Probing Co/Si interface behaviour by X-ray photoelectron spectroscopy （XPS） and atomic force microscopy （AFM）

In this work, we investigate the Co-Si reaction, the Co growth mode at room temperature, diffusion behaviour as well as morphology evolution during annealing on both H-terminated and clean Si（001） and Si（lll） surfaces. From in-situ X-ray photoelectron spectroscopy （XPS） investigation, ＂Co-Si＂ reaction appears to occur on both H-terminated and clean surfaces at room temperature （RT） and the silicide crystallinity is improved upon annealing. Co growth mode on H-terminated Si surfaces occurs in a pseudo layer-by-layer manner while small close-packed island growth mode is observed on the clean Si surface. Upon annealing at different temperatures, Co atom concentration decreases versus annealing time, which in part is attributed to Co atoms inward diffusion. The diffusion behaviour on both types of surfaces demonstrates a similar trend. Morphology study using ex-situ atomic force microscopy （AFM） shows that the islands formed on Si（001） surface after annealing at 700℃ are elongated with growth directions alternate between the two perpendicular [110 ] and [ 110] directions. Triangular islands are observed on Si （111） surface.     

H原子在Zr(0001)表面吸附的第一性原理研究

结合蒙特卡洛（MC）模拟和第一性原理密度泛函理论（DFT）方法,从Zr-H体系微观结构、吸附概率、吸附能、Mulliken电荷布居数以及电子态密度等方面对H原子在α-Zr(0001)表面的吸附位点和吸附机理等进行计算分析。结果表明:H原子在Zr(0001)表面首先产生物理吸附,然后由物理吸附转变为化学吸附,吸附过程中电荷不断由 Zr(0001)表面原子向H原子转移,最后趋于稳定。另外,稳定吸附后的H原子直接与 Zr(0001)表面最表层原子生成化学键,且主要由H(s)、Zr(s)和Zr(d)轨道的电子态做贡献。综合分析得到H原子在Zr(0001)表面的吸附位点优先级顺序为密排六方间隙位（hcp位）＞面心立方间隙位（fcc位）＞桥位（bridge位）,顶位（top位）不会产生吸附。      

Si原子在CeO2(111)表面吸附与迁移的第一性原理研究

为探究Si原子在CeO₂(111)表面吸附的微观行为,采用第一性原理的方法研究了Si原子在CeO₂(111)表面的吸附作用、电子结构和迁移过程,计算了Si原子在CeO₂(111)表面的吸附能,最稳定及次稳定吸附位置的电子态密度与电荷密度分布、迁移激活能。计算结果表明：Si原子最易吸附于基底表层的O原子上,其中O桥位（O_bri）吸附作用最强,O顶位（O_t）和O三度位（O_h）吸附强度次之。Si原子仅对其最邻近的表层O原子结构影响较大,这与Si原子及其最邻近的O原子间电荷密度重叠程度增强的结果一致。Si原子最易围绕着O_t位从O_bri位向O_h位迁移,迁移所需激活能为0.849 eV。     

Vacancy formation and solid solubility in the U-Zr-N system

For the purpose of developing a nuclear fuel with enhanced thermophysical properties and better irradiation performance density functional theory calculations are used to explore UN, ZrN and (U, Zr)N. Negative deviation of ground state energy from the ideal solution model as well as energetically favourable maximal distance between substitutional metal atoms in respective nitrides indicate mutual solubility of UN and ZrN at all temperatures. Nitrogen vacancy formation energies in UN (1.81 eV) and ZrN (1.40 eV) are considerably lower than metal vacancy formation energies. A substitutional Zr atom in UN has little effect on nitrogen vacancy formation energies (∼1.79 eV), while U in ZrN decreases the value by ∼0.1 eV (∼1.30 eV) due to elastic stress and charge density redistribution in the material. The relative distance between a substitutional metal atom and a vacancy in UN has little influence over the radially declining displacement pattern induced by the substitutional atom, while in ZrN the relaxation of atoms is governed by the position of the vacancy. The calculated vacancy formation energies indicate a lower surface energy of ZrN in comparison with UN.     

