Si(111)解理面氧吸附后Si L2,3VV俄歇谱形的分析

对Si(111)解理清洁表面及其在氧吸附后测得的SiL_2,3VV俄歇谱进行数字积分、背底扣除及退自卷积后,得到了解理清洁表面在氧吸附前后的部分跃迁态密度。分析这两者之间的差别,结合别人的实验及理论计算结果表明:氧同时以分子形式及原子形式进行吸附,在吸附过程中,还同时形成氧化硅的价态。 关键词：     

The lineshape of the L2,3 VV Auger spectrum of silicon

An attempt is presented to understand the details of the lineshape of the Si L_2,3 VV Auger spectrum from the (111) surface in the 7 × 7 superstructure. In the experiments we have followed the variation of the lineshape induced by adsorption of O₂, H₂O, CO and by bombardment with 3 keV Ar⁺ ions, over a range from a small perturbation of the surface to major changes in surface structure. For small perturbations from the clean surface we were able to resolve changes in the local density of states at surface silicon atoms. By unfolding the experimental spectra, effective transition densities of states result, which compare quite closely with calculated densities of states, apart from a certain enhancement of surface features in the experiments. All peaks in the experimental spectra can be explained, based on densities of states at the surface of pure Si(111) (7 × 7) (91.8 and 84.8 eV), Si(111) + adsorbed oxygen (70.6 eV), SiO₂, (78.9 and 64.5 eV) and plasmon losses, at 71.0 and 57.5 eV for the clean surface.     

Electron beam induced effects on gas adsorption utilizing auger electron spectroscopy: Co and O2 on Si: I. Adsorption studies

The effect of electron beam monitored gas adsorption on the clean Si surface is studied using Auger electron spectroscopy. It is shown that the beam affects the AES adsorption signal of CO and O₂ on Si by dissociating the adsorbed molecules on the surface and subsequently promoting diffusion of atomic oxygen into the bulk. A qualitative explanation of the adsorption data is presented and the initial sticking probability of O₂ on Si (111) surface is estimated to be S0 = 0.21.     

Low-dimensional nanostructures and fullerene films on semiconductor surface

The morphology and atomic structures of C₆₀ fullerene films on a Bi(0001)/Si(111)-7 × 7 surface and adsorption of fluorofullerene C₆₀F _x molecules on a Si(111)-7 × 7 surface have been studied by scanning tunneling microscopy/spectroscopy and low-energy electron microscopy under ultra high-vacuum conditions. It has been shown that initial nucleation of C₆₀ islands on the surface of an epitaxial Bi film occurs on double steps and domain boundaries, while tunnel spectra do not exhibit any significant charge transfer to the lowest unoccupied molecular orbital states. Fluorofullerene molecules allow local (at the nanoscale level) modification of Si surface through local etching.     

Si(111)表面吸附氢会形成双氢化相吗?

用热脱附谱研究了原子氢在Si(111)表面的吸附,得到了两个吸附状态。从脱附谱特性同Si(100)/H系统的相似性,可以推测氢在Si(111)表面也存在单氢化相和双氢化相两种状态。单氢化相主要是顶位吸附所形成的,而双氢化相的形成则可以用McRae所提出的Si(111)(7×7)表面原子结构的三角形二聚物层错模型来解释。 关键词：     

A molecular beam study of the adsorption and desorption of oxygen from a Pt(111) surface

The adsorption and desorption of O₂ on a Pt(111) surface have been studied using molecular beam/surface scattering techniques, in combination with AES and LEED for surface characterization. Dissociative adsorption occurs with an initial sticking probability which decreases from 0.06 at 300 K to 0.025 at 600 K. These results indicate that adsorption occurs through a weakly-held state, which is also supported by a diffuse fraction seen in the angular distribution of scattered O₂ flux. Predominately specular scattering, however, indicates that failure to stick is largely related to failure to accommodate in the molecular adsorption state. Thermal desorption results can be fit by a desorption rate constant with pre-exponential ν_d = 2.4 × 10^?2 cm² s^?1 and activation energy E_D which decreases from 51 to 42 kcal/mole^?1 with increasing coverage. A forward peaking of the angular distribution of desorbing O₂ flux suggests that part of the adsorbed oxygen atoms combine and are ejected from the surface without fully accomodating in the molecular adsorption state. A slight dependance of the dissociative sticking probability upon the angle of beam incidence further supports this contention.     

Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces

Optical second-harmonic generation (SHG) from silicon surfaces may be resonantly enhanced by dangling-bond-derived surface states. The resulting high sensitivity to hydrogen adsorption combined with unique features of SHG as an optical probe has been exploited to study various kinetical and dynamical aspects of the adsorption system H₂/Si. Studies of surface diffusion of H/Si(111)7×7 and recombinative desorption of hydrogen from Si(111)7 × 7 and Si(100)2 × 1 revealed that the covalent nature of hydrogen bonding on silicon surfaces leads to high diffusion barriers and to desorption kinetics that strongly depend on the surface structure. Recently, dissociative adsorption of molecular hydrogen on Si(100)2×1 and Si(111)7×7 could be observed for the first time by heating the surfaces to temperatures between 550 K and 1050 K and monitoring the SH response during exposure to a high flux of H₂ or D₂. The measured initial sticking coefficients for a gas temperature of 300K range from 10^–9 to 10^–5 and strongly increase as a function of surface temperature. These results demonstrate that the lattice degrees of freedom may play a decisive role in the reaction dynamics on semiconductor surfaces.     

A first principle study on the magnetic properties of Cu2O surfaces

Spin-polarized density functional theory calculations were performed to investigate the magnetism of bulk and Cu₂O surfaces. It is found that bulk Cu₂O, Cu/O-terminated Cu₂O(111) and (110) surfaces have no magnetic moment, while, the O-terminated Cu₂O(100) and polar O-terminated Cu₂O(111) surfaces have magnetism. For low index surfaces with cation and anion vacancy, we only found that the Cu vacancy on the Cu₂O(110) Cu/O-terminated surface can induce magnetism. For atomic and molecular oxygen adsorption on the low index surfaces, we found that atomic and molecular oxygen adsorption on the Cu-terminated Cu₂O(110) surface is much stronger than on the Cu/O-terminated Cu₂O(111) and Cu-terminated Cu₂O(100) surfaces. More interesting, O and O₂ adsorption on the surface of Cu/O terminated Cu₂O(111) and O₂ adsorption on the Cu-terminated Cu₂O(110) surface can induce weak ferromagnetism. In addition, we analysis origin of Cu₂O surfaces with magnetism from density of state, the surface ferromagnetism possibly due to the increased 2p–3d hybridization of surface Cu and O atoms. This is radically different from other systems previously known to exhibit point defect ferromagnetism, warranting a closer look at the phenomenon.     

Study of Ag/Si(111) submonolayer interface: I. Electronic structure by angle-resolved UPS

Angle-resolved ultraviolet photoelectron spectra have been measured for well defined Ag/Si(111) submonolayer interfaces of (1) $\text{Si(111)(}\text{3}\text{×}\text{3}\text{)R30°-Ag}$, (2) “Si(111)(6 × 1)-Ag”, and (3) Ag/Si(111) as deposited at room temperature. Non-dispersive and very narrow (FWHM ～ 0.4–0.5 eV) Ag 4d derived peaks are found at 5.6 and 6.5 eV below the Fermi level for surface (1) and at 5.3 and 6.0 eV for surface (2). Dispersions of sp “binding” states in the energy range between E_F and Ag 4d states have been precisely determined for surface (1). Electronic structures similar to those of the Ag(111) surface, including the surface state near E_F, have been observed for surface (3).     

A density-functional theory investigation on desorption of O2 on Sn（111） and its comparison with initial oxidation on the X（111） （X=Si,Ge, Sn,Pb） surfaces

<正>The first-principles calculations are performed to investigate the adsorption of O₂ molecules on an Sn（111） 2×2 surface.The chemisorbed adsorption precursor states for O₂ are identified to be along the parallel and vertical channels, and the surface reconstructions of Sn（lll） induced by oxygen adsorption are studied.Based on this,the adsorption behaviours of O₂ on X（111）（X=Si,Ge,Sn,Pb） surfaces are analysed,and the most stable adsorption channels of O₂ on X（111）（X=Si,Ge,Sn,Pb） are identified.The surface reconstructions and electron distributions along the most stable adsorption channels are discussed and compared.The results show that the O₂ adsorption ability declines gradually and the amount of charge transferred decreases with the enhancement of metallicity.     

