过渡金属异核原子簇化学键研究 1: Ⅷ-Ⅷ, ⅥB-Ⅷ族双金属原子簇电子结构研究

本文对18个Ⅷ族双金属四面体簇和16个ⅥB-Ⅷ异金属四核簇进行了量子化学研究, 用DV-Xα方法讨论了它们的化学键、电荷转移、能级态密度。计算结果表明: Ⅷ族四面体簇需36个金属电子, 其中12个形成6个金属簇骼轨道, 24个与配体成键; ⅥB-Ⅷ异金属簇核中, 因两金属能带、电负性差异, ⅥB原子易向Ⅷ原子转移电荷, 环戊二烯基配体促进这一过程; 异金属簇能级总价带比单金属簇收缩, 而d能带比单金属簇展宽。     

Electronic structure of Cr clusters on graphite

The electronic structure of small chromium clusters deposited by evaporation onto clean polycrystalline graphite has been studied by means of Auger, X-ray Photoemission (XPS) and Electron Energy Loss (EELS) spectroscopies. The XPS results show an increase in the binding energy of both core levels and valence band reducing the cluster size. The EELS measurements show a variation of the intensity ratio of L₃-to-L₂ ionization core edges. We interpret this change as due to different redistribution, within the clusterd-band, of the empty states above the Fermi level. As a consequence the XPS results may also be attributed to sizeable change of the electronic structure of the small clusters.     

Oxygen adsorption at anionic free and supported Au clusters

The structure, stability, and O2 adsorption properties of anionic Au(n) (n=1-11) clusters either free or supported at defected MgO100 surfaces are investigated using density-functional theory. O2 adsorption is strong whenever unpaired electrons are present, except for at some small, supported, planar, high-band-gap clusters. These clusters have the unpaired electrons pinned by the Madelung potential of the support. Larger clusters (starting at Au7-Au8) become three dimensional and metallic. This ensures that while one cluster orbital is pinned to the defect, another orbital at comparable energy can undergo depletion, thus binding O2 with charge transfer.     

Real-time study of the adiabatic energy loss in an atomic collision with a metal cluster

Gas-phase hydrogen atoms are accelerated towards metallic surfaces in their vicinity. As it approaches the surface, the velocity of an atom increases and this motion excites the metallic electrons, causing energy loss to the atom. This dissipative dynamics is frequently described as atomic motion under friction, where the friction coefficient is obtained from ab initio calculations assuming a weak interaction and slow atom. This paper tests the aforementioned approach by comparing to a real-time Ehrenfest molecular dynamics simulation of such a process. The electrons are treated realistically using standard approximations to time-dependent density functional theory. We find indeed that the electronic excitations produce a friction-like force on the atom. However, the friction coefficient strongly depends on the direction of the motion of the atom: it is large when the atom is moving towards the cluster and much smaller when the atom is moving away. It is concluded that a revision of the model for energy dissipation at metallic surfaces, at least for clusters, may be necessary.     

Electronic structure of crystalline uranium nitride: LCAO DFT calculations

The results of electronic structure calculations performed for the first time for crystalline uranium nitride and using a LCAO basis are discussed. For calculations we used the density functional method with the PW91 exchange correlation potential and a variety of relativistic core potentials for the uranium atom. The calculated atomization energy of the crystal agrees well with the experimental data and with the results of calculations with the plane wave basis. It is shown that a chemical bond in crystalline uranium nitride is a metal covalent bond. The metal component of the bond is due to the 5f electrons localized on the uranium atom and having energies near the Fermi level and the bottom of the conduction band. The covalent component of the chemical bond results from an overlap between the uranium 6d and 7s valence orbitals and the nitrogen 2p atomic orbitals. Inclusion of the 5f electrons in the core of the uranium atom introduces relatively minor changes in the calculated binding energy and electron density distribution.     

