CO和N2与铌原子簇结合能的光离解测量

以激光蒸发的方法,可以产生各种过渡金属的原子簇分子与离子,并在表面吸附上不同数量的CO与N,通过超声分子膨胀,冷冻至基态。本实验以自行建立的串联飞行时间质谱仪,分离出特定质量的原子簇离子,并以激光离解的方式,通过扫描光子密度,第一次较系统地测量了CO和 N 与各种金属原子簇的结合能,并着重考察了结合能与原子簇的大小和组分、CO和N_2的覆盖度的关系,实验发现:CO和N_2在铌原子簇表面的结合能与成簇的铌原子数有十分明显的关系。     

羰基原子簇化合物M3(CO)12(M=Fe,Ru,Os) 的激光解离研究

在自制的仪器上以脉冲激光溅射铁、钌、锇的三核羰基原子簇化合物,由原位质谱观察和分析溅射产生的正负离子. 比较了解离碎片及分布发现羰基锇原子簇化合物具有特殊的结构稳定性,它们不仅具有很强的金属键,而且锇与羰基分子还形成了很强的配位键. 铁和钌的原子簇在解离中脱落羰基配体,碎裂成双核以至单核的碎片离子. 而锇原子则先离解掉氧分子,形成新的C     

一些金属原子簇羰基化合物的流变性及其电子结构的EHMO研究

近年来,关于过渡金属原子簇化合物的研究工作发展很快。这类化合物的通式是 M_pL_q,其中 M_p 是 p 个以 M—M 键相结合的金属原子所形成的原子簇,L 则是与原子簇 M_p 相结合的配位体,如羰基、卤素离子、CN~-、膦等。这类化合物有许多特点,金属原子簇具有介于单原子和金属微晶之间的一种过渡型结构。配位体和被金属表面吸附了的分子或离子相似。由     

原子簇离子产生的动力学方程与分布函数

由激光等离子体反应产生的原子簇分子与离子，经历了分子-离子反应的过程．本文尝试将Smoluchowski方程加以扩展，用于描述原子簇离子的生成过程．通过应用解析方法，求解不可逆聚合过程的动力学方程，推导出了原子簇离子统计分布的函数关系式，证实了实验中观察到的原子簇的统计分布规律．     

原子簇正负离子的统计分布与生成机理

在分析了实验记录的大量原子簇离子质谱的基础上，发现原子簇的正负离子通常表现为不同的统计分布形式．将它们的分布形式与求解动力学方程得出的分布函数相比，发现由此反映了原子簇正负离子的不同的生成机理：实验所检测的原子簇正离子在许多情况下仅经历了聚合与解离反应，主要是中性原子簇分子电离后的产物；而相应的原子簇负离子则是经历了一系列离子一分子反应后的产物．     

铜族金属与完整及氮掺杂石墨烯的相互作用

基于广义梯度密度泛函理论和周期平板模型,研究了铜族金属单原子和双原子簇与完整及氮掺杂石墨烯的结合情况.结果表明,氮掺杂后石墨烯的电子结构特性由半金属性变为金属性;铜族金属在完整及石墨型氮掺杂石墨烯上的吸附较弱,结合能约为0.5eV,而在吡啶型氮掺杂和吡咯型氮掺杂石墨烯上有较强的化学吸附,结合能一般大于1eV;吡咯型氮掺杂后的构型不稳定,金属原子及簇与包含该结构的石墨烯衬底作用时会使其向吡啶型氮掺杂转变,并最终得到基于吡啶型氮掺杂的稳定吸附构型.Mulliken电荷布居分析显示,吸附在吡啶型氮掺杂石墨烯上的金属单原子与金属双原子簇带电性质相反.态密度及轨道分析表明,Cu与吡啶型氮掺杂石墨烯空位处留有悬挂键的三个原子成键,而Au与其中两个C原子成键.     

