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Pressure-induced polymerization of carbon monoxide (CO) molecules may lead to next-generation high energy density materials. By combining structural search method, first-principles calculations and ab initio molecular dynamics (AIMD) simulations, we predict several polymeric carbon oxide ${C}_{x}{O}_{y}$, i.e., ${\mathrm{C}}_{5}{\mathrm{O}}_{2}$ $(P\text{\ensuremath{-}}4m2)$, ${\mathrm{C}}_{6}{\mathrm{O}}_{4}$ ($P\text{\ensuremath{-}}4m2$ and $I\text{\ensuremath{-}}4m2$) and ${\mathrm{C}}_{8}{\mathrm{O}}_{8}$ $(I\text{\ensuremath{-}}4m2)$, which are all layered semiconductors with high energy density, large bulk modulus, and high hardness. For the $\mathrm{C}:\mathrm{O}\phantom{\rule{0.28em}{0ex}}\mathrm{ratio}=1:1$, the ${\mathrm{C}}_{8}{\mathrm{O}}_{8}\text{\ensuremath{-}}I\text{\ensuremath{-}}4m2$ phase is energetically more stable than the reported $Cmca$ and $Cmcm$ phases above 90 GPa, and its energy density is 4.51 kJ/g, which is higher than trinitrotoluene (TNT). Considering the mechanism of ${\mathrm{CO}}_{2}$ release, ${C}_{x}{O}_{y}(xgy)$ crystals, namely ${\mathrm{C}}_{5}{\mathrm{O}}_{2}$ $(P\text{\ensuremath{-}}4m2)$ and ${\mathrm{C}}_{6}{\mathrm{O}}_{4}$ ($P\text{\ensuremath{-}}4m2$ and I-4m2) are predicted above $40\ensuremath{\sim}50\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. At 100 GPa, phonon spectrum calculations and AIMD simulations indicate that they have good mechanical and dynamic stability. At 0 GPa, AIMD simulations also show the possible phase transition path of the four ${C}_{x}{O}_{y}$ structures by releasing ${\mathrm{CO}}_{2}$: ${\mathrm{C}}_{8}{\mathrm{O}}_{8}\text{\ensuremath{-}}I\text{\ensuremath{-}}4m2\ensuremath{\rightarrow}{\mathrm{C}}_{6}{\mathrm{O}}_{4}\text{\ensuremath{-}}I\text{\ensuremath{-}}4m2$ and ${\mathrm{C}}_{6}{\mathrm{O}}_{4}\text{\ensuremath{-}}P\text{\ensuremath{-}}4m2\ensuremath{\rightarrow}{\mathrm{C}}_{5}{\mathrm{O}}_{2}\text{\ensuremath{-}}P\text{\ensuremath{-}}4m2$, and ${\mathrm{C}}_{6}{\mathrm{O}}_{4}\text{\ensuremath{-}}I\text{\ensuremath{-}}4m2$ and ${\mathrm{C}}_{5}{\mathrm{O}}_{2}\text{\ensuremath{-}}P\text{\ensuremath{-}}4m2$ can remain stable at ambient conditions. Those structures may enrich the phase diagram of high-pressure ${C}_{x}{O}_{y}$, and provide clues for synthesis and exploration of new stable energetic materials besides polymeric nitrogen.

Metal-pentazolate compounds as candidates for novel high-energy-density materials have attracted extensive attention in recent years. However, dehydrated pentazolate salts of transition metal iron are rarely reported. We predict two new iron pentazolate salts Fdd2-FeN10 and P 1 ¯ (No.1)-FeN10 using a constrained crystal search method based on first-principles calculations. We propose that the stable Fdd2-FeN10 crystal may be synthesized from FeN and N2 above 20 GPa, and its formation enthalpy is lower than the reported iron pentazolate salt (marked as P 1 ¯ (No.2)-FeN10). Crystal P 1 ¯ (No.1)-FeN10 is composed of iron bispentazole molecules. Formation enthalpy, phonon spectrum and ab initio molecular dynamics calculations are performed to show their thermodynamic, mechanical and dynamic properties. Moreover, the high energy density (3.709 kJ/g, 6.349 kJ/g) and good explosive performance indicate their potential applications as high-energy-density materials.

Abstract Tantalum disulfide (TaS2) two-dimensional film material has attracted wide attention due to its unique optical and electrical properties. In this work, we report the preparation of 1 T-TaS2 quantum dots (1 T-TaS2 QDs) by top-down method. Herein, we prepared the TaS2 QDs having a monodisperse grain size of around 3 nm by an effective ultrasonic liquid phase exfoliation method. Optical studies using UV-Vis, PL, and PLE techniques on the as-prepared TaS2 QDs exhibited ultraviolet absorption at 283 nm. Furthermore, we found that dimension reduction of TaS2 has led to a modification of the band gap, namely a transition from indirect to direct band gap, which is explained using first-principle calculations. By using quinine as reference, the fluorescence quantum yield is 45.6%. Therefore, our results suggest TaS2 QDs have unique and extraordinary optical properties. Moreover, the low-cost, facile method of producing high quality TaS2 QDs in this work is ideal for mass production to ensure commercial viability of devices based on this material. Graphical abstract TaS2 quantum dots having a monodisperse grain size of around 3 nm have been prepared by an ultrasonic liquid phase exfoliation method, it has been found that the dimension reduction of TaS2 has led to a transition from indirect to direct band gap that results in the unique and extraordinary optical properties (PL QY: 45.6%).

