.