桥基原子对金属—金属作用的影响

本文借助EHMO法得到的Mulliken集居数和经验公式,讨论了过渡金属原子簇分子构型的稳定性和桥基与金属原子形成的平面距离D对其稳定性的影响。指出可用D值与实验上得到的距离d值的差别,判断过渡金属原子簇化合物金属—金属作用的强弱。     

新型内嵌混合金属氮化物原子簇富勒烯的合成、结构与性质*

内嵌混合金属氮化物原子簇富勒烯的发现极大地扩展了内嵌富勒烯家族。内嵌混合金属氮化物原子簇富勒烯是一类新型的内嵌富勒烯,其内嵌物为由2-3种不同的金属组成的氮化物原子簇。本文首先介绍了新型内嵌混合金属氮化物原子簇富勒烯的发现、合成和分离方法,并对目前所分离出来的内嵌混合金属氮化物原子簇富勒烯进行了分类。然后总结了目前所报导的内嵌混合金属氮化物原子簇富勒烯的结构表征手段,对于不同的内嵌混合金属氮化物原子簇富勒烯的分子结构分别进行了阐述。最后着重讨论了内嵌混合金属氮化物原子簇富勒烯的特殊电子性质以及物理和化学性质。本文还对内嵌混合金属氮化物原子簇富勒烯潜在的应用前景作了展望,在内嵌具有不同物理性质的两到三种金属原子的基础上,所形成的内嵌混合金属氮化物原子簇富勒烯有可能兼具不同金属原子各自的性质,从而成为多功能综合的功能材料。     

钼铁硫原子簇化合物的结构规则

本文将过渡金属原子簇化合物的9N-L结构规则, 用于讨论Mo-Fe-S原子簇化合物, 得到了它们的结构和自旋性质之间的关系,推得的理论最大自旋值与实验事实吻合. 进而, 对各种Mo-Fe-S原子簇化合物的典型分子或模型分子, 进行了EHMO量子化学计算, 得到了与上述讨论相一致的结果, 进 一步揭示和验证了上述讨论的本质.     

锂原子簇结构的分子力学研究

本文采用包含Axilord—Teller三体势的分子力学方法,计算了锂原子簇的平衡几何构型,结果表明,锂原子簇的势能面上存在一些近简并的结构。但最稳定结构与从头算的结果基本一致,同时对气相原子簇的生长模式、簇尺寸增大原子簇平均结合能的变化进行了讨论。     

Structure and energetics of equiatomic K-Cs and Rb-Cs binary clusters

The basin-hopping algorithm combined with the Gupta many-body potential is used to study the structural and energetic properties of (KCs)(n) and (RbCs)(n) bimetallic clusters with N=2n up to 50 atoms. Each binary structure is compared to those of the pure clusters of the same size. For the cluster size N=28 and for the size range of N=34-50, the introduction of K and Rb atoms in the Cs alkali metal cluster results in new ground state structures different from those of the pure elements. In the size range N>/=38 the binary and pure clusters show not only structural differences, but they also display different magic numbers. Most of the magic Rb-Cs and K-Cs clusters possess highly symmetric structures. They belong to a family of pIh structures, where a fivefold pancake is a dominant structural motif. Such geometries have not been reported for alkali binary clusters so far, but have been found for series of binary transition metal clusters with large size mismatch. Moreover, tendency to phase separation (shell-like segregation) is predicted for both K-Cs and Rb-Cs clusters with up to 1000 atoms. Our finding of a surface segregation in Rb-Cs clusters is different from that of theoretical and experimental studies on bulk Rb-Cs alloys where phase separation does not occur.     

