Formation and properties of rocksalt-type AlN and implications for high pressure phase relations in the system Si–Al–O–N

Pressure-dependent thermodynamic properties of the ambient and high pressure phases of aluminum nitride (w-AlN and rs-AlN) were calculated from first principles in order to determine their phase boundary in the p? T phase diagram. These predictions were checked by static HP/HT experiments, using a multianvil press and an Al/N/H precursor with low decomposition temperature as educt. The experimental data show that at temperatures between 1000 and 2000 K, the boundary line between the two phases is situated between 11 and 12 GPa, which is ～1.3 GPa lower than the theoretical result and generally lower than previously assumed. The hardness of rs-AlN – measured for the first time – is ～30 GPa (Knoop indenter at loads of 25–50 g), twice as hard as w-AlN. Shock wave recovery experiments on nano w-AlN allowed testing of the chemical and thermal stability of rs-AlN, and determination of its infrared absorption and ²⁷Al NMR data. The shock wave technique will eventually enable the synthesis of larger amounts of rs-AlN, making it available for technological use. Finally, implications on the high pressure stability of phases in the Si–Al–O–N system are discussed in the light of thermoelastic properties of AlN.     

Escape of photo-detached electron from an annular nanomicrocavity

The escaped probability density of the photo-detached electron in an annular nanomicrocavity shows strong oscillations as a function of the length of the escape orbits. We present a semiclassical theory that describes theses oscillations in terms of bundles of escape orbits. Due to the interference effects of the electron waves travelling along different escaped orbits, oscillatory structures appear in the escaped probability density. In addition, the calculation results suggest that the escaped probability density of the photo-detached electron is not only related to the inner radius of the annular microcavity, but also related to the laser polarization. In order to show the correspondence between the escaped probability density and the detached electron’s escaped orbits clearly, we calculate the Fourier transformed semiclassical wave function and find that the peak positions agree well with the length of the detached electron’s orbits. We hope that our results will be useful in understanding the escape and propagation process of particles through semiconductor microjunctions or ballistic microstructure.     

Tunable charge density wave in TiS_3 nanoribbons

Recently, modifications of charge density wave(CDW) in two-dimensional(2D) show intriguing properties in quasi-2D materials such as layered transition metal dichalcogenides(TMDCs). Optical, electrical transport measurements and scanning tunneling microscopy uncover the enormous difference on the many-body states when the thickness is reduced down to monolayer. However, the CDW in quasi-one-dimensional(1D) materials like transition metal trichalcogenides(TMTCs) is yet to be explored in low dimension whose mechanism is likely distinct from their quasi-2D counterparts.Here, we report a systematic study on the CDW properties of titanium trisulfide(TiS_3). Two phase transition temperatures were observed to decrease from 53 K(103 K) to 46 K(85 K) for the bulk and 15-nm thick nanoribbon, respectively,which arises from the increased fluctuation effect across the chain in the nanoribbon structure, thereby destroying the CDW coherence. It also suggests a strong anisotropy of CDW states in quasi-1D TMTCs which is different from that in TMDCs.Remarkably, by using back gate of-30 V ～ 70 V in 15-nm device, we can tune the second transition temperature from110 K(at-30 V) to 93 K(at 70 V) owing to the altered electron concentration. Finally, the optical approach through the impinging of laser beams on the sample surface is exploited to manipulate the CDW transition, where the melting of the CDW states shows a strong dependence on the excitation energy. Our results demonstrate TiS_3 as a promising quasi-1D CDW material and open up a new window for the study of collective phases in TMTCs.     

Effect of fused silica subsurface defect site density on light intensification

The effect of defect density on the modulation of incident laser waves is investigated. First, based on the actual defect distribution in the subsurface of fused silica, a three-dimensional (3D) grid model of defect sites is constructed. The 3D finite-difference time-domain method is developed to solve the Maxwell equations. Then the electrical field intensity in the vicinity of the defect sites in the subsurface of fused silica is numerically calculated. The relationships between the maximal electrical field intensity in fused silica and the geometry of the defect sites are given. The simulated results reveal that the modulation becomes more remarkable with an increase of the defect density. In addition, the effect of the distribution mode of defects on modulation is discussed. Meanwhile, the underlying physical mechanism is analyzed in detail.     

