Orientation and structure of triple step staircase on vicinal Si(1 1 1) surfaces

The vicinal Si(1 1 1) surface, inclined towards the direction, was investigated by scanning tunnelling microscopy and spot profile analysing low energy electron diffraction. It has been established that the surface, consisting of regularly spaced triple steps and (1 1 1) terraces with a width equal to that of a single unit cell of the Si(1 1 1)-7 × 7 surface structure, has the (7 7 10) orientation. An atomic model of the triple step is proposed.     

Cu、N空位对Cu_3N的电子结构、电学和光学性能影响的第一性原理研究

本文采用基于密度泛函理论的第一性原理计算方法,研究了立方相Cu_3N晶体中含Cu、N空位体系的稳定性、电学、光学性能.研究结果表明含Cu空位和N空位体系的结构比较稳定,Cu空位和N空位降低了体系的导电性,但增加了体系的透射率;含Cu空位和N空位体系的禁带宽度均大于Cu_3N体系,说明实验中制备的Cu_3N有时表现为绝缘体的可能原因为体系中存在Cu空位或N空位.     

IR and XPS investigation of visible-light photocatalysis—Nitrogen-carbon-doped TiO2 film

To use solar irradiation or interior lighting efficiently, we sought a photocatalyst with high reactivity under visible light. Nitrogen and carbon doping TiO_2−x−yN_xC_y films were obtained by heating the TiO₂ gel in an ionized N₂ gas and then were calcined at 500 °C. The TiO_2−x−yN_xC_y films have revealed an improvement over the TiO₂ films under visible light (wavelength, 500 nm) in optical absorption and photocatalytic activity such as photodegradation of methyl orange. X-ray photoemission spectroscopy, infrared spectrum and UV-visible (UV-vis) spectroscopy were used to find the difference of two kinds of films. Nitrogen and carbon doped into substitutional sites of TiO₂ has been proven to be indispensable for band-gap narrowing and photocatalytic activity.     

Ultrafast dynamics in the excited hydrogen atom transfer states of ammonia clusters

Neutral ammonia clusters (NH₃)_m are photo-excited to the electronic state by a deep UV femtosecond laser pump pulse. Within a few hundred femtoseconds a significant fraction of the clusters rearrange to form an H-transfer state (NH₃)_m-2NH₄(3s)NH₂ with the subunit NH₄ in its 3s electronic ground state. This state is then electronically excited by a time-delayed infrared control pulse of variable wavelength. Finally, a third (probe) pulse in the UV ionizes the clusters for detection. The lifetime of the excited (NH₃)_m-2NH₄(3p)NH₂ states is found to vary between 2.7 and 0.13 ps depending on cluster size and excitation energy. It increases drastically upon deuteration. The corresponding cluster size-dependent photoelectron spectra allow us to disentangle the underlying energetics of the excitation and ionization process and reveal additional processes, such as nonresonant ionization or dissociative ionization. The experimental findings suggest that the excited H-transfer ammonia complexes with m > 2 are deactivated by an internal conversion process back to the electronically lowest H-transfer state followed by fast dissociation. Received 22 September 2001 and Received in final form 31 January 2002     

空位在金刚石近(001)表面扩散的分子动力学模拟

用分子动力学方法模拟了空位在金刚石近(001)表面的扩散过程,研究了温度对空位扩散的影响.结果表明,当温度为1000K左右时,位于近表面第二层上的空位开始向表面运动;当温度在1400—2000K时,空位完全扩散到表面.这与实验结果和其他计算结果符合得很好.同时发现,温度为1400—1800K时,空位的扩散经历了两次迁移运动,其分别对应了均方位移图中的两个极大值.在不施加任何约束的条件下得到了空位的动态扩散路径,空位在金刚石近(001)表面的扩散势垒约为042eV.并探讨了一定温度下空位数目增多及其不同排列 关键词： 金刚石 空位 扩散 分子动力学     

Mg掺杂PST薄膜的溶胶-凝胶法制备及晶相形成研究

利用溶胶-凝胶法成功制备了Mg掺杂Pb_0.4Sr_0.6Mg_xTi< sub>1-xO_3-x薄膜，利用x射线衍射仪对薄膜的物相和结构进行了分析， 用扫描电子显微镜对薄膜的形貌和断面等进行了观察.研究结果表明，薄膜以立方钙钛矿为 晶相，薄膜中晶相以团聚状颗粒存在，晶相含量受热处理条件和Mg的掺杂量所控制.Mg掺杂 对Pb_0.4Sr_0.6Mg_xTi_1-xO_3-x 薄膜晶相含量的影响与钙钛矿中的氧空位缺陷相关.在一定的掺杂范围内，由掺杂引起晶相 的晶格畸变较小时，体系掺Mg平衡了晶体内本征氧空位引入的电荷不平衡，使晶相更为稳定 ，析晶能力提高，晶体形成量随掺杂浓度的提高而提高.当掺杂浓度达到一定量时， 随着Mg 掺杂浓度增加，一方面使形成晶体时杂质浓度增加造成参与形成晶相的组成含量下降，另一 方面使进入钙钛矿结构的Mg增加，氧空位大量增加使畸变程度提高，形成的晶相不稳定，析 晶能力下降，晶体含量随掺杂Mg浓度的增加而不增反降.在相同条件下制备的Pb_{0.4Sr0.6MgxTi1-xO3-x薄膜中Mg掺量约为 x=0.01时，得到的钙钛矿相含量最高，本征氧缺陷所带入的正电荷和Mg引入时带入的负电荷 间达到平衡.此外，Mg的掺入还影响到析晶与热处理过程之间的关系.在高Mg掺量范围，Mg含 量越高，形成的晶相越不稳定，热处理时间越长，使热处理过程中分解的晶相量越多，随Mg 掺量越高和热处理时间越长，薄膜中晶相含量越低. 关键词： 溶胶-凝胶法 PST薄膜 Mg掺杂 晶相形成}     

