First-principles Study of Geometric and Electronic Structures of Si(111)-√7×√3-In Surface Reconstruction

In order to determine the structures of Si(111)-√7×√3-In surfaces and to understand their electronic properties, we construct six models of both hexagonal and rectangular types and perform first-principles calculations. Their scanning tunneling microscopic images and work functions are simulated and compared with experimental results. In this way, the hex-H3' and rect-T1 models are identified as the experimental configurations for the hexagonal and rectangular types, respectively. The structural evolution mechanism of the In/Si(111) surface with indium coverage around 1.0 monolayer is discussed. The 4×1 and √7×√3 phases are suggested to have two different types of evolution mechanisms, consistent with experimental results.     

Coexistence of √3 ×√3 and quasi-linear phases of sulfur adsorbed (Θ = 1/3) on a gold (111) substrate

We analyze with the aid of density functional theory and molecular dynamics simulations the adsorption of sulfur (S) on a Au(111) surface at different temperatures with a variety of geometries. We have found a new superficial phase in which sulfur atoms form a quasi-linear chain with energies very close to the expected . The results suggest the coexistence of both configurations at T < 300 K. At high temperatures (T > 300 K) it was shown that the sulfur atoms have high mobility which allows their migration among different adsorption sites. At low temperatures, the mobility decreases and a thermal activation barrier of 25-30 meV can be estimated.     

Adsorption and decomposition of cyclohexanone (C6H10O) on Pt(111) and the (2 × 2) and (√3 × √3)r30°-Sn/Pt(111) surface alloys

Adsorption and decomposition of cyclohexanone (C(6)H(10)O) on Pt(111) and on two ordered Pt-Sn surface alloys, (2 × 2)-Sn/Pt(111) and (√3 × √3)R30°-Sn/Pt(111), formed by vapor deposition of Sn on the Pt(111) single crystal surface were studied with TPD, HREELS, AES, LEED, and DFT calculations with vibrational analyses. Saturation coverage of C(6)H(10)O was found to be 0.25 ML, independent of the Sn surface concentration. The Pt(111) surface was reactive toward cyclohexanone, with the adsorption in the monolayer being about 70% irreversible. C(6)H(10)O decomposed to yield CO, H(2)O, H(2), and CH(4). Some C-O bond breaking occurred, yielding H(2)O and leaving some carbon on the surface after TPD. HREELS data showed that cyclohexanone decomposition in the monolayer began by 200 K. Intermediates from cyclohexanone decomposition were also relatively unstable on Pt(111), since coadsorbed CO and H were formed below 250 K. Surface Sn allowed for some cyclohexanone to adsorb reversibly. C(6)H(10)O dissociated on the (2 × 2) surface to form CO and H(2)O at low coverages, and methane and H(2) in smaller amounts than on Pt(111). Adsorption of cyclohexanone on (√3 × √3)R30°-Sn/Pt(111) at 90 K was mostly reversible. DFT calculations suggest that C(6)H(10)O adsorbs on Pt(111) in two configurations: by bonding weakly through oxygen to an atop Pt site and more strongly through simultaneously oxygen and carbon of the carbonyl to a bridged Pt-Pt site. In contrast, on alloy surfaces, C(6)H(10)O bonds preferentially to Sn. The presence of Sn, furthermore, is predicted to make the formation of the strongly bound C(6)H(10)O species bonding through O and C, which is a likely decomposition precursor, thermodynamically unfavorable. Alloying with Sn, thus, is shown to moderate adsorptive and reactive activity of Pt(111).     

Relationship between the c(4×2) and the (√3×√3)R30° phases in alkanethiol self-assembled monolayers on Au(111)

Transformation between the two well-known phases of alkanethiol monolayers on Au(111), c(4×2) and (√3×√3)R30°, has been studied using scanning tunneling microscopy in ultra-high vacuum. Among the many versions of the c(4×2) phases observed, one particular structure where a lateral shift of adsorbate by as much as 0.17 nm within the unit cell is found. This lateral shift along the [112[combining macron]] direction corresponds to the movement of one adsorbed unit, towards its nearest neighbour from one hollow site to another (fcc to hcp, or hcp to fcc).     

