Relaxation of (111) silicon surface atoms from studies of Si4H9 clusters

Self-consistent Hartree-Fock and generalized valence bond calculations have been performed on clusters modeling the (111) silicon surface. We find that the surface state is accurately described as a dangling bond surface orbital with 93% p character. We determined the optimum relaxation of the surface layer to be 0.08 Å toward the second layer. In the positive ion, the surface atom relaxes toward the second layer by an additional 0.30 Å, and for the negative ion the surface atom moves toward the vacuum 0.25 Å. The vertical ionization potential was found to be 5.78 eV (experimental values are 5.6 – 5.9 eV) while the calculated adiabatic electron affinity is 3.02 eV.     

Absolute coverage of C2H2 and C2H4 on Pt(111)

Quantitative XPS measurements have been performed in order to determine the absolute coverage of acetylene and ethylene adsorbed on Pt(111) showing a 2 × 2 LEED pattern. This LEED pattern has so far been attributed to a 2 × 2 superstructure with a coverage of 0.25. A quantitative evaluation of the C(1s) peak intensities for these adsorbed layers in comparison with adsorbed CO shows that the coverage is 0.5 instead of 0.25. Therefore the 2 × 2 LEED pattern should be assigned to a 2 × 1 superstructure in three domains rather than a 2 × 2 superstructure.     

Adsorption studies on a stepped Pt(111) surface: O2, CO,C2H4, C2N2

A comparative study of the adsorption of several gases on a Pt(S)-[9(111) × (111)] surface was performed using LEED, Auger spectroscopy, flash desorption mass spectrometry and work function changes as surface sensitive techniques. Adsorption was found to be generally less ordered on the stepped surface than on the corresponding flat surface with the exception of the oxygen, where r well ordered overlayer in registry over many terraces was found. Absolute coverages were determined from flash desorption experiments for O₂, CO and C₂N₂. Similar values were obtained as on flat Pt surfaces. Two different surface species seem to be formed upon adsorption of C₂H₄ depending on the adsorption temperature. Contrary to reports from Pt(111) surfaces conversion between the two surface species is heavily restricted on the stepped surface. Work function changes revealed nonlinear adsorbate effects where the adsorbate is electronegative with respect to the substrate. Various adsorption models are discussed in the light of complementary experimental evidence. The results of this study are compared with data available from flat Pt surfaces and possible influences of steps are discussed. No general trends, however, emerge from this comparison and it seems that eventual influences of steps have to be considered individually for every adsorbate.     

On the nature of the SO4/Ag(111) and SO4/Au(111) surface bonding

The nature of sulfate-Ag(111) and sulfate-Au(111) surface bonding has been investigated at the SCF + MP2 level of theory. Convergence of binding energy with cluster size is investigated and, unlike neutral adsorbates, large clusters are required in order to obtain reliable binding energies. In the most stable adsorption mode, sulfate binds to the surface via three oxygen atoms (C_3v symmetry) with a binding energy of 159.3 kcal/mol on Ag(111) and 143.9 kcal/mol on Au(111). The geometry of adsorbed sulfate was optimized at the SCF level. While the bond length between sulfur and the oxygens coordinated to the surface increases, the sulfur-uncoordinated oxygen bond length decreases. This weakening and strengthening of the bonds, respectively, is consistent with bond order conservation in adsorbates on metal surfaces. Although a charge transfer of 0.4 electrons towards the metal is observed, the adsorbate remains very much sulfate-like. The molecular orbital analysis indicates that there is also some charge back-donation towards unoccupied orbitals of sulfate. This results in an increased electron density around sulfur as revealed in the electron density difference maps. Analysis of the Laplacian of the charge density of free sulfate provides a suitable framework to understand the nature of the different charge transfer processes and allows us to establish some similarities with the CO- and SO₂-metal bondings.     

LEED,AES and thermal desorption studies of chemisorbed Hydrogen and hydrocarbons (C2H2, C2H4, C6H6, C6H12) on the (111) and stepped [6(111) × (100)] iridium crystal surfaces; comparison with platinum