An improved clusterion-photoelectron coincidence technique for the investigation of the ionisation dynamics of clusters

The introduction of photoion-photoelectron coincidence techniques has made it possible to investigate photoionisation properties of heavy clusters, which are not accessible by conventional mass spectrometry. This technique has been further developed in combination with a zero-volt electron energy analyser and greatly improved in performance. The method has been applied to the investigation of different homogeneous and heterogeneous clusters. This type of cluster experiment requires both a very high resolution and a large dynamic range in order to identify also clusters present in low abundance. As an example, a series of coincidence mass spectra of Xe clusters has been recorded at different wavelengths. Below a photon energy of 11.1 eV, the range of observable clusters shifts to higher cluster sizes with decreasing energy. Appearance potentials and the binding energy of different cluster ions were obtained. Intensity fluctuations, already observed in spectra with electron bombardment ionisation (magic numbers), have also been detected in the coincidence spectra and become most pronounced near the ionisation threshold. This indicates that these fluctuations are caused by the size-dependent stability of the ionic and not the neutral cluster. Furthermore, the threshold size does not change linearly with cluster size. The binding energy per particle seems to change drastically aroundn=13 which indicates the existence of a shell structure in the cluster ion.     

Activation of molecular hydrogen on platinum nanoparticles: Quantum-chemical modeling

The interaction of molecular hydrogen with platinum clusters of different size has been modeled by the density functional theory method within the generalized gradient approximation (GGA). The cluster size turns out to have little effect on the interaction energy, whereas the effect of the cluster structure is rather significant. The most efficient interaction with hydrogen is observed for clusters with a structure resembling the crystal structure of platinum metal. In such clusters, the hydrogen molecule is attached to its surface without a barrier. Configurations with the bidentate hydrogen coordination are the most stable ones. The H atoms can migrate over the cluster surface, overcoming moderate potential barriers of ∼0.3–0.4 eV.     

Shapeshifting: ligation by 1,4-cyclohexadiene induces a structural change in Ag5(+)

Relatively little is known about structural transformations of very small metal clusters that result from the adsorption of molecules. Here, the ligand-induced structural transformation of Ag(5)(+)(g) by 1,4-cyclohexadiene, which is capable of binding metal clusters as a bidentate ligand, is investigated using equilibrium mass spectrometry experiments and theory. Based on the measured sequential ligand binding free energies of Ag(n)(+)(cyclohexene)(m) and Ag(n)(+)(1,4-cyclohexadiene)(m) (n = 3 and 5; m up to 3), it is found that Ag(5)(+)(1,4-cyclohexadiene) is a particularly stable cluster relative to the other ion-molecule association complexes investigated. These results together with those from electronic structure calculations suggest that upon addition of 1,4-cyclohexadiene to Ag(5)(+), the metal cluster core undergoes a structural transformation from a "bowtie" structure(s), in which two Ag(2) units are bridged side-on by a central Ag atom, into a bidentate Ag(5)(+)(1,4-cyclohexadiene) structure that resembles a "razorback" arrangement of the five Ag atoms. These results raise the prospect of using multidentate ligands to transform larger ionic silver clusters from relatively compact 3D geometries into 2D elongated "razorback" nanowires. However, results from electronic structure calculations for clusters in which the razorback nanowire structural motif is propagated to larger sizes (up to Ag(9)(+)) indicate that the energy required to form such templated structures becomes increasingly unfavourable with increasing size. By calculating the vertical and adiabatic ligand binding energies, the competing effects that contribute to the energy required to form such structures, such as the metal cluster reorganization energy, can be quantified. These results indicate that the tendency for metal clusters to form compact shapes dominates other effects that contribute to the energy for forming templated nanowire structures, and this effect dramatically increases with increasing cluster size.     

Silicon clusters: chemistry and structure

The chemical reactions of size selected silicon cluster ions (containing up to 70 atoms) have been studied with a number of different reagents using injected ion drift tube techniques. Both kinetic and equilibrium measurements have been performed as a function of temperature, and the influence of cluster annealing on chemical reactivity explored. Unlike metal clusters, where bulk behavior appears to be approached with around 30 atoms, large silicon clusters (n up to 70) are much less reactive than bulk silicon surfaces. These results suggest that the clusters in the size range examined here are not small crystals of bulk silicon, but have compact, high coordination number structures with few dangling bonds.     