C60与吸氢金刚石(111)表面碰撞崐的模拟研究

用分子动力学方法研究了Ｃ６０与吸氢的金刚石（１１１）表面碰撞特性与注入能量的大系。原子间的相互作用采用半经验的Ｂｒｅｎｎｅｒ多体势函数。结果表明，当入射能量小于２００ｅＶ时，Ｃ６０只是完整地从表面反弹；当入射能量为２００－３００ｅＶ时，Ｃ６０可与底靶发生化学反应；当注入能量＞３００ｅＶ时，Ｃ６０可与表面形成多个化学键而吸附在衬底表面。此外，计算机模拟计算还给出了Ｃ６０－金刚石表面碰撞的微观过程，并从原子层次研究了Ｃ６０表面沉积的成键机制。     

Strong segregation gettering of transition metals by implantation-formed cavities and boron-silicide precipitates in silicon

We have mechanistically and quantitatively characterized the binding of transition-metal impurities in Si to cavities formed by He implantation and to B---Si precipitates resulting from B implantation. Both sinks are inferred to act by the segregation of metal atoms to pre-existing low-energy sites, namely surface chemisorption sites in the case of cavities and bulk solution sites in the case of the B---Si phase. These gettering processes exhibit large binding energies, and they are predicted to remain active for arbitrarily small initial impurity concentrations as a result of the segregation mechanisms. Both appear promising for gettering in Si devices.     

Surface and sub-surface oxidation of α-Cu-(17 at.%)Al(1 0 0) studied by X-ray photo-electron spectroscopy and low energy He scattering spectroscopy

In this study the initial stages of oxidation of the α-Cu-(17 at.%)Al(1 0 0) single crystal oriented alloy surface was investigated by X-ray photo-electron spectroscopy and low energy He⁺ scattering spectroscopy. It was found that oxygen adsorption can be divided in two sequential stages: (i) a fast process, up to 15–20 L exposure, in which oxygen adsorbs on the alloy’s surface resulting mostly in the formation of Al–O chemisorbed bonds; and (ii) a slower process during which oxygen adsorbs forming Cu–O and Al–O chemisorbed bonds concurrent with diffusion of O to the sub-surface and Al segregation to the surface region. The surface oxidation rate is much higher than the sub-surface one. The rate of Al segregation increases with oxygen exposure and involves both surface and sub-surface regions. Annealing of the oxidized alloy surface results in a pronounced segregation of Al and formation of an aluminum oxide layer.     

Ab initio energetics of some fission products (Kr, I, Cs, Sr and He) in uranium dioxide

A computational study of some fission products (FP) energetics in uranium dioxide is presented. Krypton, iodine, caesium, strontium and helium are considered. Calculations are made within the density functional theory in the local density approximation with the plane wave pseudopotential method. Three insertion sites are considered: the octahedral interstitial position and the oxygen and uranium substitution sites. The importance of atomic relaxations is estimated on the He and Kr cases. They prove quantitatively important but can be neglected to draw qualitative trends. For each fission product incorporation and solution energies are calculated. The obtained values of the solutions energies of the various FP are in good agreement with their experimental behaviour: Kr, Cs and I atoms are insoluble in uranium dioxide, Sr solubility depends on the stoichiometry of urania. He atoms are found to have little interaction with their environment in uranium doxide.     