Phase transitions and hysteresis in a cellular automata-based model of opinion formation

A particular case of a cellular automata-based model of two-state opinion formation in social groups with a strong leader is studied. We consider a 2D Euclidian geometry of social space and mutual interactions 1/r ⁿ . The model shows an interesting dynamics which can be analytically calculated. There are two stable states of the system: a cluster around the leader and unification. Unstable clusters may also appear. A variation in parameters such as the leader's strength or the social temperature can change the size of a cluster or, when they reach some critical values, make the system jump into another state. For a certain range of parameters the system exhibits bistability and hysteresis phenomena. We obtained explicit formulas for the cluster size, critical leader's strength, and critical social temperature. These analytical results are verified by computer simulations.     

Defect properties of CuCrO2: A density functional theory calculation

Using the first-principles methods,we study the formation energetics properties of intrinsic defects,and the charge doping properties of extrinsic defects in transparent conducting oxides CuCrO2.Intrinsic defects,some typical acceptortype,and donor-type extrinsic defects in their relevant charge state are considered.By systematically calculating the formation energies and transition energy,the results of calculation show that,V Cu,O i,and O Cu are the relevant intrinsic defects in CuCrO2 ;among these intrinsic defects,V Cu is the most efficient acceptor in CuCrO2.It is found that all the donor-type extrinsic defects have difficulty in inducing n-conductivity in CuCrO2 because of their deep transition energy level.For all the acceptor-type extrinsic defects,substituting Mg for Cr is the most prominent doping acceptor with relative shallow transition energy levels in CuCrO2.Our calculation results are expected to be a guide for preparing promising n-type and p-type materials in CuCrO2.     

Ru(0001) 表面BaO吸附层的原子结构和氮分子的吸附性质

采用密度泛函理论研究了Ru(0001) /BaO表面的原子层结构和氮分子的吸附性质. 研究结果表明, 在低覆盖度下氧化钡倾向于以相同的构型形成Ru(0001) 表面原子层. 在此构型中, 氧原子位于表面p(1× 1) 结构的hcp谷位, 而钡原子则位于同一p(1× 1) 结构的顶位附近. 钌氧键键长等于0.209 nm, 比EXAFS的实验值大0.018 nm. 在Ru(0001) /BaO表面氮分子倾向吸附于钡原子附近. 相应位置的氮分子吸附能位于0.70到0.87 eV之间, 大于氧原子附近的氮分子吸附能. 钡原子附近的钌原子对氮分子具有更强的活化性能. 相应位置的氮分子拉伸振动频率等于1946 cm^{- 1}, 比氧原子附近的最大分子振动频率小约130 cm^-1. Ru(0001) /BaO表面氮分子键强度介于清洁Ru(0001) 和Ru(0001) /Ba表面之间. Ru(0001)/BaO表面不同位置的氮分子吸附性质差异是由钡和氧原子化学性质不同造成的. 表面钡原子的作用能够减少吸附氮分子的σ^*轨道电子密度, 增加π^*轨道电子密度, 从而增强氮分子和钌原子间的轨道杂化作用, 弱化氮分子键.     

Sr2 CeO4电荷迁移发光的光谱结构规律研究

利用高温固相反应法分别合成了不同物相形成机理的Sr2CeO4，Sr2CeO4：Ca^2 和Sr2CeO4：Ba^2 样品，并对其光谱特性进行了研究．结果发现，对于由SrO和CeO2直接反应生成的Sr2CeO4(Ⅰ)，激发主峰位于256nm左右；而对于SrCeO4和SrO反应生成的Sr2CeO4(Ⅱ)，激发主峰位于279nm左右．在Sr2CeO4(Ⅰ)中掺入Ca^2 ，其激发光谱随着Ca^2 离子浓度的增加逐渐接近于Sr2CeO4(Ⅱ)的激发光谱．激发主峰带应属于CeO6八面体终端Ce^4 -O^2-键的电荷迁移带．对于激发光谱中340nm左右的弱激发峰，其峰值波长不受形成机理及Ca^2 掺杂的影响，只是其强度随着激发主峰的红移而增加，它可能属于CeO6八面体平面上Ce^4 -O^2-键的电荷迁移带．形成机理及Ca^2 掺杂对发射光谱没有影响．Ca^2 在Sr2CeO4(Ⅱ)与Ba^2 在Sr2CeO4(Ⅰ)和(Ⅱ)中均难于替代Sr^2 的位置．