腺嘌呤-5-溴尿嘧啶复合物中的卤键

利用从头算和密度泛函理论研究了腺嘌呤(A)-5-溴尿嘧啶复合物中(T+)中的键合模式. 研究结果表明，T+中的Br原子同时与A分子中的氨基氢和氮原子存在弱的相互作用，在这种结合模式中，Br原子与亲核基团H正面结合，同时与亲电基团N侧面结合，分别形成氢键和卤键.静电势分析发现：T+中的Br原子与A中的N7 (或N1)是通过静电相互吸引的. Br与N原子之间的相互作用通过分子中的原子理论得以证实. 关键点的拓扑参数显示卤键是闭壳层相互作用. 自然键轨道分析说明，A中N原子上孤对电子的电荷主要转移到C{Br的反     

Data and modeling of negative ion transport in gases of interest for production of integrated circuits and nanotechnologies

We review techniques to prepare, evaluate and apply sets of cross section and transport data for negative ions that are required for the modeling of collisional non-equilibrium plasmas used for processing of microelectronic circuits. We collect and discuss the transport coefficients and cross section sets.We have compiled data for negative ions in CF₄ and CF₄-related negative ions in rare gases. In addition, we consider data for F⁻ and CF₃⁻ in rare gases. Furthermore, we analyze the cross sections of halogen negative ions in rare gases and other molecules. This is followed by the data for SF₆ related ions in SF₆ and in rare gases. The cross section for scattering of O⁻ in O₂ has been derived from the transport data and used to make calculations of the transport properties. Finally we give a brief discussion of the availability of the data for H⁻ ions in H₂. We have derived cross sections in several cases but the basic aim is to show the basic features of transport coefficients. In particular we discuss the need to represent properly some details such as the non-conservative nature of transport coefficients and the anisotropy of diffusion. Application of approximate theories and representations of cross sections are also discussed.     

The influence of boron concentrations on structural properties in disorder silicon films

In this work we present a detailed structural of a series of B-doped hydrogenated microcrystalline silicon (μc-Si:H) films deposited by plasma-enhanced chemical vapor deposition (PECVD) and B-doped polycrystalline silicon (poly-Si) films produced by step-by-step laser crystallization process from amorphous silicon. The influence of doping on the structural properties and structural changes during the sequential crystallization processes were monitored by Raman spectroscopy. Unlike μc-Si:H films, that consist of a two-phase mixture of amorphous and ordered Si, partially crystallized sample shows a stratified structure with polycrystalline silicon layer at the top of an amorphous layer. With increasing doping concentration the LO-TO phonon line in poly-Si shift to smaller wave numbers and broadens asymmetrically. The results are discussed in terms of resonant interaction between optical phonons and direct intraband transitions known as a Fano resonance. In μc-Si:H films, on the other hand, the Fano effect is not observed. The increase of doping in μc-Si:H films suppressed the crystalline volume fraction, which leads to an amorphization in the film structure. The structural variation in both μc-Si:H and poly-Si films leads to a change in hydrogen bonding configuration.     

金刚石中氦、氢含量的变化及在金刚石生长中的意义

天然金刚石自中心至边缘的显微傅里叶变换红外光谱研究表明:氮和氢在金刚石中的分布是不均匀的,这说明金刚石在整个生长过程中的物质环境是有差异的;中心至边缘的含氮总量、C-H键含量的总体降低趋势表明金刚石的生长过程是一个氮、氢的消耗过程,而中部的升高变化说明金刚石生长环境中存在氮、氢的补充,但氮补充得比氢更早一些.据此,可以将金刚石的生长过程划分为早期成核与长大、中期长大及末期长大三个阶段,其中早期和末期是氮和氢的消耗阶段,中期需要进行氮和氢的补充,且氮应该更早补充.氢对金刚石的生长是有利的,氢和氮不是以氮氢化合物的形式存在于金刚石生长的物质环境中,这暗示着在高温高压合成金刚石中欲引入氢,应当避免氮氢化合物的形成.     

Thermal and photochemical reactivity of oxygen atoms on gold nanocluster surfaces

Reduction of oxidized gold nanoclusters by exposures to foreign gases and irradiation of UV photons has been investigated using X-ray photoelectron spectroscopy. Gold nanoclusters with narrow size distributions protected by alkanethiolate ligands were deposited on a TiO₂(1 1 0) surface with dip coating. Oxygen plasma etching was used for removal of alkanethiolate ligands and oxidization of gold clusters. The oxidized gold clusters were exposed to CO, C₂H₂, C₂H₄, H₂, and hydrogen atoms. Although, C₂H₄ and H₂ did not show any indications of reduction of oxidized gold clusters, CO, C₂H₂, and hydrogen atoms reduced the oxides on gold cluster surfaces. Among them, hydrogen atoms were most effective for reduction. Irradiation of UV photons around 400 nm could also reduce the oxidized gold clusters. The photochemical reduction mechanism was proposed as follows. The photo-reduction was initiated by electronic excitation of gold clusters and oxygen atoms activated reacted with carbon atoms at the surfaces of gold clusters. Carbon species were likely absorbed in gold clusters or remained at the boundaries between gold clusters when gold clusters agglomerated during oxygen plasma exposures. As the photochemical reduction progressed, carbon atoms segregated to the surfaces of gold clusters.     

