Probing into the theory of impact sensitivity: propelling the understanding of phonon-vibron coupling coefficients.

The determination of impact sensitivity of energetic materials traditionally relies on expensive and safety-challenged experimental means. This has instigated a shift towards scientific computations to gain insights into and predict the impact response of energetic materials. In this study, we refine the phonon-vibron coupling coefficients ζ in energetic materials subjected to impact loading, building upon the foundation of the phonon up-pumping model. Considering the full range of interactions between high-order phonon overtones and molecular vibrational frequencies, this is a pivotal element for accurately determining phonon-vibron coupling coefficients ζ. This new coupling coefficient ζ relies exclusively on phonon and molecular vibrational frequencies within the range of 0-700 cm-1. Following a regression analysis involving ζ and impact sensitivity (H50) of 45 molecular nitroexplosives, we reassessed the numerical values of damping factors, establishing a = 2.5 and b = 35. This coefficient is found to be a secondary factor in determining sensitivity, secondary to the rate of decomposition propagation and thermodynamic factor (heat of explosion). Furthermore, the relationship between phonon-vibron coupling coefficients ζ and impact sensitivity was studied in 16 energetic crystalline materials and eight nitrogen-rich energetic salts. It was observed that as the phonon-vibron coupling coefficient increases, the tendency for reduced impact sensitivity H50 still exists.

A Method for Predicting the Melting Temperature of Ionic Compounds.

Predicting the melting temperature of materials has always been a topic of great concern. This article proposes an alternative model for determining the melting temperature of materials based on the main idea of the Lindemann melting criterion combined with the first-principles calculations of density functional theory. To verify the accuracy of the melting model, this article selected typical ionic crystals of MgO and 10 alkali metal halides as the validation objects. The calculation results indicate that the melting temperature of the MgO crystals and I-VII compounds is in good agreement with the experimental results.

Molecular Structures, Dipole Moments, and Electronic Properties of ß-HMX under External Electric Field from First-Principles Calculations.

In order to investigate the impact of an external electric field on the sensitivity of ß-HMX explosives, we employ first-principles calculations to determine the molecular structure, dipole moment, and electronic properties of both ß-HMX crystals and individual ß-HMX molecules under varying electric fields. When the external electric field is increasing along the [100], [010], and [001] crystallographic directions of ß-HMX, the calculation results indicate that an increase in the bond length (N1-N3/N1'-N3') of the triggering bond, an increase in the main Qnitro (N3, N3') value, an increase in the minimum surface electrostatic potential, and a decrease in band gap all contribute to a reduction in its stability. Among these directions, the [010] direction exhibits the highest sensitivity, which can be attributed to the significantly smaller effective mass along the [010] direction compared with the [001] and [100] directions. Moreover, the application of an external electric field along the Y direction of the coordinate system on individual ß-HMX molecules reveals that the strong polarization effect induced by the electric field enhances the decomposition of the N1-N3 bonds. In addition, due to the periodic potential energy of ß-HXM crystal, the polarization effect of ß-HMX crystal caused by an external electric field is much smaller than that of a single ß-HXM molecule.

Triggering the mechanism of the initial reaction of energetic materials under pressure based on Raman intensity analysis.

The Raman intensity and other stoichiometric calculations of nitromethane (NM) and 2-nitrimino-5-nitro-hexahydro-1,3,5-triazine (NNHT) have been made by using first-principles density functional theory. We propose a method to judge the initial reaction mechanism of NM and NNHT under pressure based on the Raman intensity. Both the resulting NM and NNHT undergo hydrogen transfer and conventional trigger bond cleavage. And the results obtained from the Raman peak intensities infer a reaction path that is not inferior to the traditional C-NO2 and N-NO2 bond cleavage, thus verifying our results.

Atomic mean square displacement study of the bond breaking mechanism of energetic materials before explosive initiation.

Understanding and predicting the bond breaking mechanism of energetic materials before explosion initiation is one of the huge challenges in explosion science. By means of the mean square displacement of the atom from the equilibrium position and theoretical bond breaking tensile change of the chemical bond, we establish a new criterion to judge whether the chemical bond is broken. Further, α-RDX is used as the verification object to verify the accuracy of this model. We obtained an initial decomposition temperature of 434-513 K for α-RDX at 0 GPa, and the initial bond breaking type was N-NO2. Finally, based on this model, we discussed in detail the breaking of chemical bonds of solid nitromethane near the detonation pressure. We think that the high temperature and high pressure caused by the shock wave may break all the chemical bonds of nitromethane near the detonation pressure.