Templating an organic array with Si(111)-7×7

We demonstrate that nearest neighbor molecular adsorption can be sterically hindered on the Si(111)-7×7 surface reconstruction. This breaks the energetic equivalence of corner and edge di-σ attachment geometries and allows a translationally ordered organic layer to be templated directly on the 7×7 reconstruction.     

Molecular simulation of alkyl monolayers on the Si(111) surface YUAN

The preparation of monolayers on silicon surface is of growing interest for potential applica-tions in biosensor or semiconductor technology[1—5]. The alkyl modified Si(111) surfaces[6—10] can be obtained using the thermal, catalyzed, or photochemical reaction of hydrogen-terminated sili-con with alkenes, Grignard reagents, and so on. At the same time, the monolayer properties on Si(111) surface have been studied by a variety of experimental methods[8—10] such as X-ray photo-electron spect…     

(Me3Si)2NBCl2和(Me3Si)2NB(Cl)NHSiMe3的合成及其成环反应

自从氮化硼陶瓷纤维用聚合物先驱体法[1]合成以后,硼氮六元环化合物越来越为人们所重视[2],文中在制备氮化硼纤维先驱体的过程中,合成了其中间产物,通过热缩合成环反应,合成了含有硼氮六员环的化合物,并采用色-质联用技术对其进行了表征。1　实验部分参照文献[3],改用液氮和丙酮为冷冻剂,控制温度在-50到-40℃,搅拌下向三氯化硼的正戊烷溶液中缓慢滴加三乙胺的正戊烷溶液,滴完后Ｎ2气氛下-50℃加丙酮冷凝回流2ｈ,慢慢由-50℃升至室温,抽滤即得到白色产物Ｅｔ3ＮＢＣｌ3(Ａ)。将Ａ溶于适量苯中[4,5],加入等物质的量的三乙胺,加热回流搅拌…     

(Me3Si)2CH]2(i)PrSi(NHC)Si═Si(Me)Si(i)Pr[CH(SiMe3)2]2]+: a molecule with disilenyl cation character

Reaction of the disilyne-NHC complex 1 [RLSi═SiR: (R = Si(i)Pr[CH(SiMe(3))(2)](2), L = NHC)] with MeOTf gave the cation 2 [RLSi═SiRMe](+), which is the first example of a base-stabilized heavy group 14 element analogue with vinyl cation character. Cation 2 has been fully characterized by multinuclear NMR spectroscopy and X-ray diffraction analysis. The molecular structure indicates that there are significant contributions from the NHC-stabilized cationic resonance structure 2A, the disilene-like structure 2B, and even some contribution from the silylene-like structure 2C.     

Si(CH_3)_(3)Cl对NaY沸石改性的研究

用Si(CH_3)_3C1处理Y型沸石时,选择合宜的处理条件可获得高结晶度的产物。XRD,IR,比表面和孔径分布测定表明:用Si(CH_3)_3C1处理沸石,脱铝和硅烷化作用同时发生,导致沸石比表面和孔径改变,催化反应性能提高。     