The adsorption of hydrogen, ethylene, acetylene, cyclohexane and benzene was studied on both the (111) and stepped [6(111) × (100)] crystal surfaces of iridium. The techniques used were low energy electron diffraction, Auger electron spectroscopy, and thermal desorption mass spectrometry. At 30°C, acetylene, ethylene and benzene are adsorbed with a sticking probability near unity. The sticking probability of cyclohexane is less than 0.1 on both surfaces. Heating the (111) surface above 800°C, in the presence of the hydrocarbons, results in the formation of an ordered carbonaceous overlayer with a diffraction pattern corresponding to a (9 × 9) surface structure. No indication for ordering of the carbonaceous residue was found on the stepped iridium surface in these experimental conditions. The hydrocarbon molecules form only poorly ordered surface structures on both iridium surfaces when the adsorption is carried out at 30°C. Benzene is the only gas that can be desorbed from the surfaces in large amounts by heating. Ethylene remains largely on the surface, only a few percent is removed by heating while acetylene and cyclohexane cannot be desorbed at all. When adsorption is carried out at 30°C and the crystal is subsequently flashed to high temperature, hydrogen is liberated from the surface. The hydrogen desorption spectra from the iridium surfaces exposed to C₂H₄, C₂H₂, or C₆H₆ exhibit two hydrogen desorption peaks, one around 200°C and the second around 350°C. The temperatures where these peaks appear vary slightly with the type of hydrocarbon. The relative intensities of these two peaks depend strongly on the surface used. Arguments are presented that decomposition of the hydrocarbon molecules (C-H bond breaking nd possibly also C-C bond breaking) occurs easier on the stepped iridium surface than on the (111) surface. Hydrogen is desorbed at a higher temperature from an iridium surface possessing a high density of surface imperfections than from a perfect iridium (111) surface. The results are compared with those obtained previously on similar crystal surfaces of platinum. It appears that C-H bond breaking occurs more easily on iridium than on platinum.     

N2H4在NiFe(111)合金表面吸附稳定性和电子结构的第一性原理研究

采用基于色散校正的密度泛函理论进行了第一性原理研究, 详细分析了肼(N₂H₄)在Ni₈Fe₈/Ni(111)合金表面稳定吸附构型的吸附稳定性和电子结构及成键性质. 通过比较发现, 肼分子以桥接方式吸附在表面的两个Fe原子上是最稳定的吸附构型, 其吸附能为-1.578 eV/N₂H₄. 同时发现, 肼分子在这一表面上吸附稳定性的趋势为: 桥位比顶位吸附更有利, 且在Fe原子上比在Ni原子上的吸附作用更强. 进一步分析了不同吸附位点上稳定吸附构型的电子结构、电荷密度转移以及电子局域化情况. 结果发现: 相同吸附位点的电子态密度图基本一致, 并且N原子的p轨道和与之相互作用的表面原子的d轨道之间存在态密度上的重叠; 吸附后电荷密度则主要从肼分子转移到表面原子之上; 在电子局域化函数切面图中也发现吸附后电子被局域到肼分子的N原子和相邻的表面原子之间. 这些电子结构的表征都充分说明肼分子与表面原子之间通过电荷转移形成了强烈的配位共价作用.     

Auger spectra of CO,C2H4, C2N2 and C6H6 adsorbed on Pt(111)

Electron excited carbon KVV Auger spectra of CO, C₂H₄, C₂N₂ and C₆H₆ adsorbed on Pt(111) are compared. By estimating the effective Coulomb interaction between the final-state holes it is possible to associate some features with transitions observed in free molecule spectra, but others must involve at least one electron with energy within the conduction band of the metal. Such “cross-transitions” are associated with strong 2π* character of filled states in the presence of a core hole in molecules such as CO.     

Equation of state of silicon nitride carbodiimide Si2CN4

The lattice parameters of silicon nitride carbodiimide Si₂CN₄ have been measured up to 8 GPa at room temperature using energy-dispersive X-ray powder diffraction with synchrotron radiation. A fit of the experimental p-V data to the Birch-Murnaghan equation of state yields the values of the bulk modulus of 8.8(2) GPa and its first pressure derivative of 3.4(1). The compression is found to be anisotropic, with the b-axis being significantly more compressible than the a-and c-axes.     

Thermal evolution of C2H2 and C4H2 on Pd(111) studied by high-resolution electron energy loss spectroscopy

The thermal evolution of acetylene and ethylene on a palladium (111) surface has been studied by high-resolution electron energy loss spectroscopy in the temperature range 150K–500K. Formation of ethylidyne ( CCH₃) near room temperature is important for both molecules, whereas CH is the major surfaces hydrocarbon species formed at high temperatures.     

Reexamine structures and relative stability of medium-sized silicon clusters: Low-lying endohedral fullerene-like clusters Si30-Si38

We report improved results of lowest-lying silicon clusters Si₃₀-Si₃₈. A large population of low-energy clusters are collected from previous searches by several research groups and the binding energies of these clusters are computed using density-functional theory (DFT) methods. Best candidates (isomers with high binding energies) are identified from the screening calculations. Additional constrained search is then performed for the best candidates using the basin-hopping method combined with DFT geometry optimization. The obtained low-lying clusters are classified according to binding energies computed using either the Perdew-Burke-Ernzerhof (PBE) functional or the Becke exchange and Lee-Yang-Parr correlation (BLYP) functional. We propose to rank low-lying clusters according to the mean PBE/BLYP binding energies in view that the PBE functional tends to give greater binding energies for more compact clusters whereas the BLYP functional tends to give greater binding energies for less compact clusters or clusters composed of small-sized magic-number clusters. Except for Si₃₀, the new search confirms again that medium-size silicon clusters Si₃₁-Si₃₈ constructed with proper fullerene cage motifs are most promising to be the lowest-energy structures.     