Electronic states and structural stability of supported Au clusters

The quantized energy levels of electrons in supported nanometer-size Au clusters have been resolved at room temperature using field emission techniques. By studying the time dependence of the electron emission current from an individual supported cluster, information about the structural stability of the cluster can also be obtained. Studies show abrupt jumps between different emission rates that are revisited as time progresses. This phenomenon can be attributed to a rearrangement of the cluster structure and/or orientation on the substrate and provides new evidence of multiple ‘isomeric’ structures for small clusters of metallic atoms.     

Comparison of the periodic slab approach with the finite cluster description of metal–organic interfaces at the example of PTCDA on Ag(110)

We present a comparative study of metal–organic interface properties obtained from dispersion corrected density functional theory calculations based on two different approaches: the periodic slab‐supercell technique and cluster models with 32–290 Ag atoms. Fermi smearing and fixing of cluster borders are required to make the cluster calculation feasible and realistic. The considered adsorption structure and energy of a PTCDA molecule on the Ag(110) surface is not well reproduced with clusters containing only two metallic layers. However, all clusters with four layers of silver atoms and sufficient lateral extension reproduce the adsorbate structure within 0.04 Å with respect to the slab‐supercell structure and provide adsorption energies of ( 0.08 eV) consistent with the slab result of −4.47 eV. Thus, metal–organic adsorbate systems can be realistically represented by properly defined cluster models. © 2018 Wiley Periodicals, Inc.     

Designing New Materials Using Atomic Clusters

Designing principles for forming stable metallic clusters whose chemistry mimics different atoms of the periodic table are discussed. It is shown that while bulk Al is a metal, the chemistry of an Al₁₃ resembles that of a halogen atom, a CAl₁₂ resembles an inert atom, while a NAl₁₂ resembles an alkali atom. The feasibility of making new materials using clusters as the building blocks is discussed. An ionic solid made out of Al₁₃ (or BAl₁₂) and Cs is shown to be metastable and marked by a large gap at the Fermi energy even though bulk Al and Cs are metals.     

非晶态合金Fe80P20局域结构和性质的量子化学研究

According to the structure features of Fe80P20, A series of clusters Fe4P were designed and focused on studying the stability of local structure, charge distribution and chemical bond. Using the DFT method,energy and structure of Fe4P clusters were optimized and analyzed. The computational results showed that the energy of cluster 1(2) has the lowest energy, and the possibility of its existence in the Fe8oP20 is high. Analyzing the transition states among the clusters, it was found that the clusters in the doublet state are more stable than those in the quartet state. The numbers of the Fe-P bond in the clusters play important roles in the cluster stability and electrons transfer properties. The more numbers of Fe-P bonds in the clusters, the higher the cluster stability, and the weaker the ability of P atom to get electron. The number of Fe atoms, which has bonding interactions with the P atom, is direct proportional to the average 3d orbit population of Fe atom. Basing on the orbital population, average magnetic moments of each Fe atom in the Fe4P clusters were calculated, and they are all smaller than that of single metal Fe atom. This suggests that all Fe4P clusters have soft magnetic property and they are expected to be perfect material for preparing soft magnetic apparatus.     

Simplified semiclassical theory for the electronic shell structure of metallic clusters

We present a simplified theory for the electronic shell structure of large metallic clusters by extending the semiclassical analysis by Gutzwiller, Balian and Bloch. Thus analytical results are given for the shell effect in the Fermi energy and cohesive energy. In particular, results for the temperature dependence of the shell structure are given. New results are presented for the shell structure in the ionization potential and photoyield and for the existence of electronic- versus atomic shell structure as a function of cluster size and temperature. We obtain good quantitative agreement with previous numerical calculations and with experiments on Na clusters.     

