Surface diffusion of dysprosium on the W(1 1 1) facet

Diffusion of dysprosium on the (1 1 1) facet of a tungsten micromonocrystal was investigated by means of spectral analysis of field emission current fluctuations. The experimental spectral density functions of the current fluctuations were analysed by using Gesley and Swanson’s theoretical spectral density function, which enables to determine the surface diffusion coefficient D for dysprosium. Derived from the temperature dependence of D, the diffusion activation energy E is presented for some Dy coverages θ_(1 1 1). In the temperature range 400–600 K, the E first drops from 1.25 eV per atom at θ₍₁₁₁₎≈0.25 ML to 0.48 eV per atom at θ₍₁₁₁₎≈1 ML (corresponding to the minimum of the work function of the system), then increases to 1.03 eV per atom at θ₍₁₁₁₎≈1.3 ML. The results are discussed from the aspects of the substrate structure and interaction in the adsorbed layer.     

(CH3)2和(NH2)2基团修饰的齐聚苯乙炔分子电子输运性质研究

采用密度泛函理论和非平衡格林函数相结合的方法研究了S原子作为单、双端基的(CH₃)₂-OPE (齐聚苯乙炔)和(NH₂)₂-OPE分子在金电极间的电子输运性质. 通过第一性原理优化计算获得分子部分稳定结构, 再置于Au电极之间构成两极系统, 然后再优化整个两极系统获得稳定结构. 另外, 通过非平衡格林函数方法计算了两极系统的电子输运性质. 计算结果表明, 不同的修饰基团和桥接方式可以导致两极系统的开关效应、负微分电阻行为和整流行为等不同的电子输运性质. 通过计算不同偏压下的分子体系投影轨道电子分布、透射谱、态密度, 对这些新异的电输运性质出现的机理进行了解释.     

有机分子的结构与排列方式对原子电荷分布及静电作用的影响

对由两个相同的长直链分子(CH3(CH2)5—R(R=COOH,CH3,OH)、CH3(CH2)4—COOH))呈镜面对称分布组成的四种模型,及由两个CH3(CH2)5COOH分子平行分布组成的模型进行了量化计算,研究了分子间距、功能团、链长及排列方式对原子电荷分布及分子静电相互作用的影响.结果表明:1)分子中不同位置的亚甲基团(—CH2—)的C原子电荷各不相同.2)原子电荷不仅受到分子链长及功能团的影响,同时,当分子间距及排列方式发生改变时,原子电荷也发生改变;双分子模型较单分子模型的原子电荷变化较大.3)分子间静电作用由尾基功能团的极性决定,由强到弱为—COOH—OH—CH3,分子中其他原子对静电作用的贡献较小;分子链长的增加导致尾基功能团中电荷减少,从而使得分子间静电作用减弱.     

石墨烯在Al_2O_3(0001)表面生长的模拟研究

基于密度泛函理论的广义梯度近似法,对用化学气相沉积法在蓝宝石(α-Al_2O_3)(0001)表面上生长石墨烯进行理论研究.研究结果表明:CH_4在α-Al_2O_3(0001)表面上的分解是吸热过程,由CH_4完全分解出C需要较高能量及反应能垒,这些因素不利于C在衬底表面的存在.在α-Al_2O_3(0001)表面,石墨烯形核的活跃因子并不是通常认为的C原子,而是CH基团.通过CH基团在α-Al_2O_3(0001)表面上的迁移聚集首先形成能量较低的(CH)_x结构.模拟研究(CH)_x对揭示后续石墨烯的形核生长机理具有重要意义.     