Additional file 1: Fig. S1. (a) and (c) show the HR-TEM images of the TaS2 QDs after centrifugation at 16000 rpm for 30 minutes. (b) Particle size distribution of TaS2 QDs. (d) The line profile of the TaS2 QDs diffraction fringes. Fig. S2. (a) UV-Vis absorption spectra of TaS2 QDs after centrifugation at 16000 rpm for 30 minutes. (b) UV-Vis absorption spectra of TaS2 QDs compared between 16000 rpm for 30 minutes and 7000 rpm for 25 minutes. (c) and (d) show normalized PL spectra of TaS2 QDs with excitation wavelength (λEx) of 250 nm and 270 nm, respectively.

Understanding the structure-property relationships of energetic compounds is challenging. Herein, by including the experimental data, we systematically evaluated the microscopic characteristics of a series of transition metal carbohydrazide perchlorate (TMCP) complexes (MnCP, FeCP, CoCP, NiCP, ZnCP, and CdCP) by first-principles calculations. The calculated properties, i.e., lattice enthalpy, bulk modulus, and electronic structures, were correlated with their thermal decomposition temperatures and impact sensitivities, which indicated that the stability and sensitivity of the TMCP complexes were greatly changed through coordination with different metal ions. The trend was that a large lattice enthalpy indicated a better thermal stability. Complexes with a high impact sensitivity tended to have a smaller bulk modulus and pseudo-gap. The ultra-high impact sensitivity of FeCP may have been related to the unstable spin state with respect to the volume change in the lattice. The calculated bond order and bond dissociation energy did not fully reflect the impact and friction sensitivities in this study. In addition, the combination of crystal properties and local bond information may better describe the sensitivity trend for the TMCP energetic compounds. This analysis can be applied to other energetic compounds and may provide clues for the synthesis and assessment of novel energetic compounds.

By doing first-principles based simulations, a reaction of zinc cation catalyzing the carbonyl hydrazine C N bond cleavage was verified by investigating the flash pyrolysis processes of carbohydrazide, carbohydrazide perchlorate and zinc carbohydrazide perchlorate (GTX) crystals. We propose a mechanism of nitrogen gas generation that, perchlorate anions release oxygen radicals to seize hydrogen atoms of carbohydrazide, then coordination with zinc facilitates the C N bond cleavage in carbohydrazide. The center single zinc cation of GTX decreases the barrier of C N bond cleavage. The lowest energy barrier of breaking C N bond is 127.33 kJ mol−1. The simulations reveal how the oxidizing perchlorate anion and the center metal contribute to the flash pyrolysis of GTX via the zinc cation catalyzed carbonyl hydrazine C N bond cleavage. The catalytic mechanism of transition metal in primary explosives opens new windows for designing coordination energetic compounds.

InN has received considerable attentions due to its small band-gap and unique properties in the III-nitride family. Understanding the effects of defects on optoelectronic properties of InN nanowire is essential for exploring its applications in future nanodevices. In this work, we have systematically calculated defect states in InN nanowires based on density-functional theory. Hydrogen passivation and several potential intrinsic point defects are considered in various sizes of nanowires, as well as charged defect states. For small-sized hexagonal nanowires, V N at N-poor condition or N In at N-rich condition is the most stable defect. Whereas for larger-sized nanowires, V N and InN defects are competing when the N chemical potential changes, showing obvious size effect of the defect stability on the nanowire surface. Those defect states change the electronic structure of the nanowires drastically by introducing empty bands or deep level and provide possibility to tailor the optical properties in terms of forming different stable defects.

Neutron diffraction studies have been carried out on carbon monoxide (CO) in the temperature range of 14–68 K. From the data of the temperature dependent crystal parameters, the volume thermal expansion β (T), C–O bond length, and intermolecular distance of CO have been determined and compared with calculations. The volume evolution of cubic CO solids indicates a normal thermal expansion. However, the bond length of CO molecules contracts with the increase in temperature. Correspondingly, the distance between CO molecules increases much faster with an increase in the temperature. We find that intermolecular and intramolecular interactions account for these abnormal temperature behaviors of CO molecular crystals. The abnormal change observed in the curve β (T), evolution of C–O bond length, and distance between two CO molecules indicate an order–disorder phase transition induced by head-to-tail reorientations of CO dipoles.