The effect of 3d-metal intercalation on the electronic structure of metallic and semiconducting nanotubes

The electronic structure of 3d-metal-intercalated metallic (5,5) and semiconducting (10,0) nanotubes has been studied by quantum-chemical methods. The total and partial densities of states of nanotubes as a function of metal concentration and nature and the carbon-shell structure have been calculated by the linear augmented-cylindrical-wave method. Metalized nanowires based on armchair (5,5) and zigzag (10,0) nanotubes with one, two, three, and four metal atoms in the cross-section have been calculated. The introduction of the metal is accompanied by a sharp increase in the density of states at the Fermi level of the nanowire, which determines the concentration of free electrons involved in charge transfer in the nanotube. The 3d electrons of the metal and the carbon shell are nearly equally involved in electron transport in intercalated wires. Both the 3d electrons of a metal and the carbon shell should be nearly equally involved in electron transport in intercalated wires. The introduction of metals not only affects the conductive state of the carbon nanotube but also changes the entire pattern of its valence band, in particular, increases the valence band width of the nanotube by 5–10 eV owing to the low-energy shift of the 2s(C) states.     

1S core-level shifts of Al and Ar atoms in aluminum clusters

Theoretical 1s core-electron binding energies are presented for Al and Ar atoms in free space and in AlAl₁₂, AlAl₁₂Al₆, and ArAl₁₂ clusters. The binding energies have been calculated by the self-consistent field Xα scattered-wave (SCF Xα SW ) method using various exchange parameters and different atomic-sphere overlaps. The atom/cluster binding-energy shifts have been obtained both from the Slater's transition-state energies and from the total-energy differences; these values are in better agreement with each other if calculated with proper overlapping than if with nonoverlapping spheres. A comparison with available experimental and theoretical data is given as well.     

Density Functional Theory Study on the Optical Properties of Mn-doped GaN

The absorption and emission spectra of the wurtzite Mn-doped GaN were calculated with cluster models.The predicted lattice parameters become slightly larger than those of undoped cluster.The average bond length of Mn-N is longer than that of Ga-N.Spin density shows that one Mn atom in these clusters has four single electrons with the same direction of the spin polarity.The new energy level with light Mn-doping appears at 1.37 eV above the valance band.The absorption spectra of Mn-doped GaN cover the visible light region.The calculated emission spectra show that the green luminescence of GaN material in experiment did not result from Mn dopant.With the increase of Mn doping,the emission intensity of yellow or blue band increases to different extent and the band-to-band emission band shows red shift from peak at 3.34 to 3.24 eV.     

Experimental Studies and Observations of Clusters of Rydberg Matter and Its Extreme Forms

A review is given of experimental studies of clusters of Rydberg matter (RM) with applications. This is a metallic type of matter with angular momentum l ≥ 1 for the conduction band electrons. The atoms in RM can be one-electron atoms like K, H and D which are most used in the laboratory, or two-electron atoms like He. Mixed clusters of one-electron atoms are easily studied. This material is stable in a vacuum both in the laboratory and in interstellar space. The large stability is valid for the lowest excitation level for each atom type, at l = 1 for H(1) which is the lowest energy state for protium. Typical clusters of RM are planar with magic numbers 7, 19, 37, 61 and 91. Small planar cluster can form stacks of clusters. Close-packed clusters exist for H(1) with magic numbers 4, 6, and 12, and chain clusters mainly formed by D–D “beads” are common for D(1). Several methods to study RM clusters are reviewed. Stimulated spectroscopy methods like stimulated emission and stimulated Raman are emphasized. Cluster properties like large polarizability and giant magnetic dipoles are described. Results on rotational levels, vibrational levels and electronic levels are reviewed. The observational evidence for RM clusters in space is discussed in relation to the fundamental experimental studies of the specific cluster properties.     