Computer simulation of a carbon-deposition plasma in CH4

A transport and reaction model of a low-pressure, high-frequency (13.56 MHz) CH₄ plasma used for diamondlike carbon (a-C:H) deposition was developed. The model includes reactions among four molecular species (CH₄, C₂ H₆, C₂H₄, and H₂), five radicals and atom (CH₃, CH₂, CH, C₂H₅, and H), and four ions (CH₄⁺ , CH₃⁺, CH₅⁺, and C ₂H₅⁺). It also accounts for the influence of the sticking coefficient of species at the walls. Calculated values of the dissociation degree for several flow rates are in good agreement with experimental measurements made by quadrupole mass spectroscopy. A simple surface-model based on the hydrogen coverage of surface and ion flux and energy at the substrate surface was established. This model permitted the calculation of the deposition rate on the powered electrode as a function of the power applied to this electrode. Good agreement between experimental and calculated growth rates was obtained when CH₃, C₂H₅, and CH₂ were assumed to participate in film formation, and when hydrogen removal by ion bombardment with variable energy as a function of the power was included in the model     

The synthesis of allyl ether functionalized siloxane monomers under ultrasonic irradiation at ambient conditions

The novel class of cationic UV-curable allyl ether functionalized siloxane monomers type CH₂=CH---CH₂---O---R---O---Si(CH₃)₃ and type CH₂=CH---CH₂---O---R---O---Si(CH₃)₂---O---R---CH₂---CH=CH₂ have been synthesized in excellent yields in short times at ambient temperature conditions in the absence of any added catalyst by the classical silylation reactions of homoallyloxyalcohols with chlorosilanes under ultrasonic irradiation.     

NMR study of monomethylammonium cation in (CH3NH3)5Bi2Cl11 ferroelectric polycrystal

Spin-lattice relaxation times T₁ and T_1ρ are determined for protons in three polycrystals (CH₃NH₃)₅Bi₂Cl₁₁, (CD₃NH₃)₅Bi₂Cl₁₁ and (CH₃ND₃)₅Bi₂Cl₁₁. The temperature dependencies of the relaxation times obtained for (CH₃NH₃)₅Bi₂Cl₁₁ and (CD₃NH₃)₅Bi₂Cl₁₁ are interpreted as a result of correlated motions of the three-proton groups of the monomethylammonium cation. The minimum of the T_1ρ relaxation time is explained as a result of the oscillations of the symmetry axis of the whole cation.     

Long-distance super-exchange and quantum magnetic relaxation in a hybrid metal–organic framework

The hybrid metal–organic framework [(CH3)2NH2]Fe(HCOO)3with a perovskite-like structure exhibits a variety of unusual magnetic behaviors at low temperatures. While the long-distance super-exchange through the Fe-O–CH-O–Fe exchange path leads to a canted antiferromagnetic ordering at TN～ 19 K, a second transition of magnetic blocking develops at TB～ 9 K. The stair-shaped magnetization hysteresis loops below TBresemble the behaviors of resonant quantum tunneling of magnetization in single-molecular quantum magnets. Moreover, the magnetic relaxation also exhibits several features of resonant quantum relaxation, such as the exponential law with a single characteristic relaxation time, and the nonmonotonic dependence of relaxation rate on the applied magnetic field with a much faster relaxation around the resonant fields. The origin of quantum tunneling behaviors in the [(CH3)2NH2]Fe(HCOO)3metal–organic framework is discussed in terms of magnetic phase separation due to the modification of hydrogen bonding on the long-distance super-exchange interaction.     

Electrical transport and superconductivity in the Al2O3-Bi2Sr1.8Ca1.2Cu2Oy and MgO-Bi2Sr1.8Ca1.2Cu2Oy composites

We have measured the resistivities of Al₂O₃-Bi₂Sr_1.8Ca_1.2Cu₂O_y and MgO-Bi₂Sr_1.8Ca_1.2Cu₂O_y composites with the nominal Bi₂Sr_1.8Ca_1.2Cu₂O_y volume fraction, ₂₂₁₂, ranging from 0.15 to 1.00. For the Al₂O₃-Bi₂Sr_1.8Ca_1.2Cu ₂O_y composites, we find for the samples with ₂₂₁₂≥0.6 that the superconducting transition temperature, T_c, is not disturbed by the addition of Al₂O₃. For ₂₂₁₂<0.3, no zero-resistivity state is observed. For the MgO-Bi₂Sr_1.8Ca_1.2Cu₂O_y composites, T_c is barely disturbed for the samples with ρ₂₂₁₂≥0.7. No superconducting state is observed for the samples with ρ₂₂₁₂<0.35. The variation of (300 K) with ρ₂₂₁₂ indicates a three-dimensional percolating Bi-Sr-Ca-Cu-O matrix occurring at ρ₂₂₁₂≈0.19 and ≈0.15 in Al₂O₃-Bi₂Sr_1.8Ca_1.2 Cu₂O_y and MgO-Bi₂Sr_1.8Ca_1.2Cu₂O_y, respectively. Both resistivity and magnetization measurements suggest that the reactions of Bi₂Sr_1.8Ca_1.2Cu₂O_y with MgO are weaker than with Al₂O₃.     

