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.     

Adsorption and thermal decomposition of C60 on Co/Si(111)-7 × 7

We present a study on the adsorption and thermal decomposition of C₆₀ on Co covered Si(111)-7 × 7 using scanning tunneling microscopy and X-ray photoelectron spectroscopy. Co-induced magic clusters grown on Si(111)-7 × 7 are identified as a possible adsorption site where 51 ± 3% of C₆₀ molecules adsorb at room temperature. On Co/Si(111)-7 × 7, C₆₀ molecules start to decompose at 450 °C, and are completely dissociated to form SiC by 720 °C. This temperature is significantly lower than 910 °C at which C₆₀ completely dissociates on clean Si(111)-7 × 7. This is a possible low temperature method for growing crystalline SiC films using C₆₀ as a precursor molecule.     

Site selective SiH4 adsorption on Si(111)(7 × 7) surfaces

Scanning tunneling microscopy (STM) was used to investigate room temperature adsorption and dissociation of SiH₄ on Si(111)(7 × 7) surfaces. The data show a pronounced site selectivity for this process. Initially the reaction involves exclusively the corner holes and the adjacent Si adatoms of the (7 × 7) reconstruction, with preferential adsorption of SiH₃ groups in the corner holes and of H atoms on one of the adjacent corner adatoms. For higher SiH₄ exposures the reactivity of the corner adatoms is significantly reduced, hydrogen adsorption occurs preferentially on the center adatoms. Deposited SiH_x groups (x = 2, 3) nucleate now in small clusters on the terraces. A higher density of these SiH_x clusters on domain boundaries or at steps indicates a higher reactivity of these defect sites.     

Temperature-Dependent Solid-State 119Sn- MAS NMR of Nd2Sn2O7, Sm2Sn2O7, and Y1.8Sm0.2Sn2O7. Three Sensitive Chemical-Shift Thermometers

The ¹¹⁹Sn MAS NMR resonances of the paramagnetic stannates Ln₂Sn₂O₇ (Ln = Nd, Sm, and Y_1.8Sm_0.2) have been found to be extremely sensitive to temperature, the isotropic resonances varying, at room temperature, by 14 and 1.1 ppm K^−l for Nd₂Sn₂O₇ and Sm₂Sn₂O₇, respectively. This sensitivity has been exploited to develop three chemical-shift thermometers with reciprocal dependences on temperature. Sm₂Sn₂O₇ is suitable for temperature calibration of standard MAS probes and the solid solution Y_1.8Sm_0.2Sn₂O₇ is a shift thermometer with an internal reference. Nd₂Sn₂O₇ is proposed as a high-temperature shift thermometer. These thermometers were used to determine the changes in sample temperature, in a Bruker 7 mm double-bearing probe, as the MAS speed was varied. A change of 8.5 K was observed on increasing the MAS speed from 1 to 5 kHz. The short relaxation times of the ¹¹⁹Sn nuclei enabled short recycle times to be used, and spectra of Sm₂Sn₂O₇ could be obtained in seconds.     

H2在Al7-团簇解离吸附的理论研究

利用高精度从头计算方法研究了H₂分子在Al₇^-阴离子团簇上的吸附及解离过程, 确定了分子吸附及解离吸附的稳定结构,并分析了各结构的光电子能谱. 计算表明H₂在Al₇^-上为弱的物理吸附,吸附能约为0.02 eV;解离过程的能垒约为0.75 eV. 对团簇及解离吸附结构的态密度与实验得到的光电子能谱的比较表明二者能够很好地符合, 确定H₂与激光烧蚀产生的团簇直接反应时能在Al₇^-上发生解离. 关键词： 7^-')" href="#">Al₇^- 2')" href="#">H₂ 解离吸附 从头计算     

Polarized IR and Raman spectra,temperature dependence of phonons and lattice dynamic calculations for M′2M″Ge2O7 pyrogermanates (M′ = Sr,Ba; M″ = Mg,Zn)

Infrared and Raman spectra of the polycrystalline samples and single crystals of Sr₂ZnGe₂O₇, Ba₂ZnGe₂O₇, Ba₂MgGe₂O₇ and Sr₂MgGe₂O₇ were measured. The temperature dependence of phonons was studied in the range 4–295 K. The discussion of the results is based on the factor group approach for the tetragonal space group with Z = 2. The discussion of the internal vibrations of the Ge₂O₇ unit and external modes is made on the basis of the literature data and phonon calculations. The results obtained for the spontaneous Raman scattering were used in the discussion of the stimulated Raman spectra of the studied materials. Copyright © 2010 John Wiley & Sons, Ltd.     