氧化锌有序结构在Au(111)和Cu(111)上的生长

利用NO₂或O₂作为氧化剂,研究了氧化锌在Au(111)和Cu(111)上的生长和结构。NO₂表现了更好的氧化性能,有利于有序氧化锌纳米结构或薄膜的生长。在Au(111)和Cu(111)这两个表面上,化学计量比氧化锌都形成非极性的平面化ZnO(0001)的表面结构。在Au(111)上,NO₂气氛下室温沉积锌倾向于形成双层氧化锌纳米结构;而在更高的沉积温度下,在NO₂气氛中沉积锌则可同时观测到单层和双层氧化锌纳米结构。O₂作为氧化剂时可导致形成亚化学计量比的ZnO_x结构。由于铜和锌之间的强相互作用会促进锌的体相扩散,并且铜表面可以被氧化形成表面氧化物,整层氧化锌在Cu(111)上的生长相当困难。我们通过使用NO₂作为氧化剂解决了这个问题,生长出了覆盖Cu(111)表面的满层有序氧化锌薄膜。这些有序氧化锌薄膜表面显示出莫尔条纹,表明存在一个ZnO和Cu(111)之间的莫尔超晶格。实验上观察到的超晶格结构与最近理论计算提出的Cu(111)上的氧化锌薄膜结构相符,具有最小应力。我们的研究表明,氧化锌薄膜的表界面结构可能会随氧化程度或氧化剂的不同而变化,而Cu(111)的表面氧化也可能影响氧化锌的生长。当Cu(111)表面被预氧化成铜表面氧化物时,ZnO_x的生长模式会发生变化,锌原子会受到铜氧化物晶格的限域形成单位点锌。我们的研究表明了氧化锌的生长需要抑制锌向金属基底的扩散,并阻止亚化学计量比ZnO_x的形成。因此,使用原子氧源有利于在Au(111)和Cu(111)表面上生长有序氧化锌薄膜。     

n-GaAs(100)、Si(111)表面修饰卟啉分子的光致界面电荷转移特性的研究

利用表面光电压谱研究了四碘化四-(4-三甲胺苯基)卟啉(TTMAPPIH₂)修饰n-GaAS(100)和n-Si(111)半导体表面的光致界面电荷转移特性,结果表明,n-GaAs(100)表面修饰TTMAPPIH₂分子的光致界面电荷转移效率远比n-Si(111)表面修饰的高,并且发现在该卟啉分子的非吸收区也有明显的光致界面电荷转移现象,而与n-Si(111)间则没有这种转移特性。用电化学测量和UV光谱确定了TTMAPPIH₂相对于n-GaAs(100)、n-si(111)的能级位置关系,对TTMAPPIH₂分子与n-GaAs(100)和n-Si(111)间的不同光致界面电荷转移特性进行了解释。     

Direct-space analysis of the Si–Si bonding pattern in the π-bonded chain reconstructed Si(111)(2 × 1) surface

Atomic and bond properties of silicon atoms in the buckled π-bonded chain reconstructed Si(111)(2 × 1) system were investigated by applying the quantum theory of atoms in molecules to a number of wavefunctions from periodic ab initio calculations using a slab model for the surface and geometries from experiment. Reconstruction involves much larger surface-cell charge distortions than in the unrelaxed surface and drastic changes in the atomic polarizations of the surface layer atoms. The effect of buckling is to largely differentiate the properties (charge, energy, volume, atomic polarizations) of the two unique atoms of each surface layer. The direction of electronic charge transfer in the topmost chain (from the “up” to the “down” atom) was found to be opposite to what was claimed previously. The π conjugation is not strictly localized along the topmost layer chains (where it is also largely incomplete), but rather it extends over a 2D array of bonds between the topmost and the lower surface layers. Received: 19 July 2000 / Accepted: 2 October 2000 / Published online: 23 January 2001     

苯在Si(111)-7*7表面化学吸附的理论研究

采用两种大小不同的原子簇模型Si_(30)H_(28)和Si_(13)H_(16),分别用两层 ONIOM方法（对较大原子簇）和普通量子化学方法（对较小原子族）考察了苯分子 在Si(111)-7 * 7表面的化学吸附。对三种可能的吸附物种分别用DFT或HF方法进行 了计算。通过大小原子簇吸附物种的吸附能以及几何构型优化参数的比较发现,对 于稳定的吸附物种,较小的原子簇基本上可以代替较大的原子簇进行计算,而对于 不太稳定的吸附物种,就不得不考虑周边原子的影响。计算结果表明苯在Si(111)- 7 * 7表面的主要吸附种是双σ成键的1,4加成产物,不稳定的单吸附物种可能是 1,4加成物种的前驱态。     

NH3在Ni(111)面吸附的理论研究

The bonding and configuration of the chemisorption of NH_3 at the on-top and hollow sites of Ni(111) surfaces are calculated and discussd by use of MO theory and energy band theory. The results show that the density of state and crystal orbital overlap population of the adsorption state at the on-top site have superiority over those of hollow site.     