Magnetic susceptibility of (IPA)2FeCl4, (IBuA)2FeCl4 and (IPA)2CuCi4 (IPA = isopropylammonium ion and IBuA = isobutylammonium ion)

The magnetic susceptibility of (IPA)₂FeCl₄, (IBuA)₂FeCl₄ and (IPA)₂CuCl₄, where IPA = (CH₃)₂CHNH⁺₃ and IBuA = (CH₃)₂CHCH₂NH⁺₃, were measured in the temperature range form 80 K up to a temperature near the melting points of the samples. The results obtained were interpreted in termw of a canted two-dimensional antiferromagnet. The variation of the magnetic susceptibility with the change in the composition was also discussed.     

Preparation of LiMn2O4 thin films by a sol–gel method

LiMn₂O₄ thin films were prepared by a sol–gel method using spin-coating and annealing processes. Anhydrous Mn(CH₃COCHCOCH₃)₃ (manganese acetylacetonate) and LiCH₃COCHCO–CH₃ (lithium acetylacetonate) were chosen as source materials. The film electrochemical properties depended on the drying temperature even when subjected to the same annealing conditions. The discharge capacity of annealed film increased as the drying temperature was increased. However, the rate of capacity fading during cycling increased as the drying temperature was increased.     

A quantum chemical study of ZrO2 atomic layer deposition growth reactions on the SiO2 surface

Zirconium oxide (ZrO₂) is one of the leading candidates to replace silicon oxide (SiO₂) as the gate dielectric for future generation metal-oxide-semiconductor (MOS) based nanoelectronic devices. Experimental studies have shown that a 1–3 monolayer SiO₂ film between the high permittivity metal oxide and the substrate silicon is needed to minimize electrical degradation. This study uses density functional theory (DFT) to investigate the initial growth reactions of ZrO₂ on hydroxylated SiO₂ by atomic layer deposition (ALD). The reactants investigated in this study are zirconium tetrachloride (ZrCl₄) and water (H₂O). Exchange reaction mechanisms for the two reaction half-cycles were investigated. For the first half-reaction, reaction of gaseous ZrCl₄ with the hydroxylated SiO₂ surface was studied. Upon adsorption, ZrCl₄ forms a stable intermediate complex with the surface SiO₂–OH^* site, followed by formation of SiO₂–O–Zr–Cl^* surface sites and HCl. For the second half-reaction, reaction of H₂O on SiO₂–O–Zr–Cl^* surface sites was investigated. The reaction pathway is analogous to that of the first half-reaction; water first forms a stable intermediate complex followed by evolution of HCl through combination of a Cl atom from the surface site and an H atom from H₂O. The results reveal that the stable intermediate complexes formed in both half-reactions can lead to a slow film growth rate unless process parameters are adjusted to lower the stability of the complex. The energetics of the two half-reactions are similar to those of ZrO₂ ALD on ZrO₂ and as well as the energetics of ZrO₂ ALD on hydroxylated silicon. The energetics of the growth reactions with two surface hydroxyl sites are also described.     