Experimental and theoretical studies of interactions between Si<Subscript>7</Subscript> clusters

The possibility of using magic Si₇ clusters to form a cluster material was studied experimentally and theoretically. In experiments Si₇ clusters were deposited on carbon surfaces, and the electronic structure and chemical properties of the deposited clusters were measured using X-ray photoelectron spectroscopy (XPS). A non bulk-like electronic structure of Si₇ was found in the Si 2p core level spectra. Si₇ is suggested to form a more stable structure than the non-magic Si₈ cluster and Si atoms upon deposition on carbon surfaces. Theoretically it was possible to study the interaction between the clusters without the effect of a surface. Density functional theory (DFT) calculations of potential curves of two free Si₇ clusters approaching each other in various orientations hint at the formation of cluster materials rather than the fusion of clusters forming bulk-like structures.     

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.     

YBa2Cu3O7-x涂层导体的外延生长和性能对CeO2缓冲层的依赖性

利用倾斜衬底沉积法在无织构的金属衬底上生长了MgO双轴织构的模板层,在这一模板层上实现了YBa₂Cu₃O_7-x薄膜的外延生长.在外延YBa₂Cu₃O_7-x薄膜前,依次沉积了钇稳定的立方氧化锆和CeO₂作为缓冲层.利用X射线衍射2θ扫描、扫描、Ω扫描和极图分析测定了这些膜的结构和双轴织 关键词： 2Cu₃O_7-x镀膜导体')" href="#">YBa₂Cu₃O_7-x镀膜导体 2缓冲层')" href="#">CeO₂缓冲层 厚度依赖性 外延生长     

Structures and stability of Al<Subscript><Emphasis Type="Bold">7</Emphasis></Subscript>C and Al<Subscript><Emphasis Type="Bold">7</Emphasis></Subscript>N clusters

We report results of the atomic and electronic structures of Al₇C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is found to be the one in which carbon atom occupies an interstitial position in Al₇ cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al₇C^- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al₇C^- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al₇N which has the same number of valence electrons as in Al₇C^- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al₇N)₂ to study interaction between magic clusters. Received 28 July 2001     

非晶态离子导体Li2B2O4的核磁共振研究

本文报道了非晶态离子导体Li₂B₂O₄的⁷Li核磁共振研究。测量了⁷Li核磁共振谱与温度的关系。实验中发现,Li₂B₂O₄的晶态、非晶态和部分晶化样品的⁷Li核磁共振谱有很大的不同,且在部分晶化样品的⁷Li核磁共振谱上有附加的小峰,它与LiCl(Al₂O₃)的⁷Li核磁共振谱上附加的小峰相类似。我们也对非晶态离子导体B₂O₃-0.7Li₂O-0.7LiCl进行了⁷Li核磁共振研究,其结果与上面的类似。研究结果表明,它们都起因于非晶母体与微晶的界面效应。 关键词：     

Bi<Stack><Subscript>3</Subscript><Superscript>+</Superscript></Stack> cluster primary ions in SIMS depth profiling of YBaCuO high-temperature superconductor films

SIMS depth profiling of YBa₂Cu₃O₇ high-temperature superconductor films was performed using a TOF.SIMS-5 instrument. A new approach was proposed to obtain additional information on the phase composition of YBa₂Cu₃O₇ films based on detection of cluster secondary ions.     

Bi2Ti2O7/Si薄膜的制备及C-V特性研究

采用化学溶液分解法(CSD)在Si衬底上制备了Bi₂Ti₂O₇薄膜.X射线双晶衍射和原子力显微镜检测表明,所制备的薄膜主要为Bi₂Ti₂O₇相的多晶材料.同时还研究了AuBi₂Ti₂O₇/n-Si(100)结构的电容电压(C-V)特性,结果表明,在Bi₂Ti₂O关键词： C-V特性 2Ti₂O₇薄膜')" href="#">Bi₂Ti₂O₇薄膜 电荷迁移     

Mg-TM-H (TM=Sc,Ti, V,Y, Zr,Nb)晶体形成能力和电子性能

基于密度泛函理论的第一性原理的Vienna Ab initio Simulation Package (VASP)软件系统研究了Mg₇TMH₁₆ (TM=Sc, Ti, V, Y, Zr, Nb)的晶体结构和电子性能. 分析了结合能和形成焓,计算得到的结合能显示Mg₇TiH₁₆和Mg₇NbH₁₆的稳定性最强,形成焓的变化表明Mg₇TMH₁₆具有比MgH₂更低的分解温度,电子态密度显示Mg₇TMH₁₆表现出明显的金属特性. 关键词： 第一原理 7TMH₁₆ (TM=Sc,Ti,V,Y,Zr,Nb)')" href="#">Mg₇TMH₁₆ (TM=Sc,Ti,V,Y,Zr,Nb) 结合能 形成焓     

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.     