Desorption of dimethylformamide from Zn4O(C8H4O4)3 framework

Both dimethylformamide (DMF) and diethylformamide (DEF) are important solvents for the synthesis of Zn₄O(C₈H₄O₄)₃ framework (MOF-5). It is generally recognized that DMF molecules can be completely displaced by CH₂Cl₂ during the synthesis of MOF-5. Herein, however, it was found that the DMF molecules inside the pores of the MOF-5 framework cannot be displaced by CH₂Cl₂. The desorption of the DMF molecules from the pores, which requires a temperature of 100 °C or above, is the first order with activation energy of 56.38 kJ/mol. In contrast, DEF molecules can be completely displaced by CH₂Cl₂ during the synthesis of MOF-5, because DEF molecules cannot penetrate into the pores of the MOF-5 paste.     

AP/(N2 + C2H2 + C2H4) gaseous fuel diffusion flame studies

Counterflow diffusion flame experiments and modeling results are presented for a fuel mixture consisting of N₂, C₂H₂, and C₂H₄ flowing against decomposition products from a solid AP pellet. The flame zone simulates the diffusion flame structure that is expected to exist between reaction products from AP crystals and a hydrocarbon binder. Quantitative species and temperature profiles have been measured for one strain rate, given by a separation of 5 mm, between the fuel exit and the AP surface. Species measured include C₂H₂, C₂H₄, N₂, CN, NH, OH, CH, C₂, NO, NO₂, O₂, CO₂, H₂, CO, HCl, H₂O, and soot volume fraction. Temperature was measured using a combination of a thermocouple at the fuel exit and other selected locations, spontaneous Raman scattering measurements throughout the flame, NO vibrational populations, and OH rotational population distributions. The burning rate of the AP was also measured for this flame’s strain rate. The measured eighteen scalars are compared with predictions from a detailed gas-phase kinetics model consisting of 105 species and 660 reactions. Model predictions are found to be in good agreement with experiment and illustrate the type of kinetic features that may be expected to occur in propellants when AP particles burn with the decomposition products of a polymeric binder.     

Scanning tunneling microscopy of N2H4 on silicon surfaces

The chemistry of N₂H₄ on Si(100)2 × 1 and Si(111)7 × 7 has been studied using scanning tunneling microscopy. At low coverages on Si(100)2 × 1 at room temperature the adsorption sites are distributed randomly on the surface and are imaged as dark spots in the dimer row by the STM. Upon annealing the substrate at 600 K, both isolated reaction products, as well as clusters of reaction products are formed on the surface. The STM images show that the majority of the isolated reaction products are adsorbed symmetrically across the dimers. Based on previous HREELS data, these are most likely NH_x groups. However, the clusters are not well resolved. Because of this we speculate that they are not simply symmetrically adsorbed NH_x groups, but likely have a more complicated internal structure. At higher coverages, the STM images show that the predominant pathway for adsorption is with the N---N bond parallel to the surface, in agreement with HREELS studies of this system. On Si(111)7 × 7, the molecule behaves in a manner which is similar to NH₃. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms.     

The chemisorption of CO,CO2, C2H2, C2H4, H2 and NH3 on the clean Fe(100) and (111) crystal surfaces

The chemisorption of small molecules (CO, CO₂, C₂H₂, C₂H₄, H₂ and NH₃) has been studied on the clean Fe(110) and (111) crystal faces by low-energy electron diffraction (LEED) and thermal desorption. C₂H₄ and C₂H₂ yield the same sequence of surface structures that change with temperature and crystal orientation. CO and CO₂ chemisorption similarly results in the formation of the same types of surface structures that change with surface temperature and crystal orientation. Ammonia forms several ordered surface structures on both iron crystal faces. All of the molecules decompose as a function of temperature on the iron surfaces as indicated by the Auger and thermal desorption spectra.     