中性和阳离子丁酮团簇的结构及稳定性的理论研究

使用密度泛函B3LYP方法,在6-31G*和6-311+G**基组水平上计算中性和阳离子丁酮团簇(CH_3COC_2H_5)_n和(CH_3COC_2H_5)_n~+(n 7)的稳定结构,并比较不同尺寸团簇之间的相对稳定性.中性和阳离子丁酮团簇的结构具有相似性:n=3—7时,组成团簇的丁酮的平均几何参数基本相同,单环结构最稳定;随着团簇尺寸的增加,双环结构的稳定性逐渐上升.通过平均结合能、一阶差分能、HOMO-LUMO能隙等计算分析可知:在所研究的各种尺寸团簇中,(CH_3COC_2H_5)_3是最稳定的中性团簇,与实验中的最强峰对应;(CH_3COC_2H_5)_4~+是最稳定的阳离子团簇.通过电离能计算得到丁酮分子的垂直电离能为9.535 eV与实验值相符,同时证明中性和阳离子丁酮二元团簇的结构变化较大.研究结果为实验中丁酮团簇碎片离子的形成机理提供一定的理论依据,并且为进一步研究酮类分子团簇的生长规律提供有价值的信息.     

Substrate-induced freezing of alkane monolayers adsorbed on Au(1 1 1) dependant on the alkane/gold misfit

The molecular structure of the interfaces between pure C_nH_2n+2 (n=10, 12, 14, 16) and Au(1 1 1) surface has been studied by scanning tunneling microscopy. At 291 K, self-organized monolayers with a lamella structure are formed with these alkanes. The ordered monolayers are melting at temperature higher by 46, 28, 15, 5 K than the melting point of bulk C_nH_2n+2 crystals with n=10, 12, 14, 16, respectively. Two kinds of melting process were observed: (i) a direct solid/liquid phase transition within the monolayer for C₁₀H₂₂, C₁₂H₂₆ and C₁₄H₃₀ molecules, (ii) an intermediate phase for C₁₆H₃₄ molecules. This mesophase corresponds to a two-dimensional liquid crystal formed by molecules moving along their axis and along Au1 1 0. These results agree well with calculations using a geometric model taking in account the misfit between the CH₂–CH₂–CH₂ period along alkyl chain and the gold lattice along 1 1 0 direction.     

Oxyanion derivatives of cuprate superconductors superconducting 123 phosphates

While YSr₂Cu₃O₇ cannot be prepared under ambient conditions, partial substitution of the phosphate group for copper, as in YSr₂Cu_2.8(PO₄)_0.2O_y, stabilizes this phase in the orthorhombic structure, but the material is not superconducting. Superconductivity in YSr₂Cu_2.8(PO₄)_0.2O_y is obtained by increasing the hole concentration through partial substitution of Y by Ca, as in Y_0.7Ca_0.3Sr₂Cu_2.8(PO₄)_0.2O_y (T_c≈40 K). By incorporating the phosphate group in orthorhombic YBaSrCu₃O ₇, a stable tetragonal derivative of the formula YBaSrCu_2.8(PO₄)_0.2O_y (T_c≈ 47 K) has been prepared; the T_c increases to 70 K by partial substitution of Y by Ca as in Y_0.7Ca_0.3BaSrCu_2.8(PO₄)_0.2O_y.     

Adsorption and thermodynamic studies of Cu(II) and Zn(II) on organofunctionalized-kaolinite

Kaolinite-bearing clay samples from Perus, São Paulo state, Brazil, were used for chemical modification process with dimethyl sulfoxide and organofunctionalized with the silyating agent (RO)₃Si(CH₂)₃NH(CH₂)₂NH₂ in the present study. The resulting material and natural kaolinite were subjected adsorpion process with Cu(II) and Zn(II) from aqueous solution at pH 6.0 and controlated temperature of 298 K. The Langmuir adsorption isotherm model has been applied to fit the experimental data. The results showed that the chemical modification process increases the basal spacing of the natural kaolinite from 0.711 to 0.955 nm. The energetic effects caused by Cu(II) and Zn(II) interactions were determined through calorimetric titration at the solid–liquid interface and gave a net thermal effect that enabled the calculation of the exothermic values and the equilibrium constant.     