Effect of NH3 adsorption on the atomic structure of Si(111) √3 × √3 - Al and Si(111) √3 × √3 - Ag surfaces

The room temperature (RT) adsorption of ammonia (NH₃) on Si(111)√3 × √3-Al and Si(111)√3 × √3-Ag surfaces has been studied using LEED and AES. The transformation from Si(111)√3 × √3-Al surface structure to Si(111)1 × 1-(Al, H) upon NH₃ exposure has been found to be similar to the previously observed structural transformation induced by exposure in the atomic hydrogen. It has been demonstrated that the transformation is caused by hydrogen atoms which are generated by NH₃ dissociation on the Si(111)√3 × √3-Al surface. It has been estimated that about 0.1 ML of ammonia molecules is needed to complete the structural transformation. No interaction of NH₃ with the Si(111)√3 × √3-Ag surface has been found. The dissociation of NH₃ molecules is believed to be impossible on this surface     

Ab initio study of Yb on the Ge(111)–(3 × 2) and Si(111)–(3 × 2) surfaces

The surface reconstruction, 3 × 2, induced by Yb adsorption on a Ge (Si)(111) surface has been studied using first principles density-functional calculation within the generalized gradient approximation. The two different possible adsorption sites have been considered: (i) H₃ (this site is directly above a fourth-layer Ge (Si) atom) and (ii) T₄ (directly above a second-layer Ge (Si) atom). We have found that the total energies corresponding to these binding sites are nearly the same, indeed for the Yb/Ge (Si)(111)–(3 × 2) structure the T₄ model is slightly energetic by about 0.01 (0.08) eV/unitcell compared with the H₃ model. In particular for the Ge sublayer, the energy difference is small, and therefore it is possible that the T₄, H₃, or T₄H₃ (half of the adatoms occupy the T₄ adsorption site and the rest of the adatoms are located at the H₃ site) binding sites can coexist with REM/Ge(111)–(3 × 2). In contrast to the proposed model, we have not determined any buckling in the Ge = Ge double bond. The electronic band structures of the surfaces and the corresponding natures of their orbitals have also been calculated. Our results for both substrates are seen to be in agreement with the recent experimental data, especially that of the Yb/Si(111)–(3 × 2) surface.     

Scanning tunneling microscopy of fluorinated fullerene molecules on silicon surfaces

Scanning tunneling microscopy (STM) under ultra-high vacuum conditions is used to study the initial stages of adsorption of C₆₀F₁₈ and C₆₀F₃₆ fluorofullerene molecules on Si(111)-7 x 7 and Si(100)-2 x 1 surfaces. Spatially resolved STM images of individual molecules and ab initio calculations show that the fluorofullerene molecules interact with an Si surface, with the F atoms oriented toward the surface. The large electric dipole moment of the molecules induces strong polarization on the surface, but the charge transfer is weak. The presence of C₆₀F₃₆ isomers with different symmetry—T, C ₃, and C ₁—is revealed in STM images for the first time.     

Mo(CO)6在清洁的和氧化的Si(111)表面上吸附的对比

采用高分辨电子能量损失谱对比研究Mo(CO)₆在清洁的、预吸附氧的和深度氧化的Si(111)表面上的吸附行为. 吸附Mo(CO)₆的C-O伸缩振动模式向低频方向移动,说明Mo(CO)₆与清洁Si(111)和SiO₂/Si(111)表面发生了不同的相互作用,前者较弱而后者较强. 与SiO₂/Si(111)表面的强相互作用可能引起Mo(CO)₆部分解离,形成部分分解的羰基钼物种.     

清洁Si(111)表面电子结构的理论研究

本文报道在原子集团模型下用CNDO-SCF方法对清洁Si(111)表面电子结构的系统研究结果:(1)计算了表面上的净电荷分布、电荷转移以及局域在各原子轨道上的电荷;发现T₃⁰,T₃⁺和T₃^-式的表面悬挂键结构较难存在;表面原子趋于形成带有分数电荷的悬挂键,而实际上这些悬挂键彼此结合成弯键;表面原子及其悬挂键上有净电荷积累,且有很强的定域性和取向性。(2)计算了原子集团模型的静 关键词：     

Dynamics of interaction of H2 and D2 with Ni(110) and Ni(110) surfaces

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H₂ and D₂ on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s₀, irrespective of surface temperature T_s and mean kinetic energy normal to the surface 〈 E_⊥ 〉. The desorbing molecules exhibit a cos θ_e angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s₀ is only about 0.05 if both the gas and the surface are at room temperature. s₀ is again independent of T_s, but increases continuously with 〈 E⊥ 〉 up to a value of ～0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos⁵θ_e angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.     