A simulation study on the phase transition behavior of solid nitrogen under extreme conditions.

There are numerous examples of materials that exhibit interesting phenomena at extremely low temperatures, but the difficulty of obtaining absolute zero at high pressure in experiments is sometimes a hurdle to reveal the exact explanation of these low temperature phenomena. Based on the calculations of the phonon spectrum and Gibbs free energy of α-N2 and γ-N2 under different pressures, we found that solid nitrogen at 0 K showed a re-entrant phase transition under continuously increasing pressure. The extremely low temperature in this pressure range turned out to be the main external condition for inducing phase transition as well as phase reversal.

Composition and structural characteristics of compressed alkaline earth metal hydrides.

The metallization of alkaline earth metal hydrides offers a way to achieve near-room temperature superconductivity. In order to explore the metallization mechanism of these hydrides under pressure, a detailed understanding of the property changes of alkaline earth metal hydrides is required. Based on first-principles calculations, we have systematically investigated the dihydrides (XH2, X = Be, Mg, Ca, Sr, Ba) and tetrahydrides (XH4, X = Mg, Ca, Sr, Ba) of alkaline earth metals, respectively. By applying external pressure, we show that the structures of these alkaline earth metal hydrides undergo a series of phase transitions. Moreover, we investigate how the size of the bandgap decreases and eventually closes and reveal the role of electronegativity of metal elements in the critical pressure of hydride metallization. Remarkably, the hydrogen units (H6 or H8) formed in XH4 can accelerate the metallization process. The increase of the energy level difference in hydrogen units promotes the electroacoustic coupling effect, which is conducive to realization of high superconducting transition temperature (Tc). Our theoretical findings identify MgH4-I4/mmm as having potential to be a high-temperature superconductor and provide unusual ideas for the search of unknown high-temperature superconducting materials.

Electrons and phonons of the discharge products in the lithium-oxygen and lithium-sulfur batteries from first-principles calculations.

Batteries have become a ubiquitous daily necessity, which are popularly applied to mobile phones and electric vehicles according to their size. Improving the battery cycle life and storage is important, but unexpected discharge products still restrict the upper limit of batter performance such as Li2O2, LiO2, and Li2S. In this study, we calculated electrons and phonons presenting the basic energy states in crystal using the first-principles calculations. The Li2O2 and Li2S are almost insulating due to the wide bandgap from their electronic structure, and doped-active p-orbital may be one of the pathways to improve crystal conduction due to the tendency of the density of states. The LiO2 is metallic, and the electronic structure and phonons show that the discharge products have an ionic feature. In addition, the ionic crystal can produce a larger DC permittivity because it possesses macroscopic polarisation. For Li2O2 and Li2S, the Raman peak of the O-O bonding is strong, while the Raman peak of the S-ion is very weak. The enhanced Raman peak of the S-ion presents a possibility to prevent the shuttle effect in Li-S batteries.

Theoretical study on the correlation between the structure, excess energy, surface energy, electronic structure, nitro charge, and friction sensitivity of N,N'-dinitroethylenediamine (EDNA).

The sensitivity of energetic materials along different crystal directions is not the same and is anisotropic. In order to explore the difference in friction sensitivity of different surfaces, we calculated the structure, excess energy, surface energy, electronic structure, and the nitro group along (1 1 1), (1 1 0), (1 0 1), (0 1 1), (0 0 1), (0 1 0), and (1 0 0) surfaces of EDNA based on density functional theory. The analysis results showed that relative to other surfaces, the (0 0 1) surface has the shortest N-N average bond length, largest N-N average bond population, smallest excess energy and surface energy, widest band gap, and the largest nitro group charge value, which indicates that the (0 0 1) surface has the lowest friction sensitivity compared to other surfaces. Furthermore, the conclusions obtained by analyzing the excess energy are consistent with the results of the N-N bond length and bond population, band gap, and nitro charge. Therefore, we conclude that the friction sensitivity of different surfaces of EDNA can be evaluated using excess energy.

Initial Decomposition of DATB Induced by an External Electric Field.

1,3-Diamino-2,4,6-trinitrobenzene (DATB), a nitro aromatic explosive with excellent properties, can be detonated by an electric field. Using first-principles calculation, we have investigated the initial decomposition of DATB under an electric field. In the realm of electric fields, the rotation of the nitro group around the benzene ring will cause deformation of the DATB structure. Furthermore, when an electric field is applied along the [100] or [001] direction, the C4-N10/C2-N8 bonds initiate decomposition due to electron excitation. On the contrary, the electric field along the [010] direction has a weak influence on DATB. These, together with electronic structures and infrared spectroscopy, give us a visual perspective of the energy transfer and the decomposition caused by C-N bond breaking.