原子簇NixFe和NiFex(x=1-5)电子性质的DFT研究

More than one hundred models were designed to reflect the local structure and electronic property of Ni-Fe amorphous alloys. After calculating by DFF method, a series of configurations of clusters NixFe and NiFex （x = 1 - 5） were gained. The configurations, which possessed the lowest energies and non-imaginary frequencies, were considered the most stable optimized structures. The catalytic activity, charge and magnetic properties were analyzed and discussed. The different Fe content changed the catalytic properties of clusters through altering the value of Fermi level of every cluster. However the density of state （DOS） nearby Fermi level and average 3d orbital population of atom Ni, which were also important properties related to the catalytic activation, were little changed. Based on the Fermi level, the activity of catalyst toward hydrogenation reaction would be considered best when the ratio of Ni to Fe was close to 1. The Fermi level of clusters was far distant to the level of nitrogen in singlet state. It would be the reason why the reaction condition in ammonia synthesis and nitrogen fixation process was rigorous. When Fe atom contents were higher than 75% （NiFe3）, the electrons transferred from atom Fe to Ni, but when the ratio was decreased, the transfer was reversed. The ratio of atoms of local structure also played an important role in the aspect of electron transition. On the average 3d orbital population of atom Fe, the average magnetic moments of Fe atoms in clusters were calculated. When Fe atom contents were 50% nearly, the average magnetic moment achieved the highest point.     

The electronic structures and bonding of some transition-metal monoborides

The band structures and densities of states for the isostructural monoborides of Ti, Mn, Fe, and Co have been calculated by a LCAO-TB method. These results are related to the physical and spectroscopic properties of the materials. A satisfactory account of the ESCA spectra and a qualitative correlation with the electrical and magnetic properties is afforded by the calculations. A second series of calculations by the periodic cluster method is also carried out in order to reveal more details of the bonding and the effects of diffusion of electrons into the unoccupied levels above the Fermi edge.     

First-principles investigation of the electronic and field emission properties of C-doped ZnO nanotube

In order to search for novel field emitter nanomaterial, a density functional theory investigation is performed to understand electronic structures and field emission properties of carbon doped–ZnO nanotube. It has been revealed that electron transport through ZnONT is significantly increased in the presence of the carbon atom due to the reduced HOMO–LUMO energy gap, which makes the electrons easily excited from HOMO to LUMO, and then the electrons can easily emit. Comparing the ionization potentials of the pure and doped ZnONT, at the same external electric field strength, the ionization potential of C–doped ZnO nanotube is lower than that of pure one. Also, after the doping of carbon atom, the Fermi level of ZnONT increases, which indicates that the Fermi level shifts toward the conduction band. These results indicate that the field emission properties of ZnONT can be enhanced by the doping of ZnO nanotube with the carbon atom.     

Density functional study of small neutral and charged silver cluster hydrides

Small neutral, anionic, and cationic silver cluster hydrides AgnH and anionic HAgnH (n=1-7) have been studied using the PW91PW91 density functional method. It was found that the most stable structure of the AgnH complex (neutral or charged) does not always come from that of the lowest energy bare silver cluster plus an attached H atom. Among various possible adsorption sites, the bridge site is energetically preferred for the cationic and most cases of neutral Agn. For anionic Agn, the top site is preferred for smaller Agn within n相似文献   

Metal-oxide interfacial reactions: encapsulation of Pd on TiO2 (110)

The model system Pd/TiO2 (110) was used to evaluate the correlation between metal encapsulation and electronic structure of TiO2 crystals. We observed encapsulation of Pd clusters supported on TiO2 crystals, which were heavily Ar+ sputtered, Nb-doped, or reduced by vacuum annealing. In contrast, encapsulation was not observed on unreduced, undoped, or slightly sputtered TiO2 crystals. Our results indicate a strong dependence of the encapsulation process on the electron density in the conduction band of TiO2 and on the space charge formed at Pd/TiO2 interfaces. This behavior is controlled by the initial position of the Fermi energy level (EF) of the metal and the oxide before contact is established. We proved that encapsulation reactions are favored by n-type doping of the oxide and a large work function of the metal. On the basis of this mechanism, we conclude on general trends controlling encapsulation reactions of oxide-supported metal clusters and the strong metal-support interaction (SMSI).