Resonant ion stimulated desorption of O from the TiO2(1 1 0) surface: Effects of oxygenation and ion bombardment

The O⁺ desorption from reduced, oxygenated, and ion-bombarded TiO₂(1 1 0) surfaces has been investigated during He⁺ irradiation. The O⁺ desorption is initiated by creation of an antibonding O 2s core hole state via quasi-resonant charge exchange with the He⁺ 1s state, followed by the intra-atomic Auger decay of the O 2s hole. Upon oxygenation of the reduced TiO₂(1 1 0) surface, the O⁺ yield increases by one order of magnitude. The O₂ molecule is dissociated at the vacancy site of bridging oxygen and the oxygen atoms either fill a vacancy site or chemisorb at a fivefold-coordinated Ti⁴⁺ site as an adatom. The latter is detected with much higher efficiency than the former. The O⁺ yield is increased during He⁺ bombardment of the reduced TiO₂(1 1 0) surface due to formation of lower coordinated oxygen atoms. The oxygen species thus formed by ion bombardment or oxygenation are unstable on the surface and tend to diffuse into bulk vacancy sites or higher coordination surface sites even at room temperature.     

The Behavior of Iodine in Adsorption and Desorption by Graphite

Using tracer technique, studies were undertaken on the adsorption of iodine on natural graphite at temperatures ranging from 27° to 900°C under pressures from 0.03 to 1.4 mmHg, and on the desorption of adsorbed iodine from the same specimens into either vacuum or Ar stream. The specimens were heated up either isothermally, stepwise, or at constant rate of temperature rise, up to 1,100°C. The resulting data accorded well with a theoretical treatment presuming desorption from heterogeneous surface, considering the desorption as an n-th order irreversible reaction having distributed activation energies. The iodine is considered to be held on the graphite by physical adsorption below about 450° C, and above that temperature, by chemisorption through bond formation between iodine and carbon atoms. The heat of adsorption and the activation energy of desorption obtained was found to be a function of surface coverage. The presence of Ar gas was observed to inhibit chemisorption of iodine and to decrease the activation energy of desorption of physically adsorbed iodine.     

高温超导体EuBa_2(Cu_(1-x)Fe_x)_3O_(7-y)的穆斯堡尔谱学新研究

利用~(57)Fe和~(151)Eu穆斯堡尔效应对EuBa_2(Cu_(1-x)Fe_x)_3O_(7-y)高温超导体作了研究。在液氮温区,分别测量了样品中有和无电流通过时的~(57)Fe穆斯堡尔谱,结果表明取代了Cu(2)位置的Fe的同质异能位移和四极分裂与通过超导体的电流无关,而取代Cu(1)位置的Fe的穆斯堡尔谱参数则与电流有关。不同Fe含量样品的穆斯堡尔谱分析表明,超导电性和穆斯堡尔谱参数都与Fe的含量有关。     

放射性核素U在针铁矿(010)表面吸附的第一性原理研究

采用基于密度泛函理论的第一性原理研究了有水与无水环境下放射性核素U在针铁矿(010)表面的吸附。结果表明,由于水分子与(010)表面的初始构型不同,其吸附能为-0.56~-0.85eV。同时发现在缺水的情况下,U原子可吸附在针铁矿(010)表面的替代位(T)、八面体间隙位(O)及两个O原子的桥位(B)上。其吸附能为负,大小按吸附T、B、O的顺序依次增大。水对U原子吸附能的影响较明显。当U原子占据替代位和八面体间隙位时,在其附近的水分子可增强U原子在针铁矿(010)表面的吸附,可使U原子在针铁矿(010)表面的吸附能降低1~2eV;当U原子在桥位时,水分子既可增强也可减弱U原子在针铁矿(010)表面的吸附能力,这依赖于U原子与水分子作用后其价态的变化。所有这些吸附均为放热反应。计算结果显示,由于水分子与U原子的吸附引起的表面晶格畸变较小,所有涉及的表面键长变化均不大于0.055nm。所有涉及到的反应过程中均有H、U、Fe原子上的部分电子转移到O原子上。     

Dynamic dependence of the interaction potentials for grazing scattering of fast atoms from metal and insulator surfaces