The effect of pressure on the structural, electronic and vibrational properties of solid carbon dioxide phases.

The structural, electronic and vibrational properties of solid carbon dioxide phases (I, II, III, and IV) under high pressure are studied using first-principles calculations. The calculated structural parameters are in good agreement with the experimental values. The third-order Birch-Murnaghan equation of state is fitted, and the corresponding parameters are obtained. We obtained the phase boundary points of each phase and plotted the phase diagram of solid carbon dioxide. The influence of pressure on the band structure and density of states is studied. The vibrational properties of the four phases of carbon dioxide were studied in detail, and the infrared and Raman spectra of the four phases were obtained. It can be seen from the calculated spectrum that the number and frequency of vibration peaks are in good agreement with the experimental values. And, we also analyze the influence of pressure on the frequency of vibration mode.

Active Sites in Single-Layer BiOX (X = Cl, Br, and I) Catalysts for the Hydrogen Evolution Reaction.

Although few-layer bismuth oxyhalides (BiOX, X = Cl, Br, and I) have been shown to be appropriate for photocatalytic hydrogen production, the hydrogen evolution reaction (HER) activity of BiOX is unrevealed. Herein, the origins of catalytic activity on single-layer BiOX are investigated by using the density functional theory. The grand potential calculations show that the Bi- and BiO-terminations of single-layer BiOX are stable in O-poor and O-rich environments, respectively. The Bi- and BiO-terminations of single-layer BiOX are found to have obviously active sites for HER, whereas the (001) basal planes are inert. The Gibbs free energies for the adsorption of hydrogen atoms on the Bi- and BiO-terminations are close to the optimal value of 0 eV, indicating that single-layer BiOX possess favorable HER performances. The enhanced HER activities on the Bi- and BiO-terminations are attributed to the localized edge states around the Fermi level, which are caused by the Bi 6p-orbital density of the fringe bismuth atoms and O 2p-orbital density of the fringe oxygen atoms, respectively. The results of this work suggest that single-layer BiOX are a family of promising catalysts for water splitting.

Facile Synthesis of Carbon Dots@2D MoS2 Heterostructure with Enhanced Photocatalytic Properties.

To better utilize carbon dots (CDs) as efficient photocatalysts, an excellent strategy of constructing CDs@MoS2 heterostructure is presented. Here a facile sonication-hydrothermal method is utilized to synthesize CDs@MoS2. Such heterostructure regulates the energy level configuration, and visible light absorption and the separation and transfer of photogenerated charges are enhanced remarkably, which is propitious for the production of more photoinduced charges and improvement of the heterogeneous photocatalytic activity. Meanwhile, the photocatalytic performance of CDs@MoS2 was obviously improved in methylene blue degradation. On the basis of a series of contrast experiments, the possible mechanism of the photocatalytic reaction is proposed. Therefore, this work offers a facile route for the design of a zero-dimensional/two-dimensional heterojunction for the adjustment of the energy level structure and the improvement in photocatalytic performance.

Excitation transfer and trapping kinetics in plant photosystem I probed by two-dimensional electronic spectroscopy.

Photosystem I is a robust and highly efficient biological solar engine. Its capacity to utilize virtually every absorbed photon's energy in a photochemical reaction generates great interest in the kinetics and mechanisms of excitation energy transfer and charge separation. In this work, we have employed room-temperature coherent two-dimensional electronic spectroscopy and time-resolved fluorescence spectroscopy to follow exciton equilibration and excitation trapping in intact Photosystem I complexes as well as core complexes isolated from Pisum sativum. We performed two-dimensional electronic spectroscopy measurements with low excitation pulse energies to record excited-state kinetics free from singlet-singlet annihilation. Global lifetime analysis resolved energy transfer and trapping lifetimes closely matches the time-correlated single-photon counting data. Exciton energy equilibration in the core antenna occurred on a timescale of 0.5 ps. We further observed spectral equilibration component in the core complex with a 3-4 ps lifetime between the bulk Chl states and a state absorbing at 700 nm. Trapping in the core complex occurred with a 20 ps lifetime, which in the supercomplex split into two lifetimes, 16 ps and 67-75 ps. The experimental data could be modelled with two alternative models resulting in equally good fits-a transfer-to-trap-limited model and a trap-limited model. However, the former model is only possible if the 3-4 ps component is ascribed to equilibration with a "red" core antenna pool absorbing at 700 nm. Conversely, if these low-energy states are identified with the P700 reaction centre, the transfer-to-trap-model is ruled out in favour of a trap-limited model.

