Synthesis of novel transition metal nitrides IrN2 and OsN2

Two new transition metal nitrides, IrN2 and OsN2, were synthesized at high pressures and temperatures using laser-heated diamond-anvil cell techniques. Synchrotron x-ray diffraction was used to determine the structures of novel nitrides and the equations of states of both the parent metals as well as the newly synthesized materials. The compounds have bulk moduli comparable with those of the traditional superhard materials. For IrN2, the measured bulk modulus [K0 = 428(12) GPa] is second only to that of diamond (K0 = 440 GPa). Ab initio calculations indicate that both compounds have a metal:nitrogen stoichiometry of 1:2 and that nitrogen intercalates in the lattice of the parent metal in the form of single-bonded N-N units.     

Stacking stability of MoS_2 bilayer: An ab initio study

The study of the stacking stability of bilayer MoS2 is essential since a bilayer has exhibited advantages over single layer MoS2 in many aspects for nanoelectronic applications. We explored the relative stability, optimal sliding path between different stacking orders of bilayer MoS2, and （especially） the effect of inter-layer stress, by combining first-principles density functional total energy calculations and the climbing-image nudge-elastic-band （CI-NEB） method. Among five typical stacking orders, which can be categorized into two kinds （I： AA＇, AB and II： AA＇, AB＇, A＇B）, we found that stacking orders with Mo and S superposing from both layers, such as AA＇ and AB, is more stable than the others. With smaller computational efforts than potential energy profile searching, we can study the effect of inter-layer stress on the stacking stability. Under isobaric condition, the sliding barrier increases by a few eV/（uc.GPa） from AA＇ to ABt, compared to 0.1 eV/（uc.GPa） from AB to [AB]. Moreover, we found that interlayer compressive stress can help enhance the transport properties of AA＇. This study can help understand why inter-layer stress by dielectric gating materials can be an effective means to improving MoS2 on nanoelectronic applications.     

配合物Benzene-I2结构的从头算研究

采用分子轨道从头算方法,对苯用6-311+G**基组,对碘分子用相对论的有效核实势(RECP5s25p5),以及用密度函数方法(B3LYP),计算了配合物 Benzene-I2可能构形(7种)的结构,总能量和振动频率.经过筛选,同属Cs点群的两种结构是Benzene-I2的稳定结构.自然键轨道 (NBO)分析表明,配合物Benzene-I 2主要是由于苯环的π电子和碘分子的最低空轨道(LUMO)σ*轨道之间的相互作用形成的.本文还给出了这两种结构的势能曲线,并且用含四项Morse函数和一项C6R-6 的势能函数进行曲线拟合,给出了相应的拟合参数.     

An ab initio study of the vibrational properties of alkaline-earth metal nitrates and their crystallohydrates

The vibrational frequencies and corresponding intensities have been calculated ab initio for the center of the Brillouin zone of crystalline magnesium, calcium, strontium, and barium nitrates; magnesium nitrate hexahydrate; and calcium nitrate tetrahydrate. The calculation has been performed within the electron- density functional theory using the PBE exchange-correlation functional in the basis of localized atomic orbitals with the aid of the CRYSTAL14 software. The calculated values and the experimental IR and Raman spectral data on strontium and barium nitrates are shown to be in satisfactory agreement. The frequencies of normal long-wavelength vibrations in the nitrates become red-shifted with an increase in the cation atomic mass. The occurrence of several peaks due to the vibrations of hydrogen atoms in water molecules with different dynamic charges is predicted in the IR spectrum of hexahydrate magnesium in the frequency range above 3430 cm^–1.     

Defect stability in thorium monocarbide:An ab initio study

The elastic properties and point defects of thorium monocarbide(ThC) have been studied by means of density functional theory based on the projector-augmented-wave method. The calculated electronic and elastic properties of ThC are in good agreement with experimental data and previous theoretical results. Five types of point defects have been considered in our study, including the vacancy defect, interstitial defect, antisite defect, schottky defect, and composition-conserving defect. Among these defects, the carbon vacancy defect has the lowest formation energy of 0.29 eV. The second most stable defect(0.49 eV) is one of composition-conserving defects in which one carbon is removed to another carbon site forming a C2 dimer. In addition, we also discuss several kinds of carbon interstitial defects, and predict that the carbon trimer configuration may be a transition state for a carbon dimer diffusion in Th C.     