First-principle study on optical properties of N-La-codoped anatase TiO

The electronic structures, deformation charge density, dipole moment, and optical properties of N-La-codoped anatase titanium dioxide (TiO2) are studied using the plane-wave ultrasoft pseudopotential method based on the density functional theory (DFT). The optical properties of two-ion-doped TiO2 are analyzed via electronic structures, deformation charge density, and dipole moment. For the model of N-La-doped TiO2 , a smaller atom fraction of N and La atoms induces better optical properties. The absorption edges of two doped TiO2 models redshift to the visible-light region.     

Structure and detonation performance of a novel HMX/LLM‐105 cocrystal explosive

Intermolecular interactions and properties of octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐ tetrazocine (HMX) / 2,6‐diamino‐3,5‐dinitropyrazine‐1‐oxide (LLM‐105) cocrystal were studied by using the dispersion‐corrected density functionals (ωB97XD, B97D) and meta‐hybrid functional (M062x) methods. Binding energies, heats of formation, thermodynamic properties, atoms in molecules, and natural bond orbital analysis were performed to investigate HMX/LLM‐105 complexes. Results show that the main intermolecular interactions between HMX and LLM‐105 are CH…O, NH…O, N…O, and O…O interactions. In addition, Monte Carlo simulation was employed to predict the crystal structure of HMX/LLM‐105 cocrystal. The HMX/LLM‐105 cocrystal is most likely to crystallize in C2/c space group, and its corresponding cell parameters are Z = 8, a = 41.63 Å, b = 6.77 Å, c = 45.63 Å, ß = 164.55°, and ρ = 1.99 g/cm³. Detonation velocity and pressure of HMX/LLM‐105 cocrystal are 8.95 km/s, 37.69GPa, a little lower than those of HMX (9.10 km/s, 37.76GPa). However, according to the net charges of nitro group, HMX/LLM‐105 cocrystal exhibits less sensitive than HMX. Finally, bond dissociation energy calculation shows that HMX/LLM‐105 complexes are thermally stable. Considering thermal stability, sensitivity, and detonation performance, HMX/LLM‐105 cocrystal meets the requirements of insensitive high energy density materials. Copyright © 2013 John Wiley & Sons, Ltd.     

基于发射光谱及成分分析的激光除漆机理研究

基于激光诱导击穿光谱和X射线能谱技术,测量了激光与油漆作用时发射光谱及作用前后元素成分的变化,以此研究了激光除漆的机理。实验测量了不同纳秒激光能量下激光诱导击穿油漆表面的光谱,计算了等离子体的电子密度和温度。通过扫描电子显微镜对油漆烧蚀形貌进行了分析,采用X射线能谱仪测量了烧蚀前后油漆成分的变化。研究结果表明,等离子体电子密度、温度以及烧蚀区域大小都随着入射激光能量的增加逐渐增加。在激光作用前后油漆中碳(C)含量明显降低,原子百分比从78.25%降低到67.07%,说明激光与油漆作用过程中发生了烧蚀。通过对比钛(Ti)元素、C元素和铝(Al)元素的相对原子比例,表明更高的激光能量下油漆烧蚀的更剧烈。该工作对深入研究激光除漆机理有重要意义。     

吩噻嗪表面增强拉曼光谱的密度泛函理论研究

吩噻嗪是含有N、S的杂环化合物,具有大的离域π键。用Gaussian 09程序,在B3LYP方法下,Ag原子使用LANL2DZ赝式基组,C、H、N、S等原子使用6-31++G(d,p)基组,优化并计算了吩噻嗪和吩噻嗪银配合物(PTH-Ag)的拉曼光谱,并利用GaussianView对PTH分子进行归属,为食品和产品中吩噻嗪残留的定性、定量测定提供理论依据。计算结果说明连接Ag原子越多,增强效果越明显。