Schottky barrier engineering via adsorbing gases at the sulfur vacancies in the metal-MoS2 interface.

Sulfur vacancies (S-vacancies) are common in monolayer MoS2 (mMoS2). Finding an effective way to control rather than abolish the effect of S-vacancies on contact properties is vital for the application of mMoS2. Here, we propose the adsorption of gases to passivate the S-vacancies in Pt-mMoS2 interfaces. Results demonstrate that gases are stably and preferentially adsorbed at S-vacancies. The n-type Schottky barriers of Pt-mMoS2 interfaces are reduced significantly upon the adsorption electron-donor gases, especially Cl2. The n-type transport character of the Pt-mMoS2 interface can be changed to p-type by the adsorption of electron-acceptor gases. As the adsorption concentration increases, both n- and p-type Schottky barriers are further reduced, and the lowest n- and p-type Schottky barriers are 0.36 and 0 eV, respectively. Note that the variations in Schottky barriers are independent of the oxidizing ability of gases but relative to the average number of valence electrons per gas atom. Analysis demonstrates that although gases at S-vacancies cannot cause gap states to vanish, and can even enhance Fermi level pinning, they modulate charge redistribution and the potential step at the interface region. Moreover, with increasing adsorption concentration, the valence band maximum of mMoS2 shows the opposite variation tendency to that of the potential step. Our results suggest that adsorption of gases is an effective way to passivate S-vacancies to modulate the transport properties of Pt-mMoS2 interfaces.

Non-invasively improving the Schottky barriers of metal-MoS2 interfaces: effects of atomic vacancies in a BN buffer layer.

Using first-principles calculations within density functional theory, vacancies in the BN buffer layer have been predicted to improve the Schottky barrier of the metal-MoS2 interface without deteriorating the intrinsic properties of the MoS2 layer. Here, the effects of concentrations, sizes and types of vacancies on the contact properties of metal/BN-MoS2 sandwich interfaces are comparatively studied. The results show that vacancies in the BN buffer layer not only don't deteriorate the charge scatterings and electronic properties of the MoS2 layer at the metal/BN-MoS2 interface, but also improve the charge density and contact resistance between the metal surface and the BN layer. Although these vacancies have a negligible influence on the Fermi level pinning effect of the metal/BN-MoS2 interface, both N-vacancies and B-vacancies significantly change the position of the Fermi level of the metal/BN-MoS2 interface and then tune the Schottky barriers. Moreover, the Schottky barriers of metal/BN-MoS2 interfaces can decrease at first with the increasing concentrations and sizes of vacancies. When the concentration of vacancies increases to 4%, the Schottky barriers of metal/BN-MoS2 interfaces can reduce to the minimum value. The lowest n-type and p-type Schottky barriers of Au/BN-MoS2 and Pt/BN-MoS2 interfaces can reduce to -0.16 and 0.28 eV, respectively. However, the Schottky barriers are deteriorated when the sizes and concentrations of vacancies continue to increase because vacancies with large sizes and concentrations obviously change the interfacial structures of metal/BN-MoS2 interfaces and disarrange the directions of interface dipoles. The predictions in this work provide a non-invasive method to achieve high performance metal-MoS2 interfaces with low Schottky barriers.

Characteristics of lateral and hybrid heterostructures based on monolayer MoS2: a computational study.

Novel MoS2/(MX2)n lateral and (MoS2)/(MX2)n-BN hybrid heterostructures have been designed on monolayer MoS2 to extend its applications. The electronic, interfacial and optical properties of the lateral and hybrid heterostructures have been investigated comparatively using first-principles calculations. It was found that the charge distributions, band gaps, band levels, electrostatic potentials, and optical absorption of the MoS2/(MX2)n lateral heterostructures depend greatly on the width n of MX2, irrespective of the size of the lateral heterostructures. The CBM states of the MoS2/(MX2)n lateral heterostructures dominated by the dz2 orbitals are localized around MoS2, whereas the VBM states of the MoS2/(MX2)n lateral heterostructures are dominated by the MX2 region. Through regulating the width n of the MX2 region in the MoS2/(MX2)n lateral heterostructures, the optical absorption of the lateral heterostructures under visible light can be increased, and the CBM and VBM states of the lateral heterostructures can be located above the hydrogen reduction potential and below the water oxidation potential, respectively. The similar characteristics were observed in the MoS2/(MX2)n-BN hybrid heterostructures, indicating that BN is a good substrate for the MoS2/(MX2)n lateral heterostructures. The analysis implies that forming the lateral and hybrid heterostructures is an effective way to extend the applications of monolayer MoS2 in photocatalytic water and photovoltaic devices.

