用低能电子衍射研究GaAs(110)表面的弛豫

用低能电子衍射研究了GaAs(110)表面的弛豫。发现当理论与实验之间符合得最好时,得到的结构是,保持表面上As—Ga键长不变用一个27.32°±0.24°的旋转角(ω),使As原子向外移动0.10±0.02?,Ga原子向内移动0.55±0.02?,而从Ga到第二层时空间为d₂=1.45±0.01?,从第二层Ga到第三层的空间为d₃=2.01±0.01?。对此结构As的背键长l_As=2.43±0.01?(收缩0.56％),而Ga的背键长l_Ga=2.253±0.004?(收缩8.0％)。 关键词：     

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     

乙烯在Ni（110）表面吸附的几何结构

用理论计算的方法研究了不同覆盖度的乙烯在Ni（110）表面吸附的位置.乙烯的吸附几何结构在团簇计算中进行了局部优化.在低覆盖度下，单个乙烯分子占据了短桥位和顶位之间的中间位置.乙烯分子的C—C轴大致沿衬底的Ni原子链排列（即沿<110>晶向），C—C轴与衬底Ni（110）表面有10°的倾斜角.乙烯分子的C—C键的键长为0151nm.在高覆盖度下(05ML)，乙烯在Ni (110)上形成了有序的c（2×4）相，在一个表面元胞内的两个乙烯分子的吸附位置类似于低覆盖度时的结果，但乙烯分子的C—C 键键长分别为0142和0143nm.     

多重散射团簇方法对吸附系统SO2/Ag(110)的理论分析

利用多重散射团簇方法(MSC)对吸附系统SO₂/Ag(110)的S原子K边X射线吸收精细结构谱(NEXAFS)作了理论分析.研究表明,覆盖度为0.5时,吸附的SO₂的S—O键长比气体状态时增长了(0.014±0.006)nm,OSO键角减小了15°±5°;SO₂分子的S原子处于芯位,但两个O原子处于不对称的位置;分子平面与(110)的夹角约为52°,同时分子平面相对衬底表面法线有一小角度的倾斜.MSC计算证实了该吸附系统存在一介于π^{关键词： X射线吸收精细结构 2/Ag(110)')" href="#">吸附系统SO₂/Ag(110) 多重散射团簇方法}     

偏磷酸钙玻璃陶瓷的Raman光谱分析

测量了Ca/P摩尔比为0.45的偏磷酸钙玻璃及其在300~700℃下结晶化处理获得玻璃陶瓷的拉曼光谱,对谱峰的振动模式进行了指认,利用拉曼振动频率计算了P—O键长。光谱分析发现偏磷酸钙玻璃在300℃已经开始了结晶相变,不同温度下,sν(PO2)和sν(POP)谱带的振动频率基本不变,随着温度提高,谱峰强度增加,并出现了精细的指纹结构;玻璃陶瓷的表面和内部的结构不同,表面结晶化程度高于内部;结晶过程中,P—O键平均键长会发生变化,随着结晶化程度的提高,骨架上(POP)的P—O键平均键长减小,由159.96 pm变为158.27 pm,侧链(PO2)上的P—O键长增大,由147.90 pm变为149.02 pm。     

单晶锗表面键合光敏染料的研究

本文在烯丙醇单体上进行了两种一甲川菁的合成,并用一种新的化学键合法将两种一甲川菁染料键合在抛光的半导体单晶锗表面。将键合有光敏染料的锗片进行了激光Raman光谱及XPS谱测试,结果表明,与对照锗片相比,键合后的锗片表面,锗衬底的一级拉曼峰强度减少,并在600～3200cm~(-1)范围内出现了与键合颜料分子相应的拉曼频移;在XPS谱中,分别进行了C,N,O,S,卤素等原子的谱图分析,证实了键合颜料后半导体单晶锗表面增加了C—N,S—C,C—O等键,结果与键合的颜料分子结构相符,表明两种光敏染料通过锗氧键共价键合于锗表面。     