For scattering of fast atoms from metal and insulator surfaces under axial channeling conditions pronounced peaks in the angular distributions of scattered projectiles are interpreted in terms of rainbow scattering. The angular position of such “rainbow peaks” are closely related to the interaction potential and its corrugation in the topmost surface region. We have scattered N and O atoms, with energies ranging from 10 to 70 keV, from clean and flat Al(0 0 1) and LiF(0 0 1) surfaces along low index axial directions in the surface plane and studied the positions of the rainbow peaks as function of the kinetic energy of the atomic projectiles normal to the surface. For the insulator surface the rainbow angle does not depend on projectile energy for constant normal energy, whereas for the metal surface we find pronounced dynamic effects. We interpret this different behaviour as arising from a projectile energy dependent contribution to the underlying interaction potentials owing to embedding the projectiles into the free electron gas in the selvedge of the surfaces, which is present for the metals but absent for insulators.     

The use of low energy alkali ion scattering as a probe of surface structure

An overview is given of the use of low energy ion scattering as a probe of surface structure with emphasis on work done using alkali ions. Various schemes for extracting structural information from the ion energy and angle distributions are discussed in terms of advantages and disadvantages of each. The scattering potential, which is the primary nonstructural parameter needed for analysis, is discussed in terms of recent experimental results. The structures of clean and reconstructed surfaces are discussed, with examples of measurements of layer relaxations on the Mo (111) surface and missing row reconstructions on the Au (110) and Pt (110) surfaces. Studies of adsorbate covered surfaces are presented with respect to location of the adsorbate and its effect on the structure of the underlying substrate. Finally, examples are given which demonstrate the sensitivity of ion scattering to surface defects and disordering on reconstructed Au (110) and Pt (110) surfaces and unreconstructed Mo (111) surfaces, and to ordering of adsorbates on Mo (001).     

First-principles study of hydrogen behavior in V-Cr-Ti alloys

V-Cr-Ti alloys are considered as one of the most promising candidate structural materials for fusion reactor. Hydrogen impurities originated from bombarding with reaction plasma as well as various nuclear transmutation reactions have a considerable influence on the performance and stability of V-Cr-Ti alloys. To understand the microscopic mechanism, we have simulated the behaviors of H atoms inside the pure vanadium solid as well as V-based alloys (V-4Cr-4Ti and V-5Cr-5Ti) using first-principles approach. The binding energies or absorption energies for H atom inside different interstitial sites were compared to determine the optimal trapping sites. Interaction between two interstitial H atoms was calculated and fit into a screened Coulomb potential. H atom prefers to diffuse along path between two nearest interstitial sites with low energy barriers of 0.20-0.34 eV.     

水与三甲基硅烷醇羟基的氢同位素交换反应的理论研究

以(CH_3)_3SiOH羟基模拟Li_4SiO_4陶瓷表面羟基,研究了H_2O与(CH_3)_3SiOH羟基H的氢交换反应机理.采用HF, MP2方法,在3-21G和6-311G++H~(**)水平上优化了(CH_3)_3SiOH, H_2O, (CH_3)_3SiOH-H_2O复合物及氢交换反应过渡态的结构.计算了生成(CH_3)_3SiOH-H_2O复合物的反应热,探讨了氢交换反应的路径.结果表明,可以形成2种形式的(CH_3)_3SiOH-H_2O复合物,一种是H_2O的O原子与(CH_3)_3SiOH羟基的H原子作用形成的复合物,另一种是H_2O的H原子与(CH_3)_3SiOH羟基的O原子作用形成的复合物.MP2/6-311G++~(**)水平上,对基组重叠能(BSSE)进行校正后,上述2种复合物的反应热分别为20.046 5 kJ/mol和21.630 7 kJ/mol.有利的氢交换反应路径为:H_2O的H原子与(CH_3)_3SiOH羟基的O原子作用形成的复合物,然后H2O提供1个H原子、1个O原子,(CH_3)_3SiOH提供1个O原子、1个Si原子形成由O, H, O, Si 4个原子构成的四元环过渡态,最后H_2O的O原子与(CH_3)_3SiOH 的Si原子成键形成新的(CH_3)_3SiOH,而(CH_3)_3SiOH的Si-O键断裂,由(CH_3)_3SiOH的羟基和H_2O的1个H原子形成新的H_2O分子,MP2/6-311G++~(**)水平上,BSSE校正后,此路径的反应活化能为186.898 4 kJ/mol.     