From crystalline to glassy gallium fluoride materials: an NMR study of Ga and Ga quadrupolar nuclei

Owing to the implementation of acquisition techniques specific for nuclei with very large quadrupolar interaction (full shifted echo and variable offset cumulative spectra (VOCS)), NMR spectra of ⁶⁹Ga and ⁷¹Ga are obtained in crystallised (PbGaF₅, Pb₃Ga₂F₁₂, Pb₉Ga₂F₂₄ and CsZnGaF₆) and glassy (PbF₂–ZnF₂–GaF₃) gallium fluorides. Simulations of both static (full echo or VOCS) and 15 kHz MAS spectra allow to obtain consistent determinations of isotropic chemical shifts and very large quadrupolar parameters (ν_Q up to 14 MHz). In the crystalline compounds whose structures are unknown, the number and the local symmetry of the different gallium sites are tentatively worked out. For the glassy systems, a continuous Czjzek's distribution of the NMR quadrupolar parameters accounts for the particular shape of the NMR spectrum.     

(C_5H_(11)N_2)~+和(C_7H_(12)N_2)~(2+)的NMR研究

结合1H NMR,13C NMR谱,分别对钨、钼配合物{WO2(C10H6O2)2(C5H11N2)2[H2N(CH2)3NH2]}3(1),{(C5H11N2)2[H2N(CH2)3NH2][MoO2(C10H6O2)2]}(2),{(C7H12N2)2[MoO2(C10H8O2)2]}(3)晶体结构中小分子环进行了归属.其中,配合物1和2中(C5H11N2)+的NMR研究证实了六元环由1,3-丙二胺和乙腈化合而成,配合物3中(C7H12N2)2+的NMR谱图证实了七元环由乙二胺和乙酰丙酮化合而成,并且推导出这些亲核加成-消除反应的反应机理.配合物1~3中的小分子环的合成在其它体系中尚未见报导,而在合成它们的反应中作为新产物随主体晶体析出,并由晶体结构解析和NMR得到了证实.     

Study on the structure dependent ultrafast third-order optical nonlinearity of GeS2–In2S3 chalcogenide glasses

The results of the femtosecond time-resolved optical Kerr at 820 nm in GeS₂–In₂S₃ chalcogenide glasses indicate that the response time in GeS₂–In₂S₃ glasses is subpicosecond, which is predominantly due to the distortion of the electron cloud. The value of χ⁽³⁾ in 0.95GeS₂–0.05In₂S₃ glass is also as large as 2.7 × 10⁻¹³ esu, and it reduces with the addition of In₂S₃, which may be ascribed to the microstructure evolution of GeS₂–In₂S₃ glasses. It is deduced that the intrinsic [Ge(In)S₄] tetrahedral structure units that possess the high hyperpolarizability may do great contribution to the enhancement of third-order optical nonlinearity while [S₃Ge–GeS₃] ethane-like molecular units make no considerable contribution to that in femtosecond time scale. These GeS₂–In₂S₃ and GeS₂–In₂S₃-based chalcogenide glasses would be expected to be the promising materials for all-optical switching devices.     

Mechanical anisotropy and electronic properties of X2N2 (XH2 ): first-principles calculations*

The mechanical anisotropy, structural properties, electronic band structures and thermal properties of C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are detailed and investigated in this work. The novel silicon nitride phase Si₂ N₂ (SiH₂ ) and germanium nitride phase Ge₂ N₂ (GeH₂ ) in the Cmc 2₁ structure are proposed in this work. The novel proposed Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are both mechanically and dynamically stable. The electronic band calculation of the HSE06 hybrid functional shows that C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are all wide band gap semiconductor materials, and C₂ N₂ (CH₂ ) and Si₂ N₂ (SiH₂ ) are direct band gap semiconductor materials, while Ge₂ N₂ (GeH₂ ) is a quasi-direct band gap semiconductor material, the band gap of C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are 5.634 eV, 3.013 eV, and 2.377 eV, respectively. The three-dimensional and plane distributions of Young’s modulus, shear modulus and Poisson’s ratio of C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) show that these materials have different degrees of mechanical anisotropy. The order of Young’s modulus of Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) in different directions is different from that of C₂ N₂ (CH₂ ). When the tensile axis is in a particular direction, the order of the Young’s modulus of Si₂ N₂ (SiH₂ ): E _[110] <E _[120] <E _[111] <E _[101] <E _[010] =E _[100] <E _[011] <E _[001], and the order of the Young’s modulus of Ge₂ N₂ (GeH₂ ): E _[110] <E _[111] <E _[101] <E _[120] <E _[100] <E _[010] <E _[011] <E _[001] .