Superconducting NbB2: An ab initio study of elastic constants

The five different elastic constants of the superconducting NbB₂ are calculated for the first time by ab initio density functional method with both correlation and exchange potentials. In the absence of experimental data, the results are compared with those of other related diborides. The fully relaxed and isotropic bulk moduli are also estimated and the implication of their comparison is made.     

自由基OH（X^П,A^2∑^＋）的从头算研究

用分子轨道从头算方法，研究了OH分子的基态(X^П)和激发态(A^2∑^+)。结果表明，对于基态，在QCISD(T)／6—311++G(3df,3pd)理论水平上，键距是0．09704nm，与实验值0．09706nm完全吻合。对于激发态，使用完全活性空间方法(CASSCF)和大基组6—311++G(3df,3pd)，键距是0．10098nm，与实验值0．10121nm基本吻合。从激发态A^2∑^+(v＝0)到基态X^П(v＝0)的垂直跃迁能量是4．4692eV，与实验值4．3980eV也吻合较好。     

Quantum mechanical ab initio calculations of the structural,electronic and optical properties of bulk gold nitrides

In the present work, the atomic and the electronic structures of Au₃N, AuN and AuN₂ are investigated using first-principles density-functional theory (DFT). We studied cohesive energy vs. volume data for a wide range of possible structures of these nitrides. Obtained data were fitted to a Birch-Murnaghan third-order equation of state (EOS) so as to identify the most likely candidates for the true crystal structure in this subset of the infinite parameter space, and to determine their equilibrium structural parameters. The analysis of the electronic properties was achieved by the calculations of the band structure and the total and partial density of states (DOS). Some possible pressure-induced structural phase transitions have been pointed out. Further, we carried out GW₀ calculations within the random-phase approximation (RPA) to the dielectric tensor to investigate the optical spectra of the experimentally suggested modification: Au₃N(D0₉). Obtained results are compared with experiment and with some available previous calculations.     

Interactions of H with tungsten carbide surfaces: An ab initio study

Adsorption of atomic hydrogen at several coverages on three WC surfaces with occupation of W and C sites is studied using a DFT formalism and generalized gradient approximation. In all three cases, at low coverage, the H adsorption over the W on top site is clearly favoured. Beyond this coverage, the surface occupation of both W and C sites become possible. The calculated adsorption energies ΔE indicate chemisorption. The evolution of the DOS with the coverage is reported. Our results are complemented by determination of surface energies for the (0 0 0 1), (1 0 −1 0) and (1 1 −2 0) surfaces.     

Phenylazoimidazole as a possible optical molecular switch: An ab initio study

By applying nonequilibrium Green's function formalism combined first-principles density functional theory, we investigate the electronic transport properties of the phenylazoimidazole optical molecular switch. The molecule that comprises the switch can convert between the cis and the trans forms upon photoexcitation. The influence of HOMO–LUMO gaps and the spatial distributions of molecular orbitals on the electronic transport through the molecular device are discussed in detail. Theoretical results show that the current through the trans form is significantly larger than through the cis form, which suggests this system has attractive potential application in future molecular switch technology.     

Electronic and optical properties of Si and Ge nanocrystals: An ab initio study

First-principles calculations within density functional theory and many-body perturbation theory have been carried out in order to investigate the structural, electronic and optical properties of undoped and doped silicon nanostructures. We consider Si nanoclusters co-doped with B and P. We find that the electronic band gap is reduced with respect to that of the undoped crystals, suggesting the possibility of impurity based engineering of electronic and optical properties of Si nanocrystals. Finally, motivated by recent suggestions concerning the chance of exploiting Ge dots for photovoltaic nanodevices, we present calculations of the electronic and optical properties of a Ge₃₅H₃₆ nanocrystal, and compare the results with those for the corresponding Si₃₅H₃₆ nanocrystals and the co-doped Si₃₃BPH₃₆.     

贵金属氮化物晶体结构、电子结构和力学性能的实验与理论研究进展

氮化铂（Pt-N）是高温高压条件下合成的第一种块体二元贵金属氮化物,由于该化合物具有高的体弹性模量和特殊的电子结构从而吸引了世界范围内一些理论研究者的目光.Pt-N中金属原子的质量远远大于非金属原子的质量,因此X射线衍射仅能确定Pt原子的位置,拉曼光谱虽然能限制N原子的对称性,但仍不能确定其具体位置.以上因素使得确定这种新合成物质的晶体结构成了理论研究的焦点, 这些工作已经在高压科学中开拓了一个新的领域,即贵金属元素氮化物的高压合成与物性研究.