Reaction between Uranium Mononitride and Graphite Powders

The carbothermic reaction of UN with graphite in the form of loose powder mixture has been studied in vacuo with the aid of a high-temperature thermobalance in the temperature range of 1,270~1,675°C. The gravimetric measurements were conducted continuously to determine the effect of varying reaction temperature and differences in sample particle size.

The kinetics of the reaction was found to follow the diffusion—controlled kinetic equation for mixed powder, and activation energies of 76.1, 55.1 and 72.7kcal/mol were obtained for samples of (1)—400 mesh UN and—400 mesh natural graphite mixture, (2)—150 +270 mesh UN and—150 +270 mesh natural graphite mixture, and (3)—150 +270 mesh UN and—150 +270 mesh artificial graphite mixture, respectively. X-ray diffraction studies revealed the reaction products of the coarse sample to be composed of two different kinds of UC_xN_1-x solid solutions, one having a x value of 0~0.2, and the other 0.8~0.9. From the above observations, the diffusion of C and/or N atoms through the outer carbon rich UC_xN_1-x layer may be considered to be the rate determining step.     

La1-xCaxMn0.96Fe0.04O3（x=0.31、0.5、0.6）的中子衍射研究

利用Rietveld分析方法对钙钛矿锰氧化物La_1-xCa_xMn_0.96Fe_0.04O₃（x=0.31、0.5、0.6）室温下中子衍射实验数据进行拟合。结果表明，La_1-xCa_xMn_0.96Fe_0.04O₃化合物具有MnO₆八面体，空间群为Pnma，La（Ca）原子占据4c晶位，Mn（Fe）原子占据4b晶位，O原子分别占据4c和8d晶位。根据拟合结果，计算出Mn-O键的键长和Mn-O-Mn的键角，并对该系列样品结构和磁性能间的关系进行了简单讨论。     

水蒸气分子在金属钚表面吸附的量子力学计算

采用密度泛函理论（B3LYP）方法，钚原子采用相对论有效原子实势（RECP）SDD基组，氢、氧原子采用aug-cc-pVTZ全电子基组，优化了PuO₂的分子结构，得到了相应的平衡几何构型。同时优化了Pu-H₂O的4个稳定异构体。比较能量发现水分子易于解离，从而与Pu形成更稳定的结构。由于Pu与O原子的电负性相差很大易发生电荷转移，分子的稳定性主要源于Pu-O之间的相互作用。根据电子-振动近似理论，计算了不同温度下金属Pu与H₂O吸附与解离反应的生成热力学函数。计算表明，即使在低温下，H₂O蒸汽分子也无法在金属钚表面形成分子吸附，相反，在金属钚表面水分子的解离可自发进行。     

First-Principles Investigation of （001） Surface of TiO2

By means of first-principles calculations based on the ab initio pseudopotential and plane wave basis, we present self-consistent molecular dynamic calculations to determine a clean TiO2 crystal (001) surface geometry and surface electronic states structure. All calculations were performed for both rutile and anatase phases. Using a total energy minimization strategy, we acquired the general inward relaxations. The s-like surface states just above the lower valence band at (001) surfaces both in the rutile and anatase phases are found. The band bending effect occurs both in rutile and anatase (001) surfaces. It reduces the band gap by ac. 0.4 eV in rutile and ac. 0.8 eV in anatase, elevating the Fermi energy levels. Besides the band bending of the valence band, surface states also lie near the bottom of the conduction band. The lower surface energy of anatase makes nanocrystalline TiO2 is often in the anatase phase below a critical size.Our results are in good agreement with the previous work and experimental data.