Leed and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 AND C) chemisorbed on the rhodium (111) and (100) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₄, C₂H₂ and C has been studied on the clean Rh(111) and (100) surfaces. LEED, AES and thermal desorption were used to determine the surface structures, disordering and desorption temperatures, displacement and decomposition characteristics for each species. All of the molecules studied readily chemisorbed on both surfaces. A large variety of ordered structures was observed, especially on the (111) surface. The disordering temperatures of most ordered surface structures on the (111) surface were below 100°C. It was necessary to adsorb the gases at 25° C or below in order to obtain well-ordered surface structures. Chemisorbed oxygen was readily removed from the surface by H₂ or CO gas at crystal temperatures above 50°C. CO₂ appears to dissociate to CO upon adsorption on both rhodium surfaces as indicated by the identical ordering and desorption characteristics of these two molecules. C₂H₄ and C₂H₂ also had very similar ordering and desorption characteristics and it is likely that the adsorbed species formed by both molecules is the same. Decomposition of ethylene produced a sequence of ordered carbon surface structures on the (111) face as a result of a bulk-surface carbon equilibrium. The chemisorption properties of rhodium appear to be generally similar to those of iridium, nickel and palladium.     

Local order and structural transitions in the molecule-based layer ferrimagnet (n-C4H9)4N FeFe(C2O4)3

Local structural order and temperature-dependent structural variation have been studied in the molecular-based layer ferrimagnet (n-C₄H₉)₄N Fe^IIFe^III(C₂O₄)₃ by EXAFS and high resolution X-ray powder diffraction. The EXAFS spectra measured at the Fe K-edge are successfully modelled by successive O, C, O and metal shells, showing that even when there is extensive structural disorder due to stacking faults, the local structural order in this class of ferrimagnets is fully retained. In this salt, which shows remarkable negative magnetisation at low temperature (Néel class Q), the EXAFS Debye-Waller factor has a discontinuity at 40 K, corresponding to one found in the magnetisation. At the same temperature there is also a change in the expansion of the lattice as evidenced by the high resolution X-ray powder diffraction.     

Si3N4的晶体化和ZrN/Si3N4纳米多层膜的超硬效应

研究了Si₃N₄层在ZrN/Si₃N₄纳米多层膜中的晶化现象及其对多层膜微结构与力学性能的影响. 一系列不同Si₃N₄层厚度的ZrN/Si₃N₄纳米多层膜通过反应磁控溅射法制备. 利用X射线衍射仪、高分辨透射电子显微镜和微力学探针表征了多层膜的微结构和力学性能. 结果表明，由于受到ZrN调制层晶体结构的模板作用，溅射条件下以非晶态存在的Si₃N₄层在其厚度小于0.9 nm时被强制晶化为NaCl结构的赝晶体，ZrN/Si₃N₄纳米多层膜形成共格外延生长的柱状晶，并相应地产生硬度升高的超硬效应. Si₃N₄随层厚的进一步增加又转变为非晶态，多层膜的共格生长结构因而受到破坏，其硬度也随之降低.     

Linear-chain antiferromagnetism in Ni(N2H5)2(SO4)2

The compound dihydrazinium bis(sulfato) niccolate(II), Ni(N₂H₅)₂(SO₄)₂, containing sulfato-bridged chains of Ni(II) ions, can be described as an antiferromagnetic Heisenberg linear-chain system. A reasonable agreement of susceptibility measurements in the temperature region 2–80K, with a theory developed by Weng for antiferromagnetic Heisenberg linear chains with spin S=1, is obtained for a value of the intra-chain interaction $\text{J}\text{k}\text{=?3.35K}$. Preliminary results of specific heat measurements, on the other hand, do not fit quite well using this model. The origin of this discrepancy is suggested to be a zero-field splitting of the single ion.     

乙烯和乙炔基在Ni(110)表面上吸附结构的研究

采用平面波赝势方法，利用基于从头计算的软件包，对乙烯和乙炔基在Ni(110)表面上吸附的问题进行了计算. 在低覆盖度时，孤立的乙烯分子的吸附能比密集时高，乙烯分子的C-C 轴大致沿衬底的Ni原子链方向(即沿［110］晶向)，C-C轴与衬底Ni(110)表面有12°的倾斜角，乙烯分子的C—C键的键长为 0.147nm. 乙烯分子中接近顶位的C原子与衬底中距离最近的Ni原子为0.199nm. 在高覆盖度时，乙烯分子在Ni(110)表面上形成c(2×4)再构，每个表面二维元胞中有两个乙烯分子，每个乙烯分子的吸附位置与低覆盖度时相似，而C—C键长比低覆盖度时要短. 乙炔基是乙烯在Ni(110)表面上分解的产物. 关于乙炔基的计算结果表明：乙炔基的两个C原子的间距为0.131nm，比乙烯分子中C原子的间距更短. 与乙烯分子相比，乙炔基的吸附位置更靠近顶位. H原子与吸附在顶位上的C原子相连接，C—H键也大致沿衬底的Ni原子链方向，与Ni表面呈45°的倾斜角. 关键词： 乙烯和乙炔基 平面波赝势方法 吸附几何结构