胺三乙酸银晶体和分子结构的测定

(CH₂COOH)₂NCH₂COOAg属于单斜晶系,单位晶胞中包含有四个分子,晶胞周期为α=7.61₁?,b=4.98₄?,c=23.25?,β=104°52′。空间羣为P2₁/c(C_2h⁵)。用柏特森及电子密度函数方法获得了原子坐标。键长C—C=1.51?,N—C=1.48?,C—O=1.28?及Ag—O=2.23—2.73?。指出了在分子之间具有较强的氢键O…O=2.54?。     

c(2×2)O吸附Cu(001)表面结构、电子态与STM图像的研究

用投影子缀加波和CP分子动力学方法研究了贵金属Cu(001)面的表面结构、弛豫以及O原子的c(2×2)吸附状态. 研究结果得出在这种吸附结构中，O原子与衬底Cu原子之间的垂 直距离约为0069nm，Cu—O键长为0.194nm，功函数约为5.29 eV；吸附O原子形成金属性能带结构，由于Cu—O的杂化作用,在费米能以下约6.7 eV处出现了局域的表面态.用Tersoff-Hamann途径计算了该表面的扫描隧道显微镜图像，并讨论了与实验结果之 间的关系. 关键词： Cu(001)-c(2×2)/O 电子态 STM图像     

利用扫描隧道显微镜研究水分子在Cu(110)表面的吸附与分解

利用扫描隧道显微镜研究水分子在吸附有氧原子的Cu(110)表面的吸附与分解过程.室温条件下,氧原子(O)在Cu(110)表面吸附并自组装形成规则的沿[001]方向的(2×1)Cu-O链状结构.将吸附有氧原子的Cu(110)样品置于77 K低温条件下观察水分子的吸附与分解,发现在低温下水分子通过氢键与Cu-O链中的氧原子键合而吸附于Cu-O链的顶部和周围,吸附于Cu-O链周围的水分子自组装形成规则的六边形网状结构.通过针尖隧穿电子激发,六边形网状结构中的水分子与氧原子发生化学反应,反应生成的羟基与未参与反应的水分子键合在裸露的Cu(110)表面形成蜂窝网状结构.研究结果表明,Cu(110)表面吸附的氧原子有助于水分子在金属表面的吸附和组装,同时可以催化金属表面水分子的分解反应,对水汽转换实验研究具有一定的指导意义.     

GaAs(110)面的电子结构

本文研究GaAs(110)面旋转弛豫的电子结构,采用一个原子集团来模拟GaAs(110)面,在其内边界上用一些“类Ga”和“类As”原子来钝化伸向体内的悬挂键,以消除由于有限模型而引起的多余边界效应。用EHT方法计算集团的总能量,由能量极小定出GaAs(110)面最稳定的弛豫位置为表面旋转角ω=18°,表面Ga原子向体内下降0.33?,As原子上升0.13?,这与Pandey等人从光电子部分产额谱所得的结果基本一致。本文还计算了理想和弛豫的GaAs(110)面的态密度,发现对于理想的(110)面禁带中确实存在一个空的表面峰。弛豫后,该峰向上移动进入导带,禁带中不再出现表面峰,与实验结果相符。 关键词：     

Cu/CeO2(110)界面特性的第一性原理研究

Cu-CeO₂体系因其特殊的催化能力而在固体氧化物燃料电池和水煤气转化反应等多个催化领域有重要应用. 采用基于密度泛函理论的第一性原理方法, 在原子和电子层面上系统地研究了单个Cu原子及Cu小团簇在CeO₂(110)面上的吸附构型, 价键特性和电子结构, 结果表明: 1) 单个Cu原子的最稳定吸附位是两个表面O的桥位; 2) Cu团簇的稳定吸附构型为扭曲的四面体结构; 3) Cu原子及Cu团簇的吸附在CeO₂(110)面的gap区域引入了间隙态, 这些间隙态主要来自于Cu及其近邻的O和表层还原形成的Ce³⁺, 间隙态的出现表明Cu的吸附增强了CeO₂(110)表面的活性; 4) 吸附的单个Cu原子及Cu团簇分别被CeO₂(110)面表层的Ce⁴⁺离子氧化形成了Cu^δ+和Cu₄^δ+, 并伴随着Ce³⁺离子的形成, 这个反应可归结为Cu_x/Ce⁴⁺→Cu_x^δ+/Ce³⁺; 5) Cu团簇的吸附比Cu单原子的吸附引入了更多的Ce³⁺离子, 进而形成了更多的Cu^δ+-Ce³⁺催化活性中心. 结合已报道的Cu/CeO₂(111)界面特性, 更加全面地探明了Cu与CeO₂(111)和(110)两个较稳定低指数表面的协同作用特性, 较为系统地揭示了Cu增强CeO₂催化特性的原因及Cu与CeO₂协同作用的内在机理. 关键词： 2')" href="#">Cu/CeO₂ U')" href="#">DFT+U 吸附 电子结构     

The synthesis of methanol and the reverse water-gas shift reaction over Zn-deposited Cu(100) and Cu(110) surfaces: comparison with Zn/Cu(111)