The modulation of Schottky barriers of metal-MoS2 contacts via BN-MoS2 heterostructures.

Using first-principles calculations within density functional theory, we systematically studied the effect of BN-MoS2 heterostructure on the Schottky barriers of metal-MoS2 contacts. Two types of FETs are designed according to the area of the BN-MoS2 heterostructure. Results show that the vertical and lateral Schottky barriers in all the studied contacts, irrespective of the work function of the metal, are significantly reduced or even vanish when the BN-MoS2 heterostructure substitutes the monolayer MoS2. Only the n-type lateral Schottky barrier of Au/BN-MoS2 contact relates to the area of the BN-MoS2 heterostructure. Notably, the Pt-MoS2 contact with n-type character is transformed into a p-type contact upon substituting the monolayer MoS2 by a BN-MoS2 heterostructure. These changes of the contact natures are ascribed to the variation of Fermi level pinning, work function and charge distribution. Analysis demonstrates that the Fermi level pinning effects are significantly weakened for metal/BN-MoS2 contacts because no gap states dominated by MoS2 are formed, in contrast to those of metal-MoS2 contacts. Although additional BN layers reduce the interlayer interaction and the work function of the metal, the Schottky barriers of metal/BN-MoS2 contacts still do not obey the Schottky-Mott rule. Moreover, different from metal-MoS2 contacts, the charges transfer from electrodes to the monolayer MoS2, resulting in an increment of the work function of these metals in metal/BN-MoS2 contacts. These findings may prove to be instrumental in the future design of new MoS2-based FETs with ohmic contact or p-type character.

Designing high performance metal-mMoS2 interfaces by two-dimensional insertions with suitable thickness.

Thickness has been proved to have significant influence on the physical properties of two-dimensional (2D) materials and their corresponding devices. Understanding the effect of the thickness of 2D insertions on the contact properties of metal-monolayer MoS2 interfaces (viz. metal-mMoS2 interfaces) is vital to designing high performance mMoS2 devices. In this work, the electronic structures, Schottky barriers, contact resistance, and tunneling barriers of scandium-mMoS2 (Sc-mMoS2) interfaces with BN and graphene insertions have been comparatively studied by density functional theory. No Schottky barriers are found at Sc-mMoS2 interfaces with monolayer 2D insertions. Although the contact resistance and charge injection efficiency of Sc-mMoS2 interfaces with monolayer insertions deteriorate relatively to those of the Sc-mMoS2 interface, they are still sufficient to realize high-performance mMoS2-based devices. Note that, upon increasing the number of layers of 2D insertions, these contact properties are further deteriorated with the increasing number of layers of insertions. Moreover, additional significant Schottky barriers are introduced into Sc-mMoS2 interfaces with multilayer BN; the nature Dirac points of graphene insertions are opened, suggesting low performances of Sc-mMoS2 interfaces with multilayer BN and graphene insertions. These variations can be understood on the basis of the orbital hybridization and charge redistribution between the Sc slab and mMoS2 layer. In addition, these characteristics are expected to occur in other metal-mMoS2 interfaces with two-dimensional material insertions. Overall, monolayer rather than multilayer two-dimensional insertions can be used to improve the transport properties of mMoS2-based devices.

Influences of S, Se, Te and Po substitutions on structural, electronic and optical properties of hexagonal CuAlO2 using GGA and B3LYP functionals.

The effects of X-doping (X = S, Se, Te and Po) on the structural, electronic and optical properties of hexagonal CuAlO2 were studied using first-principles density functional theory. The calculated results showed the obtained lattice constants to increase with increasing atomic number, and the X-doping to be energetically more favorable under Al-rich conditions. The calculated electronic properties showed decreased bandgaps with increasing atomic number, which was due to the better covalent hybridizations after sulfuration doping. The enhanced covalency was further confirmed by calculating the Mulliken atomic populations and bond populations. The density of states indicated the increase of the contribution to antibonding from the X-p states to be a benefit for p-type conductivity. Moreover, the X-doping induced a red shift of the absorption edge.