Fe3O4(111)面上的水煤气变换反应机理

水煤气变换是一重要的制备氢气的反应, 采用密度泛函理论(DFT), 对水煤气变换反应在Fe₃O₄ (111)面上的催化反应作了深入研究. 结果表明: 氧化还原机理的活化能明显高于结合机理的活化能, 中间步骤应以结合机理进行|氢原子结合生成氢气为整个反应的速控步骤, 理论计算其活化能与水煤气变换实验数据一致, 高达1.29 eV, 还对迄今有关水煤气变换机理研究中的各种实验现象作了合理的解释, 并对Fe₃O₄ (111)催化剂的改性设计作了讨论.     

NH_3在Ni(111)面吸附的理论研究

氨在金属表面上能够被氧化成工业用量很大的硝酸,这是制硝酸最经济的方法,所以NH_3在各种金属面上的吸附一直受到实验和理论化学家的重视。早期的研究主要集中于NH_3在金属面上的吸附,分解与合成;最近则主要讨论NH_3在金属单晶上的吸附性质。Bauschlicher,Jr研究NH_3吸附在Cu(100)和Al(111)面时,认为NH_3主要吸附在顶位,即NH_3中N原子正对着Cu面上的一个原子,计算的吸附能约为0.93eV(实验值为0.66eV)。由此推出NH_3在Ni,Fe等大部分金属面上的吸附是顶位。但是Baetzold却认为NH_3在金属面上是孔位吸附,即NH_3中N正对着表面上三个邻接原子间的空位。本文旨在探讨NH_3在Ni(111)面上的吸附位置,几何构型和轨道间相互作用本质。我们的结果完全支持NH_3主要在顶位吸附的结论。     

一氧化氮在Cu_3Pt(111)表面的吸附行为

运用密度泛函理论中广义梯度近似(GGA)的PW91方法,结合周期性平板模型,探讨了NO分子在Cu3Pt(111)表面上不同吸附位的吸附行为.结果表明:NO分子以N端朝下方式吸附在top-Pt以及hcp1和fcc2位(分别为表面Cu2Pt和Cu3簇)的吸附模式最稳定,吸附能分别为101.8、124.5和118.1kJ·mol-1.对于hcp1和fcc2位的吸附,NO中的N原子分别与底物的Cu2Pt和Cu3簇成键.吸附前后的电荷布居、态密度和振动频率的分析结果表明,净电子从底物合金表面转移到NO,N—O键伸长,频率发生红移.合金Cu3Pt和纯贵金属Pt对NO的吸附性质相似.     

苯在Si(111)-7×7表面化学吸附的理论研究

采用两种大小不同的原子簇模型Si_(30)H_(28)和Si_(13)H_(16)，分别用两层 ONIOM方法（对较大原子簇）和普通量子化学方法（对较小原子族）考察了苯分子 在Si(111)-7 * 7表面的化学吸附。对三种可能的吸附物种分别用DFT或HF方法进行 了计算。通过大小原子簇吸附物种的吸附能以及几何构型优化参数的比较发现，对 于稳定的吸附物种，较小的原子簇基本上可以代替较大的原子簇进行计算，而对于 不太稳定的吸附物种，就不得不考虑周边原子的影响。计算结果表明苯在Si(111)- 7 * 7表面的主要吸附种是双σ成键的1，4加成产物，不稳定的单吸附物种可能是 1，4加成物种的前驱态。