Self-assembly of 2,6-dimethylpyridine on Cu(1 1 0) directed by weak hydrogen bonding

Sequential stages of formation of a self-assembled monolayer of flat-lying 2,6-dimethylpyridine molecules on a single crystal Cu(1 1 0) surface have been observed by low-temperature scanning tunneling microscopy (LT-STM). At an adsorption temperature of 10 K, all of the molecules are randomly distributed at low coverage upon adsorption. The isolated molecules align their molecular axes parallel to the 〈0 0 1〉 azimuth of the Cu lattice. The nitrogen atom in the molecule is located at the four-fold hollow site. Upon annealing to 100 K, the molecules associate to form head-to-head dimers. The dimer units involve a pair of weak hydrogen bonds between methyl group-hydrogen atoms and N moieties on adjacent molecules, forming a core structure for further growth. In a later stage of self-assembly, single head-to-tail weak hydrogen bonds between ring C-H bonds and N moieties form in chains on the periphery of the central cores, leading to larger domains with a c(6 × 2) overlayer structure.     

Inhibition of hydrogen chemisorption on uranium surfaces by traces of water vapor

Traces of about 2% water vapor are sufficient to inhibit hydrogen dissociation and chemisorption on uranium surfaces, under low pressure exposures, at room temperature. The efficiency of the inhibition increases with temperature in the range of 200 - 400 K. The inhibition effect is also influenced by the extent of residual strain of the sample, with increasing inhibition efficiencies exhibited by a less strained surface. O₂, in contrast to H₂O, is not an inhibitor to surface adsorption and dissociation of hydrogen. Three types of mechanisms are discussed in order to account for the above inhibition effect of water. It is concluded that the most probable mechanism involves the reversible adsorption of water molecules on hydrogen dissociation sites causing their “blocking”.     

Vibration-induced structures in scanning tunneling microscope light emission spectra of Ni(1 1 0)-streaky (1 × 2)-H

We have measured the scanning tunneling microscope (STM) light emission spectra of Ni(1 1 0)-streaky (1 × 2) surfaces. When the tip was fixed over atomic hydrogen adsorbed on the surfaces, two types of vibration-induced structure were observed in the STM light emission spectra. One is the periodic fine structures that were already reported in our previous paper [Y. Uehara, S. Ushioda, Phys. Rev. Lett. 92 (2004) 066102] and the other newly found in the present experiments is a stepwise structure that is located at the vibrational energy of hydrogen below the cutoff energy of the STM light emission. They are ascribed to different excitation mechanisms of the vibration in the STM light emission process; the periodic fine structures appear when the vibrating motion is directly excited by the electrons injected from the tip. Conversely, the stepwise structure is observed when it is excited by the electromagnetic fields confined in the tip-sample gap, i.e., by localized surface plasmons.     

Organic contaminants removal by oxygen ECR plasma

Recently electron cyclotron resonance (ECR) plasma have been explored for wafer cleaning applications, since it is known to do less damage to silicon surface than conventional plasma. Organic contaminants removal efficiency and plasma radiation damage of the ECR plasma cleaning have been investigated. In oxygen ECR plasma cleaning, the plasma exposure time needed to remove the organic contaminants on the silicon surface down to the detection limit is 40 s, but the one to reach the lowest surface roughness is 10 s. The leakage current level of the MOS capacitor made using the Si substrate exposed to oxygen ECR plasma for 40 s is 8 × 10⁻⁹ A. The optimum exposure time determined by considering the contaminants removal efficiency and the plasma radiation damage (or the leakage current level) is 40 s. Organic contaminants seem to be removed through both sputter-off mechanism by oxygen ion bombardment and evaporation mechanism by chemical reactions with excited oxygen atoms.     

Transient desorption of HD and D2 molecules from the D/Si(1 0 0) surfaces exposed to a modulated H-beam

We have studied the direct and indirect abstraction of D adatoms by H on the Si(1 0 0) surfaces by employing a pulsed H-beam. Desorptions of HD molecules is found to occur promptly as a result of direct abstraction at the beam on-cycles. In contrast, we find that D₂ desorption induced by adsorption of H atoms, i.e., the so-called adsorption-induced desorption (AID), occurs even at the beam off-cycles. The D₂ rate curves measured with the pulsed-H beam are decomposed into four components characterized with the reaction lifetimes of ?0.005, 0.06 ± 0.01, 0.8 ± 0.1, and 30 ± 5 s. We propose that the fastest and the second fastest AID channels are related to the thermodynamical instability of (1 × 1) dihydride domains locally formed on the (3 × 1) monodeuteride/dideuteride domains. The 0.8 s AID channel is attributed to the desorption occurring at the stage when (3 × 1) monodeuteride/dideuteride domains are built up upon H adsorption onto the (2 × 1) monohydride surface. The 30 s AID path is attributed to the thermal desorption accompanied by the shrinkage of the (3 × 1) domains which were excessively formed during the beam on-cycles on the (2 × 1) monohydride surface. Atomistic mechanisms are proposed for these three AID pathways.