The catalytic activity of Zn vapor-deposited Cu(100) and Cu(110) surfaces for methanol synthesis by the hydrogenation of CO₂ and the reverse water-gas shift reaction were studied using an XPS apparatus combined with a high-pressure flow reactor (18 atm). At a reaction temperature of 523 K, no promotional effect of Zn was observed for the methanol synthesis on both Zn/Cu(100) and Zn/Cu(110). The results were quite different from those for Zn/Cu(111), on which a significant promotion of methanol synthesis activity appeared to be due to the deposition of Zn, indicating that the promotional effect of Zn was sensitive to the surface structure of Cu. However, hysteresis was observed in the catalytic activity for methanol synthesis over the Zn/Cu(110) surface upon heating above 543 K in the reaction mixture. The activity became twice that measured before heating, which was close to the methanol synthesis activity of Zn/Cu(111) at the same Zn coverage. On the other hand, no such hysteresis was observed for the reverse water-gas shift reaction on Zn/Cu(110), indicating that the active site for methanol synthesis was not identical to that for the reverse water-gas shift reaction. In the post-reaction surface analysis, formate species was detected on both Zn/Cu(100) and Zn/Cu(110), whose coverage increased with increasing Zn coverage at 0<Θ_Zn<0.2. No correlation between the formate coverage and the methanol synthesis activity was obtained, which was in contrast to the results for Zn/Cu(111). Thus, the structure sensitivity observed in the catalytic activity of methanol synthesis over Zn-deposited Cu surfaces is ascribed to the significant difference in the reactivity of the formate intermediate.     

A reversible reaction forming (---Cu---O---) strings and (Cu)6-clusters on Ag(110) shown by STM

An artificial new surface of (---Cu---O---) chains grown on Ag(110) surface was prepared by reacting a surface with Cu atoms, where the (---Cu---O---) chains grow in the [1 0] direction and are self-assembled on the Ag(110) surface in a (2 x 2)-p2mg structure. When the Cu---O/Ag(110) surface was heated in vacuum, the (---Cu---O---) chain decomposed to uniform cluster dots arranged along the [1 0] direction, where the cluster dots were composed of six Cu atoms. When the Ag(110) surface with the Cu---cluster dots was exposed to O₂, the (---Cu---O---) lines were redrawn along the [1 0] direction by reacting a s in the [1 0] direction with O₂. This is a reversible chemical reaction in one dimensional regime proved in atomic resolution.     

Methanol and formaldehyde molecular states on copper surfaces at 300 K?

In a recent paper, Kojima, Sugihara, Miyazaki and Yasumori concluded that methanol and formaldehyde adsorb molecularly (non-dissociatively) on polycrystalline copper at 300 K. Methanol and methyl formate were also found to produce adsorbed formaldehyde. We demonstrate that the “ formaldehyde” UPS spectrum in their study was incorrectly assigned, and is identical to that of adsorbed formate generated during dissociative exposure of formaldehyde to a Cu(110) surface. We have measured the He II spectra of formaldehyde (120 K) and formate (300 K) on clean Cu(110) and show that they are distinctly different. No evidence is found in the present work for stabilization of molecular formaldehyde, methanol or methyl formate on the Cu(110) surface at 300 K.     

Imaging of atomic-scale structure of oxide surfaces and adsorbed molecules by noncontact atomic force microscopy

Atom-resolved images of a TiO₂(110)-(1×1) surface and individual formate and acetate ions adsorbed on the surface were obtained by noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum. In contrast to previous scanning tunneling microscopic studies imaging five-fold coordinated Ti atoms, outermost atoms of bridge-bound oxygen ridges of the surface were resolved as protruding rows by NC-AFM. High-resolution image of the surface revealed that the bridging oxygen atoms on terraces ordered in a (1×1) periodicity. Randomly distributed point and multiple defects of oxygen atoms were also imaged as dark spots. The (2×1) overlayer of formate and acetate ions were resolved as ordered bright spots. Dispersed formate ions at a low coverage were also observed as bright spots between the bridging oxygen ridges along the [001] direction.     