First-principles study of Ga7As7 ionic cluster and influence of multi-charge on its structure

This paper investigates the structures and stabilities of neutral Ga7As7 cluster and its ions in detail by using first-principles density functional theory. Many low energy structures of Ga7As7 cluster are found. It confirms that the ground state structure of neutral Ga7As7 cluster is a pentagonal prism with four face atoms like a basket structure, as reported by previous works. The ground state structures of positive Ga7As7 cluster ions are different from that of the neutral cluster. These investigations suggest that Ga atoms occupy the capping positions more easily than As atoms. Mulliken population analyses also show that Ga atoms can lose or obtain charge more easily than As atoms. It finds that the neutral Ga7As7 cluster can become more stable by gaining one or two additional electrons but further more electrons would cause the decrease of binding energy. The ionisation energy increases with the increase of the number of the removed electrons. These calculated results indicate that the net magnetic moment of the neutral Ga7As7 cluster is zero because all electrons are paired together in their respective molecular orbits. But for the ionic Ga7As7 cluster with odd number of electrons, the net magnetic moment is 1.0 μB due to an unpaired electron.     

Palladium-ytterbium bimetallic electrocatalysts supported on carbon black,titanium suboxide,or poly(diallyldimethylammonium chloride)-functionalized titanium suboxide towards methanol oxidation in alkaline media

Palladium-ytterbium (Pd-Yb) bimetallic catalysts with different Pd/Yb ratios supported on carbon black (20%Pd-x%Yb/C, x?=?0, 1, 5, 10, and 15) were prepared by a sodium borohydride reduction method. The 20%Pd-5%Yb/C catalyst exhibited the best electrocatalytic activity towards methanol oxidation in alkaline media. The improved electrocatalytic activity and stability of 20%Pd-5%Yb/C can be explained by a bi-functional mechanism. In addition, the higher content of metallic palladium caused by the addition of ytterbium also contributes to the better catalytic activity of the 20%Pd+5%Yb/C catalyst. In view of the good electrocatalytic performance of 20%Pd+5%Yb/C, the 20%Pd+5%Yb catalyst supported on titanium suboxide (20%Pd+5%Yb/Ti₄O₇) was prepared. However, the Pd-Yb particles supported on Ti₄O₇ were seriously agglomerated. To improve the dispersion status of alloy particles, the Ti₄O₇ was functionalized with poly(diallyldimethylammonium chloride) (Ti₄O₇-PDDA). Electrochemical characterizations showed that no matter Ti₄O₇ or Ti₄O₇-PDDA as supports, Pd-Yb catalysts exhibited better catalytic activity than 20%Pd-5%Yb/C. The improvement mainly results from the further increase of metallic Pd due to the presence of Ti₄O₇.     

Ac-Josephson effect in YBa2Cu3O7/YBa2Cu3O7 point contact junctions

The ac-Josephson effect could be demonstrated up to dc voltages of 20 mV on the I–V characteristic of adjustable YBa₂Cu₃O₇/YBa₂Cu₃O₇ point contacts at 4.2 K. A detailed analysis of the rf power dependence of the size of microwave-induced constant voltage steps is given for microwave frequencies of 70 and 90 GHz. The results prove that electron pairing is responsible for the superconductivity in YBa₂Cu₃O₇.     

SrB4 O7 : Eu磷光粉的制备及其发光性能

采用高温固相法合成了SrB₄O₇ : Eu荧光粉,并研究了不同原料、掺杂浓度、煅烧温度等因素对其发光性能的影响。发射光谱测试结果表明:SrB₄O₇ : Eu荧光粉的最佳Eu掺杂浓度为2%左右,进一步增大掺杂浓度会导致浓度猝灭。煅烧温度对基质组成影响较大,随着温度的升高,基质中BO₄四面体所占比例增大,有利于Eu³⁺离子的还原。以水合硼酸锶为原料制得样品的发光强度高于以SrCO₃和H₃BO₃为原料制得样品的发光强度。     

Electron tunneling into thin films of Y1Ba2Cu3O7

We have fabricated tunnel junctions on thin films of Y₁Ba₂Cu₃O₇ which were epitaxially grown by magnetron sputtering on (100) and on (110) oriented SrTiO₃ substrates. These junctions of the type Y₁Ba₂Cu₃O₇/barrier/Pb or In showed with high reproducibility in the conductance a gap-like structure with the maxima near±16mV. We supplied experimental arguments that this structure reflects properties of the quasi particle excitation spectrum of Y₁Ba₂Cu₃O₇. The gap-like structure was found to disappear atT _c mainly by weakening and not by a shift of the peaks to lower voltages.