Theoretical study of hydrazine adsorption on Pt(111): Anti or cis?

Hydrazine (N₂H₄) adsorption on metal surface is important due to its application in the direct hydrazine fuel cell technology. First principles DFT calculations have been carried out to understand the structure and mechanism of hydrazine adsorption on Pt(111). Calculations revealed that configuration with hydrazine adsorbed on its anti-conformation yields the largest adsorption energy suggesting it to be the most stable structure on Pt(111). This result was found to be in disagreement with available XPS results which favor the adsorption on cis-conformation as the most stable configuration. However, by taking into account the energy cost for orbital re-hybridization and internal rotation involves in the adsorption, it was found that the interaction strength between adsorbate and substrate is comparably equal for adsorption on both anti and cis-conformations that indicates the feasibility of the adsorption in cis-conformation to occur. Charge transfers from lone-pair orbitals belong to the highest occupied molecular orbital (HOMO) and second highest occupied molecular orbital (S-HOMO) were found to be important in the formation of the bonding. The π-anti-bonding HOMO lone-pair transfers its charge to the surface which stabilizes the internal structure of the molecule and responsible for the stable anti-conformation adsorption structure. The interaction of the π-bonding S-HOMO lone pair with the surface was found to be dative type and plays an important role in the stabilization of cis-conformation adsorption structure.     

The adsorption of PH3 on the Si(111)-(7 × 7) surface: an example of autocatalytic dissociative chemisorption

The dissociative chemisorption of phosphine, PH₃, on the Si(111)-(7 × 7) surface has been examined employing supersonic molecular beam techniques. The initial probability of reaction, S_R,0, has been found to be sensitive to substrate temperature, T_s, where S_R,0 increases sharply by approximately a factor of 4–5 as T_s is increased above 800°C, which corresponds well with the (7 × 7) ↔ “(1 × 1)” phase transition. The reaction probability, S_R, measured as a function of dose for PH₃ reacting on Si(111)-(7 × 7) at T_s < 800°C, exhibits a dramatic increase as the surface is exposed to the PH₃ molecular beam. This unique autocatalytic behavior is consistent with a mechanism in which submonolayer coverages of P(a) are capable of lifting the (7 × 7) reconstruction thus giving rise to a more reactive “(1 × 1)-like” phase. The reaction probability of Si₂H₆ on Si(111)-(7 × 7) is also observed to pass through a maximum with increasing P(a) coverages, and can be explained by considering similar changes in surface structure and reactivity.     

Scanning tunneling microscopy/spectroscopy study of adsorption of C<Subscript>60</Subscript>F<Subscript>36</Subscript> molecules on the 7 × 7-Si(111) surface

Spatially resolved images of an individual C₆₀F₃₆ fluorofullerene molecules on Si(111)-7 × 7 surface have been obtained by means of scanning tunneling microscopy/spectroscopy (STM/STS). The presence of isomers with different symmetry (T, C ₃, C ₁) has been revealed in STM investigation of initial adsorption stage of C₆₀F₃₆ on silicon surface Si(111)-(7 × 7). The adsorbed fluorofullerene molecule can occupy any adsorption site of silicon surface (corner site, faulted half, unfaulted half) that indicates for strong molecule-substrate interaction. The HOMO-LUMO gap of the adsorbed C₆₀F₃₆ molecules have been estimated from current image tunneling spectroscopy (CITS) and z(V) with engaged feedback measurements. The value of HOMO-LUMO gap observed experimentally was 3 eV. The C₆₀F₃₆ molecules adsorption on Si(111)-(7 × 7) surface was stable and kept equilibrium configuration during several hours.     

Structures near the L2,3 core edges of clean and: Oxygen covered Si(111) studied with low-energy electrons

The L_2,3 core excitation spectra of clean and oxygen covered surfaces of Si(111), measured in low-energy electron energy loss Spectroscopy reproduce previous results obtained with highenergy electrons or synchrotron radiation. It is inferred that among the different transition channels, the matrix elements for the dipole term dominate. Spectra on oxidized surfaces indicate that oxygen adsorption on Si(111) induces the formation of SiO₄ structural units.