用低能电子衍射研究ⅢA-ⅤA和ⅡB-ⅥA化合物(110)和(1010)表面的弛豫

用低能电子衍射研究了ⅢA-VA和ⅡB-VIA化合物(110)和(1010)表面的弛豫,发现当理论计算与实验符合得很好时其结构是:保持表面上A-B键长不变,用一个旋转角ω,使B原(离)子向外移动,A原(离)子向内移动,第一表面原子层间距d₁=0.610-0.810?[对ⅢA-VA(110)],0.536—0.825?[对ⅡB-VIA(110)]和0.633-1.060?[对ⅡB-VIA(1010),第二表面原子层间距d₂=1.300-1.610?[对ⅢA-VA(100)],1.430-1.700?[对ⅡB-VIA(110)]和0.820-0.930?[对ⅡB-VIA(1010),而第三表面原子层间距d₃=1.410-2.440?[对ⅢA-VA(110)],2.020-2.250?[对ⅡB-VIA(110)]和1.910-2.440?[对ⅡB-VIA(1010)]。对此结构,弛豫率α是:0.24±0.02[对ⅢA-VA(110)],0.25±0.02[对ⅡB-VIA(1010)]和0.33±0.03[对ⅡB-VIA(1010)]。 关键词：     

NEXAFS study of HCOO/Ag(110): Evidence for dynamic bending

The structure of the surface formate on Ag(110) at 300 K was studied by near-edge X-ray absorption fine structure (NEXAFS) at the carbon K-edge. Interpretation of the NEXAFS spectra of the formate intermediate with a localized bond picture is inconsistent with the findings of previous vibrational studies. Rather, the resonances must be assigned to transitions to delocalized molecular orbitals, whereupon the results agree with the bidentate configuration deduced from the vibrational experiments. A bonding geometry is determined with respect to the surface normal in which the O-O direction lies parallel to the plane of the surface; no azimuthal ordering was detected. The molecular plane appears to be inclined at an angle of 30 ± 15°. This apparent tilt may be due to a dynamical motion of the species about the surface normal in which the molecular plane oscillates and rotates about the surface normal. These results indicate different bonding geometries for formate on Ag(110) and Cu(110).     

Two-dimensional unoccupied electronic band structure of clean Cu(110) and (1 × 2) Na/Cu(110)

Using the technique of angle-resolved inverse photoemission, we have measured the dispersion of an unoccupied Cu(110) surface state for the clean Cu(110) surface and for the (1 × 2) reconstructed Na/Cu(110) surface along the symmetry lines. The dispersion of the crystal-induced surface state of clean Cu(110) at 2.05 eV above the Fermi energy at the point of the SBZ is free-electron-like with an effective mass of (1.0 ± 0.2)m_e at the point, which is in good agreement with other experimental results as well as a theoretical calculation. This surface state shifts to 2.5 eV above the Fermi energy for the (1 × 2) phase of Na/Cu(110) with a coverage of 0.25 ML, and the dispersion along the direction is considerably reduced compared to the clean surface. On the other hand, the dispersion of this state for (1 × 2) Na/Cu(110) (0.25 ML) along the direction is close to that of clean Cu(110). We account for these results within a missing-row picture of the Na-induced reconstruction.     

A spectroscopic study of the adsorption and reactions of methanol,formaldehyde and methyl formate on clean and oxygenated Cu(110) surfaces

The adsorption and reaction of methanoi (CH₃OH), methyl formate (CH₃OCHO) and formaldehyde (H₂CO) on clean and oxygen-covered Cu(110) surfaces has been studied with EELS, UPS and thermal desorption spectroscopy (TDS). The clean surface is relatively unreactive but adsorbed oxygen readily attacks the hydroxyl proton and formyl carbon atoms to generate the intermediate methoxy (CH₃O) and formate (HCOO). Methyl formate is split into two intermediates, methoxy and formate. By correlating the three techniques we analyse (a) the condensed multilayer at 90 K; (b) the weakly bound molecular monolayer states prior to dissociation or reaction and (c) the reactive intermediates at higher temperatures. Formaldehyde forms the surface polymer polyoxymethylene [(CH₂O)_n] in the monolayer on Cu(110) which subsequently reacts with oxygen to generate formate. No molecular formaldehyde was observed above 120 K. Correlation of the EELS and UPS results for polyoxymethylene shows that an earlier interpretation by Rubloff et al. [Phys. Rev. B14 (1976) 1450] of anomalous shifts in the formaldehyde UPS spectrum on surfaces is incorrect, and due simply to the new polymeric structure of surface formaldehyde. Methyl formate coordinates to copper via the carbonyl lone pair orbital and methanol via the oxygen lone pair orbital. No evidence was found for methyl formate synthesis by dimerization of formaldehyde (the Tischenko reaction) or dehydrogenation of methanol on the clean Cu(110) surface. These latter reactions are facile over copper catalysts at atmospheric pressure. The success of the oxidation experiments and the failure of the synthesis reactions in UHV is a consequence of the pressure dependence of the equilibrium constants for the different reactions. As found previously in Fischer-Tropsch studies, condensation reaction equilibria are pressure dependent and product formation is considerably suppressed at UHV pressures.