Size and charge effects on the binding of CO to late transition metal clusters

We report on the size and charge dependence of the C-O stretching frequency, nu(CO), in complexes of CO with gas phase anionic, neutral, and cationic cobalt clusters (Co(n)CO(-0+)), anionic, neutral, and cationic rhodium clusters (Rh(n)CO(-0+)), and cationic nickel clusters (Ni(n)CO(+)) for n up to 37. We develop models, based on the established vibrational spectroscopy of organometallic carbonyl compounds, to understand how cluster size and charge relate to nu(CO) in these complexes. The dominating factor is the available electron density for backdonation from the metal to the CO pi* orbital. Electrostatic effects play a significant but minor role. For the charged clusters, the size trends are related to the dilution of the charge density at the binding site on the cluster as n increases. At large n, nu(CO) approaches asymptotes that are not the same as found for nu(CO) on the single crystal metal surfaces, reflecting differences between binding sites on medium sized clusters and the more highly coordinated metal surface sites.     

银团簇结构与特性的分子动力学研究

用分子动力学方法模拟了银团簇的结构与力能学.计算模拟中使用了一种基于第一性原理的原子间互作用多体势函数.通过分子动力学模拟确定了银微团簇(原子个数3～13)的稳态结构;模拟了原子个数为13～141的银FCC晶体结构理想球形团簇的力能学,发现球形银团簇形成三雏紧密结构;计算了平均结合能,给出了结合能随团簇原子数N的变化图,发现随N增大团簇结合能逐渐接近块材的数值.     

Superbase route to supertetrahedral chalcogenide clusters

Supertetrahedral Tn clusters are exact fragments of a cubic ZnS type lattice. Thus far, Tn clusters up to T4 with 20 metal sites can be synthesized in a discrete molecular form. Yet, synthesis of larger discrete supertetrahedral clusters still remains a great challenge, likely due to the rapidly increasing negative charge on the cluster as the size goes up. By using organic superbases (DBN and DBU) to help stabilize the negative charge, a family of discrete supertetrahedral chalcogenide clusters with sizes spanning from T3 (10 metal sites) to T5 (35 metal sites) have been made. The T5 cluster represents the largest molecular supertetrahedral Tn cluster known to date.     

Adsorbate binding energies for ammonia on cobalt and nickel clusters

Equilibrium reactions of ammonia with cobalt and nickel clusters are analyzed to determine cluster-adsorbate binding energies. The temperature dependence of reaction equilibrium constantsK _eq are measured and ?ΔH ⁰ values obtained from plots of lnK _eq vs 1/T. We find that binding energies generally decrease with increasing ammonia coverage, and that for a given number of NH₃ molecules binding energies increase with increasing cluster size. The pattern of binding energies is found to be consistent with proposed geometrical structures for Co₁₉ and for clusters in the 55-atom size range. Cluster-ammonia binding energies are generally somewhat higher than for bulk metal surfaces, an expected result considering the character of the cluster surface and the nature of the NH₃-metal interaction.     

Pushing up the size limit of chalcogenide supertetrahedral clusters: two- and three-dimensional photoluminescent open frameworks from (Cu(5)In(30)S(54))(13-) clusters

Direct band gap copper indium chalcogenides are of great technological importance in part because of their high photovoltaic conversion efficiency. Covalent superlattices constructed from copper indium chalcogenide clusters are of particular interest because they may combine open framework architecture with semiconducting properties. Here two photoluminescent covalent superlattices built from core-shell type copper indium sulfide supertetrahedral clusters are reported. Each cluster consists of 35 metal cations and is so far the largest known supertetrahedral cluster with a metal-to-metal distance of 1.6 nm. In addition, this is the first example of supertetrahedral clusters in heterometallic copper indium chalcogenides. The preparation of these large clusters has narrowed down the size gap between colloidal nanoclusters and small supertetrahedral clusters and revealed new possibilities in the construction of nanoporous semiconducting superlattices with tunable pore size. Through the combination of metal ions with different oxidation states to provide both overall and local charge neutrality, an effective approach has been demonstrated in the rational synthesis of chalcogenide open framework materials with large and unprecedented supertetrahedral clusters.